Studies of the influence of alloying elements on the growth of ferrite from austenite under decarburization conditions: Fe-C-Nl alloys

We have evaluated controlled decarburization as a method for probing the effect of alloying elements on ferrite growth from austenite. The technique permits the exploration of longer-time ferrite layer growth; it minimizes the effects of interface structure on ferrite growth; and it permits the isolation of the effects of temperature and alloying element concentration on ferrite/austenite interface motion. The study of the decarburization of initially homogeneous Fe-C-Ni alloys was complemented by experiments using specimens with a controlled nickel concentration gradient. Although the decarburization method yields consistent results at longer times, it is found to be less appropriate for the study of initial ferrite growth. Nucleation in the gas/solid interface region, coupled with uncertainties about the precise time of decarburization, leads to large relative errors at the earliest times. For these reasons, the method is considered a valuable complement to studies based on precipitation boundary conditions. This article is based on a presentation given in the symposium “The Effects of Alloying Elements on the Gamma to Alpha Transformation in Steels,” October 6, 2002, at the TMS Fall Meeting in Columbus, Ohio, under the auspices of the McMaster Centre for Steel Research and the TMS-ASM Phase Transformations Committee.     

An instability in Fe-C-M alloys

Some aspects of phase transformations controlled by carbon diffusion in Fe-C-M alloys, M being a substitutional alloying element, are discussed. The rapid carbon controlled reaction comes to an end when the carbon activity has become uniform all over the material. It is found that this state may be unstable if the alloying element M is nonuniformly distributed. A portion of the interface may then move farther into the region of untransformed material. It is proposed that the ferrite formed inside austenite grains during cooling of intercritically annealed dual-phase steels is formed by this mechanism.     

Calculation of super component model modified for X100 pipeline steel

It was reported that super component thermodynamic model could be modified by considering the influence of the interactions among the alloying elements. The influence of alloying elements and deformation energy on the ferrite transformation parameters in X100 pipeline steel was calculated based on this modified model. The results show that the transformation temperature of austenite to ferrite (A3) calculated by this model is higher than it calculated by Thermo- calc software; the deformation energy storage increases the ferrite phase change driving force and nucleate driving force, but it is not significant with temperature change; with the increase of alloying element content, Mo can significantly reduce the phase change driving force of ferrite and pearlite more than Ni and Cr.     

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.     

Morphological stability of γ/α interface formed by carburization in Fe-C-X alloys

Effect of alloying elements on the morphological stability of austenite/ferrite interface formed by carburization of Fe-X alloys at 850 °C and 800 °C was investigated. Planar interfaces were found when the alloying elements added were from among the following: Ti, V, Nb, Ta, Cr, Mo, W, Co, and Cu. Nonplanar interfaces with Widmanstätten-like structures and/or an isolated phase were observed when the alloying elements were from the following group: P, Al, Sb, Ni, Mn, Si, and Ge. The degree of supersaturation of C in the α phase adjacent to the γ phase front was analyzed using the concept of local equilibrium. It was confirmed that there was indeed a close correlation between the morphological stability and the degree of C supersaturation, which in itself depended on whether the alloying element added was an α or γ stabilizer and how strongly it bonded with C in the ferrite phase.     

A Conversional Model for Austenite Formation in Hypereutectoid Steels

An on-heating conversional model has been proposed to calculate the volume fraction of austenite in hypereutectoid steels from dilation strain. The transformation strain equation for austenite formation from ferrite and cementite was developed with lattice parameters modified to include alloying element effects. The conversional model was verified by comparing calculations with experimentally measured transformation temperatures and constituent volume fractions in the transformation temperature range for various heating rates. A continuous heating transformation (CHT) diagram was produced from model results.     

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.     

Banded Microstructure Formation and Its Effect on the Performance of 06Ni9 Steel

Focusing on the banded microstructure formed during the production of 06Ni9 steels for cryo-LNG,this paper examines its formation,distribution of alloying elements,structure,hardness,and low-temperature property.The results show that the banded microstructure formed after hot-rolling and cooling of the steel binct in which the element segregation occurred during solidification.The phase change during heat treatment also can cause the formation of the banded microstructure of 06Ni9 steel.The white bands are mainly composed of ferrite and reversed austenite,and the black bands are mainly composed of reversed austenite and a certain amount of ferrite.Element segregation and formation of more carbide caused some black regions to appear.Grain refinement of 06Ni9steel is beneficial to the formation of reversed austenite,the redistribution of alloying elements,improving the stability of austenite and the low-temperature toughness of steel.This steel easily undergoes nickel segregation;thus,undergoing a secondary quenching and tempering process is recommended.The refinement of martensite quenching above A c3,the martensite that is rich in nickel and carbon,residual austenite and a few little of ferrite after secondary quenching lower than A c3 are beneficial to the formation of high stability austenite.Thus,this can meet the strength and toughness requirement of the low temperature 06Ni9 steel.     

