【作者】 王凯；

【导师】 李天庆；

【作者基本信息】 江苏大学 ， 材料科学与工程， 2022， 硕士

【摘要】 超级双相不锈钢2507具有优秀的力学性能和耐腐蚀性能,在海洋工程、化学化工、能源等领域具有广泛应用前景。合理的铁素体-奥氏体两相比例决定了超级双相不锈钢2507性能。焊接制造是超级双相不锈钢2507的重要成形工艺。然而,焊接时会导致焊接接头中两相比例失衡,影响超级双相不锈钢2507焊接接头性能。调控焊接接头中的两相比例是获得超级双相不锈钢2507优质焊接接头的关键。针对超级双相不锈钢2507的焊接,国内外焊接工作者已经开展了一些研究,取得了一些结果;但有关薄板超级双相不锈钢2507高能量密度焊接(激光焊接、等离子弧焊接)时,如何有效提高焊缝氮含量、奥氏体含量?氮元素向焊缝的过渡机制是怎样的?这些关键问题都亟待探索。开展超级双相不锈钢2507的高能量密度焊接研究,对于丰富超级双相不锈钢2507焊接理论和指导超级双相不锈钢2507的焊接具有重要的理论意义和工程价值。本研究将采用激光焊接和气流再压缩等离子弧焊接对超级双相不锈钢2507进行焊接,获得超级双相不锈钢2507焊接接头。对焊接接头进行金相组织分析、EBSD分析、TEM分析,对焊接接头微区进行EPMA点扫,采用氧氮氢分析仪分析焊缝氮含量,对焊接接头进行显微硬度测试,对焊接接头进行电化学腐蚀,采用SEM观察电化学腐蚀后的点蚀坑。本文将对比研究纯氩和氩氮联合保护下的激光焊接接头组织和性能;分析氮气比例变化对激光焊接接头组织和性能的影响;对比研究不添加氮气的常规等离子弧焊接和通过压缩气通道添加氮气的气流再压缩等离子弧焊接接头组织和性能;分析氮气流量对气流再压缩等离子弧焊接接头组织和性能的影响;对比分析激光焊接和气流再压缩等离子弧焊接接头的形貌和焊缝中氮含量;初步探究激光焊接和气流再压缩等离子弧焊接中氮向焊缝的过渡机制。本研究得到以下主要结论。(1)与不添加氮气的激光焊接相比,添加25%氮气(氮气流量5 L/min)的激光焊接:焊缝中奥氏体含量增加了10.7%;焊缝氮含量增加了0.012 wt%;焊缝两相取向、焊接接头显微硬度、焊接接头耐腐蚀性能均无明显变化。(2)激光焊接中,在氮气流量为5-20 L/min的范围内,随着氮气比例的增加:焊缝奥氏体含量先增加后减小;焊缝两相取向情况不变;焊缝奥氏体中氮含量、焊接接头显微硬度、焊接接头耐腐蚀性能均无明显变化。氮气比例为75%(氮气流量15 L/min)时,焊缝奥氏体含量最高,达到40.8%。(3)与不添加氮气的常规等离子弧焊接相比,添加0.11 L/min氮气的气流再压缩等离子弧焊接:焊缝奥氏体含量提高了23.0%;奥氏体形貌改变,除了晶界奥氏体、晶内奥氏体外,出现了魏氏奥氏体;焊缝两相取向情况不变;焊缝氮含量增加了0.033 wt%;焊缝奥氏体显微硬度升高,铁素体显微硬度降低;焊接接头耐腐蚀性能提高。(4)气流再压缩等离子弧焊接中,在氮气流量为0.07-0.18 L/min的范围内,随着氮气流量的增加:焊缝奥氏体含量增加;焊缝两相取向情况不变;焊缝奥氏体中氮含量增加;焊缝奥氏体显微硬度增加,铁素体显微硬度先减小后增加;焊接接头耐腐蚀性能提高。氮气流量为0.18 L/min时,焊缝奥氏体含量为57.6%。(5)激光焊接时,氮气对焊缝奥氏体量的提升程度有限,对焊接接头性能的改善作用有限,即使是100%氮气(氮气流量20 L/min)下,激光焊接中的氮气作用效果仍然不显著;气流再压缩等离子弧焊接时,通过压缩气通道加入少量的氮气(氮气流量0.11 L/min),就可以明显提高焊缝上表面的奥氏体量,改善焊接接头性能。(6)与激光焊接相比,气流再压缩等离子弧焊接时熔池氮元素溶解速率更大,熔池内部氮含量上升区宽度更宽,熔池氮元素逸出速率、熔化前沿氮含量下降区宽度、凝固前沿氮含量下降区宽度基本不变,更容易实现焊缝奥氏体量、焊缝氮含量、焊缝奥氏体中氮含量的提升。更多还原

