The evolution of microstructure and texture during cold rolling of commercial-purity titanium (CP-Ti) was studied with particular reference to deformation twinning and dislocation slip. For low to intermediate deformation up to 40% in thickness reduction, the external strain was accommodated by slip and deformation twinning. In this stage, both compressive ( { 1 1 2 ¯ 2 } 〈 1 1 2 ¯ 3 ¯ 〉 ) and tensile ( { 1 0 1 ¯ 2 } 〈 1 0 1 ¯ 1 ¯ 〉 ) twins, as well as, secondary twins and tertiary twins were activated in the grains of favorable orientation, and this resulted in a heterogeneous microstructure in which grains were refined in local areas. For heavy deformation, between 60 and 90%, slip overrode twinning and shear bands developed. The crystal texture of deformed specimens was weakened by twinning but was strengthened by slip, resulting in a split-basal texture in heavily deformed specimens.

Effect of deformation twinning on microstructure and texture evolution during cold rolling of CP-titanium

Thermomechanical processing of advanced high strength steels

In order to improve the understanding of the dynamic and post-dynamic recrystallization behaviours of AISI 304 austenitic stainless steel, a series of hot torsion test have been performed under a range of deformation conditions. The mechanical and microstructural features of dynamic recrystallization (DRX) were characterized to compare and contrast them with those of the post-dynamic recrystallization. A necklace type of dynamically recrystallized microstructure was observed during hot deformation at 900 °C and at a strain rate of 0.01 s−1. Following deformation, the dependency of time for 50% recrystallization, t50, changed from “strain dependent” to “strain independent” at a transition strain (ɛ*), which is significantly beyond the peak. This transition strain was clearly linked to the strain for 50% dynamic recrystallization during deformation. The interrelations between the fraction of dynamically recrystallized microstructure, the evolution of post-dynamically recrystallized microstructure and the final grain size have been established. The results also showed an important role of grain growth on softening of deformed austenite.

Recrystallization in AISI 304 austenitic stainless steel during and after hot deformation

The microstructure evolution and nucleation mechanisms of dynamic recrystallization in hot-deformed Inconel 625 superalloy

The hot deformation behavior of a 304 austenitic stainless steel was investigated to characterize the evolution of the dynamically recrystallized structure as a starting point for studies of the postdeformation recrystallization behavior. The effect of different deformation parameters such as strain, strain rate, and temperature were investigated. The flow curves showed typical signs of dynamic recrystallization (DRX) over a wide range of temperatures and strain rates (i.e., different Zener–Hollomon (Z) values). However, under very high or very low Z values, the flow curves’ shapes changed toward those of the dynamic recovery and multiple peaks, respectively. The results showed that while DRX starts at a strain as low as 60 pct of the peak strain, a fully DRX microstructure needs a high strain of almost 4.5 times the initiation strain. The DRX average grain size showed power-law functions with both the Zener–Hollomon parameter and the peak stress, although power-law breakdown was observed at high Z values.

https://ro.uow.edu.au/cgi/viewcontent.cgi?article=2012&context=engpapers

Hot deformation and recrystallization of austenitic stainless steel. Part I: dynamic recrystallization

The dynamic recrystallization (DRX) of AISI 304 stainless steel was studied with torsion test in the temperature range of 900–1100°C and the strain rate range of 5.0×10 −2 –5.0×10 0 s −1 . The initiation and evolution of DRX were investigated with microstructural analysis and then the critical strain for DRX initiation could be confirmed by analysis of flow stress. The volume fraction of DRX ( X DRX ) as a function of processing variables, such as strain rate ( e ), temperature ( T ), and strain ( e ) were established. For the exact prediction of X DRX , the variation of flow stress with strain was analyzed in detail. The calculation of grain size was based on X DRX and the investigated microstructure at a few points. It was found that the calculated results agreed with the microstructure of the alloy at any deformation conditions.

Dynamic recrystallization behavior of AISI 304 stainless steel

In this paper, we have developed the equations for predicting austenite grain size (AGS) of microalloyed medium carbon steel during hot rolling Dynamic recrystallization, which plays a major role in reducing the flow stress and AGS, was described by modifying Avrami's equation In addition, for better AGS prediction of material during rolling, the fraction of dynamic recrystallization was expressed as a function of strain together with the Zener–Hollomon parameter Torsion tests were performed in the temperature range of 900–1100 °C and the strain rate range of 50×10 −2 –50×10 0 s −1 to study the softening behavior To examine the validity of the developed equation, we have then applied it to a four-pass continuous deformation process simulated by hot torsion test For the exact prediction of microstructural evolution during hot torsion test, the change of temperature was calculated using finite difference method It was found that the calculated grain size of the alloy is in good agreement with the observed grain size It is expected that a fine grained steel showing high strength and high toughness can be obtained by controlling deformation conditions on the basis of the newly developed equation

Modeling of AGS and recrystallized fraction of microalloyed medium carbon steel during hot deformation

The high-temperature deformation behavior of AISI 430 ferritic stainless steel was studied by torsion tests. The deformation tests were performed in the temperature range of 900–1100°C and strain rate range of 5.0×10−2 −5.0/sec. The evolutions of flow stress and microstructure show the characteristics of continuous dynamic recrystallization (CDRX). The flow stress gradually decreased with strain over the peak stress without steady state. Below the 100% effective strain, grains appeared in small angle grain boundaries with a misorientation of 3–9°. In addition, when heavy deformation (>300%) was applied, higher misorientation (∼15°) was achieved. The tendency of CDRX increased with increasing strain rate and decreasing temperature. The dependence of CDRX grain size on the strain rate and temperature was discussed.

Continuous dynamic recrystallization of AISI 430 ferritic stainless steel

Determination of “no-recrystallization” temperature of Invar alloy by fractional softening measurement during the multistage deformation

The torsion tests of 304 stainless steel were performed in the temperature range of 900~1100°C and strain rate range of 5.0 × 10-2~5.0 × 10°/sec to study the high temperature softening behavior. The flow curves and microstructures showed the behavior of dynamic recrystallization (DRX). The dependence of temperature(T) and the stain rate(ɛ) on the flow stress was expressed by the hyperbolic sine law, ɛ=2.75 × 10 (sinh 0.076σ)5.26 exp(-379000 J/mol/RT). The dynamically recrystallized grain size(dDRX) was related to the value of the Zener-Hollomon parameter Z. Under the Z value of 1017, it was found that the grain size ranged from 10 to 20 μm. The relationship between the dynamically recrystallized grain size and Z parameter was found as follows: dDRX=l 39.48-7.33 log Z and the calculated grain sizes were well matched with the experimental values. To determine the optimum condition of plastic deformation, the deformation map was made from the efficiency of power dissipation. The maximum efficiency was 0.41 at the condition of 1100°C, 0.5/sec, 100%.

Sung-Il Kim

Papers

Dynamic recrystallization behavior of AISI 304 stainless steel

Modeling of AGS and recrystallized fraction of microalloyed medium carbon steel during hot deformation

Continuous dynamic recrystallization of AISI 430 ferritic stainless steel

Determination of “no-recrystallization” temperature of Invar alloy by fractional softening measurement during the multistage deformation

The effects of deformation variables on hot workability of austenitic stainless steel