Fractional Softening Research Articles

Metadynamic and static recrystallization softening behavior of a bainite steel

The metadynamic recrystallization (MDRX) and static recrystallization (SRX) softening behavior of a bainite steel was investigated by two-pass isothermal compression experiments at temperatures of 1173, 1273, 1373, and 1473 K and strain rates of 0.01, 0.1, 1, and 10 s−1 with inter-pass times of 1, 5, 10, and 30 s on a Gleeble-1500 thermo-mechanical simulator. Kinetic equations were developed to evaluate the softening fractions caused by MDRX and SRX. A comparison between the experimental and predicted softening fractions showed that the proposed kinetic equations can provide a precise estimation of the MDRX and SRX behavior of the studied steel. The results based on the kinetic equations indicated that the MDRX and SRX softening fraction increases with the increase in strain rate, deformation temperature, inter-pass time, and pre-strain; the activation energy of MDRX is much smaller than that of SRX; and the no-recrystallization temperature of the investigated steel is 1179.4 K.

Constrained groove pressing, cold-rolling, and post-deformation isothermal annealing: Consequences of their synergy on material behavior

Stress-relieved Al-Mn-Si specimens were constrained groove pressed (CGPed) and in the following, cold-rolled under different strains of 0.47, 0.8 and 1.27. Dual strained sheets were isothermally heat treated at 150, 250, and 350 °C. Microstructure survey revealed that generated shear-bands by CGP acted as talent sites for further strain-induced grain boundary migration (SIGBM) during annealing. SEM micrographs pointed out that coarse particles (1 μm <) had not preferential positions within the aluminum matrix and often comminuted into fine dispersoids (0.5 μm >) under heavy strains. Assessment of the softening fraction (Rrec) depicted that greater accumulated strains along with higher post-annealing temperatures accelerated the thermal recovery and hence, reduced the Rrec. Tensile testing was performed repeatedly while fluctuation range for obtained stress-strain curves for each specimen as a criterion of structural uniformity was considered. It was found that post-deformation treatment of severely deformed sheets at 250 °C supplied maximum structural uniformity due to perfect restoration and evolution of inhomogeneity agents such as shear-bands, and precipitates within the matrix. In this regard, processing route contained cold-rolling by 0.8 strain of two-pass CGPed specimen and subsequent annealing at 350 °C ascertained to be an adequate combination of deformation and heat treatment with respect to achievement of the maximum uniform elongation (UE) of 22.96% and contribution of uniform deformation energy (UD) in total mechanically absorbed energy equal to 84.18% that provided the optimal formability for the applied alloy. Fracture analysis was performed by fracture angle measurement and SEM fractography. Thermal annealing of CGPed sheet at 150 and 250 °C declined its fracture angle from 55° to 50° and 46.5°, respectively. However, annealing at 350 °C changed the dominant fracture mode from shear to necking by incidence of fibrous morphology and enhanced the angle to 61°. Likewise, it was revealed that in spite of fracture angle increment by rolling reduction, post-rolling prevented the necking fracture in CGPed sheet treated at 350 °C and this might be shifted to higher temperatures.

Microstructure and Mechanical Properties of a High-Mn TWIP Steel Subjected to Cold Rolling and Annealing

The structure–property relationship was studied in an Fe-18Mn-0.6C-1.5Al steel subjected to cold rolling to various total reductions from 20% to 80% and subsequent annealing for 30 min at temperatures of 673 to 973 K. The cold rolling resulted in significant strengthening of the steel. The hardness increased from 1900 to almost 6000 MPa after rolling reduction of 80%. Recovery of cold worked microstructure developed during annealing at temperatures of 673 and 773 K, resulting in slight softening, which did not exceed 0.2. On the other hand, static recrystallization readily developed in the cold rolled samples with total reductions above 20% during annealing at 873 and 973 K, leading to fractional softening of about 0.8. The recrystallized grain size depended on annealing temperature and rolling reduction; namely, it decreased with a decrease in the temperature and an increase in the rolling reduction. The mean recrystallized grain size from approximately 1 to 8 μm could be developed depending on the rolling/annealing conditions. The recovered and fine grained recrystallized steel samples were characterized by improved strength properties. The yield strength of the recovered, recrystallized, and partially recrystallized steel samples could be expressed by a unique relationship taking into account the fractional contributions from dislocation and grain size strengthening into overall strength.

