Effects of Cr and Nb addition on high-temperature tensile properties in austenitic cast steels used for turbo-charger application

In automotive turbo-chargers, excellent high-temperature properties are needed for retaining their structures at extremely high temperature of exhausted gas, and thus austenitic cast steels have been actively developed. In this study, high-temperature tensile properties of austenitic cast steels containing different W, Nb, and Cr were interpreted by deformation and fracture mechanisms related with carbides and austenite matrix. Replacement effects of W by Nb or Cr were also investigated because of expensive alloying cost of W. The high-temperature tensile test results revealed that the Cr-added steel containing many M7C3 carbides showed the higher strength than the Nb-added steel containing hard MC carbides. This was because the strength could be enhanced by distributing carbides having slightly lower hardness such as M7C3 carbides in the strengthened matrix as it could hold hard carbides, like in the Cr-added steel. The Cr-added steel (1wt% W) also showed the higher strength and elongation at 900 °C than the basic-composition steel (2wt% W), which indicated that the partly replacement of W by Cr was successfully achieved.