Sessions Target Improved Processes.

Of the 20 sessions presented by the AFS Cast Iron Committee (Div. 5), the majority focused on iron's two most significant market sectors: ductile and gray iron. Twelve presentations focused on improving ductile iron casting processes and machining, while four others concentrated on gray iron metallurgy and microstructure. The remaining speakers covered topics ranging from "Monday morning" iron and optimizing current casting processes to inoculation practices.

In their presentation (089), A. Javaid, K.G. Davis and M. Sahoo, CANMET/Materials Technology Laboratory, studied the most important variables affecting the mechanical properties and carbide formation in thin-wall ductile iron castings. The high cooling rate in thin-wall ductile iron results in increased carbides with a corresponding loss in mechanical properties. Finding the most economical ways of reducing or eliminating carbide formation is important to meeting the increased demand for these components.

Laboratory heats of 220 lb were produced in an aluminalined l00-kW medium-frequency induction furnace. The samples for mechanical property evaluation were obtained from 3-and 12-mm sections of a step block casting. Variables studied were composition, inoculation type and practice, base iron preconditioning, pouring temperature, and mold materials.

The presenters found that section size was the main factor influencing tensile properties. While many samples from the 3-mm sections showed low elongation values, likely caused by a high pearlitic content or presence of carbides, many others showed higher elongation and superior strength above those required by ASTM A536 grades. At moderate-to-high elongations, the thin-wall samples were significantly stronger than samples from the identical irons in the 12-mm sections.

Toughness in the two sections was roughly equivalent, with the main difference between the impact properties in the two section sizes being the relative insensitivity to either melt chemistry or molten metal processing values, they said.

Silicon had the greatest effect on tensile properties of the thin-wall sections but had little effect on impact toughness. Any variable that led to an increased nodule count led to greater ductility, lower strength and improved toughness. The greatest effect came from late inoculation, base iron pre-conditioning, and using an inoculant containing bismuth and rare earths.

In their presentation (013), C.R. Loper, Jr., R.D. Kendrick and 0. Suarez, Univ. of Wisconsin Madison, discussed the effect of small additions of sulfur on post inoculated high carbon-equivalent (CE) ductile iron melts. To explore how sulfur would affect solidification characteristics, they used thermal analysis and microstructural characterization on three post-inoculated hyper eutectic nodular iron melts (4.74% CE) with both iron containing 0.001 wt% sulfur and iron that had subsequent sulfur additions to reach 0.0049 and 0.0059 wt%. To detect characteristic temperatures in the solidification and eutectic transformation ranges, raw cooling curves, their numerical derivatives and inverse rate cooling methods were employed.

They reported higher primary graphite arrest in irons treated with sulfur while the eutectic and recalescence parameters remained unchanged. Also, as a result of the additions, higher nodule counts were measured while the graphite morphology was not modified noticeably.

In their thermal and image analysis results, the three observed that late sulfur additions promoted the formation of graphite nodules and that higher nodule count in sulfur treated irons aids in the formation of additional ferrite in the microstructure, as they expected.

In their presentation (075), N. Enright, S.Z. Lu, A. Hellawell, and J. Filling, Michigan Technological Univ., discussed the relationship between fractal analysis of graphite and mechanical properties in gray iron. The authors conducted a feasibility study to correlate the morphologies and distributions of graphite with the mechanical properties in commercial gray iron by using a combination of a fractal description of graphite shape and an automated image analysis technique. Two parameters, fractal dimension (roughness) and shape factor, were used to numerically characterize the graphite morphologies.

They found that graphite morphologies associated with different properties (tensile strength values from less than 30,000 psi to more than 50,000 psi) can be identified as separate groups in the fractal map, and the measurements are close as the samples have much less variation in properties (tensile strength less than 2500 psi) with similar graphite microstructures. In addition, they found that the high tensile strength is associated with larger fractal dimension values (finer graphite) and the smaller shape factor values (shorter graphite).