Study On Microstructure And Mechanical Properties Of 441 Stainless Steel Welded Joints

441 FSS(Ferritic stainless steel)has excellent corrosion resistance and mechanical properties,and is widely used in automotive,construction,chemical and other fields due to its low cost compared with austenitic stainless steel.As a single-phase ferritic stainless steel,the toughness of the 441 FSS welded joint is reduced,and the welded joint is easy to crack for the welded structure which needs large deformation plastic processing such as automobile exhaust pipe.Improving the microstructure and properties of welded joints and reducing residual stress are effective measures to prevent cracking.In this paper,different batches of 441 FSS sheets were butt welded by TIG(Tungsten argon arc welding)and LBW(Laser welding)methods.The influence of welding process parameters on the forming quality of 441 FSS sheets welded joints was studied,the welding process parameters were optimized,and the influence of chemical composition on microstructure and properties was analyzed.The effects of UIT(Ultrasonic impact treatment)and low temperature HT(Heat treatment)on the microstructure and residual stress of welded joints were discussed.The plastic forming property of welding test plate was studied by cupping test.The cracking reason and fracture mechanism of welded joints were analyzed.The results show that:(1)When the welding current was 50 A and the welding speed was 25 cm/min,the TIG weld width and the HAZ range was small,the weld was penetrated,and there were no defects such as weld depression and bite edge.The coarse columnar grain and HAZ grain growth of TIG welded joint of 441 FSS led to the decrease of local toughness,which made the welded joint become the weak link of welded structure.The fracture mode of 441 FSS sheet welded joint under working load was mainly longitudinal fracture along the weld direction and transverse fracture perpendicular to the weld direction.The fracture mode was related to the loading mode,the microstructure and properties of the joint and the residual force distribution.The fracture mode of TIG welded pipe during processing was mainly related to the loading mode,and longitudinal cracks were mainly generated when expanding the pipe,and transverse cracks were mainly generated when bending the pipe.The crack source at the crack location was related to the size,distribution and number of inclusions in the material.The longitudinal cracks of TIG welded pipe joints mainly originated in the vicinity of the fusion zone,and the transverse cracks originated in the columnar crystal zone of the weld.Inclusions,second phase particles or other brittle phases were used as crack sources,and most of them were located near the surface.The longitudinal crack initiation zone was the cleavage fracture of transgranular fracture,and sometimes a small amount of dimple aggregation fracture occurred in the expansion zone.The initiation zone of transverse crack was cleavage fracture of transgranular fracture,and more dimples gathered in the propagation zone than in the longitudinal crack to form mixed fracture.The strain distribution of the cup hemisphere tended to be uniform under the loading of the cup test.The fracture mode was mainly related to the microstructure and properties of the welded joint and the residual stress state.Due to the composition segregation,weld and HAZ embrittlement of TIG welded joint of thin plate,the main fracture mode of cup crack was longitudinal cracking,which was located in the center of the cup semisphere.(2)The nano-fine grain layer with thickness of 5 ? 25 ?m was produced on the surface of TIG welded joint by UIT,and the microhardness of the surface layer at 100?m increased by 19.5% on average.The forming performance of the joint increased with the increase of UIT width.The low temperature HT can improve the elongation of TIG welded joint to 108.05% and the forming performance to 102.13%.(3)When the laser power was 1.3 KW,the welding speed was 25 mm/s and the defocusing amount was 0 mm,the width of LBW weld was small and the microstructure was uniform.There were no defects such as weld depression,hole and bite edge,and the welding deformation was small.The chemical composition of different batches of441 FSS has great influence on the mechanical properties of LBW welded joints.In the standard range of 441 FSS,the content of Si,Ti and Nb was higher,the content of Mn was suitable,the content of S and P was less and the grain was smaller,the mechanical properties of the welded joint of thin plate were better.Compared with TIG welded joints,441 FSS laser welded joints had fine microstructure,fine grains and less precipitates.In addition to longitudinal cracks,transverse cracks were also included,which were located in the center of the cup semisphere.(4)The UIT can increase the tensile strength of the base metal to 101.07%,and reduce the elongation to 97.28%.The average microhardness of the surface layer at 100?m of the LBW welded joint increased by 18.1%,and the tensile strength increased to102.54% without reducing its elongation.The UIT reduced the maximum longitudinal tensile stress of the welded specimen by 78.4%,and the transverse tensile stress was transformed into compressive stress.The cupping value was increased to 120%,and the fracture was mainly transverse fracture.The UIT changed the fracture position of the transverse crack,and the weld in the middle of the cup hemisphere was transformed into the weld zone at the edge of the sphere.The crack initiation source of the transverse crack was transformed from the surface layer to the inside of the plate.The crack initiation source area was the cleavage fracture of transgranular fracture,and many dimples gathered in the expansion area to form a mixed fracture.Low temperature HT can significantly improve the elongation of LBW welded joints.The maximum longitudinal tensile stress of the welded specimen was reduced by 83.5%,the transverse tensile stress was reduced by 80.2%,and the cup value was increased to 131%.After low temperature HT,the crack position of the laser welded specimen was changed from the weld zone to the base metal zone,and the fracture was dominated by dimples.