Formation of aligned two-phase microstructures by applying a magnetic field during the austenite to ferrite transformation in steels

Application of a magnetic field during the ferrite to austenite transformation in Fe–C alloys was found to yield a two-phase microstructure with the paramagnetic austenite grains aligned as chains or columns along the direction of the field in the matrix of ferromagnetic ferrite phase. The underlying mechanism of dipolar interactions suggests that similar alignment of microstructures should take place during the austenite to ferrite transformation under a magnetic field. In the present investigation, an experimental setup has been designed to study the magnetic alignment. Its concept is characterized by deforming steels prior to the austenite to ferrite transformation to introduce ample nucleation sites in addition to applying magnetic fields up to 12 T. Experiments have revealed successful conditions for aligned two-phase microstructures in carbon steels. The formation mechanism of the aligned structures is discussed from the viewpoint of the nucleation and growth of ferrite grains in austenite phase under a magnetic field. Furthermore, it is shown that the shape of the aligned ferrite grains is determined by a balance of the magnetostatic and the interfacial energies.     

强磁场对硅锰铸钢等温珠光体粒化的影响

对硅锰铸钢在相变温度(A1)附近进行等温处理的研究表明,经磁场处理后的试样组织中存在更多的粒状珠光体组织。此粒状珠光体由两部分构成:一部分来自块状铁素体中直接析出粒状的不连续渗碳体;一部分来自等温过程中片状渗碳体的熔断。磁场通过促进铁素体的形核与长大,使较多的碳原子"陷落"在铁素体内以沉淀析出方式形成粒状渗碳体,处理温度越低,此作用越明显。此外,磁场下珠光体组成相磁致伸缩率存在差异,引起珠光体两组成相之间的应变能变化,有利于渗碳体以球状析出于奥氏体或铁素体中,也对粒状珠光体分数的增加作出贡献。     

The alignment,aggregation and magnetization behaviors in MnBi–Bi composites solidified under a high magnetic field

Bi–6 wt% Mn alloy is solidified under a high magnetic field and its microstructures and magnetic properties have been investigated. Microstructure results show that three kinds of morphology of MnBi phase appear in different temperature zones. In all these cases, the grains are orientated with the 〈001〉-crystal direction along the magnetic field direction and aggregated. Magnetic measurement shows a pronounced anisotropy in magnetization in directions normal and parallel to the fabrication field, resulting from this alignment. The effect of the magnetic field on the Mn_1.08Bi/MnBi (paramagnetic/ferromagnetic) phase transformation has been studied and the result shows that the phase transformation temperature T_C increases with the increase of the external magnetic field and under a field of 10 T, a typical increase of T_C is 20 °C during heating and 22 °C during cooling. The change in the morphology and in the magnetic properties of MnBi phase is discussed from the phase transformation and the crystal structure change in magnetic field.     

Grain refinement and mechanical properties of asymmetrically rolled low carbon steel

The formation of fine ferrite grains by the asymmetric rolling of low carbon steel and their mechanical properties were studied. Super-cooled low carbon austenite was deformed by asymmetric rolling at 750 °C with a roll size ratio of 1.5 and immediately cooled at various cooling rates ranging from 3 °C/s to 15 °C/s. Fine ferrite grains (∼2 μm) were formed after asymmetric rolling, preferentially at the prior austenite grain boundaries. The volume fraction of the fine ferrite grains increased with increasing rolling reduction. A ferrite plus pearlite microstructure was obtained at smaller strains and slower cooling rates. However, after heavy deformation, a fine ferrite grain structure with carbide particles dispersed at the ferrite grain boundaries was obtained and the pearlite structure was not observed even after very slow cooling, which implies that most of the ferrite grains were formed dynamically, i.e. during deformation. The yield strength of the asymmetrically rolled steel plates increased with increasing deformation; however, the yield ratio also increased with increasing rolling reduction. The best combination of strength and yield ratio was obtained by using a low level of deformation and a high cooling rate, in which case a portion of the untransformed austenite transformed to martensite.     

Dynamic Ferrite Transformation Behaviors in 6Ni-0.1C Steel

Phase transformation from austenite to ferrite is an important process to control the microstructures of steels. To obtain finer ferrite grains for enhancing its mechanical property, various thermomechanical processes followed by static ferrite transformation have been carried out for austenite phase. This article reviews the dynamic transformation (DT), in which ferrite transforms during deformation of austenite, in a 6Ni-0.1C steel recently studied by the authors. Softening of flow stress was caused by DT, and it was interpreted through a true stress–true strain curve analysis. This analysis predicted the formation of ferrite grains even above the Ae₃ temperature (ortho-equilibrium transformation temperature between austenite and ferrite), where austenite is stable thermodynamically, under some deformation conditions, and the occurrence of DT above Ae₃ was experimentally confirmed. Moreover, the change in ferrite grain size in DT was determined by deformation condition, i.e., deformation temperature and strain rate at a certain strain, and ultrafine ferrite grains with a mean grain size of 1 μm were obtained through DT with subsequent dynamic recrystallization of ferrite.     

