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Technical Information Sheets Our technical knowledge is something we are proud to share with you. The Technical Information sheets are presently available in English, German and Russian. Problems opening the documents? Please your local Elkem representative or our Headquarters. English German Russian

Title

01 Oxidation of Ferrosilicon Alloys During Storage 02 Inoculation of Cast Iron 03 Inoculant Alloy Composition 04 Inoculation Practices 05 Inoculation Mechanisms 06 Fading of Inoculation 07 Magnesium Contents in Ductile Iron 08 Sampling of Liquid Cast Iron 09 Magnesium Treatment Processes 10 Tundish Cover Ladle Nodularization 11The "Sandwich Pocket Process" 12 Effects of Minor and Trace Elements in Cast Iron 13 Compacted Graphite Iron 14 Ferroalloy Storage Hopper Design 15 Selection of Inoculants for Grey Cast Iron 16 Selection of Inoculants for Ductile Cast Iron 17 Recommended Target Analysis for Grey Cast Iron 18 Recommended Target Analysis for Ductile Cast Iron 19 Aluminium in Cast Iron 20 Selection of Nodularizers 21 Heat Conservation in Liquid Iron Revised! 22 Late Metal Stream Inoculation 23 Factors Influencing the Recovery and Addition of Magnesium in Ductile Iron Ladle Treatment Processes 24 Partition of Slag Phases in the Treatment and Pouring of Ductile Iron 25 Poor Nodularity in Ductile Iron 26 Fading Nodularity in Ductile Iron 27 Alternative Tundish Ladle Design 28 Magnesium versus Sulphur in Ductile Iron 29 Nitrogen Blowholes 30 Hydrogen Pinholes 31 Carbon Monoxide Blowholes in Grey Iron

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32 Magnesium Slag Defects in Ductile Iron 33 Slag Defects in Grey Iron 34 Internal Shrinkage Porosity 35 In-the-Mould Nodularisation 36 Inoculation of Heavy Section Castings 37 Characterisation of Molybdenum Containing Phases in SiMo Ductile Iron The files are best viewed in Adobe Reader 6 or higher.

Last modified: 2010-05-12

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Technical Information 1 Oxidation of Ferrosilicon Alloys During Storage Ferrosilicon alloys, such as those used for inoculation or the production of nodular graphite irons, are subject to surface oxidation if exposed to moisture or high levels of humidity during transport and storage. The oxidation can become a serious problem particularly with the more finely divided, crushed and graded materials, leading to loss of efficiency when added to liquid iron. This may mean the use of larger additions and, consequently, increased costs and possible slag problems. The level of oxidation can be related to the oxygen content of the alloy. The oxygen contents of inoculants and MgFeSi-alloys have been determined as a function of storage time when stored under dry conditions and under high humidity. The figure below is typical of the results obtained. Very small changes in the oxygen content occurred with alloys stored under dry conditions while alloys subject to wet storage showed severe tendencies to oxidation.

Degree of oxidation for ferrosilicon stored in dry and humid storage.

Important Precautions for Storage of Ferrosilicon Alloys: 1. Transportation of ferroalloys should always be carried out in sealed containers or other watertight units such as big bags or steel drums. Loads should be well sheeted to protect the units from rain and spray. 2. If ferroalloys have to be stored outside they should be kept in closed, watertight drums or bags to avoid exposure to water or rain. Large changes in temperature (e.g. exposure to sunlight) should be avoided in order to minimize the risk of condensation. 3. The best way to avoid the oxidation of ferrosilicon alloys is to store them in a dry warehouse having constant temperature.

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Office address Hoffsveien 65B Oslo Norway

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Revision No. 2.1 20.03.2004

Technical Information 2 Inoculation of Cast Iron Inoculation is a means of controlling the structure and properties of cast irons by increasing the number of nucleation sites available for the growth of graphite flakes in grey irons or graphite nodules in ductile irons. This reduces undercooling during eutectic solidification thereby minimizing the risk of forming hard iron carbides or "chill" in the structure, particularly in thin sections. An inoculant is a material added to the liquid iron just before casting that provides suitable sites for the nucleation of graphite. The most effective inoculants are ferrosilicon alloys containing small amounts of one or more of the elements Ca, Ba, Sr, Zr and/or Ce. The micrographs and table below show examples of structures and properties obtained in grey and ductile iron with and without the addition of an inoculant. As seen from the micrographs, the uninoculated castings (left) contain large quantities of hard, brittle iron carbides (cementite, Fe3C) and very poor graphite structures. The inoculated castings (right) contain uniform structures of small, random oriented flakes (grey iron) and a large number of small graphite nodules in a ferrite/pearlite matrix (ductile iron).

Graphite structure of uninoculated grey cast iron (100X).

Graphite structure of inoculated grey cast iron (100X).

Microstructure of uninoculated ductile cast iron (100X).

Microstructure of inoculated ductile cast iron (100X).





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Technical Information 2

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Typical properties of uninoculated compared to inoculated ductile iron. Property

Uninoculated

Inoculated

Rp0.2

Not detected

200 - 400 MPa

Tensile Strength

Rm

< 300 MPa

350 - 800 MPa

Elongation

A5 HB

Not detected

3 - 30 %

> 600

140 - 300

Proof Strength

Brinell Hardness Nodule Count

10 mm section

< 50 per mm

Microstructure

ASTM Classification

Carbidic

2

> 150 per mm

2

Ferritic and/or Pearlitic

Important Benefits of Inoculation: 1. Eliminate the formation of hard, brittle iron carbides (cementite) often referred to as "chill" in the structure and promote the formation of graphite during eutectic solidification. 2. Improve machinability and mechanical properties and reduce variations due to changes in section size. 3. Increase the number of graphite nodules in ductile irons thereby producing finer, more uniform structures over a range of section thicknesses. Such structures promote improved mechanical properties, a reduction in the segregation tendency of some alloying or trace elements in the iron and give better machinability. Note that certain iron conditions, for example initial sulphur content (grey iron), temperature and total “fade” time will affect the selection of a proprietary inoculant. Reference should be made to Elkem Technical Information Sheets No. 15 and 16 before selecting an inoculant for use. For more detailed information on inoculation and proprietary inoculants see Elkem Brochures: "Cast Iron Inoculation", "Foundrisil® Inoculant", and "Superseed® Inoculant".

Technical Information 3 Inoculant Alloy Composition Ferrosilicon alloys for use, as inoculants for cast irons must contain small amounts of one or more elements in order to obtain the required effects in the iron. Generally, inoculants may contain controlled amounts of several alloying elements in order to improve their effects but the most important elements in commercial inoculants are: Primary elements

Beneficial elements

Calcium

Zirconium

Barium

Cerium (Rare Earth’s)

Strontium

Aluminium Sulphur Oxygen

Strontium differs from the other elements in that it is only fully effective in the absence of calcium and aluminium, whereas the other alloys can benefit from being in combination. No FeSi-based inoculant will be effective without balanced additions of one or more of these elements. The figure below shows a schematic example of inoculation effect as a function of total reactive element content in the inoculant (i.e. Ca, Ba, Sr, etc.), and shows that optimum effect is obtained at concentrations above 0.5 per cent.

Inoculation effect (e.g. nodule count or chill reduction) as a function oftotal reactive element content in a ferrosilicon inoculant.





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Important Factors Regarding Inoculant Composition: 1. Contents of strontium, calcium or barium between 0.6 to 1.5 per cent normally give the required level of inoculation in cast irons under most foundry conditions. 2. When the strontium content or the sum of calcium plus barium content falls below about 0.5 per cent the inoculating effect will be significantly reduced and carbides or chill may occur. 3. Alloy contents above about 1.5 per cent may give improved inoculation under some conditions but may also give a greater tendency to produce slag or dross. 4. Grey irons with sulphur contents below about 0.05 per cent may only respond to certain specialised inoculants (e.g. strontium plus zirconium containing). 5. Inoculants alloyed with strontium are extremely effective for treating most grey irons but may be less effective in ductile irons containing high levels of rare earth’s as part of their nodularizing treatment. For more detailed information on inoculant composition please refer to Elkem Brochures : "Cast Iron Inoculation", "Foundrisil® Inoculant", and "Superseed® Inoculant".

Technical Information 4 Inoculation Practices Inoculants are generally added to cast irons at one or more of three stages during the casting procedure: 1. To the pouring ladle during filling. 2. To the stream of metal as it enters the mould. 3. Using an insert placed strategically in the mould runner system. Factors influencing the choice of inoculating method are: 1. The time from filling the ladle to pouring the last casting, commonly known as the fade time. 2. Metal temperature. 3. Ability to add the inoculant at a particular point in the process. 4. Suitability of the casting system to late stream inoculation. Inoculation to the Ladle Due to the unavoidable lengths of time involved in handling ladles, it is necessary to add relatively large amounts of inoculant to offset the fading losses which occur. Addition rates vary from 0.2% for the majority of grey irons to 0.75% for the most critical ductile irons. Inoculant alloys should be selected according to ladle size and be dust free thus avoiding losses due to oxidation or thermal air currents. Generally, ladles up to 300 kgs can use a 0.5-3 mm grading, and for ladle sizes above this a 1-6 mm material is recommended. In order to obtain the highest efficiency from the inoculant, simple addition rules should be followed: 1. Add the inoculant to the stream of metal entering the ladle, not as an addition prior to filling. 2. Trickle the inoculant into the metal stream as the ladle is between 25% and 75% full. This ensures good mixing and solution. 3. Ensure that the metal is slag free before tapping into the ladle. Inoculant trapped within the slag is wasted. 4. When several transfers of metal between ladles are involved, add the inoculant during the last transfer before pouring to minimise fade. NOTE; Inoculant should never be added to the bottom of the ladle prior to tapping, particularly if the ladle is red hot or if there is a small amount of metal remaining from a previous cast.





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When inoculating ductile irons, it is essential to add inoculant only when the magnesium reaction is finished. Adding inoculant with the nodularising agent or during the reaction will significantly reduce the effectiveness of the inoculant and may result in increased carbides in the castings. In cases where it is necessary to add nodulariser and inoculant in the same ladle, the tap should be halted when about 2/3 of the iron has been poured onto the nodulariser. Then wait until the reaction has finished and then add the inoculant to the stream of remaining metal as described above. Inoculation in the Casting Stream Late metal stream inoculation, addition of inoculant to the stream of metal entering the mould, virtually eliminates fade. As such, the addition rates are greatly reduced compared to conventional ladle treatment, 0.02-0.05% for grey irons, 0.05-0.2% for ductile being common. The inoculant has to be specially graded in 0.2-0.7 mm normally to ensure rapid solution in the iron and good flowability through the application machine. Specialist application machines are commercially available, however many foundries have designed and built screw feed mechanisms to give consistent addition rates during pouring. Late stream inoculation is most easily applied to static pouring stations or ladle transporters, application to a moving ladle is not readily achieved. Inoculation in the Mould Use of an insert made from pressed or cast inoculant can be used as insurance, rarely is this type of treatment used as the primary source of inoculation. Different size and composition tablets are available and prove particularly valuable when the fade time is long, acting as a secondary inoculation, or when late metal stream treatment is not possible. The possibility of human error in failing to add the tablet to a mould does necessitate a high degree of post casting inspection, usually in the few cases where tablets are used as the only inoculant. Reference should also be made to Elkem Technical Information Sheets; No 2, “Inoculation of Cast Irons”. No 5, “Inoculation Mechanisms”. No 6, “Fading of Inoculation”.

Technical Information 5 Inoculation Mechanisms Several theories have been developed in the past to explain the mechanisms of graphite nucleation during solidification of cast iron. Most theories are based on the assumption that graphite is formed as a result of heterogeneous nucleation from non-metallic substrates during solidification and that minor elements such as Ca, Ba and Sr play an important role in the nucleation process. Ductile Iron In magnesium treated cast irons, micro-inclusions after treatment contain mainly magnesium, calcium, sulphur, silicon, and oxygen. These are primary reaction products of the magnesium treatment. The inclusions are composed of a sulphide core and a faceted outer silicate shell. The sulphide core contains both MgS and CaS, while the outer shell consists of complex magnesium silicates (e.g. MgSiO3, Mg2SiO4). These phases will not act as potent nucleation sites for graphite during solidification because of a large nucleus/graphite interfacial energy barrier. After inoculation with a Ca-containing ferrosilicon, hexagonal silicate phases of the CaSiO3 and the CaAl2Si2O8 type will form at the surface of the existing sulphide/oxide inclusions produced during nodularization. These calcium silicates will then act as very favourable sites for graphite nucleation during solidification, due to their hexagonal crystal structure that matches the graphite crystal lattice very well (i.e. low energy interface). The figure below to the left shows a typical micro-inclusion in ductile cast iron that is formed after nodularization, while the figure to the right gives a schematic representation of the inclusion composition after inoculation with ferrosilicon containing either Ca, Ba or Sr. Major constituent phases: XO SiO2 or XO Al2O3 2SiO2

Shell: MgO SiO2 2MgO 2SiO2 Core: MgS CaS

Where X = Ca, Sr or Ba

Duplex sulphide/oxide micro-inclusion in ductile cast iron

Schematic representation of an inclusion after inoculation with a X-containing ferrosilicon inoculant (X denotes Ca, Ba or Sr).

