Simple Solutions That Work! Issue 7

For several years, the iron casting environment is driving the need for stronger cast irons with lower weight than gray iron parts, but with improved machinability, thermal-fatigue resistance, damping capacity, casting mold yield, and castability compared to ductile iron parts. Compacted graphite cast irons (CGI) provide a cost-effective solution to meet these challenges. Automotive components, such as disc brake rotors, are prime candidates for conversion to compacted graphite iron production, especially where lighter weight and higher strength are important issues to design engineers. Unfortunately, producing consistent quality compacted graphite iron requires even more stringent controls than ductile iron production. The most popular method of producing CG iron requires the use of complex thermal analysis techniques. The equipment and foundry controls needed to produce CGI as well as associated licensing costs have prevented widespread use of CGI. A review of worldwide research investigations and foundry experience involving different liquid metal treatment procedures to produce compacted graphite iron was the topic of a 2002 AFS Casting Congress Compacted Graphite Iron Panel. From these panel discussions, the presentation by D. Kelley generated a considerable interest in the “Resulfurizing after Magnesium Treatment”. Kelley showed that in a production environment, using a 0.015 to 0.025% sulfur addition (after magnesium addition) to denodulize magnesium treated iron, he was able to consistently produce acceptable CG irons with less than 20 percent nodularity. The key to Kelley’s success was the use of a new iron sulfide briquette (Resulf 30), which allowed consistently high sulfur recoveries (85 to 90%). Prior to using these briquettes, granular iron sulfide (iron pyrites) was used with sporadic and inconsistent CGI results; typical recoveries of were only 30 to 40%. Since sulfur is used to denodulize the irons, there was little concern about contamination of foundry returns. This is not the case when another anti-nodularizing elements, such as titanium, is used in CGI production. This production research was a response to the desire of many foundries to add alternative elements such as titanium to “denodulize” magnesium treated ductile iron. The simultaneous use of sulfur with inoculating agents is not a new concept. The use of sulfur added with potent oxy- sulfide forming elements was first demonstrated by Naro and Wallace (1970). Naro showed that balanced ratios of rare earths and sulfur, without the presence of ferrosilicon provided drastic reductions in undercooling, completely eliminated chill and promoted favorable graphite shapes in gray irons. In a 1984 study, Strande showed that calcium silicide based inoculants along with increased sulfur additions provided vastly improved machinability in gray iron castings compared to proprietary ferrosilicon based inoculants and similar late sulfur additions. DR. R.L. (ROD) NARO & D.C. WILLIAMS ASI INTERNATIONAL, Inc. ARTICLE TAKEAWAYS: 1. Precise control of sulfur recovery for ductile iron & compacted graphite iron 2. Amount of sulfur addition in magnesium-treated iron needed to obtain a critical nodular graphite/compacted graphite (NG/CG) ratio depends many factors. INEXPENSIVE METHOD TO PRODUCE COMPACTED GRAPHITE IRON WITHOUT COSTLY THERMAL ANALYSIS 28