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.     

Effect of Austenite Deformation on Continuous Cooling Transformation Microstructures for 22CrSH Gear Steel

 The effect of compressive deformation of austenite on continuous cooling transformation microstructures for 22CrSH gear steel has been investigated using a Gleeble 1500 thermal simulator. The experimental results show that the deformation of austenite promotes the formation of proeutectoid ferrite and pearlite, and leads to the increase of critical cooling rate of proeutectoid ferrite plus pearlite microstructure. The grain boundary allotriomorphic ferrite occupies the austenite grain surfaces when the prior deformation takes place or the cooling rate is decreased, which causes a transition from bainite to acicular ferrite. The deformation enhances the stability of transformation from austenite to acicular ferrite, which results in an increase of M/A constituent.     

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

The present article is concerened 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 austente-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid temperature, where allotriomorphic ferrite is the only austenite transormation product, thesoft-impingement effect should be taken into account in the modeling. In that case, the Gilmouret 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/λ interface remains the same as the overal 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 inR ²) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of temperature studied. C. CAPDEVILA, Research Associate, formerly with the Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalurgicas (CENIM), Consejo Superior de Investigaciones Cientificas (CSIC), 28040 Madrid, Spain     

New insights into the widmanstätten proeutectoid ferrite transformation: Integration of crystallographic and three-dimensional morphological observations

The crystallography and three-dimensional (3-D) morphology of Widmanstätten proeutectoid ferrite precipitates are examined in an Fe-0.12 wt pct C-3.28 wt pct Ni steel isothermally reacted at 650 °C, 600 °C, and 550 °C. This article integrates new orientation mapping (OM) results with the findings of a companion article to this one on the 3-D morphology of proeutectoid ferrit^[1] and an earlier transmission electron microscopy (TEM) study which is reanalyzed here in light of the new OM and 3-D results. All of these studies were performed for the same alloy and heat treatments. The 3-D morphologies and distributions of proeutectoid ferrite precipitates are now known to often be quite different from those deduced by conventional two-dimensional (2-D) microscopy techniques. The present crystallographic studies indicate that “primary” ferrite (nucleated directly on prior austenite grain boundaries) forms monolithic single crystals and can be approximated as elongated triangular pyramids. “Secondary” ferrite morphologies can be described as laths and plates branching into the austenite from a thick and/or broad allotriomorphic ferrite base. These secondary Widmanstätten branches are composed of many misoriented crystals with ferrite: ferrite boundaries between them and appear to approach a common orientation as they extend into the austenite grain. Implications of the current findings on existing growth and crystallography models are discussed, and a preliminary hypothesis or mechanism of ferrite formation has been proposed to account for the present observations.     

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.     

中碳Cr-Mo-V钢连续冷却时的组织转变规律

采用Formastor-FⅡ全自动相变仪实现不同冷却速度,利用金相显微镜、扫描电子显微镜和透射电子显微镜,研究了45CrMoV钢在不同冷却速度下的组织转变规律以及回火温度对组织的影响。结果表明,随着冷却速度的变慢,45CrMoV钢的组织由马氏体变为马氏体、先共析铁素体、下贝氏体和粒状贝氏体的混合物。冷却速度进一步变慢,先共析铁素体数量增多,下贝氏体和粒状贝氏体总量减少,材料硬度不断下降;45CrMoV钢中的粒状贝氏体为岛状、颗粒状,也有不规则形状,下贝氏体铁素体板条比低碳钢和超低碳钢中的板条更宽,分布更分散,板条形态不规则;随着回火温度的升高,45CrMoV钢中的渗碳体由细针状变为细条状,最后长大为椭球状,材料强度下降,韧性上升。     

层流冷却过程低碳钢相变模型的适用性分析

选取了现有典型的C-Mn钢相变过程的物理冶金模型,包括5组孕育期模型、7组相变动力学方程模型、5组相变后铁素体晶粒尺寸模型.利用自行开发的组织性能预报系统软件模拟计算了在3组实际冷却工艺条件下各模型的奥氏体转变过程,并对各模型进行了评价.结果表明,对于所设定的成分和工艺条件,适用性较好的孕育期模型是Kwon所提出的模型...     

