Thermal Processing of the Injection Moulded M2 High Speed Steel (M2 HSS) Investigated by Positron Annihilation Spectroscopy

摘要

In order to optimise the sintering process of high speed steel (M2) samples produced by powder injection moulding, we have applied positron annihilation Spectroscopy to investigate the binder and debinding effect on the densification and microstructural characteristics induced by sintering. Positron lifetime experiments have been performed on green and sintered samples prepared from three different types of feedstock. The injection moulded samples are debound following three different extraction methods: thermal, solvent and wick debinding, and subsequently sintered in the temperature range 1493 - 1623 K. The positron lifetime spectra from the sintered material reveal different microstructural characteristics depending on the feedstock and debinding method. It is found that the mean positron lifetime and the density of the sintered samples are correlated with sintering temperature. The highest densities are attained when only one type of effective positron trap is present in the sintered samples. This occurs after sintering at 1623 K. Injection moulded samples from feedstock free of stearic acid, debound by a thermal treatment and subsequently sintered for 60 min at 1623 K yield a fractional density of 0.99 and a defect microstructure characterised by the presence of a single type of positron traps. The positron lifetime for these traps is significantly shorter than the one found in samples obtained from feedstock prepared adding stearic acid. This indicates that the addition of stearic acid produces some effect on the microstructural defects generated by sintering at 1623 K.