A combined model for the description of austenitization,homogenization and grain growth in hypoeutectoid Fe–C steels during heating

A combined model which allows one to simulate all the steps of the reaustenitization process of ferrito-pearlitic plain carbon steel has been developed. The dissolution of pearlite, the transformation of ferrite into austenite and the homogenization of the carbon distribution is described with a finite volume method. The simulation is performed on a bidimensional domain where ferrite (α), pearlite (P) and austenite (γ) grains are represented. The dissolution of pearlite is described by the growth of spherical grains and simple nucleation and growth laws. The movement of α/γ interfaces is calculated by solving the diffusion equation for carbon in the α and γ phases and accounting for the solute flux balance at the interface using a pseudo-front tracking method. The diffusion model is coupled with a Monte Carlo simulation which describes the grain growth occurring in austenite at a later stage of austenitization. The evolution of the volume fractions of pearlite and ferrite, the maximum and minimum carbon concentrations in the domain and the mean austenite grain size are represented as a function of the temperature for a typical case of constant heating rate. The influence of the different steps of the austenitization process on the global kinetics is discussed.     

Microstructural development of cobalt ferrite ceramics and its influence on magnetic properties

The microstructural evolution and its influence on magnetic properties in cobalt ferrite were investigated. The cobalt ferrite powders were prepared via a solid-state reaction route and then sintered at 1200 °C for 1, 2, and 16 h in air. The microstructures from sintered samples represented a bimodal distribution of grain size, which is associated with abnormal grain growth behavior. And thus, with increasing sintering time, the number and size of abnormal grains accordingly increased but the matrix grains were frozen with stagnant grain growth. In the sample sintered for 16 h, all of the matrix grains were consumed and the abnormal grains consequently impinged on each other. With the appearance of abnormal grains, the magnetic coercivity significantly decreased from 586.3 Oe (1 h sintered sample) to 168.3 Oe (16 h sintered sample). This is due to the magnetization in abnormal grains being easily flipped. In order to achieve high magnetic coercivity of cobalt ferrite, it is thus imperative to fabricate the fine and homogeneous microstructure.     

Abnormal ferrite in hyper-eutectoid steels

The microstructural characteristics of ultra-high carbon hyper-eutectoid Fe–C and Fe–C–Cu experimental steels have been examined after isothermal transformation in a range just beneath the eutectoid temperature. Particular attention was paid to the formation of so-called “abnormal ferrite”, which refers to coarse ferrite grains which can form, in hyper-eutectoid compositions, on the pro-eutectoid cementite before the pearlite reaction occurs. Thus it is confirmed that the abnormal ferrite is not a result of pearlite coarsening, but of austenite decomposition before the conditions for coupled growth of pearlite are established. The abnormal ferrite formed on both allotriomorphic and Widmanstätten forms of pro-eutectoid cementite, and, significantly, it was observed that the pro-eutectoid cementite continued to grow, despite being enclosed by the abnormal ferrite. Under certain conditions this could lead to the eventual formation of substantially reduced amounts of pearlite. Thus, a model for carbon redistribution that allows the pro-eutectoid cementite to thicken concurrently with the abnormal ferrite is presented. The orientation relationships between the abnormal ferrite and pro-eutectoid cementite were also determined and found to be close to those which have been reported between pearlitic ferrite and pearlitic cementite.     

Magnetic domain memory in multiferroic Ni2MnGa

Lorentz microscopy is employed to study the evolution of magnetic domain structures across ferromagnetic and ferroelastic martensitic transformations in near stoichiometric Ni₂MnGa. In situ transmission electron microscopy studies have revealed a new phenomenon of magnetic domain memory of the austenite across both ferroic transformations. The reversible magnetic domain structure is attributed to pinning of the magnetic domain walls by antiphase boundaries in the Heusler phase. Two types of martensite domain structures, herringbone and stripe domain, have been observed. The type of domain arrangement is found to be dependent on the relative orientation of the easy axis of magnetization with respect to the thin foil normal.     

