Controlling Lustrous Carbon Defects. (Molding Methods & Materials Silver Anniversary Paper).

In his Silver Anniversary Lecture titled, "Formation and Control of Lustrous Carbon Surface Defects in Iron and Steel Castings" (02-154), R. Naro, ASI International, updated the presentation he gave 25 years ago. Naro highlighted the changes in the industry since 1977, specifically foundry closures, an increase in imports, decreased ferrous casting production and the rapid growth of phenolic urethane binder use.

Phenolic urethane binder use led to outbreaks of lustrous carbon surface defects on castings, prompting the initial research in 1971, he explained. Lustrous carbon defects in castings can be characterized as:

* wrinkles such as elephant skin, crow's feet or alligator skin;

* surface laps such as cold shuts, seams, soot or kish;

* internal laminations and external blisters;

* surface porosity;

* lost foam defects.

Naro stated that updated objectives for research on this topic include evaluating new binder systems, comparing alternative binders, examining the effects of venting and positive exhausting, and determining methods to eliminate surface reactions when casting steel. To reach these objectives, a test casting was developed, and pouring temperature, time, and binder levels and type were assessed, both in 1971 and in 2002. Venting also was tested in 2002.

Naro compared new binder systems with the phenolic urethane binder results from 1971. Biodiesel and aromatic binder systems (from 2002) emerged successful with less wrinkling than both the standard 2002 binder formulation and the 1971 formulation. He also established that venting, either vacuum-assisted or through drilling holes, reduces the amount of lustrous carbon defects.

Naro's recommendations that emerged from the testing in 2002 include:

* avoid pouring at low temperatures;

* pour as quickly as possible, but with the least turbulence;

* vent the molds;

* add red iron oxide to prevent lustrous carbon films from forming;

* investigate new binder forms stemming from biodiesel-based sand systems.