δ-γ phase equilibria in iron-rich iron-chromium-nickel alloys at temperatures between 1200 and 1350 °C

The δ-γ phase equilibria of iron-rich iron-chromium-nickel alloys with 23 to 38% Cr, 7 to 22% Ni are established at temperatures between 1200 and 1350 °C. The effect of chemical compositions and annealing temperatures on equilibrium δ ferrite fraction is discussed.     

Die Thermokraft von Aluminiumnitrid bei Temperaturen zwischen 1350 und 1650 °C in Argon- und Stickstoffatmosphäre

Beschreibung der Versuchsapparatur zur Messung der Thermospannung der Kette Rh |_{T + Δ} AIN |_T Rh für Temperatur-differenzen bis zu ± 60 °C zwischen 1350 und 1650 °C. Ermittlung der Thermokraft und ihres homogenen Anteils. Berechnung der Überführungswärme der wandernden Ionen aus der homogenen Thermokraft. Erörterung zum Leitungsmechanismus im Aluminiumnitrid AlN.     

Influence of Nanometric Ceria Coating on Oxidation Behavior of Chromium at 900 ℃

Isothermal and cyclic oxidation behaviors of chromium samples with and without nanometric CeO2 coating were studied at 900℃ in air. Scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, and high-resolution electron microscopy （HREM） were used to examine the morphology and microstructure of the oxide film. It was found that ceria coating greatly improved the oxidation resistance of Cr both in isothermal and cyclic oxidizing experiments. Acoustic emission （AE） technique was used in situ to monitor the cracking and spalling of oxide film, and AE signals were analyzed in time-domain and number-domain according to related oxide fracture model. Laser Raman spectrometer was also used to study the stress of oxide film formed on Cr with and without ceria. The improvement in oxidation resistance of chromium is believed mainly due to that ceria greatly reduced the growth speed and grain size of Cr2O3. This fine grained Cr2O3 oxide film might have better high temperature plasticity and could relieve parts of the compressive stress by means of creeping and maintained ridge character and relatively lower level of internal stress. Meanwhile, ceria application reduced the size and number of interfacial defects, remarkably enhanced the adhesive property of Cr2O3 oxide scale formed on Cr substrate.     

Solubility prediction of malachite in aqueous ammoniacal ammonium chloride solutions at 25 °C

The solubility of malachite in the presence of ammonia, ammonium chloride and their mixed solution is calculated by a geochemical modeling code and is measured in a series of dissolution experiments using synthetic malachite at 25 °C. The simulated results show a good agreement with the experimental data gained at 25 °C. The predicted and experimental results indicate that the precipitate CuO limits the copper solubility in aqueous ammonia and Cu(OH)_1.5Cl_0.5 in aqueous ammonium chloride. For a mixed solution containing ammonia and ammonium chloride, highest copper solubility can be achieved by adjusting the [NH₃]/[NH₄Cl] ratio to about 2:1. The thermodynamic model presented rationalises the interactions between the different components and predicts the influence of changes in the concentration of ammonia and ammonium chloride on the copper solubility of malachite.     

Research on Microstructure and Mechanical Property of S30432 Supreheater Pipe Aged at 700°C

The microstructure and performance of S30432 supreheater pipe were investigated before and after it is aged 3000 h at 700°C. A lot of twins disappeared after aging, M₂₃C₆ particles precipitated along the grain boundary and MX particles precipitated near the dislocation. After the endurance experiment of the aged pipe, M₂₃C₆ precipitations with the size of about 50 nm pinned dislocations and the cavity appeared on the grain boundary. The aged S30432 supreheater pipe showed the ductile fracture. Increasing temperature made the inter‐granular fracture transfer the quasi‐cleavage fracture and the equiaxed morphology change the parabola morphology. The strength of the untreated specimen decreased with the temperature increasing. The hardness and the impact energy of the aged specimen were larger than those of the untreated specimen. Tensile strength and elongation of untreated specimen were larger than that of aged specimen.     

