核电大型锻件是核电装备的核心构件，其质量关系到核电站的安全与效益。热处理工艺与装备直接影响到核电大型锻件的最终性能。本文结合实验测试与数值模拟，分别研究了核电压力容器用A508-3钢加热和冷却过程中的相变动力学，优化了相变量计算模型，建立并验证了热处理过程的多场耦合数值计算方法，应用于两种典型核电大型锻件热处理过程的模拟。同时，对大型淬火水槽流场进行FVM数值模拟，进而提出工装设计与水槽优化方案。 首先，利用连续加热膨胀实验和等转变量方法，研究了A508-3钢的奥氏体化动力学，获得了有效激活能E(γ)、修正指前因子lnA'(γ)与相变量γ的关系，开发了奥氏体相变量数值计算方法，并采用变速加热工艺对该算法的合理性及模型参数的准确性进行了验证。针对实际加热过程，对CFD软件ANSYSFLUENT进行了二次开发，在温度场的模拟中嵌入了奥氏体相变量计算模块和PID控温热源项模块，开发了温度场-组织场耦合模拟数值算法。 A508-3钢立方体加热验证实验结果表明:模拟结果与实测升温曲线基本符合，能反映奥氏体化转变对温度场的影响，可用于实际加热过程的模拟。 其次，结合膨胀实验和金相组织观察，研究了A508-3钢的过冷奥氏体分解动力学，确定了相变临界点(Ac1=720℃，Ac3=810℃，Bs=552℃， B〓=390℃， Ms=398℃，M〓=195℃)和 CCT、TTT 曲线。建立了奥氏体-先析铁素体和奥氏体-珠光体转变量计算模型；提出了描述贝氏体不完全转变动力学的改进模型，获得了贝氏体最大转变量〓与等温温度T的关系(〓 = 1-exp[-0.0577(Bs-T)]);提出了描述贝氏体不完全转变动力学的改进模型；马氏体相变采用K-M方程描述，确定了参数σ〓与温度T之间的关系；定量分析了先于马氏体相变的扩散型相变对马氏体相变开始温度(Ms)的影响，即Ms(℃)=f〓×[399.31183-1.76228exp(11.72424× f〓)]+f〓×[393.96898-59.44244×f〓] + f〓×[395.27349-109.85704×f〓]。开发了过冷奥氏体分解过程的相变量计算方法和淬火过程流场-温度场-组织场耦合模拟数值算法，通过对ANSYS FLUENT软件进行二次开发，实现了淬火过程流场-温度场-组织场的耦合模拟，可用于大型锻件实际淬火过程模拟。 再次，对两种典型核电大型锻件的热处理工艺过程(AP1000稳压器下封头锻件加热、蒸发器筒体锻件淬火)进行了模拟研究。模拟结果显示， API000稳压器下封头锻件加热过程的温度模拟结果与实测升温曲线基本吻合；锻件温度场和奥氏体组织场存在较为严重的不均匀性，不同位置的到温时间和奥氏体化进程显示出较大的时间差异。在低、中温区多段式阶梯加热有利于减小不同位置的温度差。对蒸发器筒体锻件淬火流场的CFD数值模拟结果表明，水槽内流场分布较为均匀，锻件表面高度方向和圆周方向上可获得较为均匀的流场，外壁面流速高于相应内壁面，下端面流速略高于上端面。根据流速分布确定锻件表面的换热系数，预测了该锻件在淬火过程中的温度场和组织场演变。模拟结果与实测冷却曲线基本一致，锻件内部组织以贝氏体为主，马氏体仅出现在筒体外表面和上、下端面的边角位置，整个锻件未出现铁素体，取样位置处的冷却速度约为0.1℃/s～0.5℃/s。 最后，对大型淬火水槽空载和加载典型核电大型锻件CAP1400核电蒸发器上封头的流场进行了 CFD模拟研究，多重参考坐标系模型的引入可反映带有螺旋桨搅拌系统的大型水槽内部流场，研究结果表明:水槽内部流场存在严重的不均匀性，下半区流速高，上半区流速低且出现大范围“流动死区”。下半区底部螺旋桨的搅动起主要作用，侧面螺旋浆仅可提高淬火工作区外围流速。低位回流口结构及侧面螺旋桨对底部螺旋桨的干扰作用抑制了水流向上流动是造成水槽上部“流动死区”的重要原因。研究了 CAP1400核电蒸发器上封头三种不同淬火方案的流场，研究结果表明:碗口向上的入水方式可使封头内表面与水直接接触，透孔型料盘及镂空式吊臂的设计可形成水流通道，可减少工装对锻件外表面水流的阻挡；在封头内腔安置抽水泵，可提高封头内腔和碗口处的水流速度，改善内表面流场的均匀性；安装水泵导流罩的同时提高抽水泵流量，可进一步改善碗内流速不足的问题；沙漏型导流罩可以改善锻件外表面的流场均匀性，圆锥台形导流罩可进一步改善沙漏型导流罩因自身高度原因削弱锻件外表面流速的问题；圆锥台形导流罩配合窗口式回流内筒的设计可有效地提高锻件外表面的流速和均匀性。本文研究结果成功应用于大型锻件的热处理工艺设计和设备优化。 关键词:核电大型锻件，A508-3钢，多场耦合模拟，大型淬火水槽，相变动力学

 Nuclear power large forgings are core component of nuclear power equipment. Qualified large forgings are crucial for ensuring the safety and efficiency of nuclear power plants. The heat treatment process and the equipment used in the process directly affect the final performance of nuclear power large forgings. With the aid of combining experimental tests and numerical simulation, this paper studied phase transformation kinetics of A508-3 steel used in nuclear power pressure vessels during heating and cooling process respectively. The research developed the algorithm to get phase volume fraction, established and verified multi-field coupled numerical model in the heat treatment, and applied the models to the simulation of heat treatment process for two typical nuclear power large forgings. Also, flow field distribution in the large quenching tank was simulated with FVM, and the optimizing design schemes for the. fixture and large tank structure were proposed further. Firstly,the austenitization kinetics of A508-3 steel during heating was studied using the isoconversional method combined with the continuous heating dilatometric tests. The effective activation energy E(γ) and the modified pre-exponential factor lnA'(γ) as functions of transformed austenite fraction γ were also obtained. The numerical algorithm of austenite transformation fraction was