Structure and Hardness of Martensite in Quenched Fe-C Steels

The exceptional high hardness of lath martensite in quenched Fe-C steels is explained by the Engel-Brewer valence electron theory for crystal structures. The theory predicts the transformation sequence FCC-HCP-BCC with FCC iron as Fe-3v, HCP iron as Fe-2v, BCC iron as Fe-Iv and carbon as C-4v. Electronic compatibility requires transformation from FCC to HCP to form two separate components. Carbon-rich clusters of C-4v with 8 Fe-3v atoms are distributed uniformly in a carbon-free matrix of HCP Fe-2v atoms. The carbon-iron clusters are viewed as particle-like, calculated as 0.63 nm in size, and is responsible for the high strength of martensite. The carbon-free region experiences shear deformation during FCC to HCP transformation leading to work hardened fine grains Subsequent transformation to BCC iron maintains the same size carbon cluster with additional shearing deformation during HCP to BCC formation in the carbon-free region Tempering studies of quenched martensite are shown to support the carbon-iron cluster model.