中国腐蚀与防护学报, 2023, 43(4): 713-724 DOI: 10.11902/1005.4537.2023.157

中国腐蚀与防护学会杰出青年成就奖论文专栏

激光淬火对AISI 4130钢微观组织结构及腐蚀、磨损行为的影响机制

肖檬1, 王勤英,1, 张兴寿1, 西宇辰,1, 白树林2, 董立谨1, 张进1, 杨俊杰3

1.西南石油大学新能源与材料学院 成都 610500

2.北京大学材料科学与工程学院 北京 100871

3.成都中原总机石油机械有限公司 成都 610400

Effect of Laser Quenching on Microstructure, Corrosion and Wear Behavior of AISI 4130 Steel

XIAO Meng1, WANG Qinying,1, ZHANG Xingshou1, XI Yuchen,1, BAI Shulin2, DONG Lijin1, ZHANG Jin1, YANG Junjie3

1.School of New Energy and Materials, Southwest Petroleum University, Chengdu 610500, China

2.School of Materials Science and Engineering, Peking University, Beijing 100871, China

3.Chengdu Zhongyuan Petroleum Machinery Co., Ltd., Chengdu 610400, China

通讯作者: 王勤英,E-mail:wangqy0401@swpu.edu.cn,研究方向为激光增材再制造,油气田材料腐蚀与防护;西宇辰,E-mail:xycsony3@126.com,研究方向为油气田材料腐蚀与防护

收稿日期: 2023-05-11   修回日期: 2023-06-05  

基金资助: 国家自然科学基金.  52174007
国家自然科学基金.  51801167
四川省省院省校科技合作研发项目.  23SYSX0127

Corresponding authors: WANG Qin-ying, E-mail:wangqy0401@swpu.edu.cn;XI Yu-chen, E-mail:xycsony3@126.com

Received: 2023-05-11   Revised: 2023-06-05  

Fund supported: National Natural Science Foundation of China.  52174007
National Natural Science Foundation of China.  51801167
Science and Technology Cooperation Research Project of Sichuan Province and University/Institution.  23SYSX0127

作者简介 About authors

肖檬,女,1999年生,硕士生

王勤英,1987年生,2015年7月毕业于北京大学,获理学博士学位,后于加拿大阿尔伯塔大学从事博士后研究工作。现就职于西南石油大学,教授,院长助理。王勤英博士长期从事油气装备激光修复研究。率先引入SHAP和ALE可解释方法阐明了环境因素间的交互效应及其对油气田材料点蚀的作用规律,揭示了复杂油气钻采环境对激光修复层不同熔覆区表面钝化膜稳定性的影响机制,形成了油气装备高耐蚀耐磨激光修复全流程工艺。先后主持国家自然科学基金面上项目、青年基金等科研项目10项,成果在npjMat.Degrad.等期刊发表学术论文45篇,授权专利3件。成果应用于油田及油服企业,解决了油气装备修复难、耐蚀耐磨性差的技术难题,获中国石油和化工自动化行业科技进步二等奖等科研奖励共3项。入选中国科协“青年人才托举工程”、澳大利亚教育部“奋进”学者、四川省学术和技术带头人后备人选,现为校青年科技创新团队带头人。兼任中国腐蚀与防护学会青年工作委员会委员,中国腐蚀与防护学会磨蚀与防护技术专委会、非金属耐蚀材料及先进制备技术专委会委员,四川省腐蚀与防护学会理事,《中国腐蚀与防护学报》等期刊的编委或青年编委。2023年获得中国腐蚀与防护学会杰出青年成就奖。

摘要

为改善AISI 4130钢表面硬度和腐蚀磨损性能,用高功率激光器在AISI 4130钢表面制备淬火层,利用扫描电镜 (SEM) 和X射线衍射仪 (XRD) 研究不同功率淬火试样的微观结构,利用电化学工作站分析淬火前后试样的耐蚀性能,利用维氏显微硬度仪对淬火试样截面进行硬度测试,采用往复摩擦磨损试验机,测试不同淬火试样的耐磨损性能。结果表明,激光淬火后AISI 4130钢表面主要为马氏体组织和富Cr碳化物颗粒。基材、2.0 kW淬火试样及2.2 kW淬火试样的维钝电流密度依次为60.00、102.28 和108.58 µA/cm2,淬火试样的耐蚀性降低。 激光处理后,淬火层表面硬度提高了85%以上,AISI 4130钢基体与2.0和2.2 kW激光淬火试样的平均摩擦系数分别为0.366、0.293和0.195,摩擦系数下降,淬火试样的耐磨性提高。

