Effect of Sintering Atmosphere on the Transformation of Surface Oxides during the Sintering of Chromium Alloyed Steel

Journal article, 2010

Chromium is the one of the most attractive alloying elements for PM steels. Nevertheless, its high affinity to oxygen during sintering, especially during the heating stage, has to be considered. The present study relates surface products formation and their transformation to the employed sintering atmosphere. The study consists of the investigation of the type, morphology and distribution of reaction products on the fracture surfaces of specimens pressed and sintered at different temperatures between 700 and 1200°C, for various times, as well as the evaluation of the metal surface – sintering atmosphere interactions in terms of gas composition and temperature. Specimens were sintered in three different atmospheres - nitrogen, nitrogen/10% hydrogen and vacuum. Fractographic investigation of fracture surfaces of specimens sampled at different temperatures, by means of HR SEM combined with EDX analysis, show that the presence of hydrogen has clearly a positive effect on the early reduction of surface iron oxide during the heating stage, which is reflected in lower amounts of oxides present as well as lower carbon loss after sintering. The amount of oxide inclusions is higher after high temperature sintering in the case of vacuum due to pronounced development of inter-particle connections during the heating stage, above 900°C, which led to enclosure of the surface oxide layer inside the necks. A change in the morphology and size of oxide inclusions was observed during the heating stage and after sintering the oxides were mainly encountered as small (<1 μm) spherical inclusions and rarely coalesced into larger agglomerates. At higher temperatures, inclusions are stoichiometric Cr-Mn spinel oxides that have among the highest thermodynamic stabilities for this system. Metallographic study of sintered specimens verifies the extent of the oxide reduction and the carbon dissolution