【作者】 齐勇田；

【导师】 邹增大；

【作者基本信息】 山东大学 ， 材料加工工程， 2009， 博士

【摘要】 半钢轧辊广泛应用于冶金行业的轧机中,由于其工作条件非常恶劣,轧辊的表面上经常会出现磨损和剥落等情况,严重地影响热轧产品的品质和质量,若轧辊只能报废或替换,这将会给企业带来巨大的经济损失。为了修复己报废的铸造合金ZUB160CrNiMo半钢轧辊,针对这种轧辊的主要失效行为,本文利用激光熔覆技术在轧辊基材表面制备原位自生金属陶瓷复合层,这种熔覆层可以实现金属材料与陶瓷材料两优异性能的结合,该领域研究应用前景广阔。鉴于此,本文在以下方面开展了研究工作:本文选择横流二氧化碳激光束作为热源,利用激光熔覆技术在ZUB160CrNiMo半钢轧辊表面进行耐磨熔覆层的工艺研究,通过激光功率、扫描速度、光斑直径等三个主要激光熔覆工艺参数对熔覆层几何形状与稀释率的影响,分析了激光熔覆工艺参数对熔覆层质量的作用效果,研究指出:随着激光功率的增加,熔覆层宽度加大,厚度减小,熔深增加,形状系数增大,稀释率增加,晶粒粗化。随着扫描速度的增大,熔覆层宽度、厚度、熔深减小,形状系数和稀释率也减小,熔覆层晶粒细化,硬度增加,表面粗糙,气孔产生的倾向变大。对本研究采用的自制的合金粉末,当单层单道焊时,预涂层厚度为1mm时,激光功率选3000瓦,扫描速度为300mm/min,光斑直径3.0mm为宜。根据ZUB160CrNiMo半钢轧辊表层对熔覆层性能的要求,自行研制了FeCrBSiMo自熔性铁基合金粉末。选择TiN陶瓷粉与石墨C（摩尔比为1:1）混合粉作为陶瓷粉末,以其质量百分数30%与Fe基合金粉末机械混合均匀,构成激光熔覆预涂粉,通过丙酮稀释的有机粘结剂粘涂在半钢轧辊表面,采用适当的激光工艺可获得质量良好的熔覆层。然后利用现代分析测试手段（OM、SEM、TEM、EPMA、EDAX、XRD等）对熔覆层组织结构进行了分析,结果表明:激光熔覆铁基金属陶瓷复合层的组织由α—Fe及大量形状不规则的稳定相Ti(C_yN_1-y)（0≤y≤1）共同组成。Ti(C_yN_1-y)是预涂粉中加入的TiN和石墨在激光熔覆过程中通过原位反应合成的新强化相,其尺寸在0.1～40μm,弥散分布在马氏体基体中。经TEM观察,Ti(C_yN_1-y)（0≤y≤1）与熔覆层基体结合紧密,具有洁净的相结构,界面无孔洞和其它析出相。依据热力学理论,对TiN与C原位合成Ti(C_yN_1-y)的反应进行了热力学分析,得出在激光熔覆过程中TiN发生分解TiN=[Ti]+[N],分解出来的[Ti]原子将和石墨C优先结合,发生[Ti]+[C]=TiC,同时[Ti]也会同高温分解出的N原子反应[Ti]+[N]=TiN。TiC和TiN两种陶瓷颗粒不仅结构均为体心立方晶格,而且它们的晶格常数非常接近,因而它们具有很好的互溶性,冷却凝固时发生yTiC+（1-y）TiN=Ti(C_yN_1-y)固溶反应,形成复合颗粒硬质相。当激光熔覆进行时,TiN和石墨C在高温下还会直接发生置换反应,C会置换TiN中的部分N,生成Ti(C_yN_1-y)颗粒相且原位析出。对Ti(C_yN_1-y)颗粒增强的铁基熔覆层的耐磨性及其磨损机制进行了研究,结果表明,原位合成的Ti(C_yN_1-y)显著地提高熔覆层的显微硬度和耐磨性,其强化机制除了第二相弥散强化和细晶强化外,还包括C、Mo、Cr等元素的固溶强化和马氏体组织强化。在半钢轧辊表面形成含Ti(C_yN_1-y)颗粒强化的激光熔覆层,在熔覆层的内部易产生平行于熔合线的横向裂纹和沿树枝晶方向的纵向裂纹,特别是当进行多层多道熔覆时,熔覆层的裂纹敏感性增大。熔覆层内部裂纹多起源于熔覆层和半钢基体结合处,然后向铁基熔覆层的表层扩展,裂纹呈现穿晶断裂和沿晶断裂两种形态。熔覆接头的热影响区未发现有裂纹产生。优化工艺参数可减少熔覆层裂纹的发生倾向,也可在半钢基底母材上熔覆—Ni基梯度过渡层,来减小铁基熔覆层产生裂纹的敏感性。更多还原

