Showing papers on "Austenite published in 1990"

Precipitation reactions and strengthening behavior in 18 wt pct nickel maraging steels

Abstract: The crystallography, structure, and composition of the strengthening precipitates in maraging steels C-250 and T-250 have been studied utilizing analytical electron microscopy and computersimulated electron diffraction patterns. The kinetics of precipitation were studied by electrical resistivity and microhardness measurements and could be described adequately by the Johnson-Mehl-Avarami equation, with precipitate nucleation occurring on dislocations and growth proceeding by a mechanism in which the dislocations serve as collector lines for solute from the matrix along which pipe diffusion occurs. The strengthening of the Co-free, higher Ti T-250 steel is caused by a refined distribution of Ni3Ti precipitates. High strength is maintained at longer times from the combined effect of a high resistance of these precipitates to coarsening and a small volume fraction of reverted austenite. In the case of the Co-containing, lower Ti C-250 steel, strengthening results from the combined presence of Ni3Ti (initially) and Fe2Mo precipitates (at longer times). Loss of strength at longer times is associated, in part, with overaging and mainly from the larger volume fraction of reverted austenite. The resistance to austenite reversion is dependent on the manner in which the relative nickel content of the martensite matrix is affected by the precipitating phases, and the difference in the reversion tendency between the two steels can be explained on this basis.

A perspective on the morphology of bainite

Abstract: The morphology of continuously cooled bainite in steels is examined in this review and compared with that of isothermally transformed bainite. Some experimental observations of microstructures in continuously cooled commercial steels are also presented; these results demonstrate the presence of microstructures which are not easily defined in terms of any of the “classical” bainitic morphologies. The numerous terms created over the last 50 years to describe specific bainite morphologies have led to some confusion, and it is suggested that the commonly used terminologies do not adequately describe the full range of bainitic microstructures which are observed. A general definition for the bainite transformation is proposed. A classification system for bainitic microstructures is also developed, encompassing both isothermally transformed and continuously cooled bainites. It is suggested that primary bainite morphologies be classified as B1, B2, or B3, depending on whether the acicular ferrite is associated with (1) intralath precipitates, (2) interlath particles/films, or (3) discrete regions of retained austenite and/or secondary transformation product(e.g., martensite or pearlite), respectively. Possible ambiguities with the descriptions of autotempered martensite and Widmanstatten ferrite are also discussed, and a number of areas are identified where further work would be helpful.

Model for transition from upper to lower bainite

Abstract: An attempt is made to model the transition from upper to lower bainite in steels, based on the hypothesis that bainitic ferrite grows with a supersaturation of carbon in solid solution. The theory involves a comparison between the time required to reject the excess carbon into the residual austenite by diffusion and the time required to obtain a detectable degree of cementite precipitation in the bainitic ferrite. If the precipitation process is relatively rapid, then it is assumed that lower bainite is obtained. The results are found to be in broad agreement with published experimental data.MST/1095

Continuous cooling transformations and microstructures in a low-carbon, high-strength low-alloy plate steel

Abstract: A continuous-cooling-transformation (CCT) diagram was determined for a high-strength low-alloy plate steel containing (in weight percent) 0.06 C, 1.45 Mn, 1.25 Cu, 0.97 Ni, 0.72 Cr, and 0.42 Mo. Dilatometric measurements were supplemented by microhardness testing, light microscopy, and transmission electron microscopy. The CCT diagram showed significant suppression of polygonal ferrite formation and a prominent transformation region, normally attributed to bainite formation, at temperatures intermediate to those of polygonal ferrite and martensite formation. In the intermediate region, ferrite formation in groups of similarly oriented crystals about 1 μm in size and containing a high density of dislocations dominated the transformation of austenite during continuous cooling. The ferrite grains assumed two morphologies, elongated or acicular and equiaxed or granular, leading to the terms “acicular ferrite” and “granular ferrite,” respectively, to describe these structures. Austenite regions, some transformed to martensite, were enriched in carbon and retained at interfaces between ferrite grains. Coarse interfacial ledges and the nonacicular morphology of the granular ferrite grains provided evidence for a phase transformation mechanism involving reconstructive diffusion of substitutional atoms. At slow cooling rates, polygonal ferrite and Widmanstatten ferrite formed. These latter structures contained low dislocation densities and e-copper precipitates formed by an interphase transformation mechanism.

