Pct H2 Research Articles

Hydride Formation Process for the Powder Metallurgical Recycle of Zircaloy from Used Nuclear Fuel

A Zircaloy recycle process is being investigated based on the underlying principle that Zr reacts with H2 to form ZrH2, which is the same reaction that produces performance limiting ZrH2 formations in reactor cladding. However, in the proposed application, hydride formation is an enabling phenomenon that will embrittle and crumble Zircaloy as a precursor for milling and dehydration to form Zircaloy metal powder. Hydride formation experiments were performed to quantify the primary process variables of time and temperature. These experiments were performed by hydriding nuclear grade Zircaloy-4 tubes under flowing gas (Ar-5 pct H2) for various times and temperatures. The results were used to create a correlation for the formation of zirconium hydride as a function of time and temperature. Further, it was observed that it was much more effective to hydride the Zircaloy-4 tubes at temperatures below the α-β-δ eutectoid temperature of 818 K (545 °C), presumably related to the high hydrogen solubility of β-Zr. Samples treated below this temperature readily crumbled during the hydride formation reaction and were subsequently easily ground to powder, making this the ideal temperature range for the proposed recycle method. Hydrogen pickup was faster above 818 K (545 °C), but the samples were generally tougher.

Temperature Measurement Deviation during Annealing of Multiphase Steels

A pyrometric analysis of the temperature of continuous annealed strip material is an essential component of industrial production. However, despite calibration, the development of composition-dependent oxides during annealing prevents an accurate pyrometric measurement of actual strip temperature. An Al-based and a Si-based transformation induced plasticity (TRIP)-assisted steel were pyrometrically analyzed during annealing under a reducing atmosphere (5 pct H2 and 95 pct Ar). Oxide development on both steels led to a deviation between pyrometer readings and simultaneous thermocouple measurements. The developing oxides influenced the pyrometer readings to different extents because of unique emissivities and different growth kinetics.

Selective Oxidation of a CMnSi Steel during Heating to 1000 °C: Reversible SiO2 Oxidation

Many multiphase cold-rolled high-strength steels require complex thermal processing, including a (partial) austenitization to reach the desired microstructure. However, of importance for their galvanizability, the selective oxidation of these steels in the austenitic region has not been studied extensively. The present work aims at characterizing the selective oxidation of a CMnSi steel during heating up to 1030 °C by X-ray photoelectron spectroscopy (XPS) surface analysis. A phenomenological description of the evolution of the surface composition during heating and soaking in a N2-5 pct H2 atmosphere with a dew point of −50 °C is presented. Four distinct stages of selective oxidation were identified during heating; manganese oxidation, SiO2 film growth, a transition stage, and a steady state with boron nitride (BN) passivation. During the transition stage, the previously formed SiO2 film disappears, resulting in increased iron concentration at the surface. Simultaneously, a BN film forms on the uncovered metallic iron, inhibiting any further oxidation of the surface. It is demonstrated that the SiO2 oxidation is reversible and temperature controlled.

Effect of Firing Temperature and Reducing Gas Composition during Low-Temperature Reduction of Nanocrystalline Fe2O3

Pure nanocrystalline hematite (40 to 100 nm) compacts were prepared and sintered at various temperatures (300 °C to 600 °C) and then reduced with 100 pct H2 at 500 °C. On the other hand, fired compacts at 500 °C were reduced with a H2-Ar gas mixture containing different concentration of hydrogen (100, 75, 50, and 25 pct) at 500 °C using thermogravimetric techniques. Nanocrystalline Fe2O3 compacts were characterized before and after reduction with X-ray diffraction, scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and reflected light microscope. It was found that the fired compacts at 400 °C to 600 °C have relatively faster reaction behaviors compared to that at lower firing temperature 300 °C. By decreasing the firing temperature to 300 °C, partial sintering with grain growth was observed clearly during reduction. Also, it was found that the reduction rate increased with increasing hydrogen content in the reducing gas. Comparatively, grain growth and partial coalescence took place during reduction with 25 pct H2 due to long reaction time.

