Characterization of creep properties and the microstructure of a service-exposed low alloy CrMoV steel steam pipe

The effect of service degradation on the microstructure and creep properties of a 0.5%Cr-0.5%Mo-0.3%V steel steam pipe after an accumulated service time of 77,900 h was investigated and analysed. For comparison pur -poses, additional experiments and analyses were carried out on the same pipe in the as-received (service unused) conditions. The results of the microstructural analysis and the short-term creep testing confirmed that the high creep resistance of the investigated steel is caused predominantly by the dispersion of fine vanadium carbides, providing an effective precipitation hardening of the steel matrix. However, the microstructure is not in an equilibrium state, and as the size of the vanadium carbides and the accompanying mean interparticle spacing increase during the creep, the detrimental effect of vanadium carbide coarsening leads to a significant weakening in the precipitation hardening and, consequently, a decrease in the creep resistance. No creep cavitation was observed in the service-exposed steel. The short-term creep testing of the service-exposed and unused states of the steel was carried out in the power-law (dislocation) creep regime. Analysis of the creep data did not reveal any change in the operating creep deformation mechanisms as a response to the structural processes occurring during the long-term service. Instead, changes in the kinetics of creep flow and fracture were found.