Effects of Silicon in High-Cr White Cast Irons

To improve their performance, this research characterized the effects of Si on various attributes of high-Cr white cast irons. Hypoeutectic alloys of 15 and 25% Cr white irons were produced with Si levels varying of 0.4, 1 and 1.6%Si and poured into 3-inch (75-mm) Y-block sand molds. Various tests were used to determine the optimum hardening temperature and the kinetics of pearlite transformation. In addition, image analysis was performed to determine eutectic carbide fraction. Furthermore, energy-dispersive spectroscopy analysis was conducted to determine the distributions of the alloying elements among the various solidification constituents. Raising Cr from 15 to 25% caused the optimum hardening temperature to rise from 1680 to 1920 °F (915 to 1050 °C). In the 15% Cr series, an increase of 1.2% Si raised the optimum hardening temperature by 80 °F (50 °C), whereas, for the 25% Cr series, the optimum hardening temperature was unchanged. Peak hardness was significantly higher (by 50 HB) in the leaner Cr alloys and the peak hardness in both the 15% and 25% Cr alloys decreased slightly (by 20 HB) when the Si content was increased from 0.4 to 1.6%. However, the hardenability (the time to the onset of pearlite transformation) increased with increasing Si content. The influence of Si on % eutectic saturation was not clear. When analyzing carbide fraction, Si had a negligible effect in the 15%Cr series, but seemed to cause the 25%Cr series to develop less eutectic carbide, that is, become more hypoeutectic. Cr was richest in the eutectic carbides and leanest in the eutectic austenite matrix. Si was largely rejected from the eutectic carbides and richest in the eutectic austenite.