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%.