Static Softening Research Articles

Effect of deformation parameters on microstructure of martensitic stainless steel during static recrystallization

In this paper, the static recrystallization behavior of X20Cr13 martensitic stainless steel was studied based on the hot compression test. The experimental results show that when the strain of the first pass is less than the critical strain of dynamic recrystallization, the main static softening mechanism is static recrystallization. The effects of prestrain, initial austenite grain size, deformation temperature and strain rate on the microstructure during static recrystallization were analyzed in detail.

Study of Nb Solubility in Hot Charging Process

The aim of this work is to investigate the dissolution behavior of Nb in hot charging hot rolling configurations. To do so, an indirect experimental procedure is used to quantify the amount of Nb present in solution before rolling. The method is based on the effect of dissolved Nb on static recrystallization kinetics due to its solute drag effect. After different thermal cycles, simulating cold and hot charging conditions, double hit torsion tests have been performed with a 0.23%C steel microalloyed with 0.03% Nb. By means of these tests, the static softening behavior has been determined. Comparison of the recrystallization times allows indirect evaluation of the amount of Nb in solid solution after each treatment. The results have been correlated with the precipitation state of the samples.

Microstructure Evolution and Kinetics of Static Recrystallization of Medium Mn Steel in the Two‐Hit Isothermal Compression

The thermal deformation behavior and microstructure evolution of medium Mn steel (0.15C–7Mn) are investigated by the two‐hit compression tests. The results reveal that different static softening dynamics are the functions of deformation temperature, interval time, and strain rate. The short static softening plateaus are exhibited in the static softening curves during post‐deformation holding at 950 °C and 0.1, 1 s−1. Based on the electron backscatter diffraction (EBSD) analysis, the progresses of static recovery (SRV) and static recrystallization (SRX) are enhanced by the increment of deformation temperature. The frequencies of medium angle boundaries (MAGBs) and high angle boundaries (HAGBs) increase gradually with an increased interval time at 1000 °C and 0.1 s−1, which suggests that both SRV and SRX contribute to static softening, but the predominant softening mechanism is SRV. In addition, the influence of temperature, interval time, and strain rate on the distribution and average magnitude of grain size is investigated. The deformation temperature and strain rate have a significant influence not only on average grain dimension, but also on the distribution of grain size. Interval time has great influence on average grain size, but has slight effect on distribution of grain size.

Study of static recrystallization behavior of austenite in a Ti–V microalloyed steel

The static recrystallization (SRX) behavior in Ti–V microalloyed steel was studied through two-pass compression tests using a Gleeble-3500 thermo-mechanical simulator. Under the experimental conditions with the deformation temperatures of 950–1050 °C, strain rates of 0.01–1 s–1, pass strains of 0.1–0.2 and inter-pass time of 1–100 s, respectively. The influence of various processing parameters on the static softening were discussed seriously and the results showed that the softening fractions raise quickly with the deformation temperature, the strain rate, the pass strain and the inter-pass time; Oppositely, it is reduced as the initial austenite grain size raises. In addition, the kinetic equations have been adopted and the required parameters have been determined based on the experiments, the calculated results are consistent with the experimental results, indicating the SRX behavior and microstructural evolutions of Ti–V microalloyed steel could be estimated by the determined kinetics parameters.

Static Softening Behavior and Modified Kinetics of Al 2219 Alloy Based on a Double-Pass Hot Compression Test

In this paper, the static softening mechanism of a 2219 aluminum alloy was studied based on a double-pass isothermal compression test. For the experiment, different temperatures (623 K, 723 K, and 773 K), strain rates (0.1/s, 1/s, and 10/s), deformation ratios (20%, 30%, and 40%), and insulation periods (5 s, 30 s, and 60 s) were used. Based on the double-pass flow stress curves obtained from the experiment, the step rate expressed by the equivalent dynamic recrystallization fraction is dependent on the deformation parameters, which increases with the increase in strain rate and insulation time, while it decreases with the increase in temperature and strain. Based on the microstructure observed using electron backscattered diffraction (EBSD), the static softening mechanism of the Al 2219 alloy is mainly static recovery and incomplete static recrystallization. A new expression for the static recrystallization fraction is proposed using the reduction rate of the sub-grain boundary. The dependent rule on the deformation parameters is consistent with the step rate, but it is of physical significance. In addition, the modified static recrystallization kinetics established by the new SRX fraction method was proven to have a good modeling and prediction performance under given deformation conditions.

