Understanding Emissivity and Thermocouple Test Blocks

A recent process development test relating to carburizing illustrated the need to better understand the effect of surface emissivity and the proper use of dummy thermocouple test blocks.  The testing involved carburizing areas of a partially copper plated alloy steel part.  The copper plating covered areas of the part that were not to be carburized.  Since the configuration of the part made it impossible to place a thermocouple within the part, a dummy test block made of carbon steel with the approximate same cross-section was used for the process thermocouple without proper consideration of the surface condition of the test block.

Using the test block as the control, carburizing was initiated at the proper temperature based on the test block having reached that temperature.  At the completion of the test, the part was examined for carburizing results and found in the non-copper plated areas, the depth of the carburized case to be shallow.  This indicated that the cycle performed did not initially hold the part long enough at the correct temperature prior to carburizing.  This resulted in the conclusion that when using dummy test blocks for controlling process times and temperatures, many factors must be considered including surface emissivity.

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