We resume our discussion of furnace gases (see part 1).

  • Carbon dioxide: At elevated austenitizing temperatures, carbon dioxide will react with the carbon in the steel surface and produce carbon monoxide as shown in the following the reaction: (C) + CO2 = 2CO

    This reaction will continue to occur until there is no carbon dioxide available in the furnace atmosphere or until the steel surface has completely lost its carbon. In other words, it can be seen that the carbon dioxide is an oxidizing gas.
  • Hydrogen: Hydrogen is considered to be a reducing gas, which will reduce iron oxide to iron or copper oxide to copper. At temperatures above 1300°F, hydrogen will have a decarburizing effect on the surface of the steel. Below 1300°F, the decarburization is almost negligible. If water vapor is present, the decarburizing effect will increase because of its dissociation and provide atomic nitrogen and oxygen. Part of the reaction will be with carbon on the steel surface to form methane in the following manner: (C) + 4H = CH4
  • Water vapor: If an endothermic gas generator is being used, the air that is being used to form the endothermic reaction contains water vapor from the atmosphere. Water vapor will oxidize the iron in the surface of the steel and will combine also with the carbon in the steel surface to form carbon monoxide and hydrogen: (Fe) + H2O = FeO + H2 (Reaction 1); (C) + H2O = CO + H2

    This is why steam is often used as a bluing agent for motor laminations because it will oxidize the surface of the steel. If the temperature is around 700°F, a blue color will be seen.
  • Hydrocarbons: Hydrocarbon gases are rich in carbon and can be easily cracked, as follows: (CH4) = Methane; (C3H8) = Propane; (C2H6) = Ethane; (C2H2) = Acetylene

    These gases will produce carbon-rich atmospheres within the furnace process chamber. The chemical activity that will take place at the surface of the steel will depend on the surface temperature of steel in order to decompose the carbon-rich gas into nascent carbon. One needs to be very careful in the selection of the carbon-rich gas in order to minimize the risk of sooting occurring within the process chamber.
  • Ammonia: Ammonia is often used for one or both of its elemental gases (nitrogen and/or hydrogen). The ammonia can be used as a source of nitrogen for the nitriding process or a source of hydrogen for a reducing process. The ammonia can be produced as a generated gas, a bottled gas or a bulk-storage gas.
  • Argon:Argon is truly an inert gas that will not react with a metal surface. Its use is more often seen in the aerospace industry, providing a truly nonreactive atmosphere gas. The major drawback of using argon is the cost.