Elemental partitioning and microstructural development in duplex stainless steel weld metal

A thermodynamic analysis which is capable of estimating the austenite/ferrite equilibria in duplex stainless steels has been carried out using the sublattice thermodynamic model. The partitioning of alloying elements between the austenite and ferrite phases has been calculated as a function of temperature. The results showed that chromium partitioning was not influenced significantly by the temperature. The molybdenum, on the other hand, was found to partition preferentially into ferrite phase as the temperature decreases. A strong partitioning of nickel into the austenite was observed to decrease gradually with increasing temperature. Among the alloying elements, average nitrogen concentration was found to have the most profound effect on the phase balance and the partitioning of nitrogen into the austenite. The partitioning coefficient of nitrogen (the ratio of the mole fraction of nitrogen in the austenite to that in the ferrite) was found to be as high as 7.0 around 1300 K. Consequently, the volume fraction of austenite was influenced by relatively small additions of nitrogen. The results are compared with the experimentally observed data in a duplex stainless steel weld metal in conjunction with the solid state δ → δ + γ phase transformation. Particular attention was given to the morphological instability of grain boundary austenite allotriomorphs. A compariso between the experimental results and calculations indicated that the instability associated with irregular austenite perturbations results from the high degree of undercooling. The results suggest that the model can be used successfully to understand the development of the microstructure in duplex stainless steel weld metals.     

Towards an austenite decomposition model for TRIP steels

The current status of developing a fundamental model for describing the overall austenite decomposition kinetics to ferrite and carbide‐free bainite in low carbon TRIP steels alloyed with Mn and Si is reviewed. For ferrite growth, a model is proposed where both interface and carbon diffusion‐controlled ferrite formation are considered in a mixed‐mode approach. The kinetic model is coupled with Thermocalc to obtain necessary thermodynamic information. Spherical geometry with an outer ferrite shell is assumed to capture in a simple way the topological conditions for growth. The mixed‐mode modelling philosophy has been identified to permit a rigorous incorporation of the solute drag effect of substitutional alloying elements, in particular Mn. The Purdy‐Brechet solute drag theory is adopted to characterize the interaction of Mn with the moving austenite‐ferrite interface. The challenges of quantifying the required solute drag parameters are discussed with an emphasis on a potential solute drag interaction of Mn and Si. The model is extended to non‐isothermal processing paths to account for continuous and stepped cooling occurring on the run‐out table of a hot strip mill or on a continuous annealing line. The transformation start temperature during cooling is predicted with a model combining nucleation and early growth which had previously been validated for conventional low carbon steels. The overall model is evaluated by comparing the predictions with experimental data for the ferrite growth kinetics during continuous cooling of a classical TRIP steel with mass contents of 0.19 % C, 1.49 % Mn and 1.95 % Si. Extension of the model to include bainite formation remains a challenge. Both diffusional and displacive model approaches are discussed for the formation of carbide‐free bainite. It is suggested to develop a combined nucleation and growth model which would enable to capture a potential transition from a diffusional to a displacive transformation mode with decreasing temperature.     

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.     

Conversion Model for the Martensitic Transformation of Banded Austenite in a Ferrite Matrix

A model is proposed for the calculation of the volume fraction of martensite formed during the transformation of banded austenite in a hot-rolled AISI 430 stainless steel. The proposed model includes the strain resulting from the difference in coefficient of thermal expansion of austenite and ferrite and the effect of alloying elements on the lattice parameters. The model was verified by comparing the calculated values of the transformation strain of martensite with the experimental measurements.     