developed, and the rationality of the algorithm and the accuracy of the model parameters were verified in the heating process with non-constant heating rates. For the actual heating process, the CFD software ANSYS FLUENT was re-developed. By embedding the austenite transformation fraction calculation module and the PID-controlled temperature source term module in the temperature field simulation, the temperature filed-microstructure field coupled numerical algorithm was developed. In this research, the verification experimental results of the heated A508-3 steel cube indicated that the simulated results were basically consistent with the measured heating curves, and can reflect the influence of austenitizing transformation on the temperature field, thus it can be used to simulate the actual heating process. Secondly,the austenite decomposition kinetics of A508-3 steel during cooling process has been investigated by the dilatometric experiments and microstructure observation. The kinetic parameters such as critical temperature of phase transformation (Ac1=720℃,Ac3=810℃,Bs=552℃,B〓=390℃， Ms=398℃，M〓=195℃) and the CCT and TTT curves were obtained further. The calculation models for austenite-proeutectoid ferrite and austenite-pearlite transformation fraction were established, and an improved model for bainite incomplete transformation kinetics was proposed. The relationship between the maximum bainite transformation fraction, f〓 and isothermal temperature T was obtained: fh〓=1-exp[-0.0577(Bs-T)]. The martensite phase transformation was described by the K-M equation, and the relationship between the parameters σ〓 and the temperature T was determined. Also, the influence of diffusive transformation prior to martensitic transformation on the phase transformation starting temperature (Ms) was investigated with quantitative analysis, the concluded relationship is: Ms(℃) =f〓×[399.31183-1.76228exp(11.72424×f〓)]+f〓×[393.96898-59.44244×f〓]+ f〓×[395.27349-109.85704×f〓]. Numerical calculation methods of transformation fraction for austenite decomposition process and a flow-temperature-microstructure multi-field coupled numerical algorithm were proposed. Software ANSYS FLUENT was re-developed to simulate the coupled flow field-temperature field- microstructure field in the quenching process, and the model can be used to simulate the large forgings during the actual quenching process. Thirdly,the heat treatment process of two typical nuclear power large forgings (heating of AP1000 nuclear power steam pressurizer lower closure head forging, quenching of evaporator cylinder forging) were simulated. The simulation results for the heating process of AP1000 nuclear power steam pressurizer lower closure head forging showed that the temperature simulation results were basically consistent with the measured heating curves, and obvious nonuniformity existed in the temperature and austenite field, with large time gap for different locations to reach the same temperature and austenitizing process. Multi-step heating procedure in low and middle temperature zones favored uniform temperature distribution at different positions. The CFD numerical simulation results indicated that, when the large forging was quenched in the large tank, more uniform flow field distribution appeared in the height direction