Fractional Softening Research Articles

Dynamic and Static Recrystallization Behavior of Low Carbon High Niobium Microalloyed Steel

The recrystallization behavior of a low carbon high niobium microalloyed steel was investigated using continuous and interrupted hot compression tests. The results showed that the initiation of dynamic recrystallization (DRX) could be detected from inflection in the plot of the strain hardening rate against stress regardless of the stress peak appearance. According to the Zener-Hollomon parameter equation, the activity energy of DRX ( Q def) was obtained and a new modified expression calculating Q def was proposed in consideration of the chemical composition of experimental steel. Applying the 2% offset method, the static softening fraction was determined. The graphic representation of the softening fraction vs interruption time gave the information about the non-static recrystallization temperature (about 1000 °C) and the relationship of precipitation-time-temperature. Static recrystallization kinetics followed the Avrami's law at high deformation temperature, and different values of the exponent m were given to illustrate the effect of niobium element on static recrystallization at different deformation temperatures.

Modeling the Flow Curve Characteristics of 410 Martensitic Stainless Steel Under Hot Working Condition

The hot deformation behavior of AISI 410 martensitic stainless steel was investigated by conducting hot compression tests between 1173 K (900 °C) and 1423 K (1150 °C) and between strain rates of 0.001 s−1 to 1 s−1. The hyperbolic sine function described the relation well between flow stress at a given strain and the Zener–Hollomon parameter (Z). The variation of flow stress with deformation temperature gave the average value of apparent activation energy as 448 kJ/mol. The strain and stress corresponding to two important points associated with flow curve (i.e., peak strain and the onset of steady-state flow) were related to the Z parameter using power-law equations. A model also was proposed based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation to estimate the fractional softening of dynamic recrystallization at any given strain. This model can be used readily for the prediction of flow stress. The values of n and k, material constants in the JMAK equation, were determined for the studied material. The strains regarding the peak and the onset of steady-state flow were formulated in term of applied strain rate and the constants of the JMAK equation. A good agreement was found between the predicted strains and those obtained by the experimental work.

Work hardening and kinetics of dynamic recrystallization in hot deformed austenite

The work hardening behaviors of 4 steels deformed under dynamic recrystallization (DRX) conditions were analysed. The h and r parameters necessary to describe the work hardening or dynamic recovery (DRV) curves in the absence of DRX were determined from the experimental flow curves. The fractional softening X attributable to DRX, defined as the difference between the calculated DRV and experimental DRX curves, was used to derive the Avrami kinetics of DRX. Some conclusions are drawn about the effects of deformation temperature, strain rate and steel composition on the kinetics of DRX.

Static softening characteristics and static recrystallization kinetics of aluminum alloy A6082 after hot deformation

The static softening behavior of aluminum alloy A6082 was investigated by interrupted hot tests conducted on Gleeble-1500 simulator at deformation temperatures from 573 to 773 K and strain rates from 0.1 to 10 s−1, with a pre-strain from 0.3 to 0.7 and variable inter-pass delay times. The offset method was applied to convert the changes in flow stress between two passes to static softening fraction. The microstructural changes were characterized by the quantitative metallography of quenched specimens. The results showed both static softening and static recrystallization curves exhibited a simple sigmoidal shape; the static softening is related to the static recrystallization in a nonlinear manner with 50% static recrystallized volume fraction corresponding to 80% static softening fraction; an increase in temperature, strain rate or pre-strain yields a decrease in the time for 50% static recrysallized volume fraction, on which the temperature has the most remarkable influence; Si and Mn additions accelerate the process of static recrystallization. Finally, the equations of static recrystallization kinetics of this alloy were developed with a good agreement between the predicted and experimental results.

