The kinetics of carbide precipitation in silicon-aluminum steels

The kinetics of carbide precipitation in a fully processed 2.3 wt Pct silicon, 0.66 wt Pct aluminum electrical steel with carbon contents of 0.005 to 0.016 wt Pct were investigated over the temperature range from 150 to 760 °C and times from 30 seconds to 240 hours. The size, morphology, and distribution of the carbide phases, as functions of aging time and temperature, were determined by optical and transmission electron microscopy. The 1.5T core loss was also evaluated and correlated with the changes in precipitation. Distinct C curves were observed for the formation of grain-boundary cementite at temperatures above 350 °C and a transition carbide ({100} _α habit plane) at temperatures below 350 °C. Grain-boundary cementite had a relatively small effect on core loss. The large increases in core loss that accompanied transition carbide precipitation peaked at specific aging temperatures depending on the carbon content of the steel. Once a transition carbide dispersion was initially established at a given aging temperature, particle coarsening and core loss changes were generally insensitive to aging time. The influence of a combined addition of silicon and aluminum on the solubility of cementite and the transition carbide in iron was estimated and discussed. This paper is based on a presentation made at the symposium “Physical Metallurgy of Electrical Steels” held at the 1985 annual AIME meeting in New York on February 24–28, 1985, under the auspices of the TMS Ferrous Metallurgy Committee.     

The composition and structure of chromite during reduction with carbon

Examination of grains of chromite, subjected to varying degrees of reduction by heating chromite-graphite mixtures, using a scanning electron microscope and electronprobe microanalyser. Initially reduction of iron from chromite to form metallic iron, with a small chromium content, at the surface. Iron diffusion from within the grains to the surface with continued reduction and contraction of the core of unaltered chromite. Reduction of chromium as the main reaction when the core has disappeared and observation of a second skrinking core, this time of chromium. Increase of the chromium content of the metal phase and forming of the carbide (Cr, Fe)₇C₃ during this period. Reduction within the grains when the chromium core has disappeared. MgAl₂O₄ containing up to 6 percent chromium as final residue of the original chromite for the experimental conditions.     

Recovery of Chromium from AOD‐Converter Slags

Slags from the production of high‐alloyed steel contain both chemically bound chromium (mainly as Cr₂O₃) in the mineral fraction and elemental chromium in the metallic remainders. Thermochemical post treatment of the slag in an electric arc furnace under reducing conditions enables the nearly complete recovery of the total amount of chromium in form of a metallic alloy. The best results were achieved by resistance melting (submerged electrodes) with addition of a reducing agent into the melt. The efficiencies of the reducing agents carbon, aluminium, silicon (as ferrosilicon) and silicon carbide were investigated and compared. As aluminium is the strongest reducing agent, it is less selective and reduces much more SiO₂ than Cr₂O₃. While SiC shows only low reactivity because of its high thermal resistance, carbon and silicon had the highest reducing potentials: More than 97% of the chemically bound chromium can be recovered by application of these reducing agents. Due to the high temperature required for the reduction of the chromium compounds, the reduction of SiO₂ as an undesired side reaction cannot be avoided. However, compared with mechanical procedures that are limited to the recovery of the metallic remainders, the total chromium recovery can be significantly increased by the described reductive melting procedure.     

Thermodynamics of iron alumino-chromite spinel inclusions in steel

The effect of chromium on the oxygen concentration of iron melts in equilibrium with various spinel reaction products has been determined. Alumina crucibles were used and experiments were performed at 1550, 1600, and 1650°C. Thermodynamic relationships between the equilibrium concentrations of chromium and oxygen in the iron melts have been established for chromium concentrations ranging up to 20 wt pct. Results from X-ray and electron microprobe analyses for the composition of the deoxidation products, together with solute activity relationships, indicate that the composition of the equilibrium spinel phase changes progressively from iron aluminate in the absence of chromium, through a series of aluminate-chromite solid solutions, FeO (Al _x Cr_1−x )₂O₃, (<0.5 pct chromium), to a complex chromite spinel, Fe₂Cr₇O₁₂, (0.5 to 3 pct chromium), and finally chromium oxide, Cr₃O₄ (>3 pct chromium). Deoxidation diagrams have been constructed and the effects of small amounts of alloying elements on the deoxidation behavior of aluminum interpreted in terms of buffered reactions which maintain oxygen concentrations in the melt at levels in excess of those normally associated with aluminum killed steel in equilibrium with alumina alone.     

