Alloy 718 Subsea Bolt in Relation to Surface Cracking: A Microstructural Perspective

The brittle fracture mode close to the surface of Inconel 718 subsea bolts is closely related to their nanostructure. When the bolts are used subsea under cathodic protection, hydrogen evolves. Intergranular precipitates and points of intersection of slip lines always held responsible for hydrogen enhanced decohesion. However, thus far, little attention has been paid to the bolts manufacturing method and to the characterization of the role of subsurface oversized nanoprecipitates on transgranular surface cracking. As-received subsea bolts were analyzed using multi-scale observation techniques such as focused ion beam milling, scanning electron microscopy, transmission electron microscopy, and in-air strain rate tensile tests. The results further demonstrated that under identical API aging, there was a difference in the morphological precipitation and strength of 718 as a bolt and rectangular billet. For the 718 rectangular billets, discrete intergranular stable δ/MC carbides precipitate and only γ' of 10 - 20 nm was observed for the bulk and subsurface. Whereas, for the CRA subsea bolt, γ' was approximately 30 nm, γ" was 30 - 50 nm for the bulk (370 HV);γ' was approximately 50 nm and γ" was 50 - 100 nm at subsurface (400 HV). As-received subsea bolts were investigated at subsurface (10 μm from the surface) at the thread and shank region, which revealed transgranular sheared oversized ~50 nm γ' (with dislocation networks) and metastable γ" > 100 nm particles, respectively. Thus, an effort was made to develop the hydrogen-assisted surface cracking theory for bolts.