Hydrogen cracking associated with armament structures has become more prevalent in recent times [1–3]. In this work, a thick-walled, autofrettaged pressure vessel was manufactured from ASTM A723 Grade 2 steel and heat-treated to yield strength of 1170 MPa. An outside diameter keyway was then machined.The keyway was exposed to concentrated sulfuric acid, leading to apparent cracking within 20 hours of exposure.
Investigation of the affected keyway in the pressure vessel indicated that localized hardened areas were present. The base material possessed hardness values of Rc 37–39, while areas near the keyway possessed a local hard zone up to Rc 44. These zones extended to a depth of approximately 4 mm. The different hardness layers suggest that the environmental cracking incubated and propagated in two separate stages. It is speculated that cracking in the hard layer incubated quickly and propagated to approximately the 4 mm depth, then arrested itself once it encountered the more ductile base material. Previously published  crack growth (da/dt) test data, and new data verify that this process of incubation and propagation could have occurred in a matter of seconds. The cracking then resumed in the softer base material after approximately 300 hours of incubation time. Additional da/dt testing of this condition has been performed over a range of yield strengths and verifies that incubation times and crack propagation rates are similar to those observed in this pressure vessel.