Decarb; here it is on a piece of our infamous 5hour 01:
Decarb is like germs, it is very ubiquitous, present in almost every piece of steel we heat in a normal forge or oven without shielding gases or other barriers, but also like germs, it is mostly so small and minimal that it is of no great consequence. The best way to deal with it is to assume that you will grind 5-10 thousandths off all of your forgings, and be careful about your austenitizing heats for hardening. Using heat sources with extreme atmospheres and long term exposure to the air can give problems in the long run. Decarb occurs in steps from the very outside inward and is not readily detectible, requiring careful examination. Normal analysis will detect the overall chemistry of the steel and if there is but .003 in patches on the outside will not register it like it would if massive carbon was lost throughout. Such massive carbon loss would require very big screw ups in the heating of the steel.
As has been pointed out, scale is not decarb, it is iron oxide. In fact scale can counteract decarb by removing iron at the same rate or greater and thus maintaining the ratio of Fe to C on the surface. For more information on this I recommend reading chapter of Tool Steel Simplified By Palmer and Luerssen which covers extensive testing done by Carpenter on the affects of highly oxidizing atmospheres on many steel types. There is also much discussion on the much greater threat moisture in furnace atmospheres poses in decarburization than oxygen. I can verify the information in this book with my observations and experience both in knife work and at the metallograph.
The O.P. of this thread involves a confusion between decarb and overall hardenability of the steel. One should quench as soon as possible in order to avoid any diffusional processes that will result in loss of hardness simply due to failure to under-cool the piece to avoid non-martenistic transformations. Air cooling will result in some decarb but in such minute thickness before the steel will cool bellow required temperatures that it shouldnt cause too much issue, otherwise air hardening alloys would require significant stock removal before use.
As we have been discussing it takes considerable time and temperature to remove significant carbon from the steel, and thus it should be obvious that much of the old theories surrounding putting carbon into the steel via forging processes is for the most part myth. Significant carburization requires temperatures in excess of 1600f for extended times with the aid of an activating agent, and will first form a high carbon skin that will then be diffused deeper into the material over time. Much of this skin will be lost with exposure to the atmosphere or any stock removal, just as the decarb would. Carburizing compounds include activators, in the old days cyanide bearing materials would help in this regard but today gas carburizing techniques are the most common for case hardening.
