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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The fractured surface of the un-charged and hydrogen charged tensile specimens are shown in Fig. 4. Ductile fractured surface with dimples formation is observed in un-charged specimens, whereas flat brittle like fractured surface with very less dimple formation is observed in hydrogen charged specimens. The tensile test was conducted to study the extent of hydrogen charging of the subject LAS with the hydrogen charging parameters mentioned in Section 2. The main motive of this research paper is to study the effect of absorbed hydrogen on short fatigue crack propagation of the subject LAS.

dimples s

(a) (b) Fig. 4. Fractured surface of (a) un-charged and (b) hydrogen charged SA508 Gr. 3 Cl. I LAS after tensile testing

The comparison of short fatigue crack growth rate da/dN as a function of crack length a in hydrogen charged and un-charged SA508 Gr. 3 Cl. I LAS is shown in Fig. 5. The short fatigue crack growth rate in hydrogen charged condition is one order higher as compared to that of un-charged condition under the same fatigue loading conditions as mentioned in Section 2. This is attributed to the difference in the interaction of short fatigue crack with the microstructural features of the subject LAS under hydrogen charged and un-charged conditions. The SEM analysis of un-charged SENT specimens after short crack propagation revealed that the short fatigue cracks highly interact with PABGs, inclusions and M/A islands present in SA508 Gr. 3 Cl. I LAS. Also, trans-granular short fatigue crack propagation through the prior austenite grains took place in un-charged subject LAS. Whereas, the SEM analysis of hydrogen charged SENT specimens after short crack propagation revealed that both trans-granular and inter granular short crack propagation occurred in hydrogen charged subject LAS. Trans-granular crack propagation is observed in smaller grains whereas inter-granular crack propagation is observed along the PAGBs of larger size grains. PAGBs offered negligible resistance to short fatigue cracks in hydrogen charged samples. Due to this reason less fluctuation in short fatigue crack propagation rate are observed in hydrogen charged condition as compared to un-charged conditions as shown in the inset of Fig. 5. The occasional fluctuations in the growth rate of short cracks in hydrogen charged condition is due to the change in the short crack growth behavior from trans-granular to inter granular. Apart from PAGBs other microstructural features such as inclusions and M/A islands present in the subject material effect the short fatigue crack propagation rate. These M/A islands and inclusions are randomly distributed in the materials and are occasionally encountered by the short crack path. Multiple samples need to be tested to capture the difference in the role of all the microstructural features present in the material under hydrogen charged and un-charged conditions. The role of these M/A islands and inclusions on short crack fatigue crack propagation in the hydrogen charged subject LAS is still to be evaluated and is also under progress. In the present study, only the role of PAGBs and fine carbides distributed within the prior austenite grains was identified.