Advancing ductile iron production.

This 20 presentations in this year's Cast Iron Div. covered a wide range of topics. In addition to panels on gating/risering, solving casting defect problems, manganese (Mn) and Mn/sulfur (S) ratios in gray irons, the division featured AFS research reports on fractal methods for assessing ductile iron nodularity, age strengthening of gray iron, status of the ASTM A48 "B" test bar mold and fatigue properties.

The division's offerings were nearly split between gray and ductile iron - the ferrous casting industry's most promising sector in a changing environment. In fact, two papers addressed the recent initiative of raising foundries' ability to produce lighter, thin-wall (2.5-5 mm) iron castings.

The Univ. of Alabama's J.F. Cuttino, J. Andrews and T. Piwonka collected 65 references that dealt with the production of thin-wall iron castings (189). Through a literature search (broken out by processes, general casting parameters, mold parameters, core parameters and miscellaneous contributions), they found several trends.

Noting that most thin-wall castings are ductile or compacted graphite iron, their findings indicated: high mold densities are most advantageous for precision; mold densities increase with the use of compensating squeeze heads or jolt/squeeze heads; short gating systems reduce hydrogen pinholing problems; casting accuracies decrease with increasing number of cores; and core assembly and placement techniques are critical to accurate castings. Shakeout, they said, continues to be a problem and should be further researched.

Two key references stood out in the authors' minds in regard to producing accurate, thin-wall castings. K. Hornung's 1995 paper outlined procedures and analysis of a 2.5 mm thick exhaust manifold. Nearly 1 million of these manifolds have been produced in green sand with automatic pressure pouring. Also, they noted that "the FM (a low-pressure countergravity process) casting method shows tremendous potential for thin-wall iron casting." By stacking coreboxes in layers of five (with each box holding two molds stacked symmetrically), the process implemented by Valfond Co. at France's Fumel Foundry is capable of producing 1500 castings/day with wall thicknesses of 2.8 mm.

Another presentation on thin-wall casting production (074) by A. Javaid, J. Thomson and M. Sahoo, MTL/CANMET, studied the factors affecting the formation of carbides in thin-wall ductile iron. Because of the high cooling rate of thin-wall ductile iron castings, they said special practices and procedures are needed to assure the development of a carbide-free microstructure. "By optimizing the chemical composition, postinoculation and preconditioning along with a precise control of the melting process and the use of high-purity charge materials, carbide-free, thin-wall castings can be produced," they said.

In addition to rigorous control of charge materials (with minimal carbide-forming elements) and subsequent processing, the following practices, they said, minimize or eliminate carbides:

* a base iron with a high carbon equivalent (4.5-4.7%);

* an effective preconditioning of the base iron before treatment with 0.2% of silicon carbide and/or 75% ferrosilicon (FeSi);

* an effective (but not excessive) treatment with 0.030.045% magnesium, depending on base S content;

* an effective and relatively large postinoculation (0.71.00% FeSi) is essential;

* late inoculation with 75%FeSi containing 0.006-0.009% cerium and bismuth and use of fade-resistant and potent inoculants for a higher nodule count (thereby minimizing the formation of carbides in thin sections);

* use of appropriate grain size of molding materials, certain binders and moldable insulating materials were shown to decrease the tendency toward carbide formations in thin sections by decreasing the cooling rate.

Another ductile iron presentation by W.A. Henning, Miller and Co., focused on head-to-head comparisons of ductile iron recarburizers (108). Despite acceptance in ductile iron foundries, some controversy still remains in certain segments over noncrystalline carbon's effects on nucleation. To shed light on the issue, an extensive test program at a large Midwest ductile iron producer compared the ductile irons produced with crystalline (crushed electrode) and noncrystalline recarburizers (low-sulfur petroleum coke).

The results of the 5-day production test, said Henning, showed the use of a low-sulfur, low-ash petroleum coke produced equivalent results to ductile iron produced using crystalline graphite. "These findings add credibility to the continued use of petroleum cokes, thus allowing foundries to continue to take advantage of their costs with no deterioration in final iron quality," he said.

A presentation on the quantitative description of the solidification morphology of lamellar and ductile iron (060) by Belgium's F. Mampaey, WTCM Foundry Center, was reported to offer a new understanding of solidification. His experiments, he said, showed that computer models ignore the presence of dendrites present in both eutectics, and that the models do not account for the different graphite precipitation rates in gray and ductile iron. "The models fail to predict the well-known difference in feeding behavior of both types of cast iron," he said.