2020-12-02

热处理温度和润滑条件对saf2507超双相不锈钢微动磨损性能的影响

Super duplex stainless steel (SDSS) has excellent mechanical properties and corrosionresistance. However, currently, there are few researches conducted on its fretting wear performance. This paper studies the influence of different heat treatment temperatures andmedium environment on the fretting wear performance of SAF 2507 SDSS. 超双相不锈钢(SDSS)具有优良的机械性能和耐腐蚀性能。然而，目前对其微动磨损性能的研究很少。这篇论文研究了不同热处理温度和介质环境对saf2507 SDSS微动磨损性能的影响。

Results showthat the combined effect of the sigma phase and seawater lubrication can significantlyimprove the wear resistance of SAF 2507 SDSS. After treated with different heat treatmenttemperatures, different contents of sigma phases are precipitated out of SAF 2507 SDSS,which improves the wear resistance of the material to different degrees.结果表明:sigma相和海水润滑的共同作用能显著提高saf2507 SDSS的耐磨性。经过不同热处理温度处理后，从SAF 2507 SDSS中析出不同含量的sigma相，不同程度地提高了材料的耐磨性。

In addition, the fret-ting wear performance of SAF 2507 SDSS also relates to the lubrication medium. In air, thefriction and wear performance of SAF 2507 SDSS is poor, while in seawater, solution andcorrosion products that acted as a lubricant dramatically improve the wear resistance of thematerial. Under the combined action of heat treatment and seawater lubrication medium,the friction coefficient and wear reduce by 70% and 91 %, respectively.此外，saf2507 SDSS的微动磨损性能也与润滑介质有关。在空气中，SAF 2507 SDSS的摩擦磨损性能较差，而在海水、溶液和腐蚀产品中作为润滑剂，极大地提高了材料的耐磨性。在热处理和海水润滑介质的共同作用下，摩擦系数和磨损系数分别降低70%和91%。

[DOI: 10.1115/1.4044081]

Keywords: super duplex stainless steel, heat treatment temperature, sigma phase, fretting,artificial seawater关键词:超双相不锈钢，热处理温度，sigma相，微动，人工海水

1 Introduction

Super duplex stainless steel (SDSS) is composed of two phaseswith approximately equal volume fractions of ferrite and austenite,and it has a combination of some characteristics of ferritic stainlesssteel and austenitic stainless steel.

1介绍

超双相不锈钢(SDSS)是由铁素体和奥氏体体积分数近似相等的两相组成，具有铁素体不锈钢和奥氏体不锈钢的一些特性。

In contrast to pure ferritic stainless steel or austenitic stainless steel, SDSS has better mechanicalproperties and corrosion resistance, which leads to their widespreadapplications in petrochemical industries andmarineindustries [1-8].与纯铁素体不锈钢或奥氏体不锈钢相比，SDSS具有更好的力学性能和耐腐蚀性能，这使得其在石油化工和海洋工业中得到广泛应用[1-8]。

However, some undesirable phases such as nitrides, carbides, andintermetallic phases may appear in these steels with several pre-cipitation reactions, and these undesirable phases will extremelyinfluence the working properties due to microstructural modifica-tions if the manufacturing condition cannot be properly controlled.

然而，一些不需要的相，如氮化物、碳化物和金属间相，可能会出现在这些钢中，经过多次的预合成反应，如果不能适当控制生产条件，这些不需要的相会由于微观结构的改变而极大地影响其工作性能。

Among these phases, the sigma phase with fast formation kineticshas been particularly studied. It can be formed in the temperatureinterval from 600 to 1000C [2,5,9-11]. Therefore, the tempera-tures of this study are selected from this temperature range.其中，对具有快速生成动力学的sigma相进行了重点研究。它可以在600 ~ 1000C的温度区间内形成[2,5,9-11]。因此，本研究的温度就是在这个温度范围内选取的。

