Effect of matrix hardness on the creep properties of a 12CrMoVNb steel

Abstract

Using a creep-ductile 12CrMoVNb steel, constant-load creep tests were conducted in air at 650 degrees C, and the effects of matrix hardness on the creep properties were investigated. Specimens with a matrix hardness (Rc) of 30, 25, and 20 were prepared using different tempering conditions. The creep behaviors were well described by the power-law creep equation, with the stress exponents of strain rate (n) and rupture time (chi) decreasing with matrix hardness. Rupture-time analyses showed that creep rupture occurred by the nucleation of creep cavities on second-phase particles and growth by creep flow of the surrounding matrix. A hardness decrease tends to lower the rupture time and increase the strain rate (epsilon), and the effect of hardness was quite distinct at high applied stresses due to the short creep times, but not so at low applied stresses due to elongated creep times. After 10(4) hours, there were almost no effects. The hardness decrease during the creep test was more severe for the specimens with higher hardness and was also more severe in the gage section than in the head section, the latter due to the stress-assisted diffusion in the coarsening of carbides. Microstructural examinations showed that subgrain boundaries grew during creep, and equiaxed carbide particles coarsened during the creep test, the rates of coarsening being greater for specimens with a higher hardness.