Effect of Austenite Deformation on Recrystallisation Behaviour in an X-70 Microalloyed Steel

In the present study, the effect of austenite deformation on the recrystallisation behaviour in terms of recrystallisation-stop and recrystallisation-limit temperatures (T5% and T95%) of an X70 niobium microalloyed pipeline steel have been investigated by interrupted plane strain compression tests. The extents of recrystallisation are calculated using a modified fractional softening parameter. And the 20% and 60% of fractional softening were correlated to T5% and T95%. Quantitative optical metallography indicates that this method provides for a convenient and reliable experimental measurement of the critical temperatures associated with the recrystallisation of austenite. The recrystallisation kinetics and the precipitation kinetics of Nb(CN) were calculated using two widely applied models. The experimental results from this study suggest that the current model of precipitation kinetics might overestimate the precipitation start time.

Silicon Steel: Hot Working Constitutive Behaviour and Multistage Rolling Simulation

From torsion testing of silicon steel over the range 600 to 900 °C and 0.1-5 s-1, the constitutive analysis was performed by the sin h law to establish activation energies. As the temperature rose and strain rate declined, the fracture strain increased to as much as 14.3 and the subgrains in the elongated grains became larger. These results are shown graphically to agree with data from a wide collection of published reports on Si steels. The strengths were intermediate between those of 409 (11Cr) and 434 (16Cr-1Mo) that indicated Si has a strengthening effect almost 4 times that of Cr per unit weight; however, the activation energies of 409 and 434 were higher. Simulation of multistage rolling was done with T declining from 900 to 600 °C, pass strains of 0.2 or 0.3, strain rate of 1 s-1 and intervals of 20 or 40 seconds. Because of the short intervals, the pass flow curves did not exhibit the yield points of isothermal tests so that it was not possible to calculate fractional softening in the intervals at a declining temperature. Two new measures were suggested: a) the relative softening during an interval that decreased as temperature fell and b) relative reduction of the maximum pass stress relative to isothermal peak stress that increased at a decreasing rate as temperature declined.

Study of fractional softening of AZ31 magnesium alloy under multistage hot deformation

In the present study flow softening behaviour of AZ31 magnesium alloy was investigated by double hit compression tests in the temperature range 250–400°C and strain rate range 10–3–10–1 s–1. The tests were conducted with delay times varying 4–250 s after achieving the critical strain ϵc in each deformation condition. It has been found that static restoration processes (recovery and recrystallisation) were intensely depended on strain rate and deformation temperature. Fractional softening values increased with increasing strain rate and deformation temperature. Accordingly the softening curves were divided into three regions. The softening in the short interpass times was attributed to the occurrence of static recovery and followed by static recrystallisation and grain growth as dominant softening mechanisms for the second and third regions respectively. In addition static recrystallisation kinetics was interpreted by Avrami equation. Analysis of the results indicated that Avrami constant was changed by varying temperature.

Assessment of austenite static recrystallization and grain size evolution during multipass hot rolling of a niobium-microalloyed steel

Double-deformation isothermal tests and multipass continuous-cooling hot torsion tests were used to study the evolution of austenite microstructures during isothermal and non-isothermal hot deformation of an Nb microalloyed steel. These tests, coupled with microstructural characterization, have verified that the no-recrystallization temperature (T nr ) corresponds roughly to the temperature where recrystallization starts to be incomplete during rolling. An accurate method to estimate the recrystallized fraction during hot rolling based on stress-strain data, and which does not require metallographic studies, is proposed. The results of this method have been successfully compared to metallographic measurements, the values of non-isothermal fractional softening and the accumulated stress measured in the plots of mean flow stress (MFS) versus the inverse of temperature. A remarkable austenite grain refinement occurs in the first hot rolling passes after reheating. The correlation of isothermal and continuous cooling tests is better understood if the effect of grain size on recrystallization and precipitation is taken into account.

