Evaluation of the post-welding heat treatment effect on compatibility with hydrogen.

The article covers effect of post-weld heat treatment (PWHT) on the properties of thermo-mechanically treated (TMT) pipeline steel in interaction with hydrogen gas. The object of the study were specimens of the base metal and longitudinal welded joints of X65 large diameter pipes in the initial state (after TMT) and after PWHT. Tests in the hydrogen environment showed that PWHT conducted under ASME B 31.12 modes leads to a decrease in the plastic properties of X65 pipe steel when exposed to hydrogen gas, which is due to the fact that PWHT contributes to a signifi-cant increase in the number of hydrogen traps and concentration in the surface layer. Microstructure of the specimens after PWHT is characterized by a significant amount of highly dispersed precipitates, presumably carbides, which can act as low-energy hydrogen traps, increasing the amount of diffusionally mobile hydrogen and contributing to its movement into crack development zones. The specimens with PWHT showed a significant change in fracture pattern after hydrogen testing, indicating the embrittlement effect of hydrogen: multiple laminations, cracks, fracture consists mainly of quasi-cleavage facets. Thus, the post-welding heat treatment of X65 steel leads to a significant increase in the negative effect of hydrogen on steel: embrittlement of heat-treated steel when exposed to hydrogen gas is observed. Since the post-welding heat treatment negatively affects compatibility of X65 steel with hydrogen gas, PWHT may be cancelled in accordance with GR-3.7.1 of ASME B 31.12.