The Effect On Microstructure And Properties Of Cementite In Nanostructured Bainitic Bearing Steel

High-end bearings are the key components to ensure the smooth operation of high-end equipment.The nanostructured bainitic bearing steel developed by our research group has excellent comprehensive properties and it has attracted much attention in the bearing industry.The undissolved cementite in bearings steel is to ensure the wear resistance,while the existence of cementite is harmful to rolling contact fatigue life.Therefore,the cementite has a great influence on use performance of bearing.In this paper,GCr15Si1 Mo highcarbon nanostructured bainitic bearing steel is the main research object.The effects of cementite refinment and complete dissolution on the on the microstructure and properties of bearing steel are investigated systematicly by using microstructure characterization methods and property testing methods,such as XRD,SEM,TEM,tensile,impact,wear,rolling contact fatigue.Finally,the method of preparing matrix component steel is used to compare and study the effect of cementite on the microstructure and properties of bearing steel matrix.The conclusions are as follow.Through the pretrement of quenching and high temperature tempering,the cementite size in the microstructure of GCr15Si1 Mo steel was refined from 0.49 μm to 0.20 μm.Then,the austenite grain size is refined from 5.7 μm to 2.8 μm.Eventually,the microstructure of nanostructured bainitic bearing steel has achieved double refinement.After refining cementite,the incubation period of bainitic transformation was significantly prolonged.And the exspansion rate and the volume fraction of bainite were also reduced.The hardness of the steel maintained at HRC60,while the toughness decreased significantly.After the cementite was refined,the crack nucleation work increased significantly.However,the impact process was fast and violent.The distance between fine cementite was short,and the distance and time during crack propagation were greatly shortened.The crack propagation work was greatly reduced,and the specimen showed a lower absorption work.Finally,the impact toughness decreased.After refining cementite,the cementite was easy to dissolve back during wear process.And a nanocrystalline layer was formed on the surface of the worn specimen,and the surface hardness increased.Thus,the specimen showed excellent wear resistance.Meanwhile,the ability of crack nucleation was decreased after refining cementite,and the rolling contact fatigue life of bearing steel improved 2.5 times.After the cementite was completely eliminated by solution treatment,the carbon content of matrix increased significantly.Thus,bainite transformation process is greatly inhibited.The volume fraction of bainitic ferrite decreased from 85.6% to 62.3%,and the volum fraction of retained austenite increased from 7.8% to 37.7%.Due to the complete dissolution of cementite,the growth of the bainitic ferrite lath is not hindered.And then,the bainitic ferrite lath showed a long and straight distribution with uniform orientation.Meanwhile,the hard crack source disappeared and retained austenite was easy to transform into martensite under the cyclic load.Due to the fact that martensite occurred in the microstructure,the surface hardness of specimen improved.Crack propagation of subsurface was delayed.The wear resistance and rolling contact fatigue of test steel were improved.However,high volume fraction of retained austenite significantly reduced the toughness of the bearing steel and the dimensional stability of the bearing.Using the method of smelting the component steel of the matrix,the influence of cementite on the matrix microstructure of the bearing steel was studied.It was found that austenite grain size decreased from 11.6 μm to 5.7 μm and the apparent thickness of bainitic laths also decreased from 78 nm to 62 nm.Due to the fact that the cementite hindered the growth of the bainite lath,the shape of the bainite lath changed from long and straight to short and small.And the orientation of bainite lath also changed from regular to random.The test steel had high strength,high hardness and low toughness under the strengthening effect of cementite and finer bainite lath.During the wear process,the cementite was easy to spalling off,which will adversely affect the wear resistance.The research in this paper can provide basic theoretical support for the control of cementite in nanostructured bainitic bearing steel.The next step should focus on the volume fraction of refined cementite and retained austenite after complete dissolution.It provides support for improving the comprehensive properties of nanostructured bainiti bearing steel.