On the 25%Cr-28%Ni-2%Mo austenitic heat-resisting steel containing up to 0.67% nitrogen, 2% vanadium, 5.7% wolfram, and 0.05% boron, the high temperature strengths were investigated by tensile creep rupture testing and short time tensile testing, and the correspondence of these properties with the microstructure of the steel was discussed.
Addition of nitrogen to the steel containing vanadium and boron increases not onlythe short time tensile strength at the temperature up to 700°C, but also remarkably the 1000 hr rupture strength at 700°C.
The 25% Cr-28%Ni-2%, Mo steel, added with nitrogen, wolfram, and boron, has showed the most excellent creep rupture strength in the range of the present work that was carried out at both the temperature of 700°C and 800°C; for example, the steel containing 5.7% wolfram, 0.05% boron, and 0.67% nitrogen has showed 26.5 kg/mm2 of 1000 hr rupture strength at 700°C. Further, in the lower range of nitrogen con tent, the 700°C-1000 hr rupture strength of the steel was increased by about 9 kg/mm2due to the addition of 5% wolfram and 0.05% boron, and this value corresponds to about 100% increase of the strength com paring to that of wolfram-boron free steel.
With addition of boron to this type of steel, the high temperature ductilities have been improved remarkably, but the creep rupture strength has not so much.