核电用316LN奥氏体不锈钢热变形组织演变与断裂行为

发布时间：2018-02-16 07:58

本文关键词： 奥氏体不锈钢热锻再结晶模型有限元模拟断裂　出处：《北京科技大学》2016年博士论文　论文类型：学位论文

【摘要】：超低C控N的316LN奥氏体不锈钢,具有良好的加工性能、优良的综合力学性能以及耐晶间应力腐蚀性能,被选用作第三代AP1000压水堆核电站的一回路主管道材料。不同于第二代压水堆核电站的铸造主管道,AP1000主管道采用管体和管嘴整体锻造成形,制造难度很大。AP1000主管道锻造工艺的关键是有效地解决锻造过程中的混晶问题,使晶粒细匀化；在钢锭镦粗、拔长等压实阶段,有效地控制裂纹尤其是表面裂纹的产生。西屋公司对AP1000主管道的晶粒度提出了严格要求,要求其晶粒度整体大于ASME 2级,而316LN钢是单相奥氏体不锈钢,不能通过热处理改善组织,故热锻完成后的组织对主管道的最终力学性能起着决定性作用。因此,掌握316LN钢在多道次热锻过程中的微观组织演变规律和断裂行为对AP1000主管道的制造技术控制至关重要。本文针对AP1000核电站主管道的锻造过程,通过大量的Gleeble实验和热处理实验,研究了316LN钢在多道次热变形过程中的动态再结晶、静态再结晶和亚动态再结晶组织演变规律、奥氏体晶粒长大规律和断裂行为,建立了再结晶、晶粒长大和韧性断裂模型。实验结果将有助于主管道在锻造过程中的组织控制及裂纹预防,并可为主管道加工工艺优化设计提供参考数据和数值模拟模型。通过大量的Gleeble单道次和双道次热压缩实验,研究了主管道材料316LN钢在热变形过程中动态再结晶、静态再结晶以及亚动态再结晶行为。研究发现,在热变形过程中,316LN钢较难发生动态回复,其主要的软化机制是动态再结晶,动态再结晶临界应变与峰值应变的比值仅为0.38;动态再结晶优先在晶界、三叉晶界和孪晶界形核,也可在形变带附近形核,再结晶主要形核机制为形变诱导晶界迁移机制。在多道次热变形过程中,当变形施加的真应变小于动态再结晶临界应变时,随后的间歇过程中的软化行为表现为静态回复和静态再结晶；当变形施加的真应变超过亚动态再结晶临界值时,道次间歇过程中发生亚动态再结晶。有效地解决了由于DEFORM-3D有限元模拟软件在计算的过程中以等效应变和等效应变速率为变量,而引起的再结晶组织演变模拟结果与实验值偏差较大的问题。根据热模拟实验数据,建立了再结晶动力学、晶粒尺寸与真应变、真应变速率之间的经典关系方程。将这些模型直接应用于DEFORM-3D软件中,模拟316LN钢的变形过程组织演变行为时会产生较大偏差,因此必须对再结晶模型进行修正,采用等效应变和等效应变速率替代真应变和真应变速率。借助DEFORM-3D软件将真应变和真应变速率转换为相应的等效应变和等效应变速率,以等效应变和等效应变速率为参数重新构建了再结晶模型,利用重建后的再结晶模型进行DEFORM-3D有限元模拟时,模拟结果和实验值之间符合地很好。通过大量的热处理试验研究了316LN钢900～1200℃下保温0.25～10h的奥氏体晶粒长大规律。当温度低于1000℃时,316LN钢的奥氏体晶粒长大速度较慢;当温度高于1050℃时,奥氏体晶粒长大速度较快。当保温时间短于0.5 h时,316LN钢的奥氏体晶粒长大速度较快；由于晶粒长大驱动力的不断减小,保温时间长于0.5 h时,奥氏体晶粒长大速度明显放慢。316LN钢长时间高温下保温后的晶粒尺寸基本不受初始晶粒尺寸的影响。根据热处理实验数据,建立了316LN钢奥氏体晶粒尺寸随温度、时间和初始晶粒尺寸变化的方程,奥氏体晶粒长大常数n为2.47。在900～1200℃、0.01～1s1-的热拉伸变形参数范围内,当颈缩发生后,颈缩附近的变形比较剧烈,形变量较大。当变形温度为1200℃、应变速率为1s-1时,断口附近的再结晶形核率和再结晶晶粒长大速率都较高,在形核和长大两个过程的共同作用下,断口附近的再结晶组织最为均匀细小：此变形条件下试样的断面收缩率也最高,塑性最好。在900～1100℃下,316LN钢的断面收缩率在应变速率为0.1s-1时出现峰值。316LN钢基于正则化的Cockcroft Latham准则的临界断裂因子与其Zener-Hollomon参数的对数之间存在线性关系。
[Abstract]:316LN austenitic stainless steel with ultra low C N control, has a good processing performance, excellent comprehensive mechanical properties and resistance to intergranular stress corrosion, was used as a main material pipeline loop reactor nuclear power plant third generation AP1000 water pressure. The casting main pipeline is different from the second generation PWR nuclear power plant, the main AP1000 the pipeline pipe body and nozzle whole forging, the key.AP1000 main pipe forging manufacturing difficulty is effectively solve the problem of mixed crystal in the forging process, the grain refining and homogenizing; in the ingot upsetting, stretching compaction stage, effectively control the crack especially the surface crack. Westinghouse the grain size of AP1000 main pipe made stringent requirements, the grain size of the whole is greater than 2 ASME, and the 316LN steel is austenitic stainless steel, not through the heat treatment to improve the organization, so the hot forging after the completion of the organization in charge of the road The end plays a decisive role in the mechanical properties. Therefore, mastering the microstructure during multipass hot forging process of 316LN steel evolution and fracture behavior of AP1000 manufacturing technology of the main pipeline control is very important. In this paper, the forging process for AP1000 nuclear power plant main pipeline, through a lot of experiments and Gleeble experiments of heat treatment of 316LN steel was studied, and then the crystallization in deformation in the dynamic process of multi pass hot, static recrystallization and meta dynamic recrystallization microstructure evolution, austenite grain growth behavior and fracture behavior, established recrystallization, grain growth and ductile fracture model. The experimental results will be helpful to the main pipeline in the forging process of the organization control and crack prevention, and can the main pipeline processing technology optimization design provides reference data and numerical simulation model. Through a large number of Gleeble single and double pass hot compression experiment of primary pipe material 316LN Steel during hot deformation