激光熔化沉积低活化钢中M23C6析出长大行为及过程调控

Laser melting deposition is a quite promising method in manufacturing complex shaped parts of reduced activation steel. Controlling the precipitation behavior to obtain fine dispersed M23C6 particles is ultimate to the performance properties of reduced activation steel during its service at high temperature..In this work, a continuum model for nucleation, growth, and coarsening of carbides, based on the Cahn-Hillert non-classical nucleation theory and Langer–Schwartz theory, was developed for the investigation of M23C6 precipitation behavior. Firstly, the physical relationship among the laser parameters, such as the energy density, the time interval and the characteristics of the thermal cycling curve (temperature, cooling rate, and so on), was established. Secondly, the controlling factors of precipitation and the growth behavior of M23C6 were put forward. Finally, the average size and distribution of M23C6 could be adjusted to realize the precise control of the microstructure and properties..The physical nature of interface and segregation of alloying element in the vicinity of precipitation would be studied in reduced activation steel, which is in service condition at high temperature. A kinetic model for coarsening behavior of M23C6 particle was proposed as well. It is the theoretical basis to control the precipitation behaviors of M23C6 experimentally. Moreover, the model is able to provide guidance in suppressing the coarsening behavior of M23C6 , and this is vital for the improvement of high temperature service performance.The current project could reveal the physical essence and provide guidelines for the laser melting deposition technology, as well as the optimization of microstructure and mechanical properties in reduced activation martensitic steel.

激光熔化沉积技术有望解决低活化钢复杂结构零部件研制的难题。控制激光成形过程M23C6的析出长大行为关系到低活化钢的高温安全服役。因此，研究激光成形过程中M23C6析出机制，以及激光成形低活化钢高温服役过程中M23C6粗化机制，具有重要的科学价值和现实意义。. 本项目拟采用Cahn-Hillert非经典形核理论结合Langer-Schwartz模型，阐明低活化钢中M23C6析出长大行为。通过建立能量密度、时间间隔等激光参数与热循环曲线特征量（温度、冷速、Ac1以上循环次数等）的物理关系，分析M23C6析出长大行为的控制因素，调控M23C6平均尺寸与形态分布，实现激光成形组织性能的精准调控。. 研究激光成形低活化钢高温服役过程中板条界面元素偏聚、界面特征分布等界面特征对M23C6粗化行为的影响，建立高温服役过程中M23C6粗化动力学模型，为高温服役安全提供理论依据。

激光熔化沉积技术有望解决低活化钢复杂结构零部件研制难题。为了保证低活化钢零部件的高温服役安全，需要控制激光成形组织及其性能，尤其是控制激光成形过程M23C6的析出行为，具有重要科学价值和现实意义。本项目利用InfraTec红外热像仪测温结合Abaqus有限元模拟分析温度场的变化，利用生死单元法以及Fortran语言编程对移动激光热源进行控制并建立了激光成形过程的三维温度场分布数值模型，建立能量密度、时间间隔等激光参数与热循环曲线特征量（温度、冷速、Ac1以上循环次数等）的物理关系。采用经典形核理论（CNT）和SFFK模型，基于Materials Calculator计算软件，利用有限元模型计算获得的激光熔化沉积过程温度场数据，构建了低活化钢在激光熔化沉积过程中M23C6的形核和长大模型，并且验证了不同工艺下M23C6形核和长大行为。研究结果表明：单壁墙中靠近顶部激光熔化沉积组织中的M23C6长大行为比底部更加明显，析出相平均尺寸大于底部。这是因为在激光多次热循环过程中，靠近顶部的组织温度峰值高于底部且冷却速率低于底部，所以其组织内M23C6能获得更多的能量以形核和生长。激光熔化沉积组织中M23C6的形核、长大和回溶取决于在热循环过程中吸收的能量，能量由温度曲线的峰值、峰宽和冷却速率决定。由于激光熔化沉积过程热累积的作用，相对于底部组织，其经历的冷却速率高，因此晶粒尺寸小，细晶强化效应明显；随高度增加冷速降低导致形核速率速率降低，引起顶部组织的奥氏体长大，主要由于应变储能不足以诱发奥氏体形核，激光熔化沉积过程的多次热循环不能激发奥氏体的再结晶，因此热循环过程中奥氏体只能长大。由于过饱和的C析出和马氏体板条的退化导致强度的显著下降，而奥氏体化阶段完全释放了马氏体板条残余应力状态并消除位错缠结，导致淬火回火热处理试样强度下降而冲击韧性改善。此外，在激光熔化沉积热循环过程中形成的高密度的细小弥散M23C6颗粒的钉扎作用能够有效提高组织高温热稳定性。