Microstructure and mechanical property of Q690C HSLA steel joint welded by multipass metal active gas (MAG) welding process

To increase productivity and improve joint property, 8 mm thick high-strength low-alloy structural steel of Q690C supplied in TMCP condition was welded with 3 pass metal active gas (MAG) welding at low heat input (10 kJ/cm) using equal-matched welding wire without preheating. The joint macrostructure, microstructure and mechanical property were investigated. The results show that no crack and defects were observed, and joint tensile, bending and impact properties are qualified. Each weld metal was acicular ferrite, which had excellent impact toughness. Although intercritical heat affected zone (ICHAZ) became soft due to temper of the base metal, the resultant harmful effect on tensile property was inhibited due to its smaller width (0.2-0.4 mm for one pass). The joint not only at the location further away from fusion line, but also exhibited a passed impact toughness at 0℃, even at coarse grained heat affected zone (CGHAZ) with lath beam microstructure of coarse upper bainite and M-A constituent. The former can be attributed to the lower heat input, which can reduce softening degree. The latter can be attributed to (i) the lower heat input which can reduce the effective grain size (length of lath bainite) of CGHAZ and the fusion zone (consisting of coarse upper bainite lath and M-A constituent) and (ii) temper of bainite by subsequent welding passes.