Abstract: Gas carburizing is carried out in a subsantially closed furnace in an atmosphere initially containing hydrogen with just enough methane (or other suitable hydrocarbon gas) to provide sufficient carbon for carburization at a temperature above the austenitic transformation temperature of a ferrous metal workpiece. For each volume of methane that decomposes at the surface of the workpiece in the carburization process, two volumes of hydrogen are generated. A small amount of natural gas is fed into the furnace to maintain the carbon content of the atmosphere, and a small quantity of the predominantly hydrogen carburizing mixture is bled from the furnace to maintain a desired pressure.