多场耦合作用下长输天然气管线在役焊接烧穿失稳机理及判据研究

The problems of aging and corrosion have severely restricted the safety production of natural gas pipelines. It is imminent to research and improve the safety repair technology of pipelines. In-service welding has the advantages of high efficiency and low environmental pollution. Burn-through is the first problem to be solved. In-service welding is influenced by pipeline structure, welding parameters, and medium pressure, which is a high-temperature failure process under the coupling of structure field, temperature field and stress field. This project explores the multi-field coupling mechanism of in-service welding by building structure-thermal-force multi-field coupled constitutive model, and studying the relationship of multi-field coupling. The mechanism of burn-through during in-service welding is studied from macroscopic-microscopic multiscale, which is based on the experiment of in-service welding, finite element simulation, molecular dynamics simulation and theoretical analysis. The residual strength of pipe wall is calculated by three-dimensional integral, and the burn-through pressure is calculated combined with high-temperature plastic failure criterion. Finally, a criterion of burn-through during in-service welding will be proposed. The study will promote the development and application of in-service welding technology, and it has important theoretical and practical value to enrich the welding theory under harsh conditions and ensure the safe delivery of natural gas.

管道的老化、腐蚀等问题，已严重制约天然气管道的安全生产，研究和提高管道的安全修复技术迫在眉睫。在役焊接具有修复时间短、速度快、对环境污染小等优点，烧穿失稳是其首先要解决的问题。在役焊接烧穿受到管道结构、焊接工艺及输送介质压力等多因素的影响，是一个结构场、温度场、应力场多场耦合作用下的高温失效过程。本项目通过建立结构-热-力多场耦合本构模型，研究在役焊接多场耦合作用关系，探究在役焊接的多场耦合作用机理；利用在役焊接烧穿失稳试验、有限元数值模拟、分子动力学模拟及理论分析相结合的方法，从宏观-细观-微观多尺度研究多场耦合作用下的在役焊接烧穿失稳；通过三维积分计算焊接区管壁剩余强度，结合高温塑性失效准则，计算在役焊接的烧穿失稳应力，建立在役焊接烧穿失稳判据。该研究将推动在役焊接技术的发展和应用，同时对于丰富严酷条件下的焊接理论，保障天然气的安全输送，具有重要的理论和实际应用价值。

长输天然气管线在役焊接修复技术具有修复时间短、速度快、对环境污染小等优点，而烧穿失稳是首先要解决的问题。目前有关在役焊接烧穿的研究均是基于宏观表象，缺乏对在役焊接损伤失效的微观机理的研究及相应的科学理论支撑。本项目通过通过三维有限元数值模拟、分子动力学微观模拟、宏观损伤演化的有限元分析及实验验证相结合的方式，研究多场耦合下的在役焊接过程，揭示了烧穿失稳机理；探讨温度、应力等对材料微缺陷萌生的影响，结合焊接热过程、介质压力等因素，研究宏观缺陷的扩展致失效过程及其影响规律；基于改进的剩余强度三维积分法及高温塑性失效准则计算失稳压力，进一步建立了科学的在役焊接失稳安全判据。这对于丰富严酷条件下的焊接理论，指导在役焊接安全修复，保障天然气安全输送，具有重要的理论和实际意义。主要结论如下：.（1）高温下晶界弱化，晶界滑移严重，微裂纹易在弱化的晶界（熔合区）以及三晶粒交界的应力集中处（粗晶区）萌生，多处微裂纹在垂直于拉应力的方向沿弱化的晶界向内壁扩展，并相互汇合形成贯穿管壁的宏观裂纹，最终导致烧穿失稳。.（2）近管道内壁区主要发生Rachinger滑移，且位错回复引起的“老化”加快了孔洞的萌生过程；完全奥氏体化区出现了动态再结晶，有效消除大量位错、空位等微缺陷，避免位错塞积及孔洞的出现；熔合区激发动态再结晶的临界应变更小，约为2.0 %，且在温度与应力应变共同作用下诱发了晶界预熔。.（3）在役焊接修复时剩余壁厚的最大损伤值位于最大熔深处后方1mm-2mm区域内，损伤值在壁厚方向上由熔合线向内壁递减，且损伤更容易在熔合线上最大熔深处后方萌生。.（4）高温塑性失效准则适用于结构因素简单、易获取最危险路径的工况。而对于耦合场环境和结构因素较复杂的工况，剩余强度三维积分法具有更大优势。