Study On Fatigue Properties Of SW400 High Strength Steel Partial Lap T-joint

SW400 high strength steel local lap T-joint is formed by lap welding of L-shaped plate and rectangular plate,which is essentially a lap joint of T-joint.At present,this joint is widely used in the synthetic welds of the steel rim of the steel assembled wheel.It bears complex radial load,irregular bending moment and torque in service,and is prone to fatigue damage,which seriously threatens the life safety of the driver.Therefore,it is of great significance to study the fatigue performance of this joint.In this paper,the SW400 high strength steel partial lap T-joint is taken as the research object,and the effects of submerged arc welding process and welding gun angle on the geometric characteristics and fatigue performance of the weld under the radial load perpendicular to the joint plate are tested and analyzed.On this basis,the influence factors of welding process and weld shape and size on the fatigue crack propagation are studied and analyzed,and the fatigue crack propagation law is revealed;The mechanism of fatigue crack initiation and propagation is analyzed based on the micro fracture morphology,which provides a theoretical basis for improving the fatigue life of local lap T-joints and improving the welding process.The finite element analysis technique is used to simulate the stress distribution of SW400 high strength steel partial lap T-joint under external load,which provides an effective method for predicting fatigue fracture for engineering application.The results show that:The welding process parameters have a great influence on the geometric characteristics of SW400 high strength steel local overlap T-joint.With the increase of welding line energy,the weld thickness of the joint increases from about 3000μm gradually increases to about 3500μm and eventually increased to more than 4000μm.The shape of weld was changed from concave weld to flat weld and convex weld,and the fatigue performance was increased by 21.2% and 19% respectively;On this basis,increasing the included angle between the welding gun and the plate will make the weld thickness continue to increase,but the fatigue performance will be less improved.When the joint weld thickness is less than 4000μm,the fracture form of the joint is weld fracture.The crack of Weld Fracture originates from the weld root and finally extends to the weld surface.There are many secondary cracks in the expansion area and a large number of micro voids in the transient fracture area;When the weld thickness of the joint is greater than 4000μm,the fracture mode of the joint is mainly the fracture of the base metal.The base metal crack is first generated in the middle of the weld,and the initiation time at the end of the weld is late.There are multiple crack sources.The fatigue crack extends and meets from the crack source and leaves a step at the intersection.The transient fracture area is 45° shear oblique fracture and the area of the end transient fracture area is large.Stress relieving annealing heat treatment and shortening the bending edge length of L-shaped plate can effectively improve the fatigue performance of SW400 high strength steel local lap T-joint.Using laser backing and submerged arc filling welding to prepare the joint can improve the root formation and avoid welding defects.The results of finite element analysis show that the extreme stress region is mainly distributed near the weld root,and its distribution law is consistent with the fatigue crack initiation position.Therefore,the fatigue crack initiation position and propagation direction of local lap T-joint of SW400 high strength steel can be predicted by the finite element analysis method.The residual stress and tensile load are superposed by superposition method.The results show that the existence of welding residual stress will worsen the stress distribution of the joint and make the joint end have a higher tendency to produce weld cracks.