Fracture toughness of a high carbon and high silicon steel

A high carbon and high silicon (HCHS) steel containing about 1% carbon and 2.5% silicon has been developed. This steel has been synthesized using concepts from austempered ductile cast iron (ADI) technology. The influence of austempering temperature on the microstructure and the mechanical properties of this steel in room temperature and ambient atmosphere was examined. The influence of microstructure on the plane strain fracture toughness of this HCHS steel was also investigated. Compact tension and round cylindrical specimens were prepared from this steel. These specimens were then austenitized at 927°C for 2 h and then austempered at several temperatures between 260 and 399°C for a fixed time period of 2 h to produce different microstructures. The microstructures were characterized by X-ray diffraction and optical metallography and correlated to the mechanical properties. The test results showed that the maximum fracture toughness is obtained in this steel with a upper bainitic microstructure when the microstructure contains about 35% austenite and the carbon content in the austenite is about 2%. The retained austenite and its carbon contents increased with austempering temperature, reaching a peak value at 385°C and then retained austenite decreased with increasing temperature. The carbon content of the austenite also showed a similar behavior. The fracture toughness was found to depend on the parameter (XγCγ/d)1/2 where Xγ is the volume fraction of the austenite, Cγ is the carbon content of austenite and d is the mean free path of dislocation motion in ferrite.