| With the rapid development of transportation,especially high-speed railway,aerospace,atomic energy and other industries,bearing steel is used for severe service environment,such as high temperature,high speed,heavy load,corrosion resistance,wear resistance,fatigue resistance and so on in recent year.Countries in the world have developed a series of corrosion-resistant high-temperature bearing steels to adapt to these special needs.40Cr15Mo2VN and 8Cr4Mo4V high-temperature bearing steels are the typical steels.The service temperature of 40Cr15Mo2VN is higher than that of 8Cr4Mo4V.The high temperature performance of 40Cr5MoVN is improved by reducing carbon and increasing nitrogen which causes better toughness and significantly enhanced corrosion resistance.But its strength and hardness are lower than 8Cr4Mo4V.And the inclusion control level of 8Cr4Mo4V is significantly improved because of VIM and VAR.Fatigue performance is one of the important measurement criteria of bearing steel quality parameters.Failure caused by loss of bearing precision due to friction and wear cannot be ignored.Theoretically,there will be extremely little wear of bearing if the lubrication condition is excellent.In this paper,it was studied the effects of carbides and inclusions on the wear and fatigue properties of the two steels.Firstly,better grease was prepared and applied to the wear test.There are various wear forms in bearing service,such as sliding,rolling and fretting.This paper mainly focus on sliding and fretting in which the relations between carbides&inclusions and wear properties is analyzed.In the meanwhile,the wear mechanism of high temperature bearing steel under the condition of lubrication is studied.The rotating bending fatigue test of 40Cr15Mo2VN was carried out at 300?and 400?.After the test,the high temperature fatigue failure mechanism was analyzed.It provides certain reference to the research on high temperature fatigue failure for the future.Through ultrasonic dispersion and mechanical agitation,the nano-additives in lithium-based greases are well dispersed.When the mass fractions of MoS2 and CNT in the grease are all 0.8%,the wear scar diameter is 0.348mm.And the lubrication performance is the best.In the sliding wear test,the coefficients of friction of the steels are not continuous.It has a small coefficient of friction in the early stage and then increases.The average friction coefficients of 40Cr15Mo2VN and 8Cr4Mo4V are 0.162 and0.166,respectively.It is found in the test that the wear performance is related to the type of carbides.The inclusions have an adverse effect on the wear resistance of the steel.The oxidation phenomenon of 8Cr4Mo4V in the wear process is more serious than 40Cr15Mo2VN.In the 40Cr15Mo2VN fretting wear test,when the stress is constant and the sliding velocity V increases from 0.028m/s to 0.112m/s,the friction coefficient increases from 0.115 to 0.130.The volume wear rate decreases from 0.0066×10-3mm3/?N*m?to 0.0033×10-3mm3/?N*m?.When the sliding velocity V=0.028m/s and the contact stress increases,the friction coefficient increases from about 0.118 to0.126.The volume wear rate increases from 0.0063×10-3mm3/?N*m?to 0.1345×10-3mm3/?N*m?.In 8Cr4Mo4V fretting wear test,when the contact stress is 2.047GPa and the sliding velocity V increases from 0.014m/s to 0.112m/s,the friction coefficient increases from about 0.124 to 0.128.The volume wear rate decreases from 0.0059×10-3mm3/?N*m?to 0.0055×10-3mm3/?N*m?.When the sliding velocity V=0.028m/s and the contact stress increases,the friction coefficients are all about 0.122 and have little changes.The volume wear rate increases from 0.0048×10-3mm3/?N*m?to0.0129×10-3mm3/?N*m?.In the rotating bending fatigue test of 40Cr15Mo2VN bearing steel at high temperature,the fatigue limit strength at 300?and 400?are 787MPa and 860MPa,respectively.The safety fatigue limit at 300?decreases by 8.5%by comparison with400?.Fracture surfaces are observed by SEM.Fatigue failure types are surface defects,inclusions and voids of the matrix.Because of the higher degree of creep voids aggregation,it is easy to connect into micro cracks and lead to fatigue failure at300?so that the fatigue limit decreases than 400?. |
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