【Abstract】 Super duplex stainless steel 2507 is a kind of stainless steel with excellent mechanical properties and corrosion resistance.And therefore,super duplex stainless steel 2507 has a wide range of applications in marine engineering,chemical industry,energy and other fields.A reasonable ratio of ferrite content to austenite content determines the performance of super duplex stainless steel 2507.Welding manufacturing is an important manufacturing process of super duplex stainless steel 2507.However,welding will lead to an imbalance in the ratio of the content of two phases in the welded joint,which will affect the performance of super duplex stainless steel 2507 welded joints.Regulating the ratio of the content of two phases in the welded joint is the key to obtaining a high-quality welded joint of super duplex stainless steel 2507.For the welding of super duplex stainless steel 2507,domestic and foreign welding researchers have carried out some research and achieved some results.However,during high energy density welding(laser welding,plasma arc welding)on thin plate super duplex stainless steel 2507,how to improve the nitrogen content and austenite content in the weld metal effectively? What is the transition mechanism of nitrogen to the weld metal? These key issues need to be explored urgently.The research on high energy density welding of super duplex stainless steel 2507 will enrich the welding theory of super duplex stainless steel 2507 and guide the welding of super duplex stainless steel 2507,which has important theoretical significance and engineering value.In this study,laser beam welding and gas focusing plasma arc welding are used to weld super duplex stainless steel 2507 to obtain the welded joint of super duplex stainless steel2507.The metallographic structure analysis,EBSD analysis and TEM analysis are carried out on welded joints.EPMA spot scanning is carried out on the micro-area of the welded joints.The nitrogen content of the weld metal is analyzed by oxygen-nitrogen-hydrogen analyzer.The microhardness test and the electrochemical corrosion test are carried out on the welded joints,and the pits after electrochemical corrosion test are observed by SEM.In this paper,the microstructure and properties of laser beam welded joints shielded by pure argon and shielded by combined argon-nitrogen are comparatively studied,while the effect of nitrogen proportion change on the microstructure and properties of laser beam welded joints is analyzed.The microstructure and properties of welded joints of conventional plasma arc welding without nitrogen addition and gas focusing plasma arc welding with nitrogen addition through focusing gas channel are compared and studied,while the effect of nitrogen flow rate on the microstructure and properties of welded joints of gas focusing plasma arc welding is analyzed.The morphology of welded joints and nitrogen content in weld metal of laser beam welding and gas focusing plasma arc welding are compared and analyzed.And the transition mechanism of nitrogen to weld metal in laser beam welding and gas focusing plasma arc welding is preliminarily explored.The main conclusions of this study are as follows.(1)Compared with laser beam welding without nitrogen addition,laser beam welding with 25% nitrogen addition(nitrogen flow rate of 5 L/min),the austenite content in the weld metal increases by 10.7%,and the nitrogen content in the weld metal increases by 0.012wt%,and the orientation of two phases in the weld metal,microhardness of welded joints,and corrosion resistance of welded joints do not change significantly.(2)During laser beam welding,in the range of nitrogen flow rate of 5-20 L/min,with the increase of nitrogen proportion,the content of austenite in the weld metal increases firstly and then decreases,and the orientation of two phases in the weld metal remains unchanged,and there is no significant change in nitrogen content of weld metal,microhardness of welded joints,and corrosion resistance of welded joints.When the nitrogen proportion is 75%(nitrogen flow rate of 15 L/min),the austenite content of the weld metal is the highest,reaching 40.8%.(3)Compared with conventional plasma arc welding without nitrogen addition,gas focusing plasma arc welding with 0.11 L/min nitrogen addition,the austenite content in the weld metal increases by 23.0%,and the morphology of austenite changes,and Widmanst(?)tten austenite appears,while the grain boundary austenite and intragranular austenite also exist,and the orientation of two phases in the weld metal remains unchanged,and the nitrogen content of the weld metal increases by 0.033 wt%,and the microhardness of austenite in weld metal increases,and the microhardness of ferrite in weld metal reduces,and the corrosion resistance of welded joint improves.(4)During gas focusing plasma arc welding,within the range of nitrogen flow rate of0.07-0.18 L/min,with the increase of nitrogen flow rate,the austenite content in the weld metal increases,and the orientation of two phases in the weld metal remains unchanged,and the nitrogen content of austenite in the weld metal increases,and the microhardness of austenite in the weld metal increases,and the microhardness of ferrite in the weld metal decreases firstly and then increases,and the corrosion resistance of the welded joint increases.When the nitrogen flow rate is 0.18 L/min,the austenite content of the weld is 57.6%.(5)During laser beam welding,nitrogen has limited effects on the increase of austenite content in weld metal and the improvement of performance of welded joint.Even shielded by 100% nitrogen(nitrogen flow rate of 20 L / min),the effect of nitrogen in laser beam welding is still not significant.During gas focusing plasma arc welding,adding a small amount of nitrogen(nitrogen flow rate of 0.11 L/min)through focusing gas channel can significantly increase the content of austenite in the upper surface of the weld metal and improve the performance of the welded joint.(6)Compared with laser beam welding,during gas focusing plasma arc welding,the dissolution rate of nitrogen in the molten pool is larger,and the width of the rising zone of nitrogen content in the molten pool is wider,while the escape rate of nitrogen in the molten pool,the width of the falling zone of nitrogen content at the melting front,and the width of the falling zone of nitrogen content at the solidification front are basically unchanged.It is easier for gas focusing plasma arc welding to increase the content of austenite in the weld metal,the nitrogen content of the weld metal and the nitrogen content of austenite in the weld metal.更多还原