Second phases in steel: a review of the concept, origin, and their relevance for properties

Steels are multiphase alloys with an increasingly complex constitution. This complexity of steel microstructures has been recognized since the birth of steel physical metallurgy. Non-metallic inclusions have also been very early recognized as relevant to the understanding of steel behavior. With the advances in precipitation hardening and grain size control, many precipitate phases gained importance in steel design. Around 1950-70 the term “second phases” was coined as an all-encompassing definition that would cover non-metallic inclusions as well as fine precipitates such as nitrides and carbonitrides even in steels that already had a multi-phase constitution. While this classification may be practical in some cases, we argue that it hinders the proper understanding of the origin and effects of particles in steel and unduly complicates the understanding of the phenomena in which they take part. In this work, we briefly review the origin of the second phase particle concept and discuss the critical properties of particles with respect to their influence on steel behavior. Through several examples, we propose that size and volume fraction are the main variables in evaluating how particles affect steels. While chemical composition is key to understanding the origin of the particles, we suggest that these variables are, together with interface properties, the most relevant to understand the effect of particles on steel behavior.