An Alternative Route for the Production of Compacted Graphite Irons, 2004-10-21

8/9/2019 An Alternative Route for the Production of Compacted Graphite Irons, 2004-10-21

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An Alternative Route for the Production of Compacted Graphite Irons

C.M.Ecob and C.Hartung, Elkem ASA, Norway

Abstract.

This paper reviews the properties desired from a compacted graphite iron (CGI),these being an intermediate between grey and ductile irons. The two commonlyaccepted production methods are discussed, undertreatment with magnesium anda Mg + Ti addition, which creates and then suppresses nodule formation. Theadvantages and disadvantages of these methods are reviewed and compared to anew alloy based process specifically designed for CGI production. The newprocess gives a wider production window and does not have the disadvantages ofreturns contaminated with titanium.

Other considerations in the production of CGI, such as base metal composition,oxygen levels, inoculation and preconditioning are discussed.

Introduction

Changes to environmental legislation, predominantly in Europe, are affecting theway automotive manufacturers are planning car and truck design. By the year2008, average emissions must be reduced from 180 grams of carbon monoxideper kilometre to 140 g/km and average fuel consumption must be reduced from 7litres per 100 km to 5 l/100km.

This means that fuel has to be burned more efficiently to generate increased powerper litre of fuel. Inevitably this means that engines will have to burn fuel at hottertemperatures and therefore the engine must intrinsically have increased thermalstability and strength.

One solution to meet these standards is to introduce a higher proportion ofturbocharged diesel engines.

Both grey iron and aluminium would struggle to meet the properties required,

aluminium in terms of thermal stability, grey irons in terms of thermal conductivityand strength.

Compacted graphite will certainly meet the desired mechanical and physicalproperties, provided that historic production difficulties, particularly sectionsensitivity, can be overcome.

Compacted Graphite Iron (CGI) has been known now for many years, although it isonly recently that the material has become accepted as a serious engineeringmaterial.


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The production of ingot moulds, slag pots and casting trays has for many yearsbeen in compacted graphite irons, although these are all thick section castingswithout the intricacy of automotive casting design.

In recent times, cylinder blocks, heads, brake drums and discs, manifolds,

turbochargers and even piston rings have been produced in compacted graphiteirons. Most of the major automotive manufacturers have either producedcomponents in CG iron or are at the prototyping stage, although this varies acrossthe globe and some producers are considering alternative methods of increasingvehicle performance.

The list of components above have mainly been traditional grey iron castings, yetthe foundries most suited to the production of CGI are ductile iron foundries, wherelow sulphur levels are normally more easily achieved.

Until now undertreatment with MgFeSi or subverting the nodularity of a ductile type

iron with additions of titanium, have been the most common production methodsfor CGI.

The former has a dangerously narrow production window and the latter can lead tocontamination of ductile/grey iron returns with undesirable titanium. In this paper,the desired properties of CGI are reviewed and a third method of production isdescribed which give a greater production window and more consistency in theproduction process.

Properties of CGI

The properties of ductile iron are controlled by the matrix, whereas the graphiteflake form and size govern the properties of grey iron.

This determines such properties as the ductility in nodular irons and the thermalconductivity of grey iron. Compacted graphite irons make use of the vermiculargraphite form to give properties intermediate between grey and ductile. Anexample of this is the thermal conductivity and is shown in Figure 1.

Figure 1: Comparison of heat conductivity of grey, compacted (CGI) and ductile iron as afunction of operating temperature. [1]


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With CGI having a higher strength than grey iron, this has enabled thinner wallsections to be produced, partially explaining why the majority of castings switchingto CG iron come from the grey iron sector. A general guide to the mechanicalproperties achievable compared to grey and ductile irons are given in Table 1.

Relative damping capacity of grey, compacted and ductile iron is presented inFigure 2.

Table 1: Comparison of mechanical properties of grey (GI), compacted (CGI) and ductile iron(DI).

