Diffusion in growth of bainite

Edgewise growth rates for Widmanstätten ferrite and bainite in low alloy steels can be represented with an empirical equation showing proportionality to the square of the supersaturation of the austenite. The proportionality constant has a value in reasonable agreement with the assumption of rate control by carbon diffusion. The growth rates are too low to give a noticeable supersaturation of carbon in the growing ferrite. The experimentalB _s for low alloy steels does not seem to be related to theT _s line, nor doesBs evaluated from the incomplete transformation to bainite for an alloy steel. By assuming rate control by carbon diffusion, the empirical equation can be used to calculate the growth rate under paraequilibrium or no partition, local equilibrium (NPLE) conditions. Experimental growth rates for a similar steel falls in-between. The fact that paraequilibrium does not seem to apply is taken as an indication that the α/γ interface for Widmanstätten ferrite and bainite is not of a purely martensitic type.     

Formation mechanism of bainitic ferrite in an Fe-2 Pct Si-0.6 Pct C alloy

The bainite transformation at 723 K in an Fe-2 pct Si-0.6 pct C alloy (mass pct) was investigated with transmission electron microscopy (TEM) and quantitative metallography to clarify the growth mechanism of the ferritic component of bainite. In early stages of transformation, the bainitic ferrite was carbide free. The laths of bainitic ferrite within a packet were parallel to one another and separated by carbon-enriched retained austenite. The average carbon concentration of the bainitic ferrite was estimated to be 0.19 mass pct at the lowest, indicating that the ferrite was highly supersaturated with respect to carbon. The laths did not thicken during the subsequent isothermal holding, although they were in contact with austenite of which the average carbon concentration was lower than the paraequilibrium value. In the later stage of transformation, large carbide plates formed in the austenite between the laths, resulting in the decrease in the carbon concentration of the austenite. Subsequently, the ferrite with a variant different from the initially formed ferrite in the packet was decomposed for the completion of transformation. The present results indicate that the bainitic ferrite develops by a displacive mechanism rather than a diffusional mechanism. Formerly Graduate Student, Kyoto University, Kyoto 606-01, Japan This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

The Effect of Nb Micro-alloying on the Bainitic Phase Transformation Under Strip Casting Conditions

The effect of Nb concentration on the transformation from austenite to bainitic ferrite has been examined under simulated strip casting conditions. Nb concentration was found to delay the nucleation of bainite, but accelerated its growth. It is suggested that the delay in nucleation increases the driving force for transformation, which results in an increase in the growth rate of the bainite. The bainite/austenite interfaces are proposed to move too quickly to suffer appreciable solute drag.     

Paraequilibria in the Fe-Cr(Ni,Mn)-C systems and their effect on phase transformations

Modern achievements in the thermodynamics of alloys are used to calculate phase equilibria and to construct Fe-Cr-C, Fe-Ni-C, and Fe-Mn-C paraequilibrium phase diagrams. The reactions that can occur when austenite decomposes in the temperature range where only carbon atoms can be redistributed between transformation products are discussed. It is shown how the limited diffusion mobility of substitutional atoms can affect austenite transformation processes and increase the variety of related reactions. One of the most interesting results of our paraequilibrium analysis is the prediction of the possibility of complete austenite decomposition into a ferrite + cementite paraequilibrium mixture, which should transform into an equilibrium mixture of the α and γ phases upon long-term holding. In other words, the following unusual (at first glance) phenomena can occur during austenite decomposition: the disappearance and, then, appearance of austenite and, in contrast, the precipitation and, then, complete dissolution of cementite.     

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.     

Incomplete reaction phenomenon

Bainite forms by a displacive transformation mechanism and exhibits an incomplete reaction phenomenon. The kinetics and mechanism of bainite transformation have been studied in high- strength Fe-C-Si-Mn and Fe-C-Si-Ni steels using high-speed dilatometry. The new experiments reported here confirm the incomplete reaction phenomenon, with the bainite transformation stop- ping well before paraequilibrium is achieved. These results show that bainite probably grows without diffusion, but soon afterwards, excess carbon is partitioned into the residual austenite.     

The Morphology and Mechanical Properties of Bainite Formed from Deformed Austenite

Deformation of austenite containing 0.85 pct C is shown to significantly increase the ten-sile strength of bainite formed during subsequent transformation. Quantitative metallo-graphic measurements indicate that strengthening is due primarily to an increased dislo-cation density in the ferrite and reduced carbide size, with consequent finer distribution, compared with the bainite formed from undeformed austenite. It is also shown that de-formed austenite transforms to upper bainite at temperatures at least as low as 200°C due to enhanced nucleation and/or growth at slip band heterogeneities generated by the defor-mation process while the only effect on the formation of lower bainite is a retardation of the transformation and reduction of ferrite plate size.     