Bainite viewed three different ways

Abstract: The present status of the three principal definitions of bainite currently in use is reviewed. On the surface relief definition, bainite consists of precipitate plates, producing an invariant plane strain (IPS) surface relief effect, which form by shear,i.e., martensitically, at temperatures usually aboveMs andMd. The generalized microstructural definition describes bainite as the product of the diffusional, noncooperative, compctitive ledgewise growth of two precipitate phases formed during eutectoid decomposition, with the minority phase appearing in nonlamellar form. This alternative mode of eutectoid decomposition is thus fundamentally different from the diffusional, cooperative, shared growth ledges mechanism for the formation of pearlite developed by Hackney and Shiflet. The overall reaction kinetics definition of bainite views this transformation as being confined to a temperature range well below that of the eutectoid temperature and being increasingly incomplete as its upper limiting temperature, the kineticBs, is approached. Recent research has shown, however, that even in steels (the only alloys in which this set of phenomena has been reported), incomplete transformation is not generally operative. Revisions in and additions to the phenomenology of bainite defined in this manner have been recently made. Extensive conflicts among the three definitions are readily demonstrated. Arguments are developed in favor of preference for the generalized microstructural definition, reassessment of the overall reaction kinetics definition, and discarding of the surface relief definition.

HVEM studies of the effects of hydrogen on the deformation and fracture of AISI type 316 austenitic stainless steel

Abstract: The mechanisms of hydrogen embrittlement in AISI type 316 austenitic stainless steel have been investigated by in situ straining in a high-voltage electron microscope (HVEM) equipped with an environmental cell. Hydrogen effects on strain-induced phase transformations, the generation rate and velocity of dislocation, and crack propagation rates were studied. The salient features of the fracture were similar for cracks propagating in vacuum and in hydrogen gas. In each case, e and α′ martensite formed at the crack; the e phase extended ahead of the crack while the α′ phase was restricted to high stress regions near the crack tip. The principal effect of hydrogen was to decrease the stress required for dislocation motion, for phase transformation of the austenite, and for crack propagation.

The Incomplete transformation phenomenon in Fe-C-Mo alloys

Abstract: The overall kinetics of isothermal transformation of austenite to bainite were studied with quantitative metallography and transmission electron microscopy (TEM) in a series of highpurity Fe-C-Mo alloys containing 0.06 to 0.27 wt pct C and 0.23 to 4.28 wt pct Mo at reaction temperatures mainly below that of the bay in the time-temperature-transformation (TTT) curve for initiation of transformation. Ferrite growth kinetics were also determined at temperatures slightly below that of the bay. The incomplete character of the bainite transformation was found to depend upon both the C and Mo concentrations; below threshold combinations of these elements, the incomplete reaction is absent. In cases in which the bainite reaction was incomplete, transformation of austenite resumed and went to completion following the initiation of Mo2C precipitation at α: γ boundaries. The time of transformation stasis increased with the proportions of C and Mo in the alloy. Pearlite was not observed anywhere in the time-temperature-composition (TTC) region investigated. Ferrite growth kinetics at temperatures near the bay do not exhibit simple time laws; this behavior is attributed to a solute drag-like effect (SDLE). Extensive sympathetic nucleation of ferrite is observed at temperatures below that of the bay. The temperature and composition dependence of the incomplete reaction can be explained by a combination of the SDLE and the variation in the sympathetic nucleation rate of ferrite with temperature and the amount of transformation.

