Influence of Gas Atmosphere Oxygen Partial Pressure on the Selective Oxidation during Intercritical Annealing of a 6Mn-2Si Advanced High Strength Steel

Selective oxidation of a 6Mn-2Si advanced high strength steel at an intercritical annealing temperature (IAT) of 665ȗC as a function of IAT holding time and process atmosphere oxygen partial pressure was investigated. Based on the modified Wagner model for the prediction of the transition between external/internal oxidation, the effect of process atmosphere oxygen partial pressure on the alloy selective surface oxidation was assessed by employing dew points (dp) of –50ȗC, –30ȗC and +5ȗC in a N2-5%H2 process atmosphere such that different modes of oxidation could be studied. SEM surface observations of the samples annealed under the –50ȗC dp process atmosphere clearly showed external enrichment of the alloying elements in the form of a coarse-grained oxide film covering the surface, particularly at extended holding times. At the intermediate oxygen partial pressure range (–30ȗC dp), a 200 – 500 nm oxide particles were detected on the surface along with smaller nodule-like particles formed between them. This changed to a rather oxide-free surface after annealing at the +5qC dp, indicative of internal oxidation. However, sparse surface enrichment was revealed at longer holding times at this oxygen partial pressure. Auger elemental maps acquired from the –30qC dp + 600s sample provided a much better understanding of the surface oxides, showing enrichment of Si in some areas of the aforementioned samples.