Surface Oxide Transformation during HIP of Austenitic Fe-19Mn-18Cr-C-N PM steel

Paper in proceeding, 2014

The Fe-19Mn-18Cr-C-N austenitic stainless steel is very attractive material due to its superior strength based on the C+N alloying concept (developed by LWT at the Ruhr-University, Bochum) in a combination with high toughness and ductility and at the same time low price. In conventional casting the low solubility of nitrogen in the liquid limits its alloying content. Manufacturing of such high interstitial steels (HIS) utilizing powder metallurgy route seems to be reasonable as it allows nitrogen alloying of the powder with following preserving its content during HIP process. High contents of Cr and Mn that are characterized by high oxygen affinity, bring significant risk of contamination of powder surface by thermodynamically stable oxide phases during powder manufacturing, handling and further consolidation. Amount, composition and distribution of oxide phases in consolidated material determine its mechanical performance. Hence, characteristics of surface oxide on initial powder surface and mapping of its transformation during consolidation are of vital importance. Advanced surface sensitive analysis techniques like X-ray photoelectron spectroscopy and high resolution electron microscopy combined with EDX analysis were applied to investigate the characteristics of the surface oxide on the base powder and its changes after HIP. Results indicate that the base powder is covered by a heterogeneous surface oxide layer, formed by thin iron oxide layer (~4 nm) with the presence of fine particulates of thermodynamically stable Cr-Mn-based oxides, sizing about 30 nm, that also form larger agglomerates in some sites. Material after HIP at 1150°C and 1000 bar for 4 hours is fully dense, with no visible precipitates in the microstructure and fine grain size (~20 μm). Analysis of the fracture surface by XPS and SEM+EDX indicate low amount of the oxide phases present. Oxide inclusions found were of spherical shape, sizing up to 500 nm, and were mostly Mn-Si-O-oxides.