Reheat furnaces are responsible for bringing cold metal to the correct temperature for hot-forming applications such as rolling, extruding or forging. To optimize quality and reduce wastage, the temperature needs to be consistent throughout the product, which requires accurate temperature monitoring. In addition, temperature measurements are key to providing optimized heating trajectories for the metal, which results in significant energy savings, consistent metallurgical properties and minimal surface scaling. The most effective and accurate method of achieving these measurements is through thermal imaging.
Most steel mills and metal forging operations rely on reheat furnaces to ensure that metal slabs or billets reach a uniform, repeatable temperature prior to being sent to the rolling mills and forge presses. The purpose of the reheat furnace is to bring the product temperature up to the working level so that it can be rolled, extruded or forged.
There are two major types of reheat furnaces: continuous and batch. Each can use various heating methods, but the most common use either natural gas/methane or natural gas with oxygen enrichment.
Continuous Reheat Furnaces
Continuous reheat furnaces have evolved from simple pusher furnaces — where stock is moved sequentially through the furnace as each new product is added — to the more advanced walking-beam furnaces. The walking-beam design — where stepping, alternating skids “walk” the product through the process — produces a more uniform underside temperature that results in higher quality finished products.
Reheat furnaces typically are used in hot-rolling mills. The slab or billet is loaded into the cool end of the furnace. From there, it passes through preheat, heating and soaking zones before being discharged for rolling.
In such a system, the preheat zones warm up the material before the heating zone delivers the main heating for the product. The soaking zone allows the heat to homogenize throughout the steel so that when the slab or billet is discharged, it has uniform temperature distribution throughout its thickness, length and width. The aim of this method is to create a tightly controlled heating trajectory as the stock travels through the furnace. For maximum fuel efficiency, the stock should achieve the desired temperature shortly before discharge.
Issues arise when the steel is heated too quickly, however, particularly in the preheat zone. This is fuel inefficient and, as such, wastes energy and increases costs. It also can affect the metallurgical properties of the steel and increase surface scaling.
Batch Reheat Furnaces
Typically, batch reheat furnaces are used in forging operations, where the metal is hammered or pressed. If the metal temperature drops below a specific value, it can damage the forging machinery as well as negatively affect the forging quality and the metal part itself.
To prevent such problems, a part that has cooled too much must be returned to the batch reheat furnace and brought up to temperature before further forging occurs. Once the reheated part is ready, it is quickly transported back to the forge for further work. This pattern of heating, forging and return to the furnace for reheating can be repeated several times during the forging process. Again, the heating needs to be uniform to ensure that the entire product is at the correct temperature for forging to continue.
Regardless of the furnace design or fuels used, it is important to accurately measure the temperature of the stock as it progresses through the furnace.
FIGURE 1. Using a furnace thermal-imaging system together with the advanced software packages allows the background compensation to be done in the same thermal camera frame/video online, continuously measuring the correct stock temperature.
There are a number of measurement principles and methods to determine the spectral emissivity of a metal or stock surface. The spectral emissivity mainly depends on the material and surface conditions.
The measuring-system readings are a sum of the emitted and the reflected radiation from the stock. To consider the reflected background radiation and subtract it from the system reading, thermocouples or pyrometers can be used, measuring the background refractory temperature. By measuring and calculating the background signal correctly, an accurate temperature reading of the stock is provided.
Based on the behavior of the furnace, an appropriate measuring system is necessary before proceeding to achieve the most accurate temperature. Once the correct system is chosen, accurate stock temperature values can be fed to the furnace control model to improve quality and reduce fuel costs.
Measuring Furnace Temperature with Thermocouples
Typically, thermocouples were installed through the furnace roof or walls to measure the furnace temperature in each zone. For example, preheat zones are used to warm up the stock and heating zones to deliver the main heating for the stock and soaking zone, which allows heat to homogenize.
One thing to keep in mind is that the thermocouples do not measure the exact stock temperature. They only measure the furnace atmosphere and surroundings. Mathematical heating models then are used to deduce the stock temperature. This historical, “blind” method of temperature profiling provides an approximation of the metal’s surface and bulk temperature during its movement through the furnace, assuming that movement is steady and repeatable.
