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交流电场增强粉末法硼铝共渗研究

作者:润色论文网  来源:www.runselw.com  发布时间:2019/11/1 9:41:36  

摘要:粉末法硼铝共渗是一种强化钢铁材料表面性能的化学热处理方法,不仅能显著提高硬度和耐磨性,而且能增强工件的抗氧化性和耐蚀性,具有工艺简单,设备成本低等优点。但传统的粉末法所存在的处理温度高、渗扩时间长、能耗高等问题仍未解决。本论文采用自行设计的装置对45钢和20钢进行交流电场增强粉末法硼铝共渗,并与对应的常规硼铝共渗做对比,以金相显微观察、显微硬度测试和X射线衍射分析等手段,研究电场强度、渗剂配比等因素对试样共渗层组织、相结构、厚度、显微硬度分布和抗氧化性能的影响。

实验结果表明:交流电场增强硼铝共渗与常规硼铝共渗一样,当共渗剂中硼含量较高、铝含量较低时,共渗层以渗硼为主,共渗过程中消耗的主要是硼,铝的消耗量很小,同时,随着渗剂中铝含量的增加,渗层厚度呈现先增加后减少的趋势,适量进硼的渗入;当共渗剂中铝含量较高时,共渗层以渗铝为主,共渗过程中消耗的主要是铝,硼的消耗量很小,硼对铝的渗入起到强烈阻碍作用,且随着渗剂中硼含量的增加阻碍作用增大。交流电场对硼铝共渗有明显的促进作用,在800℃下施加2A的交流电场进行4h的交流电场增强硼铝共渗,渗层厚度相比常规硼铝硼铝的提高1倍以上。经过电场增强硼铝共渗的试样在700℃下长时间保温渗层厚度几乎无变化,渗层硬度不降低,抗氧化性显著提升,远优于常规硼铝共渗及电场渗硼的试样。此外,在渗剂中加入铁粉进行交流电场增强共渗不仅能降低电压,稳定电流,还能对含N、Si相的产生起到抑制作用。

分析认为:在硼铝共渗过程的化学反应阶段,部分Al粉作为还原剂能置换出活性硼原子,提高活性硼原子的数目,促进渗硼;在共渗的扩散阶段,铝含量较低时,铝原子渗入表面造成晶格畸变,为硼的渗入提供扩散通道;铝含量较高时,铝势高,铝的扩散能力大于硼,阻碍硼的渗入。交流电场的促渗作用有:提高渗剂的反应温度,促进活性原子的生成;增加试样的渗扩温度,促进原子在基体内的扩散;提高试样内部的空位浓度,为活性原子的扩散提供更多的通道。而无论硼含量是高还是低,硼在硼铝共渗中总是阻碍铝的渗入,这是因为活性硼原子的生成会消耗作为还原剂的铝,同时硼原子渗入基体表面后会抑制铝原子的渗入。在进行氧化实验时,硼铝共渗层中的铝原子会与氧气反应生成致密的Al2O3薄膜,阻止了氧原子向基体的扩散,起到保护试样的作用;而交流电场能促进渗层的生长,提高渗层中的铝原子数量,改善渗层中硼原子和铝原子的分布。因此交流电场硼铝共渗渗层在高温氧化条件下既能保持高硬度,又能保持渗层组织形态和厚度基本不变。

Powder-pack boron-aluminizing is a chemicalheat treatment for strengthening steels’ surface properties. Boro-aluminisingcan improve steels’ surface hardness, wear resistance, oxidation resistance andcorrosion resistance. The technology has some advantages, for example,simplicity in treating process, low investment and low cost. However, there aresome problems like high processing temperature, long processing time and highenergy consumptions in conventional powder-packing boro-aluminizing (CPBA). Inthe present work, alternating current field enhanced powder-packboron-aluminizing(ACFPBA) on 45 and 20 carbon steels were carried out withself-designed apparatus. As a comparison, the CPBA was also carried out.Metalloscope, microhardness testing and X-ray diffraction were employed forrevealing experimental results which phases, micro-structure, thickness,micro-hardness distribution, oxidation resistance of the treated layer. Theeffects of the ACF intensity and the pack agent composition on the treatedlayers were studied.

Experimental results indicate that boridepredominates diffusion layer when the content of B-Fe in pack agent is high ,regardless of ACFPBA or CPBA. The consumption of the process is mainly boron,and the consumption of aluminum is very low. With the increase of aluminumpowder in the pack media, the thickness of the boron-aluminizing layersincreases firstly and decreases later.  Namely,proper amount of aluminum promotes boron infiltration. When thecontent of aluminum powder in pack agent is high, the microstructure of thetreated layer is an aluminizing structure mainly, and aluminum is consumedheavily in boron-aluminizing, the consumption of boron is small. boron has astrong inhibiting effect on the infiltration of aluminum. With the increase ofB-Fe content, the hinderance of boron increases. The alternating current fieldplays an important role in promoting boron-aluminizing. Compared withconventional boron-aluminizing, applying AC electric field of 2A at 800℃ for 4hincrease the treated layer thickness by 2 times. The sample’s treated layerthickness of ACFPBA is hardly decreased, microhardness of the treated layernever lower. The oxidation resistance of samples treated by ACFPBA is greatlyimproved, which is much better than that of CPBA and alternating current fieldenhanced powder-pack boriding (ACFPB). In addition,usingpack agent with iron powder in ACFPBA can not only steady current by reducingvoltage, but also inhibit the production of silicide and nitride.

According to the analysis, in the chemicalreaction stage of the process of boron-aluminizing, some Al powder can be usedas reducing agent to replace the active boron atom, increase the number ofactive boron atom and promote boronizing. During the diffusion stage ofboron-aluminizing, when the content of aluminum powder is low, the penetration of aluminum atoms cause latticedistortion and providing the diffusion channel for boron atoms. When aluminumcontent is high, the concentration of active aluminum atom is  also high, and the diffusion of aluminum isgreater than boron, which can hinder boriding. Regardless of high or low  boron content, boron atom always hinderaluminium during boron-aluminizing. Because the production of activated boronatoms consume aluminum powder which is also used as a reducing agent.Meanwhile, the boron atom infiltrates the substrate and inhibits theinfiltration of the aluminum atoms. There are a lot of reasons whichalternating current field enhance pack boron-aluminizing. Firstly, the reactiontemperature of pack agent is increased, which promoting the production ofactive atom. Secondly, the diffusion temperature of the sample is increased,and the diffusion of atoms in the matrix was promoted. Definitively, ACFincrease the vacancy concentration inside the sample to provide more channelsfor the diffusion of active atoms. In the high temperature oxidation test, thealuminum

关键词:粉末法硼铝共渗;交流电场;共渗层组织;性能

Powder-pack boron-aluminizing;Alternatingcurrent field;The treated layer structure;Performance

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