【Abstract】 Adamite steel is being widely used to make rollers in the field of metallurgy because of its better hardness, such as, continuous hot casting and steel rolling. At operating temperature, the working surfaces usually suffer from normal abrasion, corrosion and crack and so on in the procedure of service, which leads to the decrease of production quality and reduction of productivity. At the same time, the user may encounter heavy economic burden if they change new rollers frequently. It is certain that the cost will be improved if you always abandon old mechanical components that are not in good conditions. It becomes necessary for us to study how to maintain and prolong their service life by applying thin hard coating at their surfaces. You know that laser surface cladding process is a promising surface treatment technology for industry applications, due to both an excellent metallurgical bonding and less defects between the layers and substrates. Hence, the technologies of laser cladding composite coating reinforced by ceramic particles have been investigated in the paper. In order to solve above problems and decrease the production cost. The main results involved in the research are as follows.Laser cladding process was studied on the surface of Adamite steel through CO₂ laser modified technology. It was found that laser cladding parameters affected the microstructure and properties of the layers obviously, especially for laser scanning speed （v）, laser power （p） and laser beam diameter （d）. Under the condition of proper parameters, Fe-based alloy composite coating reinforced by Ti (C_yN_1-y) particles was fabricated on the surface of Adamite steel. It was shown that macroscopic and microscopic qualities are related to the cladding materials and processing parameters of laser cladding. The results showed that width, depth, shape coefficient, dilution and crystal grain of the laser cladding coatings increase with laser power, but thickness of the layer decrease at same parameters. It is also shown that width, thickness, depth, shape coefficient, dilution and crystal grain of the laser cladding coatings decrease with laser scanning speed, but hardness and sensitivity of pores of the layers increase under the same condition. By selecting proper process parameters, such as laser power with 3000w, laser beam diameter with 3.0mm, the prefabricating thickness with 1.0mm, laser scanning speed with 300mm/min. FeCrBSiMo and [TiN+C] powders （the ratio of TiN:C is 1:1） are used to form single laser cladding layer, which possess better appearance and high strength between layer and substrate.Pre-fabricate alloy powder was mixed before laser cladding according to surface properties requirement of Adamite steel. The self-made organic adhesive was used to paste the powders. With optimum parameters, Fe-based alloy composite coating reinforced by Ti (C_yN_1-y) particles was fabricated on the surface of Adamite steel through CO₂ laser cladding technology. The microstructure of laser cladding coating was observed by optical microscopy （OM）, x-ray diffractionmetry （XRD）, scanning electron microscopy （SEM）, electron probe microscopy analyzer （EPMA）, and transmission scanning electron microscopy （TEM）. The distribution of elements and phases in the coatings were identified using EDS and x-ray diffractionmetry （XRD）. Relationship between the microstructure and properties was analyzed in the paper in detail. The results show that Titanium carbonitride Ti（C, N） particles are introduced by an in-situ metallurgical reaction between TiN particles and graphite powders during laser cladding process. Titanium carbonitride particles existed in the layer are fairly fine, ranging from 0.1μm to 40μm, and evenly dispersed in the metal matrix. Most of them take on nearly round shape, and some of them are irregular in shape. The bonding zone is a distinct zone between the layer and substrate, which illustrates that the perfect metallurgical bonding is achieved through this zone The interface between Ti（C, N） particles and the matrix remains clean and is free from deleterious phases by TEM, which insures that the carbide-matrix has a strong interface bond. The in-situ hard carbonitride grains have high bonding strength with the matrix, and form a good carrier capacity system together.According to the basic thermodynamic analysis of△G_T, it is feasible for the formation of Ti（C, N） that metallurgical reaction may happen between titanium nitride （TiN） and graphite （C） during laser cladding process. First of all, a great deal of energy is absorbed by the raw cladding material during laser cladding process, which causes the complete dissolution of the original titanium nitride （TiN） particles coming into being nitrogen and titanium atoms: TiN→Ti + N. Then, these nitrogen, carbon, and titanium atoms begin to diffuse in the cladding matrix. It is known that diffusing speed of carbon atoms is faster than that of other atoms in the melt pool. There are strong chemical reactions and metallurgy processes between the titanium atoms and graphite powders in the molten metal. At the same time, chemical combination reaction between Ti and N atoms can also reoccur in the cladding coating because of active chemical property of Ti and N atoms. Thus, lots of hard anti-wear phases of carbides TiC and nitrides TiN are respectively formed after the melt pool is rapidly solidified. The formation process can be characterized as follows: [Ti]+[C]→TiC, [Ti]+[N]→TiN. It is known that titanium carbon （TiC） and titanium nitride （TiN） characterize by the same NaCl-type crystal structure and approximately equal lattice constants. They may unlimitedly mix each other and form a solid solution named titanium carbonitride. Finally, Ti(C_yN_1-y) particles are synthesized by a solid-solution metallurgical reaction between TiN and TiC particles in the process of laser cladding: yTiC+（1-y） TiN→Ti(C_yN_1-y). Meanwhile, it is possible to form Ti(C_yN_1-y) that the exchange reaction between TiN and C may happen directly in the cladding coating.Wear behavior and wear-resistant property of laser cladding composite coating reinforced by Ti（C,N） particulates were tested on a tester named M-2000 without lubrication at room temperature. It is shown that dispersive strengthening effect and refining effect of the Ti（C,N） particles evenly distributed in the matrix greatly contribute to increasing the microhardness and wear-resistance of the Fe-based composite coating. On the other hand, solid solution strengthening effect of C, Mo and Cr elements and martensite strengthening effect play an important role in theα-Fe metal matrix.Horizontal and longitudinal cracks occur in the layer on the surface of Adamite steel while laser cladding, especially, it is easy for them to appear in the Fe-based alloy multilayer reinforced by Ti(C_yN_1-y) particles. Inner cracks propagate nearby fusion transition zone in theα-Fe metal matrix. These cracks usually end in the bond zone between layer and substrate. Some of them are inter-granular and others are trans-granular. The cracks which lie in the laser layer are brittle fracture. The susceptibility of occurring crack can be decreased by selecting and optimizing processing parameters. In order to keep from cracks in the Fe-based alloy multilayer reinforced by Ti(C_yN_1-y) particles, the second transition Ni-based layer was built up between the working layer and substrate. The method of transition layers may solve the problem of cracking.更多还原