Z-phase Formation during Creep and Aging in 9–12% Cr Heat Resistant Steels

Abstract

The precipitation behavior of the Z phase was investigated after long-term creep exposure in ASME-T91, T92, T122 without δ-ferrite, and T122 with δ-ferrite through elemental mapping using EF-TEM. The Z phase was identified by comparing the Cr map with the V map. Most of the Z phase was observed around prior austenite grain boundaries and/or packet boundaries in all of the steels examined. In T122 with δ-ferrite, the Z phase also precipitates around the δ-ferrite. In particular, the number of MX carbonitrides was very small in T122 with a large amount of the Z phase. The main metallic composition of the Z phase in T91 was the same as that in T92. In T122, the Z phase contained a lower Nb content. The main metallic composition of the Z phase around the δ-ferrite was the same as that in the other areas. There was no large difference in the size distribution of the Z phase among the steels. The mean diameter of the Z phases for T122 with δ-ferrite was relatively large in spite of a shorter creep exposure in contrast with T91 and T92. The number density of the Z phases increased with increasing creep exposure time except in the case of T91. The order of the number density was T122 with δ-ferrite, T91, T122 without δ-ferrite, and T92. In crept samples, the amount of Z phase in the gauge portion was higher than that in the grip portion, meaning that stress and/or strain promotes the formation of a Z phase during creep exposure.