Dynamic Recovery Behavior Research Articles

Influence of Grain Size on Work-Hardening Behavior in 12Cr Stainless Steel

Grain refinement is attracting attention as a strengthening method which does not depend on the alloying elements added to steels. Many reports have described the manufacturing methods and mechanical properties of ultra-fine grained steels. In ultra-fine grained steels, it is well known that yielding stress drastically increases in accordance with the Hall-Petch relationship, while uniform elongation significantly decreases. These tendencies imply that grain size affects not only yielding but also work-hardening behavior. However, the influence of grain size on work-hardening behavior has not been clearly understood. Therefore, in this study, we investigated the work-hardening behavior during tensile deformation of 12Cr stainless steel with various grain sizes. Grain refining was conducted by cold-rolling of annealed and quenched steel specimens, followed by recrystallization annealing. The grain size of the specimens decreased as the cold-rolling reduction rate increased. The minimum grain size obtained by this method was approximately 5 μm. With decreasing grain size, 0.2% proof stress increased and the strain which reached the plastic instability condition decreased. In the session, we report the dislocation accumulation behavior estimated by grain hardness and XRD and the dynamic recovery behavior assessed by the Kocks-Mecking model.

Physics-Based Constitutive Model to Predict Dynamic Recovery Behavior of BFe10-1-2 Cupronickel Alloy during Hot Working

AbstractThe hot deformation behavior of BFe10-1-2 cupronickel alloy was investigated over wide ranges of deformation temperature and strain rate. The physics-based constitutive model was developed to predict the dynamic recovery (DRV) behavior of BFe10-1-2 cupronickel alloy at elevated temperatures. In order to verify the validity of the developed constitutive equation, the correlation coefficient (R) and average absolute relative error (AARE) were introduced to make statistics. The results indicated that the developed constitutive equation lead a good agreement between the calculated and experimental data and can accurately characterize the hot DRV behaviors for the BFe10-1-2 cupronickel alloy.

Unexpected Effect of Nb Addition as a Microalloying Element on Mechanical Properties of δ-TRIP Steels

The concept of microalloying was applied to the δ-TRIP (transformation-induced plasticity) steel to investigate the feasibility of increasing the mechanical properties and understanding the effect of microalloying on the morphology and structure of the steel. A hot rolled δ-TRIP steel with three different contents of Nb (0, 0. 03, 0. 07 mass%) was subjected to the microstructural and mechanical examination. The high Al and Si concentration in these steels guaranteed the presence of the considerable δ-ferrite phase in the microstructure after the casting and the subsequent hot rolling. The obtained results showed that Nb dramatically affects the microstructure, the dynamic recovery and recrystallization behavior, as well as the grain shape and thus the stability of austenite after the thermomechanical process of hot rolling. The results also revealed an unexpected effect of Nb on the mechanical properties. The addition of Nb to the δ-TRIP steel led to a significant decrease in the ultimate strength (from 1144 to 917 MPa) and an increase in ductility (from 24% to 28%). These unconventional results could be explained by the change in the steel microstructure. The work-hardening behaviors of all samples exhibit three stages of the work-hardening rate evolution. At the stage 2, the work-hardening rate of the studied steels increased, being attributed to the TRIP effect and the transformation of austenite to martensite.

Constitutive modeling of hot horming of austenitic stainless steel 316LN by accounting for recrystallization in the dislocation evolution

Hot compression test data taken from Zhang [1] of metastable austenitic stainless steel AISI 316LN over a range of strain rates and temperatures shows typical dynamic recovery and recrystallization behavior. It is proposed to model this behavior by incorporating not only the hardening and recovery into the Bergstrom dislocation evolution equation, but also the recrystallization. It is shown that the initial mechanical response before recrystallization can be accurately represented by assuming that the mean free path evolves as the microstructure evolves from homogeneously spaced dislocations to cell-pattern. Results show that this novel continuum mechanical model can predict the observed behavior, showing a good match to the experimental data and capturing the transition from recrystallization to (almost) no recrystallization.

Improved dislocation density-based models for describing hot deformation behaviors of a Ni-based superalloy

Abstract

Constitutive Modeling of High-Temperature Flow Behavior of an Nb Micro-alloyed Hot Stamping Steel

