Internal hydrogen supersaturation produced by dislocation transport

A quantitative model is presented for the generation of internal supersaturations of hydrogen by kinetic rather than thermodynamic means. The maximum brittleness and slow strain rate limit is treated. The process invoked assumes hydrogen transport to deposition sites by dislocation drag, and reemission of the hydrogen by diffusion. The kinetic supersaturation is calculated from a balance between hydrogen arrival and departure rates, for internal and external hydrogen sources, and for smooth section and crack geometries. When applied to ferritic and austenitic steels, the model predicts that the kinetic supersaturations are extremely small in all realistic situations.     

The effect of alloying elements on pearlite growth

The model developed by Sharma 1 and Sharmaet al. 2 for the mechanism of pearlite growth in ternary Fe-C-Cr alloys has been extended to other systems. Detailed thermodynamic and semi-empirical kinetic calculations have been carried out to determine pearlite growth rates for a number of Fe-C-X alloys (X = Mn, Cr, Ni, Si) and compared with the experimental data from the literature. It has been shown that the pearlite growth is, in general, controlled by the alloying element boundary diffusion at low supersaturations and carbon volume diffusion at high supersaturations, as predicted by the model. The transition temperature between the two mechanisms is a function of the amount and type of alloying element. Formerly a at the Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur, India     

Kinetics of the low-temperature hydrogen reduction of single-crystal magnetite

The hydrogen reduction of plates made from synthetic magnetite single crystals is studied in the temperature range 400–570°C. A mathematical model is developed and applied to describe the kinetics of gaseous reduction of plane finite objects. The following time contributions of individual process stages are distinguished with the least squares method and numerical integration: the nucleation of an iron phase, a chemical interaction of magnetite with hydrogen, and external and internal mass transfer. The diffusion and kinetic reduction parameters are calculated. The activation energy of the chemical stage (87.5 kJ/mol) is determined. An equation for the rate constant of the interaction of magnetite with hydrogen is proposed for temperatures below 570°C. A significant effect of the processes of consolidation and sintering of reduced iron on the internal diffusion parameters is detected in the low-temperature range.     

Synergism between creep ductility and grain boundary bubbles

The marked decrease in creep ductility that can be caused by internal pressure in grain boundary pores is modelled to treat the interaction between boundary diffusion, power law creep, and bubble pressure. The application to 2.25 Cr−1 Mo steel in high pressure hydrogen is treated numerically using a computer program since here the internal methane pressure in pores is known, and kinetic data exists. Reasonable agreement is obtained between the predictions and the observed loss in ductility in the presence of 21 MPa of hydrogen. With methane pressurized bubbles, the model suggests intergranular fracture is powerlaw creep limited at essentially all temperatures and stresses. Thus one predicts the hydrogen attack resistance in service to be strongly influenced by the creep strength of the alloy. In the absence of hydrogen (methane), intergranular fracture should be limited by diffusion creep and thus much more sensitive to pore density and boundary diffusion rate than strength. Possible application to recent high temperature steamline failures in welded pipe and to helium effects in nuclear reactor materials are also indicated.     

Alumina Nucleation,Growth Kinetics,and Morphology: A Review

The reaction between aluminum and the dissolved oxygen in liquid steel yields the precipitation of alumina crystals with characteristic morphologies and size distributions. Alumina precipitation and growth involve a wide spectrum of time, length, velocity, and thermodynamic supersaturation scales throughout the refining processes. The smallest length and time scales are those quantifying the nucleation kinetics and the initial growth at the highest supersaturation. Further growth and crystal morphology depend on the concentration of surface tension elements which inhibit the crystal's faces suffering further modifications due to washing effects provided by turbulent flows. This step involves longer time, velocity, length, and moderate supersaturations. The final step involves the growth of alumina inclusions through collision–agglomeration–sintering, and capillary processes on the surfaces of argon bubbles under low supersaturations. Jumps of supersaturation, due to late aluminum additions or steel reoxidation, lead to alumina crystals changing their morphology to dendritic-type growth. The present contribution reviews each one of these kinetic phenomena closing with the consequences that alumina precipitation has on the continuous casting process. The content of this review is useful to practitioners and theorists alike.     

