Abstract
In order to rapidly accumulate data on the hydrogen compatibility of materials, a low- cost yet effective alternative to testing in high pressure hydrogen gas is in demand. We have developed a novel method for evaluation of the hydrogen compatibility of materials, which was a four-point bending fatigue test with circulation of the ammonium thiocyanate solution (H-charging solution) into a pipe-shaped specimen over an entire period of the test. Using this continuous-hydrogen-charging method (CHC-method), the effect of the concentration of the H-charging solution on the hydrogen-accelerated fatigue crack growth rate have been investigated. The consistency in the degree of hydrogen embrittlement between the CHC-method and the test in high-pressure hydrogen gas was also investigated.
The material was JIS-SCM435, Cr-Mo low alloy steel that is used for pressure vessels. Three types of solutions with concentrations of 0.2, 2 and 20 mass% were used. The ΔK (stress intensity factor range)-increase test with a constant frequency and ΔK - constant test with a variety of frequencies ranged from 0.001 to 5 Hz were performed.
In all continuous-hydrogen-charged specimens, the fatigue crack growth rate was accelerated compared to non-charged specimen, and the crack growth acceleration was more pronounced with increase in concentration of H-charging solution. Under ΔK -constant tests, the hydrogen effect on the crack growth rate became marked when the frequency was decreased from 5 Hz to 0.02 Hz, and it roughly saturated in the frequency lower than 0.02 Hz.
The similar frequency dependence of the crack growth acceleration was observed also in the test in high-pressure hydrogen gas, and the degree of the crack growth acceleration was determined by the gas pressure.
Comparing the results in between these two methods, it was revealed that the crack growth accelerations in the CHC-method with 0.2 – 20 mass% H-charging solutions were consistent with those in the test in 10 – 90 MPa hydrogen gas. This implies that it is possible to reproduce the hydrogen embrittlement in high-pressure hydrogen gas by controlling the concentration of H-charging solution in the CHC-method.