A study on inhomogeneous distribution of temper graphite particles in strip-cast Fe-C-Si alloys

This study examined the relationship between solidification structure and graphitization characteristics of white cast iron strips produced by strip casting. Experimental results showed that there was an unusual distribution of temper graphite particles along the through-thickness direction of the graphitized strips in comparison with gravity-cast chill plate. In particular, the graphite-free zones appeared in the vicinity of the strip surface after the completion of graphitization, especially in the strip with low carbon and silicon content. There were abnormally straight interfaces between matrix and eutectic cementite with a strong preferred [001]_c growth direction caused by the effect of directional solidification found in the near-surface regions of the strips. The interfaces did not form a site for the graphite to nucleate and gave rise to the graphite-free zones close to the strip surface. An increase in carbon and silicon content could significantly increase the number of temper graphite particles and shorten the time for the completion of graphitization, but an inhomogeneous distribution feature of graphite particles was still observed in strips with a higher carbon equivalent value (CE). Furthermore, variations in carbon and silicon content resulted in transitions in carbide morphology and composition, which had a tremendous effect on the graphitization characteristics of the cast iron strips.     

Production of silicon carbide from microsilica waste by reduction with lignite semicoke

Silicon carbide may be produced from fine-grain batch consisting of two main components: microsilica waste; and semicoke obtained from Berezovsk lignite (Kansko-Achinsk Basin). The physicochemical properties of the silicon carbide are certified in the present work. Two forms of microsilica are considered: (1) microsilica formed in the production of silicon (containing 93.41–95.33% SiO₂; 1.96–3.28% C_free; 0.30–0.34% Si_free; and 1.25–1.45% CaO + Fe₂O₃ + MnO); (2) microsilica formed in the production of high-silica ferrosilicon: (containing 91.72–93.63% SiO₂; 0.56–1.18% C_free; 0.18–0.20% Si_free; and 1.38–2.32% CaO + Fe₂O₃ + MnO). Its specific surface is 21000–24000 m²/kg. The microsilica is inclined to form spherical aggregates measuring 200–800 nm. The aggregates consist of spherical particles ranging in size from 30 to 100 nm. The lignite semicoke contains 94.05% carbon, 9.2% ash, 0.2% sulfur, and 0.007% phosphorus; its specific surface is 264000 m²/kg. The composition of the silicon carbide is investigated, along with its specific surface; the size and shape of the carbide particles are determined. In both cases, the predominant phase is cubic silicon carbide (β-SiC), with an accompanying glassy phase consisting of silicates of calcium, magnesium, and iron. When the batch containing microsilica from ferrosilicon production, the silicon carbide is accompanied by α iron. In synthesis at 1923 and 1973 K for 50 and 90 min, respectively, polymorphic conversion of β-SiC to α-SiC_p is observed. The content of silicon carbide in the products is 82.52–84.90%. Chemical enrichment of silicon carbide proves expedient. The optimal enrichment conditions are as follows: the action of hydrochloric acid (concentration no less than 35%) at 353 K for 3 h, with a 1:2 solid/liquid ratio. The enrichment characteristics are as follows: the content of silicon carbide in the products is 90.42–91.10%; and 87–95% of the impurities (metal oxides and iron) are removed. The silicon carbide is obtained as micropowder consisting of irregular particles (size 0.2–1.0 μm) with specific surface 8000–9000 m²/kg.     

Carbide- matrix interface mechanism of stress corrosion cracking behavior of high-strength CrMo steels

The effects of tempering temperature and carbon content on the stress corrosion cracking (SCC) behavior of high-strength CrMo steels in 3.5 pct NaCl aqueous solution have been studied by means of Auger electron spectroscopy (AES) and scanning and transmission electron micros- copy (SEM and TEM). Experimental results show that the specimens with higher carbon content and tempered at lower temperatures have a higher tendency for intergranular fracture and lower threshold stress intensity K_ISCC The SCC behavior is significantly affected by the distribution of carbide particles, especially carbide coverage on prior austenitic grain boundaries, through a carbide-matrix interface mechanism as the interface is the preferential site for the nucleation and propagation of microcracks because of its strong ability to trap hydrogen atoms. In low- temperature tempered states, there is the serious segregation of carbon in the form of carbide particles at prior austenitic grain boundaries, causing low-stress intergranular fracture. After tempering at high temperatures (≥400 °C), both the coalescence of the carbide particles at the grain boundaries and the increase of carbide precipitation within grains cause the decrease of the tendency for intergranular fracture and the rise of K_ISCC. The higher the carbon content in steels, the more the carbide particles at the grain boundaries and, subsequently, the higher the tendency for low-stress intergranular fracture. The carbide effect on K_ISCC makes an important contribution to the phenomenon that K_ISCC decreases with the rise of yield strength of the steels.     

