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实验材料为退火态40Cr钢,使用平板材料的长宽高分别为:100mm×50mm×8mm,主要元素成分如表 1所示。为了减少材料表面污染物对实验精度的影响,实验前用粗砂纸打磨40Cr表面,去除表面氧化层及油污,并放入无水乙醇中在超声波清洗仪中清洗。
Table 1. Chemical composition of 40Cr steel (mass fraction)
elements C Si Mn Cr Ni P Fe content 0.0037~0.0044 0.0017~0.0037 0.0050~0.008 0.0080~0.0110 ≤0.003 ≤0.00035 balance 相变表面示意图如图 1所示。硬质相占比50%[13],未加工表面长宽分别为100mm×50mm,矩形条的面积为300mm2;矩形条分配至100mm×50mm表面,可以计算矩形条间距为3mm(制备软硬耦合表面),光斑面积为2.25πr2;光斑分配到长宽分别为100mm×50mm的表面。调节示教器的坐标位置完成实验;其中灰色表示基体40Cr钢,白色表示激光加工区域,实物图如图 2所示。
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所考虑激光相变硬化工艺参数变量包括激光功率和扫描速率,不变量包括光斑直径3mm和离焦量0mm。实验参数如表 2所示,实验中采用光纤激光器(nlIGHTCFL-2000)与机械手(ABB2600)及其它外围辅助设备协同完成,保护气使用工业氩气(Ar),相变形式如图 1a和图 1b示。
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磨损是机械运动过程产生不希望的材料移除,不会完全消除,但可以减小。织构理论中影响摩擦学性能主要因素包括:面积率、形状尺寸[14],所以通过激光相变工艺(获取硬质层)+软硬、形状耦合表面(织构理论)方案解决40Cr钢在服役期间磨损问题。
首先,采用线切割技术沿垂直于激光扫描方向切割下长宽高分别为10mm×10mm×8mm的试样,分别编号为1#、2#、3#、4#试样;其次,对试样进行镶嵌,并使用金相砂纸逐级打磨试样截面,打磨使用砂纸顺序依次为120μm,38μm,……,6.5μm,5μm,然后使用抛光机进行抛光;最后,使用体积分数为0.04硝酸酒精溶液对试样腐蚀10s,并放入无水乙醇在超声波清洗仪中清洗,金相试样制备完成。
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通过场发射扫描电镜(scanning electron microscopy, SEM; 设备型号:NovaTEMNanoSEM450)、X射线衍射仪(设备型号:D/max-2400)、显微硬度计(设备型号:HV-1000A-DC200)分别观察淬硬层组织、物相及完成硬度测试,摩擦磨损试验机(设备型号: MS-T3000)评估软硬耦合表面抗磨性能,采用超景深显微镜(设备型号:VX-6000)观察磨损后试样表面形貌。
选择性激光相变40Cr钢摩擦磨损性能研究
Study on the friction and wear properties of 40Cr steel by selective laser transformation
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摘要: 为了解决40Cr钢零部件表面磨损问题, 结合表面织构理论, 采用激光相变硬化工艺制备分布规则的不同形状(圆形、条状)软硬耦合表面。通过扫描电镜、X射线衍射、摩擦磨损实验和超景深显微镜分别分析相变区微观组织、物相, 评估表面抗磨损性能、磨损表面形貌。结果表明, 圆形相变区截面硬度高于条状, 平均介于(720±3)HV0.1, 且淬硬层较深; 相变后生成马氏体、Cr7C3、Fe7C3; 对比40Cr钢未处理表面与软硬耦合表面摩擦系数, 软硬耦合表面摩擦系数均低于0.5且波动小, 具备好的摩擦稳定性; 磨损表面损伤小, 源于磨损过程硬质相能够阻碍磨屑运动、减少表面损伤, 软相区可以缓存能量和碎屑, 软相区以梨削、粘着损伤为主, 硬相区以小点蚀坑为主, 900W圆形软硬耦合表面减磨耐磨效果最好; 塑韧好、表面规律分布、一定比例(50%)硬质相能够有效提升和改善材料工作表面抗磨损性能, 改善对磨副接触表面, 进而能够稳定摩擦系数。该研究可为改善40Cr零部件表面磨损提供参考。Abstract: To solve the surface wear problem of 40Cr steel parts, incorporated with the theory of surface texture, laser phase transformation hardening technology was applied to preparate various shapes (spot and striation) of hard-soft coupling surfaces with regular distribution. Scanning electron microscopy, X-ray diffraction, friction and wear machine and ultra-depth of field microscopy were adopted to analyze the microstructure, physical phase, anti-wear performance, and wear surface morphology of the phase transformation zone. the results show that the cross-section hardness of the spot phase transformation zone is higher than the striation, with the average of (720±3) HV0.1, and the hardening layer is deeper. Martensite, Cr7C3, and Fe7C3 are generated after phase transformation. By comparing the friction coefficient between the untreated surface and the hard-soft coupling surface of 40Cr steel, the friction coefficient of the hard-soft coupling surface is inferior to 0.5 and its fluctuation is smaller which demonstrates great friction stability. In addition, the wear surface damage of the hard-soft coupling surface is small, and the reason of this is that the hard phase could resist the deformation while the soft phase could release the stress concentration and the accumulation of plastic deformations. The soft phase field is mainly pear peeling and adhesion damage, and the hard phase area is mainly small pitting pits. The 900W spot soft-hard coupling surface with the first-rate anti-wear performance. Above all, the conclusion is that the hardening phase with great plastic toughness and a certain proportion (50%) of inerratic surface distribution can markedly increase and promote the anti-wear performance of the material working surface, improve the contact surface of the fraction pair as well as stabilize the friction coefficient, this study can provide a reference for improving the surface wear of 40Cr parts.
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Table 1. Chemical composition of 40Cr steel (mass fraction)
elements C Si Mn Cr Ni P Fe content 0.0037~0.0044 0.0017~0.0037 0.0050~0.008 0.0080~0.0110 ≤0.003 ≤0.00035 balance -
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