The Hydrogen Bake-out Problem

When repairing or modifying a vessel, a welding engineer is faced with the task of developing a safe welding procedure so as not to introduce conditions in the weld region that increase its propensity to cracking. In developing a safe welding procedure, the control of atomic hydrogen is a primary concern. In combination with the weld residual stress and the susceptible HAZ microstructure, even a small concentration of hydrogen in the weld region can cause cracking at levels of hardness much lower than those typical for HAZ cracking. While hydrogen content introduced during the welding process is always a concern when developing a welding procedure, the pre-existing state of the vessel can exacerbate the issue when the vessel under repair has been operating in a hydrogen environment. In this scenario, prior hydrogen will be present in the steel due to the in-service conditions, and must be taken into consideration when assessing the likelihood of hydrogen-induced cracking after repair.