Mechanism Of Ferrite Formation Research Articles

Control of Austenite Characteristics and Ferrite Formation Mechanism by Multiple-Cyclic Annealing in Quenching and Partitioning Steel

Control of Austenite Characteristics and Ferrite Formation Mechanism by Multiple-Cyclic Annealing in Quenching and Partitioning Steel

Examination of phase transformation kinetics during step quenching of dual phase steels

Employing step quenching method for producing dual phase (DP) steels, kinetics of austenite to ferrite phase transformation was studied. In this regard, a series of samples were austenitized at 960 °C for 600 s. These specimens were then annealed at different intercritical temperatures, for different times between 0 and 7200 s, followed by water quenching. Detailed microstructural analysis was employed to study the variation of volume fraction as well as morphology of ferrite phase with intercritical annealing time. Moreover, mechanism of ferrite formation, by intercritical annealing treatment, was studied. It was seen that at initial annealing times a thin layer of allotriomorphic ferrite was formed at the primary austenite grain boundaries. Increasing the annealing time led to growth of this layer and formation of DP steel incorporating martensite islands distributed within polygonal ferrite matrix, after water quenching. Employing Johnson-Mehl-Avrami-Kolmogorov approach, a model was then successfully developed to predict formation of ferrite during intercritical annealing treatment. The Avrami exponent was found to be 0.56, indicating that the austenite to ferrite phase transformation, within the intercritical temperature region, was diffusion controlled, governed by one-dimensional growth.

Structure and magnetic properties of Ni0.64Zn0.36Fe2O4 nanoparticles synthesized by high-energy milling and subsequent heat treatment

High energy ball milling and subsequent annealing were applied to synthesize nanocrystalline Ni0.64Zn0.36Fe2O4 ferrite from a powder mixture of pure metal Zn, Fe2O3 and NiO in an oxygen atmosphere. The structural and phase evolution of powder particles after different milling times were studied by X-ray diffractometry. The XRD results showed that a Ni–Zn ferrite was formed with some residual Fe2O3 by annealing a 30-h-milled sample at as low as 400 °C for 2 h. The average crystallite size of the 30 h-milled powder was estimated to be about 15 nm which grew to 21 nm after annealing at 500 °C for 2 h. TEM image showed an agglomerated state of particles for 30 h-milled powders. FT-IR analysis indicated two absorption bands in the Ni–Zn ferrite structure related to octahedral and tetrahedral sites, respectively, in the range of 400–600 cm−1. Thermogravimetric analysis showed a mass loss about 2 % for as-received powder mixture below 400 °C; after that, it was almost stable. The Ni–Zn ferrite formation mechanism was detected to be in three stages: oxidation of zinc, diffusion of ZnO in Fe2O3 and the formation of ZnFe2O4, and diffusion of NiO in ZnFe2O4 and the formation of Ni–Zn ferrite. Vibrating sample magnetometery results revealed that a saturation magnetization of the 30 h-milled sample was about 5 emu/g which increased to 16 emu/g after annealing at 400 °C due to a reduction in density of lattice imperfections and strain.

Molten Salt Synthesis of Bismuth Ferrite Nano‐ and Microcrystals and their Structural Characterization

Bismuth ferrite nano‐ and microcrystals were prepared by a facile molten salt technique in two kinds of molten‐salt‐based systems (NaCl–KCl and NaCl–Na2SO4). In the NaCl–KCl salt system, a systematic study indicating the effects of process parameters (e.g., calcination temperature, holding time as well as the molten salt ratios) on the bismuth ferrite formation mechanism and structural characteristics is reported. The results show that almost pure phase BiFeO3 powders with minimum impurity phase of Bi2Fe4O9 were synthesized at temperatures of 700°C–800°C, whereas high calcination temperature (e.g., 900°C) resulted in the formation of almost pure phase Bi2Fe4O9 powders. The prolonged holding time increased the particle size via the Ostwald ripening mechanism; however, there was little effect on the particle morphology. Similar phenomenon occurred as increasing the molten salt ratios. In the NaCl–Na2SO4 salt systems, it is found that low NP‐9 (nonylphenyl ether, NP‐9) surfactant content (0–5 mL) led to the formation of almost pure phase BiFeO3 powders, whereas high NP‐9 surfactant content (e.g., 20 mL) resulted in pure phase Bi2Fe4O9 powders. The average particle size of the BiFeO3 powders was decreased as increasing the NP‐9 surfactant content, whereas their morphologies did not change significantly. Because of the simplicity and versatility of the approach used, it is expected that this methodology can be generalized to the large‐scale preparation of other important transitional metal oxides with controllable sizes and shapes.