A kinetic model for the competitive reactions of ozone with amino acid residues in proteins in reverse micelles

Lysozyme and 10 other proteins are solubilized in reverse micelles formed by 0.1 M sodium di-2-ethyl-hexylsulfosuccinate and 2.0-2.5 M water (pH 7.4) in isooctane solvent. Exposure of the protein-containing reverse micellar solutions to ozone causes oxidative damage to the proteins, as assessed by the oxidation of tryptophan residues. The oxidation product of the protein-bound tryptophan has a molar absorption coefficient of 3275 +/- 81 M-1 cm-1 (mean +/- S.D., n = 6) at 320 nm. The product is suggested to be a Criegee ozonide or a tautomer of the Criegee ozonide and not N-formylkynurenine. Ozonation of lysozyme in reverse micelles results in the formation of hydrogen peroxide in yields of only approximately 0.07 mol/mol of tryptophan residues oxidized. The recovery of hydrogen peroxide added as an internal standard to the lysozyme-containing reverse micellar solutions ranges from 84 to 88%, whether or not the samples are subjected to ozonation. This suggests that hydrogen peroxide is neither destroyed during the process of ozonation nor consumed by the protein to a significant extent in an adventitious reaction. A kinetic model for the overall reaction of ozone with the proteins is developed, taking into account the concentrations and the reactivities of individual amino acid residues toward ozone. The model predicts the fractional reaction of ozone with tryptophan residues in the proteins, despite differences in amino acid composition, molecular weight, and tertiary structures. The lack of influence of protein structure is confirmed further by the observation that the native lysozyme (with and without external S-carboxymethylcysteine) and S-carboxymethylated lysozyme give identical values of the fractional reaction of ozone with tryptophan residues. The kinetic equations for the competitive reactions of ozone with amino acid residues in proteins, with some minor modification, are applicable to ozonations on complex mixtures of lipids, proteins, and antioxidants.     

氧化钨氢还原动力学的研究进展

超细钨粉是目前制备多种硬质合金和结构材料的重要原料,其主要制备方法为氢还原法.在对大量文献资料进行查阅及分析的基础上,综述了氧化钨氢还原动力学的研究现状,包括氧化钨氢还原的基本原理、不同动力学条件下的还原历程以及物相转换.介绍了国内外学者在不同钨氧化物的氢还原过程中,通过改变动力学参数对产物钨粉的粒度、形貌、结构和性能的影响,以及实际工业生产的现状和存在的问题.重点介绍了目前氢还原动力学的几种模型及其方程,并对可能适用于氧化钨氢还原动力学研究的动力学模型进行了展望.      

Kinetics of Liquid-Phase Hydrogenation of Toluene Catalyzed by Hydrogen Storage Alloy MlNi_5

Asacleanandhigh efficiencyenergyresource ,hydrogenisthoughttobethemostpromisingsubstituteoffossilfuels .Storingandutilizingtheenergybyus ingthehydrogenasenergycarrieristhemostfavor ablemethodtosolvetheenergycrisis ,whichattractsthefocusoftheworld .Therehavebeenconsiderableprogressesinthetechnologyofhydrogen energyinre centyears .Reillyetal .[1] firstproposedtheconceptofthemetalhydrideslurryin 1 980s ,inwhichthede formationorruptureofthevesselsduetotheparticlepulverizationandthepoorheattransfer…     

Hydrogen transport by dislocations

A kinetic model is presented for the transport of hydrogen, as Cottrell atmospheres on dislocation, at a rate appreciably in excess of that for lattice diffusion. The particular destinations for the hydrogen which are modeled here are ductile fracture initiation sites such as inclusions and microvoids. The functional predictions of the mechanism are shown to be consistent with available experimental evidence on ductile fracture behavior in the presence of hydrogen.     

