Intragranular Nucleation Research Articles

New microstructural features occurring during transformation from austenite to ferrite under the kinetic influence of magnetic field in a medium carbon steel

The effects of magnetic field on nucleation barrier of the phase transformation from austenite to ferrite at different cooling rates in 42CrMo steel have been investigated. The microstructures of ferrite and pearlite aligned along the magnetic field direction (parallel to the hot-rolling direction) are obtained at a cooling rate of 10°C/min, resulting from the kinetic effects of the applied magnetic field during cooling and the microstructural influences of an inhomogeneous deformation occurring during the previous hot rolling. In this case, the formation of ferrite grains at higher temperatures is attributed mainly to the preferential nucleation at austenite boundaries. However, a fairly uniform microstructure of randomly distributed ferrite and pearlite is formed at a high cooling rate of 46°C/min in the magnetic field of 14T, as a result of both intergranular and intragranular nucleation at relatively low temperatures. Probing into this issue is helpful to gain a better understanding of kinetic influences of magnetic field on the phase transformation from austenite to ferrite.

The mechanism of acicular ferrite in weld deposits

Research has shown that the acicular ferrite microstructure in steel weld metal, which provides an optimum combination of strength and toughness, is indeed intragranularly nucleated bainite. It is possible to maximize the content of acicular ferrite by increasing the intragranular nucleation sites while maintaining a critical weld metal cooling rate and the steel hardenability. This paper highlights recent research related to nucleation and growth of acicular ferrite during decomposition of austenite.

The Role of Inclusions and Austenite Grain Size on Intragranular Nucleation of Ferrite in Medium Carbon Microalloyed Steels

The beneficial aspects of intragranular ferrite formation on mechanical properties of welds have been reported in the literature for decades. In recent years, this concept has been successfully extended to medium carbon forging steel to refine the microstructure and optimise ductility and toughness. The aim of this work is to demonstrate that intragranular formation of ferrite could be enhanced by increasing the austenite grain size and/or optimising the nature of the inclusions. In this sense, the isothermal decomposition of austenite in allotriomorphic and idiomorphic ferrite for two medium carbon steels microalloyed with vanadium and titanium have been studied. The experimental results reported in this work allows to conclude that austenite grain size and the nucleation potency of inclusions are two parameters that should be considered linked to promote the full decomposition of austenite into intragranularly nucleated ferrite.

Effect of thermomechanical parameters on the critical strain for ultrafine ferrite formation through hot torsion testing

A C–Mn–V steel was used to study ultrafine ferrite formation (1–3 μm) through dynamic strain-induced transformation (DSIT) using hot torsion experiments. A systematic study determined the critical strain for the start of DSIT ( ε C,DSIT), although this may not lead to a fully ultrafine microstructure. Therefore, the strain to produce an ultrafine ferrite (UFF) as final microstructure ( ε C,UFF) during deformation was also determined. In addition, the effect of thermomechanical parameters such as deformation temperature, prior austenite grain size, strain rate and cooling rate on ε C,DSIT and ε C,UFF has been evaluated. DSIT ferrite nucleated on prior austenite grain boundaries at an early stage of straining followed by intragranular nucleation at higher strains. The prior austenite grain size affected the distribution of DSIT ferrite nucleation sites at an early stage of transformation and the subsequent coarsening behaviour of the grain boundary and intragranular ferrite grains during post-deformation cooling. Also, ε C,DSIT and ε C,UFF increased with an increase in the prior austenite grain size and deformation temperature. The post-deformation cooling had a strong effect not only on ε C,UFF but also the UFF microstructure (i.e. final ferrite grain size and second phase characteristics).

Analysis of different acicular ferrite microstructures in low-carbon steels by electron backscattered diffraction. Study of their toughness behavior

Acicular ferrite formation, promoted by the intragranular nucleation of ferrite plates, is well known to be beneficial for achieving a good combination of mechanical properties. However, the set of microstructures that can be obtained during the subsequent development of the transformation from the primary plates generated at particles can be quite complex and depends on a certain number of variables: steel composition, temperature range, prior austenite grain size, and particle density. In the present work, acicular ferrite microstructures have been produced by isothermal treatments in three different steels with different active particle types and densities. The morphology of the obtained intragranular microstructures has been found to depend on the steel composition, the prior austenite grain size, and the density of particles able to promote intragranular nucleation. Electron backscattered diffraction (EBSD) techniques have been used to define the microstructural unit controlling toughness in these types of microstructures.

