Analysis of thermal stability of ausferrite obtained by means of cyclic heat treatment of Cu–Ni (Mn, Mo) ADI

Abstract

The objective of this study was to analyse an innovative, cyclic heat treatment used to obtain austempered ductile iron (ADI) in laboratory conditions by means of quenching dilatometer.The relationship between the physical properties of ADI – hardness HV30 and fraction of carbon-enriched austenite Va, considered as parameters forecasting the mechanical properties of ADI – tensile strength Rm and elongation A, was compared with the relationship between two basic dimensional effects revealed in differential dilatometric curves of ausferrite decomposition. The mutual compatibility between these relationships was established. Heat treatment optimization method based on the analysis of differential dilatometric curves of the ausferrite decomposition has been developed.The both approaches enabled selection of the optimal parameters of cyclic austempering and prediction of the mechanical properties which were unattainable by means of standard one-step ADI heat treatment.A kinetic analysis of decomposition after 1-step and cyclic ADI heat treatments was performed. The activation energy values for decomposition of ausferrite obtained by cyclic heat treatment 270 °C ∧∨ 390 °C and 270 °C ∧∨ 350 °C – 156 and 181 kJ/mol, respectively were found. The comparison with the activation energy for decomposition of ausferrite obtained by standard, 1-step heat treatment – 249 kJ/mol (270 °C), 230 kJ/mol (310 °C), 174 kJ/mol (350 °C) and 170 kJ/mol (390 °C) indicates that the cyclic heat treatment does not improve thermal stability of ausferrite.