We machine some parts from 4140A. For wear resistance, they need a harder face. Obviously there are probably better materials and processes for face hardening but these parts are less than 1" cubed so hard to not full harden then anyway. We originally made these from 4140HT which is HRC 28-32. That is really not hard enough but we were trying to avoid the process. We found that we needed the Annealed condition for easier machining and further have been hardening them with a torch and oil. each is carefully heated to just when they glow a bright orange. Th process has been getting them to about HRC 50 which is about spot on.
It is my understanding that 4140 can acheive HRC55 but we really don't need it. Our target is HRC48-53. On our first batch, the parts were tempered back at 400F. On all other runs, we have not tempered because of some data we found that indicated that there could be structural concerns if you don't temper back far enough and better to just leave un tempered. We have destructively tested the parts and are working great.
However, we want to refine the process and get better. We have a kiln and I am looking to convert it with digital control for now to try it out. However, the largest problem we have that is hurting us on these is the carbon scale that forms on the parts. It does not come off in the tumber and requires that we bead blast each part which is a huge pain. We really need a clean part from HT. IIRC, some propane or inert gas injection in an oven to remove the oxygen. Propane makes me nervous though.