Non-metallic inclusion and intragranular nucleation of ferrite in Ti-killed C–Mn steel

The influence of Ti addition on the development of acicular ferrite microstructure during the γ/α transformation in C–Mn steels has been studied. The optical microstructures of the heat-treated specimens with different Ti concentrations were characterized. Transmission electron microscopy analysis with thin foil specimens was carried out to investigate the phase composition of non-metallic inclusions and the local variation of chemical composition around the inclusions. It has been found that an acicular ferrite dominant microstructure could be produced when the Ti concentration is higher than about 50 ppm. The transition from the conventional bainitic microstructure to the interlocking acicular ferrite microstructure occurs in response to the change in the main-component of the non-metallic inclusions from Mn–Si oxide to Ti 2O 3. The Mn depleted zones around Ti 2O 3 particles were detected, which could explain how the intragranular nucleation is facilitated on Ti 2O 3 particles.

Relevant aspects of allotriomorphic and idiomorphic ferrite transformation kinetics

Recently, the grain boundary - intragranular nucleation of ferrite has received much attention in the search for an improvement in toughness of steel, especially when conventional austenite grain refinement techniques are not enough. The present paper deals with some relevant aspects of the competitive process of allotriomorphic and idiomorphic ferrite transformation in a medium carbon V - Ti microalloyed steel, for two different isothermal temperatures and values of prior austenite grain size. It is concluded that consideration of proeutectoid ferrite (allotriomorphic plus idiomorphic) as a single transformation might lead to wrong conclusions in an analysis of the isothermal decomposition of austenite.

Nucleation of Proeutectoid Ferrite on Complex Precipitates in Austenite

Effect of various intragranular inclusions or precipitates (MnS, VC and V(C, N)) on the microstructure and kinetics of intragranular ferrite transformation at the temperatures between 973 and 823 K was studied using various Fe-2Mn-(0.13, 0.2)C(mass%) alloys with the small addition of sulfur, vanadium, and nitrogen. In Fe-2Mn-0.13C-50ppmS and Fe-2Mn-0.2C-470ppmS alloys, MnS particles, mostly incoherent in austenite, do not act as effective nucleation sites of ferrite. V addition slightly improves the potency of MnS as ferrite nucleation site by forming MnS+VC complex precipitates. The addition of both V and N largely enhances the intragranular nucleation of ferrite idiomorph on the MnS+V(C, N) complex precipitate. It is considered that two factors, i.e., (1) the advantage in the balance of interphase boundary energy and (2) the increase in the fraction of V(C, N) precipitate by the addition of nitrogen, are mainly responsible for the promotion of intragranular ferrite formation on the MnS+V(C, N) complex precipitate.

Static recrystallization mechanisms in a coarse-grained Nb-microalloyed austenite

Static recrystallization mechanisms have been studied in a coarse-grained Nb microalloyed austenite. An austenite with a coarse grain size of 800 µm, typical of thin slab casting processes, has been deformed in torsion at a temperature of 1100 °C. After deformation, the specimens have been held for different times at this high temperature and then water quenched. The microstructural changes occurring during static recrystallization were characterized by metallographic evaluation. It has been observed that new recrystallized grains nucleate preferentially on parent austenite grain boundaries and tend to form in clusters. Once all the boundaries have been consumed, intragranular nucleation is actived at late stages of recrystallization. Clustered nucleation allows impingement to take place early during the recrystallization process, favoring grain-coarsening phenomena to occur behind the recrystallization front, which is denoted by the significant reduction in the number of grains per unit volume observed during early stages of recrystallization. Static recrystallization proceeds heterogeneously, as a result of a nonuniform distribution of stored energy in the deformed material. A continuous decrease of the average migration rate of the recrystallization front is observed, which can be ascribed to the reduction of the driving force for migration as recrystallization advances.

Modelización de la formación isotérmica de ferrita idiomórfica en aceros de medio carbono microaleados con vanadio-titanio

A theoretical model is presented in this work to calculate the evolution of isothermal austenite-to-idiomorphic ferrite transformation in a medium carbon vanadium-titanium microalloyed steel. This model has been developed on the basis of the study of the nucleation and growth kinetics of idiomorphic ferrite, considering the influence of the nature, size and distribution of the inclusions, which are responsible of the intragranular nucleation of idiomorphic ferrite. Moreover, the influence of the austenite grain size on the isothermal decomposition of austenite in idiomorphic ferrite has been thoroughly analysed. An excellent agreement (85 % in R) has been obtained between experimental and predicted values of volume fraction of idiomorphic ferrite.