High-temperature environmental embrittlement of thermomechanically processed TiAl-based intermetallic alloys

Thermomechanically processed TiAl-based intermetallic alloys with various alloy compositions and microstructures were tensile tested in various environmental media, including air, water vapor, and a gas mixture of 5 vol pct, H2 + Ar, as functions of temperature and strain rate. All the TiAl-based intermetallic alloys showed reduced tensile fracture stress (or elongation) in air, in water vapor, and in a gas mixture of 5 vol pct H2 + Ar, not only at ambient temperature (RT ∼ 600 K), but also at high temperature, from 600 to 1000 K (and sometimes at temperatures higher than 1000 K). The high-temperature environmental embrittlement of TiAl-based intermetallic alloys depended upon the microstructure. The factors causing the high-temperature environmental embrittlement may include hydrogen atoms decomposed from water vapor (H2O) or hydrogen gas (H2), similar to those causing the low-temperature environmental embrittlement. Also, it is demonstrated that the oxidized scale is effective in reducing high-temperature environmental embrittlement.

The physical chemistry of thermal decomposition of South African chromite minerals

Natural chromite minerals form extended solid solutions with binary spinels of FeCr2O4, MgCr2O4, FeAl2O4, and MgAl2O4. The decomposition of natural spinels strongly depends upon the chemical potential imposed in the forms of temperature, pressure, and pH difference in aqueous media (e.g., during natural weathering). In this investigation, we studied the thermal decomposition behavior of South African chromite ores in order to relate the influence of oxygen potential with the likely product phases formed. The decomposition is also a generic step in the understanding of the formation of sodium chromate during soda-ash roasting and the reduction of chromite ores for ferrochrome alloy making. The phase equilibria in South African chromite minerals were investigated by isochronal thermal analysis and isothermal heat treatment of chromite mineral in air, argon, and Ar-5 pct H2 atmospheres over a temperature range from 473 to 1473 K. The effects of the oxygen partial pressure and temperature on the phase constituents of the heat-treated product are discussed by referring to the results of X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray detector (EDX), and electron probe microanalysis (EPMA). The structure of phases formed and the morphology of phase-separated regions in chromite appear to be strongly dependent on the oxygen partial pressure. The mechanism of the decomposition of complex spinel phases is described under the influence of oxygen partial pressure and temperature.

Reactive wetting of SiO2 substrates by molten Al

The reactive wetting behavior of SiO2 substrates by molten Al was investigated at temperatures between 800 °C to 1250 °C in a purified Ar-3 pct H2 atmosphere of about 0.11 MPa using an improved sessile drop method. The time dependence of the changes in contact angle and droplet geometry was monitored and the wetting kinetics was identified. The initial equilibrium or quasi-equilibrium contact angles are generally larger than 90 deg and do not significantly vary with temperature. The subsequent remarkable decrease in the contact angle mainly results from the progressive decrease in the droplet volume rather than the advance of the solid-liquid interfacial front. The significant effect of temperature on the wetting kinetics is essentially related to its effect on the reaction and molten Al penetration progress. For systems with a considerable decrease in the droplet volume during reactive wetting, a criterion for evaluation of the true wetting improvement was proposed.

Kinetics of carbide formation from reduced iron in CO-H2-H2S mixtures

Gravimetric measurements were made for the kinetics of the carbide formation from reduced iron by preventing the decomposition of iron carbides and the deposition of carbon by utilizing the effect of sulfur. Particles of a hematite ore were completely reduced in the H2-H2S mixture and converted to iron carbide in a CO-H2-H2S-CO2-H2O-Ar or CO-CO2-COS-N2 mixture at 873 to 1073 K. The carbide formation kinetics was drastically accelerated by the addition of a few percent H2 to CO and reached the maximum at about 20 pct H2. From morphology observation, it was assumed that the carburization reaction on the surface of pores in the reduced iron determines the carbide formation kinetics, and the carburization rate, dfθ/dt, is expressed at the product of a function of carburization rate, g(pi, T), and the relative reaction surface area at the time t, 1 − fθ. Applying the integrated rate equation, -ln (1−fθ)=g(pi, T)t, to the experimental results, the value of g(pi, T) was obtained. By analyzing the carbide formation rate, it was assumed that the elementary reactions determining the carbide formation rate are CO+H(ad) → [C]+OH(ad) and OH(ad)+H2= H2O+H(ad), except in the low ratio of \(p_{H_2 } /p_{CO} \), where the rate determining elementary reaction shifts to O(ad)+CO → CO2. The overall rate Eq. [22] has been proposed for the carbide formation from the reduced iron in CO-H2-H2S-CO2-H2O mixtures.

