Microstructure and Properties of Undermatching Welded Joint of 700 MPa Grade High Strength Low Alloy Steel

700 MPa grade high strength low alloy steel was welded by gas metal arc welding( GMAW) process on the principle of a undermatching design. The microstructure and properties of the welded joint were investigated by OM,SEM and mechanical property tests. The results show that the microstructure of the weld zone consists mainly of acicular ferrite and a small amount of proeutectoid ferrite,the microstructure of coarse grain heat affected zone is upper bainite,and that the microhardness of the weld zone is significantly lower than that of the base metal,the impact toughness( AKV) of the weld zone is 58 J at-40 ℃,and the tensile strength of welded joint is as high as 92. 4% of that of the base metal,the welded joint providing good comprehensive mechanical properties.