氮碳复合化学热处理表面硬化层的时效工艺及强化机理研究

发布时间：2019-04-22 07:47

【摘要】：化学热处理是重要的金属材料表面强化工艺之一,在工业界有着非常广泛的应用。但是,传统的化学热处理工艺也存在着诸多缺点,比如渗碳工艺较高的淬火温度导致工件变形、氮化工艺的表面硬化层承载能力不足、表面硬化层的后续热处理强化工艺较少等。因此,开发一种适用材料广泛、表面硬化层强度高、热处理工件变形小、表面具有残余压应力等众多优点的复合化学热处理工艺就显得尤为重要。本文采用低碳钢、中碳合金钢以及多元合金铸铁等材料,设计了一类复合化学热处理工艺,即高温以渗碳为主、低温以渗氮为主,并在加热盐浴炉中辅以淬火、时效等工艺。而且,该工艺借鉴了淬火-分配-回火(Q-P-T)工艺的机理,目的是要进一步提升表面硬化层的强度。在实验过程中,通过金相显微镜和扫描电镜观察了样品的微观组织,通过显微硬度计研究了样品表面硬化层的硬度变化,通过X射线衍射仪表征样品中的晶体结构,并结合文献研究了表面硬化层的时效析出和强化机理。结果表明:(1)氮碳复合化学热处理工艺获得的渗层组织从金属部件的外层到心部依次为化合物层→马氏体+残留奥氏体层→过渡层→心部组织。马氏体+残留奥氏体层在回火过程中发生复杂变化,也是在渗层组织中硬度最高的微观组织,42CrMo材料的马氏体+残留奥氏体层最大硬度可达到将近900 HV;(2)调整氮碳复合化学热处理工艺中氮碳共渗阶段工艺参数,可以获得无化合物层的渗层组织,无化合物层渗层最外层是马氏体+残留奥氏体层,显微硬度平均值约为650 HV,而有化合物层渗层最外层是化合物层,显微硬度平均值约为500 HV;(3)时效强化机制主要发生在化合物层,化合物层主要成分是ε相,时效过程ε相中弥散析出γ′,起到强化作用,Q235材料表面硬化层中化合物层经时效处理后显微硬度增大超过100 HV;(4)多元合金铸铁淬火后,形成的马氏体和残留奥氏体在后续保温过程中发生转变包括:碳分配及碳化物析出(Q-P-T工艺两个主要成分与结构变化过程)、残留奥氏体转变等,所以样品在420℃、570℃和620℃回火后,硬度先增大后减小,分别为43.5 HRC、44.3 HRC和38.3 HRC。
[Abstract]:Chemical heat treatment is one of the most important surface strengthening processes for metal materials and has been widely used in industry. However, the traditional chemical heat treatment process also has many shortcomings, for example, the high quenching temperature of Carburizing process leads to the deformation of the workpiece, and the load carrying capacity of the surface hardening layer of nitriding process is insufficient. The strengthening process of surface hardening layer by subsequent heat treatment is less. Therefore, it is particularly important to develop a composite chemical heat treatment process, which is suitable for a wide range of materials, high strength of surface hardening layer, small deformation of heat-treated workpiece and residual compressive stress on the surface. In this paper, low carbon steel, medium carbon alloy steel and multi-alloy cast iron are used to design a kind of composite chemical heat treatment process, that is, carburizing is the main process at high temperature, nitriding is the main method at low temperature, quenching and aging are used in the heating salt bath furnace. Moreover, the mechanism of quenching-distribution-tempering (Q-P-T) process is used for reference in order to further improve the strength of surface hardened layer. During the experiment, the microstructure of the sample was observed by means of metallographic microscope and scanning electron microscope, the hardness change of the hardening layer on the surface of the sample was studied by microhardness tester, and the crystal structure of the sample was characterized by X-ray diffractometer. The aging precipitation and strengthening mechanism of the surface hardening layer have been studied in combination with the literature. The results show that: (1) the microstructure of the nitriding layer obtained by nitrogen-carbon composite chemical heat treatment is the core of the transition layer of the martensite residual martensite layer from the outer part of the metal component to the core. The martensite residual austenite layer has complex changes during tempering and is also the microstructure with the highest hardness in the microstructure of the infiltration layer. The maximum hardness of martensite residual austenite layer in 42CrMo material can reach nearly 900 HV;. (2) the microstructure of carburizing layer without compound layer can be obtained by adjusting the parameters of nitrocarburizing stage in the process of nitrogen-carbon composite chemical heat treatment. The outermost layer without compound layer is martensitic residual austenite layer, and the average microhardness is about 650HV,. The outer layer of the compound layer is the compound layer, and the average microhardness is about 500 HV;. (3) the aging strengthening mechanism mainly occurs in the compound layer, the main composition of the compound layer is 蔚 phase, 纬'is dispersed out of the 蔚 phase during the aging process, which plays a strengthening role. After aging treatment, the microhardness of the compound layer in the surface hardening layer of Q235 material increases by more than 100 HV;. (4) the transformation of martensite and retained austenite after quenching of multi-alloy cast iron includes carbon distribution and carbides precipitation (two main composition and structure change processes of Q-P-T process). So after tempering at 420C, 570C and 620C, the hardness of the sample increases first and then decreases, which is 43.5 HRC,44.3 HRC and 38.3 HRC., respectively.
【学位授予单位】：江苏大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG156.82

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