关键词: 激光淬火 ; AISI 4130钢 ; 微观组织 ; 耐蚀性 ; 耐磨性

Abstract

Laser quenching technology is widely used as a means for the strengthening in the field of metallic materials. Laser quenched materials have the advantages of high precision, small heat affected zone, uniform carbide dispersion and finer grains. In order to improve the surface hardness and wear resistance of AISI 4130 steel used in petroleum field, a high hardness and high wear resistance quenching layer was prepared on the surface of AISI 4130 steel by laser quenching technology. The effect of quenching power on the microstructure evolution, corrosion resistance, microhardness and wear resistance of AISI4130 steel were investigated. AISI4130 steel samples of 10 mm×10 mm×8 mm (L×W×H) were prepared by wire cut electric discharge machine. The quenching layer was prepared on the surface of AISI 4130 steel by high power laser. The microstructure and element distribution characteristics of the steel quenched with different power was studied by scanning electron microscope (SEM) with EDS and X-ray diffractometer (XRD). The corrosion resistance of the steel before and after quenching was assessed by electrochemical workstation and immersion test. The hardness of quenched steels was measured by Vickers microhardness tester. The wear resistance of different quenched steels was tested by reciprocating friction and wear tester, while the wear scratch morphology was analyzed by three-dimensional optical microscope. After laser quenching, the surface microstructure of AISI 4130 steel was obviously refined and composed of mainly martensite and Cr-rich carbide particles. The thickness of the heat affected zone of the steels of laser quenched at 2.0 and 2.2 kW was 501.5 and 553.6 μm, respectively. The impedance arc radius of the bare AISI 4130 steel and two quenched steels may be ranked as the following: 2.2 kW quenched >2.0 kW quenched >substrate. The passive current density of the bare steel, 2.0 kW- and 2.2 kW-quenched steel was 60.00,102.28 and 108.58 μA/cm2, respectively. The passivation current density of the two quenched steels was about 1.7 times that of the bare one. After quenching, the surface hardness of the steel increased by more than 85%. The average friction coefficient of the bare AISI 4130 steel and 2.0 kW- and 2.2 kW-laser quenched ones was 0.366, 0.293 and 0.195, respectively. Compared with the bare steel, the volume wear rate of 2.0 kW- and 2.2 kW-laser quenched ones was reduced by 25% and 36%, respectively. The wear resistance of quenched steels increased by 20% and 47%, respectively. The corrosion resistance of the quenched steels is reduced, but the corrosion resistance of the 2.0 kW quenched steel is better than that of the 2.2 kW ones. The precipitation of Cr-rich carbide particles in the steel will aggravate the destruction of the corrosion product film, resulting in a decrease in the corrosion resistance of the quenched steel. The higher the carbide content on the surface of the quenched steel, the more difficult it is to cut the convex surface of the abrasive into a tough phase and the wear rate of the sample surface decreases, thereby improving the overall wear resistance of the material.

Keywords: laser quenching ; AISI 4130 steel ; microstructure ; corrosion resistance ; wear resistance

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本文引用格式

肖檬, 王勤英, 张兴寿, 西宇辰, 白树林, 董立谨, 张进, 杨俊杰. 激光淬火对AISI 4130钢微观组织结构及腐蚀、磨损行为的影响机制. 中国腐蚀与防护学报[J], 2023, 43(4): 713-724 DOI:10.11902/1005.4537.2023.157

XIAO Meng, WANG Qinying, ZHANG Xingshou, XI Yuchen, BAI Shulin, DONG Lijin, ZHANG Jin, YANG Junjie. Effect of Laser Quenching on Microstructure, Corrosion and Wear Behavior of AISI 4130 Steel. Journal of Chinese Society for Corrosion and Protection[J], 2023, 43(4): 713-724 DOI:10.11902/1005.4537.2023.157

