In this paper, the effect of strain rate (in the domain of 0.001 to 10 s−1) on dynamic recrystallization (DRX) kinetics in a nitrogen-enhanced 316L(N) austenitic stainless steel during high temperature [≥1123 K (≥850 °C)] deformation is reported. In the low strain rate domain (i.e., <0.1 s−1), the DRX is predominantly governed by higher growth of DRX grains resulting in a higher DRX fraction and larger DRX grain size. On the other hand, DRX at higher strain rates (i.e., ≥1 s−1) is mainly controlled by higher nucleation resulting in higher DRX fraction with a finer grain size. In the intermediate strain rate regime of 0.1 s−1, sluggish kinetics of DRX is observed since neither the nucleation nor the growth of DRX grains is predominant. The annealing twinning event, which may accelerates the DRX kinetics, is also observed to occur more frequently during the low and high strain rate deformations. © 2014, The Minerals, Metals & Materials Society and ASM International.

V. Subramanya Sarma

Department of Metallurgical and Materials Engineering

Austenitic stainless steel

Deformation

Dynamic recrystallization

Grain growth

Grain size and shape

Growth kinetics

Kinetics

nitrogen

Nucleation

ANNEALING TWINNINGS

Effect of strain

FINER GRAINS

High strain rate deformation

High temperature

INTERMEDIATE STRAIN RATE

Low strain rates

SLUGGISH KINETICS

Strain rate

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Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

In the present work, the role of deformation stress state (uniaxial compression (UC) and plane strain compression (PSC)) on work hardening, softening and dynamic recrystallization (DRX) behaviour of Ni during hot deformation is investigated. Hot deformation was performed through UC and PSC to a von Mises equivalent true strain of 0.7 at 973 K, 1023 K and 1073 K at strain rates of 0.01 s−1, 0.1 s−1 and 1 s−1 in a thermomechanical simulator. Analysis of flow curves revealed rapid work hardening followed by softening in PSC samples. In UC samples, continuous work hardening at a slower rate was observed and at 1123 K, steady state was achieved. Analysis of the work hardening parameter ‘h’ and dynamic recovery parameter ‘r’ from the flow curve data showed that the h and r values of PSC deformed samples are higher than UC deformed samples. The DRX behaviour is dependent on temperature and strain rate and significant difference in DRX fraction was observed between UC and PSC samples deformed at strain rate of 0.1 s−1. Evaluation of microstructure from electron back scatter diffraction maps showed that DRX fraction is significantly higher in PSC deformed samples. Samples deformed through PSC have higher annealing twin density and number of twins per grain. It is reasoned that annealing twin formation is activated by the presence of shear stress component during PSC mode of deformation. Annealing twin formation and growth in turn facilitate DRX and explain the observed differences between UC and PSC modes of deformation. © 2019 Elsevier B.V.

Materials Science and Engineering A

Role of stress state on dynamic recrystallization behaviour of Ni during hot deformation: Analysis of uniaxial compression and plane strain compression

The hot workability, deformation behavior and microstructure evolution of a Cu alloyed super-304H stainless steel is studied over a wide range of temperatures (1173–1423 K) and strain rates (0.001–10 s−1). The processing map and activation energy map are developed for delineating the stable and unstable domains of processing. Microstructural analysis have revealed that continuous dynamic recrystallization (cDRX) is the primary restoration mechanism at lower strain rates (&lt; 0.5 s−1). The occurrence of cDRX is confirmed by the evidence of progressive conversion of low angle boundaries (LABs) to high angle boundaries in where few LABs exhibit a typical value of strain-induced boundary misorientation. However, a sharp transition from cDRX to discontinuous dynamic recrystallization (dDRX) is observed following deformation at higher strain rates (0.5–10 s−1) and temperatures (1350–1423 K). The dDRX domain is identified as optimum for processing due to its higher efficiency levels (30–35%), lower activation energy (450–550 kJ mol−1) and evolution of defect-free fine and equiaxed grains. Unstable regions are observed in 1173–1300 K and 0.03–1 s−1 which is characterized by higher activation energy for deformation (650–950 kJ mol−1) and presence of micro-cracks in the product microstructure. © 2018 Elsevier B.V.

