Ah the dreaded AL-12, the problem with changing the (read expensive) IGBT's is if it was caused by the firing board, then it will blow immediately on power up.
Or if the fault also caused the firing board to fail also, the result will be the same.
Been there, done that.
The silicon heat sink compound should not be confused with silicon dielectric grease.
Different composition.
I recommend sending them in to someone that can check the firing board for correct operation, otherwise it can be an expensive non-productive experience.
Minder.