Study on weakest link of welding joint of a 980 MPa high strength steel

In this paper,the properties and fracture mechanisms of MAG and TIG welding joint of a new 980MPa high strength steel were studied by impact test,tensile test and scanning electron microscopy(SEM) observations.The weak link of welding joints was determined,and effects of weak link on the whole impact toughness of joints were obtained.By a series of experiments,the following results were obtained:the tensile strength had few discrepancies between the two weld metals and base metal,but the percentage of area reduction was lower than base metal.Weld metal(especially original weld metal) is the weakest links on the welding joints.The proportions of the original weld metal and weld metal had an effects on impact toughness,Charpy toughness of MAG welding joints was smaller than that of TIG welding joints.For MAG welding joints,quasi-cleavage fracture with many cleavage facets dominated the whole fracture surface.For TIG welding joints,ductile fracture with many dimples dominated the whole fracture surface.The phenomena was attributed to two reasons:on the one hand,the amount of nickel in welding stick of TIG welding joints were more than that of MAG welding joints,so its fracture toughness was better than that of MAG welding joints.On the other hand,there were many inclusions in MAG weld metals,and the size of these inclusions are larger than that of TIG welding weld metal.Cracks were initiated from these large inclusions easily,which induced the early brittle quasi-cleavage fracture.