X80管线钢与其焊接接头耐蚀性研究
发布时间：2019-05-28 14:33
【摘要】：X80管线钢具有较高的强度、韧性及较好的焊接性,在世界范围内获得了广泛的应用。我国西气东输二线段干线是目前世界上成功应用X80管线钢的最长管线。焊接是连接管线钢的主要技术手段,而焊接区域是管线的薄弱区域,因此管线钢的成功焊接是关系到管线的使用寿命和安全的最关键的加工技术。管道大量使用的实践表明腐蚀已成为管线失效的最重要原因之一,管道爆裂、泄漏等失效不仅造成严重的经济损失,还有可能造成严重的灾难性后果,因此有必要对管线钢与其焊接接头的耐蚀性进行重点深入研究。本文首先研究了X80管线钢母材在高pH及H2S环境中的腐蚀行为,并分析了其腐蚀机理。然后研究了埋弧焊工艺下焊接接头各区域的组织及在不同环境中的耐蚀性,分析对比了耐蚀性的薄弱环节。利用焊接热模拟技术深入研究了焊接热输入对X80管线钢粗晶区组织与在不同环境中耐蚀性的影响,以求从提高粗晶区耐蚀性的角度,得出较好的焊接热输入范围。同时通过焊接热模拟技术研究了二次焊接热循环峰值温度对粗晶区组织及不同环境中的耐蚀性的影响。通过对X80管线钢在0.5MNa2C03-1 M NaHC03溶液中的腐蚀行为的研究,发现在25～95℃温度范围内,基体均会发生明显的钝化现象,但随着温度升高钝化膜的稳定性下降,且腐蚀倾向及腐蚀速率升高；在溶液中加入一定含量的Cl-会引起点蚀现象,一定程度上升高温度有利于抑制点蚀现象的发生,研究认为是溶液中OH-浓度增加的缘故；在自然浸泡过程中,基体表面逐渐生成一层腐蚀产物,其成分主要是FeC03；在稳定钝化区选取的阳极极化电位越高,所产生的钝化膜的稳定性越好；在该溶液中X80管线钢存在严重的应力腐蚀倾向,阴极保护虽然能够有效的抑制金属溶解,但由于氢的引入使得阴极保护效果降低；而阳极保护能够促进钝化膜的产生及修复并抑制裂纹扩展,获得了较好的效果,且在稳定钝化区选取的电位越大,其保护效果越好。X80管线钢在50℃的通饱和H2S的蒸馏水中及NACE A溶液中的腐蚀行为研究结果表明,两种环境中基体表面均会产生一层硫化物腐蚀产物膜,腐蚀产物的形状、尺寸、化学组成及晶体结构均随浸泡时间的变化而变化,四方硫铁均作为初生产物出现,立方FeS为次生产物,在NACE A溶液中腐蚀产物存在明显的分层现象,并建立了腐蚀产物分层现象的模型。比较而言,基体在NACE A溶液中溶解速率较高。同时在NACE A溶液中,在浸泡的最初阶段,腐蚀产物较难附着在基体表面；但腐蚀产物厚度达到某临界值后,腐蚀产物便快速沉淀积累,其速度远大于在蒸馏水中。试验发现腐蚀产物膜对基体存在一定的保护作用,可降低基体的腐蚀倾向及腐蚀速率。通过测试在两种腐蚀介质中,新鲜电极与腐蚀产物覆盖电极之间的电偶电流,发现两者之间存在较强的电偶效应,两者偶合时前者充当阳极而发生优先溶解。在偶合后的最初阶段,电偶电流较大,且其随着阴、阳极面积比增加或腐蚀产物覆盖电极浸泡时间的增加而增加。但电偶电流并不是一直不变的,其随着偶合时间的增加而逐渐减小,并最终减小到接近于0,这主要是新鲜电极表面状态发生改变所致,其表面产生了一层新的腐蚀产物,即H2S环境中腐蚀产物膜具有较好的自我修复功能。因此在该环境中,腐蚀产物膜发生局部脱落后不会引起基体的严重的局部腐蚀。研究了X80管线钢埋弧焊焊接接头各区域的组织及耐蚀性,发现其焊缝组织以针状铁素体为主,粗晶区原奥氏体晶界明显可见且组织以粒状贝氏体为主,细晶区组织以多边形铁素体为主,在铁素体晶界处可见呈块状分布的M/A组元,焊接接头硬度最高、低值分别位于焊缝及靠近母材的热影响区。在25℃的0.5MNa2CO3-1M NaHCO3溶液中,母材、焊缝、粗晶区及细晶区均会发生明显的钝化现象,母材及焊缝的耐蚀性及钝化膜稳定性相差不大,粗晶区耐蚀性及钝化膜稳定性最差,而细晶区的最好。在55℃的通饱和H2S的5wt.%NaCl溶液中浸泡96 h后,各试样表面均会产生一层腐蚀产物,且腐蚀产物的化学组成及晶体结构基本不存在差别,但其形貌差异较大,母材及细晶区试样表面腐蚀产物规则、致密性较好,因此其对基体保护效果较好,而粗晶区试样表面腐蚀产物存在较多孔洞,保护效果较差,因此粗晶区耐蚀性最差,为焊接接头耐蚀性的薄弱环节。利用焊接热模拟技术,采用Gleeble 3500试验机获得了焊接热输入为10～55kJ/cm、峰值温度为1300℃的X80管线钢热模拟粗晶区试样。发现粗晶区组织以粒状贝氏体为主,随焊接热输入增加,粗晶区中粒状贝氏体板条宽度逐渐增加,且逐渐有铁素体析出,M/A组元含量先增加后减少,形状逐渐从针状变为粒状。在25℃的0.5MNa2CO3-1MNaHCO3溶液中,虽然母材及粗晶区均会发生钝化,但粗晶区钝化膜稳定性较差,且耐蚀性均差于母材；随着焊接热输入的增加,粗晶区耐蚀性及钝化膜稳定性先增强后降低,在焊接热输入为30 kJ/cm时粗晶区耐蚀性最好。在55℃的通饱和H2S的5wt.%的NaCl溶液中,粗晶区耐蚀性均差于母材,且随着焊接热输入增加,粗晶区耐蚀性逐渐增强,这归因于其表面腐蚀产物中孔洞的逐渐减少。在25℃通持续5% C02/N2的NS4溶液中,粗晶区耐蚀性依然均差于母材,浸泡2天后,在母材及粗晶区试样的M/A组元处可观察到大量孔洞,这是由于M/A组元或其周边区域的优先溶解造成的点蚀；随着焊接热输入的增加,点蚀孔的数目及面积比逐渐减少,在焊接热输入为55 kJ/cm时,仅在少量粗大的M/A组元表面可见到较小的点蚀孔,因此随着焊接热输入的增加,粗晶区耐蚀性逐渐增强。由以上结果可知,为提高粗晶区的耐蚀性,应根据不同的腐蚀环境来选择焊接工艺规范。