Abstract
The thin foil technique for examining metals by transmission electron-microscopy has been used to study precipitation of carbides in the grain boundary and the matrix of 304 type and 316 type stainless steels. Based on the observation, the behaviour of molybdenum in austenitic stainless steel is considered and the differences of grain boundary corrosion and of creep properties between both stainless steels are discussed. Molybdenum suppresses the diffusion of chromium in austenitic stainless steel. Molybdenum suppresses, therefore, the grain boundary precipitation of chromium carbides, and the grain boundary corrosion resistance increases by addition of molybdenum to austenitic stainless steel. Futhermore, as a result of suppression of the vacancy diffusion by molybdenum, the climbing motion of dislocation is interrupted and the deformation rate of materials is reduced. Improvement of creep properties results, therefore, from addition of molybdenum.