The surface shell contains hexagonal calcium silicates formed during inoculant addition, while the bulk particle is a product of the nodularization treatment. Hence, the inoculation does not increase the total number of nuclei particles in the melt, but rather modifies the surface of the already existing products from nodularization. This explains the important link between magnesium treatment and inoculation, and that the basis for effective ductile iron inoculation is laid during nodularization. The resulting nodule number density will also differ greatly due to the inclusion surface modification. Elkem ASA, Foundry Products Postal address P.O.Box 5211 Majorstuen NO-0302 Oslo Norway




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When inoculation is carried out with a strontium or barium containing ferrosilicon inoculant, hexagonal silicates equivalent to the calcium silicates will be formed (i.e. SrSiO3, SrAl2Si2O8, BaSiO3, BaAl2Si2O8). Grey Iron In grey iron the nucleation mechanisms differ somewhat from the situation in ductile iron, primarily due to the fact that magnesium is not added prior to inoculation. Consequently, other substrates will play the important role as sites for the Ca, Ba or Sr silicates formed during inoculation. In principal, the same types of inoculants can be used for both grey and ductile irons, and the inoculation mechanisms from hexagonal silicate phases at the surface of primary inclusions are equal for all types of irons. However, in grey cast iron the lack of primary magnesium sulphides and silicates means that other particles have to take the primary role. It is assumed that the presence of a high number of manganese sulphides (MnS) replaces the magnesium containing particles found in ductile iron. The relation between manganese and sulphur in grey cast iron should be as follows: %Mn = 1.7 x %S + 0.3 When this balance is correct, the optimum conditions for the formation of small MnS particles is obtained. Furthermore, to have a highest possible number density of MnS particles as a basis for effective inoculation, the sulphur content of grey irons should be significantly higher than for ductile irons. Normally, a sulphur content between 0.05 and 0.15% is recommended. Some Important Considerations •

Pure ferrosilicon has no inoculation effect whatsoever.

•

Inoculation does not increase the number of potential nucleation sites in grey and ductile iron but modifies existing sites to a beneficial form.

•

The important consideration in effective inoculation is the formation of a high number of non-metallic micro-particles (sulphides and oxides) during magnesium treatment of ductile iron and the formation of numerous manganese sulphide particles in grey iron.

•

Minor elements such as Ca, Ba and Sr as well as silicon in inoculants are critical for powerful effectiveness of an inoculant material.

•

The base metal oxygen content is also critical in the formation of effective oxide substrates for graphite nucleation.

Technical Information 6 Fading of Inoculation Inoculants for cast irons produce their effects by creating a marked increase in the number of nucleation sites suitable for graphite growth during solidification of the eutectic. These become apparent as an increase in the number of eutectic cells in grey irons and in the number of graphite nodules in ductile irons. This results in a reduction in undercooling thus minimizing the risk of forming iron carbides or "chill" particularly in thin, rapidly cooled sections. The effects of inoculation are at a maximum immediately after making the addition and they fade with time. The rate of fading depends on: •

The inoculant composition;

•

The type of iron to which it is added;

•

Temperature;

•

Surface energies;

•

Diffusion rates.

Fading may be very rapid with much of the effect lost within the first few minutes after addition. Fading of inoculation can be explained by the coalescing and re-solution of the nuclei population which causes the total number of potential nucleation sites to be reduced (i.e. growth or coarsening of nuclei particles according to the so-called Ostwald Ripening effect). This behaviour is in close agreement with experimental observations of fading as illustrated in the following figures.

Reduction in nuclei population (micro-inclusions) with time



Fading characteristics of inoculation.



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Principal Results of Fading: 1. To cause greater undercooling to take place during eutectic solidification thus leading to an increased tendency to chilling in grey and ductile irons particularly in thin section. 2. To reduce the numbers of eutectic cells or graphite nodules in the structure and to cause deterioration in graphite form. Severe fading can promote the formation of undercooled graphite with associated ferrite in grey irons and significant amounts of non-nodular graphite in ductile irons, both of which can adversely affect the properties of the iron. Some Well Established Facts Concerning Inoculant Fade: •

The effects of all inoculants fade with time.

•

There is no period after inoculation during which zero fading occurs. To obtain maximum effect the metal must be cast as soon as possible after inoculation, the ultimate being addition of the inoculant to the pouring stream.

•

The effects of some inoculants fade more slowly than others depending on their composition and conditions of use.

•

An inoculant which gives a high eutectic cell number is not necessarily the most effective in reducing chill. In grey iron, Strontium-based inoculants are recognised to give the best combination of a coarse cell structure, low shrinkage tendency, and very low chill level.

•

Under any particular set of conditions it is not possible to predict the fading characteristics of an inoculant from its composition. Foundries therefore should carry out tests to determine which is the most suitable inoculant for their purpose. These tests should be made under careful control to avoid the spurious effects of factors such as metal temperature, inoculant storage, etc.

Technical Information 7 Magnesium Contents in Ductile Iron Magnesium is usually introduced into cast iron melts by the addition of a magnesium ferrosilicon alloy (MgFeSi or FeSiMg). When magnesium is added its first effect is to combine with any sulphur and oxygen present in the iron to form sulphides and oxides. No "free" magnesium can occur in solution in the iron to promote the formation of spheroidal graphite until all the sulphur and oxygen have been consumed. In order to cope with variations in the oxygen and sulphur contents of the base iron, a higher addition of magnesium is normally made than is strictly needed. This is in addition to that made to compensate for losses by evaporation during addition. Only a fraction of the magnesium is dissolved in the iron after the nodularizing reaction is complete. The total analytical or residual magnesium content of liquid iron immediately after treatment is comprised of: •

Dissolved magnesium;

•

Micro-inclusions of magnesium compounds (oxides and sulphides)

•

Larger, magnesium containing slag particles

These contributions to total magnesium will react in different ways during subsequent holding of the iron. A schematic example of the fading characteristics of the magnesium content on holding is given in the figure below. It is not possible to separate between these three contributions to the residual magnesium by conventional analytical methods that will only give the total magnesium content of the iron.

Figure 1: Fading of magnesium during holding of treated ductile iron (left), and schematic representation of magnesium losses from a treatment ladle (right).





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Important Characteristics of Magnesium Analysis and Magnesium Fading on Holding: •

The total residual magnesium content of ductile iron as analyzed is not the same as the dissolved magnesium content.

•

Fading of the magnesium content on holding treated iron may be the result of slag separation, inclusion flotation and evaporation loss of dissolved magnesium. In some instances magnesium fading can make a positive contribution to the metal cleanliness and freedom from slag entrapment since harmful slag particles will float to the bath surface with holding time and can hence be removed.

•

Only the total magnesium content (slags + micro-particles + dissolved) in a sample can be analyzed by ordinary analytical methods.

•

It has been shown that losses of dissolved magnesium on holding generally are small and that the degeneration of the spheroidal graphite structure often attributed to magnesium fade, is actually the result of fading of inoculation. Fully spheroidal graphite structures can often be regained by a small, late addition of inoculant.

Figure 2: Schematic representation of fading of graphite nodularity on holding. A second addition of inoculant can regain fully spheroidal graphite structures even though the analytical magnesium content is falling continuously.

Technical Information 8 Sampling of Liquid Cast Iron When taking a sample from liquid cast iron it is common to use a sampling spoon from which the metal is poured into a chill mould, which ensures a uniform sample for subsequent analysis. Obtaining a representative sample from the iron is not so simple, since the bulk liquid from which it is taken is not homogeneous with respect to minor and alloying element concentrations. Reactive elements in cast iron will be present both dissolved in the iron and combined with other elements to form particles such as oxides, sulphides, nitrides, etc. Such non-metallic particles will normally be lighter than the liquid iron, which means that there will be constant movement upwards of inclusions as the lighter particles float to the surface. Dissolved elements will also segregate in the liquid due to the difference in atomic weight with iron. Lighter elements and gas forming elements have a tendency to move upwards, while heavier elements may accumulate further down. For instance, Carbon (light) has a tendency to segregate upwards (even at temperatures well above the liquidus), while Lead (heavy) will be accumulated in the lower part of a vessel. The schematic figure below shows a container (e.g. furnace, ladle, holding furnace, pouring vessel, etc.) with a segregated liquid metal composition. Two sampling positions with examples of accompanying analyses are indicated. There will be a significant difference in composition at the top and bottom of the container due to the flotation and segregation phenomena.

Particle Flotation According to Stoke’s Law

. . .. ... ... . ....... .. . .... .. . . .. . . . . . . . . .. . . . . . . . . . .

.

. .

.

3.7 %C 2.6% Si 0.02% P 0.015% S 0.050% Mg 0.020% Al 3.6 %C 2.5% Si 0.02% P 0.010% S 0.030% Mg 0.010% Al

Schematic example of sampling from two different positions in a container with liquid iron. Some elements vary due to segregation with time.





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Segregation effects of the common elements: Element

Segregation effects

Carbon C

Present mainly as dissolved element above the liquidus temperature. Some graphite, carbides or supersaturated CO gas may however contribute to the analysis. A relatively strong segregation (flotation) effect occurs, especially in induction furnaces and in cold iron where free graphite particles form (below the liquidus temperature).

Silicon Si

Also present mainly as dissolved element in liquid iron. A significant part may however be oxides and silicates that segregate due to flotation. In ductile iron magnesium silicates will float and cause some silicon segregation

Sulphur S

In grey iron, Sulphur is present mainly dissolved and as manganese sulphides. In ductile iron, there will be no dissolved sulphur, and all is present as magnesium sulphides or calcium and rare earth sulphides. All sulphides segregate due to flotation

Magnesium In ductile iron Magnesium is present both as dissolved element and as sulphides and oxides/ silicates. Dissolved magnesium will segregate somewhat due to a low vapour pressure and Mg losses to the air. Combined magnesium as sulphides and oxides will segregate due to flotation. Aluminium Al

Will be present both as dissolved element and combined as oxides. In both grey and ductile iron various complex oxides and aluminates will all segregate due to flotation.

Flotation according to Stoke’s Law Typical non-metallic particles or inclusions in cast iron have a specific density between 2 and 4 g/cm3. Iron has a density close to 8 g/cm3, which means that most particles are less than half the density of iron. This will force them to float, and the main driving force determining the velocity is the size of the particles. Stoke’s law can be expressed as follows: v=

d m2 ( ρ s − ρ )g 18µ

where v is the flotation speed (m/s), dm is the diameter of the particle (m), ρs is the specific density of the liquid (iron) and ρ is the specific density of the particle phase (both in kg/m3), g is the gravity constant (9.81 m/s2), and µ is the viscosity of the liquid iron (Ns/m2 or kg/ms). A typical viscosity for liquid iron is about 0.007 kg/ms Important Consideration for Sampling •

The sample should always be taken in the same position and at the same time.

•

Stirring in induction furnaces or from pouring will equalise segregation. Samples should be taken during or soon after mixing actions.

•

Chemical analysis can be used to calibrate structure and properties, but cannot be compared to other foundries due to the sampling and segregation variables.

•

Analyses should be used with care and not trusted blindly. Variations from sample to sample must be accepted not only as a process variable, but also as a sampling variable.

•

During pouring of furnaces, lip-ladles, tea-pot ladles and bottom poured holders there will be analytical variations due to segregation. There exists no absolute analysis for a given volume of liquid metal.

•

Due to the flotation and segregation effects, bottom poured vessels as tea-pot ladles and some autopours will show an inverse fading effect for elements that accumulate in the upper, last part of the vessel to be poured.

Technical Information 9 Magnesium Treatment Processes This information sheet gives a comparison of the various magnesium treatment processes in common use in iron foundries. The important characteristics of the different processes are listed below for various treatment agents such as Ni/Mg alloy, MgFeSi alloys, Mg metal and Mg/Fe briquettes. The various treatment processes considered are: overpour or sandwich, tundish cover, in the mould, flow through, plunging, converter and cored wire. Overpour or Sandwich Treatment agent suited to process

Tundish Cover

In the Mould

Flow Through

Plunging

Converter

Cored Wire

Ni base alloy

MgFeSi alloy

MgFeSi alloy

MgFeSi alloy

MgFeSi alloy

MgFeSi or Mg/Si

Mg/Fe briquette

Mg

Mg/Si or Mg

Mg-content (%)

4 - 15

3 - 10

3 - 10

3 - 10

3-5

10 - 45

15 +

100

20 - 100

Mg-recovery (%)

45 - 90

35 - 70

50 - 80

70 - 80

30 - 50

30 - 60

30 - 50

30 - 50

30 - 50

Low

Medium

Medium

High

Medium

Cost of plant General process flexibility (6=best) Fume emitted

1)

Nil

Nil

Low

Nil

6

6

5

2

3

3

2

1

2

Low

Nil

Medium

High

High

High

High

3)

Yes

Yes

Yes

Yes

Medium

Med/high

Fume-collection required

Yes

Yes

No

Restrictions on base metal sulphur (%)

0.04

0.03

0.03

0.01

0.03

0.04

0.1

No

No

Treatment location point

Furnace

Furnace

Furnace

Mould

To suit

To suit

To suit

To suit

To suit

Treatment weight restrictions

None

None

None

< 500 kg

None

> 500 kg

> 500 kg

> 500 kg

> 500 kg

Inoculation effect from treatment

Nil

Very high

Med/high

Low

Nil

Nil

Nil/low

Violence of reaction Possible risk of excessive silicon

No

Low/med Med/high

2)

Low/med Med/high Low/med Very low

No

Medium

Yes

Yes

Yes, but unlikely

Small to large

Small to large

Medium to large

Small to medium

No

No

No

Yes

No

Size of production Small unit suited to process (alloyed) Licence/royalty

2)

No

Med/high Med/high Very high

High

Yes

Yes

No

No

No

Medium to large

Medium to large

Large

Medium to large

No

No

Yes

No

4)

1)

In the mould process requires specially designed running systems for each pattern. Tundish cover and in the mould are the two processes least likely to require the installation of fume extraction. 3) General fume extraction in the treatment area recommended. 4) Royalties for the process, not for the MgFeSi-alloy. 2)

The table is designed to help foundries starting production of ductile iron to select the most suitable process and treatment alloy for their specific foundry conditions. For many applications the tundish cover ladle process is likely to be the simplest to operate and to give the best consistency and highest magnesium recoveries combined with the lowest operating and maintenance costs and should therefore be a good choice for many small and large foundries. However, under certain conditions such as large plants dedicated to specific products, one of the other processes may be more suitable. Hence, each individual foundry must select carefully a process that will give the best overall consistency, economy and environmental control for their situation. Elkem ASA, Foundry Products Postal address P.O.Box 5211 Majorstuen NO-0302 Oslo Norway




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The figure below shows a schematic representation of how the most common treatment processes affect parameters as reaction violence, magnesium recovery, fume and slag formation, and inoculation effectiveness. There is a clear correlation between these four parameters, and all commercial treatment processes will be located along the shaded line-area of the figure. At the upper left end of the range, processes applying high magnesium containing materials such as the converter and cored wire process will be located, while at the lower right end highly efficient processes as the tundish cover and in mould process can be found. Typically, processes having a high degree of violence show a low magnesium recovery, high fume and slag formation and an iron that is difficult to inoculate. On the contrary, processes having a low degree of violence show a high magnesium recovery, low fume and slag formation and conditions of very good inoculation performance. No process will be located in the upper right or lower left corner of the diagram.

