Double-hit Compression Tests Research Articles

Analysis of Recrystallization Kinetics Concerning the Experimental, Computational, and Empirical Evaluation of Critical Temperatures for Static Recrystallization in Nb, Ti, and V Microalloyed Steels

Recrystallization kinetics and two critical temperatures—the non-recrystallization temperature TNR and the static recrystallization critical temperature TSRCT—of five Nb, Ti, and V microalloyed steel grades are evaluated. The experimental examination is realized by employing isothermal double-hit compression tests and continuous hot torsion tests, both performed on a Gleeble® 3800 thermo-mechanical simulator. The experimental results are used for the critical assessment of predicted TNR using four empirical equations from the literature, and for the validation of simulated TNR and TSRCT. The thermokinetic computer simulations are realized using the mean-field microstructure modeling software MatCalc. Analysis shows that higher microalloying contents increase both critical temperatures, TNR and TSRCT, whereby the effect of recrystallization retardation of Nb is more pronounced than that of Ti or V. The most accurate reproduction of the experimental recrystallization behavior of the five examined steel grades is realized by the employed physics-based simulation approach.

Recrystallisation Characteristics of a Cu Bearing HSLA Steel Assessed Through High Temperature Compressive Deformation

Dynamic (DRX) operative duringdeformation and static recrystallization (SRX) operative after deformation are considered responsible for the changes in microstructure and texture of deformed materials. Especially in the case of advanced steels that are required in the form of plates of various thicknesses, hot rolling is the main manufacturing process during which the steel undergoes DRX under the rolls and SRX between rolling passes/strands. Knowledge on DRX and SRX characteristics of such steels is crucial for optimisation of hot rolling parameters,achieving the desired microstructure and consequently the targeted mechanical properties.In this study, certain key aspects of both dynamic (DRX) and static recrystallisation (SRX) behaviour of aCu-bearing HSLA steel, which was developed at DMRL, have been explored through high temperature deformation studies using Gleeble thermo-mechanical simulator. Through uniaxial compression testing in the austenitic regime, domains of continuous and discontinuous DRX prevalent in the steel were identified and critical parameters for initiation of dynamic recrystallisation viz., critical strain (ec), critical stress (sc), peak stress (sp) and peak strain (ep) were determined as a function of temperature and strain rate. SRX characteristics were assessed through uniaxial double hit compression tests with fixed strain rate and strain per hit but at different temperatures and with different imposed intermediate static recrystallisation (ISRT) times. From the fractional softening data, parameters such as time for 50% recrystallisation, t0.5, temperature for 50% recrystallisation, T0.5, and activation energy, QSRX have been estimated.Although the steel exhibited good plastic deformation characteristics, the results suggest that the role of copper in retarding recrystallisation is significant.

Effect of Sample Geometry on Strain Uniformity and Double Hit Compression Tests for Softening Kinetics Determination

Static softening is an essential process during the hot rolling of steel to refine grain size and improve mechanical properties. The double hit test is used to measure the static softening volume fraction from the flow stress curves. Herein, the influence of different sample geometries on the determination of softening fraction from the double hit test for conditions where 0–99% softening is expected. The double hit tests are modeled in DEFORM for conventional sample geometries in both uniaxial compression and standard plane strain compression tests, and an additional simulation of a modified plane strain compression test sample geometry, designed to provide more uniform strain distributions, is also performed. A user routine is incorporated into the model to predict the localized softening volume fraction from localized strain based on known softening equations and set back the localized strain value to zero while the softening volume fraction reaches 85%. The standard plane strain compression test geometry shows a strong strain gradient resulting in inaccurate softening fraction volume predictions from flow stress, whereas the uniaxial compression test and modified plane strain compression test geometries show better predictions of softening volume fraction from the flow stress data.

Coupling crystal plasticity and cellular automaton models to study meta-dynamic recrystallization during hot rolling at high strain rates

Predicting microstructure and (micro-)texture evolution during thermo-mechanical processing requires the combined simulation of plastic deformation and recrystallization. Here, a simulation approach based on the coupling of a full-field dislocation density based crystal plasticity model and a cellular automaton model is presented. A regridding/remeshing procedure is used to transfer data between the deformed mesh of the large-strain crystal plasticity model and the regular grid of the cellular automaton. Moreover, a physics based nucleation criterion has been developed based on dislocation density difference and changes in orientation due to deformation. The developed framework is used to study meta-dynamic recrystallization during double-hit compression tests and multi-stand rolling in high-resolution representative volume elements. These simulations reveal a good agreement with experimental results in terms of texture evolution, mechanical behaviour and growth kinetics, while enabling insights regarding the effect of nucleation on kinetics and crystallographic texture evolution.

