Abstract
This paper describes the microstructure, deformation, and fracture of low-temperature-tempered (LTT) martensitic steels. The microstructural reasons for the ability of these steels to achieve ultrahigh strengths and the factors controlling ductility and toughness are described for low-carbon, medium-carbon, and high-carbon LTT martensitic steels. The key strengthening mechanism of LTT martensitic steels is the strain hardening provided by the transition carbide/dislocation substructure of the martensite crystals. In low- and medium-carbon steels, LTT microstructures fail by ductile fracture mechanisms, and ductility decreases as strain hardening rates increase with increasing carbon content. In high-carbon LTT steels, quench embrittlement associated with phosphorus segragation and cementite formation at austenite grain boundaries limits toughness and fatigue resistance. Approaches which permit the application of the high strengths of high-carbon LTT steels and minimize the effects of quench embrittlement are discussed.
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