PSI - Issue 13

Vishal Singh et al. / Procedia Structural Integrity 13 (2018) 1427–1432 Author name / Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction Hydrogen has been identified as one of the cleanest energy resources capable of reducing our dependency on fossil fuels. Many energy applications are already being explored to replace fossil fuels with hydrogen-based energy solutions (Kanezaki et al. (2008)). Despite knowing the effectiveness of hydrogen energy, its use on larger scale (that requires hydrogen storage and transportation) is restricted due to the well-known phenomenon of hydrogen embrittlement (HE). HE is an insidious failure mode that causes loss of structural integrity of steels (Murakami and Matsuoka (2010)). Eextensive research on the effect of hydrogen on steel performance is available describing the hydrogen embrittlement behavior of steels based on slow strain rate test (SSRT), linearly increasing stress test (LIST), fracture toughness testing and long fatigue crack growth testing (Chatzidouros et al. (2014); Venezuela et al. (2015); Tsay et al. (2008)). For designing steel components capable of delivering satisfactory performance over the years under hydrogen environment, reliable fatigue data of steels under hydrogen environment is most desired. Total fatigue life of a material can be divided into fatigue crack initiation and long fatigue crack growth phase. Literature available on the effect of hydrogen on fatigue behavior of steels generally discusses their long crack propagation behavior that leads to the change in crack growth rate and the threshold stress intensity factor range ( ∆ �� ) (Tsay et al. (2008); Ogawa et al. 2018). However, major portion of fatigue life is consumed in crack initiation phase that includes fatigue crack nucleation and short fatigue crack propagation. The size of short fatigue cracks is of the order of grain size (few tens of microns), that are generally not detectable by conventional crack detection techniques applicable for the long fatigue crack growth behaviour (Krupp, (2007)). Short fatigue crack growth behaviour is characteristically different from long crack growth behaviour showing high propagation rate at much lower values than threshold stress intensity factor range as well as strong dependency on the microstructural features such as grain boundaries, phase boundaries, and inclusions. The short fatigue crack growth behaviour can provide fundamental understanding and correlation of the metallic microstructure with hydrogen embrittlement phenomenon. Effective microstructural engineering capable to arrest the propagation of these short fatigue cracks in steels under hydrogen environment needs detailed investigation regarding their interaction with various microstructural features such as phase/grain boundaries, grain orientation, inclusions etc. (Obrtlık et al. (1997); Mikulich et al. (2006); Mazánová et al. (2018)). Such investigations can help to improve the choice of steels and can open new trends toward the material engineering of steels with enhanced fatigue performance under hydrogen environment. Hence, a framework capable to explore the role of microstructural features on short fatigue crack propagation under hydrogen environment is desired. To this end, a novel experimental framework is developed to investigate the short fatigue crack behaviour of hydrogen charged steels involving in-situ observation of propagating short cracks coupled with image processing to obtain their da/dN vs curves. Microstructural dependent behaviour of short fatigue cracks elucidated in the present work will help to improve the material design for better performance under hydrogen environment. SA508 Grade 3 Class I low alloy steel applicable in nuclear reactor pressure vessel; two API line pipe steel grades X65 and X80, and a steel versatile in applications from fuel cell, nuclear power plant to line pipes for chemical, petrochemical i.e. austenitic stainless-steel grade 316L were studied in the present work. This paper consists of four sections. Following this brief introduction, Section 2 describes the experimental procedure. Results and discussions are provided in Section 3 followed by conclusions in Section 4. 2. Experimental procedure Chemical composition of investigated steels is presented in Table 1. To investigate the role of hydrogen on microstructural interaction of short fatigue crack growth, fatigue specimens of investigated steels were electrochemically charged by exposing to an electrolyte containing 1 N H 2 SO 4 solution + 1.4 g/L Thiourea (charging promoter) under current density of 20 mA/cm 2 for 4 hours. Specimen as cathode and platinum mesh as anode were used during hydrogen charging. In-situ investigations of short fatigue crack growth on single edge notch tension (SENT) specimen (with an initial notch size of ~70 µm for 316L and SA508 steel, and notch size ~50 µm for X65 and X80 steel) was conducted using digital microscope . Fatigue experimentations were conducted with R ratio 0.1 at frequency 35 Hz.