Formation of silicon carbide in the reaction of reduction of silica by carbon

Conclusions In the initial stages of heat treatment of H₂SiO₃ and sucrose a mixture of highly dispersed defective SiO₂ particles and carbon material is formed. Then as the result of contact interparticle interaction of a radical character disintegration (activation) of the carbon particles and envelopment of them by a layer of SiO₂ accompanied by deformation of the Si-O-Si bonds occur. Filling of the pores of the carbon material with silicon oxide creates in subsequent higher temperature treatment favorable conditions for formation of SiC. The particles formed as the result of the relatively low-temperature solid-state reaction are non-uniform in composition. Their core consists of uninteracted carbon and after it follow a layer of silicon carbide and an outer layer of SiO₂. A switch to the area of high synthesis temperature makes it possible to approach a stoichiometric composition of the silicon carbide.Translated from Poroshkovaya Metallurgiya, No. 9(321), pp. 57–62, September, 1989.     

Matrix/reinforcement interactions in shock-wave consolidated titanium aluminide reinforced with SiC

Explosive shock-wave consolidation has been used to fabricate a composite consisting of gamma phase titanium aluminide matrix reinforced with paniculate silicon carbide. Although the consolidation process takes less than a microsecond to complete, melting of the surfaces of the titanium aluminide powder particles results in a reaction with the silicon carbide. Titanium suicide and titanium carbide form in the molten zone, depleting it of titanium to the extent that its final composition is TiAl₃. In addition, a second titanium suicide phase forms on the surfaces of the silicon carbide particles. This paper is based on a presentation made in the symposium “Interfaces and Surfaces of Titanium Materials” presented at the 1988 TMS/AIME fall meeting in Chicago, Illinois, September 25–29, 1988, under the auspices of the TMS Titanium Committee.     

Influence of tempering temperature on stability of carbide phases in 2.6cr-0.7mo-0.3v steel with various carbon content

The present work evaluates the influence of the bulk carbon content (0.1, 0.006, and 0.005 wt pct) and tempering temperature (823, 853, and 913 K) on stability, chemical composition, and size of carbide particles in 540 ks tempered states of 2.6Cr-0.7Mo-0.3V steel. The scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDXS) and electron diffraction methods were used to analyze the carbide particles. A characteristic energy-dispersive X-ray (EDX) spectrum can be attributed to each of the identified carbides. The MC carbide is stable in all experimental states. The phase stability of Fe-Cr-rich carbides increased in the order ε, Fe₃C → M₃C → M₇C₃, with tempering temperature increasing. In steels with higher carbon content tempered at low temperature, M₂₃C₆ carbide was also noted. The Mo₂C and M₆C carbides were not observed. It was shown that the decrease of the bulk carbon content has the same influence on the carbide phases stability as the increase of the bulk vanadium content at the unchanged Cr, Mo, C bulk contents and tempering temperature. Similarly, the decrease of tempering temperature has the same influence on the carbide phases stability as the decrease of the bulk Cr content at the unchanged V, Mo, and C bulk contents.     

Carbon Solubility and Phase Composition of Silicomanganese

This paper examines silicomanganese produced in the electric ferroalloy furnaces at Tasmanian Electrometallurgical Company (TEMCO). Its carbon solubility and the phase composition were determined. Silicon content is the major factor affecting carbon concentration. Temperature also plays an important role in the carbon concentration. Silicomanganese consists of three phases: the silicides (Mn,Fe)₅Si₃ with approximately 0.7 wt% dissolved carbon and (Mn,Fe)₃Si with a carbon content of 0 ‐ 0.6 wt%, and carbide (Mn,Fe)₁₇Si₄C₃ with 3.3 wt% C. The fraction of carbide phase increased directly with increasing total carbon in the alloy. Silicon carbide was not detected. Reasonable estimates of carbon content are possible using the carbon‐silicon equilibrium relationship to the SiMn production process of TEMCO.     

Modelling of ferrosilicon smelting in submerged arc furnaces

Abstract

Ferrosilicon alloys are commonly manufactured in submerged electric arc furnaces with little slag. In the presence of iron, silica will be reduced by carbon to give a maximum of ~ 22 wt% silicon in the liquid alloy. The rest of the carbon will be consumed to transform silica to silicon carbide at ~ 1810 K. Higher grades of ferrosilicon alloy may be produced owing to the reactions occurring between silicon carbide and silica at temperatures above 1810 K. Thermodynamic data on the standard free energy of formation of species are used in the study to calculate the required smelting temperatures at various silicon contents of the alloy. The sum of partial pressures of carbon monoxide and silicon monoxide must equal the applied pressure of 1 atm at the smelting temperature. It is important to know the activity coefficient of silicon in the alloy as a function of temperature, and the silicon content of the alloy using literature data. Mass and enthalpy balances are used to determine the carbon and electricity requirements of the process. The recycling of silicon monoxide is promoted by maintaining a bed of a certain height so that evolved gases are cooled owing to heat exchange between the gas and solid phases. It might result in a saving of more than 3000 kWh/t of Fe-80Si alloy. The reduction of silica is found to account for just 47·6% of the total energy that is added via the calorific value of carbon and the electricity in producing the alloy. Further improvement in the performance is visualised by reducing electrical losses and recovering as much as possible the calorific value of outgoing carbon monoxide.     