Kinetics of iron oxidation with water vapour at temperatures between 1300 and 1450°C

In this paper measurements of the kinetics of iron oxidation in H₂O-containing gas mixtures of various compositions in the temperature range of 1300 to 1450°C, and metallographic examinations are described. The reaction product is solid wustite below and liquid iron saturated iron oxide above 1377°C, this temperature being the incongruent melting temperature of iron saturated wustite. The experiments were carried out by measuring the weight increase during the oxidation of an iron specimen connected to a thermo-balance with a platinum wire. The water vapour was generated by means of a water vapour saturator. The rate law is linear in the beginning of the single experiment and later becomes parabolic. The linear law was interpreted as being caused by two resistances connected in series: the transport of the oxidising gas through the adjacent gas boundary layer, and the phase boundary reaction at the oxide interface. The parabolic law was interpreted as being determined by the transport of iron ions and vacancies through the growing oxide layer. The resistance of gas transport becomes negligible above a certain critical gas velocity which is, for example, 23 cm/s at 1342°C. The temperature-dependent values of the phase boundary reaction rate constant were calculated with the help of known theories from the results of those experiments in which the gas velocities were above the critical value for gas transport at the respective temperatures. The parabolic law did not apply, when the oxidation product was liquid as under these circumstances the formed oxide dropped off the specimen during the experiment and hence, became no thicker than 22 μm. For all the experiments the oxide layer was composed of wustite, even when the oxygen potential of the reaction gas was far higher than that for equilibrium of the gas with wustite and magnetite. The surface structure of the oxide layer and the grain sizes varied with temperature. At lower temperatures the grains were relatively small while at higher temperatures they became extremely large up to a diameter of 6 mm.     

Finite element modelling of the creep deformation of T91 steel weldments at 600°C

Finite element modelling of the creep deformation of T91 steel weldments, welded using the manual metal arc (MMA) and submerged arc (SA) welding processes, was carried out to predict creep curves for both of the weldments under different stresses and compared with the experimental data. The stress and strain redistribution across the length of the transverse-weld specimens has also been predicted. Data of creep tests at 600°C at stresses between 90-130 MPa for the base metal, the MMA and SA weld metals, and the simulated heat-affected zone were used to determine Garofalo's equation for creep strain. Finite element meshes for both of the weldments were constructed after calculating the HAZ locations using Rosenthal's heat flow equation.     

Elevated-pH bioleaching of a low-grade ultramafic nickel sulphide ore in stirred-tank reactors at 5 to 45 °C

This study is a continuation of previous work designed to assess the effect of elevated-pH bioleaching on a low-grade ultramafic nickel sulphide ore from Manitoba, Canada. The ore contains 21% magnesium and 0.3% nickel. Nickel is the only significant metal value, and is present primarily as pentlandite. A substantial fraction of the magnesium is present as lizardite, making processing of the ore difficult with conventional pyro- and biohydrometallurgical techniques. This work has two objectives: to maximize nickel extraction, and to minimize magnesium mobilization. Five-week stirred-tank bioleaching experiments were conducted with finely ground ore (− 147 µm) at three pH levels (3, 4 and 5) and five temperatures (5, 15, 22.5, 30, and 45 °C). The initial rate of nickel extraction from pentlandite was observed to be inversely correlated to acidity at all temperatures, while the final extraction of nickel after 5 weeks was determined to be moderately correlated to acidity at high temperatures and negatively correlated to acidity at low temperatures. The advantage of elevated-pH bioleaching was most evident at 5 °C, in which the final extraction of nickel at pH 5 was approximately 250% greater than at pH 3. Electron probe X-ray microanalysis of the post-leach residues revealed that the un-reacted lizardite was enriched with nickel during experiments conducted at pH 5, and that the extent of enrichment was a strong function of temperature. The undesirable extraction of magnesium exhibited a strong negative pH–temperature interaction and the consumption of sulphuric acid directly tracked the extraction of magnesium over all experimental conditions. Bioleaching at elevated pH substantially increased the ratio of nickel to magnesium in the leachate, and resulted in a substantial reduction in sulphuric acid consumption.     

Influence of alumina and magnesia on phase equilibria in the system Fe‐CaO‐FeOn‐SiO2 at 1600 °C

The phase relations in the systems CaO‐FeO_n‐SiO₂‐Al₂O₃ and CaO‐FeO_n‐SiO₂‐MgO at 1600 °C saturated with liquid iron are described. The phase conditions in the quaternary system with aluminium oxide are so far rather unknown. An addition of aluminium oxide makes the calciumsilicates and lime saturation areas move towards the CaO corner thus enlarging the homogeneous liquidus space accordingly. Aluminium oxide can be considered as a flux in this case. The fields of different solid solutions in the system CaO‐FeO_n‐SiO₂‐MgO were taken into account. Some additional (2 to 4)‐phase areas occur with increasing MgO content. This causes the precipitation of magnesiowustite which dominates the system accordingly. The changed phase relations strongly influence the direction of tie‐lines and their length. Therewith, the correlation between liquid phase and solid phase, calculated via lever rule, is influenced, too.     

