Precipitation behavior during aging and creep in 18Cr-9Ni-3Cu-Nb-N steel

Microstructural changes during aging and creep were investigated in 18Cr-9Ni-3Cu-Nb-N steel (KA-SUS304J1HTB, ASME Code Case 2328), focusing on copper phase, MX carbonitride and Z phase. The creep strength of 18Cr-9Ni-3Cu-Nb-N steel was much higher than that of conventional SUS304 steel in the short term. However, the difference of these two steels was small in the long term, because the creep strength of 18Cr-9Ni-3Cu-Nb-N steel degraded in the long term. Niobium rich MX particles precipitated in grain interior after aging for just one hour at 650 to 800 °C. Modified Z phase [Cr(Nb, V)N] particles were observed after aging accompanied by the disappearance of MX particles. The time required for precipitation of modified Z phase was about 2500 h at 650 °C and 1050 h at 700 °C. Fine copper particles were detected in grain interior after aging for one hour. Large copper particles were located at the interface between the σ phase and the matrix after long-term aging at 650 °C and 700 °C.