Hydrogen-induced internal-stress plasticity in titanium

Internal-stress plasticity is a deformation mechanism resulting from the biasing of internal mismatch stresses by an external stress.[1] These internal stresses can be produced by a temperature change in materials with grains (or phases) exhibiting different thermal expansion coefficient[2] or different density during a phase transformation[3]; alternate sources of internal stresses include compressibility mismatch during pressure changes[4] and swelling mismatch during irradiation.[5] The most common type of mismatch generation is thermal, as it easily lends itself to cycling during which tensile strains well in excess of 100 pct can be accumulated; this phenomenon is known as internal-stress superplasticity.[1] The purpose of the present article is to report, for the first Fig. 1—Titanium-hydrogen phase diagram[14] showing chemical cycles time, internal-stress plasticity where internal stresses are used in the present study of chemically induced internal-stress plasticity induced by a reversible change of chemical composition (at at 805 or 860 8C ( pH2 5 0 } 4.1 kPa) as well as typical thermal cycles constant temperature and pressure) rather than a change used in the literature for thermally induced internal-stress plasticity in of temperature or pressure (at constant composition), as hydrogen-free Ti (T 5 860 } 900 8C). illustrated in Figure 1. We use the Ti-H system to demonstrate this effect, because hydrogen diffuses rapidly in and out of titanium where it produces mismatch strains both directly (by lattice swelling in the presence of concentration gradients) and indirectly (by inducing the hcp (a-Ti) to bcc (b-Ti) transformation), as demonstrated in a recent study of hydrogen-induced ratchetting.[6] We note that this chemically induced internal-stress plasticity is fundamentally different from thermochemical processing of titanium[7] where hydrogen alloying is used to produce a weaker, more easily workable b-Ti phase, and where internal stresses are unimportant. Titanium plates (99.7 pct pure, from Alpha Aesar, MA) were machined into dog-bone specimens with a gage 28 to 52 mm in length, 5.4 to 6.0 mm in width, and 2 mm in thickness. Specimens were tested in a low-stress creep apparatus described earlier,[8] modified to allow for a gas atmosphere flowing at a rate of 1.22 L/min, which could be changed from Ar (99.999 pct pure) to an Ar/4 vol pct H2 mixture. This 4.1 kPa hydrogen partial pressure corresponds to an equilibrium hydrogen concentration in titanium well within the b-Ti field (Figure 1). The specimen strain was continuously measured at the cold end of the load train and the temperature was maintained at a constant value of 805 8C 6 2 8C or 860 8C 6 2 8C by a thermocouple contacting the gage section or the pull head. Figure 2 shows the creep rate as a function of the applied Fig. 2—Isothermal creep rate at 805 8C and 860 8C for hydrogen-free astress for hydrogen-free a-Ti and for fully hydrogenated bTi and fully hydrogenated b-Ti. Ti at temperatures of 805 8C and 860 8C. The stress exponents

Surface tension of molten silicon measured by the electromagnetic levitation method under microgravity

The surface tension of molten silicon was measured using the electromagnetic levitation method under microgravity. The atmosphere was Ar-3 pct H2 purified by Pt asbestos and magnesium perchlorate. The partial oxygen pressure is estimated to be less than 1.1 × 10−14 Pa. The spherical droplet shape was controlled by changing the current ratio between a quadrupole coil and a dipole coil. The surface tension of molten silicon can be expressed by the equation $$\gamma = 735 - 0.074 (T - 1687)$$ where γ is the surface tension (in mN/m) and T is the temperature (in K). The value was measured over a wide range of temperatures, from 230 K below the melting point (1687 K) to 1890 K. The scatter of the measured surface-tension values is much less than that measured by conventional methods.

Thermodynamics on the formation of spinel nonmetallic inclusion in liquid steel

The nonmetallic spinel inclusion, MgOzAl2O3, has a high melting point and produces an undeformed C-type inclusion in steel products. Therefore, it is very harmful in the production of high grade wire, spring, and bearing steels. Thus, it is necessary to develop a methodology by which we can predict the occurrence of such defects. The thermodynamics of the formation of spinel nonmetallic inclusions during co-deoxidation with aluminum and magnesium is discussed in the present work. Previous literature thermodynamic values for strong deoxidizers such as aluminum, magnesium, and calcium are not in good agreement with the observed results and measurements. This discrepancy between predicted and measured results is due to an incomplete expression of the activities of dissolved oxygen and deoxidizers in current literature. Namely, only first-order interaction parameters are available despite the very strong interactions among the dissolved elements. In the present work, the deoxidation equilibrium with magnesium in liquid iron was studied in order to evaluate the activities of oxygen and dissolved magnesium and, include in the thermodynamic description, the firstand second-order interaction parameters, including the cross-product terms. A dolomite crucible was used to enhance the magnesium content in liquid iron in the present experiment on thermodynamics of spinel formation with reference to the study on desulfurization with a dolomite crucible undertaken by Ototani et al.[1] The assessment of aluminum-oxygen equilibrium in liquid iron reported by other investigators was also reviewed to develop values for the second-order interaction parameters. Finally, thermodynamic data on calcium deoxidation equilibrium were obtained. The iron sample was melted by a high frequency induction furnace. Iron of about 90 grams was charged into a crucible. The added weight of a deoxidizer such as magnesium and calcium was 1 mass pct of the weight of iron. The experimental procedure is briefly shown as follows. After the iron melted under a gas flow of a mixture of Ar-50 vol pct H2, hydrogen was impinged onto the melt surface to reduce dissolved oxygen content until it was less

