Issue 52

A. Laureys et alii, Frattura ed Integrità Strutturale, 52 (2020) 113-127; DOI: 10.3221/IGF-ESIS.52.10 113 Focussed on the 1st Benelux Network Meeting and Workshop on Damage and Fracture Mechanics Initiation of hydrogen induced cracks at secondary phase particles A. Laureys, M. Pinson, L. Claeys, T. De Seranno, T. Depover, K. Verbeken Ghent University, Department of Materials, Textiles and Chemical Engineering, Technologiepark 46, 9052 Zwijnaarde, Belgium aurelie.laureys@ugent.be, margot.pinson@ugent.be , lisa.claeys@ugent.be, tim.deseranno@ugent.be tom.depover@ugent.be (https://orcid.org/0000-0002-8856-1122 ) kim.verbeken@ugent.be ( https://orcid.org/0000-0002-5190-016X ) A BSTRACT . The goal of this work is to propose a general mechanism for hydrogen induced crack initiation in steels based on a microstructural study of multiple steel grades. Four types of steels with strongly varying microstructures are studied for this purpose, i.e. ultra low carbon (ULC) steel, TRIP (transformation induced plasticity) steel, Fe-C-Ti generic alloy, and pressure vessel steel. A strong dependency of the initiation of hydrogen induced cracks on the microstructural features in the materials is observed. By use of SEM-EBSD characterization, initiation is found to always occur at the hard secondary phase particles in the materials. K EYWORDS . Blisters; Hydrogen induced cracking; particles; Secondary phase; Scanning electron microscopy. Citation: Laureys, L., Pinson, M., Claeys, L., De Seranno, T., Depover, T., Verbeken, K., Initiation of hydrogen induced cracks at secondary phase particles, Frattura ed Integrità Strutturale, 52 (2020) 113-127. Received: 18.12.2019 Accepted: 26.01.2020 Published: 01.04.2020 Copyright: © 2020 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION xperimental electrochemical charging of steels with hydrogen mimics, on the one hand, well hydrogen generation and entry in the material from sources such as corrosion or applications such as arc welding [1]. Small amounts of hydrogen in a metal in combination with external or residual stresses can lead to hydrogen embrittlement (HE) or hydrogen assisted cracking (HAC), which results in subcritical failure of the materials [2, 3, 4]. On the other hand, intensive electrochemical charging can induce both surface and internal damage in a material [5]. The hydrogen charging procedure can consequently be applied to simulate situations where hydrogen induced cracking (HIC) takes place. Hydrogen induced internal cracking and blistering can occur in metals subjected to high fugacity hydrogen environments, such as high pressure hydrogen gas environments or under extreme cathodic charging conditions, even without the application of an external load or residual stress, though such stresses can contribute to HIC [5, 6]. HIC occurs, for instance, in oil and gas sour service pipelines where hydrogen ingress into steel from H 2 S occurs [7]. Also hydrogen flakes as found in reactor pressure vessels [8] are an example of HIC and could be artificially reproduced by intensive electrochemical hydrogen charging [9]. HIC occurs when the hydrogen concentration in the steel matrix exceeds a threshold hydrogen concentration. The threshold hydrogen concentration might be considered as a parameter unique to a given material and strongly depends on the type, shape and amount of inclusions and segregation in the matrix [10, 11, 12, 13] . The internal pressure theory [14, 15, 16] explains the phenomenon of HIC in high fugacity hydrogen environments. The theory states that HIC results from the formation of high pressure hydrogen gas bubbles in internal voids and microcracks. When an alloy is exposed to a hydrogen E