Decomposition Of Ferrite Research Articles

Zinc Ferrite Decomposition by Sodium Carbonate and Calcium Oxide at Roasting of Steelmaking Dust

When re-melting scrap in electric arc furnaces (EAF), dust is formed. This dust is a rich zinc-containing raw material. The composition of the EAF-dust from JSC “Pervouralsk New Pipe Plant” was investigated. Dust contains 18 wt.% of zinc, more than 1/3 of zinc is a part of the slightly soluble in acids and alkalis of zinc ferrite ZnFe2O4. The aim of the work is to study the effect of roasting temperatures of 700, 850 and 1000 °C and the effect of CaO and Na2CO3 additives on the decomposition of zinc ferrite. Thermodynamic analysis of chemical transformations in dust during roasting showed that the additions of CaO and Na2CO3 provide destruction of zinc ferrite and promote the transfer of zinc into the acid-soluble form of ZnO. The effect of the roasting temperature, the duration of roasting and the amount of calcium oxide and sodium carbonate additions on the degree of zinc transition into acid-soluble form was studied experimentally. The best results were obtained at 1000 oС. The increase in the roasting time in the interval from 1 to 3 hours promotes an increase in the concentration of acid-soluble zinc in the cinder. The combined additions of 36 wt.% of Na2CO3 and 19 wt.% of CaO above the dust mass are optimal. The proportion of acid-soluble zinc in the cinder increased to 97–99 %, as a result of roasting under optimal conditions.

Microstructural and Corrosion Properties of Cold Rolled Laser Welded UNS S32750 Duplex Stainless Steel

The main goal of this work was to study the effect of plastic deformation on weldability of duplex stainless steel (DSS). It is well known that plastic deformation prior to thermal cycles can enhance secondary phase precipitation in DSS which can lead to significant change of the ferrite-austenite phase ratio. From this point of view one of the most important phase transformation in DSS is the eutectoid decomposition of ferrite. Duplex stainless steels (DSSs) are a category of stainless steels which are employed in all kinds of applications where high strength and excellent corrosion resistance are both required. This favorable combination of properties is provided by their biphasic microstructure, consisting of ferrite and austenite in approximately equal volume fractions. Nevertheless, these materials may suffer from several microstructural transformations if they undergo heat treatments, welding processes or thermal cycles. These transformations modify the balanced phase ratio, compromising the corrosion and mechanical properties of the material. In this paper, the microstructural stability as a consequence of heat history due to welding processes has been investigated for a super duplex stainless steel (SDSS) UNS S32750. During this work, the effects of laser beam welding on cold rolled UNS S32750 SDSS have been investigated. Samples have been cold rolled at different thickness reduction (ε = 9.6%, 21.1%, 29.6%, 39.4%, 49.5%, and 60.3%) and then welded using Nd:YAG laser. Optical and electronical microscopy, eddy’s current tests, microhardness tests, and critical pitting temperature tests have been performed on the welded samples to analyze the microstructure, ferrite content, hardness, and corrosion resistance. Results show that laser welded joints had a strongly unbalanced microstructure, mostly consisting of ferritic phase (~60%). Ferrite content decreases with increasing distance from the middle of the joint. The heat-affected zone (HAZ) was almost undetectable and no defects or secondary phases have been observed. Both hardness and corrosion susceptibility of the joints increase. Plastic deformation had no effects on microstructure, hardness or corrosion resistance of the joints, but resulted in higher hardness of the base material. Cold rolling process instead, influences the corrosion resistance of the base material.

Microstructural and magnetic investigations of duplex steel

In this work, the eutectoid decomposition of δ-ferrite was investigated in 2507 type super duplex stainless steel. Samples were cold rolled in different extents and then heat treated at different temperatures. The ferrite contents of the specimens were determined by magnetic measurements and electron backscatter diffraction. The microstructure was examined by optical microscope and electron backscatter diffraction. The results showed that the cold rolling before the heat treatment significantly increases the rate of eutectoid decomposition and decreases the starting temperature of the ferrite decomposition. The measured ferrite contents with various methods showed diverse results because of the different principles of the measuring methods.

