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交流电场增强45钢中温粉末法硼铝共渗特性

  • 谢飞1,2

    机 构：

    1. 常州大学 材料科学与工程学院, 常州 213164

    2. 常州大学 江苏省材料表面科学与技术重点实验室, 常州 213164

    ×
  • ， 王晓娟1

    机 构：

    1. 常州大学 材料科学与工程学院, 常州 213164

    ×
  • ， 潘建伟3

    机 构：

    3. 常州大学 怀德学院, 常州 213016

    ×

1. 常州大学 材料科学与工程学院, 常州 213164；
2. 常州大学 江苏省材料表面科学与技术重点实验室, 常州 213164；
3. 常州大学 怀德学院, 常州 213016；

Characterization of Alternating Current Field Enhanced Pack Boron-Aluminizing at a Medium Temperature for 45 Steel

  • XIE Fei1,2

    Affiliation：

    1. School of Materials Science and Engineering, Changzhou University, Changzhou 213164 , China

    2. Key Laboratory of Materials Surface Engineering of Jiangsu Province, Changzhou University, Changzhou 213164 , China

    ×
  • ， WANG Xiaojuan1

    Affiliation：

    1. School of Materials Science and Engineering, Changzhou University, Changzhou 213164 , China

    ×
  • ， PAN Jianwei3

    Affiliation：

    3. Huaide College, Changzhou University, Changzhou 213016 , China

    ×

1. School of Materials Science and Engineering, Changzhou University, Changzhou 213164 , China；
2. Key Laboratory of Materials Surface Engineering of Jiangsu Province, Changzhou University, Changzhou 213164 , China；
3. Huaide College, Changzhou University, Changzhou 213016 , China；

通讯作者:

谢飞(1964—),男(汉),教授,博士;研究方向:金属材料热处理、表面工程;E-mail:xiefei@cczu.edu.cn

中图分类号:TG156.8

文献标识码:A

文章编号:1007-9289(2020)04-0121-07

DOI:10.11933/j.issn.1007-9289.20200617003

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闫梦龙,谢飞,潘建伟.交流电场对45钢粉末法渗铝的影响[J].材料热处理学报,2015,36(2):203-208.YAN M L,XIE F,PAN J W.Effects of alternating current field on pack aluminizing of 45 steel [J].Transactions of Materials and Heat Treatment,2015,32(6):203-208(in Chinese).
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目录contents

    摘要

    为克服传统粉末法硼铝共渗存在的处理温度高、渗速慢以及渗剂利用率低的缺点,研究以交流电场加速中碳 45 钢中温粉末法硼铝共渗。 分别采用光学显微镜、X 射线衍射仪、扫描电镜能谱仪和显微硬度计等观测共渗层厚度、组织、相结构、成分分布及硬度分布,研究电场对共渗的影响,分析硼、铝的交互作用。 研究发现:交流电场对硼铝共渗的促进程度与渗剂配比有关;当渗剂中铝粉的质量分数低于 3%时,共渗以渗硼为主,共渗层组织主要为表层含铝的锯齿状硼化物,铝促进硼化物生长,在施加交流电场时表现更为显著,当电场电流为 2 A 时,铝的促渗作用在铝粉的质量分数为 2%时达到峰值,渗层厚度约为相应不加铝粉的 3 倍,而当电场电流增至 6 A 时,渗层厚度约为相应不加铝粉的 10 倍, 增加电场电流会增加渗层次表层的硬度;当渗剂中铝粉的质量分数≥3%时,共渗以渗铝为主,渗层组织表现为渗铝特征,但渗层厚度远比相应单一渗铝的薄,交流电场的促渗作用不显著。

