Ferrite Nucleation Research Articles

Effect of Inclusion Size on the Nucleation of Acicular Ferrite in Welds.

Low-carbon steel weld with a high density of oxide inclusions prepared using a experimental metal-cored wire has been examined to study the effect of inclusion size on the formation of acicular ferrite, and to understand the role of inclusion in the nucleation of ferrite lath. Depending on the ferrite morphology associated with inclusions, a total of 282 inclusions observed under TEM could be classified into two groups, i.e. the non-nucleant and the nucleant. Experimental results showed that the group of inclusions acted as nucleant were appreciably larger in size compared with those of non-nucleant resulting in the increased probability of nucleation with the increase of inclusion size, even though the chemical and structural natures appeared to be the same. The group of nucleant-inclusion was further divided into two types depending on the degree of nucleation, which was evaluated by the number of ferrite lath nucleated. Statistical analysis performed on inclusion size indicated that the larger the inclusion size is the more ferrite laths could be nucleated. Those laths nucleated from a large single inclusion have grown in many different radial directions and mostly had a different crystallographic orientation from those of adjacent ferrite laths. As a result of this study, it is demonstrated that larger inclusions are indeed more potent nucleation sites when compared with those of smaller size. Thus it could be concluded that the provision of the inclusion surface as for the inert surface for the heterogeneous nucleation of acicular ferrite lath would be the principal role of inclusions playing in the weld metal of low alloy steels. Other possible mechanisms were also considered, but they were unlikely to be operated in the present weld metal system.

鉄系合金における拡散変態に及ぼす強磁場の影響

Effects of a strong magnetic field of 10 T on the microstructure and kinetics of ferrite transformation behaviors in Fe-based alloys were investigated systematically using a vacuum electric furnace installed in a helium-free superconducting magnet. It was found that a magnetic field of 10 T significantly stimulates ferrite and pearlite transformations. The rates of ferrite nucleation and growth were measured with and without a magnetic field. It was found that a magnetic field of 10 T increases the nucleation rate of ferrite and increases the parabolic rate constant above a certain temperature, between 973 K and 993 K, but decreases below this temperature. A possible mechanism is discussed.

Effect of Ti Addition on the Potency of MnS for Ferrite Nucleation in C-Mn-V Steels.

The variation of precipitation behavior of MnS has been thermodynamically calculated in V-bearing C-Mn steels by substituting Ti for V. The use of two different set of solubility data for Ti 4 C 2 S 2 and TiS reported in literatures leads to a completely reversed precipitation behavior of MnS. The result of experimental observations is in qualitative agreement with the calculated result using the solubility data of Liu and Jonas. The calculated result showed, contrary to the case of V-containing steel, that the precipitated amount of MnS in steels bearing Ti-only rather decreases with decreasing temperature and that this is due to the formation of Ti 4 C 2 S 2 Controlled cooling experiments with interrupted quenching technique showed that MnS directly acts as the nucleation site for ferrite and that its nucleation potency depends on the precipitation behavior of MnS. Namely, MnS rarely acts as a nucleation site for ferrite in steels where its precipitated amount decreases with decreasing temperature.

Nucleation of intragranular ferrite at Ti 2O 3 particle in low carbon steel

Systematic experiments have been performed to study the intragranular nucleation phenomena of ferrite in Ti-bearing low carbon steels. The fine intragranular acicular ferrite structure in Ti-bearing low carbon steels is induced by heterogeneous nucleation of ferrite plates at fine dispersed Ti 2O 3 particles, which are thermodynamically stable in the steels. From a steel–Ti 2O 3 diffusion bonding experiment, it is clearly observed that a local Mn-depleted zone (MDZ) has developed in the vicinity of the steel–Ti 2O 3 interface. The MDZ formation around Ti 2O 3 particles is believed to be a dominant driving force for the heterogeneous nucleation of intragranular ferrite. It is also confirmed that the development of the MDZ is associated with the absorption of Mn into the Ti 2O 3 phase at high temperatures. The width of the MDZ decreases with decreasing bonding (austenitizing) temperature and influences greatly the intragranular nucleation ability of acicular ferrite at Ti 2O 3 particles.