Magnetic-field-induced phase separation via spinodal decomposition in epitaxial manganese ferrite thin films

ABSTRACTIn this study, we report about the occurrence of phase separation through spinodal decomposition (SD) in spinel manganese ferrite (Mn ferrite) thin films grown by Dynamic Aurora pulsed laser deposition. The driving force behind this SD in Mn ferrite films is considered to be an ion-impingement-enhanced diffusion that is induced by the application of magnetic field during film growth. The phase separation to Mn-rich and Fe-rich phases in Mn ferrite films is confirmed from the Bragg’s peak splitting and the appearance of the patterned checkerboard-like domain in the surface. In the cross-sectional microstructure analysis, the distribution of Mn and Fe-signals alternately changes along the lateral (x and y) directions, while it is almost homogeneous in the z-direction. The result suggests that columnar-type phase separation occurs by the up-hill diffusion only along the in-plane directions. The propagation of a quasi-sinusoidal compositional wave in the lateral directions is confirmed from spatially resolved chemical composition analysis, which strongly demonstrates the occurrence of phase separation via SD. It is also found that the composition of Mn-rich and Fe-rich phases in phase-separated Mn ferrite thin films deposited at higher growth temperature and in situ magnetic field does not depend on the corresponding average film composition.

Features of the Decomposition of Delta Ferrite in Nitrogen-Containing Austenitic Steels

Features of the decomposition of the ferromagnetic δ ferrite with the formation of nonferromagnetic constituents (γ and σ phases) at temperatures of 900–1200°C in a nitrogen-containing austenitic steel have been determined. The transformation of δ ferrite at high temperatures (1100–1200°C) occurs with the formation of Widmanstaatten austenitic crystals in the ferritic matrix. At lower temperatures (900–1000°C), the transformation of δ ferrite develops by discontinuous decomposition with the formation of colonies of alternating plates of paramagnetic γ and σ phases. In the course of formation of the nonferromagnetic state as a result of the decomposition of δ ferrite at 900–1000°C (for 1 h), a subgrain structure with an increased density of dislocations, which is typical of thermomechanical treatment, is retained in the steel previously strengthened by high-temperature deformation.

Long-Term Effects of Temperature Exposure on SLM 304L Stainless Steel

Austenitic stainless steel is extensively used in industries that operate at elevated temperatures. This work investigates the high-temperature microstructure stability as well as elevated-temperature properties of 304L stainless steel fabricated using the selective laser melting (SLM) process. Significant microstructural changes were seen after a 400°C aging process for as little as 25 h. This dramatic change in microstructure would not be expected based on the ferrite decomposition studied in conventional 304L materials. The as-built additively manufactured alloy has much faster kinetic response to heat treatment at 400°C. An investigation of the structures which occur, the kinetics of the various transformations, and the mechanical properties is presented. The impact of this on the application of SLM 304L is discussed.

THERMODYNAMIC ANALYSIS OF ZINC FERRITE DECOMPOSITION IN ELECTRIC ARC FURNACE DUST BY LIME

The paper studies the scientific basis of the pyrometallurgical treatment process for electric steelmaking dust containing zinc ferrites. Thermodynamic analysis of zinc ferrite decomposition by lime was performed. According to the calculated data analysis, dust requires adding at least 46 % of CaO to decompose more than 90 % of ZnFe2O4, and at least 60 % of CaO to decompose more than 95 % of ZnFe2O4. The calculation results were verified by the laboratory furnace experiments. Experimental dust calcination in air with lime added up to 60 % of dust mass at a temperature of 1000 °C and a holding time of 4 h confirmed that zinc ferrite is decomposed by calcium oxide with the formation of zinc oxide and dicalcium ferrite. In addition, 50 kg of sublimates per 1 ton of dust were obtained containing 29 % of lead and 15 % of zinc. Dust calcination with the addition of lime can be used to transform zinc from ferrite to a soluble oxide form. Intermediate products resulting from calcination can be used for zinc and lead recovery. After zinc leaching it is possible to obtain the iron-containing product applicable in ferrous metallurgy. The approach has a variety of technological advantages in comparison with the known Waelz process. In particular, calcination with lime requires lower temperature (1000 °C) than the known technology (1250 °C), it eliminates the second stage of Waelz treatment necessary to purify zinc oxide fed for leaching from halides, significantly reduces coke consumption and simplifies gas cleaning from dust due to the 6–8 times lower quantity of sublimates.