AISI 4130钢 (35CrMo) 是ASTM29标准中的一种低合金钢,其中含有质量分数为1%Cr和0.2%Mo作为增强元素,能够阻止奥氏体向珠光体转化,显著提高强度、硬度、耐磨性及淬透性[1]。AISI 4130钢广泛用于制造锻造油气阀体、连杆和石油深井钻杆接头等[2],由于温度、腐蚀介质以及较大冲击造成工件的磨损占失效的90%以上[3]。因此,提高AISI 4130钢工件表面的抗磨损性能势在必行。通过表面改性技术,如热喷涂、激光熔覆和激光喷丸[4]等来强化金属材料性能已经成为国内外学者广泛关注的研究方向。其中,激光淬火 (LQ) 已经被广泛用于提高曲轴、齿轮和气缸等工件的耐磨性,该方法是利用高能激光束迅速加热材料表面,当材料表面温度到达相变温度后快速冷却,形成晶粒细化的马氏体组织淬硬层,使得工件表面得到强化的过程[5]。同时,该技术具备高精度、热影响区小、碳化物分散均匀以及晶粒更加细化的优势[6]。克服了传统淬火持续时间长,需要整个构件同时淬火,最终获得的表面硬化层无规律及工件变形大等缺点[7]。此外,激光表面淬火不改变工件尺寸和表面粗糙度,可提高其工件耐磨性和抗疲劳性[8],较传统淬火具有更高的表面硬度[9]。Chen等[10]对高铬铸铁进行淬火回火 (QT) 和LQ处理,结果表明LQ对试样显微硬度的强化效果最优,与QT相比,LQ加热冷却速率快,易形成温度梯度,产生更细的马氏体组织和更多的碳化物,大幅度增强高铬铸铁的耐磨性;LQ试样的摩擦系数下降9.70%,磨损率下降11.32%。激光工艺参数,如激光功率、光斑尺寸及扫描速率是决定材料淬火质量的关键,有研究结果显示激光功率对材料的奥氏体化程度起决定性作用,从而进一步影响材料表面硬度[11, 12]。杨骏龙等[3]对35CrMo钢进行激光淬火处理,研究了激光功率对35CrMo钢奥氏体化程度的影响机制。研究表明激光功率为3.2 kW的试样表面硬度提高了20%,耐磨性比功率为1.6和2.4 kW处理的材料明显提高。Li等[13]对1.0C-1.5Cr钢进行了传统淬火和激光表面淬火研究,与传统淬火相比,LQ试样的表面硬度提高约20%,有益于增强其耐磨性能。单淬火与双淬火后的冲击吸收能量相似,约为淬火前的25%。Pantelis等[14]采用4种不同搭接率对CK60结构钢进行了激光表面处理,结果表明,使用0%和38%搭接率处理的试样具有更优越的耐磨性。在所有研究的试样中,搭接率为0%处理的试样具最优的耐蚀耐磨性能。此外,LQ工艺也存在其他问题,如激光功率密度及激光不均匀性会影响淬火工艺的稳定性,工件初始状态对淬火质量影响较大,淬火材料表面存在应力不均匀的现象和激光表面淬火难以保证大面积硬化层。

因此,本文研究了激光功率对AISI 4130钢表面淬火的影响,并对淬火层深度、组织演变特征、显微硬度和耐蚀耐磨性等进行了研究,揭示了激光功率对AISI 4130钢的微观组织结构及性能的影响机制,为探究高质量LQ技术提供理论依据。

1 实验方法

本实验采用的基体材料为AISI 4130合金结构钢,其化学成分 (质量分数,%) 为:C 0.28~0.33,Si 0.15~0.35,Mn 0.40~0.60,Cr 0.80~1.10,Mo 0.15-0.25,Ni≤0.03,Cu≤0.03,Fe余量。

LQ工艺是采用高功率激光器(P 6000)对AISI 4130钢进行淬火处理,激光扫描速率为0.00567 m/s,光斑尺寸为25 mm×20 mm,搭接率为0%。樊湘芳等[15]对40CrNiMoA钢进行表面LQ处理时,采用0.02 m/s扫描速率及1.6 kW激光功率的试样组织和耐磨性能最佳。基于此,为研究激光功率对AISI 4130钢淬火硬化层微观组织演变规律的影响机制,激光功率确定为2.0和2.2 kW。搭接接头的形式和LQ工艺的原理如图1所示。

