| Austenitic stainless steel is widely used in petrochemical,liquefied natural gas,nuclear power and other fields because of its good corrosion resistance,plastic toughness and weldability.Due to its low thermal conductivity and high linear expansion coefficient,welding is easy to produce residual stress,thermal crack,brittle phase precipitation and other defects,so it is necessary to study the high-quality and high-efficient welding procedure of austenitic stainless steel.The low yield strength of austenitic stainless steel can lead to the increase of container materials and increase the manufacturing cost.The strain strengthening technology can significantly improve the yield strength of austenitic stainless steel and save engineering materials.Currently,the research on strain strengthening technology mainly focuses on the base metal,and there are few reports on strain strengthening of joints,and there is a lack of analysis on strengthening effect of different areas of strain strengthening joints and welding heat input on strain strengtheningjoints.In this paper,the butt welding test of 304 stainless steel sheet(7mm)was carried out by combined welding of PAW and TIG with different welding heat input,and strain strengthening at room temperature with a strain variable of 9%was applied on joints along the direction perpendicular to weld seam.The microstructure of thin plate combined welding joint was studied in detail,and effect of welding heat input on the macroscopic morphology,microstructure,microhardness and strength of combined welding joint was analyzed.Besides,effect of strain strengthening on joints were studied,and influence of welding heat input on the strength of strain strengthened joint was discussed.The results show that the microstructure of weld metal were ferrite and austenite.In middle district of TIG layer,lathy ferrite was changed to skeleton-shaped ferrite from in the middle was transferred from the fusion zone(FZ)to center area.The junction of the welding bead was composed of upper columnar crystal zone and lower cellular crystal zone,with more austenite in upper layer and more ferrite in lower layer.In the side of this district,the grain size were the smallest and the content of ferrite was highest.From FZ to inside of weld metal in PAW layer,cellular crystal was changed to columnar crystal,and dotted and short-rod shaped ferrite was changed to skeleton-shaped.During the process of solidification,the grain at the bottom of weld metal was preferentially grown and then grew alternately with the grain at both sides.The microstructure of the FZ was ferrite and austenite,and the width of the fusion zone in PAW was larger than that in TIG.In TIG layer,the width of FZ was about 50 ?m,and strip-shaped ferrite was densely distributed between the meshy austenite and extended to heat affected zone(HAZ).The grain size of FZ was smaller than that of weld metal,and the ferrite content of lower part is higher than that of upper part.At junction of welding bead,the grain size of from TIG layer to PAW layer,the grain size was first decreased and then increased,and reached the maximum at PAW layer.In PAW layer,ferrite of ferrite-denced district nearby weld metal was in the shape of dot and short-rod,and ferrite of ferrite-extended district nearby HAZ was parallel in the shape of line and extended to HAZ.Upward from bottom of weld metal,width of FZ was decreased,and the ferrite density was increased.The austenite grain of HAZ was slightly coarser than that of base metal,and there were short-linear ferrite at the grain boundary.The amount of ferrite in HAZ of PAW layer was relatively large,but the grain size was smaller.With the increase of heat input of PAW,the shape of ferrite was changed from lath to skeleton in the junction of welding bead,and the grain size of lower layer was increased.The amount of ferrite at the side of junction of welding bead was decreased,and the massive ferrite accumulation area and meshy austenite gradually appeared in district near TIG layer,which was the possible fracture source of the joint.With the increase of welding heat input,the austenite grain size of PAW weld metal was increased,amount of ferrite was decreased and ferrite more ferrite existed in the shape of skeleton.In FZ,width of ferrite-denced district and ferrite-extended district were decreased,and change of ferrite-extended district was more obvious.The variation trend of pool width,depth of penetration and reinforcement in each district of combined welding metal with the change of welding current.arc voltage and other technological parameters were in line with the general rule.With the increase of welding heat input,the sectional area of PAW weld metal increased from 18.5mm2 to 25.0mm2.The room temperature tensile strength of combined welding joint was higher than that of the base metal,and the maximum tensile strength was 750MPa.With the increase of PAW heat input(14.6?17.6kJ/cm),the tensile strength of the combined welding joint was first increased and then decreased,and maximum tensile strength was reached with E=15.5kJ/cm.When the welding heat input value was 14.6kJ/cm and 17.6kJ/cm,the fracture was occurred at the weld metal,and the weld forming coefficient was lower than 0.60.In between,fracture was occurred at HAZ,and the weld forming coefficient was relatively high.The welding heat input of PAW ranged from 15.5 to 17.0kJ/cm,and the strength of weld metal was higher and the welding quality was better.The tensile test results match the microstructure and microhardness of joint.The microhardness of FZ was higher than that of weld metal and HAZ in TIG layer,and the microhardness distribution of PAW layer is more uniform.With the increase of welding heat input,the microhardness of PAW layer was first increased and then decreased,and the width of HAZ was extended.The microhardness reached maximum when the welding heat input was 16.9kJ/cm.When the welding heat input was 14.6kJ/cm and 17.6kJ/cm,the microhardness of the weld metal was lower than that of the HAZ,and when welding heat input was between them,the microhardness of HAZ was lower than that of the weld metal.After the pretension test,the PAW+TIG combined welding joint for 304 stainless steel sheet was subjected to nonuniform deformation.The deformation rate of weld metal was 3.9%,and deformation rate of HAZ was above 9%.There was no new phase formation,grain was coarsed and orientation was changed,and ferrite was refined.Width of FZ was decreased.In HAZ,the deformation-induced martensite was formed in parallel and cross arrangement,and the brittle ?-phase precipitate out along the grain boundary of the coarse austenite.With the increase of welding heat input,the deformation rate of each area was increased.The austenite grain coarsening degree of the strain strengthened joint was more obvious,the grain rotation Angle was decreased,ferrite morphology was finer,and formation of deformation-induced martensite was promoted.In the case of large heat input,the strain strengthening had little effect on the microhardness of the weld metal,but it had obvious effect on HAZ. |
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