    Abstract

    Alternating current field (ACF) enhanced pack boron-aluminizing (PBA) at a medium temperature for medium carbon 45 steel was studied to overcome the drawbacks of high treating temperature, slow diffusiong speed and low utilization of pack media in conventional PBA. The thickness, microstructure, phases, element distribution and hardness distribution of the coating were revealed with optical microscope, X-ray diffractormeter, energy dispersive specmeter in a scanning electron microscope and micro-hardness tester, respectively. The effects of the ACF on PBA were studied, and the interaction of boron with aluminium during the PBA was analyzed. Results show that the promotion of ACF on PBA is related to the ratio of the pack media. When the content of Al powder in the pack media is less than 3%, boriding is predominant in coating. Boride with sawtooth morphology is predominant in the PBA case, in the surface layer of which Al dissolves. The Al powder in the pack media promotes the PBA, especially when an ACF is applied. When a 2 A ACF current is applied, the promotion to PAB is maximized by employing 2% Al. The coating thickness is about three times as thick as the case obtained without employing Al powder. While when the ACF current is increased to 6 A, the coating thickness is about ten times as thick as the case obtained without employing Al powder. The hardness of the subsurface is increased with the increase of the ACF current. When the content of Al powder in the pack media is more than or equal to 3%, aluminizing is predominant in the PBA. The PBA case shows a feature of aluminizing, while its thickness is much thinner than that by corresponding single aluminizing. And the enhancing effect of ACF for producing such case is not remarkable.

  • 0 引言

  • 普通碳钢经渗硼后表面硬度可以达到1400~2300 HV,比经渗碳、渗氮等处理后的碳钢具有更高的表面硬度和耐磨性。渗硼虽然也能在一定程度上提高碳钢的耐蚀性和抗高温氧化性等,但逊于渗铝,渗铝虽然可以显著提高钢的耐蚀性与抗高温氧化性,但在提高硬度和耐磨性方面则远不如渗硼[1]。硼铝共渗可在一定程度上弥补单一渗硼和渗铝之不足,获得较单一渗硼及单一渗铝渗层更佳的综合性能[2-5]。在各种硼铝共渗方法中,粉末法硼铝共渗具有设备简单、操作方便、渗层厚度均匀且易于控制等优点。但与其他多种粉末法渗扩相似,现行传统粉末法硼铝共渗目前尚存在渗扩处理温度高、渗扩时间长和中低温渗扩时渗速慢等缺点。

  • 为提高粉末法渗扩速度、降低渗扩温度,研究人员开展了许多研究。有的通过在渗剂中加入诸如稀土等催渗剂促进渗剂反应、活化渗扩件表面[6-8]; 有的利用微波提高需渗扩原子的活性及通过微波与已形成渗层的作用使工件表面和界面得到充分活化,加速渗扩[9-10];有的利用高强度磁场对渗扩中的化学反应和扩散的强化作用来促进渗扩[11];有的在渗扩前或渗扩过程中对渗扩件表面进行一定程度的塑性变形来细化表面晶粒、增加位错和空位密度以增加渗扩通道,从而降低渗扩温度,提高渗扩速度[12-14]。研究发现,通过在渗罐中设置一对电极,对渗剂和渗扩件施加直流或交流电场能显著促进粉末法渗硼、渗铝、渗铬、渗硅等[15-21];合理运用交流电场,充分利用其电热和电磁效应,不仅能够加快渗扩速度,提高渗扩效率,还能更经济、便捷地优化渗硼、渗铝层的相结构,改善渗层性能。

  • 为拓展交流电场增强粉末法渗扩技术,在前期研究交流电场增强粉末法单一渗硼、渗铝的基础上[18,20-21],文中以中碳45 钢为处理对象,在粉末法硼铝共渗过程中对处理试样和共渗剂施加50 Hz工频交流电场,研究电场电流和渗剂组份等对中温硼铝共渗渗层生长、组织、相结构、成分分布及硬度分布等的影响规律,分析共渗中硼、铝两种元素对共渗的交互作用,以及交流电场对粉末法硼铝共渗的作用机理。

