Understanding and Controlling the Ductile-Brittle Transition in High-Strength Martensitic Steel

技术摘要：本研究涉及微观结构对板条马氏体钢延性-脆性转变的影响。它具体阐述了马氏体“块”作为板条马氏体钢中有效晶粒尺寸的核心作用，以及块尺寸对强度和解理断裂抗力的明显不同的影响。这项研究在技术上具有重要意义，因为现在使用或提议用于低温、北极或低温用途的高强度钢是板条马氏体钢。这些钢的性能和成本方面的技术进步需要了解和利用控制低温强度和韧性的微观结构机制。这项研究具有重要的科学意义，因为它有望完成控制马氏体钢延性-脆性转变的微观结构-性能关系的新兴机械理论，这一理论是经过四十多年的国际冶金研究慢慢发展起来的。该研究将研究一系列具有板条马氏体显微组织的Fe-C、Fe-Ni和Fe-Mn钢。它们的颗粒、块状和块状结构将被表征，并使用已知可细化有效块尺寸的热处理进行修改。将进行适当的强度和韧性测试，并利用高分辨率表征技术（包括定向成像和剖面断口分析）进行研究，以确定块体结构对强度、韧性和解理断裂路径的影响。研究结果将为板条马氏体的块状结构及其对强度和韧脆转变的具体影响提供新的、验证性的信息，从而提出设计具有优异性能的先进合金的新冶金方法。非技术摘要：由于脆性断裂导致灾难性失效的危险，高强度钢在低温、北极或深海环境中的使用受到严重限制。相关的冶金问题是韧脆转变；在高温下坚韧且具有延展性的钢在低于其延性-脆性转变温度时使用时几乎变得玻璃脆。为了避免这个问题，低温结构和设备的设计者使用了相对低强度的钢，这会影响工程效率，或者使用高合金钢，这会增加成本。钢的韧脆转变温度很大程度上受到其内部结构（“微观结构”）的影响，即其原子在微米和纳米尺度上的排列方式。通过世界各地许多实验室约五十年的定向研究，冶金学家已经了解了高强度钢微观结构的许多特征，以及如何设计该微观结构来控制延性-脆性转变。目前的研究项目旨在通过应用最好的现代工具对在受控条件下断裂的钢材进行微观结构表征，来充实这一理论理解。研究结果将为控制脆性转变的基本机制提供新的、验证性的信息，提出新的冶金方法来设计具有优异性能的先进合金，用于低温、北极或深海服务。首席研究员将继续教授新生关于武器历史的研讨会课程，他之前曾用这门课程来激发学生对冶金领域的兴趣。 PI 还将鼓励女性和代表性不足的群体参与他的研究项目，就像他过去所做的那样。

TECHNICAL SUMMARY: This research concerns the influence of microstructure on the ductile-brittle transition in lath martensitic steels. It specifically addresses the central role of the martensite "block" as the effective grain size in lath martensitic steel and the apparently very different influence of the block size on the strength and the cleavage fracture resistance. The research is technologically important since the high-strength steels now used or proposed for low-temperature, arctic or cryogenic service are lath martensitic steels. Technological advances in the properties and cost of these steels require understanding and exploiting the microstructural mechanisms that control low-temperature strength and toughness. The research is scientifically important since it promises to complete the emerging mechanistic theory of the microstructure-property relations that control the ductile-brittle transition in martensitic steels, a theory that has slowly developed through some forty years of international metallurgical research. The research will study a series of Fe-C, Fe-Ni and Fe-Mn steels with lath martensitic microstructures. Their grain, packet and block structures will be characterized, and modified using thermal treatments that are known to refine the effective block size. Appropriate strength and toughness tests will be done, and studied with high-resolution characterization techniques, including orientation imaging and profile fractography, to determine the influence of the block structure on the strength, toughness and cleavage fracture path. The results will provide new, probative information on the block structure of lath martensite and its specific influence on the strength and the ductile-brittle transition, suggesting new metallurgical approaches to design advanced alloys with superior properties.NON-TECHNICAL SUMMARY: The use of high-strength steel in low-temperature, arctic or deep-sea environments is severely restricted by the danger of catastrophic failure through brittle fracture. The relevant metallurgical problem is the ductile-brittle transition; steels that are tough and ductile at high temperature become almost glassy brittle when used below their ductile-brittle transition temperature. To avoid the problem the designers of low-temperature structures and devices have used relatively low-strength steels, which compromises engineering efficiency, or highly alloyed steels, which raise cost. The ductile-brittle transition temperature of steel is strongly influenced by its internal structure ("microstructure") - the way its atoms are arranged on the micro- and nanoscale. Through some fifty years of directed research in many laboratories around the world metallurgists have come to understand many features of the microstructure of high strength steels and how that microstructure can be engineered to control the ductile-brittle transition. The present research project is intended to fill out that theoretical understanding by applying the best modern tools for microstructure characterization to steels that have been broken under controlled conditions. The results will provide new, probative information on the fundamental mechanisms that govern the brittle transition, suggesting new metallurgical approaches to design advanced alloys with superior properties for low-temperature, arctic or deep-sea service. The PI will continue to teach a freshman seminar course on the history of weaponry, which he has used previously to interest students in the field of Metallurgy. The PI will also encourage the involvement of women and underrepresented groups in his research projects, as he has done in the past.