Deformation Passes Research Articles

Ferrite Grain Size Distributions in Ultra-Fine-Grained High-Strength Low-Alloy Steel After Controlled Thermomechanical Deformation

Plane-strain compression testing was carried out above, around, and below the A r3 temperature with the deformation temperature, T def, varying between 1323 K and 973 K (1050 °C and 700 °C), using Gleeble 3500, to develop uniform distribution of ultra-fine ferrite (UFF) grains. Prior austenite (γ) grain structure, developed after soaking at 1473 K (1200 °C), was mixed in nature, comprising both coarse- and fine-γ-grain sizes. Applying heavy deformation in a single pass, just above the austenite-to-ferrite (α) transformation temperature (A r3), and cooling to room temperature resulted in the formation of UFF grain sizes (average α-grain size ~2 to 3 μm), with the largest grain sizes extending up to ~10 to 12 μm. Water quenching just after deformation prevented the coarsening of UFF grains and restricted the largest grain sizes to under 6 μm. Although the ferrite grain structures appeared homogeneous in slowly cooled samples (cooling rate (CR) 1 K/s), careful observation revealed the presence of alternate bands of coarse- (5 to 10 μm) and fine-α grains (<1 to 3 μm). The final α-grain size distributions were explained in view of the starting γ-grain size variation, dynamic recrystallization (DRX) of γ, dynamic strain-induced γ-to-α transformation (DSIT), and DRX of α and grain growth during slow cooling. Electron backscattered diffraction analysis (EBSD) revealed the presence of a large fraction (70 to 80 pct) of high-angle boundaries, having misorientation ≥15 deg. Compared to the use of the single, heavy deformation pass, the application of a number of lighter passes between A e3 and A r3 temperatures is more suitable in industrial rolling conditions, and also has the potential of developing UFF grains with high-angle boundaries.

Grain Refinement in Constrained Groove Pressing of 7050 Aluminum Alloy

Constrained groove pressing is a simple and effective method of grain refinement. Using the experimental data obtained by regression analysis, this paper analyzes the simulation of the four pass constrained groove pressing deformation of 7050 aluminum alloy. The simulation results show that the grain size of the billet is refined significantly after four pass constrained groove pressing deformation and decreases from the original 90 μm to a minimum of 14.0 μm. With the increase of the number of deformation passes, refinement effect becomes weakened gradually, the grain size tends to stabilize and the organization is more uniform.

Anisotropic mechanical properties of equal channel angular pressed Al–5% Zr alloy containing platelet particles

Aluminum alloys containing platelet particles are known by their anisotropic mechanical properties. This anisotropy can be influenced by the change in the shape or the arrangement of those platelet intermetallics. In this study, the effect of processing Al/Al3Zr alloy by equal channel angular pressing (ECAP) on its anisotropic mechanical properties has been investigated. Al–5mass% Zr alloys processed by ECAP using routes A and BC were used for the investigation. ECAPed samples showed a notable decrease in the size of the Al3Zr platelets and an increased tendency to align parallel to the deformation axis with increasing the number of passes. Some anisotropy in compression strength was observed at 4 passes of deformation; however, this anisotropy became negligible after 8 passes of ECAP. The wear tests showed that the ECAPed samples have almost isotropic wear property.

High-strain-rate response of ultra-fine-grained copper

The high-strain-rate response of ultra-fine-grained (UFG) copper processed by equal channel angular pressing (ECAP) was characterized by three different dynamic testing methods: reverse Taylor impact, cylindrical compression specimens, and hat-shaped specimens in Hopkinson bar experiments. Upon recovery after impact, the specimens were found to undergo dynamic recrystallization at a calculated temperature of 360 K, indicating that the UFG copper is thermally unstable. Reverse Taylor tests were conducted on as-received oxygen-free high-conductivity copper rod and ECAP specimens with 2 and 8 sequential deformation passes. The dynamic deformation of the samples was modeled using AUTODYN-2D, and a modified Johnson–Cook constitutive equation was found to closely capture the dynamic response. Both the dynamic experiments and analysis from the reverse Taylor tests indicate enhanced strain-rate sensitivity in comparison with conventional polycrystalline copper, in agreement with predictions of reduced activation volume. The shear band thickness, as obtained in forced localization tests, showed a marked decrease, in comparison to conventional polycrystalline copper; this effect is interpreted as due to an accelerated thermal softening and inherent instability exhibited by the UFG structure.

