X65管线厚板控冷过程的相变效应研究与数值模拟

【作者】 张德丰；

【导师】 陆建生； Thomas Antretter；

【作者基本信息】 昆明理工大学 ， 材料学， 2012， 博士

【摘要】 当今,在石油、天然气的输送中,管道运输因具有经济性、安全性、连续性等明显优势而获得广泛应用。由于高强韧性的X65管线厚板是现今乃至今后较长一段时间内长输管道建设的主体材料,因此,其生产工艺已成为相关领域内最具活力的一个研究热点。控制冷却是X65管线厚板常见的热处理工艺,目前普遍采用的控制冷却工艺常为三段式控冷,即管线厚板要先后经历“无水空冷”、“层流水的连续冷却”和“出水空冷”三个阶段方完成控冷过程。由于X65管线厚板控冷时因上下表面的不对称控冷会在板厚方向产生不对称分布的高值残余应力,进而形成严重的横向翘曲,受到板宽较窄的限制,残余应力和横向翘曲难以用矫直方式彻底消除,这不仅严重影响了该产品的板型质量,同时由于长输管道常处于低温、高压、易腐蚀等极端恶劣环境,较高的残余应力还为X65管线在后续使用时诱发应力腐蚀开裂埋下安全隐患。因此,研究并有效控制X65管线厚板控冷时产生的残余应力,对提高该产品的板型质量、延长使用寿命,将具有十分重要的意义。鉴于X65管线厚板控冷过程的高温、连续等特性,将生产中断进行现场实验研究,成本过高；以及受到板厚的限制,X射线衍射、中子散射等传统检测手段又无法对板内的温度、应力和应变进行连续而整体的分析；且目前也没有相对成熟的理论可对控冷过程进行直接计算；因此,本研究采用基于有限元的数值模拟方法对X65管线厚板的控冷过程进行研究。尽管此前的材料科学工作者对同类问题进行了大量有益的尝试,但他们常将控冷过程中由相变引起的相变潜热、相变膨胀、TRIP效应(TRIP-Transformation Induced Plasticity)等相变效应予以一定程度的简化甚至忽略。因此,他们研究时使用的模型并不完全适用于X65管线厚板控冷过程。本研究首先确定了X65管线厚板控冷时由奥氏体A-针状铁素体F相变诱发的相变潜热、相变膨胀和TRIP效应等相变效应的影响,进而通过理论研究明确了用于描述各相变效应的理论模型,并通过开发USDFLD、HETVAL、UEXPAN和TRIP子程序拓展了ABAQUS软件分析相变效应的功能；接着开发了考虑上述相变效应的2个有限元模型,并进行15组模拟实验初步明确了模拟控冷时考虑相变效应的必要性；接着用根据X65管线厚板实际的控冷工艺开发的热力耦合有限元模型,具体研究并明确了相变潜热、相变膨胀和TRIP效应等相变效应对控冷的影响和机制；最后通过将参照的与实际的控冷工艺的模拟结果进行比较分析,确定了实际使用的不对称控冷工艺对控冷的影响,进而提出优化的控冷工艺,并评估了优化工艺减小残余应力的效果；主要结论如下：(1)开发的热力耦合有限元模型因考虑了X65管线厚板控冷时涉及的弹塑性变形、热膨胀、传导、对流、辐射和相变效应(包括相变潜热、相变膨胀(?)TRIP效应)等目前所知的所有相关物理影响因素,可用于研究X65管线厚板的控冷过程。(2)相变效应对X65管线厚板的控冷具有重要影响,模拟时必须考虑。相变潜热使相变期间的板温升高52.7℃、使中部和下表面的温降分别减缓50%、25%,使控冷结束的板温升高44℃。相变膨胀通过产生(723MPa,-479MPa)的组织应力影响板厚方向的应力,该应力甚至可逆转中部和下表面附近的应力状态,而相变潜热主要通过小幅减小应力峰值、TRIP效应主要通过迁移压应力峰分别影响板厚方向的应力。相变膨胀和相变潜热通过产生均值为正的总体应变以减小板厚方向的负总体应变,而TRIP效应则相反,但均使总体应变趋于均匀。(3)实际生产使用的不对称控冷工艺是产生高值残余应力的重要原因,而优化的控冷工艺可显著降低残余应力。实际生产使用的不对称控冷工艺使X65管线厚板形成了不对称分布的温度、应力、应变,进而在板厚方向产生峰值为(350MPa、-272MPa)的残余应力,而优化的对称控冷工艺使厚板中部的压应力峰值和下表面的拉应力峰值分别减小114MPa、116MPa)。更多还原

