Takai Kenichi

Abstract The effect of hydrogen-enhanced strain-induced lattice defects formed during the incubation period on hydrogen embrittlement fracture of iron was clarified by quantitative evaluation using low-temperature thermal desorption spectroscopy (L-TDS) from −200 ºC. Tensile testing of iron specimens was conducted in solution with various concentrations of ammonium thiocyanate as a catalyst poison. Cathodic electrolysis was employed to establish conditions of low and high hydrogen content to examine the fracture characteristics of the iron specimens. The fracture elongation of the hydrogen-charged iron specimens was lower than that of the hydrogen-free specimens, although the elongation was the same regardless of the hydrogen content. In contrast, the flow stress during the deformation process increased with increasing hydrogen content. Specimens were prepared under the same hydrogen charging conditions and unloaded within a uniform elongation range. L-TDS was used to detect lattice defects with hydrogen re-charged as a probe under equilibrium conditions with dislocation cores and strain fields around the cores and vacancies in the specimens. The formation of vacancy-type defects was promoted in the presence of hydrogen during plastic deformation, and the extent of promotion was similar regardless of the hydrogen content. The concentration of hydrogen-enhanced strain-induced vacancies may thus affect the decrease in ductility due to the presence of hydrogen, and the hydrogen coordination number to its vacancies is responsible for the increase in flow stress.

Abstract The effects of aging treatments on the annihilation and accumulation behavior of hydrogen-enhanced strain-induced vacancies (HESIVs) formed in tempered martensitic steel were investigated. The vacancies were formed by applying plastic deformation in the presence of hydrogen. The tracer hydrogen content and peak temperature under various aging treatment conditions were measured using low-temperature thermal desorption spectroscopy (L-TDS). Aging treatments were performed in the absence and presence of hydrogen at 30 °C for 0, 2, 5, and 9 d. The spectra measured by L-TDS were divided into two peaks by using Gaussian curves: Peak 1H corresponding to hydrogen desorption from dislocations and Peak 2H corresponding to hydrogen desorption from vacancies. The amount of Peak 2H, i.e., the amount of vacancies decreased and the peak temperature of Peak 2H, i.e., clustered vacancy size increased with increasing aging time. The change in the amount and the peak temperature of Peak 2H was smaller than that in previous studies for pure iron. Furthermore, the change was greater for aging in the absence of hydrogen than for aging in the presence of hydrogen. Therefore, the impurities in the steel such as solid solute carbon and hydrogen probably stabilize vacancies, decrease the diffusion coefficient of vacancies, and then partially suppress the annihilation and accumulation of vacancies.