图1

图1   激光表面淬火示意图

Fig.1   Schematic diagram of laser surface quenching


激光表面淬火试样被切割成10 mm×10 mm×8 mm (L×W×H) 的立方体。首先采用600~2000#的SiO2砂纸对试样截面进行打磨,随后用金刚石膏抛光为镜面,然后采用无水乙醇冲洗并吹干,最后采用体积分数为4%的硝酸溶液蚀刻约10 s。利用配备能量色散光谱 (EDS) 的扫描电镜 (SEM,EV0 MA15) 观察试样的微观组织结构特征,并利用EDS在加速电压20 kV、探针电流500 pA和采集时间240 s下研究试样截面元素分布特征。采用X射线衍射仪 (XRD,X Pert PRO MPD) 对试样进行物相组成研究。

采用CS310H电化学工作站,将试样、铂片 (Pt)和饱和甘汞电极分别作为工作电极、对电极和参比电极,对淬火试样进行电化学行为测试。电化学测试前,每个试样底部和周边均用环氧树脂密封,腐蚀液为3.5% (质量分数) NaCl溶液,每种条件下试样重复测量3次。截面硬度采用维氏显微硬度仪 (HV-1000STA)测量,测试方法为距淬火表面约50 μm开始测试,0.05~1.05 mm之间每隔0.04 mm测量一个点,1.05 mm后每隔0.25 mm测量一个点。载荷为200 gf,停留时间为15 s。采用UMT-2摩擦磨损试验机在试样表面进行往复磨损,行程为5 mm,摩擦速率为5 mm/s,载荷为80 N。此外,使用三维光学显微镜 (布鲁克轮廓GT-K) 获得磨损划痕的三维形态。

2 结果与分析

2.1 激光淬火AISI 4130钢的微观组织结构特征研究

图2为AISI 4130钢基材与2.0和2.2 kW激光淬火试样的表面微观结构。由图可知,淬火后表层组织出现较明显细化,淬火试样表面主要为高温回火马氏体 (TM) 和碳化物,而2.2 kW淬火试样出现较多二次马氏体(FM)[16]

图2

图2   AISI 4130钢基材与淬火试样的表面微观结构

Fig.2   Surface microstructure of AISI 4130 steel substrate (a1, a2) and quenched samples with laser power of 2.0 kW (b1, b2) and 2.2 kW (c1, c2)


34为AISI 4130钢在2.0和2.2 kW激光淬火后的截面微观组织。两种处理试样的热影响区厚度分别为501.5和553.6 µm,激光硬化层厚度随激光功率的增加而增加。将两种处理试样的淬硬区 (HAZ) 根据微观组织特征划分为6个不同的区域 (d~i)。近表面d区由于快速淬火过程造成的垂直温度梯度导致形成了TM。由表至内激光加热温度逐渐降低,次表层e区组织主要为马氏体及未溶解碳化物。f和g区为不完全淬火层,出现奥氏体及渗碳体,且越靠近基体铁素体含量越多。h区为高温回火区,组织主要为铁素体、珠光体及析出渗碳体 (PC),以及未溶渗碳体 (UC)[17, 18]。i区到达基体,组织为铁素体及少量珠光体。与基材 (图3i4i) 相比,淬火试样 (图3d4d) 内部产生较多马氏体,保留少量残余奥氏体,晶粒细化明显,碳化物分散均匀。

图3

图3   AISI 4130钢2 kW激光淬火后截面的微观组织

Fig. 3   Cross-sectional microstructure of AISI 4130 steel after 2 kW laser quenching: (a) cross-sectional quenching layer thickness, (b) Near-surface quenching zone, (c) hardened zone and matrix transition zone, (d-i) corresponding zones in Fig.3b and c


图4

图4   AISI 4130钢2.2 kW激光淬火后截面的微观组织

Fig.4   Cross-sectional microstructure of AISI 4130 steel after 2.2 kW laser quenching: (a) cross-sectional quenching layer thickness, (b) Near-surface quenching zone, (c) hardened zone and matrix transition zone, (d-i) corresponding zones in Fig.4b and c


