铁基合金等离子体稀土氮碳共渗组织超细化与深层扩散机制

发布时间：2018-05-14 22:07

本文选题：稀土 + 氮碳共渗　；参考：《哈尔滨工业大学》2015年博士论文

【摘要】：M50Ni L钢是新一代的高强轴承钢,广泛应用于航空制造业等高端装备制造业。对于轴承而言,其失效形式主要为接触疲劳失效和磨损失效,因此要求其具有优异的表面性能。然而目前对于M50Ni L钢的表面改性技术却鲜有报道。如何在轴承钢表面获得超细化的组织且较深的改性层一直以来是化学热处理的研究热点。本文针对M50Ni L钢稀土氮碳共渗层的高强韧性的性能要求,将等离子体稀土氮碳共渗技术应用于M50Ni L钢的表面改性。基于循环相变超细化思想,设计了变温循环稀土氮碳共渗工艺,以期实现共渗组织超细化;同时研究了不同铁基合金低温稀土共渗过程中的稀土元素催渗机制。在不同相区对M50Ni L钢进行稀土氮碳共渗,研究温度、氮氢比及共渗时间对共渗层组织结构的影响。结果表明:M50Ni L钢a相区稀土氮碳共渗层无化合物层,共渗层微结构为粗大的板条马氏体。共渗层的相组成主要为a′N、g′-Fe4N及e-Fe2-3N,且相结构随温度变化较大,随氮氢比变化不明显。g相区稀土氮碳共渗层组织同样无化合物层,相结构受温度及氮氢比影响较大。相同氮氢比下,g′-Fe4N含量随温度升高而减少;当氮氢比为0.3:0.1 L/min时,650°C下共渗1h后,开始形成g-Fe N0.076,说明奥氏体化开始。以不同相区M50Ni L钢稀土氮碳共渗层相结构演变规律为基础,通过热力计算设计了变温循环共渗工艺,在共渗层表面上获得了纳米级的超细组织,而在共渗层内部30mm处局部得到了超细化的组织。其中,g?a相区(降温)循环共渗表面超细化组织由a′N+g′-Fe4N或单一的g′-Fe4N组成,g相区(升温)循环共渗表面超细化组织由a′N和非晶组成。M50Ni L钢变温循环稀土氮碳共渗超细化机制为:首先,g相区共渗过程中发生奥氏体化,形成g-Fe N0.076,其次在后续的降、升温循环过程中反复发生g-Fe N0.076#174;a′N+g′-Fe4N转变以及马氏体相变(g#174;a′N)。与此同时合金元素导致e和g′氮化物的稳定性下降,使其在共渗过程中不易长大,最终形成超细化共渗层组织。M50Ni L钢经不同相区稀土氮碳共渗后,硬度及耐磨性均大幅提高。磨损机制随磨损速度由氧化疲劳磨损逐渐转变为磨粒磨损和粘着磨损。其中g?a相区(降温)循环2次共渗层具有最优的耐磨性。而稀土La的加入能够抑制共渗层的脆性,提高共渗层的强韧性,增强共渗层的耐磨性。超细化共渗层耐磨性提高得益于其特殊的表面组织结构。细小的含氮马氏体+弥散析出的g′-Fe4N组织有利于提高共渗层的强韧性和耐磨性。稀土元素在化学热处理中被证实具有明显的催渗效果,然而其催渗机制尚未得到很好的揭示,尤其在等离子体低温稀土共渗中。本文的实验和热力学计算结果证实:等离子体低温稀土共渗过程中La与N之间的作用是相互吸引的。通过实验和理论计算提出等离子体低温稀土共渗过程中稀土催化机制:首先,稀土共渗过程中,在La和La Fe O3的共同作用下,使共渗表面变得粗糙,比表面积增大,有利于N的吸附;其次La对N的吸引提高了表面N的活度,与此同时通过La Fe O3对O的吸附,使得共渗表面N原子与La分离而向内深层扩散。基于对含合金元素La Fe O3晶体氧空位形成能的计算,解释了高合金钢不催渗的原因,同时提出了深层渗氮钢的设计思想,即:深层渗氮钢的成分选择上应含有适量的Ni元素,同时应尽量减少Cr、Mo、V等元素的含量。
[Abstract]:M50Ni L steel is a new generation of high strength bearing steel, which is widely used in high end equipment manufacturing industry such as aviation manufacturing. For bearings, its failure forms are mainly contact fatigue failure and wear failure, so it is required to have excellent surface properties. However, there are few reports on the surface modification technology of M50Ni L steel at present. In this paper, the plasma rare-earth nitrocarburizing technology is applied to the surface modification of the M50Ni L steel for the performance requirements of the high strength and toughness of the rare earth nitrogen carbon co permeable layer of M50Ni L steel. Based on the idea of cyclic phase transition superfine, this paper designs a variable temperature evidence-based process. The rare-earth nitrocarburizing process is expected to realize the superfining of the co permeable tissue. At the same time, the rare-earth element infiltration mechanism of different iron base alloys in the process of low temperature rare earth co permeation is studied. The effects of rare-earth Nitrocarburizing on M50Ni L steel in different phase regions, the influence of temperature, nitrogen and hydrogen ratio and the time of CO infiltration on the microstructure of the co permeable layer are studied. The results show that M50Ni L steel a There is no compound layer in the rare-earth nitrocarburizing layer, and the microstructure of the co impermeable layer is large lath martensite. The phase composition of the co impermeable layer is mainly a 'N, G' -Fe4N and e-Fe2-3N, and the phase structure changes with the temperature. The phase structure is affected by the temperature and the nitrogen hydrogen ratio with the change of the nitrogen and hydrogen ratio in the.G phase. With the same nitrogen and hydrogen ratio, the content of G '-Fe4N decreases with the increase of temperature. When the ratio of nitrogen to hydrogen is 0.3:0.1 L/min, the g-Fe N0.076 is formed after the co infiltration