【作者】 唐丽娜；

【导师】 闫牧夫；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2013， 博士

【摘要】 本文对淬火态30CrMnSiA和42CrMo钢进行了不同气氛、温度和时间的等离子体渗氮及有无稀土氮碳共渗工艺处理，利用金相组织观察（OM）、X射线衍射（XRD）、扫描电镜观察（SEM）及能谱分析（EDS）、透射电镜观察（TEM）等分析测试方法研究了各工艺参数对改性层的组织结构、力学性能、耐磨性及耐腐蚀性能的影响。30CrMnSiA和42CrMo钢淬火后组织为马氏体，等离子渗氮及氮碳共渗层均由化合物层和扩散层组成。在460oC以上进行等离子体氮碳共渗时稀土的添加能够改善渗层组织；560C添加0.5L/min稀土共渗时渗层组织由“三层”结构组成，即表面5μm化合物层、中间40μm耐蚀白层以及传统的扩散层。等离子体渗氮及氮碳共渗层主要由ε-Fe_2-3N和γ′-Fe4N(ε-Fe_2-3（N,C）和γ′-Fe4N)组成，处理温度升高，改性层中ε-Fe_2-3N(ε-Fe_2-3（N,C）)相对含量减少。等离子体渗氮时，400oC和560oC渗氮时渗层中出现含氮马氏体’-FeN相；460oC渗氮时，随着渗氮时间延长渗层中ε-Fe_2-3N相含量先增多后减少，长时间（16h）渗氮后γ′-Fe4N-（200）发生择优取向。等离子体氮碳共渗时，γ′-Fe4N相在（200）晶面的择优取向随着温度的升高和时间的延长而减弱。利用稀土添加量可以调控共渗层相组成，适当添加有助于ε-Fe_2-3（N,C）相形成，同时使γ′-Fe4N的（200）晶面择优取向现象增强；560oC共渗稀土添加量达到0.5L/min时表面形成Fe3C相。渗氮气氛对表面形貌影响不大，表面氮化物颗粒随着渗氮温度的升高聚集长大，表面粗糙度增大。氮碳共渗时，稀土添加提高了表面氮化物颗粒密度，促进氮化物聚集长大；560oC×8h添加0.5L/min稀土共渗时，表面形貌由花瓣片状组织转变为平均尺寸为100nm³00nm的细小短棒状组织（Fe3C+少量氮化物），表面粗糙度显著降低。而EDS结果表明稀土元素可以渗入到材料内部一定深度，且能够促进氮元素向内扩散。淬火态30CrMnSiA钢等离子体渗氮及氮碳共渗改性层内主要为纳米尺寸的氮化物以及少量纳米级含氮马氏体α’-FeN组织（50nm¹00nm）；而稀土的添加有助于形成小尺寸含氮碳的马氏体组织，并在一定范围内随着稀土添加量的增多纳米化程度提高。纳米晶形成机制是高过饱和淬火组织在氮、碳渗入过程中表层析出纳米级氮化物，并在固有合金元素和渗入的氮、碳、稀土等共同作用下引起的渗层内应力增大，在位错密度较高的地方使位错亚结构移动、合并或者重排形成亚晶或纳米晶，实现表面纳米化。淬火态30CrMnSiA硬度为HV500，经等离子体渗氮及氮碳共渗后表面硬度显著提高，最高可达HV1122.6，表面和心部硬度随渗氮温度升高而降低。从EDS检测和有效硬化层结果可知，稀土的添加可以促使氮、碳向内扩散，但不同温度共渗时稀土较佳添加量有所不同，500oC稀土添加0.05L/min时有效硬化层最厚，而560oC×8h共渗稀土添加0.3L/min时氮向内扩散最远，且有效硬化层最厚。淬火态30CrMnSiA和42CrMo钢经等离子体渗氮及氮碳共渗处理后，心部硬度同时发生变化，与常规调质后效果相当，实现表面改性与心部组织回火同时进行。拉伸试验结果表明，与调质态30CrMnSiA钢经等离子体氮碳共渗后相比，淬火态试样共渗处理后强度升高而韧性有所下降，而稀土共渗后强度不降低但韧性增加，表明30CrMnSiA钢“淬火+稀土共渗”处理可以代替传统的“调质+氮碳共渗”复合工艺，且能够满足性能指标要求。等离子体渗氮及氮碳共渗处理降低了材料的摩擦系数，而30CrMnSiA钢渗氮层、氮碳共渗层及稀土共渗层的体积磨损率较淬火态试样分别降低了74%、78%和82%，材料耐磨性显著提高。淬火态试样的磨损机制主要是以粘着磨损为主，局部有塑性变形，伴随着表面疲劳及氧化；试验载荷的大小并不改变磨损机制，只影响磨损程度；渗氮层及氮碳共渗层的主要磨损机制均为程度不一的粘着磨损，没有明显塑性变形；而稀土共渗表层主要为轻微的粘着磨损，稀土的添加进一步提高了材料的耐磨性。等离子体渗氮及氮碳共渗工艺提高了材料的耐腐蚀性能，使材料从严重的全面均匀腐蚀转变成点蚀。30CrMnSiA钢460C等离子体渗氮层耐蚀性较好，而42CrMo钢560C等离子体渗氮层耐蚀性较好。氮碳共渗层的耐蚀性较渗氮层进一步提高，42CrMo钢的腐蚀机理从渗氮层的层内严重间隙腐蚀转变为浅表层腐蚀。而稀土的添加可以拓展腐蚀的钝化区，进一步提高耐蚀性。更多还原

【Abstract】 In this paper, quenched30CrMnSiA and42CrMo steel were plasma nitridedand nitrocarburized in different atmosphere at different temperature for differenttime with and without rare earths （RE） addition. The effects of processingparameters on the microstructure, mechanical properties, wear and corrosionresistances of the surface layers of the treated steels were characterized by opticalmicroscopy （OM）, X-ray diffraction （XRD）, scanning electron microscopy （SEM）equipped with an energy dispersive X-ray analyzer （EDS） and transmission electronmicroscopy （TEM）.The microstructure of quenched30CrMnSiA and42CrMo steels issupersaturated carbon-bearing ’-Fe. The modified layers of both plasma nitridedand nitrocarburized samples consist of a compound layer and a diffusion layer.When the nitrocarburizing temperature is higher than460oC, the RE addition couldimprove the microstructure of surface layer. The surface layer nitrocarburized at560C with0.5L/min RE can be divided into three sub-layers （i.e. out