Microstructure Evolution and Hot Cracking in Arc Welds

非技术总结：热裂纹发生在焊接过程中（当焊缝仍然很热时），在焊接熔池凝固期间或之后。避免不锈钢焊缝开裂至关重要，因为各种等级的不锈钢可在市场上买到，并广泛用于发电厂和化工厂。该项目旨在解决两个基本问题：1）为什么某些等级的不锈钢比其他一些等级的不锈钢更能抵抗热裂纹，以及2）焊缝内部的微观特征（称为微观结构）如何在焊接过程中的高温下抵抗热裂纹。该项目预计将促进对不锈钢焊接热裂纹的理解，目前不锈钢焊接热裂纹是基于室温微观结构，这种微观结构在焊接过程中甚至在高温下都不存在，以抵抗热裂纹。该项目的实验结果有望指导寻找更好地抵抗热裂纹的不锈钢填充金属，帮助了解不锈钢增材制造中的热裂纹，并作为不锈钢冶金教学的优秀新材料。该项目旨在雇用一名西班牙裔研究生。 技术概要：该项目涉及不锈钢电弧焊的热裂纹，不锈钢主要是铁铬镍合金。目的是建立对不锈钢焊缝中凝固裂纹和塑性浸渍裂纹的基本认识。提出了良好的抗凝固裂纹性（其发生在沿着在焊池后面的糊状区中生长的枝晶之间的边界）是由在凝固期间早期形成显著量的枝晶间相以将枝晶结合在一起引起的。还提出，良好的抗延性-浸渍裂纹性（其发生在紧邻糊状区后面的熔合区区域中的完全生长的枝晶之间的沿着边界）是由不存在裸柱状枝晶引起的，所述裸柱状枝晶提供了容易裂纹萌生和扩展的长直路径。这些机制将通过以下方式进行验证：1.在焊接过程中，用液态伍德金属（70 ℃）对各种等级的不锈钢进行淬火，使其在糊状区及其周围的高温显微组织中凝固，2.检查由淬火引起的裂纹的程度并将其与高温显微组织相关联，以及3.对不锈钢进行凝固开裂试验。建议的不锈钢研究将应用于高熵合金FeCrNiCoCux（也含有铁，铬和镍一样的不锈钢），以更好地了解其焊接性。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.