铬矿粉制块直接炼钢的研究

用铬矿粉作原料,在实验室内完成了用作合金剂的两种制块工艺的研究。①碳质铬块(％):Cr_2O_3 34.98～36.14、FeO10.66～10.97、P<0.01、C 9.36～10.94。③硅质铬块(％):Cr_2O_3 33.99～35.01、FeO 10.62～18.29、p<0.01、Si 10.56～12.51。用本产品作合金剂,在100 kVA中频感应炉上对40Cr、15CrMo等钢种的冶炼工艺进行研究。试验证明,炼钢操作无困难,浇铸正常,铬回收率:碳质铬块为82.27～95.03％,平均为91.46％;硅质铬块为98.74～98.85％,平均为98.80％。试验是成功的。     

Carbides in high-speed steels containing silicon

The effects of silicon additions up to 3.5 wt pct on the as-cast carbides, as-quenched carbides, and as-tempered carbides of high-speed steels W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V were investigated. In order to further understand these effects, a Fe-16Mo-0.9C alloy was also studied. The results show that a critical content of silicon exists for the effects of silicon on the types and amount of eutectic carbides in the high-speed steels, which is about 3, 2, and 1 wt pct for W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V, respectively. When the silicon content exceeds the critical value, the M₂C eutectic carbide almost disappears in the tested high-speed steels. Silicon additions were found to raise the precipitate temperature of primary MC carbide in the melt of high-speed steels that contained d-ferrite, and hence increased the size of primary MC carbide. The precipitate temperature of primary MC carbide in the high-speed steels without d-ferrite, however, was almost not affected by the addition of silicon. It is found that silicon additions increase the amount of undis-solved M₆C carbide very obviously. The higher the tungsten content in the high-speed steels, the more apparent is the effect of silicon additions on the undissolved M₆C carbides. The amount of MC and M₂C temper precipitates is decreased in the W6Mo5Cr4V and W9Mo3Cr4V steels by the addition of silicon, but in the W3Mo2Cr4V steel, it rises to about 2.3 wt pct.     

Structure and Properties of Cast Near-Congruent Copper-Manganese Alloys

Microstructure development in the casting of copper-manganese alloys based on the congruent point at 34.6 wt pct Mn and 1146 K (873 °C) has been studied. The alloys were prepared by induction melting of electrolytic Cu and Mn in clay-graphite crucibles in open air. Under conventional casting conditions, the alloys exhibit fine cellular (non-dendritic) solidification morphology with a distinct absence of solidification shrinkage microporosity, and they maintain these attributes over a composition range of approximately 3 wt pct Mn about the congruent point. The high Mn concentration in the alloy admits carbon into solution in the melt, resulting in formation of manganese carbide Mn₇C₃ particles having two different forms (globular and angular) in the cast microstructure. The Mn carbide was eliminated or controlled to low levels by melting in an alumina or a silicon carbide crucible, or in a clay-graphite crucible at lower temperatures. Microstructure development in casting the alloy was analyzed in terms of the available phase diagrams and thermochemical data. Hardness and tensile testing indicated a potent solid solution strengthening effect of Mn and high ductility in the as-cast condition, with additional hardness (strength) when the alloy contains the Mn carbide phase.     

