Decarburization as a tool to explore parallels between solid-solid and liquid-solid transformations: Fe-C-Mn-Si steels

The study of single-interface transformations under controlled conditions offers insights into the similarities and distinctions between solidification and solid-solid transformations. In this contribution, we consider some parallels between the formation of a layer of ferrite on an originally austenitic steel bar and the growth of the columnar zone in the solidification of an alloy ingot. The necessary conditions for solid-solid interfacial breakdown of a decarburization front are explored and the evidence to date reviewed. The response of some Fe-C-Mn-Si steels to controlled decarburization is considered, and we present a first report of the morphological instability of a ferrite/austenite decarburization interface, and compare it with the interfacial breakdown in the solid-liquid interface. The instability is tentatively ascribed to the effects of grain-boundary nucleation and/or grain-boundary diffusion of alloying elements in austenite. Other similarities (and differences) between the growth of a ferrite layer on an alloy steel and the development of the columnar zone of an ingot are discussed.     

The growth of ferrite in Fe-C-X alloys: The role of thermodynamics, diffusion, and interfacial conditions

The growth of allotriomorphic ferrite from austenite in Fe-C-X alloys is studied. Two systems have been selected: the Fe-C-Ni system, in which the substitutional alloying element is expected to have a weak interaction with both the C and the moving interface, and the Fe-C-Mo system, in which these interactions are expected to be non-negligible. The ferrite growth kinetics was measured using two types of experiments: classical isothermal heat treatments and decarburization experiments. All of the experimental observations can be quantitatively rationalized using a model that describes an evolution in interfacial conditions from paraequilibrium (PE) to local equilibrium with negligible partitioning (LENP) during growth. This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,” December 2–3, 2004, organized by the The Royal Institute of Technology in Stockholm, Sweden.     

Self-Consistent Model for Planar Ferrite Growth in Fe-C-X Alloys

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

Development of high toughness in austempered type ductile cast iron and evaluation of its properties

In order to increase the toughness of austempered ductile cast irons, we attempted to strengthen the fracture initiation sites such as graphite-matrix interfaces and eutectic cell boundaries in a way of the microsegregation of alloying elements. For instance, the retained austenite which is stable under external stresses may be introduced preferentially into these sites by the addition of Ni, which segregates to a graphite periphery and of Mn, which partitions mainly to eutectic cell boundaries. Following this concept, the effects of various austempering processes on toughness are also in-vestigated. The cast iron alloying with Ni and Mn shows the best fracture toughness when it is heat-treated by either QB' or B' process; here, the QB' means the oil-quenching from an austenite γ phase range followed by austempering from a ferrite α plus γ range and the B' means austempering from a (α + γ) range. In the newly developed iron, there is a mixed microstructure composed of the ferrite, bainitic ferrite, and austenite. Abnormal elongation due to the TRIP effect in the austenite phase is found to have occurred at about 198 K. Moreover, it is shown that this TRIP effect may be caused by the formation of deformation twins.     

The incomplete transformation phenomenon in steel

The incomplete transformation (ICT) phenomenon is defined as the temporary cessation of ferrite formation (in the absence of carbide precipitation at α:γ boundaries) before the fraction of austenite transformed to ferrite predicted by the Lever rule is attained. The ICT phenomenon is central to the “overall reaction kinetics” definition of bainite but plays lesser roles in the quite different groups of phenomena comprising the “surface relief” and “generalized microstructural” definitions. Experimental generalizations that can be made about the ICT are briefly noted. Effects of alloying elements, X, upon various aspects of the nucleation and growth of ferrite are listed in order of apparently increasing strength. The ICT is seen to be one of the stronger effects in the latter spectrum. Theories of the ICT are then critically examined. The currently most promising theories involve (1) the cessation of growth induced by the coupled-solute drag effect (C-SDE), accentuated by the overlap of the carbon diffusion fields associated with adjacent ferrite crystals; and (2) the concepts of item (1) plus local alloying element partition between ferrite and austenite (LE-NP), thereby making any further ferrite growth require volume diffusion of X in austenite and thus to take place exceedingly slowly. Distinguishing between these theories will require use of an Fe-C-X system in which the temperature-carbon concentration paths of the paraequilibrium (PE) Ae₃ and of the “no partition” boundary are well separated. Although the Fe-C-Mo system has proved convenient for studying many aspects of the ICT phenomenon, it does not fulfill this specification. Fe-C-Mn alloys do so and should be particularly useful subjects for future investigations of the ICT phenomenon. This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,” December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden.     

