核电压力容器用SA508-3钢准确定氢及热处理组织研究

IMR OpenIR

	核电压力容器用SA508-3钢准确定氢及热处理组织研究
	郝露菡
学位类型	博士
导师	李殿中
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料加工工程
关键词	Sa508-3钢准确定氢扩氢热处理有限元模拟组织演化 Sa508-3 Steel Accurate Hydrogen Determination Anti-flake Heat Treatment Fem Simulation Microstructure Evolution
其他摘要	" SA508-3钢是第三代核电机组的关键结构材料，其构成的反应器、蒸发器和稳压器是保障核电机组安全、稳定运行的核心部件。由于核电压力容器长期在高温、高压和热中子辐照等苛刻条件下服役，因此对SA508-3钢的化学成分、加工工艺和力学性能有着极高的要求，特别是为了避免由于辐照肿胀而引起的材料脆化，要求SA508-3钢中的氢含量小于0.8ppm。为达到这一指标，绝大多数制造企业除了在冶炼、浇注时采取真空脱气外，在锻后还设置长时间的扩氢热处理（长达数百小时）。即使如此，在成品检验中仍存在定氢数据离散、单个数据不达标的现象，不但造成产品大量报废，而且个别由于定氢不准而判定为合格的产品将严重威胁核电的使用安全。本研究针对当前工业生产中的核电压力容器锻件无法准确定氢这一现状，紧紧围绕定氢试样的制备与检测过程开展研究，通过对比分析多种试样的取制方法，建立了定氢试样表面的氢吸附模型，表征了不同加工状态试样由表及里的相对氢浓度梯度，揭示了影响定氢准确性的本质因素，阐明了钢中夹杂物对形成白点损伤的核心作用。在此基础上，制定了准确定氢的操作规范，并在企业应用，获得了准确、稳定的定氢数据。此外，本研究还表征了当前工业生产中多种工艺条件下SA508-3钢的热处理组织，阐明了关键析出相的结构及其对材料力学性能的影响。论文的主要研究内容和结论包括： 1) SA508-3钢高温材料数据库的建立。实测了SA508-3钢的应力—应变曲线、热物性参数及氢在其中的扩散系数等数据，并对热变形过程中材料的回复、再结晶规律进行研究，将应力、应变等材料的宏观特征与动态回复、动态再结晶等微观特征结合，建立该钢种的高温本构关系模型及热物性参数数据库，为采用有限元方法模拟锻造成形及氢扩散提供了有效的材料参数。 2) SA508-3钢中痕量氢的准确测定。针对当前工业检测中痕量氢测试结果离散的问题，系统研究了定氢试样的加工方法、保存时间、温度及表面质量对定氢结果的影响，发现样品表面会吸附一定数量的氢，由表面粗糙度不同引起的表面氢含量不同是造成定氢结果离散的本质原因。相应的理论计算表明，当样品表面粗糙度为Ra 6.3μm时，表面氢的存在对定氢结果的影响最大可达100ppm，在定氢检测前对样品表面进行有效预处理是获得准确数据的关键。通过比较6种试样表面预处理方式，结合飞行时间二次离子质谱仪(TOF-SIMS)对样品表面氢浓度进行分析，发现超声波清洗是最简单且有效的手段，并据此制定了标准的定氢试样制取方法。 3) SA508-3钢的氢损伤机制。探索了铁素体钢的充氢方法，并针对SA508-3钢的氢损伤机制及氢对其力学性能的影响进行系统的研究。研究发现SA508-3钢的氢损伤机制为：在拉伸变形时，钢中的过饱和氢跟随位错一起运动，在固有缺陷处发生位错塞积，造成氢的局部富集，其中部分氢从基体中析出而形成分子氢，在局部产生较大的氢压，结合氢的局部富集使该位置铁原子之间键合力降低的作用，使得裂纹首先从氢富集的缺陷部位起裂，并向外扩展，形成不可逆的氢损伤—白点。 4) SA508-3钢的析出相特性。研究表明，调质态SA508-3钢中含有针状碳化钼(Mo2C)和颗粒状的合金渗碳体两种析出相，且针状Mo2C会导致杂质元素P和S的偏聚，应予以避免；SA508-3钢对淬火冷速很敏感，为保障调质后钢的强度和韧性，淬火时应避免粒状贝氏体形成；SA508-3钢在缓冷至室温的组织中除了在粒状贝氏体内部形成的渗碳体外没有其他的脆性析出相出现，在高温锻造时可不必考虑脆性析出相的影响。" ; "SA508-3 steel is the key structural material which is extensively used in the large components of the third generation nuclear power plant, such as reactor pressure vessels (RPV), steam generator (SG) and pressurizer. Nuclear pressure vessel is usually used under the environment of high temperature, high pressure and neutron radiation, hence the requirement of SA508-3 steel in chemical composition, hot-working process and mechanical properties is highly demanding. Particularly, in order to avoid the radiation embrittlement aggravated by hydrogen, the hydrogen concentration in SA508-3 steel must be reduced below 0.8ppm. However, the hydrogen concentration in steels cannot be accurately determined in the steel-making industry in China nowadays. Even after the anti-flake heat treatment with 300~700 hours, the tested hydrogen content is still discrete and frequently above 0.8ppm. The uncertainty of the exact hydrogen content in SA508-3 steel not only leads to the waste of time and energy for anti-flake annealing, but also constitutes a serious threat to the security of nuclear power. Therefore, how to get the accurate data of hydrogen in SA508-3 steel and keep the hydrogen content below 0.8ppm efficiently is a puzzle raised by the steel-making industry. In this study, we carried out our work about the accurate determination of hydrogen in SA508-3 steel from comparing the effect of machining parameters, surface quality, and pre-treatment methods on hydrogen analysis result. And then hydrogen adsorption model was built and the hydrogen concentration gradient from the surface to