红外吸收法测定铝碳化硅碳砖中碳化硅量

根据铝碳化硅碳砖的组成特点，对各种常用炭素材料和碳化硅的分解温度进行试验，找出了各种常用炭素材料与碳化硅的分解温度的不同，并据此通过高温灼烧法将炭素材料中各种形式的碳与碳化硅中碳分离。应用HCS－140型红外碳硫分析仪，对碳化硅中碳的测定条件进行了研究，在选定的工作条件下，用红外吸收法测定碳化硅中碳的含量，从而换算出碳化硅量，取得了满意的结果。     

Synthesis of Fine Vanadium-Carbide (VC<Subscript>0.88</Subscript>) Powder Using Carbon Nanofiber

The synthesis of fine vanadium-carbide (VC_0.88) powder is considered. To produce the vanadium carbide, vanadium(III) oxide is reduced by means of carbon nanofiber in an induction furnace with an argon atmosphere. The carbon nanofiber is produced by the catalytic decomposition of light hydrocarbons. The specific surface of the carbon nanofiber is very high: ~150000 m²/kg, as against ~50000 m²/kg for soot. The impurity content in the carbon nanofiber is 1 wt %. By analysis of the phase diagram of the V–C system, the batch composition and the upper temperature limit in carbide formation may be determined such that vanadium carbide is formed as powder. Thermodynamic analysis yields the initial temperature at which the vanadium( III) oxide is reduced in a furnace with different CO pressures. The characteristics of the vanadium carbide are determined by the following methods: X-ray phase and elementary analysis; pycnometric analysis; scanning electron microscopy with local energy dispersion X-ray microanalysis (EDX); low-temperature nitrogen adsorption with subsequent determination of the specific surface by the BET method; sedimentation analysis; and synchronous thermogravimetric analysis and differential scanning calorimetry (TG/DSC). The material obtained with optimal reduction parameters consists of a single phase: vanadium carbide VC_0.88. The powder particles are predominantly clumped together in aggregates. The mean size of the particles and aggregates is 9.2–9.4 μm, with a broad size distribution. The specific surface of the samples is 1800–2400 m²/kg. Oxidation of vanadium carbide begins at about 430°C and is practically over at 830°C. Optimal synthesis requires stoichiometric proportions of the reagents in the production of vanadium carbide VC_0.88 at 1500–1600°C, with 20-min holding. In this process, carbon nanofiber effectively produces the carbide as the reduction product. The vanadium(III) oxide is reduced practically completely to VC_0.88.     

不同碳源对硅微粉制备碳化硅的影响

探究了不同碳源对以硅微粉为原料通过碳热还原法制备碳化硅粉体的影响,采用Fact Sage软件对制备SiC的反应过程进行了热力学计算,得出理论反应起始温度;探究了分别以石油焦、活性炭、石墨粉和蔗糖为还原剂对冶炼效果的影响。研究表明:以硅微粉为硅源通过碳热还原反应制备碳化硅的冶炼温度以1 450~1 650℃合理,当以石油焦为还原剂时,冶炼产物中SiC含量最高,品质最好。     

Precipitation in a liquid phase sintered WNiFe alloy

The precipitate phases formed within samples of a W-5 wt% Ni-5 wt% Fe alloy following sintering and controlled, post-sintering heat treatments have been characterized using techniques of transmission electron microscopy. Interphase boundary precipitation of a complex eta carbide has been confirmed in an as-sintered, commercial alloy of high impurity carbon content, and reproduced on a reduced scale in an experimental alloy of reduced carbon content by isothermal ageing (10 h, 900–950°C of solution treated (1 h, 1350°C) and quenched specimens. The experimental alloy contained no evidence of third phase precipitation in the as-sintered condition, but fine particles were noted along tungsten grain boundaries in solution treated and quenched specimens and were found to have a structure similar to that of the matrix γ phase. Isothermal ageing of solution treated specimens at temperatures of 750–850°C produced local colonies of cellular precipitation within the matrix phase, the discontinuous phase of essentially pure tungsten arising in response to a retained supersaturation of tungsten in matrix solid solution in the quenched material. A competitive precipitation of the eta carbide, (Ni, Fe)₆W₆C, was observed in the matrix phase under similar heat treatment conditions, with the most common form of the precipitate being Widmanstätten laths of (111)₇ habit plane. The present study produced no evidence for the formation of an intermetallic phase in the WNiFe alloy containing a weight ratio of Ni:Fe of 1:1.     

