Predicting times to low strain for a 1CrMoV rotor steel using a 6-θ projection technique

Abstract

The θ projection method of creep analysis is known to produce the poorest predictions of creep properties at low strains. This paper applies a recently suggested modification of the θ concept to 1CrMoV rotor steel where long term data exists to enable an assessment of this modification to be made. The modification takes the form of two additional θ terms that allow the initial stages of any creep curve to be modelled more accurately. The paper shows that the resulting 6-θ approach produces predictions of long-term failure times and minimum creep rates that are as good as those obtained using the traditional 4-θ approach. Unlike the 4-θ approach, the 6-θ approach is also shown to be capable of accurately predicting times to very low strains (0.05% and 0.1%) at stress levels as low as 77 MPa (well below the lowest stress of 230 MPa used in the theta analysis). For times to 1.0% strain or more the 4-θ and 6-θ techniques give similar short and long-term predictions.