Depth carbonitridings with high nitrogen content

Mechanical steel parts treated by case-hardening or carbonitriding followed by quenching and tempering are used on mechanical systems stressed by fatigue and/or contact fatigue. Since the end of the 20th century, we have seen a decrease in the weight/power ratio of vehicles to comply with the Kyoto agreements in terms of CO₂ emissions. The reduction in the size of engine transmission parts and the increase in mechanical torques (downsizing) are leading to an increase in operating stresses and a possible rise in system temperatures, mainly for parts subject to contact fatigue. It is therefore necessary to work with heat-treated steels, which will have improved mechanical characteristics and residual stresses that are stable at working temperatures (e.g. 250°C-300°C for contact pressures of 3.5 to 4 GPa).

Faced with this technical direction, dictated by ecological imperatives, new thermochemical treatments have been developed to improve the mechanical characteristics obtained after conventional carburizing or carbonitriding, while remaining economically competitive with long-duration treatments such as nitriding and nitrocarburizing. For these reasons, deep carbonitriding with nitrogen over-enrichment has made its appearance in heat treatment workshops. They can be found under various names such as "deep carbo", "carbo HN" (High Nitrogen), "carbo HD" (High Diffusion)... The aim of this article is firstly to describe the differences between these new thermochemical treatments and standard carburizing or carbonitriding, and secondly to detail the means of regulating these new atmospheres in production.

At the end of the article, readers will find a glossary and table of acronyms, notations and symbols.