Suppression of hydrogen-induced damage in friction stir welded low carbon steel joints

•The low carbon steel joints show greatly refined microstructure in the stir zone.•Stir zone shows the highest resistance to hydrogen damage among the specific zones.•Hydrogen damage was suppressed by changes of grain size and low angle boundary.•Hydrogen charged joint shows much less ductility loss in stir zone than base metal.

Hydrogen-induced damage including blister and cracking in friction stir welded low carbon steel were evaluated by the cathodic hydrogen charging method. After hydrogen charging for 2 h, irreversible dome-shaped blisters and internal cracking began to appear on the surface of the base metal. However, after hydrogen charging for 16 h, cracking formed along the thermo-mechanically affected zone boundary, while the blisters or cracking were hardly observed in the stir zone. In addition, the stir zone showed a plasticity reduction from 38% to 28%, much less than that of the base metal reduced from 48% to 2%.