Prediction of the Formation Region of Eutectic NbC in a Large 12Cr-Mo-V-Nb Steel Ingot

The purpose of this study is to predict the formation region of eutectic NbC in large ingots and to eliminate its detrimental effects. The correlation between the formation region of eutectic NbC and the ratio of the height H to the diameter D (H/D) was investigated experimentally using ingots of 2 t and 10 t and the temperature gradient at the end of solidification in these ingots was calculated by a finite difference method.The plots of the logarithm of the critical temperature gradient for the formation of eutectic NbC which was obtained experimentally against the logarithm of the calculated average cooling rate during solidification yielded a straight line. It was predicted from this result that when casting a 100 t ingot eutectic NbC would not be formed if the value of H/D is less than 0.6 and magnetic particle test indication would not be detected if the value of H/D is less than 1.0. A 100 t ingot having ratio H/D of 0.96 was made and a turbine rotor shaft was produced from this ingot. The center core portion of the rotor shaft had satisfactory mechanical properties and no indication of defects was detected by magnetic particle testing.