Effect of Charge Materials on Slag Formation in Ductile Iron Melts

Effect of Charge Materials on Slag Formation in Ductile Iron Melts C. Labrecque, M. Gagné and E. Planque Rio Tinto Iron & Titanium Inc. Sorel-Tracy, Quebec, Canada ABSTRACT The formation of an oxide slag on the liquid bath is unavoidable during the processing of Ductile Iron melts. Although the slag acts as a protective layer against the oxidation of the iron, it usually represents a significant cost item, depending on its composition and the quantity that is present. For example, an excessive amount of slag may result in increased furnace refractory wear, more slag transfer to the pouring ladle (increased probability of defective castings) and higher melting and disposal costs. The slag formed during Ductile Iron processing has many origins, some of them being inevitable. This is the case of the magnesium oxide and sulphide that evolve from the nodulization treatment. However, a significant amount of slag may also be generated by the charge materials used for the production of Ductile Iron castings. By knowing the impact of each charge material on slag formation, the foundryman can then include the cost related to slag formation in its economical analysis and optimize the total cost of its Ductile Iron castings. In this study, the effect of the various charge materials, namely high purity iron, Ductile Iron returns and steel scrap, and of their physical and chemical characteristics on slag formation was studied. It was shown that HPI has a very small contribution to slag formation when compared to the other materials. The results also indicate that the chemistry of the slag, which dictates its reactivity and aggressivity vis-à-vis refractory linings, is strongly influenced by the charge materials. INTRODUCTION The reaction of molten metal with its surrounding atmosphere, particularly with oxygen, is unavoidable, unless vacuum is maintained above the metal surface. As a result, a layer of liquid oxides usually forms and floats on the surface. The composition and the extent of this layer are, however, strongly influenced by the composition and the nature of the materials melted. Per example, melting pure aluminium results in the formation of a thin Al 2 O 3 layer floating at the surface. However, when an alloyed material such as Ductile Iron is processed the composition and volume of the slag formed depend on the reactivity of the elements present and on the volume of non-soluble phases entrapped in the solid charge materials. Moreover, wear and dissolution of furnace lining by the slag further contribute to increase the volume of slag. During the manufacture of Ductile Iron, the slag floating at the surface of the liquid bath may be the result of oxidation of alloying elements, such as silicon, which are present in the melt or of floatation of compounds, usually complex oxides and sulphides, found in the charge materials. The objective of this study was to isolate the effect of the different charge materials used in the manufacture of Ductile Iron, namely high purity iron units, Ductile Iron returns and steel scrap, on the formation of slag, independently of other sources of slag such as Mg treatment. By knowing the individual impact of each charge material on slag formation and its related cost, the foundryman can then optimize its process, keeping in mind that the cheapest charge material may not result in the lowest cost finished casting. EXPERIMENTAL PROCEDURES Charge Materials The formation of slag by each of the three major components of a Ductile Iron charge was characterized; these materials included two high purity iron (HPI) grades, ferritic Ductile Iron returns and two steel scrap grades. High Purity Iron Sorelmetal F-10 and S-110 grades were evaluated during this study; their typical chemical composition is listed in Table 1. The major difference between the two materials is the silicon content which varies from 0.15 0.20% in F- 10 to 0.90 1.10% in S-110. In addition, the influence of the inherent variability of the appearance of the ingots (due to the manufacturing process) on the formation of slag was evaluated. The test variables used were the surface roughness of the ingots and the thickness and weight of the surface oxide layer. In the paper, the HPI materials with apparently higher than usual surface roughness and oxide content are referred to as "irregular HPI".

