An Investigation into the Effects of Different Oxy-Nitrocarburizing Conditions on Hardness Profiles and Corrosion Behavior of 16MnCr5 Steels

This work is aimed to develop and optimize a gas ferritic oxynitrocarburizing treatment applied to an industrial 16MnCr5 component. The main effort has been to reduce the treatment time and the gas consumption by using a nitriding atmosphere that comprises nitrous oxide and ammonia. Special attention has been paid to improve the salt spray corrosion resistance with respect to the hardness requirements and the whole compound zone depth. Several changes have been applied to a base treatment by modifying the time and the temperature of each process phase, as well as the gas mixture and the flow of the nitrocarburizing atmosphere. Microhardness investigations and measurements of the compound layer's depth have been carried out on the treated specimens in order to evaluate the influences due to the variations of the process variables during the nitrocarburizing process. The introduction of nitrous oxide and ammonia allows reducing the nitriding time from 3 to 2 hours, and the purge time can be significantly reduced. Moreover, the increase of the nitrocarburizing temperature from 550 to 580°C allowed reducing the time of this phase from 4 to 3 hours. The post-oxidation time was moved from 3 to 4 hours in order to increase the thickness of the surface oxide layer and to improve the corrosion resistance.