Microstructure Modeling for Welding Process Design

该项目旨在开发焊缝熔合区和热影响区微观组织演变的细观模型。这种模型对于现代发电厂的焊缝设计和控制是至关重要的，在现代发电厂中，焊缝的热裂和耐腐蚀性限制了工厂的性能。细观模型将基于元胞自动机、蒙特卡罗和相场模型，并结合焊接结构的热机械宏观模型。耦合将通过中观模式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的动力学决定了焊接质量，并将包含在细观模型中。在低合金低合金钢熔合区，初生铁素体在奥氏体晶界形核，并开始向柱状晶核心长大。在稍低的温度下，有时会形成魏德曼斯特陶粒。当温度低于此温度时，针状铁素体在夹杂物上形核，形成细晶铁素体组织，提供低温韧性，这在管线钢中非常重要。在核电厂和火力发电厂至关重要的奥氏体不锈钢中，如果柱状晶以三角洲铁素体的形式长大，磷和硫等元素的溶解度就会更高，这些元素在柱状晶界的聚集就会减少，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，导致晶核贫化，降低了焊缝的耐腐蚀性。所提出的细观模型将使模拟焊接组织的形成和优化焊接参数成为可能，从而产生具有最小不利偏析影响的最佳组织。

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.