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

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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.