Microstructure and mechanical properties of 921A steel welded joint by laser-MAG hybrid welding

For 921A steel plates with thickness of 6 mm, butt welding test was carried out by laser-MAG hybrid welding process, and microstructure, hardness, tensile properties and corrosion resistance of welded joints were analyzed. The results showed that continuous and beautiful welded joints without defects such as incomplete fusion, crack and porosity, could be obtained by laser-MAG hybrid welding process. Microstructure of weld was acicular ferrite, a small amount of pre-eutectoid ferrite precipitated along grain boundaries and long strip bainite. Microstructure of heat affected zone was martensite. Tensile and impact properties of welded joints were in line with national standards. Strength of the weld was higher than that of base metal, while plastic and toughness of weld were lower than those of base metal. The peak hardness 315 HV was in heat affected zone, and hardness of weld was about 280 HV, which met requirements of the maximum hardness value that should not exceed 410 HV. Electrochemical corrosion resistance of weld was the strongest, followed by base metal, and heat affected zone was the lowest. Microstructure distribution in the interaction zone of laser and MAG arc was more uniform, and hardness and corrosion resistance were obviously improved in comparison with zone of laser action alone.Highlights: Butt welding of 6 mm thick 921A high strength steel plate was achieved without defects by laser-MAG hybrid welding. Grains of weld were finer and more evenly distributed. Tensile strength, hardness and electrochemical corrosion properties of weld were higher than those of base metal, and impact absorption energy met requirements of Classification Society.