Microstructure Modeling for Welding Process Design

提出的项目旨在建立一个细观模型，用于熔合区和焊缝热影响区的微观结构演变。这种模型对于现代发电厂的焊缝设计和控制至关重要，因为焊缝的热裂和耐腐蚀性限制了电厂的性能。细观模型将基于元胞自动机、蒙特卡罗和相场模型以及焊接结构的热力学宏观模型。耦合将通过中观模型边界处的热和力学条件产生。凝固过程中的偏析和扩散效应以及固态相变动力学决定了焊接质量，并将包括在细观模型中。在低合金HSLA钢的熔合区，初生铁素体晶粒在奥氏体晶界处成核，并开始向柱状晶粒的核心生长。在稍低的温度下，有时会形成Widmanstätten颗粒。当温度低于该温度时，针状铁素体晶粒在夹杂物上形核，形成细晶铁素体组织，提供管道钢非常重要的低温韧性。如果奥氏体不锈钢的柱状晶粒以δ铁素体的形式生长，对核电站和火电厂至关重要，则磷和硫等元素的溶解度更高，并且这些元素在柱状晶界的积累减少，从而降低了热裂的风险。然而，在这一过程中，Mo等替代元素会偏析到柱状晶界，使晶心枯竭，降低焊缝的耐腐蚀性。提出的细观模型可以模拟焊缝微观组织的形成，优化焊接参数，从而产生具有最小有害偏析效应的最佳微观组织。

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.