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作者简介:

聂翔宇,男,1998年出生,硕士。主要研究方向为激光表面强化。E-mail:niexiangyu@buaa.edu.cn;

何蓓(通信作者),男,1989年出生,博士,副研究员,硕士研究生导师。主要研究方向为激光增材制造、激光表面强化。E-mail:hebei@buaa.edu.cn

中图分类号:TG111

DOI:10.11933/j.issn.1007−9289.20210420002

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目录contents

    摘要

    航空发动机传动部件服役过程易磨损失效,为提高其寿命和可靠性需进行表面强化。在 40CrNiMo 合金钢表面进行激光相变强化处理,通过调控扫描速度获得不同激光相变强化区组织,对其显微硬度和摩擦磨损性能进行表征。结果表明,随扫描速度降低,硬化层宽度和深度增大,显微组织变粗,马氏体含量增加。不同扫描速度下,硬化层表面显微硬差异小,为 77~789 HV,相比基材(330 HV)提升 135%以上。激光相变强化处理后,试样耐磨性大幅提升,硬化区组织为孪晶马氏体+回火索氏体的试样耐磨性最优,摩擦因数相比基材降低 24.9%,磨损体积减少 94.3%。研究表明,由高强的细小孪晶马氏体和韧性较好的细小回火索氏体组成的复相组织,能有效阻碍裂纹形成和扩展,显著提升耐磨性能。调控激光相变强化工艺参数, 获得高强马氏体+韧性相的复相组织,能获得优异的耐磨性能。

    Abstract

    Aero-engine transmission components are easy to wear and fail in service. In order to improve their service life and reliability, surface strengthening is needed. Laser transformation hardening treatment is carried out on the surface of 40CrNiMo alloy steel. The microstructure of different laser transformation hardening areas is obtained by adjusting the scanning speed, and its microhardness and friction and wear properties are characterized. Results show that with the decrease of scanning speed, the width and depth of hardened layer increase, the microstructure coarsens and the content of martensite increases. Under different scanning speeds, there is little difference in the surface microhardness of the hardened layer, which is 776-789 HV, which is more than 135% higher than that of the substrate (330 HV). After laser transformation hardening treatment, the wear resistance of the sample is greatly improved. The wear resistance of the sample with twin martensite + tempered sorbite in the hardened zone is the best, the friction factor is reduced by 24.9%, and the wear volume is reduced by 94.3%. Results show that the multiphase structure composed of fine twin martensite with high strength and fine tempered sorbite with good toughness can effectively hinder the formation and propagation of cracks and significantly improve the wear resistance. By adjusting the process parameters of laser transformation hardening, the multiphase structure of high-strength martensite + ductile phase can be obtained, and excellent wear resistance can be obtained.

  • 0 前言

  • 激光相变强化(Laser transformation hardening, LTH)是通过激光束照射金属工件表面,使工件表面的温度在极短时间内迅速升高到奥氏体转变温度以上、金属熔点以下,依靠金属工件自身的传热,表面温度快速下降实现自淬火,从而在工件表面制备具有细小马氏体组织的相变硬化层[1],以提高工件表面硬度,形成残余压应力[2],并提高工件耐磨性、疲劳强度等性能的表面强化技术[3]。目前,激光相变强化技术不仅在钢的表面处理中应用,还在铸铁[4]和钛合金[5]的表面处理中应用。

