A New Approach to Ductile Iron Inoculation

A New Approach to Ductile Iron Inoculation T. Skaland Elkem ASA, Research Kristiansand, Kristiansand, Norway

Copyright©2001 American Foundry Society

ABSTRACT The objective of the present paper is to describe a new approach to inoculant design that has proven successful in improving casting performance and properties. The described inoculant represents a unique new generation of products developed for powerful cast iron inoculation. The ferrosilicon alloy contains levels of Calcium and Cerium that are adjusted to minimize chill formation and neutralize subversive trace elements in the iron. In addition, the inoculant contains small and controlled amounts of Sulphur and Oxygen in a form that make them available for reaction with the Calcium and Cerium during introduction into liquid iron. This special composition is designed to give highly powerful graphite nucleation conditions in ductile irons along with very effective chill and shrinkage reduction. Examples from foundry testing are reviewed, and the unique characteristics of this new inoculant concept described. The new inoculant is found to be more powerful than conventional ferrosilicon based inoculants, and give rise to very effective reduction in the shrinkage tendency of ductile irons. Special effectiveness has been observed in irons of low sulphur or irons of a “dead” nature from prolonged holding times. Also, results show improvements in both tensile properties as well as machinability for ferritic ductile irons. The new inoculant concept represents a patent protected design (Int.Pat.No.WO99/29911), and is available under a special trade name, 1 see footnote .

INTRODUCTION Based on years of laboratory work with experimental inoculants of various compositions, a new concept for inoculation of  ductile iron has been developed. The concept is novel in the sense that it involves not only an alloyed ferrosilicon-based material, but also introduction of non-metallic powders with the ferrosilicon alloy to give its special characteristic. The background for development of this product has been based on new fundamental understanding of graphite nucleation mechanisms in ductile iron, where the main body of nucleation sites was found to be comprised of complex but very stable sulphides and oxides (Skaland 1992). Figure 1 shows an example of such nucleation site in ductile iron both as a high magnification micrograph and a schematic representation of its phase composition. In conventional ductile iron production the availability of such sulphide and oxide nucleation sites are determined by the purity of base metal and its additives, holding times and temperature as well as metallurgical treatment processes and additives. Traditionally, commercial inoculants have been based on ferrosilicon alloys containing metallic additives such as Calcium, Barium, Strontium, Aluminum, Zirconium, Rare Earth’s, etc., with the main objective of these reactive elements to combine with Sulphur and Oxygen in the iron and form potent heterogeneous nucleation sites for graphite. However, with restricted availability of Sulphur and Oxygen in the iron, the metallic inoculant additives may reach a performance limit where their effectiveness are restricted by the number of potent nucleation sites that can be formed after treatment. Thus, the primary objective of the new inoculant concept has been to introduce controlled concentrations of non-metallic elements such as Sulphur and Oxygen with the metallic inoculant. From balanced and controlled inclusion engineering, this will deliberately produce a higher number density of nucleation sites for graphite from a reaction taking place between the highly reactive metallic ingredients (Ca and Ce) and the non-metallic ingredients (S and O) of the inoculant. These additional nucleation sites will then perform in parallel to the traditional sites formed during reactions between nodulizer, inoculant and the base metal. The outcome will be a remarkable improvement in conditions for controlled graphite precipitation and growth, with all possible benefits this may introduce to the final iron quality. Several researchers have proven the benefits of Sulphur to graphite nucleation (Chisamera 1994, Lalich 1976, Mercier 1969). Also, it has been proposed that Oxygen may play a vital role in the inoculation process (Tartera 1980, Nakae 1992, Podrzucki 2000). However, the combined use and performance of both elements through post-inoculation is a novel approach that has been designed to get the benefits from both Calcium, Cerium, Sulphur and Oxygen simultaneously in the graphite nucleation process. Calcium is used as the primary reactive element in inoculation, and has proven crucial for eutectic graphite 1

