Gas carburizing of steel with furnace atmospheres formedIn Situ from propane and air: Part III. Control of furnace atmosphere composition with a zirconia oxygen sensor

Gas carburizing experiments were conducted in a batch-type sealed-quench furnace using furnace atmospheres produced by the reaction of propane and air within the furnace. The air-propane ratio of the inlet gases was automatically controlled to maintain a constant oxygen potential, as measured by a zirconia oxygen sensor, within the furnace. The results of carburizing trials at 843 and 927 °C are described. The effect of inlet gas flow rate on furnace atmosphere composition and the amount of carburizing is illustrated.     

Gas carburizing of steel with furnace atmospheres formedIn Situ from propane and air: Part I. The effect of air-propane ratio on furnace atmosphere composition and the amount of carburizing

Gas carburizing experiments were conducted in a batch-type sealed quenched furnace at 843 and 927 °C using furnace atmospheres produced by reacting propane and air within the furnace chamber. With low, constant gas flow rates it is shown that the amount of carburizing varies regularly with air-propane ratio. Furnace atmosphere composition was monitored as a function of temperature and air-propane ratio and compared with the composition expected at thermodynamic equilibrium.     

Decreased gas consumption of a fluidized bed furnace

The feasibility of utilizing a closed circulatory system to generate gases for a fluidized bed furnace was investigated with the primary concentrations of both economizing on the raw materials used for producing furnace atmospheres and decreasing the air pollution caused by exhaust gases. Air humidified with water vapor was first introduced into a charcoal furnace for causing a reaction with hot charcoal to form a carburizing atmosphere. This atmosphere was then introduced into a fluidized bed furnace to carburize steels. The exhaust gases from the fluidized bed furnace were recycled by repassing them through the hot charcoal layer in the charcoal furnace with a gas pump. The charcoal furnace and the fluidized bed furnace formed a closed circulatory system during the carburization of steels. Experiments were performed with various parameters of this system, including content of water vapor in the humid air, temperature of the charcoal, rate of recirculation of the atmosphere,etc. The effect of each parameter on the carburizing behavior in the fluidized bed furnace was investigated on the basis of the rate of carburization and the carbon potential of the atmosphere. The feasibility of applying this system to a fluidized bed furnace was assessed from the aspects of the fluidization of A1₂O₃ powder, the result of carburizing steel, and the rate of consumption of charcoal. The closed system employed in generating atmosphere was demonstrated by the experimental results to have enabled the fluidized bed furnace to operate normally and to have significantly decreased both the consumption rate of charcoal and the environmental pollution.     

Analytical models for the gas carburizing process

Mathematical models for carburizing in batch and continuous furnaces are described. Both the steady-state model for continuous furnaces and the time-dependent model for batch furnaces are based on material balances, with thermodynamic equilibrium of the constituents of the furnace atmosphere assumed. The instantaneous rate of carburizing is taken to be proportional to the difference between the carbon potential of the furnace atmosphere and the surface carbon content of the work load. Computer programs incorporating these models were written which predict furnace operating characteristics for any assumed process. The continuous furnace model predicts the pattern of internal gas flow within the furnace and computes the natural gas (or air) additions to each zone needed to achieve the desired carbon potentials and satisfy the carbon demand. The batch furnace model describes how the furnace atmosphere changes in composition during carburizing as a result of the interaction of the instantaneous carbon demand and the rate of supply of carburizing gases to the furnace. Examples of the use of these programs are given, and the limitations of the predictions are discussed.     

Gas carburizing of steel with furnace atmospheres formedIn Situ from propane and air: Part II. Analysis of the characteristics of gas flow in a batch-type sealed quench furnace

Gas flow dynamics in a batch-type sealed quench carburizing furnace were studied for operations utilizing low inlet gas flow rates. By analyzing the rate of change of furnace atmosphere composition when a sudden change is made in the inlet gas composition, it is shown that a significant amount of gas circulation occurs between the hot furnace chamber and the unheated vestibule. This circulation has the effect of increasing the mean residence time of gases within the furnace. A long mean residence time is advantageous for carburizing when the inlet gases consist of an airJhydrocarbon blend rather than prereacted endothermic gas.     