Integrated physically based modeling for the multiple static softening mechanisms following multi-stage hot deformation in Al-Zn-Mg-Cu alloys

Modern aluminum industries need an in-depth understanding and a more accurate prediction of the flow stress and microstructure evolution during multi-stage thermomechanical processing. The underlying mechanisms of Al alloys are distinct from that of other metallic materials (e.g., steels and copper) owing to the very high stacking fault energy. In the hot forming process of ultra-high strength Al-Zn-Mg-Cu alloys, the high alloying element additions have a more complex effect on multiple static softening mechanisms during post-deformation, i.e. the coupled recovery, recrystallization, precipitation and their interactions, which have not been well revealed and included in the existing plasticity models. In the present work, an integrated physically based model based on the observed microstructural characteristics and static softening behavior was developed to unravel the multiple static softening mechanisms following multi-stage hot deformation of Al-Zn-Mg-Cu alloys. By incorporating the multicomponent effects, i.e. process variables and chemical compositions, into static recovery, static recrystallization and precipitate coarsening models, the evolutions of stress, microstructure and static softening fraction could be accounted reasonably during post-deformation holding. A special attention was paid to model the functions of various alloy solute contents (i.e. Zn, Mg, Cu and Zr) on the precipitation thermodynamic, recovery and recrystallization kinetics in Al-Zn-Mg-Cu alloys. After validating by experimental data, the integrated physically based model could predict the effects of alloying elements on microstructural evolution, recrystallization and static softening kinetics of Al-Zn-Mg-Cu alloys. It was found that the different precipitation behaviors due to the addition of various alloying elements have a considerable influence on recovery, recrystallization and coupled static softening process. The solid solution atoms remaining in the matrix basically contributed to grain boundaries mobility and then slow recrystallization process, which depended on the interaction parameter, binding energy and diffusion rates of various alloying elements. This work offers an in-depth understanding of static softening mechanisms and provides a potential way for future development of advanced models and design strategies for multi-stage thermomechanical processes in Al-Zn-Mg-Cu alloys.

Static Softening Behavior of Low Carbon High Niobium Microalloyed Steel

The MMS-200 thermal simulation testing machine was used to study the static softening behavior of low carbon high niobium microalloyed steel. The effect of niobium to the static recrystallization softening behavior of the microalloy steel had been analyzed by establishing the kinetics model of static recrystallization and the micro-morphology of precipitates. The results indicated that: the static softening behavior of the tested steel significantly influenced by the deformation temperature and the interval pass time of the rolling processing. At relatively high deformation temperature and long interval pass time, the ratio of static softening was increased. Then the deformation temperature was lower to 950°C, and the static softening behavior of the test steel was ceased. But when the deformation temperature was higher than 1000°C, the static softening behavior of the test steel completely occurred. The activation energy of the test steel was 325·mol-1 by the established model calculated.

Static Recrystallization Behavior of Non-Quenched and Tempered Steel 38MnSiVS

Refinement and uniform austenite grains are essential to obtain excellent and homogenous properties for non-quenched and tempered steel, which is mainly affected by static recrystallization of the rolling process. Using the Gleeble-3500 thermal simulation test machine, 20% compression test was carried out for two passes at 850~1050 °C (interval of 50 °C) and different pass interval time conditions to study the static softening and recrystallization behavior of 38MnSiVS non-quenched and tempered steel during deformation process. The effects of strain rate, deformation temperature and interval time on static softening rate and austenite recrystallization fraction were analyzed. The results showed that the increase of deformation temperature and the increase of pass interval time had more significant impact on the static recrystallization volume fraction of 38MnSiVS steel, while the influence of strain rate was relatively smaller. When the deformation temperature was 950 °C or higher, the non-conditioning steel 38MnSiVS could undergo complete recrystallization, and partial recrystallization occurred in the temperature range of 850-950 °C. A static recrystallization volume fraction model of non-regulatory steel 38MnSiVS was established. The static recrystallization activation energy was 296.7 kJ·mol-1, and the static recrystallization volume fraction model had a relative error of 2%.