The new inoculant is available under the trade name Ultraseed ® inoculant, produced by Elkem. 1

nucleation (Bilek 1972). Cerium is introduced for several reasons. First, Cerium will contribute in neutralizing subversive trace elements in the base iron, forming stable inter-metallic compounds (Park 2000, Udomon 1985). Cerium will also have strong affinity to Sulphur and Oxygen, resulting in the formation of highly stable Cerium oxides, sulphide, and oxy-sulphides (Kozlov 1991, Warrick 1966). These Cerium compounds appear to be very beneficial in the inoculation process, resulting in improved nucleation effectiveness throughout the entire solidification range.

(a)

(b)

 Figure 1. (a) Example of duplex sulphide/oxide nuclei particle in ductile iron at large magnification in a  transmission electron microscope (70,000X). (b) Schematic representation of a nucleus particle containing  complex sulphide and oxide phases after nodularizing and inoculation of the iron (Skaland 1992).

EFFECTS OF INOCULATION ON CAST IRON PROPERTIES The principal effects of cast iron inoculation can be described as follows: • • • • • • • •

Avoid the formation of hard carbides (cementite) Promote the formation of graphite and ferrite Reduce the segregation tendency of alloying and trace elements Reduce the solidification shrinkage tendency Improve the machinability of castings Reduce the hardness Increase the ductility Give more homogeneous structures and properties in different sections of complex castings

The new inoculant concept is found to improve most all of these properties to a greater extent than other ferrosilicon based inoculant alloys. Especially, improvements in ferrite formation, shrinkage minimizing, machinability, and microstructure homogeneity have been observed through extensive testing in various foundry conditions. In the following, some of the unique features will be described in more detail, also including examples from foundries. A series of case studies will be reviewed to illustrate the performance characteristics through realistic examples from the industry.

UNIQUE FEATURES OF THE NEW INOCULANT CONCEPT The new inoculant concept provides formation of extra nucleation sites in ductile iron in addition to those initially generated by the magnesium treatment. This will increase nodule count and improve nodularity thus reducing carbide and shrinkage tendency. The balanced cerium content also neutralizes subversive elements that may prevent the formation of nodular graphite. Due to the higher nodule count obtained, the inoculant also provides formation of more ferrite in ductile irons. This can be an advantage when producing the higher ductility and impact resistant grades of ferritic iron (e.g. grade 40.3). The powerful nucleation characteristics are based on the formation of special cerium-calcium-sulphides and oxides that will act as effective nucleation sites for graphite during solidification of the iron. These nucleation sites will contribute together with the primary magnesium-silicon oxides to give powerful graphite nucleation with the outcome being a very high nodule number density. The inoculant is found to be especially potent in ductile irons of relatively low sulphur content and in irons treated with magnesium metal in a converter or wire injection process. The introduction of Cerium (Rare Earth metal) through the inoculant can also replace the need for Rare Earth’s to be added through the nodularizing process. The inoculant has also proven highly successful in providing fresh nucleation sites to ductile irons of long holding time where the base iron 2

