During gas carburizing of steel, there is a low partial pressure of oxygen. Under these conditions, alloying elements with a strong thermodynamic affinity for oxygen, such as silicon, chromium, and manganese, tend to undergo selective oxidation that propagates along grain boundaries. This results in the inward growth of oxide phases from the surface into the steel substrate, often referred to as intergranular oxidation (IGO) (Ref. 1). This leads to localized depletion of alloying elements in the vicinity of the oxides formed, with a resultant reduction in the hardenability in these regions. When the steel is quenched, this reduction in hardenability increases the likelihood of forming non-martensitic microstructures in the affected areas, see Figure 1.

Furthermore, the reduced hardenability near the surface means that the transformation of the surface layer occurs earlier than in the underlying case during quenching. As the deeper regions of the case subsequently transform martensitically, accompanied by volumetric expansion, tensile residual stresses are introduced at the surface.

The net effect of the oxide and the low-strength surface structure, in combination with tensile residual stresses, is to adversely affect fatigue performance and increase susceptibility to crack initiation.