Magnetic micro- and nanostructures of unalloyed steels: Domain wall interactions with cementite precipitates observed by MFM

The morphology and content of a cementite phase controls the macroscopic, mechanical and magnetic properties of steels. The influence of the cementite content on the bulk magnetic properties in unalloyed steels was observed in hysteresis loop and Barkhausen noise signals. Globular cementite embedded in a ferrite matrix is characterized by atomic force microscopy and magnetic force microscopy. Size, shape and orientation of the grains influence the domain configuration. When an external magnetic field is applied, the magnetization process occurs mainly in the ferrite matrix. The Bloch walls in the ferrite matrix move, and they are pinned by the cementite precipitates. This microscopic observation correlates with the macroscopic magnetic properties of the investigated material.     

Theory and Modeling of Phase Transformations under Stress in Steel

Thermodynamic prediction of the increment of the formation temperature of proeutectoid ferrite by applied stress is nearly consistent with the experimental data. Kinetics models for ferrite，pearlite and bainite transformations can be shown as modified Johnson-Mehl-Avrami equation in which parameter b(σ) varies with the level of applied stress.The effects of tensile and compressive stresses on enhancement of the ferrite/pearlite and bainite transformations are discussed. The necessity and approach of modification of additivity hypothesis are introduced and the results from modified equation in which some parameters are obtained by regression of two experimental results or taken from TTT and CCT diagrams of a certain steel are superior than that from Scheil‘s equation. Thermodynamic calculation of Ms and nucleation kinetics equations of martensitic transformation under stress are suggested. Modeling of phase transformations under stress in ferrous alloys is briefly described.     

Dynamic Phase Transformation and Spheroidization of Cementite of Hypereutectoid Steel Containing Aluminum during Deformation

利用Gleeble 1500热模拟试验机进行单轴热压缩实验, 研究了合金元素Al对过共析钢缓冷相变和 过冷奥氏体动态相变组织的影响. 结果表明: 在缓冷相变时, Al的加入抑制网状渗碳体形成, 细化珠光体 片层间距; 在过冷奥氏体形变过程中, 动态转变经历动态相变和相变所得珠光体中渗碳体球化及铁素体动 态再结晶等过程. 在动态相变过程中, 没有形成晶界网状渗碳体, 而直接产生珠光体. Al的加入使动态相变过程中奥氏体的稳定性提高、珠光体转变推迟, 进一步细化了珠光体片层间距. 在相变所得珠光体中渗碳体球化及铁素体动态再结晶的过程中, Al阻碍渗碳体粗化, 使渗碳体颗粒和铁素体晶粒尺寸细化.     

Effect of Stable Magnetic Field on the Phase Transformation of Sr3 Steel

The experimental equipment designed by the author was used to carry out quenching treatments on Sr3 steel,with and without magnet it field in different quenching mediums. The effect of steady magnetic field on the phase transformation of Sr3 steel was studied by metallographic microscope and scanning electron microscope. The result shows: the application of magnetic field can obviously increase the volume fraction of ferrite during the austenite to ferrite transformation of Sr3 steel, promote the ferrite grains refining and homogenization, and get the pearlite beam much homogeneously and much compact, when Sr3 steel is quenched in the water.     

Microstructural evolution in a commercial low carbon steel by equal channel angular pressing

In the present investigation, both macroscopic and microscopic microstructural changes in a plain low carbon steel with a mixed structure of ferrite and pearlite were examined during equal channel angular pressing . During equal channel angular pressing, the sample was rotated 180°around its longitudinal axis between the passages. By repeating the pressing up to the accumulated effective strain of ∼4, ferrite grains with a submicron size of 0.2–0.3 μm were obtained. Optical observation revealed that hard pearlite phase deformed macroscopically in a similar manner to soft ferrite phase due to a considerable capability of pearlitic cementite for plastic deformation under the present equal channel angular pressing conditions. An examination of micrographs taken by transmission electron microscopy showed that the microstructural change in ferrite phase was similar to those in single phase Al solid solution alloys in that the non-equilibrium configurations of grain boundary were observed. In pearlite phase, the morphological change of cementite lamellar plates was represented by the existence of either severely necked fragments parallel to each other or curled wavy fragments. Microstructural evolution in a plain low carbon steel subjected to equal channel angular pressing was discussed by comparing it to that of equal channel angular pressed Al solid solution alloys and heavily cold drawn pearlitic steel wire, which exhibited similar microstructural change.     

