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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 683; pp. 195 - 206
• Main Authors: Zhang, Y.H., Li, M., Godlewski, L.A., Zindel, J.W., Feng, Q.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 23-01-2017; Elsevier BV
• Subjects: Alloy development; Alloy steels; Austenitic cast steel; Austenitic stainless steel; Austenitic stainless steels; Automotive engines; Automotive parts; CALPHAD; Carbonitride; Casting; Computer simulation; Delta ferrite; Exhaust gases; Ferrite; Fuel economy; Heat resistance; Heat resistant steels; Mathematical analysis; Mathematical models; Mechanical properties; Microstructure; Morphology; Phase diagrams; Phase transformations; Phase transitions; Structural steels; Studies; Tensile properties; Tensile property; Carbonitride; CALPHAD; Ferrite; Austenitic cast steel; Tensile property
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2016.12.023

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.