Kinetic and morphological development of coke formation on heat-resistant alloys

The deposition of coke from a propylene-hydrogen mixture on to a range of austenitic Fe-Cr-Ni base alloys has been compared with deposition on pure nickel and pure copper. Nickel was catalytic to coking at temperatures below 900 °C, but at higher temperatures became encapsulated by pyrolytic coke. Copper showed no catalytic activity at any temperature in the range 600–1000 °C, and was instead covered with carbon deposited by homogeneous gas phase pyrolysis. Heat-resistant alloys formed pyrolytic coke at the same rates as copper and nickel at high temperatures, but displayed coking rates between those of the pure metals at lower temperatures. The alloys formed a chromium-rich carbide layer on their surface. This layer was initially protective, but eventually developed defects from which coke filaments grew. The formation of these filaments was catalysed by the chromium-depleted metal which became accessible to the gas following failure of the carbide layer. Once catalytic coke formation commenced, it was maintained by the presence within the coke of small metal particles rich in iron and nickel. The addition of steam to the gas slowed pyrolytic coking slightly through gas phase dilution. In the case of heat-resistant alloys, catalytic coking was largely suppressed by the formation of a surface scale containing chromium carbide and oxide. This scale was noncatalytic and was more successful than a simple carbide scale in preventing gas access to the underlying metal.     

Behaviour of ferritic stainless steels subjected to dry biogas atmospheres at high temperatures

The objective of this study is to understand the high temperature corrosion behaviour of the ferritic stainless steel type AISI 441 (18CrTiNb), a candidate for SOFC interconnectors, under dry synthetic fermentation biogas (CH₄ + CO₂ mixtures), possibly used at the anode side of the cell. Thermodynamic analysis showed that, in such mixtures, the partial pressure of oxygen lies in the range of 10^?23 to 10^?20 bar for temperature between 700 and 900 °C and that the formation of solid carbon may take place in several conditions. XRD results confirmed the formation of Cr₂O₃ and Mn‐Cr spinel, with a mixture of internal carbides. In this temperature range, kinetic experiments showed linear mass change. Comparing with the linear rate constants of 441 oxidised in pure CO₂, corrosion in biogas was larger and increased with increasing the methane content in the biogas. The surface morphology of the corroded specimens showed a dense oxide scale at temperatures less than 800 °C, serving as an efficient barrier to carbon penetration. However, when the temperature reaches 900 °C, cracks and pores appear in the oxide scale, carbon can precipitate and diffuse easier than at 800 °C and may lead to internal carbide formation. In such biogas atmospheres, 800 °C seems the maximum operating temperature of devices containing this ferritic stainless steel.     

Fatigue behavior of X40CrMoV 51 at high temperatures

The fatigue characteristics of the hot work tool steel X40CrMoV 51 (UNS number T20811) at high temperatures is presented. The temperatures for the experiments were chosen between 50–600 °C. The experiments used B-type cylindrical specimens machined from X40CrMoV in an R.R. Moore rotating bending type fatigue test machine obeying the high cycle principle. The temperature intervals for the experiments were chosen as 50, 100, 200, 300, 400, 500, and 600 °C, for which the fatigue limit for each interval at 2 × 10⁶ cycles was determined. The fatigue limit of the material at room temperature (RT) is observed as 432 MPa, whereas it drops to 383 MPa at 400 °C. On the other hand, it stays almost constant between 400–600 °C, indicating that the material includes the elements forming strong carbides such as V, Mo, and Cr, which prevent the decrease of the fatigue limit due to higher temperatures.     