Simulation of radial forging conditions by third hits hot compression tests

Radial forging conditions of ultra-high strength 32CrMoV1210 steel was investigated by using multi-pass hot compression tests. Compression continuous cooling transformation (CCCT) diagram after final deformation pass was plotted and compared with CCT diagram in order to identify the effects of deformation on transformation behavior. Comparison between these two diagrams showed that prior deformation increased martensite start temperature, M s from 45 to 85 °C. The microstructural observation illustrated that application of prior deformation leads to finer lath martensite. It also showed that when deformation temperatures decreased, the grain size and its distributions became finer and inhomogeneous, respectively.

The role of the friction during the equal channel angular pressing of an IF-steel billet

It is well known that high levels of friction induce adherence effects in materials processed by equal channel angular pressing (ECAP) promoting some degree of heterogeneity along the deformation zone. In the present paper, the role of the friction in relation of die geometry considering frictionless, ideal lubrication and severe friction conditions of an interstitial free (IF) steel deformed by ECAP technique using plane strain finite element models was investigated in details. The analysis of adherence at the billet–die contact region during only one pass of deformation was carried out in a quasi-static form at room temperature. Independent of the die channels intersection angle (90° or 120°) analyzed an adherence phenomenon was observed under determined friction conditions. It can be concluded that it is necessary to establish an upper limit to the friction coefficient in order to avoid the adherence effect in two-dimensional finite element simulations.

Synthesis of Bulk Materials by Equal Channel Angular Consolidation of Particles

Pure aluminium and titanium powders were successfully synthesised into bulk materials using equal channel angular (ECA) consolidation. Powders were used directly without the need to cold-compact them into green bodies. The processing temperatures were significantly lower than the usual sintering temperatures for aluminium and titanium. Fully dense bulk samples were achieved after one pass of ECA deformation through a 90 degree die. Mechanical properties of the as-ECA processed materials were comparable to those of wrought aluminium and titanium through ingot metallurgy. Multiple passes of ECA deformation resulted in refined microstructure and improved mechanical properties. The new process has many advantages over conventional powder sintering and is capable of producing bulk nanomaterials of high integrity.

Ab-Initio Lattice Instability Analysis on Ni and Ni3Al Single Crystals

The purpose of the present study is to elucidate the “ideal strength” of the Ni and Ni3Al single crystals, the main compositions of Ni-based superalloy, from the viewpoint of the lattice stability. The unit lattices of Ni and Ni3Al, fcc and L12 ordered alloy, are subjected to the [001] uniaxial tension/compression and hydrostatic tension/compression by using the Vienna Ab-initio Simulation Package (VASP) with the generalized gradient approximation (GGA) and ultrasoft pseudopotential. The elastic stiffness matrix is numerically evaluated at each point in the applied deformation pass, then the lattice stability is discussed based on the positiveness of the matrix. Both Ni and Ni3Al reach the Born’s stability criteria against the bifurcation to the anisotropic Poisson’s contraction in the [001] uniaxial tension, while they do the spinodal criteria against the structural transformation in the [001] uniaxial compression and hydrostatic tension. The hydrostatic compression increases the stability and shows no limit, however, it is also suggested that the spinodal instability appears when the ideal isotropy was broken. The “ideal strength” is evaluated with these stability limits and indicated as “yield curve” on the normal strain-lateral strain or normal stress-lateral stress planes.

