Ductile iron stays in spotlight.

Continuing the trend of recent years, work on the processing and properties of ductile iron received the lion's share of Cast Iron program topics. Of particular interest this year was work aimed at overcoming the unique challenges of producing heavy section ductile iron, along with improving the mechanical properties and machinability of iron components.

Researchers K. Metzloff, L. Fang and C. Loper, Univ. of Wisconsin-Madison, laid the groundwork for their ductile iron research by pointing out that "One of the few drawbacks with conventional ductile iron is that the Young's modulus (stiffness) of the material is about 25% less than that of steel. Therefore, a ductile iron component of the same geometry is more prone to flexure. In some cases, this necessitates a heavier, larger part in order to obtain the same mechanical properties."

They demonstrated through their research that lowering the carbon content of ductile iron substantially improves the modulus of elasticity to levels approaching those of steel. The same was true for the mechanical properties of the lower carbon material.

According to the Wisconsin researchers, obtaining "a low carbon equivalent (CE) high modulus cast iron is feasible using a 1.0% carbon, 2.5% silicon alloy with magnesium treatment and post inoculation. The Young's modulus of this alloy is about 28.6 million psi average (pearlitic) and the other mechanical properties are well above the ductile iron quality index curve."

Despite the desirable properties of the material, the authors also determined that the alloy "exhibits inferior feeding ability, limited by long austenite dendrites." They also concluded that higher pouring temperatures and heat treatment are necessary to reach the ultimate properties.

Production of heavy section ductile iron castings was also addressed by Loper and A. Javaid, also of the Univ. of Wisconin-Madison. Producing sound heavy section ductile iron parts presents a special challenge to metalcasters because of the extended solidification time required to assure development of spheroidal graphite in the microstinctures. "Even then," they reported, "obtaining desired graphite nodule count and uniform distribution of those nodules" remains difficult.

To counteract the extended solidification times required to produce desirable mechanical properties in heavy section ductile castings, the authors offered a list of procedures that will minimize or eliminate degenerate graphite in heavy sections. Among these are:

* controlled charge materials to insure reproducibility and introduction of minimal amounts of subversive and carbide forming elements;

* control of base iron to achieve a CE that will not result in graphite flotation when the desired casting sections are poured;

* carrying out the treatment procedure so that the required Mg residual is maintained at the lowest practical value that will yield a fully spheroidal graphite structure. In addition, the researchers also recommended effective post inoculation, rapid melt handling, and the use of a variety of molding media and chills to speed heat extraction.

Many process issues continue to surround ductile iron treatment methods. One technique showing good potential is wire feeding. During a panel presentation on the subject, M. Firman, Burnham Foundry, discussed the development of ductile iron treatment using wire feed and his foundry's experience with the technique. According to Firman, cored wire was developed by the steel and alloy industries for the production of steel. Its use for treating ductile iron was developed in Europe and brought to the U.S. nearly 10 years ago.

Burnham Foundry uses a powdered nodulizing alloy and treats batches of up to 3000 lb. Firman reports that their experience using cored wire demonstrates that the temperature of the iron is more of a limiting factor than is fade time. He also reported that the foundry is achieving a nodularity level of over 95%.

L. Tandon, Beloit Corp., also reported on his foundry's experience with cored wire treatment of ductile iron. Beloit has used the process since 1993 and has gained a variety of benefits, said Tandon. Using wire containing 25% Mg powder blended with 75% FeSi fines, both desulfurization and ductile iron treatment were performed in the same process. Post inoculation was accomplished in mold using a semi-sintered FeSi block inserted at the bottom of the downsprue. "The process resulted in improved nodule shape and increased nodule count. Mechanical properties and matrix structure were also comparable to those obtained in the old process," according to Tandon.

Even more importantly, using the wire injection process provided the flexibility needed to operate both gray and ductile iron foundries using the same base iron and cope and drag molding unit. Tandon also pointed out that switching from their old tundish system to wire feed did not require any additional capital expenditures, as the foundry was able to use its existing facilities.