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DISCUSSION The chromium coating thicknesses used in this work were comparable to those used commercially, being between 70 and 170 micrometres approximately. Even after oxidation for the temperatures and times stated the chromium concentrations at the metal‐oxide interface were between 20% and 60%. These concentrations fell steadily to approximately 13% over the approximate depth stated above before reducing sharply to zero at what was the ferrite‐austenite transformation boundary during the coating process. This is contrary to the structure observed in aluminized stainless steels where a complex structure is produced due to the existence of intermetallic phases. Hence during all the oxidation experiments performed the chromium level of the surface offered for oxidation was never below 13% and complete oxidative breakdown therefore did not occur, excluding spalling effects. Many workers have shown that the oxidation rate of iron‐chromium alloys initially drops sharply with increasing chromium but eventually reaches a minimum of about 20% chromium and then rises for more chromium rich alloys. From the graph of oxidation rate in pure oxygen against chromium content given by Mortimer et al., from 13% chromium to 100% chromium the oxidation rate increases by approximately 6 × 10−9 g.cm−2 sec.−1 It is reasonable to assume that for a diffusion coating the oxidation behaviour will be markedly affected by the composition at its outer surface layer and much less by the composition gradient. If oxidation was continued for sufficiently long periods the latter could affect the general availability of chromium ions for the oxidation process. Over the first 5?m the average chromium levels were between 63% and 20% for the chromised and chrome‐aluminized respectively. From the figures given by Mortimer et al the oxidation rate of the 63% chromium coating would be expected to be 0.5 × 10−9 g.cm−2 sec−1 greater than the 20% chromium coating on the chrome‐aluminized specimens at 600°C, on the basis of the chromium content alone. The results obtained here vary in this manner, hence it is reasonable to conclude that the general oxidation behaviour of the coatings will be very similar to that of pure iron‐chromium alloys containing the same chromium content as in the outer few micrometres of the respective coatings. Even though the true surface area is greater with diffusion treated specimens their oxidation rates are lower that for the corresponding pure alloys.

THE variation in weight gains of the binary (and ternary) iron alloys with change in the atmosphere composition clearly demonstrates the sensitivity of oxidation behaviour to conditions. In particular it can be seen from Figs. 3 and 4 that the presence of atmospheric pollutants (sulphur and nitrogen oxides, water vapour) markedly increases the oxidation rate in air. This is supported by the further marked increase in oxidation in flue gases produced by the presence of sulphur oxides. Oxidation in flue gases at 700°C is far greater than in air, Figs. 7 and 10 and Table 3. This is due to the formation of wustite which was not present in air‐formed oxide scales.

Due to the special service environment of superalloys, this paper aims to obtain effects of temperature and Ti addition on high temperature oxidation behavior of Co-Al-W-B alloys.

Isothermal oxidation experiment of Co-Al-W-based alloys were carried out at 800°C, 900°C and 1000°C for different times (3, 5, 10, 20, 50 and 100 h) referring to the method of HB5258-2000. Oxidation weight gain curves and oxidation products were detected.

The results showed that the average oxidation rates of Co-Al-W-B alloy at 800 °C and 900 °C were 0.489 g·m⁻²·h⁻¹ and 0.888 g·m⁻²·h⁻¹, respectively, which belonged to an antioxidant grade. However, the average oxidation rate at 1000 °C was 2.068 g m⁻²·h⁻¹, belonging to the secondary oxidation resistance class. In the alloy with Ti addition, dense Ti oxides film were formed at the early oxidation stage and then gradually diffused later, which can increase the oxidation resistance of the alloys to some extent. By analyzing the oxidation products of Co-Al-W-B alloy, it was found that a dense Al₂O₃ layer could be formed when the alloy was oxidized at 800°C. The continuous Al₂O₃ layer would prevent the oxygen from further spreading and make the alloy into the stable oxidation stage. However, only a non-dense Al₂O₃ layer were observed with 900°C oxidation.

It can provide references for the composition design, preparation process optimization and protective coating selection of the γ′ phase strengthened cobalt-base superalloys.

This study aims to investigate the insights of soot formation such as rate of soot coagulation, rate of soot nucleation, rate of soot surface growth and soot surface oxidation in ethylene/hydrogen/nitrogen diffusion jet flame at standard atmospheric conditions, which is very challenging to capture even with highly sophisticated measuring systems such as Laser Induced Incandescence and Planar laser-induced fluorescence. The study also aims to investigate the volume of soot in the flame using soot volume fraction and to understand the global correlation effect in the formation of soot in ethylene/hydrogen/nitrogen diffusion jet flame.

A large eddy simulation (LES) was performed using box filtered subgrid-scale tensor. A filtered and residual component of the governing equations such as continuity, momentum, energy and species are resolved and modeled, respectively. All the filtered and residual components are numerically solved using the ILU method by considering PISO pressure–velocity solver. All the hyperbolic flux uses the QUICK algorithm, and an elliptic flux uses SOU to evaluate face values. In all the cases, Courant–Friedrichs–Lewy (CFL) conditions are maintained unity.

The findings are as follows: soot volume fraction (SVF) as a function of a flame-normalized length for three different Reynolds number configurations (Re = 15,000, Re = 8,000 and Re = 5,000) using LES; soot gas phase and particulate phase insights such as rate of soot nucleation, rate of soot coagulation, rate of soot surface growth and soot surface oxidation for three different Reynolds number configurations (Re = 15,000, Re = 8,000 and Re = 5,000); and soot global correction using total soot volume in the flame volume as a function of Reynolds number and Froude number.

