Effect of salt bath temperature on microstructure and mechanical properties of an austempered alloyed cast iron

An alloyed cast iron was prepared by horizontal continuous casting. To study the salt bath temperature on microstructure and mechanical properties, the alloyed cast iron was firstly austenitized at 950 °C for 3 h and then austempered in salt bath at various temperatures (250 °C, 300 °C and 350 °C) for another 3 h. The scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and X-ray diffraction (XRD) were employed to observe the microstructure and test the mechanical properties of the alloyed cast iron. Results show that the microstructure of the alloyed cast iron is mainly composed of acicular or feathery ferrite (bainite), retained austenite, carbide, and graphite. When austempered in salt bath at 250 °C, 300 °C and 350 °C for 3 h, the volume fractions of retained austenite are 33.1%, 41.7%, and 57.2%, the thickness of acicular ferrite are 0.25 µm, 0.3 µm, and 0.8 µm. As the salt bath austempering temperature increases, the mechanical properties decrease due to the increase of the volume fraction of retained austenite and the thickness of acicular ferrite. The highest tensile strength of the alloyed cast iron is achieved when it is austempered at 250 °C in a salt bath. Under these conditions, the tensile strength of the alloyed cast iron can reach 1,429 MPa. Tensile test results indicate that the fracture mechanism is predominantly brittle fracture.