Thermodynamic Aspects of Powder-Pack Boronizing

During powder-pack boronizing, an alloy is enveloped by a mixture comprising a source of boron, a diluent and an activator. If the activity of B in boriding media exceeds that in alloys, then this difference is a driving force, which, at elevated temperatures, may result in single- or multi-layered protective coatings. If it is intended to model and optimize this diffusion-controlled process, then an ability to calculate and control boron activity in both currently employed and prospective diluent-source-activator blends would be indispensable. In literature, such calculations are seldom performed, which likely reflects a reluctance or trepidation to deal with systems routinely containing five or more components. Consequently, it is commonly yet mistakenly believed that the activity of B can be smoothly changed by gradually changing a fraction of its source. In reality, a multiphase nature of boronizing powders is manifested in intricate concentration dependencies of boron activity with intervals where it remains constant. Another puzzling feature is the constancy of boron activity in phase fields which ostensibly have a non-zero number of degrees of freedom. In this work, a seeming inconformity with Gibbs' phase rule is addressed. Although it is not unreasonable to expect that an equilibrium phase assemblage at high temperatures would differ from ingredients mixed at room temperature, it is instructive to realize how drastically dissimilar they can be in some cases. If a boriding medium is utilized several times (a routine industrial practice), then its evolution caused by multiple heating and cooling cycles should not be overlooked.