Development and Evaluation of a Workpiece Analyzer for Industrial Furnaces
The traditional method of determining the temperature of heat-treated workpieces in industrial furnaces is to measure surface temperature with radiation pyrometers or local temperature with contact thermocouples. Contact thermocouples have limited use because they damage the workpiece in a continuous process or require drilling of a hole in the workpiece for batch operation. Radiation pyrometers are used for noncontact surface temperature measurements, but suffer from inherent inaccuracies because of interference by radiation from furnace hot walls and gases in the furnace atmosphere and varying emissivity of the workpiece during thermal processing. Dual-wavelength pyrometers are designed to provide independence from emissivity variations. However, they do not perform on non-gray-bodies, they have difficulty looking through non-gray windows, and they tend to measure background temperature when the background is hotter than the target. Multicolor pyrometers use algorithms to eliminate the variable emissivity problems with limited success and applicability. In addition, traditional nondestructive temperature measuring instruments lack the capability to measure temperature gradients in a workpiece, which are critical for uniform phase transformations. Moreover, the knowledge of the time at which this uniformity is achieved is important for process cycle time optimization. There is a need for an instrument which can measure surface temperature as well as average bulk temperature to provide knowledge of temperature uniformity in the workpiece. An instrument like that could provide substantial cost savings and near-term payback. The Surface Combustion, Inc. and Textron Defense Systems research team, under DoE support, has conceptualized such an instrument to be used as Workpiece Analyzer (WPA). 1,2 This paper describes results of an analytical study to determine the feasibility of this concept and preliminary experimental data of a bench scale demonstration.
KeywordsMeasure Surface Temperature Industrial Furnace Emissivity Variation Impulse Laser Radiation Pyrometer
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