Effects of Mn and Mo addition on high-temperature tensile properties in high-Ni-containing austenitic cast steels used for turbo-charger application

Since turbo-chargers require more excellent high-temperature properties to maintain their structures at further higher exhaust gas temperatures up to 1050 °C, a 20 wt%-Ni-containing austenitic cast steel (N20 steel) has been suggested as a promising candidate cast steel. However, this steel is very expensive because it contains a large amount of expensive Ni. In order to partly replace expensive Ni by inexpensive Mn and to improve high-temperature tensile properties in the N20 steel, three austenitic cast steels were fabricated by replacing 6 wt% of Ni by 6.9 wt% of Mn or by adding 2-4 wt% of Mo. Thermodynamically calculated fractions of equilibrium phases (austenite, ferrite, and M7C3 carbide) were matched with experimentally measured fractions, although they were somewhat overestimated. The N14 steel where 6 wt% Ni was replaced by 6.9 wt% of Mn did not contain any ferrite, and showed comparable or more excellent high-temperature tensile properties than those of the N20 steel, which indicated the successful replacement up to 6 wt% Ni by Mn, together with alloying cost reduction of 10%. The Mo addition also favorably affected high-temperature properties because Mo worked for increasing both M7C3 fraction and austenite matrix hardness. Simultaneously considering mechanical properties and alloying costs, therefore, these Mo-containing N14 steels can be fully adopted for high-performance turbo-chargers requiring excellent high-temperature properties.