The effect of the nitrogen potential on the coarsening kinetics of VN precipitates

An Fe-1.06 pct V alloy was used to study the kinetics of coarsening of VN precipitates. Nitriding was carried out at 600 °C in purified NH₃ gas. Nitrided specimens were then annealed at 820 °C in furnace atmospheres of different NH₃/H₂ ratios. The transmission electron microscopy technique was used to measure the precipitate sizes. The data on the precipitate sizes indicate that the coarsening of the plate shaped VN precipitates seems to be diffusion controlled. The precipitate coarsening rate appears to be lowered by the increase in NH₃ content in the furnace atmosphere. The particle size distributions were found to be broader than the predictions of the LSW theory.     

Oxidation of low carbon steel in multicomponent gases: Part II. Reaction mechanisms during reheating

Oxidation behavior of low carbon steel during reheating in an industrial walking-beam steel reheat furnace was investigated. It was observed that scaling (oxidation) rates were reduced by reducing the input air/fuel ratio to the furnace, thereby lowering concentrations of free oxygen in the combustion products from about 3 to 1.5 pct. Laboratory experiments involving isothermal and nonisothermal oxidation were carried out in atmospheres consisting of oxygen, carbon dioxide, water vapor, and nitrogen. A general equation for the prediction of weight gains due to oxidation during reheating, using isothermal oxidation rate constants, was developed. The prediction of weight gains from nonisothermal oxidation conducted in the laboratory was poor, owing to a separation of the scale from the metal substrate which took place at about 900 °C. The predicted weight gains during reheating in the industrial reheat furnace indicated that oxidation rates during reheating were intermediate between linear and parabolic, especially during reheating with high air/fuel ratio. However, the linear mechanism predominated. Laboratory isothermal experiments for oxidation in atmospheres containing free oxygen showed that the magnitude of the linear oxidation rates was determined by the oxygen concentration in the atmosphere. It was concluded that the observed reduction in scaling rates during reheating of low carbon steel in the industrial reheat furnace was a result of the lower free oxygen level in the furnace atmosphere.     

Oxidation of aluminum-magnesium melts in air,oxygen, flue gas,and carbon dioxide

Oxidation rates of aluminum—1 to 14 pct magnesium melts in air, oxygen, flue gas, and carbon dioxide at temperatures from 600 to 1100°C were measured with an automatic recording balance. For most conditions, a protective amorphous film on the melt surface kept the oxidation rate low initially. After an interval that was shortened by increasing temperature or magnesium content, the film crystallized to magnesium oxide and magnesium aluminate, accompanied by a sudden increase in oxidation rate. Known as breakaway oxidation, this phenomenon could be produced by adding crystalline magnesium oxide or magnesium aluminate seed to melts protected with amorphous oxide. Flue gas or carbon dioxide in the atmosphere, sodium or beryllium in the alloy, or boron dusted on the surface delayed crystallization unless seed was added. Beryllium was the most effective. Flue gas from burning natural gas delayed breakaway oxidation of unseeded melts containing up to about 4 pct magnesium at normal melting furnace temperatures near 750°C. Slow melting of solid aluminum-magnesium alloy concentrated magnesium in the first liquid to melt, sharply decreasing the interval before breakaway oxidation after melting. Rapidly melting the solid increased the protective interval. Homogenizing the solid just below the melting range before rapid melting further extended the interval before the onset of breakaway oxidation.     

Effect of soaking temperature and annealing atmosphere on magnetic properties of semi-processed nonoriented electrical steel containing 0.4 % silicon

In order to investigate the effect of soaking temperature and annealing atmosphere on magnetic properties of the semi-processed nonoriented electrical steel containing 0.4 % silicon, customer annealing was carried out at a temperature ranging from 730 to 950 and in different annealing atmospheres. As the soaking temperature increases from 730 to 950 in the dry N2 atmosphere, the core loss at 1.5 Tesla decreases continuously, whereas the permeability at 1.5 Tesla increases up to 890 and then decreases significantly at 950 °C. Among DX atmospheres having a dew point of -23 °C, the air to LPG ratio of 19.5 is found to reveal the best magnetic properties due to the lowest carbon content. At a dew point of +5 °C and an air to LPG ratio of 18.5 the best magnetic properties are to be achieved regarding both carbon content and internal oxidation layer.     