形变诱导铁素体逆相变过程中的组织及取向变化

 通过对普碳钢Q235在Gleeble1500热模拟机上变形后的微观组织分析,研究了组织中形变诱导的铁素体在变形后保温阶段转变为奥氏体的逆相变现象;并利用背散射电子衍射(EBSD)技术分析了晶粒取向变化。结果表明,在变形后的保温过程中,形变诱导的铁素体先逆相变为奥氏体,同时伴随着诱导铁素体晶粒的长大;然后随着变形后保温时间的延长,逆相变后的奥氏体由马氏体相变逐渐过渡到铁素体的平衡转变,相应地铁素体由具有少量亚结构的形变诱导铁素体逐渐转变为具有较多亚结构的先共析铁素体。     

Influence of magnetic field on the kinetics of proeutectoid ferrite transformation in iron alloys

The kinetics of proeutectoid ferrite transformation in Fe-C base alloys in a strong magnetic field (7.5 T) were studied. The transformation kinetics were accelerated at temperatures not only below but also significantly above the Curie temperature. The free energy and equilibrium ferrite/austenite phase boundaries in applied magnetic fields were calculated using the reported experimental magnetic susceptibility and Weiss molecular field theory. The persistence of magnetic field effects above the Curie temperature can be attributed to the rapidly diminishing difference in relative stability between ferrite and austenite toward the Ae ₃ temperature of iron.     

Growth Kinetics of Proeutectoid Ferrite in an Fe-0.09C-1.5Mn-0.2Si Steel

Growth kinetics of proeutectoid ferrite, including grain boundary face nucleated ferrite, grain boundary edge nucleated ferrite allotriomorph and intragranular ferrite idiomorph, were experimentally measured in an Fe- 0.09C-1.5Mn-0.2Si steel and compared with theoretical calculation in local equilibrium and paraequilibrium modes. Grain boundary edge nucleated ferrite exhibited larger growth rate than grain boundary face nucleated ferrite and in- tragranular ferrite idiomorph. Experimental kinetics of proeutectoid ferrite was within the window defined by the lo- cal equilibrium and paraequilibrium limits. A transition of growth kinetics from paraequilibrium to local equilibrium was observed in the temperature range of 650--750 ℃, which can be explained in terms of solute drag.     

热变形和冷却对700 MPa级Mo-Nb微合金化钢组织的影响

通过Gleeble 1500热模拟试验机和光学显微镜，研究了变形及冷却对700MPa级0．04C-0．27Mo-0．047Nb微合金化钢组织和硬度的影响。得出该钢的静态(不变形)和动态(变形)奥氏体连续冷却转变(CCT)曲线，高温转变区，相变产物为先共析铁素体和粒状贝氏体；中温转变区，相变产物主要为贝氏体。热变形促进了铁素体和贝氏体相变，扩大了形成铁素体的冷却速度范围，推迟了羽毛状贝氏体的形成。     

Effects of TMCP Parameters on the Microstructure and Mechanical Properties of Nb-Bearing Spring Steel

The effects of TMCP parameters,such as finish rolling temperature and cooling rate on the microstructure and mechanical properties of Nb-bearing spring steel were investigated by thermal simulation,quantitative metallography and tensile test.And the precipitation in Nb-bearing spring steel was analysis by electron microscopy.Experimental results indicate that the higher finish rolling temperature or the more rapid cooling rate in a given range,the less the proeutectoid ferrite content and the thinner the interlamellar spacing is.Reasonably higher finish rolling temperature followed by properly higher cooling rate is suggested to improve the mechanical properties of Nb-bearing spring steel.Micro-addition of niobium decreases the proeutectoid ferrite content and the interlamellar spacing and leads to forming degenerated pearlite.The precipitation of size range ～20-50 nm in Nb-bearing spring steel occurred at the lamellar ferrite of pearlite and the proeutectoid ferrite.     

高温变形对含钼钢显微组织的影响

 为了寻求变形温度、变形量对含钼钢显微组织、先共析铁素体量及其晶粒度的影响规律,利用Gleeble 1500热模拟实验机、光学显微镜、XL 30扫描电镜和IA32图像分析仪,分析了不同变形量、不同变形温度下含钼钢的组织。结果表明：在一定条件下,随着变形量的增加,显微组织由铁素体+贝氏体转变为铁素体+贝氏体+珠光体;同时先共析铁素体含量及其晶粒度级别均呈现出提高的趋势。     

Effect of Niobium on Isothermal Transformation of Austenite to Ferrite in HSLA Low-Carbon Steel

Using thermomechanical simulation experiment,the kinetics of the isothermal transformation of austenite to ferrite in two HSLA low-carbon steels containing different amounts of niobium was investigated under the conditions of both deformation and undeformation.The results of optical microstructure observation and quantitative metallography analysis showed that the kinetics of the isothermal transformation of austenite to ferrite in lower niobium steel with and without deformation suggests a stage mechanism,wherein there exists a linear relationship between the logarithms of holding time and ferrite volume fraction according to Avrami equation,whereas the isothermal transformation of austenite to ferrite in high niobium steel proceeds via a two stage mechanism according to micrographs,wherein,the nucleation rate of ferrite in the initial stage of transformation is low,and in the second stage,the rate of transformation is high and the transformation of residual austenite to ferrite is rapidly complete.Using carbon extraction replica TEM,niobium carbide precipitation for different holding time was investigated and the results suggested that NbC precipitation and the presence of solute niobium would influence the transformation of austenite to ferrite.The mechanism of the effect of niobium on the isothermal transformation was discussed.     

