Effect of Nb on the tempering kinetics of low carbon steels

The effect of temperature and time on tempering kinetics was studied by using a low carbon steel microalloyed with Nb (0,031) and another steel with a similar chemical composition, however without Nb. After determining the Vickers hardness values, the kinetics law of tempering was studied by the Jonhson-Mehl-Avrami equation using a variation of temperatures and time for both steels. The hardness of the steel without Nb did not change with the tempering time at 600 C and tended to saturate at high tempering times, for the different tempering temperatures used. For the tempering temperatures of 500 and 600 C, the steel with Nb has a greater tempering resistance and this effect is associated to the precipitation of very fine NbC particles dispersed in ferrite matrix. The activation energy of the tempering transformation of the steel without Nb is 130kJ/mol and it could be inferred that the mechanism that controls the tempering kinetics of this steel is the interstitial diffusion of carbon in the ferrite. In the steel with Nb the activation energy is 180kJ/mol and the mechanism that controls the tempering kinetics is the Nb diffusion in the ferrite.