搅拌摩擦焊高强管线钢环缝的复相组织演化与增韧机制研究

Presently, fusion welding was widely used to join girth welds for oil and gas transportation pipelines. However, fusion welding easily resulted in coarsening and embrittling of the heat affected zone (HAZ), reducing toughness of the HAZ. Friction stir welding (FSW), as a solid state welding technology, can improve toughness of the HAZ due to lower heat input. In recent some studies, relatively high heat input conditions with higher peak temperatures of >> A3 were exerted to join the pipeline steels, causing a reduction in toughness of the welds due to coarsened microstructure. In this proposal, by means of using medium heat input, the peak temperatures were controlled within the austenitic non-recrystallized range, and austenite was transformed to fine bainite/ferrite multi-phase structure due to the action of severe deformation and cooling, increasing toughness. In this study, the thermal torsion was used to simulate the FSW process to build temperature and strain fields and uncover the effect of FSW on austenitic deformation at elevated temperature, deformation induced ferrite transformation and following continuous cooling transformation. Through studying microstructure and toughness of welds, the influence of phase composition, effective grain size, grain and phase boundaries, size and distribution of M-A constituent and carbonitride on both initiation and propagation of crack was revealed for fine multi-phase structure. Finally, the mechanism responsible for enhancement in toughness was elaborated, providing important reference in theory and technology for joining oil gas transportation pipelines.

目前油气管线的连接主要采用熔焊，但熔焊的高热输入易引起热影响区（HAZ）组织的粗化和脆化，降低接头韧性。搅拌摩擦焊（FSW）属固相焊，其较低的热输入可显著改善HAZ韧性。现有FSW管线钢研究普遍采用峰值温度>>A3的高热输入参数，因而导致焊缝组织粗化，韧性降低。本申请拟采用中等热输入进行FSW高强管线钢，即峰值温度被控制在奥氏体的未再结晶区，奥氏体在强变形和焊后冷却作用下转变成细小的贝氏体/铁素体复相组织，可显著改善焊缝韧性。研究中拟采用热扭转模拟FSW变形过程，揭示FSW过程中的奥氏体高温变形行为、形变诱导铁素体相变和连续冷却相变机制，并建立FSW过程的应变场和温度场。通过研究焊缝组织和韧性，揭示复相组织中的相组成、有效晶粒尺寸、晶界和相界特征、M-A组元和碳氮化物的尺寸和分布对裂纹萌生和扩散的作用机制，阐明FSW焊缝的增韧机理，为FSW在油气管线连接中的应用提供理论和技术参考。

传统熔焊高的热输入极易引起熔合区附近热影响区（HAZ）组织的粗化和脆化，严重削弱接头韧性。搅拌摩擦焊（FSW）是一种热输入较低的固相连接技术，可有效抑制粗大晶粒和马氏体-奥氏体（M-A）组元在HAZ形成。本项目以X65和X80管线钢为研究对象，在不同转速、不同冷却介质以及预热条件下对6 mm和11 mm厚的管线钢板进行FSW。系统研究了不同热输入及冷却路径对焊核区（NZ）再结晶机制、晶体学特征以及力学性能的影响。.首先，在400-700 rpm的宽泛焊接参数和100 mm/min的固定焊接速度下对6 mm厚的X65管线钢进行FSW。同时，利用多道次热压缩试验模拟了FSW的热机械加工过程，获得了奥氏体未再结晶温度(Tnr)。当转速为400 rpm时，NZ发生奥氏体的未在结晶，导致晶粒明显细化。明确了不同焊接参数对FSW管线钢过程中NZ再结晶行为的影响。其次，在300、400和600 rpm的不同焊接转速和100 mm/min的固定焊接速度下对6 mm厚的X80管线钢进行FSW。利用热模拟设备建立了不同变形条件下的连续冷却相变曲线。随着转速的降低，NZ的原奥氏体细化且伴随着Z参数的增加，并在300 rpm下得到板条贝氏体（LB）、粒状贝氏体（GB）和铁素体的复相组织。揭示了不同变形量和峰值温度对NZ再结晶行为和相转变行为的影响。第三，在空气、水、水+固态CO2和固态CO2不同冷却介质下对6 mm厚的X80管线钢进行FSW。随着冷却介质温度降低，LB和板条马氏体比例逐渐增加。特别的，在水+固态CO2冷却下，有铁素体和马氏体双相组织在NZ生成。此外，FSW的强变形和低的冷却介质温度共同抑制了变体选择，有利于高取向差变体对生成。明确了强变形和外加冷却介质对NZ晶体学特征的影响。第四，采用600 rpm-100 mm/min的焊接参数，在预热条件下对11 mm厚的 X80管线钢进行双面FSW。与GMAW ICCGHAZ相比，FSW ICCGHAZ韧性明显改善，这归因于晶粒和M-A组元的细化以及ICCGHAZ在接头中的面积较小。另外，第一道次的NZ在第二道次焊接时被重新加热，发生了铁素体的静态再结晶，组织明显细化。在FSW NZ韧性最佳，达到BM的112%。