HIGH HIGH

Fume and slag formation

LOW LOW

Converter

*

Hi gh /pu

Violence of reaction

Plunging Cored Wire re M

g

Magnesium recovery

Overpour/Sandwich Flow through Tundish Lo w

LOW LOW

Inoculation effect

In mould Mg all oy s

*

HIGH

HIGH

Schematic representation of the important correlation between “violence of reaction”, “magnesium recovery”, “fume and slag formation”, and “inoculation effect” for a range of commercial ductile iron treatment processes.

Technical Information 10 Tundish Cover Ladle Nodularizing Magnesium treatment for nodularization of ductile iron can be made by several different treatment processes. The tundish cover ladle process will under most conditions, be a convenient, effective and reliable process with good economy. The process provides good consistency and high recoveries of magnesium over a wide range of treatment sizes. A well-operated system will give between 60 – 80 % recovery of magnesium while the operating and maintenance costs normally fall well below most other commercial processes. Tundish ladle treatments also offer virtually no flare, about 90 % fume reduction, no metal splashing and minimum carbon and temperature losses. The figure below shows an example of a tundish cover ladle.

Figure 1: Schematic representation of a tundish cover ladle with dual alloy pockets.

Calculation of Filling Hole Dimensions The following formula can be used to calculate the diameter of the tundish lid filling hole: d = 0.07

W t ⋅h

where d is the filling hole diameter in centimetres (cm), W is the liquid iron batch weight in grams (g), t is the pouring time in seconds (s), and h is the ferrostatic height of metal in the tundish basin in centimetres (cm). Note: h is the height of metal, not the height of the basin itself. Base metal sulphur contents should preferably not exceed 0.02% before treatment to ensure maximum efficiency of the tundish ladle. If sulphur levels are higher, a desulphurizing step is recommended prior to nodularization.





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Magnesium Alloy Selection and Addition The tundish cover process may be used with most magnesium ferrosilicon alloys containing between 3 to 12 % magnesium but generally alloys containing 4 – 6% Mg are used. Typically an alloy with the composition given in the table would be used to treat base iron composed of a mixture of steel scrap, pig iron and returns. Alloy size grading of about 1 – 10 mm is most suitable for small treatments while sizes up to 4 – 35 mm are preferable for larger treatments. See Elkem Technical Information Sheet No. 20 for more details on selection of nodularizers in ductile iron.

Element Contents

The amount of alloy added usually lies between 1.2 and 1.8 weight % depending on the base sulphur content, the metal temperature, the magnesium content of the alloy and the consistency with which the process is carried out.

Si

45%

Mg

6%

Ca

1%

RE

1%

Al

max. 1%

Fe

Balance

Cover Material Selection It is recommended to use a cover material over the magnesium alloy in the reaction chamber in order to obtain maximum treatment economy. The cover should retain the alloy in the chamber for as long as possible before the reaction starts. Covers commonly consist of clean steel plate or clippings of a grade similar to that used in the melt charges. Cast iron cover plates can be cast from the spare metal left at the end of a cast thus avoiding the carbon dilution due to the steel cover. However, the best results are obtained using a ferrosilicon alloy as a cover material. Example of Recovery Improvements

Figure 2: Schematic representation of recovery improvements as a function of sulphur content, treatment temperature, and ladle design modifications. The initial case of 2.0 wt% addition rate represents a situation with 0.03% S and 1520°C treatment temperature.

Technical Information 11 The “Sandwich Pocket Process” The widely used "Sandwich" process for the production of Ductile Cast Iron requires the use of magnesium ferrosilicon alloy that is carefully graded to match the size of the treatment. Even then, variations in magnesium yield can occur due to sporadic, explosive ejection of alloy from the reaction chamber early in the filling cycle thus leading to excessively large additions of alloy being required to compensate for these variations. The "Sandwich Pocket" process, developed by Elkem, has been subjected to extensive foundry trials and found to minimize many of the problems associated with the normal "sandwich" process. At the same time the economics are improved by the use of slightly smaller amounts of finely divided magnesium ferrosilicon alloy over a wide range of treatment sizes.

Figure 1: Schematic example of the Ladle design; (a) Pocket located in the centre of the ladle bottom, (b) Tea-Pot ladle alternative, (c) Charging funnel.

The advantages of the process arise principally from the use of the specially designed, cylindrical pocket in which the magnesium alloy and its cover material are placed. The pocket may be formed in the bottom of the ladle as shown in Figure 1(a) or, more frequently, it may be built at the periphery of the bottom as in Figure 1(b). The latter is essential when using the process in conjunction with a tundish cover (see information sheet No. 10). The cylindrical pocket must have a height to diameter ratio of at least 1:1 and must be large enough to contain the entire magnesium alloy and the cover material. No material must lie above the level of the ladle bottom after loading. Figure 1 indicates a method of filling the pocket through a steel tube (funnel) to avoid spilling the alloy on the ladle bottom. A thick layer of refractory material surrounds the pocket in order to avoid rapid heat penetration to the bottom of the pocket as the ladle is filled. Elkem ASA, Foundry Products Postal address P.O.Box 5211 Majorstuen NO-0302 Oslo Norway




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The magnesium alloy preferably has a relatively fine size grading. Optimum results are obtained with alloy sizes ranging from about 0.1 to 10 mm. Regular alloy qualities containing 3 to 7 per cent magnesium and normal levels of calcium, aluminium and rare earth’s may be used with this process. Figure 2 shows the alloy pocket and the table suggested pocket dimensions (diameter and height) for various treatment sizes and alloy addition rates. Typical alloy sizings suited to the process are 0.1-1 mm, 0.1-5 mm, 0.4-4 mm, 0.5-5 mm. 1.0-10 mm. All these sizings will have a bulk density of about 2 kg/dm3. The unique feature of using finely graded alloy is that it has a high packing density in the pocket. As the intense heat from the metal above the pocket penetrates the cover, the alloy tends to sinter into a briquette so that, when the reaction commences, it proceeds slowly and gently downwards through the sintered alloy. This leads to very good process consistency, high magnesium yields and minimum fume and smoke emission, particularly when used in conjunction with a tundish cover. Suggested pocket dimensions for various treatment sizes and alloy addition rates. Diameter (d) and hight (h) dimensions in centimetres (cm), respectively.

Figure 2: Sandwich pocket with alloy and cover material.

Addition rate, wt%

Treatment size, kg

0.9

1.0

1.1

1.2

1.3

1.4

100

9/10

10/10

9/12

10/12

10/13

10/13

300

13/15

13/15

13/18

14/18

14/18

15/18

500

14/20

15/20

16/20

16/20

17/20

18/20

800

18/20

19/20

20/20

20/22

21/22

21/22

1000

18/25

20/25

20/25

21/25

22/25

22/25

1500

22/25

23/25

24/25

25/25

26/25

27/25

The maximum process yields are obtained by using regular ferrosilicon alloy as the cover material. 45 or 75 per cent alloy with size gradings of 1 - 10 mm or 5 - 12 mm produce an effective cover for the fine magnesium alloy below. The ideal amount of cover depends on the temperature of the metal being treated but will normally be about 10 to 15 per cent by volume of the magnesium alloy. If the use of ferrosilicon as the cover material leads to excessive silicon pick-up, satisfactory results can also be obtained from the use of clean, high quality steel punchings as the cover. When using the "Sandwich Pocket" process it is important to keep the ladle in the inverted position when empty in order to avoid slag and metal beads draining into the pocket and reducing its capacity.

Technical Information 12 Effects of Minor and Trace Elements in Cast Iron Small quantities of many elements can occur in cast irons and have a marked influence on the structure and properties of the castings. Some are present as deliberate additions while others arise from trace impurities in the raw materials. Several of these elements have beneficial effects, particularly in grey irons, while others are very damaging and must be avoided as far as possible. The following table lists the common sources of these elements, the levels at which they are likely to occur and their principle effects. The use of some of these elements (e.g. chromium) as major alloying elements is not included in the table.

Element Aluminium Al

Antimony Sb Arsenic As Barium Ba Bismuth Bi Boron B Calcium Ca Cerium Ce Chromium Cr Cobalt Co Copper Cu Hydrogen H

Normal Levels (%) Effects in Cast Irons Promotes hydrogen pinholes in light sections Al-killed steel scrap, above about 0.005 %Al. Neutralizes nitrogen. inoculants, ferro-alloys, light Up to Promotes dross formation. Detrimental to nodular alloy components, additions 0.03 graphite above approx. 0.08%. May be neutralized of aluminium. by cerium. Strong graphite stabilizer. Steel scrap, vitreous Up to Strong pearlite and carbide promoter. enamel scrap, bearing 0.02 Inhibits nodularity in absence of rare earth’s. shells, deliberate additions Up to Strong pearlite and carbide promoter. Pig iron, steel scrap. 0.05 Improves nodular graphite shape. Barium containing Up to Improves graphite nucleation and reduces fading. 0.003 Reduces chilling tendency and promotes graphite inoculants. Promotes chill and undesirable graphite forms. Rarely Deliberate additions, mould Increases nodule number in ductile irons above coatings containing Bi. containing rare earth’s (cerium). Excessive nodule 0.01 counts may cause shrinkage problems. Above 5 ppm promotes ferrite. Above 10 ppm proVitreous enamel scrap, Up to motes carbides particularly in ductile irons. 20 ppm 0.01 deliberate additions as FeB. improves annealing of malleable irons. Improves spheroidization of graphite nodules. Ferro-alloys, nodularizers, Up to Improves graphite nucleation. 0.01 inoculants. Reduces chilling tendency and promotes graphite. Normally not used in grey irons. Most magnesium alloys or Up to Suppresses deleterious elements in ductile irons. added as mishmetall or 0.02 Improves spheroidization of graphite. Carbide other rare earth sources. stabilizing due to segregation. Alloy steel, chromium plate, Promotes chill and pearlite. Increases strength. Up to Form carbide segregates in ductile irons above some pig irons, ferro0.3 0.05%. chromium. Up to Tool steel No significant effects in cast irons. 0.02 Copper wire, copper-based Promotes pearlite. Improves strength. Up to Impairs ferritization in ductile irons. alloys, steel scrap, 0.5 No harmful effects. deliberate additions of Cu. Produces sub-surface pinholes. Has a mild chillDamp refractories, mould promoting action. Promotes "inverse chill" when materials and humid there is insufficient manganese present to additions. neutralize sulphur. Promotes coarse graphite. Common Sources





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Element

Common Sources

Lead Pb

Old paints, some vitreous enamel, free-cutting steel, terne plate, solder, petrol engine deposits.

Magnesium Mg

Additions of magnesium alloys (nodularizers).

Most pig irons, steel scrap, additions of ferro manganese lump or briquettes. Refined pig irons, alloy Molybdenium steels, ferro-molybdenum Mo additions. Nickel Nickel plate, steel scrap, Ni refined irons, NiMg alloy. Coke, carburizers, core Nitrogen binders, steel scrap, N additions of nitrided ferromanganese. Manganese Mn

Phosphorous Phosphoric pig iron and P scrap, additions of FeP. Silicon Si

Ferro-silicon alloys, steel scrap, pig iron.

Sulphur S

Coke, carburizers, pig iron, scrap iron, additions of iron sulphide.

Strontium Sr

Strontium containing inoculants.

Tellurium Te

Free-cutting copper, mould coatings, cooling curve carbon samples.

Tin Sn

Titanium Ti

Solder, tin plated steel scrap, bronze components, tin additions. Some pig irons, some paints and vitreous enamels, CG-iron returns, additions of titanium or ferro-titanium

2 Normal Levels (%) Effects in Cast Irons Causes spiky and undesirable graphite structures in grey irons and severely reduces strength at levels above 0.004 %. Promotes pearlite and Up to carbides. 0.005 Cause degenerated nodular graphite forms. Effects on graphite in ductile irons are neutralized by rare earth’s (cerium). Promotes nodular graphite in ductile irons. 0.03 - 0.08 Carbide stabilizing effect in ductile irons. Not used in grey irons. Neutralizes sulphur by forming MnS. Promotes pearlite formation. Forms carbide 0.2 - 1.0 segregates in ductile irons. Promotes gas holes at high levels in conjunction with high sulphur levels. Mild pearlite promoter. Up to Increases strength. 0.1 Can promote shrinkage and carbides. Up to Small amounts have little effects. 0.5 Graphitizing effect in larger quantities. Compacts flake graphite. Promotes pearlite. Increases strength. High levels causes fissure Up to defects in heavier sections. Can be neutralized by 0.015 Al, Ti and Zr. Has little effect in ductile irons. Increases CEV. Increases fluidity. Forms Up to phosphide eutectic. Damaging in ductile irons 0.1 above 0.05 %. At levels above 0.04%, can cause metal penetration. Promotes graphitization, reduces chill, stabilizes 0.8-4.0 ferrite, improves castability. Very damaging to structure and properties unless Up to 0.15 balanced by manganese. Improves grey irons (grey response to most inoculants. Increases Mg irons) requirements in ductile irons. Should be below 0.03 % in ductile irons. Up to Improves graphite nucleation in grey and ductile. 0.003 Strongly reduces chilling tendency in grey irons. Strongly promotes carbides. Causes many undesirable forms of graphite. Effects observed as Up to low as 0.0003 %. Effects reduced by combination 0.003 with Mg and Ce in ductile irons. Strongly promotes pearlite. Improves strength. Up to Embrittles ductile irons above 0.08 %. 0.15 No other harmful effects.

