Microstructures and Properties of Bainitic Nodular Cast Iron Treated by Controlled Cooling and Austempering Treatment

The mixed structures of bainite matrix, austenite and nodular graphite were obtained in nodular cast iron treated by controlled cooling and austempering in electric oven instead of nitrate salt bath. The relations between bainitic morphology and mechanical properties were investigated. Experimental results show that, the nodular cast iron treated by controlled cooling in water bath and austempering has microstructures of upper bainite and austenite, the mechanical properties fluctuate greatly due to its relatively narrow temperature region in processing. The consistency of mechanical properties of lower bainite nodular cast iron is superior to upper bainite nodular cast iron. The comprehensive mechanical properties of lower bainite nodular cast iron are improved by tempering. However, the brittleness increases greatly when martensite appears in mixed structures, while good mechanical properties can still be obtained by tempering at elevated temperatures.