Effects of Transformation Stress and Deformation before Austenitization on Nucleation of Intragranular Bainite

Abstract

Intragranular bainite is known to be effective to develop toughness greatly in steels. Effects of aging below eutectoid temperature before austenitization, the deformation before solution treatment and the duration of solution treatment on nucleation of intragranular bainite have been examined. In the austenite (γ) region where carbide pre-existed, the stress due to inverse transformation, ferrite (α) to γ, would be introduced into fine austenitized region with lower transus temperature to γ, resulting in the introduction of stable small-angle boundary at high temperatures acting as the nucleation site for intragranular bainite lath. Deformation of α by cold rolling before solution treatment enhances the nucleation of intragranular bainite, suggesting that the above γ region through the inverse transformation is easily deformed by the hardened neighboring α. In the deformed γ region, small-angle boundary would be more easily introduced, acting as nucleation site for intragranular bainite. Intragranular bainite lath grew and hardness decreased by prolonging the duration of solution treatment, indicating that transformation stress introduced by the pre-treatment was influenced by solution treatment. The effect of sulfur addition on formation of intragranular bainite was also analyzed.