Fretting is a form of surface damage that occurs between contact-ing surfaces with a small amplitude oscillatory movement [12,13].Itinduces friction and wear on the contact surface. At the same time, itaccelerates the initiation and expansion of fatigue cracks, which cangreatly reduce the fatigue life of components. Fretting damage hasbecome one of the main causes of the destruction of many devices.It generally occurs in a variety of machine elements, such as bearingraces, hub and shaft, keys and key ways, bolted and riveted joints,etc.[ 14- -21]. The surface damage caused by fretting may has unin-tended consequences, andthus, controlling the surface damagecaused by fretting is significant for industrial applications.微动是一种发生在具有小振幅振荡运动的接触表面之间的表面损伤形式[12,13]。它引起接触表面的摩擦和磨损。同时加速了疲劳裂纹的萌生和扩展，大大降低了构件的疲劳寿命。微动损伤已成为许多设备损坏的主要原因之一。它通常发生在各种机械部件上，如轴承、轮毂和轴、钥匙和键槽、螺栓和铆接接头等。[14 - -21]。微动所引起的表面损伤可能会产生不可逆的后果，因此，控制微动所引起的表面损伤对工业应用具有重要意义。

Super duplex stainless steel is widely used in marine engineeringowing to their superior corrosion-resistant and excellent comprehen-sive mechanical properties. 超双相不锈钢以其优异的耐腐蚀性能和综合力学性能，在船舶工程中得到了广泛的应用。

The sigma phase precipitated after heattreatment has a great influence on the wear resistance and corrosion resistance of SDSS. 热处理后析出的sigma相对SDSS的耐磨性和耐腐蚀性有很大的影响。

A large number of researches have been conducted onthe influence of the sigma phase on corrosion resistanceand wear behavior of duplex stainless steels [9,22- -27]. sigma相对双相不锈钢耐蚀性和磨损性能的影响已开展了大量研究[9,22- -27]。

However,the effect of the sigma phase on the fretting behavior of SDSS hasnot been well studied. Furthermore, the relationship between microstructure of the SDSS and frettingwear has not been thoroughlyestablished. 然而，sigma相对微动磨损行为的影响还没有得到很好的研究，此外，微动磨损与微动磨损之间的关系也没有得到很好的研究。

Therefore, in this study, SAF 2507 SDSS is chosen tostudy the evolution of the fretting behavior with different amountsof the sigma phase after treating at four different temperatures ranging from 800'C to 950'C at a regular interval of 50C for30 min. In addition, the effect of lubricating conditions on the frettingwear behavior of the SAF 2507 SDSS is further investigated.因此，在本研究中，我们选择了SAF 2507 SDSS来研究在800 - 950'C的四种不同温度下，每隔50℃处理30分钟，不同数量sigma相下的微动行为的演变。此外，进一步研究了润滑条件对saf2507 SDSS微动磨损行为的影响。

2 Experimental Procedure实验的程序

2.1Materials. 材料

SAF 2507 SDSS with a dimension of 12 mm x12.5 mm x 4 mm was purchased from Sandmeyer Steel Company.Its chemical composition (wt%) is 25.15 Cr, 6.74 Ni, 3.43 Mo,0.27 N, 0.022 C, 0.55 Si, 0.69 Mn, 0.029 P, 0.002 S, 0.13 Cu,and balance Fe.

SAF 2507 SDSS尺寸为12毫米x12.5毫米x4毫米，购自Sandmeyer钢铁公司。其化学成分(wt%)为:25.15 Cr、6.74 Ni、3.43 Mo、0.27 N、0.022 C、0.55 Si、0.69 Mn、0.029 P、0.002 S、0.13 Cu、平衡Fe。

Prior to fretting tests, each specimen was groundusing a series of SiC papers up to 2000 grit and then polished.After that, all the specimenswere degreased and cleaned withacetone and dried with N, gas. 在微动试验之前，每个样品都用一系列高达2000粒的SiC纸碾压，然后抛光。对标本进行丙酮脱脂清洗，氮气干燥处理。

AISI 52100 steel ball with a diameterof 6 mm and a hardness of 61-66 HRC was purchased from Shang-hai Steel Ball Plant Co., Ltd. The surface roughness of the steel ballwaslower than0.02 um.The artificialseawater waspreparedaccording to the standard ASTM D 1141-98.AISI 52100钢球直径6mm, HRC硬度61-66，购自上海钢球有限公司。钢球表面粗糙度低于0.02 um。人工海水是按照标准ASTM D 1141-98制备的。