Hot-workability of super-304H exhibiting continuous to discontinuous dynamic recrystallization transition

Dynamic recrystallization (DRX) kinetics and its association with annealing twins in alloy 617 has been studied in a range of strain rates (0.001 s−1 to 10 s−1) and temperatures (1173–1473 K). In both low (&lt; 0.1 s−1) and high (≥ 1 s−1) strain rate domains, DRX fraction is high as compared to intermediate strain rate (~ 0.1 s−1). The lower critical strain for DRX initiation with higher growth of DRX grains is responsible for large DRX fraction with coarse grains microstructure in the low strain rate domain. In contrast, DRX at higher strain rates is mainly controlled by the large stored energy along with adiabatic temperature rise which leads to higher DRX nucleation. It is also noticed that the evolution of twins is greatly influenced by deformation temperature at all imposed strain rates. Here, the evolution of twins is quantified by measuring twin boundary length fraction and length density. The twin-length density increases with increase in DRX fraction at low temperatures (&lt; 1423 K) but decreases with increasing DRX fraction at high temperatures (≥ 1423 K). An exponential rise in recrystallized grain size with DRX fraction is observed at a temperature ≥ 1423 K. This suggests that rapid migration of high-angle boundaries happened at ≥ 1423 K which may not be favorable for nucleation of twins during DRX. © 2017 Elsevier B.V.

Influence of processing parameters on dynamic recrystallization and the associated annealing twin boundary evolution in a nickel base superalloy

Hot deformation behavior of a phosphorous-modified super austenitic stainless steel was studied in the temperature range of 1173–1423 K and strain rate range of 0.001–10 s− 1 employing thermomechanical simulator. The apparent activation energy for deformation in the above processing regime was estimated to be 482 kJ mol− 1. The deformation parameters were modeled using Arrhenius equation and Zener–Hollomon parameter (Z). Peak stress, critical stress for dynamic recrystallization, stress at which flow softening is maximum as well as steady state stress was found to exhibit a linear relationship with ln(Z/A). Strains corresponding to these stresses were also found to exhibit the relation ε=CZAp. Processing maps were developed at different plastic strains employing dynamic materials modeling. Microstructures corresponding to the different processing conditions were characterized employing electron back scatter diffraction. Based on the analysis of microstructure and processing map, the optimum processing domain for hot deformation is identified as strain rate range of 0.01–0.1 s− 1 and temperature range of 1300–1350 K. Although a significant recrystallization was observed following hot deformation in the strain rate ranges of 1–10 s− 1 and temperature ranges 1373–1423 K, this domain was marked as unstable in the processing map. © 2016 Elsevier Ltd

Materials and Design

Hot deformation characteristics and processing map of a phosphorous modified super austenitic stainless steel

The hot deformation behavior of alloy 617 has been studied by performing hot compression in a range of temperatures (1173-1473 K) and strain rates (0.001-10 s-1). The peak flow stress found to increase with Zener-Hollomon parameter (Z) following a hyperbolic-sine function relationship whereas peak strain followed power-law type relationship with Z. The average activation energy for the entire hot deformation domain was estimated to be 481 kJ mol-1. The experimental stress-strain data has been used to develop processing map employing dynamic material model. The material exhibits a wide stable domain below 0.1 s-1 spanning over 1250-1473 K, with a peak efficiency of ~45%. Based on the processing map and subsequent microstructural observation, the optimum hot deformation domain of alloy 617 is identified as 1323-1423 K and 0.001-0.1 s-1. Furthermore, microstructural observation alone has revealed that a significant DRX with grain refinement could also be obtained at high temperature (&gt;1373 K) and high strain rate (&gt;1 s-1) domain although processing map has marked this region as unstable domain. © 2016 Elsevier B.V.

Characterization of hot deformation behavior of alloy 617 through kinetic analysis, dynamic material modeling and microstructural studies

This article discusses the role of twinning on dynamic recrystallization (DRX) and microstructural evolution during moderate to high strain rate (0.1 to 100 s 1) hot deformation (1173 to 1373 K (900 to 1100 °C) range) in a Ti-modified austenitic stainless steel (alloy D9). The extent of DRX increased with increasing strain rate and temperature in the range of hot working parameters employed in the present study. The acceleration of DRX with strain rate is attributed to increased rate of dislocation accumulation during high strain rate deformation as well as adiabatic temperature rise. The DRX grains were found to be twinned and a linear relationship was observed between the area fraction of DRX grains and the fraction of σ3 boundaries. Analysis of misorientations revealed that the majority of these σ3 boundaries are newly formed coherent twin boundaries during DRX. Interaction of pre-existing σ3 boundaries that may regenerate new σ3 boundaries did not seem to occur frequently during DRX. The majority of the twin boundaries are found within theDRXgrains, signifying that these annealing twins are mainly formed by "growth accidents" during the expansion of the DRX grains. It is suggested that annealing twins play an important role during nucleation and subsequent expansion of the DRX process in alloy D9. © The Minerals, Metals &amp; Materials Society and ASM International 2012.

