Jose Manuel Prado

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}.

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.

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.

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.

Effect of rare-earth metals on the hot strength of HSLA steels

Abstract An experimental study was done on the effect of rare-earth metals on a high-strength low-alloy steel. The work was focused in deriving the influence on Ce and La on the hot-working behavior. For this purpose, uniaxial hot-compression tests were carried out in a wide range of temperatures and strain rates. The effect of the rare-earth metals was determined by comparison of the characteristic parameters, describing the constitutive equations of the high-temperature response of the steel with, and without, rare-earth metals. The results showed that rare-earth metals were playing a major significant role on hardening mechanisms rather than on softening by dynamic recovery. On the contrary, rare-earth metals were able to delay the onset of dynamic recrystallization. All the present experimental results suggested that the latter roles are played by solid solution strengthening, through a solute drag effect, and not by precipitated particles.

Effects of precipitation during dynamic recrystallization of copper with different oxygen levels

Previous research works assert that the observed increase in hot flow stress of commercially pure copper is attributed to the interactions between solute atoms and dislocations, specifically by interstitial oxygen. This work shows TEM images of the formation of Cu2O precipitates after warm working temperatures that in part help explain the increase of stress during hot compression of 99.9% pure copper. Three commercially pure large-grained coppers with 26, 46 and 62ppm of oxygen were tested at different temperatures (600°C-950°C) and strain rates (0.3s-1- 0.001s-1). At temperatures below 850°C, the stress differences between coppers, tested at same the strain rate, became increasingly higher. A correlation between stress increase and oxygen content was found. Precipitation of nanometric Cu2O did not show any difference in dynamically recrystallized grain size; however hardness tests showed that the final properties were modified. This work discusses the effect precipitation of Cu2O has on the hot flow curve and the final microstructure of hot formed 99.9% pure copper with different oxygen levels.

Modelling the Hot Working of Simple Geometries Employing Physical-Based Constitutive Equations and the Finite Element Method

The prediction of the final microstructure by simulation of the hot plastic deformation usually gives unsatisfactory results. This is partially due to the inadequate constitutive equations employed by the conventional and commercial software available to describe the hot flow behaviour. In this work the latter limitation is overcome by using physical-based constitutive equations where account of the interaction between microstructure and processing variables is taken. The hot forging behaviour of a simple piece (a small gear) was simulated in this way employing the software ABAQUS. Additionally it was assumed that strain rate and temperature are not constant during the process. Special attention was paid to the prediction of the final grain size and the appearance of dynamic recrystallization, facts traditionally ignored in numerical modelling.

Characterization of hot flow behaviour and deformation stability of medium carbon microalloyed steel using artificial neural networks and dynamic material model

Abstract Artificial neural network (ANN) and dynamic material model (DMM) are considered to be powerful methods to characterize the flow behaviour of metallic materials. The aim of this study is to analyze the performance of these two methods in the characterization of flow behaviour and deformation stability of medium-carbon microalloyed steel. Flow curves obtained from hot compression tests have been used to describe the flow behaviour of the studied steel using an ANN model. Good correlation between experimental and predicted data was observed. To characterize the deformation stability of the studied steel, experimental processing maps are generated using DMM. Finally, in order to verify the accuracy of ANN results, processing maps based on the DMM have been developed using ANN predicted data. It has been found that these maps agree closely with those obtained using experimental data.

On the Onset of Dynamic Recrystallization in Steels

The knowledge of the flow behavior of metallic alloys subjected to hot forming operations is of particular interest for designers and engineers in the practice of industrial forming processes simulations (i.e. rolling mill). Nowadays dynamic recrystallization (DRX) is recognized as one of the most relevant and meaningful mechanisms available for the control of microstructure. This mechanism occurs during hot forming operations over a wide range of metals and alloys and it is known to be as a powerful tool which can be used to the control of the microstructure and properties of alloys. Therefore is important to know, particularly in low stacking fault energy (SFE) materials, the precise time for which DRX is available to act. At constant strain rate such time is defined by a critical strain, εc. Unfortunately this critical value is not directly measurable on the flow curve; as a result different methods have been developed to derive it. Focused on steels, in the present work the state of art on the critical strain for the initiation of DRX is summarized and a review of the different methods and expressions for determining εc is included. The collected data is suitable to feeding constitutive models.

Study of the nanometric grain size distribution in iron compacts obtained by mechanical milling

A study has been carried out on the grain size distribution of cylindrical compacts obtained by consolidation of iron powder severely deformed by mechanical milling. Consolidation has been performed in two consecutive steps: cold and hot conditions. The hot one was done at two temperatures, namely 425 and 475°C. After milling, the iron powder has a grain size of 8 nm (± 4 nm) with an average hardness of 800 HV. After hot compaction the grain size increases up to 50 nm, especially at 475°C where a small fraction of grains reach larger values than the average. The grain size was evaluated by two different techniques, X-Ray Diffraction and Transmission Electron Microscopy. Results showed some differences between both methods. The advantage of using TEM is that grain size distribution, and not only the average size, can be obtained. Small discs were also obtained from the compacted specimen in order to fracture them on a “ball on three balls” equipment. The fracture behaviour of the samples was then studied by SEM.