核电大锻件“块状组织”形成机制及其对强韧性影响

低合金钢大锻件广泛应用于核电、石化等重大工程中，随着锻件向一体化、复杂化方向发展，常出现冲击韧性偏低或严重分散问题，严重影响大锻件服役安全性。申请人前期研究发现核电大锻件实物解剖中常出现一种“块状组织(Bc)”，但实验室热模拟试块中却不出现该组织。进一步研究发现，一定的热力耦合条件可诱发Bc在实验室材料中出现，同时粗大的Bc会恶化材料冲击韧性。但目前影响Bc形成的关键因素、相变特征及其对强韧性的影响机制尚不明确。本项目以核电SA508-3钢大锻件为应用背景，通过热力耦合模拟和原位观察研究SA508-3钢中Bc形成和演化规律，明确影响Bc形成的关键因素；利用EBSD和TEM等表征Bc微区成分、精细结构和晶体学特征，揭示Bc相变机制；结合细观断口学分析和晶体塑性有限元模拟，阐明Bc对SA508-3钢强韧性的影响机制。本研究不仅有利于丰富低合金钢相变机制，也为低合金钢大锻件组织调控提供理论支撑。

Low-alloy steel heavy forgings are widely used as critical components in nuclear power and petrochemical equipment. With the development of equipment in terms of integration and complexity, the impact toughness of these heavy forgings has lower values or large scatter. It seriously affects the service safety of heavy forgings. The applicant recently discovered that a kind of “massive microstructure (Bc)” often appeared in the complex heavy forgings used for nuclear power. Our preliminary results showed that it is difficult to form Bc in the laboratorial material during thermal simulation conditions, whereas adopting a suitable thermal-mechanical simulation can induce the formation of Bc. We also proved that coarse Bc significantly decreases the impact toughness of the material. However, the specific formation conditions and transformation characteristics of Bc and its influence mechanism on the mechanical properties of the material are unclear. Therefore, this project takes SA508-3 steel heavy forgings used for nuclear power as the requirement background. Based on in-situ experimental investigations and thermal-mechanical simulation experiments, the formation and evolution of stress induced Bc will be investigated during the solid-state phase transformation of SA508-3 steel. Furthermore, the composition, crystallographic variants, and fine structures of Bc will be thoroughly characterized through electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The transformation mechanism of Bc will be elaborated. Finally, by combining the sub-macroscopic fractography and crystal plastic finite element methods, the influence mechanism of Bc on the strength and toughness of SA508-3 steel will be also discussed. These study not only enrich the understanding of phase transformation mechanism of low-alloy steels, but also provide theoretical references for the microstructural adjustment of low-alloy steel heavy forgings.