The kinetics of carburization of γ-iron in CO-He and CO-H2 atmospheres at 920°C

Investigation of the carburization of thin iron foils in CO-He or CO-H₂ gas mixtures at 920°C in a flow apparatus by a resistance method. Discussion of the kinetics, mechanism of the chemical surface reaction steps in the carburization. Influence of the carbon diffusion in the solid iron on the kinetics of the carburization of thick samples.     

On the kinetic characteristics of the reduction and sublimation of phosphorus from melts

It is shown that the literature data on the mechanism of the carbothermal reduction and sublimation of phosphorus from oxide melts are contradictory. The results of our studies are presented. An experimental procedure is proposed to calculate the reaction rate on the unit surface of the reducing agent. The regime and kinetic characteristics of the process change with the concentration of P₂O₅ in the oxide melt, the temperature, and the presence of impurities and metallic phase in the melt.     

Kinetics of iron oxidation with CO2 between 1300 and 1450 °C

The paper describes measurements of the kinetics of iron oxidation in CO₂-CO mixtures of various compositions in the temperature range of 1300 to 1450 °C. The reaction product is solid wustite below and liquid iron saturated iron-oxygen melt above 1377 °C, the incongruent melting temperature of iron saturated wustite. The experiments were carried out by measuring the weight increase during the oxidation of an iron specimen connected to a thermo-balance with a platinum wire. The specimen had the shape of a flat plate, and the oxidising gas flowed past this plate with a defined velocity. Among the experiments the flow velocities were varied. The rate law is linear in the beginning of the single experiment and later becomes parabolic. The linear law was interpreted as caused by two resistances connected in series: the transport of the oxidising gas through the adjacent gas boundary layer, and the phase boundary reaction at the oxide interface. The parabolic law was interpreted as determined by the transport of iron ions and vacancies through the growing oxide layer. The resistance of gas transport becomes negligible above a certain critical gas velocity which is 24 cm/s at 1400 °C. The critical gas velocity increases with rising temperature. The gas transport resistance was described by known theories of mass transfer in front of a solid wall. The temperature-dependent values of the phase boundary reaction rate constant were calculated with the help of known theories from the results of those experiments where the gas velocities were above the critical value for gas transport. From the parabolic law the diffusion coefficient of vacancies in wustite was calculated. The parabolic law does not appear, when the oxidation product is liquid as under these circumstances the formed oxide drops off the specimen during the experiment and hence, becomes not thicker than 22 μm. All the results join well with older results for lower temperatures known from literature.     

Phase equilibria in the system Na2CO3-CaCO3-CO2 at 750 to 900°C

Phase diagram of the binary system Na₂CO₃-CaCO₃. Determination of solid state phase equilibria by thermal analysis at 750 to 900°C and bar.     

Microstructural Evolution and Change in Hardness of S30432 Heat‐resistant Steel during Creep at 650 °C

The microstructural evolution of S30432 heat‐resistant steel during creep at 650 °C and its effect on the change in hardness was investigated. The change of hardness during creep of S30432 at 650 °C can be divided into three stages. These are related to the precipitation and coarsening of ε‐Cu and M₂₃C₆ carbides, decrease in the number of twins and increase in grain size. The precipitation of ε‐Cu dominantly contributes to the significant hardening at stage I, and the coarsening of ε‐Cu is the key factor to decrease the hardness at stage II. At stage III, the hardness hardly changes since the microstructure of S30432 tends to be stable in the long‐term creep range.     

Dependence of Accelerated Creep Rate at 580 °C and Room Temperature Yield Stress for Two Creep Resistant Steels

Two creep resistant steels, P91 and X20, were tempered for 17520 h at 650 °C or 8760 h at 750 °C to study the growth and redistribution of carbide precipitates in martensite. On specimens annealed for a different time, yield stress at room temperature and accelerated creep rate at 580 °C were determined. With increasing yield stress in the range from 350 to 650 MPa the accelerated creep rate decreased continuously by about 2 orders of magnitude from 8·10^?7 s^?1 to 5·10^?9 s^?1. For equal yield stress, the creep rate was slightly lower for the steel P91 than for the steel X20.