PSI - Issue 68

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

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Procedia Structural Integrity 68 (2025) 708–714

European Conference on Fracture 2024 Investigation of microstructural evolution in nano-structured bainite during Paris law regime of fatigue crack growth Bhawesh Chhajed 1 , Sudharm Rathore 1 , Avanish Kumar 2 , Aparna Singh 1* 1 Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Mumbai 400076, India 2 Department of Fuel, Minerals and Metallurgical Engineering, Indian Institute of Technology (ISM) Dhanbad, Dhanbad 826004, India Abstract Two different blocks of nano-bainitic specimens, NSB_250 and NSB_350 were prepared by austenitization at 950°C for 40 min, followed by austempering at 250°C and 350°C respectively. Lower austempering temperature led to thinner bainitic laths, smaller fraction of retained austenite (RA) and higher fatigue crack growth rates. This study investigated the microstructural evolution in both specimens close to the cracked surface when the crack growth was in the Paris law regime. Samples were extracted using focussed ion beam (FIB) milling and the microstructure was examined using transmission electron microscopy (TEM) and compared with that of undeformed specimens. Formation of deformation twins in RA were observed in NSB_350 while severe deformation of bainitic ferrite and micro-twin formation in RA was observed for NSB_250. © 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 Keywords: Nano-structured bainite; Fatigue; Transmission Electron Microscopy; Paris law regime 1. Introduction Nano-structured bainitic (NSB) steel demonstrates high mechanical strength, ductility, toughness, wear resistance, torsional strength and high cycle fatigue properties, Caballero and Bhadeshia (2004); Chhajed et al. (2022); Kumar et al. (2022); Kumar and Singh (2021); Kumar and Singh (2020)a; Singh and Singh (2018). The remarkable strength is achieved due to numerous interfaces between bainitic ferrite (BF) and RA as both the phases are present in a nanometric length scale. RA exhibits two different morphologies, namely, film type (FA) and blocky type (BA). Several studies such as Garcia-Mateo and Caballero (2005); Liu et al. (2019); Shen et al. (2015); Zhou et al. (2013) have shown that BA leads to drastic reduction in the toughness of NSB steel owing to the tendency to European Conference on Fracture 2024 Investigation of microstructural evolution in nano-structured bainite during Paris law regime of fatigue crack growth Bhawesh Chhajed 1 , Sudharm Rathore 1 , Avanish Kumar 2 , Aparna Singh 1* 1 Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Mumbai 400076, India 2 Department of Fuel, Minerals and Metallurgical Engineering, Indian Institute of Technology (ISM) Dhanbad, Dhanbad 826004, India Abstract Two different blocks of nano-bainitic specimens, NSB_250 and NSB_350 were prepared by austenitization at 950°C for 40 min, followed by austempering at 250°C and 350°C respectively. Lower austempering temperature led to thinner bainitic laths, smaller fraction of retained austenite (RA) and higher fatigue crack growth rates. This study investigated the microstructural evolution in both specimens close to the cracked surface when the crack growth was in the Paris law regime. Samples were extracted using focussed ion beam (FIB) milling and the microstructure was examined using transmission electron microscopy (TEM) and compared with that of undeformed specimens. Formation of deformation twins in RA were observed in NSB_350 while severe deformation of bainitic ferrite and micro-twin formation in RA was observed for NSB_250. © 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 Keywords: Nano-structured bainite; Fatigue; Transmission Electron Microscopy; Paris law regime 1. Introduction Nano-structured bainitic (NSB) steel demonstrates high mechanical strength, ductility, toughness, wear resistance, torsional strength and high cycle fatigue properties, Caballero and Bhadeshia (2004); Chhajed et al. (2022); Kumar et al. (2022); Kumar and Singh (2021); Kumar and Singh (2020)a; Singh and Singh (2018). The remarkable strength is achieved due to numerous interfaces between bainitic ferrite (BF) and RA as both the phases are present in a nanometric length scale. RA exhibits two different morphologies, namely, film type (FA) and blocky type (BA). Several studies such as Garcia-Mateo and Caballero (2005); Liu et al. (2019); Shen et al. (2015); Zhou et al. (2013) have shown that BA leads to drastic reduction in the toughness of NSB steel owing to the tendency to © 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.: +91-22-2576-7605 E-mail address: aparna_s@iitb.ac.in * Corresponding author. Tel.: +91-22-2576-7605 E-mail address: aparna_s@iitb.ac.in

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

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.2025.06.119