an introduction to Heat Treating Furnaces

Heat-Treating Furnaces

Various types of furnaces have been designed for heat-treating ferrous and nonferrous metals. It is difficult to classify completely the many types of furnaces used in heat-treating processes. The size and shape of the parts, the volume of production to be handled, the type of treatment needed, as well as considerations of economy and efficiency, are all factors to be considered in the selection of heat-treating furnaces.

Classification
In general, heat-treating furnaces can be classified with respect to
the following:
• Source of heat, as gas-fired, oil-fired, and electric.
• Method of applying heat, as overfired, underfired, recuperative, and forced convection.
• Method of heat control, as manual or automatic.
• Protection of the work, as a controlled-atmosphere type of furnace.
• Batch or continuous operation

Types of Furnaces
Many types of furnaces are designed for a special application or feature, such as high-speed, pusher, rotary-hearth, tilting-oven, pit, pot, box, tool, air-tempering, continuous high-temperature, and carburizing furnace. Furnaces can also be classified with respect to the maximum temperature for which the furnace was designed as:
(1) low (600°F, 315.55°C)
(2) medium (1200°F, 648.88°C)
(3) high (1800°F, 982.22°C)
(4) extra high (2600°F, 1426.66°C). 
These temperature ratings are not standard ratings, but they are compiled from a large number of manufacturers’ ratings for many types of furnaces. 
A simple production line might consist of four furnaces, . The furnaces are  illustrated in order of use are preheat, high heat, quench, and draw or tempering.
These furnaces would be selected according to the temperature requirements of the work being heat-treated. Other installations might include oil or water quench, soak, and/or rinse tanks

Gas-Fired Oven Furnaces
 Essentially, these furnaces have a substantial frame or casing mounted on cast-iron legs; heavy steel channels are provided to prevent sagging. Many furnaces have a counterbalanced door and lifting mechanism.
Depending on the size of the furnace, the interior walls consist of 4.5- to 5-inch fire brick backed up with insulation. The hearth is made of flat preburned forms of fire clay.
The burners are placed properly to fire underneath the working hearth without impinging on the refractory or workpiece. This type of burner arrangement is used for heat
treatment of the low-carbon steels. For high-speed temperatures, one set of burners is placed to fire
underneath the hearth, and the opposite set is placed to fire directly underneath the roof or arch of the furnace. This burner arrangement provides a rotary action for the hot products of combustion
and insures rapid and uniform heating. It also prevents buildup of excessive temperatures beneath the hearth. According to the type of burner equipment provided, the operating temperatures are from 1600°F (871.11°C) to 2400°F (1315.55°C).

Electrically Heated Furnaces
Heat-treating furnaces can be heated with electricity. This is convenient, does not require a flue, is more accurately controlled, and, in certain cases, can provide higher temperatures. Electrically heated
furnaces can be divided into four classes:
• Open element, where the heating element is exposed to the atmosphere of the furnace
• Closed element, with the heating element sealed into the walls and/or floor of the heating chamber
• Immersion type, which has a sealed resistance heating element immersed in a molten bath (usually limited to 1100°F- 593.33°C)
• Electrode type, consisting of electrodes suspended into an electrically conductive salt
The current passes through the salt between the electrodes. The electrical currents cause the salt bath to circulate and this tends to maintain a uniform temperature.
The number of electrodes depends on the size of the furnace, the temperature required, and the amount of work to be processed.

Pit Furnaces
Vertical furnaces of the pit type are used for heating long, slender work. These are sunk into the floor like a hole with tops that can be swung off to open the furnace. Warpage can be minimized by suspending long pieces vertically. Pit-type furnaces can be used to heat batches of small parts, which can be loaded into a basket and lowered into the furnace

Pot-Hardening Furnaces
The pot furnace is designed for indirect heating, the materials being placed in a liquid heat transmitting medium. This is the immersion method of heat-treating small articles. The immersion method is adapted to lead hardening, liquid carburizing, liquid nitridizing, drawing, and reheating. The pot furnace has become more popular as a tool for industrial production because it is convenient, clean, accurate, economical, speedy, reduces warpage, and eliminates scale.
Pot furnaces are built for either gas, oil, or electrical firing. The basic parts of a pot furnace are the furnace, pot, drain, and hood.
The burners are arranged to fire tangentially through accurately-formed combustion tunnels into the combustion chamber.
The basic design of an electrically heated pot furnace is to form a single continuous helix of heavy-duty nickel-chromium rod, either round or square in shape (depending on the size of the furnace),
around the lining tile of the chamber so that a uniform diffusion of heat is achieved.
Recent developments have been the improvement of the electrodetype pot furnace and the salt baths used in them. Industry is using more of these furnaces because of their many advantages and low
maintenance costs. They are especially useful in the heat-treatment of high-speed steel.

