A kinetic and morphological study of the coking of some heat-resistant steels

The coking behaviour of a range of austenitic, heat-resistant steels has been examined in the temperature range 700-1000°C. At and below 800°C, catalytic coke in the form of bundles of filaments formed at localized defect sites in the carbide scales. A wide range in weight-gain kinetics resulted from the differing efficacy of the non-catalytic carbide scales in excluding carbon from the catalytically active substrate. At and above 900°C, catalytic coke formation gave way to pyrolytic coke formation and internal carburization became significant. Parabolic kinetics resulted from the fact that internal carburization was rate-determining. Carburizing alloys gained weight an order of magnitude faster than did alloys protected by oxide films. This was a consequence of dissolution of carbon into the alloy directly from the gas stream being much faster than the rate of coke formation on the alloy surface. Oxide-protected alloys all gained weight at a similar rate, the rate being that of coke deposition on coke. Oxide films containing aluminium were more effective in excluding carbon from the alloy than chromium-containing oxides. However, under reducing conditions, preformed oxide films were not beneficial in limiting carburization in the longer term, because they were prone to spalling, cracking and conversion to non-protective carbide.     

Coke deposition on and removal from metals and heat-resistant alloys under steam-cracking conditions

The kinetics of coke formation and its removal from nickel, copper and alloys have been studied and the results correlated with the morphology of the deposits. Coke deposition from hydrogen-propylene-steam was fast on nickel and slower on copper and the alloys. The deposition on nickel was filamentary in appearance, resulting from a heterogeneously catalysed process, whereas the deposit on the alloys and on non-catalytic copper was essentially uniform. On the alloys, protection appeared to result from the production of scales, containing predominantly Cr₃C₂ and Cr₂O₃, on the surface. At the same time, carburization of the alloy was reduced, apparently as a result of the formation of a silicon-enriched layer at the base of the scale. Steam was found to have little effect on coke formation. However, in the absence of gas-phase hydrocarbons, coke gasification by steam was rapid for nickel but several orders of magnitude slower for the alloys. The main effect of steam is to aid in the formation of scales that are protective against coking and carburization, by promoting oxide formation.     

Superplastic deformation characteristics of two microduplex titanium alloys

A comparison of the superplastic deformation behaviour of Ti-6Al-4V (wt%) between 760 and 940‡ C and Ti-6Al-2Sn-4Zr-2Mo between 820 and 970‡ C has been carried out on sheet materials possessing similar as-received microstructures. High tensile elongations were obtained with maximum values being recorded at 880‡ C for Ti-6Al-4V (Ti-6/4) and at 940‡ C for Ti-6Al-2Sn-4Zr-2Mo (Ti-6/2/4/2), under which conditions both alloys possessed aΒ phase proportion of approximately 0.40. For a given deformation temperature the Ti-6/4 alloy had a slightly lower flow stress than the Ti-6/2/4/2, and this was attributed to the lowerΒ phase proportion in the latter alloy. However, at the respective optimum deformation temperatures the Ti-6/2/4/2 alloy had the lower flow stress. The results show that suitably processed Ti-6/2/4/2 alloy is capable of withstanding substantial superplastic strains at relatively low flow stresses, although the optimum deformation temperature is higher for this alloy than for Ti-6/4 material possessing a similar microstructure.     

The nitridation of austenitic Fe-29.8 Mn-7.4 Al-0.92 C and mainly ferritic Fe-32.3 Mn-8.8 Al-0.04 C alloys

The nitridation of the austenitic Fe-29.8% Mn-7.4% Al-0.92% C and mainly ferritic Fe-32.3% Mn-8.8% Al-0.04% C alloys was studied. Plate-shaped AIN was observed in both alloys after heating in air and nitrogen at 1000 C. The scale formed on Fe-Mn-Al alloy with prolonged heating in air at 1000 C consists of several layers, from the innermost layer are first, AIN with Al-depleted austenite matrix; second, AIN with manganese-depleted ferrite matrix; and third, intermixed oxide which was only observed on alloys heated in air. Mechanisms of the formation of AIN scale were discussed.     

The effect of carbon on the high temperature oxidation of Fe-31 Mn-9Al-0.87C alloy

The alloy with the composition Fe-31Mn-9Al-0.87C was employed to investigate the effects of carbon on the oxidation behaviour at 800, 900 and 1000° C in dry air. Electron and optical microscopy were applied to examine the morphology and elemental redistribution in the oxide scale. Oxidation kinetics of the alloy oxidized at 800 and 900° C exhibited three-stage and two-stage parabolic rate laws, respectively. For the alloy oxidized at 1000° C, a carbon-induced breakaway three-stage oxidation mechanism developed. The carbon addition had a detrimental effect on the oxidation resistance and resulted in a porous initial oxide layer, which was favourable to the oxidation of manganese as well as the formation of a uniform and bulky oxide. As the oxidation temperature was increased, the diffusion rates of the metallic elements and the healing ability of oxide scales were enhanced. However, when the carbon content in alloy was above the saturation value, a breakaway scaling may have occurred due to the carbon-induced oxidation.     

