Effective design of new austenitic cast steels for ultra-high temperature automotive exhaust components through combined CALPHAD and experimental approaches

Designing engines with higher power densities to comply with ever increasing emissions and fuel economy regulations requires the development of cost-effective alloys with superior properties than existing high-performance alloys. A new class of Nb-bearing austenitic heat-resistant cast steels showed promise to withstand exhaust gas temperatures of as high as 1000°C. This paper describes the development of this new class of alloys using combined CALPHAD (CALculation of PHAse Diagrams) and experimental approaches to investigate the effect of C and N additions on the microstructure and tensile properties. Composition ranges were first established based on CALPHAD predictions to achieve the desired phases, and then four alloys of varying N/C ratios were cast and experimentally characterized to determine quantitatively the microstructure and mechanical properties. Microstructural characterization revealed that the NbC/Nb(C,N) phase transformed from “Chinese-script”, a mixed flake-blocky morphology to faceted-blocky morphology as the N/C ratio increased. The area fractions of the different phases were quantified and agreed favorably with the calculated results. Alloys with the “Chinese-script” NbC/Nb(C,N) showed favorable tensile properties at room temperature and 1000°C. Excessive δ-ferrite diminished the tensile resistance at 1000°C, while limited quantity of (Cr, Fe)23C6 did not show adverse effects.