Annealment and Thermal Treatment of Expansion Metal Alloys

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Thermal Treatments Data

Although the nickel-iron low expansion alloys are not hardenable, annealing and stress relief thermal treatments are at times necessary to promote structure uniformity and dimensional stability. The relief of stresses introduced by severe forming, bending, or machining, can be accomplished by annealing at temperatures of approximately 1400 to 1800 F (760 to 982 C) for a time sufficient, to thoroughly heat through the section. However, the nickel-irons will oxidize readily at these high temperatures. When annealing cannot be done in a non-oxidizing enviroment (vacuum, dry hydrogen, dissociated ammonia, etc.) sufficient material must be allowed on work pieces to clean-up by light grinding, pickling, etc., after annealing. Stress relief for sections having light finishing cuts or grinding performed after annealing is accomplished by heating to 600 to 800 f (316 to 427 C), for a time to uniformly heat through the work piece.

All of the nickel-iron controlled expansion alloys can be hot forged, upset, formed, etc., at temperatures between 1800 to 2100 F (982 to 1149 C). However, heating for hot working must be done uniformly at slow rates from relatively cold furnaces (black) to minimize the potential for cracking through large sections as a result of differential thermal expansion stresses. Large forgings should be annealed to promote uniformity after hot working.