MICROSTRUCTURE EVOLUTION AND PHASE TRANSFORMATIONS IN MICROALLOYED ARMOX 500T STEEL DURING A DILATATION PROCESS

Abstract

With this work we successfully developed two modified Armox 500T alloys using microalloying with different amounts of niobium (Nb) and boron (B) to obtain a finer grain size, which in return enhances the remnant properties. Furthermore, different heat-treatment cycles were designed and performed using a quenching dilatometer to study the combined effect of thermal cycling and microalloying with Nb and B on the microstructure and transformation temperatures including the start and finish temperature of the austenite transformation (Ac1, Ac3) and the martensite start and finish temperature (Ms, Mf) of the investigated alloys. Dilatometry results show that increasing the content of Nb from 0.07 w/% to 0.13 w/% and B from 0.0035 to 0.0046 w/% increases the temperature range between Ac1 and Ac3 by 55 °C, indicating a broader range for changing heat-treatment temperatures. In addition, the Ms temperature is reduced by 13 °C due to austenite refinement caused by the microalloying of Nb and B. The effect of the annealing temperature at a constant heating rate showed a significant impact on the austenite grain size and hardness. Furthermore, the kinetics of phase transformations were theoretically studied using Thermo-Calc, and the numerical predictions were confirmed experimentally with dilatometry results. Metallography investigations using a scanning electron microscope (SEM) and an optical microscope (OM) were conducted to evaluate the microstructure evolution of the developed alloys. Hardness tests were performed to evaluate the effect of the grain refinement of martensite lathes caused by microalloying with Nb, B, and heat-treatment thermal cycling. It is found that the hardness of the modified Armox alloys in this research was improved by 14 % in comparison with the conventional Armox 500T.