Conditions Of Dynamic Recrystallization Research Articles

Determining the Conditions for Dynamic Recrystallization in Hot Deformation of C–Mn–V Steels and the Effects of Cr and Mo Additions

The refinement of microstructure and its homogeneity during controlled hot strip rolling is primarily achieved by controlling the austenite recrystallization before its transformation during accelerated cooling. The paper describes the methodology to determine the deformation conditions favorable for dynamic recrystallization (DRX). Using this methodology it becomes possible to delineate the conditions for postdeformation static and metadynamic recrystallization as well. The work is based on viscoplastic power law formalism applied to steady state flow within wide range of deformation temperatures and strain rates. Two equations of the same form but with different coefficients can be used depending on whether the steady state flow is controlled by dynamic recovery (DRV) or DRX. The transition from DRV- to DRX at the corresponding value of Zener-Hollomon parameter Zt can be viewed as the demarcation between static and metadynamic recrystallization occurring after deformation. The approach is illustrated using low carbon Mn–V steel. Alloying with Cr and especially with Mo suppresses DRX and MDRX as manifested by increasing Zt.

The Effect of Secondary Metalworking Processes on Susceptibility of Aircraft to Catastrophic Failures and Prevention Methods

Abstract The causes of plane crashes, stemming from the subcritical growth of fatigue cracks, are examined. It is found that the crashes occurred mainly because of the negligence of the defects arising in the course of secondary metalworking processes. It is shown that it is possible to prevent such damage, i.e. voids, wedge cracks, grain boundary cracks, adiabatic shear bands and flow localization, through the use of processing maps indicating the ranges in which the above defects arise and the ranges in which safe deformation mechanisms, such as deformation in dynamic recrystallization conditions, superplasticity, globularization and dynamic recovery, occur. Thanks to the use of such maps the processes can be optimized by selecting proper deformation rates and forming temperatures.

Critical Condition of Dynamic Recrystallization of IN690 Using Strain Hardening Rate

It had been studied that thermal deformation properties of super-alloy Inconel690(IN690) by thermal simulation on Gleeble 3800 Simulator, which the ranges of temperature was 1000～1200°Cand strain rates was 1.0～80/s. According to the experiment data, the critical conditions of super-alloy Inconel690 had been researched by strain hardening rate. Zener-Hollomn parameters (Z) had been introduced into establish the critical conditions of dynamic recrystallization, such as critical strain and critical stress and peak strain. The results showed that the critical strain model which established using strain hardening rate was in good agreement with that of Sellar’s model.

Materials modeling and simulation of isothermal forging of rolled AZ31B magnesium alloy: Anisotropy of flow

Isothermal forging of a rib–web shape in AZ31B magnesium alloy in the rolling direction was conducted at speeds of 0.01–10 mm s −1 in the temperature range of 300–500 °C with the purpose of validating the results of materials models involving kinetic analysis and processing map. The process was also simulated using finite element method DEFORM to obtain the local values of strain and strain rate. Forging parallel to the rolling direction in the range 375–550 °C and 0.0003–0.3 s −1 under the conditions of dynamic recrystallization (DRX) resulted in a symmetrical cup-shape while at other conditions an elliptical boat-shape was produced with the major axis coinciding with the transverse direction and the minor axis aligned with the normal direction. This anisotropy of flow has been attributed to the strong basal texture in the rolled plate and the dominance of prismatic slip at lower temperatures. In the DRX domain on the other hand, pyramidal slip dominates along with cross-slip as the recovery mechanism, which destroys the initial texture and restores the symmetry of flow. The grain size variation for forgings done in the DRX domain validated the predictions of the material models.

Structure and mechanical properties of iron subjected to surface severe plastic deformation by friction: I. Structure formation

The severe plastic deformation of armco iron by friction is experimentally studied, and the results obtained are used to show that efficient grain refinement is possible in the temperature ranges of warm and hot deformation. A nanocrystalline structure forms only under dynamic recrystallization conditions during hot deformation, which is ensured by deformation in different directions at a rate higher than 102 s−1.

Determination of critical conditions for dynamic recrystallization of a microalloyed steel

A low carbon Nb–Ti microalloyed steel was subjected to hot torsion testing over the range of temperatures from 900 to 1100 °C and strain rates from 0.01 to 1 s −1 to characterize its hot deformation behavior. The initiation and evolution of dynamic recrystallization were investigated by analyzing of hot flow curves. Two important dynamic recrystallization parameters, the critical strain and the point of maximum dynamic softening, derived from strain hardening rate- stress curves. These parameters then were used to predict the dynamic recrystallized fraction. The results showed that the critical stress and strain increase with decreasing deformation temperature and increasing strain rate. The hot deformation activation energy of the steel investigated in the present work is 375 kJ/mol, and the expression for steady state flow stress is σ SS = 0.07 Z 0.14 = 0.07 ⋅ ε ˙ exp 375,000 R T 0.14 The volume fraction of dynamic recrystallization as a function of processing variables was established. It was found that the model used for predicting the kinetic of dynamic recrystallization is in good agreement with the data directly acquired from experimental flow curves.