Kinetics analysis of mullite formation reaction at high temperatures

A detailed kinetics analysis was performed on the mullite (3Al₂O₃·2SiO₂) formation reaction occurring at high temperatures ranging from ∼1600 to 1800°C. The counterdiffusion model of Al³⁺ and Si⁴⁺ fluxes was used to derive the kinetics equation of the mullite formation reaction. From the parabolic kinetics between the thickness of the mullite layer and time, the reaction rate constant (k) for the mullite formation was determined to be a function of the average diffusion coefficient of Si⁴⁺ ions. This kinetics equation can be used to estimate the mullite formation at any temperature. By substituting previous experimental data into the present kinetics equation, average diffusion coefficient values of Si⁴⁺ ions in the mullite layer were calculated and these values are in good agreement with the diffusion coefficient values calculated using Aksay’s interdiffusion equation for mullite formation. The activation energy values for the diffusion of the Si⁴⁺ ions were estimated to range from 730 to 780 kJ/mol, which are close to those obtained from previous diffusion and creep experiments.     

The measurement of grinding temperatures at high specific material removal rates

Developments in measuring surface temperatures during grinding at high specific material removal rates using PVD low melting point coatings are reported. This paper considers the use of this type of coating to determine the finished surface temperature and the thermal gradient below the workpiece surface under aggressive grinding conditions. Theoretical surface temperatures derived from mathematical modelling are compared to those measured by the PVD coating technique. Theoretical temperatures showed good correlation to the measured temperatures. Metallographic examination including Vickers microhardness testing was used to validate the measured thermal gradients and depth of phase transformations. The effect of the heating rate due to the grinding conditions was shown to have a significant influence on the A3 austenitic transition temperature. The technique of using low melting point PVD coatings is shown to offer a robust measurement technique for the determination of the finished surface temperature of a ground component, within aggressive environments and at high specific material removal rates.     

Cyclic carburizing behaviour of Al modified high Si-containing HP40 alloy

The chromia layer, which is formed during the pre-oxidation process, can suffer from the reduction during the carburizing. However, the oxide coating retards the growth rate of the external carbide, and thus permits the development of the inner silica layer. Nevertheless, wedging crack of the silica layer during cyclic carburizing facilitates the internal carburization. For the comparison, the high grain boundary density within the Ni, Al rich layer of aluminium adopted alloy promotes the severe internal carburization. However, as the ion diffusion has been blocked by the integrity outer alumina layer, the pre-oxidized Al-adopted alloy almost unaffected during cyclic carburizing.     

Oxidation of copper at high temperatures

The kinetics and mechanism of copper oxidation have been measured over the temperature range 900–1050°C and the pressure range 5×10^?3 to 8×10^?1 atm. It has been shown that, at the pressures lower than the dissociation pressure of CuO, the oxide scale formed on flat fragments of the copper specimens is compact and composed of a single layer, adhering closely to the metallic base. Growth of the scale proceeds under these conditions by outward diffusion of metal. The rate of the process under the conditions for which single-phase scales are formed increases with increasing oxygen pressure according to the equation: $${\text{k''}}_{\text{p}}^{} = const {\text{p}}_{{\text{O}}_{\text{2}} }^{{\text{1/3}}{\text{.9}}} $$ . the activation energy for oxidation is 24 ± 2 kcal/mole. On the basis of theFueki-Wagner method and the method proposed in the present work, the self-diffusioncoefficients of copper in cuprous oxide were calculated as a functionof oxygen pressure and temperature. It has been shown that distribution of thedefect concentration in the growing layer of the scale is linear.     

Internal stress in TiAlBN at high temperatures

Hard TiAl(B)N coatings were deposited by radio-frequency magnetron sputtering in reactive mode in an argon and nitrogen environment using a TiAlB target with 12 at.% of boron. The deposition was carried out under ion bombardment at various negative bias voltages in the range of 0 to 170 V, and at substrate temperatures between 453 and 523 K. The internal stress in the coatings was studied at room temperature as a function of annealing temperatures in ambient air up to 1123 K. The heating duration was 2 h followed by annealing for 1 h. The microstructure, phase composition and hardness were also studied prior to and after annealing.We found that the TiAlBN coatings consist of TiAl₃ and TiN phases. With increasing ion bombardment, the structure of the coatings changes from columnar to nano-scale features. Prior to annealing we also observed a correlation between the residual stress and hardness. After annealing, the compressive stresses of the TiAl(B)N coatings decreased from 1.0 GPa to less than 0.2 GPa, while the hardness remained constant or increased from ∼ 10 GPa to ∼ 25 GPa. The hardness increase of the coatings after annealing is related to a self-hardening effect.     

