CREEP RESISTANCE AND STRUCTURE OF 10% CR-3% СО-2% W-0.29% CU-0.17% RE STEEL WITH LOW NITROGEN AND HIGH BORON CONTENTS USED IN UNIT COMPONENTS FOR COAL POWER PLANTS

Abstract

The creep resistance and structure of 10% Cr-3% Co-2% W-0.29% Cu-0.17% Re steel with 0.1% carbon, low nitrogen content, and high boron content were investigated in a long-term fracture strength test at 650°C under loads from 200 to 100 MPa applied in increments of 20 MPa. For comparison, 9% Cr steel with 0.1% carbon, 0.05% nitrogen, and 0.005% boron was studied. The steels were subjected to preliminary heat treatment including normalization 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 steel and 9% Cr steel 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 long-term tensile strength of 10% Cr steel was 13% higher compared to 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.