or magnesium treated iron have been held for prolonged times before addition of the post inoculant. Such long hold times are well known to reduce the overall nucleation capabilities of the iron prior to inoculation resulting in so-called “dead” iron. The new inoculant concept will thus re-install good nucleation effectiveness from reactions with its deliberate Sulphur and Oxygen content forming additional, new nucleation sites. Due to the powerful effects on raising nodule count and improving chill protection, it has been found that the tendency to shrinkage cavity formation is also greatly reduced with this inoculant. Especially, the type of shrinkage that often occur as small porosities in hot-spot sections of complex castings appear to be effectively reduced or even eliminated by this inoculant concept. It has been found that a characteristic bi-modal size distribution of nodules often will occur from a secondary, late precipitation event in the last part of the solidification sequence. Such late graphite expansion effects will effectively counteract shrinkage contraction in the last part of solidification, when risers have stopped functioning and graphite expansion is most needed to counteract shrink. It appears that the new inoculant concept is effectively distributing the graphite nucleation and growth phenomena throughout the entire solidification range. Conventional inoculants, however, have a tendency to give massive and early expansion effects and very little contribution in the last part of solidification when really most needed. Strong nucleation effect and high nodule count is also a prerequisite to maximize the ferrite content when producing as-cast ferritic grades of ductile iron. Particularly when there are limitations on the final silicon content of the iron, the high nodule count obtained with this inoculant has proven effective to ensure the required minimum content of ferrite in such castings. The bi-modal size distribution of nodules, and the fact that the smaller and late precipitated nodules are formed in the last liquid to freeze, also aid in formation of more ferrite by acting as effective carbon sinks in these segregated areas enriched in pearlite promoting elements. This is also indirectly improving the machinability of ductile iron, and the inoculant should therefore be the preferred choice when good machinability is an important requirement. The special inoculant composition including additions of finely dispersed oxides and sulphides with the ferrosilicon based alloy, causes a specific appearance of this product. Figure 2 shows a comparison of the new inoculant design and a conventional ferrosilicon based alloy. It is clear that the new concept appears with characteristic black particle surfaces due to its sulphide and oxide content. Table 1 gives the specifications and typical composition of the new inoculant concept.

( a)

( b)

 Figure 2. Physical appearance of (a) conventional ferrosilicon inoculant with metallic, glinsing surface  characteristics, (b) new inoculant concept with black surface characteristics.

Table 1. Specifications and typical composition of the new inoculant concept. % Silicon

% C a lc iu m

% C e rium

% A lu mi nu m

% Sulp hur

% Oxygen

Specifications

70 - 76

0.7 5 – 1. 25

1.5 – 2.0

0. 75 – 1. 25

Max. 1.0

Max. 1.0

Typicals

73

1.0

1.75

1. 0

T ra c e

T ra c e

3

RESULTS FROM FOUNDRY TESTING The new inoculant concept has now been tested and implemented in numerous foundries Globally. More than 80 foundries have conducted testing so far, and of these above 60% have reported some kind of successful results. Foundries have different criteria and objectives in testing, and are looking for individual types of property improvements. The following specific features may be mentioned from foundry testing: • • • • • • •

Especially effective to reinstall powerful inoculation conditions in “dead” base irons Improved performance both in pure Magnesium and Magnesium ferrosilicon treatment applications Especially effective used as a late in-stream inoculation Especially effective in eliminating shrinkage porosity in complicated hot-spot sections Reduces the section sensitivity of nodule structures in castings of variable thickness May not be that efficient as an all-round gray iron inoculant Successful applications also observed in compacted graphite iron (reduced section sensitivity)

In the following, four case studies from foundry testing will be reviewed. The intention with these case studies is to show some typical examples of performance characteristics observed in different foundry conditions with the new inoculant concept. CASE STUDY 1

