Plasticity Steel Research Articles

A variational material model for transformation-induced plasticity in polycrystalline steels

AbstractThis work presents a variational material model for transformation-induced plasticity in steels. We will use the principle of the minimum of the dissipation potential to develop a coupled material model for plastic deformations and phase transformations that simultaneously accounts for the hardening effects that play an important role. We will use a polycrystalline approach and introduce a combined Voigt/Reuß bound and a coupled ansatz for the dissipation functional to model the simultaneous effects of plastic deformations and phase transformations. Finally, we will present the first numerical results for a tension/compression test.

Fatigue Crack Growth Behavior of High Manganese Austenitic TWIP Steels

Fatigue crack growth (FCG) behavior of three high manganese austenitic twin-induced plasticity (TWIP) steels with different stacking fault energy (SFE) was investigated, aiming at studying the correlation between the FCG resistance and the SFE of the steels. FCG tests were performed using three-point bending specimens at room temperature at stress ratio of 0.1 under the control of stress intensity factor range. Test results showed that the fatigue threshold values of these steels decrease with increasing the SFE. However, in the Paris regime, the crack growth rates of the steels do not appear to correlate directly with SFE. These results are discussed according to the degree of fatigue crack closure and the deformation mode of crack tip zone.

Оценка эффективности режимов термообработки сварного соединения при помощи Sysweld

The article presents the procedure to determine the field of residual stress after welding and heat treatment in a typical weld joint with FEM Sysweld software. Changes of steel elasticity and plasticity as a result of heating, as well as changes in the structural composition of steel in the process of welding and heat treatment, and speed and trajectory of welding were considered. Three different heat treatment modes used after welding have been examined: underannealing, normalization followed by air cooling, full annealing followed by slow cooling of the construction. The levels of residual stresses formed after welding and after each mode of heat treatment proved to be different as expected. The normalization and underannealing modes allowed to reduce considerably the level of residual stresses, however, the most effective heat treatment in terms of residual stress reduction is full annealing followed by slow cooling of the construction. The data analysis testifies that for the most effective reduction of residual stresses, the structural and phase recrystallizations are required that take place when the construction is heated above critical temperature Ас3.

Influence of the In-Service Factors on the Serviceability of Graphitized Steel

We compare the regularities of the influence of high (up to 800°С) and low (down to – 40°C) temperatures and corrosive media on the mechanical characteristics of graphitized steel and known 2 and T wheel steels. It is shown that the decrease in the temperature of austenitization and rapid growth of the high-temperature plasticity of steels create more favorable conditions for the formation of slider-type defects on the rolling surfaces of railroad wheels. It is also established that, at low temperatures (down to – 40°C), the graphitized steel is not susceptible to low-temperature embrittlement and the corrosive media do not affect its cyclic crack resistance.

Modeling of the Mechanical Response During Reversal Shear Loading: Application to Steels

A hardening model that considers the dislocation reversal‐related mechanisms is applied to predict the mechanical behavior of low carbon (LC) and twinning‐induced plasticity (TWIP) steels under forward–reverse shear test with various pre‐strain levels. The predicted results are presented in terms of stress–strain response and texture evolution. It is shown that the proposed model, embedded in the visco‐plastic self‐consistent (VPSC) framework, well captures the changes in the reloading yield stress and the hardening evolution due to the strain path reversal. The contributions of the different mechanisms are also discussed.

Prediction of forming limit diagram using Marciniak–Kuczynski and Swift-Hill models via yield criteria for TRIP800

The present paper is devoted to the investigation of the prediction capabilities of the forming limit diagrams via frequently used instability criteria, Marciniak–Kuczynski (M–K) theory and Swift–Hill model. Both models consist of a core algorithm which can be modified by using different yield and hardening functions. In the study, the forming limit characteristics of advanced high strength transformation induced plasticity (TRIP) steel are investigated experimentally and numerically. In the instability criteria, Barlat-89 and YLD2000 anisotropic yield functions are implemented and results are compared. Results reveal that the M–K model is acceptable using the YLD2000-2d anisotropic yield function.

