【作者】 蒋元凯；

【导师】 郭正洪；

【作者基本信息】 上海交通大学 ， 材料学， 2019， 硕士

【摘要】 目前,我国正大力发展核电事业,深入研究核电压力容器用钢在制造和服役过程中的显微组织与力学性能演变规律显得格外重要。本文研究了核电压力容器用SA508 Gr.3钢过冷奥氏体转变动力学,采用示波冲击实验和高温蠕变实验方法研究了材料在复杂环境下的组织演变规律及其与力学性能之间的关系,为理解大型锻件的热处理工艺及其与组织性能间的关系提供参考。首先,采用热膨胀法研究SA508 Gr.3钢的过冷奥氏体转变动力学,获得了包含相变开始/结束温度、转变量和转变时间等相变基础数据的TTT和CCT曲线;并结合光学显微镜,扫描电子显微镜和硬度仪等手段研究了过冷奥氏体分解过程的组织演变规律,研究发现:随等温温度降低,SA508 Gr.3钢依次形成先共析铁素体、针状铁素体、贝氏体铁素体、上贝氏体和下贝氏体;连续冷却转变组织随冷速加快依次为先共析铁素体+贝氏体+马氏体、粒状贝氏体、贝氏体+马氏体、板条马氏体。其次,对炉冷态试样、空冷态试样和油冷态试样进行不同温度下的示波冲击实验,得到了不同试样的韧脆转变曲线,结果显示:随着冷却速度的提高,材料的上、下平台冲击功逐渐增大,韧脆转变温度降低。通过力-位移曲线、宏微观断口形貌和二次裂纹特征研究了不同显微组织的冲击断裂机制,研究发现束区尺寸和大角晶界密度是影响微裂纹萌生与扩展的关键因素,它决定了材料的冲击性能。最后,研究了贝氏体组织在不同温度和应力条件下的高温蠕变性能,结果表明,低温时蠕变难以发生,材料表现出较好的蠕变抗力;随着温度升高或应力增大,蠕变现象愈发明显。SEM和EBSD分析显示蠕变后的组织产生细化,晶粒内部出现了大量的亚结构,说明蠕变变形机制主要为位错运动机制,而在高温下点缺陷扩散和晶界滑动也是蠕变变形的主要原因。更多还原

【Abstract】 At present,China has been devoting major efforts to developing the nuclear power industry,thus the research on the microstructural evolution and mechanical property in the process of manufacturing and servicing for nuclear reactor pressure vessel has significant importance.In this dissertation,the phase transformation kinetics and microstructural evolution of the SA508 Gr.3 steel for nuclear reactor pressure vessel were studied.The relationship between microstructure and mechanical property was established by analyzing the impact fracture and high temperature creep behavior.The purpose of the dissertation is to provide guidance for understanding the relationship between heat treatment process,microstructure and mechanical properties for nuclear large forgings.Firstly,the phase transformation kinetics of the SA508 Gr.3 steel was studied by means of dilatometer method,the TTT and CCT curves including the start/finish temperature,transformation amount and time kinetics information were established and the microstructural evolution during the undercooled austenite isothermal transformation and continuous cooling transformation was researched.The result showed that as the isothermal temperature decreases,the isothermal transformation microstructure are proeutectoid ferrite,flake ferrite,carbonless bainite,upper bainite and lower bainite respectively.As the cooling rate increases,the continuous cooling transformation microstructure are proeutectoid ferrite and bainite and matrensite,bainite,martensite and bainite,lath martensite respectively.Secondly,the instrumented impact experiment at different temperature of furnace cooling sample,air cooling sample and oil cooling sample was conducted.The result showed that as the cooling rate increases,the impact energy increases and the ductile-brittle transition temperature decreases.According to the load-displacement curve,the macro and micro fracture surface observation and the feature of secondary cracks,it can be concluded that the block size of microstructure and the density of big angle boundaries are the key factors affecting the crack initiation and propagation for nuclear reactor pressure vessel.Finally,the creep tests at different temperature and stress showed that creep behavior is difficult to occur at low temperature.As the temperature or stress increases,the creep strain increases and the creep behavior becomes more and more obvious.The scanning electron microscope and electron backscattered diffraction analysis showed that the microstructure is refined and plenty of sub-structures appear in the grain after creep test,which reveals the dislocation movement mechanism is the main creep deformation mechanism.In addition,the point defect diffusion and grain boundary sliding make contribution to creep deformation at high temperature.更多还原