MatrixTensile

[MPa]

Hardness

[HB]

Elongation

[%]

GIPearlitic 200 270 175 230 0 1

Ferritic 330 410 130 190 5 10CGIPearlitic 420 580 200 250 2 5

Ferritic 400 600 140 200 15 25DI

Pearlitic 600 700 240 300 3 10

0 0,5 1

Grey

CGI

Ductile

Relative Damping Capacity

Figure 2: Comparison of the relative damping capacity of grey, compacted (CGI) and ductileiron.

Production Challenges

The principal challenge in producing a satisfactory compacted graphite ironremains the problem of section sensitivity. Most foundries will gauge the structureof the iron from a standard test bar, however in CGI; this is unlikely to reflect theactual properties of the casting. This is due to the unfortunate fact that if, forexample, the test bar contains 100% of compacted graphite forms, thinner sectionsin the actual casting will contain a proportion of graphite nodules, whereas a thickersection may contain graphite flakes as found in a grey iron. This also assumes thatthe test bar is cast in the same moulding medium as the actual castings, again aninfluence often overlooked in foundries.


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Figure 3 shows some microstructures containing various proportions of graphitenodules and the effect of increased nodule proportions is given in Figures 4, 5 and6. Factors influencing the graphite shape formed during casting are discussed laterin this paper.

5% 15%

30% 55%Figure 3: Various proportions of graphite nodules in CGI microstructures.

Figure 4: Effect of increasing nodularity on some of the properties of Compacted GraphiteIron.


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Figure 5: Effect of increasing nodularity on the mechanical properties of CGI. [2]

Figure 6: Effect of increasing nodularity on the thermal conductivity of CGI for variousoperating temperatures. [2]

Generally, CGI producers, and those starting in the production of CGI, will measurethe percentage of nodules in the matrix and this method is widely accepted. Anassumption is always made that flake graphite is not present other than at thesurface. A new ISO standard is currently nearing completion where the structurewill be classified based on nodularity.

It may seem strange to describe a material based on unwanted features, but sinceall mechanical and physical properties have been linked to nodularity the standardwill also be based on nodularity.

The standard will cover 5 grades of CGI with tensile strength from 300 to 500N/mm2 and elongation from 2.5 to 0.5%.

When only looking at the nodule count the shape, distribution and thickness of thecompacted graphite is not fully taken into consideration when classifying CGI.Shape, distribution and thickness of the compacted graphite will however have a

significant influence on the thermal and mechanical properties.


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Elkem has developed a programme for measuring the compactness of an ironusing image analysis techniques and this is shown in Figure 7. Whatever method isused to classify CGI it is however important to remember that this material willdepend on visual control by trained personnel as there is no good method todifferentiate between compacted graphite and degenerated graphite forms as

chunky, exploded and flake just using image analysis. Roundness or aspect ratio isoften used to classify the different graphite structures, but still a visual controlwould be needed to rule out flake graphite since this graphite form would beclassified as compacted graphite using roundness.


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Figure 7: Elkem Microstructure Report for CGI.


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Production Routes.

Compacted graphite irons may be produced from any one of several treatmentmethods. The most common are undertreatment by magnesium (compared to a

ductile iron process) and by a Mg + Ti treatment which creates and thensuppresses the nodules to the compacted form. In theory, treatment with cerium ornitrogen is possible, but the authors are unaware of any foundry makingcommercial castings by these processes and, for the purposes of this paper, theyare ignored.

Undertreatment with magnesium.

Typical ductile iron will have a residual magnesium of 0.025-0.06% Mg, dependingon the casting section thickness and the type of castings being made. Forcompacted graphite irons, the residual magnesium tends to be in the range of

0.01-0.03% and standard MgFeSi alloys can be used. This is, however, a difficultprocess to control with only a narrow residual Mg window giving satisfactorycompacted structures, too high a Mg will give an excess of nodules whilst too lowMg will lead to the formation of grey iron flake structures, particularly in thickersections. In castings of multiple section thicknesses, this method is practicallyimpossible to control and is not widely used. The process becomes even moredifficult with pure magnesium treatments, particularly wire, and often involvesexpensive trimming treatments, which incur royalty or licence fees.

Magnesium plus titanium treatments.

