Growth kinetics of grain boundary ferrite allotriomorphs in Fe−C alloys

Bradley, John R.; Rigsbee, J. M.; Aaronson, H.I.

doi:10.1007/bf02661647

Cited by 124 publications

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“…The aspect ratio h of the allotriomorphs is considered constant, because the lengthening and thickening processes are actually coupled. Consistent with experimental evidences [21], h is assume to have a value of 3. The value of a 1 can be obtained by numerical solution from the equation [22 ], …”

Section: Resultsmentioning

confidence: 77%

Isothermal allotriomorphic ferrite formation kinetics in a medium carbon vanadium-titanium microalloyed steel

2001

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IntroductionA great number of attempts, from purely empirical to semi-empirical models, have been made to predict the kinetics of ferrite transformation [1,2]. However, models recently developed are becoming less empirical since they rely on thermodynamic and phase transformation theories [3]. Unemoto et al. [4] developed a methodology to simulate the allotriomorphic ferrite transformation under isothermal conditions. Reed and Bhadeshia [5 On the other hand, several authors have reported the role of acicular ferrite in the improvement of mechanical properties in medium carbon forging steels [ ] reported a thermodynamic model couple with a simplified kinetic theory. This model can reproduce the C-curve behavior typical of those parts of the time-temperature-transformation diagrams that are due to allotriomorphic ferrite in low-carbon multicomponent steel. In principle, most of these models are able to predict the kinetics of allotriomorphic ferrite for low-carbon low-alloy steels. But, the level of agreement between predicted and calculated allotriomorphic ferrite kinetics is less satisfactory for medium carbon microalloyed steels. 6,7,8]. Likewise, recent works have studied the role of the allotriomorphic ferrite to promote the formation of acicular ferrite to the detriment of bainite [9,10,11,12 The present paper is concerned with the theoretical and experimental description of the growth kinetics of allotriomorphic ferrite in medium carbon vanadium-titanium microalloyed steel. The theoretical model presented in this work allows to calculate the kinetics of isothermal austenite-to-allotriomorphic ferrite transformation at temperatures at which allotriomorphic ferrite might not be the only austenite decomposition product.]. Thus, the amount of acicular ferrite increases, as allotriomorphic ferrite is present along the austenite grain boundaries of medium carbon microalloyed steels. Therefore, a deep understanding of the decomposition of austenite in allotriomorphic ferrite is needed in order to control the total amount of acicular ferrite present in the microstructure. Materials and Experimental ProcedureThe chemical composition of the steel studied is presented in Table 1. The material was supplied in the form of 50 mm square bars, obtained by conventional casting as a square ingot (2500 kg) and hot rolling to bar. The isothermal decomposition of austenite has been analyzed by means of a high resolution dilatometer DT 1000 Adamel-Lhomargy described elsewhere [13]. Cylindrical dilatometric specimens of 2 mm in diameter and 12 mm in length were machined parallel to the rolling direction of the bar, were used for these tests. The change in length of the specimen is transmitted via amorphous silica push-rod. This variation is measured by a linear variable differential transformer (LVDT) sensor in a gas-tight enclosure enabling testing under vacuum or an inert atmosphere with an accuracy lower than 0.1 mm. The dilatometric curve (relative change in length (dL/L o ) vs. time (t)) is monitored with the help of a comp...

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Section: Resultsmentioning

confidence: 77%

Isothermal allotriomorphic ferrite formation kinetics in a medium carbon vanadium-titanium microalloyed steel

2001

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“…It is accepted that in Fe-C alloys the growth of grain boundary ferrite allotriomorphs is controlled by carbon diffusion in austenite, although the measured growth rates are often smaller than those calculated from the theory. 1,2) The principal reason for this discrepancy was thought to be the development of facets at ferrite/austenite interphase boundaries. The migration of planar ferrite/austenite boundaries in a diffusion couple was indeed explained very well by carbon diffusion in austenite.…”

Section: Introductionmentioning

confidence: 99%

“…7,9) Particularly in Fe-C-Cr and Mo alloys the influence of solute drag is considered to be so strong that the TTT-curves exhibit the characteristic shape, known as the bay, at intermediate temperatures. 10) Measured growth rates of grain boundary allotriomorphs were compared with calculations assuming that the particle shape is a sphere, an oblate ellipsoid 2) or even assuming planar growth. 7,8) The former approximations yield a considerably faster growth rate than planar growth at large undercoolings.…”

Section: Introductionmentioning

confidence: 99%

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Three-dimensional Morphology and Growth Kinetics of Intragranular Ferrite Idiomorphs Formed in Association with Inclusions in an Fe-C-Mn Alloy.

