Relationship among process parameters, microstructure, and mechanical properties of austempered ductile iron (ADI)

The study investigated the relationship among process parameters, microstructure, and mechanical properties of austempered ductile cast iron (ADI). ADI was composed of spheroidal graphite, acicular ferrite, and high-carbon austenite. Austempering process parameters affected the microstructure including the ferrite size, the volume fraction of austenite and its carbon concentration, and resulted in varying degrees of mechanical response. High carbon concentration in prior austenite obtained at high austenitizing temperature improved its strength, resulting in the formation of thin acicular ferrite. The high-carbon austenite and acicular ferrites of ADI were coarsened under increased austempering temperature. The austempering time affected the microstructure significantly at durations less than 60 min but affected little when the holding time was further extended. ADI exhibited a strong combination of mechanical properties with an ultimate tensile strength (UTS) of 1348 MPa and impact energy of 70 J when austenitized at 900 °C for 90 min and austempered at 300 °C for 60 min. High strength was mainly attributed to fine acicular ferrite, and the robustness was related to fine microstructure with appropriate levels of high-carbon austenite. • The relationship among austempering parameters, microstructure, and mechanical properties of ADI were investigated. • The strength of ADI is first increased and then decreased with the increase of austenitizing temperature. • The strength of ADI is decreased, while the ductility is increased with increasing austempering temperature. • Impact toughness of ADI was proportional to strength-ductility balance.