Hydrogen transport by dislocations

A kinetic model is presented for the transport of hydrogen, as Cottrell atmospheres on dislocation, at a rate appreciably in excess of that for lattice diffusion. The particular destinations for the hydrogen which are modeled here are ductile fracture initiation sites such as inclusions and microvoids. The functional predictions of the mechanism are shown to be consistent with available experimental evidence on ductile fracture behavior in the presence of hydrogen.     

Theory for growth of needle-shaped particles in multicomponent systems

A solution is presented for the growth of needle-shaped particles (paraboloids of revolution) in multicomponent systems that obey Henry’s law. Interface kinetics and capillarity effects are incorporated, and it is demonstrated that the maximum velocity hypothesis cannot be sustained if it is assumed that there is local equilibrium at the interface. The particle is unable to grow with equilibrium for small supersaturations when capillarity effects are prominent and for large supersaturations when the interface kinetics effect is large. A method to obtain the lengthening rate and tip radius is provided.     

The role of dislocations during transport of hydrogen in hydrogen embrittlement of iron

In order to assess the role of hydrogen transport in hydrogen embrittlement, one of the kinetic aspects of hydrogen embrittlement, the strain rate dependence, was analyzed in terms of hydrogen transport. The results have been analyzed in terms of a new model which takes into account atomic and macroscopic diffusion in describing dislocation-hydrogen interactions. It was found that the strong strain rate dependence of hydrogen embrittlement can be explained by a dynamic trapping effect, which is associated with an increase in the population of dislocations during deformation.     

Kinetic Model for Removal of Phenol by Horseradish Peroxidase with PEG

A kinetic model has been developed to simulate horseradish peroxidase-catalyzed polymerization of phenol in the presence of polyethylene glycol based on the following kinetics. The phenol conversion expression is a second-order Michaelis–Menten equation with respect to the concentrations of phenol and hydrogen peroxide. The enzyme inactivation is attributed to the polymer and hydrogen peroxide simultaneously. The inactivation by polymer is an apparently second-order reaction, first-order in each of enzyme and phenol, whereas the inactivation by peroxide is also an apparently second-order reaction, first-order in each of enzyme and hydrogen peroxide. The rates of consumption of hydrogen peroxide and polyethylene glycol are directly proportional to the rate of conversion of phenol. Experimental data show that the model output can predict the phenol removal and activity depletion realistically under a variety of reaction conditions. The model has been verified by predicting some independent experimental results on reaction stoichiometry, horseradish peroxidase dose effect, and semibatch operation with respect to hydrogen peroxide.     

The influence of stirring method on hydrogen removal during ladle treatment

A mathematical model based on fundamental transport equations for vacuum degassing in an Asea‐SKF ladle furnace was developed earlier and predicted hydrogen values from the model were found to agree well with experimental measurements from plant trials. In the present study, the mathematical model was used to predict the hydrogen removal for four different stirring methods: 1–2) combined gas and induction stirring with the inductive stirrer working upwards or downwards and 3–4) gas stirring with two different locations for two porous plugs. Plant data was used as input to the calculations. The hydrogen calculation was validated by simulation of an actual heat and comparison with measured data. After 44 minutes of stirring the predicted and measured hydrogen contents were 1.6 and 1.8 ppm, respectively. This agreement was considered satisfactory and the model was then used to study the effect of different stirring methods on hydrogen refining. A change in the stirring method was found to affect the hydrogen refining. Combined gas and induction stirring using downward stirring was found to result in the lowest final hydrogen content in the steel as it rendered the most favourable kinetic conditions for hydrogen refining.     

Reduction Kinetics of Metal Oxides by Hydrogen

In this paper, a new kinetic model has been developed for reduction of metal oxides with hydrogen under both isothermal and non‐isothermal conditions. This model describes the kinetics of single reductive reaction and double reductive reactions by considering the diffusion and chemical reaction controlling mechanisms. In particular, the model is in the analytic form of expressing the reduction extent as an explicit function of time, temperature, radius of the particle, and hydrogen partial pressure, which is convenient for using and theoretical analysis. The reduction kinetics of nickel oxide, natural ilmenite, and Fe₂MoO₄ agree well with the theoretical results by the present model.     

