Some important considerations in creep modelling of ferritic steels and nickel-based superalloys

Trends in life with respect to stress can vary significantly based on the form of the chosen Arrhenius equation, and this in turn is fundamentally influenced by the nature of the rate controlling microstructural obstacle. Recognising this crucial aspect is important since shortcomings in creep models are in many cases attributed to myriad microstructural changes, while in fact, it is the basic creep mechanism which needs to be properly accounted for. It is proposed that nano-scale vanadium carbonitrides and dislocation nodes are the rate controlling obstacles in ferritic steels and superalloys, respectively, and this defines the basic rate equation to be used for these materials. With these definitions in place, simple constitutive equations, similar in spirit to the Larson-Miller parameter are derived. The ability of these equations to predict long-term behaviour and remaining life is demonstrated using data on P91 boiler tubes and IN738 superalloy.