Research On Microstructure And Properties For Welding Joints Of700Mpa Grade High Strength Low Alloy Steel

High strength low alloy (HSLA) steels have been widely used in engineering machinery,energy, vehicles and vessels industries etc., due to their excellent strength and toughness. Butthe welding of high strength steel is complicated. Cold cracking, mismatching of toughness andstrength, poor performances of heat affected zone (HAZ) may be encountered in joint duringwelding. In this Paper, the yield strength700MPa grade HSLA steels used in vehicles wasresearched in cold crack sensitivity, strength toughness matching, and the influence of differentheat input on microstructure and joint performance. The steel was welded by gas metal arcwelding (GMAW) processes. The tensile, bending, impact, microhardness test were carried forthe weld joint. The microstructure and mechanical properties of welded joints were investigatedto reveal the relationship of welding parameters, microstructure and performance of joints. Y-groove crack tests were carried out to study on its weldability.The results show that: because of the low carbon equivalent and the sensitivity exponent ofcold crack, crack sensitivity of experimental steel is very low, and weld preheating is notrequired. Both of GHS70welding wire and WH80-G welding wire have good crack extensionresistance. Furthermore, Results indicated that， with the increase of strength of welding wires,the creaking sensitivities of joints increased, and become more sensitive to the change of heatinput.GHS70wire and WH80-G wire were selected for "undermatching weld joint" and"equalmatching weld joint". Microstructure and mechanical properties were analyzed for thetwo joints. The microstructure on weld zone of undermatching weld joint are mainly acicularferrite and some grain boundary proeutectoid ferrite, the tensile strength reached775MPa,92.4%of base metal, fracture position in weld. The microstructure on weld zone ofequalmatching weld joint are mainly acicular ferrite and low bainite and proeutectoid ferrite, thetensile strength reached815MPa,92.4%of base metal, fracture position in HAZ. There is noobvious difference of impact toughness between two joints from20℃to-20℃."equalmatchingweld joint" is more appropriate for the apply requirement.The effects of welding heat input on the microstructure and mechanical properties of jointwere investigated used WH80-G wire. With the increasing of heat input, Bainite reduced andproeutectoid ferrite increased in the weld area, the tensile strength is declined, Akvof the weldarea increase firstly and then decrease in environmental temperature-20℃. The joint got thebest comprehensive strength and toughness properties at the14.66kJ·cm-1heat input. Tensile fracture morphology is mixed type indicating dimple and cleavage steps, crack started in the"V" groove roots, Martensite was observed in this area according to metallographic.