TTP Diagrams of Z Phase in 9–12% Cr Heat-Resistant Steels

Abstract

Time-Temperature-Precipitation diagrams of Z phase were investigated in crept samples of 9–12% Cr heat-resistant steels. The Z phases were formed around the prior austenite grain boundaries at 823 K, 923 K, and 973 K—temperatures similar to the 873 K reported in a previous paper. The time required for Z phase formation was shorter in higher Cr steels, indicating that to avoid Z phase formation the Cr content of the steel should be reduced as far as possible. The beginning of Z phase formation did not consist with an inflection point on the stress–rupture curve in all steels. Z phase formation could promote the degree of creep strength degradation at long-term region. The average chemical composition in the Z phase depended on the temperature: the Nb content of the Z phase was lower at lower temperatures. The average chemical composition of the Z phase changed during creep; the Cr and Nb contents of the Z phase increased and decreased, respectively, during creep exposure. The size and number density of the Z phase at 823 K, which is the maximum operating temperature of the next generation of fast breeder reactor components, were small in contrast with those at 873 K. The influence of Z phase formation on creep strength would not be large up to about 50 000 h at 823 K since most of MX cabonitrides would not disappear.