Converting Gray Iron to Ductile: Three Foundries' Experiences.

By examining three plants' production practices, foundries can enhance their own 'best' practices for producing quality ductile iron components.

One of the best ways for foundries to measure the accuracy and quality of their production practices is to examine those of their colleagues. Through this examination, foundries may be able to learn a new "trick of the trade" or method of production that will improve their own casting operation.

This article examines three foundries' production practices in the conversion of their base gray iron melt to base ductile iron. The foundries examined are Brillion Iron Works, Inc. (BIW), Brillion, Wisconsin, Burnham Foundry, Zanesville, Ohio, and Giddings & Lewis Castings (G&L), Menominee, Michigan. As the most critical phase in ductile iron production, melt preparation determines the ultimate metallurgical success or failure of the castings. By examining the methods utilized by the three firms in this article, other ductile iron foundries will have points of comparison with which to determine the efficiency of their melt conversion methods.

BRILLION IRON WORKS

BIW has two plants capable of producing 1200-1300 tons of iron/day. Plant 1 has five 9-ton and one 1 5-ton coreless induction furnaces serving six automated molding lines. The breakdown of production for this plant is 80% gray iron and 20% ductile. Plant 2 has five 23-ton coreless induction furnaces and two 65-ton channel holding furnaces serving four automated molding lines. The breakdown of production for this plant is 80% ductile iron and 20% gray. Between the two plants, BIW produces all standard grades of ductile iron and all classes of gray iron from class 25-45.

Scheduling production in the two plants is based on 3-day rolling windows as well as iron type and demand. Supervisors schedule each individual shift based on customer due date, manpower requirements and iron limitations. BIW has the capability to move patterns between the molding lines to accommodate job demands, however, there is a preferred line for each job.

Concerns

BIW has four main concerns in the conversion of base gray iron to ductile iron.

Sulfur CS) Levels--Plant 1 is a gray iron shop that sometimes pours ductile. During gray iron production, the base metal's S levels run from 0.07-0.08%. The base ductile iron S starts at 0.040.05% and will gradually decrease to 0.02-0.025% after eight charges are through the furnace. Plant 2 is a ductile iron plant that sometimes pours gray iron. Its base ductile iron S runs from 0.015-0.02% and its base gray iron S runs from 0.02-0.03%. There is little change in the S levels when converting from ductile iron to gray and vice versa.

Hardness and Tensile Strength--For BIW, the key to these two factors is the stability and predictability of its conversion system. The foundry knows what is happening when the conversion sequence is in process. The foundry practices two- or three-stage inoculation. Any changes in the S level mean the foundry has to monitor its inoculation closer.

The hardening agents, manganese (Mn), chromium (Cr), copper (Cu), molybdenum (Mo) and nickel (Ni), must be predictable during the conversion cycles so appropriate actions can be taken. BIW doesn't make alloy additions to the furnace on ductile iron to gray conversions.

Although BIW has not had any problems with tensile strength drop-off, it has seen drop-off in hardness with some jobs poured during conversion.

Nodularily--Predictability of the conversion system and process are key with nodularity. Plant I utilizes a procedure to step down wire additions as the S drops. Additions are dropped based on initial experiment results as BIW has been able to correlate furnace charges with S levels, reducing the dependence on lab results. In Plant 2, all ductile iron is duplexed through two channel holding furnaces. The result has been no nodularity problems.

Porosity--BIW has seen isolated instances of porosity on lower S level gray iron charges in Plant 1. Iron pyrite additions were used at one time to rectify the problem, but were dropped due to S contamination of the furnace linings. This problem currently is addressed by scheduling restrictions on individual jobs. Plant 2 doesn't experience any such problems.

Conversion Procedure

Plant 2's conversion procedures will be examined first due to their simplicity when compared to Plant 1.

In Plant 2, the ductile-to-gray conversion charge is 60% steel and 40% gray remelt (which is the same as the standard gray iron charge). This conversion charge will have varying amounts of graphite and silicon (Si) carbide based on the chemistry required. Due to a desire to eliminate the pig iron (softening) influence in the melt, upon conversion to gray the first casting cycle can be for casting thin-section class 30 or lower, the second cycle class 35 or lower, and the third cycle to any approved gray iron.

In the conversion, S levels increase from 0.015-0.02% to 0.02-0.03%. This S is from residual in the steel as the ladle alloy does not change between conversion iron and standard gray iron. In terms of inoculation, strontium-bearing inoculants work best for low S applications.

In Plant 2, gray iron is poured direct from the furnaces while ductile iron is duplexed through a holder.