【Abstract】 Today, pipeline transportation is widely available due to its economic, security, continuity and other obvious advantages in the oil and gas transportation. X65pipeline heavy plate with favorable mechanical properties is the main long-distance pipeline material in now and future, the pipeline manufacturing process is becoming a hot research area in industrial field. Controlled-Cooling is a common heat treatment technology during X65pipeline heavy plate production. The technology is composed of three stages:Air Cooling without water, Laminar Cooling, Air Cooling. Serious transverse warping is a normally phenomenon during the plate manufacturing process due to asymmetrical high residual stresses distribution via thickness direction of plate during asymmetrical Controlled Cooling of the plate on top and bottom surfaces. It should be noted that the residual stress is difficult to eliminate by straightening method due to insufficient width of plate. This phenomenon seriously affects quality of plate shape, but also limits the subsequent utilization because of crack-formation induced by stress corrosion when long-distance pipeline is used in extreme environments, e.g. low temperature, high pressure, corrosive circumstance, and so on. Therefore, it is very important to study and control the residual stresses during Controlled Cooling of X65pipeline heavy plate for improving the quality of shape and lifetime of the pipeline.Because Controlled Cooling of X65pipeline heavy plate is a continuous process in high temperature, it is not worthy of interrupting the process to do experimental study, and experimental techniques such as X-ray or neutron diffraction methods give only a rough idea about the temperature, stresses and strain distribution in the plate, and generally do not allow scanning the entire plate. Up to now, there is also no effective theory can calculate and optimize the process. Therefore, a numerical simulation method based on finite element method (FEM) is carried out to investigate the Controlled Cooling of X65pipeline heavy plate in this study.Although others researchers obtained some valuable experiences, they often simplified or ignored the phase transformation effect during Controlled Cooling, which included latent heat, transformation dilation, TRIP effect (TRIP-Transformation Induced Plasticity). The models used in those studies don’t fit well with the Controlled Cooling of X65pipeline heavy plate. Firstly, effect of Austenite-Ferrite phase transformation during Controlled Cooling of X65pipeline heavy plate, such as latent heat, transformation dilation, and TRIP effect, is confirmed in this study. The theoretic models related to effect of phase transformation are built with a base on theoretic study. The functions of ABAQUS, which used to describe effect of phase transformation, are developed by coding the subrotines, including USDFLD, HETVAL, UEXPAN and TRIP. Secondly, two finite element models related with above phase transformation are established. It is found that the effect of the phase transformation is necessary for simulation of Controlled Cooling through15sets of simulation experiments by using the two models. A thermo-mechanical coupled FEM model based on the actual Controlled Cooling technology of X65pipeline heavy plate is established. The influence and mechanism of effect of phase transformation on X65pipeline heavy plate, such as latent heat, transformation dilation, and TRIP effect, are studied and confirmed. Finally, the effect of the actual asymmetric Controlled Cooling technology on residual stresses of X65pipeline heavy plate is detailed discussed by comparing to the result from ones of reference. Consequently, an optimizing Controlled Cooling technology is proposed. The reduction of residual stress in the plate is estimated through optimizing Controlled Cooling technology. In this study, the following conclusions are obtained:(1) All relevant physical phenomena so far, such as elastoplastic deformation, thermal expansion, conduction, convection, radiation and effect on phase transformation, including latent heat, transformation dilation, TRIP effect, are considered in the developed thermo-mechanical coupled FEM model. The model is suitable to the Controlled Cooling of X65pipeline heavy plate.(2) The influence of transformation effect on Controlled Cooling of X65pipeline heavy plate is important, and it must be considered in the simulation.Latent heat enhances about52.7℃during phase transformation and the cooling speed of inner part delays50%, and25%slow for the plate bottom surface, which also increases the temperature of plate44℃finally.The structural stresses from transformation dilation with peak value,723MPa and-479MPa, affect strongly residual stresses along thickness direction of plate, and the stresses status near the inner and bottom surface can even be reversed by the stresses. The stresses along thickness direction of plate are decreased slightly by latent heat and the stresses are changed by shifting compressive stress peak by TRIP effect.Because transformation dilation and latent heat produce positive total strain, they reduce the negative total strain along thickness direction of plate, however, TRIP effect works on the contrary. All of them make the total strain to be more even.(3) During plate industrial manufacturing, high residual stress within the plate mainly depends on the actual asymmetrical Controlled Cooling parameters. The optimizing Controlled Cooling technology in this study can reduce residual stress significantly.The asymmetrical distribution of temperature, stresses, and strain are caused during the actual asymmetrical Controlled Cooling of X65pipeline heavy plate. Therefore, high residual stresses,350MPa and-272MPa are generated. Compressive peak stress of inner and tensile peak stress of bottom surface are decreased114MPa and116MPa respectively by the optimized technology.更多还原