Microstructural characterization of controlled forged HSLA-80 steel by transmission electron microscopy

In this study, transmission electron microscopy (TEM) was used to evaluate the effect of controlled forging followed by cooling at various rates on microstructure of an HSLA-80 steel. The observations demonstrate that water-quenched steel has finer multiphase constituents of lath martensite, bainite and twined martensite, whereas air-cooling has resulted in a mixture of bainitic ferrite, retained austenite or MA constituents along with some Widmanstatten ferrite. When the steel is cooled in sand, the maximum volume fraction of polygonal ferrite (PF) was produced which, in turn, increased volume fraction of MA constituents. Precipitation of fine ε-Cu, Nb and Ti carbides and carbonitrides was observed and identified using energy dispersive spectrometric analysis (EDS) and electron diffraction.