Thermal Ageing of Heat-Resistant Steels

Abstract

The microstructural evolution during short-term thermal exposure of 9/12Cr heat-resistant steels is described, as well as mechanical properties after exposure. The tempered martensitic lath structure, as well as the precipitation of carbide and MX-type carbonitrides in the steel matrix is stable. On thermal exposure, with increase of cobalt and tungsten contents, cobalt could promote the segregation of tungsten along the martensite lath to form Laves phase, and large size and high density of Laves phase precipitates along the grain boundaries could lead to the brittle intergranular fracture of the steels. In addition, the long-term thermal ageing effect on China Low Activation Martensitic (CLAM) steel is discussed. The microstructural evolution, including the growth of M23C6 carbides and the formation of Laves phase precipitates as well as the evolved subgrains, leads to changes in the mechanical properties. The upper shelf energy of the thermally aged CLAM steel decreases with the extension of ageing time, while the yield strength changes slightly. After long-term thermal ageing, the MX-type precipitates remain stable. The growth of M23C6 and the formation of Laves phase are confirmed. The Laves phase was the main factor leading to the increase in the ductile-brittle transition temperature.