Effect of chemical composition and heat treatment on hydrogen embrittlement susceptibilities of 4340M Martensitic and Bainitic steels

高强度材料常因其在有限空间内具有较高的比强度而被使用。其中，低合金钢表现出高强度，易于生产和可制造性的组合，使其成为航空航天和制造业中许多结构部件（例如起落架，紧固件和执行器）的经济和有价值的选择。尽管低合金钢具有各种有益的性能，但一个常见的问题是氢脆（HE），即在电镀过程中经常引入氢的情况下，材料的机械性能会恶化，随后会出现过早和延迟的脆性断裂。在对抗HE时，有必要对氢气捕获和HE敏感性有准确的材料特定知识，以便采取适当的措施（例如，烘烤以去除氢气和确定在役负荷水平）。AISI 4340钢一直是航空航天应用中常用的起落架材料。近年来，AISI 300M （4340M）低合金钢和贝氏体钢因其提高的强度、优异的断裂韧性和冲击强度而被开发作为替代材料。它们的机械性能可以通过额外的热/机械处理进一步调整。然而，这些材料的氢捕获特性和HE敏感性仍然未知。本项目旨在通过详细研究确定导致氢捕获的关键显微组织指标，并建立4340M和贝氏体钢的阈值曲线，来解决这一知识缺口。此外，将开发一个微观结构的数值模型，以便对低合金钢的氢捕获和HE敏感性进行通用数值分析，并进行原子模拟，以获得合金碳化物在低合金钢HE中的作用的基本理解。该项目的成果将为工业合作伙伴提供更好的、定量的微观结构对低合金钢氢捕获和HE敏感性的影响，并随后帮助他们在预防HE的实践中做出明智和可靠的决策。

High strength materials are often used for their high specific strength in limited space. Among those, low alloy steels exhibit a combination of high strength, ease of production and manufacturability, making them an economical and worthwhile choice for many structural components (e.g., landing gear, fasteners and actuators) in aerospace and manufacturing industries. Despite the various beneficial properties offered, one common problem low alloy steel suffers is hydrogen embrittlement (HE), i.e., the material experiences deterioration in mechanical properties and subsequent premature and delayed brittle fracture, in the presence of hydrogen that is often introduced during the plating process. In combating HE, it is necessary to have accurate material-specific knowledge of hydrogen trapping and HE susceptibility so that appropriate measures (e.g., baking to remove hydrogen and determination of in-service loading levels) can be taken. AISI 4340 steel has been the prevailing landing gear material used in aerospace applications. Recently, AISI 300M (4340M) low alloy steel and Bainitic steel were developed as alternative materials due to the improved strength, superior fracture toughness and impact strength. Their mechanical properties can be further tuned by additional thermal/mechanical treatments. However, hydrogen trapping characteristics and HE susceptibility of those materials remain unknown. This project aims to address such knowledge deficit by conducting a detailed study to identify key microstructural metrics responsible for hydrogen trapping and establish threshold curves for 4340M and Bainitic steel. Further, a microstructure-informed numerical model will be developed to enable generic numerical analysis of hydrogen trapping and HE susceptibility for low alloy steel, and atomistic simulations will be performed to gain fundamental understanding of the role of alloy carbides in HE of low alloy steel. Outcomes from this project will provide the industrial partners better and quantitative knowledge of the effect of microstructure on hydrogen trapping and HE susceptibility of low alloy steel, and subsequently help them take informed and reliable decisions in practices towards HE prevention.