Ferrite Formation Mechanism in Boron Contained Pearlitic Spheroidal Graphite Cast Iron

Ferrite Formation Mechanism in Boron Contained Pearlitic Spheroidal Graphite Cast Iron

Intragranular Ferrite Formation Mechanism and Mechanical Properties of Non-quenched-and-tempered Medium Carbon Steels

The type and size distribution of inclusions in non quenched-and-tempered medium carbon steels were investigated quantitatively. The effect of Ti containing complex inclusions on the formation of intragranular ferrite was studied. The continuous cooling transformation (CCT) diagrams of the tested steels were obtained for determining the cooling rate range of the formation of intragranular ferrite. The mechanical properties of the tested steels were determined at room temperature. The results show that with increasing Ti content, the fraction of Ti containing complex inclusions, which could act as nuclei for intragranular ferrite, increased and the inclusion size became smaller. In the cooling rate range of 0.5~2.5°C/s, plenty of intragranular ferrite formed, while at the rate of 2°C/s, the microstructure was mainly acicular ferrite. With the formation of intragranular ferrite, the toughness was enhanced by about 50% keeping the same strength level for the studied steels.

Kinetics and mechanism of nickel ferrite formation under high temperature ultrasonic treatment

The effect of simultaneous ultrasonic and thermal treatment on kinetics and mechanism of nickel ferrite formation was studied. It was established that sonication leads to notable increase of the mean rate of this reaction and decrease of effective activation energy from 190 ± 5 to 125 ± 7 kJ/mol. XRD data show that ultrasonic treatment significantly affects the microstructure of both initial reagent (Fe 2O 3) and reaction product (NiFe 2O 4) thus promoting formation of well developed reaction zone. A general model of ferrite formation mechanism under high temperature ultrasonic treatment was proposed.

Synthesis of 3BaO·2CoO·12Fe 2O 3 powder using chemical coprecipitation

The titration order effect on the 3BaO·2CoO·12Fe 2O 3 (Co 2Z) ferrite formation mechanism is obtained using the coprecipitation method. This process is investigated using thermogravimetry (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and superconducting quantum interference device (SQUID) magnetometer. It was found that Co 2Z phase prepared using coprecipitation was obtained by the reaction of two previous formed BaFe 12O 19 (BaM) and Ba 2Co 2Fe 12O 22 (Co 2Y) phases. For the sample obtained using back titration, the BaM phase was obtained directly by reacting BaCO 3 and amorphous iron hydroxide with no α-Fe 2O 3 and BaFe 2O 4 formed as intermediates. However, owing to the existence of coarsened α-Fe 2O 3 and BaFe 2O 4 in the calcined sample obtained using direct titration, the crystallite size of the formed BaM for the sample obtained using direct titration was larger than that by back titration. Consequently, the reactivity of the formed BaM and Co 2Y phases for the samples obtained using direct titration were lower than those by back titration due to the larger crystallite sizes, which led to the Co 2Z phase formation temperature for the sample obtained by direct titration having a higher temperature than that by back titration.

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 Widmanstatten 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 Widmanstatten 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.

Preparation and Properties of Barium Ferrite Using Hot‐Rolled Mill Scale

Commercial‐quality barium ferrite was made on a laboratory scale by the solid‐state reaction of raw hot‐rolled mill scale and barium carbonate. The quality of the isotropic magnets reached coercivity Hc 1750 Oe, remanence Br 2200 G, and maximum energy product (BH)max 1.0 × 106 G · Oe and that of the anisotropic magnets formed by dry powder die pressing Hc 2000 Oe, Br 3500 G, and (BH)max 2.6 × 106 G · Oe. The amount of the additives was in the range 0.4 to 0.8 wt%. The processing parameters were reported. The mechanism of ferrite formation was proposed.

The mechanism of ferrite formation from iron sulfides during zinc roasting

The formation of zinc ferrite (ZnFe2O4) during the fluidized-bed roasting of zinc concentrates presents subsequent processing difficulties both for zinc recovery and for iron separation and disposal. A major source of iron in these concentrates is from the iron sulfides — pyrite and pyrrhotite. This study examined the changes undergone by these iron minerals when roasted together with sphalerite at 1223 K in a fluidizing gas mixture of 3 pct oxygen and 97 pct nitrogen. Optical microscopy and electron microprobe analysis were employed to identify the three stages that lead to ferrite formation and to examine the processes that occur within each stage. The first stage is oxidation of the sulfides to highly vesicular, amorphous magnetite particles containing small amounts of zinc. The second stage involves both densification of these particles by sintering and counterdiffusion of iron and zinc cations to form a continuous phase of homogeneous zinc-rich spinel and a precipitate of hematite. In the third stage, continuation of cation diffusion and increasingPo 2 results in the formation of stoichiometric zinc ferrite. These observations have been interpreted by reference to the established phase relationships that occur in the Zn-Fe-O system, and a detailed, solid state reaction mechanism for the formation of zinc ferrite has been proposed.

Mechanism of ferrite formation in a mixture of metallic oxide powders

It has been established that the mechanism of formation of new crystalline structures can be studied by determining the variation of the most probable crystallite size as a function of the ratio between the amounts of the starting phases. It is shown that magnesium and nickel ferrites with spinel structures form on the bases of the thermally-more-stable magnesium and nickel oxide lattices, with preferential diffusion of iron cations. To the initial stages of formation of these ferrites there will correspond a nonstoichiometric solid solution of increased divalent cation concentration. The crystal lattices of such spinel solid solutions are deficient in oxygen and contain interstitial cations.