On the roles of clusters during intragranular nucleation in the absence of static defects

In some alloy systems, e.g., Cu-Co, prior formation of clusters does not affect the nucleation of precipitates, whereas in others, e.g., Al-Cu-X, where X can be Mg, Ag, Cd, In, Sn, Si, or Ge (singly or in combination), clusters can not only accelerate the nucleation kinetics of the first phase to form but can also change the identity of this phase. When solute atoms in a cluster are strongly bound to one another but none has a high-binding energy to vacancies, clusters will tend to dissolve and to cease forming as a result of the loss of quenched-in vacancies before the incubation time for nucleation has elapsed unless the ambient vacancy concentration is high enough to sustain the clustering process. When solutes are also strongly bound to vacancies, however, clusters can survive through and beyond the incubation time. Two mechanisms are examined through which such clusters can assist nucleation. (a) One or more of the atom species constituting the clusters adsorbs at the interfaces of the nuclei and thereby reduces their interfacial energy. (b) At plate- (or lath-) shaped nuclei formed with an appreciable shear-strain energy, segregation of large atoms to the regions under tension within the local strain field and of small atoms and/or vacancies to local regions under compression can markedly reduce this strain energy. Nucleation taking place in the absence of static defects, such as aggregates, dislocations, internal boundaries, or particles, is clearly homogeneous when clusters do not assist the nucleation process. However, atom probe results have demonstrated the existence of clusters surviving long enough to assist nucleation and in so doing suggest that the definition of “heterogeneous nucleation” be extended to include even these continuously fluctuating heterogeneities.

Formation of acicular ferrite and influence of vanadium alloying

The effect of vanadium microalloying in promoting a tough, acicular ferrite microstructure in C-Mn steels has been investigated. The microstructure obtained consisted of fine interlocking ferrite plates and was indistinguishable from acicular ferrite developed in steel weld metals, apparently from the intragranular nucleation of ferrite at inclusions. A number of variables were examined in high purity experimental steels including composition and heat treatment conditions, and related to a metallographic examination of the microstructure by high resolution micro-analytical transmission electron microscopy and surface analysis. A comprehensive study of the inclusions in the steels, containing different ratios of oxygen and nitrogen concentration, did not find any significant evidence that inclusion assisted nucleation was the sole determining factor in producing acicular ferrite. Moreover, no evidence could be found to relate vanadium alloying to significant vanadium nitride precipitation, either separately, or associated with the inclusions. Thus, in the present steels, any possible alternative influence of vanadium on intragranular ferrite nucleation is not obscured by effects associated with the inclusion population. The vanadium concentration appeared to be the most important influence in developing an acicular ferrite microstructure in these experimental steels, and this is not inconsistent with previous reports in the literature of a beneficial 'vanadium effect'. Evidence for vanadium segregation in the microstructure was found, which may be related to the effect of vanadium in encouraging the formation of acicular ferrite. Even when there is good evidence that inclusions are responsible for intragranular ferrite nucleation (as, for example, in steel weld metals), a 1 :1 inclusion-ferrite relationship has been difficult to establish. Thus, even an inclusion activated nucleation theory is likely to require additional intragranular ferrite formation without inclusion assistance, such as sympathetic or autocatalytic nucleation, and this could be reflected in the present study by vanadium atom clustering facilitating an alternative intragranular ferrite nucleation mechanism.

Oxygen induced chemical ordering and ultrafine lamellar structure formation in a Ti-48 at. % Al alloy

ABSTRACTInfluence of oxygen on the microstructure development of a Ti-48 at. % Al alloy has been investigated by means of transmission electron microscopy and 1D atom probe. Oxygen is found to significantly increase the temperature of the α → α2 chemical ordering reaction. As a consequence, above a critical oxygen content, the α → α2 transformation is substituted to the α → γm massive transformation when the Ti-48 Al alloy is quenched from the single a-phase field. In such a case, our pervious work has shown that the alloy exhibits a fully (α2 + γ) ultrafine lamellar structure. The present work gives a complete description of the ultrafine lamellar structure formation which, in opposition to the classical lamellar structure formation, involves an intragranular nucleation and growth of the γ phase within the α2 matrix.