A Study of Recrystallization Behavior of Two Ni-Cu–Containing High-Strength Steels with Different Mn Contents

Abstract The Defence Metallurgical Research Laboratory (DMRL) has been investigating several high-strength and high-toughness steels in the past few decades for various defense applications. As a part of this work, two novel low-carbon steels—each containing 4–6 wt% Ni + Cu of similar strength level, one with high Mn (∼1 wt%) and one with low Mn (∼0.3 wt%)—were investigated. In order to optimize the rolling schedules during production, it is necessary to understand the recrystallization behavior of these two steels during hot deformation. Usually, the recrystallization behavior of materials that do not undergo phase transformations (viz. aluminum, copper, etc.) is commonly studied using microstructural techniques for identification of fraction of recrystallized grains. But, as the steels in the present study undergo phase transformation wherein the austenitic structure at deformation temperature is transformed to martensitic/bainitic structure during cooling, it is challenging to track the recrystallization process directly from the microstructure. Therefore, the recrystallization behavior was investigated by conducting double-hit compression tests (DHCTs) using Gleeble. Fractional softening values were determined from DHCT data using various methods, such as the offset method, back-extrapolation method, and mean flow stress method. The mean flow stress method was found to be more reliable and thus was adopted in this study. The effects of different deformation temperature (850°C–1,050°C) and intermediate static recrystallization time (ISRT) (10, 100, and 500 s) on fractional softening were studied. Temperature for 50 % recrystallization (T0.5), determined as the temperature for 50 % fractional softening, was seen to reduce with an increase in ISRT. Steel with lower manganese content showed relatively lower fractional softening values and thus relatively higher T0.5, as compared with that for the steel with higher manganese content, for all three ISRTs.

Effect of Strain-Induced Precipitation on the Austenite Non-recrystallization (Tnr) Behavior of a High Niobium Microalloyed Steel

The non-recrystallization temperature (Tnr) of high niobium microalloyed steel was determined from both multihit and double-hit compression tests obtained under plane strain condition. The Tnr was determined to be in the range of 985 °C to 1010 °C for multihit conditions. The double-hit tests carried out at an interpass time of 5 seconds gave very low Tnr in the range of 860 °C to 900 °C. In order to understand this, double-hit experiments were carried out for different interpass times (2, 15, 100 seconds) at three different temperatures (950 °C, 1050 °C, and 1150 °C). The negative softening behavior was observed at 950 °C for higher interpass times of 15 and 100 seconds. This implies that the Tnr of this steel is well above 950 °C. This was due to the high amount of strain-induced precipitation of niobium carbonitrides at higher interpass times as revealed from TEM investigation. The precipitate size evolution considering the Dutta and Sellars nucleation condition in TC-PRISMA agrees well with the experimentally observed precipitate sizes. However, the evolution of Zener pinning forces considering TC-PRISMA nucleation condition is similar to both reported and experimentally determined values. The evolution of tensile properties also correlates well with the observed austenite recrystallization softening behavior. Therefore, a minimum interpass time of 15 seconds is required during double-hit compression tests to effectively precipitate Nb(C, N) and delay the static recrystallization softening behavior of this steel. This lead to the determination of comparable Tnr values between double-hit and multihit methods.

Role of Precipitates in Recrystallization Mechanisms of Nb-Mo Microalloyed Steel

Recrystallization behavior of a Nb-Mo microalloyed steel has been investigated using double-hit compression tests. A suitable reheating temperature and soaking time were established for the complete dissolution of the microalloying precipitates prior to hot deformation. The influence of hot deformation conditions on the flow behavior and the effect of alloying elements, Nb in particular, on austenite recrystallization kinetics are highlighted. The strain-induced precipitation has been found to play an important role in hindering recrystallization kinetics. Post-deformation microstructural analysis indicates that strain-induced grain boundary migration (SIMB) is one of the mechanisms for the formation of recrystallized grains. Retardation of recrystallization has been explained by estimating pinning force of Nb(C, N) precipitates and recrystallization driving force at 1000 °C, which is in good agreement with the experimental observations in the present study as well as other reported data. The study brings out a better understanding on the influence of strain-induced precipitates on recrystallization behavior.

