Embrittling Components in Sintered Steels: Comparison of Phosphorus and Boron

In ferrous powder metallurgy, both boron and phosphorus have been known to be sintering activators for a long time. However, the use has been widely different: while P is a standard additive to sintered iron and steels, boron has been frequently studied, but its use in practice is very limited. Both additives are also known to be potentially embrittling, though in a different way. In the present study the differences between the effects of both elements are shown: while P activates sintering up to a certain threshold, in part by stabilizing ferrite, in part by forming a transient liquid phase, boron is the classical additive enhancing persistent liquid phase, being virtually insoluble in the iron matrix. The consequence is that sintered steels can tolerate quite a proportion of phosphorus, depending on composition and sintering process; boron however is strongly embrittling in particular in combination with carbon, which requires establishing a precisely defined content that enhances sintering but is not yet embrittling. The fracture mode of embrittled materials is also different: while with Fe-P the classical intergranular fracture is observed, with boron a much more rugged fracture surface appears, indicating some failure through the eutectic interparticle network but mostly transgranular cleavage. If carbon is added, in both cases transgranular cleavage dominates even in the severely embrittled specimens, indicating that no more the grain boundaries and sintering necks are the weakest links in the systems.