and circumferential direction of the forging surface. The flow velocity near the outer surface is higher than that near the internal surface, and the flow velocity near the bottom end face is slightly higher than that near the top end face. Heat transfer coefficient of forging surface was determined based on flow velocity, and then the temperature field and micro structure field in quenching process were predicted. The simulation results were basically anastomotic with the measured cooling curves. The microstructure in the forging mainly consisted of bainite, and martensite only appeared on the outer surface and the outer edges on the end faces of the cylinder forging, with no ferrite appeared. The cooling rate at the sampling position was approximately from 0.1℃/s to 0.5℃/s. Finally,the CFD simulation was conducted to study the flow field distribution in the large quenching tank typical nuclear power large forging with and without loading -CAP1400 evaporator upper closure head. Multiple Reference Frame (MRF) model was introduced into the simulation, which can reflect the actual flow field in large tank with propellers stirring system. The results showed serious inhomogeneity of flow field in the large tank, Compared with the high flow velocity in the lower half zone, low flow velocity and a large area of "flow dead zone" appeared in the upper half zone. In the lower half zone of the tank, the agitation of the propellers at the bottom played a major role for flow velocity distribution, whereas, the propellers located at the wall of the tank only improved the peripheral flow velocity in the quenching work area. Lower inner backflow slot and the interference effect on the bottom propellers by the propellers at the wall were the main reasons for the "flow dead zone" in the upper half zone of the quenching tank, for the upflow of water was inhibited. CFD simulation of three different quenching schemes were carried out for the CAP1400 nuclear power evaporator upper closure head as well. The results showed that the inner surface of the upper closure head can be directly contacted with water when the forging quenched with bowl positioned upward, and the water flow passage between the through-hole tray and hollowed suspension arm can reduce the restriction of water flow around the outer surface of the forging. A pump installed within the closure head can increase the flow velocity and improve uniformity within the inner chamber and rim of the bowl. Increasing the flow of the pump and installing pump deflector can solve the problems of insufficient flow velocity and poor flow uniformity in the bowl. The hourglass-type deflector can improve the flow filed uniformity near the outer surface of the forging while the design of truncated cone-type deflector and window-type backflow inner slot can further solve the problem of weakened flow rate due to the height of hourglass-type deflector, and can effectively increase the flow velocity and improve uniformity on the outer surface of the closure heat. The results in this research were successfully applied to the heat treatment process design and equipment optimization for large forgings. KEY WORDS:Nuclear power large forgings, A508-3 steel, Multi-field coupled numerical simulation, Large quenching tank, Phase transformation kinetics