Q: We are an automotive OEM, and for some new engine projects we are contemplating fracture-split con rods. What are the comparisons of mechanical properties, including fatigue strength, for this material versus conventional chrome-moly carbon steel? What is the optimum microstructure for this material?
A: There are basically two methods for forging fracture-split con rods. The PM pre-form method used by at least one firm, for Ford and others, uses atomized powder compacted in a pre-form die followed by sintering and subsequent forging from an atmosphere furnace, followed by rapid cooling to develop the properties in some rods; others are heat treated with 4140-type cycles.
The other method is to forge the con rod from bar steel having slight notches in place corresponding to the fracture zone. This method is offered by at least one forge shop that I know of: Impact Forge of Indiana. They supply rods for high-performance engines. There are papers written on the subject (available online from the Society of Manufacturing Engineers; visit www.sme.org), which I suggest you review.
I have some experience at forging PM parts, but not the Cr-Mo alloys you listed. Also, I cannot advise on the microstructures that might be shown in papers on the subject.
For more than 40 years H. James Henning held key technical positions in the forging industry, including as director of technology for the Forging Industry Association, and as president of Henning Education Services, a Columbus, OH, firm specializing in customized education and training in forging technologies.
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