Effects of V or Cu Addition on High-Temperature Tensile Properties of High-Ni-Containing Austenitic Cast Steels Used for High-Performance Turbo-Charger Housings

For maintaining high-performance turbo-charger housings at higher exhaust-gas temperatures, e.g., 1050 °C, a high-Ni(20 wt%)-containing austenitic steel (ASTM HK40 steel) has been actively modified. As a promising method, the solid-solution hardening obtained from the V or Cu addition was utilized in this study. Five austenitic cast steels were made by adding V or Cu in the modified HK40 steel (N16 steel; 4%-Ni replacement by 4.6%-Mn), and the high-temperature property improvement was explained by detailed microstructural evolutions coupled with thermodynamically calculated phase diagrams. The V- or Cu-added steels showed improvements in high-temperature properties over the N16 steel because the addition of V or Cu raised both austenite matrix hardness and volume fraction of M₇C₃ carbide. Considering that only 3.6–4.8 vol% of M₇C₃ was present in the five austenitic cast steels, the strengths were affected more by the matrix hardness. When the V or Cu content was quite high, however, the ferrite or Cu-coring is formed, thereby leading to the serious deterioration of high-temperature properties. The V- or Cu-added steels within proper contents are applicable well to automotive turbo-charger housings requiring or emphasizing high-temperature tensile properties.