Research On The Key Technologies Of Laser Transformation Hardening For Bearing Surface Of GCr15 Steel

There are many means to strengthen the material surface.The surface modification technique is an advanced way to improve the reliability and the service life of working surface of bearing.The laser transformation hardening (LSH) is a newly-developed technology,which is now used for the material surface treatment .In terms of the key project of the Natural Science Research Foundation of Department of Education of Anhui province (No:KJ2009A021)"Research on the key technologies of laser transformation hardening for bearing surface of GCr15 steel",this dissertation aims to study on the key technologies in the LSH for bearing surface and samples of GCr15 steel.This dissertation will explore the novel approaches for the surface hardening of bearing rings, in which the foundamental theory and practice are combined with actual meaning.The LSH proccess of GCr15 steel bearing rings is conducted using the CO2 laser heat source.The laser processing parameters are optimized using the orthogonal experimental design method.The macromorphology and microstructure of the hardened layers are investigated by the optical microscopy(OM) and scanning electron microscopy(SEM, FEI ESEM XL30).The laser-quenching parameters selected should ensure the quenched zones to provide not only sufficient case depth and higher hardness,but also a suitable profile and finer microstructure.The experimental results show that the hardness of bearing surface is significantly improved after the LSH process,and the depth of harden layer and the hardness increase with the increasing of laser power.The depth of laser tracks of about 0.8~0.9 mm has been obtained. The hardness enhancement of hardened layer is attributed to the grain refining and the formation of supersaturated hidden crystal martensite during the laser quenching.The wide-band laser surface quenching for GCr15 steel treated conventionally is carried out by the CO2 laser equipment.Then the content of retained austenite and the distribution of residual stress are analyzed by using the X-ray diffraction(XRD).The tested results imply that the quenched zones have more retained austenite and finer carbides due to a higher degree of carbide dissolution.Compressive stress are detected on the surface of harden layer after the LTH,and it increases with the increasing of laser power.Subsequently the laser-quenched samples are tempered at various levels of temperatures.The results show that they have the good microstructure stability at the relatively high temperature compared with those of conventionally hardening GCr15 steel specimens.The reason to enhance the tempering stability is analyzed by means of SEM observation.In this work, the temperature field of laser surface hardened layers for race surface is also analysed numerically in detail. The influences of laser parameters on the widths and depths of LTH zones of GCr15 steel are discussed and the results obtained are proved to coincide with the experiments.From these theoretical results,it is clear that in close proximity to the boundaries(such as beginning and ending boundaries),the temperature field has an obvious change when the laser mean power is kept the constant,which leads to the hardness discrepancy and the case discrepancy between the beginning and ending boundaries of the bearing rings.The paper puts forward feasible methods to improve distribution uniformity of the hardened case and the hardness uniformity of steel surface.The characteristics of dry friction wear resistance of hardened GCr15 steel specimens are studied and compared with those of conventionally hardening specimens to evaluate the tribological behaviors.Laser surface hardening GCr15 steel specimens exhibit superior wear resistance to conventionally hardened specimens due to the effects of hardening of microstructure,high hardness, and toughness.The results show that the acting wear mechanism for both the laser quenched layer and the hardened layer with conventional heat treatments are predominantly adhesive ,material transfer ,oxidation and abrasion.The microstructural thermal stability is increased after laser surface treatment. The structures with higher microstructural thermal stability exhibit a good stability to the heat effect of sliding friction and wear.The wear resistance under lubricated sliding conditions is compared between specimens treated with laser and those of conventionally hardened.The tribological properties of laser surface-quenched GCr15 steel specimens are slightly better due to the effects of microstructure hardening, high hardness and toughness,with the wear rate (in the order of 10-6mg/N·m) lower than those of the conventionally treated specimens.At the steady state, the frictional coefficient of laser-treated samples has no obvious difference from that of the conventionally treated samples.The wear mechanism for both cases is similar, generally involves surface fatigue wear and slight abrasion wear.The effect of laser powers on the final microstructure and the hardness after laser quenching and the wear resistance of the bearing steel GCr15 under sliding wear in air is investigated.The difference of microstructure is produced by changing the laser power, which result in the bigger difference of surface hardness and the wear rate. When the laser power is increased, the amount of undissolved carbides of the laser hardened layer surface is decreased, the average content of carbon of the laser hardened layer surface is increased, thus leads to the increase of the surface hardness. In addition, the retained austenite and its stress induced transformation into martensite takes place in the wear test on the hardened zone surface. The wear resistance of laser hardened layer is increased when the laser power is increased both under the dry sliding wear conditions and lubricated sliding conditions. The influence of laser surface hardened layers on the bending strength and flexural deflection of the samples is studied through the three-point bend tests.In the tests,the material plasticity is characterized by the bending deflection.The results show that the bending strength and flexural deflection of the samples are decreased obviously after the laser quenching .Moreover ,the failure mechanism also changes,which causes the decrease of fracture toughness and plasticity.The fractured surfaces of laser surface hardening specimens are analyzed and compared with those of conventionally hardening specimens.Fracture toughness characteristic of dimples is observed on fracture surface of conventionally hardened specimens. The fractured surfaces of the laser surface hardened layers exhibit the brittle fracture in crack initiation zone. The mode of brittle fracture is mainly the intergranular fracture and many cleavage steps in crack initiation zone.In conclusion ,the results demonstrates that the LSH can produce a predominantly martensitic microstructure with more retained austenite and finer carbides,with high hardness and wear resistance and residual compressive stress at the surface of GCr15 steel.The wear resistance of bearing assembly is increased after the LSH. In order to apply the LSH process to actual production for the surface hardening process of bearing rings , further experiments and studies requires to be done.