Observation of an adiabatic shear band in AISI 4340 steel by high-voltage transmission electron microscopy

Abstract: Adiabatic shear bands, formed in a hollow AISI 4340 steel cylinder subjected to dynamic expansion by means of an explosive charge placed in its longitudinal axis, were characterized. The adiabatic shear bands formed in this quenched and tempered steel were of the classical “transformed” type. Scanning electron microscopy (SEM) of etched surfaces revealed that alignment of the lamellae along the direction of shear seems to be the event that precedes shear localization. The transmission electron microscopy of a “white”-etching shear band having undergone a shear strain of approximately 4 revealed that it containedX (Fe5C2) carbides in a martensitic structure. These carbides were observed to form on (112) internal microtwins. Grains could not be resolved inside of the shear band, but they could be observed in the surrounding matrix material. A traverse of the shear band was made, and there existed no definite boundary between the matrix and the shear band. No evidence of a transformation to austenite was observed. Heat transfer calculations were conducted to help explain the features observed inside of the shear band. It is concluded that the “white”-etching bands, commonly referred to in the literature as “transformed” bands, do not exhibit a transformation at values of shear strain of up to 4. The enhanced reflectivity is an etching artifact and is possibly due to microstructural changes, a very small grain size, and carbide redissolution in the bands.

Atom probe and transmission electron microscopy study of aging of cast duplex stainless steels

Abstract: The microstructural evolution of Mo bearing and Mo free cast duplex stainless steels, induced by long term aging in the range 350–4507deg;C has been studied using atom probe and field ion microscopy and transmission electron microscopy. The salient features of this evolution at 350 and 400°C are spinodal decomposition and G phase precipitation in the ferrite. The nucleation of the G phase is enhanced by spinodal decomposition. Other intermetallic phases observed at higher temperatures may also be formed at 400°C. In the ferrite/ austenite interface, an intermetallic film, possibly an icosahedral phase, is formed after aging for 30 000 h at 400 and 450°C in Mo bearing steels, whereas interfacial M23C6 carbides precipitate in the Mo free materials. The observed evolution of the spatial and compositional parameters of the spinodal decomposition in Mo bearing steels is described and its relationship with the evolution of mechanical properties is outlined.MST/1185

Phase Equilibria in Fe-Mn-Al-C Alloys

Abstract: Phase constitutions of Fe-(20-30)wt%Mn-(0-10)wt%Al-C alloys have been investigated by electron probe microanalysis and transmission electron microscopy. The phase relations between austenite and perovskite carbide, (Fe, Mn)3AlC in the temperature range of 900-1 200°C have been carefully examined. An L12-type ordered structure, which was reported to be formed in rapidly solidified alloys as a metastable phase has not been detected in specimens aged at temperatures above 600°C.

An investigation of the generality of incomplete transformation to bainite in Fe-C-X alloys.

Abstract: The overall kinetics of the isothermal transformation of austenite to bainite and to pearlite in high-purity Fe-C-3 at. pct X alloys (X = Mn, Si, Ni, or Cu) containing 0.1 wt pct C and 0.4 wt pct C were investigated with quantitative metallography and transmission electron microscopy (TEM) to ascertain the presence or absence of the incomplete reaction phenomenon. The incomplete transformation of austenite to bainite was not observed in the Fe-C-Si, Fe-C-Ni, Fe-C-Cu, or Fe-0.4C-Mn alloys. It was found, however, in the Fe-0.1C-Mn alloy. Transmission electron microscopy results indicate that sympathetic nucleation of ferrite without carbide precipitation is a necessary but not a sufficient condition for the development of the incomplete reaction phenomenon. Transformation resumes following stasis in the low-carbon Fe-C-Mn alloy with the formation of a nodular bainite. The results support the view that the incomplete transformation of austenite to bainite is a characteristic of specific alloying elements and is not an inherent trait of the bainite reaction.