Experiment and simulation of static softening behavior of alloyed steel during round bar hot rolling

Static softening is a crucial mechanism during the hot rolling of steel to relax residual stress and strain, refine microstructure, and improve mechanical properties. In this study, double hit tests with varying temperature, strain rate, interpass time, and pre-strains were performed using a Gleeble machine to investigate static softening behavior. Based on experimental results, a kinetic model of static softening was developed to represent interpass softening behavior during hot rolling. An explicit static softening model was implemented as a subroutine into a three-dimensional finite element model of round bar hot rolling and static softening was simulated. Results show that static softening occurs quickly at the beginning of the interpass time and then slows down. Also, the effects of temperature and rolling speed on static softening were simulated and the results show that temperature has a more significant influence on static softening than rolling speed.

Static softening behavior and modeling of an Al–Cu–Mg–Zr alloy with various pre-precipitation microstructures during multistage hot deformation

The complex precipitates have been found to work on the hot workability of heat treatable Al alloys considerably. In this work, to include the functions of interrupted holding following multistage hot working, double-stage hot compression tests were performed on an Al–Cu–Mg–Zr alloy with different pre-precipitation microstructures which were tailored through air cooling, water quenching and furnace cooling after solution heat treatment. Microstructural characterizations and physically-based modeling were employed to investigate the static softening behaviors during interval holding. The results indicated that static softening fraction increased with rising deformation temperature, strain rate and holding time. Under the same deformation conditions, the furnace-cooled alloy presented the highest static softening, while the lowest in water-quenched alloy. Particularly, double plateaus were presented in the static softening curve of water-quenched alloy when deformed at 300 °C and 0.1 s−1, which was interpreted by static recovery, static precipitation and its coarsening as well as the complete depletion of stored strain energy during post-deformation holding. When deformed at 450 °C and 0.1 s−1, higher static softening fraction and longer plateaus were observed in all alloy specimens due to the remarkable static recovery within a very short holding time after first-pass deformation. In addition, the established recovery model could generally rationalize the experimental results, and the direct static softening contribution of precipitates coarsening was indicated to be slight according to the results of integrated model.

Dynamic and static softening behavior of a titanium added ultra-low carbon steel during hot and warm deformation

In this work, the mechanical and metallurgical behavior of titanium added ultra-low carbon (ULC-Ti) steel under hot and warm working conditions was studied by means of dilatometer compression tests. Double compression schedules were performed at temperatures between 700 °C and 1000 °C, inter-pass times of 0,5−100 s, and deformations of 0.5 and 0.3 in the first and second stages, respectively, with strain rate of 1 s−1. The analysis of the flow curves obtained allowed the evaluation of the softening characteristics of the steel. The softening and microstructure features indicate different restoration mechanisms depending of the temperature range. Static recrystallization with rapid microstructure restoration is observed at temperatures within the austenitic and intercritical region. Strain induced boundary motion and recovery mechanisms apparently control the restoration process in the ferrite region. In general, flow stress curves showed dynamic recovery behavior. Only the deformation in the intercritical region produced a peak in the flow stress curves. Isothermal holding tests associated with single hit compression indicated the presence of dynamic strain induced transformation, accelerating the softening kinetics at the intercritical region.

Delay in the static softening kinetics of a Nb-N-bearing austenitic stainless steel

In this work, the retardation in the static softening kinetics of an austenitic stainless steel ASTM F-1586 (ISO 5832-9) used in the manufacture of orthopedic implants is investigated. Double-pass deformations were applied by means of torsion testing under isothermal conditions. The samples were initially heated to 1250 °C using an induction furnace and were held for 300 s to allow for complete solubility. This was followed by cooling down to the deformation temperatures (Tdef) in the range 1000 to 1200 °C. Pass strains of 0.30, strain rate of 1.0 s −1 and interpass times ranging from 0.1–1800 s were employed. These process parameters were selected to simulate the industrial-scale hot forging of this alloy to produce orthopedic implants. The results of the thermomechanical physical simulations indicated that 60% of the softening mechanisms occurred by means of static recovery (SRV). The calculated stacking fault energy (γSFE) of the present material is around 69 mJ m−2, which explains the incubation time for the onset of static recrystallization (SRX) to be greater than 10 s at temperatures below 1100 °C. The static softening curves, i.e. softening fraction (Xs) versus time (tp) plots, indicate that the delay in the static recrystallization (SRX) kinetics probably resulted from the effect of Z-phase precipitates (NbCrN) located in the grain boundaries which were induced by deformation.

