PSI - Issue 14

Rajwinder Singh et al. / Procedia Structural Integrity 14 (2019) 930–936 Author name / Structural Integrity Procedia 00 (2018) 000–000

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HE Hydrogen Embrittlement SENT Single Edge Notch Tension EDM Electrical Discharge Machining SEM Scanning Electron Microscope ∆σ Stress range R Cyclic loading ratio f Cyclic loading frequency M/A Martensite/Austenite PAGB Prior Austenite Grain Boundary

2. Experimental procedure SA508 Gr. 3 Cl. I LAS was provided by Bhabha Atomic Research Centre, Mumbai, India in the form of pipe. The chemical composition of the material is given in Table 1. The samples for material characterization and testing were machined from the pipe using EDM and milling machine. Table 1: Chemical composition of SA508 Grade 3 Class I low alloy steel Element C S P Mn Si Cr Ni Mo Cu Al Ti Fe Weight % 0.19 0.002 0.018 1.3 0.23 0.17 0.7 0.44 0.13 0.02 -- 96.8 The samples for microstructural evaluation under SEM were polished using emery paper and then etched with 10% Nital solution. Dog bone specimens as shown in Fig. 1(a) were used for tensile testing. The tensile testing of hydrogen charged and un-charged dog bone specimens was carried out at room temperature and at strain rate of 3.33×10 -4 sec -1 . The electrochemical hydrogen charging of the specimens was carried out using 1N H 2 SO 4 electrolyte and constant current of 20 mA/cm 2 is applied for 4 hours. Thiourea is added to the electrolyte to prevent the recombination of the hydrogen atoms during the electrochemical process. The specimen is made as cathode and platinum wire is made as anode during this electrochemical hydrogen charging. The hydrogen charged and un charged SENT specimens with an initial notch of 85 µm to 90 µm as shown in Fig. 1(b) were used to investigate the short fatigue crack propagation in the subject LAS. The short fatigue crack growth experiments were carried out at room temperature using 25 KN load capacity servo hydraulic fatigue testing machine at applied ∆ =475 MPa, ( ⁄ ) =0.1 and =35 Hz. Moving digital microscope fitted near to the fatigue testing machine was used to track the initiation and propagation of short cracks from the notch of the SENT specimens subjected to cyclic loading. The SENT specimens were polished with emery paper before conducting short fatigue crack propagation experiments. The cyclic loading was interrupted at regular intervals to take pictures of this short crack using digital microscope. The SENT specimens after short crack propagation were etched with 10% Nital solution and then observed under SEM to investigate the interaction of short fatigue crack with the microstructural feature of the subject LAS. The details of experimental procedure for short fatigue crack propagation can be found elsewhere (Singh et al. (2018)).

notch

Fig. 1. (a) Dog bone samples for tensile testing and (b) SENT specimens for short fatigue crack growth testing. (all dimensions are in mm) (a) (b)