  • 国内外专家学者对激光相变强化的硬化区组织、硬度、耐磨性进行了大量试验研究。激光加热过程中,材料表面微区在高能量密度激光辐照下,能够实现温度的急剧升高,同时热影响区域较小,在随后的自冷却过程中,冷却速度快,能够实现表层组织的超细化。因此,相较于传统淬火方式,激光相变强化能获得更高的表面硬度和耐磨性。SHIUE等[6]对AISI 4340钢激光表面硬化组织进行了研究,结果表明,硬化区组织主要由板条马氏体(板条边界处有残余奥氏体)和孪晶马氏体组成。CHEN Z K等[7]研究了40Cr钢的激光相变强化组织,根据组织特征不同,硬化区可分为高温区、中温区、低温区。40Cr基体组织为珠光体+铁素体,高温区组织为马氏体+残余奥氏体,中温区组织为马氏体+贝氏体+残余奥氏体+索氏体+铬的碳化物,低温区组织为铁素体+屈氏体。淬火区显微硬度由基材的230HV提高到淬火层的693HV,同时耐冲击磨损性能提高。CHEN Z Y等[8]通过对激光硬化处理17-4PH不锈钢显微组织和硬度的研究,发现硬化层厚度为1.75mm,硬化层组织主要为典型板条马氏体、粗NbC以及细小面心立方富铜相。显微硬度为446HV,其硬度提高的原因主要是位错密度高、组织细化以及残余奥氏体转变为马氏体。AQIDA等[9]采用CO2 激光器实现了H13钢激光表面改性,在改性层中形成纳米尺寸和超细尺寸的晶粒结构,相比未改性试样,其硬度提高了超过2.5倍。曹熙等[10]对轮轨材料进行了激光离散淬火,车轮和钢轨试样表面硬度均大幅提高,磨损率降低超过20%。

  • 工艺参数将影响激光相变强化的组织和性能。SHIUE等[6]对AISI 4340钢进行了扫描式激光表面硬化,结果表明,在完全奥氏体化条件下,扫描速度越快,硬化层的马氏体尺寸越小,自回火效应越低,试样表层硬度越高。SYED等[11]对C-Mn低碳汽车钢板进行了不同扫描速度的激光相变强化处理,研究发现硬化深度随着扫描速度的降低而增加。MORADI等[12]对AISI 4130碳钢进行激光相变强化,研究发现提高激光功率,降低扫描速度和焦平面位置有利于降低铁素体的含量,提高硬度。宋瑞宏等[13]对45钢进行了不同功率的激光淬火处理并研究了其耐磨性能,研究发现一定范围内,淬火后试样磨损量和摩擦因数随着激光功率的增大而降低。当功率过大时,表面过热区范围大,可能出现脆性剥落,导致摩擦因数变大。

  • 40CrNiMo合金钢是一种中碳低合金高强钢,具有较高的强度、韧性,良好的淬透性及热稳定性,为航空发动机传动部件常用材料[14]。航空发动机传动部件服役环境具有高温、高载荷、高速的特点,必须进行表面强化处理以提高其寿命和可靠性[15]。相比传统的渗碳、渗氮、火焰淬火等表面强化技术,激光相变强化技术效率高、硬化层组织超细化、工件表面硬度高、耐磨性高、变形小、表面质量好,对于高精度、大尺寸航空齿轮的表面强化具有极大优势[16]。孔德军等[17-19]对调制态40CrNiMo钢进行了激光相变强化,得到晶粒度为7至8级的细小马氏体组织,抗拉强度提升24.4%,屈服强度提升25.3%,疲劳强度提升14.2%,且不同载荷下的耐磨性能均大幅提高。杨振等[20]研究了激光相变强化40CrNiMo钢的耐磨性能,发现磨损量仅为基体的36.1%,其磨损机制为磨粒磨损,对基体的剥落磨损和黏着磨损有大幅改善。上述研究基本阐明了激光相变强化40CrNiMo钢硬化区顶部组织的形成机制和耐磨性改善的机理,但未对硬化区域不同位置的微观组织进行深入探究,工艺参数对强化区组织和耐磨性的影响规律尚不清楚。

  • 本文从材料显微组织调控角度出发,设计了不同的激光相变强化工艺参数,获得不同的激光相变强化组织,研究激光相变强化过程中奥氏体转变、马氏体转变动力学行为并揭示了硬化区显微组织形成机制,探究不同组织之间显微硬度和耐磨性等性能差异。