The thermal deformation behavior and constitutive models of an Nb micro-alloyed 22MnB5 steel were investigated by conducting isothermal uniaxial tensile tests at the temperature range of 873-1223 K with strain rates of 0.1-10 s−1. The results indicated that the investigated steel showed typical work hardening and dynamic recovery behavior during hot deformation, and the flow stress decreased with a decrease in strain rate and/or an increase in temperature. On the basis of the experimental data, the modified Johnson-Cook (modified JC), modified Norton-Hoff (modified NH), and Arrhenius-type (AT) constitutive models were established for the subject steel. However, the flow stress values predicted by these three models revealed some remarkable deviations from the experimental values for certain experimental conditions. Therefore, a new combined modified Norton-Hoff and Arrhenius-type constitutive model (combined modified NH-AT model), which accurately reflected both the work hardening and dynamic recovery behavior of the subject steel, was developed by introducing the modified parameter ke. Furthermore, the accuracy of these constitutive models was assessed by the correlation coefficient, the average absolute relative error, and the root mean square error, which indicated that the flow stress values computed by the combined modified NH-AT model were highly consistent with the experimental values (R = 0.998, AARE = 1.63%, RMSE = 3.85 MPa). The result confirmed that the combined modified NH-AT model was suitable for the studied Nb micro-alloyed hot stamping steel. Additionally, the practicability of the new model was also verified using finite element simulations in ANSYS/LS-DYNA, and the results confirmed that the new model was practical and highly accurate.

A unified physically based constitutive model for describing strain hardening effect and dynamic recovery behavior of a Ni-based superalloy

Abstract

Constitutive Relationship Modeling and Characterization of Flow Behavior under Hot Working for Fe–Cr–Ni–W–Cu–Co Super-Austenitic Stainless Steel

The hot deformation behavior of a Fe–22Cr–25Ni–3.5W–3Cu–1.5Co super-austenitic stainless steel was investigated using isothermal compression tests with a wide range of temperatures (1173–1373 K) and strain rates (0.1–10 s−1). The results showed that all the flow curves gradually turned to balanced stress state without notable peak stress characteristics during the entire deformation, which indicated that the dynamic recovery behavior played a main restoration mechanism in the steel. Modeling constitutive equations relating to the temperature, strain rate and flow stress were proposed to determine the materials constants and activation energy necessary for deformation. In order to give the precise predicted values of the flow behavior, the influence of strain was identified using polynomial functions. The relationship of flow stress, temperature and strain rate was represented by the Zener-Hollomon parameter including the Arrhenius term. The predicted results validated that the developed constitutive equations can describe high temperature flow behavior well. Furthermore, a modified Zener-Hollomon parameter map of the studied steel was developed to clarify the restoration mechanism based on the constitutive modeling data and microstructural observation.

New constitutive model for high-temperature deformation behavior of inconel 718 superalloy

Abstract Hot deformation behavior of an aged inconel 718 superalloy is investigated by isothermal compression tests over wide ranges of strain rate and deformation temperature. In this study, the effects of hot deformation parameters (deformation temperature and strain rate) on flow stress are analyzed. It is found that the flow stress of the studied superalloy is significantly affected by deformation temperature and strain rate. The flow stress decreases with the decrease of strain rate or the increase of deformation temperature. Based on the experimental results, a new constitutive model is developed to describe the hot deformation behavior. For the work hardening-dynamic recovery stage, an isotropic internal variable is used to represent the plastic deformation resistance, and a viscoplastic constitutive model is developed to describe the work hardening and dynamic recovery behavior. For the dynamic softening stage, the phenomenological constitutive models, which consider the coupled effects of deformation temperature, strain rate and strain on hot deformation behavior, are developed. The predicted flow stresses are in a good agreement with the experimental ones, indicating that the developed models can accurately characterize the hot deformation behavior of the studied Ni-based superalloy.

On the hot working of FeSi ferritic steels

The high temperature deformation behavior of FeSi electrical steels (ferritic at the testing temperature) was studied under hot compression conditions. The flow curves of these steels exhibit a conventional dynamic recovery behavior. The analysis of the hot working data was carried out by applying the classical hyperbolic-sine equation to derive the peak stress in the flow curve by determining the apparent stress exponent, n, and the activation energy, Q. A modified hyperbolic-sine equation showed a better correlation with experimental data. The modified equation assumes that the rate controlling mechanism is the glide and climb of dislocations. Consequently, Q can be taken equal to the iron self-diffusion in ferrite (Q=239kJ/mol) and the creep exponent can be set equal to 5. The whole flow curves were also characterized by the one variable approach which in turn requires deriving characteristic dynamic recovery and work hardening terms at any processing conditions. A direct relationship between the hardening and softening terms and the Si content of the steels was obtained. Finally all the flow curves were discussed and modeled as a function of the processing parameters and chemical composition. Finally, a universal constitutive equation to describe the high temperature deformation of these steels taking account of the chemical composition is proposed.

Slab Analysis of Large Cylindrical Shell Rolling

Considering the characteristics of large cylindrical shell rolling, such as double driving rolls, asymmetrical rolling and huge workpiece, a slab method was developed to establish the rolling force model. In this model, the non-uniform normal and shear stresses and the upper and lower surface temperatures of the workpiece were taken into account. Moreover, the flow stress model, considering the dynamic recovery and dynamic recrystallization behaviors of the material, was established. The rolling pressure distribution, the rolling force, the rolling torque and the neutral points could be calculated quickly and easily by the rolling force model. The predicted results were shown to be in good agreement with the measured values, which indicated that the model can satisfy the requirement of industrial application.