Effects of Si and Cr on Complete Decarburization Behavior of High Carbon Steels in Atmosphere of 2 vol.% O2

The effects of chemical compositions,especially silicon and chromium contents,on the complete decarburization behaviors of steels in atmosphere of 2 vol.% O_2 and 98vol.%N_2 were investigated by using a simultaneous thermal analyzer.Complete decarburization was observed at both 750 and 800 ℃ for 60Si2 Mn A steel,while 750 ℃ only for 92 A steel.For GCr15 steel,no decarburization was found at 750 or 800 ℃,and only partial decarburization was observed at 850 ℃.It indicates that silicon promotes while chromium prevents the complete decarburization of steels in atmosphere with 2 vol.%O_2.The main reason is that silicon increases while chromium reduces the equilibrium concentration of ferrite at the interface of ferrite and austenite,which results in the complete decarburization.     

Diffusional growth limitation and hardenability

It has long been recognized that the effects of alloying elements on the hardenability of steels is related directly to their effects on the nucleation and growth kinetics of proeutectoid and eutectoid austenite decomposition products. In the present paper, theoretical and experimental studies of proeutectoid-ferrite and pearlite growth are reviewed for systems of the form Fe-C-X, where X is a substitutional alloying element such as Co, Cr, Mn, Mo, Ni, Si, and so forth. Of principle interest is the limitation which an alloying element X imposes on the corresponding diffusional growth kinetics. Although the agreement between theory and experiment is reasonable for most systems, there remain areas of considerable controversy (e.g., the role of interface diffusion, whether or not local equilibrium is maintained at interfaces and solute segregation to interfaces). This paper is based on a presentation made at a symposium on “Hardenability” held at the Cleveland Meeting of The Metallurgical Society of AIME, October 17, 1972, under the sponsorship of the IMD Heat Treatment Committee. Note added in proof: Boswellet al. have initiated a theoretical treatment of the “impurity drag” effect of carbide formers on proeutectoid ferrite growth.     

Fe-C-Mn-Al系TRIP钢两相区奥氏体转变模拟

为了研究Fe-C-Mn-A1系TRIP钢两相区奥氏体化过程中合金元素在奥氏体和铁素体中的分布,利用热膨胀仪、金相显微镜、电子探针等仪器,在对TRIP钢两相区奥氏体化过程进行热力学与动力学分析的基础上,建立了两相区奥氏体化过程的扩散模型,采用显式有限体积法对800℃与840℃的奥氏体化过程进行了数值求解.模拟结果表明:奥氏体转变初期受C元素在奥氏体中的扩散控制达到亚平衡,奥氏体转变速率较快;此时A1元素在奥氏体与铁素体界面处的浓度差较显著,Mn元素在奥氏体与铁素体界面处的浓度差不显著.奥氏体转变后期受Mn元素在铁素体内的扩散控制,转变速率较慢;此时A1元素在铁素体内已大量富集,Mn元素在奥氏体与铁索体界面处有较显著的浓度差.     

