Influence of interfacial structure upon carbide precipitation at austenite: Ferrite boundaries in an Fe-C-Mo alloy

During conventional isothermal transformation of an Fe-0.11 pct C-1.95 pct Mo alloy, eutectoid decomposition o°Curs by the interphase boundary carbide precipitation and the fibrous carbide Mechanisms at 770° to 825 °C. When proeutectoid ferrite is formed and then recrystallized within the α+ γ region, and subsequently further transformed at 770° to 825 °C, however, both of these eutectoid decomposition Mechanisms are rendered inoperative. Carbide precipitation o°Curs instead entirely as isolated particles. This result supports the deduction that carbide precipitation at austenite: ferrite boundaries can o°Cur only when these boundaries are locally immobilized by a partially coherent interfacial structure. A general approach to explaining the development of planar and curved interphase boundary precipitation, fibrous structure, and pearlite is developed in terM_s of two crystallographic factors. T. OBARA, formerly Research Associate in the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 and the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. G. J. SHIFLET, formerly Graduate Student, Michigan Technological University, and Visiting Graduate Student, Carnegie-Mellon University. H-I AARONSON, formerly Professor at Michigan Technological University.     

Simulation of Nucleation of Proeutectoid Ferrite at Austenite Grain Boundaries during Continuous Cooling

The nucleation kinetics of proeutectoid ferrite during continuous cooling in three Fe-C-Mn-Si steels, measured in-situ by three-dimensional X-ray diffraction microscope, are compared with numerical simulation that takes into account differences in the activation energy of nucleation among grain boundary faces, edges, and corners. The essential feature of ferrite nucleation in the 0.21 pct C steel, i.e., nucleation occurred just below Ae₃ and ceased at a small undercooling, is reproduced taking into account the site consumption, primarily at grain corners and overlap of solute diffusion fields in the grain boundary region or the matrix and assuming a very small or almost null activation energy of nucleation. In the 0.35 and 0.45 pct C steels, small activation energy, as reported by Offerman et al., was not unequivocally obtained because ferrite nucleation occurred at considerably large undercoolings, even below the paraequilibrium Ae₃ in these steels. The increasing rate of the observed particle number with decreasing temperature is considerably smaller than calculation. This article is based on a presentation given in the symposium entitled “Solid-State Nucleation and Critical Nuclei during First Order Diffusional Phase Transformations,” which occurred October 15–19, 2006 during the MS&T meeting in Cincinnati, Ohio under the auspices of the TMS/ASMI Phase Transformations Committee.

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Crystallographic Analysis of Proeutectoid Ferrite/Austenite Interface and Interphase Precipitation of Vanadium Carbide in Medium-Carbon Steel

To clarify the mechanism of interphase precipitation of vanadium carbide (VC) in a medium-carbon steel, orientation relationships (ORs) and plane orientations of ferrite/austenite interfaces were investigated. It was found that a large part of grain boundary ferrite holds near-K-S OR with at least one side of austenite adjacent to grain boundary regardless of V addition. By the V addition, a fraction of grain boundary ferrite holding near the K-S OR with both sides of austenite is decreased remarkably. Furthermore, only non-K-S ferrite/austenite interfaces migrate dominantly in the V-added alloy in contrast to the V-free alloy. Ferrite/austenite interface orientations are not fixed crystallographically but are randomly distributed in terms of ferrite and austenite orientations. Those results do not agree with the ledge mechanism originally proposed by Honeycombe. Thus, it is proposed that the ledge mechanism is extended to the non-K-S interface, which partially consists of coherent and less-mobile interfaces.     

Influence of Deformation on the Precipitation Behavior of Nb(CN) in Austenite and Ferrite

The evolution of Nb precipitates in a low-alloyed steel at 973 K (700 °C) as a function of strain and subsequent dwell time is studied via atom-probe tomography (APT) and transmission electron microscopy. The results show that the volume fraction of the precipitates increases with increasing accumulated deformation because deformation-induced dislocations act as nucleation sites. The chemistry of these precipitates which are Nb carbonitrides changes with the dwell time after the deformation step. With increasing time, the C fraction increases. The precipitation analysis by APT in the austenite and the ferrite reveals that precipitates in the ferrite are larger and exhibit a higher C fraction compared to the precipitates in the austenite after the same thermo-mechanical treatment. The investigations also show that the volume fraction of Nb carbonitrides in the ferrite is higher than in the austenite.     

Precipitation of Nb in Ferrite After Austenite Conditioning. Part I: Microstructural Characterization

While the role of Nb during the processing of austenite is quite clear, what happens in subsequent stages to the concentration of this element left in solution is subject to some debate. In particular, some uncertainty still subsists concerning the eventual homogeneous precipitation in Nb supersaturated polygonal ferrite. The present work was aimed at clarifying the precipitation sequence of Nb during coiling, through a systematic work and a careful selection of the processing conditions in order to produce different scenarios concerning the initial state of Nb. A Nb-microalloyed steel was thermomechanically processed in the laboratory followed by simulated coiling at different temperatures in the 873?K to 1023?K (600?°C to 750?°C) range. Transmission electron microscopy (TEM) showed interphase precipitation of NbC at high coiling temperatures, while at 873?K (600?°C), homogeneous general precipitation took place in ferrite and followed a Baker?CNutting orientation relationship.     

