Comparison of fracture test data for some high strength steels stressed in a saline medium and in gaseous hydrogen

In this study, a comparison is made of the results of fracture tests conducted on four steels of high yield strength ( YS ≥ 1200 N/mm 2) stressed either in gaseous hydrogen at a pressure of 1 atmosphere or in a saline solution of 0.6 M NaCl. The steels involved were NiCrMo (35 NCD 16), Si (61S7), CrMo (45 CD4) and SiCrMo (45 SCD 6) grades. The tests were conducted with C.T. type fracture mechanics notched specimens. It is shown that: 1. (1) the non-propagation thresholds of the cracks are substantially higher in gaseous hydrogen (1 atm) than in the saline solution, 2. (2) if one considers that this threshold may be used as a criterion of the ultimate strength of steels in the presence of hydrogen, the rating obtained for the four steels investigated is the same whether the tests be carried out in the aqueous or gaseous media, 3. (3) the life of the specimen, other conditions being equal, is about 10 times shorter in H 2 than in the 0.6 M NaCl solution, 4. (4) the fracture appearances are not esactly the same in the two media: in addition to grain boundary decohesion zones, many quasi-cleavage zones and characteristic asperities on the exposed grain boundaries are found in hydrogen.