Role of the particle–matrix interface on the nucleation of acicular ferrite in a medium carbon microalloyed steel

The inclusions responsible for the nucleation of acicular ferrite in a medium carbon microalloyed steel have been studied using optical and transmission electron microscopy. The acicular ferrite microstructures for such studies were obtained by isothermal treatments following austenitization at 1250°C. The samples were maintained for different times at 450°C and then water quenched, permitting observation of the evolution of the microstructure. Intragranular nucleation is enhanced by the presence of MnS inclusions covered by a CuS layer leading to an improvement in the global energetic balance for the transformation. This better energetic balance is obtained by replacing a high-energy interface between inclusion and austenite with a lower energy interface between inclusion and ferrite. The result is a fully acicular microstructure.

Grain Boundary Nucleation of Proeutectoid Ferrite in an Fe-C-Ni Alloy.

The nucleation kinetics of proeutectoid ferrite at austenite grain boundaries were studied in an Fe-C-Ni alloy by observing ferrite particles on fracture facets along prior austenite grain boundaries. Whereas a large scatter in the ferrite particle numbers was observed among grain boundary facets, the average particle number appeared to increase with isothermal holding time. The particle number measured on the polished surface of specimens applying Schwartz-Saltykov analysis also increased with holding time. Diffusion-controlled growth of a ferrite nucleus was simulated incorporating Gibbs-Thomson capillary effect to indicate that the time for a nucleus to grow to a detectable size was insignificantly small. These results seem to show that ferrite nucleation occurred in a sizable time range and continuous nucleation, rather than site saturation (instantaneous nucleation), may account better for ferrite nucleation kinetics at temperatures of measurement. The difference in ferrite particle numbers between the two methods of measurement is discussed in terms of the possible omission of small particles and non-sphericity of ferrite particles etc.

Nanostructured oxide films by ferrite plating with sonochemistry

Applying power ultrasound waves (19.5 kHz, 600 W) to 300 ml of FeCl 2 aqueous solution (pH=7.0) at 70 °C, we successfully encapsulated polyacrylate spheres of 250 nm in average diameter with magnetite ferrite coatings. This broke the previous lower limit (300 nm) of the size of the particles to be encapsulated; without application of the ultrasound waves, the ferrite coating became discontinuous or insular. From TEM observations of the cross sections, the polymer spheres were covered with uniform columnar crystallites of 30–40 nm at the bottom and 60–70 nm at the top in diameter. The ultrasound waves produce on the polymer surfaces OH groups, which work as ferrite nucleation sites; this improved the quality of the ferrite coatings. The ferrite-encapsulated particles will greatly improve the performance of the enzyme immunoassay as a cancer test reagent.

In-situ Observation of Intragranular Ferrite Nucleation at Oxide Particles.

"In-situ" real time observation of the ferrite nucleation at nonmetallic inclusions has been successfully made at elevated temperatures with a confocal scanning laser microscope. The nucleation site, nucleation temperature, and/morphological change of the precipitates and ferrite can be sequentially determined as the phenomena occurring on the surface of steel. A polygonal ferrite starts to precipitate at an oxide particle at 1010 K and a Widmanstatten ferrite starts to precipitate at a MnS particle at 967 K. The degree of super cooling for austenite-ferrite transformation is found to be in good agreement with the prediction by a thermodynamic calculation.

Competition between nucleation and early growth of ferrite from austenite—studies using cellular automaton simulations

A model for the nucleation of ferrite on austenite grain boundaries and the growth of these nuclei along the grain boundary and into the grain, is developed. A cellular automaton algorithm, with transformation rules based on this model, is used to simulate the decomposition of austenite into ferrite. When performed under continuous cooling conditions, the simulations give an insight into the competition between nucleation and early growth, which determines the variation of ferrite grain size with the cooling rate and with austenite grain size. The number of ferrite grains per austenite grain, ferrite grain size and the kinetics of ferrite formation are obtained as a function of the cooling rate and austenite grain size. Contour plots of the volume fraction of ferrite in the cells at different times, enables visualization of the ferrite growth process.