Effect of Zn content on the static softening behavior and kinetics of Al–Zn–Mg–Cu alloys during double-stage hot deformation

Zn content has significant influence on the hot workability and final properties of Al–Zn–Mg–Cu alloys. In the present work, single and double stage hot compression tests have been performed on Al–Zn–Mg–Cu alloys with various Zn contents at temperatures of 300 °C and 400 °C, and strain rates of 0.01 s−1 and 0.1 s−1, respectively. The results showed that the static softening curve plateau was observed, both the duration of static softening plateau and softening fraction decreased with increasing Zn content at deformation temperature of 300 °C. When deformation temperature increased to 400 °C, the static softening curves appeared approximate typical sigmoidal shape with slight influence from Zn content. By combining additional in-situ electrical resistivity measurement, hardness testing and microstructural observations, the static softening mechanisms were found to be the functions of static recovery and precipitates coarsening at lower temperature, and static recovery and static recrystallization at higher temperature. The increased Zn addition mainly affected static softening mechanisms at 300 °C by forming precipitates. The simplified static softening kinetics were also investigated based on Johnson-Mehl-Avrami-Kolmogorov model which coupling static recovery and static recrystallization.

The role of various Zr additions in static softening behavior of Al-Zn-Mg-Cu alloys during interval holding of double-stage hot deformation

Double-stage interrupted uniaxial hot compression tests were conducted to investigate the static softening behavior in Al-Zn-Mg-Cu alloys with various Zr additions at temperature of 300 °C and 400 °C and strain rates of 0.01 s−1 and 0.1 s−1. The microstructural evolution was also characterized by utilizing electron backscatter diffraction and transmission electron microscopy. The results revealed that static softening fraction of the studied alloys increases with increased Zr additions. Different static softening kinetics and plateaus were indicated to be the functions of deformation temperature, strain rate and Zr addition. At 300 °C, the observed short static softening plateaus were attributed to static recovery, static precipitation and its coarsening during post-deformation holding. High static softening fractions and long plateaus were presented when deformed at 400 °C and 0.1 s−1, which were resulted from rapid static recovery and recrystallization within very short time.

Study on Static Recrystallization Behavior of 38MnVTi Non-Quenched and Tempered Steel

The static recrystallization behavior of 38MnVTi non-quenched and tempered steel within the interval of passes was investigated by double-pass compression experiments using Gleeble-1500 thermal simulation machine. The true strain-stress curves of the steel were obtained, and the effects of deformation temperature, strain rate and pre-strain on the static recrystallization behaviours were studied. Single-pass compression experiments for 38MnVTi non-quenched and tempered steel were finished with the same conditions to the double-pass compression experiments. The static recrystallization dynamics model and the static recrystallization grain size model can be established. The predicted results of the model accord well with experimental ones, which indicate the proposed model, can give an accurate prediction of the static softening behaviour for 38MnVTi non-quenched and tempered steel.

Modeling and Simulation of the Static Recrystallization of 5754 Aluminium Alloy by Cellular Automaton

To study the factors that affect the mechanical properties of materials, double-pass hot compression tests were performed under different deformation parameters using a Gleeble-3500 thermo-simulation machine. The static softening behavior of 5754 aluminium alloy during testing was analyzed by the 0.2% offset-stress method. The results show that the static softening fraction was greatly influenced by deformation parameters and rapidly increased with increasing delay time, strain rate and deformation temperature. In addition, a mesoscopic cellular automaton (CA) model was employed to simulate the microstructural evolution of the static recrystallization (SRX) during the double-pass hot compression test of the 5754 aluminium alloy. The results show that the SRX nuclei first formed along the grain boundaries, where the energy was sufficient, and deformation parameters had a significant influence on the SRX of the 5754 aluminium alloy. The recrystallized volume fraction increased with increasing temperature, strain rate, and delay time among deformation stages. The mean recrystallized grain size increased with increasing deformation temperature and delay time. However, the mean grain size was slightly reduced with an increase in the strain rate from 0.1 s−1 to 1 s−1 at constant temperature and delay time.

