Nitriding behavior and strengthening mechanism of Ti added steels in a rapid nitriding process

The nitriding process is one of the common methods for surface hardening, and consists of heat treatment in a furnace for many hours. The nitriding behavior and strengthening mechanism of Ti added steels in the nitriding process, which is applicable to a high temperature and rapid process such as the continuous annealing of steel strip, were investigated. Cold rolled Ti added steel sheets were annealed for recrystallization and nitriding in electric furnaces. Then the hardness distributions in the cross section were measured. The sheets were hardened only near the surface. The maximum hardness depended on the Ti content, and the thickness of hardened layer depended on the nitriding time and the flow rate of NH₃. The optimum temperature was 750°C in this experiment. Observation by means of electron microscopy showed contrasts due to fine particles with a size of 1 to 2 nm which were considered to be Ti nitrides or Ti-N clusters. These contrasts were observed only near the surface. This suggests that the hardening is caused by the small particles. The diffusion model of N that considered the precipitation of TiN was used for the simulation of the nitriding behavior. The result showed that N entering into steel immediately precipitates as TiN, then supersaturated N diffuses to the inside. The simulation result agrees with the experiment. The estimation of the amount of strengthening was carried out. It indicated that the strengthening mechanism is mainly the precipitation hardening of TiN that could be Ti nitrides or Ti-N clusters.