【作者】 郭伟；

【导师】 郭宁；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2015， 硕士

【摘要】 随着核电事业的快速发展,对核安全以及核维护技术的需求日趋迫切。水下焊接作为核电维护以及应急抢险的关键技术,其重要性尤为突出。从水下焊接技术和理论本身来看,应用于核电修复时除辐照条件外其特殊性主要体现在硼酸水下环境,尤其目前针对硼酸水环境下的水下焊接传质过程、环境介质与焊接过程的交互作用机制等理论问题尚属空白。因此,本文针对核电硼酸水下环境开展水下焊接相关基础理论研究工作,重点研究了硼酸环境对水下湿法焊接传质过程、焊接过程稳定性及焊接质量的影响规律和机理。本文研究内容的开展不仅填补了水下湿法焊接相关理论空白,且为核电水下修复技术的开发与推广提供理论支撑。提出并建立了基于X射线高速成像的水下焊接过程监控方法及系统,结合数学建模及分析,研究了水下湿法焊接熔滴过渡特性,建立了熔滴受力模型,揭示了飞溅形成机制,阐明了硼酸环境对水下湿法焊接熔滴过渡的影响规律。结果表明水下湿法焊接熔滴过渡存在两种典型形式,即“排斥过渡和固体短路过渡的复合形式”和“排斥过渡和表面张力过渡的复合形式”。熔滴以这两种方式过渡时,常分别伴随着“爆炸型”飞溅和“熔滴排斥型”飞溅的产生。当硼酸浓度较小时,熔滴主要按前者方式进行过渡;随着硼酸浓度的提高,使水下电弧气泡上浮所产生的气体拖拽力LF和电磁力增大,因此熔滴所受排斥力增大,按后者方式过渡的熔滴所占比例提高。此外,“熔池震荡型”飞溅会在整个焊接过程中不断产生。采集了不同浓度硼酸溶液中水下湿法焊接的电弧信号。在计算机统计分析信号数据后,通过波形图、概率分布图、U-I图和变异系数四个不同的评价指标研究了硼酸浓度对水下湿法焊接电弧稳定性的影响。结果发现硼酸溶液浓度增大后,水下湿法焊接过程中短路和断弧的比重增大,即电弧电压和焊接电流的波动程度变大,电弧燃烧稳定性降低。研究了不同浓度的硼酸对焊接接头宏观成形、微观组织、成分以及强度、韧性、硬度等力学性能的影响。结果表明,随着硼酸浓度的增大,水下湿法焊接焊缝宽度增大、熔深减小;焊缝的物相结构无变化,但硼元素含量逐渐升高;此外,焊缝区等轴的块状铁素体晶粒尺寸先细化后略粗化。焊接接头的力学性能测试结果表明,硼酸浓度增大后,焊接接头的抗拉强度、断后延伸率的变化程度不大,而焊缝区平均硬度逐渐增大,且焊缝冲击韧度呈现先提高后降低的特点。更多还原

【Abstract】 With the rapid development of nuclear industry, the demand for nuclear security and nuclear maintenance technology become pressing gradually. As a key technology for nuclear maintenance and emergency rescue, underwater wet welding is very significant. Considering the theory of underwater welding, the particularity of underwater welding which is applied to repair nuclear plant is mainly embodied in the boric acid underwater environment except for irradiation conditions. Especially, many issues of underwater welding in boric acid solution, such as the metal transfer process and the interaction mechanism of environmental media and the welding process, are not clear so far. Therefore, in this paper, the fundamental theoretical research of underwater wet welding in boric acid environment is carried out. The study focuses on investigating the effect of boric acid on metal transfer process, stability of welding process and weld quality of underwater wet welding. It is believed that this research can fill up the blank of related theory for underwater wet welding, and supply theoretical foundation for the development and popularization of the underwater repair technology in the nuclear plant.The monitoring method and system for underwater wet welding process are proposed and established based on X-ray high-speed imaging. Through this method and system combined with the mathematical modeling and analysis, the characteristics of metal transfer during underwater wet welding is researched. Besides, the force model of drops is built, and the mechanism of welding spatter is revealed. Furthermore, the influence of boric acid environment on metal transfer is illuminated detailedly. The research results show that the mixed repelled globular transfer and solid short circuit transfer mode and the mixed repelled globular transfer and surface tension transfer mode are two kinds of main metal transfer modes during underwater welding in boric acid solution. Respectively, the droplet repelled spatter and explosive spatter are observed when drops transfer by these two modes. If the concentration of boric acid solution is low, the main transfer mode is the former. However, the proportion of the latter mode increases with the concentration of boric acid solution, which is caused by the larger gas flow drag force and electromagnetic force because of higher rising speed of the bubbles. In addition, the molten pool shock spatter is formed in the whole underwater welding process.The arc signal of underwater wet welding in boric acid solution with different concentration is collected, and then be processed by computer. Four assessment methods, which refer to the oscillogram, probability distribution, U-I diagram and variation coefficient, are adopted to explore the influence of boric acid on the arc stability in underwater wet welding. The results suggest that the proportion of short circuit and arc interruption in the underwater wet welding increase with the concentration of boric acid. So the fluctuation of arc voltage and welding current increase, and the arc stability is deteriorated gradually.The effect of boric acid concentration on the macro forming, microstructure and composition as well as mechanical properties(strength, toughness and hardness) of welded joint is studied. The result shows that with the increasing of boric acid concentration, the weld width enlarge, but weld penetration lessens. Although the phase composition of weld is unchanged, the content of boric element in weld is increased. Besides, the equiaxial block ferrite will refine before coarse. The test results of mechanical properties indicate that the tensile strength, elongation of welded joint only have a little change even increasing the concentration of boric acid solution. Moreover, the average hardness of weld zone increases gradually, and the impact toughness of weld metal will rise first and then fall.更多还原