Austempered ductile iron exhibits excellent mechanical properties and wear resistance. The iron obtained by conventional austempering process, however, contains a large amount of retained austenite. The poor machinability due to strain-induced transformation of the retained austenite has made it difficult to extend the commercial application. When the desired quality requirements are primarily wear resistance and static and fatigue strengths, the reducing of the retained austenite might be applied. An austempering process has been investigated to improve the machinability and secure the required mechanical properties of the iron by the reducing the amount of the retained austenite. Though the retained austenite is significantly effective to improve elongation and toughness, the perfect bainitic transformation of austenite is the most effective to improve the machinability.
As a two-stepped austempering process is used usually to improve the cooling rate for heavy parts, the special two-stepped austempering was developed. In this austempering, the desired hardness is secured in the first step to improve the wear resistance and strength, and then in the second step the retained austenite is transformed to bainite perfectly to improve the machinability in a short time.
The austempered iron exhibits the excellent static and fatigue strength similar to those of the conventional ADI containing the retained austenite. Although the elongation and impact strength are somewhat inferior to those of the conventional ADI, the desired properties are obtained. The iron, even the high hardness of Hv 300, shows more excellent machinability than FCD70 of the same hardness and has an excellent machinability equivalent to FCD70 of low hardness.
