What is the thickness of the parts in question? Any details on the quench process (pressure, gas composition, time to room T, etc., ?). Was delta ferrite observed in the specimens with low TS? My first impression is that Type 403 with air quenching will not result in full martensitic transformation, especially in thick sections. I don't think delta ferrite should be a problem unless the original bar/billet/etc. had delta ferrite.