The Influence of Initial Microstructure on the Microstructure and Mechanical Properties of Ductile Iron After Nanostructurisation Heat Treatment

In this study, the microstructure and mechanical properties of ductile cast iron subjected to a novel austempering treatment were investigated. The methodology was based on prolonged isothermal annealing within the low-temperature range of the bainitic transformation. The experiments were carried out on samples with two different initial microstructures: pearlitic–ferritic and ausferritic. Two long-term austempering treatment variants were designed and tested: single (AT-1) and double (AT-2). In both cases, the heat treatment led to the production of a nanoausferritic microstructure, characterized by exceptionally thin plates of bainitic ferrite separated by thin layers of retained austenite. Compared to conventional austempering, the new method produced a much finer microstructure and a reduced amount of retained austenite. Moreover, the AT-2 variant further enhanced the homogeneity of the microstructure, promoting a higher fraction of thin ferrite plates. Mechanical tests revealed that the new heat treatment significantly improved the performance parameters of the material: hardness increased from 27 HRC to 32–35 HRC, tensile strength rose from 1027 MPa to 1220–1296 MPa, and yield strength from 683 MPa to 1033–1054 MPa. It was proved that, regardless of the initial microstructure of ADI, new processes exhibited comparable, enhanced mechanical properties, confirming their efficiency and universality.