Evolution of austenite static recrystallization and grain size during hot rolling of a V-microalloyed steel

Laboratory double-deformation isothermal tests and multipass continuous cooling hot torsion tests were used to study the static recrystallization of austenite under isothermal and anisothermal conditions as well as to simulate the hot rolling of a 0.13% V-microalloyed steel. Characterization of the evolution of austenite microstructure was carried out. It has been verified that no-recrystallization temperature ( T nr) approximately corresponds to the temperature where recrystallization starts to be incomplete during rolling. However, incomplete recrystallization is visually evident at temperatures 25–50 °C below T nr, where grain elongation and increase in aspect ratio with temperature drop start to be significant. An accurate method to estimate the recrystallized fraction during hot rolling from stress–strain data and with no need of metallographic studies has been designed. The results of this method have been compared to metallographic measurements, the values of anisothermal fractional softening and the accumulated stress measured in the MFS plots at T < T nr. A pronounced austenite grain refinement has been detected in the first hot rolling passes after reheating, as grain size decreases from 155 μm to 27 μm in six passes. If the effect of grain size on recrystallization and precipitation is taken into account, the correlation of isothermal and continuous cooling tests as well as the relationship between SRCT and T nr or RLT temperatures can be better understood.

Recrystallization during Thermomechanical Processing of IMI834

The microstructure development of the near-α titanium alloy IMI834 displaying an initial bimodal α + β microstructure has been characterized with respect to dynamic and static β recrystallization, through quantitative metallography. The investigation was supplemented by interrupted compression testing, to evaluate the applicability of fractional softening in the determination of static recrystallization kinetics for titanium alloys. Micrographs are presented that indicate that dynamic recrystallization occurred under test conditions (temperatures of 975 °C, 1000 °C, 1025 °C, 1060 °C, and 1100 °C and strain rates of 0.01, 0.1, and 1 s−1), although complete grain refinement and homogeneity were only achieved following static recrystallization. Measurement obtained via image analysis revealed recrystallization kinetics that increased with temperature, for single-phase β predeformation microstructures (1060 °C to 1100 °C). However, bimodal α + β microstructures (1000 °C to 1025 °C) displayed more rapid recrystallization rates with decreasing temperature, which was attributed to a corresponding increase in α phase fraction. This increase yielded a more refined initial β grain size and an increase in favorable nucleation sites. In two-phase α + β microstructures (975 °C) with secondary α in the β matrix, recrystallized grains could not be directly observed with optical microscopy. However, interrupted compression testing indicated that static recrystallization may be comparable to that observed at 1000 °C. At all temperatures, an increasing strain rate accelerated recrystallization kinetics.

Prediction of metadynamic softening in a multi-pass hot deformed low alloy steel using artificial neural network

The metadynamic softening behaviors in 42CrMo steel were investigated by isothermal interrupted hot compression tests. Based on the experimental results, an efficient artificial neural network (ANN) model was developed to predict the flow stress and metadynamic softening fractions. The effects of deformation parameters on metadynamic softening behaviors in the hot deformed 42CrMo steel have been investigated by the experimental and predicted results from the developed ANN model. Results show that the effects of deformation parameters, such as strain rate and deformation temperature, on the softening fractions of metadynamic recrystallization are significant. However, the strain (beyond the peak strain) has little influence. A very good correlation between experimental and predicted results indicates that the excellent capability of the developed ANN model to predict the flow stress level and metadynamic softening, the metadynamic recrystallization behaviors were well evidenced.

AVALIAÇÃO DO AMACIAMENTO ESTÁTICO A QUENTE E A MORNO DE UM AÇO SILICIOSO

Double hit compression tests were performed on an ultra low carbon Si-Al alloyed steel. The true strain in both deformation steps was kept constant (0.2), while varying the strain rate (1 s -1 , 10 s -1 and 30 s -1 ), the temperature (1,050°C-austenite phase, 950°C-intercritical region and 850°C-ferritic phase) and the interpass time (0.5; 1; 5 and 50 s). The temperatures used are within the industrial range applied in the finishing rolling for this steel. The analysis of the flow curves obtained allowed the evaluation of the static softening characteristics of the steel. It was shown that the steel has a similar softening condition between passes at the three temperatures studied, with the softening fraction increasing with the increasing time and strain rate. The characteristics of the softening curves and microstructural features indicate that at the two higher temperatures (austenite phase and upper part of the intercritical region) the softening process is governed by recrystallization, with the nucleation and growth of subgrains. At the lower temperature, however, the softening is mainly due to strain induced boundary movement (SIBM) and recovery.