Microstructural evolution of a superaustenitic stainless steel during a two-step deformation process

Single- and two-step hot compression experiments were carried out on 16Cr25Ni6Mo superaustenitic stainless steel in the temperature range from 950 to 1150°C and at a strain rate of 0.1 s-1. In the two-step tests, the first pass was interrupted at a strain of 0.2; after an interpass time of 5, 20, 40, 60, or 80 s, the test was resumed. The progress of dynamic recrystallization at the interruption strain was less than 10%. The static softening in the interpass period increased with increasing deformation temperature and increasing interpass time. The static recrystallization was found to be responsible for fast static softening in the temperature range from 950 to 1050°C. However, the gentle static softening at 1100 and 1150°C was attributed to the combination of static and metadynamic recrystallizations. The correlation between calculated fractional softening and microstructural observations showed that approximately 30% of interpass softening could be attributed to the static recovery. The microstructural observations illustrated the formation of fine recrystallized grains at the grain boundaries at longer interpass time. The Avrami kinetics equation was used to establish a relationship between the fractional softening and the interpass period. The activation energy for static softening was determined as 276 kJ/mol.

Mechanical Behavior and Microstructure Evolution of Al-Cu-Li Alloy during Multi-Pass Hot Deformation

Continuous, discontinuous, double-pass and multi-pass hot isothermal compression tests were conducted to understand the mechanical behavior and microstructure evolution of the Al-2.5Cu-1.58Li-0.3Mn-0.12Zr-0.06Mg-0.05Ti (wt%) alloy. The tests were conducted at temperature of 420 °C and strain rates of 0.001, 0.1 and 10 s −1 on Gleeble-1500. EBSD and TEM were used to investigate microstructure evolution at different deformation stages of the alloy. Double-pass hot deformation results show that the extension of interval time between two passes is conductive to promoting static softening. It is possible to control the grain size in the test alloy by changing the strain rate at different deformation stages. During multi-pass hot deformation process, static softening between the deformation pass is beneficial to the dynamic softening, vice versa. Dynamic precipitation of T 1 and θ′ phase is observed during the multi-pass deformation process ( T =420 °C, ɛ ˙ = 0.1 s −1 ). In the initial stage of multi-pass deformation, strain-induced dislocations provide the nucleation site for θ′ and T 1 phase, promoting their precipitation and coarsening. With increasing of strain, the size of the precipitates T 1 and θ′ phase decreases and then tends to be stable, and the density of δ′ phase decreases. Flow stress of the test alloy during multi-pass hot deformation is influenced by dynamic softening, static softening and dynamic precipitation.

Static recovery of an AlMg4.5Mn aluminium alloy during multi-pass hot-rolling

Abstract In multi-pass hot-rolling of aluminium alloys stored energy is created by the plastic deformation in the form of dislocations and sub-grains. The energy dissipates due to sub-grain growth and the annihilation or rearrangement of dislocations. Both mechanisms lead to a reduction of dislocation density and hence a decrease in yield stress, i.e. to recovery and softening. Reliable quantitative results on these aspects, experimental as well as theoretical, are of high technological importance. Literature provides many results on the hardening and softening behaviour of aluminium alloys during plastic deformation. Few quantitative investigations, however, deal with the static softening after hot deformation. In this contribution we share recent quantitative results on the static softening behaviour of an AlMg4.5Mn aluminium alloy and its implications for the industrial hot-rolling process. We show the static softening behaviour derived from stress relaxation tests on a Baehr DIL805 dilatometer. The results agree well with a microstructure model, which is based on thermally activated dislocation climb. The activation energy for static recovery, however, is found to be outside the range predicted by theory. The relaxation results together with additional thermomechanical characterization are used to calibrate a material model based on the Kocks-Mecking equation, which has been implemented in a commercial implicit Finite-Element software. Numerical results from a three-dimensional Finite-Element model of the industrial multi-pass hot-rolling process show how static recovery affects the distribution of dislocation density across the rolling ingot between passes.

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.