Up to 0.10

Neutralizes nitrogen in grey irons. Promotes hydrogen pinholing due to aluminium. Promotes undercooled graphite in grey iron. Suppresses nodular graphite in CG-irons.

Tungsten W

High speed tool steel

Up to 0.05

Rarely found in significant amounts. Mild pearlite promoter.

Vanadium V

Steel scrap, tool steel, some pig irons, ferrovanadium additions.

Up to 0.10

Promotes chill formation. Refines flake graphite. Markedly increases strength.

Technical Information 13 Compacted Graphite Iron Compacted graphite irons (CG-irons) are a range of cast irons having mechanical and physical properties intermediate between those of flake graphite and nodular graphite cast irons. They are of interest to engineers because of their useful combination of strength, thermal conductivity and thermal stability. Compacted graphite irons may be produced from many different treatment methods. The production requires controls similar to those applied in the manufacture of ductile iron castings. This Information Sheet summarises a successful production route for CG-iron that has been developed by Elkem, as well as examples of structures and properties for such materials. Treatment is based on a high rare earth containing magnesium-ferrosilicon alloy designed specially for compacted graphite iron production. No introduction of such elements as titanium or nitrogen is necessary with the present alloying concept. TM

Recommended composition of treatment alloy; CompactMag : % Si

% Mg

% Ca

% RE

% Al

% Fe

44 - 48

5.0 – 6.0

1.8 – 2.3

5.0 – 6.5

max. 1.0

balance

Recommendations for Compacted Graphite Iron Production Base Iron Composition Base iron composition should preferentially be hypereutectic with a carbon equivalent (CE) of about 4.3 - 4.5. Suggested base iron composition: %C

% Si

%S

3.5 – 3.8

1.5 – 1.9

0.007 – 0.012

Other elements are less important, but should not be higher than for ductile iron production. Generally a higher level of pearlite and carbide promoting elements may be tolerated, as long as the sulphur level is kept low and constant. Alloy Addition The addition rate of the alloy described above has to be adjusted according to base metal composition, treatment process and casting requirements for each individual foundry. Base metal sulphur level is the main factor strongly affecting the required alloy addition rate. Experiences with the base iron composition given above have shown best results for addition rates between: 0.30 – 0.45 wt% addition of alloy specified above. The addition method may be an ordinary ladle treatment process as for ductile iron production (sandwich or tundish ladle).


Office address Hoffsveien 65B Oslo



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Inoculant Sandwich Cover It is recommended to add a moderately powerful inoculant as sandwich cover. Barium containing ferrosilicon inoculants are found to give good results e.g. Foundrisil® inoculant. The inoculant cover has been found to decrease the tendency to chill formation and to give a more homogeneous graphite structure over different casting sections. In many cases subsequent inoculation is not needed, but for chill prone sections adequate post inoculant should also be used. Addition rate of the sandwich cover should be fixed and in the range 0.2 to 0.3 wt%. Microstructures The Figures below show examples of microstructures and final iron composition obtained for a CG-iron casting produced from a high RE-containing MgFeSi-alloy. Structures in both thin (5 mm) and thick (35 mm) section sizes are given to show the limited section sensitivity obtained. Homogeneous CG-structures throughout all section sizes are a characteristic feature obtained by the high RE-containing alloy in comparison to ordinary lower RE-containing MgFeSi-alloys.

Microstructure of compacted graphite iron produced from a high RE-containing MgFeSialloy, 5 mm section size (left), 35 mm section size (right) (100X).

Final CG-Iron Composition After treatment the final iron composition should be in the following range: %C

% Si

%S

% Mg

% Ce

3.3 – 3.6

2.0 – 2.5

0.005 – 0.012

0.005 – 0.015

0.005 – 0.015

Fade time and treatment temperature From laboratory and field testing, fade times up to 20 minutes were found not to have a negative influence on the microstructure control. Treatment temperatures in the range 1400 – 1520°C have been tested without any negative effect on the microstructure. Choice of post inoculant has to be adjusted according to treatment temperature used. Pearlitic grades of CGI Pearlite promoting elements (Mn, Cu, etc) may have to be applied to produce pearlitic grades of CGI. For more information, see the Elkem Product Data Sheet “CompactMag”.

Technical Information 14 Ferroalloy Storage Bin Design This Technical information sheet describes a ferroalloy storage bin designed to minimise segregation effects of alloys during processing in the foundry. Segregation may cause erratic variations in alloy performances and recoveries when used in cast iron production. How Do Alloys Segregate? Ferroalloys are often shipped in big-bag packaging. These are provided in a variety of styles and sizes. When alloy is filled into bags, segregation tend to occur which causes more coarse particles to concentrate along the periphery of the bag and fine particles in the centre. This phenomenon is shown schematically in Figure 1. As alloys are subject to the motions involved with transportation, the finer sizes may also tend to segregate to the bottom of the bag. This effect can be magnified when alloy is removed from the top of the big-bag, with the last material being rich in finer sizes. This is shown schematically in Figure 2. When alloy is discharged from the bottom of the bag using a discharge spout built into the bag, or from a hopper knife located in the centre of the bag, it is possible that further segregation of sizes may occur. This is illustrated in Figure 3. In this case the finer sizes tend to discharge first and the coarser sizes later. If alloy is discharged into another container, such as a bottom discharge hopper, the same effect shown in Figure 1 will be repeated, only the effect will be magnified. Red bands indicate coarser alloy and blue bands indicate finer sizes in Figures 1 through 3.

Figure 1: Segregation phenomena occurring during filling of big-bag giving coarser particles along the periphery of the big-bag and finer particles in the centre.

Figure 2: Finer sizes may segregate to the bottom of the package. This effect can be magnified when alloy is removed from the top, and the last material will be rich in finer sizes.

Figure 3: Bottom discharge causing finer material to discharge first and the coarser material last.

Alloy handling to minimise segregation It is recommended that bottom discharge steel bins be used to store and dispense MgFeSi alloys. Figure 4, side view, shows the re-blending effect during big-bag discharge that can be gained by using multiple knives in the top of the bin. These are placed midway between the centre and the side of the bag, in both side and end view directions. This makes 5 knives in total, which open 5 discharge holes in the bottom of the big-bag when lowered onto the knives by a crane or a forklift truck. Figure 4, front view, shows how alloy can be further re-blended by using a rake to pull alloy out across the full length of the discharge trough. The re-blended material falls into a container, which sits on a scale below the trough. This requires considerably less effort than shovelling alloy from a bin discharge trough, whilst minimising spillage and segregation.


Office address Hoffsveien 65B Oslo



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Figure 4: Re-blending bin discharge. Side view (left), front view (right).

Figures 5 and 6 shows photos of a ferroalloy bin designed to minimize segregation effects.

Figure 5: Upper picture shows example of bin design for minimised segregation. Dimensions are approx. 1250mm wide and 2000mm high. Capacity is approximately 1500 kg MgFeSi. Lower picture shows knives for cutting open the bottom of the big-bags.

Figure 6: Upper picture showing discharge opening. Lower left picture showing a big-bag being lowered into the hopper. Note that knives will open the bag and eliminate the need for the operator to open bottom discharge spout while the bag is suspended. Lower right picture shows operator raking alloy out of the trough. Each stroke should be the full length of the trough.

Technical Information 15 Selection of Inoculants for Grey Cast Iron An inoculant must serve several purposes in grey irons: •

to eliminate iron carbides or “chill”;

•

to modify the graphite morphology to a uniform “A” type structure;

•

to reduce the section sensitivity between thin and thick sections within the same casting;

•

to be effective over the length of the ladle pouring cycle.

All commercially available inoculants are based either on a ferrosilicon alloy, a blend of graphite and ferrosilicon or a mixture of ferroalloys. Increased demand by the casting ends for consistency has led most foundries to abandon blends in favour of quality controllable specialist ferrosilicon based alloys. These alloys invariably contain either 45-50% silicon or 60-75% silicon, both with additions of property enhancing elements and a balance of iron. The most common effective added elements are: Element

Considerations

Aluminium Al

Normally present in ferrosilicon alloys, but has little inoculating effect. High aluminium contents typically found in cheap uncontrolled alloys can cause hydrogen pinhole problems in greensand moulding systems and a maximum of 1.5% is advised. Some in-the-mould alloys have high aluminium (>4%) and it is claimed that this is effective in this niche application.

Barium Ba

A powerful graphite promoting element which also provides good fade resistance. Up to about 3% can be used beneficially, however excess can create slag defects. Care should be taken in certain castings containing sharp radii or where the sand is slightly soft as the high eutectic cell number generally found with barium containing materials may cause inter-cell shrinkage.

Calcium Ca

A medium potency inoculating element, often added in conjunction with other alloyed elements. A combination of barium and calcium can be particularly effective in irons of lower sulphur content (0.03-0.05%).

Strontium Sr

The most powerful inoculants for grey irons of medium/ high sulphur level contain about 1% strontium. Both in of chill control, particularly in thin casting sections, and modification of structure, strontium containing alloys are found to be very effective. Good fade resistance is found with these alloys. Peculiar to strontium containing ferrosilicons is the property of powerful chill reduction whilst maintaining a much lower eutectic cell number than found with other proprietary inoculants.

Zirconium Zr

A medium potency, all-purpose element that gives good chill reduction and, in particular, will control the graphite morphology in high carbon equivalent irons. Zirconium has the added advantage of controlling nitrogen in heavily cored or shell systems, creating a harmless ZrN2 inclusion.





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Several factors have to be taken into consideration in the selection of an inoculant for grey iron: •

the sulphur content of the base iron;

•

the fade time, i.e. the total time taken from adding the inoculant to pouring the final casting from the ladle;

•

the carbon equivalent of the iron.

The carbon equivalent (CE), given in weight percent, relates the combined effects of different alloying elements used in the making of cast irons to an equivalent amount of carbon. This value can be calculated using a mathematical equation, and the following formula is commonly applied for cast iron: CE = %C +

%Si + %P 3

Addition Methods Consideration should also be given to the method of adding the inoculant. In-stream methods eliminate much of the inoculant fade and reduce the addition rates encountered with conventional ladle treatments. In these cases, attention must be given to the inoculant particle size, a 0.2 – 0.7 mm grading being suited to most in-stream applications. For ladle inoculation, a 0.5 – 2 mm grade is deal for small ladles, up to 250 kgs capacity and 2 – 6 mm material should be used for ladles above this. Inoculants containing excessive fines should be avoided as these contain higher levels of oxides and create dust during addition. Elkem is happy to provide further information to help in the correct selection of inoculants and inoculating practises.

Technical Information 16 Selection of Inoculants for Ductile Cast Iron Careful selection of charge materials and nodularisers is often negated by the use of the incorrect inoculant. Due consideration should be given to this essential part of the process and the following points should be taken into : •

Which kind of nodulariser and treatment process has been used.

•

The fade time of the metal, that is the time from adding the inoculant to pouring the last metal from the ladle.

•

The Rare Earth content of the nodulariser (or otherwise added RE).

Pure Mg processes, such as plunging, cored wires or converters, reduce the number density of inherent nuclei in the iron making the iron difficult to inoculate. MgFeSi processes have a net effect of adding nuclei to the iron. Typically, a higher inoculant addition will be required when pure Mg processes are employed. Rare Earth's serve to neutralise the effects of some subversive elements found in steel scrap used in the furnace charge, however, they can have the same effect on certain elements added as integral parts of the inoculant.

Inevitably, inoculation of ductile iron requires greater amounts of treatment agent than grey iron, principally due to the carbide stabilizing properties of the magnesium used during nodularisation. Whereas the graphite flakes govern the properties of grey iron, ductile iron characteristics are dominated by the matrix. Formation of even, rounded nodules is therefore essential to obtain the best properties. Four main groups of inoculants are commonly available, all based on ferrosilicon plus deliberately added property enhancing elements. Element

Considerations

Calcium Ca

Foundry Grade ferrosilicons (FG FeSi) containing balanced amounts of calcium and aluminium. Care should be taken in the selection of these materials as many are supplied with very high levels of aluminium (>3%) which can cause severe pinholing problems in the casting. Good FG FeSi will give satisfactory nodule counts and iron properties in many medium section castings.

Barium Ba

Barium containing inoculants are especially useful where the fade time of the iron is long or where the solidification of the casting is slow (e.g., heavy sections). Total barium contents in the inoculant above 3% are unnecessary and serve no purpose but may cause slag generation.

Strontium Sr

Strontium containing inoculants may only be used under certain conditions in ductile iron. Strontium will give an excellent chill removal and nodule count in iron treated with pure Mg, RE free processes or in many MgFeSi situations where the RE content of the nodulariser is less than 1%. High RE contents will neutralise the effects of Strontium.

Zirconium Zr

Zirconium containing inoculants are excellent medium potency and fade resistant materials. Zirconium has the added advantage of tying up any N2 from the melting process or cores.

Other inoculants are commercially available, containing a variety of elements, rare earth’s, bismuth and manganese for example and details of the properties of these can be obtained from the manufacturers. All of the types noted above are available in both ladle and stream gradings, details can be obtained from your local Elkem sales representative. Elkem ASA, Foundry Products Postal address P.O.Box 5211 Majorstuen NO-0302 Oslo Norway




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Technical Information 17 Recommended Target Analysis for Grey Iron The table shows suggested target analysis for the six ISO standard grades of grey cast iron. Recommended composition ranges for carbon, silicon, manganese and sulphur, as well as maximum level of phosphorus are given in the table. Also, the calculated carbon equivalents (CE) are shown in the table. Recommended target composition for grey cast iron according to the ISO standard grades. ISO Grade

Element contents

100

150

200

250

300

350

%C

3.5 – 3.8

3.4 – 3.7

3.2 – 3.5

3.1 – 3.4

3.0 – 3.2

2.9 – 3.1

% Si

2.3 – 2.8

2.1 – 2.6

1.8 – 2.3

1.6 – 2.1

1.3 – 1.9

1.1 – 1.5

% Mn

0.4 – 0.8

0.5 – 0.8

0.6 – 0.8

0.6 – 0.8

0.7 – 0.9

0.8 – 1.0

%P

max. 0.20

Max. 0.20

max. 0.20

Max. 0.15

max. 0.10

max. 0.10

%S

0.06 – 0.15

0.06 – 0.15

0.06 – 0.15

0.06 – 0.12

0.06 – 0.12

0.06 – 0.12

CE

4.2 – 4.6

4.0 – 4.3

3.8 – 4.1

3.6 – 3.9

3.4 – 3.7

3.2 – 3.5

It is important to note that following the recommended composition ranges given above will provide a good basis for obtaining the respective ISO grade properties. However, correct properties are not guaranteed unless several other important parameters are handled properly. Among these are: •

Correct choice of inoculant material and proper addition procedures (see Elkem Technical Information Sheets No. 4 and 15).

