Microstructure Modeling for Welding Process Design

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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.