Hydrogen Induced Stress Cracking of Inconel 718 under Cathodic Polarization - Effects of Hydrogen on the Initiation and Propagation of Cracks
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This work has examined the susceptibility to hydrogen induced stress cracking (HISC) of Inconel 718 (UNS N07718) by stepwise tensile loading tests with in situ cathodic polarization. Specifically, the effect of hydrogen on crack initiation and propagation in Inconel 718 has been investigated by use of scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). Materials with two different grain sizes were examined in order to explore the effect of grain size on HE in Inconel 718. An attempt was made to develop a novel procedure for measuring local strains in tensile samples tested with in situ cathodic polarization. This procedure was based on using digital image correlation software to analyze series of images obtained with an optical microscope during tensile testing. Inconel 718 material of both grain sizes were found to be susceptible to HISC, based on fracture surfaces exhibiting transgranular cleavage fractures and severe secondary cracking on the sample surface. No secondary cracking was observed for samples tested in air. Qualitative examinations of specimen surfaces in SEM suggested that crack initiation happens preferentially at grain boundaries (GBs) and twins. This was supported by EBSD results indicating higher dislocation density at GBs and twins. Crack propagation happened transgranularly, as evidenced by the fracture surfaces. Hydrogen induced localized plasticity (HELP) is assumed to be the dominating mechanism for HISC, as suggested by significantly deformed regions near the crack edge and surrounding the crack tip. Particle content did not appear to affect crack initiation. A method for using digital image correlation technology to map local strains from in situ tensile testing was developed, although more work is necessary to refine the method and setup.