PSI - Issue 79

Jorge Luis González-Velázquez et al. / Procedia Structural Integrity 79 (2026) 526–533

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© 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 IGF28 - MedFract3 organizers

Keywords: Pipeline steels; hydrogen induced cracking; microprint technique; non-metallic inclusions; microstructure

1. Introduction In the late 1970’s the need to exploit oil and gas fields in Antarctic regions led the pipeline industry to introduce high strength grades of API 5L steels [1, 2]. By the late 1990’s it was evident that the metallurgy of pipeline steels could be classified as Old Generation (OG) steels, having a ferrite-pearlite microstructure and mid-strength grades (from X36 up to X60), and New Generation (NG) having martensite, bainite or dual martensite-bainite microstructures and high-strength grades, such as X70 and up to X100. Another important characteristic of NG steels is the use of nonmetallic inclusion control technologies to reduce susceptibility to hydrogen induced cracking (HIC) of low carbon steel pipeline transporting H2S containing hydrocarbons [3]. Hydrogen-induced cracking (HIC) in OG pipeline steels has been extensively studied for the last 40 years, and some basic aspects of its mechanisms and the role of microstructural parameters are well known. For instance, it has been widely demonstrated that type 2 elongated MnS non-metallic inclusions act as irreversible hydrogen traps, and therefore they have a key role in the nucleation of HIC, where the size, shape, and spacing between inclusions are among the main influencing factor while high pearlite banding degrees have a higher susceptibility to HIC, as compared to steels with randomly distributed pearlite [4]. Ohmisawa et al. [5] used HMT to observe the distribution of hydrogen traps in pearlitic steels and found that strain concentration regions and grain boundaries are the main sites for hydrogen concentration. New generation pipeline steels were introduced in the late 1990’s with the aim to improve the mechanical strength and other characteristics such as the ductile-to-brittle transition and weldability. This was accomplished by adding carbide-forming alloying elements like vanadium (V), titanium (Ti), and niobium (Nb) and applying thermomechanical process to develop high strength microstructures of fine grains and fine particles as the main strengthening mechanisms [7-8]. Martensite and bainite are the main second phases in NG pipeline steels. Several studies report that dual-phase (martensitic-bainitic) pipeline steels are more susceptible to HIC due to the inherent brittleness of martensite [9-11]. It is assumed that the hydrogen atoms dissolved in the lattice reduce the cohesive strength of ferrite-martensite interfaces, providing low energy paths for HIC [12,13]. Another parameter that has been investigated on its effect on the HIC behavior in pipeline steel is the crystallographic texture Masoumi et al. [6] observed that the HIC resistance in pipeline steel was increased by the dominant <011>//ND-oriented grains and a small number of <001>//ND-oriented grains while, the hot rolled specimen with sharp {001}//ND texture were highly susceptible to HIC. Based on the previous findings, the aim of this paper is to further understand the role of microstructure and strength grade on the HIC nucleation and growht of old and new generation pipeline steels, and with this knowledge improve the Fitness-For-Service assessments of existig pipelines that perform under HIC damage service conditions. Rectangular plates of API 5L pipeline steel of grades X46, X52 and X56 (OG) and X70-1, X70-2 and X80 (NG), were taken from pipeline segments withdrawn from service, ultrasonic examination was done to ensure the absence of internal cracks and other defects. The chemical composition and mechanical properties of the tested plates are shown in Tables 1.and 2. 2.2 Cathodic hydrogen charging, hydrogen permeation and potentiodynamic tests The cathodic charging (CC) experimental setup is schematically depicted in Fig. 1. Rectangles 18 cm X 12 cm and the original pipe thickness were mechanically cut from the pipes, away from the seam weld. Both plate surfaces were polished with 600-grit SiC sandpaper. The face of the plate corresponding to the internal side of the pipe was glued to an acrylic cell to receive an electrolyte solution of 0.4 Wt.% sulfuric acid in distilled water, that gave a pH of 1.2 to 2. Experimental procedure 2.1 Material