IGO is a surface phenomenon that is most often associated with atmosphere gas carburizing (Fig. 1). The consequence of IGO (and the concentration gradients that develop during oxide formation) is that the material adjacent to the oxides has modified transformational behavior. Instead of forming martensite on quenching, steels with this condition develop non-martensitic transformation products (e.g. bainite, pearlite), which adversely affect mechanical properties (e.g. hardness, residual stress, bending fatigue)..
The rate of diffusion of oxygen into a steel surface is dependent on the oxygen potential of the furnace atmosphere and the process variables (i.e. the depth of oxide penetration is influenced by case depth, time at carburizing temperature, carbon potential and the chemical composition of the steel). During the carburization process, the oxygen atoms (which are about 35% smaller than the iron atoms) are released as a direct result of the presence of water vapor and carbon dioxide in the gas. Oxygen diffuses slowly into the steel surface (as does carbon and hydrogen, albeit more quickly) and migrates to the grain boundaries. Once in the steel, oxygen combines chemically with the elements already present (e.g. chromium, titanium, manganese) that have an affinity for oxygen.