•

Avoidance of superheating and prolonged holding times.

•

Careful choice of raw materials in order to avoid excessive concentrations of minor and alloying elements that may interfere with the mechanical properties. (see Elkem Technical Information Sheet No. 12).

•

Proper slag removal to avoid inclusion defects.

•

Consideration to the effect of moulding medium on cooling rate and solidification structure.

It should also be noted that for special purpose grey irons it may be advantageous to deviate from the recommended analysis in order to improve specific properties (e.g. better thermal conductivity, lower shrinkage tendency, improved damping capacity, etc.)





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The effect of carbon content and carbon equivalent (CE) value on tensile strength can be illustrated in simple diagrams. The figures below show the connection between these parameters in grey iron.

Effect of increasing carbon content on the tensile strength in grey irons

Tensile strength as a function of carbon equivalent value (CE value) in grey irons

Technical Information 18 Recommended Target Analysis for Ductile Iron The table shows suggested target analysis for production of the ISO standard grades of ductile cast iron. Recommended composition ranges for carbon, silicon and manganese are given in the table. Also, recommendations to maximum levels of some minor elements are given as footnotes to the table. Recommended target composition for ductile cast iron according to the ISO standard grades. Average casting section [mm]

ISO grade(s) 800/2, 700/2, 600/3 %C

%Si

%Mn

500/7 %C

%Si

450/10, 400/15, 400/18 %Mn <0.3

%C

%Si

3.6-3.8 2.6-2.8

350/22

%Mn <0.2

%C

1)

%Si

13

3.6-3.8 2.6-2.8

<0.5

3.6-3.8 2.6-2.8

13-25

3.5-3.6 2.2-2.5

<0.6

3.5-3.6 2.2-2.5 <0.35 3.5-3.6 2.2-2.5 <0.25 3.5-3.6 2.0-2.5 <0.15

25-50

3.5-3.6 2.1-2.3

<0.7

3.5-3.6 2.2-2.4

<0.4

3.5-3.6 2.2-2.4

50-100

3.4-3.5 1.9-2.1

<0.8

3.4-3.5 2.0-2.2

<0.5

3.4-3.5 2.0-2.2 <0.35 3.4-3.5 1.8-2.0

100

3.4-3.5 1.8-2.0

<0.8

3.4-3.5 1.8-2.0

<0.6

3.4-3.5 1.8-2.0

<0.3 <0.4

3.6-3.8 2.0-2.5

%Mn <0.1

3.5-3.6 2.0-2.4 <0.15 <0.2

3.4-3.5 1.8-2.0 <0.25

1) Maximum 2.5% Silicon when impact properties are required.

Important notes: •

For Grades 800/2, 700/2 and 600/3 additions of 0.5% Cu or 0.1% Sn may be made.

•

Base metal Sulphur content should be restricted to maximum 0.020%.

•

Final ductile iron Sulphur content should be maximum 0.015%.

•

Phosphorus, in all grades of ductile iron, should be maintained below 0.03%.

•

Chromium levels should be maintained below 0.05%.

•

Residual Magnesium levels should be in the range of 0.03-0.06%.

It should be noted that following the recommended composition ranges given above provides a good basis for obtaining the respective ISO grade properties. However, correct properties are not guaranteed unless several other important parameters are handled properly. Among these are: •

Correct choice of inoculant material and proper addition procedures (see Elkem Technical Information Sheets No. 4 and 16).

•

Avoidance of superheating and prolonged holding times.

•

Careful choice of raw materials and ferroalloys in order to avoid excessive concentrations of minor and alloying elements that may interfere with the mechanical properties (see Elkem Technical Information Sheet No. 12).

•

Correct choice of magnesium treatment process and nodularizing agent for the actual purpose and conditions.

•

Proper slag removal to avoid inclusion defects.





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Technical Information 18 •

2

Consideration to the effect of moulding medium on cooling rate and solidification structure.

Finally, it should also be noted that for special purpose ductile irons it may be advantageous to deviate somewhat from the recommended analysis in order to alter specific properties (e.g. improved strength or ductility, reduced graphite flotation, special alloyed grades, etc.)

Technical Information 19 Aluminium in Cast Iron Aluminium is normally found in cast irons as a mainly harmless residual element. The major sources for aluminium are steel scrap, contaminated cast scrap (engine blocks etc with pistons included), the ferroalloys consumed and inclusions of non-ferrous metals in the charge materials. A common occurrence in foundries is the pinhole problem from hydrogen gas evolution, which often can be attributed to excessive aluminium contents. It is accepted that aluminium has an influence on the surface tension of the liquid iron, a consequence of which could be susceptibility to pinholing defects. The figure below shows the relationship between aluminium in the iron and the tendency to pinholing. It is shown that grey iron is more sensitive to pinholing than ductile iron due to the overall lower surface tension for grey iron. Above a certain level (approximately 0.2% Al), the susceptibility for pinholing is reduced as the surface tension again increases. The most critical range is 0.05 – 0.2% for ductile iron and 0.008 – 0.2% for grey iron. Consequently, the contents of aluminium should always be kept low, preferably below this range where the risk will be highest.

Influence of Aluminium Content on Surface Tension and Pinhole Susceptibility of Grey and Ductile Irons

It should also be noted that iron temperature will influence the surface tension and thus well insulated ladles are of importance (refer to Elkem Technical Information Sheet No. 21 for more data). Aluminium will also add to the slag formation, resulting in poor furnace performance, more ladle and holder maintenance, and increased risk for slag inclusions in castings. Aluminium has virtually no inoculating effect as such, but it may add to the hardness of the iron, and it can also be harmful to the nodularity of ductile iron. It is also important to note that titanium will play the same role as aluminium to a certain extent, although normally present in smaller amounts than aluminium. Furthermore, the two elements will have an aggregated effect, and both elements should be monitored and controlled at all times. Elkem ASA, Foundry Products Postal address P.O.Box 5211 Majorstuen NO-0302 Oslo Norway




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It is also well known that many elements can interact with aluminium to affect the iron properties, either by enhanced inoculation potency or by detrimental effects such as the combination of aluminium and titanium. The presence of even minor traces of titanium means that the tolerable aluminium levels will be dramatically reduced. The figure below shows the combined effects of aluminium and titanium on the hydrogen pinholing tendency in ductile iron. Above the curve there will be a significant risk for such defects to occur.

Combined effects of aluminium and titanium on hydrogen pinholing tendency in ductile iron.

Example of pinhole defect in grey iron.

For ductile iron the permissible aluminium is roughly 5 – 10 times that of grey iron. No data is available concerning the combined effects of aluminium and titanium in grey iron, but there are reasons to believe the interaction is about as for ductile iron and both elements should therefore be closely watched. Since both nodularizers (magnesium-ferrosilicon) and inoculants will contain various amounts of aluminium and titanium, it is important that choice of alloys is being made with full awareness of its total chemical composition. At higher aluminium contents, ferrosiliconbased alloys will tend to improve solubility, but the increased slag formation and pinholing tendency should call for caution. High aluminium containing alloys should hence only be used where low addition rates are applicable (i.e. stream inoculation). Special attention should be paid to large amounts of ferrosilicon used as a furnace charge material. It is worth noting that hydrogen pinhole defects often will have similar characteristics as other type of gas defects, such as nitrogen porosity. A characteristic feature for pinholes is the graphite lining covering the inner pore surfaces. An example of such a hydrogen pinhole defect is shown in the figure above. This defect characteristic can also occur for nitrogen defects, and it is therefore often difficult to separate between such gas defects. A thorough investigation into nitrogen, aluminium and titanium levels will be necessary to determine the type of gas involved, since a high Aluminium and Titanium level may promote hydrogen pinholes but at the same time effectively neutralize nitrogen by forming TiN and AlN inclusions. Choice of core binder system and green sand humidity level is also vital for the avoidance of hydrogen and nitrogen pinhole defects.

Technical Information 20 Selection of Nodularizers Among foundries there is always a debate about the selection of nodulizer for their particular process. The two diagrams below show some variables that will affect the selection of magnesium ferrosilicon alloy. The diagrams can help you to find the right alloy composition for a given process and equipment. On the next page, recommendations to alloy composition for special processes like in the mould, flow through, and sandwich pocket processes are also given. Effects of Process Variables on Magnesium Ferrosilicon Alloy Selection: > 1480oC

Lower Mg-recovery and increased violence. Use high Ca in alloy.

< 1480oC

Better Mg-recovery. Low Ca in alloy can be used.

Temperature

Ladle dimensions Process Variables Tundish cover

Treatment weight

1:1 H:D

Violent reaction and low Mg-recovery. Use medium Ca in alloy.

2:1 H:D

Less violent reaction and better Mg-recovery. Low Ca possible.

No

Fume and flare. Use low Mg and increased Ca in alloy. Normally, use 4-32 mm sizing.

Yes

Very little fume and flare. Better Mg-yield. Higher Mg and lower Ca in alloy possible. All sizes possible.

< 500 kg

Use more fine sized nodularizer. Less than 12 mm sizing recommended.

> 500 kg

Use coarser nodularizer. Typically 4-32 mm or 1-20 mm.

Effects of Iron Composition on Magnesium Ferrosilcon Alloy Selection:

Silicon content

Base Iron Composition

Sulphur content

Charge Materials

Carbon Content

0.5-1.5%

Use a 5% Mg-FeSi alloy

1.5-1.8%

MgFeSi/low Si nodularizer mixture or 10% Mg-FeSi alloy

> 1.8%

Low Si nodularizer

< 0.025%

All type of nodularizers can be used.

0.025-0.04% > 0.04%

Desulphurization before Mg-treatment

High purity steel scrap

Use nodularizer with low RE-content or low Mg with increased RE.

Low quality steel scrap

Requires increased RE-content to balance subversive tramp elements.

3.2-4.0%

Not generally significant in product selection, but Mg-recovery slightly lower at high carbon contents.



High Mg/RE alloys recommended. Increased slag formation. Desulphurization recommended.



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Nodularizers for Continuous Treatment: The following are typical specifications for MgFeSi alloys for different treatment processes: Treatment process Specification

In the mould

Flow through

Silicon

44-48%

44-48%

44-48%

Magnesium

5.0-6.0%

2.8-3.5%

3.5-4.0%

Calcium

0.4-0.6%

1.1-1.6%

1.3-1.8%

Aluminium

0.8-1.2%

0.5-1.0%

0.5-1.0%

Lanthanum

0.25-0.40% 1.1-1.4%

1.1-1.4%

2-12 mm

2-12 mm

Rare earth’s Sizing

1-4 mm

Important notes: •

Calcium is included to reduce reactivity and give optimum Mg-recovery.

•

Low aluminium decreases the tendency to give slag related defects.

•

These are typical analyses, and other alloy compositions are also available to meet individual requirements.

•

For details on the sandwich pocket process, see Elkem Technical Information Sheet No. 11.

Technical Information 21 Heat Conservation in Liquid Iron When processing liquid iron in ladles and holders there will be a continuous loss of temperature from radiation and heat conduction through refractories. Such energy losses can be minimised by using insulating materials and covered vessels for holding and transportation of metal. This sheet gives some examples of how temperature can be conserved in a ladle. Some of the important consequences of heat conservation are also listed in the end. Heat transfer - refractory linings: (T − T ) (T − T2 ) dT Q& = − k = −k 2 1 = k 1 dx L L

(1)

where Q& is heat transfer per unit area (W/m2), k is thermal conductivity (W/m·K), T1 is temperature of the hot face (K), T2 is temperature of the cold face (K), and L is refractory thickness (m). The equation is negative because heat transfer is contrary to the direction of the temperature gradient. Refractory conduction, single component: Example: k = 1 W/m·K for high alumina lining T1 = 1480°C (1753 K), T2 = 38°C (311 K) L = 0.051 m (51 mm) 2 Q& = 1 (1753 - 311)/0.051 = 28.3 kW/m For each square meter of single component alumina refractory, the rate of heat loss is approx. 28 kW. Heat transfer, multiple layers: Q& =

(T1 − T3 ) L1 L2 L3 + + k1 k 2 k 3

(2)

where k1 is the conductivity and L1 is the thickness of material 1, etc. Refractory conduction, multiple components: Example: T1 = 1480°C (1753 K), T3 = 38°C (311 K) High alumina: L1 = 25 mm, k1 = 1 W/m2·K Insulating brick: L2 = 25 mm, k2 = 0.5 W/m2·K Ceramic paper: L3 = 6 mm, k3 = 0.05 W/m2·K 2 Q& = (1753-311) / (0.025+0.050+0.12) = 7.4 kW/m For each square meter of multiple component refractory, the rate of heat loss is about 7 kW. Elkem ASA, Foundry Products Postal address P.O.Box 5211 Majorstuen NO-0302 Oslo Norway


© Copyright Elkem AS Telephone +47 22 45 01 00 Telefax +47 22 45 01 52


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2

Refractory conduction, savings from multiple components: Refractory consept

Heat losses

Single component Multiple components Energy savings

28.3 kW 7.4 kW 20.9 kW

About 75% of the heat loss from a single component refractory can be saved by using a multiple component alternative including insulating bricks and ceramic paper (fibre). Heat radiation: Q& = εσ (T14 − T24 )

(3)

where Q& is heat radiation per area (W/m2), ε is emissivity (number between 0 and 1), σ is the Stefan-Boltzmann constant (5.67·10-8 W/m2·K4), T1 is temperature of the radiating material (K), and T2 is temperature of the receiving media (K), the latter usually room temperature (25 °C). Some common emissivity values: Surface

Temperature

Emissivity

Sheet steel

25 - 50 °C

0.81-0.83

Molten iron

1400-1600 °C

0.25-0.40

high Al2O3

1000-1500 °C

0.45-0.60

low Al2O3

1000-1500 °C

0.65-0.80

Al2O3-SiO2 refractories

Radiation from metal surface: Q&

= 5.67*10-8 x 0.33 x (17534-2984) = 176.6 kW/m2 @ 1480 °C

Radiation from empty ladle refractory between fills: Q&

= 5.67*10-8 x 0.45 x (17534-2984) = 240.8 kW/m2 @ 1480 °C

Hence, for a 0.17 m2 metal bath surface the heat loss will be about 30 kW, while for a 1 m2 refractory area in an empty ladle the loss will be approx. 240 kW. Important effects of heat conservation: •

Less melting energy required. Thus, lower tapping temperature.