Analysis of the Effect of Mn on the Recrystallization Kinetics of High Nb Steel: An Example of Physically-based Alloy Design

Strip casting and thin slab casting are new near net-shape technologies, which have successfully emerged in steels because of significant energy savings, substantial reduction in green house gas emissions and cost benefits. Extensive research was undertaken aimed at integrating microalloying technology with near net-shape casting technology, using a base chemistry of low carbon (0.03 wt%) and high niobium (up to 0.1 %wt), which is a well established chemistry for higher grade pipeline steels.In the present contribution, physically-based modelling is used to optimize the Mn content and the processing conditions for the application of strain-accumulation in the new steels. The results of the modelling confirm the distinct advantage of the low Mn chemistry for the application of plate rolling which is carried out in the high temperature window using small to medium deformation passes. The high Mn chemistry is found to be more advantageous when the rolling is carried out in the low temperature window and using large pass reductions. This result is in agreement with recent rolling simulations and mill trials which show that the high Mn chemistry is superior to the low Mn chemistry for the application of near net-shape strip-rolling.

Ab-initio Lattice Instability Analysis on Ni and Ni3Al Single Crystals

The purpose of the present study is to elucidate the “ideal strength” of the Ni and Ni3AI single crystals, the main compositions of Ni-based superalloy, from the viewpoint of the lattice stability. The unit lattices of Ni and Ni3Al, fcc and L I2 ordered alloy, are subjected to the [001] uniaxial tension/compression and hydrostatic tension/compression by using the Vienna Ab-initio Simulation Package (VASP) with the generalized gradient approximation (GGA) and ultrasoft pseudopotential. The elastic stiffness matrix is numerically evaluated at each point in the applied deformation pass, then the lattice stability is discussed based on the positiveness of the matrix. Both Ni and Ni3Al reaches the Born's stability criteria against the bifurcation to the anisotropic Poisson's contraction in the [001] uniaxial tension, while they do the spinodal criteria against the structural transformation in the [001] uniaxial compression and hydrostatic tension. The hydrostatic compression increases the stability and shows no limit, however, it is also suggested that the spinodal instability appears when the ideal isotropy was broken. The “ideal strength” is evaluated with these stability limits and indicated as “yield curve” on the normal strain-lateral strain or normal stress-lateral stress planes.

The Influence of Chemical Composition on the Kinetics of Dynamic Recrystallization of IF Austenite

Dynamic recrystallization, DRX, has become an increasingly important softening mechanism both from fundamental and industrial points of view. During finishing rolling of strips or wire rods, strain is accumulated from pass to pass so that DRX can be triggered. The time need for 50% of material to recrystallize, t50DRX, is strongly dependent on temperature and to a lesser extent on strain rate at which deformation occurs. Few studies report results on the kinetics of DRX and how this softening mechanism can be predicted for a given set of hot deformation conditions, namely strain, strain rate and pass temperature. The purpose of this paper was to investigate how the chemical composition of IF austenite can affect the kinetics of DRX by measuring the apparent activation energy for DRX, QDRX, for alloys with additions of Ti and a combination of Ti-Nb contents. Predicted and measured values of t50DRX, were compared and an empirical expression was proposed to model measured values.

Modelling Work-Roll Temperature Variations in Hot Strip Rolling

Prediction of temperature distribution within work-rolls during the hot strip rolling process is of great importance to mill designers because of the significant role it plays in the product's dimensional accuracy and roll life. The present article employs the unsteady state heat transfer equation with time-dependent boundary conditions, as well as a two-dimensional finite element method, to determine work-roll temperature variations during continuous hot strip rolling. To achieve an accurate temperature field, the authors consider the effects of different factors, including work-roll and metal thermal relationship, idling work-roll revolutions, and the effects of various process parameters such as rolling speed, interface heat transfer co-efficient, and the amount of thickness reduction at each deformation pass. A comparison of the predicted and published experimental results shows the validity of the mathematical model.