  • 1 试验方法

  • 试样以热轧态45 钢制作,尺寸为 Φ10 mm× 5 mm,共渗前表面经80 μm(180 目)、38 μm(400 目)和13 μm(1000 目)水砂纸依次打磨。渗剂组成(质量分数)为:5%KBF4 +0%~6%Al粉+1%NH4Cl+1%木炭+使用过1 次的旧渗硼剂(余量), 旧渗硼剂中的原供硼剂为硼铁。渗扩处理在自行研究设计的交流电场增强粉末法渗扩处理(以下简称电场渗扩,ACFEPT)装置上进行,装置基本结构特点与渗扩工艺过程参见文献[18],渗扩保温温度为750℃,保温时间4 h,其他参数详见试验结果中的相应说明。作为对比,同时进行相应的常规粉末法渗扩处理(以下简称常规渗扩,CPT)。

  • 采用金相显微镜观察分析渗扩试样截面组织并测量渗扩层厚度,以试样表面至渗层与基体交界处距离的平均值来确定渗层厚度。以显微硬度计(载荷50 g,加载时间15 s)测量渗层硬度; 采用X射线衍射仪(XRD,Cu靶,Kα,100 mA)分析渗层表层物相; 以扫描电镜( SEM) 及能谱仪(EDS)分析渗层中Al和Fe的分布。由于硼的原子序数较小,当其含量较低时,EDS难以较准确地检测其含量,故文中未分析硼在共渗层中的分布。

  • 2 试验结果

  • 2.1 渗层厚度

  • 图1 给出了采用不同铝粉含量(质量分数,下同)渗剂得到的电场渗扩和常规渗扩渗层的厚度。在750℃进行单一渗硼,渗层很薄(约20 μm),施加了2 A的交流电场对渗层厚度(约23 μm)增加也影响不大;加入铝粉,随铝粉含量增加,常规共渗的渗层厚度先微增后又减少,铝粉含量高于3%后,厚度减少较多;施加交流电场后,渗层厚度随铝粉含量增加也呈现先增后减的规律,但渗层厚度增加显著,在铝粉含量为2%时达峰值,约为不加铝粉常规渗硼渗层厚度的3 倍,之后随铝粉含量进一步增加,共渗层厚度迅速降低,在3%铝粉含量时达最低值,随后随铝粉含量进一步增加,共渗层厚度又缓慢增加,与常规共渗相比,厚度差别不大。

  • 图1 常规与交流电场增强硼铝共渗层厚度与渗剂铝含量关系

  • Fig.1 Variation in coating thickness of CPT and ACFEPT samples as a function of Al content in pack media

  • 当渗硼剂中铝粉含量为2%时,交流电场电流对共渗层厚度的影响规律如图2 所示。由图可见,交流电场对共渗层生长速度影响巨大,在6 A的交流电场作用下,经4 h渗扩就获得约200 μm渗层,而相应常规渗扩渗层的厚度仅约23 μm。

  • 2.2 显微组织

  • 光学金相观察发现无论电场共渗还是常规共渗,其渗层组织随渗剂铝粉含量由0%增至6%,组织形态总体由典型的锯齿状渗硼层特征变为较为平直的渗铝层组织特征。但电场共渗组织形态还一定程度受电场电流高低的影响。当施加电流为2 A的交流电场时,若铝粉含量小于1.5%,渗层呈现锯齿状单相Fe2B特征,但当渗剂中铝粉含量在1.5%到3%之间时,渗层表现为双相FeB+Fe2B特征。渗剂中铝粉含量为2%时,当电场电流增至4 A,共渗层与心部基体间出现在中碳钢深层渗硼中常见的渗硼过渡区,过渡区充满珠光体,这是由于碳不溶于硼化物中,而渗入基体中的硼又具有抑制铁素体析出的作用, 所以在硼与由表层排出的碳的联合作用下,在渗硼缓冷中形成伪共析组织[1, 22]。图3 给出了几种不同试样的渗层截面组织。

