Aging and creep behaviour of 15%Cr-15%Ni-Mo-Ti-B steel

Factors influencing the creep behaviour of a commercial 15%Cr-15%Ni-Mo-Ti-B steel at 650°C have been investigated, using a combination of aging, hot tensile, and creep studies. At stress levels near those recommended for pressure vessels, the aging process was not significantly affected by creep. Two stages of weakening were observed in material air cooled from 1050°C or 1150°C, the first being attributed to a general reduction of carbon supersaturation, and the second to a general loss of titanium solute concentration. The initial strengthening by carbon and titanium in solution was not sufficiently persistent to be of much practical advantage. To obtain any long-term benefit from the titanium present in the steel, it was necessary to produce fine titanium carbide precipitate–dislocation arrays. After heat treatment at 1150°C the initial reduction in carbon supersaturation during aging rapidly destroyed the ability of the steel to nucleate further strain-induced titanium carbide. Thus, for long-term service applications at subyield stresses it is important to introduce strain prior to service.