  • 1 材料与方法

  • 1.1 试验材料

  • 试验材料为调质态40CrNiMo合金钢,其化学成分(质量分数)如表1所示,其组织为回火索氏体,即片状铁素体基体中分布着细小弥散的碳化物的复相组织,如图1所示。试样尺寸为60mm×20mm×15mm,在进行激光相变强化前使用铣床铣平并使用无水乙醇超声清洗干净。

  • 表1 40CrNiMo钢化学成分(质量分数)

  • Table1 Chemical compositions of 40CrNiMo steel

  • 图1 40CrNiMo合金钢微观组织

  • Fig.1 Microstructure of 40CrNiMo steel

  • 1.2 激光相变强化试验

  • 激光相变强化试验是在北京航空航天大学大型金属构件增材制造国家工程实验室自主研制的激光成型加工设备上进行的,该设备配备了YLS-12000光纤激光器和三轴联动四坐标数控机床等,扫描式激光相变强化试验原理如图2所示。激光相变强化试样编号及工艺参数如表2所示。

  • 图2 激光相变强化试验示意图

  • Fig.2 Schematic diagram of laser transformation hardening experiment

  • 表2 激光相变强化工艺参数

  • Table2 Experimental parameters of different sample groups

  • 1.3 表征

  • 采用Zeiss Lab A1型光学显微镜(OM)进行金相组织观察;采用JSM 6010型扫描电子显微镜 (SEM)进行高倍显微组织分析;采用FEI Tecnai F30型透射电子显微镜(TEM)中进行高倍组织分析;采用恒一FM 800型显微硬度计对激光相变强化处理后的硬化层截面硬度进行测试,施加载荷为5N,保载时间为15s,垂直方向上每间隔100 μm选取多个点进行测试,同一水平位置测试5个点并计算平均值作为该位置的显微硬度值,根据所测数据绘制激光相变强化区截面显微硬度分布图;采用布鲁克UMT-2型磨损试验机进行线性往复摩擦磨损性能测试,实验组试样为调质态40CrNiMo钢激光相变强化处理后试样,对照组试样为调质态40CrNiMo钢基材试样,选择直径为4mm的Si3N4 陶瓷球为对磨副,实验参数为加载时间10s,振幅2.5mm,试验载荷10N,试验频率2Hz/s,单次试验时长30min;采用JSM 6010型扫描电子显微镜分析磨损形貌;采用ZYGO NEX view三维白光干涉表面形貌仪测量试样磨损体积。

  • 2 结果与讨论

  • 2.1 显微组织

  • 图3 为不同扫描速度下激光相变强化40CrNiMo钢的截面形貌,硬化区呈月牙形,硬化区与基材区之间无裂纹缺陷,组织致密且存在明显的过渡区即高温回火区。随激光扫描速度的降低,硬化区宽度和深度均增大。

  • 图4是S-840试样硬化区截面的显微组织和硬化区顶部的透射电镜形貌。硬化区(Hardened zone)顶部组织为孪晶马氏体[21-22]+细小的回火索氏体,两者组成了均匀的复相组织。由于组织中仍然存在回火索氏体,硬化区为不完全淬火区。硬化区底部组织为细小回火索氏体。过渡区(Transition zone)组织也为回火索氏体,但与基材区(Substrate)回火索氏体相比明显粗化,说明过渡区回火索氏体经历了高温回火。

  • 图3 激光相变强化40CrNiMo钢的截面形貌

  • Fig.3 Section feature of 40CrNiMo steel by laser transformation hardening

  • 图4 试样S-840截面显微组织和硬化区顶部的透射电镜形貌

  • Fig.4 Section microstructure and TEM morphology of S-840

  • 图5 是试样S-600硬化区纵截面的显微组织和硬化区顶部的透射电镜形貌。硬化区(Hardened zone)顶部组织为板条马氏体[23-24]+孪晶马氏体+ 细小回火索氏体,仍是一种不完全淬火区组织。回火索氏体含量较试样S-840大幅减少,只有少量细小的回火索氏体均匀的分布在马氏体晶粒之间。硬化区底部组织为孪晶马氏体+细小回火索氏体。过渡区(Transition zone)组织为高温回火形成的回火索氏体,比基材区组织(图4d)更粗大。

