Decomposition Of Ferrite Research Articles

Magnetic and ageing behaviour of 7MoPLUS and the viability of monitoring ferrite decomposition using AC magnetic susceptibility

The objectives of this work were to assess the possibility of adopting 7MoPLUS as a new Feroplug/Sigmaplug alloy for temperature measurement and investigate the viability of using AC magnetic susceptibility to monitor ferrite decomposition. A series of 7MoPLUS samples were subjected to thermal ageing between 500 and 800 °C and the attendant changes in their microstructures and magnetic properties were studied by using optical microscopy, XRD, vibrating sample magnetometry (VSM) and AC susceptibility. 7MoPLUS can be used satisfactorily as a Feroplug/Sigmaplug alloy. AC magnetic susceptibility is a viable tool for monitoring ferrite decomposition and so it can serve as a complement to XRD and VSM for this purpose. Plastic deformation will enhance ferrite decomposition and thus the attendant changes in magnetic properties will be accelerated. Magnetic measurements and XRD have shown that cold rolling will not induce martensitic transformation in the austenite of 7MoPLUS. Extension of the use of the Feroplug and Sigmaplug techniques from long-term temperature measurement (several thousand hours) to intermediate-term temperature measurement (a few hundred hours) may be achievable by using plastically deformed 7MoPLUS.

Complex magnetic and microstructural investigation of duplex stainless steel

Heat treatment induced microstructural processes were studied by different non-destructive magnetic methods in the present work. A commercial SAF 2507 type superduplex stainless steel was investigated. This alloy contains about 40% ferrite and 60% austenite phases in its initial condition. The ferrite phase is not stable, therefore it can decompose to sigma phase and secondary austenite due to heat treatment. All the mechanical, corrosion resistance and magnetic properties are strongly influenced by the microstructural changes. This study had two aims: to understand better the kinetics of the ferrite decomposition process and to study the application possibilities of the applied magnetic measurements. The results, which were supported by electronmicroscopic method (electron back scattered diffraction, EBSD), help us to understand better the microstructural changes in duplex stainless steels caused by heat treatment.

Structure and properties of the heat-affected zone of duplex steels welded joints

Influence of the secondary austenite γ 2-precipitation in a heat-affected zone of welded joints of duplex steels, on hardness and corrosion resistance, have been analysed. The secondary austenite was created as a product of ferrite decomposition α → γ 2 as an effect of the welding thermal cycle and an aging following the welding process. Correlation of the thermal cycle, γ 2-phase morphology, kind of precipitations and hardness, as well as a corrosion resistance were described. Thermal cycle has a significant influence on the transformation α → γ 2, and morphology of the created γ 2-phases. The main mechanism of the secondary austenite creation is the diffusive transformation, as a result of which the Widmanstätten structure is formed. The increase of the γ 2-phase quantity in the tested steel heat-affected zone causes an essential influence of the joint hardness and corrosion resistance.

2相ステンレス鋳鋼中のフェライトの593 Kにおける相分離

A duplex stainless steel composed of 23% ferrite and austenite has been aged at 593 K. A change in the mean internal magnetic field of Fe in ferrite has been confirmed by Mössbauer spectroscopy with increasing aging time. The result is attributed to the compositional change owing to the phase decomposition of ferrite. A mottled structure characteristic of the spinodal decomposition was observed by TEM at the aging time of 180 ks(50 h). After aging 360 ks(100 h), precipitates were observed as black spot in the bright field images of TEM. The precipitate is interpreted as Cr-rich phase by HAADF-STEM observation. The volume fraction of the Cr-rich phase has been measured as a function of aging time by Mössbauer spectroscopy and TEM observation. Both result showed a fairly well agreement with each other. The decomposition fraction of ferrite y is expressed by a sigmoidal function, y=1-exp (-ktn) where n=0.67.

Effect of mechanical milling on the structural, magnetic and dielectric properties of coprecipitated ultrafine zinc ferrite

Nanosized ZnFe 2O 4 particles containing traces of α-Fe 2O 3 by intent were produced by low temperature chemical coprecipitation methods. These particles were subjected to high-energy ball milling. These were then characterised using X-ray diffraction, magnetisation and dielectric studies. The effect of milling on zinc ferrite particles have been studied with a view to ascertaining the anomalous behaviour of these materials in the nanoregime. X-ray diffraction and magnetisation studies carried out show that these particles are associated with strains and it is the surface effects that contribute to the magnetisation. Hematite percentage, probably due to decomposition of zinc ferrite, increases with milling. Dielectric behaviour of these particles is due to interfacial polarisation as proposed by Koops. Also the defects caused by the milling produce traps in the surface layer contributes to dielectric permittivity via spin polarised electron tunnelling between grains. The ionic mechanism is enhanced in dielectrics with the rise in temperature which results in the increase of dielectric permittivity with temperature.