  • 图2 共渗层厚度与交流电场电流关系

  • Fig.2 Relationship of ACFEPT coating thickness and ACF curren

  • 图3 不同渗扩试样截面组织

  • Fig.3 Cross-sectional microstructures of differently treated samples

  • 2.3 相结构

  • 图4(a)和图4(b)分别给出了不同铝含量渗剂常规渗扩和电场渗扩处理试样表层的X射线衍射分析结果,虽然两类渗扩渗层表层相均总体以Fe2B → FeB+Fe2B→ Al2Fe+AlFe3 顺序演变, 但也存在差异。常规共渗在铝粉含量达1%时渗层表层即有少量FeB出现,由于量很少,光学显微观察难以分辨(图3(g)); 常规共渗在铝粉含量达2%时渗层表层除Fe2B外还探测到Fe3 Si。电场共渗在铝粉含量1%时渗层表层无FeB,但经铝含量2%的渗剂处理后表层则完全由单相FeB构成,说明此时FeB层已很厚,在实验衍射条件下已无法探测到其下的Fe2B;经铝含量为6%渗剂处理的电场共渗试样表层的Al2Fe衍射峰的相对强度高于相应常规共渗的。电场共渗与常规共渗表层相的差异应该是交流电场对共渗过程中渗剂反应的促进和对渗层内扩散的促进两方面综合作用造成的。

  • 图4(c) 给出了电场电流对经含2%铝粉渗剂处理试样表层相结构的影响规律。根据FeB和Fe2B对应衍射峰相对强度的变化,可以看出当电场电流由0 A增至6 A时,表层FeB先逐渐增加、变厚,当电场电流至4 A时,表层FeB已厚至足以阻止X射线透入探测内部Fe2B,Fe2B峰完全消失,但当电场电流增至6 A时,FeB相又大幅度减少。

  • 图4 不同工艺渗扩试样渗层表面X射线衍射图谱

  • Fig.4 X-ray diffraction patterns from surface of samples treated by different processes

  • 2.4 渗层元素分布

  • 图5 给出了以EDS线扫描检测出的Al和Fe在几种不同工艺渗扩渗层中的分布情况。由图5 可知:对常规共渗或电场电流2 A的电场共渗(图5( a) 和图5( b)),试样表面的Al浓度很高,进入渗层后其浓度迅速降低;增加渗剂中的铝粉含量,试样表层和次表层的Al浓度均很高( 图5( c));增大电场电流( I =6 A),Al更多更深地渗入渗层(图5( d))。与铝浓度分布相对应,渗层中铁浓度分布呈现出彼增此减或彼减此增的对应关系(图5( d)~图5( h))。 EDS结果说明,图4 给出的共渗样品的FeB和Fe2B实际应为( Fe1-xAl x) B和( Fe1-xAl x) 2B。

  • 2.5 渗层硬度分布

  • 图6 给出了不同工艺处理后渗层的硬度分布曲线。在渗剂中铝粉含量低于3%时,渗层表层硬度低于次表层硬度,硬度最高值出现在次表面,铝粉含量达2%时, 硬度峰值最高, 高达1800 HV0.05 以上。这一方面与表层存在微孔缺陷、次表层组织致密有关;另一方面应该是由于表层硼化物为部分铁被铝置换的( Fe1-xAl x) B/(Fe1-xAl x)2B,而非FeB/Fe2B [23-24]。经含6%铝粉渗剂处理的渗层表层硬度仅约300 HV0.05,与XRD分析结果一致,表明其非硼化物。在渗剂中铝粉含量为2%时,增加电场电流显著增加渗层深度,提高内侧渗层硬度,但当电场电流高于2 A后,对渗层硬度峰值影响不大。

  • 图5 不同工艺渗扩试样铝和铁含量沿渗层深度分布的能谱分析

  • Fig.5 Depth distribution of aluminium and iron detected with EDS of differently treated samples