In this case, the iron is treated similarly to a ductile iron in terms of the magnesiumaddition, the difference being an addition of titanium to the process, either as anaddition of FeTi or using the Ti as an integral part of the MgFeSi. A residual 0.08-0.12% Ti would be typical. This method gives a wider production window than theMg undertreatment and reasonable CG structures can be obtained in both thin andthicker sections.

The major disadvantage of this method, apart from the high costs of the treatmentalloys, is the exceptionally poor machinability of the castings. A further concern isthe contamination of returns with titanium making them unsuited for use in eithergrey (promotion of type D graphite) or ductile irons.

Alternative methods for producing CGI are summarised in figure 8.


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Figure 8: Different production methods for CGI.

The Elkem alternative, CompactMag alloy

Elkem has developed a new alloy for the production of CGI, which is free fromharmful elements such as titanium and yet retains a good production window.

The alloy contains 5-6% Mg and 5.5-6.5% Rare Earth (RE) in a normal ferrosiliconnodularising base alloy.

It has previously been noted that magnesium is the common factor in the twodescribed methods of making CGI and research by Elkem has shown that rareearths have a beneficial effect on the section sensitivity, resulting in less variationof microstructure between thin and thicker sections. Rare Earths are also easier tocontrol than magnesium in that better and more predictable recoveries areobtained.

Using this Mg + RE alloy, good CGI structures can be obtained using an alloyaddition rate of only 0.3-0.45% as either a ladle or in-the-mould treatment. Thiscompares to 1-2% MgFeSi + Ti in the Mg+Ti method or 0.5-1.0% MgFeSi in theundertreatment method using standard commercially available alloys. In either

case this represents a substantial alloy cost saving. Whilst this figure will vary fromfoundry to foundry, an example of the treatment cost is given in Table 2.

Table 2: Comparison of treatment cost per ton treated iron (2003 numbers).

1.3 wt% MgFeSi US$ 13 0.35 wt% CompactMagTM alloyTM US$ 5

0.25 wt% FeTi US$ 6 US$

0.3 wt Inoculant US$ 5 0.2 wt% Inoculant US$ 3

Total US$ 24 Total US$ 8

It is interesting to compare the Mg+Ti route and the undertreatment method to theMg+RE (CompactMagTM alloy) route.


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Figure 9 shows microstructures obtained with a 0.35% addition of a standardMgFeSi alloy (6%Mg, 1% RE) and CompactMagTM alloy. This shows that a 5mmsection made with the standard MgFeSi contains predominantly nodules whilst a35mm section from the same casting is mainly grey iron. The same addition ofCompactMagTM alloy (0.35%) gives mainly compacted graphite in both sections,

although inevitably more nodules are seen in the 5mm section. This is in line withthe earlier discussion on the narrow production window with the magnesiumundertreatment.

5 mm Section 35 mm Section

MgFeSi with 1% RE MgFeSi with 1% REAddition rate: 0.35 wt% Addition rate: 0.35 wt%

CompactMagTM

alloy CompactMagTM

alloyAddition rate: 0.35 wt% Addition rate: 0.35 wt%

Figure 9: Example comparison Mg undertreatment and CompactMagTM alloy.

Table 3 shows a comparison of properties obtained in a foundry that ran the Mg+Tiand Mg+RE (CompactMagTM alloy) systems. The Mg + Ti additions were 1.3%MgFeSi and 0.5% FeTi compared to 0.35% CompactMagTM alloy in a sandwichladle process.


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Table 3: Example comparison Mg + Ti and CompactMagTM

alloy.

Example Example

PropertyGrey Iron(ISO 100)

Compacted byTitanium

Compacted byCompactMagTM

Ductile Iron(ISO 400-12)

Yield Strength

[MPa]- 290 330 min. 250

TensileStrength[MPa]

min. 100 365 380 min. 400

Elongation

[%]ca. 0.5 4.5 5 min. 15

Better yield strengths and tensile strengths are noted, whilst the lower alloyaddition rates not only gave a huge cost saving, but far less slag on the surface ofthe metal as shown in Figure 10.

MgFeSi with 1% RE: 1.5 wt% CompactMagTM

alloy: 0.35 wt%FeTi: 0.25 wt%

Figure 10: Slag on the surface of treatment ladle.


