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感应重熔对高速轧机轴承热喷涂层微观组织性能的影响

2021-09-09解芳翟长生燕松山胡瑞许春霞

河北科技大学学报 2021年4期

解芳 翟长生 燕松山 胡瑞 许春霞

摘 要:為了改善高速轧机轴承热喷涂层的微观组织性能,采用感应重熔技术对GCr15轴承钢表面预制备的高能火焰喷涂Ni60A涂层进行感应重熔处理,并利用金相显微镜、扫描电镜、X射线衍射仪、显微硬度计对感应重熔前后涂层的孔隙率、微观组织、显微硬度等进行对比研究,探讨感应重熔对涂层以及界面微观组织、显微硬度的影响。结果表明,高能火焰喷涂镍基涂层的孔隙率高达5.09%,且表面孔洞缺陷较多,涂层与基体结合的界面处存在较明显的界面裂纹和孔隙缺陷,呈现典型的机械结合,界面结合特性较差;而经感应重熔后其涂层组织致密,孔隙率仅为0.27%,涂层缺陷明显减少,与基体结合的界面处呈现出强冶金融合特性,且涂层中硬质相数量显著增多,涂层及界面的显微硬度均得到较大提升。因此,感应重熔技术可改善涂层的表面组织性能,在高速轧机轴承表面强化方面有一定的应用价值。

关键词:材料表面与界面;感应重熔;高速轧机轴承;镍基涂层:微观组织

中图分类号:TG174.4   文献标识码:A

doi:10.7535/hbkd.2021yx04004

收稿日期:2021-03-25;修回日期:2021-05-28;责任编辑:冯 民

基金项目:国家自然科学基金(51605230,51765044);河南省高等学校重点科研项目(21A460021);河南省高等学校青年骨干教师培养计划项目(2016GGJS-148)

第一作者简介:解 芳(1981—),女,江苏滨海人,副教授,博士,主要从事摩擦学理论及其工业应用方面的研究。

E-mail:xiefang811222@163.com

解芳,翟长生,燕松山,等.感应重熔对高速轧机轴承热喷涂层微观组织性能的影响[J].河北科技大学学报,2021,42(4):345-351.XIE Fang,ZHAI Changsheng,YAN Songshan, et al.Effect of induction remelting on microstructure and properties of hot spraying coating for high speed rolling mill bearings[J].Journal of Hebei University of Science and Technology,2021,42(4):345-351.

Effect of induction remelting on microstructure and properties of hot spraying coating for high speed rolling mill bearings

XIE Fang1,ZHAI Changsheng1,2,YAN Songshan3,HU Rui4,XU Chunxia4

(1.School of Intelligent Manufacturing,Nanyang Institute of Technology,Nanyang,Henan 473004,China;2.Henan Hangong Machinery Remanufacturing Technology Company Limited,Nanyang,Henan 473000,China;3.School of Mechanical and Electronic Engineering,Wuhan University of Technology,Wuhan,Hubei 430070,China;4.School of Mechanical and Electrical Engineering,Nanchang Institute of Technology,Nanchang,Jiangxi 330029,China)

Abstract:In order to improve the microstructure and properties of hot spraying coating for bearing of high speed rolling mill,induction remelting was carried out on the Ni60A coating which was prepared on the surface of GCr15 Bearing Steel by high energy flame spraying.The porosity,microstructure and microhardness of the coating before and after induction remelting were analyzed comparatively by metallographic microscope,scanning electron microscope,X-ray diffractometer and microhardness tester.The effect of induction remelting on the microstructure and microhardness of the coating and interface was investigated.The results show that the porosity of nickel based coating prepared by high energy flame spraying is as high as 5.09%,and there are many surface pore defects,as well as there are obvious interface cracks and pore defects at the interface between the coating and the substrate,showing typical mechanical bonding and poor interface bonding properties.However,after the induction remelting,the microstructure of the coating is dense,the porosity is only 0.27%,the defects of the coating are obviously reduced,and the bonding interface between the coating and the substrate shows strong metallurgical fusion characteristics.Additionally,the amount of hard phase in the coating increases significantly,and the microhardness of the coating and interface is greatly improved.Therefore,the induction remelting process can not only improve the surface quality of hot spraying coating for high-speed rolling mill bearings,but also improve the interface bonding characteristics of the coating.The application of induction remelting technology can improve the coating surface structure property,and it may be used in the surface strenthening of high-speed rolling mill bearings for some point.

