Temperature measurement during the rod heat treatment

May 2017 — The more precise process temperatures can be controlled in production, the more valuable the result. Required for this is suitable measuring technology. Ideally, pyrometers are suitable which measure the temperature from the radiated heat according to Planck's law.

Steel bars and wire rods meet higher demands in view of the product quality. During the thermo-mechanical treatment (TMT) of bars the temperature control has a great importance of the rolling and quenching process. The thermo-mechanical treatment of wire rod steel leads to an improved performance characteristics. The thermo-mechanical rolling ensures solidification of the steel and provides mechanical properties which cannot be achieved just only through heat treatment. The subsequent cooling controls the transformation- and precipitation kinetics. In comparison to normalized steel, better combinations of solidity-, hardiness- and bending characteristics as well as weldability can be achieved in this way.

To meet the high requirements of the temperature observance during the production process, appropriate measurement technology solutions are required. Infrared thermometers or so-called pyrometers are ideally suited to determine the temperature contactless from the radiation emitted by the measuring object according to Planck’s law. Because of the adverse production conditions in wire rod mills due to extreme steam and dust, as well as the surface characteristics due to the scale and oxidation, appropriate measuring methods are required in order to achieve a precise, reliable and interference-free temperature measuring to control the production process. 

For this purpose, the temperature of the billet is often controlled directly after the heating furnace to adjust the feed rate. Because of the high heat radiation, pyrometers with high optical resolution are used to detect the billet temperature from a safe distance of several meters. As the surface of the billets are heavily scaled, pyrometers are used which operates according to the two-colour measurement method. The infrared radiation is measured simultaneously at two wavelengths. The ratio of these two radiations is proportional to the target temperature. The different radiation properties of clean and slagged surfaces are recognized by modern two-colour or ratio pyrometers. The Clean Surface Detection (CSD) function is able to sift out the correct temperature of the clean surface. To align the pyrometers, the devices are equipped either with a through-the-lens sighting or a video camera. The video camera provides the possibility to control the measurement location permanently on the monitor in the control room. 

Temperature control in the rolling mill

Another process decisive measurement location is the temperature control in the rolling mill. The oscillating of the wire makes the measuring more difficult. To solve these problems, so-called Panorama pyrometers have been developed. They are equipped with a rectangular measuring area, which is produced in an optical way without moving parts. As long as the wire moves within the measuring area, the temperature will be detected reliably. Previously, pyrometers with swivel mirrors have been used to deflect the measuring spot. The disadvantages of swivel mirrors are the maintenance-and-susceptible motors and moving components, due to the extreme ambient conditions. Furthermore, the temperature can only be detected periodically and not continuously when using oscillating mirrors.

Depending on the object size and the required measuring distance, Panorama pyrometers of the series” Cella Temp PKL 63” or “Cella Temp PA 43” are used. The compact pyrometer Cella Temp PKL 63 with a fixed focus distance of one meter has a LED spotlight which permanently illuminates the measurement area. The green light is clearly visible on the red glowing object. The devices of the models Cella Temp PA 43 have a focusable optics and can therefore be used variably from the measuring distance depending on the mounting situation in the rolling mill. For checking the alignment and focusing you can choose either the through-the-lens sighting or the video camera. Because Panorama pyrometers are also based on the ratio measurement method, precise measurements are made even in case of heavy steam. A Smart Contamination Monitoring (SCM) function ensures a continuous monitoring of the signal strength in modern microprocessor-controlled devices. In case that the infrared radiation decreases to a critical value, it will be detected by the device and an alarm will be triggered.

After rolling, the controlled quenching of the wire rod is performed. The control of the cooling down temperature is therefore very important. Low temperatures upon measurements of metals are metrologically difficult, because the wire is sometimes cooled down to 100 °C. New kinds of pyrometers have been developed for this purpose. They are able to detect the heat radiation despite the wire’s low infrared radiation. In the “Cella Temp PA 28” a light gathering optical system combined with a high-resolution electronics and a patented signal processing ensure stable measurement values, even at a high feed rate of the wire rod.

Designed for harsh ambient conditions

The measurement systems are supplemented with a mounting especially designed for the harsh ambient conditions. A high efficient air purge minimizes the hazard of the contamination of the pyrometer’s lenses. The air is blown forward through a ring gap. In combination with a flow-optimized aperture a high cleaning effect is achieve with a minimum of air consumption.For further processing of the measurements in modern plant control systems, more and more digital interfaces are being used. Compare to the use of the common analogue current signal, the digital communication provides a noise-free signal transmission in case of high electromagnetic fields. In addition to this, further information, alarm- and diagnostic data are being transmitted via the digital interface. The remote control of the devices in case of different production materials during operation is also possible during the operation via the PLC.

The “Cella Port PT 183” portable Panorama pyrometer can be used to carry out a rapid temperature check along the production process from the oven to the rolling mill. The Automatic Temperature Detection (ATD) function makes the measuring very simple. The measuring starts automatically as soon as the pyrometer is recognized the hot object. An audible alarm signals the end of the measurement after three seconds. Moreover, the rectangular measuring area makes it easy to align the portable device to the measuring object even from a save distances. The ratio measuring method in the handheld device also ensures an interference-free measuring under adverse conditions.

Keller HCW GmbH
Carl-Keller Strasse 2-10
49479 Ibbenbueren/Germany
Contact person is Mr. Albert Book
albert.book@keller-msr.de
www.keller-msr.com

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