APFIM and AEM investigation of CF8 and CF8M primary coolant pipe steels

Abstract: Atom probe field ion microscopy, analytical electron microscopy, and optical microscopy have been used to investigate the changes that occur in the microstructure of cast CF8 and CF8M primary coolant pipe steels after long term thermal aging. These cast steels have a duplex microstructure consisting of austenite with approximately 15 vol. -% ferrite. In material aged at 300 or 400°C for up to 70 000 h, the ferrite had spinodally decomposed into a modulated fine scale interconnected network consisting of an iron rich a phase and a chromium enriched α′ phase with a periodicity of between 2 and 9 nm. Roughly spherical G phase precipitates 2 to 10 nm in diameter were also observed at concentrations of more than 1021 m−3. The degradation in mechanical properties of these materials is a consequence of the spinodal decomposition and G phase precipitation in the ferrite.MST/1184

Microstructure and stability of FeCrC hardfacing alloys

Abstract: The microstructure and high temperature stability of an iron-based hardfacing alloy of nominal composition Fe-30Cr-38C (weight per cent) deposited by manual metal arc welding has been investigated using microscopy, microanalysis, dilatometry and thermodynamic modelling In the as-deposited condition the undiluted alloy was confirmed to consist of a mixture of M 7 C 3 carbide and metastable austenite containing a high chromium concentration Since the properties of the alloy can depend on the stability of the austenite, annealing experiments were carried out to investigate the decomposition of the austenite into a mixture of ferrite and carbides The results demonstrate that at temperatures around 750 °C the austenite starts to decompose rapidly, beginning with the precipitation of M 23 C 6 carbides, although the final equilibrium phase mixture is simply chromium-depleted ferrite and M 7 C 3 The implications of these results are discussed with respect to the potential applications of the alloy

Transition from bainite to acicular ferrite in reheated Fe–Cr–C weld deposits

Abstract: Factors controlling the transition from acicular ferrite to bainite in Fe–Cr–C weld metals have been investigated. It appears that the presence of allotriomorphs of ferrite at austenite grain boundaries has the effect of suppressing the formation of bainitic sheaves. This in turn allows the acicular ferrite plates to develop on intragranular nucleation sites. A theoretical analysis indicates that bainitic transformation is prevented from developing at the allotriomorphic ferrite/austenite boundaries by the carbon concentration field present in the austenite at the allotriomorphic ferrite/austenite interface. This field does not homogenise within the residual austenite during the time scale of the experiments.MST/1217

Effect of Dynamic Recrystallization on Microstructural Evolution during Strip Rolling

Abstract: In the absence of static recrystallization, dynamic recrystallization is the principal softening mechanism operating during strip rolling. This process takes place when the interpass time is short, there is little strain-induced precipitation, and the presence of alloying elements in solution retards static recrystallization. It can lead to austenite grain sizes below 5 μm and ferrite grain sizes of about 3 μm when cooling is carried out at 10°C/s. Increasing the roughing-to-finishing delay time or the delay time between successive passes leads to an increase in the density of Nb(CN) precipitates, which in turn promotes the formation of pancaked austenite. When the latter structure is cooled, 7 μm ferrite grain sizes are produced, which are coarser than the ferrite structures obtained from dynamically recrystallized austenite rolled over the same temperature range.

A mechanism for the formation of lower bainite

Abstract: A diffusional mechanism for the formation of lower bainite is proposed based primarily on transmission electron microscopy (TEM) observations of isothermally reacted specimens of Fe-C-2 pct Mn alloys. The mechanism involves the initial precipitation of a nearly carbide-free ferrite“spine,” followed by sympathetic nucleation of“secondary (ferrite) plates” which lie at an angle to the initial“spine.” Carbide precipitation subsequently occurs in austenite at ferrite: austenite boundaries located in small gaps between the“secondary plates.” An“annealing” process then occurs in which the gaps are filled in by further growth of ferrite and additional carbide precipitation; the annealing out of ferrite: ferrite boundaries between impinged“secondary plates” completes this process. This annealing stage contributes to the final appearance of lower bainite sheaves as monolithic plates containing embedded carbides. The present mechanism accounts for the single variant of carbides oriented at an angle to the sheaf axis repeatedly reported in lower bainite; it is also consistent with the previous observation of one“rough” side and one“smooth” side of lower bainite“plates.”

