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Shipbuilding is heavily reliant on welding as a primary fabrication technique. Any high performance naval steel must also possess good weldability. It is therefore of great practical importance to conduct weldability testing of naval steels. Among various weldability issues of high-strength steels, hydrogen-induced cracking (HIC) in the heat-affected zone (HAZ) following welding is one of the biggest concerns. As a result, in the present work, research was conducted to study the HAZ HIC susceptibility of several naval steels. Since the coarse-grained heat-affected zone (CGHAZ) is generally known to be the most susceptible to HIC in the HAZ region, the continuous cooling transformation (CCT) behavior of the CGHAZ of naval steels HSLA-65, HSLA-100, and HY-100 was investigated. The CGHAZ microstructure over a range of cooling rates was characterized, and corresponding CCT diagrams were constructed. It was found that depending on the cooling rate, martensite, bainite, ferrite and pearlite can form in the CGHAZ of HSLA-65. For HSLA-100 and HY-100, only martensite and bainite formed over the range of cooling rates that were simulated. The constructed CCT diagrams can be used as a reference to select welding parameters to avoid the formation of high-hardness martensite in the CGHAZ, in order to ensure resistance to hydrogen-induced cracking. Implant testing was conducted on the naval steels to evaluate their susceptibility to HAZ HIC. Stress vs. time to failure curves were plotted, and the lower critical stress (LCS), normalized critical stress ratio (NCSR) and embrittlement index (EI) for each steel were determined, which were used to quantitatively compare HIC susceptibility. The CGHAZ microstructure of the naval steels was characterized, and the HIC fracture behavior was studied. Intergranular (IG), quasi-cleavage (QC) and microvoid coalescence (MVC) fracture modes were found to occur in sequence during the crack initiation and propagation process. This was rationalized using Beachem's model. Based on the implant test results, it can be concluded that with respect to HAZ HIC susceptibility, the four steels from the most susceptible to the least, are HY-100, BA-160, HSLA-100 and HSLA-65. Increasing diffusible hydrogen content showed that HSLA-100 has better tolerance to the increase in hydrogen levels than BA-160 and HY-100, with HY-100 exhibiting the least tolerance to hydrogen increase in weld joint. For the BA-160 steel, the effect of welding parameters on HAZ HIC susceptibility was investigated. It was shown that both increasing heat input and using preheat can improve the HAZ HIC resistance of BA-160. It was also found that using a PWHT at 650°C for 1 hour to reduce HIC susceptibility of BA-160 steel is also beneficial for the strength recovery in the softened as-welded CGHAZ. This is attributed to the re-precipitation of strengthening phases during the PWHT process that are dissolved in the CGHAZ during heating to the high temperature and do not re-precipitate completely during cooling.

The mechanical behavior of hydrogen-charged 316L stainless steel has been studied in samples containing sharp fatigue-induced pre-cracks, and compared with material that had been carburized at low temperatures following pre-cracking. The hydrogen charging involved exposure to high pressure (~138 MPa) H2 at 573 K for 24 days.Non-treated samples either stayed elastic in low stress intensity range or exhibited plasticity at the crack tip at high range; no hydrogen-induced cracking could be observed. On the carburized sample, however, the main crack extended during loading. The variation of crack depth along the crack was verified and reveals that the crack becomes shallower as it approached the newly extended crack tip. The threshold stress intensity factor for the hardened case, Kth, is calculated based on this observation. Plastic zone evolution at the crack tip suggests the presence of hydrogen facilitates the motion of dislocations at the room temperature.