Kinetics of austenitisation of ductile irons containing two different contents of manganese and copper

Abstract

This work deals with the study of austenitisation behaviour of two ductile irons: alloy A [3·18C–2·64Si–0·45Mn (wt-%)] and alloy D [3·18C–3·0Si–1·04Mn–1·13Cu (wt-%)]. Samples were austenitised at 850, 900 and 950°C with varying times and then quenched in water. Following special etching techniques, the possible nucleation sites for austenitisation (transformed to martensite on quenching) have been identified as the ferrite/cementite interfaces within the pearlite matrix as well as the pearlite colony boundaries. Austenitisation also occurs in an epitaxial manner beyond the ferrite rims already existing around the graphite nodules. The kinetics of austenitisation is analysed from the volume fraction of austenite/martensite formed after known intervals of time at constant temperature using Avrami type relationship. In alloy A, the beginning of austenitisation is characterised by a dimensionality of <1, followed by an unidirectional growth in the later stages. Austenitisation in alloy D conforms to a dimensionality of 1 followed by two-dimensional growth at later stages. The activation energies measured from Arrhenius plots range between 50 and 115 kJ mol⁻¹ K⁻¹ and between 45 and 53 kJ mol⁻¹ K⁻¹ in alloys A and D respectively.

Keywords:

• Ductile iron
• Austenite
• Martensite
• Austenitisation kinetics
• Dimensionality
• Activation energy
• Diffusion

The financial support by National Institute of Technology, Durgapur, India is gratefully acknowledged.