Chuck;
thanks for posting the tempering scale.
KyJon;
To now answer your question, this scale pretty well sums up my thoughts. You will note the highest tempering temp this scale went ot is 1,000F which produces a hardness of about 31Rc. To twmper at 1200F or above is going to drop that hardness well down into the Rc 20s. This is simply not a good use of 4140. 4140 is a great steel, but if I had a gun frame made of it I would want it given a proper heat treat & blued, just forget the color. If one wants color then use a grade of low carbon steel & case harden it. This gives the advantage of that "Glass Hard" skin which is wear resistant. You will definitely feel a difference in working a dropping block action or closing a double which cocks the ejectors by pushing them in as they rub down the breech face, between that hard case & a part in the Rc 20's. 8620 is a more modern alloy which I understand has a much higher strength than the old standard 1018 etc.