  • 图6 不同处理试样渗层层深的显微硬度分布和交流电场频率对渗硼层硬度分布的影响

  • Fig.6 Microhardness distribution of coating depth of differently treated samples and influence of alternating current frequency on microhardness of boronizing layer

  • 3 分析与讨论

  • 交流电场在粉末法单一渗硼、渗铝中已被证实具有优异的促进渗扩的作用,交流电场的促渗作用一般认为是通过对渗剂和渗扩件两方面的综合作用实现的[18-21,27]。首先,交流电场通过直接作用于渗剂使其发热,通过交流电场的电磁效应直接作用于渗剂使渗剂分子振动增强,两方面作用均加剧了渗剂反应,克服常规粉末法渗扩中渗剂单纯依靠外热产生活性硼、铝原子及含硼、铝的活性基团之不足,促进活性硼、铝原子及含硼、铝的活性基团的产生;同时,交流电场的电磁搅拌作用与热作用对渗剂中活性硼、铝原子及含硼、铝的活性基团向渗扩件表面的扩散也有一定的促进作用。其次,正如通过金属的直流电可以提高金属内部空位的浓度及其迁移能力[25-26],交流电场在位于电场中的试样内产生感生交流电, 感生交流电同样应具有提高试样内部空位浓度及其迁移能力的作用,从而降低渗入硼、铝在钢中的扩散激活能,加速渗入的硼、铝原子向基体内扩散[27]。在对渗扩速度的影响方面,交流电场的热作用效果远弱于交流电场的电磁作用效果[27]。交流电场加速活性硼、铝原子及含硼、铝的活性基团的产生能够提高渗扩样表面硼和铝原子的浓度,当浓度高至一定程度时形成富硼相FeB、富铝相Al2Fe,而交流电场导致的试样基体内扩散的增强,则又有降低渗扩元素在表层富集的作用,二者综合,导致了图4(c)所示结果。

  • 以上分析的交流电场对硼、铝原子向基体内扩散的促进作用主要受控于交流电场的强弱。文中研究以交流电场电流来衡量电场的强弱。随电场电流增加,交流电场的热作用和电磁作用均增加,促渗作用增强,共渗层厚度因此增加(图2)。

  • 但是,图1 所示结果表明在硼铝共渗中,交流电场对共渗层生长的作用与渗剂配比有非常大的关系,渗剂中不含铝粉采用旧渗硼剂作为硼源进行单一渗硼时,即使施加交流电场,对渗扩的促进作用有限,但渗剂中加入适量铝粉进行以渗硼为主的交流电场增强共渗,硼的渗入速度大幅度增加。对此结果,用交流电场在粉末法渗扩单一元素中的作用机制难以说明,必须考虑硼铝共渗中硼、铝两种元素的相互作用。