A micromechanical model for the transformation induced plasticity in polycrystalline steels

AbstractDue to the effect of transformation induced plasticity (TRIP) , TRIP‐steels are very promising materials, e.g. for the automobile industry. The material behavior is characterized by very complex inner processes, namely phase transformation coupled with plastic deformation and kinematic hardening. We establish a micromechanical model which uses the volume fractions of the single phases, the plastic strain and the hardening parameter in every grain of the polycrystalline material as internal variables. Furthermore, we apply the Principle of the Minimum of the Dissipation Potential to derive the associated evolution equations. The use of a coupled dissipation functional and a combined Voigt/Reuss bound directly results in coupled evolution equations for the internal variables and in one combined yield function. Additionally, we show numerical results which prove our model's ability to give a first prediction of the TRIP‐steels' complex material behavior. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of interstitial carbon, nitrogen, and hydrogen on the plasticity and brittleness of steel

The introduction of carbon, nitrogen, and hydrogen in steel is analyzed in terms of the electron structure, dislocation properties, hardening, and failure of the steel. The similarity and differences in the mechanical properties of the corresponding solution solutions are discussed in relation to the influence of these elements on the density of electron states at the Fermi level of iron and correspondingly on the concentration of free electrons. Carbon reduces the concentration of free electrons, while nitrogen and hydrogen have the opposite effect. Hence, the atomic interaction is changed: specifically, its covalent or metallic component will be intensified. The dislocation rate in deformation is analyzed in the approximation of mobile and immobile interstitial atoms. In the first case, the interstitial atoms obstruct dislocational slip; the mobility of the dislocations is determined by the binding enthalpy of the dislocations with impurity atoms. If the interstitial atoms may accompany dislocations, the atomic bond is locally changed in dislocational atmospheres. That affects the unit energy of the dislocations and the distance between them in the slip planes. On the basis of the research results, the significant similarity between the hydrogen brittleness of austenitic steel and the ductile–brittle transition in alloying with nitrogen is explained.

Analysis of Long-Term in-Service Degradation of the Shukhov Tower Elements

The condition of old and repair steels of the Shukhov tower elements that were in service for ~110 and 70 years, respectively, was analyzed. They were assessed with several mechanical characteristics: hardness, static and impact strength, and plasticity. Both steels are shown to exhibit extra low hardness and brittle fracture resistance, which cannot be explained only by the quality of rolled metal of that time, with lower mechanical characteristics of old steel. It was assumed that in-service degradation of steel properties took place due to the evolution of scattered damage. Larger elongation of old steel at lower hardness and impact strength also points to this fact. It was also found that strength and plasticity of both steels, determined in the medium of synthetic acid rain, were lower than in air. The presence of in-service damages in the bulk of the metal was confirmed fractographically, in the case of possible hydrogen absorption by the specimens from the medium, they initiate brittle fracture in the central part of their cross-section.

External Stress Effect on Microstructure and Transformation Plasticity of Hot Work Tool Steel During Quenching

External Stress Effect on Microstructure and Transformation Plasticity of Hot Work Tool Steel During Quenching

Numerical analysis of concrete filled Buckling Restrained Braces

This paper is on the numerical analysis of concrete filled Buckling Restrained Braces (BRBs), which are typically used as diagonal bracing members of steel braced frames designed for dissipative behavior and lateral load resistance under seismic action. BRBs are typically composed of a slender steel core which is continuously supported by a concrete casing against buckling. The complex behavior of the BRB devices is usually studied by experimental tests. The numerical analysis is rarely applied, it requires advanced tools, such as modeling of cyclic steel plasticity with combined hardening, cyclic concrete behavior, plastic buckling, contact and friction. In the paper a novel Chaboche based cyclic steel material model is presented, which is developed in ANSYS finite element environment. The model is able to describe all the important features of the cyclic behavior of structural steel, including low cyclic fatigue capacity. The William–Warnke model is used for the concrete material with combined kinematic hardening. The computational model has a special focus on the contact/friction phenomena between the steel core and supporting concrete. The structure and approximations of the numerical models, the calculation of model parameters on the basis of its own test results are detailed. The features of the behavior are demonstrated by the application of the developed 3D solid Buckling Restrained Brace model.