Effect of process variables on formation of dynamic strain induced ultrafine ferrite during hot torsion testing

Ultrafine grain sizes were produced using hot torsion testing of a 0.11C-1.68Mn-0.20Si wt- steel, with ultrafine ferrite <1 m nucleating intragranularly during testing by dynamic strain induced transformation. A systematic study was made of the effect of isothermal deformation temperature, strain level, strain rate, and accelerated cooling during deformation on the formation of ultrafine ferrite by this process. Decreasing the isothermal testing temperature below the Ae3 temperature led to a greater driving force for ferrite nucleation and thus more extensive nucleation during testing; the formation of Widmanstatten ferrite prior to, or early during, deformation imposed a lower temperature limit. Increasing the strain above that where ferrite first began 0.8 at 675C and a strain rate of 3 s1 increased the intragranular nucleation of ferrite. Strain rate appeared to have little effect on the amount of ferrite formed. However, slower strain rates led to extensive polygonisation of the ferrite formed because more time was available for ferrite recovery. Accelerated cooling during deformation followed by air cooling to room temperature led to a uniform microstructure consisting of very fine ferrite grains and fine spherical carbides located in the grain boundaries regions. Air cooling after isothermal testing led to carbide bands and a larger ferrite grain size.

Nucleation of Intragranular Accicular Ferrite in a Ti-Containing Low Carbon Steel of High N Content

Formation of acicular ferrite grains by dispersing non-metallic inclusions which can act as intragranular nucleation sites within austenite grain is proposed as a method to obtain refined grain structure in steels. The microstructure of acicular ferrite steel consists of fine interlocking ferrite plates nucleated on dispersed non-metallic inclusions. This acicular ferrite structure provides a desirable combination of high strength and good toughness because of its small plate thickness and interlocking microstruture. in Ti-containing low carbon steels, researchers proposed that Ti2O3 or MnS, among inclusions, is the most effective nucleant for intragranular ferrite because of the formation of Mn depleted zone(MDZ) around Ti2O3 or MnS particles. TiN is also reported as a nucleation site owing to the coherency between ferrite and TiN crystals. But It has not been a simple task to identify which inclusions and what mechanisms are really effective in the formation of intragranular acicular ferrite.

Modeling of kinetics of isothermal idiomorphic ferrite formation in a medium-carbon vanadium-titanium microalloyed steel

The present article is concerned with the theoretical and experimental study of the growth kinetics of idiomorphic ferrite in a medium-carbon vanadium-titanium microalloyed steel. A theoretical model is presented to calculate the evolution of isothermal austenite-to-idiomorphic ferrite transformation with time for a given temperature. Moreover, the nature, size, and distribution of the inclusions that are responsible for the intragranular nucleation of idiomorphic ferrite have been characterized by scanning electron microscopy (SEM). Finally, the influence of austenite grain size in the isothermal decomposition of austenite in idiomorphic ferrite has been thoroughly analyzed. An excellent agreement (higher than 90 pct in R2) has been obtained between the experimental and predicted values of the volume fraction of idiomorphic ferrite.

Nucleation sites for ultrafine ferrite produced by deformation of austenite during single-pass strip rolling

An austenitic Ni-30 wt pct Fe alloy, with a stacking-fault energy and deformation characteristics similar to those of austenitic low-carbon steel at elevated temperatures, has been used to examine the defect substructure within austenite deformed by single-pass strip rolling and to identify those features most likely to provide sites for intragranular nucleation of ultrafine ferrite in steels. Samples of this alloy and a 0.095 wt pct C-1.58Mn-0.22Si-0.27Mo steel have been hot rolled and cooled under similar conditions, and the resulting microstructures were compared using transmission electron microscopy (TEM), electron diffraction, and X-ray diffraction. Following a single rolling pass of ∼40 pct reduction of a 2mm strip at 800 °C, three microstructural zones were identified throughout its thickness. The surface zone (of 0.1 to 0.4 mm in depth) within the steel comprised a uniform microstructure of ultrafine ferrite, while the equivalent zone of a Ni-30Fe alloy contained a network of dislocation cells, with an average diameter of 0.5 to 1.0 µm. The scale and distribution and, thus, nucleation density of the ferrite grains formed in the steel were consistent with the formation of individual ferrite nuclei on cell boundaries within the austenite. In the transition zone, 0.3 to 0.5 mm below the surface of the steel strip, discrete polygonal ferrite grains were observed to form in parallel, and closely spaced “rafts” traversing individual grains of austenite. Based on observations of the equivalent zone of the rolled Ni-30Fe alloy, the ferrite distribution could be correlated with planar defects in the form of intragranular microshear bands formed within the deformed austenite during rolling. Within the central zone of the steel strip, a bainitic microstructure, typical of that observed after conventional hot rolling of this steel, was observed following air cooling. In this region of the rolled Ni-30Fe alloy, a network of microbands was observed, typical of material deformed under plane-strain conditions.