Study on Static Recrystallization Behavior of Medium-Carbon Cr-Ni-Mo Alloyed Steel During Hot Deformation

A series of two-pass hot compression tests were conducted on Gleeble-1500 thermo-mechanical simulator to investigate the static recrystallization (SRX) behavior of a medium-carbon Cr-Ni-Mo alloyed steel 34CrNiMo. The compression tests were performed at a deformation temperature range of 950-1150 °C, a strain rate range of 0.1-3.5 s−1, and an interval time range of 1-100 s. The experimental flow stress curves and microstructural observation indicate that deformation temperature, pass interval time, strain rate, and pre-strain have significant influences on the recrystallization behavior of 34CrNiMo steel. It is identified that the softening fraction increases with the increasing deformation temperature, pre-strain, and interval time, while it decreases with the increasing strain rate. Based on the experimental data, the SRX kinetics equations of 34CrNiMo steel were developed. And the calculated results are in good agreement with the experimental ones, which demonstrates that the established equations can be used to describe the SRX behavior of 34CrNiMo steel at elevated temperatures.

An Investigation on the Softening Mechanism of 5754 Aluminum Alloy during Multistage Hot Deformation

Isothermal interrupted hot compression tests of 5754 aluminum were conducted on a Gleeble-3500 thermo-mechanical simulator at temperatures of 350 °C and 450 °C, and strain rates of 0.1 s−1 and 1 s−1. To investigate the metadynamic recrystallization behavior, a range of inter-pass delay times (5–60 s) was employed. These tests simulated flat rolling to investigate how softening behaviors respond to controlled parameters, such as deformation temperature, strain rate, and delay times. These data allowed the parameters for the hot rolling process to be optimized. The dynamic softening at each pass and the effect of metadynamic recrystallization on flow properties and microstructural evolution were analyzed in detail. An offset yield strength of 0.2% was employed to calculate the softening fraction undergoing metadynamic recrystallization. A kinetic model was developed to describe the metadynamic recrystallization behaviors of the hot-deformed 5754 aluminum alloy. Furthermore, the time constant for 50% recrystallization was expressed as functions related to the temperature and the strain rate. The experimental and calculated results were found to be in close agreement, which verified the developed model.

Metadynamic recrystallization of Nb–V microalloyed steel during hot deformation

Abstract

Static Softening in a Ni-30Fe Austenitic Model Alloy After Hot Deformation: Microstructure and Texture Evolution

In the current study, the microstructure and texture characteristics of a model Ni-30Fe austenitic alloy were investigated during hot deformation and subsequent isothermal holding. The deformation led to the formation of self-screening arrays of microbands within a majority of grains. The microbands characteristics underwent rather modest changes during the post-deformation annealing, which suggests that limited dislocation annihilation occurs within the corresponding dislocation walls. The fraction of statically recrystallized (SRX) grains progressively increased with the holding time and closely matched the softening fraction measured from the offset flow stress approach. The corresponding texture was weak and preserved its character with the holding time. There was no pronounced temperature effect on the grain boundary character distribution after the completion of SRX. The Σ3 and Σ9 coincidence site lattice boundaries were characterized as (111) pure twist and (1−14) symmetric tilt types, respectively. Nonetheless, the recrystallization temperature slightly affected the grain boundary network.

Modelling the Static Recrystallization Kinetics of Microalloyed TWIP Steels with Different Alloying Contents

During hot rolling, austenite recrystallization determines the grain size evolution and the extent of strain accumulation, and therefore, it can be used to tailor the microstructure and mechanical properties of the final product. However, at the moment, models describing the recrystallization kinetics of high-Mn steels are scarce and they do not take into account the effect of the alloying elements present in these steels. The aim of this work is to provide a quantitative model for the determination of the static recrystallization kinetics valid for a wide range of high-Mn steel compositions. Softening data determined for steels with different Mn (20 to 30%), Al (0 to 1.5%) and C (0.2 to 1%) levels at different strain, strain-rate and temperature conditions were analyzed. Static recrystallization of the investigated high-Mn steels follow Avrami’s law, with n Avrami exponents which are temperature dependent and lower than those determined for low C steels. A dependence of the t0.5 (time for 50% fractional softening) on the carbon content has been also observed and it was incorporated into an equation for the calculation of this parameter.

