Changes of Microstructures and Mechanical Properties of Type 321 and 347 Austenitic Steels after Long Aging

Abstract

18-8 series stainless steels, which being used widely for big boiler plants recently, reveal some changes ofmechanical properties during their use at high temperature, resulting fromthe changes of structures, i. e., precipitation of carbide particles and their coalescence.
Type 321 and 347 stainless steels were aged for up to 10000 hr at approximate service temperature in useof 650°C. The changes of structure, especially, the changes of observed shapes of precipitated carbides, were examined by optical and electron microscopies and those of mechanicalproperties were also investigated.
As results, precipitated carbides had many shapes, i. e., massive, fiborid, dendric, net-like and rod-likeM₂₃C₆ and spheroidal TiC on the grain boundaries and fine spheroidal TiC, needle-like TiC and fine spherroidalM₂₃C₆ within the grains in type 321 steel, and also massive and spheroidal carbides (NbC) onthe grain boundaries and spheroidal and massive carbides (M₂₃C₆+NbC), rosettes of NbC and needle-likecarbide within the grains in type 347 steel. The Charpy impact value at room temperature and the creeprupture strength at 650°C of both steels became lower owing to long time aging.