'How we began pouring ductile iron ... and did it right the first time.' (Blackhawk Foundry and Machine Co.)
Editor's Note: If your foundry is examining the possibilities of producing the most promising metal of the ferrous casting industry, this article may help chart your path. From this first-person account, you'll see how Blackhawk Foundry & Machine Co., a 72-year-old gray iron foundry in Davenport, Iowa, made the successful leap into ductile iron.
It was at a board meeting in June 1991 that we took our first steps into the ductile iron (DI) market. While the 180-employee foundry had produced gray iron (GI) for the agricultural, heavy equipment, diesel engine, hydraulic pump and power transmission industries, the firm's Board of Directors, managers and salesmen initiated the conversation on DI because:
* growth in DI markets and decline in GI;
* a desire to get the shop on two shifts by going after work we currently could not bid on;
* many customers appeared to prefer dealing with foundries able to pour both GI and DI.
Because of the sheer impact of such a decision, sales was charged with surveying current customers about our pursuing DI. The rest of that meeting was spent discussing requirements in equipment, manning, new processes, etc., to produce DI in our shop. That was an important discussion, since only three of the 10 people in the room had any prior DI manufacturing experience.
After several weeks, sales returned and said many customers were excited about buying both types of iron. In fact, the foundry's most promising large growth customer said 80% of its new designs were coming out as DI. Another customer said it had decided to only source to foundries pouring both GI and DI, and if Blackhawk was not pouring DI in three years, we would not be in their future as a casting supplier. On the other hand, a major customer was worried because it already had enough DI suppliers and was concerned about the process hurting GI capacity.
Blackhawk melted in an acid-slag, cold-blast cupola with oxygen enrichment capability. Plant layout ruled out any possibility of adding electric furnaces for melting ductile, so the cupola it was. Basic slag practice to achieve a low-sulfur (S), high-carbon (C) base metal was impractical, since we had to melt GI out of the same unit. This meant adding external desulfurization.
While there was room between the cupola spout and the furnace receiver to sandwich in a porous plug dwell unit, typical maximum spout temperatures of 2730-2750F (1499-1510C), coupled with limited superheat capability in our holding furnaces wouldn't allow us to overcome the additional temperature loss of 50-75F (10-23C) from desulfurization. The cupola had to be redesigned to achieve spout temperatures of 2780-2800F (1527-1537C).
Adding hotblast capability to the cupola seemed to be a good solution, since it could be used every day that we poured GI. An externally fired unit could do the job, but would require fuel. A recuperative unit would cost more, but would use already-generated stack gases for fuel.
For magnesium (Mg) treatment methods, upright designs were limited due to Blackhawk's horizontal drum-shaped delivery ladles, monorails and floor conveyors. In-the-mold treatment was considered, but reduction of mold yield by the reaction chamber and a customer desire to test large numbers of castings for nodularity were deterrents. We decided that external stream treatment, in which the reaction chamber box is external to the mold and placed between a holding furnace and iron delivery ladle, would work.
Sales returned to customers, searching for data on grades and volumes of work that would be available in our size range. In GI, our weight range was 1-100 lb, which put weight limits on all three of our mold sizes. Recognizing DI's propensity for mold wall movement (all of our molding is flaskless), we set limits for mold weights at 75% of what we worked with in GI.
Suppliers were used extensively. For desulfurization, we received dwell unit designs, material costs and typical usage rates. Our alloy supplier showed us treatment box design, alloy costs and usage rates. Suppliers of ductile pig iron and steel scrap were contacted in regard to availability and cost. Sand suppliers were contacted about pricing issues, and as to whether they could deliver a specified amount of S in the seacoal premix. The hotblast designer projected the impact on fuel use and melt cost, and materials for dwell units and treatment boxes. Our refractory supplier was also contacted about flow and usage rates of nitrogen for the desulfurizer.
We intended to operate both holding furnaces - base GI in one and base DI in the other. With equipment additions having been made since that was last done in the foundry, we brought in our local utility to study our power transformer situation.