Hardening Through an Ultrafine Carbide Precipitation in Austenite of a Low-Carbon Steel Containing Titanium and Tungsten

Metallurgical and Materials Transactions A - In this study, a significant hardening effect was obtained in a low-carbon steel containing Ti and W by performing a 20 pct hot compressive...     

Influence of Crystallography on Ferrite Nucleation at Austenite Grain-Boundary Faces,Edges, and Corners in a Co-15Fe Alloy

The nucleation of ferrite precipitates at austenite grain faces, edges (triple lines), and corners (quadruple points) was studied in a Co-15Fe alloy in which the matrix phase was retained upon cooling to room temperature by serial sectioning coupled with electron backscatter diffraction analysis. Nearly half of the edges and corners were vacant at an undercooling of 60 K from the γ/(α + γ) boundary where the precipitation occurred significantly at grain faces. A significant proportion of precipitates had Kurdjumov–Sachs (K–S) and to a lesser extent Nishiyama–Wassermann (N–W) orientation relationships with more than one grain at all boundary sites. Vacant edges and corners were readily observed, of which the misorientations of matrix grain boundaries would permit a precipitate to have a specific orientation relationship with multiple grains. Small differences in the nucleation activation energy among the grain faces, edges, and corners may lend support to a view proposed from experiments of nucleation in Fe-C base alloys that ferrite nuclei are more or less surrounded by low-energy facets of α/γ phase boundary.     

形变及冷却条件对35钢铁素体组织的影响

利用热模拟单道次压缩形变试验研究了35钢在临界奥氏体区(Ae3～Ar3)形变后铁素体组织的变化规律。结果表明,形变后水淬,小形变时铁素体首先在原奥氏体晶粒的界隅和晶界上形核,随着形变温度的降低和应变量的增大,原奥氏体内部出现大量细小、等轴的铁素体。在此温度区间形变后缓慢冷却,能够得到超平衡数量(共析转变温度的铁素体转变量)的铁素体,并且随形变温度的降低和形变量的增大,铁素体晶粒明显细化和等轴化。当形变温度降到690℃(Ar3附近),真应变0.92时,铁素体晶粒细化到2～4μm,其体积分数达到76.86%。     

硼在奥氏体晶界的两种偏聚形式

用PTA方法研究了Fe-30%Ni合金中硼在晶界的偏聚行为。实验表明在550-1200℃保温后用不同方式冷却的试样中存在平衡与非平衡两类硼偏聚,它们各自的形成机制不同,试验条件对它们的影响不同。平衡偏聚在保温时形成,在低温区淬火时起主要作用。在高温加热后,用通常冷却速度淬火时,晶界偏聚主要来源于冷却过程中产生的非平衡偏聚,实际瘁火试样中观察到的硼偏聚是这两类偏聚的叠加。试验指出,Fe-30%Ni合金中偏聚方式有一个转折温度区,这温度受冷却速度影响,在通常冷却速度下,这个转折温度在650~750℃之间。     

Abnormal Grain Growth in Austenite Structure Reversely Transformed from Ferrite/Pearlite-Banded Structure

The grain growth behavior of austenite reversely transformed from ferrite/pearlite (F/P)-banded and non-banded steels has been studied. It was found that the grain-coarsening temperature [the temperature at which abnormal grain growth (AGG) occurs] of the initially banded F/P structure is quite low compared with that of the non-banded sample. In the F/P-banded sample, the abnormal grains always originate from the former ferrite region. The occurrence of AGG is essentially attributable not to the austenite nucleation process during heating but to the grain growth process after the completion of austenizing. It was proposed that the lowered grain-coarsening temperature in the banded structure is due to the non-uniform pinning-effect of AlN precipitates between former ferrite and pearlite regions.     

铈对Fe Cr Ni Nb Ti Al W合金持久寿命及晶界析出相的影响

 研究了稀土元素铈对铁基合金Fe Cr Ni Nb Ti Al W晶界析出相的数量、形貌、分布的影响及其与持久寿命的关系。结果表明：铈含量较低时，晶界析出相主要是大片状相及少量颗粒相；当铈含量增加时，晶界中颗粒相数量明显增加，片状相减少，且析出相分布较均匀，当铈含量过高时，晶界中析出大量片状相及颗粒相，且析出相总量增加。这些析出相的数量、形貌以及分布等特征的变化规律与合金持久寿命的变化规律一致。     

The Role of Interfacial Coherency in the Kinetics of Austenite to Ferrite Transformation in Fe-C-M Alloys: A Phase Field Study

Metallurgical and Materials Transactions A - The role of interfacial coherency in the kinetics of austenite to ferrite transformation in Fe-C-M (M is a substitutional element) alloys is studied...     

Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Ferrite in Low Carbon Steels

 Abstract Effects of prior austenite deformation and non-metallic inclusions on the ferrite nucleation and grain refinement of two kinds of low carbon steels have been studied. The ferrites nucleation on MnS and V(C,N) is observed. The combination of thermal-mechanical processes with adequate amounts of non-metallic inclusions formed in low carbon steels could effectively refine the grain size and the microstructure. Ferrite nucleated on the single MnS or V(C,N) inclusions and complex MnS+V(C,N) inclusion. The proper addition of elements S and V could effectively promote the formation of ferrite and further refinement of ferrite grains.     

Influence of Prior Austenite Deformation and Non-Metallic Inclusions on Ferrite Formation in Low-Carbon Steels

Effects of prior austenite deformation and non-metallic inclusions on the ferrite nucleation and grain refinement of two kinds of low-carbon steels have been studied. The ferrite nucleation on MnS and V(C,N) is observed. The combination of thermomechanical processes with adequate amounts of non-metallic inclusions formed in low-carbon steels could effectively refine the grain size and the microstructure. Ferrite nucleated on the single MnS or V(CN) inclusions and complex MnS+V(C,N) inclusion. The proper addition of elements S and V could effectively promote the formation of ferrite and further refinement of ferrite grains.     

Precipitation of Nb in Ferrite After Austenite Conditioning. Part II: Strengthening Contribution in High-Strength Low-Alloy (HSLA) Steels

Often, Nb contributes to the strength of a microalloyed steel beyond the expected level because of the grain size strengthening resulting from thermomechanical processing. Two different mechanisms are behind this phenomenon, and both of them have to do with the amount of Nb remaining in solution after hot rolling. The first of them is the increase of the hardenability of the steel as a result of Nb, and the second one is the fine precipitation of NbC in ferrite. Three Nb microalloyed steels were thermomechanically processed in the laboratory and coiled at different temperatures to investigate the effect of Nb content on the tensile properties. The extra strength was linearly related to the Nb remaining in solution after the hot working. The maximum contribution from Nb was reached for a coiling temperature of 873?K (600?°C).     

Influence of Heating Rate on Ferrite Recrystallization and Austenite Formation in Cold-Rolled Microalloyed Dual-Phase Steels

Compared with other dual-phase (DP) steels, initial microstructures of cold-rolled martensite-ferrite have scarcely been investigated, even though they represent a promising industrial alternative to conventional ferrite-pearlite cold-rolled microstructures. In this study, the influence of the heating rate (over the range of 1 to 10 K/s) on the development of microstructures in a microalloyed DP steel is investigated; this includes the tempering of martensite, precipitation of microalloying elements, recrystallization, and austenite formation. This study points out the influence of the degree of ferrite recrystallization prior to the austenite formation, as well as the importance of the cementite distribution. A low heating rate giving a high degree of recrystallization, leads to the formation of coarse austenite grains that are homogenously distributed in the ferrite matrix. However, a high heating rate leading to a low recrystallization degree, results in a banded-like structure with small austenite grains surrounded by large ferrite grains. A combined approach, involving relevant multiscale microstructural characterization and modeling to rationalize the effect of the coupled processes, highlights the role of the cold-worked initial microstructure, here a martensite-ferrite mixture: recrystallization and austenite formation commence in the former martensite islands before extending in the rest of the material.     

Fe-C-Si-Mn-Mo合金贝氏体—奥氏体界面位错的观察

本文用STEM电镜观察了Fe-0.48C-0.80Mn-1.30Si-0.45Mo合金的贝氏体-奥氏体界面位错的形貌及晶体学特征。结果表明,贝氏体-奥氏体界面的位错组态很接近于Rigsbee和Aaronson所描述的魏氏铁素体的界面结构。本文还讨论了贝氏体、板条马氏体、魏氏铁素体的界面结构及界面迁移机制的异同。     

Influence of Martensitic Structure and Carbide Precipitation Behavior on Mechanical Properties of 25CrMoNbB Alloy Steels with Mo Contents of 0.25 and 0.5 Pct

In this article, the effects of Mo contents of 0.25 and 0.50 pct on the martensitic structure and carbide precipitation behavior of the experimental steels were investigated and their effects on strength, toughness, and fatigue strength were studied. The results showed that the martensitic blocks and laths were refined and the dislocation density increased with the addition of Mo contents of 0.25 and 0.50 pct. Meanwhile, the amount of carbides increased and the size of carbides decreased in tempered steels. The refinement of carbides is due to the increment of nucleation sites resulting from martensitic structure refinement and the dislocation density increment. Besides, the improvement of thermal stability of M₂₃C₆ carbides enriched with Mo also contributes to carbide refinement. With the addition of 0.50 pct Mo, the strength was improved and the toughness did not deteriorate compared with the steel 0.25 pct Mo. Meanwhile, the fatigue strength was also significantly improved with the addition of 0.50 pct Mo.