Influence of hydrogen produced during steam oxidation on creep deformation and rupture ductility of Gr.92 steel

Abstract

The influence of hydrogen produced during steam oxidation on the creep deformation and rupture ductility has been investigated for Gr.92 at 650 °C by comparing creep data in steam with those in air. The total strain ε_r is definitely smaller in steam than in air, while the time to minimum creep rate t_m, strain to minimum creep rate ε_m, minimum creep rate $\dot{\epsilon }$ _min and time to rupture t_r are approximately the same between in steam and in air. Hydrogen introduced into the specimens from the surface oxide scale formed during creep test in steam is expected to be always uniformly distributed throughout the creep specimens with 5 mm gauge diameter during creep tests at 650 °C because of its high diffusion rate. The smaller ε_r in steam than in air and substantially the same ε_m between in steam and in air are reasonably explained by the mechanism of hydrogen-enhanced deformation-induced vacancy formation model. Large strains in the later stage of accelerating creep region, together with the presence of hydrogen during creep in steam, enable the formation of high density of vacancies and microvoids, which reduces the total strain ε_r. The amount of strain seems to be too small in the transient creep region, suggesting that vacancies scarcely form and hence no effect of steam environment on the t_m and ε_m.