q: What bolting material should be used with Inconel 625 flanges? We used ASTM A193 Gr B7 however noticing some corrosion issues.
A: This is a very common problem but to answer it accurately, it depends on the environment that your flanges see and also to a certain extent what media is inside the flange.
If we focus on corrosion only, the reason why the media contained by the flange is of concern is there are normally small fugitive emissions. Depending on the corrosiveness of the medial it could certainly attack the B7 bolts given the fact that you are probably are trying to contain a fairly corrosive media by using Inconel 625. Additionally you will accelerate corrosion via galvanic effects if the bolting get wet.
Beyond corrosion, you need to consider the strength of the bolts as well. B7 has a min yield of 105ksi while Inconel 625 has a typical yield of 85ksi. So while Inconel 625 will certainly work as bolting from a corrosion point of view, you need to see if it has enough strength for you pressure rating. If you need higher strength you might want to consider Inconel 718 as it has both excellent strength and corrosion resistant properties.