Influence of Microstructure and Heat-Treatment on Impact and Creep Properties of a Low Alloy CrMoV Turbine Steel

Abstract

This investigation concerns the strongly scattering mechanical properties of a 17 MoV 84 creep resistant steel. Emphasis is laid on the effect on microstructure of heat-treatment. The mechanical tests made after various austenitizing treatments, cooling rates, and tempering temperatures revealed that a complex relationship existed between the mechanical properties and microstructure in the steel precipitation hardened by vanadium carbides. In spite of constant tensile strength, the impact and creep properties in the steel therefore differ widely, due to relatively small microstructural effects. The study shows that the microstructural constituents, i.e., martensite, lower bainite, upper bainite in aciculare and globulare forms, as well as pre-eutectoid ferrite, all influence the room temperature ductility in a similar way to the creep ductility. However, in the case of creep strength, similar microstructural constituents have almost the reverse effect. As a result of these contrasting effects, it is not possible to obtain good ductility and creep properties at the same time. Any heat-treatments recommended can therefore only be a compromise between the properties desired.