Roasting of Nickel Concentrates

The oxidation of three nickel concentrates from two Canadian smelters was studied by thermogravimetric analysis. Concentrate samples were heated to 1223 K (950 °C) in inert or oxidizing atmospheres to determine the reaction behavior. By recording the mass change as well as the SO₂ content in the outlet gas, the oxidation behaviors were quantified. Isothermal roasting tests were carried out on the concentrates over the temperature range of 673 K (400 °C) to 1123 K (850 °C). When heated in air, the samples gain mass as a result of sulfate formation at temperatures up to approximately 873 K (600 °C) to 973 K (700 °C), whereas at higher temperatures, the samples exhibit a large mass loss attributed to sulfate decomposition as well as direct SO₂ formation by oxidation. In a 4 pct O₂ gas atmosphere, significantly less sulfates were formed. Mixed reactions take place, in which some lead to mass loss and SO₂ generation, and others lead to mass gain and SO₂ consumption. The relative importance of the various reactions depends on the experimental conditions.     

THE IMPORTANCE OF ATMOSPHERE CONTROL IN HARD-METAL PRODUCTION

Abstract

The furnace atmospheres used in the manufacture of hard-metal from the pressed compact to the sintered component are discussed.

The very fine size (0·5–8·0 μm) of the powder particles makes the compacts particularly prone to react with furnace atmospheres. All these reactions affect the carbon content of the alloys, which must be controlled within extremely close limits to ensure good quality.

The removal of pressing lubricant, presintering, and final sintering all involve heating the components to temperatures at which reactions with the furnace atmosphere can occur. Both hydrogen and vacuum furnaces are used and care is required to maintain a quality of atmosphere that will not lead to a deleterious change in carbon content.     

Phase equilibria in the system Fe-Zn-O at intermediate conditions between metallic-iron saturation and air

The phase equilibria in the FeO-Fe₂O₃-ZnO system have been experimentally investigated at oxygen partial pressures between metallic iron saturation and air using a specially developed quenching technique, followed by electron probe X-ray microanalysis (EPMA) and then wet chemistry for determination of ferrous and ferric iron concentrations. Gas mixtures of H₂, N₂, and CO₂ or CO and CO₂ controlled the atmosphere in the furnace. The determined metal cation ratios in phases at equilibrium were used for the construction of the 1200 °C isothermal section of the Fe-Zn-O system. The univariant equilibria between the gas phase, spinel, wustite, and zincite was found to be close to pO₂=1 · 10⁻⁸ atm at 1200 °C. The ferric and ferrous iron concentrations in zincite and spinel at equilibrium were also determined at temperatures from 1200 °C to 1400 °C at pO₂ = 1·10⁻⁶ atm and at 1200 °C at pO₂ values ranging from 1 · 10⁻⁴ to 1 · 10⁻⁸ atm. Implications of the phase equilibria in the Fe-Zn-O system for the formation of the platelike zincite, especially important for the Imperial Smelting Process (ISP), are discussed.     

Influence of Gas Atmosphere Dew Point on the Galvannealing of CMnSi TRIP Steel

The Fe-Zn reaction occurring during the galvannealing of a Si-bearing transformation-induced plasticity (TRIP) steel was investigated by field-emission electron probe microanalysis and field-emission transmission electron microscopy. The galvannealing was simulated after hot dipping in a Zn bath containing 0.13 mass pct Al at 733 K (460 °C). The galvannealing temperature was in the range of 813 K to 843 K (540 °C to 570 °C). The kinetics and mechanism of the galvannealing reaction were strongly influenced by the gas atmosphere dew point (DP). After the galvannealing of a panel annealed in a N₂+10 pct H₂ gas atmosphere with low DPs [213 K and 243 K (?60 °C and ?30 °C)], the coating layer consisted of δ (FeZn₁₀) and η (Zn) phase crystals. The Mn-Si compound oxides formed during intercritical annealing were present mostly at the steel/coating interface after the galvannealing. Galvannealing of a panel annealed in higher DP [263 K and 273 K, and 278 K (?10 °C, 0 °C, and +5 °C)] gas atmospheres resulted in a coating layer consisting of δ and Г (Fe₃Zn₁₀) phase crystals, and a thin layer of Г ₁ (Fe₁₁Zn₄₀) phase crystals at the steel/coating interface. The Mn-Si oxides were distributed homogeneously throughout the galvannealed (GA) coating layer. When the surface oxide layer thickness on panels annealed in a high DP gas atmosphere was reduced, the Fe content at the GA coating surface increased. Annealing in a higher DP gas atmosphere improved the coating quality of the GA panels because a thinner layer of oxides was formed. A high DP atmosphere can therefore significantly contribute to the suppression of Zn-alloy coating defects on CMnSi TRIP steel processed in hot dip galvanizing lines.     

