Hardening process of welded joints of 400 MPa ultra fine grained steel

The joints of manual arc welding of 400 MPa ultra fine grained steel were treated by means of surface mechanic hardening and sub-zero treatment, the microstructure and mechanical properties of untreated welded joints were compared with those of the treated welded joints. The results show that the grain size increasing and strength decreasing obviously do exist in the HAZ of manual arc welded joints of 400 MPa ultra fine grained steel. The microstructure in surface layer is refined by surface mechanic hardening, and the mechanical properties of welded joints can be improved greatly by surface mechanic hardening. The strength and toughness of welded joints increase by the sub-zero treatment, but the results are not satisfactory.