Abstract: The transformation of upper bainite of normalized 55SiMnMo steel during tempering has been studied by optical microscopy,X-ray diffraction,transmission electron microscopy and differential scanning calorimetry.Ferrite plate surface is parallel to ferrite{110}and is the coherent interface for α and γ,the crystallographic relationship is(111)_γ,∥(011)_α.The small plate surface of Fe_3C precipitated during tempering is parallel to(100).The strengthening mechanism for carbon-rich austenite in normalized condition is primarily uniform solid so- lution hardening,however,in the tempered state,it is strengthened chiefly by non-uniform one.

Key words: SiMnMo steel, bainite, transformation