Research On Microstructure Evolution And Mechanism Of Phase Transformation During Cryogenic Treatment Of High Carbon High Alloy Steel

Cryogenic treatment is well-known as a clean and efficient supplementary processing in the recent years,which could been used to enhance the overall mechanical properties of high carbon alloy steel.However,the previous studies which aimed at clarifying the mechanism of cryogenic treatment are basiclly based on the performance of cryogenic treated materials under room temperature.The lack of process controled experimental data and theoretical guidance resulted in a huge difference of the service life of parts after cryogenic treatment.So,the application of cryogenic treatment was limited severely.The present project aims to explore the microstructure evolution and phase transition in the process of cryogenic treatment and optimize the processing parameters for high carbon high alloy steel SDC99(0.83C-0.57Si-0.51Mn-9.37Cr-0.28V-1.46Mo-87 Fe,wt.%).The SDC99 steel was treated at different temperatures between-80℃ and-196℃ for different time before and after tempering.Subsequently,mechanical properties of these cryogenic treated steels were measured,such as Rockwell hardness,impact energy,and abrasion resistance.Effects of the cryogenic treatment on the morphology and distribution of retained austenite,martensite matrix and carbides were studied using SEM,XRD,TEM,internal friction and modulus test system,three dimensional atom probe and resistivity meter were used in the study as well.The relationship of the macroscopic properties and microstructure after cryogenic treatment was established.The microstructure evolution and transformation mechanism of high carbon alloy steel were revealed.The major conclusions are as following:(1)The macroscopic mechanical properties of the samples such as Rockwell hardness,wear resistance increased after cryogenic treatment,but the impact energy is slightly decreased.The results of experiments on the overall mechanical properties,friction and wear of cryogenic treated SDC99 steel shows that the best procedure of cryogenic treatment for SDC99 steel is-196℃ for more than 12 hours among the experimental parameters used in this research.In addition,it is better that the temper treatment was carried out before cryogenic treatment than that it was carried out after the cryogenic treatment.Compared with the conventional treated SDC99 steel,the wear-resistance of sample was improved about 43.8% when quenching at 1030℃,tempering at 210℃ for 2 hours,DCT at-196℃ for 24 hours and tempering at 210℃ for 2 hours.(2)SEM and TEM images show that the retained austenite subjected to conventional heat treatment is block-shape,but it is laminar-shape located around carbides after cryogenic treatment.Cryogenic treatment cannot make retained austenite transform to martensite completely.About 4% austenite retained is left in the matrix after the cryogenic treatment,which is well matched with simulating results of DEFORM?.The results of XRD shows that the tetragonal degree of martensite decrease after cryogenic treatment,lower the degree of supersaturation of carbon in martensite.In addition,cryogenic treatment immediately after quenching is best to reduce the lattice distortion.The widths of martensite in QT and DCT samples are 200 nm and 10 nm respectively,so as to refine the martensitic matrix..Low-angle XRD test results show that isothermal martensite formed during cryogenic treatment.(3)Three-dimensional atom probe(3DAP)results show that the average carbon content of martensite matrix is reduced after tempering.Most carbon atoms distributed uniformly in SDC99 steel after quenching from 1030℃ to room temperature.But the carbon atoms segregated at the new formed twin-boundaries of martensite after cryogenic treatment at-196℃ for 8 hours.With the decreasing of temperature and the extension of holding time,the carbon atoms will be further clustering,which result in the formation of laminar zone with 5~10 nm in thickness of carbon segregation but no carbide precipitation.The carbon atoms segregation at the new formed twin-boundaries of martensite will become stronger to form C-rich phase with thickness about 10 nm or form carbides M23C6 with Cr and Mo during the subsequent tempering at 210℃.(4)High temperature internal friction and modulus of elasticity test results indicate the SKK peak of SDC99 steel after fitting and deduct background increases after cryogenic treatment,which confirms that carbon atoms segregation to dislocation during cryogenic treatment.Thermodynamic calculation results show that the energy change of cooling the sample from room temperature(25℃)to liquid nitrogen temperature(-196℃)is about-3269313×10-14 J.The energy can increase the dislocation density by 1.03×1013 cm-2,so that the cryogenic treatment when the strain induced carbon slant to be together.The thermodynamic calculation results agree with the experimental data obtained from the IF analysis.The current study confirmed the formation of carbon segregation and the unusual precipitation of carbides during cryogenic treatment,which is significant to understand the phase transformation of high carbon high alloy steel at low temperature.And these results are helpful to find the optimum procedure of cryogenic treatment for certain materials.