Keywords:

material surface and interface;induction remelting;bearing of high speed rolling mill;nickel based coating;microstructure

高速轧机轴承是中国重点发展的高端轴承之一,其在高摩擦磨损和重载冲击的条件下工作,工况环境十分恶劣,是高速轧机的主要易损件[1-2]。随着用户对轧制产品质量要求的提高,轧机轧制力与轧制速度不断增加,对轧机轴承的性能也提出了更高要求。因此,提高高速轧机轴承的可靠性与寿命是轧机轴承制造业急需解决的关键问题。现有研究表明,起源于轴承表面或亚表面的裂纹、磨损、腐蚀等损伤,是影响高速轧机轴承可靠性和寿命的关键因素[3-6]。如何改善高速轧机轴承的表面组织结构,增加高速轧机轴承的表面硬度和耐磨性,是提高其可靠性和寿命需要突破的技术难题[7-9]。

热喷涂层的重熔技术是提高轴承等零部件表面性能的有效方法之一[10-11]。目前,常用的重熔技术主要包括钨极氩弧重熔[12-13]、电子束重熔[14-15]、激光重熔[16-17]、感应重熔[18-19]等。上述几种重熔技术均可以改善热喷涂层的耐磨、耐蚀、抗氧化等性能。然而,钨极氩弧重熔难以对低熔点和易蒸发的金属(如铅、锡、锌)表面涂层进行重熔;电子束重熔受到真空室空间的限制,仅适用于加工尺寸较小的零部件;激光重熔的生产效率较低,且生产成本较高,不适用于大型零部件的表面强化;而感应重熔技术加工效率高,引起的金属基体热损伤和变形小,特别适用于大型柱状零部件的表面强化[11]。

基于此,为了探讨利用感应重熔技术对高速轧机轴承进行表面强化的可行性,本文采用感应重熔技术对GCr15轴承钢表面预制备的高能火焰喷涂Ni60A涂层进行感应重熔处理,研究感应重熔对高速轧机轴承热喷涂层微观组织性能的影响,为感应重熔技术在高速轧机轴承表面的强化应用提供理论依据。

1 试样制备与试验方法

1.1 试样制备

本试验的涂层材料为武汉汉工智造新材料科技有限公司研制的NF201材料[20],该材料为镍基自熔性合金粉末,SEM形貌如图1所示。由图1可知:该粉末呈圆球形状,粒度分布为45~106 μm,流动性好,易控制送粉量,适宜自动喷涂操作。

利用高能火焰喷涂设备[21],在经过表面喷砂粗糙活化处理的高速轧机轴承的GCr15钢基体上制备厚度为1 mm的涂层,高能火焰喷涂工艺参数见表1。

为了研究感应重熔对高速轧机轴承热喷涂层微观组织性能的影响,利用自主研制的高效智能感应重熔系统,对制备的高能火焰喷涂镍基涂层进行感应重熔[20],感应重熔过程中,涂层材料的预热温度为300 ℃,重熔温度为1 012 ℃,精度为±3 ℃。重熔完成后,空冷至室温。

1.2 试验方法

1)利用线切割电火花机床将高能火焰喷涂镍基涂层在感应重熔前后的2种涂层试样切割成断面为7 mm×7 mm和表面为10 mm×7 mm的试样;利用XQ-1型金相试样镶嵌机对试样进行镶嵌,经砂纸打磨后,使用金刚石抛光膏将试样抛至镜面,按体积比3∶1将浓盐酸和浓硝酸配制成腐蚀剂对试样进行腐蚀处理。