Investigations on the influence of internal nitridation on creep crack growth in alloy 800 H

For two batches of Alloy 800 H with Al contents of 0.02 and 0.34 pct, creep crack growth was investigated at 1000 °C in air and in an Ar-5 pct H2 atmosphere. TheK concept, the net-section stress concept, and theC* concept of creep fracture mechanics were applied when plotting the experimental results. TheC* concept proved to give the best correlation between load parameter and crack growth rate. A good agreement was observed between the experimental results and the model calculations for crack extension by constrained diffusive cavity growth. Strong internal nitridation, which occurred in the air tests and which had been shown to increase the creep strength in creep rupture tests, did not show any significant influence on the creep crack growth rate in comparison with tests in an Ar-5 pct H2 atmosphere, in which no internal nitridation was observed. Also, the differences in the Al contents of the two batches did not play a role. This behavior is explained by the fact that neither the nitride particles nor the particle matrix interface is a particularly weak site in the material. It also becomes obvious that theC* concept can be rather insensitive to precipitation strengthening effects, if these only affect the parameterB in Norton's creep law.

Hot desulfurization of coal gas with copper

The hot desulfurization of coal gas (syngas) with copper has been investigated on a laboratory scale. The copper was in the form of particles embedded in pellets of alumina. It was found that gas with about 66 pct CO, 34 pct H2, and 0.3 pct H2S can be desulfurized at 750 °C, in a packed bed reactor to about 0.03 pct H2S. The regeneration was carried out with air at 900 °C yielding a gas with about 10 pct SO2. The process appears to be attractive for industrial applications because it has several advantages compared to other proposed hot desulfurization processes.

Corrosion of engineering alloys in simulated eastern oil shale retorting environments

Eastern United States oil shales usually contain sulfur levels considerably higher than those encountered in their Western counterparts. Because the emphasis in the development of Eastern oil shale retorting technologies is rather recent, very little is known about the requirements for materials of construction for various high temperature components. In this investigation, engineering alloys, including carbon steel, 410 ss, and 304 ss, have been exposed in the laboratory to environments approximating those encountered in indirectly heated Eastern oil shale retorts. These included temperatures in the neighborhood of 455 °C (850 °F) and gases with typically 1 pct H2, 1 pct H2S, and 10 pct H2O. Special runs were also made where samples were packed in spent shale to determine how such contact would affect performance. The corrosion rates and the corrosion products observed in the laboratory experiments resembled the trends and features encountered in pilot plant retort coupons exposed to comparable environment and temperature conditions. These experiments were extended to exposure times exceeding those of the longest retort alloy tests. The results are used to evaluate mechanisms of alloy deterioration and to aid in providing criteria for the selection of materials of construction.

Deterioration of electromotive force of chromel-alumel thermocouples in reducing atmospheres at high temperatures

The deterioration of electromotive force (emf) of Chromel-Alumel (CA) thermocouples in 80 pct H2 + 15 pct CO + 5 pct CO2 has been analyzed in terms of the corrosion behavior of Chromel. Emf of the CA thermocouple deteriorated drastically in 80 pct H2 + 15 pct CO + 5 pct CO2. After exposure for about 1000 hours at 900 °C, the decrease of emf was about 16 mV. The deterioration process could be separated into three terms. The first term, which has the smallest time constant of about 20 hours, was attributed to carbon deposition on the Chromel surface in the temperature range of 600 to 700 °C. The second term, which has a time constant of about 100 hours, was attributed to the severe internal oxidation of chromium in the temperature range of 500 to 800 °C. The third term, having the largest time constant of several thousand hours, might be attributed to the moderate and gradual preferential oxidation of chromium in Chromel in the range 800 to 900 °C. Boron nitride (BN) coating on CA thermocouples could reduce this deterioration of emf; the decrease of emf was improved to about 3 °C during 700 hours test at 900 °C.

