A study of internal oxidation in carburising steels

The phenomena known as `internal oxidation' can play a major detrimental role in the failure of carburised components such as bearings and gears. Internal oxidation leads to a degradation of the surface layer often leading to surface break up and fatigue. This work is concerned with a detailed understanding of the formation of internal oxidation leading to modifications to composition or process parameters to eliminate or reduce internal oxidation.

Experimental steels for the most part have been used in this study with Si content varying from 0.11 to 0.77 mass percent. A commercial carburising process at David Brown Heatch Ltd. consisting of a number of process stages with varying C potential and treatment temperature has been used in the study.

Scanning Electron Microscopy (SEM) , Cross sectional Transmission Electron Microscopy (TEM), Energy Dispereive Spectroscopy (EDS) and Electron Probe Microanalysis (EPMA), Glow Discharge Optical Emission Spectroscopy (GDOES), Quantitative Image Analysis techniques were applied to characterise the specimens.

A model has been proposed that explains the formation of the characteristic morphology observed in internal oxidation and is based upon competitive processes concerning the diffusion rate of alloy species within the bulk material and the differences in the free energy of formation of the various oxides types. The observed morphology of the internal oxidation zone at different carburised exposure time, in range of 0.25-16.6h, related to the penetration depth and density of the internal oxides in the internal oxidation zone, with particular emphasis on the relative importance of oxygen partial pressure at reaction and free energy of formation of oxides have been studied. The research indicated that the internal oxides grew fast in the base process. Three elements Cr, Mn and Si were oxidised at this stage and formed complex oxides within the grains and on the grain boundaries. Further, as carburising time increased, existing oxides grew and new oxides nucleated again along grain boundaries. In the boost process, only Si was oxidised. Si oxides penetrated to a greater depth along the grain boundaries. The generally two-zone morphology characteristic was found in the internal oxidation zone of carburised steel. Outer zone: larger size complex oxides which contain higher concentrations of Cr, Mn and some Si on the grain boundaries or within the grain; Inner zone: intergranular Si oxides on the grain boundaries. Small dispersed oxide paticles were observed in both zones.

Different oxides were formed in the internal oxidation zone as complex oxides, sometime as agglomerated oxide phases, and intergranular oxides. These complex oxides were identified as Cr1.5Mnj.5 O4C, rMnO4, Mn2SiO4 and MnSiO3. The intergranular oxidation was mainly Si oxides, such as Si02. The agglomerated oxide phases were observed usually as the Cr-Mn complex oxides with Si oxides or Mn-Si complex oxides growing around them.

The role of Si is critical in that its solid state diffusion coefficient in Fe is considerably higher than that for Mn and Cr whilst the free energy of formation of Si oxide is lower than that for Mn and Cr. It affects internal oxidation, not only on the morphology but also the rate of penetration. For the specimen with low Si bulk content, the internal oxidation zone consisted of larger complex oxides elongation close to the surface, with intergranular oxidation remote from surface. The penetration depth of the internal oxides increased with increasing bulk Si content. In this case, oxygen diffused not only through the metal lattice but also at an enhance rate along the internal oxides/metal matrix interface. There was a peak value, after which as Si increased in the bulk metal, more intergranular oxidation was formed instead of the larger oxides. A continuous layer was formed parallel to the surface that reduced further diffusion of oxygen. The penetration depth of the internal oxides decreased with further increases of bulk Si content. The higher the carbon potential, the lower was the penetration depth of the internal oxides and the less dense the internal oxidation zone.