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Abstract
The variation with tempering temperature of the mechanical properties of the bainite and the bainite/martensite duplex structure in a low-carbon highstrength steel has been investigated and compared with that of the martensite. Both the bainite and the duplex structure exhibited much lower impact transition temperatures than the martensite and this can be explained in terms of the unit crack path, which is the mean cleavage crack path.
The 350° C embrittlement was more pronounced in the tempered martensite than in the tempered bainitic structures. Tempering at temperatures higher than 600° C greatly improved the toughness of each structure but decreased the strength. It is believed that this improvement in toughness by tempering is attributable to the ferrite matrix properties rather than the effect of the unit crackpath. The optimum combination of toughness and strength can be achieved by producing the tempered bainite/martensite duplex structure at all strength levels.