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 (1 wt% W) also showed the higher strength and elongation at 900 °C than the basic-composition steel (2 wt% W), which indicated that the partly replacement of W by Cr was successfully achieved.