The Effect of Retained Work Hardening on the Driving Force for Dynamic Transformation

Abstract

The driving force for dynamic transformation during double-hit hot compression of an as-cast medium-carbon low-alloy steel was done at 1473 K at strain rates of 0.25 s -1 and 0.50 s -1 .Dynamically transformed ferrite was detected using the Kernel Average Misorientation (KAM) technique. The time interval between deformations affects the retained driving force, which decreases at a rate of 65-75 J/mol per second. This rate decreases with decreasing temperature due to a lower rate of recovery.

Microstructural manifestation and kinetics of dynamic transformation in a W-modified 9%Cr steel

“…In order to assess the occurrence of DT, the ‘double-derivative’ criterion [2-5] was applied, the salient points of which are described here. In a strain-controlled deformation experiment, strain ( ) is the independent variable and stress ( ) is the dependent variable.…”

“…This quantity is termed the ‘work-hardening rate ( )’, where . The changes in work-hardening rate have been correlated with storage and dissipation of energy during hot deformation [2-5,19]. During hot deformation, energy is considered to be stored or dissipated through the evolving dislocation structures.…”

“…DT was originally demonstrated in C-Mn steels [2,3]. The transformation has recently been applied to medium-carbon steels [4] and a 9%Cr-1%Mo high temperature steel [5]. Several classes of steels, therefore, are amenable to microstructural modification using DT.…”

“…Unlike DRX, the fundamental and technological aspects of DT are still being resolved [2,3,5]. For instance, DT is currently understood to be a displacive transformation [3,4,8], but the role of defect-enhanced diffusion is also asserted [2,9]. The driving force for DT is the resistance of work-hardened austenite to further deformation [3], but the precise parameter controlling the transformation is not identified.…”

“…Since DT- α is metastable, the possibility of reverse transformation during thermal exposure must be considered. This is relevant to multiple-stage processing, where brief thermal exposure is possible during interpass times [4] as well as during intermediate heat treatment. The formation and decomposition of DT- α are examined through controlled experiments and analysis of the resultant microstructures.…”