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 conc epts from austempered ductile cast iron (ADI) technology. The influence of austempering temperature on the microstructure and the mechanical propertie s of this steel in room temperature and ambient atmosphere was examined. Th e influence of microstructure on the plane strain fracture toughness of thi s HCHS steel was also investigated. Compact tension and round cylindrical s pecimens were prepared from this steel. These specimens were then austeniti zed at 927 degreesC for 2 h and then austempered at several temperatures be tween 260 and 399 degreesC for a fixed time period of 2 h to produce differ ent microstructures. The microstructures were characterized by X-ray diffra ction and optical metallography and correlated to the mechanical properties . The test results showed that the maximum fracture toughness is obtained i n this steel with a upper bainitic microstructure when the microstructure c ontains about 35% austenite and the carbon content in the austenite is abou t 2%. The retained austenite and its carbon contents increased with austemp ering temperature, reaching a peak value at 385 degreesC 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 (XgammaCgamma/d)(1/2) where X-gamma is the volu me fraction of the austenite, C-gamma is the carbon content of austenite an d d is the mean free path of dislocation motion in ferrite. (C) 2001 Elsevi er Science B.V. All rights reserved.