Non-contact infrared temperature control in alumium rolling process

February 11, 2025  233  233   

During this process, aluminium slabs or billets undergo either cold or hot working, depending on the desired material characteristics. Cold rolling increases the strength and hardness of the aluminium by altering its microstructure, though it also makes the material more brittle. Conversely, hot working, conducted at temperatures between 260 °C and 510 °C depending on the alloy, maintains the aluminium’s ductility by preventing work hardening. As the aluminium passes through the rolling mill, its temperature varies significantly—from an initial preheating stage at around 200 °C, rising to approximately 450 °C upon entering the rolling mill, and then dropping below 100 °C as it exits.

One of the key challenges in this process is the accurate temperature measurement of aluminium, mainly due to its highly reflective surface. Aluminium is notoriously difficult to measure with infrared cameras because of its low emissivity and high reflectivity in the infrared spectrum. Especially when polished or bright, aluminium’s emissivity is very low, typically ranging from 0.02 to 0.1 for long-wavelength infrared devices. This low emissivity means that aluminium emits very little infrared radiation compared to other materials, making it challenging for long-wavelength infrared cameras to provide accurate temperature readings. Additionally, aluminium’s emissivity can vary with temperature, further complicating precise measurement. Short-wavelength infrared cameras, which typically have a higher evaluated temperature range, are often incompatible with the temperatures used in aluminium processing, making them less suitable for these applications.

Harnessing the Wedge Effect: Overcoming Emissivity Challenges in Infrared Temperature Measurement

One effective way to address the emissivity challenge in temperature measurement is to focus on the point where the coil forms a wedge, creating a nearly enclosed cavity for infrared radiation. In the deepest part of this wedge, the cavity acts as a virtual blackbody with a stable emissivity close to one. This approach, known as the wedge method, is particularly practical for measuring the temperature of polished steel strips, which typically have a low emissivity of around 0.37. The wedge naturally forms within the coiled strip, making it an optimal location for accurate temperature readings.

The wedge effect in infrared thermography explains why surfaces within a wedge-shaped structure exhibit higher apparent emissivity compared to flat surfaces. This increased emissivity results from multiple reflections of infrared radiation within the wedge, where the radiation bounces off the walls, enhancing the likelihood of absorption and re-emission. This process creates an environment that closely mimics a perfect blackbody, with an emissivity approaching 0.998, regardless of the material’s natural emissivity. This phenomenon significantly reduces the impact of emissivity variations and background radiation on temperature measurements. For the wedge effect to be effective, the depth of the wedge must significantly exceed its opening.

The Xi410 infrared camera can autonomously leverage the wedge effect despite being a long-wavelength device. The thermal imager can accurately identify the highest temperature by configuring a region of interest (ROI) in the PIXConnect software to capture the maximum temperature within the wedge of the aluminium coil. The Xi410 can then transmit thermal data directly to a programmable logic controller (PLC) via analogue outputs or digital interfaces, ensuring precise and reliable temperature monitoring in real-time.

Achieving Optimal Aluminium Cold Rolling with Affordable Temperature Monitoring Using Wedge Measurement Techniques

When rolling high-grade products, the decisive difference is precise temperature control. If the required temperatures are precisely maintained, the product quality can be assured, and damage to the roll stand will be avoided. This application is suitable for the wedge measurement technique used at the coiler or between the roll and the product during the rolling process, which eliminates the challenges posed by low temperatures and variable emissivity, and also eliminates background reflection influences. This allows the manufacturer to implement continuous process temperature monitoring. It ensures that the system consistently tracks temperature variations throughout the production process, allowing for real-time adjustments and supporting the documentation of process and tool temperatures, providing insights into the production process, and enabling closed-loop control for automated adjustments that maintain optimal conditions and prevent deviations.

Optris long wavelength camera can exhibit the wedge effect to accurately measure the strip temperatures in cold rolling and coiling applications. In this application, the autonomous thermal imager from the Xi Series provides multiple interface options, enabling the camera to be easily integrated into new or existing process control systems. Unlike other solutions from the competition, there is no need for special software or unique sets of wavelengths in this challenging application, as Optris infrared cameras will measure the wedge effect at an affordable price.