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 longterm thermal ageing effect on China Low Activation Martensitic (CLAM) steel is discussed. The microstructural evolution, including the growth of M 23 C 6 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 M 23 C 6 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. Microstructure Heat Treatment, Martensitic Lath and SubgrainStructure, M 23 C 6A normalising treatment can give the desired martensitic microstructure for the China Low Activation Martensitic (CLAM) steel (Table 6.1) and provide good solubilisation for carbide into the matrix with a homogeneous microstructure. The treatment of air cooling from 980 °C leads to martensitic transformation with lath sections between 0.14 μm and 0.42 μm. The subsequent tempering softens the steel and promotes the formation of M 23 C 6 and fine MX precipitates. Dislocations