Intergranular embrittlement of iron-carbon alloys by impurities

It is shown that plain Fe-C alloys can be embrittled by certain heat treatments if they are doped with Sb, Sn, As or P. Embrittlement occurs when these elements are rejected from precipitating carbides during cooling, and it results from a piling-up of the elements ahead of the carbide and the concomitant lowering of cohesion along the carbide-ferrite interface. It is a transient (non-equilibrium) condition and disappears upon continued holding at elevated temperatures. Embrittlement by equilibrium segregation of these impurities in unalloyed ferrite apparently does not occur. A model for this transient embrittlement is proposed and tested. This model is relevant to the phenomenon of “500°F” embrittlement in alloy steels, and it is also consistent with most aspects of temper embrittlement.