【作者】 王冠；

【导师】 闫牧夫；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2014， 硕士

【摘要】 本文以新型高合金低碳轴承钢M50NiL钢的实际应用为背景。针对在该钢表面如何快速产生深度较深且力学性能良好的改性层问题进行研究。由于M50NiL钢不适用于在高温、高氮氢比、长时间的条件下进行等离子体氮碳共渗的问题，因此提出了循环氮碳共渗和渗碳加渗氮复合改性两种方法。对循环工艺的各项参数以及不同表面改性方法的复合对共渗层深度，相结构，硬度梯度，耐磨性的影响进行研究，并给出对应的机理分析。借助OM、SEM，XRD和EDS等手段对共渗层组织结构和渗入元素进行分析，并对改性层的显微硬度、耐磨性进行表征。研究结果表明当循环共渗温度区间较低且循环周次较少时，氮氢比对共渗层的深度影响不大。共渗层深度主要取决于在高温段的保温时间，在低温段的充氮量也是一个重要的因素。增加循环周次后的试验表明，在充入氮元素总量相近的情况下，保温温度区间较低的工艺下氮元素渗入的距离反而最远，对应的表面硬度最低。循环氮碳共渗后试样表面相结构中主要是γ’-Fe4N相和ε-Fe2-3N相，还有少量的α’-Fe相，在试样表面并不能看到白亮层，这是由于循环方法所采用的特殊工艺造成的。循环共渗方法试样表面上观察到的氮化物外形不规则且化合物之间存在很多空隙。经循环氮碳共渗改性后试样表面的硬度均可达到1100HV0.1以上，磨损率只有2.49×10-5mm3/Nm，对应的磨损机制主要为粘着磨损，还存在一定程度的氧化磨损和磨粒磨损。循环氮碳共渗加激光表面淬火的方法并不适用于对M50NiL钢进行表面改性，会造成试样表面和改性层的开裂，导致材料失效。气体渗碳加氮碳共渗方法对于M50NiL钢的表面改性取得了成功。复合改性试验表明后续的氮碳共渗可以使改性层从表面至内部400μm的范围内形成一个高硬度平台，此范围内的平均硬度值达到900HV0.1以上，对应金相组织照片中的分层区域，对于改性层深度以及试样表面的硬度影响不大，且复合改性后试样的表面硬度小于单纯进行循环氮碳共渗获得的试样的表面硬度。表面相结构中最主要的相为含氮，碳的α’-Fe相，还有一些低氮化合物相以及合金碳化物相，不过不会析出硬质的高氮化合物相。经过气体渗碳加氮碳共渗复合改性后试样表面的耐磨性同样有显著的提高，磨损率相比于未处理态有大幅度的下降，摩擦系数曲线较为平稳，对应的磨损机制主要为粘着磨损。更多还原

【Abstract】 The background of this study is the application of a kind of novel ball bearing steelwhich called M50NiL with high alloying element. The target of this study is how toprepare modified layer with enough depth and well mechanical property in the steel.Because this steel can not be treated with high temperature, high flow ratio of nitrogenand hydrogen and long time, so we propose two new methods, one of which is calledcyclic nitrocarburizing, the other is called carburize and nitride composite treatment. Inthis study, it discusses the effect of the technological parameter in cyclicnitrocarburizing and different composite treatments on the layer depth, phase structure,microhardness gradient and abrasive resistance. At the same time, it provides themechanism. With the help of OM, SEM, XRD and EDS, it analyzes the phase structureand elements which diffuse into the material in the modified layer and represents themicrohardness and abrasive resistance simultaneously.The outcome shows that when the temperature range is low and the cycle time isshort, the flow ratio of nitrogen and hydrogen has little effect on the layer depth. Thelayer depth mainly depends on the maintain time in high temperature stage, but the flowof nitrogen in low temperature stage could also be an important issue. After increase thecycle time of the experiments, the results indicate that on the condition that the totalamount of nitrogen is same, nitrogen could diffuse into the deepest distance in relativelylow temperature range, but the microhardness of the surface is lowest among all theexperiments. The phase of γ’-Fe4N, ε-Fe2-3N and a little α’-Fe consist the phasestructure of the modified layer. However, we can not found white layer on the surface. Itattributes to the special technological parameter of the cyclic method. The shape of thenitrides which found on the surface of the specimens is not regular, between which therehave a lot of gaps. The surface microhardness of the specimens which treated with thecyclic method can reach1100HV0.1. The wear rate of the specimens is only2.49*10-5mm3/Nm. The main worn mechanism is adhesive wear and accompany with alittle oxidation and abrasive wear.Cyclic nitrocarburizing accompany with laser quenching is not a proper surfacetreatment method for M50NiL steel. It will cause crack both on the surface and in themodified layer, then the material become invalid. However, gas carburizing accompanywith nitrocarburizing obtains well effect when apply on the M50NiL steel. A high microhardness platform which last400μm from surface is formed when the compositemethod is applied on the steel. The average microhardness of the platform is higher than900HV0.1. We could see distinct layers in the metallograph. But this composite methodhas little effect on the layer depth and surface microhardness. At meanwhile, the surfacemicrohardness which obtains from the composite method is lower than the one obtainsfrom the cyclic nitrocarburizing method. The main phase exists in the surface ismartensite that contain some nitrogen and carbon atoms, expect that, there still havesome low nitrogen compounds and alloy carbide. However, there is no high nitrogencompounds exist in the surface phase structure. The abrasive resistance of thespecimens also improves after the composite treatment. Comparing with the untreatedspecimens, the wear rate of the treated specimens decreases a lot. The curve whichreflects the frictional coefficient is stable. The main worn mechanism is adhesive wear.更多还原