Ferrite Nucleation Research Articles

Microscopic observation of inclusions contributing to formation of acicular ferrite in steel weld metal

The strength and toughness in low carbon low alloy steel welds is improved by the refinement of microstructures resulting from the formation of acicular ferrite. The behaviour of the nucleation and growth of acicular ferrite has been extensively studied and the weld metal of refined acicular ferrite is practically used in industry. It is known that inclusions in weld metal strongly contribute to the nucleation of acicular ferrite. The present authors made dynamic observations of the nucleation and growth of acicular ferrite at inclusion sites using a high temperature laser scanning microscope. In the steel weld metal of a Ti–B system, the formation of acicular ferrite is reported to be controlled by the weight ratio of aluminium and oxygen (Al/O ratio) which is an index of oxygen potential after the completion of deoxidisation by aluminium. The proper control of the Al/O ratio results in the formation of MnAl2O4 oxide of a spinel structure whose capability of the acicular ferrite nucleation is governed by a specific lattice misfit between nucleated ferrite and this oxide. It is considered that the addition of titanium is essential because titanium assists the formation of MnAl2O4. 8 However, there are reports depicting that there is no specific lattice orientation between ferrite and oxides of a spinel structure. As to the role of titanium, the researchers reported that it brings about catalysis or acts as the first nuclei for acicular ferrite nucleation. In spite of the fact that the acicular ferrite production technology is practically used in industry, the mechanisms of the acicular ferrite nucleation have been not completely clarified yet and thus, the development of nucleation control technologies for a further improvement of weld metal toughness is hindered. In the present study, the inclusions were formed by varying Al/O ratios. Thus formed inclusions were directly sliced into thin foils and crystallographic analyses were performed using a transmission electron microscopy (TEM). The formation of a multiphase type of inclusions and the existence of titanium enriched layers on the interface between the inclusions and nucleated ferrite were clarified by an electron diffraction analysis and an energy disperse X-ray spectroscopy (EDS). Experimental

Effect of Vanadium on Development of Acicular Ferrite Microstructure in Low Carbon Steel

The influence of vanadium on the development of an acicular ferrite microstructure has been investigated in a low carbon steel. Optical and electron microscopy were carried out to identify the precipitates, inclusions and constituents of the acicular ferrite microstructures. By the addition of vanadium, the main microstructural constituent was changed from a side plate ferrite to an acicular ferrite. VCN precipitates, which were known to favor the nucleation of acicular ferrite, were formed on the (Mn,Si) oxide and MnS particles. The presence of vanadium in alloys suppressed the formation of a side plate ferrite and reduced the transformation of ferrite during an isothermal transformation. Nucleation of intragranular ferrite and a subsequent sympathetic nucleation of ferrite within austenite grains were favored in the vanadium containing steel and an acicular ferrite microstructure was developed.

Nucleation of Intragranular Ferrite on B1-Type Non-Metallic Inclusions

A nucleation of intragranular ferrite grains by B1-type non-metallic inclusions was investigated using electron microscopes. Intragranular ferrite grains surrounding a TiN particle is observed in Ti-containing steel. On the other hand, no intragranular ferrite nucleated on NbN and ZrN. It is deduced that the NbN and the ZrN particle can not promote a nucleation of an intragranular ferrite for their high interfacial energy with ferrite. The TiN particle which has the BN relationship with the intragranular ferrite is an inclusion that effectively promotes an intragranular ferrite nucleation. The acicular ferrite grains formed in the austenite grains were trigged by the intragranular ferrite grain(s) formed around TiN.

Ferrite Transformation during Deformation of Super-cooled Austenite

The research results achieved by the Korean national project, HIPERS-21, on the ferrite transformation during the deformation of super-cooled low carbon austenite were summarized. Fine ferrite grains formed during the deformation of austenite, i.e. dynamically. The rate of ferrite nucleation was estimated to be accelerated several hundred times by the deformation of austenite. However, the grain refinement could not be explained by the accumulated strain alone. The application of stress during the ferrite transformation is known to effectively weaken the orientation relationship of the ferrite grains with austenite, making the coalescence of the grain difficult during the growth of ferrite and effectively increasing the nominal ferrite nucleation rate. The application of the dynamic ferrite transformation to industrial hot rolling or plate milling was also summarized. The multi-pass rolling technique was introduced to produce a fine ferrite grain structure and controlled cooling was adopted to produce a multi-phase structure in order to improve the work hardening rate of fine grained steels.

