Here are photos of a 4340 steel part before and after heat treating.
The part was attacked and eaten away during the 1600 F autenitization cycle.
I coated the part with Brownell's PBC scale prevention compound (AKA KeepBrite) before
heat treating.  I think what happened was that the part was contaminated
with sulfurized cutting oil and acetone when I heated and dunked it in the
PBC powder.  Something nasty came out of this brew in the furnace.  The
lesson, I think, is to clean parts with detergent, rinse and dry them well before
dunking them in PBC anbd putting them in the furnace.