Morphologies of coke deposited on surfaces of pure Ni and Fe-Cr-Ni-Mn alloys during pyrolysis of propane

Morphologies of coke deposited on pure Ni and Fe-Cr-Ni-Mn alloy surfaces during pyrolysis of propane at 750–1000°C have been investigated in detail. It is found that surface scales developed initially on pure Ni and Fe-Cr-Ni-Mn alloy surfaces have no catalytic effect on deposition of filamentary coke. But metal or alloy substrates under cracked scales strongly catalyze nucleation and growth of filamentary coke along the cracks. Ni is more efficient to catalyze the growth of filamentary coke than Fe-Cr-Ni-Mn alloys. The structure of oxide scales has marked influence on distribution and size of filamentary coke deposited on alloy surfaces. Coking morphology is closely dependent of coking temperature and time. Either increasing coking temperature or prolonging coking time results in coking morphology changes from filamentary to spherical. Both dissolution/precipitation mechanism and direct nucleation and growth mechanism may make a contribution to the development of graphitic film coke.     

Influence of the Heat Treatments on Creep Behaviour of Nickel Aluminide with Boron

The nickel aluminide with boron alloy is being considered for elevated-temperature structural application in particular for jet turbine engine components. The alloy is attractive due to its ease of production, the low cost of its components, and its property advantages relative to superalloys. Therefore, if alloys based on Ni₃Al are successfully developed, substantial increases in engine performance and efficiency may be realized.The creep characteristics of an intermetallic Ni₃Al alloy containing boron produced by hot isostatic pressing were investigated in the temperature range 800 to 900^°C. Various heat treatments were used to produce different initial grain sizes of this alloy.Parameters studied were steady state strain rate, time to fracture, ductility and Larson-Miller parameter. The stress exponent, activation energy for creep and grain size exponent were calculated.It was found that by increasing the temperature of the heat treatment, the grain size increased. The results showed that the creep behaviour for this alloy improved as grain size increased. Furthermore, a comparison of the resulting creep data with data obtained from references is discussed.     

Evolution of oxide scale on a Ni–Mo–Cr alloy at 900 °C

The cyclic oxidation behavior of a Ni–Mo–Cr alloy was studied in air at 900 °C for exposure periods of up to 1000 h. The morphology, microstructure and composition of the oxide scale was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Oxidation kinetics was determined by weight gain measurements. The results show that steady state oxidation was achieved within 1 h of exposure. During transient oxidation, the alloy grain boundaries intersecting the alloy surface became depleted in Ni and enriched in Mo and Cr. The scale initially formed at the surface was NiO which grew outwardly. However, a protective Cr₂O₃ layer developed, rapidly retarding the rate of oxidation. Formation of NiMoO₄ was also observed. The presence of Mo in the alloy facilitated the formation of a Cr₂O₃ layer at an early stage of oxidation. The alloy exhibited considerable oxide spalling during prolonged exposure.     

Developing aluminium–zinc-based a new alloy for tribological applications

In order to develop aluminium–zinc-based a new alloy for tribological applications, six binary Al–Zn and seven ternary Al–25Zn–(1–5)Cu were prepared by permanent mould casting. Their microstructure and mechanical properties were investigated. Dry sliding friction and wear properties of the ternary alloys were investigated using a pin-on-disc machine. Surface and subsurface regions of the wear samples were studied with scanning electron microscopy (SEM). The highest hardness and tensile strength were obtained with the Al–25Zn alloy among the binary ones. The microstructure of this alloy consisted of aluminium-rich α and eutectoid α + η phases. Addition of copper to this alloy resulted in the formation of θ (CuAl₂) phase. The hardness of the ternary alloys increased with increasing copper content. The highest tensile and compressive strengths and wear resistance and the lowest friction coefficient were obtained from the ternary Al–25Zn–3Cu alloy. The dimensional change measured on ageing (stabilization) of this alloy was found to be much lower than that obtained from the copper containing zinc-based alloys. Microstructural changes were observed below the surface of the wear samples of the Al–25Zn–3Cu alloy. These changes were related to the heavy deformation of the surface material due to normal and frictional forces, and smearing and oxidation of wear material. Adhesion was found to be the main wear mechanism for the alloys tested.     

