A method of hardening a planetary gear shaft includes carbonitriding an outer peripheral surface of the planetary gear shaft and quenching the planetary gear shaft in oil at a temperature between approximately 120 and 150° C. The method also includes quenching the planetary gear shaft in a liquid at a temperature between approximately −70 and −120° C., and tempering the planetary gear shaft. After tempering, the outer peripheral surface of the planetary gear shaft includes a surface hardness of HV 832 or more and with the shaft material maintaining a hardness of at least HV 513 to a depth of at least 0.5 mm. High temperature tempering and induction hardening steps may be added to obtain soft ends of the shaft suitable for a staking operation.