Effects of Niobium on the Strength and Toughness of Low-Chromium High-Boron Cast Iron

Fe-4.2Cr-2.0B-0.5C-(0~3.0)Nb cast alloys were melted in an induction furnace. The properties and microstructures of the cast alloys were studied. The results showed that the structures consisted of martensitic primary phases and boron carbide network eutectic structures, which were composed of α' -Fe, Fe 2 B, Nb carboboride and other phases. With increasing Nb content, the hardness decreased from 65.8 HRC to 52.2 HRC, the bending strength increased from 1304 to 1731 MPa, and the fracture toughness increased from 29.8 to 32.9 MPa.m 1/2 . The volume percentage of eutectic phases in the alloy decreased from 39 to 28 pct. NbC was distributed in the eutectic structures, which resulted in a broken eutectic network and a decrease in the carbon solid solution in the primary matrix phases. The changes in macrohardness, bending strength and fracture toughness were related to the volume percentage of eutectic phases and the formation of NbC compounds .