与2.0 kW淬火试样相比,2.2 kW淬火试样马氏体相变转化率较高,保留残余奥氏体较少,马氏体组织分布相对均匀,淬火效果相对较好。原因是在淬火过程中,激光功率越高,其对表面的升温速率越快,使得奥氏体形核并长大速率加快,冷却后形成的马氏体组织将更加稳定。由此表明,随着功率增加,淬火层厚度与马氏体含量逐渐增加,这与Liu等[19]研究结果一致。

图5为AISI 4130钢淬火试样的截面元素分布特征。由图可知,2.0 kW淬火试样O含量从表面到40 µm处显著增加,2.2 kW淬火试样O含量从表面到75 µm处含量显著增加,但波动较大。Fe由内部至表面逐渐降低,2.0 kW及2.2 kW淬火试样分别降低13.9%和13.5%。这可能是由于淬火过程马氏体转变引起热影响区O含量变化,导致Fe和O含量的相对变化。当高功率激光束照射试样表面时,由于试样的导热性,表层将会形成温度梯度。因此,在LQ过程中试样表面温度剧烈增加,导致Fe与O2发生反应;距离试样表面越近,O含量及温度越高,加速反应的进行,导致Fe由内至表面含量逐渐减少[20]。两种试样Fe变化的区域厚度,都处于前图中的f区及以上,由此可知随功率的增加,Fe含量的降低越不明显。图6为AISI 4130钢基材与LQ试样表面的XRD谱,可以看出,淬火并不会明显改变试样的物相。

图5

图5   AISI 4130钢淬火后截面元素分布特征

Fig.5   Cross-sectional element distribution of AISI 4130 steel after quenching: (a, b) 2.0 kW, (c, d) 2.2 kW


图6

图6   AISI 4130钢基材与LQ试样的XRD谱

Fig.6   XRD patterns of AISI 4130 steel substrate and laser quenching samples


2.2 激光淬火AISI 4130钢的耐蚀性研究

图7为AISI 4130钢基材与两种LQ试样在3.5% NaCl溶液中测试的电化学阻抗谱 (EIS)。由图可知,阻抗弧半径从小到大依次为2.2 kW淬火试样、2.0 kW淬火试样和基材,表明淬火试样的耐蚀性较基材更低,且功率越高,耐蚀性越差。根据图7b可知,较宽的频率范围下存在较高的相位角,表明在所有试样表面存在腐蚀产物膜[21]。且|Z|0.01 Hz从小到大依次为2.2 kW淬火试样、2.0 kW淬火试样和基材。

图7

图7   AISI 4130钢基材与2.0 kW、2.2 kW功率LQ试样的EIS图

Fig.7   EIS diagram of AISI 4130 steel substrate and two quenched samples in 3.5% NaCl solution: (a) Nyquist diagram and corresponding equivalent circuit, (b) Bode diagram


考虑到AISI 4130钢、3.5% NaCl溶液以及试样的EIS图特征,选择图7a等效电路来拟合EIS图。其中,Rs为3.5% NaCl溶液电阻,n为弥散系数,CPEdl为恒相位角元件,Rct为电荷转移电阻,L为活性物质在钝性膜上的吸附电感,Rf为腐蚀产物膜的电阻。上述参数的拟合结果见表2。由表可知,基材、2.0 kW淬火试样及2.2 kW淬火试样Rct与Rf的电阻之和Rc+f分别为:5862、4331和3627 Ω·cm2,表明淬火试样的耐蚀性降低,且淬火功率越高,耐蚀性越差。

表2   AISI 4130钢在不同激光功率下阻抗拟合数据

Table 2  Fitting parameter results of the EIS test of AISI 4130 steel under different laser powers obtained by the proposed equivalent circuit model

Laser power / kWRs / Ω·cm2Y / Ω-1·cm-2·s-nnRf / Ω·cm2L / H·cm2Rct / Ω·cm2
As-received2.8885.36×10-50.80112443714738
2.02.34128.00×10-50.79300533431326
2.22.33145.70×10-50.78102935582598

