中国科学院力学研究所机构知识库

H13钢(4Cr5MoSiV1)由于具有较高的韧性，较好的耐磨性和耐腐蚀性，被广泛用于模具的制造。在工业生产中，通常对H13钢模具表面进行渗氮处理，以进一步提高其性能，从而减少模具的维修频率以及更换次数。激光熔覆作为一种先进的表面改性技术，在表面强化和修复领域有着广泛的应用。本文针对渗氮H13钢模具激光修复中氮容易析出并形成气孔的问题，基于冶金与材料制备的热力学与动力学原理研究了N的分布与释放机制，设计了适用于渗氮H13钢的激光熔覆涂层；另外，由于模具的损伤形态呈现出多样性，提出了模块化修复方法。研究方法和结果如下：

（1）根据热力学理论设计了含Ti、含Co两种熔覆材料，研究了激光熔覆渗氮H13钢时N的析出和释放行为及其影响因素，如熔化和凝固速度、熔覆材料的成分及含量。为了解决N析出和气孔形成的问题，结合激光重熔过程的仿真模拟结果试验了基板渗氮表层经激光重熔预处理后再熔覆H13与Stellite合金混合粉末的方案（方案一）；根据吉布斯自由能分析熔池中可能发生的反应，设计熔覆粉末的成分配比，提出先以H13与Ti混合粉末打底再熔覆H13与Stellite合金混合粉末的方案（方案二）。通过改变激光线能量密度和混合粉末的配比进行工艺优化和成分优化，在渗氮H13钢表面获得了无明显孔洞、裂纹缺陷且性能良好的激光熔覆层。研究结果表明，通过激光重熔预处理减少渗氮层中的N含量或通过添加金属粉末提高N在钢中的溶解度可以减少甚至避免渗氮H13钢在激光熔覆时形成氮气孔。

（2）通过光学显微镜、扫描电子显微镜、X射线衍射、背散射电子衍射等材料表征手段观测涂层的微结构，进行显微硬度测试、摩擦磨损实验和电化学腐蚀实验，研究激光熔覆试样微结构和性能之间的关系。方案一，对基板渗氮表层进行激光重熔预处理，激光重熔预处理会促使N释放，再熔覆H13与Stellite合金混合粉末，经成分和工艺优化后的H13+50%Stellite涂层内组织以马氏体为主，存在大片残余奥氏体，涂层硬度约550 HV，在500 ℃下磨损机制主要为磨粒磨损，腐蚀电流密度为7.57×10^–7 A/cm²；方案二，先以不同配比的H13与Ti混合粉末打底，随着H13与Ti混合粉末中Ti元素含量由0%增加至10%，涂层与基体冶金过渡区存在明显差异，H13+10%Ti涂层与基体冶金结合良好，在H13+10%Ti涂层上激光熔覆H13与Stellite合金混合粉末，优化后的H13+50%Stellite涂层内组织均匀致密，包含马氏体和少量残余奥氏体，试样形成了硬-软-硬的结构，H13+50%Stellite涂层的硬度约575 HV，主导的磨损机制为粘着磨损，涂层表面的腐蚀电流密度为2.48×10^–6 A/cm²。由于固溶强化和第二相强化的作用，H13+50%Stellite涂层具有较高的硬度和较好的耐磨性，方案二所制备试样的表面耐磨性优于方案一所制备试样。与渗氮H13钢相比，激光熔覆试样表面的耐腐蚀性均显著提高。

（3）设计了几种典型的修复形状，明确其最优激光工艺，形成模块化的修复工艺，即：针对模具的面、边、角上损伤形态，设计典型的缺口形状，进行机加工减材和激光熔覆增材，再精加工恢复原始尺寸。分析了模块几何形状及尺寸的设计、激光熔覆扫描策略的选择对修复冶金质量和表面精度的影响。

综上，本文通过激光熔覆技术制备了无明显冶金缺陷且具有较高硬度和较好耐磨、耐腐蚀性能的涂层，解决了渗氮H13钢模具强化与修复中氮气孔形成以及涂层与基体结合区成分偏析的问题，实现了模具的模块化修复。

