厚大断面核电压力容器SA508-3钢锻件低温冲击韧性研究

发布时间：2018-05-22 16:40

本文选题：SA508-3钢 + 显微组织　；参考：《沈阳理工大学》2017年硕士论文

【摘要】：SA508-3钢作为一种低碳低合金钢以较高的强度和良好的低温冲击韧性等优点,广泛应用于核电压力容器中。核电压力容器是核电机组的关键部件,决定了核电站的安全性和服役年限。压力容器锻件质量对反应堆安全运行起着重要的作用。随着反应堆压力容器壁厚的增加,壁厚中心部位常出现低温冲击韧性偏低且波动的现象,严重降低了核电压力容器服役安全性。针对此问题,本研究系统分析了核电压力容器锻件沿壁厚方向的显微组织变化规律和SA508-3钢连续冷却过程的相变特征,通过在实验室使用不同的冷却方式模拟实际锻件壁厚不同位置的冷却条件,对比分析实验室不同冷却条件下得到试样与实际锻件不同位置试样的显微组织和力学性能,提出了影响锻件壁厚中心位置低温冲击韧性的原因,在此基础上设计了在不依赖于提升淬火过程冷却速率的前提下,优化锻件壁厚中心位置粒状贝氏体中马氏体-奥氏体岛(M-A岛)的热处理新工艺,提高了材料的低温冲击韧性。本论文的主要研究内容和结论如下:1.厚大断面SA508-3钢锻件实际解剖分析从纯净度、显微组织、力学性能三个方面对实际锻件进行了解剖分析,结果表明,在锻件化学成分、气体含量、夹杂物控制良好的情况下,锻件外壁的强度和冲击韧性良好,组织为马氏体和下贝氏体;而锻件心部低温冲击韧性较差,其粒状贝氏体组织是低温冲击韧性较差且出现波动的主要原因。2.SA508-3钢相变特征研究利用Thermo-Calc热力学软件计算了SA508-3钢的平衡相图,使用热膨胀仪、显微硬度计测试了SA508-3钢平衡相变点和连续冷却转变曲线,利用金相显微镜、扫描电镜对不同冷速下的SA508-3钢微观组织进行表征。结果表明,SA508-3钢平衡态析出相主要有KSI碳化物、合金渗碳体、MC_SHP、M7C3等;连续冷却过程中,当冷速在0.35~20℃/s时,组织以贝氏体为主,并随冷速的增加,先后出现粒状贝氏体、上贝氏体和下贝氏体;在实验室使用不同冷却方式模拟实际锻件壁厚不同位置的冷却条件,通过显微组织对比分析,选择实验室水冷模拟实际锥形筒体的表面状态,砂冷模拟实际锥形筒体的心部状态。发现砂冷试样的平均冲击功显著低于水冷试样,随着温度的降低,两者的差距变大;砂冷试样的显微组织为粒状贝氏体,主要由贝氏体铁素体基体和呈不规则块状或细条状的M-A岛组成,水冷试样的显微组织为马氏体+下贝氏体。3.SA508-3钢亚温淬火+回火热处理工艺研究研究了亚温淬火对SA508-3钢相变规律的影响,与正常奥氏体化的CCT曲线相比,800℃亚温淬火提高淬透性。当冷速在0.35~15℃/s时,组织以贝氏体为主,并随冷速的增加,先后出现粒状贝氏体、上贝氏体和下贝氏体。分别研究了不同亚温淬火奥氏体化温度和不同回火温度下采用实验室砂冷模拟实际锻件心部时的组织和性能,发现800℃亚温淬火+650℃回火后砂冷试样粒状贝氏体中M-A岛尺寸和数量明显减少,显著提高了材料的低温冲击韧性。
[Abstract]:As a low carbon low alloy steel, SA508-3 steel is widely used in nuclear power pressure vessel as a kind of low carbon low alloy steel, which has the advantages of high strength and good low temperature impact toughness. The nuclear power pressure vessel is the key part of the nuclear power plant. It determines the safety and service life of the nuclear power plant. The quality of the forgings of the pressure vessel plays an important role in the safe operation of the reactor. With the increase of the wall thickness of the reactor pressure vessel, the low and fluctuating phenomenon of the low temperature impact toughness at the center of the wall is often appeared, which seriously reduces the service safety of the nuclear pressure vessel. In this study, the change law of the explicit microstructure along the wall thickness of the nuclear Pressure Vessel Forgings and the continuous cooling of the SA508-3 steel are systematically analyzed. By using different cooling modes in the laboratory to simulate the cooling conditions of the different positions of the wall thickness of the actual forgings in the laboratory, the microstructure and mechanical properties of the specimens at different positions in different cooling conditions in the laboratory are compared and analyzed, and the original impact toughness of the center position of the forgings is proposed. On the basis of this, a new process of heat treatment for Martensitic austenite Island (M-A Island) in granular bainite, which is not dependent on the cooling rate of the hardening process, is designed to improve the low temperature impact toughness of the material. The main contents and conclusions of this paper are as follows: 1. thick section SA508-3 steel forgings The actual anatomical analysis of the actual forgings is analyzed from three aspects of purity, microstructure and mechanical properties. The results show that the strength and impact toughness of the outer wall of forgings are good under the condition of the chemical composition, gas content and inclusion control of the forgings. The microstructure is martensite and lower bainite, and the low temperature impact toughness of the forgings is low. Poor, its granular bainite structure is the main cause of low temperature impact toughness and the main cause of fluctuation of.2.SA508-3 steel. Study on the equilibrium phase diagram of SA508-3 steel using Thermo-Calc thermodynamic software, using thermo dilatometer and microhardness tester to test the equilibrium phase change point and continuous cooling transition curve of SA508-3 steel, using Jin Xiangxian. Microscopes and scanning electron microscopy (SEM) are used to characterize the microstructure of SA508-3 steel at different cooling rates. The results show that the main precipitates in the equilibrium state of SA508-3 steel are KSI carbides, alloy carburized bodies, MC_SHP, M7C3 and so on. In the continuous cooling process, when the cooling rate is 0.35~20 C /s, the microstructure is mainly bainite, and the granular bainite appears successively with the increase of cooling speed. Bainite and lower bainite are used in the laboratory to simulate the cooling conditions of the different positions of the wall thickness of the actual forgings in the laboratory. Through the comparison and analysis of the microstructure, the surface state of the actual cone cylinder is simulated by the water cooling in the laboratory, and the sand cold is used to simulate the heart state of the actual cone cylinder. With the decrease of temperature, the gap becomes larger with the temperature decreasing, and the microstructure of the sand cold specimen is granular bainite, mainly composed of bainite ferrite matrix and the irregular lump or fine strip M-A island. The microstructure of the water cooled sample is studied by the martensitic + subbainite.3.SA508-3 steel and the tempering and tempering heat treatment process. The effect of subtemperature quenching on the phase transformation of SA508-3 steel was compared with the CCT curve of normal austenitizing. At 800 C, the temperature quenching increased the hardenability. When the cooling rate was at 0.35~15 /s, the microstructure was bainite, the upper bainite and the lower bainite with the increase of cooling speed. The austenitizing of different subtemperature quenching was studied respectively. The microstructure and properties of the core in the actual forging are simulated at the temperature and the tempering temperature at different tempering temperatures. It is found that the size and quantity of the M-A island in the granular bainite of the sand cold sample at 800 C after tempering at +650 C is obviously reduced and the low temperature impact toughness of the material is greatly improved.
【学位授予单位】：沈阳理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TL351.6;TG142.1

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