PSI - Issue 35

Kpemou Apou Martial et al. / Procedia Structural Integrity 35 (2022) 254–260 Kpemou A. M. et al. / Structural Integrity Procedia 00 (2019) 000 – 000

255

2

1. Introduction In the context of climate change and the increase in global energy consumption, linked to population growth, there is a constant search to limit greenhouse gas emissions and thus preserve our planet. The use of hydrogen is a viable solution in this context. Hydrogen, the simplest chemical element, and a very light gas is very abundant on Earth. It is a component of water and living matter and can be produced by the electrolysis of water. This abundance could be a major asset in the race for sustainable energy. Hydrogen combustion is carbon-free and is very efficient for electricity storage. However, the development of hydrogen technologies requires the overcoming of several obstacles. One of these is the transportation and distribution of hydrogen. The most common means of distributing hydrogen is by pipelines, which are used over very long distances. For transport, the introduction of hydrogen into the existing pipeline network has been proposed as blended with natural gas [1]. This method requires separation from end-users and leads to other challenges to be overcome. Pipeline steels used for transport are subject to hydrogen embrittlement and see their mechanical properties decrease. This phenomenon can lead, under conditions related to the operating temperature, these pipeline steels to operate in the brittle domain, where a sudden rupture is inevitable. Indeed, Johnson [2] has discovered at the end of the 19th century that hydrogen alters the mechanical properties of steels by hydrogen embrittlement (HE). Failure elongation is greatly reduced but yield stress and ultimate strength are less affected. The fracture resistance is also reduced. Many mechanisms can explain HE phenomenon, such as atomic bonds of metals weakening [3], enhancement of plasticity, emission of dislocations / decohesion competition [4] and stress triaxiality. One of the mechanical proprieties, which can be affected by HE, is the DBTT or the Ductile to Brittle Transition Temperature. This temperature is conventionally defined and compares to the operating temperature as an acceptability criterion for codes. Pipe design is made at different level of safety in several codes like API 579-1 ASME FFS-1 [5] for pipes in general and ASME B31.12 [6] for hydrogen piping and pipelines. Design at level 1 is based on the following concept: the material has sufficient ductility to prevent cleavage triggering. This condition is fulfilled if the operating temperature Ts or the Minimum Allowable Temperature (MAT) is higher than the Ductile to Brittle Transition Temperature (DBTT) Tt. The operating temperature or the MAT are defined by codes. In France, for example, Ts = -20°C or -29°C for carbon steel see according to [6]. ASME B31.12 [6] also gives the evolution of the MAT value with the thickness of the pipe. (Eq. 1) ( ) ( ) 0.5 2 20 for 0 0.394 135.9 284.85 171.26 for 0.394 6.0 1.0 1.7971 0.17887 MAT t inch t t MAT t inch t t   = −       − + + =     + −   (1) This paper aims to quantify the effect of hydrogen embrittlement on the transition temperature of API 5L X65 pipe steel, using two types of Charpy specimens: the Standard Charpy and the ½ Mini – Charpy. Hydrogen charging of the specimens was performed by electrolysis in NS4 solution. The data obtained by tests were fitted with the Oldfield equation and the transition temperature was obtained. A comparison was made between the value of the transition temperature obtained for tests on specimens with and without hydrogen. The DBTT shift, induced by hydrogen embrittlement is explained by comparison of fracture process volume and critical hydrogen volume. 2. Material The pipe steel that is the subject of our investigation is API 5L X65, with yield stress equal to 465.5 MPa. Its typical chemical composition is given in Table 1 as indicated by Coseru et al [7]. Table 2 shows mechanical properties at room temperature of the pipe steel, and Fig. 1, its microstructure, characteristic of ferrito-pearlitic steel. Specimens are extracted from a pipe with an external diameter of 355 mm and thickness of 19 mm.