Aspects of smelting reduction of chromite ore fines in CaO-FeO-Cr2O3-SiO2-Al2O3 slag system by carbon dissolved in high carbon ferrochromium alloy bath

Abstract

Chromite reduction by carbon dissolved in a high carbon ferrochromium alloy melt has been investigated in the temperature range 1580-1640°C using a slag system based on CaO₂-FeO-Cr₂O₃-SiO₂-Al₂O₃. Although the reduction is essentially first order with respect to Cr₂O₃ concentration, it exhibits both zero order and first order reaction kinetics. The zero order period is occupied by the preferential reduction of iron oxide, during which time there is no significant change in the concentration of Cr₂O₃. The predominance of the divalent chromium oxide in the slag phase is seen to provide further evidence that the reduction of chromite occurs by a stagewise process, involving the thermodynamically stable CrO species. While high basicity slags may be recommended to minimise the generation of CrO, and hence improve reaction kinetics and the extent of Cr₂O₃ reduction, there is a limitation imposed by chemical erosion of the alumina crucible as the slag basicity is increased above unity, with the dissolving Al₂O₃ further retarding the reduction kinetics. There is also evidence to suggest the participation of a reductant other than carbon (possibly silicon) in the reduction of chromite.     

Kinetics of chromium oxide reduction from a basic steelmaking slag by silicon dissolved in liquid iron

The reduction of chromium oxide from a basic steelmaking slag (45 wt pct CaO, 35 wt pct SiO₂, 10 wt pct MgO, 10 wt pct A1₂O₃) by silicon dissolved in liquid iron at steelmaking temperatures was studied to determine the rate-limiting steps. The reduction is described by the reactions: (Cr₂O₃) + Si = (SiO₂) + (CrO) + Cr [1] and 2 (CrO) +Si = (SiO₂) + 2 Cr [2] The experiments were carried out under an argon atmosphere in a vertical resistance-heated tube furnace. The slag and metal phases were held in zirconia crucibles. The course of the reactions was followed by periodically sampling the slag phase and analyzing for total chromium, divalent chromium, and iron. Results obtained by varying stirring rate, temperature, and composition defined the rate-limiting mechanism for each reaction. The rate of reduction of trivalent chromium (reaction [1] above) increases with moderate increases in stirring of the slag, and increases markedly with increases in temperature. The effects of changes in composition identified the rate-limiting step for Cr⁺³ reduction as diffusion of Cr⁺³ from the bulk slag to the slag-metal interface. The rate of reduction of divalent chromium does not vary with changes in stirring of the slag, but increases in temperature markedly increase the reaction rate. Thus, this reaction is limited by the rate of an interfacial chemical reaction. The reduction of divalent chromium is linearly dependent on concentration of divalent chromium, but is independent of silicon content of the metal phase.     

Kinetics of chromite ore reduction from MgO-CaO-SiO2-FeO-Cr2O3-Al2O3 slag system by carbon dissolved in high carbon ferrochromium alloy bath

Abstract

Kinetic experiments were performed in an induction furnace to investigate the reduction of chromite ore by carbon dissolved in a high carbon ferrochromium alloy melt under conditions of varying Cr₂O₃ concentration, slag basicity, and temperature. The results obtained show that chromite reduction by dissolved carbon in slag systems of the type MgO-CaO-SiO₂-FeO-Cr₂O₃- Al₂O₃ occurs principally by a stagewise process encompassing an intermediate reaction in which the divalent chromium oxide species is involved. During the fast period, Cr₂O₃ reduction is controlled by the diffusion of oxygen species in the slag for which a mass transfer coefficient of 0·003 cm s^-1 was calculated. An activation energy value of 117 kJ mol ^-1 obtained for the reduction of Cr₂O₃ implies the rate controlling step is mass transfer of Cr₂O₃ from the slag to the slag/metal interface, since activation energies for metal phase control are typically <70 kJ mol ^-1. The second period represents a pseudo-equilibrium condition with respect to Cr₂O₃ reduction that is probably under thermodynamic control by a step or mechanism involving the reduction of divalent chromium oxide to chromium.     