compoundlayer of5μm, middle “white” layer of40μm, and inner diffusion zone）.The phase composition of the plasma nitrided or nitrocarburized layer mainlycontains ε-Fe_2-3N and γ′-Fe4N (or ε-Fe_2-3（N,C） and γ′-Fe4N). The relative content ofε-Fe_2-3N (ε-Fe_2-3（N,C）) decreases with increasing the treating temperature. Thereexists the diffraction peaks of ’-FeNwhen the nitriding is conducted at400oC and560oC. The relative content of ε-Fe_2-3N in surface layer nitrided at460oC increasesfirst and then decreases with prolonging the treating time, and the γ′-Fe4N possesses（200） preferred orientation when the treating time is16h. However, the （200）preferred orientation of γ′-Fe4N becomes weaker as the nitrocarburizing temperatureincreases and treating time prolongs. The RE flux could control the relative contentsof the phases produced in surface layer. The proper RE addition favours theformation of ε-Fe_2-3（N,C） and the （200） preferred orientation of γ′-Fe4N. Thecarbide Fe3C phase occurs at the surface employing a flux of0.5L/min RE during560oC nitrocarburizing.Nitriding atmosphere has little effect on the surface morphologies of nitridedsample. The nitrided granules assemble and grow up at the surface with increasingthe treating temperature, resulting in an increasing of the surface roughness. For thenitrocarburized samples, the incorporation of RE increases the density of surfacenitrides and helps to the assembling and growing of nitrides. The morphology ofFe3C formed at the surface is changed from petal-like type to fine stick-like one with a mean size of100nm³00nm when the sample is nitrocarburized at560oC for8hwith0.5L/min RE flux. EDS results show that the RE elements can diffuse into thesurface layer to a certain depth, which helps a diffusion of nitrogen element.TEM results show that the microstructure in the surface layer of nitrided andnitrocarburized30CrMnSiA steel are mainly comprised of nano-sized nitrides and asmall quantity of nano-sized martensite ’-FeNwith a scale of50nm¹00nm.Significantly, the involvement of RE is beneficial to the formation of nano-sized’-FeN, and under this experimental conditions, the more the RE addition intoatmosphere, the more degree of nanocrystallization phenomenon. The localnanocrystallization in surface layer is based on the following fact: firstly, thenano-sized nitrides precipitate in the surface layer of30CrMnSiA steel with highsupersaturated microstructure, and secondly, the internal stress in the modified layerincreases resulting from the synergistic effect of the penetration of nitrogen, carbonand big sized La atom and the solid soluted alloy elements. And the nano-crystallineof ’-FeNis formed by the moving, amalgamating and rearranging of the dislocationsubstructures.The hardness of the quenched30CrMnSiA steel is HV500, and the surfacehardness can be improved evidently, up to HV1122.6by nitriding with NH3. Bothsurface and core hardness decreases as the nitriding temperature increases. From theresults of EDS and