Influence of Widmanstätten ferrite on mechanical properties of microalloyed steels

Abstract

Using thermal cycles typical for the heat-affected zone (HAZ) of a welded joint, Widmanstätten ferrite structures have been generated in microalloyed steels. These structures have been characterized by the prior austenite grain size d_γ, the width h of the ferrite border in the austenite grain boundaries, and the distance λ between side plates in the Widmanstätten ferrite. These parameters increase with increasing cooling time between 800° and 500°C. The results confirm previous observations on austenite grain size but demonstrate also that the width of the ferrite border and the side-plate distance behave in the same way. The austenite grain size also increases when the peak temperature of the weld thermal cycles is raised. If holding times in the interval between 650° and 700°C are introduced the width of the ferrite border is coarsened. A model is proposed where the yield stress and hardness are linearly related to the volume fraction of ferrite border 3h/d_γ and to the inverse square root of the distance between the side plates λ −1/2. The Charpy V impact transition temperature (ITT) linearly decreases as a function of the square root of the maximum possible inscribed crack length in the ferrite boundary. Additions of, respectively, 0·09%V, 0·3%Mn, and 0·03%Nb to a 0·15C–1·30 Mn steel raise the yield stress by 50–150 MNm⁻². The additions of Nb, Mn, and V increase the ITT by 20–40K, 10–25K, and 0–10K, respectively. The influence of alloying elements is attributed mainly to particle and solution hardening and only to a lesser extent to variations in the structural parameters. The influence of the welding parameters, on the other hand, is ascribed to variation in the coarseness of the microstructure.