A model for the development of microstructure in low-alloy steel (Fe-Mn-Si-C) weld deposits

Experimentally observed microstructural variations in a series of low-alloy steel weld deposits containing different carbon concentrations (produced using experimental electrodes) are discussed in terms of a phenomenological model based on phase transformation theory. The model requires an input of austenite grain size, chemical composition and the cooling curve of the fusion zone; this allows the calculation of isothermal transformation diagrams and quantities necessary to define the start and finish temperatures of various reactions. Allotriomorphic ferrite growth is assumed to occur by a paraequilibrium transformation mechanism; its formation is found to determine the development of both Widmanstätten and acicular ferrite. It seems possible to rationalise the microstructural variations in terms of phase transformation theory, and although the presence of inclusions for the heterogeneous intragranular nucleation of acicular ferrite seems necessary, the inclusions, when present in a concentration beyond a limiting value, do not seem to control the overall development of the microstructure.     

Transformation of austenite into bainitic ferrite and martensite

The stress induced martensitic transformation in the upper metastable intermediate state of γ-α transformation in ferrous materials, structured as ferritic bainite, is discussed. The fibrous structured ferritic bainite consists of retained austenite and ferrite platelets growing in the [111]α//[101]γ direction. The ferrite growth Induces carbon enrichment of the adjacent austenite at the phase boundaries. Strengthening at high stress levels up to the yield point causes dislocation tangles in the ferrite fibre and the formation of shear bands crossing each other in the retained austenite. At lower carbon contents of the austenite, lath martensite precipitates at the shear band intersections and at high shear band densities martensite blocks are observed. In carbon enriched austenite martensite lenses formed by shear processes have been observed. At alternating loading conditions, exceeding the stress level for athermic martensite formation, various shear planes are activated forming characteristic patterns of plate martensite.     

Continuous Cooling Transformation Behavior and Kinetic Models of Transformations for an Ultra-Low Carbon Bainitic Steel

 The aim was to investigate transformation behavior and transformation kinetics of an ultra-low carbon bainitic steel during continuous cooling. Continuous cooling transformation (CCT) curves of tested steel were measured by thermal dilatometer and metallographic structures at room temperature were observed by optical microscope. Then transformation kinetic equation of austenite to ferrite as well as austenite to bainite was established by analyzing the relationship of lnln［1/(1-f)］ and lnt in the kinetic equation on the basis of processed experimental data. Finally, the measured and calculated kinetic behaviors of the steel during continuous cooling were compared and growth patterns of transformed ferrite and bainite were analyzed. Results showed that calculated result was in reasonable agreement with the experimental data. It could be concluded that the growth modes of transformed ferrite and bainite were mainly one dimension as the Avrami exponents were between 1 and 2.     

Effect of Thermo‐Mechanical Process on Phase Transformation Behaviors of V–Ti–N Microalloyed Steel

Thermo‐mechanical simulation tests were performed on V–Ti–N microalloyed steel under three hot working conditions by using Gleeble‐3800 thermo‐mechanical simulator to study the effects of hot deformation and post‐deformation holding process on the continuous cooling transformation behaviors of overcooled austenite. The continuous cooling transformation diagrams (CCT diagrams) were determined by thermal dilation method and metallographic method. The effects of the hot deformation, post‐deformation holding, and cooling rate on the microstructure evolution were analyzed. The results show that deformation promotes ferrite and pearlite transformation. In addition, deformation leads to an increase in bainite start temperature, which becomes more markedly with the increase in cooling rate. The post‐deformation holding process is much favorable to promote carbonitride precipitation of the microalloying elements, which contributes to ferrite nucleation and smaller austenite grains. As a result, an increase in ferrite quantity and a decrease in ferrite grain size can be observed. And further more, the post‐deformation holding process reduces the effect of hot deformation on the bainite start temperature.     

Bainite Growth Behavior at Early Stage of Transformation in a High Carbon Silicon-Containing Steel

 The growth behavior at the early stage of bainitic transformation was investigated using optical microscopy, X-ray diffraction analysis and transmission electronic microscopy. The bainite was obtained by isothermal transformation at 200 ℃ only for a short time in a high carbon silicon-containing steel after austenitization at 200 ℃ only for 20 min. Transmission electronic microscopy shows that the bainite appears in the form of plates with a width of about 30 nm, and that the interface of the bainite leading tip is wedge shaped. X-ray diffraction analysis reveals that the bainite plates consist of single ferrite phase, with absence of carbides. The results confirm the occurrence of the moiré which suggests the existence of austenite grain boundaries at the bainite leading tip. Both the lateral growth and longitudinal growth of bainite have weak ability to traverse the lattice-distortion strain fields and austenite grain boundary. The austenite grain boundary impedes the longitudinal growth of the bainite plate, i. e. , the growth of bainite plate stops at the austenite grain boundary. The longitudinal growth of bainite associated with the features of shear mechanism can not completely be in accordance with that of martensitic transformation.     

