Control of billet temperature is a key factor in hot forging processes. Other-than-optimal temperature results can adversely affect die wear, metallic microstructures, metal flow within dies and yield incomplete die fill. There are numerous methods used in the forging industry to measure billet temperature in hot forging processes, including thermocouples, touch probes, infrared pyrometers and thermal-imaging cameras. This first of two articles examines the principles of operation, advantages and disadvantages, selection and proper operation of infrared pyrometers.
Infrared pyrometers have long been a very popular selection in the forging industry to fulfill the need for billet temperature control. That said, an industry-wide lack of understanding exists relating to the proper use and selection of a pyrometer. This knowledge deficit can prove quite costly because the improper use and selection of an infrared pyrometer can lead to significant temperature measurement error.
The first step in ensuring consistent and precise temperature measurement with an infrared pyrometer is to understand how it functions. An infrared pyrometer determines billet temperature by measuring the thermal energy radiated by a high-temperature workpiece. All objects emit thermal energy in an amount proportional to their temperature. This energy is emitted at its greatest intensity in what is known as the infrared spectrum of light. The infrared spectrum is characterized by the range of all wavelengths between 700 nanometers and 1 millimeter.
A pyrometer uses a specially designed lens to focus the measured infrared energy at a particular wavelength onto a sensor much the way a traditional camera focuses visible light onto film. An infrared pyrometer, however, only measures infrared energy within a certain area known as the spot size. This spot size is a physical characteristic of the pyrometer and is determined by both the lens installed on the pyrometer and the distance between the pyrometer and the measured object. The spot size through which an infrared pyrometer measures should always be smaller than the surface of the workpiece. This ensures that only the temperature of the workpiece is being measured.
Emissivity and Apparent Temperature
Temperature measurements with an infrared pyrometer are adjusted to accurately measure specific materials based on a number of measurement parameters, the most important of which is workpiece emissivity. Emissivity is a material characteristic defined as the efficiency at which a real object radiates thermal energy compared to a theoretically perfect emitter referred to as a blackbody. A blackbody is unique in that nothing can emit more energy at any wavelength or temperature. Emissivity, often represented with the symbol epsilon (ε), quantifies the efficiency (given as a value ranging between zero and one) where the closer the emissivity value is to one, the more efficiently an object emits thermal energy. All real objects undergo a loss of efficiency during the radiation of thermal energy. If this phenomenon is unaccounted for, an infrared pyrometer will output what is known as the apparent temperature, which will always be lower than the actual workpiece temperature.
To ensure accurate temperature measurements with an infrared pyrometer, the correct emissivity for the measured workpiece must be used. To properly select an emissivity value for a given application, it is important to understand the factors that determine the emissivity of a material. For metals, the most important characteristic affecting workpiece emissivity is the surface condition. The more smooth and reflective the surface of the measured workpiece, the lower the efficiency at which it can emit thermal energy. Conversely, dark, rough and oxidized surfaces will always have a higher emissivity value. Notice in Table 1 how the polished aluminum, polished steel and stainless steel have significantly lower emissivity values than their oxidized counterparts.
The second parameter that plays an important role in pyrometer measurement is wavelength. The majority of pyrometers measure thermal energy at wavelengths ranging from 1-14 micrometers. Wavelength, in the context of infrared pyrometers, is analogous to the window through which an infrared pyrometer measures the thermal energy radiated by a workpiece.
The measurement wavelength of an infrared pyrometer determines the range of temperatures that can be measured with the unit. The longer the measurement wavelength, the lower the minimum temperature that can be measured. However, the use of a longer-wavelength infrared pyrometer to measure a lower minimum temperature does result in a trade-off. The temperature measurement error that results from the use of an incorrect workpiece emissivity will be larger in longer-wavelength infrared pyrometers than in shorter-wavelength infrared pyrometers. Because of this trade-off, the common rule of thumb is to use the shortest-wavelength infrared pyrometer capable of measuring the lowest necessary temperature for the application.
Not only is the measurement wavelength of an infrared pyrometer responsible for determining the minimum temperature measured, but in some scenarios the emissivity of the workpiece material is also dependent on the wavelength at which it is measured. The majority of materials used in the forging industry have an emissivity value that varies depending on the measurement wavelength.
There are a select few materials described as graybodies, which do not exhibit a variable emissivity with respect to the wavelength at which they are measured. Of the common forging materials, only oxidized steel exhibits graybody characteristics. The emissivity of oxidized steel remains constant at 0.85 for all measurement wavelengths. The remaining metals used in forging applications do not behave in a graybody fashion, and their emissivity setting must be selected for the specific wavelength at which it is measured by an infrared pyrometer.
Pyrometer Advantages and Limitations
Infrared pyrometers offer several benefits over other temperature-measurement devices used in the forging industry. These include noncontact temperature measurement, portability and fast response time. The most significant is the noncontact aspect of infrared pyrometers. Because of this characteristic, workpiece temperature measurement can be performed without interfering with the forging process. Infrared pyrometers eliminate the need for exposed thermocouple wires and can be used at safe distances from hot forging presses and equipment. Infrared pyrometers are also quite portable and can easily be permanently mounted as part of a forging operation or temporarily set up on a portable tripod. Lastly, infrared pyrometers have an extremely fast temperature-measurement response time – usually around 10 milliseconds. Therefore, an infrared pyrometer can quickly determine the temperature of a hot workpiece the instant it leaves a furnace, just prior to a forging operation or at various points in the forging process.
Even though there are numerous benefits to the use of infrared pyrometers in hot forging processes, several limitations do arise. These include difficulty measuring small workpieces at distances greater than 10 feet, measurement range, measurement characteristics and cost. In some scenarios where the workpiece is small or is greater than 10 feet away from the pyrometer, it is difficult to maintain a small enough measurement spot size.
Infrared pyrometers also cannot measure a wide temperature range with a single unit. Longer-wavelength pyrometers can generally measure temperatures up to 1500°F (816°C), while shorter-wavelength pyrometers cannot measure much below 500°F (260°C). It is also important to understand that infrared pyrometers only measure the temperature at the surface of a workpiece and not the core. If a workpiece is being heated in a conventional furnace, it will take substantial time for its core to reach hot forging temperatures. Lastly, higher-end infrared pyrometers can cost significantly more than other temperature-measurement methods currently available to the forging industry.
This article offered a general overview of how infrared pyrometers operate, as well the advantages and disadvantages associated with their use. In a second article to follow, guidance for proper use and selection of infrared pyrometers for some commonly forged materials will be provided.
I would like to add a note of thanks to the Forging Industry Educational and Research Foundation (FIERF), particularly Carola Sekreter, for the funding of the project at Marquette University that resulted in much of the information shared within this article. In addition, I would like to thank my academic advisor, Dr. Joseph Domblesky, as well as Ron Finstad of Ircon Corp. and Matthew Carlton of Raytek for their continued support and resources contributing to my work.