Table 1 Chemical Composition of HPI Materials. Element Sorelmetal F-10 Sorelmetal S-110 Regular and Irregular Regular Irregular C 4.39 3.97 3.90 S 0.006 0.010 0.004 P 0.010 0.011 0.009 Si 0.17 0.93 1.04 Mn 0.005 0.006 0.005 Ductile Iron Returns Ductile Iron returns were obtained from a commercial foundry. They consisted mostly of ferritic castings that were rejected for structural defects. Typical rejected castings are shown in Figure 1 while Table 2 lists their chemical composition. and 0.014% Cr for the bushelings and 0.16% Cu and 0.13% Cr for the frag. Table 3 Chemical Composition of the Steel Scrap Materials. Element Bushelings Frag C 0.10 0.10 S 0.002 0.015 Si 0.0* 0.15 P 0.001 N.A. Mn 0.36 0.45 *: Aluminium killed steel. Characterization of HPI Ingots As indicated previously, the physical appearance of HPI ingots was classified as regular and irregular; Figure 2 presents the typical appearance of both types of ingots. Note that the irregular ones were hand-selected to represent the worst surface quality of the produced ingots. a) Figure 1 : Typical Rejected Castings Used as Ductile Iron Returns. Table 2 Chemical Composition of Ductile Iron Returns. Steel Scrap Element wt % C 3.77 S 0.013 P 0.012 Si 2.69 Mn 0.20 Mg 0.040 Two grades of steel scrap were used in the course of this evaluation program. One consisted of top quality "Busheling" scrap and the other of industrial quality "Frag" scrap. Table 3 presents the chemical composition of representative samples of these two materials. Note that the "Frag" scrap was tested under two conditions: asreceived and after selecting the cleanest components of the material. A major difference between the two types of steel scrap is the Cr and Cu levels; these were 0.018% Cu b) Figure 2: Illustration of a) Regular and b) Irregular HPI Ingots. The quantity of iron oxide present on the surface of the ingots was measured with two different techniques. Two typical ingots of each category were used in each case. In the first series of tests, cross section of the ingots including the surface were cut, mounted in bakelite and metallographically polished. Samples were classified as originating from a surface of the ingots solidifying in contact with the mold or in contact with the atmosphere. The thickness of the oxide skin covering the surface was

measured with a computerized image analyzer. A total of Table 5 700 measurements per type of HPI ingots were taken. Number of Test Heats. The weight of oxide covering the ingots was also evaluated. After brush cleaning, the ingots were weighed and then placed in a phosphoric acid solution to remove the surface oxide. They were taken out of the solution at regular time intervals, brushed and inspected for detectable remaining oxide. This was repeated until no oxide remains on the surface. The ingots were then dried and weighted. Finally the total oxygen content of the ingots was measured at their center; two samples were taken per ingot (2 ingots per grade). Evaluation of Slag Formation Different charges, whose compositions are listed in Table 4, were melted in a 230 kg capacity coreless induction furnace lined with an alumina crucible. The number of test heats per material is given in Table 5. For each test, a total metallic charge of 150 kg was melted and adjusted with the necessary amounts of graphite and FeSi 75 in order to obtain an iron bath containing 3.5 3.9% C and 1.5 2.0% Si. High purity Sorelsteel billet slices were used as additive to HPI and D.I. returns heats to dilute the carbon or silicon contents with minimum effect on the overall composition of the iron. The charges were melted, heated to 1530 C and kept 20 minutes at that temperature. Note that for HPI heats the concentration of oxygen in solution was measured at different temperatures using CELOX LAB DATA CAST 2000. After the 20 minutes plateau, the slag present at the surface of the metal was collected manually. After sampling for chemical analysis, the iron was discarded in a ladle and the wall of the furnace was cleaned of the adhering slag which was collected The slags from the metal surface and from the crucible were