the base of steel was characterized by Time-of-Flight Secondary Ion Mass Spectroscope (TOF-SIMS). At last, we found the essential factor that influencing the determination of hydrogen and eventually get the stable and accurate hydrogen content in steel. Besides, hydrogen embrittlement sensitivity, microstructure and precipitation of SA508-3 steel under different heat treatment states were systematically studied. The main contents and results in current study include: 1) The thermo-physical data of SA508-3 steel, such as thermal conductivity, specific heat and thermal diffusivity, were measured from room temperature to 1200℃. And the high temperature constitutive equations of SA508-3 steel were established based on measured stress-strain curves which provide basic data for FEM simulation. 2) Surface hydrogen introduced by the adsorption of water or oil on the rough surface of SA508-3 steel is the key factor which influences the accurate determination of hydrogen. Theoretical analysis indicates that the surface hydrogen adsorbed by the rough surface can reach 100ppm. TOF-SIMS was used to characterize the hydrogen concentration gradient from the surface to the base of the SA508-3 steel, and the result confirmed the existence of hydrogen on the rough surface of the sample. Through the comparison of different surface pre-treatment methods, it’s found that ultrasonic cleaning is the most effective way to get the stable and accurate hydrogen content in materials. 3) The mechanism of hydrogen embrittlement for SA508-3 steel was studied through the slow tensile test. During the deformation, the hydrogen in steel is transported with the movement of dislocation and then accumulated around the defects (mainly inclusions) along with the dislocation tangle. Hydrogen gathering leads to the supersaturation of hydrogen around inclusions, and then some hydrogen would run out from the steel matrix to the tiny space around inclusions and become hydrogen gas (H2). The huge pressure caused by H2 at the interface between inclusion and matrix, combined with the decreased bonding force between Fe atoms caused by the existence of H atoms lead to the formation of crack around the inclusions. It’s found that 1.0ppm’s hydrogen in SA508-3 steel can lead to the formation of flake and the sharp drop of elongation upon the tensile test. 4) The microstructure and precipitation of SA508-3 steel under different heat treatment processes were characterized. It’s found that two sorts of carbide precipitated in the ferrite matrix during the tempering process， which were indentified to be alloyed cementite and molybdenum carbide (Mo2C). Needle-like Mo2C tends to bring the segregation of impurities, so it’s better for Mo to solute in the matrix; And SA508-3 steel is extremely sensitive to the cooling rate during the quenching process. Increasing the cooling rate above 5℃/s to avoid the transformation of granular bainite is crucial for SA508-3 steel in order to get the qualified mechanical properties. Besides, only cementite is precipitated from SA508-3 steel during the slow cooling process, therefore there is no need to consider the influence of brittle precipitation on forging process."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64486
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	郝露菡. 核电压力容器用SA508-3钢准确定氢及热处理组织研究[D]. 北京. 中国科学院金属研究所,2012.