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38CrMoAl钢和H13钢离子渗氮白亮层形成与调控研究

作者:润色论文网  来源:www.runselw.com  发布时间:2019/11/1 9:38:17  

摘要:离子渗氮是将零件置于真空容器中,在高压电场下渗氮气体介质发生辉光放电现象,电离成电子和离子,并以较高速度轰击零件表层,将表层加热使其温度升高,然后活性氮离子以扩散的形式渗入零件并逐渐形成渗氮层的过程。离子渗氮技术因其易操作且可控性强、渗速快、渗层质量好、零件渗氮后不易变形等优点而得到普遍应用。零件经常规工艺离子渗氮处理后,会在表面形成一层白亮层,白亮层具有较高的硬度,能提高零件的耐磨性、疲劳强度等。但也存在不足之处,白亮层脆性较大,对于承受冲击载荷的零件,如38CrMoAl 液压柱塞和H13热作模具等零部件当受到较大的冲击载荷时,容易开裂、脱落,导致零件的耐磨性、疲劳强度等大大降低,影响零件使用寿命。同时,脱落的白亮层碎片还可能影响整个机械系统,造成巨大的经济损失。因此,针对承受冲击载荷零件,如何避免离子渗氮白亮层的生成,只形成性能优良的扩散层是值得深入研究的。

本文选用调质态的38CrMoAl 钢和H13钢作为研究材料,系统的研究了温度、氮氢比对离子渗氮的影响,探讨分析了渗氮机理。采用金相显微镜、X射线衍射仪、维氏硬度计、磨损测试机等分析仪器,对不同工艺参数离子渗氮处理后的38CrMoAl钢和H13钢试样的截面组织、物相组成、硬度、脆性及耐磨性分别进行了测试与分析,并自行设计了热疲劳实验对H13钢热疲劳性进行了测试。

38CrMoAl钢离子渗氮研究表明,白亮层厚度随着渗氮温度的降低而逐渐变薄,当低温离子渗氮工艺为450 ℃×6 h(N2:H2=1:3),38CrMoAl钢表面在不生成白亮层的同时,能得到高表面硬度、高韧性和耐磨性。当渗氮温度超过450 ℃或保温时间超过8 h时,38CrMoAl钢表面开始逐渐形成白亮层。同时,在较高渗氮温度下,白亮层厚度随着氮氢比的降低而减薄,当低氮势离子渗氮工艺为540 ℃、N2:H2=1:5(6 h)时,38CrMoAl钢表层在不生成无白亮层的同时,能获得较好的表面性能。但是,低氮势离子渗氮的渗氮温度不能太高,当渗氮温度超过540 ℃时,即使将氮氢比降至1:6,试样表面仍会有白亮层形成;同时,当保温时间超过为6 h时,经540 ℃、N2:H2=1:5离子渗氮后,38CrMoAl钢表面也会形成白亮层。因此,450 ℃×6 h(N2:H2=1:3)和540 ℃、N2:H2=1:5(6 h)为38CrMoAl钢无白亮层离子渗氮的最佳工艺。

H13钢离子渗氮研究表明,当低温离子渗氮温度为470 ℃(N2:H2=1:3)时,经保温6 h和8 h后,H13钢表面均无白亮层生成,但保温8 h的试样具有更高的表面硬度、韧性、耐磨性和抗热疲劳性能。当渗氮温度超过470 ℃时,H13钢表面开始逐渐形成白亮层。同时,在较高渗氮温度下,当低氮势离子渗氮温度为540 ℃(6 h)时,当N2:H2=1:4和N2:H2=1:5时,H13钢表层均无白亮层生成,但经N2:H2=1:4离子渗氮的试样具有更好的表面性能。因此,H13钢无白亮层离子渗氮的最佳工艺为470 ℃×8 h(N2:H2=1:3)和540 ℃、N2:H2=1:4(6 h)。

Plasma nitriding is an important techniquefor strengthening metallic materials, which is widely used because of its easyoperation and controllability, high nitriding speed, good quality of nitridinglayer and small deformation of workpiece after nitriding. Plasma nitriding iscarried out in a vacuum container. Driven by a high voltage electric field, theelectrons and ions with high energy from the glow discharge will bombard thesurface of workpiece in a high speed and releases energy for heating thesubstrate to a required temperature. And the active nitrogen ions is adsorbedand diffused into the surface of the workpiece to form a nitriding layer.