Thermodynamic Analysis of Zinc Ferrite Decomposition in Electric Arc Furnace Dust by Lime

Scientific basis of pyrometallurgical processing technology of dusts of electric steelmaking containing zinc ferrites was investigated. The thermodynamic analysis of the decomposition of zinc ferrite with lime was carried out. The analysis of calculated data has shown that, in order to decompose more than 90% ZnFe2O4, it is necessary to add no less than 46% CaO for dust, while to decompose more than 95% ZnFe2O4, no less than 60% CaO is necessary. The results of the calculation were verified experimentally using a laboratory furnace. The experimental calcination of dust in air with the addition of lime in an amount of 60% of the dust weight at 1000°C with a holding time of 4 h confirmed that the decomposition of zinc ferrite with calcium oxide with the formation of zinc oxide and dicalcium ferrite occurs. In addition, sublimates were also formed in an amount of 50 kg per 1 t of dust containing 29% of lead and 15% of zinc. Dust calcination with lime can be applied to transform zinc from ferrite into a soluble oxide form. Intermediate products for the recovery of zinc and lead can be obtained by the calcination. After zinc leaching, it is possible to obtain an iron-containing product, which can be used in ferrous metallurgy. This approach has a series of process advantages compared with the well-known Waelz technology. In particular, calcination with lime requires lower temperature (1000°C) than the known technology (1250°C), it eliminates the second stage of the Waelz treatment necessary to purify zinc oxide fed for leaching from halides, considerably reduces coke consumption, and significantly simplifies gas cleaning from dust due to a decrease in the amount of sublimates by a factor of 6–8.

Influence of Microstructure on the Corrosion Resistance of AISI 317L (UNS S31703)

The AISI 317L stainless steel is an austenitic grade with at least 3%Mo. Recently, this steel has replaced AISI 316L in many applications in chemical and petrochemical industries, due to its higher pitting corrosion resistance. The microstructure of the hot rolled and annealed material studied in this work consists of austenitic grains and 4.0% of delta ferrite (δ) with elongated islands morphology. This microstructure was modified by three processes: cold rolling with 87% of reduction, aging at 450oC for 400 h, and welding by gas tungsten arc welding (GTAW) process. The corrosion resistance was evaluated by anodic polarization tests (ASTM G-61) and critical pitting temperature tests (ASTM G-150). Cold rolling produced a microstructure consisting of elongated grains of austenite and martensite α', high dislocation density and the elongated islands of delta ferrite present in the annealed material. Welding produced a dendritic microstructure with 7.0% of delta ferrite and some σ precipitated in the ferrite. Finally, the aging at 450oC for 400 h provoked the decomposition of ferrite. The results show that these microstructural changes affected the pitting corrosion resistance, as detected by ASTM G-61 and ASTM G-150 tests.

Effect of Thermal Aging at Low Temperature on the Mechanical Properties and Corrosion Resistance of LDX 2404 Duplex Stainless Steel

It is well known that when duplex stainless steels (DSS) are subjected to temperatures ranging from 300 to 1000 °C they may undergo precipitation of several phases, which can seriously impair their mechanical properties and corrosion resistance. The present work studied the effect of thermal aging (up to 2000 h) at 475 °C on the corrosion and mechanical properties of the newly developed 2404 DSS. The evaluation was based on potentiodynamic polarization in 3.5% NaCl solution and on Charpy tests. The pitting corrosion resistance was found to decrease significantly with aging time at 475 °C. In addition, Charpy tests revealed that after 100 h of thermal aging the material becomes brittle due to the spinodal decomposition of ferrite.