Sulphidation of the Fe-40 at.% Al alloy in H2/H2 mixtures at high temperatures

Sulphidation of an ordered bee alloy having the nominal composition Fe-40 at.% Al was studied at temperatures of 1073-1273 K in hydrogen/hydrogen sulphide mixtures at sulphur pressures of 10^?5 – 10^?2 Pa. The sulphidation runs were followed thermogravimetrically. Phase and chemical composition of the sulphide scales and the scale morphologies were determined by means of XRD, EDX, EPMA and SEM. The sulphidation runs showed a relatively fast uptake of sulphur at the beginning followed by a gradual slowdown. The estimated parabolic rate constants for the second stage of the reaction were of the order of 10^?12 – 10^?11 kg²/m⁴s, i. e. comparable to the oxidation rates of alumina formers in the same temperature range. The initial faster growth of the sulphide scale was attributed to the formation of iron-aluminum sulphospinel whereas the second stage to the development of aluminum sulphide at the alloy/scale interface.     

Scale development on impure nickel at high temperatures

The growth of scales on impure Ni at 1373 and 1473 K was examined by microscopy and by oxygen-tracer-imaging techniques. The outward transport of cations was the basic rate-controlling step, and on the simple initial columnar scales such cation movement was the only kinetic process. However, oxygen movement was essential for the continued growth of more complex scales. Three types of sub columnar oxide were observed: (1) relatively coarse equiaxed crystals, (2) fume-like crystals, and (3) oxide that filled the pores between the crystals of the first two types. The oxygen for the first two types was provided by dissociation of the base of the columnar layer and also from voids that developed in the columnar layer. Oxygen for the third type came directly from the atmosphere following scale cracking. Such cracks healed after forming. The origin of the major features of scales was accounted for.     

Sulfidation of manganese in H2S-H2 atmospheres at temperatures between 1073 and 1273 K

The sulfidation behavior of Mn in H₂S-H₂ atmospheres, atm, maintained at a total pressure of 1 atm was investigated at temperatures between 1073 and 1273 K. The reaction kinetics obeyed the parabolic rate law; the parabolic rate constant was proportional to where n=6.1±0.3. A value of 21,000±2000 cal/mole was estimated for the activation energy of the growth of the sulfide scale under constant sulfur pressure. Polycrystalline columnar -MnS scales were formed which exhibited texture with preferred (111) orientation. The parabolic rate constants at different sulfur pressures from this and previous investigations were used to evaluate the manganese self-diffusion coefficient in -MnS at .     

Reaction in Al-Ti-C powders and its relation to the formation and stability of TiC in Al at high temperatures

By heating Al, Ti and C powders in a DSC and by observing and analysing the microstructures produced, it has been found that, for Ti and C concentrations, equivalent to those observed in metal matrix composites, TiC forms from Al₃Ti and Al₄C₃ above 890°C. Microstructural evidence suggests that the formation of TiC occurs by reaction between Ti dissolved in Al and Al₄C₃ through the dissolution of Al₃Ti. It is, therefore, reasonable to assume that TiC particles react in Al below 890°C to form Al₃Ti and Al₄C₃.     

耐热钢HP40Nb的中温粉末法渗铝及其抗渗碳特性

以铝铁为供铝剂、氯化氨为活化剂、碳化硅为填充剂,研究了耐热钢HP40Nb在800～900℃间的粉末法渗铝特性,采用固体强化渗碳方式来研究不同状态试样的抗渗碳能力。研究表明:渗层表面质量很高,渗层与基体结合良好,渗层主要由沉积区和过渡区两部分组成,较之高温渗铝,渗层厚度较薄;采用直流电场增强法对HP40Nb粉末法渗铝渗层相组成种类影响不大,但能使其800℃时的渗铝速度提高近7倍。所试验渗铝层在1000℃强化渗碳条件下表现出优良的抗渗碳特性。