For Plant 2 conversions from gray iron to ductile, the charge is 50% steel and 50% pig iron. BIW uses this charge to raise the pig iron content in the bath immediately. There aren't any restrictions on the ductile jobs that can be poured initially after conversion as the pig iron reduced S to normal ductile levels and all the ductile base is duplexed through a holding furnace.

In Plant 1 for the conversion of gray iron to ductile, the first step is a carbon (C) raiser conversion by switching from pet coke with 2% S to graphite with 0.025% S. Performed 16 hr prior to running ductile, the result is a S drop from 0.08 to 0.05%.

For the gray iron to ductile conversion charge, the furnace is taken down to two charges and then a 5400-lb pig iron addition is made. This is significantly different from Plant 2 because the S must be reduced to 0.035-0.04%. Also, Cr, Cu, Mo, Mn and Ni must be reduced to acceptable levels for ferritic/pearlitic iron. As a result of this melt difference, remelt segregation is critical in Plant 1 due to the drastic difference in S and pearlite stabilizers.

For the ductile iron to gray conversion in Plant 1, the charge is 100% steel, which will vary from 2700-3200 lb. BIW also switches back to pet coke. Three to eight charges are required to stabilize the melt back to normal gray iron levels. BIW tried elemental additions to level the pearlite stabilizers faster, but the foundry encountered too many variation problems. Iron pyrite also was tested, but the foundry found problems with S saturating the furnace lining and negatively affecting the ductile production.

BIW has been successful with its conversions in both plants by keeping the processes as simple and consistent as possible.

BURNHAM FOUNDRY

Located in Zanesville, Ohio, Burnham Foundry casts gray iron components for heating boilers and ductile iron exhaust manifolds for automobiles. The foundry uses a water-cooled cupola (18 tons/hr) to melt and it duplexes through a 30-ton resistance furnace.

The metal mix for Burnham is 85% gray iron (class 20 and 25) and 15% ductile (grades 65-45-12, 6040-10 and 58-38-15). The first two grades of ductile iron have the same internal chemistry ranges and are heat-treated to the proper hardness and mechanical property levels. The last grade of ductile iron requires lower amounts of residual elements (Cr and Mn). Other than C and Si, these residuals have the most impact on Burnham's iron properties, and are used as key process indicators for the foundry's controls.

Ductile iron is poured intermittently on one of the foundry's three molding lines. The gray iron is poured on the other molding lines at the same time using the same base iron but with additional alloying as needed. The key to the shared molding lines is scheduling so that the number of gray iron castings poured from the base ductile iron is minimized due to the iron cost. The shakeout for the ductile iron castings is 15 mm, however, all these castings then are heat-treated to maximize property uniformity and allow more latitude in residual element concentration ranges.

Base Gray Iron

The normal base gray iron for Burnham is a class 20 iron with a C equivalent of 4.35-4.55% and C levels from 3.60-3.85%. Normal S levels run from 0.065-0.085%. This base can be converted quickly to a variety of ductile iron grades by altering key residual elements, including Si, Mn, Cr, Cu, V and Mo. Other elements that are carbide formers/stabilizers usually are not found in Burnham's scrap. The scrap's Cr and Mn content are used as a guide in charge makeup.

In Burnham's experience, rapid changeovers to ductile iron from gray require several measures, including:

* excellent communication between melting, molding and lab personnel;

* use chemically reliable charge materials (low-alloy steels and pig iron);

* develop and maintain iron chemistry and inoculation/alloying standards for all affected pouring lines. These standards must be set up so that suitable mechanical properties are produced over the entire range of the standards during both changeover and normal operations;

* use the proper cupola charge material mixture to quickly adjust the duplex furnace bath to the suitable chemistry. It may be necessary to have a two-stage cupola charge change. The first stage could drastically reduce Si, Mn and Cr and the second stage could be a charge of lower "purity" while maintaining the standardized levels of the critical process elements;

* for changeovers, minimize the iron level in the duplexing furnace so that the incoming changeover cupola charge will have the maximum impact on changing furnace chemistry;

* use timely sampling of the cupola and holding furnace iron to determine the appropriate gray iron alloy/inoculation additions for all lines producing gray iron castings during and after the changeover to base ductile iron;

* use the same technique for changeovers of other furnaces supplied by the main holding furnace but with different iron chemistry requirements. As a result, the iron transfer ladles are alloyed as necessary for the process requirements of the other lines.

