7124 Heat Treatment

Heat treatment of Ni-Mo, Ni-Cr-Mo, and superaustenitic stainless steels is relatively simple and straightforward but does require close attention to temperature, rate of heat up, time at temperature, cleanliness, and method of cooling.

The annealing is designed to optimize both mechanical and corrosion resistance characteristics. Following all hot-working/forming and most cold-forming operations, a full solution anneal is done. Stress relief at intermediate temperatures is generally not recommended due to danger of precipitation, which can be detrimental to mechanical behavior and/or corrosion resistance. Prior to annealing, cleanliness (i.e., removing grease, oil, and carboneous materials and foreign substances) is a must because of the possible diffusion of carbon, thus lowering the corrosion resistance. It is recommended to charge the alloy component (specially Ni-Mo alloys) in a hot furnace as opposed to charging cold and gradually raising the furnace temperature, unless there is a compelling reason to do so. Holding time of 5-10 min at temperature is sufficient, and this "counting of time at temperature" should only be started after the entire cross section of the part being heat treated is at the proper heat treatment temperature. Hence proper placement of thermocouple or thermocouples to ensure this is very critical. It is also important to avoid any stagnant condition during the heat treatment due to the possibility of accelerated scale formation and high-temperature attack especially in Ni-Mo alloys, which do not contain chromium. Rapid cooling is very important and essential to prevent any detrimental phase precipitation. Depending on section thickness, the component can be either rapid air-cooled or water quenched. Table 7.22 gives the solution annealing temperatures for various alloys.

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