Abstract
The structure changes of AISI 321 stainless steel during creep rupture testing at 650°C have been investigated with and electron microscope to know the effect of the heat-treatment and cold-working upon the creep rupture properties. The conclusions were as follows: (1) The changes of various carbides and other phases during creep rupture testing, especially those of TiC precipitated in the matrix, were examined. Titanium carbide precipitated in the matrix in fine granular form at an early stage and in needle-like form after a long period. (2) When the solution-treatment temperature is high and the solubility of carbide forming elements increases, the nucleation and the coarsening is considered to have much effect upon the increase in the creep-resistance precipitates at a later stage. As a result, the long-time creep rupture strength becomes large. (3) The recrystallization in a cold-worked material during creep rupture testing accelerates the coarsening of carbides and other phases. Consequently, the creep rupture strength becomes small and the rupture elongation increases. (4) If the solution-treatment before cold-working is performed at a high temperature, the lowering of the creep rupture strength is delayed, because the needle-like titanium carbide grains precipitating in the matrix and the slip bands inhibit the recrystallization of the material. (5) Also, if the degree of working is small, the lowering of the rupture strength is retarded, because the recrystallization is inhibited.