钢中氢热脱附谱线影响因素的数值模拟

热脱附谱线分析已广泛用于研究高强钢中氢原子的吸收与扩散行为,以及氢原子与氢陷阱之间的反应动力学参数等。通过分析热脱附谱线峰值温度可获取氢陷阱的最重要参数：氢陷阱与氢原子之间的结合能。采用McNabb-Foster模型系统模拟研究钢中氢的热脱附谱线的影响因素。结果表明：除氢原子与氢陷阱之间的结合能之外,热脱附实验加热速率以及初始实验温度,样品尺寸、形状,氢原子在氢陷阱中的初始占有率,以及在样品中的分布等均能影响热脱附谱线的峰值温度以及形状,从而对氢陷阱与氢原子之间的结合能分析产生影响。同时基于局域平衡模型的模拟结果发现热脱附过程中氢原子从低结合能氢陷阱解析后可再次被高结合能陷阱捕获。     

Kinetic enrichment of hydrogen at interfaces and voids by dislocation sweep-in of hydrogen

A rigorous treatment is presented to answer the question of whether the dislocation sweep-in of hydrogen can result in enrichment of hydrogen at voids and boundaries. It is concluded that significant enrichment can occur in both low and high hydrogen diffusivity situations whether the enrichment is contributed by a dislocation annihilation process or by stripping of hydrogen atoms from dislocations by the internal traps. The model presented is based on enrichment controlled by long-range diffusion. It calculates the diffusion leakage current for the case where unsaturated trap interfaces simultaneously fill and leak hydrogen atoms into the matrix by diffusion, and compares this amount with the arrival current of hydrogen to the traps by dislocation sweep-in. The relative magnitudes of the two currents are found to depend on appropriate, calculable time constant values. These values are evaluated for physical situations of interest.     

Hydrogen Transport and Rationalization of Porosity Formation during Welding of Titanium Alloys

The transport of hydrogen during fusion welding of the titanium alloy Ti-6Al4V is analyzed. A coupled thermodynamic/kinetic treatment is proposed for the mass transport within and around the weld pool. The modeling indicates that hydrogen accumulates in the weld pool as a consequence of the thermodynamic driving forces that arise; a region of hydrogen depletion exists in cooler, surrounding regions in the heat-affected zone and beyond. Coupling with a hydrogen diffusion-controlled bubble growth model is used to simulate bubble growth in the melt and, thus, to make predictions of the hydrogen concentration barrier needed for pore formation. The effects of surface tension of liquid metal and the radius of preexisting microbubble size on the barrier are discussed. The work provides insights into the mechanism of porosity formation in titanium alloys.     

Neurological deterioration following head injury: the eyes had it

We present a semiclassical treatment for the inclusion of kinetic energy-dependent variations in the moment of inertia for certain molecules exhibiting internal rotational motion. The model is applied to trifluoroacetyl bromide and the resulting dynamically modified transition energies yield an improved spectral deviation in contrast to the rigid model. Copyright 1998 Academic Press.     

Role of Mass Transfer Resistance in Overall Substrate Removal Rate in Upflow Anaerobic Sludge Bed Reactors

A kinetic model (incorporating intrinsic Haldane kinetics) and an empirical model (incorporating apparent Haldane kinetics) for phenol degradation in upflow anaerobic sludge bed (UASB) reactors were used. UASB-reactor performance data were also generated for model verification. From the independent batch study together with statistical analyses, the apparent Haldane kinetic constants k′ and Ki′ did not differ from the intrinsic k and Ki, but the apparent Ks′ was significantly larger than the intrinsic Ks (i.e., greater internal mass transfer resistance). From the calculated results (of overall effectiveness factor, Thiele modulus, and Biot number) together with parametric sensitivity analyses, the internal mass transfer resistance would play a more influential role than the external mass transfer resistance on the overall substrate removal rate in UASB reactors. The simulated residual phenol concentrations using the empirical model were in good agreement with the experimental data; the simulated results using the empirical model were also close to those using the kinetic model. Accordingly, the empirical model that can properly describe the overall substrate removal rate should be acceptable for process design of UASB reactors.