High-silicon Iron Research Articles

Spheroidal graphite coalescence during thermal cycling in the ferritic domain of a high-silicon cast iron studied by optical microscopy and X-ray computed tomography

High-silicon cast irons remain ferritic at higher temperature than usual cast irons and present both better oxidation resistance and higher mechanical strength. Such a high-silicon spheroidal graphite cast iron was heat treated up to 50 h in total at 800 °C in two different manners: under isothermal and cyclic conditions. Under the isothermal condition, the distribution of graphite inclusions did not vary significantly. On the contrary, temperature cycling led to spectacular coalescence whose evolution was observed after 1000, 2000 and 3000 cycles. Quite unexpectedly, the largest graphite particles - that were growing because of coalescence - showed an irregular outer surface after 1000 cycles, which evolved in impressive dendritic instabilities and then large protuberances after 2000 and 3000 cycles. Optical microscopy and X-ray computed tomography (XCT) were used to quantitatively study these microstructural changes.

Mesoporous manganese silicate composite adsorbents synthesized from high-silicon iron ore tailing

Iron ore tailing (IOT) as a solid waste has attracted attention worldwide owing to the potential safety hazards and waste of resources. However, few studies focused on exploiting the structural characteristics of IOT. IOT is a special silicate mineral mixture composed of, for example, quartz, anorthite, microcline, and clinochlore which contain adjustable silica tetrahedron structures. In our research, Mn2+ was introduced to hydrothermally react with the adjustable silica tetrahedrons of IOT with the aid of sodium silicate, to provide IOT with reactive Mn-OH groups and high specific surface areas by constructing functional value-added mesoporous manganese silicate (MS) composites. It was found that the specific surface area (SBET) of IOT improved from 6.3 to 448.2 m2g−1 and achieved ideal pore size distributions after being transformed into MS composites. The composite obtained exhibited an excellent selective adsorption performance that could rapidly remove 100% methylene blue (MB) molecules from the mixed cationic dye solutions within 1min. The adsorption behavior was well described by pseudo second-order model and Freundlich isotherm model, indicating the chemical adsorption and multi-layer adsorption. The maximum adsorption capacity was up to 217mgg−1 (25°C, pH=7, adsorption time: 120min, MB initial concentration: 400mgL−1). Through investigation, it was found that the selective adsorptive ability is based on the hydrogen bonding and the suitable mesoporous structure for MB molecules.

Creep of the Matrix During Coalescence and Overgrowth of Graphite Precipitates in a High-Silicon Spheroidal Graphite Iron Submitted to Thermal Cycling in the Ferritic Domain

Thermal cycling of a high-silicon spheroidal graphite cast iron within the ferritic domain leads to coalescence of graphite particles with dendritic protuberances on the largest ones. This phenomenon occurs if there is some dissolution of graphite at the maximum temperature of the thermal cycle. Upon cooling, carbon redeposits in preferred locations, leading to the protuberances. This communication demonstrates that the ferritic matrix around the overgrowths can creep and remain in close contact with graphite.

Influence of Carbide-Promoting Elements on the Microstructure of High-Silicon Ductile Iron

Because of its low cost, steel scrap is one of the most important raw materials for the production of ductile iron (DI). The amount of carbide-promoting elements in steel scrap, such as chromium, manganese, molybdenum, niobium and vanadium, is expected to increase in the future. Most of these elements have a negative impact on the microstructure and mechanical properties of DI. The solubility of carbide-promoting elements in solid solution-strengthened DI materials, standardized in DIN EN 1563:2011, is modified by the high silicon content. For these new materials, the tolerance limits for carbide-promoting elements and their mutual influence must be known to ensure a sustainable production process. To investigate the individual and combined impact of carbide-promoting elements on the carbide content in high-silicon ductile iron EN-GJS-500-14, experimental investigations and thermodynamic–kinetic microstructure simulations were carried out. Microstructure was characterized using metallographic analysis, and quantitative relations between chemical composition and microstructure were developed by means of regression analysis. Besides this quantitative analysis, it was found that the formation of grain boundary carbides can be detected via thermal analysis. Furthermore, experiments and simulations showed that vanadium promotes the formation of chunky graphite in high-silicon DI castings.