Effects of Ce on Inclusions, Microstructure, Mechanical Properties, and Corrosion Behavior of AISI 202 Stainless Steel

The sizes and morphologies of nonmetallic inclusions, microhardness, tensile strength, and Charpy impact toughness in AISI 202 stainless steel with different Ce contents were synthetically analyzed by means of SEM, TEM, microhardness tester, and tensile and Charpy impact tests. Effects of Ce addition on the corrosion behavior were investigated in 5 wt.% H2SO4 solution for different periods of time through measuring AC impedance. The EIS measurements indicate that the steels with Ce addition exhibit higher R p values than those without Ce, which illustrates the relative resistance to uniform corrosion is accompanied by an increasing Ce addition. Ce addition to AISI 202 stainless steel improves its uniform corrosion resistance owing to metamorphic inclusions and the improvement of electrode potential in matrix. Upon increasing Ce addition, the indentation morphology of samples transfers from sink-in types to pile-up types, explaining good machinability of steels containing Ce. It is witnessed from the fracture mode that Ce refines the grain size of steels, significantly increasing the strength; in the meantime, its plasticity is improved, thereby solving the contradiction between the strength and the plasticity of steels. It is concluded that AISI 202 stainless steel with 0.016 wt.% Ce addition in the mass fraction has the best mechanical properties and the uniform corrosion resistance.

Transformation plasticity in boron-bearing low carbon steel

The transformation plasticity (TP), which indicates that permanent strain remains after solid-solid phase transformation, even under much smaller stress than the yield stress, has been described by a vacancy diffusion mechanism in the migrating interface during diffusional phase transformation. In this study, the influence of boron (B) addition on the TP of low carbon high strength steel was investigated through the observation of the B segregation in the phase interface between primary austenite phase and ferrite phase using secondary ion mass spectroscopy. The B segregation at the austenite-ferrite phase interface was confirmed to cause drastic decrease of the TP strain by comparison of the dilatation behavior of B-bearing and B-free steels under a tensile force during slow cooling, where the diffusional phase transformation occurs in B-bearing steel. Furthermore, it was also confirmed that the velocity of B diffusion is larger than the migration velocity of interface at the given temperature through a calculation based on Fick’s law.

Study on fibre laser spot welding of TRIP980 steel

A 980 MPa transformation induced plasticity (TRIP) steel was fibre laser spot welded by different Argon (Ar) shielding conditions, laser power (1000 up to 2500 W) and defocusing distances (−8 up to +8 mm). The surface appearance, cross-section macrostructure, microstructure, hardness, tensile shear properties and fatigue properties of laser spot welds were evaluated. The results showed that the welds with Ar shielding had larger weld appearance and bonding sizes, better tensile shear properties compared with the welds without Ar shielding. With the increase in laser power, the laser welding mode changed from conduction to keyhole, which improved the bonding size and mechanical properties. The bonding size and mechanical properties increased in the order of defocusing distances of +8, −8, +4, −4 and 0 mm. During the fatigue tests of laser spot weld, the fusion zone pullout and sheet transverse fracture failure modes were observed.

Implementation of a Constitutive Model for the Mechanical Behavior of TWIP Steels and Validation Simulations

High manganese content TWinning Induced Plasticity (TWIP) steels are promising for the production of lightweight components due to their high strength combined with extreme ductility, see [1]. This paper deals with the implementation of a constitutive model for the macroscopic deformation behavior of TWIP steels under mechanical loading with the aim of simulating metal forming processes and representing the behavior of TWIP-steel components – for example under crash loading - with the Finite Element code LS-DYNA® and refers to our recently published papers: [2],[4],[5]. Within the present paper we focus on the implementation of the model formulated in [2] and its extension to stress dependent twinning effects.

Dislocation core reconstruction induced by carbon segregation in bcc iron

Dislocations are central to the understanding of plasticity in metals. Using density functional theory the authors show that interstitial carbon atoms in ferritic steels stabilize a dislocation core configuration called a hard core which is otherwise unstable. Since, as they also show, at equilibrium all dislocations are expected to be fully decorated by carbon atoms, this has important implications for the understanding of plasticity in ferritic steels. It is expected that the picture developed here could also be applicable to other solutes besides carbon and to other metals besides iron.

