Atmospheric Furnaces - Product Temperature is Critical
Heat treating of metal parts in an atmospheric furnace is a very common practice. One such furnace is called a mesh-belt furnace, in which the parts are heated in an atmosphere and then eventually quenched. The type of parts heat treated includes fasteners, stampings, hand tools, bearings, automotive parts and machined components. The processes include clean hardening, light case carburizing and carbonitriding, and austempering and martempering. The control of the furnace is usually fully automatic, and the control of the temperature during the entire process is very critical. In addition to the temperature control, a record of the part temperature is usually an absolute necessity for the end customer.
Why is temperature control so critical? If the part is overheated, it will have undesirable grain structure. If the part is not heated enough, however, the material will not be at the required austenitizing temperature and will not have the proper microstructure.
How is the Temperature Measured and Controlled?In metals applications, the most common sensor is a thermocouple. Infrared thermometers are also used to measure the final temperature. Which is the best sensor? They both have advantages and disadvantages, but when used in the correct manner, they do a very good job of temperature measurement.
When a thermocouple is installed in a furnace (Fig. 1), what is the sensor measuring? It is really indicating the temperature of the tip of the thermocouple. If the line speed changes or if the amount of material is increased or decreased, the thermocouple cannot detect the temperature change in the material.
Infrared ThermometersWhen installed properly, infrared thermometers will not see the environment but will measure the real temperature of the product. Since they do not touch the product, infrared thermometers are ideal for moving targets because they do not interfere with the actual process.
As you might guess, we will concentrate on the use of infrared thermometers to measure the part temperature in the mesh-belt furnace. The selection of the correct instrument and the installation of the sensor are very critical.
What is the right wavelength instrument to use for metals applications? The basic rule is to use the shortest wavelength instrument that will measure the target temperature. Why? As you work with shorter wavelengths, the emissivity of the metal is higher. The problem is that as you work with shorter wavelength instruments, the lowest temperature that can be measured is limited. For example, a 1-micron instrument can measure down to about 900°F (480°C), a 1.6-micron instrument can measure 500°F (260°C) minimum and an instrument that operates at 2-2.6 microns can only measure to 150°F (66°C). To go lower in temperature requires using a longer wavelength instrument.
The second reason for using the shortest wavelength is that a change in emissivity has the least effect on measurement accuracy for short-wavelength instruments. Figure 2 shows four instruments viewing a target at 1830°F (1000°C). If the emissivity changed 10%, the error for a 1-micron instrument is only 10°C. If you used an instrument that operated at 8-14 micron wavelength, the error would be 80°C. These errors are dictated by the laws of physics and affect all infrared thermometers the same. Because the temperatures of the parts are 1700-1800°F (925-980°C) at the exit of the furnace, the ideal wavelength is a 1-micron thermometer, which can measure a range of 1000-2400°F (540-1315°C).
InstallationNow that an instrument has been selected, it must be installed correctly. The first parameter to be considered is optical resolution. Figure 3 shows the optics of a typical instrument that has through-the-lens focusable optics. To determine the spot size of the instrument, the formula is d=D/F, where d is the spot size of the instrument at the focal point, D is the distance from the sensor to the focal point and F is the focal factor that is a characteristic of each instrument. This value is always provided in the instrument specifications or user manual.
How does this work? Suppose the distance from the sensor to the center of the mesh belt is 60 inches (Fig. 3). If the focal factor was 100, the smallest spot the instrument can measure is d=60/100=0.6 inches. This means the target must be larger than 0.6 inches or it will not fill the spot the instrument is measuring and an error will be created. If the target is smaller than this, an instrument with a higher resolution factor will be needed. The most typical instrument used for this application has F=150. So for the above example, d=60/150=0.4 inches. Because of the smaller spot-size requirement, the instrument selection becomes one that operates at 1.6 micron rather then the 1.0-micron wavelength as previously suggested. At 1.6 microns the F factor is 150.
The best way to install the sensor is to aim it across and just above the mesh belt (Fig. 4). We also suggest focusing the instrument at the center of the belt. This will provide the best cone of vision across the entire belt. A point to make is that the target does not have to be at the focal point. It can be anywhere along the cone of vision, and as long as it is big enough to fill the cone of vision, the instrument will measure the correct part temperature.
Figure 5 shows the typical installation of a sensor on the oven. The sensor system consists of a water-cooling assembly, an air purge and the sensor. We do not suggest using a window because they are very hard to keep clean, and if the window gets dirty, the instrument will measure the wrong temperature value. The purge gas can be the gas used in the furnace, thus preventing air from being introduced into the furnace.
Measuring Part TemperatureThe instrument is installed so that it measures the parts just as they are about to fall off the mesh belt into the quench. Why not use an infrared thermometer to measure the part temperature in the furnace? Inside of the furnace, the heaters are hotter than the parts. Since a typical part could have an emissivity of 0.8, it is a 20% reflector. The infrared thermometer will see infrared energy from the hot part but will also see 20% of the energy from the heaters reflecting off the surface of the material. This reflected energy added to the energy from the hot part will cause the instrument to measure too high of a target temperature.
With some of the newest infrared thermometers, it is possible to correct for any errors caused by reflected background radiation. This can be accomplished by measuring the furnace walls or heater temperature with either a thermocouple sensor or another infrared thermometer. With software, the reflected energy is subtracted from the primary sensor, thus providing a real part temperature. Because of the expense of a second sensor and the difficulty in the installation of two sensors, the thermocouple becomes the choice of sensors for the main portion of the furnace.
At the exit of the furnace and just before the quench there is a short space where the parts are in an enclosure but are not heated. This means the hottest item in the area is the heated parts, and the infrared thermometer only sees radiant energy from the hot parts not the oven walls or any heating element. As a result, an accurate part temperature is achieved at this point. This is the final temperature of the part just before it is quenched, which determines the microstructure of the part. It is also the temperature that the end customer wants to have recorded.
End users often inquire about the use of a two-color thermometer, which does not require that the cone of vision be filled with the hot target. This makes it easy to work with small parts. The two-color instrument is not ideal, however, if there is any reflected energy around. This reflected energy has a greater effect on the two-color instrument than it will on a single-wavelength instrument. So, for this reason it is usually not suggested that a two-color instrument be used for this application.
ConclusionInside a furnace, an infrared thermometer with active background compensation is needed. Alternatively, thermocouples should be used to eliminate possible errors caused by reflected energy in the furnace. At the exit of the furnace the infrared thermometer is by far the best instrument because it truly indicates the part temperature and is not affected by the air temperature in the furnace. If the line speed or loading of the furnace changes, the temperature changes will be quickly detected. These values can be used as a final control input to the heating process, thus guaranteeing high-quality parts for the final customer.
Additionally, installation of an infrared thermometer is simple in that the instrument must be focused, the lens must be kept clean and the right wavelength has to be selected. The thermometers are rugged and last for many years without repair. IH
A special thanks goes to Can-Eng Furnaces (Niagara Falls, Ontario), who provided the technical photos and information. They can be reached at 905-356-1327; www.can-eng.com
For more information: Vern Lappe is vice president, technical services for Ircon Inc., 7300 N. Natchez Ave., Niles, Ill. 60714; tel: 847-967-5151; fax: 847-647-0949; e-mail: email@example.com; web: www.ircon.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: mesh belt furnace, infrared thermometer, wavelength, emissivity, background compensation, thermocouple