2)采用LEXT OLS4100奥林巴斯激光共焦显微镜观察上述2种涂层表面及断面的微观组织,计算涂层表面的孔隙率。

3)采用德国布鲁克D8型XRD衍射仪对2种涂层试样的物相组成进行测定,扫描速度为2°/min,扫描范围为30°~80°,步长为0.02°。

4)采用FEI Quanta 650 FEG扫描电镜观察涂层的微观组织形貌,并利用X射线能谱仪进行EDS分析。

5)利用Innovatest Falcon 50维氏显微硬度仪测试涂层试样断面显微硬度。

2 结果与讨论

2.1 喷涂粉末及涂层的物相组成

HG201喷涂粉末及高能火焰喷涂镍基涂层在感应重熔前后的XRD图谱见图2。由图2可知,HG201粉末以Ni,Cr3Si,NiB,Ni3Si2,Cr3Ni5Si2,Ni2Si和Ni16Cr6Si7等非硬质相为主,并存在着少量的Fe23(C,B)6,Cr7C3硬質相,如图2 a)所示。经过高能火焰喷涂后(感应重熔前),涂层中出现了Fe3(C,B)6,Fe7C3,Cr7C3等硼化物和碳化物硬质相,以及Fe5SiB2,FeSi2,Ni4B3等非硬质相,而原喷涂粉末中NiB,Cr3Ni5Si2非硬质相消失,如图2 b)所示,说明高能火焰喷涂促进了粉末的部分熔化,形成了新的物相体系。对高能火焰喷涂镍基涂层进一步进行感应重熔后,其物相以Ni固溶体及Fe7C3,Fe23(C,B)6,(Cr,Fe)7C3,Cr7C3,CrB4硬质相为主,并存在着少量的Ni3Si2,Ni4B3,Cr3Ni5Si2非硬质相,如图2 c)所示,由此可见感应重熔促进了硬质相的增加。

2.2 感应重熔前后镍基涂层的金相组织

图3 a)、图3 b)所示分别为感应重熔前后高能火焰喷涂镍基涂层的表面金相组织图。由图3可知:感应重熔前,高能火焰喷涂镍基涂层的表面含有较多尺寸较大的孔洞、氧化夹杂和裂纹缺陷,分别如图3 a)中箭头1,2,3所示。感应重熔后,高能火焰喷涂镍基涂层的表面组织变得非常致密,仅存在少量尺寸相对较小的孔洞,如图3 b)中箭头4所示,而几乎未见裂纹缺陷。

图4 a)、图4 b)所示分别为感应重熔前后高能火焰喷涂镍基涂层的断面金相组织图。由图4可知:感应重熔前,高能火焰喷涂镍基涂层断面的界面处存在较明显的孔隙缺陷和界面裂纹,分别如图4 a)中箭头1,2所示,呈现典型的机械结合,界面结合强度较低。如图4 b)所示,感应重熔后,高能火焰喷涂镍基涂层与基体在界面处发生融合,形成了紧密的冶金结合,界面处的孔隙缺陷和界面裂纹显著减少。

利用Image-proplus6.0金相分析软件对上述2种涂层的孔隙率进行测试,结果如表2所示。由表2可知:感应重熔前高能火焰喷涂镍基涂层的平均孔隙率为5.09%,孔隙率较高;而该涂层经过感应重熔后其平均孔隙率仅为0.27%,说明对高能火焰喷涂镍基涂层进行感应重熔可以有效降低其孔隙率,减少涂层孔隙缺陷,提高组织致密性。

2.3 感应重熔前后镍基涂层的SEM形貌观察及EDS分析

图5为感应重熔前后高能火焰喷涂镍基涂层的表面SEM形貌。由图5可知:感应重熔前高能火焰喷涂镍基涂层中存在尺寸较大、数量较多的孔洞和裂纹,如图5 a)中箭头1,2,3所示。感应重熔后,高能火焰喷涂镍基涂层中的孔洞及裂纹显著减少,涂层中主要弥散分布着白亮色组织(Ni固溶体)、黑色组织(碳化物、硼化物等硬质相)和灰色组织(Ni,Cr合金的软质共晶组织)[22],分别如图5 b)中箭头4,5,6所示。说明感应重熔能够有效促进涂层晶粒细化,使其组织变得更加致密。