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It is clear from the examples shown that a third commercially acceptable method ofproducing CG iron is available and this offers several advantages over the MgFeSi- undertreatment and MgFeSi+Ti route.

4 No Ti contamination of returns.

4 CGI returns can be safely mixed with ductile returns.

4 Less Si introduction to the ladle allows for higher Si in the furnaceleading to improved lining life.

4 Minimum of slag and dross.

4 Better machinability by avoiding hard titanium carbide and titaniumcarbonitride inclusions.

4 Lower treatment costs.

4 Greater flexibility due to wider production window compared toMgFeSi - undertreatment.

4 Less section sensitivity compared to MgFeSi undertreatment.

4 No royalty or licence fees normally found with wire adjustmentsystems.


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Other considerations.

Whilst a correctly applied Mg/RE treatment process offers the best option to thefoundryman for the treatment process, some other factors have to be considered

before good CGI can be produced.These include the iron composition, preconditioning and inoculation on themetallurgical concerns chill, shrinkage, microstructure and section sensitivity.

Iron Composition

As with the successful production of ductile irons, probably the most importantconsideration is the preparation of the base iron for subsequent treatment. Whilstthe compacting process and inoculation are important, many foundries needlesslywaste alloy and time trying to correct the iron after treatment when this can be

done before the metal enters the treatment/casting cycle.

When looking at the base iron composition it is most important to control thefollowing three elements:

% C 3.5 - 3.8

% Si 1.5 - 1.9

% S 0.007 - 0.012 (preferred)

All other elements have less importance, but should not be significantly higher than

for ductile iron production. Generally a higher level of pearlite and carbidepromoting elements can be tolerated, as long as the S-level in the base iron is keptlow and the CompactMagTM alloy addition is kept below 0.40 wt%.

After treatment the final iron composition should be in the following range:

% C 3.3 - 3.6

% Si 2.0 - 2.5

% S 0.005 - 0.012 (preferred)

% Mg 0.005 0.015

% Ce 0.005 0.015

Typically the Mg- and Ce-content will be in the same range in the final iron. It isrecommended to aim for low C- and Si-content in the final iron, because this willgive a more consistent process although inoculation may be needed.Experimentation has shown that nodule count increases with increasing Si-contentand it becomes difficult to get a good compacted graphite structure.

Many foundries wish to use the same base charge for both ductile and compactedgraphite irons, however it should be noted that the low addition rates of


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CompactMag and Foundrisil inoculant required may necessitate additionalsilicon units being added to the base charge.

Preconditioning

Some foundries pre-treat their iron to create the same conditions prior to everytreatment. This preconditioning can be a controlled introduction of either S and/orO.

The most successful foundries now measure the base iron oxygen levels with anoxygen probe to determine active oxygen. Through additions of low stabilityoxygen source the level is maintained between 50-80 ppm of total oxygen. One ofthe popular preconditioners is the Elkem product Ultraseed inoculant, whichprovides both oxygen and sulphur in addition to other nucleating elements at thisvital stage of the process. The preconditioning with Ultraseed inoculant can bedone either through addition to the furnace or to the stream as the iron is poured

from the furnace to the tundish ladle or other treatment vessel. Care should,however, be taken as preconditioning can provide sufficient nuclei to generateexcess nodules. It must be remembered that the use of Mg/RE CompactMagTM

alloy is not as violent as typical ductile reactions and the destructive effect onpotential nuclei is not as great.

Sulphur Content in the Base Iron

The base metal sulphur content plays a critical role in the production of CG iron. Inmany cases, castings being converted to CGI have traditionally been made in greyiron, hence the interest of some grey iron foundries in producing CG irons. Thisdoes require a change of thinking in such foundries to produce a base ironsatisfactory for CGI.

The sulphur content in the base iron should be in the range 0.007 0.015%. It ispossible to produce good CGI with a base iron sulphur level as high as 0.02%, butgenerally the process becomes harder to control as the base iron sulphur levelincreases. While there seems to be a linear correlation between base iron sulphurand addition of CompactMagTM alloy needed for the sulphur range 0.007 0.02%,this is not the case for sulphur levels above 0.02%. Here it looks like there is anexponential correlation, hence more compacting agent will be needed and the

compacting process becomes unpredictable.Figure 11 shows that CompactMagTM alloy will give highly satisfactory structures atthe higher sulphur levels and that the addition rate of the alloy does not have to beincreased significantly.