This foundry uses electric induction melting and a tundish ladle process for preparing ductile iron. The treated ductile iron is transferred into a fairly large channel induction holding and pouring unit where iron may sit for a while. The foundry has experienced problems with carbides and excess shrinkage in complex castings of very thin sections. A key challenge is to avoid big shrinkage porosity in a distant hot spot knob section that has to be drilled and need a smooth inner hole surface. The foundry normally uses an in-stream late post inoculation addition of a (Zr,Mn,Ca)-bearing ferrosilicon inoculant. When first testing the new inoculant concept, this was directly compared to the (Zr,Mn,Ca)-bearing ferrosilicon alloy as an in-stream addition. Table 2 shows the typical average nodule number densities obtained for the two inoculants, w hile Figures 3 through 5 shows examples o f the effects on nodule structure, carbide formation, and hot-spot shrinkage formation tendency. It is evident from the results that a pronounced difference in nodule number density and size distribution occur for the two inoculants. The new inoculant concept gives about double the nodule number density of the (Zr,Mn,Ca)-bearing inoculant, and the nodule size distribution shows a shift to numerous smaller and better shaped nodules. Figure 4 also shows the effects on carbide formation tendency in a very thin, 2-3 mm section of the same casting. The (Zr,Mn,Ca)-bearing inoculant with its lower nodule count reveals the appearance of intercellular carbides, while the (Ca,Ce,S,O)-inoculant has effectively eliminated these thin section carbides. Finally, Figure 5 shows the effects of  inoculation on the critical and difficult hot-spot section. The large region of micro-shrinkage occurring with the (Zr,Mn,Ca)bearing inoculant has been effectively minimized with the new (Ca,Ce,S,O)-inoculant. The extension of micro-porosity in Figure 5b has proven small enough avoiding rough inner surfaces after drilling of this critical knob section. Table 2. Average nodule number density for the (Zr,Mn,Ca)-bearing ferrosilicon inoculant  and the new inoculant concept. (Zr, Mn, Ca)-inoculant

N e w i n o c u la n t c o n c e p t

315 nodules per mm2

602 nodules per mm2

Conclusively, it can be said that this foundry is very happy with the new inoculant performances. A complete transition from the (Zr,Mn,Ca)-bearing inoculant to the (Ca,Ce,S,O)-inoculant has been taking place for ductile iron production. Significant improvements in reject rate and final casting quality has been obtained. The present addition rate of the new inoculant concept is about 25% less than the previous (Zr,Mn,Ca)-bearing alloy, still providing the significant improvements.

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(a )

( b)

( c)

( d)

 Figure 3. Examples of microstructure results from Test Foundry 1. (Zr,Mn,Ca)-containing (Zr,Mn,Ca)-containing inoculant, (a) polished condition, (b) etched in Nital.  New (Ca,Ce,S,O)-containing inoculant, (c) polished condition, (d) etched in Nital. (100X)

(a )

(b )

 Figure 4. Examples of carbide conditions in thin 2 mm flange from Test Foundry 1. (Zr,Mn,Ca)-containing inoculant revealing intercellular carbides. (Ca,Ce,S,O)-containing inoculant showing carbide free conditions.

(a )

(b )

 Figure 5. Examples of hot-spot micro-shrinkage porosity formation tendency from Test Foundry 1. (a) (Zr,Mn,Ca)-containing inoculant causing massive shrinkage porosities. (b) (Ca,Ce,S,O)-containing inoculant giving only traces of micro-shrinkage porosity. 5

CASE STUDY 2

This foundry is also an induction melting, sandwich treatment operation, and here the objective has been to test a series of  different generic inoculants in order to find the optimum product for their autopouring and in-stream inoculation on a DISA molding line. Output parameters evaluated include nodule- and microstructure, microstructure, mechanical properties and shrinkage tendency for the different inoculants.

(a)

(b)

(c )

(d)

 Figure 6. Examples of graphite nodule structure in plate castings from Test Foundry 2.  5 mm section size: size: (a) Sr-containing inoculant, (b) (Ca,Ce,S,O)-contai (Ca,Ce,S,O)-containing ning inoculant.  40 mm section size: (c) Sr-containing inoculant, (d) (Ca,Ce,S,O)-containing (Ca,Ce,S,O)-containing inoculant (100X). This foundry normally uses a Strontium-bearing ferrosilicon inoculant, and Figure 6 shows an example of the effects of SrFeSi and (Ca,Ce,S,O)-FeSi inoculation on the final nodule structure in a thin 5 mm and a thick 40 mm plate section casting. A quite remarkable difference is observed, especially for the heavier 40 mm plate section, where the (Ca,Ce,S,O)-inoculant appears to give a very strong increase in nodule count. Figure 7 shows a quantitative comparison of nodule counts for the two inoculants in sections of 5, 10, 20 and 40 mm thickness. The Sr-FeSi inoculant gives a quite normal and expected behavior 2 2 with a falling nodule count from about 300 per mm to 150 per mm when the section thickness increases from 5 through 40 mm. This behavior is normally observed for most commercial inoculants when increasing the section thickness. The (Ca,Ce,S,O)-containing inoculant on the other hand, shows a complete different and quite remarkable behavior. The histograms in Figure 7 and also the micrographs in Figure 6, shows an almost unaffected nodule count for the spread in 2 section thickness. About 300 nodules per mm are measured for both the 5 and 40 mm sections of the same test casting. In 2 2 fact, the 40 mm section contains an even higher nodule number density of 340 per mm versus only 312 per mm for the much thinner 5 mm section. This is clearly confirmed by the micrographs in Figures 6b and 6d.

