Hydrogen Desorption Behavior Trapped in Various Microstructures of High-Strength Steels Using Thermal Desorption Analysi
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AND for high-strength steel with tensile strength over 1200 MPa has been increasing in recent years. With an increase in tensile strength of steels, there has been a critical challenge concerning delayed fracture caused by hydrogen uptake from a corrosive environment.[1] Susceptibility to delayed fracture for highstrength steel closely correlates with hydrogen trapping states in the metal. The states are classified into two types: diffusible hydrogen detrimental to the susceptibility to delayed fracture and non-diffusible hydrogen innocuous to it.[2–5] Trapping states change according to different microstructures.[4–12] Thermal desorption analysis (TDA) is an effective method for analyzing hydrogen trapping states in steel; TDA enables us to identify trapping states based on the
KEI SAITO is with the Graduate School of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan and now with the Nissan Motor Co., Ltd., Kanagawa 243-0192, Japan. KENICHI TAKAI is with the Department of Engineering and Applied Sciences, Faculty of Science and Technology, Sophia University, Tokyo 102-8554, Japan. Contact e-mail: [email protected] Manuscript submitted on July 22, 2020; Accepted November 1, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
peak temperatures of hydrogen desorbed from steels upon heating. The hydrogen desorption temperature found by TDA strongly depends on both the hydrogen diffusion coefficient and binding energies of hydrogen trapping sites. Thus, the temperature differs depending on steel microstructures. Hydrogen present in martensitic steel is in a state where it is released in a range from room temperature to 473 K is denoted as Peak 1H.[13] Peak 1H is diffusible and is released at room temperature. It was observed that cold-drawn pearlitic steel displays not only Peak 1H but also another peak denoted as Peak 2H, representing hydrogen desorbed at temperatures over 473 K.[4,5,14] It has been reported that Peak 2H for cold-drawn pearlitic steel is non-diffusible hydrogen that is not desorbed at room temperature.[4,5] One of the authors previously reported that hydrogen in a face-centered cubic (fcc) crystal structure,[15] e.g., SUS316L austenitic stainless steel and the nickel-based alloy Inconel 625, is a kind of diffusible hydrogen. The reason is that hydrogen was omitted when specimens were exposed at room temperature for a long time, even though hydrogen desorbed in the same temperature range as Peak 2H for cold-drawn pearlitic steel. It has been reported that the rates of hydrogen diffusivity for steels containing retained austenite with the fcc crystal structure affected their hydrogen desorption profiles irrespective of the hydrogen trapping strength because
the diffusion rate was lower than that of a single phase steel with the bcc crystal structure.[16–18] In view of these previous reports, when analyzing hydrogen desorption curves obtained by TDA, it is difficult to identify the state of hydrogen present (i.e., diffusible or non-diffusible) in different microstructures as well as
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