Effect of the Microalloying Elements on Nucleation and Growth Kinetics of Allotriomorphic Ferrite in Medium Carbon-Manganese Steels

The influence of alloying elements on nucleation and growth kinetics of allotriomorphic ferrite has been studied using dilatometric techniques and microstructural analysis in four medium carbon-manganese steels (0.3%C - 1.4% Mn). A careful comparison between a C-Mn steel and microalloyed steels containing V, Ti and Mo subjected to isothermal transformation confirmed that all these elements delay the allotriomorphic ferrite transformation. More significant effect has been found by adding Mo, compared with the effect of the V and Ti.

Modelling the Austenite Decomposition in Steel on a Physical Basis

This paper presents some recent models [1-3] for modelling the γ → α transformation kinetics of Fe-C alloys on a physical basis. The models illustrate the effect of the austenite grain size distribution and the ferrite nucleation density, the effect of surface tension during early growth and the effect of a finite interface mobility during later stages of growth. Although these models are not yet capable of modelling the transformation behaviour of lean steels let alone that of HSLA steels, the concepts presented here may be useful in the development of more advanced models applicable to real steel grades.

Comparison of the Deformation Characteristics of a Ni-30wt% Fe Alloy and Plain Carbon Steel

A major barrier confronting researchers studying the hot deformation of plain and low carbon steels is the inability to directly observe the deformation microstructures of hot worked austenite due to the unavoidable transformation to martensite on quenching to room temperature. Various model materials, such as austenitic stainless steels have been used to overcome this difficulty. However, these materials have markedly different stacking fault energies from plain and low carbon steels and this will affect the evolving deformation structure. In this work, a model austenitic Ni-30wt%Fe alloy, calculated to have a stacking fault energy similar to that of low carbon steel, has been tested using hot compression. Stress-strain curves obtained during hot deformation show characteristics similar to those generated during identical tests on a 0.15wt%C steel. This suggests that the two materials behave similarly during deformation under similar experimental conditions. An application of the Ni-Fe alloy in the study of microstructural changes in austenite during hot deformation is demonstrated. A series of hot torsion experiments on a 0.11wt%C steel have been found to produce deformation-induced, intragranular nucleation of ferrite from austenite when a single deformation pulse is applied at 675°C. A similar set of experiments have also been performed on the Ni-Fe alloy at 750°C. Transmission electron microscopy carried out on the Ni-Fe alloy torsion specimens has revealed that likely preferred sites for intragranular ferrite nucleation appear to be microbands produced in the austenite during deformation.

Synthesis of uniform cobalt ferrite particles from a highly condensed suspension of β-FeOOH and β-Co(OH)2 particles

Abstract Uniform cobalt ferrite particles were prepared by conversion from β-FeOOH and β-Co(OH)2 particles as an application of the recently developed ‘gel-sol method’. A concentrated NaOH solution was added to a highly condensed suspension of β-FeOOH and β-Co(OH)2, containing small amounts of FeCl3 and CoCl2, to adjust the pH to 13, and it was aged at 40°C for 30 min to obtain nucleation of the cobalt ferrite. After the nucleation stage, the pH was lowered to 11 and the temperature was raised to 80°C, followed by aging at this temperature for a week to grow the ferrite particles. The arrangement of pH and use of β-FeOOH and β-Co(OH)2 of low solubilities as reservoirs of Fe3+ and Co2+ ions contributed to the formation of uniform particles by preventing the renucleation and coagulation of the particles during the growth stage.