Microstructure of Sputtered CoFe2O4 Film

The growth morphology of CoFe2O4 (Co ferrite) film deposited by the sputtering method was investigated at substrate temperatures of 330–870 K. Co ferrite films were grown on (001) MgO single crystals epitaxially. Results, however, showed that the increase of substrate temperatures leads to the formation of Co precipitates in a Co ferrite matrix. Careful inspection of X-ray diffraction patterns revealed that γ-Fe2O3 formed in the films deposited at the substrate temperature of 330 K, which indicates that decomposition of Co ferrite had took place during deposition. The crystal structures of Co in the matrix were the hcp and fcc structures at the substrate temperatures of 670 and 870 K, respectively. Co was elongated in a filament-like fashion in the direction of growth in the film deposited at 670 K, but had a particle-like appearance in the film deposited at 870 K. The growth morphology of Co in the Co ferrite matrix can be understood by the interfacial energy and the anisotropic structure of hcp Co.

Microstructure of Sputtered CoFe2O4 Film

The growth morphology of CoFe 2 O 4 (Co ferrite) film deposited by the sputtering method was investigated at substrate temperatures of 330-870 K. Co ferrite films were grown on (001) MgO single crystals epitaxially. Results, however, showed that the increase of substrate temperatures leads to the formation of Co precipitates in a Co ferrite matrix. Careful inspection of X-ray diffraction patterns revealed that γ-Fe 2 O 3 formed in the films deposited at the substrate temperature of 330 K, which indicates that decomposition of Co ferrite had took place during deposition. The crystal structures of Co in the matrix were the hcp and fcc structures at the substrate temperatures of 670 and 870 K, respectively. Co was elongated in a filament-like fashion in the direction of growth in the film deposited at 670 K, but had a particle-like appearance in the film deposited at 870 K. The growth morphology of Co in the Co ferrite matrix can be understood by the interfacial energy and the anisotropic structure of hcp Co.

Magnetic property changes during embrittlement of a duplex stainless steel

Changes in magnetic properties were used to investigate the ferrite decomposition that occurs in wrought duplex stainless steel (DSS) UNS S31803 at high (800°C) and low temperatures (475°C). At 800°C the saturation and residual induction, the coercive force and the differential permeability decrease with time, due to the increase in the austenite content. Firstly, secondary austenite (γ 2) forms in the α/α grain boundaries causing embrittlement, but not hardening. Ferrite then decomposes into σ and austenite phases (α→σ+γ 2), producing hardening and severe embrittlement. During embrittlement at 475°C, only a small increase in the coercivity was observed, even on aging up to 500 h. On the other hand, the Curie temperature increased with aging time, as a consequence of spinodal decomposition (α→α+α′). The results obtained show that the mechanical property changes of DSS UNS S31803 due to exposures at 475 and 800°C may be monitored by magnetic measurements.

EFTEM Elemental Mapping of α-α′ Interconnected Isotropic Nanostructures Formed by Spinodal Decomposition of Ferrite in Aged CF8 Cast Duplex Steel

Abstract Phase separation by spinodal decomposition of body centered cubic Fe-Cr binary alloys to form isotropic fully interconnected nanostructures of Fe-rich α and Cr-enriched α′ phases typically on a scale of 2 to 10 nm is scientifically intriguing and technologically important. In aged dual-phase steels, it is the decomposition of the ferrite to form such α-α′ composition modulations that is primarily responsible for the increased ferrite hardness that contributes to embrittlement. The α′ structures are isotropic since the lattice constants of the two phases are almost identical and consequently the strain energy is small. In other spinodal systems larger strain energy leads to triaxially modulated structures aligned along <100> in cubic materials. A review of TEM characterization of spinodal alloys revealed that aligned structures had received the most attention, because they are more common and their characterization by TEM is more tractable.

Mössbauer and atom probe studies on the ferrite decomposition in duplex stainless steels caused by the quenching rate

The structural and magnetic properties of duplex stainless steels have been studied by conversion electron Mössbauer spectrometry (CEMS) and atom probe. The results prove that the quenching rate Q is responsible for ferrite embrittlement of duplex stainless steels. This embrittlement, due to ferrite decomposition, increases when Q decreases. The Mössbauer spectrometry has been proved to be appropriate to study the early stages of the transformation. From the hyperfine field distribution of the very decomposed ferrite, the concentration of the Fe-enriched domains and the Cr-enriched domains have been estimated and compared with results obtained by atom probe.