Interaction of Fecralloy with sodium disilicate glass

Interactions of sodium disilicate glass with Fecralloy, Fe-6Cr-4Al-0.25Y, have been analysed in terms of the Pask chemical equilibrium theory. Sessile drops assumed contact angles of 38 to 47‡ at 900 to 1100‡C on polished substrates following interactions that led to significant weight losses. Up to 7% of aluminium and much smaller amounts of iron, chromium manganese and yttrium were taken into the glass. On pre-oxidized substrates, coated with alumina, near-perfect wetting was achieved but once more aluminium was taken into the glass and weight losses suffered. These data can be interpreted as resulting from redox reactions between the aluminium in the alloy and the sodium monoxide in the glass.     

Reduction of sodium cyclotriphosphate by metallic molybdenum

The products of the reaction between sodium cyclotriphosphate and powder Mo in vacuum at temperatures up to 1000‡C were characterized by x-ray diffraction and³¹P nuclear magnetic resonance. Above 650–700‡C, the reaction yields MoP and amorphous sodium phosphates (orthophosphate, pyrophosphate, and cyclotetraphosphate) containing Mo in the oxidation state 5+. At the Mo content ensuring the largest percentage of sodium cyclotetraphosphate in the reaction products, these also contain a small amount of elemental phosphorus.     

InSb-InAs alloys prepared by rapid quenching (106–108 K/s)

Homogeneous InSb-InAs alloys are prepared by rapid quenching (10⁶-10⁸ K/s) from the liquid state. As evidenced by x-ray diffraction studies, rapid quenching prevents liquid-phase segregation, so that solidification proceeds by a diffusionless mechanism. The lattice parameters of the rapidly quenched alloys are determined. The effects of melt overheating and sample thickness on the diffusionless solidification process are analyzed. The nonlinear composition dependence of the lattice parameter in the InSb-InAs system is accounted for using the calculated degrees of dissociation of InSb and InAs along the liquidus line and at 1000‡C and the covalent radii of the constituent elements.     

The determination of the diffusion constants of oxygen in nickel and α-iron by an internal oxidation method

The diffusion constants of oxygen in nickel and -iron are calculated from experimentally determined internal oxidation rates in dilute silicon-bearing alloys. A single crystal alloy of composition 0.058 wt % silicon in nickel and polycrystalline alloys of composition 0.48 wt % silicon in nickel and 0.072 wt % silicon in iron were oxidised and allowance is made for the absorption of oxygen in the reaction with silicon. For nickel the values of activation energy Q=73.9 kcal/g atom in the range 800 to 1200° C and Q=74.4 kcal/g atom in the range 900 to 1300° C are in good agreement with a previous result. The activation energy required for oxygen diffusion in -iron was found to be 39.9 kcal/g atom in the range 700 to 850° C.     

Microstructures and high temperature oxidation resistance of alloys from Nb–Cr–Si system

Oxidation behavior of Nb–30Si–(10,20)Cr alloys have been evaluated in air from 700 to 1400 °C by heating for 24 h and furnace cooling them. The lower weight gain per unit area has been observed for 20Cr alloy at 1200, 1300, and 1400 °C. Pesting has been observed at lower temperatures (700, 800, 900 °C). Analysis indicates that the powder formation at 900, 100, 1100 °C may be associated with β form of Nb₂O₅ (base centered monoclinic form). However the m-monoclinic form of Nb₂O₅ evolves at temperatures above 900 °C while o-orthorhombic Nb₂O₅ forms at below this temperature. The phases in the alloys have been calculated using the Pandat^TM software program at different temperatures using calculated Nb–Cr–Si phase diagrams.     

A novel etchant for revealing the prior austenite grain boundaries and matrix information in high alloy steels

To observe the prior austenite grain (PAG) boundaries in high-alloy steels, a novel etchant composed of oxalic acid, hydrogen peroxide, sodium bisulfite, and water has been developed. Etching with this system successfully revealed both the grain boundaries and the deformation bands within grains in high Co–Ni martensitic alloy steels. The carbon content of the alloys used in the test program varied from 0.23% to 0.35%, the cobalt content was in the range 9–13%, and the nickel content was in the range 8–11%. The specimens were austenitized at temperatures in the range 900–1200 °C. The rolling temperatures ranged from 700 °C to 1000 °C. The grain sizes in these alloys and those containing titanium were clearly revealed by optical microscopy following application of the new etchant.     