A rough list of equipment, training costs, additional in-plant processing and projected prices was compiled. Then, as a benchmark activity, sales asked to quote on customers' DI patterns. Customers found our quoted prices were competitive, confirming our systems and cost-estimates were in line.
Five months after DI was first muttered in the president's office, we had a decision to make. We were a privately held company with limited resources. During the last year we experienced several small layoffs, and were currently operating only six of our nine molding units. Was now the time for a substantial investment in equipment and processes?
It was decided to continue to look at DI by committing to the installation of the recuperative hotblast. It would be needed for DI and would make a positive immediate impact on GI. Planning for DI would continue and a final decision on timing for implementation would be made after the hotblast installation in July 1992.
Customer feedback showed most DI work would be split evenly between 80-55-06 and 65-45-12. This meant producing pearlitic and ferritic grades concurrently. Melting only pearlitic and heat treating the ferritic grades was an alternative, but heat treat and handling costs add up, and we knew that producing as-cast ferritic iron would be a good cost and marketing tool. Just as we do for class 25, 30, 35 and 40 gray irons, we decided on melting a common base iron that could be alloyed to produce any final grade desired. We worked out desired ranges for various elements and worked with our pig iron, steel and alloy suppliers to put together a projected charge makeup and alloying practice.
The projected work included lightweight castings with thin sections, so carbide-control would be an issue. We decided to use an in-mold inoculant [a pressed and sintered block of high calcium (Ca) and aluminum (Al), with a small amount of Mg] on every mold. It would eliminate the formation of carbides, and raise nodule counts for better physical properties and machinability, as well as give us a late kick for nodularity control. The inoculant would also help with the Mg fade question we had with our drum-type iron delivery ladles.
In the tooling area, we were in the process of hiring a new tooling engineer, and since no one had DI gating and risering experience, we made that a prerequisite for interviewing. Second, the question of shrink rule on DI patterns was an issue. The commonly used value for GI is 1/8 in., but we always worked at 3/32 in. due to more mold wall movement in our flaskless molding sand system. After networking with other foundries and pattern shops, and realizing DI was prone to mold wall movement, we decided on a 1/16 in. shrink rule. (Several years later, that same rule is being used on all our larger castings.)
Third, we designed shell coreboxes for test bar molds and for micro-release coupons.
Last, we had to deal with DI's greater propensity for shrinkage. Beyond the gating, we needed to retrain our people to look for shrinkage more closely in areas of castings where shrinkage typically wasn't found in GI.
Capital Equipment list
During planning/research, a more complete capital equipment list was developed. While extensive in nature and representing a substantial capital investment, it showed the interest in doing things correctly.
* two sonic test units for nondestructive auditing of nodularity and sorting of questionable castings;
* new transformer site, upgrading of several old units, placement of new units;
* cutoff saw to handle micro-release coupons;
* new polishing area to allow for quick turnaround of micro-coupons for ladle release during production;
* coreboxes for micro-release coupon and test bar mold;
* telepher car scales; only one of three iron delivery cars was equipped with scales;
* platform scales for weighing up treatment alloys;
* liquid nitrogen tank and site;
* addition of two channels to our spectrometer, Ce and La;
* a timer system to control fade time of treated iron;
* desulfurizing dwell units;
* Mg treatment (Sigmat) boxes;
* melting area engineering, for adding desulfurization and treatment process to the operation;
* segregation shakers, to isolate questionable nodularity castings;
* replacement tensile machine to handle DI bars (customers would eventually want in-house capability);
* C/S determinator to replace lab's old units;
* cutoff saws for difficult-to-knock-off gating systems;
* pneumatic tube system to loop five pouring decks, cupola and lab for timely handling of micro and chemistry samples.
Full Speed Ahead
In July 1992, the hotblast installation had gone fairly well. Customers were asking weekly about DI. On July 22, 1992, the president opened the board meeting by expressing Blackhawk's decision and desire to enter the DI market. The Board of Directors gave full commitment to whatever resources it would take to do the job right. They decided that a cross-department team would be charged with developing a plan, keeping in mind two considerations:
1. "That Blackhawk produce, from the start, only quality DI castings."