Softening Kinetics of Plain Carbon Steels Containing Dilute Nb Additions

The recrystallisation and precipitation kinetics of a plain carbon steel with 0.017 % Nb were studied using the double-hit deformation technique for interpass holding of 5 and 20s. The present study focuses on the effect of prestrain and deformation temperature on recrystallisation behaviour of the investigated steel. The fractional softening was calculated based on the percentage difference between the areas under the interrupted and uninterrupted deformations flow curves. The T5% and T95%, marking the beginning and end of recrystallisation, respectively, are determined as a function of strain. Quantitative microstructural studies validated the findings from the softening studies. The predicated results of recrystallisation regime are found to be in agreement with industrial observation and other experimental measurement for this steel. It can be seen that the dilute additions of Nb can influence the static recrystallisation of austenite under certain rolling condition which may lead to improved mechanical properties of steel.

The study on kinetics of metadynamic recrystallisation of a Nb-V microalloyed non-quenched and tempered steel

The metadynamic recrystallisation (MDRX) behaviour of a Nb-V microalloyed non-quenched and tempered steel was investigated by isothermal hot compression tests on Gleeble-1500 thermal-mechanical simulator. Compression tests were performed using double hit schedules at temperatures of 1,273-1,423 K, strain rates of 0.01-5 s−1, initial grain sizes of 92-149 μm and inter-pass time of 0.5-10 s. The effects of deformation parameters, including deformation temperature, strain rate, initial grain size and inter-pass time, on MDRX softening fraction were analysed. The experimental results show that MDRX softening fraction increases with the increase of deformation temperature, strain rate and inter-pass time, while it decreases with the increase of initial grain size. Based on the experimental results, the kinetic model of MDRX for the tested steel was established. A good agreement between the experimental and predicted MDRX softening fraction indicates that the established kinetic model can precisely predict the MDRX softening fraction for the tested steel.

Kinetics equations and microstructural evolution during metadynamic recrystallization in a nickel-based superalloy with δ phase

The metadynamic recrystallization (MDRX) characteristics of a nickel-based superalloy with δ phase are studied by isothermal two-pass hot compressive experiments. The kinetic equations are developed to evaluate the softening fractions caused by MDRX. The microstructural changes induced by MDRX are investigated using interrupt experiments and quantitatively characterized by microscopic observations. It is found that the softening fraction significantly increases with increasing the pre-strain, strain rate and deformation temperature. The evaluated softening fractions well agree with the tested ones, which demonstrates that the developed kinetic equations can accurately evaluate the MDRX fractions for the investigated superalloy. Additionally, the MDRX grain size sharply increases when the pre-strain and deformation temperature are increased, while obviously decreases with increasing the strain rate.

Physically-based constitutive modelling of hot deformation behavior in a LDX 2101 duplex stainless steel

A detailed understanding of the hot deformation and work hardening behavior of LDX 2101 dual phase steel has been obtained through a wide range of hot compression tests with strain rates from 0.01 to 50s−1 and temperatures from 900 to 1250°C. In most of the cases, the material showed typical dynamic recrystallization (DRX) behavior i.e., a peak followed by a gradual decrease to a steady state stress. The work hardening rate showed a two stage behavior i.e., a transient sharp drop at low stress values followed by a gradual decrease at higher stresses. Using the work hardening rate behavior at the latter stage, the saturation stress was calculated for different hot working conditions. Regression methods were used to develop a hyperbolic-sine equation linking the saturated stress to the deformation conditions. A physically-based Estrin–Mecking (EM) constitutive equation was then employed to model the flow behavior in the work hardening (WH)-dynamic recovery (DRV) regime. Finally, the Avrami equation to describe the evolution of the softening fraction was coupled to the EM model to extend the model to the dynamic recrystallization region. The results show that the model which is based on the stress-strain and work hardening behavior accurately predicts the flow behavior of this microstructurally complex steel.