Modelling the steady state deformation stress under various deformation conditions using a single irreversible thermodynamics based formulation

A new unified description for the steady state deformation stress in single and polycrystalline metals and for various deformation conditions is presented. The new formulation for dislocation controlled deformation stems from the field of irreversible thermodynamics. The model applies to conditions of dynamic recovery as well as dynamic recrystallization and has been validated for constant strain rate and creep loading conditions. Unlike existing approaches, the new model captures transitions between deformation mechanisms within a single formulation. For conditions of dynamic recrystallization, the average dislocation density is found to be a function of the shear strain rate and a term combining the dislocation climb velocity and the grain boundary velocity.

Microstructure Evolution Modeling during and after Deformation in 304 Austenitic Stainless Steel through Cellular Automaton Approach

A 2D cellular automaton approach was used to simulate microstructure evolution during and after hot deformation. Initial properties of the microstructure and dislocation density were used as input data to the cellular automaton model. The flow curve and final grain size were the output data for the dynamic recrystallization simulation, and softening kinetics curves were the output data of static and metadynamic recrystallization simulations. The model proposed in this work considered the effect of thermomechanical parameters (e.g., temperature and strain rate) on the nucleation and growth kinetics during dynamic recrystallization. The dynamic recrystallized microstructures at different strains, temperatures, and strain rates were used as input data for static and metadynamic recrystallization simulations. It was shown that the cellular automaton approach can model the final microstructure and flow curve successfully in dynamic recrystallization conditions. The postdeformation simulation results showed that the time for 50% recrystallization decreases with increasing strain for a given initial grain size and that dynamic recrystallization slows the postdeformation recrystallization kinetics compared to a model without dynamic recrystallization.

Prediction of the critical conditions for dynamic recrystallization in the austenitic steel 800H

Engineering approaches to model dynamic recrystallization (DRX) explicitly assume the existence of a critical strain for DRX. However, DRX is not set off at a certain “critical” amount of strain, rather is initiated by reaching critical conditions for microstructural instability. It was proposed recently that these critical conditions can be expressed in terms of a point of inflection on the θ(σ) curve, where σ is the flow stress and θ is the hardening rate. A theoretical framework is presented to predict these critical conditions on the basis of a dislocation work hardening model. Once tuned to a specific hardening curve the model allows to predict the onset of DRX for different temperatures and strain rates, and, in principle, for changing materials chemistry.

Prediction of the critical conditions for dynamic recrystallization in metals

Abstract Engineering approaches to model dynamic recrystallization (DRX) explicitly assume the existence of a critical strain for DRX. Since the strain is not a state variable of crystal plasticity, however, there is no critical strain, rather there are critical conditions for microstructural instability. It was proposed recently that these critical conditions can be expressed in terms of a point of inflection on the θ(σ) curve, where σ is the flow stress and θ is the hardening rate. A theoretical framework is presented to predict these critical conditions on the basis of a dislocation work hardening model. Once tuned to a specific hardening curve the model allows to predict the onset of DRX for different temperatures and strain rates, and, in principle, for changing materials chemistry.

Prediction of dynamic recrystallization condition by deformation efficiency for Al 2024 composite reinforced with SiC particle

Hot torsion test has been carried out for Al 2024 composite reinforced with 8 μm SiCp (15 vol.%) to suggest optimum hot working condition for dynamic recrystallization (DRX) at the temperature range of 320 to 520 °C and strain rate range of 0.1 to 3.0/sec. Flow curve and deformed microstructure have been analyzed to identify the hot restoration mechanism of DRX. Processing map showing the variation of the deformation efficiency expressed by [2m/(m + 1)], where m is the strain rate sensitivity, with temperature and strain rate has been described for the composite. The characteristics of domain of DRX and peak efficiency of the composite have been analyzed by observing deformed microstructure. The composite showed 40–50% efficiency at the DRX domain (370–460 °C, 0.1–0.5/sec). Also, the variation of deformation efficiency with Zener-Hollomon parameter (Z = έ exp(Q/RT)) were discussed to find out optimum hot working condition for DRX of the composite. It is found that the optimum temperature and strain rate condition for DRX of the composite is 430–450 °C and 0.5/sec.

Dynamic Recrystallization Textures in Ferrite and Ferritic Stainless Steels

The textures produced in two conventional IF steels, two ferritic stainless steels and α-iron were determined after deformation in torsion over the temperature range 20-1200°C. The experiments were carried out in the ferrite range and the conditions were chosen so as to eliminate the possibility of static recrystallization. The differences between the deformation and dynamic recrystallization (DRX) textures are sharp. The ideal orientations observed at room temperature are F(110) , J1(011) , J2(110) , D1(112) , D2(112) , E1(011) and E2(011) . The texture produced under dynamic recrystallization conditions is dominated by the D1, D2 and E2 components. The simulations indicate that the low stored energy nucleation mechanism plays the dominant role in the formation of bcc DRX textures. The results are also interpreted in terms of the continuous (in situ) and discontinuous mechanisms of dynamic recrystallization.

