Abstract: Interphase precipitation of nanometer-sized carbides was investigated in a low-carbon, Ti-Mo-V microalloyed steel using multiple high-resolution characterization techniques. Nanoindentation testing was conducted to evaluate local precipitation strengthening and observe any possible nano-hardness variations, which may indicate local variations of precipitation behavior. A single region of polygonal ferrite was evaluated, and nano-hardness exhibited a normal distribution without systematic spatial variations. Electron backscatter diffraction analysis was used to characterize the orientation relationship (OR) between polygonal ferrite grains and adjacent (prior) austenite regions since the OR has been reported to have a large influence on the propensity for interphase precipitation to occur. Approximately 67 pct of the polygonal ferrite grains that were evaluated did not exhibit the Kurdjumov-Sachs (K-S) OR with at least one adjacent (prior) austenite region. Growth interfaces with the non-K-S OR are believed to promote interphase precipitation. Transmission electron microscopy was used to identify and evaluate characteristic rows (sheets) of interphase precipitates through bright field imaging and selected area electron diffraction analysis. The combination of the above techniques allowed for unambiguous confirmation of interphase precipitation within polygonal ferrite after cyclic austenitization and isothermal transformation.
Authors: C.A. Felker, J.G. Speer, G. Liu, and E. De Moor
Keywords: microalloyed steel, interphase precipitation, nano-indentation, orientation relationships, electron backscatter diffraction, transmission electron microscopy