Variation of Transition Temperatures from Upper to Lower Bainites in Plain Carbon Steels

Abstract
Experimental results and explanations for the transition temperature from upper to lower bainites in carbon steels containing from 0.20 to 1.80 wt%C were presented metallographically and kinematically. The experimental results are summarized as follows : 1) Lower bainite is not formed in steels with less than 0.35 wt%C and no transition from upper to lower bainite occurs. 2) The transition temperature of steels containing from 0.54 to 1.10 %C indicates a constant temperature of 350°C and does not depend on the carbon content. It is important to note that a transition temperature of 350°C corresponds to the Ms temperature of a 0.55%C steel being the boundary of the martensite morphology between a lath and a plate. 3) Transition temperatures of steels with more than 1.10%C decrease along the a line below about 65°C from T₀-composition line. The bainitic transformation is essentially a kind of the martensitic one and its nucleation site is considered to be a carbon depleted zone in austenite by the thermal fluctuation of carbon atom at an isothermal holding temperature. The supercooling of about 65°C below the T₀-composition line at the carbon range more than 1.10 wt%C is attributed to the non-chemical free energy for the displacive growth of lower bainite.