crushed separately and the liberated iron was removed by magnetic separation. It is recognized that some metallic iron remained attached to the slag particles while tiny slag particles attached to larger iron ones were removed by magnetic separation. All possible measures were taken to minimize these phenomena. After magnetic separation, the slag was weighted and subjected to chemical analysis. Table 4 Metallic Charge Composition. Heat HPI (%) D.I. Returns (%) Sorelsteel (%) Steel Scrap (%) F-10 80 0 20 0 S-110 80 0 20 0 D.I. Returns 0 67 33 0 Steel scrap 50(F-10) 0 0 50 Principal Charge Material Number of Heats F-10 regular 3 F-10 irregular 1 S-110 regular 3 S-110 irregular 3 D.I. Returns 2 Bushelings 2 Cleaned Frag 2 Frag 3 Prior to beginning the experimental program, a preliminary series of tests were performed to validate the experimental procedures and determine the influence of furnace melting history on slag formation. Several heats of regular Ductile Iron were previously produced in the furnace, in which the Mg treatment was carried out by plunging. Three heats with Sorelmetal S-110 were produced using the procedures described above. Results are presented in Figure 3. It shows that the amount of slag collected during the test heats decreases significantly after 3 test heats and then stabilizes. Therefore, in order to avoid the influence of previous heats on the amount of slag collected wash heats were carried out with the material to be tested prior to perform the test heats. %wt Slag (Slag/Total Metallic Charge) 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 S-110 Normal Level 0 1 2 3 4 Heat Number Figure 3: Amount of Slag Collected as a Function of the Number of Heats Following Ductile Iron Production. Results and Discussion Slag Formation with HPI Materials The measured physical characteristics of the HPI ingots are listed in Table 6; these are the thickness of the oxide layer, the surface oxide weight and the total oxygen content at the centre of the ingots.

Grade (%Si) Appearence F-10 (0.15) S110 (1.10) Table 6 Characteristics of HPI Ingots. Average Oxide Thickness (mm) Surface Walls Surface Oxide (wt%) Total Oxygen (ppm) at the Centre Ingot 1 Ingot 2 Ingot 1 Ingot 2 Ingot 1 Ingot 2 Regular 0.047 0.057 0.008 0.008 0.07 0.10 123 Irregular 0.029 0.032 0.028 0.034 N.A. N.A. 182 Regular 0.042 0.067 0.041 0.064 0.05 0.11 185 Irregular 0.036 0.055 0.022 0.034 0.08 0.09 96 Silicon content, i.e. 0.15 vs 1.10% Si, showed no significant influence on the characteristics of HPI ingots, regular or irregular, the only detected difference being the very thin oxide layer found on the surfaces of low silicon regular HPI ingots solidified in contact with the mold. However, since silicon content has no effect on the other measured characteristics that are also related to oxidation of the ingots, it may indicate that the mold wash used on the ingot mold surfaces was drier during the production of that heat of HPI. This did not significantly affect the weight of surface oxide or the oxygen content at the center. The physical appearance of the ingots i.e. regular or irregular, whatever the silicon concentration, showed no influence on the characteristics of the ingots. The four types of HPI were melted as previously described to evaluate the amount of slag formed. The composition of the iron after holding 20 minutes at 1530 C (2785 F) is shown in Table 7. Note that the final silicon content of the F-10 heats was slightly lower than the one achieved in the S-110 heats, probably because of a lower than expected silicon recovery from the added FeSi 75. Otherwise, all heats are equivalent with respect to liquid iron composition. The slag materials collected on the liquid metal and on the wall of the crucible were analyzed separately to verify the possible contamination of the slag adhering to the crucible wall by the crucible material itself, i.e. Al 2 O 3. The results are presented in Table 8. Table 7 Chemical Composition of Liquid Iron of the HPI Based Heats Prior to Slag Collection. F-10 S-110 Elements (wt%) Regular Irregular Regular Irregular C 3.56 3.42 3.64 3.73 S 0.005 0.012 