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

Abstract

Since turbo-chargers require more excellent high-temperature properties to maintain their structures at further higher exhaust gas temperatures up to 1050 degrees 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.