Effect of silicon content on transformation kinetics of austempered ductile iron

The present study investigated the effect of austenitising temperature (850, 900, and 950°C) and austempering time (0–7 h) on the volume of retained austenite of a 0.3 wt-%Mn ductile iron containing two different levels of silicon, namely 2.02 wt-% and 3.31 wt-%, and austempered at 360°C. The volume fraction of retained austenite and austenite carbon content results were then correlated with microstructural changes and impact toughness results. It is shown that the austenite stability is of great significance with respect to impact toughness, and that ferrite, when present in acicular form, can increase the mechanical stability of the austenite. It is observed that decreasing the austenitising temperature increases the driving force for the stage I transformation reaction in which mother austenite transforms to high carbon austenite plus acicular ferrite. However, the austenitising temperature has only a small effect on the kinetics of the stage II reaction in which high carbon austenite transforms to bainiticferrite plus carbides. In the low silicon iron, austenitising at 950°C results in a continuous network of intercellular and low carbon austenite which reduces impact properties. Intercellular austenite is attributed to the segregation of manganese and high austenitising temperature which decrease the carbon diffusion rate and delay ferrite nucleation and growth. Decreasing the austenitising temperature to 850°C increases the rate of transformation which results in a more uniform microstructure, stable high carbon austenite, and higher impact toughness. Silicon has the effect of modifying the Fe-C phase diagram such that a higher solution treatment temperature is needed to fully austenitise the iron. Furthermore, a three phase region of austenite-ferrite-graphite is introduced in the Fe-C-Si phase diagram. Consequently, austenitising at low solution treatment temperatures produces structures containing proeutectoidferrite. Increasing the austenitising temperature to 950°C leads to a more uniform acicular microstructure of stable high carbon retained austenite andferrite and results in optimum impact properties. Following short austempering times in irons containing 2.02% and 3.31% silicon, the carbon content of the retained austenite is low and on subsequent cooling to room temperature it transforms to martensite, resulting in low impact values. Optimum properties are obtained at intermediate austempering periods when both the amount of retained austenite and the austenite carbon content are maximum. Extending the austempering time causes the high carbon austenite to decompose to ferrite plus carbides, the stage II reaction, leading to a reduction in impact toughness.

Effect of silicon content on transformation kinetics of austempered ductile iron

The present study investigated the effect of austenitising temperature (850, 900, and 950°C) and austempering time (0–7 h) on the volume of retained austenite of a 0.3 wt-%Mn ductile iron containing two different levels of silicon, namely 2.02 wt-% and 3.31 wt-%, and austempered at 360°C. The volume fraction of retained austenite and austenite carbon content results were then correlated with microstructural changes and impact toughness results. It is shown that the austenite stability is of great significance with respect to impact toughness, and that ferrite, when present in acicular form, can increase the mechanical stability of the austenite. It is observed that decreasing the austenitising temperature increases the driving force for the stage I transformation reaction in which mother austenite transforms to high carbon austenite plus acicular ferrite. However, the austenitising temperature has only a small effect on the kinetics of the stage II reaction in which high carbon austenite transforms to bainiticferrite plus carbides. In the low silicon iron, austenitising at 950°C results in a continuous network of intercellular and low carbon austenite which reduces impact properties. Intercellular austenite is attributed to the segregation of manganese and high austenitising temperature which decrease the carbon diffusion rate and delay ferrite nucleation and growth. Decreasing the austenitising temperature to 850°C increases the rate of transformation which results in a more uniform microstructure, stable high carbon austenite, and higher impact toughness. Silicon has the effect of modifying the Fe-C phase diagram such that a higher solution treatment temperature is needed to fully austenitise the iron. Furthermore, a three phase region of austenite-ferrite-graphite is introduced in the Fe-C-Si phase diagram. Consequently, austenitising at low solution treatment temperatures produces structures containing proeutectoidferrite. Increasing the austenitising temperature to 950°C leads to a more uniform acicular microstructure of stable high carbon retained austenite andferrite and results in optimum impact properties. Following short austempering times in irons containing 2.02% and 3.31% silicon, the carbon content of the retained austenite is low and on subsequent cooling to room temperature it transforms to martensite, resulting in low impact values. Optimum properties are obtained at intermediate austempering periods when both the amount of retained austenite and the austenite carbon content are maximum. Extending the austempering time causes the high carbon austenite to decompose to ferrite plus carbides, the stage II reaction, leading to a reduction in impact toughness.

