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.
展开▼
