深海用X70管线钢焊接接头耐蚀性研究

发布时间：2018-03-04 01:05

本文选题：X70管线钢　切入点：焊接接头　出处：《郑州大学》2017年硕士论文　论文类型：学位论文

【摘要】：管线运输由于其经济、安全、高效逐渐成为油气运输的首选方式。国外深海管线运输自1954年美国在墨西哥湾建立的第一条海底管道以来飞速发展,国内管线钢起步较晚。全长261 km的南海荔湾气田是我国首个深海油气项目,本文研究的深海用X70管线钢外径达691 mm,管壁厚40.5 mm,用于3500 m深海,在壁厚和服役深度上均达到国内外领先水平。深海用管线钢向着厚壁化、高钢级、大变形、微合金化方向发展。腐蚀容易导致材料的失效,从而导致经济的巨大损失及安全隐患。深海管线长期处于低温高压的复杂多变环境,对管线钢的耐蚀性要求较高,本文主要通过盐雾腐蚀试验、应力腐蚀试验、浸泡试验、失重试验、电化学试验对3500m深海用X70管线钢焊接接头的腐蚀行为进行研究。对厚规格X70管线钢焊接接头进行中性盐雾试验(NSS试验),通过金相显微镜、扫描电镜对腐蚀产物形貌进行观察,采用EDS、XRD对腐蚀产物进行面扫描和物相分析。结果表明,盐雾腐蚀过程先发生点蚀,点蚀逐步发展成全面腐蚀。Cl-引起点蚀,点蚀导致裂纹的产生。腐蚀产物成分均匀,主要由铁的氧化物、氯化物和氢氧化物组成。腐蚀速率随着腐蚀时间的延长变缓慢,腐蚀过程中Cl-易穿过外层疏松的腐蚀产物与基体发生反应,而最内层致密的腐蚀产物膜对腐蚀介质的进一步扩散起到阻挡作用,减缓了反应速率,提高了耐蚀性。对3500m深海用X70管线钢基体与焊接接头在不同温度和不同应变速率下进行拉伸试验分析其应力腐蚀行为,通过扫描电镜观察其断口形貌,通过EDS分析其断口成分,通过慢拉伸曲线分析其断裂行为,阐述了应力腐蚀机理,对深海用管线钢服役使用具有参考意义。结果表明,基体和焊接接头在3.3×10-6 s-1和1.0×10-6 s-1这两种应变速率下的应力腐蚀倾向都不明显。温度对基体和焊接接头的应力腐蚀影响不明显。通过浸泡试验、失重试验、电化学试验对深海用厚规格X70管线钢焊接接头各区域耐蚀性进行研究,通过XRD分析其钝化膜的组成,通过扫描电镜观察其显微组织。结果表明,焊缝的耐蚀性最优,自腐蚀电位最高,热影响区耐蚀性次之,自腐蚀电位介于焊缝和近热基体之间,近热基体耐蚀性最差,自腐蚀电位最负,且对于相同区域,内焊的耐蚀性优于外焊。内层Fe3O4形成致密的钝化膜能有效减缓反应的进行,外层疏松的腐蚀产物Fe2O3、FeOOH、Fe(OH)3对基体无保护作用。焊缝处微观组织多为晶内形核铁素体,M-A岛细小均匀,耐蚀性最好;热影响区组织梯度变化最大,M-A岛组元粗大,耐蚀性次于焊缝;近热基体由铁素体和贝氏体组织,贝氏体呈岛状分布,耐蚀性最差。内焊部分受外焊热循环的影响,微观组织更加细化,M-A岛体积分数较多,应力得到释放,耐蚀性好于外焊。
[Abstract]:Because of its economy, safety and efficiency, pipeline transportation has gradually become the preferred mode of oil and gas transportation. Foreign deep-sea pipeline transportation has developed rapidly since 1954, when the United States established the first submarine pipeline in the Gulf of Mexico. Domestic pipeline steel started relatively late. Liwan gas field in Nanhai, with a total length of 261 km, is the first deep-sea oil and gas project in China. The outer diameter of X70 pipeline steel in this paper is 691 mm and the pipe wall thickness is 40.5 mm, which is used in the deep sea of 3500 m. Both the wall thickness and service depth have reached the leading level at home and abroad. Deep sea pipeline steel is developing towards thickening, high steel grade, large deformation and microalloying. Corrosion can easily lead to material failure. The deep sea pipeline is in the complex and changeable environment of low temperature and high pressure for a long time, and the corrosion resistance of pipeline steel is very high. This paper mainly adopts salt spray corrosion test, stress corrosion test, immersion test, etc. The corrosion behavior of the welded joint of X70 pipeline steel used in the deep sea of 3500m was studied by weightlessness test and electrochemical test. The neutral salt spray test was carried out on the welded joint of X70 pipeline steel with thick specification, and the NSS test was carried out by metallographic microscope. Scanning electron microscope (SEM) was used to observe the morphology of corrosion products, and EDS- XRD was used to analyze the surface and phase of the corrosion products. The results showed that the pitting corrosion occurred in the process of salt spray corrosion, and the pitting corrosion gradually developed into total corrosion. Cl- induced pitting corrosion. Pitting causes cracks. The corrosion products are homogeneous and consist mainly of iron oxides, chlorides and hydroxides. The corrosion rate slows down with the prolongation of corrosion time. In the process of corrosion, the corrosion products which easily penetrate through the outer layer react with the substrate, and the dense corrosion product film in the innermost layer acts as a barrier to the further diffusion of the corrosion medium and slows down the reaction rate. The corrosion resistance of X70 pipeline steel used in 3500m deep sea was studied by tensile tests at different temperatures and different strain rates. The fracture morphology was observed by scanning electron microscope (SEM), and the fractographic composition was analyzed by EDS. The fracture behavior of the steel is analyzed by the slow tensile curve, and the stress corrosion mechanism is expounded, which is of reference significance for the service use of deep sea pipeline steel. The results show that, The stress corrosion tendency of matrix and welded joint at the strain rate of 3.3 脳 10 ~ (-6) s ~ (-1) and 1.0 脳 10 ~ (-6) s ~ (-1) is not obvious. The effect of temperature on stress corrosion of matrix and welded joint is not obvious. The corrosion resistance of deep sea X70 pipeline steel welded joint was studied by electrochemical test. The composition of passivation film was analyzed by XRD and the microstructure was observed by scanning electron microscope. The results showed that the corrosion resistance of weld was the best. The corrosion resistance of the self-corrosion potential is the highest, the corrosion resistance of the heat-affected zone is the second, the corrosion resistance of the self-corrosion potential is between the weld seam and the near-hot substrate, the corrosion resistance of the near thermal substrate is the worst, the self-corrosion potential is the most negative, and for the same region, The corrosion resistance of inner welding is better than that of external welding. The dense passivation film formed by inner Fe3O4 can effectively slow down the reaction, and the loose corrosion product Fe _ 2O _ 3 / FeOOH _ (3) has no protective effect on the substrate. The microstructure gradient of the heat affected zone is the largest, and the corrosion resistance is inferior to that of the weld. The near hot matrix is composed of ferrite and bainite, and the bainite is island distributed, and the corrosion resistance is the worst. The internal welding part is affected by the external welding thermal cycle. The microstructures are more refined, the stress is released, and the corrosion resistance is better than that of external welding.
【学位授予单位】：郑州大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TE988.2

【相似文献】