Jose María Cabrera

Hot deformation of duplex stainless steels

Abstract Duplex stainless steels (DSSs) have become established materials, successfully employed in many industrial applications. Their combination of mechanical properties and corrosion resistance is particularly appreciated in the petrochemical field. Hot deformation of these two-phase materials is still a critical point because the different mechanical response of austenite and ferrite often leads to the formation of edge cracks. In the present research, two DSSs with different nitrogen contents, i.e. EN 1.4462 and EN 1.4410, have been subjected to uniaxial hot compression tests in a wide range of temperatures and strain rates. The microstructural changes produced as a consequence of the distinct test conditions have been analyzed by means of optical and electron microscopy. The characteristics of high temperature plastic flow of both DSSs are interpreted in terms of the classical hyperbolic sine equation. The results are finally discussed considering the intrinsic two-phase nature of the materials studied.

Abnormal grain growth in a medium-carbon microalloyed steel

An experimental study on the grain growth of a medium-carbon V-Ti microalloyed steel with two levels of AIN has been carried out. A system to study grain-size distributions in order to detect the abnormal grain growth has been proposed. Log-normal distributions were verified and then properties of normal distributions were applied to distinguish normal and abnormal grains. The benefits of working with the relative difference (RD) of grain size in order to compare the grain-growth behaviour have been discussed. It was experimentally concluded that abnormal growth appears when RD is larger than 2.5. From the results a map of abnormal grain growth against time and temperature could be plotted. It was concluded that abnormal grain growth is due to the AIN dissolution when the above maps are correlated with the theoretical volume fraction of precipitates. The importance and effect of heating rate have also been shown: high heating rates can produce abnormal growth at higher temperatures than those of the equilibrium dissolution.

Characterization of the hot deformation in a microalloyed medium carbon steel using processing maps

Over the past few decades, medium carbon microalloyed steels have aroused considerable interest in physicists and metallurgical engineers because of the fact that these materials find wide application in automobile components industry. The hot workability of a microalloyed medium carbon steel is optimized using the power dissipation maps developed on the basis of the Dynamic Materials Model. The selected steel undergoes single peak dynamic recrystallization in the domain centered about 1,150 C and 10 s{sup {minus}1} with a peak efficiency of 32%, which may be considered as the optimum domain for hot working. The material undergoes dynamic recovery in the domain centered at about 900 C and 0.1 s{sup {minus}1}.

Modeling thermomechanical processing of austenite

Abstract This work is divided into two parts. First, a mathematical modeling of the austenite transformation during deformation operations at high temperature is presented. The model introduced by Estrin, Mecking and Bergstrom is employed in predicting the dynamic recovery step while the additional softening by dynamic recrystallization is modeled according to the Laasraoui and Jonas approach. Second, a prediction of the volume fraction of martensite, bainite and ferrite–pearlite formed during cooling is also presented. Most existing models can only be applied under conventional heat treatments. They cannot be used in welding or other hot working operations, where the austenite grain size is out of equilibrium conditions. The model here presented, which is a modification of the Ion, Easterling and Ashby model, is found to depend on the chemical composition and the initial austenitic grain size. Finally, a practical coupling of the latter models is presented. For this purpose, a finite element analysis of a hot forging operation is performed under different working conditions (temperature, strain rate, initial grain size) and different cooling conditions.

Modeling of the hot deformation behavior of boron microalloyed steels under uniaxial hot-compression conditions

Abstract The present study shows that the hot deformation behavior of boron microalloyed steels can be quantitatively described by constitutive equations. These equations take into account both dynamic recovery and recrystallization phenomena. They have been fitted using experimental data taken from hot compression tests of four boron microalloyed steels in order to determine their characteristic parameters. The tests were carried out over a wide range of temperatures (950, 1000, 1050 and 1100°C) and strain rates (10−3, 10−2 and 10−1 s−1). The analysis of the characteristic parameters of the constitutive equations describing the hot flow behavior of these steels shows that boron additions play a major role in softening mechanisms rather than on hardening. A quantification of the boron effect is also presented. The experimental data were compared with the predictions of the proposed model and an excellent agreement between measured and predicted values for all boron micro-alloyed steels over a wide range of temperatures and strain rates was obtained.

Effect of the Chemical Composition on the Peak and Steady Stresses of Plain Carbon and Microalloyed Steels Deformed under Hot Working Conditions

Two different behaviours are classically observed during the high temperature deformation of metals: i) power law creep and ii) exponential law creep. The first is observed at relatively low stresses and is considered as a deformation process controlled by diffusion. At higher stresses the above behaviour is converted into an exponential one, i.e. the power law breaks down. Both phenomena can be described by a single expression of the form: e = A(sinhασ) n .exp(-Q/RT) Here the parameters A, n, α and Q depend on the material being considered, and are usually referred to as apparent values because no account is generally taken of the internal microstructural state. In the particular case of microalloyed steels, a broad range of values have been reported in the literature for the latter constants, and clear trends have not always been evident. In recent work, it has been shown that the high temperature behaviour of medium carbon microalloyed steels can be accurately described by the classical hyperbolic sine relation provided the stresses are normalised by Youngs modulus E(T) and the strain rates by the self-diffusion coefficient D(T). According to this formulation, only two parameters need to be determined to characterise the hot flow behaviour: A and α (n can be set equal to 5 for carbon steels). In the present work, the latter expression is extended to plain carbon and low carbon microalloyed steels, and applied to the peak and steady stresses of the flow curve. To attain this goal, experimental results corresponding to several different steels reported by many authors are employed. The effect of chemical composition on the above constants is derived statistically.

