Moisture Measurement in Heat Treatment – The Harsh Truth

Heat treatment processes for metals represent aggressive environments for the measurement of blanket gas moisture. Ambient conditions in the furnaces have high particulate levels and acid gases either outgassing or being burned off of the metal components being processed. These issues, combined with high temperatures, make measurements in the furnace environment extremely difficult. However, the chemical composition of the ambient environment inside the furnace has a significant impact on the results of the heat treatment process.

The blanket gas composition is critical to the oxidation or reduction reactions at the metal surface, which also impacts the absorption of compounds into the metal structure. The moisture levels in the blanket gas are one of the major contributing factors impacting these processes. Over the past few years, the hot form hardening process has been adopted for many structural components in the automotive industry, facilitating the use of thinner, stronger metal alloys which meet the vehicle weight reduction goals of the industry without compromising crash test performance.
Along with other metal parameters, surface coatings are used that can react with ambient moisture, making moisture measurement and control even more critical to the process. Historically, moisture measurement in the hot form hardening process has been unreliable. Conventional moisture measurement technologies such as electro-chemical, IR and chilled mirror cannot tolerate the elevated process temperatures, acid gases and high particulate levels of hot form hardening.

Notwithstanding that, their slow response dictates the need for a sensor at each measurement point. In addition, the electro-chemical sensors do not recover quickly enough from wet up conditions when furnaces are open to external ambient conditions. The introduction of the COSA Xentaur XTDL moisture analyzer has solved all these issues. Specifically designed for the heat treatment environment, the fast response of the XTDL’s Tunable Laser Absorption Spectroscopy technology means that one instrument can be used to monitor multiple points in the furnace.

Its backscatter technique requires no optical components to be in the gas stream, thereby leaving the instrument unaffected by particulates and acid gases and making it virtually maintenance free. In independent plant level studies by a major automobile manufacturer in 2014, the XTDL out performed 5 other moisture measurement technologies and was the only method to provide continuous, consistent, and accurate measurements on a 24 hour, 7 day per week basis. The XTDL comes factory calibrated with a proprietary tuning process requiring no field calibration.

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