【作者】 陆斌锋；

【导师】 姚舜；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2010， 博士

【摘要】 摩擦磨损是材料损坏的主要形式之一,研究材料摩擦磨损机理,开发抗磨损材料或者通过在材料表面熔敷耐磨层,提高材料抗磨损性能,延长材料使用寿命,节约资源、降低能耗和环境污染,具有重要的工程应用价值和社会经济效应。本文采用真空电子束分束旋转扫描和CO2激光扫描两种方法,以Fe/Cr/C(Cr3C2/Fe)合金粉末为熔覆材料,在903钢表面原位合成碳化物表面复合层。通过对熔敷试样的表面复合层进行金相分析,扫描电镜分析和X射线衍射分析,以及显微硬度测试和室温干滑动磨损试验,对不同试样表面复合层的组织、硬度和耐磨损性能进行评价,并对表面复合层在室温干滑动条件下的磨损机理进行探讨;通过对电子束原位合成的表面复合层进行热处理,研究分析其组织和抗磨损性能的变化。通过对熔化基材的稀释率和扫描过程中元素损耗率计算可以得出表面复合层中铬、碳元素的含量范围,基于Fe-Cr-C三元相图分析,可以设计和控制预测表面熔敷复合层的基本组织结构。论文设计了四种配比的Fe/Cr/C合金粉末,考察不同成分的合金粉末对表面复合层组织和性能的影响。论文发现按照表面复合层中铬碳元素含量的高低,四种不同配比的Fe/Cr/C合金粉末制备的试样出现四种不同的组织特征。按含碳量从高到低次序分别是过共晶组织、共晶组织、亚共晶组织和马氏体组织。过共晶组织以粗大的初生碳化物为主要特征,韧性相为奥氏体和碳化物的共晶组织,显微硬度达到了基体材料的3.4倍;共晶组织的碳化物为细长粒状或杆状、呈菊蔟状分布,表面复合层的显微硬度达到了母材3倍;亚共晶组织由先析出奥氏体枝状晶和奥氏体与碳化物的共晶组织组成,碳化物的颗粒非常细小,少部分共晶碳化物相互连接形成网络状组织;当碳含量较低时,表面复合层碳化物含量很少,主要为马氏体组织。表面复合层的硬度主要与其所含的碳化物的量有关,碳化物含量越高,其硬度也越大;马氏体组织由于含有过饱和的碳原子,导致严重的晶格畸变,其硬度相比基材也有很大提高,大约为基材的2倍。此外,真空电子束扫描制备的表面复合层存在着浓度梯度和组织梯度,各种粉末配比制备的表面复合层呈现不同程度的梯度特征,这是由于电子束对熔池的搅拌作用的特点和903钢对合金稀释作用造成的。这种表面复合层纵向存在的成分和组织梯度对复合层与903钢基体结合和提高复合层表面耐磨性是有利的。采用激光扫描制备表面复合层时,由于加热速度快,时间短,熔炼时熔池中元素难以充分、均匀扩散,生成的表面复合层组织存在不均匀性,铬、碳含量较大的区域形成了奥氏体枝状晶和M7C3/γ-Fe共晶组织,共晶碳化物颗粒极为细小;铬、碳含量较少的区域,主要生成马氏体组织。激光扫描合成表面复合层时,扫描轨迹重叠的部分存在重熔区和二次加热区,二次加热区的组织主要为奥氏体组织,其硬度有所下降。电子束原位合成表面复合层的耐磨损性能跟其组织形貌密切相关。过共晶组织的表面复合层组织抗磨损性能最好,共晶组织和亚共晶组织表面复合层抗磨损性能次之,在转速为250r/min时,过共晶组织、共晶组织和亚共晶组织的表面复合层相对耐磨性分别达到903钢11.7、8.5和5.1倍。在低应力磨损状态下,过共晶和共晶组织表面复合层中大量的碳化物能够有效地阻止磨料表面微凸体刺入形成微观切削机制,而奥氏体相能有效地组织裂纹的生成和扩展,对碳化物相起到很好的支撑作用。亚共晶组织表面复合层中细小的共晶碳化物组织尺寸远大于表面微凸体能造成的犁沟尺寸,能够有效地抵御磨粒的刺入和划痕,同时又不容易剥落,其耐磨性也较好。根据存在硬质相材料的磨粒磨损模型,作者提出的含碳化物表面复合层在磨粒磨损机理下磨损体积的估算公式,理论计算与实验结果相吻合。马氏体组织表面复合层与GCr15钢球发生了严重的粘着磨损,而且马氏体基体中微量存在的碳化物,增加了裂纹敏感性,裂纹容易扩展,通过切削作用导致表面材料除去,限制了其耐磨性能,其相对耐磨性约为903钢的2.1倍。激光制备表面复合层的组织存在不均匀性,其磨损机理也比较复杂,磨粒磨损和粘着磨损各占一定的比例,摩擦系数的波动较大。对过共晶组织的表面复合层试样进行后热处理发现:在1000℃和900℃高温下,奥氏体软质相中析出更多的细粒状二次碳化物;初生碳化物和共晶碳化物没有发生明显转变,在该温度下能够稳定存在;试样经过空冷后软质相仍为奥氏体组织,奥氏体中铬元素的含量下降较明显。扩散到基体的铬元素和碳元素也大量增加,熔合线的组织由奥氏体转变为马氏体组织。过共晶组织表面复合层在800℃正火处理后,软质相奥氏体大部分转变为铁素体平衡相,由于铁素体溶碳量的下降,原软质相奥氏体中铬元素和碳元素的扩散析出了大量的二次碳化物。经过热处理后试样的硬度都略有下降,数值却更加平均。经过三种不同温度热处理后试样的耐磨性都有所提高。热处理对表面复合层耐磨损性能的影响主要通过其组织中二次碳化物的析出和晶界位错塞积的减少;晶界处位错塞积的减少比二次碳化物的析出对耐磨性具有更大的影响;经过1000℃热处理后的试样尽管其析出的二次碳化物数量较少,但其最大程度地减小了奥氏体与碳化物界面处的位错塞积,其相对耐磨性提高了16.5%;经过800℃热处理的试样析出的二次碳化物最多,其相对耐磨性提高了12.9%。论文基于金属基复合材料的理念利用电子束熔炼扫描系统和CO2激光束实现了在低碳钢表面原位合成(Cr,Fe)7C3碳化物表面复合层,为提高材料耐磨损性能的表面改性提供了新的途径。更多还原

【Abstract】 Friction and wear is the main form of material failure. The development of wear-resistant materials or fabrication of wear-resistant layer through surface cladding or surface modification has extended the service life of materials laygely. To improve the wear resistance of materials also contributes to conserve resources as well as reduce energy consumption and environmental pollution, which has an important engineering value and huge socio-economic effects.In this paper, a new type of‘four line rotation scanning’vacuum electron beam was proposed; using this electron beam scanning method and CO2 laser, carbides composite surface layer was sucessfully in situ synthesized with Fe/Cr/C (Cr3C2/Fe) alloy powder on 903 steel substrate; through optimization of scanning electron-beam parameters and adjustment of Fe/Cr/C alloy powder ratio, surface composite layers of the different properties were in situ synthesized on the low-alloy steel. The microsructure of each surface composite layer was analyzed with optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM); the hardness and wear-resistant performance of each surface composite layer was evaluated with microhardness tester and tribological tester. Dry sliding wear mechanism of the surface of composite layers was explored; surface composite layers in situ synthesized by the electron beam were also heat-treated to study the changes in its microstructure and the improvement of wear resistance. The results and main conclusion of the study are as follows:Four different kinds of Fe/Cr/C alloy powder were designed according to the Fe-Cr-C ternary phase diagram to study the influence of powder composition on the microstructure and wear resistance of surface composite layer. The microstructure of surface composite layers prepared by different ratio Fe/Cr/C powder mixtures vary significantly. Since the eutectic composition of Fe-Cr-C alloy depends mainly on the carbon content, carbon content has much more influence on the microstructure of surface composite layer compared with chromium content. In a certain range, increasing the carbon content, higher amounts of chromium carbides will be formed.The chromium and carbon content in the surface composite layer can be calculated by considering the dilution of the melting substrate and the powder loss during the scanning. Thus the microstructure of the synthesized layer is predictable using the Fe-Cr-C ternary phase diagram.Samples synthesized with four different ratios of Fe/Cr/C powder mixtures show different microstructure characteristics. With carbon content decreasing, the microstructure of the composite layers is hypereutectic microstructure, eutectic structure, hypoeutectic microstructure and martensite structure respectively. The main features of hypereutectic sample are large primary carbide embeded in ductileγ-Fe/M7C3 eutectic structure; the micro-hardness is 3.3-fold of the substrate. In eutectic structure, the carbides are granular or slender rod-shaped, showing the distribution of chrysanthemum-shaped cluster; the micro-hardness was 3-fold of the substrate. In hypoeutectic microstructure, dendritic austenite first precipated, then eutectic reaction take place to form austenite and carbides; carbide particles are very small; some are connected to form network structure; when the carbon content is too low, carbides does not appear in the surface layer, which is mainly martensite phase. The hardness of the surface composite layer is mainly related to carbide amount. Tthe higher carbide amount, the greater its hardness; the supersaturated carbon atom in martensite, result in the serious distortion of the lattice; its hardness also greatly improved compared to the substrate, reaches 2-fold of the substrate.When using CO2 laser beam to scan pre-placed powder mixture on 903 steel substrate, scanning time is too short for elements to diffuse evenly in melting pool because of the high power of laser beam. The surface composite layer shows some non-uniformity. Dendritic austenite andγ-Fe/M7C3 eutectic structure formed in the high chromium and carbon content region. As a result of fast cooling rate, the carbide particles are extremely fine; in the low chromium and carbon content region, the microstructure is mainly martensite because of the rapid cooling rate. There is overlap on the scanning track, which result in remelting zone and re-heating zone in and near the