0.007 0.012 Si 1.55 1.42 1.73 1.61 Mn 0.067 0.093 0.040 0.015 Table 8 Chemical Composition of Slag Materials Floating on the Liquid Bath and Adhering to the Furnace Wall. Element or F-10 Regular S-110 Regular S-110 Irregular Compound (wt%) Floating Wall Floating Wall Floating Wall C 0.69 0.33 0.86 0.22 0.22 0.27 S 0.07 0.05 0.05 0.04 0.11 0.18 Total Fe 11.4 10.9 22.5 14.5 13.5 10.8 SiO2 55.9 52.9 51.1 42.7 46.6 48.5 Al2O3 15.5 18.7 10.7 30.0 19.0 17.9 CaO 9.4 9.4 4.8 4.3 10.9 15.0 MgO 2.1 2.2 0.9 1.3 2.8 1.6 MnO 1.0 0.9 1.0 0.9 0.3 0.2 TiO2 0.7 0.7 1.0 1.1 1.7 1.7 As indicated earlier, the contamination of the slag samples by metallic iron was noticed by visual inspection. The occurrence of a significant concentration of iron in the slag confirms that point; it is believed that a major fraction of the iron is found as metallic iron since the high silicon and carbon contents of the liquid bath should ensure the preferential oxidation of these elements vis-à-vis iron. A significant concentration of Al 2 O 3 was analyzed in all samples. Since Sorelmetal contains 0.002% Al, the maximum amount of Al 2 O 3 that can form, assuming that aluminium completely reacts with oxygen and that Sorelsteel has the same aluminium content, is: 0.002 75g (Al2O 3) 150kg X X = 8g Al 100 28g (Al) 2 O 3 Since the typical weight of slag formed is ~ 225 g, the Al 2 O 3 originating from Sorelmetal would represent less than 5% of the slag formed. Another possible source of Al 2 O 3 is the FeSi alloy added to bring the silicon content to ~ 1.5%; in this case, assuming an aluminium content of ~ 1%, the contribution would be about 44,2 g or ~ 20%. The major fraction of the alumina found in the slag thus originates from the furnace lining. Note that the difference in Al 2 O 3 content between the floating slag and the wall slag for the S-110 regular HPI is most probably due to fragments of lining detached with the slag from the furnace wall. With this exception the compositions of the floating slag and of that adhering to the wall were comparable, indicating that the procedures used to detach the slag were appropriate and that the formed slag was not too aggressive vis-à-vis the Al 2 O 3 lining. As seen in Table 8, the HPI grade (or initial Si content) and the appearance of the ingots have no significant influence on the chemistry of the slag generated during melting. In all cases, the major constituents of the slag are silica, followed by Al 2 O 3 (most probably originating from the furnace lining) and CaO which may originate from the lining, the added FeSi and/or from desulphurizing residues in the HPI. Other oxides are in very low concentrations due to the high purity level of the charge materials.

Figures 4, 5 and 6 show the total amount of slag sampled from each heat (surface + wall) as a function of surface oxide thickness, oxide weight and total oxygen content of the HPI ingots, respectively. It clearly shows that the variation of the appearance or of the physical characteristics of the ingots has no significant effect on the quantity of slag formed. The oxide layer coating the ingots being essentially iron oxide, it is reduced by the silicon present in the melt which forms SiO 2 and constitutes, with Al 2 O 3 from the refractory lining, the major components of the slag. The contribution of the oxidation of elements such as Mn and Ti from HPI is negligeable. based on irregular HPI ingots. Results are shown in Figure 7. It shows that at 1530 C (2785 F) (the holding temperature for the test heats), the liquid bath contains only about 5 ppm oxygen in equilibrium with carbon and silicon. The results shown in Figure 7 indicates that the amount of oxygen in solution in the liquid iron increases with the iron temperature, as expected, but remains at low concentration due to the high Si and C contents of the bath. Once the chemical equilibrium between the metal, the slag and the atmosphere is reached, no more slag should be generated at the surface of the bath, except that coming from lining wear. 0.2 0.15 0.1 0.05 0 0.044 0.046 0.048 0.05 0.052 0.054 0.056 Oxide Thickness (mm) ppm Oxygen 9 8 7 F-10 6 S-110 5 4 3 2 1 0 1300 1350 1400 1450 1500 1550 1600 Temperature [ C] Figure 4. Effect of Oxide Layer Thickness on HPI Ingots on Slag Formation. Figure 5. Effect of Oxide Weight on HPI Ingots on Slag Formation. 