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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3.1. Effect of hydrogen on short fatigue crack growth in 316L steel Austenitic stainless steels are widely applicable in line pipes for hydrocarbons, chemicals and nuclear power industries. The hydrogen-based degradation of these steels is well accepted (Kanezaki et al. (2008)). Fig. 2a presents the variation of crack length ( ) with number of cycles ( ) for uncharged and hydrogen charged 316L steel obtained after fatigue loading at constant stress range ( ∆ ) of 270 MPa. Early initiation followed by accelerated crack growth in hydrogen charged specimen with increase in number of cycles can be observed in Fig. 2a. Shorter plateau regions (in vs curve, Fig. 2a) in hydrogen charged specimen than in uncharged 316L, confirmed lesser hindrances to short fatigue crack propagation from the microstructural features under hydrogen environment. Fig. 2b presents a comparison of short fatigue crack growth rate ( ) as a function of crack length ( ) for hydrogen charged and uncharged 316L steel. The deceleration in the curves correspond to the grain boundaries resisting the short cracks to propagate. Similar deceleration was provided by the grain boundaries for both hydrogen charged and uncharged specimens. However, the crack growth rate within the grain appears to be much faster in hydrogen charged specimen compared to uncharged specimen. This may be due to crack tip localized enhanced slip activities that resulted in an increased crack growth under hydrogen environment.

Fig. 2. Variation of (a) short fatigue crack length ‘ with numbers of cycles ‘ and (b) short crack growth rate ′ with crack length ‘ of AISI 316L steel at ∆ = 270 MPa For the uncharged specimen, slip bands were seen in multiple grains around the crack tip as well as away from the crack tip, as shown in Fig. 3a. On the other hand, for the hydrogen charged specimen, slip bands appeared in the vicinity of the crack tip with negligible slip bands formation away from crack tip. Some microcracks were also observed away from the crack tip for this specimen, as shown in Fig. 3b. For an uncharged specimen, the activation of slip systems led to the formation of slip bands in most of the grains. However, for the case of hydrogen charged specimen, hydrogen led to the pinning of dislocations hindering the slip activity. For this case, the stresses got relieved by microcracks generation away from crack tip. Due to localized slip activity, the crack propagation for this case appears to be more planar compared to the uncharged specimen.

Fig. 3. Optical micrographs of crack path ahead of notch in (a) uncharged and (b) hydrogen charged 316L specimens