2. "That Blackhawk move into production of DI in a well-planned and timely manner designed to augment item one, while at the same time continuing to meet all current and future business requirements for our GI operation."
Consisting of the general manufacturing manager, foundry superintendent, quality assurance manager, tooling engineer, maintenance superintendent, coreroom supervisor and plant metallurgist, the implementation team was responsible for seeing that production quantities of DI be ready by January 1, 1994.
Glitches in the hotblast installation due to poor planning and project management, prompted the team to employ every planning and project management tool or technique available for the DI project. We purchased several magnetic, washable surface planning boards and mounted them on the conference room walls. Two of them became the Gantt Planning Chart, where we listed our time frame across the top and major items down the left side. Adding known information, these charts assured nothing fell through the cracks.
The Gantt chart exercise revealed that we hadn't allowed time to try out our processes and develop operational experience to correct problems. Three months of prototyping time was set aside so that in addition to all our planning, we could attempt to develop the same expertise that competitors with 10-20 years experience have. Our customers expected that.
Next, a Process Failure Modes and Effects Analysis (FMEA) was developed so there would be no surprises later on. We started work on this document by flowcharting our current GI operation, followed by a flowchart on how DI would compare to GI with its differences and additions. Figure 1 shows the beginning of the two flowcharts. The DI flowchart also predicted additional manpower needs for DI.
The FMEA, which was 24 pages in length, detailed these processes:
* iron yard;
* cupola operation;
* holding furnace operation;
* Mg treatment:
* iron delivery;
* iron pour-off;
* ladle release;
* returns control;
* further processing.
The FMEA (Fig. 2) was the best (and the most time-consuming and nerve-wracking) part of the planning process. As the team's most-used educational tool, its completeness and accuracy is attested to by the fact that after one year of prototyping, the document required only five minor changes.
Much engineering was needed in melting, and an engineering firm was engaged to redesign problem areas. Two new processes had to be shoehorned into an area where no allowances had been made for growth. Our lime/spar supplier worked with us on a dwell unit design, and once overall dimensions were set, the cupola system was redesigned to allow for easy placement between the cupola and holding furnace. Two infinitely variable auger feeders were purchased for the lime and fluor-spar. Samples of the two materials were sent to the feeder manufacturer, and capability studies were done on feed rates in the volumes we would use. The engineers then designed a low-head height installation for the feeders and the hoppers for the two materials.
Putting the treatment box in front of the holding furnace also proved interesting. When in front of the holding furnace, the box had to be turned sideways to line up with the iron delivery car. This necessitated installing a second set of hydraulic tilt controls for the furnaces. Because of height requirements for the boxes, the delivery ladle itself needed to be below floor level. Therefore, we needed a pit between the two holding furnaces. Then, the box was put on a swiveling pedestal to allow the ladle to be placed and removed from the pit.
Training and education were part of the process at the beginning and continues today. A general orientation was held on a Saturday morning, and many employees met for several hours. We described what DI was, how it differed from GI, what it meant to the future of Blackhawk, and how we were planning for it. A three-day class in "Gray and Ductile Iron Metallurgy" was held with 25 selected salary and hourly personnel. Suppliers were brought in for training on our specific processes: desulfurization, Mg treatment, gating/risering, inoculation and metallurgy. Workers from melting and the foundry attended "Gray and Ductile Iron Inoculation Practices" courses at CMI. Numerous sessions were held with melters, iron pourers, molding machine operators, degaters and supervisors explaining their roles in the process.
Training must have reached several thousand man-hours, but it was worth it. People were excited and proud of being made part of something big and new.
By October 1993, our GI business was picking up for the first time in four years, and we restarted several idled molding lines. Instead of pouring 7-8 hr/day, we were pouring 9-10. Fitting DI into the mix was going to be more problematic than expected, as the foundry picture had completely changed.
As to schedule, our DI holder was on line the first week in October. In running our desulfurizer for the first time, we realized that our nitrogen regulators didn't provide enough flow for the best bubbling of the metal in the dwell unit. New regulators were installed and allowed us to top off the furnace two days later.
