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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3.2. Effect of hydrogen on short fatigue crack growth in SA508 Gr. 3 Cl. 1 low alloy steel Fig. 4 presents the comparison of short fatigue behavior of hydrogen charged and uncharged SA508 Gr. 3 Cl. 1 low alloy steel. Short fatigue crack growth investigations were conducted at stress range ( ∆σ ) of 475 MPa. From Fig. 4a and b, it was concluded that the crack growth in SA508 hydrogen charged specimen was significantly higher than that of uncharged specimen.

Fig. 4. Variation of (a) short fatigue crack length ‘ with numbers of cycles ‘ and (b) short crack growth rate ′ with crack length ‘ of SA508 sample at ∆ = 475 MPa SEM analysis of uncharged SA508 specimen (not presented here) revealed that the prior austenite grain boundaries (PAGBs) offer the highest resistance to the short fatigue crack propagation. Uniformly distributed carbides were found to resist the crack growth more significantly after PAGBs. In hydrogen charged specimen, role of PAGBs toward the resistance to crack growth was negligible that can be noticed by comparatively lesser decelerations under hydrogen charging condition in Fig. 4b. Transgranular crack propagation was dominating in uncharged specimen. However, under hydrogen environment, crack was found to propagate both intergranular and transgranular. Intergranular crack propagation occurred along the PAGBs of large size prior austenite grains whereas transgranular crack propagation occurred through small size prior austenite grains. 3.3. Effect of hydrogen on short fatigue crack growth in X65 and X80 steels Fig. 5 presents the effect of hydrogen on short fatigue crack growth on API line pipe grade X65 and X80 steels. Fatigue experiments for both the steels were conducted at stress range ( ∆ ) of 400 MPa. In uncharged condition, early crack initiation and comparatively slower propagation in X65 than X80 steel was observed (see Fig. 5a). Under hydrogen environment both the steels resulted in similar crack propagation rate, however like uncharged conditions early crack initiation in X65 than X80 steel was observed. The decelerations in uncharged specimen of X65 and X80 (see Fig. 5b) were in corresponding to the microstructural features such as grain boundary, phase boundary and crack branching. These decelerations were almost negligible under hydrogen charged specimens, hence confirmed the lesser resistance to crack propagation in hydrogen environment. Crack was found to propagate both inter and transgranular under charged and uncharged condition in both the steels. Decohesion at the interface of M/A stringers aligned with the direction of crack propagation was found to facilitate crack propagation under hydrogen charged and uncharged condition in both the steels. In general, the hydrogen charged specimen of both the steels showed high crack growth rate and lesser hindrance to the short fatigue crack propagation. Due to refined microstructure (in terms of grain size) not all grain/phase boundaries were capable to offer resistance to the propagation even in uncharged conditions.