H13 steel (4Cr5MoSiV1) is widely used in tool manufacturing because of its high toughness, good wear resistance and excellent corrosion resistance. In industry, the surface of H13 steel is generally nitrided to improve its properties, thus reducing the frequency of maintenance and replacement. Laser cladding, an advanced surface modification technology, is widely used in the field of surface strengthening and repairing. However, nitrogen will precipitate and may form nitrogen pores during laser repair of nitrided H13 steel. In order to solve this problem, the distribution and release mechanism of nitrogen is studied based on the thermodynamics and dynamics principle of metallurgy and material preparation. And laser cladded coatings for nitrided H13 steel were designed. In addition, this article proposes a modularized repairing method to achieve standardized repair of different tool damages. The specific details are as follows.

(1) Two types of cladding materials containing Ti and Co were designed based on the thermodynamic principle. The precipitation and release behavior of N during laser cladding nitrided H13 steel were studied. Its influencing factors include melting and solidification rate, composition of cladding material, etc. Two schemes were tested. In scheme 1, laser remelting pre-treatment was carried out on the nitrided surface of substrate based on simulation results of laser remelting process, followed by laser cladding the mixed powder of H13 and Stellite alloy. In scheme 2, using the mixed powder of H13 and Ti as the base material, which is designed according to Gibbs free energy analysis of possible reactions in the molten pool, then the mixed powder of H13 and Stellite alloy was cladded on the base material. The process was optimized by changing laser line energy density, and the composition was optimized by changing the mass ratio of mixed powder. Laser cladded coatings with good properties but no obvious pores and cracks were prepared on nitrided H13 steel. The results reveal that the formation of nitrogen pores can be reduced or even avoided by reducing N content of nitrided layer or increasing the solubility of N in molten pool by adding metal powder during laser cladding nitrided H13 steel.

(2) The microstructure of samples was characterized by material characterization techniques at different scales, such as optical microscope, scanning electron microscopy, X-ray diffraction and electron backscatter diffraction. Performance testing includes micro-hardness test, friction and wear experiment as well as electrochemical corrosion experiment. The results of scheme 1 show that laser remelting pre-treatment can induce the release of nitrogen, thereby reducing N content in the nitrided layer. After laser cladding, the microstructure is mainly martensite and there is a large quantity of retained austenite. The average hardness of the laser cladded coating is about 550 HV. The wear mechanism at 500 ℃ is mainly abrasive wear. And its corrosion current density is 7.57×10^–7 A/cm². The results of scheme 2 show that with the increase of Ti content from 0% to 10% in the mixed powder of H13 and Ti, there is a significant difference in the metallurgical transition zone between coating and substrate. Among them, H13+10%Ti coating has the best quality with good metallurgical bonding between coating and substrate. The microstructure of H13+50%Stellite coating on it is homogeneous and dense, mainly composed of martensite, including a small amount of retained austenite. The sample has a hard-soft-hard structure. The average hardness of the coating surface is about 575 HV, and its dominant wear mechanism is adhesive wear. The corrosion current density of the coating surface is 2.48×10^–6 A/cm². Due to the effect of solution strengthening and second phase strengthening, H13+50%Stellite coating has high hardness and good wear resistance. The surface wear resistance of the sample prepared by scheme 2 is better than that prepared by scheme 1. Compared with nitrided H13 steel substrate, the corrosion resistance of laser cladding samples is significantly improved.

(3) Typical repair shapes were designed and the optimum process was determined. The steps of modularized repairing are as follows. Firstly, machine the local damage area according to the shape and size of modules and repair it by laser cladding, then restore the original size through precision machining. Optimizing the geometric shape of module and choosing appropriate path of laser scanning are of great significance for improving metallurgical quality and surface precision.

In this article, laser cladded coatings with high hardness, good wear resistance, excellent corrosion resistance and no obvious metallurgical defects were prepared by laser cladding technology, which solved the problem of nitrogen pore formation and component segregation in bonding zone between coating and substrate during strengthening and repairing of nitrided H13 steel molds. Laser standardized repair of molds has achieved good results.