  • 研究人员对于铝是否溶于硼化物层中存在不同认识[23-24,28-29]。图5 的结果证实铝溶于硼化物,与文献[23]~文献[24]的结果一致。图1 结果表明在以渗硼为主的硼铝共渗中,适量铝粉能够促进渗硼层生长,并且在交流电场作用下更为显著,这应该是由铝粉促渗与交流电场促渗的综合叠加所致。铝粉的促渗作用应该来自于铝粉对渗剂中硼铁的高效还原作用(对于以硼铁作为供硼剂的渗剂,具体反应机制尚待进一步研究),使渗硼剂更多更快地释放硼,增加渗箱气氛中的硼势,在交流电场的综合作用下,硼向试样表面与内部的扩散得到大幅度促进,渗层显著增厚。当渗剂中铝粉加入量合适(例如2%),得到足够高的硼势时,不仅渗层最厚,渗层表层还出现富硼相( Fe1-xAl x) B(图3( c)、图4( b) 和图4(c)),富硼相随电场电流增加呈现先增后减规律(图4(c)),这是因为当电场电流增加,渗剂反应增强,形成更多活性硼原子,由渗剂扩散进渗层, 渗层表层硼浓度增加,富硼相增加;但电场电流增至一定程度后,交流电场又进一步增加了硼由表层向基体的内扩散,使次表层硼浓度降低。对于相应的常规共渗,由于缺乏交流电场对渗扩反应、对硼向试样表面与内部扩散的促进,铝对硼化物生长的促进作用极为有限。硼铝共渗中,部分铝也吸附于试样表面并扩散进渗层,扩散进入的铝在未达到形成含铝金属间化合物时,就如同含铝钢中的铝会阻碍硼的渗入[28-29],对硼化物的形成、生长有负面影响;当渗剂中铝粉含量进一步增加、表面吸附与渗入铝增加时,阻碍也进一步增强。共渗过程中铝粉的促渗与阻碍这两方面的综合作用不仅导致了图1 所示的渗硼层厚度与铝粉含量间的关系,还相应导致了图4( b) 所示的渗层相与铝粉含量间的关系。铝粉同样能将硅从填充剂碳化硅中还原出来,但是否能还原足量Si与Fe反应形成Fe3 Si,还受渗扩气氛中活性B、Al浓度等的影响。

  • 当渗剂中铝粉含量≥3%时,硼已经难以渗入,共渗转为以渗铝为主,但渗层厚度不仅小于前面以渗硼为主的共渗层厚度,更是远低于以相同铝含量渗剂在相同温度、相同电场电流进行的单一渗铝的渗铝层厚度[30]。这表明在以渗铝为主的硼铝共渗时,硼具有阻碍铝渗入的作用,目前对于硼产生阻碍作用的机制尚不清楚,需进一步研究。

  • 4 结论

  • (1) 45 钢在750℃ 进行交流电场增强硼铝共渗,当渗剂中铝粉含量低于3%时,渗层组织主要为表层含铝的锯齿状硼化物;当渗剂中铝粉含量≥3%时,渗层表现为渗铝组织特征;随铝粉含量增加, 渗层表层相以Fe2B/( Fe1-xAl x)2B →(Fe1-xAl x) B+( Fe1-xAl x)2B→ Al2Fe +AlFe3 顺序演变。

  • (2) 在以渗硼为主的硼铝共渗中,渗剂中的铝对共渗具有促进作用,并且在施加交流电场时表现更为显著,当电场电流为2 A时,铝的促渗作用在渗剂中铝粉含量为2%时达峰值,4 h渗扩得到渗层厚度约为相应不加铝粉常规/电场渗扩的3 倍,而当电场电流增至6 A时,4 h渗扩的渗层厚度约为相应不加铝粉常规渗扩的10 倍。增加电场电流不仅促进渗层生长,还增加硼铝共渗层次表层硬度。含铝硼化物层的表层硬度较单一渗硼层低。

  • (3) 当渗剂中铝粉含量增高至3%以上时, 硼铝共渗以渗铝为主,但硼显著抑制铝的渗入, 此时交流电场对共渗的促进程度远低于对以渗硼为主的硼铝共渗。

  • 参考文献

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  • 基本信息

    中图分类号: TG156.8
    文献标识码: A
    DOI: 10.11933/j.issn.1007-9289.20200617003
    文章编号: 1007-9289(2020)04-0121-07

    基金信息

    国家自然科学基金 (51171032)；
    Supported by National Natural Science Foundation of China (51171032)；

    引用信息

    谢飞,王晓娟, 潘建伟. 交流电场增强 45 钢中温粉末法硼铝共渗特性[J]. 中国表面工程, 2020, 33(4): 121-127.

    XIE F, WANG X J, PAN J W. Characterization of alternating current field enhanced pack boron-aluminizing on at a medium temperature for 45 steel[J]. China Surface Engineering, 2020, 33(4): 121-127.

    稿件历史

    收稿日期: 2020-06-17

  • 参考文献

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