Preparation Of Nanograined 304 Stainless Steel And The Grain Size Effect

304 stainless steel(304ss)is the widest used austenitic stainless steel due to its good mechanical properties and corrosion resistance.Grain refinement is one of the main methods to strengthen 304ss.However,it is a challenge to fabricate nanograined 304ss with extremely fine grain size(<10 nm)by plastic deformation.On one hand,plastic deformation fabricating nanograins usually require harsh processing conditions such as high strain rate,strong shear deformation,and even liquid nitrogen temperature;On the other hand,plastic deformation usually introduce a large number of defects and grain boundaries,causing the poor structural stability of nanocrystals.In this work,rolling,warm rolling,warm surface mechanical grinding(W-SMGT),surface mechanical rolling(SMRT),and high pressure torsion(HPT)were employed to explore the processes of uniform bulk nanograined 304ss and extremely fine grained 304ss;The grain size effects of thermal stability and martensitic transformation in nanograined 304ss were studied.Through experiments such as cross rolling,multidirectional rolling,repeated annealing and rolling,and warm rolling,we found that:I.Cross rolling can promote multiple twinning at the initial stage of rolling(about 10%of thickness reduction of rolling),accelerating martensitic nucleations through twin intersections in the middle stage of deformation(about 30%～50%of thickness reduction of rolling).While the grain refinements of cross rolling is similar to that of unidirectional rolling during large deformation stage of rolling(about 83%of thickness reduction of rolling),and obtained microstructures are α’ laths with an average thickness of 29 nm.II.Introducing a rolling direction in the normal direction(ND)or the transverse direction(TD)of samples is favorable for formation of equiaxed grains.III.A fully austenitic nanograined 304ss with an average lath thickness of 82 nm can be obtained when warm rolled at 250℃with a thickness reduction of 90%.Austenitic gradient nanograined 304ss samples were prepared by using W-SMGT.Grain size effects of thermal stability in nanograined 304ss were studied.In the surface layer of sample,thermal stability of nanograins with average grain sizes less than 60 nm increases as the grain size decreases,which is contrary to the traditional trend of"smaller,more unstable".The onset grain coarsening temperature for nanograins with an average size of about 30 nm is 896℃(0.7 Tm),which is about 250℃ higher than the reported coarsening temperature of 304ss in the literature.The abnormal thermal stability originates from the interaction between partial dislocations and grain boundaries below a critical grain size during plastic deformation,which causes the structure relaxation of grain boundaries.In the martensitic nanograined 304ss with the similar grain size as a contrast,no grain boundary relaxation was observed.The grain instability temperature is about 650℃(0.54 Tm),independent of grain sizes range from 26 nm to submicron,which can be contributed to the reverse martensitic transformation.The grain size effect of martensitic transformation in nanograined 304ss was studied by using tensile deformation.Generally,the smaller the grain size,the more difficult to activate martensitic transformation.However,in nanograined 304ss with grain size smaller than 60 nm,it is found that the smaller the grain size of 304ss nanograins,the easier the martensitic transformation occurred.Below 60 nm,the dominate deformation mechanism transforms from full dislocations to partial dislocations,which causes grain boundaries emitting.a large number of partial dislocations and becoming the preferred nucleation site of α’ martensite.Micro tensile experiments indicate that martensitic transformation is expected to provide an extra work hardening and ductility for nanograined 304ss below a critical grain size.Extremely fine nanograined 304ss with an average hardness of 10.1 ± 0.47 GPa was fabricated by using HPT.The sample is composed of two typical microstructures:shear band and laths.The laths are α’ martensite with an average thickness of about 26 nm.The average size of nanograins in the shear band is about 8 nm.These 8 nm nanograins exhibitted high thermal stability with a α’ structure after annealing at 800℃for 30 min.