Effect Of Grain Boundary Engineering On Microstructure And Properties Of S31254-40B Super Austenitic Stainless Steel

S31254 super austenitic stainless steel has good corrosion resistance due to its large amount of alloy elements such as chromium,nickel,molybdenum and so on.It is widely used in extreme environments such as flue gas desulfurization,seawater desalination and papermaking industry.However,there are a lot of precipitates in the hot rolled material.The addition of trace boron can inhibit the segregation of alloying elements and improve the precipitation of materials.After solid solution treatment,the precipitated phase of boron microalloyed S31254 super austenitic stainless steel will be dissolved back,but the grain size of the material will increase greatly.It is not conducive to the corrosion resistance and mechanical properties of the material.Therefore,how to optimize the structure of materials without precipitating the second phase has become the key to improve the properties of materials.Grain boundary engineering can improve the grain boundary characteristic distribution,corrosion resistance and mechanical properties of face-centered cubic crystals.The theory was increasingly mature after decades of development since 1980 s.This paper attempted to improve the microstructure and properties of boron microalloyed S31254 super austenitic stainless steel after solution treatment by grain boundary engineering treatment.Firstly,the effects of grain boundary engineering on the microstructure of the materials were studied by different cold rolling and annealing methods.The corrosion resistance and mechanical properties of the materials treated by grain boundary engineering were evaluated by electrochemical method,boiling ferric sulfate-sulfuric acid test and tensile test.Through the analysis of a series of experimental results,the following conclusions can be drawn:(1)In the cold rolling process of grain boundary engineering treatment,when the cold rolling deformation is small,the grain size of the sample after cold deformation is no much different from that of the solid solution state,and the grain deformation is relatively uniform.When the cold deformation is large,the high density dislocation appears in the sample,which is reflected in the microstructure of the wrinkled grain morphology.(2)After cold rolling with small deformation,the average grain size of the sample is improved after annealing for 5/10/20 min at 1160?,1170? and 1180?.A large number of annealing twins were formed during the annealing process;The deformation twins caused by deformation cannot be eliminated by low temperature annealing after large deformation cold rolling.The second phase precipitates when the annealing time is prolonged.After annealing at 1120? to 1180? for 10 min,the boron microalloyed S31254 super austenitic stainless steel occurred recrystallization annealing.The grain size was optimized,and the number and density of twins were increased.(3)The number of twin boundaries increased after the grain boundary engineering treatment of large/small deformation cold rolling + high temperature and short time annealing.As a result,the channels of the passivating film forming elements to the surface of the test steel are increased,the passivating film is easier to form and denser.At the same time,the twin boundaries are low energy grain boundaries.Therefore,the pitting corrosion resistance and intergranular corrosion resistance of experimental steels are improved after grain boundary engineering.(4)The grain was refined after grain boundary engineering treatment by cold rolling with small deformation and high temperature short time annealing.The increase of twinning boundaries in the sample inhibits dislocation slip,so the strength of the material increases.Moreover,after grain refinement,plastic deformation under external force can be dispersed in more grains,and the stress concentration is small.The plastic deformation of the material is uniform and enhanced.