Carbothermal Reduction of Quartz with Carbon from Natural Gas

Carbothermal reaction between quartz and two different carbons originating from natural gas were investigated in this paper. One of two carbons is the commercial carbon black produced from natural gas in a medium thermal production process. The other carbon is obtained from natural gas cracking at 1273 K (1000 °C) deposited directly on the quartz pellet. At the 1923 K (1650 °C) and CO atmosphere, the impact of carbon content, pellet structure, gas transfer, and heating rate are investigated in a thermo-gravimetric furnace. The reaction process can be divided into two steps: an initial SiC-producing step followed by a SiO-producing step. Higher carbon content and increased gas transfer improves the reaction rate of SiC-producing step, while the thicker carbon coating in carbon-deposited pellet hinders reaction rate. Better gas transfer of sample holder improves reaction rate but causes more SiO loss. Heating rate has almost no influence on reaction. Mass balance analysis shows that mole ratios between SiO₂, free carbon, and SiC in the SiC-producing step and SiO-producing step in CO and Ar fit the reaction SiO₂(s) + 3 C(s) = SiC(s) + 2 CO(g). SiC-particle and SiC-coating formation process in mixed pellet and carbon-deposited pellet are proposed. SiC whiskers formed in the voids of these two types of pellets.     

Study of Pellets and Lumps as Raw Materials in Silicon Production from Quartz and Silicon Carbide

The use of high-purity carbon and quartz raw materials reduces the need for comprehensive refining steps after the silicon has been produced carbothermically in the electric reduction furnace. The current work aims at comparing the reaction mechanisms and kinetics occurring in the inner part of the reduction furnace when pellets or lumpy charge is used, as well as the effect of the raw material mix. Laboratory-scale carbothermic reduction experiments have been carried out in an induction furnace. High-purity silicon carbide and two different high-purity hydrothermal quartzes were charged as raw materials at different molar ratios. The charge was in the form of lumps (size, 2–5 mm) or as powder (size, 10–20 μm), mixed and agglomerated as pellets (size, 1–3 mm) and reacted at 2273 K (2000 °C). The thermal properties of the quartzes were measured also by heating a small piece of quartz in CO atmosphere. The investigated quartzes have different reactivity in reducing atmosphere. The carbothermal reduction experiments show differences in the reacted charge between pellets and lumps as charge material. Solid–gas reactions take place from the inside of the pellets porosity, whereas reactions in lumps occur topochemically. Silicon in pellets is produced mainly in the rim zone. Larger volumes of silicon have been found when using lumpy charge. More SiO is produced when using pellets than for lumpy SiO₂ for the same molar ratio and heating conditions. The two SiC polytypes used in the carbothermal reduction experiments as carbon reductants presented different reactivity.     

Automatic control of the carbon potential of furnace atmospheres without adding enriched gas

An approach has been proposed in this article for controlling the carbon potential of furnace atmosphere without the need to add enriched gas for the sake of simplifying the process of carbon potential control and consequently lowering the cost of atmosphere. The atmosphere was generated by reacting an N₂ + H₂O mixture with hot charcoal. An oxygen sensor and a computer were used for controlling the carbon potential of the atmosphere through controlling the temperature of charcoal instead of adding an enriched gas to the atmosphere. The control result was evaluated from (a) the stability of oxygen partial pressure of the furnace atmosphere and (b) the equilibrium carbon content of the steel heated in the furnace atmosphere. The result of controlling carbon potential with this method was indicated by observed experiments to be satisfactory;i.e., the deviation of carbon potential was below 0.06 pct. There was especially no need to add enriched gases to the atmosphere.     