2.2Heat Treatment and Metallographic Etch. In order tostudy the effect of microstructure on the fretting behavior of SAF2507 SDSS, the specimens were subjected to a series of heat treat-ments at different temperatures in a high-temperature electric resis-tance furnace (NWTQ 12C, Luoyang, China), and then was waterquenched to room temperature. After heat treatment, the specimenswere electrolytic etched in 40% NaOH solution to observe themicrostructure of SAF 2507 SDSS.2.2热处理和金相腐蚀。为了研究显微组织对SAF2507 SDSS微动行为的影响，在高温电阻炉(中国洛阳NWTQ 12C)中对试样进行了一系列不同温度的热处理，然后再进行室温水淬。样品经热处理后，在40% NaOH溶液中电解腐蚀，观察其微观结构。

2.3Fretting Tests. The fretting wear tests were conducted onSRV-IV oscillating frictionandwear tester with a ball-on-flatcontact configuration, as shown in Fig. 1. 2.3微动磨损测试。微动磨损试验是在srv - iv振动摩擦磨损试验机上进行的，该试验机采用平面接触球结构，如图1所示。

The upper specimenwas the commercially available AISI 52100 bearing steel ball andthe lower specimen was SAF 2507 SDSS. During fretting tests,the upper specimen ball was moved with small amplitude, and thelower flat specimen waskept stationary. 上标本为市场上有售的AISI 52100轴承钢球，下标本为SAF 2507 SDSS。微动试验中，上试样球进行小幅度运动，下试样保持静止。

All the fretting tests witha normal load of 50 N, a displacement amplitude of 100 um, a fre-quency of 20 Hz, and the test duration of 1 h were carried out atroom temperature. The radius and initial Hertz contact stress are101 um and 2.34 MPa, respectively. 在正常负载50 N、位移幅值100 um、频率20 Hz、试验持续时间1 h的条件下，均在室温下进行微动试验。半径和初始赫兹接触应力分别为101 um和2.34 MPa。

Fretting tests were performed in the media of air, deionized water, and artificial seawater.Toensure the reliability of experimental results, three repetitive exper-iments were conducted for each fretting condition.在空气、去离子水和人工海水中进行微动试验。为保证实验结果的可靠性，对每种微动状态进行了三次重复实验。

Fig.1A schematic diagram of SRV-IV oscillating friction andwear tester图1a SRV-IV振动摩擦磨损试验机原理图

2.4Characterization. The microstructures of as-received andheat-treatedSAF 2507 SDSSspecimens were observed by aBX53T-12F01 optical microscopy (OM, Olympus, Japan). Sigmaphase content is calculated by IMAGE-PRO PLUS 6.0 software. Thehardness values of the specimens treated with different heat treat-ment conditions were measured using a micro-V ickers hardnesstester (MH-5-VM, China) with a load of0.5 kg for 5 s.2.4描述。采用aBX53T-12F01光学显微镜(OM, Olympus, Japan)观察了经热处理和接收的saf2507 sdss试样的显微组织。Sigma相位内容由IMAGE-PRO PLUS 6.0软件计算。用微维氏硬度计(MH-5-VM，中国)测定了不同热处理条件下试样在0.5 kg载荷下5秒的硬度值。

The friction coefficient curves were recorded automatically bythe TRIBOMETER software. After fretting tests, the morphologiesand surface chemical compositions of wear scars of SAF 2507SDSS were observed and analyzed through a scanning electronmicroscopy combined with energy-dispersive spectroscopy (SEM/EDS, Japan Electron Optics L aboratory, Japan). The wear volumevalues, 2D profile micrographs, and 3D profile micrographs ofwear scarswere measured by a noncontact optical profilometer(Nano Map500LS, AEP Technology, USA).摩擦系数曲线由摩擦学软件自动记录。通过微动试验，利用扫描电子显微镜结合能量弥散光谱(SEM/EDS，日本电子光学实验室，日本)观察和分析了SAF 2507SDSS的磨损疤痕形貌和表面化学成分。使用非接触式光学轮廓仪(Nano Map500LS, AEP技术，美国)测量了磨损量、2D轮廓和3D轮廓。

3 Results and Discussion结果与讨论

3.1Microstructures of As-Received and Heat-Treated SAF2507 SDSS. Primitive microstructure of SAF 2507 SDSS consistedof approximately equal amounts of ferrite (ox) and austenite (y), andthe distribution of a and Y phases are homogeneous (Figs. 2(a) and2(b)). Figures 2(c)-2(j) show the microstructures of SAF 2507SDSS after heat treatment at 800'C, 850'C, 900 'C, and 950'Cfor 30 min, respectively. 3.1收到的和热处理的SAF2507 SDSS的组织。saf2507 SDSS的原始微观结构由等量的铁素体(ox)和奥氏体(y)组成，a相和y相分布均匀(图2(a)和2(b))。图2(c)-2(j)分别显示了在800'C、850'C、900 ' c和950' c热处理30分钟后的SAF 2507SDSS的组织。

