Papers by Author: Chiradeep Ghosh

Abstract: High temperature flow curves were evaluated on two Nb steels in both compression and torsion and at a series of temperatures and strain rates. The critical strains for the initiation of dynamic transformation (DT) were determined by the double differentiation method. These are shown to be distinctly lower than those associated with dynamic recrystallization (DRX). It is also evident that the compression critical strains for both DT and DRX are lower than the equivalent torsion critical strains. Mean flow stresses (MFSs) were calculated by integration from the flow curves. When plotted against inverse temperature, stress drops were observed about 30 degrees above the Ae₃. These drops are shown to be caused by the dynamic transformation of austenite to ferrite, a softer phase. The characteristics of the ferrite produced dynamically are described and the transformation is shown to be displacive in nature, leading to the appearance fine Widmanstatten plates.

Abstract: Recent observations regarding the transformation of deformed austenite are reviewed. It is shown that superequilibrium ferrite and pearlite can be formed at temperatures well above the Ae₃ and Ae₁, respectively. The role of the stored energy associated with the introduction of the dislocations introduced by the deformation is discussed. It is shown that the forward dynamic transformation into ferrite and pearlite is several orders of magnitude faster than the reverse static transformation back into austenite. The retarding effect of alloying additions such as niobium is also outlined. The results are interpreted in terms of the effect of deformation on the modified phase diagrams pertaining to the transformation of deformed austenite.

Abstract: Recent observations regarding the transformation of deformed austenite are reviewed. It is shown that superequilibrium ferrite and pearlite can be formed at temperatures well above the Ae₃ and Ae₁, respectively. The role of the stored energy associated with the introduction of the dislocations introduced by the deformation is discussed. It is shown that the forward dynamic transformation into ferrite and pearlite is several orders of magnitude faster than the reverse static transformation back into austenite. The retarding effect of alloying additions such as niobium is also outlined. The results are interpreted in terms of the effect of deformation on the modified phase diagrams pertaining to the transformation of deformed austenite.

Abstract: A 0.21% C plain carbon steel was deformed in torsion to strains of ε = 0.15-3.0 at a strain rate of ε ̇= 4.5 s^-1 over the temperature range 722-822°C in a 5%H₂-Ar gas atmosphere. The experimental parameters were varied in order to study the formation of ferrite and pearlite by dynamic transformation (DT) in the intercritical region. This transformation was observed right up to the highest experimental temperature (822°C). The pearlite formed by DT contained cementite spheroids whose size distribution evolved during isothermal holding after deformation. In the first stage, corresponding to the first 800 s of holding, spheroid coarsening took place. When the holding time exceeded 800 s, the spheroids dissolved and the pearlite reverted into the original parent austenite. The results indicate that pearlite can form by DT at temperatures well above the Ae₁ and that the reverse static transformation is much slower than the forward dynamic transformation.