Microstructural Stability of Heat-Resistant Steels
The microstructural evolution of advanced 9–12 %Cr ferritic/martensitic heat-resistant steels used for power generation plants is discussed in this chapter. Despite the small differences in chemical compositions, the steels share the same microstructure of the tempered martensite. It is the thermal stability of the initial microstructure that controls the creep behaviour of these heat-resistant steels. The microstructural evolution in 9–12 %Cr ferrite heat-resistant steels includes (1) martensitic lath widening, (2) disappearance of prior austenite grain boundary, (3) emergence of subgrains, (4) growth and coarsening of precipitates and (5) formation of new precipitates, specifically Laves phase and Z phase. The first three microstructural processes could be retarded by properly disposing of the last two. Namely, improving the stability of precipitates and optimising their size distribution can effectively exert the beneficial influence of precipitates on microstructures. In this sense, the microstructural stability of the tempered martensite is in fact the stability of precipitates during the creep. The creep stress could promote the formation of Laves phase. Many attempts have been made to improve the microstructural stability of 9–12 %Cr steels and several promising heat-resistant steels have been developed.
KeywordsLave Phase Creep Strength Martensitic Lath Subgrain Boundary M23C6 Carbide
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