Senior engineer, Westinghouse Research Laboratories, Pittsburgh, Pa.
A state-of-the-art survey of literature pertaining to low alloy Cr-Mo-V steels has been completed with a view to elucidate the effects of composition, heat treatment, and microstructure on the creep strength and ductility of the steels. It appears that minor amounts of alloy additions such as boron, titanium, and cerium and impurity elements phosphorus, sulfur, tin, antimony, aluminum, and copper may affect the creep strength or ductility or both of the steels. Higher austenitizing and lower tempering temperatures lead to improved strength at the expense of rupture ductility. An upper bainite microstructure is associated with the highest creep strength and the lowest ductility, for temperatures up to 1050°F (565°C) and for times of at least up to 10,000 h. In bainite-ferrite aggregates, creep and rupture strengths increase in proportion to the amount of bainite, and the difference in strength between the various structures is maintained at least up to 10,000 h at 1070°F (575°C). Stress rupture strengths in general increase linearly with room temperature tensile strength for temperatures up to 1000°F (538°C) and times up to 10,000 h. Variation of rupture strength and minimum creep rate with temperature and time can be adequately described by the Orr-Sherby-Dorn parameter. Activation energies for both creep and for rupture are determined to be about 90 kcal/mole (375 kJ/mole). Further, it is observed that ˙ε × tr ⋍ 3.3 and that tt ⋍ 0.3 tr, where ˙ϵ tt, and tr are the minimum creep rate, time for transition from second- to third-stage creep, and time to rupture, respectively.
Paper ID: JTE10143J