Abstract
18-8 series stainless steels, which being used widely for big boiler plants recently, reveal some changes of mechanical properties during their use at high temperatures, resulting from the change of structures, i. e., precipitation of carbide particles and their coalescence.
Types 304 and 316 stainless steels were aged for up to 10, 000 hr at approximate service temperature in use of 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 mechanical properties were also investigated.
As results, there was no major change in microstructures of type 304L steel having carbide precipitates coalescing on the grain boundaries after aging for 10, 000 hr But much carbide precipitates which have many shapes were observed on grain boundaries and also within grains of type 304H steel. The Charpy impact values of both steels became lower.
The carbides precipitated mainly on grain boundaries of type 316H steel and had many shapes for example hexagonal plate-like and needle-like.
