November 2022 Volume 4

EQUIPMENT & TECHNOLOGY

this also results in an incorrect output. The process control loop is, for example, what burner controls use to maintain furnace temperature. In the case of an induction furnace, the control loop is what dictates induction coil power. II. The Need for Accurate Temperature Measurement In hot forging applications, the stock is heated to a predetermined temperature prior to forging. The temperature is selected based on the metal type and composition. A temperature too low will affect the dimensions of the forging. Having a temperature too high or holding it for too long will cause surface chemistry of the metal to change and/or alter the forging grain structure [1]. Surface decarburization from overheating is prevalent with induction heating when a temperature control loop is not present. Forging materials at temperatures other than ideal also leads to equipment problems. A forge press can be damaged due to increased force required to form under-heated stock. Overheated carbon steel stock tends to develop heavy oxide scale which accelerates die wear [1]. III. Temperature Measuring Instruments For a process control loop to work it needs a signal from the process variable. In temperature control applications this signal is provided by the temperaturemeasurement apparatus.Measuring temperature takes place by direct contact and non-contact methods. A. Direct Contact Measurement Measurement by direct contact is accomplished by bringing a probe or sensor in contact with the surface or medium. The most popular device for temperatures above 1000°F is the thermocouple. Thermocouples are typically used to measure heating furnace atmosphere temperatures, where, for example, in a box furnace application, the thermocouple is situated in a location where it senses the same temperature as the stock that is heating. In this application, contact measurement does not measure the stock temperature but accurately measures the temperature of the environment which is heating the stock. Thermocouples are ideally suited for furnace and oven temperature measurement, and the measurement of gaseous and liquid-phase fluids.

1. Thermocouple Probe How a thermocouple probe works

When two wires of dissimilar metals (i.e., nickel and nickel/ chromium alloy) are joined together to create a junction at one end then heated, a temperature dependent potential voltage is generated between the wires. The generation of this voltage is termed the thermoelectric effect. When the thermoelectrically produced voltage is fed into a transducing device, it can be converted into a temperature reading or temperature-dependent output signal. This is how a thermocouple creates a temperature measurement. Why thermocouples are not ideal unless necessary In a hot forging application, the forging stock is almost always in motion. If not in motion as is the case with ‘pigeonhole’ style induction coils, it has limited access. In these two cases, the use of a contact thermocouple to measure the forge material stock is presented with numerous challenges. The foremost is how to bring the thermocouple in contact with the stock for long enough time to acquire an accurate measurement. Pausing for too long slows down production and results in temperature loss. B. Non-Contact Measurement: The Pyrometer The non-contact method of temperature measurement, through the use of pyrometers, has become the preferred way to determine forge stock temperature prior to forging. What is a pyrometer? How does a pyrometer measure temperature? All matter with a temperature above 0 Kelvin/-273 degrees Celsius (absolute zero) radiates electromagnetic energy in the form of infrared (IR) radiation. How efficiently the surface a body radiates energy is a property called emissivity and is defined as “a ratio of the total energy radiated by a given surface at a given temperature to the total energy radiated by a blackbody at the same temperature” [2].

Figure 4: Electromagnetic energy spectrum The amount of IR energy a body will radiate at a given temperature is affected by its composition and surface characteristics. Materials like metals characteristically have a low emissivity. Many non metals with smooth/polished surfaces also exhibit low emissivity values. Conversely, non-metals and materials with rough surfaces typically have a high emissivity [3].

Figure 3: Basic thermocouple probe and symbol

FIA MAGAZINE | NOVEMBER 2022 9