Comparison between real and simulated degradation in a 1.25% Cr–0.5% Mo steel for high temperature service

Usually aspects related to the level of impurities and specific alloying elements are analyzed regarding their influence on the susceptibility to temper embrittlement of a 2.25% Cr–0.5% Mo steel. Heat treatments are employed to simulate in-service degradation on an accelerated basis. Of the possible heat treatments, step cooling has been the most widely employed. However, it is evident that, while there has been an evolution in steel manufacturing processes, the same cannot be said for the simulation techniques (accelerated tests) or the verification of their accuracy. In the present work, samples of a 1.25% Cr–0.5% Mo steel removed from industrial equipment after 250,000 h of operation at 540 °C revealed a significant loss in toughness. This same material, subjected to de-embrittlement and recovery heat treatments, showed an improvement in the level of absorbed energy on Charpy impact testing, as compared to the material in the postservice condition. This material was then utilized as the base material for further testing. The subsequent application of the conventional step cooling heat treatment to this recovered material in order to simulate the service conditions resulted in Charpy impact energy levels superior to those exhibited by the original material (degraded by actual service conditions), thereby suggesting that the step cooling was not able to simulate satisfactorily such service conditions for this material.