February 2022 Volume 4

MATERIALS

It shouldbe noted that some of the semi-austenitic gradesmay require cold working or refrigerating prior to age hardening, however, most martensitic grades can be age hardened directly following solution treatment. Age hardening typically begins around 900°F which will produce the maximum strength level. As the aging temperature is increased, the precipitates become less cohesive with the atomic matrix and thus provide less strength, so the resulting hardness is substantially lower. Thermal processing accuracy of age hardening cycles is of paramount importance. These cycles are almost always carried out using a load contact thermocouple to control the cycle time because the soak time indicated for any age treatment is assumed to be the time that the parts are at the designated temperature. If a furnace is loaded with thousands of pounds of material, it will take the workload considerable time to catch up to the furnace temperature. Failure to properly use load thermocouples for this step could result in substantial under-soaking of the workpieces and consequently failure to achieve the desired strength level. Commonly, age hardening treatments require control of the soak time to within a 30-minute window, sometimes less. If this window is exceeded, even at a carefully controlled temperature, the precipitates will begin to lose cohesiveness and the material can lose strength. Furnace uniformity is again absolutely critical for this treatment so as to ensure all parts in the load attain the desired

temperature. In aerospace practice, furnaces used for age-hardening cycle must be at least +/-10°F throughout the work zone.

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

FIA MAGAZINE | FEBRUARY 2022 37