Microstructure Modeling for Welding Process Design

该项目的目的是开发一个细观模型的焊缝熔合区和热影响区的微观组织演变。这种模型对于现代发电厂的焊缝设计和控制至关重要，其中焊缝的热裂纹和耐腐蚀性限制了工厂性能。细观模型将基于元胞自动机、蒙特卡罗和相场模型，并结合焊接结构的热机械宏观模型。通过细观模型边界处的热力条件产生耦合。 凝固过程中的偏析和扩散效应以及固态相变动力学定义了焊缝质量，并将纳入细观模型。在低合金高强度低合金钢的熔合区，初生铁素体晶粒在奥氏体晶界形核，并开始向柱状晶中心生长。在较低的温度下，有时会形成Widmanstätten晶粒。在低于该温度下，针状铁素体晶粒在夹杂物上成核，形成细晶粒铁素体显微组织，提供在管线钢中非常重要的低温韧性。 如果柱状晶粒在对核电厂和热电厂至关重要的奥氏体不锈钢中生长为δ铁素体，则诸如磷和硫的元素的溶解度较高，并且这些元素在柱状晶粒边界处的积累减少，从而降低热裂纹的风险。然而，在该过程中，诸如Mo的替代元素可偏析到柱状晶界，从而使晶粒核心耗尽并降低焊缝的耐腐蚀性。建议的细观模型将有可能模拟焊缝组织的形成和优化焊接参数，使最佳的组织与最小的有害偏析的影响，可以产生。

The proposed project aims to develop a meso-model for the microstructure evolution in the fusion and heat-affected zones of welds. Such a model is critical to the design and control of the welds for modern power plants where the welds' hot cracking and corrosion resistance limit the plant performance. The meso-model will be based on a cellular automata, Monte Carlo and phase field models coupled with a thermo-mechanical macro-model of the welded structure. Coupling will be produced through the thermal and mechanical conditions at the boundary of the meso-model. The segregation and, therefore, diffusion effects during solidification, and the kinetics of phase transformations in the solid state define the weld quality and will be included into the meso-model. In the fusion zone of low alloy HSLA steels, the primary ferrite grains nucleate at the austenite grain boundaries and begin to grow towards the core of the columnar grains. At slightly lower temperatures, the Widmanstätten grains sometimes form. At the temperatures lower than that, the acicular ferrite grains nucleate on inclusions forming a fine grained ferrite microstructure providing low temperature toughness that is very important in pipeline steels. If the columnar grains grow as delta ferrite in austenitic stainless steels that are critical to nuclear and thermal power plants, the solubility of the elements such as phosphorous and sulphur is higher and the accumulation of these elements at columnar grain boundaries is reduced, thus reducing the risk of hot cracking. However, in this process substitutional elements such as Mo can segregate to the columnar grain boundary leaving the grain core depleted and reducing the corrosion resistance of the weld. The proposed meso-model will make it possible to simulate the weld microstructure formation and optimize the welding parameters so that the optimum microstructure with minimum detrimental segregation effects can be produced.