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The consensus view is that a high carbon case gives gears the best overall properties, provided that there is no carbide network and the retained austenite has been reduced below 20 % by sub-zero treatment. This view is effectively enshrined in the SAE AMS 2759/7A standard. The sub-zero treatment usually takes place immediately after the quench to avoid austenite stabilization. However, for some parts with complex geometries that might crack during the treatment, a short low temperature temper is carried out first. Little is known on how this temper affects the subsequent sub-zero treatment. Three carburising steels used extensively in the aerospace industry were carburised to produce high retained austenite levels in the case using two different but typical carburising cycles. The retained austenite was determined by X-ray diffraction before and after sub-zero treatment was carried out in accordance with the standard and compared with that obtained when an intermediate temper was used. This study shows that for three typical carburising steels carburised using typical cycles, the efficacy of the sub-zero treatment is probably slightly reduced after the temper but not enough to be industrially significant.
sub-zero treatment, tempering, retained austenite
Mmatscribe UK, West Yorkshire,