Effect of Si and Al Additions on the Low Temperature Toughness and Fracture Mode of Fe-27Mn Alloys

Abstract: The effect of Si and Al additions on the low temperature toughness and fracture mode of Fe-27Mn (in mass%) alloys was investigated in terms of the microstructure, heterogeneous deformation and stacking fault energy. The Fe-27Mn binary alloy has two constituent phases at 77 K; Epsilon martensite (e) and metastable austenite (γ). It undergoes a ductile-to-brittle transition because of an intergranular fracture associated with heterogeneous deformation. The intergranular fracture is caused by stress concentration at grain boundaries on which large e plates impinge. Silicon addition to the Fe-27Mn binary alloy is effective for refining e plates and changes the fracture mode from intergranular to transgranular. The fracture is, however, a quasi-cleavage mode along e plates so that Fe-27Mn-Si alloys also exhibit a ductile-to-brittle transition related to the formation of e. The stress concentration at the intersection of e plates results in the formation of microcracks along the e plates leading to the quasi-cleavage fracture. Fracture modes of high manganese steels containing e largely depend on the microstructure and the deformation behavior. Silicon addition probably affects not only the nucleation of e, but also the cross slip behavior of partial dislocations at the intersection of e plates through a decrease in the stacking fault energy of γ, thus leading to the change in fracture mode.Aluminum addition to high manganese steels is so effective for suppressing the γ-e transformation that the low temperature embrittlement associated with the formation of e does not occur with even a small Al addition. In an Fe-27Mn-2.5Al alloy, for example, toughness is high even at 77 K because the γ phase is so stable that the transformation to e does not occur during deformation at 77 K.

Mössbauer study of the distribution of carbon interstitiels in iron alloys and the isochronal kinetics of the aging of martensite: The clustering-ordering synergy

Abstract: Conversion electron Mossbauer spectroscopy allows the resolution of all components in spectra of 100 pct retained austenite for varying carbon contents (>6.5 at. pct C). An off-stoichiometry ordered structure, Fe8Ci1−y, is proposed which confirms interstitial repulsion in y iron. Transmission spectroscopy is used to follow the nonisothermal kinetics of the concomitant clustering and ordering processes during aging and of the first stage of tempering in 8.5 and 8 at. pct iron martensite. Activation energies of 75 ∓ 2.5, 74 ∓ 1.5, and 121.5 ∓ 1.5 kJ/mole are observed. The synergy between the clustering and ordering within the frame of the carbon multiplet model is emphasized. The e- or η-carbide obtained by tempering is analyzed to be Fe2.4C.

Growth and overall transformation kinetics above the bay temperature in Fe-C-Mo alloys

