Nanocrystallization And Microcosmic Mechanism In Inner-surface Of AISI 304 Stainless Steel Pipe

Inner surface damage such as wear and corrosion etc.usually makes pipes failure during early stage of service.Modification of the pipe inner surface with improved strength and resistance against wear and corrosion etc.has thus been the general method to enhance the overall performance.However,the traditional methods like nesting with composite,spraying and electroplating,etc.suffer from disadvantages including complicate and expensive processing,and the weak bonding between modified layer and the matrix.It is of great scientific and technical importance to explore novel method to modify the microstructure and properties of pipe inner surface.Surface nanocrystallization（SNC）is a new method to achieve surface modification through fabricating nanostructured surface layer with high strength and hardness,good wear resistance and chemical activities.Among many SNC techniques,surface plastic deformation is able to fabricate gradient nanostructured surface layer with good bonding between matrix in a simple and cheap manner.There are extensive investigations on the SNC of plate and rod samples,however,less attention was paid to the pipe inner surface.It is thus the present investigation to focus on the SNC of pipe inner surface.An austenitic stainless steel with wide industrial applications will be chosen as the experimental materials,of which the microstructures of the processed samples was characterized by X-ray diffraction,scanning electron microscope and transmission electron microscope and the surface hardness,hardness gradient and the surface roughness will be measured.The following conclusions were reached:1)A pipe inner surface grinding（PISG）technique was developed.PISG is able to induce surface plastic deformation with high shear strain（>10）,high strain rate(¹0⁴ s^-1),and high strain gradient(>0.5μm^-1).2)PISG induces Single-phase nano-austenite structured surface layer and gradient hardened layer in AISI 304 stainless steel pipe.The surface grain size was refined down to around 20 nm,the hardness was increased by a factor over 2 and the thickness of the hardened layer can be 800μm.3)The general pattern of the structural evolution of AISI 304 stainless steel toward nanometer scale was established.With the increase of plastic deformation,structural evolution involves four stages:i)dislocation slip dominated,forming planner dislocation arrays;i)twining dominated,twin-twin intersection and deformation induced martensitic transformation;iii)shear banding dominated,including the initiation,development and substructure evolution of shear bands;iv)formation nanograin.