Predicting rupture and low strain times for a 2.25Cr-1Mo steel using a six-theta projection technique

The theta projection method of creep analysis is known to produce the poore st predictions of creep properties at low strains. This paper applies a rec ently suggested modification of the a concept to 2.25Cr-1Mo steel where lon g term data exist to enable an assessment of this modification to be made. The modification takes the form of two additional theta terms that allow th e initial stages of any creep curve to be modelled more accurately. The pap er shows that the resulting six-theta approach produces predictions of long -term failure times that are marginally batter than those obtained using th e traditional four-theta approach. However, the six-theta approach is shown to be much more capable of accurately predicting times to low strains (1 p er cent strain) at stress levels as low as 40 MPa at 838 K and 80 MPa at 81 3 K (well below the lowest stresses used in the theta analysis). Finally, w eighting each By in the six-theta approach is not recommended as it is show n to have a detrimental impact on the longer-term predictions obtained. Thi s reflects inherent numerical difficulties associated with the six-theta ap proach.