Influence of the processing route on oxide transformation in austenitic Fe-19Mn-18Cr-C-N PM steel

Abstract

The present work deals with the analysis of oxide state in the Fe-19Mn-18Cr-C-N (X40MnCrN19- 18) powder steel in densified state by surface sensitive analysis techniques. The steel powder was densified by different powder metallurgical manufacturing techniques (hot isostatic pressing (HIP), supersolidus liquid phase sintering (SLPS) and solid state sintering (SSS)). Further the surface composition was studied using X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive X-ray analysis. This was done to gain in- formation about the included oxides in the densified powder mass and their state depending on the different applied consolidation techniques. The results are discussed considering former investigations which analyzed the loose powder. Hence the development of the oxides is related to the initial composition of the powder surface. The results show large differences in consolidation between the different techniques related to the different thermodynamic conditions during consolidation and therefore oxide states of the produced samples. The samples produced by HIP and SLPS show the best densification and in contrast the sintered samples are much more porous even though a vacuum dwelling step during heating stage was applied. Focusing on oxides the outcome is that highly thermodynamically stable mixed Mn-Si-Cr-O- oxides exist in the HIP and SLPS powder masses and that their shape and size depends on the applied consolidation principle. The sintered samples contain a much higher amount of oxides with lower thermodynamic sta- bility. Only the dwelling at 700◦C under technical vacuum showed an improvement of the oxide reduction for solid state sintering and it is concluded that a dwelling temperature should not overstep this temperature. Furthermore it is estimated that HIP and SLPS consolidation overcome the initial state of the powder very well, which is not achieved by solid state sintering.