Gas Carburizing

          It is another method of extra carbon into the surface of the steel; in this case by heating the metal in a furnace into which a gas which is rich such as methane, propane, butane is introduced. In gas carburizing, the gas may be generated from suitable liquids, as done in certain processes, or the gas such as methane or propane may be led directly into the container in which the work is placed. It is necessary to maintain a continuous flow of carburizing gas into the furnace, and to extract the spent gas. Some city gasses are quite suitable. Some source of oxygen is necessary for best results, either as air or CO2, but usually it is not necessary to add any oxygen.

          There are explosive ranges of gas composition, which should be avoided. The work is placed in a gas-tight container, which can be heated, in a suitable furnace, or the furnace itself may be the container. The carburizing gas is admitted to the container, and the exit is vented. It should be apparent that there are certain advantages in the gas carburizing process. The steel does not have to be packed, and there is no handling and storage problem of the solid carburizer with which to contend. The time required to bring the work to the carburizing temperature is less than is necessary with solid carburizers. On the other hand, the pack method requires less expensive equipment, and control of the composition of the gasses is simple. Continuous furnace can be used in either process. Continuous gas-carburizing furnace have been developed only recently. In continues gas carburizing, the quenching and tempering cycles take place in the same unit, that is the carburizing, quenching, and tempering processes are carried out in sequence in the same closed furnace as the work progresses on a conveyor from one operation to the next. There are several variations of gas carburizing;

          The horizontal, rotary type of gas carburizing furnace has a retort of muffle which revolves slowly so that the parts are rotated in the steam of gas; this is suitable for smaller parts such as ball and roller bearings, chain links, pins, axels and so on. Carburizing gas led into the retort, and the exit gas is burned. Case depths in relating to time and temperature. Larger parts are usually carburized in a vertically rotary furnace, in which gas is given a swirling rotary motion so that it circulates around the parts.