Corrosion behaviors of inconel 617 in hydrogen base gas mixture

The corrosion behavior of Inconel 617, a candidate for the structural material of heat exchanger in the high temperature gas-cooled reactor (HTGR), has been investigated at elevated temperatures in the hydrogen base gas mixture (80 pct H2 + 15 pct CO + 5 pct CO2). This gas mixture simulates the reducing gas in the direct steel making system that uses heat from HTGR in Japan. This gas has relatively high oxidizing and carburizing potentials. In the temperature range of 650 to 1000 °C Inconel 617 oxidized to form a Cr2O3 scale containing titanium oxide. The activation energy for this process is estimated to be 50 to 60 kcal/mol. The time dependence of the growth of the surface oxide scale was parabolic. The aluminum in Inconel 617 was internally oxidized. The time dependence of the internal oxidation was noticed to obey a 0.4 power rate law. Carburization was noticed at 650 and 900 °C. At 900 °C, carbides containing Si, Ti, and Mo precipitated beneath the oxide scale for gas exposure times up to 200 h. After 200 h, the formation and growth of the surface scale suppresses carburization. The thermodynamic analysis of gas atmosphere proposed by Gurry could be applied successfully to the experimental results. Some inconsistency existed mainly because of the scale formation and direct gas-metal interactions.

Effects of shock loading and cold rolling on the structure and high temperature creep properties of γ′ strengthened Ni-18.6 pct Cr-4.3 pct Al

The effects of shock loading and cold rolling on the structure and high temperature creep properties of precipitation strengthened Ni-18.6 pct Cr-4.3 pct Al have been investigated. The creep properties of this alloy depend on whether the testing is conducted in forming gas (90 pct N2 + 10 pct H2) or in vacuum. It is found that cold rolling invariably increases the creep rate and decreases the creep life when testing is conducted in forming gas. This unexpected result is believed to be caused by strain concentrations at grain boundaries which are associated with cold rolling and which contribute to creep by promoting premature intergranular fracture. In order to minimize the effects of intergranular fracture and to determine the intrinsic effects of prestraining it is necessary to conduct creep tests in vacuum. Taking this precaution, we find that the creep strength of this alloy can be enhanced only slightly by cold rolling a few percent. Cold rolling by a greater amount or shock loading to about 70 kbar (6.99 × 109 Pa) results in higher creep rates than the asaged condition. It is shown that these unexpected weakening effects can be correlated with, and are probably caused by, the presence of misshaped γ′ precipitate particles. The irregular shapes of these precipitates are believed to be caused by the line tension effects of surrounding dislocations.

Gaseous reduction of iron oxides: Part IV. mathematical analysis of partial internal reduction-diffusion control

In this mathematical analysis of gaseous reduction of iron oxides, the partial internal reduction of the porous oxide and gas diffusion in the porous iron layer are considered simultaneously in deriving the rate equation. The rate equation, derived by partly analytical and partly numerical solutions, is well substantiated by the experimental results obtained previously. The following parameters, determined previously, are used in the application of the rate equation: i) specific rate constant for the gas reaction on the pore walls of wustite, ii) pore surface area of wustite, iii) effective gas diffusivity in the porous wustite formed during reduction of hematite, and iv) effective gas diffusivity in the porous iron layer. The effective depth of the internal reduction zone at the wustite-iron diffuse interface increases steeply with the progress of reduction beyond about 50 pct O removal. For reduction of 1 to 2 cm diam hematite spheroids in 100 pct H2, the gas composition at the diffuse iron-wustite interface is within 10 to 20 pct of that for the iron-wustite equilibrium; beyond about 50 pct O removal, the rate of reduction is controlled primarily by gas diffusion in the porous iron layer. From the mathematical analysis it is found that the relative depth of internal reduction increases with decreasing particle size and increasing temperature.

Constitution and mechanical properties of titanium-hydrogen alloys

Hydrogen forms a beta-stabilized system with titanium, with a beta eutectoid at about 300°C and 44 atomic pct H2. The solid solubility of hydrogen in alpha decreases from about 8 to about 0.1 atomic pct from 300°C to room temperature. Hydrogen has little effect on tensile properties, but decreases notch-bar toughness to a large degree. This latter effect appears to be the result of increased notch sensitivity.