Effects of silicon content and intercritical annealing on the manganese partitioning in dual phase steels

 Steels of constant manganese and carbon contents with 0.34-2.26 wt. % silicon content were cast. The as-cast steels were then hot rolled at 1100°C in five passes to reduce the cast ingot thickness from 80 to 4 mm, air cooled to room temperature and cold rolled to 2 mm thickness. Dual-phase microstructures with different the volume fraction of martensite were obtained through the intercritical annealing of the steels at different temperatures for 15 min followed by water quenching. In addition of intercritical annealing temperature, silicon content also altered the volume fraction of martensite in dual-phase steels. The partitioning of manganese in dual-phase silicon steels were investigated using energy-dispersive X-ray spectrometry (EDS). The partitioning coefficient, defined as the ratio of the amounts of alloying element in the austenite to that in the adjacent ferrite, for manganese increased with increasing intercritical annealing temperature and silicon content of steels. It was also shown that the solubility of manganese in ferrite and austenite decreased with increasing intercritical temperature. The results were discussed by the diffusivity and the solubility of manganese in ferrite and austenite present in dual-phase silicon steels.     

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     

Effects of Silicon Content and Intercritical Annealing on Manganese Partitioning in Dual Phase Steels

Steels of constant manganese and carbon contents with silicon content of 0.34%-2.26% were cast.The as-cast steels were then hot rolled at 1100 ℃ in five passes to reduce the cast ingot thickness from 80 to 4 mm, air cooled to room temperature and cold rolled to 2 mm in thickness. Dual phase microstructures with different volume fraction of martensite were obtained through the intercritical annealing of the steels at different temperatures for 15 min followed by water quenching. In addition to intercritical annealing temperature, silicon content also altered the volume fraction of martensite in dual phase steels. The partitioning of manganese in dual phase silicon steels was investigated using energy-dispersive spectrometer (EDS). The partitioning coefficient, defined as the ratio of the amounts of alloying element in the austenite to that in the adjacent ferrite, for manganese increased with increasing intercritical annealing temperature and silicon content of steels. It was also found that the solubility of manganese in ferrite and austenite decreased with increasing intercritical temperature. The results were discussed by the diffusivity and the solubility of manganese in ferrite and austenite existed in dual phase silicon steels.     

Determination of stacking fault energy of austenite in a duplex stainless steel

A determination of stacking fault energy (SFE) of the austenite phase of a duplex stainless steel, material no. 1.4462, has been carried out using transmission electron microscopy (TEM). Furthermore, cold rolling tests and microstructural analysis have been realized in order to allow a detailed discussion of the obtained SFE-values. The results of this Investigation indicate that the stacking fault energy of the austenite phase within the duplex stainless steel Is lower than those of single-phase austenitic stainless steels. This is justified by the chemical composition; mainly by the Cr and Ni alloying contents. Nevertheless, work hardening of the austenite during cold deformation is not as accentuated as expected by the low SFE-values, because at higher deformation levels the deformation mainly occurs within the ferrite phase.     

中锰钢两相区退火奥氏体逆转变的数值分析

根据中锰钢热轧组织结构确立两相区奥氏体化的几何模型和初始条件,利用DICTRA动力学分析软件对中锰钢马氏体基体奥氏体化过程进行计算分析.在奥氏体化初期的形核过程中,马氏体中过饱和的碳锰元素从铁素体迅速转移到奥氏体并在相界面奥氏体一侧聚集.后续的相变过程中,碳在奥氏体中快速均化,但锰在相界面奥氏体一侧的聚集加剧.相变初期奥氏体界面推移速度比中后期高出若干个数量级,但随时间推移迅速衰减.相变初期相界面推移是碳扩散主导,相变后期界面推移受到锰在奥氏体中扩散速度制约.温度升高可显著提高相界面推移速度.达到相同数量奥氏体的情况下,低温长时退火有利于锰从铁素体向奥氏体转移并提高其在奥氏体中的富集度,从而提高奥氏体的稳定性.      

Modeling diffusional growth during austenite decomposition to ferrite in polycrystalline FeC alloys

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.     

Artificial Neural Network Modeling of Microstructure During C-Mn and HSLA Plate Rolling

An artificial neural network (ANN) model for predicting transformed mierostrueture in conventional roll-ing process and thermomechanieal controlled process (TMCP) is proposed. The model uses austenite grain size and retained strain, which can be calculated by using microstrueture evolution models, together with a measured cooling rate and chemical compositions as inputs and the ferrite grain size and ferrite fraction as outputs. The predicted re-sults show that the model can predict the transformed microstructure which is in good agreement with the measured one, and it is better than the empirical equations. Also, the effect of the alloying elements on transformed products has been analyzed by using the model. The tendency is the same as that in the reported articles. The model can be used further for the optimization of processing parameters, mierostructure and properties in TMCP.