Influence of V Precipitates on Acicular Ferrite Transformation Part 2: Transformation Kinetics

A combination of thermodynamic models as well as some physical metallurgical principles has been used to analyse the influence that V(C, N) precipitates have on the transformation kinetics of acicular ferrite by isothermal decomposition of austenite. Those precipitates were found absolutely necessary for the nucleation of acicular ferrite as reported in Part 1 of this work, now in Part 2, through the proper design of a heat treatment route we have studied the effect that those precipitates have on the kinetics of acicular ferrite transformation.

Intragranular Nucleation of Ferrite on Precipitates and Grain Refinement in a Hot Deformed V-microalloyed Steel

The intragranular nucleation of ferrite has been studied in a V microalloyed steel (C� 0.103; Mn� 1.463; V� 0.139; N� 0.0100% mass). Using torsion tests and applying the double deformation method known as “back extrapolation”, the recrystrallised fraction of austenite has been determined for several deformation temperatures and two strain values (0.20 and 0.35) and has been plotted as a function of time. Recrystallisation-precipitation-time-temperature (RPTT) diagrams have then been drawn. The RPTT diagrams depict precipitation kinetics as a function of the temperature and time and this information has been used to study the intragranular nucleation of ferrite, cooling specimens from programmed temperatures and moments for which the precipitated volume and the average precipitate size (determined by TEM) are known. The results have allowed us to determine the contribution of intragranular nucleation to ferritic grain refinement, which was approximately 20%.

Effect of inclusions on the formation of acicular ferrite in Ti-bearing non quenched-and-tempered steel

Nucleation of acicular ferrite and its influence factors in non quenched-and-tempered steel was studied by using TEM and thermodynamic calculation. The results show that the complex particles with a center made of Ti oxide, Al 2O 3, and silicate and an outside made of a small quantity of mixture of TiN and MnS are able to act as ferrite nucleation nuclei. The acicular ferrite percentage changes little with Ti. When the oxygen content was 80 ppm, the volume percentage of acicular ferrite decreased due to an increase in allotriomorphic ferrite. The larger the cooling rate and the shorter the incubation time, the finer the titanium oxide and the higher the nucleation ratio of acicular ferrite.

The Role of α/γ Orientation Relationships during Ferrite Nucleation in an Fe-Cr-Ni Alloy

The role of the α/γ orientation relationships during ferrite nucleation is investigated. EBSD measurements were performed on an especially developed high purity ternary iron-based alloy with 20 wt.% Cr and 12 wt.% Ni with both austenite and ferrite present at room temperature to measure the orientation relationship between the austenite and ferrite crystallites. The experimental results are compared to the nucleation models of Clemm and Fisher and Aaronson and co-workers.

Ferrite Formation during Slow Continuous Cooling in Steel

Ferrite formation during austenite decomposition in carbon-manganese steel is studied during slow continuous cooling by three-dimensional x-ray diffraction microscopy at a synchrotron source. The ferrite fraction and nucleation rate are measured simultaneously and independently in real time in the bulk of the specimen. Thermodynamic calculations involving both ortho- and paraequilibrium have been performed to determine the driving force for nucleation. From the experiments and thermodynamic calculations the activation energies are estimated for nucleation and the transfer of iron atoms across the interface of the cluster during ferrite nucleation in steel.

The Role of Austenite Grain Misorientation on Ferrite Nucleation in an Fe-Cr-Ni Alloy

The relationship between the misorientation of the austenite crystallites and the favoured sites for ferrite nucleation has been investigated. Ex-situ EBSD measurements were performed on an especially developed high purity ternary iron alloy with 20 wt.% Cr and 12 wt.% Ni with both austenite and ferrite present at room temperature to measure the misorientation between the austenite crystallites. The experimental results are compared to the nucleation models of Clemm and Fisher and Aaronson and co-workers.

Transformation behavior and microstructural characteristics of acicular ferrite in linepipe steels

Transformation behavior and morphological characteristics of acicular ferrite in linepipe steels were investigated through the use of a dilatometer and EBSD. The results show that the volume fraction of acicular ferrite increased with an increase in the amount of hot-deformation in the austenite non-recrystallization region because acicular ferrite is formed at nucleation sites such as dislocations within austenite grains. This study found that acicular ferrite is formed at approximately 600 °C, where the transformation behavior can be characterized as a two-stage reaction: (i) nucleation of acicular ferrite and (ii) formation of polygonal ferrite between acicular ferrite grains. EBSD analyses show that an acicular ferrite grain consists of several sub-units misoriented by 1–2° and that a set of adjacent acicular ferrite grains with crystallographic misorientation below 15° makes up the crystallographic packet.