Effect of Cooling Rate on Graphite Morphology and Mechanical Properties in High-Silicon Ductile Iron Castings

This paper investigates the effects of silicon content above 3 wt% and different cooling rates on the graphite morphology and the mechanical properties of ductile cast iron. To research and determine these effects, three grades of high-silicon ductile iron were cast with silicon content of 3 wt%, 3.5 wt% and 4.5 wt%. In order to investigate the effect of different cooling rates during solidification, four sections with different thicknesses were cast in the same mold, i.e., cube with wall thickness 180 mm and standard Y2, Y3 and Y4 keel blocks. Microstructure analysis and tensile test were performed on the as-cast samples. In the second step of analysis, detailed metallographic observations on specially prepared samples were performed in order to investigate the risk of graphite degeneracy appearance, especially chunky graphite formation due to high silicon content and its effect on mechanical properties. The results showed that graphite content, ferrite fraction, yield strength and tensile strength were increased by increasing the silicon content. On the other hand, elongation decreases with increasing silicon content. Also, based on the metallographic observations of the present work, the risk of chunky graphite formation increases with increase in silicon content and with decrease in the cooling rate. Chunky graphite formation affects negatively mechanical properties, especially elongation in ductile cast irons with silicon content higher than 3.5 wt%.

Effect of Bismuth on Preventing Chunky Graphite in High-Silicon Ductile Iron Castings

The effect of Bi level on the macro- and microstructure of thick-walled high-silicon ductile iron castings has been investigated, as well as the influence of chunky graphite on the mechanical properties. Influence of cooling rate on graphite morphology was also investigated. High silicon level has caused the formation of chunky graphite, in very high amounts, along the entire cross section of the casting. Microstructure reveals that the addition of Bi affects not only the reduction of chunky graphite amount, but also the distribution along the casting cross section which is not uniform any more as in the casting without Bi. When Bi is added, the zone with chunky graphite appears independently of the cooling rate. Positive effect of Bi on the reduction of chunky graphite has been shifted towards higher levels, from 0.006 to 0.01 wt% Bi. Higher levels of Bi are more efficient also for the improvement in spheroidal morphology of graphite. The addition of 0.01 wt% Bi gave the best results, while the Ce/Bi ratio amounted to 0.6. This paper has proven that Bi acts on the increase in the strength values also in the high-silicon ductile iron casting. According to microstructure observations, it can be concluded that eutectic cells of chunky graphite are formed during bulk eutectic reaction. First, the primary graphite nodules are precipitated, followed by the initial phase of eutectic reaction during which the austenite dendrites precipitate and grow encapsulating at the same time primary graphite nodules.

The performance of porous ceramsites in a biological aerated filter for organic wastewater treatment and simulation analysis

Porous ceramsites were prepared using the high silicon iron tailings as the matrix. The specific surface area, bulk density and porosity of the obtained porous ceramsites are 5.54 m2/g, 0.78 g/cm3 and 49.8 %, respectively. Subsequently, biofilm was inoculated and domesticated on the surface of ceramsites in a biological aerated filter (BAF) for the removal of organic pollution in water. The removal rates of chemical oxygen demand (COD), ammonia (NH3-N), and total phosphorus (T-P) in the organic wastewater were 91 %, 85 %, and 80 % respectively. It was found out that during the maturation diffusion period, the pore size between ceramsites had a significant impact on the biofilm adhesion growth and the effect of degradation organic matter. In addition, in order to better understand the removal mechanism of biofilm for COD, NH3-N and T-P, an analytical model for the flow field and resistance characteristics between pores of biofilm was established, which may provide more valuable references for the practical utilization of porous ceramsites.

