Influences Of The Carbon And Nitrogen Contents On The Microstructure And Properties Of Nitrogen-alloyed High Carbon Martensitic Stainless Bearing Steel

High carbon martensitic stainless bearing steel is widely used in the fields of aerospace,medical equipment and petrochemical industries due to their outstanding strength,hardness and wear resistance.High carbon martensitic stainless bearing steels contain high carbon and chromium elements.Thus,large-sized eutectic carbides are inevitable.Nitrogen is an interstitial element that the addition of nitrogen to austenitic stainless steel and duplex stainless steel can improve the strength and corrosion resistance of steel.However,few works have addressed the role of nitrogen in high carbon martensitic stainless steel.In this thesis,the carbon of 9Cr18Mo was partially substituted by nitrogen,and the total content of carbon and nitrogen was controlled to be about 1%.Then,nitrogen-alloyed high carbon martensitic stainless bearing steels(NAHCMSBS)with the compositions of 0N(0.9%C),0.1N(0.1%N,0.8%C),0.15N(0.15%N,0.75%C)and 0.2N(0.2%N,0.7%C)were prepared.NAHCMSBs S were analyzed by optical microscope(OM),scanning electron microscope(SEM),energy dispersive spectrometer(EDS),X-ray diffraction(XRD),transmission electron microscope(TEM),Rockwell hardness tester and tensile testing performances.Furthermore,deformation and different heat treatment processes which effect the microstructure and properties of the NAHCMSBs were discussed.Under the same heat treatment process(1000?×30 min quenching+160?×4 h tempering),the microstructure of the NAHCMSBs is composed of tempered martensite and carbides.With the increase of nitrogen content,the carbide content in the steel first decreases and then increases.The carbide content is the lowest in 0.15N,which is 1.42%.The carbides in the microstructure are identified to be M₂₃C₆.The hardness of the NAHCMSBs first increases and then decreases with the increasing of nitrogen content.The hardness value of0.15N steel is the highest value of 60.5 HRC,which indicates good tempering stability.0.15N steel shows good comprehensive properties.The forged NAHCMSBs were further rolled.The grains and eutectic carbides are considerably refined,and the carbonitride content increases due to rolling.Under the same heat treatment process(1000?×30 min quenching+160?×4 h tempering),a small amount of Cr₂N particles precipitate in the rolled 0.15N steel.The microstructure includes martensite,M₂₃C₆and a small amount of Cr₂N.The nitrogen element is mainly dissolved in the matrix.Solid solution strengthening and precipitation strengthening are both strength the NAHCMSBs The hardness of 0.15N steel shows no obvious change due to rolling,but its tensile strength increases significantly to 1924 MPa,and a large number of small-size dimples form in the tensile fracture morphology.Quenching temperature has a great influence on the microstructure and properties of0.15N steel.Under the same tempering process(160?×4 h),the number of carbonitrides in0.15N steel decreases with the increasing of quenching temperature,and meanwhile,the grain size of martensitic lath increases.The microstructure is mainly composed of tempered martensite and carbonitrides due to quenching at the temperatures of 950?and 1000?.When the quenching temperature is increased to 1050?,the retained austenite is obviously observed in the metallographic structure,and the content of carbonitrides is obviously reduced.When the quenching temperature is further increased to 1100?,the carbonitrides substantially decrease.This time,the microstructure is mainly composed of coarse martensite slats,retained austenite and a small amount of carbonitrides.The hardness and tensile strength of 0.15N steel first increase and then decrease with the increasing of quenching temperature.The highest hardness and tensile strength are 60.5 HRC and 1924 MPa respectively,when quenched at 1000?.