Tensile properties and fracture toughness at service temperatures of an optimized pearlitic ductile iron alloy for automotive crankshafts
Résumé
Cast iron and forged steel have competed against each other for many years for different industrial applications. Crankshafts are one of the products where this competition is still active, both for heavy duty and light machinery applications. For the specific case of automotive crankshafts, high strength pearlitic cast iron grades have found a strong position in passenger car market niches. The mechanical performance data available for these specific ductile cast iron grades mostly come from room temperature tensile and impact testing. Nevertheless, the failure mode of cast iron crankshafts involves fracture mechanics, and their usual range of working temperatures goes from 20ºC for cold starts to 120ºC for top maximum lubricating oil temperature. This work covers this range by studying a high strength pearlitic cast iron employed to manufacture crankshafts both in terms of tensile, impact and fracture mechanics testing at 20ºC, 22ºC and 120ºC. A specific molding pattern was designed to produce the desired materials, namely crankshafts and Y2 and Y3 wedges, which were cast in an industrial crankshaft manufacturing line. The results indicate that there is a relevant difference between the low temperature and the high temperature mechanical response of the studied cast iron. This finding is interpreted in terms of safety, as the crankshafts should not fail in service after the engine has started running and the lubrication has reached its steady state working temperature.
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