Laser additive manufacturing is used to repair the damaged area of the rail, which is prone to martensite microstructure in the cladding layers, and it seriously affects the mechanical properties and threatens the safety of rail service. Therefore, in order to avoid the formation of martensite microstructure in the cladding layers, U75V steel rail cladding layer process experiment with different layers was carried out and the pearlite microstructure was successfully prepared by considering self-tempering characteristics. Moreover, combined with the finite element method to research the phase transformation behavior of U75V steel rail affected by thermal history, the mechanism of pearlite transformation in the process of laser directed energy deposition (LDED) repairing steel rail was studied. The results indicate that the cooling rate of the solid phase transformation stage is approximately 10⁰–10¹ °C/s. Two indispensable conditions for pearlite formation in U75V steel repaired by LDED were revealed: the laser thermal accumulation must enable the cladding layers to maintain the valley temperature above the martensite start temperature and the cooling rate of the cladding layers in the solid phase transformation must be lower than the critical cooling rate of pearlite transformation. Based on this theory, pearlite microstructure can be obtained efficiently by means of preheating pretreatment and Co element doped alloying and optimizing the process parameters. Preheating pretreatment is beneficial to increase basic temperature, and Co-doped alloying can change continuous cooling transformation curve of U75V steel rail.

中文翻译：

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激光定向能量沉积修复U75V钢轨热循环过程中珠光体的形成机制

激光增材制造用于修复钢轨损伤区域，其包覆层易产生马氏体组织，严重影响力学性能，威胁钢轨运行安全。因此，为了避免在包覆层中形成马氏体组织，对U75V钢轨包覆层进行了不同层数的工艺实验，并结合自回火特性成功制备了珠光体组织。并结合有限元方法研究受热历程影响的U75V钢轨相变行为，研究了激光定向能量沉积(LDED)修复钢轨过程中珠光体相变的机理。⁰ –10 ¹  °C/s。揭示了LDED修复U75V钢珠光体形成的两个必不可少的条件：激光热积累必须使熔覆层的谷温保持在马氏体起始温度以上，熔覆层在固相转变中的冷却速度必须较低比珠光体转变的临界冷却速度。基于此理论，通过预热预处理和Co元素掺杂合金化，优化工艺参数，可以有效地获得珠光体组织。预热预处理有利于提高基础温度，掺钴合金化可以改变U75V钢轨的连续冷却转变曲线。