Metallography of bainitic transformation in silicon containing steels

Abstract

The formation of carbide in lower bainite was studied in two silicon containing carbon steels by transmission electron microscopy and diffraction techniques. Epsilon carbide was identified in the low temperature isothermally transformed bainite structure. The crystallographic relationship between epsilon carbide and bainitic ferrite was found to follow the Jack orientation relationship,viz, (0001)ε l l(011)α, (101l)ε l 1(101)α. The cementite observed in lower bainite was in the shape of small platelets and obeyed the Isaichev orientation relationship with the bainitic ferrite,viz, (010) cl 1(1-11)α, (103) cl 1 (011)α. Direct evidence showing the sequence of carbide formation from aus-tenite in bainite has also been obtained. Based on the observations and all the crystallo-graphical features, it is strongly suggested that in silicon containing steels the bainitic carbide precipitated directly from austenite instead of from ferrite at the austenite/fer-rite interface as has been proposed by Kinsman and Aaronson (Ref. 1). The uniformity of the carbide distribution is thus envisaged to be the outcome of precipitation at the aus-tenite-ferrite interphase boundary.