不同冷却速率下X70管线钢铁素体相变的模拟

为更精确地控制及优化X70管线钢的目标组织,以经典相变理论模型为基础,建立了先共析铁素体周围的临界碳浓度与原奥氏体的碳浓度之间的数学模型,并采用逆向回归法确定了铁素体相变分数的关键性参数,经试验验证,模型具有良好的精度。结果表明：临界碳浓度满足C^k=1．8,关系;铁素体相变分数的关键性参数m=1．3,b1=0．026...     

New Duplex Microstructure of Grain Boundary Allotriomorphic Ferrite/Granular Bainite

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低合金钢焊接粗晶区连续冷却铁素体相变规律

 利用焊接粗晶区连续冷却淬火方法,对比分析了钛处理钢和普通C Mn钢焊接粗晶区连续冷却不同阶段的相变组织,研究了铁素体相变规律。结果表明,C Mn钢焊接粗晶区主要为晶界铁素体+魏氏组织铁素体;钛处理钢焊接粗晶区主要为晶界铁素体+魏氏组织铁素体+晶内铁素体组织。在钛处理钢中,晶界铁素体、魏氏组织铁素体和晶内铁素体的相变开始温度相同,但各自长大的动力学条件不同。当晶内铁素体和魏氏组织铁素体竞争发生相变时,晶内铁素体在晶内弥散分布氧化物夹杂上的非均质形核抑制了魏氏组织铁素体向晶内的长大。     

Precipitation of VC in ferrite and pearlite during direct transformation of a medium carbon microalloyed steel

Transmission electron microscopy of an air-cooled medium carbon (0.5 wt pct) steel containing 0.1 wt pct vanadium has shown that VC precipitates by the interphase mecha-nism during transformation to both proeutectoid and pearlitic ferrite. Depending upon the rate of transformation, a considerable proportion of the available vanadium may remain in supersaturated solid solution and can be precipitated as VC upon subsequent aging at 700°C. It was found that the proportion of proeutectoid ferrite, the interlamellae pearlite spac-ings and the VC precipitate dispersion parameters all decreased with increasing cooling rate in as-transformed material. G. FRIMODIG were formerly undergraduate students     

Predicting the onset of transformation under noncontinuous cooling conditions: Part II. Application to the austenite pearlite transformation

A detailed review of the additivity principle with respect to the incubation of the austenite decomposition was summarized in Part I of this two-part series and led to the concept of an “ideal” time-temperature-transformation (TTT) diagram. This curve is characteristic of the chemistry and austenite grain size in the steel and allows nonisothermal behavior to be described assuming additivity holds. The derivation of mathematical relationships between the ideal and experimental cooling data was presented in the first article. In this second article, an ideal curve for the austenite-to-pearlite transformation was derived from cooling data.The applicability of the ideal TTT curve for predicting the start of transformation under continuous cooling conditions was assessed for a range of cooling rates. Experiments were conducted under both isothermal and varying temperature conditions, including an industrial cooling schedule, using a Gleeble Thermal Simulator. Reasonable agreement was found between the predictions and the observed transformation start temperatures; predictions were consistent and compared favorably against other methods which have been frequently used to estimate the transformation start temperature for nonisothermal conditions.     

V-N对中碳SiMn非调质钢显微组织的影响

用光学显微镜和透射电镜研究了0.130%V-0.021 5%N、0.130%V-0.0304%N和0.001%V- 0.020 7%N三种V-N含量的(%)0.37～0.38C、0.82～0.92Si、1.78～1.81Mn、0.06 Ti、0.015Nb非调质钢的组织,用Gleeble 1500热模拟机测定了该钢的应力-应变曲线,并用JMatPro 4.1软件计算了该钢的CCT曲线以及900℃和600℃平衡状态下钢中各相的含量。结果表明,该钢的组织为珠光体+先共析铁素体,随氮含量增加,珠光体增多,晶界铁素体变粗;随钒含量减少,珠光体数量显著增加,贝氏体及铁素体变粗;共析铁素体和先共析铁素体中的析出物尺寸≤3 nm;V和/或N含量高的钢,应变时应力大。