Sustained-load crack growth of hydrogen-charged surface-hardened 316L stainless steel

The crack growth behavior of hydrogen-charged low-temperature carburized and hydrogen-charged non-carburized 316L stainless steel was studied in wedge-open loading samples containing sharp, cyclic-loading-induced pre-cracks. The hydrogen charging involved exposure to high pressure (≈138 MPa) hydrogen at 573 K for 24 days. Non-carburized samples either resisted crack growth under low stress intensity or exhibited plasticity at the crack tip at high stress intensity. However, in the samples interstitially hardened by low-temperature carburization, and containing a “case” with a “colossal” carbon supersaturation, the main crack extended during loading and several new cracks emerged around the crack tip. This observation could be attributed to the high strength of the hardened case, since an inverse relationship between yield strength and the hydrogen-induced threshold stress intensity factor, Kth, is well established in high-strength- and austenitic stainless steels. The crack extension and induced cracking were confined to the hardened case. Plastic zone evolution observations on all samples imply that the presence of hydrogen promotes dislocation mobility, whereas interstitial carbon substantially suppresses the size of the plastic zone.