Case Color Hardening?
 

My understanding is that gun parts are cae colored to make the surface "tougher". If that is the case, what happens when the case coloring is worn off? Has the "toughness" gone away, or does it remain?

John, Case color is actually a byproduct of the suface hardening process. Many machine parts including doublegun frames are case hardened and what that means is the the surface of the frame is made hard generally about .010-.015 deep while the subsurface of the frame remains soft as in the steels natural state. If frames were through hardened they would not be able to withstand the repeated pounding they take upon firing without suffering failures ie:cracking. In answer to your question if a frame has no visible case color left on it that does not mean it has become soft. A frame would have to be put in a furnace and brought up to temp to anneal it. That would be the only way to make a frame completley soft. Problem is that if this is done distortion of the frame may occur. Parts can be case hardened without achieving case color also. A perfect example of this is to heat a part made from Cold Rolled Steel cherry red and then dip it into a cyanide powder. The surface will harden to about .015 deep.

a factory case hardened receiver will stay hard for ever, regardless of what color it is.

So, if the subsurface steel is still somewhat malleable in order to withstand this repeated pounding, does the case-hardened .010" - .015" surface eventually crack from this repeated pounding, if only microscopically? It seems to me that it should crack or craze due to the fact that the softer steel beneath the surface is flexing to some degree.

Dean, The surface hardening provides wear resistance and toughness. They are not hardened to Rc-60 which is about glass hard. Most likely in the neighborhood of Rc-45 or so.

yep all that pretty engraving would wear off if the frame was soft.... charlie