FE analysis of hydrogen diffusion around a crack tip in an austenitic stainless steel

Abstract

Hydrogen-assisted cracking (HAC) is a coupled phenomenon between crack growth rate and hydrogen diffusion to the region ahead of crack tip. To understand the cracking mechanism and to control or predict the HAC rate, knowledge of hydrogen diffusion around a crack tip is essential. Metastable austenitic stainless steels can be severely embrittled by hydrogen due to strain-induced α′ martensite transformation around crack tip, as the hydrogen diffusivity in α′ martensite is much higher than in γ austenite while the solubility is lower. However, the effect of induced α′ martensite on local diffusion of hydrogen around a loaded crack tip has not been intuitively demonstrated. This study first reveals the quantitative relations of hydrogen diffusivity and solubility with induced α′ martensite amount, and then performs FE analysis of hydrogen diffusion around a crack tip in a 304L ASS cylinder storing high pressure gaseous hydrogen with considering the combined effect of α′ martensite transformation and hydrostatic stress on diffusion. The hydrostatic stress exhibits a peak value at a distance away from crack tip, while the peak α′ martensite volume fraction locates at the crack tip. Compared to the analysis only considering stress effect, the presence of induced α′ martensite can not only significantly accelerate the hydrogen transport to the critical region ahead of crack tip, but also markedly elevate the level that the peak hydrogen concentration reaches. The peak hydrogen concentration doesn't locate at the peak hydrostatic stress site as generally expected, but at the site in the γ austenite-rich region abutting the α′ martensite-rich region at crack tip vicinity, because this site possesses very high hydrogen solubility and close to the high hydrogen diffusivity region, consequently accommodating the hydrogen getting off the α′ martensite diffusion “highway”.