LATH MARTENSITES IN LOW CARBON STEELS

Abstract
The morphology and crystallography of lath martensite in low and medium carbon steels have been studied by transmission electron microscopy and diffraction. The steels have microduplex structures of dislocated lath martensite (a < b << c) with fairly straight boundaries and continuous interlath thin films of retained austenite. Stacks of laths (i.e., single crystals of martensite) form the packets which are derived from different {111} transformation variants of austenite. Microdiffraction experiments directly allow the determination of the orientation relationships between austenite and martensite. Relative orientations of adjacent individual laths cluster about common orientations from small to large angular differences all around a common <110>M direction. The overall microstructure and orientations result from minimization of the total strain and shape de formation. Considerable accommodation occurs by de formation of laths (sometimes twinned) and austenite (sometimes tripped to twin martensite). In the meantime, microchemical analyses have shown considerable carbon segregation to the martensite-austenite interface.