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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.

THIS STUDY WAS DEVELOPED in response to a component fatigue strength test which determined that if the current #3 engine bearing cap were used in the 2.5L Turbo application, it would be over stressed. Proposed solutions for solving this problem included: a redesigned grey iron cap with additional material in the highly stressed areas, or a cap made from either nodular iron or a free machining steel using current specifications. One of the manufacturing concerns about switching materials is the perceived difference in the machinability of nodular iron and steel. A single point turning evaluation was carried out by Chrysler Motors' Machinability Development Laboratory to compare the machinability of various materials proposed for use in engine bearing caps. Materials tested included: SAE G2500 grey cast iron, the current production material; SAE D45-12 nodular cast iron; and SAE steel grades 1117, 1137, 1215, 12L14, 1215, and 1215 modified (Incut 200).