Precipitation behavior of the lower bainitic carbide in a medium-carbon steel containing Si, Mn and Mo

The fine microstructure, crystallographic features and ɛ-carbides precipitation behavior of lower bainite produced by isothermal transformation in a medium-carbon steel containing Si, Mn and Mo were investigated using transmission electronic microscopy. It was found that the microstructure produced by isothermal reaction at 320 °C was composed of a large amount of plate-like lower bainite with retained austenite embedded between the plates, and ɛ-carbides precipitated within them. Midrib and subunits were readily visible in the lower bainite plate. The bainite plate kept a G-T orientation relationship (OR) with the austenite. Selected area electron diffraction patterns of “three phases in four variants” and analysis indicated that two variants of ɛ-carbides precipitated in a single bainitic ferrite plate. The two (or three) variants of ɛ-carbides can simultaneously keep a Jack OR with its “bainite matrix”, while keeping no fixed OR with the austenite. The crystallographic features of ɛ-carbides precipitated within the bainite were the same as those observed in tempered martensite. The results indicated that the bainitic transformation bore an analogy to the martensitic one in carbide precipitation.