Anomalous Fractures of Diesel Engine Bearing Cap Bolts
Sudden and unexplained bearing cap bolt fractures were experienced with reduced-shank design bolts fabricated from 42 CrMo 4 steel, quenched and tempered to a nominal hardness of 38 to 40 HRC. Fractographic analysis provided evidence favoring stress-corrosion cracking as the operating transgranular fracture failure mechanism. Water containing H7S was subsequently identified as the aggressive environment that precipitated the fractures in the presence of high tensile stress. This environment was generated by the chemical breakdown of the engine oil additive and moisture ingress into the normally sealed bearing cap chamber surrounding the bolt shank. A complete absence of fractures in bolts from one of the two vendors was attributed primarily to surface residual compressive stresses produced on the bolt shank by a finish machining operation after heat treatment. Shot cleaning, with fine cast shot, produced a surface residual compressive stress, which eliminated stress-corrosion fractures under severe laboratory conditions.