Microstructure Evolution and Hot Cracking in Arc Welds

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NON-TECHNICAL SUMMARY:Hot cracking occurs during welding (while the weld is still very hot) either during or right after solidification of the weld pool. It is essential to avoid cracking in welds of stainless steels because various grades of stainless steels are commercially available and widely used in power and chemical plants. The project seeks to address two fundamental issues: 1) why some grades of stainless steels resist hot cracking significantly better than some other grades, and 2) how the microscopic features inside the weld (called the microstructure) resist hot cracking at elevated temperatures during welding. The project is expected to advance the understanding of hot cracking in welds of stainless steels, which is currently based on the room-temperature microstructure that does not even exist at elevated temperatures during welding to resist hot cracking. The experimental results from the project can be expected to guide the search for stainless-steel filler metals that better resist hot cracking, help understand hot cracking in additive manufacturing of stainless steels and serve as excellent new materials for teaching the metallurgy of stainless steels. The project seeks to hire a Hispanic graduate student. TECHNICAL SUMMARY:The project deals with hot cracking in arc welds of stainless steels, which are essentially Fe-Cr-Ni alloys. The goal is to establish the fundamental understanding of solidification cracking and ductility-dip cracking in welds of stainless steels. It is proposed that good resistance to solidification cracking (which occurs along boundaries between dendrites growing in the mushy zone behind the weld pool) is caused by the formation of a significant amount of the interdendritic phase early during solidification to bond the dendrites together. It is also proposed that good resistance to ductility-dip cracking (which occurs along boundaries between fully-grown dendrites in the region of the fusion zone closely behind the mushy zone) is caused by the absence of naked columnar dendrites that provide long straight paths for easy crack initiation and propagation. The mechanisms will be verified by: 1. quenching various grades of stainless steels with liquid Wood's metal (70oC) during welding to freeze-in the elevated-temperature microstructure in and around the mushy zone, 2. examining the extents of cracking induced by quenching and relating them to the elevated-temperature microstructure, and 3. conducting solidification cracking tests on the stainless steels. The proposed study on stainless steels will be applied to high-entropy alloys FeCrNiCoCux (which also contain Fe, Cr and Ni like stainless steels) to better understand their weldability.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.