The sigma phase is generated after heattreatment, and its content gradually changes as the heat treatmenttemperature elevates. After heat treatment at 800'C, the sigmaphase and secondary austenite are precipitated in the ferrite phase(Figs.2(c) and 2(d)). A certain amount of sigma phase particlesare nucleated preferentially at the ferrite and austenitic interfaceand then grow into the ferrite grains [28]. 西格玛相产生于热处理后，其含量随着热处理温度的升高而逐渐变化。经过800’c热处理后，西格玛相和二次奥氏体在铁素体相中析出(图2(c)和图2(d))。一定量的西格玛相颗粒在铁素体与奥氏体界面优先成核，然后成长为铁素体晶粒[28]。

The sigma phase is an inter-metallic that is rich of Cr and Mo, and precipitation of the sigmaphase induces a decrease of ferrite formed elements Cr and Moand increases austenite formed element Ni in the adjacent ferritephase; this forces secondary austenitic to be precipitated out offerrite, that is, ox- -0 +y2. When the heat treatment temperatureincreases to 850 'c, the sigma phase precipitation increases (Figs.2(e) and 2(f)). sigma相是一种富含Cr和Mo的金属间化合物，sigma相的析出导致相邻铁相中铁素体形成元素Cr和mo的减少，增加奥氏体形成元素Ni;这使得二次奥氏体析出铁矿，即ox- -0 +y2。当热处理温度增加到850 'c时，西格玛相析出增加(图2(e)和图2(f))。

However, sigma phase precipitation is graduallyrestricted with the temperature further increasing, which reachesthe maximum when at 850'C. When the heat treatment tempera-ture is lower than 850 'C, sigma phase precipitation increases withthe increase of heat treatment temperature, which is related to theincrease in the proliferation of alloying elements. However, as theheat treatment temperature increases further, although the prolifera-tion of alloying elements also increases, the drive of the sigma phaseprecipitation decline and the nucleation becomes more difficult, andthus, the sigma phase precipitation goes down.但随着温度的进一步升高，西格玛相降水逐渐受到限制，在850’c时达到最大。当热处理温度低于850 'C时，随着热处理温度的升高，sigma相析出增加，这与合金元素扩散增加有关。然而，随着热处理温度的进一步升高，尽管合金元素增殖增多，但sigma相析出的驱动减弱，成核变得更加困难，因此，sigma相析出减少。

Sigma phase content and hardness of SAF 2507 SDSS at differ-ent heat treatment temperatures are shown in Fig. 3. Both hardnessand sigma phase content present a same trend of first increase andthen decrease with the increasing heat treatment temperature. InFig.3(b), the hardness of as-received specimen is 264 HV. Afterheat treatment from 800 to 950 'C, all the hardnesses are higherthan that of the as-received specimen. This implies that sigma .phase has a significant impact on the hardness of this steel. Sigmaphase is a hard and brittle intermetallic compound with an averagehardness of up to 800 HV [29]. Therefore, the deposition of sigmaphase can improve the hardness of SAF 2507 SDSS.图3显示了不同热处理温度下SAF 2507 SDSS的Sigma相含量和硬度。随着热处理温度的升高，硬度和西格玛相含量呈现先升高后降低的趋势。在图3(b)中，试样硬度为264 HV。经过800 - 950 'C的热处理后，所有的硬度都比接收样品的硬度高。这意味着sigma相对这种钢的硬度有显著的影响。乙叉酶是一种硬而脆的金属间化合物，平均硬度高达800 HV[29]。因此，乙胺基酶的沉积可以提高saf2507 SDSS的硬度。

3.2Fretting W ear Behavior of SA F 2507 SDSS After HeatTreatment. Figure 4 depicts the friction coefficient as a functionof the cycle number during fretting wear under different tempera-tures in air, deionized water, and artificial seawater. In Fig. 4, all the friction coefficients present obvious increase in the initial periodand get stable later. However, there is a distinct difference betweenair and aqueous environments.3.2热处理后的saf2507 SDSS的微动磨损行为。图4描述了在空气、去离子水和人工海水中不同温度下微动磨损的摩擦系数作为循环数的函数。在图4中，摩擦系数在初始阶段都有明显的增加，之后趋于稳定。然而，空气和水的环境有明显的区别。

When fretting in aqueous environ-ments, all the friction coefficients decrease as they reach a plateau(~1000 cycles). The fluctuations of friction coefficient in bothaqueous mediums show better stability, and friction coefficientvalues arelower thanthose in air. The frictioncoefficientdecreases successivelywhen the fretting tests are performed inair, deionized water, and artificial seawater.