Unidirectional solidification of Co−Cr−C monovariant eutectic alloys

Compositions on the eutectic liquidus line between Co-25 wt pct Cr-3.5 wt pct C and Co-45 wt pct Cr-2.2 wt pct C wherein the simultaneous freezing of a metal matrix and a carbide occur have been solidified unidirectionally. The composite structures formed consist of a cobalt matrix containing substantial amounts of soluble chromium with the carbide phase, (Cr, Co)₇C₃, as an aligned fibrous dispersion. The structure of one monovariant eutectic composition has been examined as a function of solidification velocity. Cellular growth was observed at rates above approximately 4 cm per hr. However, the cell cusps only gradually deepen with increasing growth rate and the carbide phase remains essentially aligned even at 50 cm per hr. The concentration of carbide fibers in the transverse section was measured at various growth rates and was found to be approximately a linear function of rate. The structure of the alloy was also examined by transmission electron microscopy. These results indicate that the preferred growth direction of the carbide is more important than the solid-solid surface energy in controlling the growth.     

A process for debismuthizing lead with magnesium

Significant amounts of bismuth can be removed from magnesium-lead alloys by crystallization of the intermetallic compounds Mg_{0. 6573}Pb_{0. 3427} and Mg_0.662Pb_0.338in the Mg₂Pb, magnesium plumbide phase field of the Pb-Mg-Bi system. The results of the previous studies have been used to develop a process for debismuthizing lead using controlled conditions for the crystallization of magnesium plumbide from alloys containing 0.03 to 0.06 wt Pct bismuth and magnesium whose concentration is determined by the relationship wt Pct Mg = 2.9 + 20 x wt Pct Bi. A flow diagram showing the sequence of operations is presented together with a material balance, which was established from data obtained from individual experiments simulating the previously mentioned unit operations, with a final product containing less than 0.0010 wt Pct Bi. The process also includes the recycling of magnesium recovered by vacuum distillation. Additional procedures are included to extend the process to treat alloys containing up to 1.5 wt pct bismuth.     

Production of cast higher chromium carbide using self-propagating high-temperature synthesis

The mechanisms of self-propagating high-temperature synthesis as they apply to cast chromium carbide have been examined. It is demonstrated that by optimizing the initial-mixture composition, it is possible to synthesize cast higher chromium carbide Cr₃C₂ with a 1.5–3 wt. % aluminum addition. Treating the final product with a hydrochloric acid solution enables the aluminum content to be reduced to 0.2 wt. %.Translated from Poroshkovaya Metallurgiya, No. 11(359), pp. 57–60, November, 1992.     

Solid state reactions between silicon carbide and (Fe,Ni, Cr)-alloys: reaction paths,kinetics and morphology

Solid state reactions between silicon carbide and various (Fe, Cr, Ni)-alloy compositions are studied. The different phases present in the reaction scale, their chemical composition and morphology are determined. The reaction path and the reaction kinetics are also measured. A typical composition distribution and morphology of the reaction scale is associated to each alloy element: iron yields broad α-iron zones with randomly distributed graphite precipitates; the presence of nickel results in typical reaction scales made of alternating bands of pure silicide and other bands of silicide containing a high density of carbon precipitates; chromium leads to large chromium carbide precipitates on the alloy side. The solid state reactions between SiC and (Fe, Cr, Ni)-alloys can be described as the dissolution of silicon carbide by iron and nickel, resulting in the formation of silicon-rich compounds (α-, τ-phase, Ni₅Si₂ or Ni₂Si) and in the precipitation of graphite flakes.     

转炉铬矿熔融还原法冶炼不锈钢母液过程的热力学

针对转炉铬矿熔融还原法冶炼不锈钢母液工艺,利用冶金热力学原理对该过程中涉及的几个重要问题进行了理论分析和计算,为在转炉中用铬矿实现钢的直接合金化的热力学可行性提供了理论依据.得到结论:①铬矿石在熔融还原转炉中是完全可以被还原的;②金属熔体中铬的活度随金属中铬质量分数的增加而增大,温度对金属铬活度的影响较小;③在1500...     

