Carbide Formation and Matrix Strengthening by Nb Addition in Austenitic Stainless Cast Steels Used for Turbo-Charger-Housing Materials

In order to increase the exhaust-gas temperature of austenitic stainless cast steels used for turbo-charger-housings up to 1050 degrees C, the more excellent high-temperature strength should be obtained because it works for keeping a sustainability of the housing shape. Three austenitic stainless cast steels were fabricated in this study by adding Nb into the Ni/Mn-replaced conventional ASTM HK40 steel, and their high-temperature strength was investigated in relation with the carbide formation and matrix strengthening. The 1-wt%-Nb-added steel showed the enhancement in high-temperature strength over the non-Nb-added steel. This indicated that the Nb addition was effective on the strength improvement because it raised both the fraction of thermally-stable hard MC-type carbides and the austenite-matrix hardness. However, the 1.5-wt%-Nb-added steel showed the lower high-temperature strength than the 1-wt%-Nb-added steel, although it contained the more MC carbides. The more MC carbides reduced the matrix hardness, enlarged the hardness difference between carbides and matrix, and deteriorated the high-temperature strength. Our results demonstrate that the strengthened matrix as well as populated carbides due to the Nb addition are desirable for further improving the high-temperature strength.