在水环境中微动时，摩擦系数在达到平稳期(约1000次)时减小。两种水介质中摩擦系数的波动表现出较好的稳定性，且摩擦系数值低于空气中。当微动试验在空气、去离子水和人工海水中进行时，摩擦系数不断降低。

This phenomenon isinduced by solution lubrication and the formation of corrosion prod-ucts in seawater [30- -32]. Artificial seawater has complex ingredi-ents in comparison with the deionized water. The active elementssuch as S, P, and Cl in artificial seawater are easy to react withthe element Fe in AISI 52100 ball to form the low shear tribo-layerduring fretting [30,31]. In addition, wear debris can be corroded byseawater, and the generated corrosion products exhibit excellentlubricant effect, which is observed by previous studies [32- -37].这一现象是由溶液润滑和海水中腐蚀突起的形成引起的[30- -32]。与去离子水相比，人工海水具有复杂的成分。人工海水中的活性元素S、P、Cl等在微动过程中容易与AISI 52100球中的Fe元素发生反应，形成低剪切摩擦层[30,31]。此外，磨屑可被海水腐蚀，腐蚀产物表现出良好的润滑效果，这是前人研究的结果[32- -37]。

Therefore, relatively low friction coefficient is observed in artificialseawater. On the contrary, in air, during the initial period of frettingprocess, the presence of thin oxide films on the metal surfacedecreases the friction coefficient. As the fretting process goes on,the friction coefficient increases rapidly within a small cyclenumber because of the breakage of thin oxide films. In air, debrisare easily produced with repetitive tangent stress, and they arehardly rejected from the fretting zone completely. 因此，在人工海水中，摩擦系数相对较低。相反，在空气中，在摩擦过程的初始阶段，金属表面氧化膜的存在降低了摩擦系数。随着微动过程的进行，由于氧化膜的破裂，摩擦系数在一个小的循环数内迅速增加。在空气中，碎屑很容易产生重复的切线应力，而且很难完全排除在微动区。

These debrishave dual effects on the fretting performance. On the one hand,the adhesion wear particles separate the metal-to-metal directcontact and reduce the friction coefficient acting as rolling balls.On the other hand, however, once the wear particles are expelledfrom the contact surface, the metallic surfaces will be in contactwith each other directly and lead to higher friction consequently[30,38,39]. The competitive two effects achieve dynamic balance,and the friction coefficient gets steady, as shown in Fig. 4(a).这些碎片对微动性能有双重影响。一方面，粘着磨损颗粒分离了金属与金属的直接接触，使摩擦系数降低，起到滚动球的作用。但另一方面，一旦磨损颗粒从接触面被驱逐出去，金属表面就会直接接触，从而导致更高的摩擦力[30,38,39]。竞争的两种效应达到动平衡，摩擦系数趋于稳定，如图4(a)所示。

To further determine the combined effect of the sigma phaseand thefretting lubricating conditionon friction coefficient, therelationship between the average friction coefficient and the heattreatment temperature in these three friction medium environmentsis illustrated in Fig. 5.为了进一步确定sigma阶段和微动润滑条件对摩擦系数的综合影响，图5说明了这三种摩擦介质环境中平均摩擦系数和热处理温度之间的关系。

In Fig. 5(a), the average friction coefficientvalues decrease when frettingtests are carried out in air, deionizedwater,andartificialseawater:0.9, 0.5, and 0.3, respectively.Figure 5(a) shows that in these three lubricating environments,theaverage friction coefficient valuesfirst decrease and then increase with the increase of heat treatment temperature. Averagefriction coefficient reaches the lowest level when specimens aretreated at 850C. 在图5(a)中，在空气、去离子水和人工海水中进行放大试验时，平均摩擦系数值分别减小:0.9、0.5和0.3。由图5(a)可知，三种润滑环境中，平均摩擦系数值随热处理温度的升高先减小后增大。平均摩擦系数在850C时达到最低水平。