When thermocouples operate at elevated temperatures for an extended period of time, however, their accuracy degrades due to tip migration. (Tip migration is when the alloys at the tip start to combine rather than remain as two separate metals.) As a result, thermocouples need to be checked on a regular basis and replaced when they become out of specification or are broken. This may happen quite often. The protective sheath around the thermocouple can be damaged when the refractory surface expands or contracts.
Some operators compensate for the problems of thermocouple measurement by overheating the product, leaving it in the soaking zone for longer. This is seen as more desirable than having to completely reheat the product. However, this wastes energy and often affects the metallurgical properties of the product, causing additional scaling on the surface
In the search for ever-higher quality, companies are switching to process pyrometers and thermal-imaging systems. These systems provide a high quality image and are fully radiometric, enabling accurate temperature measurements within the scene. While pyrometers deliver just one measuring point, thermal-imaging systems provide multiple measurement points, areas of interest and profiles as well as a live image of the process.
Challenges of Noncontact Infrared Temperature Measurement
Different fuels and the resulting hot combustion gases can affect temperature measurement, so application-specific pyrometers and thermal-imaging systems need to use advanced spectral filtering. With higher background temperatures of the furnace refractories, the background radiation influence needs to be measured continuously and subtracted accurately from the measuring signal.
Another challenge is that the reheating process needs to be controlled closely and monitored to ensure the metallurgical properties of the stock that is reheated. A clear, visible image of the stock reheated in the furnaces needs to be provided to the furnace operator teams. These systems need to be reliable for continuous operation in rough industrial environments at the high furnace and surrounding temperatures.
Noncontact radiation thermometers — also referred to as infrared pyrometers — have been used for many years to measure actual stock temperatures directly. Readings from pyrometers are a combination of the emitted and reflected radiation from the stock and furnace background. To determine the correct stock temperature, the background temperature must be measured and its effects subtracted from the pyrometer reading.
A thermocouple can be used to measure the background temperature, too. It is essential to have a good understanding of the variables involved in these measurements because these pyrometer readings are the sum of the emitted and the reflected radiation from the stock and the surroundings.
A thermal image of metal stock can be viewed using a near infrared borescope.
Traditional thermal imagers require a large hole in the furnace wall, leading to costly heat wastage and heat damage to the camera. Modern thermal imagers require only a small hole in the refractory and are installed sealed to the process.
A major advantage of thermal imaging is the live visual image that is available to operators. Once the appropriate thermal-imaging system is chosen, accurate stock temperature values can be fed to the furnace control model to improve quality and reduce fuel costs via analog and digital interfacing, providing full Industry 4.0 communications.
Some thermal-imaging systems use near-infrared imaging to create continuous, high definition thermal images that are unaffected by the furnace’s hot atmosphere or gases. They allow operators to measure up to 3 million live data points, enabling highly specific optimization of furnace temperatures and fuel usage.
Using the furnace thermal-imaging systems (figure 1) together with advanced software packages allows the background compensation to be done in the same thermal camera frame or video online. The software and imaging system continuously measuring the correct stock temperature. In this case, no additional pyrometer is needed, and existing thermocouples can be continuously checked, too.
With continuous coverage, automated alarm outputs can instantly alert the user to any problems. Thermal anomalies are easy to detect, through continuous monitoring of all positions from the safety of the control room.
This is particularly useful, for example, when reheating billets. In this example, the steel may be heated correctly, but it could have a bent end (hooked billet), which is likely to hinder the movement of the billet out of the furnace. The live image allows the operator to identify this problem and manipulate the billet out of the furnace prior to any problems occurring. It also supports the detection of any issues with surface properties, leakage or burners that could affect product quality.
In conclusion, with ever more demanding quality requirements, companies are switching to process thermal-imaging systems to provide temperature measurements of their stock. The starting point for higher product quality is knowing that the temperature is homogenous and consistent. These systems deliver a highly detailed image and are fully radiometric, enabling accurate temperature readings within their entire field of view. Measurement points, areas of interest and profiles can be configured to measure multiple areas on a myriad of targets within the scene based on the advanced thermal-imaging software.
In this way, thermal-imaging cameras allow the measurement of loads that are varied in quantity, size and location in the furnace. Thermal-imaging cameras can measure the complete product as it exits the reheat furnace, identifying any erroneous or anomalous measurements. They also provide a live visual image, which is particularly advantageous.
As a result, thermal-imaging cameras can provide accurate temperature measurement of the entire metal stock in metal reheat applications and the furnace conditions, which can deliver benefits in cost savings and product quality.