Hatem S. Zurob

Modeling recrystallization of microalloyed austenite: effect of coupling recovery, precipitation and recrystallization

Abstract In this contribution, existing models for precipitation, recovery and recrystallization have been coupled, with their interdependencies explained, to describe the microstructural evolution in a supersaturated alloy after hot deformation. Microalloyed austenite has been used as an example system and the time evolution of the precipitate diameter and the recrystallization and softening fractions are compared with the available experimental data. The model predictions are in excellent quantitative agreement with the experimental observations. Particular attention is paid to the occurrence of ‘plateaus’ or ‘humps’ in the softening and recrystallization fraction plots. In both cases, the incorporation of recovery is an essential ingredient for a quantitative description of the microstructural evolution in the hot-worked structure.

A model for the competition of precipitation and recrystallization in deformed austenite

Abstract The hot-deformation of a microalloyed steel sets the stage for the processes of static-recrystallization and strain-induced precipitation. A physically-based model was developed to describe the interaction of these two processes. The precipitates were assumed to form, exclusively, on dislocations. Dynamic effects as well as static recovery were ignored. Given the alloy composition, deformation temperature and dislocation density, the model is able to predict the recrystallized fraction as a function of time. The model may be used to construct recrystallization–time–temperature (RTT) maps as well as deformation–temperature (DT) maps. The predictions of the model are in excellent qualitative agreement with experimental observations.

Effect of chemical composition on work hardening of Fe-Mn-C TWIP steels

The work hardening behaviour of Fe-Mn-C twinning induced plasticity (TWIP) steels with a wide compositional range has been investigated. Based on the consideration that twinning provides a dynamic composite effect resulting in high work hardening rate in TWIP steels, the present work proposes a model to describe such behaviour as a function of chemical composition. The model predictions are in good agreement with experimental observations.

Self-consistent model for planar ferrite growth in Fe-C-X alloys

A self-consistent model for non-partitioning planar ferrite growth from alloyed austenite is presented. The model captures the evolution with time of interfacial contact conditions for substitutional and interstitial solutes. Substitutional element solute drag is evaluated in terms of the dissipation of free energy within the interface, and an estimate is provided for the rate of buildup of the alloying element “spike” in austenite. The transport of the alloying elements within the interface region is modeled using a discrete-jump model, while the bulk diffusion of C is treated using a standard continuum treatment. The model is validated against ferrite precipitation and decarburization kinetics in the Fe-Ni-C, Fe-Mn-C, and Fe-Mo-C systems.

Quantifying the Solute Drag Effect on Ferrite Growth in Fe-C-X Alloys Using Controlled Decarburization Experiments

The kinetics of ferrite growth in the Fe-C-Co and Fe-C-Si systems has been quantified using controlled decarburization experiments. The Fe-C-Co system is a particularly interesting system since a large range of Co contents can be considered providing a suitable data set for examination of the composition dependence of the solute drag effect. Six Fe-C-Co alloys containing Co from 0.5 to 20 pct have been considered. Three Fe-C-Si alloys have also been considered and each has been transformed at three temperatures proving a suitable data set for examining the temperature dependence of the solute drag effect. This data set, along with ferrite growth data from decarburization experiments on an Fe-C-2Cr alloy has been used to test the ferrite growth model proposed in the companion article by Zurob et al. It is shown that this model for ferrite growth, that includes diffusional dissipation due to interaction between the solute and the migrating boundary, quantitatively captures both the temperature and composition dependence of the deviation of experimental ferrite growth kinetics from the PE and/or LENP models.

Vacancy behavior and solute cluster growth during natural aging of an Al-Mg-Si alloy

The natural aging behavior of an Al-0.46Mg-1.05Si-0.14Fe (wt pct) alloy was studied with positron annihilation lifetime spectroscopy at four temperatures and with 25Mg solid-state nuclear magnetic resonance. The evolution of positron lifetime is shown to consist of three distinct stages. In an effort to understand the physical processes occurring during natural aging, a phenomenological model was developed for the first two stages and is shown to describe the experimental observations well. The description accounts for the decay in positron lifetime due to diffusion of vacancies to sinks and an increase in positron lifetime attributed to growing solute clusters. NMR measurements of 25Mg solute partitioning during natural aging support the model.