Recrystallisation-assisted creep of an austenitic Fe-Ni alloy under low stresses after hot deformation

Static and meta-dynamic recrystallisation are known to take place in some steels under low tensile stresses during thermo-mechanical processes, e.g. between rolling stands in a hot strip finishing mill. The main objective of the present work is to understand the mechanisms responsible for the recrystallisation-assisted visco-plastic deformation in a hot worked iron-nickel austenitic alloy subjected to low stress levels at high temperatures. To that purpose, the recrystallisation kinetics of the alloy was first quantified from stress relaxation, interrupted creep and double-hit compression tests, followed by a microstructural study. The evolutions of strain both in the recrystallising and in the fully recrystallised microstructures were quantified. One of the most original aspects of this work is the experimental verification that the acceleration of the viscoplastic deformation of a recrystallising material is unequivocally and intrinsically associated with the on-going recrystallisation phenomenon itself. For the austenitic alloy of interest, the most important contribution to this deformation enhancement was found to arise from the deformation of newly recrystallised grains in the primary creep regime. Finally, it is also shown that this recrystallisation-assisted deformation can be qualitatively predicted with acceptable accuracy using a relatively simple, physics-inspired constitutive model.

Assessment of EBSD Analysis and Reconstruction Methods as a Tool for the Determination of Recrystallized Fractions in Hot-Deformed Austenitic Microstructures

The microstructural evolution of hot-deformed austenite during recrystallization was studied in a non-microalloyed low carbon steel and a low carbon steel, microalloyed with niobium and titanium. Double-hit compression tests were carried out to evaluate the isothermal recrystallization behavior. Specific deformation temperatures and interpass times were combined to produce characteristic recrystallization states by quenching the samples to a martensitic microstructure. The reconstruction software Merengue 2 was used to determine the prior austenite microstructure and evaluate the recrystallized fraction in the microstructure. The reconstruction is based on EBSD measurements of the martensitic microstructure. The determination of the recrystallized fraction was realized by evaluating the grain orientation spread. This work compares the results of both methods and estimates their uncertainties. It shows the potential of EBSD reconstruction methods to characterize different recrystallization states of low carbon steels out of EBSD data.

Grain growth behaviour of an AISI 422 martensitic stainless steel after hot deformation process

ABSTRACTThe metadynamic softening behaviour and grain size refinement of an AISI 422 martensitic stainless steel in the temperature range of 950–1150°C was investigated by double-hit compression tests. The deformed specimens were held at deformation temperature with delay times of 5–300 s after achieving a strain of 0.3. Based on the experimental results, a model was established for estimation of a softening fraction at different deformation parameters, and the softening fraction was compared with a recrystallised fraction. A major deviation was observed at the beginning of interpass time denoting a significant contribution of recovery to the fractional softening (FS). However, by increasing the time and temperature, the difference between the FS and recrystallised fraction is reduced. The finer grain size was achieved by prior fine pre-austenite grain and lower secondary deformation temperature. The initial grain size of 53 µm decreased down to 32 and 19 µm at the deformation temperatures of 1020 and 940°C, respectively. The austenite grains have considerable growth at a temperature higher than 1020°C, while the grain coarsening is negligible at lower deformation temperatures.

Analysis of recrystallization behavior of hot-deformed austenite reconstructed from electron backscattering diffraction orientation maps of lath martensite

The recrystallization behavior of hot-deformed austenite of a 0.55% C steel at 800°C was investigated by a method of reconstructing the parent austenite orientation map from an electron backscattering diffraction orientation map of lath martensite. Recrystallized austenite grains were clearly distinguished from un-recrystallized austenite grains. Very good correlation was confirmed between the static recrystallization behavior investigated mechanically by double-hit compression tests and the change in austenite microstructure evaluated by the reconstruction method. The recrystallization behavior of hot-deformed 0.55% C steel at 800°C is directly revealed and it was observed that by addition of 0.1% V the recrystallization was significantly retarded.

