The Precipitation Behavior Of The σ Phase And Its Effect On Performance In 2707 Duplex Stainless Steel

As a new class of duplex stainless steel,2707 hyper duplex stainless steel（HDSS）,having higher Cr,Mo,Ni and N than other duplex stainless steels present excellent corrosion resistance,due to the ferrite and austenite content close to 1:1.They are being utilized in more severe corrosion environment,with the pitting resistance equivalent number close to 50 and the service temperature close to 90℃.In addition,2707 HDSS has excellent stress corrosion cracking resistance in chloride ion environment,high mechanical strength and good weldability.Friction stir welding（FSW）,as a new solid-phase joining technology,has been applied to low alloy duplex stainless steel due to its advantages of high cooling speed and low peak temperature.It can effectively avoid the defects of the dual-phase change and precipitation of harmful phase in the heat affected zone which would be produced in other welding processes and greatly improve the performance of the joint.However,due to the higher alloy elements,especially Mo,2707 HDSS has a faster rate of producing harmful phases（σ、χ）when it stays in the sensitive zone of precipitated phase during welding and hot working than other duplex stainless steels.In addition,theσphase is affected by both precipitation temperature and deformation.Moreover,the precipitation behavior ofσphase in 2707 HDSS and its comparison with other duplex stainless steels are still lacking.The effect of friction stir welding on theσphase precipitation and corrosion resistance of 2707 HDSS needs to be clarified.The influence of deformation induced by friction stir welding onσphase precipitation behavior is not clear.In order to solve these problems,firstly,the influence of different aging treatments on the precipitation of 2707 HDSS was studied,the TTT curve ofσphase was drawn,and the relationship between precipitation phase and corrosion resistance was clarified.Secondly,the precipitation behavior ofσphase in friction stir welding process is studied,and the correlation betweenσphase precipitation and corrosion resistance has been judged effectively.Finally,the mechanism of microstructure evolution of FSW post-weld joint and theσphase precipitation during the aging process of the obtained deformed microstructure are also evaluated clearly.The conclusions are as follows:（1）The precipitated phase of 2707 HDSS is mainlyσphase after aging at different temperature and time.The nose temperature ofσphase is 950℃.Compared with other DSS,the nose temperature of precipitated phase in 2707HDSS is higher and the precipitation rate is faster.Aging at 950℃for different time,σphase begins to form at the boundaries of ferrite and austenite.And then as the aging time is prolonged theσphase accelerated into a linear form and thenσphase occupied the ferrite in the fishbone structure inside the grain finally.Theσphase is enriched with Cr and Mo,which destroys the balance of two-phase and results in the uneven distribution of elements in the surface.（2）The precipitation ofσphase lead to the decrease of pitting potential of2707 HDSS after aging at 950℃for different time.In addition,the nucleation ofσphase leads to the formation of unstable passive film,which reduces the corrosion resistance of the material.When the precipitate phase is less,the range of pitting corrosion is smaller and the pitting corrosion only occurs at the boundary of ferrite and austenite phase.When theσphase precipitate more,the range of pitting corrosion is larger,which occurs at the secondary austenite aroundσphase and someσphases turns into pitting.At aging for 10 min the selective corrosion occurred clearly and the whole ferrite phase dropped off into pitting.（3）FSW has applied to 2707 HDSS,the grains of the welded joint in the SZ was dynamically recrystallized and refined at different rotational speeds and the grain size increased by increasing the rotational speed.However,σphase exists in the whole SZ of welded joint at 200 rpm and at 300 rpm only a fewσphases appeared at the bottom of the SZ.There was no precipitated phase in the welded joints at 400 rpm and 500 rpm.Theσphase and grain refinement have a great influence on corrosion resistance.The joints of 400 rpm shows the best corrosion resistance with finest grain compared with the BM and 500 rpm,due to more Cr₂O₃ and Fe₂O₃ formed on the surface.While the bottom of SZ at 200rpm and 300 rpm,there is less corrosion resistance with finer grains than BM due to the large number ofσphases.400 rpm is the best welding process.（4）At the rotational speed of 400 rpm,the deformation degree in different regions of the joint is different.Ferrite and austenite in the SZ undergoes different types of dynamic recrystallization,while some of them is deformed and some of them undergo dynamic recrystallization in the heat affected zone.In addition,compared with the base metal,the precipitation range ofσphase is wider and the precipitation time is shorter after aging at different temperatures,but the nose temperature is not changed.The amount of precipitated phase is different in different areas of the welded joint.The advancing side of stir zone have the largest precipitated phase due to the largest deformation.