•

Reduced temperature losses during pouring.

• • • • •

Extended furnace-lining life. Less formation and accumulation of slag in ladles. Less magnesium alloy required in ductile iron. Less maintenance of ladle refractories. Better environment (especially ductile iron) due to less smoke and fume.

Technical Information 22 Late Metal Stream Inoculation With increasing demands from end-s for higher quality castings and increasing numbers of foundries investing in highly mechanised moulding and pouring lines, the requirements for effective inoculation are becoming more difficult to meet. Non-uniform inoculation from conventional ladle treatments can arise, caused by variations in metal temperature, human error in addition and the accepted deterioration of inoculating effect with time (fade). Further, particular difficulties can arise in some pouring units where satisfactory addition of inoculant is not possible. Automatic high production moulding lines linked to either a heated or unheated stopper rod controlled pouring unit have proved to be both highly economic and efficient, particularly in the repetition section of the industry where high volumes of castings of consistent metal quality are required. A method of inoculating this iron between the stopper rod and the downsprue is therefore essential. The system must be automatic, linked to activation of the stopper rod (or in some cases activated by light emitted by the metal flow) and be highly reliable. Several types of inoculating unit are available commercially at variable quality and cost. Whilst Elkem does not supply such machinery, it is strongly recommended that a system with adequate safety systems be installed to eliminate the possible requirement for 100% inspection of castings. Specialist advice on the selection of inoculating units and recommendations on installation can be obtained through local Elkem representatives. With high investment in moulding lines, pouring units and inoculating systems, it is logical to control accurately the quality of the inoculant. This must have: •

Good consistent flowability to ensure that particles of inoculant reach the metal stream, even at low flow rates through a small orifice in the injection unit.

•

A clean cut on the bottom sieve fraction to avoid excessive fines. Undersize material can contain oxides and the heat from the metal stream tends to blow fine material into the atmosphere. Variations in the undersize fraction can also lead to inconsistency in flow rate and possible blockage of the dispensing unit.

•

No oversize particles which can block the injection unit.

•

Good solubility in the iron to give maximum inoculation effect without leaving hard, unmachinable inoculant particles which have not dissolved, even at low casting temperatures.

•

Less tendency to give slag inclusions in the iron.





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With conventional ladle inoculation, the effects of the inoculant begin to fade instantaneously and it has been shown that the number of sites available for nucleation deteriorates.

Coarsening behaviour of nuclei particles in cast iron during holding.

From this, it can be seen that some 5x105 sites per cubic millimetre of metal are available immediately after addition of the inoculant, fading to 1x105 after about 10 minutes. With late metal stream inoculation, fade is virtually eliminated and the addition rate to provide satisfactory inoculation (chill removal, graphite morphology) is dramatically reduced. See also Elkem Technical Information Sheet No. 6 on “Fading of Inoculation” for more details. Typical addition rates: Cast Iron

Ladle Inoculation

Late Stream Inoculation

Grey Iron

0.1-0.3%

0.03-0.2%, typical 0.1%

Ductile Iron

0.3-0.6%

0.06-0.3%, typical 0.2%

Elkem provides a complete range of materials suitable for late stream inoculation and reference should be made to the Data Sheets for Superseed® 50/75, Superseed® Extra, Zircinoc®, Foundrisil®, Reseed® and Vaxon® 75. These materials are available world-wide in a 0.2-0.7 mm grading for maximum flowability and solution into the iron. See also Elkem Technical Information Sheet No. 15 and 16 on “Selection of Inoculants” for more details. In certain applications a wider grading is acceptable and details are available on request. The main advantages of late metal stream inoculation are: •

Reduced addition rates compared to ladle inoculation provide an economic benefit.

•

Reduced addition rates mean lower calcium and aluminium additions thus leading to less tendency towards slag and pinholes.

•

Avoiding the promotion of high eutectic cell counts that may lead to shrinkage porosity, again because of the lower addition rates of inoculant.

•

Operator error is eliminated.

•

Only the metal entering the mould is inoculated, thus avoiding wasteful treatment of ladle “heels”.

Technical Information 23 Factors Influencing the Recovery and Addition of Magnesium in Ductile Iron Ladle Treatment Processes Residual magnesium and magnesium recovery have always been subjects for discussion amongst foundry people. This sheet summarises the most important factors that will influence the recovery of magnesium in ductile iron production. 1) Sulphur content in base iron. Sulphur has to be neutralised in order to increase the surface tension of the iron. High sulphur in the base metal means increased Mg addition. 2) Oxygen content in base iron. Oxygen has to be neutralised in order to increase the surface tension of the iron. As with sulphur, increased oxygen content requires higher Mg addition. 3) Slag from the melting or holding furnace. Slag that is transferred from the furnace will react with magnesium and reduce the recovery. Proper separation procedures to minimise slag carry over need to be in place. 4) Tapping temperature. Tapping (treatment) temperature should be kept as low as possible in order to avoid excessive reaction violence. The higher the temperature, the more vaporization and lower recovery of magnesium. 5) Time between MgFeSi addition to the ladle and tapping. Time between magnesium addition and tapping should be minimised to prevent preheating and oxidation of the alloy. At the same time, there should be no liquid metal residual from previous treatments in the ladle as this may start to react with the alloy. 6) Slag in ladle and pocket. Slag building up in the ladle and reaction pocket leads to reduced magnesium recovery, probably due to reactions between the slag and the magnesium and also as the pocket depth is reduced changing the reaction conditions. Overspill of alloy will occur if the pocket is allowed to fill with slag. Ladles should be kept tilted when empty to avoid slag clogging the pockets. 7) Alloy cover. An alloy cover in the ladle, for example fine sized FeSi or steel plates, will delay the reaction start and give better absorption of magnesium into the liquid metal. 8) Filling time. Filling rate should be high in order to achieve a high ferrostatic head in the ladle before the reaction starts. 9) Fading/Pouring time. Long holding times after treatment and long pouring times require higher initial Mg contents to compensate for fading effects. Pouring times should be minimised to overcome these effects.





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10) Inoculation. With a good inoculation, less residual magnesium is required to give good nodularity. This again means less alloy addition and better magnesium recovery. 11) Ladle design. The ratio of internal Height: Diameter should be at least 2:1 and the pocket should have space enough to carry the alloy addition and covering material (see TI sheet No.10). The ladle should also be properly insulated to minimise heat losses and consequently the required treatment temperature (see TI sheet No. 21). A tundish cover lid is also highly recommended for alloy and temperature recovery reasons. 12) Chemical composition of nodulariser. High magnesium content in the alloy will give a violent reaction and reduced recovery. High Ca will reduce the reactivity and increase the recovery, but also increase the tendency to slag formation. Rare earth’s (cerium) will assist in giving better recovery because it allows for working at lower magnesium in the alloy and lower residual magnesium in the iron (see TI sheet No. 20). 13) Alloy sizing. A wide alloy sizing gives dense bulk packing in the pocket. The alloy will then fuse and react (dissolve) slowly in a controlled manner with a minimum of pieces floating. Lumps floating and burning on the surface are a waste (see TI sheet No. 20). 14) Storage of foundry alloys. All foundry alloys will oxidise if exposed to moisture. Oxidised alloys will give a lower recovery than fresh materials. Containers of alloy should be stored in a dry place and not opened until required at the treatment station (see TI sheet No. 1). R e q u ir e d M g F e S i a d d it io n w t %

1 .9

O x id is e d M g - a llo y .

T a p p in g te m p .

1 .8

1 5 2 0 °C

1 .7 S = 0 .0 1 6

5 m in

1 .6

lo t o f S = 0 .0 1

1 4 8 0 °C

1 .5

1 .3 1 .2

som e c le a n fu rn a c e

1 .4

1 4 6 0 °C S = 0 .0 0 6

S u lp h u r c o n t.

L a d le d e s ig n d ia m e t e r : h e ig h t

T im e b e tw e e n M gFeSi add. a n d t a p p in g

S la g in fu r n a c e .

none

2 m in

som e

C a = 0 ,5 , R e = 0 ,5

1 :1

C a = 0 ,5

1 :1 ,5

C a = 1 ,0

heavy

som e

good 30 s

A llo y C over

C a = 2 ,5 1 :3

C a = 2 ,5 R e = 1 ,5

fre s h

C h e m ic a l c o m p o s it io n of M gFeSi

Factors that can influence the MgFeSi addition to a ductile iron ladle treatment process.

Appendix: Checklist Recovery and Addition of Mg in Ductile Iron Ladle Treatment Processes.

Technical Information 23 - Appendix Checklist - Recovery and Addition of Mg in Ductile Iron Ladle Treatment Processes For process improvements fill in information in the white fields. To help you detect possible causes for sudden changes in Mg-recovery and needed Mgaddition, fill in both white and grey fields.

1

2

S-Content in Base Iron Before

max. 0.020 wt% Now

Final S-Content Before

max. 0.015 wt% Now

Residual Mg-Content Before

in the range 0.03 – 0.06 wt% Now

O Content in Base Iron: Before

Now if measured

3

4

to remove slag! Any process disturbances reported? Tapping Temperature Before Treatment Size

1450 – 1500°C Now

Before Now

kg kg

5

Keep time between addition to ladle and treatment short! Any process disturbances reported?

6

Slag in ladle or pocket ?





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Technical Information 23 - Appendix

7

Cover Material Before Type Now Type

2

% addition

8

Keep the filling rate into the ladle high! Any process disturbances reported?

9

Keep holding and pouring time short! Any process disturbances reported?

10

11

Inoculation Nodule count Chill wedge

Before

nodules/mm2 mm

H : D – Ratio Before Now

12 Alloy Composition Mg-Content Ce-Content Ca-Content Al-content

Now

Lower

Before

2:1

Now

Higher

Spec wt% wt% wt% wt%

New lot? 13

14

Alloy Sizing Before Now

min

max

% undersize

Correct storage of foundry alloys In house Outdoor Weather changes?

Yes

No

Technical Information 24 Partition of Slag Phases in the Treatment and Pouring of Ductile Iron One problem encountered in automatic pouring systems incorporating a stopper rod is the build up of slag in the pouring unit, especially on and around the stopper rod, that leads to: •

Costly cleaning and maintenance of the holding unit.

•

The stopper rod not seating properly and metal dripping from the launder.

•

Inconsistent pouring rates which can interfere with consistent inoculation from dispensing units.

•

Slag entering the mould.

An investigation has been carried out into the composition of slag phases contained in a tundish ladle/unheated automatic pouring unit system. Different slag phases have been found to precipitate in various parts of the system and these are shown schematically below.

Slag constituents: Magnesium Calcium Sulphide Iron Aluminium Figure 1: Partition of slag phases in the sandwich treatment ladle (surface, wall and pocket) and in the autopour (inlet, surface, bottom, stopper and nozzle).

Ductile iron slag phases can be categorised into five principle types: 1. Magnesium containing slags: MgO (Periclase), MgO•Al2O3 (Spinel), 2MgO•SiO2 (Forsterite) and MgO•SiO2 (Enstatite). These phases tend to be distributed throughout the system, both treatment vessel and pouring unit. The very hard aluminium containing spinel is found to be concentrated around the stopper rod and pouring nozzle. 2. Calcium containing slags: complex oxides, sulphides and aluminates. Calcium phases can be found in most locations in the system, but are predominantly concentrated around the stopper rod. 3. Sulphide phases; magnesium and calcium sulphides. Sulphide phases tend to be concentrated around the stopper rod, but can be found in small quantities elsewhere in the system. Proper control of sulphur in the base metal and good deslagging practise minimises the harmful effects of this phase. 4. Iron containing slags: Fe,Al (Hercynite), Fe,Mg (Ringwoodite and Magnesoferrite). These are mostly found in the upper parts of the pouring unit and may be regarded as oxidation products generated by between the iron and atmosphere. Elkem ASA, Foundry Products Postal address P.O.Box 5211 Majorstuen NO-0302 Oslo Norway




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5. Aluminium phases: Al,Si (Mullite), Al,Mg (Sapphrinite) As with other harmful aluminium containing slags, these tend to be concentrated around the stopper rod. In general , aluminium and calcium phases have the most tendency to concentrate on the stopper rod. It is possible to alter the slag phase composition and hold the more harmful phases back in the treatment ladle. Barium This is done with the addition of barium, either as an integral part of the MgFeSi alloy or, more commonly, as part of a ferroalloy sandwich cover (e.g. by the use of a barium containing inoculant in the cover material). The competition between barium, calcium and magnesium to combine with the aluminium phases seems to be dominated by barium. Barium phases tend to settle in the treatment ladle and only a minimum of slag is transferred into the pouring unit, as shown schematically below.

Slag constituents: Barium

Figure 2: Distribution of barium containing slag phases.

