Influence of Grain Refinement of X60 Pipeline Friction-Welded Joints on Their Cracking Resistance to Hydrogen Embrittlement in a High Pressure Gaseous Hydrogen Environment

Abstract

The coarse-grained heat-affected zone (CGHAZ) of conventional fusion weld joints is particularly prone to cracking in hydrogen environments. The X60 pipeline steel rotary friction welding (RFW) joint consists of a weld zone (WZ) and a thermo-mechanically affected zone (TMAZ). RFW can effectively induces recrystallization, forming fine equiaxed grains throughout the joint with grain size consistently smaller than those in the base metal; The average grain size is 3.5 μm in the base metal whereas it is 2 μm in the TMAZ and 1.5 μm in the weld zone. This grain refinement not only enhances the impact toughness of the welded joints compared to the base metal, it also significantly reduced the hydrogen embrittlement (HE) susceptibility. Slow strain rate tensile (SSRT) tests performed in a 6.3 MPa hydrogen environment showed resistance to HE of the weld is considerably higher than the base steel: the ratio of elongation-to-failure with hydrogen charging to that of uncharged sample was 0.78 in the base metal and it is 0.98 for the RFW joint. In the base metal, the coarser grain size and a higher density of dislocations are associated with its higher susceptibility to HE. In contrast, the consumption of dislocations during recrystallization of the RFW joint and the increase in high-angle grain boundaries (HAGBs) in this weld zone reduced the hydrogen concentration at grain boundary by promoting a more uniform distribution of hydrogen in throughout the weld microstructure, thereby enhancing its resistance to HE.

Keywords: X60 pipeline steel, Grain refinement, Hydrogen embrittlement, rotary friction welding, welded joint