【作者】 李同道；

【导师】 王勇；

【作者基本信息】 中国石油大学 ， 油气储运工程， 2007， 博士

【摘要】 轧辊是轧机的主要部件,也是生产中的主要消耗部件之一,尺寸大,价格比较昂贵,降低辊耗是生产中面临的一个很重要的问题。而采用激光加热的方法对金属材料进行快速凝固加工可制备出具有优异物理化学性能的微晶、非晶、准晶等非平衡亚稳组织,从而获得成分、组织及性能完全不同于零件基材、具有细小均匀快速凝固非平衡组织特征,可显著提高材料的性能。因此,进行“轧辊激光表面快速熔凝组织与性能的研究”具有很重要的实际意义和理论价值。本文通过采用5KW横流CO₂激光器对轧辊钢进行了激光熔凝试验,对熔凝过程对材料的组织、硬度、裂纹敏感性等进行了探讨;并采用大型有限元软件SYSWELD对激光熔凝过程进行了数值模拟,分析了温度场、应力场、硬度场变化规律。激光熔凝后,改性层显微组织可分为三个区:熔凝区、热影响区和基体。熔凝区主要由胞状晶+树枝晶组成,显微组织为马氏体、残余奥氏体和弥散分布的碳化物,热影响区主要由马氏体、残余奥氏体和碳化物组成。轧辊激光快速熔凝处理,沿层深方向硬度呈现梯度分布,熔凝层硬度稍有降低,热影响区出现硬度最大值,且当功率和光斑直径一定时,熔凝层深与扫描速度成反比。激光快速熔凝处理的宏观质量受扫描速度的影响很大,随着扫描速度的增大,表面裂纹敏感性增加,熔池的深度逐渐减小,而熔池宽度逐渐减小,得到的△H/W之比逐渐增大。激光快速熔凝单道扫描在熔凝区两侧均出现裂纹,进行搭接处理,当搭接量较小时在搭接区都出现了裂纹,搭接量达到光斑直径的一半时,搭接区域组织较为均匀且无裂纹出现,搭接区存在回火软化现象,硬度值下降。建立了激光宽带三维体热源模型,利用此热源模型能够模拟出符合深/宽尺寸情况的激光熔凝熔池形状,计算得到的激光熔凝区宽度和深度与实测值符合得较好,验证了模型的适用性。该模型对激光宽带加工温度场的模拟结果更符合实际。根据数值模拟的结果,激光熔凝过程是一个快速加热、快速冷却的过程,其温度变化率可达10⁴℃/s数量级;经过激光熔凝后的材料硬度得到了显著的提升,中心部位的硬度小于熔凝区其它区域的硬度;熔凝区的残余应力为压应力,而热影响区则存在着较大的拉应力,成为形成裂纹的危险区域;碳含量的不均匀变化导致熔凝区的压应力分布不均。激光熔凝加工后得到不同工艺参数下熔凝区的不同残余应力分布。熔凝层表面和层深方向的Mises应力和平均应力分布规律相似,扫描速率过大或者过小,应力幅值均有所降低。当激光束扫描速率V=600～1000mm/min时,熔凝区得到了残余压应力分布,有利于提高工件的使用性能。激光熔凝区残余应力的产生与熔凝区材料发生组织转变,得到了硬脆的马氏体相有关。更多还原

【Abstract】 Roller is the most important part of the rolling mill, and also is one of the main consuming parts during production, which has a large size and is very expensive, so reducing the consuming of the roller is an important problem in the production. Fast solidification process on the metal material by Laser heating technology can produce the non-equilibrium and metastable microstructures such as minicrystal, amorphous, quasi-crystal and so on, which is fine, uniform and metastable, and different from the matrix in elements, microstructures and properties, and then improves the properties of the materials. Therefore, studying on "Research on the microstructures and properties of roller by laser surface melting" is provided with a large practical significance and theoretical value.In this paper laser surface melting （LSM） is performed on the roller steel with a 5kW continuous wave CO₂ laser, and the microstructures, hardness and cracking are studied; also numerical simulation was carried out about the process of laser surface melting with the software SYSWELD, what’s more, the varying regularities of temperature field, stress field and hardness field has been achieved.After laser surface melting, the sectional region on roller steel is divided into three distinctive zones, namely, laser melted zone, heat-affected zone （HAZ） and the substrate, which can not be delimited strictly. Laser melted zone is composed of cellular structure and dendritic crystals, and the microstructures are martensite, residual austenite and disperse carbide, also HAZ is mainly composed of martensite, residual austenite and carbide. Hardness of the roller after laser surface melting has a gradient distribution along the depth of melted zone, which in melted zone is lower after laser surface melting, however, the maximum value appears at HAZ. And while the laser power and the diameter are given, the depth of the melted zone has an inversely-proportional relationship with the velocity. The macroscopic quantity is influenced by scanning velocity significantly. With the increase of the scanning velocity, the cracking susceptibility increases, and the depth of melted zone decreases gradually as well as the width, so the ratio ofΔH and W increases gradually. Cracks were found on both sides of molten pool under the experimental conditions. While overlapping and with a small overlapping fraction, cracks appear in the overlapping, but when the overlapping fraction reaches half of the diameter of the spot, and also the microstructures in the overlapping are uniform and free of cracks, but the softening occurs with the hardness decreasing.There-dimension model of the laser source was established, and the shape of the laser surface melting according with the depth and width can be simulated, which is concordant with the calculated values, so the model is applicable. The temperature field can be simulated with the model concordant with practiceAccording to the results of simulation, laser surface melting is a process of quick heating and cooling, in which the temperature can reach 10⁴℃/s; the hardness of the material after laser surface melting increases, and in the center of the melted zone is larger than that of the other zones; and the residual stress is compressive in the melted zone, but which is tensile at HAZ, being the dangerous zone of cracking; the non-uniform variation of the carbon content results in the non-uniform distribution of compression stress in the melted zone.Different distributions of residual stress appear under different progress parameters by laser surface melting. The distributions of the Mises stress and average stress in the surface of melted zone are similar to that along the depth of the melted zone, that is to say, the value of stress will decrease when the scanning velocity is too larger or too small. While the scanning velocity varies from 600mm/min to 1000mm/min, compression stress can be obtained which is in favor of improving the property of the workpiece. The generation of the compression stress in the melted zone is related to the hard brittle martensite created during the microstructural transformation更多还原