Phase transformation in austempered ductile iron by microjet impact

Specimens of ductile irons austempered at 320 °C and 360 °C for 2 h were conducted with ultrasonic vibration treatment. Microstructures were characterized using optical (OM) and scanning electron microscopy (SEM) and X-ray diffraction. Vickers hardness was also measured to assess the variation of hardness affected by the microstructure. Experimental results show that specimens after cavitation erosion progressively developed phase transformation induced by plastic deformation. Some stringer-type austenite was found to precipitate carbides, while some island-like austenite was transformed into martensite. If the microstructure led to deplete carbon within the island-like austenite, this austenite readily transformed to martensite due to stress induced by microjet impact. If austenite was rich in Mn, the transformation from austenite to martensite would be retarded. The observations then indicate the coexistence of martensite and precipitated ferrite particles within the colony of island-like austenite. With increased cumulative ultrasonic time, microhardness values were enhanced both in areas near graphite and in intercellular regions. In addition, the elastic strain-energy density estimated from a microjet impact was lower than the theoretical value of homogeneous nucleation of martensite nucleus. Clearly, the transformed martensite was induced by heterogeneous nucleation.