| Austenitic stainless steel was often replaced by ferritic stainless steel in some applications which do not require high strength and corrosion resistance because of its low cost and good strength and corrosion resistance.However,during the welding of ferritic stainless steel,problems such as embrittlement of welded joints,coarse grains in heat-affected zone and the increased sensitivity to intergranular corrosion often occur.The development and application of ferritic stainless steel are greatly restricted by these problems,therefore the 430 ferritic stainless steel with 3mm thickness was welded by two welding methods and different welding parameters was choose in this paper.The change rules of microstructure,mechanical properties and corrosion resistance were studied,and the suitable welding process was obtained,which provided theoretical guidance for the production practice of thin-walled 430 ferritic stainless steel applied in pipeline.The mierostructure and tensile properties of 430 ferritic stainless steel plasma arc welded joints were studied,the range analysis of the tensile strength and the heat affected zone grain statistical results was carried out,and a regression equation was established.The results showed that the weld was composed of larger cell crystals at the center and columnar crystals at the edges,all of which were relatively coarse ferrite structure and a certain amount of lath martensite structure pinned at the ferrite grain boundary.The heat-affected zone structure consisted of coarse-grained and fine-grained areas,and a clear boundary line appeared.When the welding ceurrent was 155A,the welding speed was 260 mm/min,and the ion gas flow rate was 3.2 L/min,the tensile strength of the welded joint reached the maximum value of439.5MPa,and the tensile elongation 24.36%was obtained.The order of influence of each factor against the tensile strength was welding current>welding speed>ion gas flow.The order of influence on the grain size of the heat-affected zone was welding current>ion gas flow rate>welding speed.The microstrUctUre,tensile properties,eorrosion resistance and hardness distribution of 430 stainless steel TIG welded joints were investigated.The results showed that the weld was composed of ferrite and lath martensite grown at its grain boundary.When the welding speed was 160 mm/min,the martensite eontent in the weld was the most.With the welding speed increasing,the grain size of the coarse-grained and fine-grained areas in the heat-affected zone was decreased,and the tensile strength of the four welded joints was increased firstly and then decreased.When the welding speed was 160 mm/min,the tensile strength of TIG welded joints with elongation 15.01%was 475.33 MPa,and the tensile performance was the best.The hardness distribution curve of welded joints was "saddle shape",and the Vickers hardness of 430 ferritic stainless steel base material was about 181 HV.The hardness of weld zone was greater than 430 ferritic stainless steel base material,and the hardness near fusion line was the highest,when the welding speed was 160 mm/min,the hardness of weld was the highest,which was 210 HV.When the welding speed was 160mm/min,the self-corrosion potential of the weld was the largest and the corrosion area was smallest.Compared with plasma arc welding,the TIG welded joint had less welding deformation and higher tensile strength,and it is more suitable for welding thin-walled ferritie stainless steel.The microstructure,tensile properties,corrosion resistance and hardness distribution of 430 stainless steel welded joints after heat treatment at different temperatures were studied.The results showed that when the heat treatment temperature was 850℃ the morphology of the base metal changed obviously,and the lath martensite at the grain boundary of the weld was increased.When the temperature rose to 950℃,the grain grew obviously,and a large number of black precipitates(Fe,Cr)23C6 appeared.When the heat treatment temperature was 650℃-850℃,the tensile strength of the welded joint was similar to the tensile strength at room temperature.When the temperature was 950℃,the tensile strength was decreased significantly to 279.04 MPa and the elongation was only 5.67%.When the heat treatment temperature was 750℃ and 850℃,the number of dimples was the largest,and the diameter of dimples was smaller at 850℃.When the heat treatment temperature increases to 950℃,the number of dimples was decreased obviously,and the fracture surface was characterized by river-like brittle fracture.After heat treatment,the hardness of base metal and weld were both firstly increased and then decreased.The hardness was the highest when heat treatment temperature was 850℃,and the hardness of base metal was 247 HV and that of weld was 285 HV.When the heat treatment temperature is 650℃ and 750 C,the self-corrosion potential of the weld zone of TIG welded joint was larger,and the corrosion area surface in the corrosion morphology was larger.For the TIG welded joint of 430 ferritic stainless steel,the temperature of heat treatment should be 750 ℃,so that the welded joint with good mechanical properties and corrosion resistance can be obtained.There was a positive correlation between the heat input and the grain size of HAZ in 430 ferritic stainless steel welded joint.The dynamic formula of grain growth in HAZ of 430 ferritic stainless steel welded joints was D1.96=D01.96+t900∑t’900Aexp{-2Qapp/R(Ti+Ti+1)}△ti.Therefore,the welding process of 430 ferritic stainless steel needed to strictly control the heat input.On the premise of satisfying the welding forming,the welding process parameters with small heat input should be adopted as far as possible. |
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