  • 图5 试样S-600截面显微组织和硬化区顶部的透射电镜形貌

  • Fig.5 Section microstructure and TEM morphology of S-600

  • 图6 是试样S-360硬化区纵截面的显微组织和硬化区顶部的透射电镜形貌。硬化区(Hardened zone)顶部组织为板条马氏体+孪晶马氏体,回火索氏体全部发生相变,是一种完全淬火区组织。马氏体晶粒之间存在少量的残余奥氏体[25]。硬化区中部组织为板条马氏体+孪晶马氏体+回火索氏体,是一种不完全淬火区组织,硬化区底部组织为孪晶马氏体+回火索氏体。过渡区(Transition zone)组织与试样S-840、S-600相似,为高温回火形成的粗大的回火索氏体。

  • 图6 试样S-360截面显微组织和硬化区顶部的透射电镜形貌

  • Fig.6 Section microstructure and TEM morphology of S-360

  • 2.2 硬化层硬度

  • 显微硬度测结果如图7所示。由图可知,其他参数不变,随着激光扫描速度降低,硬化区深度增加,最大接近1mm;且随着距硬化区表面距离的增加,显微硬度减小,其中高温回火区硬度最小。试样S-840硬化区表面硬度为779HV;试样S-600硬化区表面硬度最大,其硬度值为789HV,较基材硬度(约330HV)提高了约1.4倍;试样S-360硬化区表面硬度为776HV。

  • 图7 硬化区截面显微硬度

  • Fig.7 Microhardness of cross section of hardened zone

  • 2.3 耐磨性能

  • 不同组织试样线性往复摩擦磨损试验摩擦因数结果如图8所示。由试验结果可知,基材试样摩擦因数最高,其平均摩擦因数为0.775;试样S-840、 S-600和S-360的平均摩擦因数分别为0.582、0.592和0.615。试样S-840平均摩擦因数最低,较调质态基材试样降低24.9%。

  • 图8 基材及激光相变强化试样往复摩擦磨损试验摩擦因数

  • Fig.8 Friction coefficient of reciprocating friction and wear test of substrate and laser transformation hardening samples

  • 基材及不同激光相变强化工艺处理40CrNiMo钢试样经线性往复摩擦磨损试验后磨损体积如图9所示,其中基材的磨损体积最大,为7.19×10-3 mm 3; 试样S-840的磨损体积最小,只有4.1×10-4 mm 3,比未激光相变强化处理基材磨损体积减少94.3%; 试样S-600的磨损体积为1.78×10-3 mm 3,比基材磨损体积减少75.2%;试样S-360的磨损体积为8.9× 10-4 mm 3,比基材磨损体积减少87.6%。由磨损体积损失结果可知,尽管激光相变强化工艺不同,获得的硬化区组织不同,但是硬化区耐磨性均获得了较大的提高,其中试样S-840的耐磨性提升最明显,这和其平均摩擦因数最低是一致的。

  • 图9 基材及激光相变强化试样往复摩擦磨损体积损失

  • Fig.9 Volume loss after reciprocating friction and wear test of substrate and laser transformation hardening samples

  • 基材及不同激光相变强化工艺处理40CrNiMo钢试样经线性往复摩擦磨损表面形貌如图10所示。基材(图10a)磨痕比较明显,磨损坑平均宽度约为666 μm,磨损坑内存在大片亮白色区域,说明基材摩擦磨损过程中发生大块剥落,即存在较严重的黏附现象,同时基材磨损坑可以观察到较粗糙的“犁痕”,表明同样存在较严重的磨粒磨损,基材主要磨损机制为严重的黏着磨损+磨粒磨损。试样S-840 (图10b)磨损坑最窄,平均宽度约231 μm,试样表面只存在非常浅的“犁痕”,表明试样磨粒磨损较轻,磨损坑内组织沿磨损方向发生明显塑性变形,呈现 “鱼鳞状”,磨损机制主要为剥离磨损。试样S-600 (图10c)磨损坑较基材变窄,平均宽度约406 μm,磨损坑“犁痕”非常明显,同时磨损坑内存在较多亮白色区域,磨损机制主要为磨粒磨损+黏着磨损。试样S-360(图10d)与试样S-600类似,磨损坑平均宽度约387 μm,磨损机制主要为磨粒磨损+黏着磨损。

