Investigation On Torsional Deformation Behavior And Microstructure Evolution Of Duplex Stainless Steel

Duplex stainless steel is composed by ferrite phase and austenite phase.It has the excellent performance of ferritic stainless steel,with great resistance to chloride stress corrosion resistance and high strength.It also has characteristics of austenitic stainless steel,with excellent weldability and plasticity.Duplex stainless steels have been widely used in paper making industry,petrochemical industry,marine engineering and other harsh environments.With the continuous improvement of national economy,the demand for duplex stainless steel seamless steel tube is growing.The hot torsion performance of material has great impact to production progress,but the research on torsion performance is still limited and not comprehensive enough.Bying heat treatment and hot torsion,studying the phase transition law,mechanical properties and microstructure evolution rule of 2205.so as to provide a reference to skew rolling process.Because of the material with high pressure torsion could be applied to a large amount of deformation,it could be used to produce ultra-fine material.By experiment of high pressure to research duplex stainless steel property and grain refinement mechanism.The main results obtained in this paper are as follows:(1)The solution heat treatment of the experimental steel shows:in the temperature range of 900?1200? for 2?15min,with the increase of temperature and holding time,the austenite phase changes to ferrite phase and the grain size increases.Through the study of the holding time,it is found that the microstructure of the experimental steel is close to the equilibrium structure after holding for 15min,and the extension of holding time has a small influence on the proportion between ferrite and austenite.(2)The softening behavior of the steel during hot torsion deformation can be significantly affected by deformation temperature,holding time and deformation rate.The increase of the temperature,holding time and deformation rate can improve the softening ability of the material and improve the fracture strain of the test steel under torsional shear stress.The experimental results show that when the temperature is above 1050? and the holding time with 15min,the softening process is adequate and the deformation resistance is lower.But in the range of 1050?1100?,the deformation resistance change is relatively large,in order to ensure a stable rolling process,the temperature of the steel in cross-rolling should be carried out at a temperature higher than 1100?,and holding at least 15min.(3)The microstructures after hot torsion have been examined and the results show that:increasing the deformation temperature and strain rate can improve the homogeneity of the material.The austenite equiaxed phenomenon is more obvious after deformation,and the grain size is reduced.At 1100?,during the hot torsion process,both ferrite and austenite undergo dynamic recrystallization,and twinning occurs in the austenite.(4)The results of high pressure torsion experiments show that:the hardness of the material increase with the increase of the number of turns.With the same number of torsion,the hardness increases with torsional radius increasing.The hardening process of the test steel can be divided into three stages:initial hardening stage,linear hardening stage,and no hardening stage.(5)During the high pressure torsion process of duplex stainless steel,the relationship between the long axis direction of grain and the torsion direction before deformation could influence the cross section microstructure evolution.When they are parallel,the direction of elongation is further elongated and the grain boundary is straight.When the they are perpendicular to each other,the two phase grain boundary become severe bended.(6)For duplex stainless steel,the refinements between the two phases during high pressure torsion are different.In ferrite,the dislocation cells have been found and gradually transforms into with large angle boundaries grains.The refinement mechanism of austenite contains deformation twinning and dislocation reactions.(7)Thermal stability of the steel with severe plastic deformation was studied,the result shows that:the steels possess good stability.After holding 45min at 950?,the hardness of the material tends to a stable value.In addition,after heat treatment,a large amount of ? phase precipitates in the material,with the increase of the deformation,the precipitation increases.The ? phase is dispersed in the austenite,which stabilizes the material's performance to a certain degree.