Creep fracture of 12Cr–Mo–V steel

Abstract

A study has been made of the effects of microstructure on the creep crack growth characteristics of a 12Cr–Mo–V steel. The structures investigated were those which might be expected in a weld heat-affected zone and included those which could be formed by incorrect post-weld heat treatment. In correctly heat-treated coarse-grained microstructures creep fracture occurred by a mixture of intergranular and transgranular cavitation. These cavities nucleated preferentially on sulphide, silicate, and oxide inclusions but there was also evidence for secondary nucleation on carbides on prior austenite or martensite lath boundaries. The presence of prior austenite boundary δ-ferrite biased the fracture toward the transgranular mode but had little effect on crack growth rates. Crack propagation rates were greatly increased by partial transformation direct from austenite at 700°C as a result of performing the stress relief heat treatment before cooling through the martensite transformation range. In the absence of δ-ferrite this was attributed to the formation of coarse M₂₃C₆ on austenite boundaries before the martensite transfornmtion. The presence of δ-ferrite resulted in accelerated transformation of austenite to α-ferrite plus carbide and the increased crack growth rates were then due to localized rupture of the weaker ferrite.