An experimental study of the polycrystalline plasticity of austenitic stainless steel

The development and validation of crystal plasticity models requires the ability to map deformation at the microstructural scale. Here, a new method of high-resolution deformation mapping is used to measure strain, material rotation and lattice rotation in austenitic stainless steel at sub-micron resolution. Electron back-scatter diffraction maps are used to link the deformation to the microstructure. Deformation occurs in domains, in which most of the plastic strain originates from the activation of a single slip system with high resolved shear stress. Within domains, slip is localized in lamellar regions that increase in number with strain. The deformation incompatibility between grains that develops as a consequence of this single crystal like behaviour is accommodated by either a gradient in slip intensity and the consequent development of lattice curvature at the grain boundary or the activation of an additional high stressed slip system and the consequent formation of a complementary deformation domain within the grain. In many cases, however, lattice curvature across grain boundaries is small because the deformation domains in neighbouring grains are compatible. The implications of these observations for continuum crystal plasticity modelling are discussed.

Comparison of Damage Evolution in Different Steels by Means of 3D X Ray Tomography

In this paper, the damage behavior of three different steels is studied by the means of 3D X ray tomography in tension: a Fe22Mn0.6C austenitic TWIP steel, a 316 L austenitic stainless steel, and an interstitial‐free (IF) ferritic one. The comparison gives a better insight into the damage of the twinning‐induced plasticity steel (Fe22Mn0.6C), which is a promising one due its very high mechanical properties. It is shown that the damage process of this type of steel involves not only an important nucleation of small voids but also a large growth of the biggest cavities. This last conclusion is true even if a constant macroscopical triaxiality is observed.

Influence of austenitising temperature on kinetics of phase transformations in medium carbon TRIP steel

Comprehensive, dilatometric studies of the kinetic transformations of undercooled austenite in medium carbon TRansformation Induced Plasticity (TRIP) steel during cooling from the temperature range of 750–1030°C were carried out. Selection of austenitising temperatures was based on the knowledge of the critical temperatures. The influence of the austenitising temperature on the formation ranges of new phases and microstructural constituents during continuous cooling was determined. It was also shown that the annealing of the investigated steel at the lowest temperature resulted in the strongest lowering of Ms temperature as well as in the lowering hardenability. Based on the analysis of the stated relationships, modifications of the annealing, adapted to the investigated steel chemical composition, were proposed.

Experimental investigation on electroplastic effect of DP980 advanced high strength steel

This paper investigated the influence of the electric pulses on the flow behavior and plasticity of the advanced high strength steel (AHSS) Dual Phase, DP980. In order to isolate the thermal effect of the electric pulses, two kinds of uniaxial tensile tests at the same testing temperature were carried out: (1) isothermal tensile test carried out in an environment cabinet and (2) electrically-assisted isothermal tensile test. The stress–strain curves were recorded and compared. The results indicate that at the same testing temperature, the stress–strain curves obtained by test (1) are generally lower than curves deserved by test (2). It demonstrates that electric pulses can not reduce the flow stress when compared with the case that without electric pulse which is contrary to the traditional electroplastic effect. Another result is that when the testing temperature is not more than 573K, the stress–strain curves obtained by both tests are higher than the tension curve in room temperature and the cases are opposite when testing temperature is more than 573K. In addition, the elongation improvement is not observed as well in the stress–strain curves. The difference between the two tests when compared the cross section shrinkage rate and the fracture elongation rate is not obvious. From another aspect to study the athermal effect of the electric pulse, tests with only varying the peak current density or pulse frequency at the same testing temperature were conducted. The results once again show that no evident difference between the stress–strain curves. It confirms that no athermal effect exists in DP980. However, with temperature elevated to 473K, the material is strengthened and then weakened as temperature exceeds 473K. Scanning electron microscope (SEM) analysis was adopted to better understand the observed phenomena. The results show that the decomposion of the martensite and the thermal effect are the main reason that attributes to the strength variation. Fracture surface morphology analysis indicates that the appearance of the shallower and smaller dimples as well as fracture platform represents the bad plasticity when increasing temperature. Meanwhile, fracture pattern is transformed from brittle fracture in test (1) to ductile fracture in test (2) when tensioned at 673K.