process of dynamic recrystallization, static recrystallization and meta dynamic recrystallization behavior. The study found that during hot deformation process, 316LN steel is difficult to occur in the dynamic response, the main softening mechanism is dynamic recrystallization and recrystallization critical strain and peak strain dynamic ratio value is only 0.38; dynamic crystallize preferentially at the grain boundaries, triple grain boundary and twin boundary nucleation, also can bring near nucleation during deformation, recrystallization nucleation mechanism is the main deformation induced grain boundary migration mechanism in multi pass hot deformation process, when the true strain deformation applied less than dynamic recrystallization critical strain softening behavior, the subsequent batch process the performance for the static recovery and static recrystallization; when the true strain deformation applied over metarecrystallization critical value, dynamic recrystallization occurred in sub pass batch process. Because DEFORM-3D can effectively solve the finite element simulation Software to strain in the process of calculation and the equivalent strain rate as the variable, caused by the recrystallization microstructure evolution simulation results and experimental results of large deviation. Based on the thermal simulation experiment data, establish the recrystallization kinetics, grain size and true strain, it should be classic rate between the equation. These models are applied directly to the DEFORM-3D software, simulation of deformation process of 316LN steel microstructure evolution behavior will produce larger deviations, it is necessary to revise the model of recrystallization, the equivalent strain and equivalent strain rate instead of the true strain and true strain rate. The true strain and true strain rate is converted to equivalent strain and the equivalent strain rate by means of DEFORM-3D software, with strain and strain rate parameters to construct the model of recrystallization, the reconstruction of the recrystallization model by DEFORM-3D finite element During the simulation, the simulation results and experimental values fit in well. Through the heat treatment experiments of 316LN steel were studied under 900~1200 DEG C for 0.25 ~ 10h insulation austenite grain growth behavior. When the temperature is below 1000 DEG C, grew slower austenite grain of 316LN steel; when the temperature is higher than 1050 DEG C, the austenite grain growth fast. When the holding time is shorter than 0.5 h, the austenite grain of 316LN steel grew faster; the driving force of grain growth continues to decrease, the holding time is longer than 0.5 h, the austenite grain growth slowed down significantly the grain size of.316LN steel after long time insulation under high temperature are not affected by the initial grain size according to. Heat treatment of the experimental data, the austenite grain size of 316LN steel with temperature is established, and the time of the initial grain size variation equation, the austenite grain growth constant n was 2.47. at 900~1200 C, 0 The tensile deformation parameters of 1 ~ 1s1- range when necking occurs after Necking Deformation near intense deformation is larger. When the deformation temperature is 1200 DEG C, the strain rate of 1s-1, near the fracture rate of recrystallization nucleation and recrystallization grain growth rate is higher, the interaction up to two in the process of nucleation and recrystallization, the fracture near the most uniform and fine for the organization: the deformation section shrinkage rate for the sample is the highest, the best plastic. At 900~1100 DEG C, the rate of contraction of 316LN steel at the strain rate of 0.1s-1 appeared the linear relationship between the logarithmic peak parameters of.316LN steel the critical fracture factor and Zener-Hollomon regularization Cockcroft based on the Latham criterion.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG142.71;TG316

【相似文献】