Microstructure Evolution and Hot Cracking in Arc Welds

非技术概要：热裂纹发生在焊接过程中(当焊缝仍然很热时)，或者是在熔池凝固期间或之后。避免不锈钢焊缝开裂是至关重要的，因为各种牌号的不锈钢在商业上都可以买到，并在发电厂和化工厂中广泛使用。该项目致力于解决两个基本问题：1)为什么某些牌号的不锈钢比其他牌号的不锈钢更耐热裂；2)焊缝内部的微观特征(称为显微组织)如何在焊接过程中抵抗高温下的热裂。该项目预计将促进对不锈钢焊缝热裂纹的理解，目前不锈钢焊缝的热裂纹是基于室温微结构，在焊接过程中甚至不存在高温组织，以防止热裂纹。该项目的实验结果有望指导寻找更好地抗热裂的不锈钢填充金属，有助于理解不锈钢添加剂制造中的热裂，并可作为教学不锈钢冶金学的优良新材料。该项目旨在招聘一名西班牙裔研究生。技术概述：该项目涉及不锈钢弧焊的热裂纹，不锈钢基本上是Fe-Cr-Ni合金。目的是建立对不锈钢焊缝中的凝固裂纹和塑性浸泡裂纹的基本理解。结果表明，良好的抗凝固开裂(沿熔池后糊状区中生长的枝晶间边界发生)是由于在凝固早期形成了大量的枝晶间相，从而将枝晶粘合在一起。还提出了良好的抗塑性-倾角开裂(沿熔合区紧邻糊状区的完全长大的枝晶之间的边界发生)的原因是由于没有裸露的柱状枝晶，这些枝晶提供了长而直的路径，便于裂纹的萌生和扩展。其机理将通过以下几个方面得到验证：1.在焊接过程中用液态金属(70oC)淬火，以冻结糊状区及其周围的高温组织；2.检查淬火引起的裂纹程度，并将其与高温组织联系起来；3.对不锈钢进行凝固裂纹试验。拟议的关于不锈钢的研究将应用于高熵合金FeCrNiCoCux(其中也包含铁、铬和镍等不锈钢)，以更好地了解它们的可焊性。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.