Influencing Factors on the High Purity Pig Iron - Steel Scrap Optimum Ratio in Ductile Iron Production

I. Riposan*, M. Chisamere*, S. Stan*, N.Adam**
*Politehnica University of Bucharest, Romania
**Robinete Indestirale, Bacau, Romania

1. Introduction

Ductile irons show complex chemical composition, and therefore rigorous controls are required for the group of elements (Table 1) that are generally present. The important thing to note is that the elements during solidification process segregate inside the eutectic cells (FS<1.0: Si, Ni, Cu) or outside the intercellular regions (FS>1.0: Mo, Ti, V, Cr, Mn, P) [1].

To obtain ferritic ductile irons, most important are the elements that cause pearlite stabilization, having FP – Pearlite Promoting Effectiveness Factor at high values. Such elements are Sn, Mo, P, Cu, Mn, Ni, Sb and As [2]. In all cases, Antinodulizing Factor (FAN) must also be considered, for ensuring high nodularity as required for ductile iron (>80% NG, <20% C/VG, without LG). A lot of elements such as Bi, Pb, Sb, Ti, Sn, As and Al [3] are considered harmful to achieve high nodularity.

Along with the graphite deterioration the occurrence of intercellular lamellar graphite, which is also promoted by such elements as Bi, Pb, Sb, As, Cd, Al and Sn. Thick wall castings present a typical graphite morphology (Chunky Graphite) localized in the thermal center favored by Si, Ni, Ce and Ca. Carbides have a frequent occurrence in the ductile iron structure, as eutectic carbides (high degree of undercooling), inverse chill (Cr, Mn, P, Mg, Ti, As and Ce) and intercellular carbides (Mn, Cr, Mo and V) [1]. It is evident that the production of ductile iron requires a rigorous control of over 30 elements interacting and thus requiring a strict control over the selection of the metallic and non-metallic materials used.

For many foundries, steel scrap is an important component of their charge make-up, as it is a lower cost material. However, it is also a major contributor for trace elements. High purity pig iron, despite its higher cost, is very attractive as it is the lowest contributor of trace elements and has the potential to improve the metallurgical quality of the iron melt, especially for graphite nucleation. Although the ductile iron returns are the other component of the charge makeup, it is the steel scrap to high purity pig iron ratio, which is more important in order to minimize the melt cost and maximize the quality of castings. [4,5]

Laboratory experiments and plant trials were conducted to optimize the High Purity Pig Iron (HPPI) and the steel scrap additions for the various pearlitic/ferritic grades under different melting practices.

In laboratory experiments, the effect of SC/HPPI ratio on the solidification parameters was tested. In plant trials, as cast and heat treated ferritic ductile irons were obtained, by melting in the 50 Hz-Coreless Induction Furnace (CIF). High manganese gray iron, ductile iron returns and steel scrap were the starting materials and high purity pig iron was added to adjust the chemical and metallurgical behavior of the base iron melt. The possibility to use commercial steel scrap in Medium Frequency-Coreless Induction Furnace was also considered for different SC/HPPI/DIR ratios and metal matrix make-ups.

Finally, some representative graphs were recorded for various DI Returns/ Premium Quality Steel Scrap/High Purity Pig Iron ratios for the representative ferritic and pearlitic ductile iron grades.

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2. Influence of SC/HPPI Ratio on the Solidification Parameters

In the laboratory experiments, the influence of the SC/HPPI ratio on the solidification behavior was pointed out for the same final chemical composition of ductile irons. Two representative heats are presented in Table 2: for the same ductile iron returns rate of (40%) in, which the SC/HPPI ratio was changed from, 2:1 to 1:2. The final chemical composition of the two heats is very close (CE = 4.51 and 4.54%) as a result of alloying materials added into graphite crucible induction furnace (10 kg, 8000 Hz).

