Study On Strengthening And Toughening Mechanisms And Corrosion Resistance Of High Nitrogen Martensitic Stainless Steel Based On Carbon And Nitrogen Regulation

With the rapid development of industrialization,traditional stainless bearing steels(such as 9Crl8 and 9Cr18Mo),no longer meet the requirement of market due to the existence of coarse eutectic carbides which deteriorate their properties.As a new generation of high quality bearing steel,high nitrogen martensitic stainless bearing steel has high hardness,strength and abrasive resistance,good fatigue toughness and corrosion resistance.Therefore,the study on its chemical compositions,microstructures and properties are conducive to the development of China's high quality bearing steels.In this paper,the effects of C and N contents on microstructures and properties of high nitrogen martensitic stainless steels were investigated systematically.The C and N contents of experimental steels are 0.20C-0.545N,0.35C-0.372N and 0.50C-0.163N,respectively.Microstructures of experimental steels were analysed by OM and SEM,EDS and XRD.Properties were measured by hardness tester and impact testing machine,tensile testing machine and electrochemical workstation.The main conclusions are obtained as follows:According to results of Thermo-calc and JMatPro,increasing C and N contents can expand austenitic phase area and inhibit the formation of ?-ferrite at high temperatures.With decreasing nitrogen content and increasing carbon content in experimental steels,precipitation sensitivity,precipitation temperature and maximum content of M23C6 increase gradually,but precipitation temperature and maximum content of M2N decrease gradually and its precipitation sensitivity changes little.When the carbon content is 0.5 wt.%,M7C3 carbides precipitate during the solidification.The microstructures of steels after annealing contain ferrite+M23C6+M2N.Precipitates of 0.20C-0.545N are fine granular and uniform in size.There are fine granular precipitates and a few bulk-shaped precipitates with larger size in 0.35C-0.372N.For 0.50C-0.163N,there are more bulk-shaped or long stripe-shaped precipitates with larger size.With increasing quenching temperature(1000?1060?),the grain size of 0.20C-0.545N and 0.50C-0.163N increase slightly.The grain size of 0.35C-0.372N increases slightly when temperature is increased from 1000 to 1040 ?,but its grain size increases obviously when temperature is 1060?.The hardness of experimental steels after quenching decreases with increasing temperature.After cryogenic treatment,the hardness of experimental steels is improved obviously and first increases and then decreases with increasing queeching temperature.After tempering at 200?,the matrix microstructure of experimental steels is martensite+retained austenite.With decreasing nitrogen content and increasing carbon content,the type of precipitates changed from M2N to M23C6+M2N and finally to M23C6.For 0.20C-0.545N and 0.35C-0.372N,precipates mainly distributed in grains and partially at grain boundaries,but there exist many precipitates in grains and at grain boundaries for 0.50C-0.163N.The mean equivalent diameter and volume fraction of precipitates increase in the order of 0.35C-0.372N,0.20C-0.545N,0.50C-0.163N.The content of retained austenite increases in the order of 0.50C-0.163N,0.20C-0.545N,0.35C-0.372N.Experimental steel with high carbon content and low nitrogen content has very high hardness and strength,but its toughness and corrosion resistance are poor.Experimental steel with low carbon content and high nitrogen content has high hardness,strength and excellent corrosion resistance,but its toughness is not good enough.Experimental steel with middle carbon and nitrogen content,its hardness and strength are slightly lower than those of the preceeing experimental steels,but it has excellent toughness and good corrosion resistance.Accordingly,regulating carbon and nitrogen contents in high nitrogen martensitic stainless steel can meet the requirements for its properties in different application area.