The interaction between silicon nitride and several iron,nickel and molybdenum-based alloys

Solid-solid reactions have been studied between silicon nitride and AlSl 316 and 20/25/Nb austenitic stainless steels, Fecralloy ferritic stainless steels (with and without yttrium), PE 16, Nimonic 75, Hastelloy X nickel-based alloys and a TZM molybdenum alloy. The reactant couples were heat-treated, in gettered inert gas, for up to 5161 h, at 800 to 1100° C. The temperature for the onset of measurable reaction with the iron and nickel-based alloys was between 825 and 900° C. Interaction was appreciable at 1000° C, being greatest with 20/25/Nb and least with the Fecralloy steel. The overall pattern of these reactions was similar, in that selected alloy constituents (chromium, together with iron and/or nickel where appropriate) reacted with the silicon nitride to form an adherent product, which was basically a silicide, although it also contained nitrogen. Some of the silicon and/or nitrogen released by subsequent decomposition of the primary reaction product was taken up by the alloys. In PE 16 and Hastelloy X alloys silicon was associated with molybdenum. There were several types of nitrogen pick-up: in the Hastelloy X alloy it followed a diffusion profile, while with other alloys it reacted with the constituents Ti, Al or Y to form nitrides. The surface layers on the austenitic stainless steel were denitrided, with nitrogen being transferred, via the gas phase, to a tantalum getter. With the TZM alloy no constituent was transferred to the silicon nitride. However, a silicon layer built up at the alloy surface and nitrogen was picked up, with its penetration following a diffusion profile.Trade Mark of the United Kingdom Atomic Energy Authority.Trade Mark of Henry Wiggin and Co. Ltd.Trade Mark of Union Carbide Corporation.     

Development of a low-expansion bond coating for Ni-base superalloys

Ternary NiCrAl alloys were modified by the addition of Ti and Si in order to adjust their coefficients of thermal expansion (CTE) to less than 15 × 10⁻⁶ K⁻¹ (from room temperature to 1000 °C) without sacrificing essential high temperature oxidation resistance. Vacuum induction melted cast alloy samples were investigated by dilatometry and thermogravimetry (TG). Oxidative TG was conducted isothermally at 900 and 1000 °C and cyclically between 600 and 1100 °C for 100 h respectively. The CTE is reduced mainly by Ti and secondly by Cr additions. Quinary alloys which showed optimal oxidation resistance essentially exhibit phase stability in the solid state at all temperatures. Approximately 4 at.% Si is needed to attain low oxidation rates and to prevent oxide spallation as well. The beneficial effect of Si on oxidation behavior is attributed hypothetically to its ability to initiate the formation of protective alumina and subsequently silica. High Cr contents lessen the beneficial effect of Si owing to the concurrent formation of chromia and/or titania.     

Temperature effect on the role of yttrium in oxidation behaviour of NiCrAl alloys

Temperature effect on the oxidation behaviour of NiCrAl and NiCrAl-Y alloys has been studied in the present work. Thermogravimetric analysis has been carried out at 900, 1000, 1100 °C for 210 h in order to investigate oxidation kinetics. Phase constitution of surface scales was determined by using X-ray diffraction, and microstructure was examined by using scanning electron microscope with energy dispersive spectra. Oxidation resistance of NiCrAl alloy has been found to be improved substantially by adding rare earth element yttrium at all the three experimental temperatures. But Y-rich inclusions result in higher initial oxidation rates as a result of oxidation of the inclusions.     

Reaction kinetics and mechanical properties in the reactive brazing of copper to aluminum nitride

Aluminum nitride (AlN) is an attractive substrate material for electronic packaging applications because of its high thermal conductivity and electrical resistivity. However, improved metallization of aluminum nitride is required for reliable conductivity and good adhesion to the ceramic substrate. In this study, the kinetics, microstructure, and mechanical strength of Ag–Cu–Ti/AlN reaction couples have been studied in the temperature range of 900–1,050 °C and hold time range of 0–1.44 × 10⁴ s using a eutectic silver–copper filler alloy containing titanium within the range of 2–8 wt%. The product layer thickening kinetics has been observed to change from a linear to non-linear thickening mechanism with the increase in holding time and temperature. At shorter hold times at a fixed temperature, the interfacial product layer followed a linear thickening kinetics. With the increase in the hold time, the thickening kinetics of the interface followed a non-linear thickening behavior. The non-linear thickening mechanism has been approximated as a parabolic thickening mechanism. The interface has been found to be rich in the reactive metal (Ti) content. The mechanical strength of the brazed joints has been analyzed using four-point bend tests. The strength of the brazed joints initially increased and then decreased with an increase in the hold time at a fixed temperature. A maximum strength of 196 MPa has been obtained for a brazed joint heated at 1,000 °C for 2,700 s containing 2 wt% Ti in the filler alloy. It was observed that the sample with the maximum strength had a discontinuous interface.     

Long-term oxidation behaviour of Ni3Al alloys with and without chromium additions

The oxidation behaviour of Ni₃Al alloys with and without chromium additions was studied after long-term exposure in an air furnace over a wide temperature range from 560–1300C. The chromium-containing alloy exhibited a better oxidation resistance below 1150C. Above this temperature the penetration depth of oxides into and along the grain boundaries in the chromium-containing alloy became larger than that in the alloy without chromium. Also the penetration depth increased drastically at 1150C for both alloys. Through energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy analyses, the compositions and structures of the oxide scales for both alloys were also identified.