PSI - Issue 68

Available online at www.sciencedirect.com Structural Integrity Procedia 00 (2025) 000–000 Available online at www.sciencedirect.com ScienceDirect

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Procedia Structural Integrity 68 (2025) 1115–1120

European Conference on Fracture 2024 The effect of low-temperature tempering on hydrogen diffusion and trapping in direct-quenched martensitic steel Renata Latypova a, *, Lisa Claeys b , Kim Verbeken b , Tom Depover b , Jukka Kömi a , Sakari Pallaspuro a a University of Oulu, Materials and Mechanical Engineering, Centre for Advanced Steels Research (CASR), Oulu, 90014, Finland b Ghent University, Department of Materials, Textiles and Chemical Engineering, research group Sustainable Materials Science, Ghent, 9052, Belgium Abstract Low-temperature tempering (LTT) is a common heat treatment for martensitic ultrahigh-strength steels, aimed at improving mechanical properties and enhancing resistance against hydrogen embrittlement (HE). This study investigates the effects of LTT on hydrogen (H) diffusion and trapping in a 500 HBW direct-quenched (DQ) steel with lath-martensitic microstructure. Four materials are tested: initial DQ steel auto-tempered during quenching, and the same tempered with a 1-hour holding time at 50 °C, 150 °C, and 250 °C. The given tempering temperatures are used to quantify the effects of carbon segregation (50 – 150 °C) and transformation of retained austenite to cementite (250 °C). Electrochemical hydrogen permeation and melt extraction tests were carried out to explore the impact of LTT on H diffusion and total H concentration. In addition, HE susceptibility was investigated using a novel tuning-fork test (TFT) with an incremental step loading technique. As tempering temperature increases, H concentration decreases while H diffusion accelerates. The changes in H concentration and diffusivity are correlated with the microstructure, focusing on the effects of Cottrell atmospheres and dislocation mobility, which can contribute to improved HE resistance after 250 °C tempering. © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ECF24 organizers © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ECF24 organizers

* Corresponding author. Tel.: +358503052542 E-mail address: renata.latypova@oulu.fi

2452-3216 © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ECF24 organizers

2452-3216 © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ECF24 organizers 10.1016/j.prostr.2025.06.177