I say it is brittle because it has failed twice and both times quite spectacularly. Not only did the inconel fail in the area adjacent to the welds but spider cracks formed flowing out of the main cracks and into the surrounding area. It looked like glass might look if you punch a hole in it (i.e. not tempered glass!).

 

That is interesting.  Sounds like ‘stress corrosion’ cracking; which is the reason to use inconel vs other SS alloys at these temps.  Almost makes me wonder whether the welder picked up the wrong material when he made the parts; or at least didn’t give it any annealing heat treatment after welding.

 

http://www.corrosion-doctors.org/Forms/scc.htm

 

Al