Tundish - Cover Mg – treatment was applied. According to the metallic charge make-up and SC/HPPI ratio, respectively:

• 2.2% FeSiCaMg 6 for SC: HPPI = 2:1 (L₁ heat)

• 2.0% FeSiCaMg 6 for SC: HPPI = 1:2 (L₂ heat)

Ladle inoculation was used as 0.35% FeSi75 addition.

The specific samples were cast in dry sand molds, under identical conditions of temperature and holding time:

• 25 mm Y- Block, for microstructure and mechanical properties evaluation;

• Wedge Test (W₄) and Chill Test (4C) (metallic plate, ASTM A 367-94);

• Quick-cup, Electronite type – Cooling Curve Analysis;

• Cone Sample (f 80 x 65 mm, V=110 cm³), for Shrinkage evaluation.

Metallographic analysis pointed out the same characteristics in the both variants as 80-90% Graphite Nodularity and metal matrix (Ferrite: Pearlite 1:1) despite the lower Mg – treatment alloy consumption in the second heat (SC/HPPI = 1:2).

Figure 1 illustrates the solidification parameters and mechanical properties of the ductile irons with the similar final chemical composition. According to SC/HPPI ratio decreasing:

• No different mechanical properties resulted

• Beneficial influence on the solidification was recorded:

• lower eutectic undercooling degree;

• chilling tendency (clear chill) markedly lower:

• *Wedge Test: 11 to 2 mm

• *Chill Test: 25 to 12 mm

• shrinkage level visibly lower.

3.1. Molten Cupola Iron "Heel" in 50 Hz – CIF Operation

A cast iron foundry operates both an acid cupola (1100 mm inner diameter) and an acid lined coreless induction furnace - CIF (6.3 t, 50 Hz) exclusively as a duplex system. Ductile iron is melted in induction furnace either with gray iron (200-300 grades) from the cupola or with proper charge melting in the induction furnace.

Frequently when it is necessary to change from gray to ductile iron production, the iron received from cupola is at high sulfur content (>0.15%), manganese (0.6-1.0%), phosphorus (>0.08%) and trace elements. On other times commercial steel scrap is usually used with excessive Mn content. In order to obtain ferritic ductile iron grades high purity pig iron (HPPI) is used to adjust the base iron for chemical and metallurgical control.

Low frequency coreless induction furnace (6.3 t) was used to produce as-cast 400-15 and 400-12 ferritic ductile iron grades and short annealed 400-18 grade from the base of 9-10% cupolas molten iron "heel" (0.18% S, 0.66% Mn), 26-51% DI Returns (0.33% Mn) and 18-28% Steel Scrap (0.3 and 1.15% Mn), by addition of 37-44% HPPI (0.002% Mn, 0.005% S) (Table 3-5, A….E heats) [4].

High purity pig iron (HPPI) had a high contribution in carbon (1.45-1.75%) but very low in silicon (0.06-0.07%), phosphorus (0.0007-0.0009%), sulfur (0.0019-0.0022%) and trace elements (<0.015% Cu, Cr and 0.03-0.04% Ni), without significant changes in manganese. In these conditions the manganese content in the charge is mainly due to the cupola iron "heel" (0.06-0.07% Mn), the contribution of ductile iron returns (0.08-0.16% Mn) and the steel scrap (0.08-0.2% Mn) but it was kept less than 0.3% by HPPI contribution.

Average level of silicon in the included charge was usually in the 0.9-1.0% Si range, even with the highest rate of returns (51%). Molten cupola iron "heel" gave the highest sulfur (0.016-0.018%), despite it being no more than 10% in the charge.

Sandwich technique was used for Mg – treatment (1.9-2.0% FeSiCaCeMg) and ladle inoculation was applied with Ba containing FeSi. General characteristics of the microstructure were as follows: more than 90% Nodular Graphite, more than 120 Nodules/mm², less then 60m m nodule diameter, less than 2% carbides. High Ultimate Tensile Strength (UTS) and Elongation (A) were recorded in as-cast state, with 0.30% Mn (D and E heats). In the last case, more than 550 N/mm² Ultimate Tensile Strength was obtained at UTS/BH = 3.0 ratio with no more than 40% pearlite in the structure and very good nodular graphite phase. Brinell Hardness (BH) level was according to metal matrix make-up.