In terms of cupola charge materials for both changeovers and base ductile iron melting, Burnham recommends:

* plate and structural steel with the mixed steel grades properly sized;

* low-carbon steel bushings;

* low-Mn, high-C grade pig iron. Use should be minimized for standard ductile iron;

* gray iron and lower elongation ductile iron returns such as runners, gating and scrap castings. It is important not to use significant amounts of return iron with higher alloy content such as class 30, 35, 40 and above. This is accomplished by separating higher Cr sprue and scrap from normal returns;

* high C SiC for deoxidation, coke reduction and melt improvement;

* ferrosilicon, as other ferroalloys usually are eliminated from the charge;

* foundry coke and dolomitic limestone;

The cupola charge materials that are used in the transition from gray iron to high-elongation base ductile iron include pig iron, low-alloy carbon steel bushings and high-elongation ductile iron returns. In general, charge materials with high purity (greater predictability) are used for changing over the cupola and main holding furnace chemistry. These materials are blended to maintain the proper chemistry levels.

Changeover

It is Burnham's production practice to schedule ductile iron runs during the last half of the unit's operating shift. The first half of the shift is used to heat the molding line's bottom-pour unit and changeover the cupola and main holding furnace iron chemistry. The balance of the shift is devoted to ductile iron casting production (using cored wire) and class 20 gray iron on the other two molding lines.

For the holding furnace, when an iron changeover is to be made, Burnham's method is to get the cupola changeover charges through the cupola well and then to spill the cupola. The foundry then runs the holding furnace level down near the minimum. At that point, the cupola is restarted and the holding furnace refilled. This process allows the furnace bath to be changed over quickly.

During the changeover, it may be necessary to remove some iron from the holding furnace during refilling. Also, it may be necessary to add alloy material for the iron transferred to the other molding line. The furnace bath and molding line chemistries all must be closely watched during the transition period.

Near the end of the ductile iron shift, it is necessary to begin to controllably change the iron chemistry back to class 20 gray iron. The Mn and Cr contents are raised toward the upper end of the control ranges for the ductile iron during this. process. Once melting is finished for the day, the cupola is drained and preparations for the next day are started.

The cupola startup charge material is changed to provide higher Mn and Cr in the iron to finish changing the main holding furnace back to class 20 gray iron base. The iron to the various pouring lines is alloyed as needed to maintain the proper composition and mechanical properties of the castings produced while the reverse changeover is occurring.

GIDDINGS & LEWIS CASTINGS

G&L is a nobake jobbing foundry with a yearly casting production of 10,000 tons (10% ductile iron) and casting sizes from 500-70,000 lb. It is an electric furnace operation with three 22-ton channel furnaces and one 15-ton coreless induction furnace. Most melting is performed in the coreless induction furnace, and the melt is transferred 14,000 lb at a time into the channel furnaces.

When the foundry first began its ductile iron production, each ladle of base gray iron was desulfurized using a porous plug ladle and calcium carbide. Each ladle then was slagged and subsequently treated in the same porous plug ladle by adding 5% ferrosilicon (MgFe) while nitrogen stirred the metal. This method of treatment led to tremendous variations in melt temperature and Mg recoveries. In addition, this method was labor-intensive. As a result, the foundry decided to change its method.

In its current method of ductile iron production, four 3500-lb charges are added to the coreless induction furnace and its gray iron base melt (after transferring). These charges, which are called pre-round, consist of 1000 lb shredded low-Mn steel clips, 2000-lb pig ductile iron, 500-lb ductile iron returns and a low-S recarburizer. This high-purity charge is used to dilute the melt's residual elements.

After the pre-round is melted and the iron transferred to a channel furnace, the entire gray iron base induction furnace is desulfurized using a 10-ton porous plug ladle and 1.75% calcium oxide. Upon the transfer of the iron, a Mn, Si and pyrite addition is made to the channel furnace to compensate and adjust chemistry of the gray iron.

Once the desulfurization of the induction furnace is complete, the iron is returned. The final conversion of the furnace to ductile iron is complete with the addition of four more of the 3500-lb pre-round charges. Table 1 shows the furnace chemistry comparisons for G&L with its base gray iron, the gray iron after the first pre-round and then after the conversion to ductile iron.

The majority of the ductile iron castings poured at G&L are poured directly from the 15-ton coreless induction furnace. When castings exceeding this capability are poured, the foundry must convert the coreless furnace and desulfurize the heel of the channel furnace (normally twice). The next step is either to transfer the melted ductile iron in the coreless furnace to the channel furnace or to add ductile iron charges directly to the channel furnace(s) until the necessary tap weight of the required ductile iron is reached.

This article was adapted from a panel presentation (00-137) at the 2000 AFS Casting Congress.

                        Giddings & Lewis Castings'
                      Iron Chemistry Comparisons (%)
                             Si   Mn    S    Mo   Cu
Base Gray Iron              1.39 0.74 0.065 0.12 0.29
Gray Iron After Pre-Round   1.13 0.51 0.044 0.07 0.19
After Conversion to Ductile 1.17 0.35 0.014 0.04 0.11