August 2022 Volume 4

MATERIALS

carburizing. Grain growth in most modern steels will start at a temperature above 1800°F and increase in severity exponentially above that temperature. Thus, it is typically unavoidable in forging. The only way to reset these enlarged grains is to reheat the material and cool it again via normalization to create more refined and uniform grains. In normalizing, we heat significantly higher than the upper critical temperature, which usually is 100-200°F higher than annealing. This is because there is a chemical diffusion aspect in normalizing in that we are trying to get the carbon and alloying elements in the material dispersed as uniformly as possible. This process is aided by higher temperature. For a normalizing treatment, the generally accepted practice is to air cool the parts directly from the normalizing temperature. This cools the parts much more quickly through the Ar1-Ar3 critical temperature range and results in a much more refined structure of ferrite and fine pearlite than we see in annealing. This end result is higher hardness and higher strength than we would have with annealed parts. Normalization is commonly specified with a hardness range. In order to tailor the process to a specified range, high powered fans can be used to speed up the cooling rate to raise the final hardness. If parts come out with hardness above specification after a simple air cool, we can use a tempering or a sub-critical anneal operation to lower the hardness into a specified range. Normalized parts

will respond to tempering, it is just a much less drastic hardness drop response than we see in tempering hardened steel forgings. Sometimes forgings are used in the normalized state, however, it is also commonly used as a setup operation to optimize response to a subsequent quench and temper or carburized treatment. 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. Email: chuck.hartwig@thermtech.net

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