MECHANICAL-PROPERTIES OF FE-CR-C-B CAST ALLOYS

Fe-5%Cr-/15%Cr/25%Cr-C-B alloys having hypoeutectic, eutectic and hypereutectic compositions were solidified in three types molds and their compressive strength, hardness and impact energy were examined. Based on the pseudo-binary eutectic composition, the composition of each alloy was decided in the range from hypoeutectic to hypereutectic by changing the carbon and/or boron content. The solidification rate for each mold was 8.3 x 10(-5) approximately 2.5 x 10(-4) m/s in the unidirectional mold, 4.2 x 10(-4) approximately 5.8 x 10(-4) m/s in the sand mold and almost-equal-to 5.0 x 10(-3) m/s in the metal mold. A linear relationship was found to hold between the hardness and the 0.2% proof stress in all the chromium alloy systems. In the hardness range from 50 to 65 HRC, the 0.2% proof stress increased in the order of the 5%, 15% and 25% Cr alloy systems. The effects of boron additions were mainly found on the hardness and the proof stress: particularly 15% and 25%Cr alloys containing boron showed a high hardness value of about 60 HRC and a high proof stress value of about 2000 MPa even in the as-cast condition. This improvement in hardness and proof stress was due to the change of the solidification structure and the bainitic or martensitic transformation of the matrix in the as-cast condition. It was further found that the hypoeutectic and eutectic 15% Cr alloys containing 29% boron have good mechanical properties in the rapid solidification condition for the metal mold. The reason for avoidance of pearlitic transformation due to the boron addition was thought as follows. Boron reduces the eutectic carbon content and decreases the carbon content of the matrix, so that the avoidance of pearlitic transformation occurs to lead to the increase in M(s) temperature. Such a tendency was pronounced especially in the high-chromium alloy system.