Strain induced grain boundary migration effects on grain growth of an austenitic stainless steel during static and metadynamic recrystallization

Static and metadynamic recrystallization of an AISI 304L austenitic stainless steel was investigated at 1100°C and 10−2s−1 strain rate. The kinetics of recrystallization was determined through double hit compression tests. Two strain levels were selected for the first compression hit: εf=0.15 for static recrystallization (SRX) and 0.25 for metadynamic recrystallization (MDRX). Both the as-deformed and the recrystallized microstructures were investigated through optical microscopy and electron back-scattered diffraction (EBSD) technique. During deformation, strain induced grain boundary migration appeared to be significant, producing a square-like grain boundary structure aligned along the directions of the maximum shear stresses in compression. EBSD analysis revealed to be as a fundamental technique that the dislocation density was distributed heterogeneously in the deformed grains. Grain growth driven by surface energy reduction was also investigated, finding that it was too slow to explain the experimental data. Based on microstructural results, it was concluded that saturation of the nucleation sites occurred in the first stages of recrystallization, while grain growth driven by strain induced grain boundary migration (SIGBM) dominated the subsequent stages.

Effect of Thermomechanical Processing on the Microstructure and Mechanical Properties of Nb-Ti-V Microalloyed Steel

The paper presents the results of thermomechanical treatment via forging on the microstructure and mechanical properties of newly obtained microalloyed steel containing 0.28% C, 1.41% Mn, 0.027% Nb, 0.028% Ti, and 0.019% V. The investigated steel is assigned to the production of forged elements for the automotive industry. Conditions of forging using the thermomechanical processing method were developed based on plastometric tests. Continuous and double-hit compression tests were conducted using the Gleeble 3800 thermomechanical simulator. The samples were investigated in a temperature range from 900 to 1100 °C and a strain rate of 1 and 10 s−1. To determine the recrystallization kinetics of plastically deformed austenite, discontinuous compression tests of samples using the applied deformation were conducted in a temperature range from 900 to 1100 °C with isothermal holding of the specimens between successive deformations for 2-100 s. Observations of the microstructures of thin foils were conducted using a TITAN80-300 FEI transmission electron microscope. The applied thermomechanical treatment allows to obtain a fine-grained microstructure of the austenite during hot-working and production of forged parts. These acquire advantageous mechanical properties and guaranteed crack resistance after controlled cooling from the end plastic deformation temperature and successive tempering. Forgings produced using the thermomechanical treatment method, consecutively subjected to tempering in a temperature range from 550 to 650 °C, reveal values of YS0.2 which equal from 994 to 892 MPa, UTS from 1084 to 958 MPa, KV from 69 to 109 J, KV−40 from 55 to 83 J, and a hardness ranging from 360 to 300 HBW.

Metadynamic Recrystallization Kinetics of Twin Roll Cast AZ31 Alloy during Hot Deformation

Metadynamic recrystallization takes place during isothermal annealing of materials in which an incomplete dynamic recrystallization occurred. In magnesium alloys this question has been investigated less until now. The metadynamic recrystallization behavior of twin-roll-cast and annealed AZ31 strip after hot deformation was investigated under conditions corresponding to hot strip rolling process of twin-roll-cast Mg coils. Plane strain compression tests were performed using double hit compression tests at temperatures of 250 - 400 oC, strain rates of 0.1 - 5 s-1 and interpass time of 0.5 – 10 s. 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 metadynamic recrystallization softening fractions and grain size in the two-pass hot deformed AZ31 alloy. Results also reveal that the pre-strain (beyond the peak strain) has little influence on the metadynamic recrystallization behavior in AZ31 alloy. Comparisons between the experimental and the predicted results were carried out. A good agreement between the experimental and the predicted results was obtained.

Comparison of Semi-empirical and Dislocation Density based Material Equations for Fast Modeling of Multistage Hot Working of Steel

Work hardening in metal forming can be modeled using different material model types. To capture the material response to plastic deformation semi-empirical models developed by e.g. Sellars and others compete with the physically-based – or internal variable – models developed by e.g. Bergstrøm or Kocks and Mecking. The more physical nature of the internal variable models means that they typically consist of complex systems of differential equations build upon a multitude of parameters (often > 20) to describe the various specific phenomena involved. The question thus open to debate is whether those internal variable models are more an advantage or a burden from an application point of view. In order to shed light on this question a direct comparison between semi-empirical and internal variable material models is drawn. Both model types are assessed using the following categories: model complexity, effort of model calibration, performance in compression tests, and applicability to hot rolling. The general quality of the model fitted to the same high manganese steel is demonstrated by a double hit compression tests. Additionally the material models are used to predict flow stress, recrystallized fractions and roll forces in a typical hot strip rolling schedule i.e. a complex multistage hot working operation. A difference in the best approach and necessary effort was exposed during model calibration. Validation trials reveal a good agreement between both model types. When modeling a complex rolling operation, however, profound differences in the microstructure predictions become apparent.

