Effect of Through‐Thickness Microstructure and Tempering Temperature on Hydrogen Diffusion in 110‐ksi Oil Casing Steel

The accumulation of diffusible hydrogen atoms could lead to the hydrogen‐induced damage and significantly reduce the service life of the oil casing steel. In this study, the through‐thickness hydrogen diffusion behavior of casing steel is investigated, focusing on the effect of tempering temperature on hydrogen diffusion and Charpy impact energy. The results show that the banded defects are observed in the oil casing steel pipe. Both the fraction of the banded defects and the average grain diameter are progressively decreased from the inner to the outer layer. The outer layer exhibited a superior resistance to the hydrogen diffusion compared to the middle and inner layers. With the increasing of the tempering temperature from 650 to 750 °C, the hydrogen diffusion coefficient is decreased from 1.588 × 10 −6 cm 2 s −1 to 0.991 × 10 −7 cm 2 s −1 , and the impact toughness is increased from 144.907 J cm −2 to 205.432 J·cm −2 . The volume fraction of carbides precipitated during tempering increases from 3.81 to 4.95%, respectively. Fine carbides act as irreversible hydrogen traps to pin hydrogen atoms, but large carbides may cause hydrogen‐induced cracking. Therefore, both the hydrogen diffusion resistance and the impact toughness behaviors could be promoted with a proper tempering temperature.