Formularization of softening fractions and related kinetics for static recrystallization using inverse analysis of double compression test

An inverse analysis for obtaining a flow curve equation, which accounts for inhomogeneous strain and temperature distributions, is applied to the evaluation of the softening fraction in a double compression test. To obtain the set of equations in the first and second hits, two approaches are used. One is to determine different sets of coefficients for the first and second hits, independently. The other approach is to introduce the residual strain ɛ R in the inverse analysis and to use the same set of coefficients in the first and second hits. Assuming both the rule of additivity for metallurgical strain and static recrystallization kinetics, which depends on temperature and applied strain, the residual strain after dwelling can be related to the recrystallized fraction. Under the above conditions, the effects of inhomogeneous strain and temperature distributions on softening behavior during the double compression test can be investigated. The distribution of softening behavior within a workpiece in plain carbon steel is investigated. The calculated softening fractions are compared with metallographically observed recrystallized fractions at three different observation points. Metallographically observed recrystallized volume fractions agree well with the calculated softening fractions, which accounts for inhomogeneous distributions of deformation and temperature, at higher fractions.

On the Recrystallisation Characteristics and Kinetics of a 9SMn28 Free Cutting Steel

The static and metadynamic recrystallisation characteristics of a 9SMn28 (EN 1.0715) free cutting steel have been evaluated by employing the stress relaxation technique. In the steel, the sulphur was bound in the form of numerous MnS inclusions. Fractional softening laws stating the kinetics of static and metadynamic softening behaviour were experimentally determined and compared with the existing literature data for C/C-Mn steels. The analysis of the static recrystallisation data suggested the powers of the strain and strain rate to be -2.7 and -0.13, respectively, and the apparent activation energy was estimated as 177 kJ/mol. The power of grain size was taken from a regression model developed previously that is able to predict the static recrystallisation kinetics of vast number of carbon and microalloyed steel grades. Even though a fraction of Mn was out of the solid solution in the form of sulphides, the predictions by the regression model as accounting the balance Mn in the solid solution were quite close to the experimental data, confirming the applicability of the model. As expected, the metadynamic recrystallisation behaviour showed a strong dependence on the strain rate, the power being -0.78 and the apparent activation energy 57 kJ/mol.

Effect of Initial Austenite Microstructure on the Softening – Precipitation Interaction in a Low Nb Microalloyed Steel

The influence of initial grain size on the softening-precipitation interaction in a low niobium microalloyed steel has been investigated. The study has revealed that for the largest initial grain size (1000 μm), the recrystallised fraction remains lower than the softening fraction until relatively long times are reached. In contrast, for the smallest initial grain size (166 μm) both magnitudes are similar. As a result, precipitation interacts with recrystallisation in the case of the finest austenite grain size, whereas for the coarsest one, since recrystallisation is significantly retarded, interaction with recovery process is observed. Apparently, the initial austenite grain size does not affect precipitation kinetics.

Dynamic and Metadynamic Recrystallization of a Martensitic Precipitation Hardenable Stainless Steel

The dynamic (DRX) and metadynamic (MDRX) recrystallization behaviour of an as-cast precipitation hardenable stainless steel has been investigated by conducting continuous and interrupted hot compression tests in the temperature range of 900 to 1150 °C and under strain rates of 0.001 to 1 s−1. The effects of temperature and strain rate and the Zener-Hollomon parameter (Z) on the flow behaviour and peak strain were investigated by continuous tests and the equation ϵ p = 4.3 × 10−4Z0.14 was proposed. The kinetics and fractional softening of MDRX were found to increase with temperature and strain rate. The Avrami exponent n was determined as 0.5 and the equation X = 1 – exp [−0.693 (t/t 0.5)]0.5 was proposed. From the variation of t 0.5 with deformation temperature and strain rate, the equation t 0.5 = 1.73 × 10−7 ϵ−0.33 exp(191000/RT) was proposed.