0.21 0.19 0.17 0.15 0.13 0.11 0.09 0.07 0.06 0.08 0.1 0.12 0.14 0.16 0.21 0.19 0.17 0.15 0.13 0.11 0.09 0.07 % Surface Oxide 0 0.01 0.02 0.03 0.04 0.05 %O total Figure 7. Effect of Liquid Iron Bath Temperature on the Concentration of Oxygen in Solution in Iron at Equilibrium. Slag Formation with Ductile Iron Returns and Steel Scrap Charges Table 9 lists the liquid iron composition of heats based on Ductile Iron returns (67% returns) and steel scrap (50% steel scrap). As indicated in Table 4, addition of Sorelsteel bars or Sorelmetal was made to returns and to steel scrap to adjust the C and Si contents. The silicon content is slightly higher in the heats based on Ductile Iron returns, which also contain a lower carbon concentration. However, with respect to oxidation kinetics, a 3.15% C melt acts essentially like a 3.64% C melt in presence of 1.50 1.70% Si and the heats listed in Table 9 have C and Si contents comparable to those based on high purity iron. The difference in scrap quality can be noticed by the chemical composition of the heats. The bushelings steel scrap exhibits a lower level of trace elements (for example: Cu) than the frag scrap, whose composition was slightly improved by the hand selection process. Figure 6. Effect of Total Oxygen Content of HPI Ingots on Slag Formation. As a complement to this study on the effect of HPI characteristics on slag formation, the concentration of oxygen in solution in the liquid bath was monitored as a function of the iron temperature. In order to check the most severe conditions, this was carried out on heats

Table 9 Chemical Composition of Liquid Iron for Heats Based on D.I. Returns and Steel Scrap. Element Heat Based on wt % D.I. Returns High Purity Cleaned Frags Steel Scrap Frags C 3.15 3.41 3.64 3.49 S 0.008 0.003 0.010 0.010 Si 1.73 1.53 1.58 1.57 Mn 0.23 0.24 0.23 0.23 Cu 0.068 0.033 0.069 0.076 The chemical compositions of the slag formed on the melts based on Ductile Iron returns and steel scrap are listed in Table 10. The "wall" slag typically contains a significantly lower carbon content than the "floating" slag in which a carbon concentration as high as 14.5% was analyzed. This was not observed in the HPI heats, Table 8. It indicates that a fraction of the recarburizer added to adjust the carbon content of the melt remained entrapped in the slag that readily forms when melting these materials. The "wall" slag was found to contain more Al 2 O 3 than the floating slag due to the detachment of refractory particles when collecting the slag adhering to the furnace wall. This phenomenon which was limited in the HPI heats, may indicate that the slag formed in the melts with Ductile Iron returns or steel scrap is more intrusive vis-à-vis the furnace refractory than the one formed in the HPI heats. Table 10 Chemical Composition of Slag Materials Generated in Heats Based on D.I. Returns and Steel Scrap. Element or D.I. Returns Bushelings Cleaned Frag Frag compound (wt%) Floating Wall Floating Wall Floating Wall Floating Wall C 11.05 3.95 14.50 3.16 0.72 3.41 1.49 0.89 S 0.07 0.05 0.07 0.05 0.04 0.03 0.05 0.03 Total Fe 16.30 13.50 11.26 10.20 21.43 17.55 14.00 12.20 SiO 2 44.10 36.80 50.88 55.94 45.95 35.53 45.51 33.30 Al 2 O 3 9.20 37.60 8.02 13.17 15.39 30.20 19.90 36.80 CaO 0.59 2.96 4.92 5.69 4.55 3.68 5.36 2.50 MgO 4.51 3.89 0.60 0.75 1.03 1.85 1.08 3.40 MnO 8.98 5.89 6.56 6.12 5.84 4.70 5.61 4.80 TiO 2 0.50 0.85 0.84 0.95 1.93 1.98 1.73 0.85 Major differences in slag composition between HPI melts and the other materials are seen when comparing the data of Tables 8 and 10. This is further evidenced in Table 11, which compares the composition of the floating slag collected on heats based on the different charge materials. Note that the composition of the slags from Table 10 was corrected, assuming that the typical carbon content should be < 1%. Table 11 Comparison of Floating Slag Composition of HPI and Other Materials