Static Recrystallization Kinetics Model of X70 Pipeline Steel

The main purpose of this study is to develop a numerical model to describe the static recrystallization kinetics in X70 pipeline steel. In order to investigate the static softening behavior, double-hit compression tests were performed on a Gleeble-3500 thermomechanical simulator over a temperature range of 950-1150°C. The interpass time varies from 1-500s. Different values were applied to estimate effects of strain rate and deformation on static recrystallization. The results show that factors such as high temperature, long holding time and lower strain rate will lead to the inclination of softening behavior. The static softening activation energy was obtained. The good agreement between the predicted values and experimental results indicated the validation of the developed model of this study.

Multi-stage thermomechanical behavior of AISI 410 martensitic steel

The flow softening behavior of AISI 410 martensitic stainless steel was investigated by double hit compression tests in the temperature range of 950–1100°C and the strain rates of 0.01, 0.1, and 0.5s−1. The tests were conducted with delay times varying between 1 and 50s after achieving a strain of 0.1, 0.16 and 0.2 in the first stage. The fractional softening has been predicted by the offset stress method and the kinetic equations have been proposed to predict the static recrystallization behavior of hot deformed AISI 410 martensitic stainless steel. Experimental results indicate that the flow softening is a function of the inter-pass time for different strain rates and deformation temperatures. Using analytical approach, the static recrystallization activation energy and kinetics model was obtained and built. A good agreement between experimental results and predicted values of the model was achieved.

Hot deformation behavior and softening kinetics of Ti–V–B microalloyed steels

AbstractResults of the effect of deformation parameters on the hot-deformation behavior and softening kinetics of two Ti–V–B-microalloyed steels were presented in the paper. The investigated steels are assigned to production of forged elements for the automotive industry by thermo-mechanical treatment. Research was conducted by compression test at elevated temperature. Hot compression test was performed using the Gleeble 3800 thermomechanical simulator at the temperature range from 900°C to 1100°C and strain rate of 1, 10 and 50s−1. The σ–ε curves were determined on the basis of continuous compression and double-hit compression tests.

Recrystallization behavior of a Nb-microalloyed steel during hot compression

Using a Thermecmastor-Z hot simulator, dynamic recrystallization (DRX) and static recrystallization (SRX) behavior of a Nb-microalloyed steel was investigated by single-hit compression tests and double-hit compression tests, respectively. The experimental results show that DRX will more easily occur at higher deformation temperature and lower strain rate. The deformation activation energy and stress exponent for the Nb-microalloyed steel are calculated to be 379.29 ± 23.56 kJ/mol and 5.76 in temperature range of 950 °C to 1100 °C by regression analysis, respectively. Furthermore, a semi-empirical model is developed to identify the peak stress and strain for DRX. It is found that SRX kinetics follows Avrami’s law, and the softening fraction predicted by the model agrees well with experimental results.

Control of the Austenite Recrystallization in Niobium Microalloyed Steels

The use of heavy gauge steel sheets for structural applications very often requires a combination of high yield strength and adequate toughness. The most cost effective way to realize a high yield strength and a high ductility in a low alloyed steel is grain refinement. In industrial practice, this refinement is realized by controlled processing. This process consists of controlling the slab reheating temperature, applying a large amount of hot deformation below the non-recrystallization temperature (Tnr) and accelerated cooling. A better knowledge of Tnr could optimize the process and the best mechanical properties could be reached against the lowest cost. Tnr can be raised by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at low temperatures either by solute drag or by precipitation pinning. In this study, the recrystallization behavior of five Nb-microalloyed model alloys with various Nb contents, was evaluated by double hit compression tests. Further, the precipitation state of the materials was investigated experimentally by Inductively Couples Mass Spectroscopy and X-ray Diffraction. The construction of recrystallization-time-temperature diagrams and precipitation-time-temperature diagrams showed that both mechanisms, i.e. recrystallization and precipitation, strongly influence each other.