Effect Of Recovery And Recrystallization On Low Temperature Nitriding Of Deformed Austenitic Stainless Steel

Austenitic stainless steel has good plastic toughness,weldability and excellent corrosion resistance,and is widely used in all walks of life.However,due to its low hardness(< 250HV)and poor wear resistance,its service life will be greatly reduced in wear environment.As one of the most widely used stainless steel surface modification technologies,low temperature nitriding technology can effectively improve the hardness and wear resistance of stainless steel surface by improving the microstructure and stress state of stainless steel surface.However,due to the low process temperature,the infiltration layer is generally thin.In the nitriding process,the diffusion of nitrogen atoms in materials is mainly affected by the diffusion treatment temperature and diffusion activation energy.Although a thicker nitrided layer can be obtained by increasing the temperature,it will lead to chromium deficiency in stainless steel matrix,resulting in a decline in corrosion resistance.However,a large number of structural defects such as grain boundaries and dislocations will occur in the matrix of stainless steel deformed at room temperature,which can significantly reduce the diffusion activation energy and increase the thickness of the infiltrated layer.In this paper,based on the recovery and recrystallization mechanism of deformed metal,AISI316 L austenitic stainless steel after deformation at room temperature(10%,30%,50%)is used as the matrix material,which is tempered(250?,550?,750?)and solution treated at 950?,respectively,and then subjected to glow plasma nitriding at 380?.The aim is to study the effects of structural defects,grain boundary density and second phase changes on low temperature nitriding of 316 L stainless steel.The effects of deformation,recovery and recrystallization on the microstructure and mechanical properties of austenitic stainless steel after nitriding at low temperature were studied and analyzed by optical microscope,scanning electron microscope,transmission electron microscope,X-ray diffractometer,electrochemical workstation,microhardness tester and friction and wear tester.Test results show that:At first,316 L stainless steel was subjected to tensile deformation at room temperature,followed by recovery and recrystallization.10%,30%,and 50% plastic deformation is produced by tensile at room temperature.The hardness of the deformed sample is 267HV0.05,308 HV0.05,and 324 HV0.05 respectively,while the hardness of the original sample is 220Hv0.05.The surface hardness of the deformed sample increases to more than20% of the hardness before deformation,and increases with the increase of deformation amount.After deformation,a large number of deformed structures were produced in the test samples,and the density increased with the increase of deformation.At a lower deformation(10%),deformation-induced martensite appeared,and its component increased with the increase of deformation.After 10%,30% and 50% deformation,the volume fraction of martensite in 316 L stainless steel was 12.6%,13.2% and 21.7%,respectively.The lower the recovery temperature,the higher the deformation amount,the higher the martensite component in the deformed sample and the density of deformed structure between grains,and the hardness change of the test sample also shows the same change rule.After recrystallization at the highest temperature(950 ?),the structural defects and martensite disappeared completely,and the grain boundary size decreased obviously,that is,the grain boundary density increased with the increase of deformation before recrystallization,showing complete recrystallization,and its surface hardness increased with the increase of grain boundary density.Secondly,the deformed 316 L stainless steel after recovery and recrystallization was nitrided,and the structure of the composite nitrided sample was characterized.Compared with the original nitrided sample,the thickness and hardness of the nitrided layer of the composite nitrided sample are obviously improved,and the nitrided layer is composed of supersaturated nitrogen solid solution,that is,the structural defects produced by tensile deformation at room temperature and the second phase(increase of crystal defects,increase of grain boundary density and martensite induced by deformation)can promote the growth of the nitrided layer.In the composite nitrided samples,the thickness of the composite nitrided samples treated by(30%,50%)deformation +(250?,750?)recovery+nitriding is obviously thinner,while the composite nitrided samples treated by(30%,50%)deformation +550?recovery+nitriding are thicker.It is found by transmission electron microscope that some nitrogen atoms exist in the form of Cr-N compounds at the defects in the infiltrated layer,resulting in local blockage of nitrogen atoms.With the recovery temperature of 316 L stainless steel(30%,50%)rising to 550? after deformation,the relative content of Cr-N in the nitriding layer decreases,the local blockage of nitrogen atoms is alleviated,and the thickness of the nitriding layer increases greatly When the recovery temperature reaches750?,the defect density and martensite component in the matrix structure of the sample decrease greatly,and the short-circuit diffusion effect weakens,which shows that the thickness of the infiltrated layer decreases obviously However,the recrystallization of 316 L stainless steel after recrystallization treatment at 950? after deformation is complete,and the thickness of the infiltrated layer increases with the increase of grain boundary density.In four groups of composite nitrided samples(10% deformation+recovery/recrystallization),the thickness of the nitrided layer decreases with the increase of heat treatment temperature.Finally,the properties of composite nitrided samples were characterized.The corrosion resistance,hardness and wear resistance of composite nitrided samples are all related to the deformation and heat treatment before nitriding.The AISI316 L austenitic stainless steel after glow plasma nitriding at 380? has a self-corrosion potential of-0.358 V and a self-corrosion current density of 1.145×10-6.Self-corrosion potential of 316 L stainless steel substrate is-0.365 V,and self-corrosion current density is 3.872×10-5,that is,the corrosion resistance of316 L stainless steel nitrided at 380 ? is improved.After composite nitriding of 316 L stainless steel,the self-corrosion current density of composite nitrided samples is greater than that of the original nitrided samples and less than that of the non-nitrided samples.The self-corrosion potential of composite nitrided samples recovered at different temperatures under the same deformation has little difference,but the self-corrosion current density first increases and then decreases.The self-corrosion potential of composite nitrided samples after recrystallization treatment under different deformation decreases with the increase of deformation,and the self-corrosion current density increases with the increase of deformation.In the wear test under 10 N load,the higher the hardness of composite nitrided samples,the smaller the wear mark width and wear degree.When the load force in the wear test is 20 N,the thicker the nitriding layer,the smaller the wear mark width and wear degree.