Solubility model to predict effects of aluminium and nitrogen contents on susceptibility of steel castings to intergranular embrittlement

Abstract

The precipitation of aluminium nitride at the as-cast grain boundaries of low-ralloy steel castings can be a major cause of intergranular embrittlement. A model based on the solubility of AIN in austenite is used to examine the relative effects of the aluminium and nitrogen contents on the amount of AIN available for grainboundary precipitation. Since aluminium is usually added beyond the level required for stoichiometric combination with nitrogen, it is demonstrated that variations in the aluminium content do not significantly alter the amount of grainboundary AIN formed. The most effective method of reducing the AIN levels is shown to be by close control of the nitrogen content. The sources of Al and N in steel castings are considered and the effects of precipitation temperature, deoxidation practice, and macro- and microsegregation on the susceptibility to intergranular fracture are also discussed.