Multiaxial Stress Rupture Criteria for Ferritic Steels

Summary

Many ferritic steels become ‘creep-brittle’ after long times at high temperatures. This paper is concerned with the behaviour of such steels under multiaxial stressing, which is the type of loading that they invariably encounter in service in power and process plant applications. Various criteria have been proposed for predicting failure under multiaxial stress states. In the present investigation the applicability of these criteria to ferritic steels was initially assessed using rupture data obtained on a single cast of 2\frac{1}{4}CrMo steel tested under different stress states. The best correlation between prediction and experiment was given by relatively simple formulations containing only maximum principal stress, σ₁ and von Mises’ effective stress, $\bar \sigma$, terms. Of these formulations, the one which was considered to be most consistent with the physical processes controlling creep was then applied to the available multiaxial rupture-life data for ferritic steels. These data cover a wide range of steels tested under a variety of stress states. It was found that the rupture life, t _r could be generally well expressed as:

$$t_r \, = \,M\sigma _1^{ - 2.5} \bar \sigma ^{ - m + 2.5} $$

(1)

where ‘M’ and ‘m’ can be obtained from uniaxial stress-rupture tests