Effect of Prior Martensite on Mechanical Properties of Austempered Ductile Iron

An unalloyed ductile iron, which incorporates C and Si as major and Mn as minor alloying elements, is processed by a novel austempering process, in order to obtain superior mechanical properties. The samples are initially austenitized at 890°C for 20 min, then quenched into patented water-based quenching liquid at 180°C for 0.5, 2 and 3.5 s respectively, and austempered at 220°C for 240 min in an electric furnace. Optical microscopy (OM) and scanning electron microscopy (SEM) are performed to correlate the mechanical properties with microstructural characteristics. It is found that partial martensite can be formed firstly upon quenching, which will accelerate the subsequent bainitic transformation and promote refinement of multiphase colonies during austempering. The prior martensite content increases with increasing holding time during quenching. A tensile strength of 1330MPa, an elongation of 3.13% and a hardness of 45HRC can be achieved by controlling the prior martensite content to 12%. SEM of fracture surfaces reveals a mixed ductile and cleavage rupture morphology type in all samples. The results indicate that the tensile behavior of the investigated ADI is mainly influenced by the content of prior martensite.