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Higher horsepower per liter engines have put more demand on the crankshaft, often requiring the use of forged steel. This paper examines cost reduction opportunities to offset the penalties associated with forged steel, with raw material and machinability being the primary factors evaluated. A cost model for crankshaft processing is utilized in this paper as a design tool to select the lowest cost material grade. This model is supported by fatigue and machinability data for various steel grades. Materials considered are medium carbon, low alloy, and microalloy steels; the effects of sulfur as a machining enhancer is also studied.

Material properties of niobium (Nb)-treated SAE I 141 steel connecting rod produced by control cooling were evaluated in comparison with quenched and tempered connecting rod of the same steel. The microstructure of the control cooled connecting rod is predominantly bainitic compared with tempered martensite for the quenched and tempered rod. Tensile strengths are comparable, while yield strength and ductility are lower for the control cooled connecting rod. There are small variations in hardness of the control cooled connecting rod depending on the section thickness in contrast with the uniform hardness for the quenched and tempered rod. Component high-cycle fatigue resistance is lower for the control cooled connecting rod due to the lower yield strength. Both connecting rods meet the fatigue strength performance requirements for the rod design evaluated.