Technical and environmental assessment of the incorporation of iron ore tailings in construction clay bricks

The high production of iron ore in Brazil necessarily leads to the generation of large amounts of waste from the beneficiation stage. The storage of this material in dams is associated not only to environmental problems, but also with the risks related to the safety of the population. The aim of this research was to study a new alternative for reusing high-silicon iron ore tailings, applying them in the red clay ceramic industry. For this, a mixture design of experiments was developed, using three components - the iron ore tailings and two clayey materials – with the respective incorporation ranges: 0%–40% and 30%–70% (by mass). The raw materials were characterized physically, chemically, mineralogically and morphologically. Subsequently, ten mixtures obtained from the experimental design were prepared, and cylindrical specimens were formed by uniaxial pressing. After the firing at 850 °C, 950 °C and 1050 °C, the properties of firing linear shrinkage, bulk density, apparent porosity, water absorption, compressive strength and microstructure behavior of the specimens were assessed. Both the mixture design of experiments and desirability function enabled the determination of an optimal composition which contains 29.1% (by mass) of tailings and which meets the international standards. Next, small-scale construction bricks were produced based on this ideal mixture. The specimens were characterized physically, mechanically and environmentally. The brick has presented 20.94% of water absorption and compressive strength equal to 4.27 MPa, proving its potential to be used in sealing masonry. Besides, its environmental behavior has indicated that the material can be classified as non-hazardous and non-inert, and it is not aggressive to the environment or human health both during and after the useful life. This study has shown that the proposed brick presents advantages from an environmental, economic and technical points of view, contributing to sustainability in the industrial sector and civil construction.

Some paradoxical observations about spheroidal graphite degeneracy

Differential thermal analysis experiments have been performed on samples machined from Y2-blocks cast with different high-silicon spheroidal graphite irons. Depending on magnesium, silicon, cerium and antimony content, the as-cast microstructure showed various levels of chunky graphite in the central part of the blocks. In contrast, the microstructure of the materials after remelting and resolidification during differential thermal analysis consisted of lamellar or compacted graphite. The formation of chunky graphite in the as-cast microstructure is rationalized using an index or silicon equivalent that has been recently suggested. The differences in the microstructures after differential thermal analysis are discussed in terms of available free magnesium. Emphasis is finally put on the striking differences in characteristic size of the microstructures made of compacted graphite as compared to lamellar graphite and chunky graphite. This leads to tentative conclusions about growth of compacted and chunky graphite which would be worthy of further experimental investigations.

Microstructure of As-Cast High-Silicon Ductile Iron Produced via Permanent Mold Casting

Permanent mold casting of ductile iron is an environmentally friendly alternative to sand mold casting. The high cooling rate leads to a homogeneous microstructure with low grain size, but can also lead to cementite formation. In the work presented, the as-cast microstructure of solid solution-strengthened ductile iron (SSDI) cast in permanent mold made of gray iron is analyzed. The high silicon content in SSDI decreases chilling tendency and promotes the formation of ferrite. Silicon contents in the range 3.2–4.3 wt% and mold preheating temperatures in the range 300–450 °C were analyzed for section thicknesses of 10, 20 and 30 mm. Solidification time and cooling rate at the casting sections analyzed were measured. The results show that an as-cast microstructure free from carbides is achieved in sections of 20 and 30 mm for all castings analyzed, whereas for 10 mm a partial chilled microstructure is produced. As expected, the pearlite content decreases with increasing silicon content and increasing preheating temperature. A higher cooling rate increases eutectoid undercooling and thus promotes pearlite formation. From the experiments carried out, a quantitative correlation is deduced for the pearlite content as a function of eutectoid undercooling and Si content. The nodule count is approximated as a linear dependency on solidification time.

Elevated temperature thermal conductivities of some as-cast and austempered cast irons

ABSTRACT The aim of this study was to provide insight on thermal conductivity of three cast iron groups, namely lamellar, compacted and spheroidal graphite irons at elevated temperatures up to 673 K (400°C) in as-cast and austempered states. Austempering treatments increased mechanical properties of all the studied materials while decreasing thermal conductivity across the line. The effects of austempering on conductivity were lower for grey and compacted graphite iron than for spheroidal graphite irons. The results indicate that heat treating can be a viable option in increasing cast iron performance in thermally stressed applications. One ferritic low-silicon spheroidal graphite iron surpassed lamellar graphite iron in conductivity at elevated temperatures, while high-silicon spheroidal graphite irons exhibited low conductivities.

