Multi Response Modelling and Optimisation of Austempering Parameters on ADI Microstructure, Hardness and Fatigue Strength

Nikša Čatipović, Ivan Peko, Jure Krolo, Jelena Čulić-Viskota, Andrej Razumić

DOI: https://doi.org/10.64486/m.65.2.4
Online publication date: November 6, 2025

Abstract: In this work, the influence of austempering temperature and time on the microstructure, hardness and fatigue strength of austempered ductile iron (ADI) is investigated. The experimental part was carried out on a total of eleven samples, nine of which were subjected to heat treatment at three temperatures (250 °C, 350 °C and 450 °C) and three time intervals (30 min, 75 min and 120 min). The tests included measurements of the fatigue strength, Vickers hardness and a microstructural analysis. The aim was to determine the optimum austempering parameters to achieve an ausferritic microstructure and improved mechanical properties. The results showed that austempering temperature had a significant effect on the properties of the treated samples, while austempering time did not show consistent statistical significance, although some samples deviated positively from the statistical model. The best result was obtained with the sample austempered at 350 °C for 120 minutes. Metallographic analysis confirmed the presence of an ausferritic microstructure, which contributed to the improved mechanical properties. The research highlights the importance of properly selecting austempering parameters to achieve an optimal combination of hardness and fatigue strength. Measured values were used to develop mathematical models which can be applied to predict analysed properties of ADI depending on austempering parameters.

Keywords: ADI; austempering parameters; hardness; fatigue; microstructural analysis; modelling

This article is published online first and will appear in Metalurgija, Vol. 65, Issue 2 (2026).