Chemical bonding at the Fe/TiX (X=C, N or O) interfaces

First principles molecular orbital calculations for the Fe/TiX (X=C, N or O) interfaces have been made by the use of the spin-polarized discrete-variational X α method. At the interfaces, no significant charge transfer occurs between the Fe and TiX layers. Ionic interaction is small and covalent bonding is predominant at the interface. The interfacial bond strength is stronger when Fe atoms are located on X atoms, that is, the Fe-on-X geometry of the Baker–Nutting orientation relationship. The Fe/TiX bond strength decreases with the rising atomic number of X. The antibonding of the FeO bonds at the Fe/TiO interface is noteworthy. Although the Fe/TiO interface shows the smallest lattice mismatch among three interfaces, it is expected that the chemical bonding at the interface is weakest. The interfacial bond strength by the present calculation agrees well with the potency of TiX for intragranular ferrite nucleation in steels that was recently found experimentally.

Epitaxial growth and phase formation of austenitic stainless steel weld metals near fusion boundaries: Study of solidification and transformation of Cr-Ni stainless steel weld metals (3rd Report)

Summary This paper describes an investigation of the solidification behaviour of austenitic stainless steel weld metals at fusion boundaries, focusing particularly on epitaxial growth and primary phase selection. In the AF solidification mode, the planar austenite grows epitaxially from the austenite grains in the base metal. Ferrite subsequently forms at the dendrite boundaries of the primary austenite after the austenite changes from planar to dendritic. In the FA solidification mode, the planar austenite grows epitaxially from the austenite grains in the base metal as a result of competition between ferrite nucleation and austenite growth. Ferrite nucleation then ensues through enrichment of Cr in the liquid and ferrite subsequently continues to grow as the primary phase. The ferrite establishes the Kurdjumov-Sachs crystallographic orientation relationship (K-S relationship) with the planar austenite. At the stage of primary ferrite solidification, austenite solidifies as the second phase at the dendrite boundaries and has the same crystallographic orientation as the original planar austenite. During FA mode solidification of the sample with the lowest Cr-Ni ratio, it is also possible for the austenite to solidify as the primary phase because of solute fluctuations in the liquid and sub-cooling changes due to local changes in the temperature gradient in the liquid.

低炭素鋼におけるTiN上のフェライト核生成に及ぼすTiNサイズの影響

The ferrite nucleation model based on a classical nucleation theory for a rectangular TiN was derived to clarify its nucleation potency. The calculation revealed that a ferrite nucleates most easily on the facets of TiN rather than on its edges, corners, and austenite grain boundary and also indicated that the minimum size of TiN exists being influenced by the chemical driving force. These theoretical findings were confirmed experimentally for the TiN precipitate in low C-steel. The minimum size of TiN which has the potency of a ferrite nucleation was experimentally determined for various chemical driving force conditions by measuring the maximum size distribution of TiN which existed in the not transformed austenite at transformation temperature. These values showed a good agreement with the calculated ones.

Ferrite nucleation and growth in a medium-carbon steel studied by neutron depolarization

The neutron depolarization technique is used for a simultaneous observation of the grain size and the volume fraction of ferrite and pearlite forming from austenite in a medium-carbon steel at different temperatures. The effect of the transformation temperature is mainly visible in the particle density, a property that is governed by the nucleation behaviour. Pearlite formation leads to very distinct effects in neutron depolarization, the interpretation of which, however, is not yet fully developed.

Nucleation of acicular ferrite enhanced by the precipitation of CuS on MnS particles

Nucleation of acicular ferrite enhanced by the precipitation of CuS on MnS particles