Individual and combined effects of additional elements Mo, Nb, Ti, N and B on creep rupture strength, deformation resistance, aging hardness and microstructure of 0.2C-18Cr-12Ni austenitic heat resisting steel were investigated, and the following results were obtained.
1) Creep rupture strength increased greatly by the additions of Mo, B and increased slightly by those of Ti, Nb in that order. The effect of N was little at 700°C but increased at 750°C. Moreover, there were two effective interactions on creep rupture strength under the simultaneous additions of Mo and B which suppressed the precipitation within the grains and at grain boundaries, and stabilized the structure, and under those of Mo and N which accelerated the fine and uniform precipitation within the grain sand gave good characteristics of age-hardening. Also, there was a useless interaction under the simultaneous additions of B and N.
2) Deformation resistance at high temperature was increased greatly by the addition of Mo, B and decreased slightly by Ti but was scarcely influenced by Nb. The addition of N increased the deformation resistance at 900°C, but scarcely influenced it at over that temperature. Especially, at 900°C under the simultaneous addition of Mo and B, Mo and N, increased the deformation resistance, but that of B and N, decreased it.
3) The hardness as solution-treated was increased by the addition of Mo, B, N, Nb in that order, but was decreased by Ti. In the simultaneous addition of those elements, the effects of single additions appeared cumulatively.
4) The characteristic of age-hardening was deteriorated by the addition of B, Nb, Ti but improved by N. Moreover, the simultaneous addition of Nb and N suppressed the age-hardening through the prevention of precipitation and that of Ti and N promoted the age-hardening through the acceleration of fine and uniform precipitation.
5) Finally, by the utilization of the effective interaction of added elements upon creep rupture strength, authors recommended two steels with the nominal compositions of 1) 0.2C-18Cr-12 Ni-3Mo-0.2B-0.2-0.6Ti and 2) 0.2C-18Cr-12Ni-3Mo-0.15N-0.02B-0.5-1.0Nb, which showed high creep rupture strength.