PSI - Issue 13

Jean-gabriel Sezgin et al. / Procedia Structural Integrity 13 (2018) 1615–1619 Jean-Gabriel Sezgin/ Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

In recent years, the use of hydrogen as an energy carrier has been in interest at a global scale. However, in order to develop a hydrogen-based society, further research concerning Hydrogen Embrittlement (HE), herein understood as the degradation of mechanical properties by the presence of hydrogen in the material (Brass and Chene 1998; Gangloff and Somerday 2012; Hirth 1980; Murakami et al. 2012), has to be conducted. Several embrittlement effects of hydrogen have already been identified and reported in the past for several materials. For instance, some mechanisms of HE in steels are available in the literature (Coudreuse 1992; Liu et al. 2016; Liu and Atrens 2013; Novak et al. 2010; Ramamurthy and Atrens 2013; Venezuela et al. 2016; Yamabe et al. 2012), although further investigations are still required to elucidate some underlying fundamental aspects. The present work has been performed to investigate the effects of internal hydrogen on tensile properties of Type 316L stainless steel. Some datasets concerning this alloy have already been published with external and internal hydrogen (Imade et al. 2008; Kanezaki et al. 2008). The literature suggests a degradation of mechanical as well as metallurgical and strength properties (Birnbaum and Sofronis 1994; Kameda and Mcmahon 1983; Liu, Irwanto, and Atrens 2013, 2014; Matsunaga et al. 2015; Matsuo, Matsuoka, and Murakami 2010; MATSUOKA et al. 2017; Petch and Stables 1952; Troiano 1960; Venezuela et al. 2015) resulting from hydrogen in solution. The alloy in interest has several applications (nuclear, automotive, etc…) and present s a relatively good hydrogen compatibility (Ordin 1997). For these reasons, some Slow Strain Rate Tensile (SSRT) tests have been performed in air at room temperature by using specimen exposed to hydrogen gas (Matsuo, Yamabe, and Matsuoka 2014; Yamabe et al. 2017). The reference ( e.g. non-exposed) specimen usually fails by the Micro-Void Coalescence (MVC) mechanism. In contrast, after exposure to high-pressure hydrogen gas (100 MPa at 270 ℃ for 200 h), a degradation of tensile ductility has been observed, leading to the Relative Reduction in Area (RRA) of 0.85. In addition, a cup-and cone failure has been observed with a reduced dimple size compared to the reference state. These facts have already been interpreted in the past by the hydrogen-enhanced localized slip deformation. Nevertheless, other mechanisms may contribute, especially to dimple morphology. Some alternative mechanisms are proposed in the literature and some of them are based on the internal pressure of hydrogen (Pressouyre 1979, 1982; Zapffe and Sims 1940). These mechanisms are usually in consideration in some Oil & Gas concerns and the resulting crack is usually called blisters or HIC; stating for Hydrogen Induced Cracks. For these applications, the alloy of interest is usually pipeline steel (or at least in BCC phase). This paper targets to comprehensively clarify the effect of internal pressure on the SSRT properties of H-charged Type-316L specimens by using jointly modelling and experiment. First of all, the results of the modelling will be briefly presented. These results include simulation of the pressure build-up and its impact on the tensile properties in the SSRT conditions. Then, these conclusions will be experimentally verified by performing a series of SSRT tests in controlled environments. 2. Predicted impact of internal pressure on the SSRT properties The internal pressure build-up and its impact have been assessed numerically by treating successively the diffusion desorption (finite difference method: FDM) and the fracture mechanics (finite element method: FEM) problems. To treat the diffusion problem, it has been chosen to implement the physical model presented in (Sezgin et al. 2017b, 2017a). Concerning the effect of internal pressure on void growth, a criterion of void sheet coalescence has been considered as detailed in (Pardoen and Hutchinson 2003; Tvergaard and Hutchinson 2002). The contribution of the internal pressure in voids to the failure of specimens during SSRT tests have been detailed in the past study and a comprehensive explanation of the adopted method as well as its practical implementation could be found in (Sezgin and Yamabe 2018). The considered diffusion properties of Type-316L are available in (Yamabe et al. 2017). The problem has been reduced to a 1D diffusion problem with a 500 µm simulation box. The thickness of the pre-existing void has been taken to be equal to 1 nm (conservative case). The initial content of hydrogen was 98.2 mass ppm (calculated by the Sie vert’s law). The simulation results of the pressure build-up are given in Figure 1-a). In the SSRT testing condition, the internal pressure in the voids was 1 MPa. Even if fast hydrogen diffusion related to dislocation pipe-diffusion caused a higher pressure, the maximal pressure was 133 MPa due to the balance of mass in the system.