Effect of nitrogen on creep rupture strength and toughness after aging of an austenitic heat resistant steel

Creep-rupture tests and Charpy impact tests after long-term thermal aging were performed for three austenitic heat-resistant steels of which compositions were Fe-25%Ni-23.5%Cr-1%Mo-6%W-(0.11 to 0.27)%N-0.05%Ti-0.20%V-0.25%Nb-0.004%B by wt% to reveal the influence of nitrogen contents on the mechanical properties required for boiler tubes. Creep-rupture strengths of the steels at 700 °C for 6000 h were higher than 155 MPa. The material which showed the highest strength at 700 °C for 6000 h was the 0.21N steel. Nitrogen in the steels contributes to increasing the creep-rupture strength. The effect of nitrogen under higher stress condition became larger while the effect became smaller under lower stress condition. It was supposed that nitrogen decreased the effect of precipitation strengthening by stabilizing the austenite phase for a longer time while it increased the effect of solid solution strengthening. On the other hand, the Charpy impact value of all steels after aging at 700 °C for 1000 h became less than 10 J/cm2. Nitrogen in the steels did not have a notable influence on the toughness after long-term thermal aging because precipitates of M23C6 and a tungsten-rich phase formed along grain boundaries deteriorated the toughness.