  • 图10 往复摩擦磨损表面局部形貌图

  • Fig.10 Local surface morphology after reciprocating friction and wear test

  • 2.4 讨论

  • 激光相变强化主要包括以下三个步骤:

  • (1) 通过激光将材料表层加热到高于奥氏体转变温度Ac1、低于材料熔点的温度。

  • (2) 使加热后的材料表层维持在奥氏体化温度Ac1(或完全奥氏体化温度Ac3)以上足够的时间,以使共析铁素体转变成奥氏体,渗碳体(Fe3C)溶解,并使C均匀地扩散到奥氏体相中。

  • (3) 停止加热,通过基材的热传导实现自冷却,冷至马氏体转变开始温度Ms以下时奥氏体快速转变为马氏体[26]

  • 扫描速度减小,单位面积有效能量增大,硬化区顶部维持在奥氏体化温度Ac1 以上的时间增加,最高温度升高,这个过程中可能经历不同的相区温度,发生不同的相变行为,因而得到不同的室温组织。硬化区不同区域也因热影响不同而呈现不同组织。过渡区和基材区几乎不受扫描速度的影响。过渡区温度略低于奥氏体化温度Ac1,回火索氏体组织发生了高温回火,因而组织变得粗大。基材区温度较低,组织无明显变化,仍为粗大的回火索氏体。

  • 如图11a所示,当激光扫描速度为840mm/min时,试样硬化区顶部(不完全淬火区)激光加热温度达到Ac1以上,回火索氏体中铁素体转变为奥氏体,碳化物发生分解,C原子固溶到奥氏体中,由于温度较低且在奥氏体转变温度以上停留时间较短,原始回火索氏体组织只有少量能转变为奥氏体组织。由于激光相变强化中冷速极快,C原子来不及析出,依然固溶在马氏体中,奥氏体转变为孪晶马氏体,因此,硬化区顶部室温组织为孪晶马氏体+细小回火索氏体。硬化区底部温度略高于Ac1 温度,冷速较慢,组织为细小的回火索氏体。

  • 如图11b所示,当激光扫描速度为600mm/min时,试样硬化区顶部(不完全淬火区)激光加热温度达到Ac1(甚至Ac3)以上,温度较840mm/min时高,在Ac1 温度以上停留时间也较840mm/min时长,所以原始回火索氏体转变为奥氏体含量更多,但不同奥氏体中C含量、合金元素含量以及马氏体的形成温度存在起伏,因而在极快的冷却条件下,不同奥氏体部分分别转变为板条马氏体和孪晶马氏体,所以硬化区顶部室温组织为板条马氏体+孪晶马氏体 +细小回火索氏体。硬化区底部加热温度同样达到Ac1 以上,但是温度较硬化区中心低,在Ac1 温度以上停留时间也较短,回火索氏体转变为奥氏体,随后冷却转变为孪晶马氏体,所以硬化区底部室温组织为孪晶马氏体+细小回火索氏体。