overlap region. The microstructure of re-heating zone is mainly austenite; its micro-hardness decreases compare with other region on the surface composite layer. Because of the non-uniformity of the microstructure, the wear mechanism is more complicated, abrasive wear and adhesive wear co-exist in the wearing process. The fluctuation of friction coefficient is relatively large.The wear-resistant performance of surface composite layer is directly related to its microstructure. The surface composite layer with excellent wear resistance is mainly due to the large amout of (Cr,Fe)7C3 distributed in the tough austenite phase. In low stress abrasion conditions, carides can effectively prevent the asperity penetrating the surface to form micro-cutting, and the austenitic phase can effectively prevent crack formation and expansion. Different types of carbides play different effects in wear mechnanism. In low stress abrasion conditions, the size of fine eutectic carbides is much larger than the furrows depth caused by asperity. It can effectively resist the scratches and piercing of the abrasive, which enables the surface compoaite layer have a good wear-resistant properties. As to martensite surface layer, although it can effectively resist micro-cutting of abrasive, serious adhesive wear takes place. The wear-resistant performance of martensite layer is worse than the carbides composite layer.Heat treatment of hypereutectic surface composite layer shows that the chromium and carbon atom in austenite are able to diffuse with activation energy at 1000℃and 900℃. More fine secondary carbides are precipitated. Coarse primary carbides undergo no obvious changes in the heat treatment at 1000℃and 900℃. After heat treatment the tough phase is still austenite though its chromium content reduced. The chromium and carbon diffused to fusion line and substrate significantly increased. The microstructure became martensite in the fusion line area. After normalizing at 800℃, the austenite phase in the surface composite layer transform into ferrite phase; a large amount of secondary carbide form due to diffusion of chromium and carbon atom. After heat treatment, the microstructure changed due to chromium and carbon diffusion, which reduce the lattice distortion and form more uniformly distributed secondary carbides. After heat treatment the hardness of all samples decreased slightly, but is more averaged.After heat treatment, the wear resistance of both samples improved. With large amount of fine carbides, whether the soft phase is ferrite or austenite seem to have no difference on the wear performace of the surface composite layer. And the friction coefficient is also very close. In a state of low stress abrasion, hard phase plays a major role in the process of abrasion resistance. The sample heat treatmed at 800℃precipates more amounts of secondary carbides. Its wear resistance increases accordingly. Though the sample heat treatmed at 1000℃contains less amounts of secondary carbides, the reduction of the dislocation accumulation along grain boundary make its wear resistance improve obviously.In summary, basing on the concept of metal matrix composites (MMC), M7C3 carbide surface composite layer is successfully synthesized using scanning electron beam and CO2 laser on low carbon steel. It provides a new way on fabrication of wear-resistant material.更多还原