Recrystallization, Precipitation Behaviors, and Refinement of Austenite Grains in High Mn, High Nb Steel

Through a series of experiments conducted on three kinds of high Mn steels with different Nb content, including stress relaxation tests, physical metallurgical modeling, and observation of prior austenite grains and precipitates, the effect of Nb on recrystallization and precipitation behaviors were investigated. The results indicate the existence of a novel deformation temperature range for grain refinement resulting from complete static recrystallization (SRX) in high Mn, high Nb steel, whereas slow SRX kinetics can be accelerated by a finer initial grain size. In this deformation temperature range, the effect of precipitation is too weak to prohibit SRX nucleation efficiently, but solute drag is still large enough to slow down growth rate. As a consequence, shorter incubation and homogeneous recrystallized nucleation can be realized at relative low temperature, and the coarsening rate of grains is much slower because of the high solute drag effect in the rolling of low C high Mn, high Nb line pipe steel.

A Novel Approach to Model Static Recrystallization of Austenite During Hot Rolling of Nb Microalloyed Steel. Part I: Precipitate-Free Case

A physically based model is developed to describe static recrystallization during the hot rolling of Nb microalloyed austenite. A key feature of the model is a detailed description of the recrystallization nucleation process; the model predicts the recrystallization incubation time as well as the time evolution of the recrystallization nucleation rate. In addition, the effects of static recovery and solute drag on the growth of the recrystallized grains are captured. The predicted recrystallization kinetics and recrystallized grain size are shown to be in good agreement with published data.

Effect of Low-Temperature Recovery Treatments on Subsequent Recrystallization in Al-2.5%Mg

The effect of low temperature recovery treatments on the recrystallization kinetics during subsequent high temperature annealing was investigated in three Al-2.5%Mg alloys with various Fe additions. Recovery treatments were carried out at 190oC for times ranging from 0.25 to 65 hrs. Recrystallization treatments were carried out at 280oC. The kinetics of recrystallization was followed using the techniques of hardness measurement, optical metallography and calorimetry.

A comparison of ferrite growth kinetics under denitriding and decarburizing conditions

Recent years have seen an increasing emphasis on the identification of transitions in growth modes during the diffusional decomposition of austenite to ferrite in Fe-C-X alloys. Of particular interest are transitions between the extremes of non-partitioned growth represented by the ParaEquilibrium (PE) and local equilibrium negligible partition limits. Identification of such transitions requires high-quality measurements of ferrite growth kinetics, and recent uses of the decarburization approach have allowed access to very high-precision growth kinetic measurements. However, one of the limitations of the decarburization approach is that the lower limit of its applicability is the eutectoid temperature and this has so far compromised its usefulness to probe the temperature dependence of kinetic transitions. In this contribution, analogous denitriding studies have been performed that allow access to much lower temperatures than are possible using decarburization. It is shown that the kinetics of ferrite growth in the Fe-N-Mn system become closer to the PE limit as the temperature is lowered. The growth kinetic data are interpreted quantitatively in the framework of the Zurob et al. model that includes diffusional dissipation due to Mn diffusion across the migrating interface. Furthermore, comparisons are made between decarburization and denitriding in Fe-Mn alloys of the same Mn content at the same temperature. The interface velocities are much faster under denitriding conditions, and this allows inferences to be made about the effect of the interface velocity on dissipation and contact conditions. It is also suggested that the binding energy of Mn to the migrating interface may be slightly higher in the Fe-C-Mn system than in the Fe-N-Mn system, and it is speculated that this is due to the strong segregation of C to the interface and the associated co-segregation of Mn.