Optimum parameters for wetting silicon carbide by aluminum alloys

The effect of magnesium and silicon additions to aluminum, free silicon on the SiC substrate, nitrogen gas in the atmosphere, and process temperature on the wetting characteristics of SiC by aluminum alloys are investigated using the sessile drop technique. The contribution of each of these parameters and their interactions to the contact angle, surface tension, and driving force for wetting are determined. In addition, an optimized process for enhanced wetting is suggested and validated. Results show that the presence of free silicon on the surface of SiC significantly reduces the contact angle between the molten alloy and the substrate. The positive effect of silicon on the contact angle is attributed to a chemical reaction in which both SiC and aluminum are active participants. The results also indicate that nitrogen gas in the atmosphere positively influences the liquid/vapor surface tension, and the presence of magnesium in the aluminum alloy favorably affects the overall driving force for wetting. A mechanism is proposed to explain the beneficial role that the interaction of nitrogen with magnesium plays in enhancing wetting. Magnesium significantly reduces the surface tension of aluminum melts but has a low vapor pressure. Consequently, it readily volatilizes during holding at the processing temperature and is lost from the alloy. It is proposed that a series of chemical reactions in the system Al-Mg-N are responsible for reintroducing magnesium into the melt, thus, maintaining a low melt surface tension. Interactions between the aluminum alloy and the silicon carbide substrate that may lead to the dissolution of the substrate and the formation of undesirable reaction products, particularly Al₄C₃, are examined, and means for mitigating their formation are outlined.     

Decarburlzation of silicon melt for solar cells by filtration and oxidation

In studies on the decarburization of liquid silicon, two types of carbon were identified in a silicon melt containing more carbon than the solubility. One was in the form of silicon carbide particles and the other was in the form of dissolved carbon. The silicon was filtered to remove the carbide particles. Using silica or silicon carbide as the material for the filter, the carbon content was reduced from 100 to 30 to 60 ppmw. Oxidation was done under reduced pressure to eliminate the rest of the carbon as carbon monoxide, and the final carbon content was as low as 10 ppmw. Silica was chosen as oxidizing reagent. To avoid a reverse reaction, Ar was introduced on the surface to expel extraneous CO or CO₂ gas evolved. The thermodynamics of the oxidation of carbon in silicon melts is also discussed. Formerly with the Graduate School of the University of Tokyo     

Effect of interactions between bubbles and graphite particles in copper alloy melts on microstructure formed during centrifugal casting: Part I. Theoretical analysis

Frequently, particles get associated with gas bubbles in a melt and their interaction influences the final distribution of particles and porosity in the casting. An analytical model for the separation of a particle from a bubble in melts containing dispersed particles and bubbles is proposed. During centrifugal casting of alloys containing dispersed particles, both the particles and gas bubbles present in the melt move with the centrifugal forces. Using the force balance between surface tension and net centrifugal forces (centrifugal force minus buoyancy force), the critical rotational speed of the mold for the separation of the particles and the bubbles during centrifugal casting is calculated. The critical rotational speed of the mold to separate the particle from the bubble is lower for a small particle attached to a larger bubble, as compared to the case when a large particle is attached to a smaller bubble. For a given bubble size, the critical rotational speed of the mold to separate the bubble from the particle decreases with increasing particle size. For the specific case of spherical 5-μm radius graphite particles dispersed in copper alloy melt, it was found that even at a low semiapical angle of about 9 deg, the critical rotational speed needs to be around 5000 rpm for a bubble size of 500-μm radius and 0.09-m-diameter mold. The rotational speed decreases to 1000 rpm when the graphite particle radius increases to 100 μm for the same bubble size in copper alloy melt.     

The composition of eta carbide in hastelloy N after aging 10,000 h at 815°C

The composition of the eta carbide in Hastelloy N containing 0.7 wt pct silicon in the alloy, approaches Mi₂C, rather than M₆C as indicated in the alloy literature. The silicon content of the eta phase in this case was ~25 at. pct, much higher than has been observed in less highly alloyed material. The data do not permit a definition of the limiting compositions of the phases.