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

技术总结：本研究涉及板条马氏体钢中微观结构对韧脆转变的影响。它具体地解决了马氏体“块”的中心作用，作为板条马氏体钢中的有效晶粒尺寸，以及块尺寸对强度和抗解理断裂性的明显非常不同的影响。这项研究在技术上具有重要意义，因为现在使用或建议用于低温、北极或低温服务的高强度钢是板条马氏体钢。这些钢的性能和成本的技术进步需要理解和利用控制低温强度和韧性的微观结构机制。这项研究在科学上具有重要意义，因为它有望完成控制马氏体钢韧脆转变的微观结构-性能关系的新兴力学理论，这一理论在国际冶金研究中经过大约40年的缓慢发展。本研究将研究一系列具有板条马氏体组织的Fe-C、Fe-Ni和Fe-Mn钢。它们的颗粒、包和块结构将被表征，并使用已知的细化有效块尺寸的热处理进行改性。将进行适当的强度和韧性试验，并采用高分辨率表征技术进行研究，包括取向成像和剖面断口分析，以确定块体结构对强度、韧性和解理断裂路径的影响，其结果将为板条马氏体的块体结构及其对强度和韧脆转变的具体影响提供新的证明性信息，提出了设计具有上级性能的先进合金的新的冶金方法。非技术概述：高强度钢在低温、北极或深海环境中的使用受到脆性断裂引起的灾难性失效的危险的严重限制。相关的冶金学问题是韧-脆转变;在高温下坚韧和延展性的钢在低于其韧-脆转变温度使用时几乎变成玻璃状脆性。为了避免这个问题，低温结构和设备的设计者使用相对低强度的钢，这会降低工程效率，或者使用高合金钢，这会增加成本。钢的韧脆转变温度受其内部结构（“微观结构”）的强烈影响-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近50年的定向研究，工程师们已经了解了高强度钢微观结构的许多特征，以及如何设计这种微观结构以控制韧脆转变。本研究项目旨在通过将最好的现代工具用于微观结构表征，以填补在受控条件下断裂的钢的理论认识。结果将提供新的，证明性的信息的基本机制，管理脆性转变，建议新的冶金方法来设计先进的合金与上级性能的低温，北极或深海服务。PI将继续教授一门关于武器历史的新生研讨会课程，他以前曾用这门课程来吸引学生对冶金领域的兴趣。PI还将鼓励妇女和代表性不足的群体参与其研究项目，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.