The potential of spinodal ferrite decomposition for increasing the very high cycle fatigue strength of duplex stainless steel

Duplex stainless steels (DSS) have become candidate materials for structural applications, where conventional austenitic stainless steels fail due to very high cycle fatigue (VHCF) in combination with corrosive attack. It seems that DSS exhibit a fatigue limit, which can be attributed to the two-phase austenitic–ferritic microstructure. Ultrasonic VHCF testing revealed that the phase boundaries are efficient obstacles for the transmission of slip bands and microstructural fatigue cracks up to 109 cycles and even beyond. The barrier strength is determined by the misorientation relationship between neighbouring grains but also by the strength of the individual phases. By thermal treatment at 475°C, spinodal decomposition of the ferrite phase results in the formation of Cr-rich α′ precipitates. While during static loading these precipitates give rise to a loss in ductility (475°C embrittlement), it was shown that the HCF strength can be increased and that there is also a tendency towards a beneficial effect on the VHCF behaviour. A more detailed analysis of the local plasticity sites by means of atom probe tomography (APT) revealed a dissolution of the α′ precipitates within operated slip bands. The dissolution might be an indication for a local softening mechanism that limits the VHCF strengthening effect of spinodal decomposition.

Microstructural Evolution in 2101 Lean Duplex Stainless Steel During Low- and Intermediate-Temperature Aging

The microstructural evolution of a 2101 lean duplex stainless steel (DSS) during isothermal aging from room temperature to 470 °C was investigated using thermoelectric power (TEP) measurements to follow the kinetics, atom probe tomography, and transmission electron microscopy. Despite the low Ni, Cr, and Mo contents, the lean DSS was sensitive to α-α' phase separation and Ni-Mn-Si-Al-Cu clustering at intermediate temperatures. The time-temperature pairs characteristic of the early stages of ferrite decomposition were determined from the TEP kinetics. Considering their composition and locations, the clusters are most likely G phase precursors.

Thermal stability of the solvothermal-synthesized MnFe2O4 nanopowder

Manganese ferrite nanopowder was prepared by a new solvothermal method, using 1,2 propanediol as solvent and KOH as precipitant. The as-synthesized powder, by solvothermal treatment in autoclave at 195 °C, for 12 h, consisted of fine manganese ferrite nanoparticles. The further thermal treatment of the initial manganese ferrite powder to higher temperature resulted in manganese ferrite decomposition due to Mn(II) oxidation to Mn(III), as observed by X-ray diffraction. FT-IR spectroscopy has evidenced that the oxidation takes place even at 400 °C. The oxidation of Mn(II) to Mn(III) was studied by TG/DSC simultaneous thermal analysis. It was shown that Mn(II) oxidation takes place in a very small extent up to 400 °C. The main oxidation step occurs around 600 °C, when a clear mass gain is registered on TG curve, associated with a sharp exothermic effect on DSC curve. The exothermic effect is smaller in case of the powder annealed at 400 °C, confirming the superficial oxidation of Mn(II) up to 400 °C. In order to avoid Mn(II) oxidation, the powder obtained at 400 °C was further annealed at 800 °C in argon atmosphere, without degassing, when manganese ferrite MnFe2O4 was obtained as major crystalline phase (69 %). All manganese ferrite powders showed a superparamagnetic behavior, with maximum magnetization of 51 emu g−1 in case of the as-synthesized powder, characteristic of magnetic ferrite nanopowders.

Thermodynamic and Kinetic Studies for Intensifying Selective Decomposition of Zinc Ferrite

A novel method to intensify the selective decomposition of zinc ferrite by a roasting process including reduction and magnetization stages was proposed. The relevant thermodynamic analysis with HSC [enthalpy (H), entropy (S) and heat capacity (C)] Chemistry 5.0 and experimental research on a laboratory scale were investigated. The thermodynamic calculations show that increasing the temperature and the CO amount promote not only the decomposition of zinc ferrite but also the formation of wustite, which can be converted to magnetite using sufficient CO2 at 823 K. The experimental results indicate that the zinc ferrite was decomposed into zinc oxide and wustite by reduction roasting under a gas mixture of 20% CO, 20% CO2 and 60% N2 at 1023 K for 90 min, and the decomposition degree of zinc ferrite reached 94%. Then, the generated wustite was transformed into magnetite by magnetization roasting under CO2 atmosphere at 823 K for 75 min, after which the selective extraction of zinc from zinc ferrite could be well achieved by low acid leaching. Increasing temperature and time were conducive to the magnetization within low temperature range, but when the temperature was above 823 K the zinc ferrite could be regenerated.

