Carbide Evolution Behavior of GCr15 Bearing Steel During Aging Process
The evolution behavior of carbides in GCr15 bearing steel and its influence on the impact toughness during long-term aging at 170℃ have been investigated by means of SEM,TEM,and XRD,aiming to meet the requirements of vacuum dry pump bearings.The results demonstrate that after quenching at 840℃ and the tempering at 230℃,the hardness of GCr15 steel remains above 59 HRC with minimal retained austenite,which is favorable to the enhancement of performance and dimensional stability for the steel at 170℃.During the aging process,carbon atom partitioning and carbide precipita-tion lead to a decrease in carbon concentration,lattice distortion and micro-zone stress strain of the ma-trix,while transitional carbides precipitate,coarsen and then transform into non-coherent cementite.The resultant effect of these microstructural variation is a reduction in material hardness,while the impact toughness initially increasing and then decreasing.However,the cooperative effect of the decarboniza-tion of martensite and carbide type transformation makes hardness of steels remain stable or even in-crease a little in between 1000 h and 2000 h during the aging process.To improve the microstructure and performance stability during the aging process,cryogenic treatment was conducted after quenching.The introduction of high-density defects promotes effective carbon distribution during tempering and aging,which gives rise to uniform distribution and size control of fine carbides.Cryogenic treatment reduces the carbide growth rate from 298 nm3/h to 229.5 nm3/h,which delays the performance decline effectively and makes the GCr15 bearing steel satisfied with demands of vacuum dry pump bearings.
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