Figure 5(b) shows that the average friction coef-ficient of the specimens after heat treated at 850 'C under artificialseawater is reduced by 60% compared with that under air. Underthe combined effect of seawater and the sigma phase, the friction coefficient is further decreased by 70%. These results show that thesigma phase and lubrication medium have synergistic effect on theantifriction effect of SAF 2507 SDSS.由图5(b)可知，试样在850′C温度下进行热处理后，在人工海水条件下的平均摩擦系数比在空气条件下降低了60%。在海水和sigma相的共同作用下，摩擦系数进一步降低了70%。结果表明，润滑油相和润滑介质对saf2507 SDSS的减摩效果具有协同作用。

Figure 6 presents the variations of wear volume with heat treat-ment temperatures in variousfretting wear environments. In Fig.6(a), the change of the wear volume relates to the temperature ofheat treatment. The wear volume presents a trend of first decreaseand then increases with an increase in the heat treatment tempera-ture. This tendency is much more pronounced when the frettingprocess is conducted in air. 图6显示了在不同微动磨损环境下，磨损量随热处理温度的变化。在图6(a)中，磨损量的变化与热处理温度有关。随着热处理温度的升高，磨损量呈现先减小后增大的趋势。当微动磨损过程在空气中进行时，这种趋势更为明显。

When the fretting wear test is carriedout in air, the wear volume of the as-received specimen is 26 X3

10 um'. However, it decreases to 10x

10umafter heat treatedat 850 'C. Therefore, wear resistance is improved bythe presenceof high sigma phase content. When the frettingwear tests arecarried out in deionized water and artificial seawater, the specimens treated at the same temperature show similar wear volume values.在空气中进行微动磨损试验时，试样的磨损体积为26 X3

10 um”。但是，它减少到10x

在850 ' c热处理后，因此，高西格玛相含量的存在提高了耐磨性。在去离子水和人工海水中进行摩擦磨损试验时，相同温度下的摩擦磨损体积值相近。

However,thewear volume is significantly reducedcomparedwith that of in air. This implies that the sigma phase and lubricatingmedium have a synergistic effect on improving the wear resistanceof material. Compared with the as-received specimen in air, thewear volume of SAF 2507 SDSScanbe reduced by up to 91%after heat treatment and under the lubrication of seawater.但是，与在空气中相比，磨损量明显减少。这意味着sigma相和润滑介质对提高材料的耐磨性有协同作用。与空气中试样相比，经过热处理和海水润滑后，试样的磨损量可减少91%。

Figure 7 shows the 2D profile of wear scars perpendicular to thefretting direction. When fretting tests are performed in air, ridges areobserved at the edge of wear scars and wear scar profile curves areuneven, as shown in Fig. 7(a). The wear depth of wear scars in airis deeper than those in deionized water andartificial seawater.When the fretting tests are carried out in aqueous medium environ-ment, the wear scars profile curves are smoother.' The wear scarsfretted in artificial seawater are the smoothest. 图7显示了垂直于微动方向的磨损疤痕二维轮廓。在空气中微动试验时，磨损疤痕边缘可见隆起，磨损疤痕轮廓曲线不均匀，如图7(a)所示。空气中的磨痕磨损深度比去离子水和人工海水中的磨痕更深。在水介质环境下进行微动试验时，磨损疤痕轮廓曲线较为光滑。“用人造海水浸泡的腕带是最光滑的。

Therefore, unlike in air, the wear damage in the aqueous medium is moreslightly.Compared with the as-received specimen, the wear depth of theonesheat treated at different temperatures is small. It indicatesthat both the medium environment and the generation of sigmaphase have a remarkable effect on the damage of wear scars.因此，不像在空气中，在水介质中的磨损损伤更轻。与试样相比，试样在不同温度下的磨损深度较小。这表明，培养基环境和乙炔酶的产生对磨损瘢痕的损伤均有显著影响。

3.3Fretting Wear Mechanism. Wear surface morphologyand the corresponding oxygen distribution are further characterizedto analyze the fretting wear mechanism under different conditions,asshown in Fig. 8. Under the samelubricating medium,wear scarsof SAF 2507 SDSS after heat treatment at 850 'C are shallower thanthose of as-received specimens. 3.3接触磨损机制。进一步表征了磨损表面形貌和氧的分布，分析了不同条件下的微动磨损机理，如图8所示。在相同的润滑介质下，在850 'C热处理后的saf2507 SDSS的耐磨层比接受试样的耐磨层浅。

