Improving iron quality. (Cast Iron).
In a six-presentation joint session with the AFS Molding Div. on thin-wall iron casting, the Cast Iron Div. featured a presentation by D.M. Stefanescu, C. CorbittCoburn, L. Dix, R. Ruxanda and T.S. Piwonka, Univ. of Alabama, on the properties of thin-wall ductile iron castings (02-178). The presentation discussed experiments performed by the university to establish tensile mechanical properties and hardness values for thin-wall ductile iron.
The experiments involved producing plate castings in green and nobake sand molds with a charge of 20% pig iron, 40% ductile iron returns and 40% steel scrap. Cerium-containing alloys were used for the molten metal treatment.
The tensile properties and hardness were measured on as-cast and machined plates to obtain bulk properties of the material. The typical mechanical properties of the machined plates ranged from: 60-70 ksi tensile strength, 18-25% elongation and 133-167 Brinell hardness. The properties met or exceeded the ASTM specifications for as-cast ductile iron, and compared favorably with the properties of regular section size cast ductile iron.
In a panel presentation outlining various methods for changing grades of ductile iron during melting (02-171), W. Thomas, Bay Engineered Castings, R. Leiterman, Metal Technologies, Inc. (MTI), and B. Burton, Waupaca Foundry, Inc., described their operations' melt procedures and the keys to effective melt changeover.
Thomas stated that his firm, which produces ductile iron castings from 500-30,000 1b, makes all alloy additions in the ladle and prefers to change between grades of ductile iron in an ascending order, For example, his foundry prefers a class 120 pig in the coreless induction furnace to changeover to class 100 or 80 ductile iron or a class 100 pig in the furnace for class 80 iron.
Leiterman emphasized the importance of controlling copper content in his foundry's ductile iron melt for a changeover. MTI is a high-production shop for automotive components that melts in a 35 ton/hr cupola and duplexes into 65 and 35-ton holding furnaces. Each casting job for MTI has a process sheet with a percent copper target. Each day, the metallurgist and foreman meet to discuss copper levels for that day's production.
According to Leiterman, the copper is controlled in the base iron by segregating the returns by low and high copper content. In the tundish ladle, the copper is added to a 0.05% level using a PLC-controlled vibratory conveyor. At pouring, a 6500-lb ladle of iron is evenly divided and distributed to the three molding lines where each line's iron is alloyed up to the final % copper target. At pouring, an even distribution of the iron from the ladle between the three mold lines is critical for accurate chemistry,
In a presentation on "Monday Morning Iron," M. Popescu, J. Thomson, R. Zavadil and M. Sahoo, CANMET/MTL, discussed AFS research to investigate the effect of holding an iron melt over an extended time period on the nucleation potential of the iron (02-158). In addition, the research's goal was to establish a controlled and efficient technique to restore iron held for an extended period of time.
For gray iron, the effect of holding time at three levels of temperature for up to 10 hr was investigated on a typical gray iron sample. Analysis of the data from the research concluded:
* holding time and temperature affect the chemical composition, microstructure and chilling tendency of gray iron;
* inoculation has a beneficial effect on the chilling tendency of "Monday Morning" gray iron;
* a special melting technique can be used to improve "Monday Morning" gray iron microstructure without increasing the pig iron level used in a charge.
For ductile iron, the effect of holding time was investigated at three levels of temperature for 7-10 hr on a typical ductile iron sample. Analysis of the data from the research concluded:
* holding time contributed to changes in ductile iron composition at all three temperature levels;
* trimming (performed on the base iron) before tapping for the magnesium treatment had a renucleation effect on the base ("Monday Morning") iron that was conveyed to the ductile iron;
* samples with similar carbon equivalents indicate that holding time at lower temperatures increased the iron's nodule count and encouraged ferrite location, while holding at high temperatures did not have a significant effect.
Other panel presentations from the Cast Iron Div. at the AFS Casting Congress included discussions on improving cleaning room automation and producing compacted graphite iron.
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|Article Type:||Brief Article|
|Date:||Jun 1, 2002|
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