| 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.
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