Role of twinning on dynamic recrystallization and microstructure during moderate to high strain rate hot deformation of a ti-modified austenitic stainless steel

The influence of the state of stress on the microstructure and dynamic recrystallization (DRX) in a titaniummodified austenitic stainless steel is assessed by performing plane-strain and uniaxial hot compression studies. Although the state of stress does not alter the mechanisms of DRX nucleation, the kinetics of DRX is hindered during plane-strain deformation vis-à-vis uniaxial deformation. © The Minerals, Metals & Materials Society and ASM International 2011.

Influence of state of stress on dynamic recrystallization in a titanium-modified austenitic stainless steel

Dynamic recrystallization (DRX) behavior in hot deformed (by uniaxial compression in a thermomechanical simulator in the temperatures range 1173 K to 1373 K [900 °C to 1100 °C]) Ti-modified austenitic stainless steel was studied using electron back scatter diffraction. Grain orientation spread with a "cut off" of 1 deg was a suitable criterion to partition dynamically recrystallized grains from the deformed matrix. The extent of DRX increased with strain and temperature, and a completely DRX microstructure with a fine grain size ∼4 μm (considering twins as grain boundaries) was obtained in the sample deformed to a strain of 0.8 at 1373 K (1100 °C). The nucleation of new DRX grains occurred by the bulging of the parent grain boundary. The DRX grains were twinned, and a linear relationship was observed between the area fraction of DRX grains and the number fraction of ∑3 boundaries. The deviation from the ideal misorientation of ∑3 boundaries decreased with an increase in the fraction of ∑3 boundaries (as well as the area fraction of DRX) signifying that most ∑3 boundaries are newly nucleated during DRX. The generation of these ∑3 boundaries could account for the formation of annealing twins during DRX. The role of ∑3 twin boundaries on DRX is discussed. © 2010 The Minerals, Metals & Materials Society and ASM International.

A study on microstructural evolution and dynamic recrystallization during isothermal deformation of a Ti-modified austenitic stainless steel

The article discusses the results of a study on low-strain thermo-mechanical (one-step and iterative) processing to enhance ∑3 n boundaries in a Ti-modified austenitic stainless steel (alloy D9). Solution annealed (SA) specimens were subjected to 10% thickness reduction by rolling followed by annealing at 1173, 1223, and 1273 K for 0.5, 1, and 2 h. Anomalous grain growth with moderate increase in ∑3 n boundaries was observed after annealing at 1,173 K for 0.5 to 2 h. Prolific multiple twinning with minimum deviation of ∑3 and ∑9 boundaries from ideal orientation was achieved after annealing at 1,273 K for 0.5 to 2 h. A significant disruption in random boundary connectivity was obtained in these conditions due to the presence of large number of ∑3-∑3-∑9/∑3- ∑9-∑27 triple junctions. Iterative processing (up to 4 cycles) employing 10% thickness reduction followed by annealing at 1,273 K for 0.5 h revealed fluctuations in the evolution of ∑3 boundaries. The ∑3 fraction increased after 2nd and 4th iteration and there is a drop after 3rd iteration. This was attributed to the increased driving force for grain boundary migration due to dislocation pile-up at twin boundaries during earlier iterations. A two step iterative processing comprising of 10% deformation followed by annealing at 1,273 K for 0.5 h is the recommended thermo-mechanical processing to achieve enhanced fraction of ∑3 n boundaries (∼73%) in alloy D9. © 2010 Springer Science+Business Media, LLC.

Journal	Data powered by TypesetMetallurgical and Materials Transactions A: Physical Metallurgy and Materials Science
Publisher	Data powered by TypesetSpringer Boston
ISSN	10735623
Open Access	No