Martensite in quenched Fe–C steels and Engel–Brewer electron theory of crystal structures

Abstract

The Engel–Brewer (E–B) electron theory of crystal structures has been used to develop a model of lath martensite formation in quenched Fe–C steels. Lath martensite is described as a cluster of eight iron atoms surrounding each carbon atom in a matrix of fine subgrained body centred cubic iron. The E–B theory is used to describe the valence state of the cluster and the matrix of iron. Three sequential transformations take place during quenching, involving face centred cubic austenite with valence of 3, hexagonal close packed hexagonite with valence of 2 and body centred cubic ferrite with valence of 1. The E–B theory predicts the maximum solubility of carbon in hexagonal close packed iron, i.e. hexagonite, to be 2·625 at-C, supporting the phenomenological value of 2·75 at-C (0·6 wt-C). The electron binding energies associated with the creation of hexagonite are consistent with experimental observations. The E–B theory also predicts the volume change from austenite to hexagonite and from hexagonite to ferrite that is in agreement with the values obtained from pressure–temperature studies. The exceptional hardness of lath martensite is attributed to the presence of iron–carbon clusters and subgrains and is proportional to its specific volume.