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a) 0.30 wt% CompactMagTM

alloy -addition b) 0.35 wt% CompactMagTM

alloy -addition

c) 0.35 wt% CompactMagTM

alloy -addition d) 0.40 wt% CompactMagTM

alloy -addition

e) 0.45wt% CompactMagTM

alloy -additionFigure 11: CGI structure at different sulphur levels with CompactMag

TMalloy.

It is suggested that, due to alloy consumption, slag generation, chill promotion andprocess control concerns, irons of above 0.020% base sulphur are not suited toproduction of CGI.

0.01 wt% S a) and b)0.015 wt S c) and d)0.02 wt% S e)


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Foundrisil Inoculant Sandwich Cover

In the production of ductile iron, many foundries use a steel scrap cover in thetreatment ladle to retard the onset of the magnesium reaction and enable them toget a greater head height of metal into the ladle.

The low addition rate of CompactMagTM alloy, as previously described, has twomajor benefits when compared to higher addition rates of MgFeSi based alloys.Not only does the low quantity of alloy give a low reactivity, but the knock-on effectof not destroying the nuclei inherent within the melt. Replacing the steel scrapcover with a moderately powerful inoculant, such as Foundrisil inoculant, providessufficient additional nucleation to minimise or eliminate the need for subsequentinoculation. Experience has shown that a 0.3% by weight addition of Foundrisil

inoculant is an optimum addition. Under standard conditions, the 0.3% Foundrisil

inoculant cover has been found to decrease the tendency to chill formation and togive better graphite compacts than the use of a steel cover. In some cases,

particularly in thicker section castings, the need for post inoculation can beeliminated. While for chill prone section the addition of Foundrisil inoculant ascover for CompactMagTM alloy may need to be adjusted upwards or additional postinoculation has to implemented.

Post-inoculation

Inoculation of CGI has to be considered carefully. Whilst it is desirable to produce astructure free from iron carbides, inoculation will tend to promote the formation ofgraphite nodules. It is therefore recommended that inoculants of moderate potencybe used. Interestingly, it has been found that inoculants generally associated with

grey iron, e.g. Superseed

inoculant, are very effective in CGI, as are the morecommon ductile inoculants. Typically, addition rates are intermediate between greyiron and ductile iron and are generally 0.1-0.5% for ladle applications, dependingon metal temperature, fade time and casting thickness. With good preconditioningand perhaps the use of an inoculant base material as sandwich cover in thetreatment ladle, it is often possible to eliminate totally the post-inoculation process.

Fade time and treatment temperature

Depending on casting and casting condition the fade time may vary from 5 to 20minutes without a negative influence on the microstructure obtained with

CompactMagTM alloy and Foundrisil inoculant cover. Treatment temperatures inthe range 1400C to 1520C have been tested without any negative effect on themicrostructure, but, as with all castings, the choice of post inoculant may have tobe adjusted dependent on the final pouring temperature.


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PQ-CGI - Process Control technology for production of Compacted GraphiteIron

Elkem & NovaCast is a single-source supplier of technology, equipment, software,training and alloys for efficient, safe and economical production of compacted

graphite iron.

Why is a process control system needed?

Compacted graphite (also called vermicular graphite) is an intermediate formbetween lamellar and spheroidal graphite. The requirements on process controlare therefore much more pronounced because both a minimum and a maximumlimit have to be considered. The process window for magnesium is very narrow,typically less than +/- 0.0015%. However the crystallisation of graphite into acompacted shape is not only dependent of the level of magnesium but also on themetallurgical status of the base iron. Especially important is the nucleation status,

the level of total oxygen, sulphur, nitrogen and the active carbon equivalent.Controlling the iron by spectrometer analysis is therefore not sufficient as it onlyshows the amount of each element but does not reveal anything about themetallurgical status. The illustration shows how the graphite shape varies with themagnesium level. If the amounts of nodules should be within 10-30% then the totalprocess window represented as active magnesium is 0.003%. The probability to bewithin the CGI-window with normal variations in oxygen, nitrogen, sulphur, ACELand nucleation and with traditional control methods is less than 80%. Thus inorder to be able to produce high quality castings in CGI it is essential to have ametallurgically based process control system.

