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 950uC 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 twodimensional growth at later stages. The activation energies measured from Arrhenius plots range between 50 and 115 kJ mol 21 K 21 and between 45 and 53 kJ mol 21 K 21 in alloys A and D respectively.

Investigation on Microstructure of Heat Treated High Manganese Austenitic Cast Iron

“…As for all cast irons, the Mn-Ni-resist microstructure is affected by the manganese content and cooling rate during solidification. Larger manganese content promotes the precipitation of eutectic carbide throughout austenite matrix and expected to reduce martensite start line (Ms) temperature as reported in previous report [24]. The presence of carbide in the matrix microstructure is predicted strongly detrimental to Mn-Niresist mechanical properties and processing.…”

“…Very high drop reading showed from 0.32 % to 0.06 % may result from the contribution of near to transition region line A1 which is in the partial melting region. This lower A1 effect by the presences of manganese as austenite stabilizer also lowered the temperatures A1 [24] and believed to drop near to 1000°C. This discovery supported the previous investigation by Ahmadabadi and Feghahati [34] which reported that a heat treatment involving an austenitising at temperature partial melting was sufficient and higher rate to reduce free carbides present in samples.…”