Main spring for barrel of timepiece, has metal strip whose external surface is hardened with respect to rest of strip according to preset depth in order to harden strip on level of main stress zones while maintaining low elastic modulus

摘要

The main spring has a strip (3) made of metal e.g. austenitic steel (5), in order to limit sensitivity of the spring to magnetic field. An external surface (7) of the strip is hardened with respect to rest of the strip according to predetermined depth in order to harden the strip on level of main stress zones while maintaining low elastic modulus, where the predetermined depth is represented between 5 and 40 % of total thickness (e) of the strip. The hardened external surface comprises diffused atoms of non-metal e.g. nitrogen and/or carbon. An independent claim is also included for a method for manufacturing a main spring.