Abstract: Fracture surface observation, microstructure observation, hardness distribution test and the basic stress analysis of a low carbon alloy carburized steel roller failed in roll forming process for manufacturing automobile flywheel, were investigated by means of optical microscope, scanning electron microscope, microhardness tester and finite element analysis. The results show that the carburized layer of the failed roller is mainly composed of acicular tempered martensite, small carbides and a few retained austenite. The microhardness decreases from the roller notch surface to the core. The distribution of microhardness near the failed roll notch is not uniform, the case depth of carburizing layer is lower than the design specification and low hardness microzones are formed in the carburized layer. Microcracks initiate from the roller notch bottom, then joint into a main crack and propagate from the notch bottom surface to the inner. The propagation direction of the main crack basically agrees with the result of finite element analysis. Both the heat treatment process and the complex loads in the service environment are attributed to the early failure of the roller.

Key words: 18CrNiMo7-6 alloy steel, roller, carburizing, quenching, hardness distribution, failure analysis