Nitriding behavior and strengthening mechanism of Ti-added steels in 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 nitrided in electric furnaces. Then the hardness distribution was measured in cross sections, and the sheets were found to strengthen only near the surfaces. The maximum hardness depended on the Ti content, and the thickness of hardened layer depended on the nitriding time and the flow rate of NH 3 . Observation by means of electron microscopy showed contrasts due to fine disc particles with a size of 10nm in diameter and several atomic layers in thickness, which were considered to be Ti nitrides or Ti-N clusters. These contrasts were observed only near the surfaces. This suggests that the hardening is caused by the small particles. A diffusion model of N that considered the precipitation of TiN was used for simulating of the nitriding behavior. The result showed that N introduced into steel immediately precipitated as TiN, then supersaturated N diffused to the inside. The simulation result agrees with the experiment. The amount of strengthening was estimated. It indicated that the strengthening mechanism was mainly the precipitation hardening of Ti nitride or Ti-N clusters.