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400 350 300 250      2     m     m200      /      N

Ce, S, O Sr

150 100 50 0 5

10

20

40

Section size (mm)

 Figure 7. Example of nodule count in various section thicknesses from 5 to 40 mm plate castings from Test Foundry 2 for (Ca,Ce,S,O)-containing inoculant and Sr-containing inoculant. This unusual behavior opens up for some interesting performances. First, it is ev ident that the section sensitivity for complex castings can be greatly reduced and microstructure and properties controlled and equalized for a large span in section thickness. This in itself may offer advantages to the homogeneous production of difficult castings with demanding properties in different locations. Further, the observation of an elevated nodule count in heavier sections also offer some additional benefits in relation to solidification contraction and shrinkage tendency. Figure 8 shows examples of crossbar hot spot shrinkage conditions using three different inoculants in Test Foundry 2. The inoculants are Ba-containing, Sr-containing, and the new (Ca,Ce,S,O)-containing FeSi. As the Figure shows, shrinkage tendency differs greatly for the different inoculants. Both the Ba-containing and Sr-containing alloys give massive contraction effects and large cavities in the hot-spot cross bar. The (Ca,Ce,S,O)-containing inoculant on the other hand, shows an almost complete elimination of shrinkage porosity with only one very small cavity revealed in the section cut through the parting line of the experimental cross bar casting. This dramatic effect on shrinkage tendency can be directly related to the nodule formation and the rate of graphite growth throughout the entire solidification sequence. As shown in Figures 6 and 7, the conventional inoculant behavior, represented by the Sr-inoculant, is to give fairly uniform nodule sizes and a reduction in nodule count as the section size increases. With the (Ca,Ce, S,O)-inoculant concept, there is an effect causing a bi-modal nodule size distribution and numerous smaller nodules that are precipitated very late during solidification. This late graphite expansion effectively counteract shrinkage contraction, as can be clearly seen in Figure 8c for the (Ca,Ce,S,O)-containing inoculant. Both the Sr- and Ba-containing inoculants give low nodule counts for heavier sections, around 200-220 per mm2 , while the (Ca,Ce,S,O)-containing inoculant gives about 350 nodules per mm 2 for the similar section. Since this phenomenon predominantly occur for heavier sections, this is where the bi-modal size distribution is most clearly observed and also where shrinkage control is mostly needed. There exist no clear understanding of the mechanisms of the late graphite formation and the resulting bi-modal nodule distribution effect. However, it is expected that the introduction of  Cerium in combination with sulphur and Oxygen in the inoculant, will introduce more nucleation sites, and possibly also a second type of sites that are activated later during solidification. Cerium oxides and oxy-sulphides will behave very different from the traditional Ca,Ba,Sr-type oxides and silicates know to nucleate primary graphite in the early stages of solidification (see Figure 1b).