Decomposition Kinetics of Ferrite in Isothermally Aged SAF 2507-Type Duplex Stainless Steel

Decomposition of the ferritic phase is studied in isothermally aged SAF 2507 superduplex stainless steel (SDSS) by means of different examination methods. The ferritic phase (δ) undergoes an eutectoid transformation into secondary austenite (γ2) and σ-phase between 650 and 1000 °C. Samples were treated at 900 °C because the incubation time of this transformation is the shortest at this temperature. In order to follow the microstructural changes, x-ray diffraction analysis (XRD), automated electron backscatter diffraction (EBSD), applied magnetic investigation [vibrating sample magnetometer (VSM)], micro-hardness tests, and differential thermal analysis (DTA) were used. The results of XRD and EBSD methods for phase quantification showed nearly the same amounts for all three phases. The results of applied magnetic investigation for the fraction of ferritic phase were also in good agreement with the corresponding results of XRD and EBSD methods. Decomposition of ferrite is similarly well-traceable on EBSD phase maps where the coherent ferritic areas gradually broke into pieces with increasing time of heat treatment. According to the EBSD measurements the σ-phase grains appeared and started to grow after 2 min aging time in the ferritic-austenitic matrix, usually on the boundaries of ferritic and austenitic grains. After 15 min treating time, the microstructure consisted of mainly σ- and austenitic (primary and secondary) phases with negligible amount of ferrite. Chemical composition of the σ-phase was measured by energy-dispersive x-ray spectroscopy (EDS) at different aging times. Activation energies of σ-phase precipitation and α′-phase formation were determined by the Kissinger plot, through DTA measurements; they are 243 and 261 kJ/mol, respectively. Using the results of phase quantifications, the Johnson-Mehl-Avrami equation was fitted.

Electric Arc Furnace Dust Treatment by the Combination of Pyro/Hydrometallurgical Processes

This project researched a mechanism of the decomposition of zinc ferrite (ZnFe2O4) to be zinc oxide (ZnO) and calcium ferrite (Ca2Fe2O5) by using calcium oxide (CaO) from lime as a reducing agent. Zinc ferrite was mostly found in electric arc furnace dust (EAFD), which is a by product from a steel making industry. The experiment was simple as following. The EAFD was mixed with lime in mole ratio of zinc ferrite and calcium oxide at 1:2, 1:3 and 1:4 and then compressed into pellets. Effects of mole ratio, temperatures and times on the decomposition were studied. The conditions used in these experiments were 800, 900, 1,000 and 1,100 oC for 2, 4 and 6 hours. The results showed that the suitable conditions for the decomposition of zinc ferrite by calcium oxide could be as low as 800 oC for only 2 hours with mole ratio of zinc ferrite from EAFD and calcium oxide from lime as 1:2. A solubility test was provided to processed samples at the conditions as 0.1 M hydrochloric acid (HCl) at 50 oC for 20 minutes. It was found that EAFD from pyrometallurgical treatment could be decomposed to be zinc oxide and the following by hydrometallurgical treatment could recovery high zinc yield from the remaining residue. However, EAFD treated by only hydrometallurgical process obtained lower zinc recovery due to the high stability of zinc ferrite in HCl and most of the solutions.

Electric Arc Furnace Dust Treatment Process by Iron Powder

Electric arc furnace dust is a byproduct from steelmaking, contained up to 50%wt of zinc ferrite. It also contains about 10-20%wt of zinc oxide and the other metal oxides. Approximately 100,000 tons of EAF dust are generated per annum within Thailand. All of the dust has been sent to landfill. The objective of this research is to investigate the kinetics mechanism of the decomposition of zinc ferrite in EAF dust to obtain zinc oxide and iron oxide by using iron powder as a reducing agent. The process was carried out by mixing zinc ferrite or EAF dust with iron powder then compressed them in a mold. The samples from compression were treated by pyrometallurgical process. The factors required to be concerned were temperatures, particle sizes of iron powder, and mole ratios of zinc ferrite per iron powder, respectively. The treated samples were analyzed by X-ray diffraction (XRD) in order to characterize zinc ferrite phase transformation. It was found that the quantity of zinc ferrite, both either pure zinc ferrite and zinc ferrite in EAF dust, were decreased after treating by the pyrometallurgical process when increasing treating time from 30 to 180 minutes at 600°C, the particle size of iron powder at 10 micron, and the mole ratio of zinc ferrite per iron powder at 1:3. Moreover about 70%wt of zinc ferrite was decomposed and the products obtained were zinc oxides (ZnO) and iron oxides (Fe3O4, FeO, Fe2O3). The reduction of zinc ferrite with iron powder was well-defined taking place by diffusion thru product layer control. The activation energy of the reduction process was found to be 47.21±2.83 kJ/mol. Therefore, the decomposition of zinc ferrite by iron powder could be carried out at 1 atm with low pyrometallurgical temperature (600°C) and equipped with hydrometallurgical process to obtain high zinc yield.