Modelling of work hardening

This paper presents simulation of the work hardening curves of metals deformed at a constant temperature and strain rate under conditions of dynamic recrystallization (DRX). A work hardening model has been developed based on the evolution of the dislocation structure during deformation. A statistical approach is used to predict the microstructure evolution which is consistent with DRX. The model provides a description of the stress-strain curves for some specified combinations of temperature and strain rate.

Dynamic recrystallization microstructures under hot working conditions

The characteristics of high temperature flow curves and evolution of new grain structures under conditions of dynamic recrystallization (DRX) are reviewed, together with the associated development of dislocation substructures. It is shown that the type of DRX taking place depends on the relative grain size D o/D s, where and D o and D s are the initial and stable dynamic grain sizes, and also on the nature of dispersion particles. These can be attributed to that dynamic nucleation occurs mainly at or near existing grain boundaries, and dynamic growth is controlled by not only the temperature corrected strain rate Z, but also dispersion particles. The work softening accompanied with DRX results from a change in the substructural nature from homogeneous. The dislocation substructures under full DRX are classified into the three categories: (i) DRX nuclei, (ii) growing DRX grains containing a dislocation density gradient, and (iii) large DRX grains with fairly homogeneous substructure. Finally, the metadynamic or post-dynamic restoration processes taking place in the DRX matrices are reviewed. The static softening-annealing time curve observed after DRX consists of three distinct stages with the respective inflection plateaus, followed by incomplete softening (stage 1,2 and 3). The rate of grain growth at stage 3 is much lower than that of normal grain growth. These can be caused by the stable existence of growing DRX grains having many-sided irregular shapes as well as high density dislocations.

Substructures in Copper in Conditions of Hish-temperature Deformation Instability

Substructural aspects of instability of plastic deformation are investigated in copper at constant strain rate at elevated and high temperatures. Subgrain misorientation as a measure of deformation energy stored in a subboundary is specially studied. Characteristics of the substructure-mean misorientation and size-are used to assess synergetic characteristics of the substructural evolution. The results indicate that the system is very far from equilibrium in conditions of instability and evolves chaotically, probably correlated with substructural inhomogeneities and, in certain cases, with a hierarchic substructure. In this process conditions of dynamic recrystallization evolve by which the instability is manifested

Retrograde deformation within the Carthage-Colton Zone as recorded by fluid inclusions and feldspar compositions: tectonic implications for the southern Grenville Province

The Carthage-Colton Zone (CCZ), located in the northwestern portion of the Proterozoic Adirondack terrane, forms the boundary between the Adirondack lowlands and highlands and is characterized by textures indicative of ductile deformation. Samples from three rock types have been collected from this zone: (1) pyroxene bearing syenite gneisses of the Diana igneous complex; (2) paragneiss samples from near the northwestern boundary of the Diana complex; (3) a quartz-rich metasediment. Feldspar geothermometry performed on the Diana metasyenites shows that porphyroclasts are relict igneous grains that formed atT≈900° C, while dynamic recrystallization occurred at temperatures as low as 470 to 550° C as shown by the compositions of feldspar neoblasts. All samples examined in this study from the CCZ contain a suite of CO2-rich fluid inclusions that are distinctive both texturally and in their microthermometric behavior (Th=-27.7 to-7.1° C) as compared to CO2-rich Adirondack fluid inclusions that do not lie within this zone (Th=-45.9 to +31.0° C). The results from fluid inclusion microthermometry and feldspar geothermometry restrict the conditions of dynamic recrystallization to temperatures and pressures of 400 to 550° C and 3 to 5 kbar. The retrograde pressure-temperature path must pass through these conditions. Similar fluid inclusion results have been obtained from the Parry Sound Shear Zone (PSSZ) which is a Grenvillian shear zone that is located in Southern Ontario (Lamb and Moecher 1992). However, the inferred retrogradeP-T paths for these two areas, the CCZ and the PSSZ, are different and this difference may be a result of late deformation along shear zones that are located between the two areas.

Effect of hot deformation and austenitizing on the grain size of steels 4Kh4VMFSSh and 45Kh3V3MFSSh

1. In steel 4Kh4VMFSSh with an original grain size of grade 2–3 a homogeneous austenitic structure (grade 5.5–6) is formed under conditions of dynamic recrystallization after 15–50% deformation at 1100–1150°C. In steel 45Kh3V3MFSSh with an original grain size of grade 4–5 a homogeneous recrystallized structure with grain size of grade 7.5–8.5 is formed after 18–50% deformation at 1150–1200°C and also after 50% deformation at 1100°C. At other temperatures and deformations the austenite grain size either remains unchanged or it is refined, but not through the entire volume. 2. Phase recrystallization of steels 4Kh4VMFSSh and 45Kh3V3MFSSh cooled from deformation temperature in air is not accompanied by austenite grain refining if no recrystallization induced by mechanical cold working develops. 3. Austenitizing of deformed steels induces no essential changes in the structure even at the optimal temperature −1050°C for steel 4Kh4VMFSSh and 1100°C for steel 45Kh3V3MFSSh. If a completely homogeneous structure is not formed during deformation, it cannot be obtained during austenitizing.