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(a)

(b )

( c)

 Figure 8. Examples of sh rinkage porosity formation in crossbar castings from Test Foundry 2. (a) Ba-containing inoculant, (b) Sr-containing inoculant, (c) (Ca,Ce,S,O)-containing inoculant. Since the phenomenon is most evident in heavier sections, it is likely that this second type of beneficial and late activated nucleation sites only show their characteristic effects in slower cooling conditions. This is, the second type nucleation sites need more time to become activated, and will thus only give maximum benefits in heavier sections of a casting. The net outcome appears as a uniform nodule count in different sections, combined with effective shrink elimination in the heavier and slower cooled sections. The conclusion from this extensive inoculant testing has been that the Test Foundry 2 now has converted to use the new (Ca,Ce,S,O)-containing inoculant for all their ductile iron production. Great improvements in especially shrinkage reduction have been experienced, and the inoculant addition rate also reduced to a minimum.

CASE STUDY 3

The third case study represents a foundry producing very heavy ductile iron castings. Induction melting and tundish treatment is applied also here. In this case, the foundry suffers from the classical problems of heavy section castings such as graphite flotation, segregation, shrinkage and relatively poor nodularity. The foundry uses manual transfer inoculation to large pouring ladles, and the present inoculant material applied is a Barium-containing ferrosilicon alloy. Barium inoculants are traditionally recommended for heavy casting and slow cooling applications, since Ba is recognized for its minimum of fading tendency during prolonged hold and solidification times. The new (Ca,Ce,S,O)-type inoculant was tested in parallel to the Ba-inoculant as a ladle addition, and effects on microstructure, machinability and shrinkage tendency evaluated. Figure 9 shows examples of typical microstructures obtained with the two different inoculants in a fairly heavy 50 mm section. The Ba-inoculant shows the expected relatively large and uniformly sized nodules in a ferritic/pearlitic matrix (see Figures 9a and 9b). The (Ca,Ce,S,O)-inoculant, on the other hand, shows a much wider spread in nodule sizes, and the characteristic bi-modal distribution effect is again clearly revealed. Figures 9c and 9d shows the effect of the (Ca,Ce,S,O)inoculant on nodule distribution and ferrite/pearlite ratio. Table 3 also gives the quantified microstructure data for the two respective inoculants in this heavy section application. From Figure 9 and Table 3 it is evident that the heavy section impact on microstructure for the (Ca,Ce,S,O)-inoculant is significant. The bi-modal nodule distribution effect was found to effectively minimize difficult and massive shrinkage formation in large castings. The formation of smaller nodules also gave a general improvement of 10% in the nodularity from about 80 to 90 %. Further, Figures 9b versus 9d clearly show a significant reduction in intercellular pearlite with the new inoculant concept. The reduction is quantified from 25% down to 13% pearlite. The interconnected network of pearlite at 25% and higher is broken down into only minor fragments of pearlite in a predominantly ferritic matrix. This is again due to the numerous smaller nodules arising in the segregated intercellular regions, acting as carbon sinks during the eutectoid transformation. transformation. Raising nodule counts by a general increase in the primary formed larger nodules, typically will not influence the pearlite ratio to the same great extent. This is because segregation patterns and profiles will still remain the same. When the grain boundary nodules are included, this will have a pronounced effect on scavenging the matrix for carbon, thus effectively reducing the risk for harmful segregation phenomena and formation of intercellular carbides, phosphides, and other unwanted microconstituents. A general improvement in tensile and impact properties was also experienced with the bimodal and homogeneous nodule distribution. Also, an improved tool life during machining of up to 50% was experienced with this new situation.