The originality of this study includes the following: coupling LES turbulent model with chemical equilibrium diffusion combustion conjunction with semi-empirical Brookes Moss Hall (BMH) soot model by choosing C6H6 as a soot precursor kinetic pathway; insights of soot formations such as rate of soot nucleation, soot coagulation rate, soot surface growth rate and soot oxidation rate for ethylene/hydrogen/nitrogen co-flow flame; and SVF and its insights study for three inlet fuel port configurations having the three different Reynolds number (Re = 15,000, Re = 8,000 and Re = 5,000).

To characterise the high temperature oxide scales for some plasma sprayed NiCrAlY coated Ni‐ and Fe‐based superalloys.

Ni‐22Cr‐10Al‐1Y metallic coatings were deposited on two Ni‐based superalloys; Superni 601 and Superni 718 and one Fe‐based superalloy; Superfer 800H by the shrouded plasma spray process. Oxidation studies were conducted on uncoated as well as plasma spray coated superalloys in air at 900°C under cyclic conditions for 50 cycles. Each cycle consisted of 1 h heating followed by 20 min of cooling in air. The thermogravimetric technique was used to approximate the kinetics of oxidation. X‐ray diffraction, SEM/EDAX and EPMA techniques were used to analyse the oxide scales.

All of the coated, as well as the uncoated, superalloys followed an alnost‐parabolic rate of oxidation. The NiCrAlY coating was found to be successful in maintaining its continuous contact with the superalloy substrates in all the cases. The oxide scales formed on the exposed NiCrAlY coated superalloys were found to be intact and spallation‐free. The main phases analysed for the coated superalloys were oxides of nickel, chromium and aluminium and spinel of nickel and chromium, which are expected to be useful for developing oxidation resistance at high temperatures.

The coated superalloys showed remarkable cyclic oxidation resistance under simulated laboratory conditions. However, it is suggested that these coated superalloys also should be tested in actual industrial environments of boilers and gas turbines, etc. so as to obtain more practical and reliable oxidation data.

The knowledge of the reaction kinetics and the nature of the surface oxide scales formed during oxidation is important for evaluating the alloys for their use and degradation characteristics in high temperature applications such as steam boilers, furnace equipment, heat exchangers and piping in chemical industry, reformer, baffle plates/tubes in fertilizer plants, jet engines, pump bodies and parts.

R. A. Heindl and N. H. Mohler of the U.S. National Bureau of Standards' refractories laboratory have recently completed an investigation of the relative oxidation rates of 20 domestic and foreign graphites, including both the flake and amorphous varieties. The study shows that the coarser sizes of graphites having a graphitic content of about 85% are difficult to oxidise either in air or in a stream of oxygen at 400°C. However, when fine and coarse sizes are combined, as in finished but unglazed crucible bodies, a slow oxidation begins at 400°C., and the rate increases at higher temperatures with a definite deterioration of the body. One result of the study has been the establishment of numerical constants for graphite oxidation rates in oxidising atmospheres.

This paper aims to discuss that a WC/Co-Cr alloy coating was applied to the surface of H13 steel by laser cladding.

The oxidation behavior of the WC/Co-Cr alloy coating at 600°C was investigated by comparing it with the performance of the steel substrate to better understand the thermal stability of H13 steel.

The results showed that the WC/Co-Cr alloy coating exhibited better high-temperature oxidation resistance and thermal stability than did uncoated H13 steel. The coated H13 steel had a lower mass gain rate and higher microhardness than did the substrate after different oxidation times.

The WC/Co-Cr alloy coating was composed of e-Co, CW3, Co6W6C, Cr23C6 and Cr7C3; this mixture offered good thermal stability and better high-temperature oxidation resistance.

Summary A study has been made of the oxidation of a range of chromized and chrome‐aluminized alloys in flowing air and flue gas atmospheres in the temperature range 500°C—900°C. Oxidation data for the full range of alloys are given but only chromized and chrome‐aluminized mild steel are covered in depth.

1. Introduction The oxidation of cobalt‐chromium alloys has recently been reviewed in some detail by Wright. In the temperature range 950–1350°C (1740–2460°F), alloys containing up to 15% Cr oxidize faster than pure cobalt, forming double‐layered scales: the outer layer is virtually pure CoO and the inner one is composed of Co‐Cr spinel particles in a chromium‐doped CoO matrix. For oxidation in air or in oxygen, if the specimens are heated in the gas, a sharp minimum in the rate is observed at about 25 %Cr, with the development of a continuous protective CT2O3 scale. However, if the alloys are rapidly exposed to oxygen at pressures above 100 Torr, then only a shallow minimum in rate is observed at about 25 to 30 %Cr, and the higher‐chromium alloys form a two‐layer scale similar to that found on the dilute alloys. Above 35 %Cr, the scale is always a thin single layer of Cr2O3.

The work described in this paper is part of a current programme that has two objects: (1) to investigate further the reasons for the different scaling behaviour of steel in steam and carbon dioxide, although these gases have similar oxygen potentials; (2) to provide background information for an investigation into the effect of variations in re‐heating furnace atmospheres upon scaling and scale adhesion.