Alloying Element Partition and Growth Kinetics of Proeutectoid Ferrite in Hot-Deformed Fe-0.1C-3Mn-1.5Si Austenite

Alloying element partition and growth kinetics of proeutectoid ferrite in deformed austenite were studied in an Fe-0.1C-3Mn-1.5Si alloy. Very small ferrite particles, less than several microns in size, were formed within the austenite matrix, presumably at twin boundaries as well as at austenite grain boundaries. Scanning transmission electron microscopy–energy-dispersive X-ray (STEM-EDX) analysis revealed that Mn was depleted and Si was enriched in the particles formed at temperatures higher than 943 K (670 °C). These were compared with the calculation of local equilibrium in quaternary alloys, in which the difference in diffusivity between two substitutional alloying elements was assumed to be small compared to the difference from the carbon diffusivity in austenite. Although the growth kinetics were considerably faster than calculated under volume diffusion control, a fine dispersion of ferrite particles was readily obtained in the partition regime due to sluggish growth engendered by diffusion of Mn and Si.     

Discussion on the Alloying Element Partition and Growth Kinetics of Proeutectoid Ferrite in Fe-C-Mn-X Alloys

Experimental data on alloying element partition and growth kinetics of proeutectoid ferrite in quaternary Fe-C-Mn-Si, Ni, and Co alloys were reanalyzed using an approximate method, which permits a quick evaluation of alloy partitioning to be made. The method yielded results in good agreement with DICTRA and is applicable to Fe-C base multicomponent alloys. Differences of the predicted local condition at the α/γ boundary from those previously presented in the alloys are noted.     

Growth of intragranular ferrite in Fe-Ni-P alloys

Analytical electron microscopy (AEM) techniques were used to study the growth of intragranular ferrite in Fe-Ni-P alloys. The spatial resolution of the AEM was exploited to gather microchemical information regarding elemental redistribution at ferrite/austenite interfaces in order to determine the growth mechanism for intragranular ferrite. In this alloy system, the growth kinetics are dictated by the bulk diffusion of Ni in austenite. Full equilibrium occurs during intragranular ferrite growth with full partitioning of Ni and P between austenite and ferrite, and chemical equilibrium occurs at the α/γ interface in both phases. A numerical model to simulate ferrite growth was developed based on equilibrium growth considerations. The Ni concentrations and precipitate sizes predicted by the model agree well with those measured by AEM techniques in the experimental alloys. The computer model has been extended to predict the thermal histories of iron meteorites and their parent asteroidal bodies.     

Simulation of ferrite growth in continuously cooled low-carbon iron alloys

The growth of a planar ferrite (α): austenite (γ) boundary in low-carbon iron and Fe-Mn alloys continuously cooled from austenite through the (α+γ) two-phase field and the α single-phase field was simulated by incorporating carbon diffusion in austenite, intrinsic boundary mobility, and the drag of an alloying element. At a very high cooling rate (≥ 10³ °C/s), the width of the carbon diffusion spike in austenite approaches the limit at which spikes are viable, so that the growth of ferrite in which carbon is not partitioned can occur even above the α solvus. In this context, the upper limiting temperature of partitionless growth of ferrite is the T ₀ temperature. In the presence of drag of an alloying element, e.g., Mn, both carbon-partitioned and partitionless growth of ferrite begins to occur at finite undercoolings from the Ae ₃, T ₀, or α-solvus temperature, at which the driving force for transformation exceeds the drag force. The intrinsic mobility of the α:γ boundary may play a significant role at an extremely high cooling rate (≥10⁵ °C/s). This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee.     

Nucleation kinetics of proeutectoid ferrite at austenite grain boundaries in Fe-C-X alloys

The nucleation kinetics of proeutectoid ferrite allotriomorphs at austenite grain boundaries in Fe-0.5 at. Pct C-3 at. Pct X alloys, where X is successively Mn, Ni, Co, and Si and in an Fe-0.8 at. Pct C-2.5 at. Pct Mo alloy have been measured using previously developed experimental techniques. The results were analyzed in terms of the influence of substitutional alloying elements upon the volume free energy change and upon the energies of austenite grain boundaries and nucleus: matrix boundaries. Classical nucleation theory was employed in conjunction with the pillbox model of the critical nucleus applied during the predecessor study of ferrite nucleation kinetics at grain boundaries in Fe-C alloys. The free energy change associated with nucleation was evaluated from both the Hillert-Staffanson and the Central Atoms Models of interstitial-substitutional solid solutions. The grain boundary concentrations of X determined with a Scanning Auger Microprobe were utilized to calculate the reduction in the austenite grain boundary energy produced by the segregation of alloying elements. Analysis of these data in terms of nucleation theory indicates that much of the influence of X upon ferrite nucleation rate derives from effects upon the volume-free energy change,i.e., upon alterations in the path of theγ/(α + γ) phase boundary. Additional effects arise from reductions in austenite grain boundary energy, with austenite-forming alloying elements being more effective in this regard than ferrite-formers. By difference, the remaining influence of the alloy elements studied evidently results from their ability to diminish the energies of the austenite: ferrite boundaries enclosing the critical nucleus. The role of nucleation kinetics in the formation of a bay in the TTT diagram of Fe-C-Mo alloys is also considered. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University     