effective hardening layer, the involvement of RE can help thediffusion of carbon and nitrogen in surface layer and there is an optimum amount ofRE addition into the plasma nitrocarburizing atmosphere. The samplenitrocarburized at500oC with0.05L/min RE has the thickest effective hardeninglayer. When the nitrocarburizing is carried out at560oC for8h, the thickest effectivehardening layer can be obtained with the optimum RE flux of0.3L/min. The corehardness value changed apparently when the quenched30CrMnSiA and42CrMosteels were plasma nitrided and nitrocarburized, and the value is almost equivalentto that of quenched and tempered steels. That is to say, the composite processes ofquenching+high temperature tempering and surface modification can be substitutedsuccessfully by quenching and plasma nitriding/nitrocarburizing for this type ofsteel.The tensile test results show that the strength of the nitrocarburized sample forthe quenched30CrMnSiA is higher than that of the nitrocarburized sample for thequenched and tempered one, but the toughness is lower. However, the quenched30CrMnSiA nitrocarburized with RE has the similar strength and higher toughnesscompared with the quenched and tempered one. It indicates that the quenching andnitrocarburizing treatment with RE could replace the conventional quenching+ tempering and nitrocarburizing one for the nitriding steels.Wear test results show that the wear resistance of the experimental steels can beincreased evidently by plasma nitriding and nitrocarburizing. The frictioncoefficients can be decreased markedly and the volume wear rates of the nitrided-,nitrocarburized-and RE nitrocarburized-layer of30CrMnSiA steel have beenreduced74%,78%and82%, respectively. The wear mechanism of the untreatedsample is mainly adhesive wear with local plastic deformation, as well as fatigueand oxidation on the worn surface. The changes of the load in wear test do notchange the wear mechanism of the samples, but change the wear degree. The mainwear mechanism of nitrided and nitrocarburized layers is adhesive wear withoutplastic deformation. The RE addition further improves the wear resistance of thesurface layer, with only a mild adhesive wear.The corrosion resistance of surface layer can be increased evidently by plasmanitriding and nitrocarburizing, by changing the corrosion mechanism from severelygeneral and uniform corrosion to pitting corrosion. The corrosion resistance is bestfor the surface layer of30CrMnSiA steel nitrided at460C and42CrMo steel treatedat560C, respectively. However, the nitrocarburized layer has higher corrosionresistance than the nitrided layer. The corrosion mechanism of the surface layer for42CrMo steel changes from severely crevice corrosion along the grains in thenitrided layer by nitriding to shallow surface corrosion in nitrocarburizing layer.Moreover, the involvement of RE could further improve the corrosion resistance byextending the passive regions and promoting the pitting corrosion potentials.更多还原