Nb及析出相对LNG储罐用9Ni钢焊材强韧化的影响研究

发布时间：2018-05-22 17:57

本文选题：9Ni钢焊材 + 焊材强韧化　；参考：《江苏科技大学》2017年硕士论文

【摘要】：我国是能源大国,过度依赖煤炭,随着能源结构调整和环保压力增加,清洁能源的供给迫在眉睫。液化天然气(Liquefied Natural Gas,简称LNG)因体积小,燃烧放热大且无污染等优点,其需求逐年增涨。LNG存储设备要求钢材在超低温服役环境具备足够的强度及韧性储备,目前我国LNG储罐用9Ni钢,已完全实现国产化,但配套镍基焊接材料仍然依赖进口,且焊接技术不成熟,因此开展焊材国产化研制,对天然气产业装备建设自主化、保障国家能源安全意义重大。本文在阐述了全球LNG储罐用9Ni钢及其焊接问题研究基础上,独立研制了9Ni钢焊条,总结了Nb对熔敷金属的显微组织影响规律,采用拉伸试验、弯曲、硬度和冲击试验检测力学性能,结合扫描电镜以及热力学计算等方法,研究了熔敷金属的强韧化机理,结果表明:采用镍基焊条,焊缝形成奥氏体组织,Nb具有减小枝晶宽度、细化晶粒的作用,在晶间处以碳化物的形式析出强化晶界,阻碍裂纹的萌生和扩展,随着Nb含量升高,析出相尺寸变大数量增多,塑韧性有所损伤。经检测,自主研制的2号焊条(Nb含量1.30%),熔敷金属性能优良,抗拉强度为705MPa,屈服强度为478MPa,低温冲击功Akv116J,符合LNG储罐设备的要求。为研究9Ni钢焊材的工艺适应性,分别使用焊条电弧焊和埋弧焊进行焊接试验。利用金相显微镜和扫描电镜对焊条电弧焊接头显微组织进行分析,力学性能检测使用拉伸、弯曲、硬度和冲击试验进行,同时对接头熔合区奥氏体-马氏体结晶界面凝固理论进行研究。结果表明:自行研制的焊条与国产9Ni钢焊接工艺适应性良好,焊缝组织为奥氏体,热影响区为马氏体,熔合区为狭窄的平面晶组织。焊缝强度高于母材,侧弯完好无裂纹,焊缝与母材界面处结合良好,热影响区出现硬度最高值为360HV5,焊缝至热影响区冲击韧性逐渐升高,焊缝平均冲击韧性达到115J,符合低温韧性要求。埋弧焊焊缝组织为奥氏体,存在析出相提高了焊缝强度,弯曲性能良好,热影响区硬度值最高为339HV5,焊缝平均冲击韧性达到94J,断口分析显示接头各位置均属韧性断裂。
[Abstract]:China is a large country of energy, relying too much on coal. With the adjustment of energy structure and the increasing pressure of environmental protection, the supply of clean energy is imminent. Because of the advantages of small volume, large combustion heat release and no pollution, the demand of LNG storage equipment for liquefied natural gas (LNG) is increasing year by year. It requires steel to have sufficient strength and toughness reserve in ultra-low temperature service environment. At present, 9Ni steel is used for LNG storage tank in our country. Although the welding materials based on nickel are still dependent on import and the welding technology is immature, it is of great significance to develop the domestic welding materials for the equipment construction of natural gas industry and to ensure the national energy security. In this paper, based on the study of 9Ni steel and its welding for global LNG storage tank, the welding rod of 9Ni steel has been developed independently, and the effect of NB on microstructure of deposited metal has been summarized. The tensile test and bending have been used to study the effect of NB on the microstructure of deposited metal. The mechanical properties were tested by hardness and impact test. The strengthening and toughening mechanism of deposited metal was studied by means of scanning electron microscope and thermodynamic calculation. The results showed that the austenitic microstructure and NB of weld formed by nickel base electrode could reduce the width of dendrite. The effect of grain refinement is to precipitate and strengthen grain boundaries in the form of carbides between crystals, which hinders the initiation and propagation of cracks. With the increase of NB content, the size of precipitated phases increases and the ductility is damaged. The test results show that the NB content of No.2 electrode developed by ourselves is 1.30%, the deposited metal has excellent properties, the tensile strength is 705MPa, the yield strength is 478MPa, and the low temperature impact energy Akv116J is satisfied with the requirements of LNG storage tank equipment. In order to study the process adaptability of 9Ni steel welding, welding experiments were carried out by welding electrode arc welding and submerged arc welding respectively. The microstructure of electrode arc welding joint was analyzed by metallographic microscope and scanning electron microscope. The mechanical properties were tested by tensile, bending, hardness and impact tests. At the same time, the solidification theory of austenitic-martensite interface was studied. The results show that the welding rod developed by ourselves has good adaptability to domestic 9Ni steel. The weld microstructure is austenite, the heat affected zone is martensite, and the fusion zone is plane crystal structure. The strength of the weld is higher than that of the base metal, the lateral bending is intact without cracks, the interface between the weld and the base metal is well bound, the maximum hardness of the HAZ is 360HV5, and the impact toughness of the weld to the heat-affected zone increases gradually. The average impact toughness of weld seam is 115J, which meets the requirement of low temperature toughness. The microstructure of submerged arc welding weld is austenite, and the strength of weld is improved with precipitated phase, and the bending property is good. The hardness of HAZ is 339HV5, the average impact toughness of weld is 94J, and the fracture analysis shows that the joints are ductile fracture in every position.
【学位授予单位】：江苏科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TE972

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