Barrier-free heterogeneous grain nucleation in polycrystalline materials: The austenite to ferrite phase transformation in steel

The process of grain formation during structural solid-state phase transformations has commonly been analysed in terms of the classical nucleation theory. In polycrystalline materials a new phase generally forms at grain boundaries in the parent phase microstructure. Under certain conditions the net interfacial energy to form a new phase can be relatively small due to the release of grain boundary energy from the parent phase. The nucleation process is then governed by cluster dynamics. We propose a simple model to predict the cross-over between different heterogeneous nucleation regimes and apply it to recent synchrotron X-ray diffraction experiments on ferrite nucleation in steel.

A Microstructure Evolution Model for Hot Rolling of a Mo-TRIP Steel

A comprehensive study of microstructure evolution for a Mo-TRIP (transformation-induced-plasticity) steel under hot strip rolling conditions has been conducted. This investigation includes austenite grain growth during reheating, deformation behavior, and static recrystallization kinetics of austenite as well as the effect of cooling rate and austenite conditioning on the continuous cooling transformation (CCT) behavior. The physically based Kocks–Mecking model has been employed to describe the deformation behavior of austenite, while the Johnson–Mehl–Avrami–Kolmogorov (JMAK) approach has been used to predict static recrystallization, and an empirical equation has been formulated for the recrystallized austenite grain size. Ferrite transformation start is described by a model considering early growth of corner nucleated ferrite. The fraction of ferrite transformed from austenite during continuous cooling is predicted by the semiempirical JMAK approach in combination with Scheil’s equation of additivity. The effect of carbon enrichment on ferrite transformation kinetics is explicitly included in the model. In addition, a phenomenological model for the bainite formation has been proposed. Martensite transformation start is described by an empirical equation taking into account carbon enrichment of remaining austenite. Finally, the entire hot strip rolling and controlled cooling process have been simulated by hot torsion tests, and the optimum coiling temperatures for the formation of TRIP microstructures have been determined.

Study of Laboratorial Simulation of V-N Microalloyed CSP Steel

Gleeble 1500D simulation machine was utilized to laboratorially simulate the production process of V-N microalloyed CSP steel strips. The results show that, in the incipient period of continuous casting, almost all Ti precipitates from the steel as TiN. With decreased temperature of the casting slab, V starts to precipitate with TiN as nucleation core or nucleate independently. After soaking, most of homogeneously-nucleated V(C,N) tends to re-dissolve, and the rest, which is difficult to dissolve, starts to grow up, while V in (Ti,V)(C,N) dissolve partially and thus V content in these particles reduces. During tandem rolling process, V(C,N) precipitates from the steel again, homogeneously or attachingly, all of which plays a role of preventing austenite grain from growing up and facilitates ferrite nucleation, increasing transformation ratio to refine ferrite microstructure eventually.

Historical Aspects of Thermomechanical Processing for Steels

Thermomechanical processing (TMP) involves both thermal and mechanical treatments that define both product shape and microstructure/properties. Since the industrial revolution, machines of augmented power, size and precision have given rise to TMP that challenged explanation of the crystal mechanisms. In wrought iron, lamellar ferrite exhibited high transverse crack resistance due to fine slag stringers that as flux facilitated welding of puddled bars in forging of shafts or rolling of plates for bell-welding into pressure tight pipes; the substructure developed in the iron as working continued below 900°C strengthened it. Patenting of high C steel wire led to an optimum cold-drawn structure for outstanding strength and toughness. Hot forming technology, combined with the refining potential for austenite decomposition gave rise to controlled rolling for enhanced ferrite nucleation, ausforming to refine martensite and intercritical rolling to deform the ferrite or to spheroidize the carbides. Cold rolling and annealing have been scheduled to impart suitable strength, grain size, substructure and texture.

