When I make a big lock such as will be for a military musket, I pack harden the plate for a couple of hours at a high temperature in my forge. This give good penetration of the carbon into the plate. I usually make my lock plates on these military guns out of iron so case hardening is important for wear. Because I am going to polish the lock back to a armory bright finish the colors are not important. After the plate has spent it's time in the forge I take the plate out of the box and let it air cool to room temperature so now it has a skin of high carbon steel but it is not hard. To harden the plate I heat it in the shop with a oxy/acetylene torch and quench it vertically in water so there is little or no warping. These big lock plates are pretty beefy so bracing them is not necessary.
