Carburizing is one of the most commonly performed steel heat treatments. For perhaps three thousand years it was performed by packing the low carbon wrought iron parts in charcoal, then raising the temperature of the pack to red heat for several hours. The entire pack, charcoal and all, was then dumped into water to quench it. The surface became very hard, while the interior or “core” of the part retained the toughness of low carbon steel.

Pack hardening is uncommon today, except in a few custom made sporting arms. Now, low carbon steel parts are heated in a prepared furnace atmosphere that provides the carbon that diffuses into the surface layers of the steel. Temperatures are usually around 1750°F (950°C). This atmosphere has traditionally been “endothermic”, using a catalyst to partially burn natural gas. Typical composition of an endothermic gas (Class 302) is 39.8% nitrogen, 20.7% carbon monoxide, 38.7% hydrogen and 0.8% methane, with a dewpoint –5°F (–20°C). This carrier gas is subsequently enriched by a small, controlled addition of a hydrocarbon gas, such as propane, or an easily vaporized liquid, which is the source of carbon. 100% nitrogen, from bulk tanks, may also be used as a carrier gas, with propylene or other hydrocarbon injected to provide the necessary carbon.

In vacuum, or low pressure, carburizing acetylene, C2H2 or cyclohexane, C6H12, is used as the source of carbon. The end result is that low carbon steel parts acquire a high carbon steel surface. When the steel is quenched it combines the hardness and wear resistance of this high carbon steel “case” with the toughness of the low carbon steel interior (core).

The alloy bar frame baskets, radiant tubes and other fixturing in the furnace also pick up carbon through many, many heat treat cycles. These fixtures are made of carburization resistant alloys. Even though the atmosphere is reducing to iron, it is still oxidizing to chromium, silicon, and aluminum. An oxide scale, some mixture of Cr2O3, SiO2 and Al2O3,forms on or just beneath the surface. This oxide layer is what provides most of the alloy’s resistance to carburization. Among the alloys, RA330 usually does the best job for the money.