Hi All. I am plugging away at rebuilding an old Remington Hepburn. I have a new breech block for it and am wondering if anyone here can tell me what harness it should be? It is machined from 8620 steel.

I believe that steel is considered a case hardening steel, so there may not be a significant range of hardness options through the body of the part. If it is case color hardened and the appearance of colors remain, the hardened skin may not have been tempered back too far. It might also be possible that gun parts could be case colored, but not been at a high enough temperature to case harden.

8620 will work safely as is, but casehardening will make is work smoother and more wear resistance.
If made from hardenable alloy steel (4140, etc), heat treat to about Rc 40.
Chuck

thanks chuckster.  If case hardening would you temper back to 40 hrc or just leave it as hardened?

SP,  Would leave it as casehardened because the softer core prevents brittleness.
Difficult to measure hardness on casehardened steel because of the softer core.
Nitre bluing after casehardening would make the case slightly softer after bluing but should work just fine.
Chuck

8620 is about the best case hardening steel  produced. It has about the same elements as Chrome-moly steel  just less of each. The main use is automotive gears. It can produce a glass hard exterior for wear resistance with a tough core. Yes, it should be case hardened. Tempering into the mid 40's Rc should work OK but I would like mine a little harder. The tough core is your friend.

I have installed two replacement breach blocks in Hepburns and had both color cased. They don't need to be hard all the way through but should have a case hard. The action will work much smoother if the parts are case hardened.

John Taylor wrote on Feb 15^th, 2018 at 11:51pm:

thank you John>
      I looked up some specs on 8620 and found this. Would like opinions from the experianced smiths here.

Cool From Carburizing Temperature in Carburizing Box, Reheat to Above Upper Critical Temperature of Core, Quench.
    This treatment is applied by allowing the piece to cool in the box to room temperature from the carburizing temperature of 1700F (925C) The piece is then reheated to above 1535F (835C) and quenched in oil. The core will be refined and exhibit maximum strength and hardness. The case will be hardened and somewhat coarsened.

3. Cool From Carburizing Temperature In Carburizing Box, Reheat Only To Above Lower Critical Temperature of the Case, Quench.
    This treatment is applied by allowing the piece to cool in the box from the carburizing temperature of 1700F (925C). The piece is then reheated to above 1350F (730C) and quenched in oil to harden and refine the case. The core will be unrefined, soft and machineable and the case will be hardened.

4. Cool From Carburizing Temperature in Carburizing Box. Reheat to Above Upper Critical Temperature of the Core, Quench. Again Reheat Above Lower Critical Temperature of the Case, Quench.
    This treatment is applied by allowing the piece to cool in the box from the carburizing temperature of 1700F (925C). The piece is then reheated to above 1535F (835C) and oil quenched in oil to refine the core. The piece is again reheated to 1350F (730C) and oil quenched to refine the case. Thus, this double heating and quenching method refines both the case and core. The refined core will be soft and machineable with maximum toughness and resistance to impact. The refined case will be hardened for wear and resistance.