利用焊接热模拟技术研究了二次焊接热循环对粗晶区组织及耐蚀性的影响,其中第一、二次焊接热循环热输入分别采用30kJ/cm、10kJ/cm,一次焊接热循环峰值温度为1300℃,二次焊接热循环峰值温度分别为650、780、950、1150及1300℃,分别对应再热粗晶区中的亚临界粗晶区、临界粗晶区、超临界粗晶区及不变粗晶区。研究发现二次焊接热循环使再热粗晶区形成了一系列的组织梯度,其中由于临界粗晶区在加热过程中的局部奥氏体化及碳原子的有向扩散,导致在冷却后其原奥氏体晶界处的M/A组元严重粗化,这使得临界粗晶区成为在25℃的0.5MNa2CO3-1MNaHCO3溶液及55℃通饱和H2S的5wt.%NaCl溶液中整个管线耐蚀性的最薄弱环节；由于二次焊接热循环热输入较低,不变粗晶区组织中粒状贝氏体板条宽度减小,M/A组元呈针状,使其耐蚀性差于粗晶区：亚临界粗晶区相当于对粗晶区进行了一次类回火处理,虽然组织差别不大,但耐蚀性明显提升；而超临界粗晶区组织明显细化,耐蚀性提高。
[Abstract]:X80 pipeline steel has high strength, toughness and good weldability, and has been widely used in the world. The main line of China's West-East Gas Transmission Line is the longest line in the world to successfully apply the X80 pipeline steel. The welding is the main technical means to connect the pipeline steel, and the welding area is the weak area of the pipeline, so the successful welding of the pipeline steel is the most critical processing technique which is related to the service life and safety of the pipeline. The practice of a large number of pipelines in the pipeline indicates that the corrosion has become one of the most important reasons for pipeline failure, and the failure of pipeline burst, leakage and the like not only causes serious economic loss, but also can cause serious and disastrous consequences, Therefore, it is necessary to study the corrosion resistance of the pipeline steel and its welded joint. In this paper, the corrosion behavior of X80 pipeline steel base material in high pH and H2S environment is studied, and its corrosion mechanism is analyzed. In this paper, the microstructure and corrosion resistance of the welded joints under the submerged arc welding process and the corrosion resistance in different environments are studied, and the weak links of the corrosion resistance are analyzed. The influence of the welding heat input on the microstructure and the corrosion resistance of X80 pipeline steel in different environments is studied by using the welding thermal simulation technology, so as to obtain a good welding heat input range from the viewpoint of improving the corrosion resistance of the coarse crystal region. The effect of the peak temperature of the secondary welding thermal cycle on the microstructure and the corrosion resistance of the coarse-grained region and the different environment was also studied by the welding thermal simulation technique. The corrosion behavior of X80 pipeline steel in the solution of 0.5 MNa2C03-1M NaHC03 was studied, and it was found that in the temperature range of 25 ~ 95 鈩，