Figure 14: Process window for magnesium by the production of compacted graphite iron.

NovaCast has developed a special patented process control system for productionof castings in compacted graphite iron. The system which is called PQ-CGI

(abbreviation for Prime Quality Compacted Graphite Iron) has been enhanced inco-operation with ELKEM for use in combination with alloys specially developed forCGI. The system uses a combination of advanced quantitative


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thermal- and chemical analysis to determine the metallurgical status of the baseiron. Based on the analysis, which includes total oxygen, the computer systemsuggests how to condition the base iron in order to keep its status within apredetermined window. Once the base iron is within that specification, the systemproduces a recipe for additions of alloys to the treatment ladle. The recipe is thus

optimised for the specific castings to be made and for the current status of thebase iron. Several treatments can be made using the same recipe as long as thebase iron remains the same. Thus it is a true one-step process. The PQ-CGIsystem also includes a thermal analysis system for verification (quality assurance)of the treated iron.

The PQ-CGI Ladle system for batch treatment

The PQ-CGI Ladle system uses two sampling stations. One is for metallurgicalanalysis of the base iron. The other is for testing and verifying the treated iron. Themetallurgical analysis, combined with chemical analysis, is used in order to

recommend conditioning of the base iron if needed until it is within apredetermined process window. Once the base iron is properly prepared then thePQ-CGI system recommends optimal additions to the treatment ladle in order toobtain the desired CGI structure and physical properties. The additions are specificto the current base iron. That means that the recipe can repeated until the furnacehas been emptied or until there is a change in the iron. The system is adaptivewhich means that it is gradually fine-tuned for each specific casting by means of alearning algorithm. The one-step process means that the time from treatment tostart of pouring is very fast. This minimises the need for over-heating and the timewait for iron after a stop in the moulding line. As the nucleation with the process iscarefully controlled no extra inoculation is needed. Combined with special Elkem

alloys such as CompactMag the fading is practical non-existing within 15 minutesafter treatment. The section sensitivity is less than with conventional alloys.

Figure 15: Example showing the process control window for PQ-CGI system.


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The main technical advantages of the PQ-CGI Ladle process are:

Compacted graphite iron can be produced with high repetition accuracy The process is controlled by a computer system, which logs all events The process allows very low magnesium levels to be used, which reduces

the risk for shrinkage and dross problems The one step-process improves machinability of the castings The process reduces the section sensitivity and produces a more

homogeneous structure The process shows very low fading which allows long times (


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Summary.

Although CGI has been known for a number of years, it is only now that castingsare being produced in commercially interesting quantities.

There are three principle routes for the production of CGI;

a) Undertreatment with Mg, normally MgFeSi

b) Suppression of the nodules to a compact form by using Mg + Ti

c) The use of CompactMagTM alloy Mg/RE system.

The latter system has been shown to have some significant advantages over thealternative methods,

4 Greater production window and more flexibility.

4 Low reactivity in the ladle, thus reducing the need for subsequentpost inoculation.

4 Low residual Mg and RE levels which reduces susceptibility tochill.

4 Can be used over a range of sulphur levels within the normallimits for CGI production.

4 Low slag generation.

4 No contamination of returns with Ti.

4 Used in conjunction with Foundrisil inoculant cover in thetreatment ladle minimises the need for post inoculation.

4 Long fade time.

For further information on, please contact your local Elkem representative who will

be able to demonstrate this total CG iron production package.

Elkem & Novacast can help you set up a technical solution for production of CGI,which gives you an advantage when it comes to quality, environment andeconomy.

Reference List

[1] Mekanpublikation Mekanresultat 85002 Mars 1985[2] BCIRA Broadsheet 253

Documents

An Alternative Route for the Production of Compacted Graphite Irons, 2004-10-21