Microstructure Model for a Dual-Phase Steel

The microstructural evolution has been studied for hot rolling of a dual-phase steel with a lean C-Mn-Si chemistry. This study includes the investigation of austenite grain growth during reheating, constitutive behaviour and static recrystallization kinetics of austenite, and austenite decomposition during simulated run-out table cooling conditions. To develop and validate the microstructure models for these phenomena, experimental studies have been carried out in the laboratory using a Gleeble 3500 thermomechanical simulator. The hyperbolic sine relationship between flow stress and Zener-Hollomon parameter is employed to describe the constitutive behaviour. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory is used to predict the static recrystallization kinetics. Ferrite transformation start is described with an approach that considers early growth of corner nucleated ferrite. The fraction of ferrite transformed from austenite during continuous cooling is described using the JMAK approach in combination with the additivity rule. The ferrite grain size is quantified as a function of the transformation start temperature. The overall microstructure model has been validated based on a number of laboratory simulations of the entire hot strip rolling and controlled cooling process with an emphasis on industrially relevant run-out table cooling strategies.

Ferrite Nucleation Kinetics inside Austenite Grain

The kinetics of ferrite nucleation inside an austenite grain under strong super cooling conditions are proposed by considering dislocation lines as nucleation sites based on cluster dynamics. The elastic energies within spherical space, cylindrical space and ellipsoidal space on a dislocation line have been studied, and are recognized to be approximately proportional to the cluster radius. By applying the conventional nucleation and grain growth theory on the dislocation line, the free energy change induced by a new ferrite nuclear generation on the dislocation line has been evaluated, and the ferrite nucleation rate has been formulated by considering the deformation elastic energy. According to the proposed inner-grain nucleation theory, the ferrite nucleation phenomenon inside austenite grain can be evaluated mathematically with strong super cooling conditions.

Nucleation of Acicular Ferrite on Sulfide Inclusion during Rapid Solidification of Low Carbon Steel

A fine-grained microstructure yields the optimum combination of strength and toughness of steel. Refinement of the ferrite microstructure by formation of fine intra-granular acicular ferrite nucleated on inclusion has been caused wide interesting in weld metallurgy and other metallurgy fields. It is also important to refine the as-cast microstructure during strip casting and thin slab casting process in which the possibility of refining microstructure by rolling technology decreases. In present paper, the possibility to nucleate intra-granular acicular ferrite by sulfide inclusion in steel without special alloy elements during rapid solidification process has been investigated by optical microscopy, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The orientation of the acicular ferrite clusters are also investigated by Electron BackScattered Diffraction (EBSD) analysis. Oxide inclusion and sulfide inclusion are popular in the sample and only sulfide inclusions are observed effective to assistant acicular ferrite nucleation. The possible mechanisms of formation of acicular ferrite in present sample, those are manganese depleted zone and phosphorus rich zone around inclusion in matrix, are discussed in detail.

鋼溶接金属におけるアシキュラーフェライト生成に関与する介在物の微視的観察

To clarify the microstructural evolution in low carbon low alloy steel weld metal, the inclusions related to acicular ferrite formation were investigated from the crystallographic points of view.The inclusions of each sample were prepared by focused ion beam and observed by transmission electron microscope. Further more, compositions of inclusions were analyzed by energy dispersive spectroscopy.The inclusions related to acicular ferrite formation were multi-phase and they consisted of MnS, MnAl2O4 and amorphous. It was observed that these inclusions were enclosed by Ti concentrated area.Thus it could be concluded that the Ti concentrated area of the inclusion surface would play a role for the heterogeneous nucleation of acicular ferrite.

溶接金属におけるアシキュラーフェライト生成・成長のその場観察

Acicular ferrite is regarded as being the most desirable microstructural feature, from the strength and toughness point of view, in mild and low alloy steel weld metals. Recent evolution and diversity of mechanical property for base metal demand the same property to the weld. Thus, the understanding of the formation mechanism of acicular ferrite microstructure in weld is one of the essential issues for low alloy steel welds.We have suggested that high-temperature laser scanning confocal microscopy was suitable system to make clear the nucleation site of precipitate in rapid cooling process, such as welding. In the present work, the morphological development of acicular ferrite, was in-situ observed during weld cooling. The sample designed to form acicular ferrite microstructure was heated and cooled in infrared imaging furnace and the morphological developments were directly observed using laser scanning conforcal microscopy.The nucleation and growth at inclusion, sympathetic nucleation and impingement events of acicular ferrite were directly shown in high-time resolution.