The effect of barium additions is that the stopper rod and the insides of the pouring unit remain cleaner and will have a longer working life. As the barium containing slags settle mainly in the treatment ladle, it appears that more slag is being generated. This is not necessarily true as the reaction slags are being accumulated in the ladle rather than being transferred into the pouring unit. Ladles will therefore require better skimming and cleaning of linings, however most foundries will find this a better, cleaner and cheaper option than replacing stopper rods and cleaning out pouring units. Without doubt, the best way of minimising harmful calcium and aluminium bearing slags from contaminating the pouring unit is to prevent the introduction of such elements into the iron. As this is not possible in many cases, the best alternative is to make a deliberate addition of a barium-containing alloy to the treatment ladle to control the slag partition and minimise slag fouling the pouring units.

Technical Information 25 Poor Nodularity in Ductile Iron This sheet summarises some important considerations affecting nodularity in ductile iron production. Factors causing different types of poor nodularity are described, and important criteria distinguishing between the different types of poor nodularity given. Compacted graphite: • Low residual magnesium and/or rare earth’s from poor nodularising practice, high temperature or long holding times. • Excess sulphur in the base iron not balanced by sufficient magnesium. • Excess titanium in iron from compacted graphite iron returns.

Exploded graphite: • Excess rare earth additions, particularly when high purity charges are used. Normally found in thick section castings or at higher carbon equivalents. • Sudden drop in base iron sulphur content from change of raw material lots (steel scrap, pig iron, recarburiser).

Chunky graphite: • Excess rare earth additions when high purity charges are used. • Low Sulphur to RE ratio. • Promoted by poor inoculation (pronounced segregation effects). • Similar causes as exploded graphite.





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2 Spiky graphite: • Very small amounts of lead (Pb), bismuth (Bi), or antimony (Sb) that have not been neutralized by rare earth’s. This has a catastrophic effect on mechanical properties. • Insufficient addition of rare earth’s to a contaminated charge.

Irregular graphite shape: • Poor inoculation or excessive fading of inoculation. Often combined with low nodule counts. • High holding temperatures or long holding times resulting in “dead” irons. • Excessive addition of nodulariser. • Can be improved by late addition of a powerful specialty inoculant.

Nodule alignment: • Low carbon equivalent. • Poor inoculation causing hypo-eutectic solidification and coarse dendrite structures. Nodule alignment at dendrite arms. • High pouring temperatures.

Flake graphite surface: • Excess sulphur built-up in moulding sand. Reversion to flakes as magnesium in the iron reacts with sulphur. • Can be overcome by using higher magnesium or rare earth’s in nodulariser, or a cerium containing inoculant. • Mould coating may also be useful.

Technical Information 26 Fading of Nodularity in Ductile Iron When properly treated and inoculated ductile iron is held for prolonged times, it is common to observe deterioration in the nodule shape of the graphite. This is often referred to as fading of nodularity. Fading of nodularity is typically related to one of two possible phenomena, either fading of magnesium or fading of inoculation. It is important that the correct type of fading is pinpointed, as possible cures to avoid poor nodularity during time will differ greatly between the two fading phenomena. The micrograph below shows a good ductile iron microstructure immediately after magnesium treatment and post inoculation. The two lower micrographs show examples of microstructures for the two principal fading mechanisms.

Magnesium fading

Inoculant fading

Compacted graphite from magnesium fading

Irregular shaped graphite from inoculant fading





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Magnesium fading effects: When magnesium is lost to the liquid metal surroundings (e.g. slag, refractory, evaporation) it may cause a loss in nodularizing power over time. This is typically related to a loss of residual magnesium content in the iron, or even a pick-up of sulphur or oxygen from the surroundings, causing a gradual consumption of the residual magnesium. When residual magnesium becomes insufficient, poor nodules referred to as compacted graphite or vermicular graphite will occur. An example of this is given in the left micrograph. Residual magnesium and effects of Mg-fading is described in more detail in Elkem Technical Information Sheet 7. Inoculant fading effects: When the metal is held for prolonged times after addition of post inoculant, the effects of the inoculant will gradually fade. Depending on the type of inoculant and the addition rate, the ductile iron will gradually lose its nodule count and the nodules will also lose their spherical shape. Typically irregular shaped nodules are resulting, associated with a general drop in nodule count. An example of this is given in the right micrograph. Fading of inoculation is described in more detail in Elkem Technical Information Sheet 6. When discovering a poor nodularity condition, it is important for the trained foundry operator to clearly distinguish between the two types described. Cures for the two types of poor nodularity may in some instances actually be converse to each other. Possible cures for Mg-fading: Magnesium fading causing compacted graphite shapes can be cured by the following actions: •

Increase residual magnesium and/or rare earth’s by adding more nodulariser.

•

Avoid high metal temperatures and long holding times.

•

Reduce base iron sulphur content by using purer charge materials and additives.

•

Improve slag skimming operations to avoid resulphurising of the iron.

•

Avoid metal exposure to air causing oxidation of residual magnesium.

•

Using a RE-containing post inoculant to compensate for Mg-losses.

Possible cures for inoculant fading: Inoculant fading causing irregular shaped graphite and a low nodule count can be cured by the following actions: •

Use a more powerful, fade resistant post-inoculant or add more inoculant

•

Avoid high holding temperatures and long holding times

•

Use a second, late addition of a powerful specialty post-inoculant material

•

Sometimes irregular shaped nodules and low nodule counts may be a result of excessive magnesium or nodulariser addition, – i.e. reduced Mg-addition may improve the nodularity.

Technical Information 27 Alternative Tundish Ladle Design The tundish cover process may be designed to suit a range of different foundry conditions. Examples are fixed lid, automatic lifting or manual removable lids. Low levels of fume and smoke will escape from the tundish vessel, giving a good foundry environment. The figures below shows some alternative ways of deg tundish cover ladles with fixed and removable lids. A fixed lid requires an alloy charging port for introduction of MgFeSi-alloy. Ladles with fixed lid can generally not be deslagged, and therefore slag accumulation may be a problem. Hence, low slag forming alloys (low Ca and Al) are recommended for such ladles. The tea pot ladles offers an advantage in that liquid iron can be filled through the tea pot spout. This means that the tundish cover basin is eliminated. Both fixed and removable lids can be applied for teapot ladles. Teapot ladles will generally supply cleaner metal into pouring ladles and autopours due to the slag skimming effect of the spout.

The lifting or removable tundish cover design is probably the most flexible and easiest to maintain. Lifting covers can be either integrated to the ladle with a separate lifting lug or Elkem ASA, Foundry Products Postal address P.O.Box 5211 Majorstuen NO-0302 Oslo Norway




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constructed as a fully removable lid by using for instance a fork lift truck. The lid can also be mounted onto the furnace spout. The major advantages with removable lids are easy access to deslagging and cleaning of the ladles and also that the same lid can be used for several ladles. Charging of MgFeSi-alloy is also easier when the lid is removed from the ladle top.

For more information about tundish ladle treatments, please refer to Elkem Technical Information sheets 9, 10, and 11, and also 20 and 21 for selection of alloys and heat conservation.

Technical Information 28 Magnesium versus Sulphur in Ductile Iron Magnesium is added to liquid iron through the nodularizing operation mainly to desulphurize and deoxidize the base iron. When the base iron is properly desulphurized, graphite will grow as spheres instead of flakes resulting in good ductile iron. The principal desulphurizing reaction of magnesium in liquid iron is as follows: Mg + S = MgS This directly implies that lower base iron sulphur content will minimize the requirement for magnesium addition to make good ductile iron. The curves below show a schematic relationship between the base iron sulphur content and required residual magnesium to produce ductile iron.

Figure 1. Schematic representation of the relation between base iron sulphur content and required residual magnesium to produce ductile iron.

The curves in Figure 1 can be explained as follows. The lower straight line indicates the minimum theoretical Mg required to give a 1:1 ratio between Mg and S. However, Mg also goes into solution in the iron and evaporates (fades) during time, and the upper three curves shows recommended Mg addition to for such dissolution and fading effects. When iron of a base sulphur of 0.02%S is faded for about 12-15 minutes, a residual initial Mg of about 0.05% may be required. Elkem ASA, Foundry Products Postal address P.O.Box 5211 Majorstuen NO-0302 Oslo Norway




Revision No. 2.1 20.03.2004


2 Slag Large MgO, MgS and Mg-Si-oxide particles

Mg O

Nucleation Sites Small MgO, MgS and Mg-Si-oxide particles

S

=

Desulphurising

+

Deoxidising

Figure 2. Principal effects of magnesium in tying up sulphur and oxygen to produce slag and nucleation sites.

Figure 2 above shows a schematic representation of the principal reactions between Mg, S and O forming potential slag and nucleation sites for graphite. Figure 3 below shows an example of the effects of two base iron sulphur levels and a low and constant MgFeSi addition. The higher S of 0.018% is excess for the low Mg causing poor nodularity. Base S = 0.018 % 1.0 wt% MgFeSi Final Mg = 0.033 % Final S = 0.016 % Nodularity ∼ 50 %

Base S = 0.010 % 1.0 wt% MgFeSi Final Mg = 0.030 % Final S = 0.010 % Nodularity ∼ 90 %

Figure 3. Example of the effects of base iron sulphur at 0.018% and 0.010% respectively, for a constant addition of magnesium at 1 wt% MgFeSi alloy giving a constant residual Mg around 0.03% in the treated iron.

For more information about residual magnesium in ductile iron, please refer to Elkem Technical Information sheet 7.

Technical Information 29 Nitrogen Fissures Nitrogen gas porosity defects are predominately a problem in grey iron, but can also occur in ductile irons at higher nitrogen contents. Generally nitrogen fissures are found in medium to heavy sections adjacent to resin bonded mould or core materials. The nitrogen fissures are typically smooth faced surface or sub-surface irregular shaped cavities perpendicular to the surface stretching a few millimetres into the casting. The insides of the cavities are mostly black and shiny with dendrites penetrating into the cavity. Nitrogen pinholes are normally surface or sub-surface rounded cavities also with black and shiny inner surfaces. A graphite flake depleted zone normally surrounds the holes.

Example of Nitrogen porosity defect in grey iron revealed on machining.

Close-up of defect showing inner graphite lining and flake depleted rim.

Possible causes: • Use of high steel scrap content in cupola melted iron with high coke charges. • Use of high nitrogen containing raw materials in electric melting. • Use of high nitrogen containing resins or build-up of nitrogen in the sand. • Low carbon equivalent. • Insufficient Ti- or Zr-contents to neutralize free nitrogen. • Use of recarburiser with high nitrogen content.

Possible cures: • Reduce nitrogen content to below 85 ppm in heavier sections and 120 ppm in thinner sections. • Limit the use of high nitrogen containing materials in the charge. • Avoid high nitrogen containing recarburiser. • Increase the carbon equivalent. • Add Ti or Zr to tie up excessive nitrogen. • Increase pouring temperature. • Use a lower nitrogen containing resin binder system for cores (<3%N). • Improve venting of mold cavity and cores.





Revision No. 1.1 20.03.2004

Technical Information 30 Hydrogen Pinholes Hydrogen pinholes can be found in both grey and ductile irons. They usually appear as small spherical holes just beneath the casting surface and normally will have a smooth and shiny inner surface coated with a dense graphite lining. A graphite flake or nodule depleted zone is typically observed adjacent to the hydrogen pinholes.

Example of hydrogen porosity revealed on machining.

Close-up of hydrogen pinhole revealing inner graphite lining.

Possible causes: • High moisture content in charge or alloy materials. • Rusty charge materials containing hydroxide. • Oil and emulsion residues in charge material. • Low carbon equivalent. • High content of Aluminium or Titanium (see Elkem TI sheet No. 19). • High moisture content in moulding sand. • Build-up of clay in greensand. • Wet mould or core coatings. • Use of damp refractories or repair linings. • Incorrect proportion of binder components in cores. • Cores have become old and have picked up moisture.

Possible cures: • Limit the use of moist or rusty charge materials and avoid wet alloy materials. • Reduce Al-content below 200 ppm. • Reduce Ti-content and avoid adding Tibearing alloys. • Increase carbon equivalent. • Adjust Mn-content to below 0.4 x %Si (grey irons only). • Avoid excess use of ferroalloys with high Al-content. • Use dry launders and ladles. • Increase pouring temperature. • Check mould and core systems for clay balls or excess moisture in sand. • Ensure proper drying of coatings and glue. • Improve venting of moulds and cores.





Revision No. 1.1 20.03.2004

Technical Information 31 Carbon Monoxide Blowholes in Grey Iron Slag related gas porosity in grey iron often occur as rounded or irregular shaped cavities either inside the casting or open to the surface. Typically, clusters of slag or dross are found in conjunction to the cavities. The blowhole itself is a result from gas formation (typically CO gas evolution), from reactions between the slag and the carbon content in the iron.

Example of surface slag blowhole in grey iron.

Close-up of defect showing slag cluster.

Possible causes: • Manganese sulphide or oxide separation aggravated by high sulphur and manganese contents and low pouring temperature. • Improper slag separation. • High sulphur content. • Slag contaminated ladles and improper draining leaving a cold metal heel in the ladle that oxidized, causing Mn-oxide formation. • High contents of Ca, Al or Ti.

Possible cures: • Use clean ladles and ensure that ladles are properly drained between fills. • Reduce sulphur content to below 0.12%. • Avoid excessive manganese content, maximum 0.7%. • Improve slag skimming and removal procedures. • Increase pouring temperature. • Check gating system and secure that no slag enters the mould, and avoid turbulent filling. • Use filters in the gating system.





Revision No. 1.1 20.03.2004

Technical Information 32 Magnesium Slag Defects in Ductile Iron Magnesium containing reaction products from ductile iron treatment is a severe potential source for inclusion defects in ductile iron. Slag inclusions are typically found just beneath or at the cope surface as a result of improper separation during liquid metal processing. Magnesium slag defects may also arise from turbulent mold filling, and are often found as dross like stringers in areas of the casting where metal is deadlocked.

Example of magnesium slag inclusions.

Example of magnesium dross stringers.

Possible causes: • Inadequate slag separation from treatment vessel, metal processing and pouring systems. • Insufficient time between treatment and pouring to let slags separate to the surface. • Lack of slag traps or filter in the gating system. • Low pouring temperature. • High treatment temperature causing the need for elevated magnesium additions. • Excessive base sulphur content causing the need for high magnesium additions. • Excess addition of slag forming elements with alloys, such as Ca and Al. • Turbulent mould filling.