  • 如图11c所示,当激光扫描速度为360mm/min时,试样硬化区顶部(完全淬火区)激光加热温度达到Ac3以上熔点Tm以下,进入奥氏体单相区且停留时间足够长,所有原始回火索氏体组织均能转变为奥氏体组织,即完全奥氏体化,但单相区温度停留时间不足以使C原子充分扩散,导致不同奥氏体中C含量、合金元素含量以及马氏体的形成温度仍然存在起伏,在快速冷却条件下,不同奥氏体分别转变为板条马氏体和孪晶马氏体,无原始回火索氏体残留,所以硬化区顶部室温组织全部为板条马氏体+孪晶马氏体。硬化区自顶部由上及下加热温度逐渐下降,硬化区中部 (不完全淬火区)温度达到Ac1(甚至Ac3)以上,但受动力学条件限制,原始回火索氏体组织不能完全奥氏体化,室温组织为板条马氏体+孪晶马氏体+回火索氏体;硬化区底部加热温度同样达到Ac1以上,但是温度更低,在Ac1温度以上停留时间更短,室温组织为孪晶马氏体+回火索氏体。

  • 图11 硬化区顶部热影响及组织转变规律示意图

  • Fig.11 Schematic diagram of thermal effects and microstructure transformation at the top of the hardened zone

  • 成分相同时,材料的硬度和微观结构影响着材料的摩擦磨损行为[27]。由于不同扫描速度下的硬化区表面显微硬度差别极小,影响40CrNiMo钢摩擦磨损行为的主要为微观组织。细小的弥散的韧性相有利于松弛马氏体之间和马氏体与碳化物之间界面处的应力集中从而延迟裂纹扩展,提高疲劳性能和耐磨性能[28]。随着扫描速度的降低,组织中回火索氏体的含量减少,残余奥氏体的含量增加,马氏体晶粒尺寸增加[29]。扫描速度为840mm/min时,高强、高硬的细小孪晶马氏体与塑性、韧性较好的细小回火索氏体组成的复相组织有利于实现材料强化层的强韧性匹配,这对阻碍微裂纹的形成和扩展是有益的[30]。因此,试样S-840相对其他试样具有较好的耐磨性。扫描速度为600mm/min时,回火索氏体的含量大幅降低,耐磨性较差。360mm/min时,组织中存在一定量均匀、弥散分布的残余奥氏体,一定程度上阻碍了裂纹的形成和扩展[31],耐磨性较600mm/min时有所提高。

  • 3 结论

  • (1) 随扫描速度降低,显微组织变粗,马氏体含量增加,回火索氏体含量减少。奥氏体化过程中C原子无法充分扩散,不同C含量的奥氏体冷却后,分别转变为板条马氏体和孪晶马氏体。

  • (2) 不同扫描速度下获得的最高表面硬度值相近,776~789HV,相比基材(330HV)提升135%以上。硬化层最大深度随扫描速度降低而增加且截面硬度均随距表面距离增加而减小。

  • (3) 硬化区组织为孪晶马氏体+回火索氏体的试样耐磨性最优,摩擦因数相比基材降低24.9%,磨损体积减少94.3%,其磨损机制主要为剥离磨损。由高强的细小孪晶马氏体和韧性较好的细小回火索氏体组成的复相组织,能有效阻碍裂纹形成和扩展,对基材黏着磨损和磨粒磨损改善明显。

  • (4) 调控激光相变强化工艺参数,获得高强马氏体+韧性相的复相组织,能获得优异的耐磨性能。

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    • [27] TREVISIOL C,JOURANI A,BOUVIER S.Effect of hardness,microstructure,normal load and abrasive size on friction and on wear behaviour of 35NCD16 steel[J].Wear,2017,388-389(15):101-111.

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    • [29] KHORRAM A,DAVOODI JAMALOEI A,JAFARI A,et al.Nd:YAG laser surface hardening of AISI 431 stainless steel,mechanical and metallurgical investigation[J].Optics & Laser Technology,2019,119:105617.

    • [30] 蒋庆磊,李亚江,王娟,等.Q550 高强钢焊接接头强韧性匹配[J].焊接学报,2010,31(10):65-68,116.JIANG Qinglei,LI Yajiang,WANG Juan,et al.Strength matching on mechanical properties of welded joint of Q550 high strength steel[J].Transactions of the China Welding Institution,2010,31(10):65-68,116.(in Chinese)

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