The workpiece which is nitrided byconventional plasma nitriding will produce a white layer on its surface. Andthe white layer has high hardness, which can improve the wear resistance andfatigue resistance of the workpiece. Unfortunately, the white layer is brittle,which is ready to crack and peel off from the surface due to subject to highimpact during the work, and thus will reduce the wear resistance and fatigueresistance. At the same time, the peeling of the white layer debris may affectthe whole mechanical system and causing huge economic losses. Therefore, it isof significant value for impact workpiece to avoid the formation of white layerin plasma nitriding.

In this paper, the influence oftemperature, nitrogen hydrogen ratio on plasma nitriding were studiedsystematically by using the 38CrMoAl steel and H13 steel as the researchmaterials, and the mechanism of nitriding was discussed. Cross sectionalmicrostructure, phase constituents, hardness, surface brittleness, tribologicalproperties, wear resistance and thermal fatigue resistance of forming whitelayer for 38CrMoAl steel and H13 steel were analyzed by optical microcopy,X-ray diffraction, HXD-1000TMC micro-hardness tester, HT-1000 ball-on-diskfriction, wear tester and self-designed thermal fatigue test respectively. 

The results showed that the white layerthickness of 38CrMoAl steel decreases with the decrease of nitridingtemperature. And no white layer was formed on the sample surface whilenitriding at low temperature of 450 ℃ for 6 h (N2:H2=1:3), at the same timealso can get high surface hardness, high toughness and wear resistance. Whilethe white layer began to form when the nitriding temperature exceed 450 ℃ orholding time over 6 h. Meanwhile, the compound layer thickness also decreasedwith the decrease of nitrogen hydrogen ratio, and no compound layer was formedat low nitrogen hydrogen ratio of 1:5 at nitriding temperature of 540 ℃ for 6h. However, the temperature of low nitrogen hydrogen ratio plasma nitridingshould not be too high, when the temperature exceed 540 ℃, the white layer willform on the surface of the sample even if the nitrogen hydrogen ratio isreduced to 1:6. And when the holding time more than 6h, the white layer willform even though nitriding at 540 ℃, N2:H2=1:5. Therefore, 450 ℃×6 h(N2:H2=1:3) and 540 ℃、N2:H2=1:5 (6 h) are the bestplasma nitriding technology for 38CrMoAl steel without formation of whitelayer.

The low temperature and low nitrogenhydrogen ratio plasma nitriding were investigated for H13 steel to improve itscombined properties. It was found that no white layer was formed on the samplesurface while nitriding at low temperature of 470 ℃ for 6 h and 8h (N2:H2=1:3),but the sample nitriding at 470 ℃ for 8 h had higher surface hardness, toughnesswear resistance and thermal fatigue resistance. However, the white layer beganto form when the nitriding temperature over 470 ℃. And no white layer wasformed when nitriding at 540 ℃ (6 h) with nitrogen hydrogen ratio of 1:4 and1:5. But nitriding with nitrogen hydrogen ratio of 1:4 had better surfaceproperties. Therefore, the best plasma nitriding technology of H13 steelwithout formation of white layer are 470 ℃×8 h (N2:H2=1:3) and 540 ℃、N2:H2=1:4 (6 h).

关键词:38CrMoAl钢;H13钢;白亮层;低温离子渗氮;低氮氢比离子渗氮

38CrMoAl steel; H13 steel; white layer; lowtemperature plasma nitriding; low nitrogen hydrogen ratio plasma nitriding

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