Hydrogen Desorption Spectra from Excess Vacancy-Type Defects Enhanced by Hydrogen in Tempered Martensitic Steel Showing Quasi-cleavage Fracture

Abstract

An attempt was made to separate and identify hydrogen peaks desorbed from plastic-strained, hydrogen-enhanced lattice defects from among various trapping sites in tempered martensitic steel showing quasi-cleavage fracture using thermal desorption spectroscopy from a low temperature (L-TDS) and positron annihilation spectroscopy (PAS). The amount of the lattice defects beneath the quasi-cleavage fracture surface was measured by L-TDS. The L-TDS results made it possible to separate two peaks, namely that of the original desorption and also that of new desorption from the steel specimens due to the application of plastic strain in the presence of hydrogen. The PAS results revealed that the new desorption obtained by L-TDS corresponded to vacancy-type defects. Hydrogen and plastic strain noticeably enhanced lattice defects formed within 1.5 mm from the fracture surface, where the average concentration of vacancy-type defects reached approximately 10 À5 order in terms of atomic ratio. These results indicate that the accumulation of excess vacancy-type defects enhanced by hydrogen in the local region can lead to nanovoid nucleation and coalescence in plastic deformation, resulting in quasi-cleavage fracture of tempered martensitic steel.

Numerical Interpretation of Hydrogen Thermal Desorption Spectra for Iron with Hydrogen-Enhanced Strain-Induced Vacancies

“…We label these peaks as Peak 1 and Peak 2, respectively. The shape of the spectra is similar to that in the case of the tempered martensitic steel [27] . From the comparison between the strain and H + strain specimens, it is seen that the defects corresponding to Peak 2 are increased by H charging.…”

“…However, the change of Peak 1 was not simulated in Figure 13. As for Peak 1, the peak temperature in Figure 4 is close to that of the corresponding peak in the spectra of the tempered martensitic steel [27] , and the H trapping energy evaluated in Section IV was close to that of screw dislocations. Thus, it can be considered that Peak 1 is formed by H atoms released mainly from dislocations as reported [27] .…”

“…As for Peak 1, the peak temperature in Figure 4 is close to that of the corresponding peak in the spectra of the tempered martensitic steel [27] , and the H trapping energy evaluated in Section IV was close to that of screw dislocations. Thus, it can be considered that Peak 1 is formed by H atoms released mainly from dislocations as reported [27] . Figure 7 indicates that by the aging process at ‡ 323 K, the H trap sites decrease and the H trapping energy changes from the case of 303 K. If it is supposed that the defect for Peak 1 is dislocations, although its decrease by the aging process of high temperature is well known, the increase of its H trapping energy is not so obvious.…”

“…(1) In the experimental spectra, the H desorption from defects generated by applying strain and charging with H atoms concurrently formed Peak 2 with the shoulder-like shape at temperature after that of Peak 1 corresponding mainly to dislocation. (2) From the experiment for the aging process of the H + strain specimen and the report [27] , it can be considered that the defects corresponding to Peak 2 are vacancies and vacancy clusters. (3) From the simulation for the behavior of H atoms, vacancies, and vacancy clusters in the pre-measurement processes, i.e., the degassing or annealing, polishing, and charging with tracer H atoms, we obtained the size distribution of vacancies and vacancy clusters in the H + strain specimen before the heating process of TDS, and we observed that the concentration of V 2 and V 3 became smaller than that of V 1 and V 4 -V 6 because of the faster diffusion of V 2 and V 3 .…”

“…Therefore, it is difficult to separate and identify the peak for vacancies. However, by measuring H desorption from a small-size specimen using a low-temperature TDS (L-TDS) apparatus [25,26] that can heat a specimen from a low temperature of 77 K to 473 K, thermal desorption spectra in which the peak for vacancies is separated from the peak for other defects can be obtained [27] . From the viewpoint of modeling thermal desorption spectra including a desorption peak for vacancies and vacancy clusters, the structure in martensitic steels is so complicated that it is difficult to model the influence of the structure on the diffusion of H atoms, vacancies, and vacancy clusters because of many unknown factors.…”