The transformation to austenite in a fine grained tool steel

The transformation to austenite in a fine grained tool steel has been investigated quantitatively until the disappearance of the ferrite. The initial structure of the steel consisted of ferrite and globular carbides. The nucleation starts at carbides which lie at the ferrite grain boundaries. The kinetics are in good agreement with Cahn's theory of grain boundary nucleated transformation. A constant growth rate was found up to 30 pct transformation. Site saturation occurs early in the reaction; this was confirmed by metallographic examination. The rate law is controlled by growth and is independent of the nucleation rate. The mechanism which is controlling this growth is the advancing ferrite-austenite interface reaction. The alloying elements influence the atomic mobilities, increasing the necessary activation energy to about 110 kcal/mol. Formerly Research Engineer, Metallurgical Department, Delft University of Technology     

Modeling of kinetics of austenite-to-allotriomorphic ferrite transformation in 0.37C-1.45Mn-0.11V microalloyed steel

The present article is concerned with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution of austenite-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid temperature, where allotriomorphic ferrite is the only austenite transformation product, the soft-impingement effect should be taken into account in the modeling. In that case, the Gilmour et al. analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast, since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided, and carbon concentration in austenite far from the α/γ interface remains the same as the overall carbon content of the steel. Hence, the soft-impingement effect should be neglected, and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent agreement (higher than 93 pct in R ²) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of temperature studied.     

Ferrite and bainite in alloy steels

The addition of alloying elements even in small concentrations can alter the properties and structure of ferrite and bainite. The various morphologies of ferrite-carbide aggregates are surveyed including alloy pearlite, fibrous carbide eutectoids and precipitation of fine alloy carbides atγ-α interfaces. Modern ideas on the morphology and growth kinetics of ferrite and upper and lower bainite are also summarized. Using this information, an attempt is made to rationalize subcritical transformations of austenite in low alloy steels. Basic factors influencing the strength of alloy ferrites are discussed, leading to an examination of structure-mechanical property relationships in ferrite and bainite. Finally the exploitation of the ferrite and bainite reactions to produce useful alloy steels by direct transformation of austenite is explored.     

Ferrite nucleation and growth during continuous cooling

The austenite decomposition has been investigated in two hypoeutectoid plain carbon steels under continuous cooling conditions using a dilatometer on a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary system, Fe-C-Mn. The austenite-to-ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates and small austenite grain sizes, where ferrite nucleates at austenite grain corners. The nuclei are assumed to have an equilibrium composition and a pillbox shape in accordance with minimal interfacial energy. For higher cooling rates or larger austenite grain sizes, early growth has to be taken into account to describe the transformation start, and nucleation is also encouraged at the remaining sites of the austenite grain boundaries. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium;i.e., only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute draglike effect on the boundary movement. Thus, growth kinetics are controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Thermopower method for detecting the ferrite-austenite transition: applications to surface reaction and diffusion

Difference of the thermoelectromotive force (temf.) for ferrite and austenite. Measurement of the change of thermopower during the transition ferrite–austenite, as a method for determination of rate parameters and diffusion constants. Application on the carburization and decarburization of Fe and Fe-Ti alloys and on the carbon diffusion in ferrite and austenite.