In order to obtain high elongation ductile irons (400-15 and especially 400-18) in the above foundry (after gray iron processing in the induction furnace) the other alternative was also tried: 100% solid charge (without "heel"), DI returns at relatively high Mn content (0.33% Mn), steel scrap (0.28% Mn) and HPPI (0.002% Mn).

Tables 4-5 (F, G-heats) show the test results. In the as-cast state, predominantly ferritic structure was obtained, at more than 90% Nodular Graphite, more than 100 Nodules/mm² (mainly less than 60m m size). 400-15 DI Grade resulted in an as-cast state, while a short annealing cycle led to 400-18 DI Grade. The absence of the "heel" in the 50 Hz-CIF led to difficulties in the start of the charge melting, so the first tested variant, (gray iron melt "heel") appears to be more efficient to start ductile iron production.

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As was mentioned before, the ductile iron production needs to be closely monitored to prevent segregation within intercellular regions (especially lamellar graphite promoters), and factors that promote antinodulizing, chunky-graphite, pearlite and/or carbides.

New generation medium frequency Coreless Induction Furnaces - MF- CIF (200-800 Hz) are more often used in foundries for ductile iron production especially, without the necessity to use liquid "heel" for the next melting operation (only solid charge). In this melting shop, the metallic charge in the ductile iron production should be considered as made up of two major parts:

• Ductile iron returns (DIR) at maximum utilization;

• Steel scrap (SC) and High Purity Pig Iron (HPPI) in properties determined by the ductile iron type (ferritic, ferritic/pearlitic or pearlitic), iron grade, casting characteristics, heat treatment restrictions, graphite condition, etc.

Mn, Cr and Cu are among the most important elements in ductile iron production that control pearlite and carbide formation and stability. In this respect premium quality steel scrap is usually used especially in ferritic ductile iron grades. At continuous increasing price it is usually represented by high quality stamped sheet steel. Much cheaper and more accessible is the plain carbon steel scrap, but generally not favored by the high content of Mn (0.4-0.8%), P (0.03-0.05%) and trace elements (Cr, Cu, Mo, etc).

Taking into account the high dilution of the unfavorable elements in the iron melt and to increase its metallurgical activity (especially as graphitizing potential), HPPI can be an important factor in determining the use of commercial steel scrap.

Figure 2 shows the representative level of Mn, Cu and Cr for the metallic charge including 30-55% Ductile Iron Returns of different types (from ferritic, ferritic-pearlitic and pearlitic DI grades production), 20-70% Commercial Steel Scrap and 15-70% High Purity Pig Iron. Depending on the availability and the type of the returns (DIR), the amount of HPPI necessary in the metallic charge will vary: the lower DIR rate and/or more ferrite, the higher HPPI needed.

Controlled laboratory and foundry experiments and technical literature review pointed out the multiple beneficial effects of high purity pig iron on the quality of ductile iron castings:

• Mn, P, S and trace element levels are limited in the base iron;

• C, Si and P content stabilization, especially in mass production;

• Metallurgical quality of the iron melt is improved, giving:

  • lower carbides tendency;

  • higher eutectic cell count and lower intercellular segregation intensity;

  • higher ferrite amount;

• Stabilization of the mechanical properties;

• Lower incidence of the shrinkage defects;

The beneficial action of HPPI in ductile iron contributes not only the virgin material in the charge but also as that is contained in the return scrap. On the base of previous obtained data in this field [1] and general evaluation of the specific conditions for ferritic and pearlitic/ferritic ductile iron, the graph shown in Figure 3 was recorded. Elongation was used as a representative to express the microstructure formation, from chemistry, graphite nucleation and growth and metal matrix conditions.