The Decomposition of Zinc Ferrite in Electric Arc Furnace Dust by Carbon

The objective of this research is to study the mechanism of the decomposition of zinc ferrite to obtain crude zinc oxide (ZnO) and iron oxide by using carbon (activated carbon) powder as the reducing agent. Zinc ferrite could be mostly found in electric arc furnace dust (EAFD). Carbon with a purity of 99.99 percent by weight was ground to be fine powder with an average size of 75 microns. Then the mixer between zinc ferrite with carbon powder and EAFD with carbon at the ratio of 1:1, 1:2 and 1:3 by mole was carried out. The samples were treated in the range of temperatures at 600, 700 and 800 °C in TGA. The treated samples were analyzed by XRD in order to identify the phase transformation of zinc ferrite. It was found that the quantity of zinc ferrite, both either pure zinc ferrite and the zinc ferrite in EAFD were decreased after treating by carbon when using treating time at 120 minutes with the temperatures of 600, 700 and 800 °C, and ratio of zinc ferrite with carbon powder at 1:3 by mole. Moreover about 70% of zinc ferrite was decomposed to be zinc oxide (ZnO) and iron oxides.

Kinetics of G-phase precipitation and spinodal decomposition in very long aged ferrite of a Mo-free duplex stainless steel

Evolution of spinodal decomposition and G-phase precipitation in ferrite of a thermally aged Mo-free duplex stainless steel was studied by Atom Probe Tomography (APT). Kinetics was compared to kinetics observed in ferrite of some Mo-bearing steels aged in similar conditions. This paper shows that formation of the G-phase particles proceeds via at least a two-step mechanism: enrichment of α/α′ inter-domains by G-former elements followed by formation of G-phase particles. As expected, G-phase precipitation is much less intense in the Mo-free steel than in Mo-bearing steels. The kinetic synergy observed in Mo-bearing steels between spinodal decomposition and G-phase precipitation is shown to also exist in Mo-free steel. Spinodal decomposition is less developed in the ferrite of the Mo-free steel investigated than in Mo-bearing steels: both the amplitude of the decomposition and the effective time exponent of the wavelength (0.06 versus 0.16) are much lower for the Mo-free steel. Neither the temperature of homogenisation nor quench effects or Ni and Mo contents could successfully explain the low time exponent of the spinodal decomposition observed in the Mo-free steel. The diffusion mechanisms could be at the origin of the different time exponents (diffusion along α/α′ interfaces or diffusion of small clusters).

Recovery of iron from lead-zinc metallurgical slags by bath smelting

The recovery of iron from iron sinking slag and lead smelter slag was investigated by desulfurization-reduction bath smelting. The effects of lead smelter slag (LSS) to iron sinking slag (ISS) mass ratio and temperature were investigated in desulfurization experiments. The X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses show that the optimum conditions are LSS:ISS of 3:7 and temperature of 1350°C. The composition of desulfurization products is mainly ZnFe2O4, and the desulfurization rate of 99.66% is obtained under optimum conditions. The thermogravimetric (TG) and differential scanning calorimeter (DSC) analyses demonstrate that reductant is necessary for decomposition and reduction of zinc ferrite in desulfurization product. The effects of reductant, temperature and feeding modes on iron enrichment were investigated in reduction experiments. The scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) determination show that the iron content of reduction product is up to 99.36% under optimum conditions of coke as reductant, reduction temperature of 1450°C and the feeding mode of premixing.