The Effect Of Cold Treatment On The Microstructure And Structure Of High Cr-Co-Mo Bearing Steel

The research of high-temperature bearing steel for aeroengine was increasingly urgent with the rapid development of aeronautic industry.In order to ensure the comprehensive performance of high temperature bearing steel,the composition design of high temperature bearing steel is based on the principle o f "low carbon+high alloy".The advantages of cryo-treatment includes low cost,simple process,non-pollution,etc.In addition,it can effectively reduce the conten t of residual austenite in high alloy steel,control the size of carbide in steel and promote the dispersion distribution of carbide.Therefore,adding cry o-treatment in the process of conventional heat treatment can improve the comprehensive properties of steels.In this paper,a new type of low carbon and high alloy bearing steel was used as the research material.The microstructure of steel samples with different cryo-treatment processes was characterized by XRD,SEM and TEM.Meanwhile,the mechanical properties such as hardness and impact toughness of the test steel were measured.The relationship between the mechanical properties and microstructure of the test steel also were studied.The matrix microstructure of test steel was lath martensite after te mpering and cryo-treatment,a small amount of thin film retained austenite was distributed between in the martensite laths,the orientation of residual austenite and martensite conformed to the K-S relationship.After cryo-treatment and tempering,the cont ent of residual austenite decreased significantly in quenched sample.When the cryo-treatment time was consistent,the content of residual austenite decreases with the decrease of cryo-treatment temperature,while the cryo-treatment temperature remains unchanged,the content of retained austenite decreases slightly with the extension of treatment time.The steel with different cryo-treatment existed a certain number of M 6C type carbides,reducing the temperature of the cryo-treatment can improve the carbide dispersion uniformity.When the temperature of the cryo-treatment was-80℃,basic along austenite grain boundary carbides distribution,when the single time of cryo-treatment from 2h extended to 4h,the average particle size increased by about 230 nm to 250 nm,but the carbides were distributed along the austenite grain boundary basically;When the temperature of the cryo-treatment reduced to-120℃,the carbide distribution was more even and the average particle size of carbides from about 210 nm to about 215 nm with the extension of cold treatment time;When the cryo-treatment temperature dropped to-196 ℃,the test steels in the single cryo-treatment time for 2h and 4h,the average particle size of carbides was about 280 nm and 310 nm,respectively.The martensitic lath was obtained by quenching and a few twins as sub-structures formed in some martensitic laths.The rudiment of martensitic lath sub-interfaces was formed in the high density dislocation regions after deep cryo-treatment(-196℃).Meanwhile,the number of twins increased,especially in the high-density dislocation regions.After tempering,the rudiment of sub-interface further evolved to the martensite lath boundary,thus the original martensitic laths were refined to 2~ 3 unit,the average width of martensitic lath decreased from about 390 nm to about 120 nm.The twins formed by cryogenic treatment did not disappear after tempering.In addition,small quantities of annealing twins formed in tempering process.The twins that formed in different tr eatment stages of test steel are {112}<111> twins.