Lath martensite and bainite in low alloy steels. [Fe-0. 3%C-3%Cr-2%Mn]

The morphology and crystallography of lath martensite and bainite have been studied by transmission electron microscopy (TEM) imaging and diffraction techniques. When quenched from the austenite phase region, the steels have microduplex structures of dislocated lath martensite (a < b << c) with continuous interlath films of retained austenite. Bainites form when interrupted quenching is applied at temperatures around M/sub s/ to allow isothermal transformation to take place. The microstructure of lower bainite resembles that of lath martensite, where bainite dislocated ferrite laths, which now contain unidirectional carbides, are surrounded by thin film retained austenite. In upper bainite, the boundaries are decorated by interlath carbides, a microstructure observed in martensite tempered at 240 to 400/sup 0/C. In all cases laths form packets in which all the laths transform from a (111) variant of austenite. Microdiffraction experiments reveal that bainitic ferrite laths are related to adjacent laths as well as the parent austenite through the same orientation relationships as found in martensite in the same alloys. This strongly confirms the shear aspect of bainitic transformations. Bainitic carbides, identified as cementite, are found to have the Isaichev orientation relationship with ferrite, which supports the idea that cementite has transformed directly from austenite.more » The microstructural and crystallographic characteristics of martensite and bainites in low carbon steels are very similar which confirm that these microstructures are the products of a continuous range of phase transformations from austenite where shear is the basis and the morphological features develop depending on the time allowed for carbon diffusion, and carbide precipitation.« less