新窗口打开| 下载CSV


图8为基材与两种LQ试样的动电位极化曲线。可知,基材与LQ试样均在阳极支上出现了明显的类钝化现象,淬火试样的极化曲线均向右移动,2.0 kW与2.2 kW的维钝电流密度约为基材的1.7倍。表明淬火试样的耐蚀性降低,且2.0 kW试样比2.2 kW试样的耐蚀性更优。

图8

图8   AISI 4130钢基材与2.0和2.2 kW功率LQ试样的动电位极化曲线

Fig.8   Potential polarization curves of AISI 4130 steel substrate and two quenched samples with laser power of 2.0 and 2.2 kW in 3.5% NaCl solution


2.3 激光淬火AISI 4130钢的耐磨性研究

图9为AISI 4130钢与两种功率LQ试样截面的显微硬度分布特征。可知,淬火层硬度呈现由表面向内部逐渐降低的趋势;与2.0 kW淬火试样相比,2.2 kW淬火试样热影响区硬度较大;其平均硬度如图9b所示,与基体相比,其硬度分别提高85%、95%。当未达到表面熔化的临界热输入量时,激光能量越大,奥氏体化程度越慢。因此,在冷却时,会出现更多的马氏体,使得表面硬度提高。此外,图中靠近基体的A,C点,硬度有明显上升。由于A、C点冷却速率快,未到达晶粒长大的临界值,所以其晶粒比B、D点细小,与图23表现一致。且越靠近基体,A,C点马氏体含量比B,D点少,因此晶粒细化是导致A,C点硬度小幅度提高的原因[20]

图9

图9   AISI 4130钢在2.0和2.2 kW功率LQ后的截面显微硬度分布特征

Fig.9   Microhardness of AISI 4130 steel after laser quenching at 2.0 and 2.2 kW power: (a) cross-sectional microhardness, (b) average hardness of the hardened zone specimen


图10为AISI 4130钢基材与两种功率LQ试样的摩擦系数曲线。可知,在磨损的起始阶段,试样的摩擦系数 (COF) 都在短时间内整体上升,此时处于磨合磨损阶段。当进入稳定磨损阶段大约500 s时,所有试样的COF都会略微下降,这表明磨损后表面粗糙度降低,试样表面的实际磨损面积和接触点的数量增加导致了COF的降低,随后抛光面被破坏,COF增大。稳态磨损阶段,AISI 4130钢基材与2.0 kW和2.2 kW LQ试样的平均COF如图10b,与基材试样相比,LQ试样的摩擦曲线更加平缓,COF降低,淬火试样耐磨性分别提高20%和47%,说明2.2 kW淬火试样耐磨性最好。这是因为淬火后细小的碳化物以及马氏体的形成,试样的表面硬度增加,耐磨性也得到提高[22]。这与Pantelis等[14]研究结果一致。

图10

图10   AISI 4130钢基材与2.0和2.2 kW功率LQ后试样的摩擦系数

Fig.10   Friction coefficient of AISI 4130 steel substrate and two quenched samples with laser power of 2.0 and 2.2 kW: (a) coefficient of friction varying with time, (b) average coefficient of friction


3 讨论

3.1 激光淬火对AISI 4130钢耐蚀性的影响机制

近年来,AISI 4130钢的耐蚀性主要受到微观组织及环境条件的影响,尤其是试样表面的氧化膜组成及结构起着主要作用。图11为3.5% NaCl溶液中浸泡24 h后试样表面的微观形貌及截面元素分布,腐蚀产物主要为花瓣形状的膜γ-FeOOH[23]。浸泡后基材试样表面产生的腐蚀产物相对较少,γ-FeOOH膜较为紧致,产生裂纹较细。而淬火试样表面产生腐蚀产物较多,γ-FeOOH膜较松散,产生裂纹较宽。2.2 kW试样可能由于其腐蚀产物太过疏松导致腐蚀产物剥落 (图11c)。

图11

图11   浸泡24 h后的表面微观形貌及截面元素分布

Fig.11   Surface morphologies (a1-c1) and cross-sectional element distribution (a2-c2) of AISI 4130 steel substrate (a1, a2) and two quenched samples with laser power of 2.0 kW (b1, b2) and 2.2 kW (c1, c2) after 24 h immersion in 3.5% NaCl solution