Lower ductile iron returns require, higher high purity pig iron additions especially for ferritic grades and superior level of elongation (ductility). For the lower quality steel scrap, more addition of HPPI is necessary, especially to obtain ferritic grades. If high purity pig iron is typically used in ferritic ductile iron production, low amount of this special material is also required in pearlitic grades. In both cases, various other influencing factors interact, so representative ranges could be considered for different ductile iron returns and ductile iron grades (Fig. 4).

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The quality of ductile iron castings is highly process-sensitive and is influenced by the chemical composition. Over 30 elements determine the quality of this iron, which is affected by segregation factors, pearlite promotion elements, antinodulizing action, chunky-graphite, intercellular flake graphite and carbide presence. On the other hand, physical characteristics of the iron melt; especially graphite nucleation is based on the heredity phenomena and the charge materials, respectively.

In laboratory experiments, it was found that for the same ductile iron return addition rate (DIR = 40%) and the same final chemical composition (CE = 4.5%), the ratio of the Steel Scrap (SC) / High Purity Pig Iron (HPPI) from 2:1 to 1:2 leads to the decreasing of eutectic undercooling, chill and shrinkage with a 10% lower Mg consumption and with no effect on structure or mechanical properties.

In a foundry practice, successive gray and ductile iron production is recorded, in a 50 Hz – Coreless Induction Furnace. A cupola iron "heel" and higher Mn – content charge materials (Steel Scrap, Returns) are modified by a HPPI addition to adjust chemistry and metallurgical quality of the base iron, especially in ferritic ductile iron production.

For the Medium Frequency – Coreless Induction Furnaces operation different commercial steel scrap / high purity pig iron/returns ratios are considered as Mn, Cr, Cu in the charge has an influence to obtain ferritic, ferritic- pearlitic and pearlitic ductile irons; the lower DIR rate and/or more ferrite, the higher HPPI share.

Representative HPPI ranges together with premium quality steel scrap and specific DIR usage were identified for different elongation level and ISO Ductile Iron Grades.

It was illustrated that the main influencing factors on the HPPI in the common metallic charge are as follows:

• Metal Matrix: 15-40% HPPI for Ferritic vs. 5-20% for Pearlitic / Ferritic.

• DI Returns (DIR): lower DIR, higher HPPI necessary

• Elongation (A): higher A, higher HPPI addition

• Steel Scrap (SC): lower SC quality, higher HPPI amount.

1. DI Techniques. SORELMETAL – Suggestions for Ductile Iron Production. Published by RIO TINTO IRON & TITANIUM Inc., Canada

2. J.C. Margerie. The Notion of Heredity in Cast Iron Metallurgy. The Metallurgy of Cast Iron, Georgi Publishing Co., Saphorin, 1975, pp. 545-558.

3. T. Thielman. Zur Wirkung van Spurenelementen in Gusseisen mit Kugel – graphit. Giessereitechik, No. 1, 1970, pp. 16-24.

4. N. Adam. I. Riposan, M. Chisamera. Ferritic Ductile Iron Production in Coreless Induction Furnace, by the use of High – Mn Returns and Steel Scrap. Romanian Foundry Journal (RO), No. 3/4, 2003, pp. 17-20.

5. M. Chisamera, I. Riposan, S. Stan, C. Gadarautanu, N. Adam – Chemical Composition, Structure and Mechanical Properties Relationship in 400-18 Ductile Iron Grade Production. International Conference "Ductile Iron on the 21^st Century", October 2-3, 2003, Krakow, Poland.

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*ISO 1083 and ASTM A536

Fig.1 Influence of SC/HPPI ratio on the solidification parameters and mechanical properties

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Fig. 3 High Purity Pig Iron (HPPI) – Ductile Iron Returns (DIR) relationship for different elongation level in Ferritic and Pearlitic/Ferritic Ductile Irons

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Fig.4 Representative ranges of High Purity Pig Iron (HPPI) for different Ductile Iron Grades and Ductile Iron Returns (DIR) in the charge

Fig. 5

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