Atmosphere Control in Heat Treatment

Posted By Tom Feltham on 13 May 2014

Posted in The Vintage Machinery Almanac

This article first appeared in Practical Engineering 1940 Vol1 No3. The information contained within the article is accurate as of 1940. The article highlights developments within Industry and Engineering at the time.

Gas-Fire “Char-Mo” Atmosphere Furnaces

Gas-fired "Char-Mo" atmosphere furnaces are designed for heat treating all types of steels, without decarburization, carburization or scaling. A protective coating, such as borax, or other special treatment, is not required. Although primarily developed for treating molybdenum steels, these special atmosphere furnaces are equally suited for all types of high-speed steels (tungsten, vanadium, cobalt, etc.), high carbon-high chrome, fast finishing, alloy spring steels, stainless cutlery, and especially medium and high carbon steels containing molybdenum.

The special atmosphere is produced by a patented process which consists of passing air through heated charcoal, and is composed primarily of nitrogen and carbon oxides. Such components as hydrocarbons, hydrogen and water vapour, which are commonly found in appreciable quantities in most furnace atmospheres, are present in such minute volumes in "Char-Mo" atmosphere, that they do not have any detrimental effect. This process permits easy control of the equilibrium of the carbon monoxide-carbon dioxide ratio of the furnace atmosphere to prevent decarburizing, carburizing or scaling of the most sensitive steels.

External Firing

These furnaces are of the muffle type, externally fired, with the atmosphere generator built integral with the furnace. A hopper-fed vertical generator retort is located in the combustion chamber in close proximity to the muffle. The burners used for heating the muffle also serve to heat the retort, automatically balancing the carbon-oxides equilibrium for whatever temperature the furnace is set, thus eliminating the necessity of adjusting the atmosphere for different types of steel.

In the temperature range from 1,400 deg. to 2,400 deg.F, the CO content will be automatically varied between 30 per cent, and 34.7 per cent, while the CO2 content is varied from 2.85 per cent, to 0.0 per cent, respectively. Air for supporting the reaction is tapped from the combustion air supply and is conducted through a control panel to the bottom of the retort. The atmosphere gas generated by the charcoal-air reaction is fed into the muffle, entering at the closed end and flowing toward the door end. The system is adjustable to produce a slight positive pressure inside the muffle, insuring against air infiltration and contamination of the atmosphere.

A control panel conveniently mounted on the furnace includes an air valve for controlling the rate of flow of the atmosphere through the muffle, an orifice meter and differential gauge for indicating the rate of flow; and a gauge for indicating the pressure within the muffle.

Low Operational Cost

The retort and hopper have sufficient charcoal capacity for more than twenty-four hours' continuous operation, assuring an uninterrupted supply of atmosphere gas. The cost of generating this type of atmosphere is quite low. One pound of charcoal costing about 1-1/2-d will produce approximately 100 cubic feet of atmosphere gas, sufficient to operate the average size superheat furnace for about one hour, making the atmosphere cost about 1 1/2 d. an hour. The larger furnaces will require atmosphere gas, depending on the frequency of door operation.

A novel double door arrangement, interlocked and mechanically operated, combined with automatic positive purging when either door is open, provides protection while the work is being charged or withdrawn from the muffle.

These atmosphere furnaces are available in both the horizontal and vertical muffle types and are made in different temperature ranges. The high speed superheat furnace has a temperature range of 2,000 deg. to 2,400 deg. F., and the high-speed preheat furnace has a range of 1,450 deg. to 1,550 deg. F. The general heat treat furnaces, available in three different sizes, have a temperature range of 1,400 deg. to 1,850 deg. F.

The high-speed superheat furnaces and the general heat treat furnaces have their own integral atmosphere generators. The preheat furnaces have no generators as they utilise atmosphere gas supplied from the generator on the superheat furnaces.

Automatic Proportioning Burners

All of these furnaces are heated by automatic proportioning one-valve-control burner equipment, utilising either the high-pressure or low-pressure system of firing. Self-cooling tunnel type burners fire into accurately formed refractory tunnels, confining the zone of combustion, and preventing impingement of flame or muffle on furnace refractories. Automatic proportioning inspirators maintain the correct proportion of gas and air over the entire operating range, irrespective of any variation in gas or air pressure or rate of furnace operation, resulting in maximum fuel efficiency. The one-valve-control feature simplifies operation and facilitates the application of automatic temperature control equipment.