LOW-TEMPERATURE ION-NITRIDING OF FE-MO-C SINTERED STEELS

The nitridability at low temperatures (723-773 K) of Fe-Mo-C sintered steels has been studied: the possibility to realize hardened surface layers of appropriate characteristics has been examined also to reduce surface microstructure alterations (e.g. decarburization) and size and shape variations which may occur for thermochemical treatments at higher temperatures. In a semi-industrial ion-nitriding plant Fe-Mo 1.5-C 0.3 (with martensitic matrix) and Fe-Mo 1.5-C 0.5 (with bainitic matrix) sintered steels have been ion-nitrided at temperatures included in the range 723 divided by 823 K and for treatment times of 4 divided by 24 h. As previously noticed, the first sintered steel, if ion-nitrided at 823 K for more than 8 h, presents a surface hardness of about 700 HV but also a sharp hardness fall (down to 450 HV) immediately beyond the compound layer owing to decarburization phenomena. Reducing the treatment temperature (773 K) this effect does not appear: the nitrided layer shows a regular hardness profile, with maximum hardness value of 720 HV and thickness of about 100 mu m, and consists essentially of the Fe4N gamma' cubic nitride and Fe-2-3(C,N) epsilon hexagonal carbonitride. At the same time, the ion-nitriding treatment in the temperature range 723 divided by 823 K has been applied to the second sintered steel. In this case, no decarburization has been observed also for treatments at the highest temperature; the hardness profiles show a gradual decrease from 730 HV to the matrix value (350 HV), but the hardness depth results very limited also for treatment times of 24 h.