Possible cures: • Improve slag removal practice by the use of T-pots or proper surface slag skimming. • Extend hold times between treatment and skimming to allow for proper slag separation. • Review gating system and avoid high velocities and turbulence. • Use filters in the gating system. • Avoid high treatment temperatures and excess magnesium alloy additions. • Restrict base metal sulphur content. • Avoid using high Ca and Al containing alloys. • Improve thermal efficiency and minimize the treatment temperature.





Revision No. 1.1 20.03.2004

Technical Information 33 Slag Defects in Grey Iron Slag defects in grey iron is typically found in the cope side or dispersed due to turbulent mold filling. Grey iron slag inclusions are typically a result of improper separation of base metal slag or oxidation of the metal during processing. Defects can also occur as a result of reactions between metal and mold materials. Grey iron slag inclusions are typically high in manganese, silicon and iron, and sometimes other slag forming constituents such as Ca and Al from alloy additions can be found. Slag inclusions common also coexist with sand grains from reactions with the mould.

Example of grey iron slag inclusion cluster.

Close-up of slag cluster showing various phases.

Possible causes: • Inadequate slag separation from melting and pouring systems. • Cold metal heels in ladles and receivers. • Lack of slag traps or filters. • Low pouring temperature. • Excess addition of slag forming materials. • Turbulent mould filling.

Possible cures: • Improve slag removal procedures. • Proper draining of ladles and received to avoid cold metal heels. • Review gating system and avoid turbulent mould filling. • Use filters in the gating system. • Increase pouring temperature. • Use an inoculant with low contents of slag forming elements such as Ca and Al. • Use inoculants with fast dissolution characteristics and the correct sizing. • Avoid adding silicon carbide at a late stage of processing.





Revision No. 1.1 20.03.2004

Technical Information 34 Internal Shrinkage Porosity Shrinkage porosity in grey and ductile irons are typically present as internal cavities of varying size and shape- from large isolated holes to more scattered and smaller porosity only visible under the microscope. Very often a characteristic dendritic sub-structure is revealed inside the porosities. The defect normally occurs due to contraction in the last liquid metal to solidify and is thus often associated with heavier sections, changes in section thickness or hot-spots in a complex casting geometry. These same locations are also prone to release certain gas related porosity and thus it is often difficult to distinguish between gas and shrinkage related defects.

Overview of a sample with internal shrinkage.

Possible causes: • Soft moulds or not properly cured binder. • Insufficient clamping or weighing. • Excessive pouring temperature. • Excessive inoculation giving pronounced graphite expansion early in solidification and thus mould wall dilatation. • No inoculation or under-inoculation. • Insufficient or excessive (>3.8%) carbon content or inadequate carbon equivalent. • Hot spots resulting from poorly designed gates and risering systems. • Poor casting design causing unnecessary changes in casting section sizes. • High percentage of steel scrap in charge or excessive alloying or trace elements.

Close-up of typical shrinkage porosity in grey iron.

Possible cures: • Improve mould rigidity. • Clamp or weight the moulds adequately. • Avoid unnecessarily high pouring temperatures. • Avoid over-inoculation. • Adjust carbon content or carbon equivalent. • Provide adequate feed metal by proper gating and risering. Use solidification simulation if necessary. • Minimise hotspots by improving casting and gating design. Avoid sharp radii. • Use internal or external chills to avoid hot-spots. • Use inoculant designed to minimise shrinkage effects. • Reduce concentration of alloying or trace elements.





Revision No. 1.1 20.03.2004

Technical Information 35 In-the-Mould Nodularising The objective with in-the-mould nodularising processes is to pour untreated base iron into the mould and do the nodularising treatment inside the mould, thus producing ductile iron castings in a one-step operation. A reaction chamber containing the nodularising MgFeSi alloy is incorporated into the runner system inside the mould. The treatment takes place continuously while the iron flows through the reaction chamber before entering the cavity that forms the casting. Some important advantages and disadvantages of the in-the-mould ductile iron treatment process: Process advantages

Process disadvantages

• • • • • • • • • •

• Reduced space on the pattern. • Reduced casting yield. • Potential formation of dross in the mould. • Consistent pouring rate required. • Low base sulphur level needed, max 0.015%. • Possible variation in Magnesium analyses in different parts of the casting. • 100% quality inspection normally required.

Reduced number of production steps. Low capital investment. Low variable cost. Reduced temperature loss. High Mg-recovery gives low addition rate of nodulariser alloy. No treatment fading. No post inoculation needed. Late treatment discourages undercooling and carbide formation. Excellent interfacing with autopouring operations. No holding of treated iron reduces slag problems in holding and pouring furnaces. • Easy recovery after downtime. • Minimal environmental impact. • No treatment slag for disposal.

Example of typical in-the-mould magnesium treatment process layout. Common design rules are: A = system choke, B = A+10%, C = A+12%, D = E = A+30%.





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Calculating size of in-the-mould reaction chamber using Alloy Solution Factor (ASF): Chamber Area =

Pouring Rate Alloy Solution Factor

Design of reaction chamber, common rules:

Factors affecting Alloy Solution Factor:

• Inlet in drag. • Outlet in cope. • Depth of chamber: Height needed for nodulariser + 25 mm

• • • • •

% Mg in MgFeSi. % RE in MgFeSi. Pouring temperature. Sulphur level in base iron. Flow pattern in reaction chamber.

Typical ASF values range from 0.045 kg/cm2sec to 0.060 kg/cm2sec. Typical addition rate of nodularising alloy range from 0.8% to 1.2%. Reaction chamber area at different ASF values and different pouring rates. ASF

ASF

Pouring rate, W / t [kg/sec]

2

[lb/in sec]

[kg/cm sec]

2

4

6

8

10

12

14

16

18

20

1 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45

0.070 0.067 0.063 0.060 0.056 0.053 0.049 0.046 0.042 0.039 0.035 0.032

28 30 32 33 36 38 41 44 47 52 57 63

57 60 63 67 71 76 81 88 95 103 114 126

85 90 95 100 107 114 122 131 142 155 171 190

114 120 126 134 142 152 163 175 190 207 228 253

142 150 158 167 178 190 203 219 237 259 284 316

171 180 190 201 213 228 244 263 284 310 341 379

199 210 221 234 249 266 284 306 332 362 398 443

228 240 253 268 284 303 325 350 379 414 455 506

256 269 284 301 320 341 366 394 427 465 512 569

284 299 316 335 356 379 406 438 474 517 569 632

Chamber Area [cm2]

2

Recommended MgFeSi alloys for in-the-mould ductile iron treatment: LametTM nodulariser

Elmag® MF-6113 nodulariser

Si

44 – 48%

Si

43 – 47%

Mg

5.0 – 6.0%

Mg

5.75 – 6.5%

La

0.25 – 0.4%

RE

0.35 – 0.7%

Ca

0.4 – 0.6%

Ca

0.35 – 0.6%

Al

0.8 – 1.2%

Al

0.4 – 0.75%

Conventional MgFeSi alloys when used as in-the-mould nodularising agents can have several disadvantages. Promotion of shrinkage porosity and formation of inclusion defects are known problems. Recommended in-the-mould MgFeSi alloy sizing is 1 – 4 mm. The use of LametTM nodulariser, which is very low in slag forming elements, is an effective way of making clean ductile iron castings. By using pure lanthanum in the alloy in place of the traditional rare earth mixture of elements, LametTM nodulariser promotes a lower shrinkage tendency than traditional alloys used for in-the-mould applications. For further information, please refer to the Elkem LametTM nodulariser brochure. LametTM nodularizer is a trademark and Elmag® nodularizer is a ed trademark owned by Elkem ASA.

Technical Information 36 Inoculation of Heavy Section Castings 1. General The important benefits of inoculation are to eliminate the formation of hard, brittle iron carbides (cementite) in the structure and promote the formation of graphite during eutectic solidification. In grey irons benefits include improvements in machinability and mechanical properties, and also a reduction in the variability of properties caused by differences in casting section. In ductile iron, an increase in the number of graphite nodules produces more uniform structures over a range of section thicknesses. Such structures promote improved mechanical properties, a reduction in the segregation tendency of some alloying or trace elements in the iron and give better machinability. Note that certain base iron conditions, such as the initial sulphur content (grey iron), temperature and total “fade” time will affect the selection of a proprietary inoculant. Reference should be made to Elkem Technical Information Sheets Nos. 15 and 16 to optimise the selection of an inoculant. 2. Inoculation practices, heavy sections Inoculants should generally be added to cast iron in at least two of three stages during the casting procedure: • To the pouring ladle during filling from the furnace or holder (ladle inoculation) • To the stream of metal as it enters the mould (late-stream inoculation) • Using an inoculant insert placed strategically in the mould runner system. A two-step inoculation has shown great benefits. As it may be difficult to inoculate in the stream when casting heavy section castings, step two in the inoculation system could then be to place an insert of correct size in the mould. For two step inoculation of heavy section castings, it would be preferable to use a barium containing inoculant, such as Barinoc® inoculant in step 1 and an Al-Ca rich insert, Elcast® insert in step 2. 3. Inoculation to the ladle Due to the unavoidable lengths of time involved in handling ladles, it is necessary to add relatively large amounts of inoculant to offset the fading losses that occur. Addition rates vary from 0.2% for the majority of grey irons to 0.75% for the most critical ductile irons. The size grading of the inoculant should be based on the ladle size. In order to obtain the highest efficiency from the inoculant, simple addition rules should be followed: • Add the inoculant to the stream of metal entering the ladle. • Trickle the inoculant into the metal stream as the ladle is 25% to 75% full. • Ensure that the metal is slag free before tapping into the ladle. • When several transfers of metal between ladles are involved, add the inoculant during the last transfer before pouring to minimise fade. Elkem ASA, Foundry Products Postal address P.O.Box 5211 Majorstuen NO-0302 Oslo Norway




Revision No. 1 03.09.04


2

4. Inoculation in the mould When late metal stream inoculation is not possible, an insert of cast inoculant should be used in the mould / casting system as the secondary inoculant in order to give the maximum possible inoculation effect. The inserts are made with defined dimensions as cast-to-shape inoculant pieces that may be added either inside the mould runner system, or in certain cases as an integral part of the pouring basin. Optimum performances are obtained by an addition of only 0.05 – 0.15 wt%. Reference should also be made to Elkem Technical Information Sheets No 5 and 6. 5. Insert dimensions Available insert dimensions are as follows (mm). Insert

L

W

H

l

w

D2kg

164

74

76

134

42

D5kg

222

100

110

172

52

D10kg

266

130

132

220

80

D20kg

312

170

166

260

116

6. Inserts placed in pouring basin Medium and large sized mould castings are inoculated with the Elcast® inserts in the pouring basin. The pouring basin should have a capacity of at least 20% of the total melted iron required and should always be kept full in order to avoid turbulence or splashing during rapid pouring. The size of the pouring basin is also important in order to have a successful mould inoculation. A combination of several inserts can be made when pouring larger castings. 7. Inserts placed in gating system Placing of the inserts in the gating system for medium-sized castings. The inserts are placed in a reaction chamber in the gating system below the sprue. The design of the runners is trapezoidal whilst the ingates are rectangular. The distance between the reaction chamber and the first ingate should be at least 120 mm. The dimensions of the reaction chamber are that of the insert, but multiplied by 1.5 for the height and with a factor of 2 for the other measurements. 8. Dissolution time Inserts are required to dissolve within a specific time, governed by the pouring time of the casting and influenced by the temperature of the cast iron. As an example, for a D2kg the dissolution time will be 40 s at a temperature of 1370 ± 40 °C.

Technical Information 37 Characterisation of Molybdenum Containing Phases in SiMo Ductile Iron In SiMo ductile iron most specifications and standards typically allow for a maximum of 10% pearlite in the structure. Visual and automatic image analysis of the structure often indicates a pearlite content in the range of 10 to 15% although all process control measures have been taken in order to keep the pearlite content at a minimum. Closer investigation of the grain boundary area in SiMo reveals that several phases in addition to pearlite and Mo-carbides can be found and that the true pearlite content of SiMo ductile iron is in most cases significantly lower than initially measured or observed under low magnification. An SAE paper describes “The presence of a precipitate phase identified as Fe2MoC-type phase in Si-Mo iron may offer an inherent microstructure advantage with respect to dimensional stability at elevated temperatures relative to alternative ductile iron materials containing a pearlitic microstructure” 1

Typical structure in SiMo ductile iron.

Overview of the grain boundary area displaying different phases. (Backscatter Image)

Overview of the grain boundary area indicating where EDX analysis was conducted.



© Copyright Elkem AS Telephone +47 22 45 01 00 Telefax +47 22 45 01 52


Revision No. 1 11.05.2010

Technical Information 37 Spectrum

2 Si

Mn

Fe

Mo

Phase

3.5

1.1

86.7

8.7

Fe – Mo (-Si) Inter-metallic phase of Fe and Mo and most likely also Si. Dominating phase probably ing for 50% of the “pearlite” content.

4.1

0.6

94.1

1.2

Mo-rich ferrite Iron with 4% silicon and 1,2 % molybdenum. The higher Mo-content is most likely due to the close proximity to Mo-rich phases causing an enrichment of Mo in the ferrite.

0.6

0.9

20.2

78.3

Mo-carbide Classical Mo-carbide found along the grain boundary. This phase is hard to detect using an optical microscope, but clearly visible in the scanning electron microscope. Mo-content in the carbide is around 80%.

4.3

0.9

93.1

1.7

Mo-pearlite Classical pearlite appearance with a Mo-content of around 1,7% due to either an elevated Mo level in the ferrite part of the pearlite or in the iron carbide part.

4.3

0.4

94.6

0.7

Ferrite Ferrite phase further away from the grain boundary showing a composition in line with the expected chemistry in SiMo ductile iron with 4% Si and 0,7% Mo.

1.

Brent Black et al, Microstructure and Dimensional Stability in Si-Mo Ductile Irons for Elevated Temperature Applications

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