Precipitation Behavior Of ? Phase And Microstructure Evolution During Superplastic Deformation Of Duplex Stainless Steel

Duplex stainless steels take the advantages of austenitic and ferritic stainless steel,thus they are widely appliedin industry,construction,marine engineering and other fields.At high temperature,the duplex stainless steel often precipitates the ?phase,which makes the toughness and plasticity of duplex stainless steel decrease sharply.Therefore,it is important to study the precipitation behavior of ?phase in the hot working process of duplex stainless steel for preparing high performance duplex stainless steel and improving its service performance.In this paper,the duplex stainless steels with different composition were used as experimental material.The hot-rolled the duplex stainless steel was first treated by solid solution treatment.The solution temperature was 1000-1350? and the holding time was 30?.After heat preservation,the samples were put into water for quenching;then cold rolling with reduction rates of 0-90%were carried out.The duplex phase stainless steel plates prepared by different processes were aging treated,the heating temperatures were 600-1000?,and the heat time were 1.8-86.4 ks.The microstructure and precipitation behavior of ? phase in duplex stainless steel during solution,cold rolling and aging were observed.Then,the microstructure evolution of duplex stainless steel during superplastic stretching was studied.The main results are:The stability of Cr/Ni ratio in ferrite and Ni/Cr ratio in austenite of duplex stainless steel was investigated.The results show that the Cr/Ni ratio of 2205 and 1Cr21Ni5Ti duplex stainless steel decreased with the increase of ferrite content,while the Ni/Cr ratio decreased with the increase of austenite content.The variation of Cr/Ni ratio in ferrite with ferrite content and Ni/Cr ratio with austenite content shows good correspondence.When the ratios of ferrite and austenite are 50%,the Cr/Ni ratio in ferrite of 2205 and 1Cr21Ni5Ti duplex stainless steel are equal,ranging from 5.95 to 6.31,and the Ni/r ratio in austenite are also equal,ranging from 0.30 to 0.32.The effects of aging temperature and holding time on the microstructure evolution and the precipitation behavior of ? phase in duplex stainless steels were investigated.The precipitation behavior of ? phase in 2205 and 1Cr21Ni5Ti duplex stainless steels during aging conforms to the "C" curve.The nose temperature of a phase precipitation in 2205 is 850?,and that in 1Cr21Ni5Ti is 750?.The nose tip temperature of a phase precipitation in 3207 and 2906 duplex stainless steels is 950?.Cold rolling can accelerate the precipitation of ?phase and promote the growth of ? phase in duplex stainless steel,but it does not change the nose temperature of ? phase precipitation.With the increase of cold rolling reduction,the precipitation time of ? phase is shortened.Compared with 2205 duplex stainless steel,the increase of Cr content widens the precipitation temperature range of ? phase,and the precipitation rate and maximum precipitation amount increase.When aged at 850? for 20 min,the ferrite will completely transform into ? phase.The changes of microstructure and texture in duplex stainless steel at the early stage of superplastic deformation were clarified.In the early stage of superplastic deformation,the geometrically necessary dislocation density(GNDs)in 2205 duplex stainless steel is mainly concentrated in ferrite.With the increase of superplastic strain,the density of geometrically necessary dislocations in ferrite decreases greatly.When the strain reaches 57%,the geometrically necessary dislocation density in 2205 duplex stainless steel increases and not concentrated in ferrite.The mainly orientation of ferrite is in the ?-fiber texture,while the austenite orientation is mainly in the rotating Gauss texture.With the increase of superplastic deformation strain,the texture strength in both ferrite and austenite weakens.When the strain is 14%,both ferrite and austenite show random orientation distribution.When the strain further increases,the strength of the y-fiber texture of ferrite and the rotational Gauss texture of austenite increases,but it is lower than that in the early stage of superplastic deformation.The superplastic deformation of duplex stainless steel was realized.The superplastic deformation process and the microstructure evolution during superplastic deformation were clarified.3207 duplex stainless steel shows excellent superplastic ability at 950? and strain rate of 1.5 × 10-3 s-1.The deformation process can be divided into four stages according to the true stress-strain curve:elastic deformation stage,steady deformation stage,quasi steady deformation stage and local necking stage.At the beginning of the quasi steady deformation stage,the deformation controlled by dislocation creep is the main mechanism of superplastic deformation,and a large number of small angle grain boundaries are generated.In the middle stage of quasi steady state deformation,the main deformation mechanism changes to grain boundary sliding mechanism,resulting in large angle grain boundary and overall weakening texture.In the local necking stage,the main deformation mechanism is grain boundary sliding,and the conventional plastic deformation also plays an auxiliary role.The local transfer process of necking during superplastic deformation of duplex stainless steel was studied.The microstructure and properties of different sections of duplex stainless steel are different after superplastic deformation.For 3207 duplex stainless steel,the small angle grain boundary density,dislocation density and hardness in the necking region of superplastic deformation are higher than those in the nearby region.In the process of superplastic deformation,the local hardening of necking zone causes the deformation to expand gradually from necking zone to surrounding area,which causes the local deformation to transfer back and forth in adjacent areas,and leads to the difference of microstructure in different parts of superplastic deformation.The precipitation mechanism of duplex stainless steel during superplastic deformation was investigated.A great number of the ? phase precipitated at the interface between ferrite and austenite in 3207 duplex stainless steel has the 20-25°misorientation.The ? phase precipitated in ferrite and austenite has the K-S or N-W orientation between 40-45?.The orientation of ferrite,austenite and precipitated ? phase at the interface between ferrite and austenite is(200)??(02-2)??(01-1)? and(020)??(311)??(200)?.In austenite,the orientation relationship between ? phase and matrix is(21-1)??(200)? and(01-1)??(11-1)?;in ferrite,? phase is elongated along<21-1>? direction,and(01-1)? and(101)? have 7.5° lattice dislocation.