Formation of ultra-fine ferrite in hot rolled strip: potential mechanisms for grain refinement

A novel single-pass hot strip rolling process has been developed in which ultra-fine (<2 μm) ferrite grains form at the surface of hot rolled strip in two low carbon steels with average austenite grain sizes above 200 μm. Two experiments were performed on strip that had been re-heated to 1250°C for 300 s and air-cooled to the rolling temperatures. The first involved hot rolling a sample of 0.09 wt.%C–1.68Mn–0.22Si–0.27Mo steel (steel A) at 800°C, which was just above the Ar 3 of this sample, while the second involved hot rolling a sample of 0.11C–1.68Mn–0.22Si steel (steel B) at 675°C, which is just below the Ar 3 temperature of the sample. After air cooling, the surface regions of strip of both steel A and B consisted of ultra-fine ferrite grains which had formed within the large austenite grains, while the central regions consisted of a bainitic microstructure. In the case of steel B, a network of allotriomorphic ferrite delineated the prior-austenite grain boundaries throughout the strip cross-section. Based on results from optical microscopy and scanning/transmission electron microscopy, as well as bulk X-ray texture analysis and microtextural analysis using Electron Back-Scattered Diffraction (EBSD), it is shown that the ultra-fine ferrite most likely forms by a process of rapid intragranular nucleation during, or immediately after, deformation. This process of inducing intragranular nucleation of ferrite by deformation is referred to as strain-induced transformation.

Effect of Deformation in Controlled Rolling on Ferrite Nucleation

Nucleation of ferrite either at austenite grain boundaries or within them on deformation defects has been examined experimentally and analyzed on the basis of the theory of heterogeneous nucleation. Low-carbon low-alloy steels were deformed by controlled rolling schedules to a total reduction of 50 or 68% in finish rolling at 730-800 °C. Deformation accelerates the kinetics of the γ→∞ transformation and strongly activates ferrite nucleation at grain boundaries. Both experimental and theoretical estimations showed that the rate of intragranular nucleation is much less compared to nucleation at grain boundaries. Intragranular nuclei develop notably only in the final stages of transformation in deformed austenite and affect the formation of structure only in the small separated areas.On a examiné expérimentalement et analysé la nucléation de la ferrite, soit aux joints de grain de l'faustenite, ou à l'finterieur de ceux-ci, aux défauts de déformation, en se basant sur la théorie de la nucléation hétérogéne. On a déformé des aciers peu alliés à faible carbone, en suivant des plans de laminage contrôlé, jusqu'fa une réduction totale de 50 ou 68% pour le laminage final a 800-730 °C. La déformation accélère la cinétique de transformation de γ→α et active fortement la nucleation de la ferrite aux joints de grains. Tant les évaluations expérimentales que theoriques ont montré que le taux de nucléation intragranulaire était beaucoup moins élevé en comparaison avec la nucleation aux joints de grain. Les noyaux intragranulaires se développent particuliérement seulement lors des étapes finales de transformation de l'faustenite déformée et affectent la formation de la structure seulement dans de petites zones séparées.

Effects of Si and Al on acicular ferrite formation in C-Mn steel

The influences of Si and Al on the microstructure of Ti-containing C-Mn steels have been studied. Although Si addition up to 1 wt pct did not change the crystal structure of dominant Ti2O3 particles dispersed in these steels, the formation of an acicular ferrite microstructure was suppressed at high Si contents. It has been found that the Mn content dissolved in Ti2O3 particles decreases with increasing Si content. This seems to weaken the formation of Mn-depleted zones (MDZs) around Ti2O3 particles, which act as a dominant driving force for the intragranular nucleation of acicular ferrite. It has also been found that the addition of a small amount of Al greatly reduces the formation of acicular ferrite. This is attributed to the change of dominant oxide particles from Ti2O3, which serves as an effective nucleant for acicular ferrite, to inert Al2O3. The result of the present study implies that it is advantageous to keep Si and Al contents in Ti-containing steels as low as possible when well-developed microstructures of acicular ferrite are desired.