Dynamic and Metadynamic Recrystallization of a Martensitic Precipitation Hardenable Stainless Steel

The dynamic (DRX) and metadynamic (MDRX) recrystallization behaviour of an as-cast precipitation hardenable stainless steel has been investigated by conducting continuous and interrupted hot compression tests in the temperature range of 900 to 1150 °C and under strain rates of 0.001 to 1 s−1. The effects of temperature and strain rate and the Zener-Hollomon parameter (Z) on the flow behaviour and peak strain were investigated by continuous tests and the equation ϵp = 4.3 × 10−4Z0.14 was proposed. The kinetics and fractional softening of MDRX were found to increase with temperature and strain rate. The Avrami exponent n was determined as 0.5 and the equation X = 1 – exp [−0.693 (t/t0.5)]0.5 was proposed. From the variation of t0.5 with deformation temperature and strain rate, the equation t0.5 = 1.73 × 10−7 ϵ−0.33 exp(191000/RT) was proposed.

The Strain Dependence of Post-Deformation Softening during the Hot Compression of 304H Stainless Steel

Experiments were carried out in which the dependence of the fractional softening on temperature, time and strain rate was determined in a 304H stainless steel. Three prestrain ranges were identified pertaining to three different post-deformation softening behaviors: 1) prestraining to below the DRX critical strain: strongly strain dependent softening by SRX alone with softening kinetics controlled by growth rate of the nuclei; 2) prestraining to above the DRX critical strain: SRX + MDRX softening with weaker strain dependence of the kinetics but still controlled by grain growth; 3) at a prestrain of ε* and beyond: nucleation-controlled MDRX softening with the full inhibition of SRX. The transition prestrain ε* can exceed the peak strain if the DRX grain refinement ratio g = D0/DDRX &gt; 4. The transition to MDRX-dominated softening can be attributed to a constant value of the normalized strain hardening rate independent of the preloading temperature and strain rate. The softening data from the compression tests show that at ε*, the time for half softening t50 exhibits a minimum. These data differ somewhat from observations obtained in the torsion testing of solid bars, in which no strain dependence of t50 was detected at ε* and beyond. Whether or not the strain dependence of t50 vanishes in the MDRX range is sensitive to the test method employed to study the post-deformation softening.

Prediction of steel flow stresses under hot working conditions

An austenitic stainless steel was deformed in torsion over a temperature range of 900-1200 °C using strain rates of 1, 5 and 10 s-1. The stress vs. strain curves determined were corrected for deformation heating and the flow stress was found to rise in the initial work-hardening regime, reaching a maximum before dropping to the steady state due to softening brought about by dynamic recrystallization. In order to determine the onset of dynamic recrystallization, diagrams of work-hardening rate vs. applied stress were drawn up for the hardening region of the flow stress curves. The flow stress curves were modeled by adjusting an evolution equation having one internal variable that describes the plastic behavior in the work-hardening regime to the experimental data. The flow stress after the onset of dynamic recrystallization was determined by incorporating the fractional softening into the evolution equation. Describing the effects of temperature and strain rate on the evolution equation through Zener-Hollomon parameters, a database was constructed for use in computer models to predict the roll force of rolling or forging loads under hot working conditions.

Static and Dynamic Strain Aging at High Temperatures in 304 Stainless Steel

Commercial 304 austenitic stainless steel was deformed at high temperatures. The experiments involved 2-hit hot compression and multi-pass hot torsion testing; the experimental variables included strain rate, temperature and interpass time. The relationship between these variables and the degree of interpass softening produced unexpected results. Specifically, the normal effect of temperature on the static softening kinetics was reversed at intermediate interpass times: the fractional softening decreased with increasing temperature for these times. The diffusion kinetics and segregation mechanics of the substitutional impurities in the material, combined with the experimental results, suggest that the temporary non-equilibrium segregation of phosphorus (and/or sulphur) to dislocations is responsible for the observed behaviour. Additionally, the observed trend in strain rate sensitivity with increasing deformation temperature indicates that dynamic strain aging was taking place.