【摘要】：化學(xué)熱處理是重要的金屬材料表面強(qiáng)化工藝之一,在工業(yè)界有著非常廣泛的應(yīng)用。但是,傳統(tǒng)的化學(xué)熱處理工藝也存在著諸多缺點,比如滲碳工藝較高的淬火溫度導(dǎo)致工件變形、氮化工藝的表面硬化層承載能力不足、表面硬化層的后續(xù)熱處理強(qiáng)化工藝較少等。因此,開發(fā)一種適用材料廣泛、表面硬化層強(qiáng)度高、熱處理工件變形小、表面具有殘余壓應(yīng)力等眾多優(yōu)點的復(fù)合化學(xué)熱處理工藝就顯得尤為重要。本文采用低碳鋼、中碳合金鋼以及多元合金鑄鐵等材料,設(shè)計了一類復(fù)合化學(xué)熱處理工藝,即高溫以滲碳為主、低溫以滲氮為主,并在加熱鹽浴爐中輔以淬火、時效等工藝。而且,該工藝借鑒了淬火-分配-回火(Q-P-T)工藝的機(jī)理,目的是要進(jìn)一步提升表面硬化層的強(qiáng)度。在實驗過程中,通過金相顯微鏡和掃描電鏡觀察了樣品的微觀組織,通過顯微硬度計研究了樣品表面硬化層的硬度變化,通過X射線衍射儀表征樣品中的晶體結(jié)構(gòu),并結(jié)合文獻(xiàn)研究了表面硬化層的時效析出和強(qiáng)化機(jī)理。結(jié)果表明:(1)氮碳復(fù)合化學(xué)熱處理工藝獲得的滲層組織從金屬部件的外層到心部依次為化合物層→馬氏體+殘留奧氏體層→過渡層→心部組織。馬氏體+殘留奧氏體層在回火過程中發(fā)生復(fù)雜變化,也是在滲層組織中硬度最高的微觀組織,42CrMo材料的馬氏體+殘留奧氏體層最大硬度可達(dá)到將近900 HV;(2)調(diào)整氮碳復(fù)合化學(xué)熱處理工藝中氮碳共滲階段工藝參數(shù),可以獲得無化合物層的滲層組織,無化合物層滲層最外層是馬氏體+殘留奧氏體層,顯微硬度平均值約為650 HV,而有化合物層滲層最外層是化合物層,顯微硬度平均值約為500 HV;(3)時效強(qiáng)化機(jī)制主要發(fā)生在化合物層,化合物層主要成分是ε相,時效過程ε相中彌散析出γ′,起到強(qiáng)化作用,Q235材料表面硬化層中化合物層經(jīng)時效處理后顯微硬度增大超過100 HV;(4)多元合金鑄鐵淬火后,形成的馬氏體和殘留奧氏體在后續(xù)保溫過程中發(fā)生轉(zhuǎn)變包括:碳分配及碳化物析出(Q-P-T工藝兩個主要成分與結(jié)構(gòu)變化過程)、殘留奧氏體轉(zhuǎn)變等,所以樣品在420℃、570℃和620℃回火后,硬度先增大后減小,分別為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, 緯'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.
【學(xué)位授予單位】：江蘇大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TG156.82

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