Investigation on Dynamic Recovery Behavior of Boron Steel 22MnB5 under Austenite State at Elevated Temperatures

Hot forming process of ultrahigh strength boron steel 22MnB5 is widely applied in vehicle industry. It is one of the most effective approaches for vehicle light weighting. Dynamic recovery is the major softening mechanism of the boron steel under austenite state at elevated temperatures. Deformation mechanism of the boron steel can be revealed by investigation on the behavior of dynamic recovery, which could also improve the accuracy of forming simulations for hot stamping. Uniaxial tensile experiments of the boron steel are carried out on the thermo-mechanical simulator Gleeble3800 at elevated temperatures. The true stress-strain curves and the relations between the work hardening rate and flow stress are obtained in different deformation conditions. The work hardening rate decreases linearly with increasing the flow stress. A flow stress model is derived based on Kocks model, and the derivative of the dislocation density with respect to the true strain, which is expressed by the peak stress and initial yield stress, is also deduced considering the dynamic recovery effect. A dynamic recovery efficiency factor h is defined as the ratio of the dynamic recovery effect and the dislocation accumulation effect. The effects of the deformation conditions on the h are analyzed.

Influences of Strain Rate and Deformation Temperature on Flow Stress and Dynamic Recrystalliazation of Heat Resistant Steel P91

In power station big caliber thick wall seamless tube typical steel P91for example, The hot deformation behavior and heat flow stress-strain curve of the heat resistant steel P91was investigated with a compression test on Gleeble3500 simulator at a temperature range of 1050°C～1200°C and strain rate range of 10-4～5s-1. The test curve shows that P91 materials have the dynamic recrystallization and recovery behavior in high temperature and deformation. With the rise of temperature and strain rate of decreased, flow stress is lower and the dynamic recrystallization phenomenon is easier to occur. By metalloscope observed, in the high temperature and low strain rate conditions, the dynamic recrystallization area and new nucleation grain size are larger, and microstructure is more uniformly distributed.

Shape memory effect and mechanical properties of carbon nanotube/shape memory polymer nanocomposites

Carbon nanotubes (CNT) have remarkable mechanical properties with very high elastic modulus and electrical conductivity. Shape memory polymer (SMP) as one of smart materials is characterized with its remarkable recoverability and shape memory effect, but its mechanical properties such as strength and elastic modulus is not high enough. In this study, CNT/SMP nanocomposites were developed with the typical CNTs of the vapor growth carbon fibers (VGCFs). A fine and homogeneous dispersion of VGCF throughout the SMP matrix is obtained. The specimens with different VGCF weight fraction, such as SMP bulk, 1.7 wt.%, 3.3 wt.% and 5.0 wt.%, were prepared, and their dynamic mechanical properties and shape recovery behavior were investigated. It was found that storage elastic modulus is improved obviously with increment of VGCF weight fraction, and the CNT/SMP nanocomposites showed a good shape memory effect. It is indicated that the recovery stress of CNT/SMP nanocomposites with only 3.3% weight fraction of carbon nanotubes will reach almost twice of that in SMP bulk.

Deformation behavior of 6063 aluminum alloy during high-speed compression

The deformation behavior characteristics of 6063 aluminum alloy were studied experimentally by isothermal compression tests on a Gleeble-1500 thermal-mechanical simulator. Cylindrical specimens of 14mm in height and 10 mm in diameter were compressed dynamically at temperatures ranging from 473 to 723 K and at higher strain rates from 5 to 30s−1. It is found that the flow curves not only depend on the strain rate and temperature but also on the dynamic recovery and recrystallization behavior. The results show that the flow stress decreased with the increase of temperature, while increased with the increase of strain rate. The discontinuous dynamic recrystallization (DDRX) may take place at a high strain rate of 20s−1 under the tested conditions. At 30s−1, the flow curve can exhibit flow softening due to the effect of temperature rise that raised the temperature by about 32 K in less than 0.05s.