Study of dimensional change of high-silicon ductile iron with ADI and Dual-Phase-ADI microstructures starting from different as-cast structures

In this work the percent dimensional change (DC%) and its dispersion in high silicon Ductile Iron with ADI and Dual-Phase-ADI structures obtained through heat treatment starting from different as-cast microstructures, was studied. Specimens were obtained from three different melts with Si content ranging from 3.1 to 4.2%. Dual-Phase-ADI structures were obtained using special heat treatment, starting from as-cast structures. Regarding dimensional change, DC% in ADI and Dual-Phase-ADI decrease as Si% and the proportion of ferrite phase in as-cast specimens increased. On the other hand, DC% increased with austenitizing temperature and ausferrite content in the final microstructure. A fuzzy model to predict DC% in ADI parts was adapted to include DC% prediction in Dual-Phase-ADI. The fuzzy model allowed to predict the DC% for ADI and Dual-Phase-ADI parts with an root mean square <0.05%.

On the thermal conductivity of CGI and SGI cast irons

The thermal conductivity of Compacted Graphite Iron (CGI) and spheroidal graphite iron (SGI) was established in the temperature range from room temperature up to 500 °C using the experimental thermal diffusivity, density and specific heat values. The influence of nodularity, graphite amount, silicon content and temperature on the thermal conductivity of fully ferritic high-silicon cast irons was investigated. It was found that the CGI materials showed higher thermal conductivity than the SGI materials. The thermal conductivity tended to increase with increasing temperature until it reached a maximum followed by a subsequent decrease as temperature was increased up to 500 °C. Conventional models were applied to estimate thermal conductivity and the predictive accuracy of each model was evaluated. The thermal conductivity could be estimated by the Helsing model. The Maxwell model, Bruggeman model and Hashin–Shtrikman model were also in fair agreement using the thermal conductivity value of graphite parallel to the basal planes in graphite.

Effects of CaO and Na2CO3 on the reduction of high silicon iron ores

The effects of CaO and Na2CO3 on the reduction of high silicon iron ores at 1 250 °C were studied. The experimental results showed that the metallization rate was significantly hindered by the addition of CaO and Na2CO3, particularly at the early stage of roasting, compared to the rate without additives. In the absence of additives, iron oxides were quickly reduced to metallic iron, and fayalite was difficult to form. When CaO and Na2CO3 were added, the low reducible iron-containing silicate compounds formed and melted, subsequently retarding the metallization process. The inhibition of Na2CO3 was more noticeable than that of CaO, and higher Na2CO3 doses resulted in stronger inhibition of the increased metallization rate. However, when Na2CO3 was added prior to CaO, the liquid phase formed, which facilitated the growth of the metallic phase. To reinforce the separation of the metallic phase and slag, an appropriate amount of liquid phase generated during the reduction is necessary. It was shown that when 10% CaO and 10% Na2CO3 were added, a high metallization rate and larger metallic iron particles were obtained, thus further decreasing the required Na2CO3 dosage.

Magnetic properties characterization of shear-textured 4 wt% Si electrical steel sheet

Abstract

Test research on the effects of mechanochemically activated iron tailings on the compressive strength of concrete

High-silicon iron tailings, which have been mechanochemically activated, was used for the preparation of concrete as a supplementary cementing material to substitute cement (by 10%, 20%, 30% and 40% respectively) based on their composition, particle sizes and pozzolanic activity. With the aid of tests, this paper discusses the effects of the tailings on the compressive strength of concrete. Tests show that with an increase in substitution rate of cement in the tailings, the compressive strength of concrete tends to decrease, and when the substitution rate is at 10%, 20% and 30% respectively, the compressive strength of concrete is measured up to the design requirement; if an appropriate amount of water reducing admixture is added while cement is substituted by tailings with a substitution rate of 10%, 20%, 30% and 40% respectively, the compressive strength of concrete is also measured up to the design requirement. The research result demonstrates that as far as the compressive strength concerned, it is feasible to use mechanochemically activated high-silicon iron tailings as a supplementary cementing material to partly substitute cement in concrete.

