An enclosure heated by a suitable fuel which provides an atmosphere of controlled heat. When furnaces are used for heat treating in welding operations, it is important to assure correct heat control, particularly when heat treating is applied to pressure vessels and similar equipment which are subject to high temperatures or high stress. When metallurgical requirements are rigid, the most suitable combustible fuels are natural gas, liquid petroleum gas, and oil.


The temperature required determines the method in which heat treating furnaces are fired. For those used in low-temperature operations, usually under 600oC (1100 OF), the recirculating type of furnace is used, in which gas or oil is burned in a separate chamber and the resulting products of combustion are circulated through the furnace by a fan.

For temperatures up to 1000°C (1900oF), indirect firing is generally used. In this type, the combustion chambers may be above, below or on one side, separated from the heat chamber by baffles.

The third type of furnace is the direct-fired furnace, which is used for temperatures above 1000°C (1900°F). The burner is fired directly into the heat chamber, usually above the charge. It is possible, but not usual, to design direct fired gas furnaces which will operate satisfactorily between 500 and 1000°C (1000 and 1900°F).

Electric Furnaces

Electric furnaces used for heat treating applications are usually the resistor type, and are ordinarily limited to applications for which temperatures do not exceed 1000°C (1900°F). Above that temperature the operating life of the resistor elements is greatly shortened. If carbon resistor elements are used rather than metal elements, however, the furnace may be used at temperatures up to 1300°C (2300°F). Furnaces with metallic elements and protective atmospheres have been successfully operated at 1100°C (2100°F) for brazing applications.

Electric furnaces operate quietly, cleanly and without the necessity for mufflers. They provide uniformity of temperature in applications for which electric heat is specified. The greatest advantages are the consistency of operation after initial set-up, and freedom from human error resulting from adjustments of fuel.

The main disadvantage of the electric furnace is that it is slow to heat up from a cold condition. Where it is possible, an electric furnace can be connected across the line to save the expense of a transformer. Frequently however, elements based on line voltage are too light for satisfactory life and transformers are necessary.



Operating Fuel-Fired Furnaces

Disastrous results can occur if fuel fired furnaces are not handled correctly. To operate these furnace safely, there are several precautions which must be rigidly followed. On all fuel fired furnaces, all doors are to be opened before lighting the furnace. When lighting burners, oil, gas or the two-pipe (blast) type, turn on the air first, then the fuel. When lighting burners of the low-pressure proportional mixing type, always open the gas supply valve wide open. All regulation of fuel is by air flow and the ratio adjusting screw on the mixer. When shutting off an oil or gas burner of any type, always shut off the fuel first.

Material Handling Methods

Furnaces for heat treating applications are further classified as one of several types, depending on the method of handling the material to be heated. In a batch method, the pieces are handled in groups. The furnace may be a stationary hearth solid bottom, stationary hearth roller bottom, car bottom, or a furnace with removable covers and pits. In a semi-continuous handling method, a stationary hearth or car bottom furnace is used in conjunction with cranes designed to remove the material. This reduces the time between changes to several minutes. The continuous system conveys the parts through the furnace, and consists of a pusher (direct or on pans or shoes), tunnel kilns, with cars, chain conveyor and moving finger, walking beam, roller hearth, rotary hearth, rotation retort or miscellaneous special types.

Furnace Design

For the most efficiency, furnaces should be designed by a furnace manufacturer or combustion engineer. There are many variables, and the size of a furnace required depends on the amount and size of material to be heated per hour, the heating time, and the amount of heat that can be liberated without damage to the furnace. See ANNEALING, HEAT TREATMENT and METALLURGY.