Recuperative Furnaces
A recuperative furnace is designed to recover as much heat as possible from the heated charge while it is cooling. In the common “in-and-out” or batch furnace, a cold charge is placed in the furnace,
brought to temperature, and then removed to cool in the air.
This method is usually too expensive for modern industry because almost 100 percent of the useful heat is wasted. Such a furnace is especially expensive to operate where slow cooling is necessary
because a large quantity of additional heat is required to return the lining of the furnace to operating temperature for each successive charge.

Controlled-Atmosphere Furnaces
A neutral atmosphere is essential for the correct hardening of highspeed tools. Considerable research has been conducted with a wide range of atmospheres to determine the neutral atmosphere and develop methods of achieving it. An understanding of the changes produced by different furnace atmospheres increases the appreciation of the advantages of a truly neutral atmosphere. Three surface changes that can be produced by different furnace atmospheres arescale, decarburization, and carburization.
Various types of controlled-atmosphere furnaces are designed to fulfill the special heat-treating requirements in shops where tools and dies are heat-treated in large volumes. These furnaces are also
adaptable to shops in which the volume is not large but the tools and dies are of a design that substantial loss could result from inadequate furnace equipment
An air-gas mixture is precombusted in the precombustion chamber to form a stable protective atmosphere of any desired proportion to fulfill the requirement of a particular type of steel that is being heat-treated.
This method introduces the principle of precombustion of a gas air mixture and the use of the resulting products of combustion to provide a protective atmospheric medium in the heat-treating furnace. To obtain the controlled atmosphere:
• The door of the furnace is sealed against the entrance of the outside atmosphere.
• An enveloping or furnace atmosphere of any analysis that is desired is provided for protection of the work undergoing heat treatment.

Temperature Control of Heat-Treating Furnaces
Regulation of the temperature of the workpiece being treated is the basic factor in modern industrial furnace control. The controls must regulate automatically. This must be done at the lowest possible
cost in spite of changes in production, ambient temperature supply voltage, fuel characteristics, or other changes in the heat treating process.
It is the temperature of the workpiece that is important—not the furnace temperature. The furnace is a basic component of the temperature-regulating system. The temperature of the work is controlled, either directly or indirectly, by regulating the furnace temperature.
In addition to the furnace, or heating equipment, temperature control systems include the following parts :
• The primary element or sensing device
• The instrument or controller
• The final control element, or power-control device
Most temperature controls for heat-treating furnaces are referred to (incorrectly) as pyrometers. A pyrometer is a device for measuring the degrees of heat higher than those recorded by a mercurial
thermometer. It includes the sensing device and a meter to indicate the number of degrees

Results Are Important
The results of the temperature control, rather than the absolute temperature, are more important in measuring a controlled temperature.
Correct operating temperature is determined by trial-and-error and by the results of heat-treating—regardless of the temperature indicated by the measuring device.
Absolute temperature is difficult to determine because:
• Temperature is not constant in most furnaces.
• The temperature in the various parts of the furnace or workpiece is always different because of differences in losses and distribution of heat throughout the furnace.
• The temperature-measuring device indicates the furnace temperature at a single point in the furnace rather than the temperature of the workpiece.

Summary
In general, heat-treating furnaces can be classified with respect to the source of heat (such as gas-fired, oiled-fired, and electric) and to the method of applying heat (such as overfired, underfired,
recuperative, and forced convection). The method of controlling heat (manual or automatic) and the protection of the work (such as a controlled-atmosphere type of furnace) are also important
Regulation of temperature of the workpiece being treated is the basic factor in modern industrial furnace control. The controls must regulate automatically. This must be done at the lowest possible
cost in spite of changes in production, ambient temperature, supply voltage, fuel characteristics, or other changes in the heat treating process.
Many types of furnaces are designed for a special application or feature (such as high-speed, pusher, rotary-hearth, tilting-oven, pot, box, tool, air-tempering, continuous high-temperature, and carburizing furnaces). Gas-fired furnaces have a substantial frame or casing
mounted on cast-iron legs. Heavy steel channels are provided to prevent sagging. Many furnaces have a counterbalanced door and lifting mechanism.
Much improvement has been made in the controls of heat-treating
furnaces. Most such furnaces in industry are being controlled by automatic controls with pyrometers. The desired temperature is set on the control, and the pyrometer holds the furnace at that temperature.
A recording pyrometer can also be used if a record is needed of the temperature achieved.