Abstract: The kinetics and morphology of isothermal transformation in the vicinity of the time-temperaturetransformation (TTT) diagram bay have been investigated with optical and transmission electron microscopy (TEM) in 19 Fe-C-Mo alloys at three levels of carbon concentration (approximately 0.15, 0.20, and 0.25 wt pct) and at Mo concentrations from 2.3 to 4.3 wt pct, essentially always at temperatures above or at that of the bay,Tb. Quantitative metallography yielded no evidence for incomplete transformation (stasis) in any of these alloys atT > Tb. Measurements of the thickening kinetics of grain boundary ferrite allotriomorphs (invariably containing either interphase boundary or fibrous Mo2C) demonstrated four different patterns of behavior. The customary parabolic time law for allotriomorph thickening in Fe-C and in many Fe-C-X systems was obtained only at higher temperatures and in the more dilute Fe-C-Mo alloys studied. With decreasing temperature and increasing solute concentrations, a two-stage and then two successive variants of a three-stage thickening process are found. In the most concentrated alloys and at temperatures nearest the bay, the second stage of the three-stage thickening process corresponds to “growth stasis”—the cessation of allotriomorph thickening. Sufficient prolongation of growth stasis presumably leads to “transformation stasis.” A number of models for growth of the carbide-containing allotriomorphs were investigated during attempts to explain the observed kinetics. It was concluded that their growth is controlled by carbon diffusion in austenite but with a driving force drastically reduced by a very strong solute drag-like effect (SDLE) induced by Mo segregation at disordered-type austenite: ferrite boundaries. Carbide growth in the fibrous structure appears to be fed by diffusion of Mo along austenite: ferrite boundaries, whereas carbides in the interphase boundary structure grow primarily by volume diffusion of Mo through austenite.

Modeling of the temperature field, transformation behavior, hardness and mechanical response of low alloy steels during cooling from the austenite region

Abstract: A comprehensive finite element program applicable to all axisymmetric shapes for the prediction of material behavior exposed to various heat treatments has been developed based on a thermodynamic and kinetic algorithm for predicting the hardenability of low alloy steels combined with the calculation of stress and distortion evolution. During the temperature field and stress calculations the microstructural contributions, like latent heat formation, influence on thermophysical properties, microstructural dependence of σ/e curve, transformation strain, and transformation plasticity are taken into account, which give more insight and more accurate predictions of the material response. Thus microstructural optimizations with respect to hardenability, hardness, residual stresses and distortion are made possible.

Lattice changes of iron-carbon martensite on aging at room temperature

Abstract: The aging behavior of iron-carbon martensite (5.1 at. pct C = 5.3C/100Fe) at about 297 K was studied by analyzing, in particular, the changes in the {002} and {200} diffraction line profiles obtained by X-ray diffractometry. Martensitic specimens were prepared by gaseous carburizing of pure iron in a mixture of H2 and CO, followed by quenching in brine and subsequently in liquid nitrogen. The aging process can be divided into two stages. First, a redistribution of carbon atoms in the martensite matrix occurs in four ways (aging time about 50 hours) leading to diffraction by the matrix independent of the clusters. Within the range of aging times employed (up to 2 years), the diffraction by the retained austenite did not change.

The distribution of substitutional alloying elements during the bainite transformation

Abstract: The behavior of substitutional alloying elements during and after the growth of upper bainite in Fe-Mn-Si-C and Fe-Mn-Si-C-Mo alloy steels has been examined using an atomic resolution microanalysis technique. From the results obtained, and judging from published data, it is concluded that manganese, nickel, silicon, chromium, and molybdenum do not redistribute during the growth of bainitic ferrite. Their concentrations are found to be uniform both at and in the vicinity of the transformation interface, with no indications of any segregation to the transformation interface during growth. However, prolonged annealing at the isothermal transformation temperature, after the formation of bainite has stopped, eventually stimulates the partitioning of substitutional alloying elements as the system tends toward equilibrium. The results demonstrate the existence of an atomic correspondence between the parent and product phases during transformation, the effect of substitutional alloying additions being manifestedvia a modification of the driving force for transformation.

Laser shock‐induced mechanical and microstructural modification of welded maraging steel

Abstract: The effect of laser‐induced high‐intensity stress waves on the hardness, fatigue resistance, and microstructure in the heat affected zone of welded 18 Ni(250) maraging steel was investigated. Laser‐shock processing increased the hardness and fatigue strength of the weldments. Some melting of the surface was involved during laser‐shock hardening which produced the reverted austenite phase. Microscopic analyses showed an increased dislocation density in the laser‐shocked area.

