Creep Resistance and Structure of 10% Cr–3% Сo–2% W–0.29% Cu–0.17% Re Steel with Low Nitrogen and High Boron Contents for Unit Components of Coal Power Plants

Abstract

The creep resistance and structure of 10% Cr–3% Сo–2% W–0.29% Cu–0.17% Re steel with 0.1% carbon, low nitrogen content and high boron content were investigated by creep rupture testing at a temperature of 650°C and stresses from 200 to 100 MPa applied in 20-MPa increments. For comparison, 9% Cr steel with 0.1% carbon, 0.05% nitrogen, and 0.005% boron was considered. The steels were subjected to preliminary heat treatment including normalizing at 1050°C for 1 hour, tempering at 750–770°C for 3 hours, and cooling in air. The structures of both heat-treated steels exhibited martensite laths with boundaries pinned by М₂₃С₆ carbides, and the rearrangement of dislocations was retarded by MX particles. A significant difference between 10% Cr and 9% Cr steels was the presence of fine М₂₃С₆ carbide particles characterized by orientational relationships with the ferrite matrix and MX carbonitrides, whose volume fraction was 6 times lower. Short-term tensile tests at room temperature showed no differences between the steels, while the creep rupture strength of 10% Cr steel was 13% higher than for 9% Cr steel. The creep deformation mechanism of the steels was also different. Structural analysis of 10% Cr steel after creep tests revealed no substantial changes in its lath structure: the lath width increased by only 58% and the dislocation density was reduced by a factor of 2. Comparison with 9% Cr steel showed that the good structural stability of 10% Cr steel during creep is caused by the high coarsening resistance of second phase particles, whose coarsening rate is 1-2 orders of magnitude lower than that in 9% Cr steel.