Yokomizo

Enomoto

2002

ISIJ International

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The three-dimensional (3-D) morphology and growth kinetics of intragranular ferrite idiomorphs formed in association with MnS(ϩVN) inclusions were studied in an Fe-C-Mn alloy. Ferrite idiomorphs began to be formed at a temperature ϳ40°C lower than the grain boundary allotriomorphs, and the highest temperature of the formation of intragranular ferrite plates was ϳ60°C below the Widmanstätten start temperature of grain boundary sideplates (W s ). The 3-D shape of ferrite idiomorphs was more equiaxed than grain boundary allotriomorphs. Measured growth rates were smaller than those calculated assuming spherical growth, similar to those of grain boundary allotriomorphs previously reported. The possible reasons for the retardation of growth, e.g. deprivation of carbon supersaturation by grain boundary ferrite allotriomorphs and solute drag etc., are discussed.KEY WORDS: ferrite; inclusion; growth; serial sectioning; morphology. Experimental Procedures Alloy and Heat TreatmentThe alloy was melted in a vacuum induction furnace using electrolytic iron, high purity carbon and manganese. An excess amount of sulfur, vanadium and nitrogen were added to promote the formation of intragranular ferrite. The composition of the alloy is shown in Table 1. A 50 kg ingot of the alloy was hot rolled to a plate of 50 mm thickness at 1 000°C and subsequently, the slab was cold rolled by 70 %. Specimens 10ϫ10ϫ0.35 mm 3 in size, three pieces bundled and coated with anti-oxidation ceramic paint, were austenitized at 1 250°C for 10 min under high purity argon atmosphere. This treatment produced a grain size of approximately 200-300 mm. A larger austenite grain size facilitates the formation of intragranular ferrite. The use of thin specimens and a high austenitizing temperature made the austenite grain boundaries essentially perpendicular to the broad faces of the specimen.2) After austenitizing the specimens were swiftly transferred into a salt bath for isothermal reaction to occur at temperatures ranging from 610 to 750°C for varying times, and quenched into iced brine. Serial Sectioning, 3-D Construction and Measurement of Growth RateFiducial hardness indents were put on the polished specimen surface for the purpose of image alignment and measurement of the thickness of the removed layers. The specimens were then polished by an automatic grinder polisher to remove ϳ0.5 mm per section. A digital image of each section taken by a CCD camera was stored in a personal computer. After masking the objects a stack of images were transformed into a 3-D image by means of visualization software (AVS). 11)For some ferrite particles the sizes were measured directly from 3-D constructed images and were compared with calculation. The growth rate of ferrite was measured from the variation with time of the maximum half-thickness or half-diameter of the ferrite particles in the plane of polish. The measurement of the diameter of the largest particles in the plane of polish could be representative of the thickness of one of the first formed ferrite particle...

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“…Similarly, Bradley et al [9] and Cotrina et al [10] analyzed coarsening kinetics via experimental data from optical observations. Recently, various numerical simulation techniques such as Potts-Monte Carlo models (MC) [11][12][13], phase field models [14,15] and cellular automaton models (CA) [16][17][18] have been gradually used to model the microstructure evolution on the mesoscale [19].…”

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confidence: 99%

Static coarsening of titanium alloys in single field by cellular automaton model considering solute drag and anisotropic mobility of grain boundaries

Yang

et al. 2012

Chin. Sci. Bull.

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Static coarsening is an important physical phenomenon that influences microstructural evolution and mechanical properties. How to simulate this process effectively has become an important topic which needs to be dealt with. In this paper, a new cellular automaton (CA) model, which considers the effect of solute drag and anisotropic mobility of grain boundaries, was developed to simulate static grain coarsening of titanium alloys in the beta-phase field. To describe the effect of the drag caused by different solute atoms on coarsening, their diffusion velocities in beta titanium were estimated relative to that of titanium atoms (Ti). A formula was proposed to quantitatively describe the relationship of the diffusion velocity of Ti to that of solute atoms; factors influencing the diffusion velocity such as solute atom radius, mass, and lattice type were considered. The anisotropic mobility of grain boundaries was represented by the parameter c 0 , which was set to 1 for a fully anisotropic effect. These equations were then implemented into the CA scheme to model the static coarsening of titanium alloys Ti-6Al-4V, Ti17 (Ti-5Al-4Mo-4Cr-2Sn-2Zr, wt%), TG6 (Ti-5.8Al-4.0Sn-4.0Zr-0.7Nb-1.5Ta-0.4Si-0.06C, wt%) and TA15 (Ti-6Al-2Zr-1Mo-1V, wt%) in the beta field. The predicted results, including coarsening kinetics and microstructural evolution, were in good agreement with experimental results. Finally, the effects of time, temperature, and chemical composition on grain coarsening and the limitations of the model were discussed. The mechanical properties of titanium alloys such as the creep resistance, fracture toughness and crack propagation resistance are determined by their grain sizes in the beta phase field [1,2]. However, the grains are prone to static coarsening when held in the beta phase field because of the structural characteristics of beta-titanium and relatively high stacking fault energy at high temperatures [3]; this results in a marked decrease in the mechanical properties of final products. Therefore, it is necessary to understand and to control static coarsening of titanium alloys in a single phase field to obtain the expected mechanical properties. Previously, many experimental investigations have focused on the static coarsening behaviors of titanium alloys in the beta phase field. Ivasishin et al. [4,5] investigated the effect of texture evolution on the values of the static coarsening exponent n and activation energy for Ti-6Al-4V alloy in the beta field. They found values of n ranging from 0.22 to 0.31 at different temperatures, but there were marked deviations attributed to different materials and experiments when the results were compared with those of others. Recently, Semiatin et al. [6][7][8] investigated the static grain coarsening of Ti-6Al-4V alloy via a series of heat treatments at typical temperatures. They found that the coarsening process of primary alpha particles was controlled by the volume diffusion of solute elements at lower temperatures and the growth exponent was equal to 0.33.

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Growth kinetics of grain boundary ferrite allotriomorphs in Fe−C alloys

Cited by 124 publications

References 22 publications

Isothermal allotriomorphic ferrite formation kinetics in a medium carbon vanadium-titanium microalloyed steel

Isothermal allotriomorphic ferrite formation kinetics in a medium carbon vanadium-titanium microalloyed steel

Three-dimensional Morphology and Growth Kinetics of Intragranular Ferrite Idiomorphs Formed in Association with Inclusions in an Fe-C-Mn Alloy.

Static coarsening of titanium alloys in single field by cellular automaton model considering solute drag and anisotropic mobility of grain boundaries

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