Elements or compound (wt%) HPI D.I. Returns Bushelings Cleaned Frag Frag Total Fe 11.4-22.5 18.30 13.30 21.40 14.00 SiO 2 46.6-55.9 49.50 59.90 45.90 45.50 Al 2 O 3 10.7-19.0 10.30 9.40 15.40 19.90 CaO 4.8-10.9 1.00 5.10 4.50 5.40 MgO 0.9-2.8 5.10 0.70 1.00 1.10 MnO 0.3-1.0 9.40 7.80 5.80 5.60 TiO 2 0.7-1.7 0.60 1.00 1.90 1.70 Within the limits of this experimental program, no significant difference was found between the "total Fe", silica and alumina contents of the floating slag formed by the different charge materials. With respect to "total Fe", it is speculated that the fraction of entrapped iron within the slag particles is comparable for all materials. For silica and alumina, an equilibrium appears to have been established between the silicon in solution in the iron, the refractory lining and the liquid bath. As the Si and Al contents of the bath are comparable for all materials (i.e. 1.50 1.75% Si and < 0.010% Al) and that the refractory lining is the same, the equilibrium values between the bath, the slag and the lining are not significantly influenced by the initial concentrations of these elements before melting. Note, however, that the presence of other compounds may dilute the SiO 2 and Al 2 O 3 concentrations and explain some of the differences seen. The heats based on Ductile Iron returns present two peculiarities: low CaO and high MgO contents. The low CaO concentration may suggest that the Ductile Iron castings used as returns were treated with low Ca alloys and that no slag carry over occurred during pouring of these castings. As expected, these heats contain a high MgO concentration. As shown in literature (1), MgO is the most thermodynamically stable oxide (except CaO) and any free magnesium present in the iron oxidizes to float in the slag, together with MgO/MgS inclusions present in the solid castings. The most significant difference between HPI melts and the other materials is the presence of a high concentration of MnO (5-10%) in the Ductile Iron returns and steel scrap based melts in spite of a relatively low Mn content of the liquid metal (0.23 0.24%). An increase of the Mn concentration in the metal would augment the MnO fraction in the slag. Because of the complex interactions between the different constituents, it is difficult to quantitatively determine the effect of MnO on the melting point of the slag. However, according to the MnO SiO 2 phase diagram (2), these compounds have an eutectic at 50% MnO, 50% SiO 2 ; therefore, it is reasonable to believe that MnO would contribute to reducing the melting point of

the slag and make it more aggressive for the lining material. This may explain the increased amount of adhering slag for melts based on Mn containing charge materials. The amounts of slag generated by the different charge materials expressed as the % of liquid bath weight are listed in Table 12; note that the amount of slag obtained from the different melts has been corrected for a charge composed of 100% of that material. Table 12 % Weight of Slag Formed. Material (% in the charge) Test Results Corrected Results HPI (80%) 0.12 0.12 D.I. Returns (67%) 2.90 4.0 Bushelings (50%) 0.35 0.57 Selected Frag (50%) 0.79 1.43 Frag (50%) 3 to 6 5.9 to 11.6 Of all the materials tested, HPI is the one that created the least amount of slag, with only 0.12% of the charged HPI material going to slag. Ductile Iron returns is a significant contributor to slag formation. MgO and MnO, whose presence is related to the material composition, contributed significantly to the slag formation, but the moulding material (SiO 2 sand) adhering to the casting surfaces is most probably the major contributor. The three grades of steel scrap tested gave different results, the highest quality steel scrap (bushelings), which displays a non-oxidized surface and no attached foreign material, has the smallest contribution to slag formation of this class of materials; this contribution is nevertheless five times that of HPI. It is seen in Table 12 that the cleanliness level of the frag steel scrap has a strong effect on the formation of slag; results vary from ~ 1.5% for a