The case-hardening is glass hard for what is left of it. The hardening process only go's a few thousands in to the surface of the metal. The case colors are formed when the metal is heated with the bone/leather/charcoal in the hotbox and then quenched. Those colors are just on the surface and will wear away quickly unless protected by a coating.
During the case-hardening process the gun parts have support blocks bolted to prevent warping. It is possible to harden a action and turn it into a paperweight. And some gun makers will add pieces of metal to actions/parts to enhance the colors when the parts are put into the quench.
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