Aging kinetics of CF3 cast stainless steel in temperature range 300–400°C

Abstract: Castings to the ASME SA351 CF3 specification used in modern pressurised water reactor nuclear power stations are mainly austenitic, but contain up to 20% ferrite, and as a result are subject to loss of toughness in service at about 300°C. To show adequate end of life toughness, tests are being carried out on material with accelerated aging above 300°C. Data on the kinetics of embrittlement are required, and it must also be shown that the significant metallurgical changes are the same at both temperatures. The kinetics has been investigated using Charpy impact specimens aged at 300, 350, and 400°C and it has been related to the hardening of the ferrite and to the microstructural changes in this phase. The activation energies determined for the embrittlement, for the hardening of the ferrite, and for the underlying spinodal reaction in the ferrite are consistent with that for chemical diffusion in this system.MST/1187

Kinetics of Martensitic transformation in partially bainitic 300M steel

Abstract: The kinetics of athermal martensitic transformation have been studied in a high silicon steel (300 M), beginning with samples which were first partially transformed to bainitic ferrite. It is found that the way in which the volume fraction of martensite increases with undercooling below the martensite start temperature is not greatly influenced by the presence of bainitic ferrite, when any carbon enrichment in the residual austenite is taken into account. the martensitic transformation obeys, within the limits of experimental errors, the same law irrespective of the presence or absence of bainitic ferrite prior to transformation. A new relationship, which takes some account of autocatalysis, has been derived to rationalize the athermal kinetics of martensitic reactions and, within the context of certain approximations, is found to be in reasonable agreement with experimental data. The role of chemical composition variations, of the type normally present in commercial steels, seems to be mainly to extend the temperature range over which most of the martensite reaction occurs in the heterogeneous samples, relative to samples which were given a homogenizing heat treatment.

Modeling diffusional growth during austenite decomposition to ferrite in polycrystalline FeC alloys

Abstract: A grain-boundary-nucleated, diffusional growth model of austenite decomposition to proeutectoid ferrite is developed for polycrystalline iron-carbon alloys. The diffusion equation is solved under restricted diffusion conditions using the quasi-static method and employing local thermodynamic equilibrium at the disordered austenite:ferrite interface. Decomposition kinetics for a model polycrystalline material consisting of a log-normal distribution of spherical grains are calculated numerically. Effects of temperature, overall carbon concentration, volume change, austenite grain size and carbon buildup in the centers of the austenite grains are included in the treatment. A scaling factor is deduced that enables the effect of austenite grain size on transformation kinetics to be characterized provided kinetic information is available for one grain size. Experiments carried out on a laboratory steel verified the applicability of the scaling factor, Also, partial I-T and C-T diagrams can be computed from the model and sample calculations are presented for an iron + 0.036 wt% carbon steel.

The influence of the Si and Mn concentrations on the kinetics of the bainite transformation in Fe-C-Si-Mn alloys

Abstract: Five Fe-C-Si-Mn alloys were investigated by dilatometry, optical microscopy, and transmission electron microscopy (TEM) analysis to determine the effects of Si and Mn content and austenitizing temperatures on the kinetics of bainite transformation. Segregation of Mn at prior austenite grain boundaries at a temperature below Bs was detected by the scanning transmission electron microscopy + energy dispersive spectroscopy (STEM + EDS) and secondary ion mass spectroscopy (SIMS) techniques. The fraction of intragranular ferrite increases with Mn content. The time needed for the initiation of the bainite transformation tends to decrease as the austenitizing temperature decreases. The results obtained were compared with those from Fe-0.38C-1.73Si and Fe-0.38C-3.11Mn alloys. Equilibrium and nonequilibrium segregation of Mn at prior austenite grain boundaries and the effect of the interaction between Si and Mn on the segregation of Mn are used to explain the results obtained and the large difference between the kinetics of the two ternary alloys and those of the Fe-C-Si-Mn alloys.