Microstructure of CRA bolts used subsea in relation to resistance to hydrogen embrittlement

The physical characteristics of hydrogen embrittlement failure of subsea bolts operating under CP are surface cracks at thread and shank area and for some cases they are completely ruptured from the shank area. The metallurgical reason for failure already reported by researchers (including for Inconel718) was the interaction of hydrogen atoms with grain boundary precipitates (Ni3Nb δ) resulting in hydrogen assisted intergranular cracking. Inconel 718 was received as rectangular block (RB) and bolts (Φ28mm). They were heat treated as per API 6A718 specifications. The yield strength and vickers hardness found for API aged RB specimen S2API was 840MPa and 340HV respectively. On the other hand API bolt was found with bulk yield strength of 880MPa and vickers hardness in the range 370-400 HV (bulk to edge of the bolt). The structure of bolt (bulk, shank and thread) and rectangular block was investigated, using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), to understand the reason for this difference. The experimental results were compared with theoretical predictions. The microstructure of the API bolt shows no evidence of intergranular (Ni3Nb δ) precipitates and the nano-structure revealed γ’ and Ni3Nb γ’’ precipitates. This suggest that for Inconel718 in the form of bolts the theory of intergranular cracking hydrogen enhanced de-cohesion at the grain boundary matrix precipitate interfaces is not specifically relevant instead the transgranular oversized γ’ precipitates (50nm γ’ at thread edge of the bolt) and the elongated metastable γ’’ discs (50-100nm at shank and thread edge of the bolt) could act as initiation point for HEDE. Moreover, it was also found that threading of the bolt (after heat treatment) results in shearing of γ’ precipitates at threads edge. These sheared γ’ precipitates could aid in trapping hydrogen atoms under CP in service leading to HEDE. The newly developed bolting materials Incoloy 945 and its higher strength version 945x (as rectangular and cylindrical blocks) were added for structural investigation because of lower content of Nb present in their matrix than 718. Finally, as future work, the structure of 718, 945 and 945x alloys were selected for assessing susceptibility to hydrogen embrittlement using slow strain rate testing under CP.