Predicting rupture and low strain times for a 225Cr-1Mo steel using a six-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 2.25Cr-1Mo steel where long term data exist 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 six-ϑ approach produces predictions of long-term failure times that are marginally better than those obtained using the traditional four-ϑ approach. However, the six-ϑ approach is shown to be much more capable of accurately predicting times to low strains (1 per cent strain) at stress levels as low as 40 MPa at 838 K and 80 MPa at 813 K (well below the lowest stresses used in the ϑ analysis). Finally, weighting each ϑij in the six-ϑ approach is not recommended as it is shown to have a detrimental impact on the longer-term predictions obtained. This reflects inherent numerical difficulties associated with the six-ϑ approach.