Influence Of Surface Processing On Mechanicaland Chemical Properties Of 18Mn18Cr0.6N Steel

The nitrogen and manganese in the high-nitrogen austenitic stainless steel replace the expensive nickel element in the traditional AISI300 series stainless steel.It has excellent mechanical properties,good biocompatibility and price advantages.The type of steel has very broad application prospects in medical,automotive,chemical and other fields.The final processing techniques for surface of metal parts,such as milling,shot blasting,rolling,etc.,will introduce strain layers,work hardening and residual stress.The changes of processing techniques for surface have an effect on menchanical and chemistry properties.However the impact of surface techniques for processing to the high-nitrogen austenitic stainless steel are still unknow.The article takes the 18Mn18Cr0.6N steel as the research object,and performs milling processing,shot blasting processing,and rolling processing on surface.The hardness,residual stress,cavitation resistance,polarization characteristics,AC impedance and pitting resistance of different processing conditions are studied.The aim of analyzing various processing techniques which influences stainless steels is to provide mechanical and chemical properties.The main conclusions obtained in this paper are as follows:(1)There are obvious hardening layers in the samples after milling,rolling and shot blasting.The surface hardening layer of the sample by rolling which is about 250?m;the shallowest hardening layer is the sample by shot blasting which is about 80?m;the hardening layer of the sample by milling is centered,and the layer is about 130?m;there is not much differences between the surface hardness values obtained by different processing methods,480 HV,476 HV and 487 HV respectively.(2)The surface stress of the sample by milling is the residual tensile stress,with the largest absolute value,which is 532 MPa.The surface of the sample after shot blasting and rolling processing is residual compressive stress;the residual compressive stress of the sample after rolling processing is larger,which is 257 MPa;the residual compressive stress of the sample after shot blasting processing is 213 MPa.The dislocation density which measured by XRD after the three surface processes has little difference,2.56×10¹⁹m^-2,2.50×10¹⁹m^-2and 1.84×10¹⁹m^-2respectively.(3)The damage is easily formed on the coherent twin grain boundaries after cavitation corrosion.Non-coherent twin grain boundaries and random large-angle grain boundaries,have strong resistance to cavitation.Campareing the sample with surface processing and the sample without surface hardening layer,the result that processing destroys the coherent relationship of the twin boundary and reduces the initiation position of cavitation damage is found,and cavitation resistance performance is improved.(4)After 48 hours of immersion in ferric chloride solution,it was found that pitting corrosion preferentially nucleated at the trigeminal grain boundary.Among them,the largest proportion of surface pitting to the total area is the sample after shot blasting,which is 1.73%;the smallest is the sample after milling,which is 0.57%;the middle of the sample after rolling is 1.69%.The diameter of the largest pitting was measured,and the largest diameter was the sample after shot blasting and rolling,respectively,180?m and 190?m;the smallest diameter was milling,which was 100?m.(5)The self-corrosion potential,pitting potential and self-corrosion current density of different surface processing were compared.The self-corrosion potential of the sample after milling was the largest,which was-0.436 V;the sample after shot blasting was the smallest,which was-0.458 V;the sample after rolling was the middle,which was-0.445V;the sample of rolling processing with the largest pitting potential,which was 0.461 V;the sample of milling processing was the middle,which was 0.345 V;the sample of shot blasting processing which was 0.225 V was the smallest,and it's passivation area is serrated.The self-etching current density of different processing methods is similar,between 5.047?7.85×10^-6A·cm².In the impedance spectrum fitting circuit,the maximum R_ctresistance is 18.03×10⁴?·cm²by rolling processing and 16.78×10⁴?·cm²by milling processing,and the minimum R_ctresistance is 4.45×10⁴?·cm²by shot blasting processing.The results show that the dense passive film is easy to form on the sample of rolling and milling processing,which improves the corrosion performance.(6)The electrochemical corrosion process is simulated by COMSOL.It is found that the area above the equilibrium potential is the largest in the nephogram of electrolyte potential distribution by shot blasting on the sample,the smallest by rolling and the middle by milling on the sample.The distribution of corrosion thickness shows 40?m corrosion,in which the area of shot blasting processing at 6.5?m corrosion thickness is the largest,the area of rolling processing is the smallest,and the area of milling processing is the middle.The results are in good agreement with the test results,which show that the corrosion of rolling and milling processing is better than shot blasting processing.