Modeling of Austenite Grain Size Distribution in Nb Microalloyed Steels Processed by Thin Slab Casting and Direct Rolling (TSDR) Route

A mathematical model has been developed to predict the austenite microstructure evolution of Nb microalloyed steels during “Thin slab casting” and “Hot direct rolling” (TSDR) processing. The model is based on empirical equations specifically derived for the microstructural and processing features typical in these new technologies. Its main novelty is that it works with austenite grain size distributions instead of the typical mean values as used in conventional models to represent the microstructure. This fact is particularly important in working with as-cast austenite due to the wide range of grain sizes present in this microstructure. In the model the different softening and hardening mechanisms that can operate during hot working in austenite are considered: static, dynamic and metadynamic recrystallization, grain growth after recrystallization and Nb(C, N) strain induced precipitation. The model uses the initial austenite grain size distribution as input and provides the size distribution of recrystallized and unrecrystallized grains at the entry of any rolling pass. A validation of the model has been carried out in the laboratory by multipass torsion tests. The model is capable of predicting any heterogeneities that may appear in the final microstructure after this kind of processing and that are not well predicted by using conventional models based on mean values. Additionally, it can calculate the deformation history, in terms of the strain accumulated in the austenite, and stress behavior, in terms of the mean flow stress (MFS) corresponding to each deformation pass.

Modelling the work-roll temperature variation at unsteady state condition

In this paper, the unsteady state heat transfer equations with time dependent boundary conditions are coupled with a two-dimensional finite element method to predict the work-roll temperature distribution during the continuous hot slab rolling process. To achieve an accurate temperature field, the effects of various factors including the thermal relationship of the work-roll and the metal slab, the idling work-roll revolutions, the rolling speed, the slab/roll interfacial heat transfer coefficient, and the magnitude of the thickness reduction of the slab at each deformation pass are taken into account. Comparisons between the predicted and published experimental results are used to illustrate the validity of the mathematical model.

Unsteady state work-roll temperature distribution during continuous hot slab rolling

Predicting temperature distributions in the work-rolls during the hot slab rolling process is of great importance to mill designers. This is because the temperature distribution and the dimensional accuracy of the slab being rolled are both dependent on the work-roll temperature. In addition, the life of the roll is also a function of its own temperature distribution. In this paper, the unsteady state heat transfer equations with time dependent boundary conditions are coupled with a two-dimensional finite element method to predict the work-roll temperature distribution during the continuous hot slab rolling process. To achieve an accurate temperature field, the effects of various factors including the thermal relationships of the work-roll and the metal slab, the idling work-roll revolutions, the rolling speed, the slab/roll interfacial heat transfer coefficient, and the magnitude of the thickness reduction of the slab at each deformation pass are taken into account. Comparisons between the predicted and published experimental results are used to illustrate the validity of the mathematical model.

Microstructure and deformation behaviour of submicrocrystalline 304 stainless steel produced by severe plastic deformation

The deformation behaviour and structural changes in a 304-type stainless steel were studied in uniaxial compression at temperatures of 873–1073 K (0.5–0.6 T m) and under strain rates from 10 −4 to 10 −2 s −1. The starting material, with an initial grain size of ≈0.3 μm, was produced by multiple warm deformation passes, changing of the loading direction from pass to pass. At high temperature, the resulting fine-grained steel has a low dislocation density and steady-state flow stresses lower than that of an annealed coarse-grained steel. Moreover, the strain rate sensitivity of the fine-grained material reaches high values, up to 0.3, which clearly differs from that of ≈0.1–0.2 for the coarse-grained steel. At room temperature, the initially fine-grained steel has higher hardness after warm deformation than the conventional steel.

