Hardened Temper effect after deep carbonitridings with high nitrogen content

High Nitrogen carbonitriding is characterized by :

– a percentage of surface nitrogen greater than 0.4%,

– a depth of enrichment that can exceed 600 µm,

– residual austenite and nitrogen-rich martensite.

This new thermochemical treatment enables us to meet two closely related technical challenges: downsizing to reduce the weight of on-board parts, and increasing the stresses applied. Both of these issues can lead to a rise in system temperature, degrading the mechanical characteristics of products if the temperature reached exceeds that set for tempering. We will see in this article that this treatment can also be used in the case of duplex PVD (physical vapour deposition) or PACVD (plasma-assisted chemical vapour deposition) treatments, where the substrate can be heated above stress-relieving tempering temperatures.

In the introductory article [RE 280] "Deep carbonitrides over-enriched with nitrogen", we noted that the shape of the hardness profile varies significantly as a function of :

– the amount of carbon and nitrogen absorbed,

– addition elements contained in treated steels.

Consequently, the choice of steel, the selection of working atmosphere (carbon potential and nitriding potential) and the depth of grinding are essential criteria for optimizing the fatigue life of mechanical parts using nitrogen-enriched carbonitriding.

The aim of this article on new carbonitrides is to continue the analysis by looking at the metallurgical phenomena that occur when the temperature of a carbonitrided part is raised with more or less absorbed nitrogen: during tempering, during the production of a PVD or PECVD-type coating, or in service, mainly in the case of products subjected to contact fatigue. To illustrate this thinking, it is important to look at the evolution of hardness profiles, residual stresses (estimated by X-ray diffraction (XRD)) and residual austenite content (measured by XRD) for different more or less alloyed, case-hardened or carbonitrided structural steels with different nitriding potentials (Carbo HN and Carbo HN ⁺ with KN _HN < KN _{HN ⁺} ). These results will enable us to gain a better understanding of nitrogen-enriched...