图12为AISI 4130钢的腐蚀机理图,如图12a所示,AISI 4130钢的腐蚀产物膜的形成中,金属表面的化学反应如下式:

阳极: 2Fe2Fe2++4e-
阴极: 2H2O+O2+4e-4OH-
总反应: 2Fe+O2+2H2O2Fe(OH)2

图12

图12   AISI 4130钢基材和淬火试样的腐蚀机理示意图

Fig.12   Schematic diagram of corrosion mechanism of AISI 4130 steel substrate (a) and quenched sample (b)


Fe(OH)2不稳定,可进一步与O2反应并发生相变,脱水或还原反应生成稳定的Fe2O3/Fe3O4

Cl-存在于腐蚀溶液,可以破坏腐蚀产物膜的薄弱区,促进O2及溶液对金属表面的接触面积,在金属表面发生点蚀,如下所示。

2Fe2++4OH-2Fe(OH)2
Fe(OH)2+Cl-Fe(OH)++Cl-+OH-

图12b为激光淬火后AISI 4130钢的腐蚀机理。淬火试样中析出富含Cr的碳化物颗粒[24],由于颗粒附近Cr含量相对较低,在Cr耗尽区无法形成腐蚀产物膜,或者由于Cl-破坏腐蚀产物膜渗透到金属表面,腐蚀产物膜将不连续,无保护作用[25],Cr的耗尽区将快速被腐蚀,加剧腐蚀产物膜的破坏,导致淬火试样的耐腐蚀性降低[21, 22, 26]。此外,由于多相微观组织,包括铁素体、残余奥氏体和大量碳化物的存在导致不同相间界面上的电偶腐蚀,促进腐蚀加剧。因此,淬火试样的耐蚀性降低。与2.2 kW淬火试样相比,2.0 kW淬火试样的耐蚀性较好,这可能是由于其残余奥氏体的含量较高,对降低试样的腐蚀速率具有积极作用[27]

3.2 激光淬火对AISI 4130 钢耐磨性的影响机制

图13为AISI 4130钢基材与两种LQ试样的表面磨损形貌。可知,基材磨损表面分布着较多氧化物磨损碎屑和犁沟,一些碎片从基底上剥落,新的表面暴露在空气中,进一步被氧化。与基材相比,LQ试样表面氧化磨损碎屑较少,磨损表面较光滑,没有发生剥落现象,表明粘着磨损有所改善,耐磨性有所提高。与2.0 kW淬火试样相比,2.2 kW淬火试样磨损表面较光滑,硬颗粒难以保留在淬火表面上,这可能是由于磨损表面的碳化物是影响材料耐磨性的主要因素。

图13

图13   AISI 4130钢磨损划痕及元素分布

Fig.13   Wear morphology and mapping analysis of AISI 4130 steel: (a1, a2) substrate, (b1, b2) 2.0 kW LQ sample, (c1, c2) 2.2 kW LQ sample


通过EDS表征AISI 4130钢基材与2.0 kW和2.2 kW LQ试样,如图13所示,在基材表面形成一层明显氧化层,淬火试样表面氧化层不明显。氧化层的形成是由于在摩擦过程中产生的热量,及Fe、Cr和Mo从基体扩散到试样表面引起的[28],空气中的O2与表面上的金属元素结合形成氧化层。

图14显示了AISI 4130钢磨损划痕的三维形态。如图14a所示,与LQ试样 (图14bc) 相比,基材试样的磨损划痕更宽,划痕凸起可能是磨损碎屑堆积引起。淬火试样中,2.0 kW的淬火试样磨损划痕更深。体积磨损率可通过下式计算[29]

图14

图14   AISI 4130钢磨损后3D形貌

Fig.14   3D morphologies (a-c), wear depth and volume wear rate (d) of AISI 4130 steel substrate (a) and two quenched samples with laser power of 2.0 kW (b) and 2.2 kW (c) after wear


V=LR2Sin-1ω2R-ω2R2-ω2412+
π32R3-2R2R2-ω2412
K=V/(LN)

