Effect of creep deformation on Z phase formation in Gr.91 steel

Abstract

In order to make clear the effect of creep deformation on Z phase formation in Gr.91 steel, creep interrupted tests were performed for 10 000, 20 000, 30 000, 50 000 and 70 000 h at 600°C under 70 MPa. The time to rupture was 80 736·8 h at 600°C under 70 MPa. Z phase was observed in the vicinity of prior austenite grain boundaries in the grip and gauge portions after creep deformation for 10 000 h and more. Number density of Z phase particles increased with increasing creep time in the grip and gauge portions. The number densities of MX carbonitrides particles in the grip and gauge portions clearly started to decrease after 30 000 h corresponding to increase in number density of Z phase. In the gauge portion, the number density of MX carbonitrides particles was almost the same as that of Z phase particles after creep rupture, meaning that a large number of MX carbonitrides particles were disappeared. The number density of Z phase particles in the gauge portion was 2·5 times of that in the grip portion after creep rupture. This indicates that creep deformation promotes the Z phase formation.