An assessment of the θ-projection method for the representation and extrapolation of creep data for a 1%Cr, 12%Mo, 14%V steel tested at 565°C

Constant stress creep data for a wrought 1%Cr, $\text{1}\text{2}\text{\%}\text{Mo}$, $\text{1}\text{4}\text{\%}\text{V}$ steel obtained from six tests carried out at 565°C have been fitted and extrapolated using the θ-projection method. Creep curves for high stresses were fitted well, but curves extrapolated to stress levels below those used in the θ-projection did not represent well the available data for primary-secondary creep. To enable a satisfactory extrapolation of results, it was found necessary to redefine the failure condition to be less sensitive to the shape of the latter sections of the tertiary creep curve. This was achieved by defining lifetime as the point on the curve at which the strain rate reached a fixed multiple of the creep rate at half the rupture time. The extrapolated data compare well with the results of tests carried out independently on a cast 1%Cr, $\text{1}\text{2}\text{\%}\text{Mo}$, $\text{1}\text{4}\text{\%}\text{V}$ steel. The extrapolation procedure not only predicts the ‘knee’ in the log stress-log lifetime curve but allows extrapolation of lifetimes by over a factor of thirty. In addition, acceptable predictions have been made of constant load experimental data.