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(a )

( b)

( c)

( d)

 Figure 9. Examples of microstructure in heavy section castings from Test Foun dry 3. Ba-c Ba-con onta tain inin ing g ino inocu cullant: ant: (a) poli polisshed hed con condi diti tion on,, (b) (b) etche tched d in in Nit Nital al.. (Ca,Ce,S,O)-containing (Ca,Ce,S,O)-containing inoculant: (c) polished condition, (d) etched in Nital. Table 3. Effects of Ba- and (Ca,Ce,S,O)-containing inoculants on microstructure characteristics in  heavy section casting at Test Foundry 3. Nodule count

Nodularity

Pearlite

Shrinkage

Relative

Per mm

%

%

tendency

machinability

Ba-inoculant

187

80

25

Significant

Me di u m

(Ca,Ce,S,O)-inoculant

357

90

13

Much less

Good

2

CASE STUDY 4

The final case study included here is from an automotive foundry using electric induction melting and sandwich magnesium treatment. Molding is done on BMD and D ISA lines, autopouring through Junker units. Late in-stream inoculation is applied on all pouring lines, and typically Zirconium-bearing inoculants have been applied. The foundry is suffering from some serious shrinkage problems, and the type shrink can be described as “massive” cavities in critical sections. Extensive risering has been implemented to try and overcome the problems, but still large shrinkage cavities are found to occur even adjacent to the risers. The new inoculant concept using a (Ca,Ce,S,O)-bearing ferrosilicon inoculants was tested out on the DISA line for a special test pattern involving a square cubic test piece attached to a fairly large riser. The cube and riser are cut in half, and evaluated for degree of shrinkage porosity formation and distribution of cavities in test casting and riser. Figures 10 and 11 shows examples of such test castings cut through the middle for evaluation of shrink. The examples compare two different Zirconium-containing Zirconium-containing inoculants, one (Zr,Mn,Ca)-type and one (Zr,Ca)-type, to the new (Ca,Ce,S,O)9

concept inoculant. Figure 10 shows conditions for the three inoculants, where the (Zr,Mn,Ca)-type to the left reveals a large cluster of micro-shrinkage porosity in the body of the cube test casting. The riser is virtually sound. The (Zr,Ca)-inoculant in the middle shows one large cavity in the cube sample and another in the neck of the riser, while the (Ca,Ce,S,O)-inoculant to the right shows only a very small porosity in the cube test piece. Conditions shown in Figure 11 compares the (Zr,Ca)-inoculant to the (Ca,Ce,S,O)-inoculant. The test piece for the (Zr,Ca)inoculant to the left shows again massive shrinkage in the cube sample and still quite widespread porosities also in the riser. The (Ca,Ce,S,O)-inoculant to the right has pushed the shrinkage void all the way back to the top of the riser, leaving the lower part of the riser and the cube casting itself sound and completely free from porosities. The foundry has solved a severe shrinkage problem using the new concept inoculant. Complete conversion from Zr-bearing to (Ca,Ce,S,O)-bearing inoculation has been implemented at this foundry.

 Figure 10. Example of cubic test castings with attached riser from Test Foundry 4.  Inoculants tested are: Left: (Zr,Mn,Ca)-bearing, middle: (Zr,Ca)-bearing, and   right: (Ca,Ce,S,O)-bearing. There exist numerous other cases showing similar improvements in chill situation, nodule structure, and shrinkage formation tendency using the new concept (Ca,Ce,S,O)-containing inoculant. However, from space limitations this paper is restricted to only cover a selected handful of classical situations experienced in small/thin and heavy/thick casting conditions. As shown above, effects are found to be most pronounced for nodule count and size distribution, as well as carbide restriction and shrink control. Other foundries have also reported great improvements in machinability conditions, and one foundry even eliminated their heat treatment operation after converting inoculants. Double tool life at half the addition rate of inoculant versus the previous regular calcium-bearing ferrosilicon has also been reported. Some foundries report of increased tensile strength even with a significant increase in the ferrite content. This paper most of all shows that the choice of inoculant material is not a trivial thing, and that different commercial inoculants may have dramatic effects on the final ductile iron quality. It is important to keep in mind, that sometimes the least expected inoculant is the one to actually perform best. Systematic and thoroughly controlled foundry testing will be the only sound and safe way to ensure that the optimum cost efficient alternative inoculant is being used in the individual foundry. There are too many unknown and uncontrollable factors affecting inoculation to give a general recommendation, and testing to solve specific challenges will at the end be the only safe way to find the improved or optimized ino culant solution.