式中,L为摩擦长度,m;R为硬质合金球半径,mm;ω为磨损轨道宽度,mm;K为体积磨损率,mm3·N-1·m-1N为法向载荷,N。图14d为AISI 4130钢的磨损深度及体积磨损率。可知,基材最深磨痕深度及体积磨损率最大。与基材相比,2.0 kW和2.2 kW LQ试样的体积磨损率分别降低了25%和36%,这说明淬火试样的耐磨性增强。

图15为AISI 4130钢基材和LQ试样的磨损机理。如图15a所示,在法向载荷下Al2O3摩擦副使试样表面形成不完全氧化层,当氧化层被破坏后,试样的金属表面在大气中重新氧化,氧化膜中存在Fe2+、Fe3+、Cr3+、Mo4+和Mo5+等阳离子,具有一定润滑作用[30]。当剪切应力和磨损热作用于试样表面时,碎片弯曲、氧化成为磨料颗粒。通过硬颗粒的运动,试样表面受到犁沟效应,形成犁沟,且导致氧化物不完整,从而加速氧化磨损。因此,基材试样的磨损机制主要为粘着磨损、氧化磨损和磨粒磨损,疲劳磨损。对于淬火试样,如图15b所示,磨损机理主要为轻微磨损及氧化磨损。首先,马氏体及Cr的碳化物具有极高硬度,阻止了表面被严重磨损[31]。其次,淬火试样中残余奥氏体具有较好的可塑性,可减少冲击[32]。最后,淬火试样表面碳化物含量越高,磨料上的凸面越难切割成坚韧相,因此降低了试样表面的磨损率[33],从而提高了材料的整体耐磨性。

图15

图15   AISI 4130钢基材和淬火试样的磨损机理示意图

Fig.15   Schematic diagram of wear mechanism of AISI 4130 steel substrate (a) and quenched sample (b)


4 结论

(1) 经2.0和2.2 kW LQ后,2.2 kW的淬火试样热影响区厚度较大,且马氏体相变转化率较高,淬火效果相对较好,淬火后显微硬度提高,耐蚀性下降,但耐磨性增强。

(2) AISI 4130钢基材与两种LQ试样的阻抗弧半径从小到大依次为2.2 kW淬火试样、2.0 kW淬火试样和基材。2.0与2.2 kW LQ试样的钝化电流密度约为基材的1.7倍,两种结果表明,LQ试样的耐蚀性降低,且2.0 kW试样比2.2 kW试样的耐蚀性更优。LQ试样中析出富含Cr的碳化物颗粒,及多相微观组织的存在将加剧腐蚀产物膜的破坏,导致LQ试样的耐腐蚀性降低。

(3) AISI 4130钢LQ后热影响区平均硬度明显提高,LQ试样耐磨性明显提高,且2.2 kW LQ试样耐磨性最好。由于淬火后马氏体及Cr的碳化物具有极高硬度,能够进一步提高抗剥落能力,且表面碳化物含量越高,试样表面的磨损率越低,材料整体耐磨性越好。

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The surface layer of the coiled spindle main shaft 40 CrNiMoA steel base material was tested by laser-quenching with different power. The macroscopic structure of cross-section,surface microstructure,microhardness,and friction and wear properties of the specimens were observed or tested by means of stereo microscope,optical microscope,microhardness tester,vertical universal friction and wear tester.The results show that,the surface structure of the laser hardened specimens at different powers exhibit different levels of refining,and compared with the matrix,the hardness and friction and wear properties are improved. When the power is 1600 W,the laser hardened layer can be clearly observed in the cross-section of the specimens,and the surface layer of the specimens is the most detailed,consisting of fine acicular martensite,a small amount of retained austenite and a finely distributed fine carbide,and the surface hardness can reach 640.3-706.8 HV0.2,which is about 2.8 times of that of the substrate. At the same time,the friction coefficient is stable between 0.40 and 0.60,which is about 50% lower than that of the substrate. The amount of wear is 1.3 mg,which is only 36.1% of the substrate. Under the test conditions of spot size with 12 mm×2 mm,scanning speed with 20 mm/s and laser power with 1600 W,the obtained microstructure and friction and wear properties are optimized after surface laser quenching of 40 CrNiMoA steel.

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