Influence of long-chain branching on linear viscoelastic flow properties and dielectric relaxation of polycarbonates

Abstract The dynamic viscoelastic property, creep and creep recovery behavior, and dielectric relaxation of long-chain branched Bisphenol A polycarbonates were measured in parallel plate rheometer and dielectric analyzer. The linear polycarbonate (PC-L) as reference and three branched polycarbonates (PC-Bs) have similar molecular weights and molecular weight distributions, while the PC-Bs have different branching degrees, below 0.7 branch points/chain and above twice of M c . The long-chain branched polycarbonates exhibit higher zero-shear viscosities, more significant shear shinning, higher flow activation energies, and much longer relaxation times. It was also found that long-chain branches increase the elasticity of melt characterized by the steady-state recoverable compliance and the storage modulus. The ‘dissident’ rheological behavior of long-chain branching exhibiting mainly in addition polymers such as polyolefin, is confirmed in condensation polymers. These behaviors resulted from additional molecular entanglements of long-chain branches can be understood qualitatively in terms of the tube model for topological constraints. The dielectric α-relaxation of linear polycarbonate and branched polycarbonates has been fitted with Vogel–Fulcher–Tammann–Hesse (VFTH) equation and the shape of relaxation time curves is also analyzed. The long-chain branched polycarbonates present longer relaxation times, but divergent α-relaxation temperatures, because the latter is dominated by the free volume.

Hot working of two AISI 304 steels: a comparative study

The hot deformation behavior was studied of two austenitic stainless steels (AISI 304 type) with different carbon contents (0.02 and 0.087%). An analysis of the parameters describing the hot flow curves and their quantification at each strain stage was carried out. The comparative study of the two steels showed no significant effect of the carbon content on the conditions for the onset of dynamic recrystallization or on dynamic recrystallization kinetics, although the Avrami exponent was clearly dependent on the hot forming conditions. Also, peak and steady state stresses were relatively insensitive to the amount of carbon. However, the work hardening and dynamic recovery behavior showed clear differences, depending on the alloy, especially at high temperatures and low strain rates, where the steel with the higher carbon content showed higher work hardening and dynamic recovery rates.

Estimation of interaction energies Me-(C, N) in f.c.c. iron-based alloys using thermo-calc thermodynamic database

The tensile behavior of austenitic stainless steels with different concentration of nitrogen was studied by analyzing deformation microstructure and calculating short-range ordering (SRO) as well as stacking fault energy (SFE). Increasing nitrogen concentration increased SFE and SRO, which affected the tensile behavior. From the early to intermediate deformation stages, the nitrogen addition induced slip planarity and the delay of dynamic recovery, which were brought by the increase of SRO. As a result, work hardening rate (WHR) rose with the nitrogen content at the end of the intermediate deformation stage. On the other hand, the SFE effect was noticeable at higher strain levels. With increasing nitrogen concentration, the formation of slip lines was suppressed and dynamic recovery was promoted. Hence, the reduction in WHR was accelerated with the nitrogen addition. When the overall dynamic recovery behavior was assessed with dislocation-density based constitutive modeling, it was found to depend mainly on SRO rather than on SFE. Therefore, the alloy with higher nitrogen content could achieve higher dislocation density and flow stress.

Effect of vanadium and molybdenum addition on high temperature recovery, recrystallization and precipitation behavior of niobium-based microalloyed steels

The effects of multiple microalloy additions on the dynamic recovery and recrystallization behavior of austenite were investigated in a series of 0.05% C-l.25% Mn steels. The steels contained one or more of 0.30% Mo, 0.035% Nb or 0.115% V, and a reference plain carbon steel was also tested. Constant strain rate compression tests were conducted in the temperature range 1150°–875°C, that is, above and below the relevant carbonitride solution temperatures ( T sol ). Dynamic recrystallization-time-temperature (RTT) curves were developed from the flow curves at 5.6 × 10 −3 and 3.7 × 10 −2 s −1. At the higher strain rate, only the solute retardation of recrystallization was detected for the vanadium, Nb-V, Nb-1.9 Mn and No-Mo steels. An additional delay due to the dynamic precipitation of Nb(CN) was observed in the Nb-1.25 Mn steel, where there was a break to the right in the RTT curve. These breaks were seen in the results for all the steels at the lower strain rate. They appeared about 50°–100°C below the calculated T sol for the particular carbonitride, and coincided with the intersection of the RTT and PTT (precipitation-time-temperature) curves. The solute retarding potential of each element was normalized to 0.1 at.% with a solute retarding parameter (SRP). The SRP for niobium, molybdenum and vanadium when added singly, was 63, 10 and 3% respectively. In the multiply alloyed steels, the SRP attributable to each element was smaller than when it was present alone. Possible explanations for this negative interaction are advanced. Dynamic PTT curves for the precipitation of carbonitrides are presented for the Nb-1.25 Mn, Nb-1.9 Mn, Nb-Mo, Nb-V and Nb-Mo-V steels. The addition of molybdenum, vanadium or manganese is seen to retard the dynamic precipitation of Nb(CN). These delays are explained in terms of the decrease in T sol resulting from the reduction in the carbon and nitrogen activity coefficients in the joint presence of the former elements. An empirical formulation for the effect of manganese on NbC solubility is deduced from results reported in the literature which is consistent with the present observations.