Heat-to-heat variation of creep strength and long-term stability of microstructure in Grade 91 steels

Heat-to-heat variation of long-term creep strength and microstructural changes was investigated in ASME Gr. 91 steels (heat name: MGC heat and MgC heat) by focusing on the effects of minor elements. The heat-to-heat variation of creep strength was not observed at 500 °C and 550 °C. At 600 °C and 650 °C, there was no significant difference in the creep strength up to 10,000 h in the two heats. However, at 600 °C, the creep strength of the MGC heat was clearly lower than that of the MgC heat at around 100,000 h. The effect of Ni content (MGC heat: 0.28 mass%, MgC heat: 0.04 mass%) in the range of the specification (Ni ≤0.40 mass%) on microstructural changes was examined. There was no significant difference in the dislocation structure after creep rupture in the two heats. No effect of Ni content was observed on the coarsening of M23C6 particles during the creep. In the MGC heat, a significant number of modified Z-phase particles were formed, consuming a large number of MX particles during the creep. On the other hand, the number of modified Z-phase particles of the MgC heat was lower after the creep rupture than with the MGC heat. An increase in the Ni content promotes modified Z-phase formation and eliminates MX particles during the creep, indicating that the deformation resistance can decrease after a long time in steel with high Ni content even in the range of the specification.