Long-Term Creep Ductility Minima in 12%CrMoV Steel

Abstract

Minima in the rupture ductility versus long-term rupture life relationship for 12CrMoV steel in the temperature range 525 to 675°C have been investigated by reference to previously published long-term creep data together with some new metallographic information. The observed minima are similar to those exhibited by most otner creep-resistant ferritic steels. The ductility behaviour has been modelled in terms of creep cavitation theory. The theory predicts a diffusional cavity growth controlled decrease in rupture ductility with decreasing stress at creep lives that are less than those at which the minimum occurs and a nucleation-controlled increase in rupture ductility at longer times. Insufficient materials property data are currently available to allow a rigorous quantitative verification of the model for the present materials. However certain quantitative predictions are in good agreement with the experimental data and metallographic observations. The further work that is underway to develop the model’s capability for very long-term ductility behaviour prediction of creep-resistant ferritic steels in general, is highlighted.