Grain Refinement of Pure Copper by ECAP

Pure commercial Cu of 99,98 wt % purity was processed at room temperature by Equal- Channel Angular Pressing (ECAP) following route Bc. Heavy deformation was introduced in the samples after a considerable number of ECAP passes, namely 1, 4, 8, 12 and 16. A significant grain refinement was observed by transmission electron microscopy (TEM). Tensile and microhardness tests were also carried out on the deformed material in order to correlate microstructure and mechanical properties. Microhardness measurements displayed a quite homogeneous strain distribution. The most significative microstructural and mechanical changes were introduced in the first ECAP pass although a gradual increment in strength and a slight further grain refinement was noticed in the consecutive ECAP passes.

Predicting Multiple Peak Dynamic Recrystallization of Copper

Abstract. The relationship between the initial grain size and the critical Zener-Hollomon parameter value ( D 0 -Z c ) defines the conditions for which a material will dynamically recrystallize with a single or with multiple peaks. The relationship between the stable dynamically recrystallized grain and the Zener-Hollomon parameter ( D rex -Z ) predicts the conditions for grain refinement or coarsening during dynamic recrystallization. The Relative-Grain-Size model ( D 0 -Z c and D rex -Z ) adequately predicts the type of hot flow behavior before reaching a stable dynamically recrystallized grain size. However, a model to reliably predict the stress-strain curve is still needed. Several models exist which have been shown to predict the transition from single to multiple peak stresses. Nevertheless few of them report real material parameters and in any case the computational time makes them unviable for any industrial simulation process. The present authors have devised a DRX algorithm to measure the stress due to the diminishing initial grain volume and to measure the correction stress due to recrystallizing grains. One stress contribution is produced as a result of the surrounding or percolating new grains and another stress is due to the response of deforming the initial grain volume. The present authors propose a relatively simple model that in conjunction with existing theories for dynamic recovery can quantitatively predict the transition from single to multiple peak stress behavior during dynamic recrystallization. The predicted stress-strain curves have been correlated to experimental results after compression testing (650oC-950oC) commercially 99.9% pure copper.

Characterization of Strain-Induced Precipitation in Inconel 718 Superalloy

Inconel 718 presents excellent mechanical properties at high temperatures, as well as good corrosion resistance and weldability. These properties, oriented to satisfy the design requirements of gas turbine components, depend on microstructural features such as grain size and precipitation. In this work, precipitation-temperature-time diagrams have been derived based on a stress relaxation technique and the characterization of precipitates by scanning electron microscopy. By using this methodology, the effect of strain accumulation during processing on the precipitation kinetics can be determined. The results show that the characteristics of precipitation are significantly modified when plastic deformation is applied, and the kinetics are slightly affected by the amount of total plastic deformation.

A Model for multi peak dynamic recrystallization in copper

Modelling hot flow stress during grain refinement operations of fcc metals has largely included the use of an Avrami type equation to describe the decrease in stress due to Dynamic Recrystallization (DRX). However when refining large-grained copper, the processing temperatures and strain rates often produce a multi peak behaviour, which is not predictable by an Avrami equation alone. If an initial grain size, D0, is greater than the stable dynamically recrystallized grain size, Drex, which is a function of the Zener-Hollomon Parameter, Z, then the material will tend to refine. However if the current the Zener-Hollomon value, given by current temperature and strain rate conditions, is lower than a critical value, Zc, which depends on D0, then a multi peak stress behaviour is expected while refining. The latter Relative-Grain-Size model (i.e. the D0-Zc and Drex-Z relationships plotted on the same log-log graph) is a practical model that allows determination of whether a material will grain coarsen or refine and whether the dynamic recrystallization behaviour will be monotonic or with multi peaks. The present authors devised a dynamic recrystallization algorithm to measure the stress due to the diminishing initial grain volume and to measure the correction stress due to recrystallizing grains. Analysis on the hot (600°C-950°C) compression data of a 99.9% pure copper inductively lead to the use of an Avrami type equation to describe the stress contribution produced by the deformation of the remaining initial grain volume and a damped cosine equation to describe the stress contribution of the synchronized volume of new grains. This work discusses the experimental evidence and analytical findings that inductively support the mathematical description of the stress-strain curve given by a Damped Cosine Avrami Model for discontinuous DRX.