图6为感应重熔前后高能火焰喷涂镍基涂层的断面SEM形貌。由图6 a)可知:由于高速火焰射流的冲击作用,高能火焰喷涂涂层中的颗粒明显扁平化,界面出现了贯穿裂纹,如图6 a)中箭头1所示。喷涂形成的颗粒周围发现闭合及连续的裂纹,如图6 a)中箭头2,3,4所示。该断面SEM形貌再次证明了高能火焰喷涂镍基涂层的多缺陷特征,且其界面呈现典型的机械结合,界面结合特性较差。由图6 b)可知,感应重熔后的高能火焰喷涂镍基涂层组织较致密,且其与基体结合的界面处未发现明显的孔洞及界面裂纹等缺陷。这说明感应重熔有助于涂层与基体在界面处形成良好的冶金反应,显著改善界面的结合特性。

图7为感应重熔前后高能火焰喷涂镍基涂层的断面EDS图谱。由图7 a)可知,感应重熔前的高能火焰喷涂镍基涂层中的主元素Ni和GCr15基体组织的主元素Fe在界面处呈现陡降特性,进一步验证了高能火焰喷涂过程中基体和涂层没有在界面处形成融合区,属于典型的机械结合。由图7 b)可知:高能火焰喷涂镍基涂层经感应重熔后,基体一侧以Fe元素为主,涂层一则以Ni,Cr元素为主;从基体向界面过渡时Fe元素含量逐渐降低,而Ni,Cr元素含量逐渐增加。在基体与涂层的界面结合处Fe,Ni,Cr元素大量共存,与感应重熔前涂层在界面处Fe元素的陡变现象不同,感应重熔后的涂层在界面附近Fe含量呈现层次性梯度过渡特征,说明感应重熔后在界面处形成了冶金结合的融合区,有助于改善涂层与基体的界面结合特性。

2.4 感应重熔前后镍基涂层的显微硬度

图8所示为感应重熔前后高能火焰喷涂镍基涂层断面显微硬度分布图。由图8可知,2种涂层的断面显微硬度均呈现阶梯状分布,基体部分的显微硬度均较小且基本相当;感应重熔前,高能火焰喷涂镍基涂层在界面过渡处的显微硬度略低于基体,而感应重熔后涂层在界面过渡处的显微硬度显著增加,且其涂层部分的显微硬度明显高于感应重熔前,说明感应重熔不仅显著提高了涂层的显微硬度,而且有助于涂层获得较好的断面显微硬度分布,改善界面的结合特性。

3 结 论

本文对GCr15轴承钢表面高能火焰镍基涂层进行了感应重熔,对比分析了感应重熔前后涂层表面及断面的孔隙率、金相组织、微观形貌与能谱以及显微硬度的变化情况,探讨了感应重熔对涂层微观组织以及界面特性的影响,得到如下结论。

1)感应重熔不仅可以有效减少高速軋机轴承热喷涂层的孔洞及裂纹缺陷,显著降低涂层的孔隙率,改善涂层表面的组织性能,而且可以有效促使基体中的Fe元素、涂层中的Ni和Cr等元素在界面处相互向对方层扩散,从而在界面处产生充分的冶金反应,获得良好的界面结合特性。

2)高速轧机轴承热喷涂层经过感应重熔后,晶粒明显细化,Fe7C3,Fe23(C,B)6,(Cr,Fe)7C3,Cr7C3,CrB4等硬质相大幅增加,表面及断面显微硬度均得到显著提高,具有较好的显微硬度分布特征。

采用单一的感应重熔技术对高速轧机轴承进行表面强化的效果仍比较有限,未来可探讨通过后热处理工艺进一步改善涂层的综合性能。

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