Texture Development in High-Silicon Iron Sheet Produced by Simple Shear Deformation

Sheet processing of high Si-Fe alloys (up to 6.5 wt pct Si) is demonstrated by application of highly confined shear deformation in cutting-extrusion. This alloy system, of major interest to electromagnetic applications, is characterized by poor workability. By a suitable interactive combination of simple shear, high strain rates, near-adiabatic heating, and large hydrostatic pressure in the deformation zone, flow localization, and cracking inherent to this alloy system are suppressed. This enables creation of sheet and foil forms from bulk ingots, cast or wrought, in a single deformation step, unlike rolling. The sheet is characterized by strong shear textures, described by partial {110} and 〈111〉 fibers, and fine-grained microstructures (~20 µm grain size). The orientation (inclination) of these fibers, with respect to the sheet surface, can be varied over a range of 35 deg through selection of the deformation path. In contrast to rolling textures, the current shear deformation textures are negligibly influenced by recrystallization annealing. A recovery-based continuous recrystallization mechanism is proposed to explain the texture retention. Some general implications for shear-based processing of alloys of limited workability are discussed.

Development of High-Silicon Eutectic Graphite Iron with Sperior Machinability

Development of High-Silicon Eutectic Graphite Iron with Sperior Machinability

Corrosion resistance of structural materials in high-temperature aqueous sulfuric acids in thermochemical water-splitting iodine–sulfur process

Very harsh environments exist in the iodine–sulfur process for hydrogen production. Structural materials for sulfuric acid vaporizers and concentrators are exposed to high-temperature corrosive environments. Immersion tests were carried out to evaluate the corrosion resistance of ceramics and to evaluate corrosion-resistant metals exposed to environments of aqueous sulfuric acids at temperatures of 320, 380, and 460 °C, and pressure of 2 MPa. The aqueous sulfuric acid concentrations for the temperatures were 75, 85, and 95 wt%, respectively. Ceramic specimens of silicon carbides (SiC), silicon-impregnated silicon carbides (Si–SiC), and silicon nitrides (Si3N4) showed excellent corrosion resistance from weight loss measurements after exposure to 75, 85, and 95 wt% sulfuric acid. High-silicon irons with silicon content of 20 wt% showed a fair measure of corrosion resistance. However, evidence of crack formation was detected via microscopy. Silicon enriched steels severely suffered from uniform corrosion with a corrosion rate in 95 wt% sulfuric acid of approximately 1 g m−2 h−1. Among the tested materials, the ceramics SiC, Si–SiC, and Si3N4 were found to be suitable candidates for structural materials in direct contact with the considered environments.

Microstructural characterization of high-silicon iron alloys produced by spray forming and co-injection of Si particles

Fe–Si alloys are widely used for magnetic applications. However, it is very difficult to process Fe–Si with a silicon content exceeding 3.5 wt.%Si (upper limit for products commercially available by the I/M route) due to the alloy's low ductility, which is attributed mainly to the formation of B2 and DO3 ordered phases that embrittle the material. To overcome this obstacle, the main focus of this work was to produce thin sheets of Fe–5 wt.%Si alloy in two steps: (1) as a Fe–3.5%Si + 3%Si p (Si particles) composite, using spray forming, and (2) rolling and heat treating (HT) the composite to dissolve the silicon and homogenize its content throughout the thickness of the sheet. To this end, 3 wt.%Si p were co-injected into the main stream of the Fe–3.5 wt.%Si spray, followed by hot-rolling of the billet at 850 °C to obtain 0.45 mm gauge thin sheets. The final material was heat-treated at 780/510 °C for 8 h or at 1250 °C for 1 h and then air cooled. The grain orientation was analyzed by EBSD and the distribution of iron, silicon and impurities was identified by X-ray dot mapping. The heat treatment caused diffusion and dissolution of the silicon particles and grain growth. However, the final silicon content was strongly dependent on the atmosphere of the heat treatment furnace. In the absence of oxygen, the silicon content reached 4.9 wt.% distributed homogeneously throughout the thickness of the composite. In the presence of oxygen, the silicon diffused to the surface and only 3.5 wt.% remained in the matrix.