This phenomenon is consistent withthe previous result of the two-dimensional profile of wear scars inFig.7. The results indicate that the precipitation of sigma phasecan improve the wear resistance of material. Figures8(a)- -8(d)shows SEM surface micrographs of the wear scars in air, which are different from those in deionized water and artificial seawater.In air, there are serious spalling and adhesion on the worn surfaces,which can also be observed in Figs. 9(a) and 9(b). 这一现象与之前图7中二维磨损疤痕剖面的结果一致。结果表明，西格玛相角的析出提高了材料的耐磨性。图8(a)- -8(d)为空气中不同于去离子水和人工海水中的磨损疤痕的SEM表面显微图。在空气中，磨损表面有严重的剥落和粘附，这在图9(a)和图9(b)中也可以观察到。

From the distri-bution of the corresponding oxygen element, the content of oxygenin the wear scar is significantly higher than that of the base material.The worn surface is covered with an oxide layer. Thus, the frettingwear mechanism for SAF 2507 SDSS in air is a combination ofadhesion wear, fatigue delamination, and oxidation wear. Unlikein air, when fretting is conducted in aqueous, the worn surface ismade up of ploughing parallel to the fretting direction and manymicro-cracks parallel or perpendicular to the fretting direction.

When compared with the wornsurface of SAF 2507 SDSStreated at 850C (Figs. 8(g), 8(k)and8(h), 8(l)), more severedamage and wear particles are produced on the worn surface ofthe as-received specimens (Figs. 8(e), 8(i) and 8(f), 8(j)). Somedebris partly accumulate within and around the contact zone(Figs. 8(e) and 8(i)).

The sigma phase improves the wear resistanceof specimens, which are heat treated at 850 'C, and thus, only asmall amount of debris are produced and they are easily expelledfrom the contact zone with the flow of solution during the frettingtests, since the accumulation of wear particles iS not observed onthe worn surface (Figs. 8(g), 8(k) and 8(h), 8(l)). In addition, theaqueous solution separates the worn surtace from the oxygen 1nthe air, and the content of oxygen detected from the worn surfaceis much lower.

The results indicate that aqueous environmentshave positive effects on the tribology behavior of SAF 2507.First, aqueous environment separates the friction pairs from oxida-tive air since oxygen dissolved in water is much lower than that inair. Furthermore, in aqueous environment, the friction pairs are con-tinuously cooled. The adhesion wear induced by instant high tem-perature is thus effectively suppressed. Instead, the wear loss ismainly induced by the plough effect of the counterpart.

Accordingly, the frettingwear mechanisms of SAF 2507 SDSS inaqueous environments aremild oxidative wear, abrasive wear,and fatigue wear. Figure 9 displays the 3D morphologies of wornsurfaces of SAF 2507 SDSS. The 3D morphologies further prove the assumption of wear mechanism in various test environmentsdeduced by SEM.

4Conclusions

Fretting tests on SAF 2507 SDSS treated at different tempera-tures are carried out in air, deionized water, and artificial seawater.The major conclusions resulting from the study can be summarizedas follows:

(1) Heat treatment temperature has a significant effect on the pre-cipitation amount of sigma phase, and it further influencesthe hardness of SAF 2507 SDSS.

(2) Sigma phase precipitation improves the fretting behavior ofSAF2507 SDSS. The friction coefficient and thewearvolume decrease with the increase of sigma phase precipita-tion, especially in air. The change in wear volume is morepronounced.

(3) The aqueous environment has a great effect on the frettingbehavior of SAF 2507 SDSS due to solution acted as a

lubricant reducestheseverity of bothnormal and shearstresses insolidsurface contact. Artificial seawater hasbetter lubricating effect than deionized water due to corro-sion products.

(4) Sigma phase and lubrication medium have a synergisticeffect on the fretting behavior of SAF 2507 SDSS. Comparedwith the as-received specimen in air, the friction coefficientcan be reduced by 70%, and the wear canbe reduced byup to 91 % under the combination of heat treatment conditionand the appropriate lubricating medium.

Funding Data

National Natural Science Foundation of China, China (GrantNo.51405478; Funder ID: 10.1 3039/501100001809).