Local conditions at moving α/γ boundaries of proeutectoid ferrite transformation in iron alloys

The local conditions at moving α/γ boundaries in iron alloys are examined from the data on growth kinetics, solute partitioning, and critical limit of transformation. In Fe-C alloys, local equilibrium of carbon is likely to be sustained at the majority of α/γ boundaries during the growth of allotriomorphic ferrite except at some boundaries containing immobile low-energy facets. In Fe-C-X alloys, there is experimental evidence that local equilibrium of the substitutional alloying element is established at higher temperatures. However, growth under near paraequilibrium conditions may be prevalent at lower temperatures and at early growth stages. The diffusion of alloying elements in ferrite and along the austenite grain boundary may have a significant influence on the growth of ferrite near the boundary between fast and slow growth. The growth of Widmanstätten and bainitic ferrite is likely controlled by carbon diffusion, that is, without a supersaturation of carbon, while the chemical condition of carbon near the plate edge may not be identical to that of a planar disordered α/γ boundary.     

直接热轧法制备Cu-P-Cr-Ni-Mo双相耐候钢

 在商用09CuPCrNi耐候钢化学成分的基础上,通过调整合金元素含量,研制出了可直接热轧双相化的Cu-P-Cr-Ni-Mo耐候钢。该钢种变形奥氏体的CCT曲线具有较宽的铁素体析出区,可作为热轧“可行的速度窗口”;铁素体析出区与贝氏体转变区之间存在约80℃的奥氏体亚稳区,可作为热轧“可行的卷取范围”;贝氏体转变区的右侧端部封口,可避免在卷取过程中发生贝氏体转变。根据Cu-P-Cr-Ni-Mo耐候钢的变形奥氏体的CCT曲线,制定了5种热轧双相化工艺,并采用Gleeble-3500热模拟机进行了轧制模拟,制备出了Cu-P-Cr-Ni-Mo热轧双相耐候钢。不同工艺下获得的双相耐候钢组织均为铁素体基体及其上呈岛状分布的马氏体,马氏体体积分数为17%~28%。     

Effect of Boron on the Isothermal Bainite Transformation

The role of Boron on the isothermal bainitic transformation in low-C, lean-alloyed steel was investigated. B clearly affected both the transformation kinetics and the morphology of isothermally transformed bainite. The effect of B was more noticeable in the high-temperature range of the bainitic transformation. The microstructure of bainite formed at 773 K (500 °C) consisted of a bainitic ferrite matrix and the martensite/austenite constituent. While the martensite/austenite constituent had an elongated morphology in B-free steel, the martensite/austenite constituents in the B-added steel had a granular morphology. Two types of bainite unit nucleation were considered: the initial nuclei and the nuclei formed on previously formed units. Electron backscattered diffraction (EBSD) analysis showed that the initial bainitic ferrite nuclei were formed at austenite grain boundaries with a Kurdjumov-Sachs (K-S) crystallographic orientation relationship with respect to one of the neighboring austenite grains, revealing the importance of interfacial energy reduction in the nucleation stage. The nuclei of the bainite transformation in the B-added steel were confined to the austenite grain interior, and the bainitic ferrite nuclei had crystallographic orientations limited to K-S variants within the same Bain variant. The characteristic bainite microstructure in B-added steel is due to the inhibition of the bainitic ferrite nucleation at austenite grain boundaries.     

Paraequilibria in the Fe-Si-C system and their relation to the bainite transformation in steels

The thermodynamic properties of silicon-containing cementite and ε carbide are estimated, and the results obtained are used to perform the thermodynamic calculation of paraequilibria in the Fe-Si-C system. Even a relatively low silicon content is shown to substantially change the positions of virtually all boundaries of phase fields in the Fe-C phase diagram. The relation between the paraequilibria and bainite transformation was analyzed. The maximum supersaturation of retained austenite is found to be controlled by a thermodynamic factor, namely, the solubility of the paraequilibrium cementite. The thermodynamic specific features also cause the differences in the mechanisms of carbide precipitation from the α and γ phases in the absence of silicon redistribution: only cementite can precipitate from austenite, whereas both cementite and ε carbide can precipitate from bainitic ferrite.     

Effect of Processing Parameters on Crystal Orientation of Microstructure in High Niobium-Bearing Microalloyed Steel

The present studies are aimed at understanding the effect of cooling rate and prior strain on the evolution of morphology,orientation relationship (OR) and variant selection in pipeline steel with 0.09wt% niobium.In identical prior austenite grain,all products include granular bainite(GB) by coherent transformation,keep orientation relationship (OR) with parent austenite.Fast cooling and large deformation below T nr both can raise drive force of coherent transformation and weaken variants selection,and it can generate higher frequency of high angle boundaries (HABs) (≥15degree).Moreover,large deformation by few passes below T nr can accelerate nucleation of ferrite grains at the austenite boundary by incoherent transformation.These fine grains haven’t OR with prior austenite grain in any side of boundary,and exhibit significant misorientations between themselves.     

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.     

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.