Internal friction study of decomposition kinetics of SAF 2507 type duplex stainless steel

During the last decade, super duplex stainless steels (DSSs) with increased nitrogen content have been an object of intensive studies. Present work is devoted to the study of the peculiarities of {delta}-ferrite decomposition in SAF 2507 type duplex steel, and redistribution of nitrogen between ferrite and austenite phases in a wide temperature range by means of internal fraction (IF). Unlike local methods of electron microscopy or engineering methods of hardness or impact toughness testing, which give basically information on the formation of brittle intermetallic phases, the internal friction technique allows to study the state of solid solution and kinetics of changes in the relative amounts of ferrite and austenite phases during thermal treatment.

Evidence of pre-precipitation stages during cooling of X2CrNiMoCuN 25.06.03 superduplex steel

Owing to the existence of both high residual stresses and ferrite decomposition phases over a wide range of temperatures, the forging and quenching processes of a heavy section piece made of X2CrNiMoCuN 25.06.03 superduplex steel may produce cracks, failures and loss of mechanical properties. Investigations were conducted on the influence of various cooling rates (denoted as V700, cooling down rate at 700°C) between 200 and 52 000°Ch−1 on microstructure and impact strength. The impact strength is shown to drop by more than 180 J when V700 is varied through 52 000–200°Ch−1. As shown by TEM, conventional and 3D atom probes, the embrittlement is caused by the pre-precipitation stage and the precipitation of the intermetallic phases εCu and σ and not by the α′Cr phase as often reported. The chromium nitrides and the Cr–Mo carbides also observed, seem to have a minor effect. The pre-precipitation and precipitation sequences are clearly correlated to the cooling rate and may account for the variations in the mechanical integrity of thick products, equivalent in chemistry and thermal states.

Mössbauer study of the ferrite decomposition in unaged duplex stainless steels

Mechanical properties of duplex stainless steels degrade during holding in the temperature range 300--400 C. While no evolution is observed in the austenite, the microstructural evolution of the ferritic phase is the main cause of the embrittlement. Moessbauer spectroscopy is often used to analyze ferrite decomposition above 475 C because it can easily detect {alpha}{prime} domains since there are weakly or non magnetic, whereas {alpha} is magnetic. However, for duplex stainless steels containing both ferrite ({alpha} bcc) and austenite ({gamma} fcc), the paramagnetic {gamma} phase limits the detection of the {alpha}{prime} phase and thus prevents the study of the {alpha}-{alpha}{prime} decomposition. To overcome this problem a method to extract the austenite contribution from the experimental spectrum was developed. As a result, the contribution of {alpha} phase and thus prevents the study of the experimental spectrum was developed. As a result, the contribution of {alpha} phase in experimental spectra can be deduced and the evolution of the {alpha}-{alpha}{prime} decomposition followed. This paper demonstrates that Moessbauer spectroscopy can be extended to the analysis of industrial duplex steels, and that this technique is more accurate than atom probe in characterizing weakly decomposed ferrites.

Characterization of the intermetallic G-phase in an AISI 329 duplex stainless steel

Duplex austenite–ferrite stainless steels are susceptible to a variety of decomposition processes when aged within the intermediate range of temperatures (250–500 °C). One of these phenomena is the precipitation of the intermetallic G-phase. In the present investigation, the crystal structure and the chemical composition of the G-phase, precipitated in the ferritic phase of an AISI329 duplex stainless steel, is studied by electron microdiffraction and energy dispersive X-ray spectroscopy. It is determined that the space group of the G-phase is F 3 with a lattice parameter four times that of the ferritic matrix. The precipitation mechanism of the G-phase showed a synergetic relation with the ferrite decomposition in Cr-rich and Fe-rich domains. Based on the obtained results, the structural proximity of ferrite matrix and G-phase has been studied. Further analysis allows to suggest that the spinodal decomposition leads to an interdomain of a ferritic structure which is thermodynamically unstable and serves as a precursory site to the development of the G-phase by atomic position readjustments inferior to the atomic distances.