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 Figure 11. Example of cubic test castings from foundry 4. Inoculants tested are:  Left: (Zr,Ca)-bearing, right: (Ca,Ce,S,O)-bearing. Note the differences in shrinkage  cavity distribution for the two inoculants.

When testing inoculants in cast iron it is always important not  only to look for a quick micrograph. Very attractive effects may then be overseen. A thorough evaluation of all the above discussed factors must be considered, since great savings may be found in reduced scrap rate from shrink, poor structures, machinability or even tensile and impact properties.

CONCLUSIONS The following main conclusions can be given from the present investigation: •

•

•

•

A new approach to ductile iron inoculant design has been described. The new design has proven successful in improving casting performance and properties. The new ferrosilicon based inoculant material contains levels of Calcium and Cerium that are adjusted to minimize chill formation and neutralize subversive trace elements in the iron. The new inoculant design also contains small and controlled amounts of Sulphur and Oxygen in a form that make them available for reaction with the Calcium and Cerium during introduction into liquid iron. The special composition is designed to give highly powerful graphite nucleation conditions in ductile irons along with very effective chill and shrinkage reduction. Experience from foundry testing has proven that the new inoculant concept is especially effective in re-installing powerful inoculation conditions in irons of a “dead” nature. Also, especial effectiveness in minimizing shrinkage porosity in complicated hot-spot sections has been observed. The new (Ca,Ce,S,O)-containing inoculant is also reducing the section sensitivity of nodule structures in castings of  variable thickness. A bi-modal size distribution of graphite nodules is often observed. This nodule distribution is instrumental in minimizing shrink and intercellular pearlite and carbide formation.

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REFERENCES Bilek, P.J., Dong, J.M., McCluhan, T.K., “The Role of Ca and Al in Inoculation of Gray Iron”,  AFS Transactions, pp.183188, (1972) Chisamera, M., Riposan, I., Proc. 5th Int. Symp. On the Physical Metallurgy of Cast Iron , Nancy, France, Sept. (1994) Kozlov, L.J., Vorobyev, A.P., “The Role of Rare-earth Metals in the Process of Spheroidal Graphite Formation ”, Cast   Metals, vol.4, no.1, (1991) Lalich, M.J., Hitchings, J.R., “Characterization of Inclusions as Nuclei for Spheroidal Graphite in Ductile Cast Iron ”, AFS Transactions, pp.653-664, (1976) Mercier, J-C., Paton, R., Margerie, J-C., Mascre, C., “Inclusions dans les spheroides de graphite”, Fonderie, April (1969) Nakae, H., Koizumi, H., Takai, K., Okauchi, K., “Nucleation of Graphite in Inoculated Cast Iron”, Trans. Japan Foundrymen’s Society , vol.11, pp. 34-39, (1992) Park, J., Loper, C.R., “Neutralizing of Lead in Gray Iron Melts Using Misch Metal”,  AFS Transactions, (2000) Podrzucki, C., Fras, E, Lopez, H.F., “The Inoculation Inoculation of Cast Iron: Role of Oxygen”,  AFS Transactions, (2000). Skaland, T., A Model for the Graphite Formation in Ductile Cast Iron , Ph.D. Thesis 1992:33, The University of Trondheim, NTH, Department of Metallurgy, Norway, (1992) Tartera, J., “Cast Iron Inoculation Mechanisms”,  AFS Int. Cast Metals J., pp.7-14, December (1980) Udomon, U.H., Loper, C.R., “Comments Concerning the Interaction of Rare Earths With Subversive Elements In Cast Irons”, AFS Transactions , pp.519-522, (1985) Warrick, R.J., “Spheroidal Graphite Nuclei in Rare Earth and Magnesium Inoculated Irons”,  AFS Cast Metals Research J ., pp.97-108, Sept., (1966)

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