Deformation and fracture in martensitic carbon steels tempered at low temperatures

This article reviews the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tem pered at low temperatures, between 150 degreesC and 200 degreesC. The carbo n-dependent strength of low-temperature-tempered (LTT) martensite is shown to be a function of the dynamic strain hardening of the dislocation and tra nsition carbide substructure of martensite crystals. In steels containing u p to 0.5 mass pet carbon, fracture occurs by ductile mechanisms of microvoi d formation at dispersions of second-phase particles in the matrix of the s train-hardened tempered martensite. Steels containing more than 0.5 mass pe t carbon with LTT martensitic microstructures are highly susceptible to bri ttle intergranular fracture at prior austenite grain boundaries. The mechan isms of the intergranular fracture are discussed, and approaches that have evolved to minimize such fracture and to utilize the high strength of high- carbon hardened steels are described.