selected cleaned frag scrap to as much as ~ 12% for a dirty scrap (sometimes called by foundrymen "Winter Scrap"). Typical value for such a scrap is 5 to 6%. General Discussion The formation of slag has many economical effects on the production of Ductile Iron castings. As shown earlier, depending on its composition, a slag may be more intrusive for refractory lining and affect its life. A large slag volume increases the risk of slag carry-over during pouring and of associated casting defects; filters help to prevent such defects but increase the production cost of the castings. Slag often reduces the yield of treatment and alloying materials; for example, light materials such as graphite added to trim the melt composition may remain entrapped in the slag. Therefore, using charge materials that generate a large volume of slag such as steel scrap results in significant additional cost whose quantification depends on the processes and production parameters utilized in a given foundry. There are however costs which more directly depend on the charge materials mix. For example, assuming an average quality steel scrap is used, i.e. 5% slag formed, the additional costs are; A metallic yield of 0.95; Additional energy to melt and superheat 5% of nonusable material (slag); Disposal of the slag (up to $40.00/ton for nonhazardous slag and up to $240.00/ton for hazardous slag (containing leachable elements (Cd, Pb )) (3) ; And many others. In a study published by Mullins (4) the costs of the two charges which are described in Table 13 were compared. He concluded that although the apparent cost of charge materials is higher with a charge containing HPI, reducing the steel scrap from 45% to 25% by including 24% HPI in the charge results in a total cost reduction of at least 2%. This excludes other advantages inherent to the use of HPI: More consistent chemical composition of the base iron and of final chemistry resulting in more consistent mechanical properties (5) ; Salvage heat treatment reduced or eliminated; Increased melting capacity; Increased casting yield; Less scrap castings. Table 13 Description of Charges Used for Cost Comparison (4). Material % in Charge No. 1 No. 2 Sorelmetal F 0 24 D.I. Returns 50 46 Steel Scrap 44.7 25.6 Graphite 2.4 1.3 FeSi 50 0.5 1.0 CONCLUSIONS Within the limits of this study on the effect of charge materials on slag formation in Ductile Iron melts, the following conclusions can be drawn: 1. For a given liquid iron composition, the silicon content of the HPI ingots has no influence on the quantity or the composition of the slag formed. 2. The physical characteristics of the HPI ingots (rough and/or oxidized surface) has no significant effect on the quantity or composition of the slag formed.

3.- The concentration of oxygen in solution in the liquid iron is small and depends on the temperature of the liquid metal. 4.- HPI is the charge material forming the smallest quantity of slag (typical: 0.12% of its weight). 5.- Ductile Iron returns generate a large amount of slag (up to 4% of its weight). 6.- The quantity of slag generated by steel scrap varies from ~ 0.6% to 12% of its weight, depending on the quality of the scrap; average quality steel scrap generates about 5 to 6% slag. 7.- The composition of the slag depends on the composition of the charge; D.I. returns slag contains a significant amount of MgO while slags formed by D.I. returns and steel scrap are enriched with MnO. 8.- Partially substituting steel scrap by HPI in Ductile Iron melts results in significant savings which are accompanied by quality improvement. REFERENCES 1. L.S. Darken and R.W. Gurry, Physical Chemistry of Metals, Mc Graw-Hill, New York, 1953. 2. L. Coudurier, D.W. Hopkins and I. Wilkomirsky, Fundamentals of Metallurgical Processes, Pergamon Press, Oxford, 1978. 3. E.C. Muratore, personal communication. 4. J.D. Mullins, Take Another Look at your Charge Material Cost, Rio Tinto Iron and Titanium Inc., Montréal, May 1994. 5.- M. Gagné and E.C. Muratore, Improving the Consistency of Pearlitic Ductile Iron, presented at the Ductile Iron Society meeting, Los Angeles, November 1995.