用高频红外碳硫仪测定碳化硅的纯度

本文报道了用高频红外碳硫仪测定碳化硅纯度的新方法。试样在瓷坩埚中灼烧，去除游离碳及挥发份，选铁屑与钨粒的混合物作助熔剂，在高频红外碳硫仪上测定ＳｉＣ中碳的含量，由此换算出ＳｉＣ的含量。方法简单、快速、准确。     

硅对高频燃烧红外吸收法测定高硅样品中碳和硫的影响

利用扫描电镜(SEM)和能谱仪(EDS)观测了经高频感应炉燃烧后高硅样品熔体中硅的存在形态和组成,从不同角度考察了样品中硅对熔融效果产生的影响,以及对高频感燃烧红外吸收法测定碳和硫结果的影响。结果显示:仅使用纯钨做助熔剂时,含硅样品熔体中的硅主要以球状的二氧化硅颗粒镶嵌于氧化钨和氧化铁等基体中,硅的存在使得高频燃烧红外吸收法测定碳和硫的准确度变差,且随着样品中硅含量的增加,这种影响的程度逐步增大。通过对影响机理进行分析认为:生成的二氧化硅使得熔体的流动性变差,且接受电磁感应形成感应电流的能力也同时减弱,从而对测定结果的精密度和正确度造成影响。实验在使用钨助熔剂的基础上,进行了添加纯锡或纯铁助熔剂的试验,结果表明:添加一定量纯锡或纯铁助熔剂,可以降低样品熔点,提高熔池温度,大大改善熔融效果,从而提高碳和硫测定的精密度和正确度。     

鱼雷罐用铝碳化硅碳砖中碳化硅的氟硅酸钾滴定法研究

探讨了测定鱼雷罐用铝碳化硅碳砖中碳化硅的氟硅酸钾滴定方法。用KOH和KNO₃混合熔剂在镍坩埚中熔融鱼雷罐用铝碳化硅碳砖,选取熔融温度为700 ℃,熔融时间为15 min的熔融条件,在此条件下进行浸取易于操作,减少了对镍坩埚的损害。采用自制的抽滤装置,即将普通抽滤瓶进行改装,将玻璃砂芯漏斗部分用塑料代替,只需用直径约为40 mm、相当于四分之一张普通滤纸的滤纸进行过滤,过滤时抽滤速度快,滤纸不会穿漏,且吸附的残余酸量很少,易于洗涤和中和。方法用于自制标准样品的测定,回收率在98%～101%之间,相对标准偏差小于5%,能够满足日常分析检测需要。     

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.     

Relative Influences of Aluminium and Silicon on the Kinetics of Bainite Formation from Intercritical Austenite

Bainite formation from intercritical austenite is of great practical importance for the production of TRIP‐assisted steels. Silicon and aluminium play important roles during this transformation by delaying carbide precipitation, thus favouring the carbon enrichment of untransformed austenite, which makes its stabilisation down to room temperature possible. Previous studies have shown a strong dependence of bainite formation kinetics on both chemical composition and transformation temperature. In the present work, the effect of silicon and aluminium contents on bainite formation kinetics is investigated experimentally using dilatometry combined with microscopical observations. The experimental results are analysed by comparison with thermodynamic parameters, such as the activation energy G¹ for nucleation of bainite and the carbon content C_to corresponding to the T_o‐curve. It is shown that the faster transformation kinetics induced by the substitution of silicon by aluminium can be ascribed (i) to a higher driving force for nucleation, (ii) to a higher carbon content C_to at the To‐curve and (iii) to the precipitation of carbide in austenite in steels with a low Al content.     

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.     

白钨矿直接合金化过程中炉渣泡沫化研究

通过实验研究了白钨矿还原过程中炉渣泡沫化的形成规律。用碳化硅还原白钨矿,炉渣的泡沫化程度远小于碳粉还原白钨矿时的泡沫化程度。使用碳粉或碳化硅还原白钨矿时,随着合金化量的增加,炉渣的泡沫化程度提高。使用硅铁还原白钨矿,会降低炉渣的泡沫化程度。使用碳粉还原白钨矿时,渣中添加CaF2会加大炉渣泡沫化程度。     

含金属硅的碳砖材料固定碳含量的分析

采用不同方法测定了微孔碳砖、半石墨碳砖等碳素材料中的固定碳含量，结合对砖结构的分析，认为现有的分析标准与方法难以准确测定加有金属硅粉或碳化硅粉碳砖中的固定碳含量，因此在验收类似产品时，作为产品技术指标考核的固定碳含量难以准确测定。