Nitrogen pick-up could occur if there was zero oxygen in the atmosphere during the anneal, such as during a bright anneal. This would then result in insufficient titanium to tie up the carbon, leaving a material which could be susceptible to sensitization.
mcguire ,pardon me if it is stupid. Will the carbon in 321 alloy not have ttied up the Titanium? Will there be residual free Ti available to combine with N during annealing? I ask this question as tomorrow I am pouring a few 321 grade castings
Arunmrao
Because the free energy of formation of TiN is much greater(in a negative sense) than that of TiC, TiC forms only after virtually all the N has been taken up as TiN. Incidentally, both oxygen and sulfur also combine with Ti more readily than does carbon. So, you have to make sure you have enough Ti to tie up all the oxygen, sulfur, nitrogen, if you want to prevent carbide formation.
In your case I would deoxidize with something stronger than Si/Mn and make sure to use a good de-sulfurizing slag.
If you keep exposure to the atmoshere at a minimum, you will keep nitrogen pick-up under control.
Consider using Ti scrap if you can get it. It's usually Ti/Al alloy and the aluminum protects the Ti from oxidation.
Yes, I have done it a number of times. Too much nitrogen and a good low dew point in bright anneal and you cna form a lot of CrN along the grain boundaries.
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Corrosion never sleeps, but it can be managed.
