Austenitisation kinetics of unalloyed and alloyed ductile iron

Abstract

Specimens with different microstructures of an unalloyed iron and a 0.75 wt-%Mn ductile iron were austenitised at 870°C to study the kinetics of austenitisation. The specimens were austenitised in a salt bath furnace for different periods. The wafer specimens were ground, polished and etched before metallography and hardness tests. The thermodynamic procedure was used to calculate the Ae ₃ phase boundary as a function of composition. The results were used to explain the effect of segregation on the kinetics of austenitisation. The results showed that the transformation rate in both the alloyed and the unalloyed pearlitic ductile iron is higher than that of the ferritic iron. As far as the ferritic structure is concerned, at the early stage of transformation the alloyed iron showed a higher austenitisation rate than the unalloyed iron, while after longer transformation the unalloyed iron showed the higher rate. This change in transformation rate is supposedly because of the effect of Mn segregation in the intercellular region. In the case of specimens with a pearlitic microstructure, unalloyed iron has a higher austenitisation rate than alloyed iron because of the effect of Mn on the stability of carbides in the pearlite. It was also shown that in all the specimens the cell boundaries are the most potent sites for nucleation of austenite. The Avrami equation was used to calculate k and n parameters as a function of microstructure. The results obtained indicate that n for specimens with different microstructures varies from 1.5 to 3.5. The results also show that the effect of the initial microstructure on the n and k values is greater than that of the chemical composition of the iron.