Effect of methane on sulfur dioxide and sodium sulfate-lnduced corrosion of nickel-base alloys

The corrosion of nickel-base binary alloys, with up to 20 wt pct Cr or Al, by sulfur dioxide contained in a nitrogen gas stream, has been found to be considerably enhanced between 800 and 1000°C in the presence of low concentrations of gaseous methane. The magnitude of the effect, which is related to the ability of the alloy surface to catalyze the decomposition of the hydrocarbon to solid carbon, decreases with increasing Cr and Al content of the alloy. Ni-Al alloys containing less than 10 wt pct Al are more readily attacked by SO₂ alone than are the corresponding Ni-Cr alloys, although the deleterious effects of added methane are more marked in the Ni-Cr series. The extent of high temperature corrosion of these alloys by deposited sodium sulfate at 900 to 1000°C is also generally increased when small amounts of methane are introduced, although in this case the addition of Cr to the alloy confers greater corrosion resistance than does the addition of an equivalent concentration of Al. In the presence of either sodium sulfate or SO₂, the addition of methane to the ambient nitrogen atmosphere results in localized reducing conditions and increased sulfur potentials near the alloy surface.     

High temperature oxidation of tungsten wires in water vapor-argon mixtures

The high temperature oxidation of tungsten wires in free convection was measured in water vapor-argon atmospheres in the temperature range between 2450 and 3000°K and in the H_{2 O} partial pressure range between 3.39 × 10^-5 and 1.82 × 10^-4 atm at 0.789 atm total pressure. The rate of the reaction generally decreases with increasing temperature at constant bulk water vapor pressure. This dependence is similar to that previously measured in O₂-argon and CO₂-argon atmosphere. The overall oxidation reaction is gas transport controlled as demonstrated by supplemental measurements at 0.395 atm total pressure. A volatile oxide counter diffusion model, which has been previously proposed to explain the oxidation in O2-and CO₂-argon, is in good agreement with H₂O-argon experiments. A comparison of the present results with the previous experiments shows that the model is in agreement with the relative magnitude of the oxidation rates measured in the three oxidizing atmospheres.     

Carburization and gas reactions of hydrocarbon-nitrogen mixtures at 850 °C and 925 °C

Rates of carburization of low-carbon steel by CH₄, C₂H₂, C₂H₄, C₂H₆, and C₃H₈ in N₂ have been measured gravimetrically at 850 °C and 925 °C. Methane appears to be the slowest and acetylene the fastest carburizing agent among the hydrocarbons tested. Hydrogen enhances the rates of carburizing of all hydrocarbons, probably by removing adsorbed oxygen from the steel surface. At high H₂/CH₄ ratios, H₂ will decarburize steel at 925 °C. All hydrocarbons, including CH₂, are also involved in gas phase reactions. These reactions may lead to the formation of soot at carburizing temperatures. Sooting is inhibited by the addition of H₂ to hydrocarbon-nitrogen gas mixtures. Acetylene appears to be a key intermediate for the formation of soot as the final product of hydrocarbon reactions in the gas phase.     

Simultaneous plasma treatment for carburizing and carbonitriding using hollow cathode discharge

Ion carburizing and nitriding are effective processes for saving energy and providing polutionless surface treatment but have the disadvantage of using much electric energy. A cylindric subsidiary cathode was set up around a rod-shaped workpiece with a gap, and hollow cathode discharge for ion carburizing was studied. Thus, simultaneous plasma treatments for ion carburizing and ion car-bonitriding in one workpiece were researched using Cr-Mo steel to save electric treatment power. First, the effects of the gap between the test piece and subsidiary cathode and the pressure of electric discharge gas, including methane gas, on fundamental plasma treatment conditions were experimen-tally researched. It was found that the temperature for ion carburizing in a H₂-N₂-Ar-CH₄ gas mixture was 1123 to 1193 K with a gap of 3 to 5 mm under a gas pressure of 133 to 532 Pa. Next, the test piece was ion carburized with hollow cathode discharge and carbonitrided with normal glow dis-charge simultaneously. The ion-carburized layer was formed in the area covered by the subsidiary cathode. The surface hardness was 800 Hv, the effective case depth was 0.6 mm, and the surface carbon content was 0.75 wt pct. An ion carbonitriding layer was formed in the area without the subsidiary cathode. The surface hardness was 700 Hv and the case depth was 0.1 mm. It is useful to form the different layers of ion carburizing and ion carbonitriding in one treatment process and to give different mechanical and tribological properties on one workpiece simultaneously.