Grain-boundary oxidation is a phenomenon that is caused by the diffusion of oxygen into the steel surface. In order for the oxidation procedure to commence, oxygen (in some forme) needs to be the present. For example, if the process is that of pack carburizing or atmosphere carburizing (using and endothermic generator), there will be oxygen present from air, atmospheric moisture or both. There can be oxygen in the form of surface oxide (hot-rolled scale) if the bar has not been fully machined.
Oxygen has a smaller atomic size, which is about 30% smaller than that of an iron atom. Thus, when atmosphere carburizing at temperatures around 1700°F and for extended periods of time, the oxygen atom can diffuse into the steel surface and begin the oxidation around the surface grain boundaries. As temperature is increased from ambient temperature up to the selected carburizing temperature, the speed of the oxygen diffusion will increase.
Another factor that will affect the amount of oxygen diffusion will be the carburizing-atmosphere carbon potential as well as the grain size. Remember that as the steel temperature is increased, the steel grain size will also increase.
Therefore, the degree of surface oxygen migration into the steel will be dependent on: temperature, time at temperature, presence of oxygen in the form of carbon dioxide, atmosphere carbon potential, alloying elements present in the steel (chemical composition of steel) and grain growth during the atmosphere/pack carb process cycle.
The above comments are based on the use of an endothermically generated atmosphere with a hydrocarbon enrichment gas, as well as with pack carburizing systems. We will finish this discussion next time.