A new method to predict the metadynamic recrystallization behavior in a typical nickel-based superalloy

The metadynamic recrystallization (MDRX) behaviors of a typical nickel-based superalloy are investigated by two-pass hot compression tests and four conventional stress-based conventional approaches (offset stress method, back-extrapolation stress method, peak stress method, and mean stress method). It is found that the conventional stress-based methods are not suitable to evaluate the MDRX softening fractions for the studied superalloy. Therefore, a new approach, ‘maximum stress method,’ is proposed to evaluate the MDRX softening fraction. Based on the proposed method, the effects of deformation temperature, strain rate, initial average grain size, and interpass time on MDRX behaviors are discussed in detail. Results show that MDRX softening fraction is sensitive to deformation parameters. The MDRX softening fraction rapidly increases with the increase of deformation temperature, strain rate, and interpass time. The MDRX softening fraction in the coarse-grain material is lower than that in the fine-grain material. Moreover, the observed microstructures indicate that the initial coarse grains can be effectively refined by MDRX. Based on the experimental results, the kinetics equations are established and validated to describe the MDRX behaviors of the studied superalloy.

Comparison of Two Physical Simulation Tests to Determine the No-Recrystallization Temperature in Hot Rolled Steel Plates

Abstract Two rolling simulations were conducted using a Gleeble 3500 to determine the no-recrystallization temperature, TNR on six microalloyed plate steels. Double hit deformation tests and multistep torsion tests were performed on steels containing varying amounts of Nb, V, and Ti. TNR for the double hit deformation tests were determined by finding fractional softening using the 5 % true-strain method and the intersection of the sigmoidal fractional softening curve with 20 % fractional softening. TNR for the multistep hot torsion test were determined using a mean flow stress method and finding the intersection of the two linear regions. TNR values following multistep hot torsion testing were lower than values measured after double hit deformation testing. The decrease in measured TNR values for the torsion tests occurs from the inherent multiple deformations, resulting in refined grains and an increase in nucleation sites for recrystallization during the subsequent deformation steps; thus recrystallization can continue to occur at lower temperatures.

Interaction Between Recovery, Recrystallization, and NbC Strain-Induced Precipitation in High-Mn Steels

The interaction between recovery, recrystallization, and strain-induced precipitation in two high-Mn steels, one of them microalloyed with Nb (0.1 pct) was investigated using mechanical testing and advanced microscopy techniques. Double-hit torsion tests were carried out in the 1373 K to 1173 K (1100 °C to 900 °C) temperature range in order to characterize the fractional softening behavior. Quenched specimens were analyzed using electron backscatter diffraction and transmission electron microscopy to determine the recrystallized fraction, the precipitation state, and the austenite microstructure evolution. At the highest temperature, 1373 K (1100 °C), similar softening kinetics were found in both steels. However, at temperatures lower than 1273 K (1000 °C) for the Nb steel, strain-induced precipitation was observed to take place resulting in significant softening retardation. For the base steel at all the temperatures investigated, and for the Nb steel in the absence of strain-induced precipitation, the mechanical softening corresponded well with the recrystallized fraction. However, when strain-induced precipitation took place, a major deviation was observed denoting a significant contribution of recovery to the fractional softening. Within the deformed grains, a substructure consisting of “subgrain bands” or microbands was developed. The precipitates were found mainly on the elongated subgrain boundaries, or at dislocations within the subgrains. This configuration was maintained after the migration of the recrystallization front.

