Effect Of Shot Peening On Nitriding Process And Properties Of Nitriding Layer Of Duplex Stainless Steel

In order to improve the wear resistance of stainless steel,people of stainless steel nitriding research,found that in the nitriding of stainless steel,N element will diffuse into the matrix,combined with Crelement in stainless steel CrN,resulting in poor chromium stainless steel surface,resulting in reduced corrosion resistance of stainless steel surface.It can be seen that although nitriding improves the hardness of stainless steel,it is at the expense of the surface corrosion resistance of stainless steel.In order to prevent Crelement precipitation in the nitriding process of dual-phase stainless steel,reducing the nitriding temperature is an effective strategy.However,reducing the temperature also means that the nitriding diffusion activation energy of N element can not be reached during the nitriding,which will make the nitriding effect is not ideal.Therefore,through specific surface pretreatment to promote nitriding,The research of reducing nitriding temperature has good engineering application value.Salt bath nitriding process is a heat treatment method of nitriding in salt bath.Compared with gas nitriding,it is suitable for small batch and various parts.In recent years,there have been many reports on the nitriding of stainless steel after shot peening,but the study on the salt bath nitriding after shot peening is relatively rare,especially the study on the erosion corrosion behavior of dual-phase stainless steel after shot peening in sand corrosive media.In this thesis,2205 duplex stainless steel as the research object,the sample surface shot peening treatment,and then different temperatures of salt bath nitriding treatment.Scanning electron microscope,metallographic microscope,X-ray diffractometer and transmission electron microscope were used to analyze and characterize the microstructure of the porous layer.The corrosion resistance of 2205 duplex stainless steel before and after nitriding was studied by potentiodynamic polarization curve test and electrochemical impedance spectroscopy test.The scour corrosion behavior of samples treated with different treatments in different simulated environments was studied by using a self-made simulated scour corrosion device.The friction and wear characteristics of nitriding layer were studied by sliding wear testing machine,and the following main results were obtained:1.After shot peening,the corrosion resistance of dual-phase stainless steel is increased,because after ultrasonic shot peening,a large number of crystal defects are formed on the surface of the sample,which promote the formation of the passivation film and make the passivation film more dense.2.At the same nitriding time,nitriding temperature is the main factor affecting the surface hardness of the sample.Compared with the nitriding temperature of 465℃,the hardness of the nitriding layer obtained at 565℃is higher,because N atoms are easier to penetrate into the surface of the sample.3.Friction and wear tests show that compared with the original sample,the wear resistance of the sample after ultrasonic shot peening pretreatment and then nitriding is obviously improved,which is manifested by the reduction of friction coefficient and narrower wear mark width.4.Under the nitriding temperature of 465℃and holding time of 4h,the self-corrosion potential and corrosion current density of the sample without shot peening are-0.767V and6.21μA/cm2,while the self-corrosion potential and corrosion current density of the sample with shot peening are-0.803V and 13.8μA/cm~2,respectively.The corrosion resistance of the nitriding sample with shot peening treatment is low.Under the nitriding temperature of565℃and holding time of 4h,the self-corrosion potential of the sample without shot peening is-1.03V and the corrosion current density is 3.55μA/cm~2,while the self-corrosion potential of the sample with shot peening is-1.05V and the corrosion current density is30.7μA/cm~2.The corrosion resistance of the nitriding sample with shot peening pretreatment is relatively low.5.The scour corrosion tests of sand-free NaCl solution and 3.5L NaCl solution showed that the scour corrosion of various samples was mainly corrosion in the single-phase medium of sand-free NaCl solution,while in the two-phase medium of sand-containing NaCl solution,there was a interaction between scour and corrosion of various samples.After shot peening,the corrosion resistance of stainless steel is reduced to different degrees.Therefore,a balance point should be considered between the improvement of wear resistance of stainless steel by nitriding and the damage of corrosion resistance.