Step-like and first-order characters of Tb0.2Pr0.8(Fe0.4Co0.6)1.93 with low interstitial carbon content

In this study we report detailed magnetic property of the 4f-3d pseudo-quaternary Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.88C_0.05 compound by detailed magnetization measurements. Very sharp magnetization jumps across the antiferromagnetic–ferromagnetic transition are observed below 3.0 K, and the number of jump-like transitions increases with decreasing temperature. The time-dependent magnetic relaxation, field sweep rate and cooling field dependence of magnetization jumps resemble the martensitic scenario. The number and occurrence of magnetization jumps are mainly determined by the competitions between the thermal fluctuation energy, elastic energy and Zeeman energy, and the field-induced antiferromagnetic to ferromagnetic phase transition at low temperatures is of first-order in nature.     

The β Iron Controversy Revisited

Early twentieth century Fe–C phase diagrams designated the paramagnetic BCC iron phase (ferrite) as β iron. By the third decade of the last century this designation all but disappeared from the literature. Why was this? Should the β phase of iron be brought back to phase diagrams? What effects on the phase diagram would arise if we differentiate the ferromagnetic magnetic phase (α) from the paramagnetic phase (β)? In this paper I will discuss some of the history of this β iron controversy and discuss the effects on the Fe–C binary phase diagram. I will urge that β iron should be returned to iron phase diagrams so as to better represent a proper view of magnetic phase equilibria, magnetic symmetry and magnetic phase transformations.     

Specific features of magnetocaloric effect in isotropic antiferromagnets

The molecular-field method has been used to study the magnetocaloric effect and the dependence of the magnetic entropy on an external magnetic field in a two-sublattice isotropic antiferromagnet. It has been shown that, for an isotropic antiferromagnet, the magnetization of a canted phase that arises after a spin-flop transition in an applied magnetic field is temperature-independent. Therefore, the magnetic entropy of the phase is independent of the strength of an external field and no magnetocaloric effect is observed. The dependence of the magnetic entropy on the magnetic field and the normal magnetocaloric effect appear only after the spin-flip transition of the antiferromagnet into a magnetic-field-induced ferromagnetic phase. In moderate magnetic fields, this occurs in fact only in the temperature range of T ≥ T _N, where T _N is the Néel temperature.     

In situ observation of austenite–ferrite interface migration in a lean Mn steel during cyclic partial phase transformations

High-temperature laser scanning confocal microscopy (HT LSCM) has been applied to investigate the austenite–ferrite interface migration during cyclic phase transformations in situ in a Fe–Mn–C alloy. It has been found that during the cyclic phase transformations the transformation proceeds via the migration of existing austenite–ferrite interfaces. The interfaces migrate in a retraceable way. For the first time, the so–Called stagnant stage has been observed directly. The new in situ observations show that the interface migration rates for interfaces in different grains are comparable with each other prior to soft impingement, while the equilibrium migration distances for different interfaces can be quite different, depending on the local grain size. The average interface velocities as measured by HT LSCM are in very good agreement with the velocities derived from dilatometric data, and those are predicted by a local equilibrium transformation model.     

Development of microstructure and texture of medium carbon steel during heavy warm deformation

The microstructure and texture development of a medium-carbon steel (0.36% C) during heavy warm deformation (HWD) was studied using scanning electron microscopy and electron back scattering diffraction. The spheroidization of pearlite is accelerated due to the HWD, which leads to the formation of completely spheroidized cementite already after the deformation and coiling at 873 K (600 °C). The homogeneity of the cementite distribution depends on the cooling rate and the coiling temperature. The cooling rate of about 10 K/s (ferrite–pearlite prior to HWD) and deformation/coiling at 943–973 K (670–700 °C) lead to a homogeneous cementite distribution with a cementite particle size of less than 1 μm. The ferrite softening can be attributed to continuous recrystallization. Even up to fairly high deformation/coiling temperatures of 983 K (710 °C) the texture consists of typical deformation components. During the continuous recrystallization the amount of high angle grain boundaries can increase up to 70% with a ferrite grain size of 1–3 μm. An increase of the cooling rate up to 20 K/s (ferrite–pearlite–bainite prior to HWD) deteriorates the homogeneity of the cementite distribution and the softening of ferrite in the final microstructure.     

锰质量分数和压缩比对中碳钢相变组织的影响

采用光学显微镜和电子探针观察了中碳钢显微组织形貌,分析了不同锰质量分数和轧制压缩比对相变组织的影响。研究表明：试验钢显微组织为形态不同的铁素体+珠光体;增加Mn质量分数抑制晶界铁素体形核和长大,同时细化铁素体晶粒,促进退化珠光体的形成;提高轧制压缩比有利于原奥氏体晶内蜂窝状铁素体的形成,该铁素体均匀的分割原奥氏体晶粒,与晶界铁素体具有相同的方向性;MnS或其复合夹杂物是铁素体在原奥氏体晶粒内部形核的有效位置。