of 1H at 650 degree C, indicating the beginning of austenitizing. The process of changing the phase structure of the rare-earth nitrocarburizing layer in the different phase region M50Ni L steel is based on the thermal calculation and design of the temperature variable circulation co infiltration process. The ultrafine microstructure of nanoscale layer was obtained on the surface of the co permeable layer, and the Superfine Microstructure was obtained in the 30mm part of the copermeable layer. In the G? A phase region (cooling) cycle, the superfine tissue was composed of a 'N+g' -Fe4N or single G '-Fe4N, and the ultrafine microstructure of the G phase region (Sheng Wen) circulated surface was composed of a' N and amorphous.M50Ni. The superfine mechanism of rare-earth nitrocarburizing in L steel temperature cycle is: first, austenitizing in the process of G phase co infiltration and forming g-Fe N0.076, followed by subsequent drop, and repeated g-Fe N0.076#174, a 'N+g' -Fe4N transformation and martensitic phase transition (g#174; a 'N). The qualitative decline makes it not easy to grow up in the process of CO permeation, and eventually forms the superfine copermeable layer of.M50Ni L steel. The hardness and wear resistance of the steel are greatly improved after different phases of rare-earth nitrocarburizing. The wear mechanism is gradually changed from oxidation fatigue wear to abrasive wear and adhesion wear with the wear rate. The 2 times co infiltration of G? A phase region (cooling) cycle The addition of rare earth La can inhibit the brittleness of the co permeable layer, improve the strength and toughness of the co permeable layer and enhance the wear resistance of the co permeable layer. The wear resistance of the ultra-fine co permeable layer is improved by its special surface structure. The microstructure of G '-Fe4N from the fine nitrogen martensite + diffusion precipitation is beneficial to the enhancement of the strength and toughness of the co permeable layer. The rare-earth element has been proved to have obvious effect in the chemical heat treatment, but its mechanism has not been well revealed, especially in the plasma low temperature rare earth co permeation. The experiment and thermodynamic calculation of this paper confirm that the effect of La and N in the process of plasma low temperature rare earth co permeation is mutually attractive. The rare-earth catalytic mechanism in the process of plasma low temperature rare earth co permeation is proposed by experiments and theoretical calculations. First, in the process of rare earth co osmotic, the co permeation surface becomes rough, the surface area is increased and the adsorption of N is increased under the joint action of La and La Fe O3. Secondly, the absorption of La to N improves the activity of the surface N, while La Fe O3 is used at the same time. The adsorption of O makes the N atoms of the co permeable surface separate from La and diffuse into the inner deep. Based on the calculation of the formation energy of the oxygen vacancy of the alloy element La Fe O3 crystal, the reason for the non urging of the high alloy steel is explained. At the same time, the design idea of the deep nitriding steel is put forward, that is, the composition of the deep nitriding steel should contain a proper amount of Ni elements, and at the same time, we should do the best. The content of Cr, Mo, V and other elements was reduced.

【学位授予单位】：哈尔滨工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG156.82

【参考文献】