Transformation of lower bainite in hypereutectoid steels

Abstract

The lower bainite transformation in five hypereutectoid steels (0.85 to 1.87 wt pet C) has been studied in the temperature range between 623 and 453 K by means of metallographic techniques. This paper brought conventional lower bainite into focus. The habit planes of ferrite and cementite platelets in lower bainite of a 1.10 wt pet C steel were close to (496)f and between (101)b and (112)b, respectively (subscriptsf, b, andc refer to the fee austenite, bec bainite ferrite, and bainite cementite, respectively). On the other hand, those in 1.80 and 1.87 wt pet C steels were close to (384)f and (101)b, respectively. The orientation relationship between austenite and bainite ferrite was the Kurdjumov-Sachs (K-S) relation and that between ferrite and cementite was the Isaichev relation for a 1.80 wt pet C steel. Application of the phenomenological theory of martensitic transformation to lower bainite in 1.80 and 1.87 wt pet C steels indicated that the lower bainite crystallography could be described by use of the (101) [−1−11]b system as the inhomogeneous shear, instead of the (112) [−1−11]b, system. The experimental and theoretical results suggest that lower bainite and martensite in 1.80 and 1.87 wt pet C steels have the same transformation inhomogeneity as the (101) [−1−1l]b system and their transformation mechanisms might be similar. The formation mechanism for lower bainite in a 1.10 pet C steel is still under consideration.