The need to revise the current methods to measure and assess static recrystallization behavior

Heterogeneous deformation developed during “static recrystallization (SRX) tests” poses serious questions about the validity of the conventional methods to measure softening fraction. The challenges to measure SRX and verify a proposed kinetic model of SRX are discussed and a least square technique is utilized to quantify the error in a proposed SRX kinetic model. This technique relies on an existing computational–experimental multi-layer formulation to account for the heterogeneity during the post interruption hot torsion deformation. The kinetics of static recrystallization for a type 304 austenitic stainless steel deformed at 900°C and strain rate of 0.01s-1 is characterized implementing the formulation. Minimizing the error between the measured and calculated torque–twist data, the parameters of the kinetic model and the flow behavior during the second hit are evaluated and compared with those obtained based on a conventional technique. Typical static recrystallization distributions in the test sample will be presented. It has been found that the major differences between the conventional and the presented technique results are due to the heterogeneous recrystallization in the cylindrical core of the specimen where the material is still partially recrystallized at the onset of the second hit deformation. For the investigated experimental conditions, the core is confined in the first two-thirds of the gauge radius, when the holding time is shorter than 50s and the maximum pre-strain is about 0.5.

A study of the dynamic recrystallization kinetics of V-microalloyed medium carbon steel

The Avrami kinetics of the dynamic recrystallization (DRX) of V-microalloyed medium carbon steel is studied by measuring the flow curves and evolution of grain size distribution (GSD) due to DRX and by modeling the flow softening kinetics and flow curves under various deformation conditions. The e1/2 method is proposed as a new method of modeling the Avrami kinetics and is shown to be, unlike the conventional t 1/2 method, capable of modeling the flow softening kinetics and flow curves with a single variable Z (= $$ {\dot{{\varepsilon }}} $$ exp(Q def/RT)). The time exponent “n” of the Avrami kinetics decreases from 2.3 to 1.4 with increasing Z, which results from a shift in the mode of necklace recrystallization from boundary migration dominant mode to nucleation dominant mode. The GSD method is proposed as a new method for estimating the DRX fraction in deformation conditions where it is difficult to distinguish recrystallized grains from un-recrystallized grains. The softening fractions estimated from the flow curves tend to overestimate the DRX fractions at a later stage of deformation, where DRX kinetics appear to be retarded by dynamic precipitation.

Characteristics of metadynamic recrystallization of Nimonic 80A superalloy

Abstract

Static Recrystallization Kinetics of a Twin-roll Cast AZ31 alloy

The static recrystallization (SRX) behaviour of twin-roll cast and annealed magnesium AZ31 strip during hot deformation was determined under deformation conditions corresponding to the hot strip rolling process used in the production of twin-roll cast magnesium coils. Plane strain compression tests were performed using double-hit schedules at temperatures of 250°°C°–°400°°C, strain rates of 1°s-1°–°10°s-1, and inter-pass times of 1°s°−°400°s, whereby the first deformation step had a logarithmic strain below the critical logarithmic strain necessary for the beginning of dynamic recrystallization. Based on the experimental results, the kinetic equations and grain size model were established. Results show the effects of deformation parameters, including forming temperature and strain rate, on static softening fractions and grain size in the two-pass hot-deformed AZ31 alloy. Deformation-induced precipitations do not hinder the softening processes.

A comprehensive method to identify the kinetics of static recrystallization using the hot torsion test results with an inverse solution

Many difficulties exist in directly following the static recrystallization of metals, particularly during hot working. Indirect measurement of static recrystallization has been extensively performed in the literature where, for example, the recrystallization behavior of austenite in steels has commonly been measured indirectly using the fractional softening method. This method relies on the yield stress changes during recrystallization which are physically simulated by hot torsion or compression tests. However, the inherent heterogeneity of deformation during a mechanical test leads to a non-uniform static recrystallization distribution in the test sample. This, in turn, poses a serious question concerning the reliability of the measurement since the stress calculation techniques during recrystallization are not adequately developed in the existing literature.This paper develops a computer-based method to account for heterogeneous deformation during fractional softening measurements based on the hot torsion test data. The importance of the fractional softening gradient in determining the kinetics is emphasized and deficiencies in our understanding of the basic mechanisms are highlighted. A computer-based method is introduced to generate the experimental and computational components in a cost function. The cost function is then utilized by an inverse solution to calibrate the design parameters in a static recrystallization model.A