Online monitoring and control of thermo chemical heat treatment processes by near infrared technology

Online monitoring systems of thermo-chemical heat treatment processes as nitriding, nitocarburizing , carburizing, for increasing hardness, wear resistance, endurance strength and/or corrosion resistance... are up to now available as gas/atmosphere monitoring systems. This paper presents a new inspection system based on a CCD camera system for monitoring such processes by analyzing the surface condition of the batch. As an example a nitriding process with a typical treatment temperatures range of 350 °C to 600 °C is presented. A near infrared enhanced CCD camera system equipped with specifically chosen spectral filters is used to measure spectral emittances during the surface modification. In particular the spectral operating range of 950 nm to 1150 nm of the silicon CCD camera is utilized. The measurement system is based on the principles of ratio pyrometry (dual-band method) known from non-contact temperature measurements, in which two images of the same scene, each taken at slightly different spectral bands, are used to determine the spectral light characteristics. This results in an improved relative sensitivity for spectral changes (i.e. deviations from the gray-body hypothesis) during the surface modification. In conventional heat treatment processes at the beginning of a process all parameters are prescribed firmly such as temperature, pressure, gas, time etc. The system presented enables in situ measurements and a closed a loop control system for an efficient and high-quality heat treatment processes. Heat treatment, CCD camera, spectral imaging, emissivity, surface characterization.