Development of a Novel Heat-Resistant Austenitic Cast Steel with an Improved Thermal Fatigue Resistance

In this work, the Thermo-Calc software was used to evaluate the effect of the Nb and Mn additions on the thermal phases stability for a conventional HT heat-resistant austenitic cast steel (HRACS) at high temperature. The simulations show that addition of Nb promoted the decrease in carbides (Fe, Cr)(7)C-3 and Cr23C6 as well as the formation of a stable NbC at elevated temperature. Meanwhile, the Mn addition does not modify the amount of Cr23C6 and (Fe, Cr)(7)C-3 and tends to reduce the (Fe, Cr)(7)C-3 to Cr23C6 transformation temperature. In order to validate the thermodynamic simulations, specimens of an austenitic cast steel modified with 3.3 wt% of Nb were fabricated, characterized and compared with specimens of conventional HT steel after the application of 25, 50 and 100 continuous heating-cooling cycles in non-protected atmosphere. It was found that the Nb causes the reduction of phases that promoted the nucleation of thermal microcracks, which was in good agreement with simulations. In addition, the results indicated that Nb allowed the reduction of the number, length and propagation rate of microcracks. Therefore, this novel HRACS demonstrated to be more resistant to thermal fatigue compared to conventional HT steel in a non-protected atmosphere.

Automated summary

This work used Thermo-Calc software to evaluate the impact of Nb and Mn additions on the thermal phases stability in high-temperature austenitic cast steel. The simulations showed that Nb addition reduced carbide content and formed stable NbC at elevated temperature, while Mn addition lowered the transformation temperature. Experimental validation confirmed that the addition of Nb reduced the formation and propagation of thermal microcracks, resulting in improved thermal fatigue resistance compared to conventional steel.