Study On Deformation Strengthening Process Of Nickel-free Austenitic Stainless Steel

Nickel free austenitic stainless steel has excellent strength,toughness,biocompatibility,and good cold working performance,and is widely used in medical implants and non implantable medical devices.At present,the research on deformation strengthening and post-deformation aging treatment of nickel-free austenitic stainless steel is not perfect.In this paper,the mechanical properties and microstructure of nickel-free austenitic stainless steel with different cold rolling deformation and after different process aging are studied by optical microscope,scanning electron microscope,transmission electron microscope and X-ray diffraction.Carrying out solid solution treatment（water cooling）on nickel-free austenitic stainless steel at 1100 DEG C for 1h to obtain solid solution nickel-free austenitic stainless steel,and then carrying out cold rolling deformation with 10%,30%,50%and 80%reduction to obtain nickel-free austenitic stainless steel in different cold rolling states;The solid-solution and cold-rolled experimental steels were aged at 400℃,500℃and 600℃for 0.5h,2h and 6h,respectively,and the aged nickel-free austenitic stainless steel was obtained.Specifically,the following conclusions are drawn:Perform a solid solution treatment at 1100℃for 1 hour on the forged experimental steel to obtain a fully austenitized solid solution sample steel.As the cold rolling deformation increases,the hardness,tensile strength,yield strength,and dislocation density of the material continue to increase,and the elongation after fracture first decreases and then tends to stabilize.After aging treatment at 400℃,the deformation microstructure of 10%,30%,and50%cold rolled specimens recovers,and the defect density of the deformation microstructure decreases,leading to an increase in plasticity and a decrease in strength of the material;80%of the cold-rolled samples showed significant annealing hardening,showing a slight increase in strength and a slight decrease in plasticity.After aging treatment at 500℃,the cold-rolled samples showed annealing hardening phenomenon,and the strength was significantly improved.The plasticity of the 10%and30%cold-rolled samples was significantly improved,while the plasticity of the 80%cold-rolled samples decreased significantly.There are small Cr₂N precipitates in the microstructure of the 80%-500℃-6h sample.The dispersed small precipitates in the microstructure can hinder the movement of dislocations,produce obvious precipitation strengthening effect,and also consume interstitial solid solution in austenite,resulting in a decrease in material toughness.After aging treatment at 600℃,the strength,hardness,and plasticity of the cold-rolled specimens all decreased to varying degrees,and the decrease became greater as the cold-rolled deformation increased.The main reason is that during the aging process,a large amount of Cr₂N accumulates at the defects in the sample,and the bonding force between the precipitates and the matrix is weak,leading to the easy initiation of cracks at the junction of Cr₂N and the matrix,and the deterioration of the material’s mechanical properties.The tensile fracture at 10%-600℃-6h is intergranular fracture,while the tensile fracture at 30%-600℃-6h and 80%-600℃-6h is transgranular fracture.