We hit our targets for element ranges in our base metal, and were ready to try Mg treatments. In the first test, we treated a ladle of iron, poured test samples and micro-coupons and returned the iron to the holding furnace. The final chemistry looked good. We had to make some small adjustments to the box/ladle interface. Several days later, we carried treated metal to two molding lines and poured our first castings. These first ladles tested inoculant chemistries and addition rates, along with alloy combinations and percentages for pearlitic and as-cast ferritic grades of iron.
Microstructures and Brinells looked good, and we successfully produced 100-73-03, 80-55-06 and 65-45-12 grades all from the same base iron. There were occasional slag stringers in our casting microstructures and we lowered inoculation rates from 0.5% to 0.1% to fight this problem. This, and a change to an ordinary 75% FeSi from a high potency proprietary inoculant, eliminated the slag stringers, and we still saw no carbides in the castings.
Our alloy supplier had been correct: the treated iron from the box was already highly inoculated, and it - in combination with the in-mold inoculant - was all we needed. We tried to force carbide formation by overtreating with Mg and by lowering final silicons (Si) to low levels. Only when we got to 1.90-2.00% final Si (much lower than any standard practices) did carbides appear in the microstructures.
We were batch melting and pouring, melting nine tons, filling the holding furnace, pouring it off and filling it up again. We did this once or twice a week, always around our GI schedule. Twelve to 14 hr days were common.
Tooling was experimenting with gating and risering, and something was learned every time castings were poured. We had a dozen patterns to play with, but the one major package we successfully bid on didn't have any real requirements until June of the following year. There was, however, one part that was shipped almost immediately.
With the full production date soon upon us, there weren't that many jobs to pour. A test lot of castings from a backup set of tooling was shipped to one somewhat wary customer. Shortly afterward, that customer asked if we could take over a four-part package and qualify tooling for February requirements. Now, instead of June, they wanted castings in January.
In January 1994, with the industry again booming, we had more GI and DI work coming our way than we could handle. Our first full production day of DI had to be on a Saturday, since five 10-12 hr days [TABULAR DATA FOR FIGURE 2 OMITTED] during the week were needed to handle the GI work. That first Saturday presented a challenge, since we had never melted and poured at the same time. Also, we had only 40 tons to pour in 8 hr; the cupola was designed to melt 12-15 ton/hr. The cupola would be off a lot - thank heavens for the hotblast.
We went to two shifts in March 1994 and boosted employment to 320 employees. By June, we were pouring 85-90 ton/day, one shift a week. The last two weeks in June we poured two shifts of DI each week, running every molding line we had. By August, we qualified more than 100 patterns.
In full production, we developed one major unforeseen problem. Our casting conveyors weren't capable of handling all the trees of DI castings when all molding lines were running. The GI castings would break off of the gating systems in the casting shakeouts and the casting conveyors could handle the volume of metal. On DI, the castings stayed on the trees and created one tangled mess after another. Some redesign of the system helped, but several conveyors must be replaced or drastically redesigned to completely solve the problem.
The used tensile machine died before reaching service. Spare parts weren't available. We've been using outside labs for test bars since day one, and are reviewing a quote on a state-of-the-art unit.
Also, as we increased production and put greater treatments per hour through our boxes, operational problems began to appear. Adjustment of flow rates through boxes and changing alloys has helped.
Advice on the Journey
Nearly three years after treating our first ladle of DI, I'd offer this advice for a foundry considering entry into DI:
* The team approach to planning and/or project management is a powerful tool. Two heads are better than one.
* Techniques such as Process FMEAs are arduous and time-consuming, but time spent up front in the process really does save time in not having to do things over and over again.
* We would not have changed anything that we did, except maybe having put more training and preparation in gating and risering. We are still experiencing shrinkage on several complex patterns with varying section sizes.
* Precise, methodical planning can help in doing things right the first time. The results are proof of the effort. We have qualified 170 DI patterns for production in the last two and one-half years. DI is now 8-10% of our production, and expected to double in the next year.
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|Author:||Helm, Lawrence E.|
|Date:||Aug 1, 1996|
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