Flow Softening-based Formation of Widmanstätten Ferrite in a 0.06%C Steel Deformed Above the Ae₃

Abstract

Compression tests were carried out at a strain rate of 1 s -1 on a 0.06%C-0.3%Mn-0.01%Si steel over two temperature ranges: i) 920°C to 980°C, and ii) 500 to 750°C. Optical and scanning electron microscopy images indicated that significant volume fractions of Widmanstätten ferrite were formed dynamically above the Ae3 temperature. The ferrite plates coalesced into polygonal grains during straining. The double differentiation method was applied to the stress-strain curves, providing average values for the dynamic transformation (DT) and dynamic recrystallization (DRX) critical strains of 0.12 and 0.20, respectively. These results are interpreted in terms of the flow softening-based transformation model by calculating both the driving forces promoting the transformation as well as the energy barriers that oppose it. The model predicts the temperature range over which DT can occur as well as the observed critical strains.

Dynamic Transformation of an X70 Steel under Plate Rolling Conditions

“…The first minimum identifies the start of dynamic transformation (DT), while the second specifies that of dynamic recrystallization (DRX). As shown in earlier studies using microstructural analyses, 6,7,21) the critical strain for DT is always lower than that for DRX.…”

“…These observations have now been given a sound thermodynamic interpretation by Aranas et al, 6,7) who proposed that the driving force for ferrite formation is the softening that takes place during transformation. This is evaluated from the difference between the current flow stress of the work-hardened austenite and the yield stress of the fresh Widmanstätten ferrite that takes its place.…”

“…Previous research has shown that dynamic transformation also contributes to the decreasing flow stress levels. 7,[11][12][13][14][15][16][17] …”

A metastable phase diagram for the dynamic transformation of austenite at temperatures above the Ae3

“…In the dynamic case, however, the free energy difference is of opposite sign, so that it becomes a barrier to the transformation instead. Under these conditions, it is the flow stress difference between the work hardened austenite and the much softer ferrite that replaces it that provides the driving force [8,9].The other quantities are the work of dilatation and of shear accommodation [10]. Unlike the situation at ambient temperatures, at which these energies are stored elastically, the high temperature versions are not stored reversibly, but involve dissipation by means of plastic work.…”

“…In the dynamic case, however, the free energy difference is of opposite sign, so that it becomes a barrier to the transformation instead. Under these conditions, it is the flow stress difference between the work hardened austenite and the much softer ferrite that replaces it that provides the driving force [8,9].…”