Investigation of the Microstructural Evolution During Large Strain Cold Working of Metals by Means of Synchrotron Radiation—A Comparative Overview

Dislocation densities, arrangements and long range internal stresses in cold rolled polycrystalline Cu, Ni, and Al, as well as in cold torsioned Fe, were determined by X-ray diffraction profile analysis using synchrotron radiation. At different deformation degrees, scanning measurements across single grains with a focal spot of less than 50 μm were carried out, in order to inform on the features of the deformation induced substructure. At small deformations including stage III, the dislocation densities and internal stresses are uniform within single grains while at higher deformations in stages IV and V, the dislocation densities and long range internal stresses exhibit opposite fluctuations. In stage IV these fluctuations correspond to the formation of polarized tilt walls (PTW’s) from polarized dipole walls (PDW’s). In contrast to the PDW’s, the PTW’s cause a much higher misorientation in between adjacent lattice areas. This transformation, which is the main element of the progressing fragmentation process during large strain deformation, occurs in all metals studied regardless of dislocation mobility and/or lattice type. Approaching higher strains in stage V, however, these parameters gain some importance especially when the deformation occurs in an iterative way. If the mobility is high, marked static recovery takes place between the single deformation passes, which results in decreases of both dislocation density and local internal stress, and no formation of PTW’s is observed.

Microstructural Parameters in Large Strain Deformed Ni Polycrystals as Investigated by Synchrotron Radiation

Dislocation densities, arrangements and long-range internal stresses in cold worked polycrystalline nickel were determined by X-ray diffraction profile analysis using synchrotron radiation. Torsionally deformed samples were scanned across the diameter of the specimens with a focal spot of 300 μm providing strain dependent results in good correlation with residual electrical resistivity data. On cold rolled specimens with different deformations, scanning measurements across single grains with a focal spot of less than 50 μm were carried out, in order to inform on the features of the deformation induced substructure. At small deformations including stage III, the dislocation densities and internal stresses are uniform within single grains while at higher deformations in stages IV and V, the dislocation density and internal stresses exhibit correlated fluctuations. In analogy to Cu, in stage IV these fluctuations could be identified as polarized tilt walls (PTW) formed from polarized dipole walls (PDW) whereas in stage V, as a difference to Cu, only PDWs and pile-ups of dislocations at the grain boundaries were found. It is suggested that the differences to Cu arise from the enhanced presence of deformation induced vacancies in Ni, enabling stronger effects of static recovery through dislocation climb to take place in between single passes of rolling deformation.

Microstructural Parameters in Large Strain Deformed Ni Polycrystals as Investigated by Synchrotron Radiation

Dislocation densities, arrangements and long-range internal stresses in cold worked polycrystalline nickel were determined by X-ray diffraction profile analysis using synchrotron radiation. Torsionally deformed samples were scanned across the diameter of the specimens with a focal spot of 300 μm providing strain dependent results in good correlation with residual electrical resistivity data. On cold rolled specimens with different deformations, scanning measurements across single grains with a focal spot of less than 50 μm were carried out, in order to inform on the features of the deformation induced substructure. At small deformations including stage III, the dislocation densities and internal stresses are uniform within single grains while at higher deformations in stages IV and V. the dislocation density and internal stresses exhibit correlated fluctuations. In analogy to Cu, in stage IV these fluctuations could be identified as polarized tilt walls (PTW) formed from polarized dipole walls (PDW) whereas in stage V, as a difference to Cu, only PDWs and pile-ups of dislocations at the grain boundaries were found. It is suggested that the differences to Cu arise from the enhanced presence of deformation induced vacancies in Ni, enabling stronger effects of static recovery through dislocation climb to take place in between single passes of rolling deformation.

Static Restoration of Aluminium During Multi-stage Hot Rolling Simulation

The static softening of aluminium between the deformation steps of multi-stage hot torsion tests has been investigated by analysing the temperature behaviour of the mean flow stress σ and the fractional softening x s throughout the hot rolling simulation. The numerous items of information which the σ - 1/T and x s - T diagrams contain were examined. Four distinct regions of static restoration have been found which display different softening behaviour, depending on the duration of the inter-pass time and the strain per pass. Additionally, the evolution of the microstructure during multistep deformation was examined by SEM. On the basis of the fractional softening, a retained strain e r has been derived which describes the predeformed structure entering a succeeding deformation pass due to incomplete static restoration during the preceding arrest interval.