【作者】 雷声；

【导师】 刘全坤；

【作者基本信息】 合肥工业大学 ， 材料加工工程， 2010， 博士

【摘要】 材料的表面处理有很多种方法,采用先进的表面改性手段可强化轴承工作表面,提高轴承工作可靠性,延长其使用寿命,应用激光对轴承表面实施相变硬化处理(激光淬火)则是一门新技术。本文结合安徽省教育厅自然科学重点资助项目“轴承表面的激光硬化关键技术及应用技术研究”(编号:KJ2009A021),研究GCr15钢试样及轴承滚道表面激光相变硬化的各种关键问题,为轴承滚道表面强化提供了新途径,将基础研究与生产实际相结合,具有实际意义。选用CO2激光器进行GCr15轴承滚道表面激光相变硬化处理试验。运用正交试验设计方法对工艺参数进行了优化,用光学显微镜(OM)和扫描电镜(SEM, FEI ESEM XL30)进行GCr15轴承钢激光表面改性后的显微组织和形貌尺寸特征分析。选择合适的激光淬火参数,以保证激光表面改性层有足够的硬化层深度和高的硬度值,有更加细小的马氏体组织。结果表明,经激光相变硬化后轴承滚道表面硬度得到提高,且随着激光功率的增加,淬硬层的深度与硬度增加,硬化层深度最多可以达到0.8~0.9mm。组织细化和过饱和的隐晶马氏体的形成是硬度提高的主要原因。对激光相变硬化处理后的GCr15钢试样,使用X射线衍射仪对淬硬层的残余奥氏体含量和残余应力分布进行了分析。结果显示,激光强化区由于碳的溶解形成较多的残余奥氏体和细小的未溶碳化物。激光相变硬化后试样表面为压应力,且随着激光功率的增大而变大。对激光相变硬化处理后的GCr15钢试样进行不同温度的回火处理,并与经常规热处理后的GCr15钢试样进行对比分析。结果表明,激光淬火组织的回火稳定性明显提高。借助扫描电镜观察得知,回火稳定性提高的原因是激光淬火后奥氏体和淬火马氏体的合金浓度得到了提高。对GCr15钢轴承滚道表面激光淬火处理过程进行数值模拟,讨论激光工艺参数对轴承滚道表面激光淬火处理后相变硬化区宽度和深度的影响,得到与实验比较吻合的结果。模拟结果表明激光工艺参数固定时,激光扫描开始、结束阶段的温度场有明显的不同,导致沿激光扫描方向硬度分布的不均匀性和激光扫描开始、结束阶段淬硬层深度分布的不均匀性。探讨了改善硬化层深度分布均匀性和表面硬度均匀性的方法。将激光强化后的GCr15钢和常规热处理的GCr15钢试样在干摩擦条件下的磨损行为进行比较,以评估其磨损行为。激光强化后的GCr15钢相对于常规热处理的GCr15钢,由于其微观结构造成的高的强韧性而导致磨损率的提高。常规热处理试样和激光强化试样的磨损机制相同,同为磨料磨损和氧化磨损,同时拌有粘着磨损存在。激光强化后材料的热稳定性提高,这使激光相变硬化区具有较高的抗回火软化能力。在油润滑条件下比较激光强化后试样和常规热处理试样的耐磨性。激光强化GCr15钢的摩擦学特性略好一点,激光硬化层因其组织硬化、高硬度和高韧性而导致磨损率(10-6mg/N·m数量级)低于常规热处理GCr15钢。在稳定状态,激光强化后试样的摩擦系数与常规热处理试样的摩擦系数没有明显的区别。激光强化后试样和常规热处理试样两者的磨损机制基本相同,均为轻微磨料磨损和疲劳磨损。研究了不同激光功率对硬化后的最终显微组织、硬度和耐磨性的影响。激光参数变化引起显微组织发生变化,造成表面硬度值和磨损率较大的差异。激光功率增大时,激光硬化层表面未溶碳化物量减少,从而表面马氏体中含碳量增加,表面硬度增高。在干摩擦和油润滑两种条件下,激光功率越大,激光硬化层的抗磨损性性越好。在干摩擦磨损过程中激光表面改性层发现摩擦诱发的马氏体相变。对激光相变硬化处理后的GCr15钢试样进行三点弯曲试验,研究激光硬化层对钢件抗弯强度和挠度的影响。弯曲试验时材料的塑性用各试样的最大挠度来表示。结果表明,经激光硬化处理后钢件的抗弯强度和挠度明显下降,断裂机制发生改变,导致了激光硬化层塑性和韧性的下降。对常规热处理试样和激光表面处理试样断口也进行对比分析,常规热处理试样断裂后断裂表面均表现出韧性断裂后的韧窝特征,激光硬化层起裂区呈现脆性断裂特征,断裂方式主要为准解理和沿晶断裂。综上所述,轴承表面的激光相变硬化可以产生具有较多残余奥氏体、细小碳化物以及过饱和的隐晶马氏体组织,从而提高轴承滚道表面的硬度和残余压应力,并提高轴承套圈滚道表面的耐磨性能。激光表面强化技术用于轴承滚道表面处理,是一种全新尝试,有利于提高其接触疲劳性能,但因作用机制十分复杂,尚需进一步开展大量的试验工作与理论研究。更多还原

【Abstract】 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.更多还原