本文编号：2487137
[Abstract]:X80 pipeline steel has high strength, toughness and good weldability, and has been widely used in the world. The main line of China's West-East Gas Transmission Line is the longest line in the world to successfully apply the X80 pipeline steel. The welding is the main technical means to connect the pipeline steel, and the welding area is the weak area of the pipeline, so the successful welding of the pipeline steel is the most critical processing technique which is related to the service life and safety of the pipeline. The practice of a large number of pipelines in the pipeline indicates that the corrosion has become one of the most important reasons for pipeline failure, and the failure of pipeline burst, leakage and the like not only causes serious economic loss, but also can cause serious and disastrous consequences, Therefore, it is necessary to study the corrosion resistance of the pipeline steel and its welded joint. In this paper, the corrosion behavior of X80 pipeline steel base material in high pH and H2S environment is studied, and its corrosion mechanism is analyzed. In this paper, the microstructure and corrosion resistance of the welded joints under the submerged arc welding process and the corrosion resistance in different environments are studied, and the weak links of the corrosion resistance are analyzed. The influence of the welding heat input on the microstructure and the corrosion resistance of X80 pipeline steel in different environments is studied by using the welding thermal simulation technology, so as to obtain a good welding heat input range from the viewpoint of improving the corrosion resistance of the coarse crystal region. The effect of the peak temperature of the secondary welding thermal cycle on the microstructure and the corrosion resistance of the coarse-grained region and the different environment was also studied by the welding thermal simulation technique. The corrosion behavior of X80 pipeline steel in the solution of 0.5 MNa2C03-1M NaHC03 was studied, and it was found that in the temperature range of 25 ~ 95 鈩，
本文编号：2487137
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