Visualization of Trapped Hydrogen Along Grain Boundaries and its Roles on Hydrogen-Induced Intergranular Fracture in Slow Strain Rate Tensile Testing of Pure Nickel

It has been reported that hydrogen accumulation along grain boundaries (GBs) is an important process in the hydrogen embrittlement (HE) in pure Ni. However, there are no quantitative studies that elucidate the behavior of hydrogen accumulation and its effect on HE. Consequently, the segregating behavior of hydrogen along GBs and its role in intergranular (IG) fracture in pure Ni were examined in the present research, via a combination of thermal desorption analysis, secondary iron mass spectrometry, Auger electron spectroscopy and slow strain rate tensile testing. It was successfully demonstrated that the hydrogen trapped at GBs and the sulfur segregated along GBs contributed to the hydrogen-trapping. In addition, the contribution of trapped hydrogen on the hydrogen-induced ductility loss was quantitatively investigated. The results revealed a decreased reduction in area (RA) with a concomitant increase in trap-site occupancy, implying that the trapped hydrogen controlled the hydrogen-induced IG fracture and ductility loss in pure Ni.

Effect of Tempering Temperature after Thermo-Mechanical Control Process on Microstructure Characteristics and Hydrogen-Induced Ductility Loss in High-Vanadium X80 Pipeline Steel

In this study, an optimum tempering temperature after a thermo-mechanical control process (TMCP) was proposed to improve the hydrogen-induced ductility loss of high-vanadium X80 pipeline steel. The results showed that with increasing tempering temperature from 450 to 650 °C, the size and quantity of granular bainite decreased but the spacing of deformed lath ferrite and the fraction of massive ferrite increased. The number of fine vanadium carbides increased as well. However, as the tempering temperature increased to 700 °C, the microstructure of T700 steel completely converted to massive ferrite and the grain size became larger. Additionally, the amount of nanoscale precipitates decreased again, and the mean size of precipitates evidently increased in T700 steel. The steel tempering at 650 °C, containing the most vanadium precipitates with a size less than 20 nm, had the lowest hydrogen diffusion coefficient and the best resistance to hydrogen-induced ductility loss.