November 2020 Volume 2

MATERIALS

tempering furnace capacity as compared to hardening furnace capacity. If a heat treat operation does not have a sufficient number of tempering furnaces, it is often necessary to run hardening cycles longer than necessary in order to wait for tempering cycles to end. Second, the uniformity standards applied to tempering style furnaces are more stringent than hardening furnaces. Typically, the minimum expectation is +/-25°F throughout the work zone for an austenitizing furnace and +/- 15°F for a tempering furnace. This is because tempering is a more temperature sensitive process than is hardening and poor furnace uniformity will result in unacceptable hardness spread within a given furnace load. Whereas heating for austenitizing furnaces relies greatly on powerful radiative heat transfer when parts start glowing red above ~1200°F, tempering furnaces must rely heavily on convection style heating as the temperatures involved with tempering may result in only a dull radiative glow at best. Because they generally lack radiative transfer, it is important that tempering furnaces be designed with robust fan systems to move the heated air evenly throughout the work zone and that the burner(s) are sized adequately for the amount of weight that is typically heated in the furnace. As a final note on process capability—it is vital that the tempering temperature/time used for a given part remains relatively constant over time. Once a successful outcome has been established, it should not be necessary to vary the tempering temperature by more

than 25°F from lot to lot. While heat treaters will experience slight chemistry variations from heat to heat of material, making changes to established processes ofmore than 25°F is likely an indication of an adverse event elsewhere in the operation. Using a significantly lower tempering temperature than was necessary on previous iterations of the same part can be a warning sign that quench effectiveness has deteriorated and should be investigated immediately. ■

Chuck Hartwig is the Director of Operations for Carburizing and Batch Hardening at ThermTech in Waukesha, WI. He holds a B.S. in Metallurgical Engineering from Colorado School of Mines.

FIA MAGAZINE | NOVEMBER 2020 15