Quantification of Structural Transformations during Heat Treatment in Ultra-high Carbon Steels

钢是一种以铁和碳为主要成分的金属材料。根据碳含量和工艺的不同，钢的性能会有很大的不同。超高碳水平的钢更像陶瓷，而不是金属；它们极其坚硬，非常易碎。这些钢主要用于工具部件，以成形较软的钢和其他金属，但如果它们的脆性能够降低，可能会有更大的用途。到目前为止，脆性已经通过加热和成形的循环来降低，这些方法在大规模生产中实施是具有挑战性的。仅通过简单的加热就有可能降低这些材料的脆性；然而，现有的热处理是能源密集型的，而且远远没有得到优化。该奖项支持超高碳钢在加热过程中的结构转变行为的基础研究。这项研究利用新颖的实验方法来实时观察加热过程中钢结构的变化，并开发可用于优化工艺和性能的新指标。这些基础性研究将导致改进的热处理策略，使超高碳钢的生产具有更好的性能组合，同时还可以减少能源消耗。这项研究的结果还将向材料科学本科生介绍现代炼钢的基本主题，并将向初中生和高中生介绍用于表征材料的数字图像分析和方法。与其他含碳量较低的钢相比，超高碳钢具有更高的硬度和强度。然而，这些钢中碳化物的高比例使它们对于许多结构应用来说太脆了。该项目旨在研究超高碳含量(1.7-2.3重量百分比)钢在高温下的组织转变，以及这些转变对机械性能的影响：韧性、硬度、强度和延展性。本研究将应用原位共聚焦扫描激光显微镜和非原位电子显微镜来研究和量化组织演变，特别是研究时间、温度和合金化学对碳化物在热处理钢中溶解动力学及其最终分布的影响。对重要机械性能的全面评估将与最终的微观组织状态相关。这些追求将导致一个分析模型，它可以准确地描述超高碳钢热处理温度范围内碳化物的溶解和析出行为。

Steel is a metallic material with iron and carbon as the primary components. The properties of steel can vary greatly depending on carbon level and processing. Steels with ultra-high carbon levels behave more like ceramics than metals; they are extremely hard and very brittle. These steels are primarily utilized in tool components to shape softer steels and other metals, but could see greater usage if their brittleness can be reduced. To date, brittleness has been reduced by cycles of heating and forming, methods that are challenging to implement on a large production scale. It is possible to decrease brittleness in these materials by simply heating alone; however existing heat treatments are energy intensive and far from optimized. This award supports fundamental research on the structural transformation behavior of ultra-high carbon steels during heating. The research utilizes novel experimental methods to observe changes to the steel structure in real time during heating and to develop new metrics that can be used to optimize processing and properties. These fundamental studies will lead to improved heat treatment strategies that will enable production of ultra-high carbon steels with improved combinations of properties while also reducing energy usage. Results from this research will also introduce materials science undergraduate students to fundamental topics of modern steelmaking and an outreach component will expose middle and high school students to digital image analysis and methods used to characterize materials. Ultra-high carbon steels can exhibit superior hardness and strength compared to other steels of lower carbon contents. However, the high fraction of carbides in these steels render them too brittle for many structural applications. This project seeks to investigate microstructural transformations at elevated temperatures in steels containing ultrahigh carbon contents (1.7-2.3 weight percent) and the impact of these transformations on mechanical properties: toughness, hardness, strength and ductility. This study will apply in-situ confocal scanning laser microscopy and ex-situ electron microscopy to study and quantify microstructural evolution, specifically examining the effects of time, temperature and alloy chemistry on carbide dissolution kinetics and their final distribution in heat treated steels. A comprehensive assessment of important mechanical properties will be correlated to the final microstructural state. These pursuits will result in an analytical model that can accurately describe carbide dissolution and precipitation behavior at temperature ranges employed for heat treatments of ultra-high carbon steels.