Surfactant-assisted ball milling of BaFe 12O 19 ferrite dispersion

A systematic study of the influence during mechanical processing of electrostatic interactions involving cationic and anionic surfactants on the surface and bulk properties of the complex oxide system barium ferrite is reported. Morphological examination by scanning electron microscopy shows a rapid decrease in powder particle size and homogenisation with milling time. In addition, from X-ray diffraction experiments changes in the crystal structure of the oxide were found. Barium ferrite decomposition (oxide phases separation) was linked with surfactant molecule type (ion and counterion) and oxide surface electrostatic potential. During room-temperature ball milling in aqueous dispersions a process of fast decomposition can be observed. For a cationic surfactant at pH 10 the milling terminates in BaCO 3 and magnetite Fe 3O 4 as main phases and the material remains magnetic. In the case of anionic surfactant type and the same pH value, the barium ferrite structure remains as the main phase, because surfactants do not absorb onto the oxide surface. On the basis of adsorption driven by electrostatic interactions it was possible to explain the effects of different surfactants on the milling product in dispersions with a low pH value.

A Mössbauer spectrometry study on the phase decomposition of cast duplex stainless steel

Cast CF3M duplex stainless steel having 25% of ferrite in volume fraction was aged at 723 K for time periods up to 10000 h. Phase decomposition of ferrite was investigated by Mossbauer spectroscopy. Ferrite decomposed initially via a spinodal process to finally yield the Fe-rich and the Cr-rich phase. The hyperfine magnetic field distribution obtained from the experimental Mossbauer spectrum was analyzed by assuming trinomial distribution of main constituent atoms Fe, Cr, Ni to determine Cr and Ni content of the Fe-rich phase. Main compositions of the phase were 84 at.% Fe, 11 at.% Cr, 5 at.% Ni.

Early stages of decomposition of ferrite in duplex stainless steel

The decomposition of a duplex stainless steel during aging at 600°C has been investigated using analytical transmission electron microscopy with the intention of elucidating the nucleation sites of the new phases arising from ferrite decomposition. The first transformation product to be formed is M23C6. This carbide, having a cube–cube orientation relationship with the parent γ grain, is found only on certain γ/α boundaries and its nucleation is shown to be strongly dependent on the γ boundary plane of the γ/α interface. Chromium depletion in the vicinity of γ/α boundaries caused by M23C6 precipitation initiates the α→γ transformation by migration of existing γ/α boundaries. Another mechanism for this bcc→fcc phase change is observed through the growth of Widmanstätten γ plates into α. Precipitation of σ begins after M23C6 formation has stopped. The existence of M23C6 on γ/α boundaries is crucial to σ formation because the majority of σ particles are found to nucleate on nodes in migrating γnew/α boundaries caused by M23C6 particle pinning. Another less common nucleation site for σ is migrating γnew/α boundaries. The σ particles nucleated on either of the sites have the Nenno orientation relationship with the parent γ. In the latter stages, the σ and new γ consume the α grains via a cellular growth mechanism.MST/1905

Early stages of decomposition of ferrite in duplex stainless steel

Early stages of decomposition of ferrite in duplex stainless steel

メスバウアー分光法によるFe-28Cr-5Ni合金中の2相分離相の組成決定

The effect of Ni on the internal magnetic field and the isomer shift of 57Fe in Fe-Cr alloys containing 3 to 14 at%Cr and 70 to 90 at%Cr were examined at room temperature by Mössbauer spectroscopy. A model Fe-28%Cr-5%Ni alloy was aged up to 5000 h at 723 K to investigate the phase decomposition of ferrite in a commercial duplex stainless steel. The alloy finally decomposed into a Cr-rich phase (α′) and an Fe-rich phase (α) after aging. A paramagnetic absorption peak due to the α′ phase has an isomer shift of −0.110 mm/s at room temperature. The value corresponds to a Cr content of 85(±1)at%Cr. The composition of the α phase was estimated to be 11(±1)at%Cr from the average internal magnetic field and the internal field distribution curve. The result of α phase was consistent with a result obtained by atom probe microanalysis. The latter underestimated the Cr content of the α′ phase in comparison with the Mössbauer effect.

Influence of compositional banding on grain anisotropy in 3CR12 steel

Abstract The microstructural evolution of a 12 wt-%Cr steel (3CR12) during cooling from a solution heat treatment at 1380°C has been investigated. Emphasis has been placed on determining the microstructural factors responsible for the persistent occurrence of grain anisotropy in this steel. Energy dispersive X-ray spectroscopy was used to determine the composition of the phases resulting from the solid state δ ferrite to austenite transformation. It is shown that the high temperature δ ferrite phase partially decomposes to austenite via a Widmanstatten growth mechanism and consequently a banded two phase structure is produced after hot rolling. The element partitioning, which arises during the solid state δ ferrite decomposition, leads to irreversible compositional banding. An empirical model is proposed for the events leading to the generation of the banded phase structure and the formation of an elongated ferritic microstructure in 3CR12 after subcritical annealing.MST/1500