Effect of nanocrystallisation on oxide composition of Ni-20Cr alloy at 1000°C

Abstract

A Ni-20Cr nanocrystalline coating was obtained by means of magnetron sputtering. The oxide composition of the Ni-20Cr alloy and its nanocrystalline coating after treatment in air at 1000°C was analysed. The results indicated that the oxide scale formed on Ni-20Cr alloy is composed of two layers. The external layer consists of NiO containing some Cr₂O₃ particles and the internal layer is continuous Cr₂O₃ with a little NiCr₂O₄, whereas the scale formed on the sputtered Ni-20Cr nanocrystalline coating consists mainly of Cr₂O₃ with a little NiCr₂O₄. The mechanism for the effect of nanocrystallisation on the oxidation of the Ni-20Cr alloy is discussed.     

Oxidation kinetics of zirconium in water vapour between 750 and 1250°C

The zirconium oxidation has been followed in H₂/H₂0 gas mixtures by microgravimetry and morphological observations between 750 and 1250°C. The reaction rate is governed by a mixed regime of oxygen diffusion through multiplex oxide layers of zirconia and an oxygen solid solution. After the metal core has disappeared the reaction proceeds by direct oxidation of the solid solution. Different kinetic behaviours have been shown according to the phase couples present.     

Effects of Si,Mn, and water vapour on the microstructure of protective scales grown on Fe–20Cr in CO2 gas

Abstract

Model alloys Fe–20Cr–0.5Si and Fe–20Cr–2Mn (wt-%) were exposed to Ar–20CO₂ and Ar–20CO₂–20H₂O at either 818 or 650°C. In dry gas, protective scales on Fe–20Cr–0.5Si consisted of an outer Cr₂O₃ layer and an inner SiO₂ layer. In wet gas, additional chromia whiskers were formed on top of the duplex scale. Chromia grains formed in wet gas were much smaller than those in dry gas. A TEM analysis revealed that phase constitutions of the protective scale on Fe–20Cr–2Mn were not uniform: Mn₃O₄ and MnCr₂O₄ above alloy grain boundaries and Mn₃O₄, Cr₂O₃ and MnCr₂O₄ on alloy grains. Formation of different oxides and morphologies are discussed in terms of changes in diffusion paths and thermodynamics caused by the presence of carbon and hydrogen.     

Mechanisms of chromia scale failure during the course of 15–18Cr ferritic stainless steel oxidation in water vapour

Abstract

Breakaway oxidation of 15–18 % Cr ferritic stainless steels occurring in water vapour is described in the temperature range 800–1000°C. The failure of the protective chromia scale leads to iron oxide(s) nodule formation with accelerated kinetics. Characterisation of the (Fe,Cr)₂O₃ initial oxide scale by Raman spectroscopy and photoelectrochemistry shows chemical evolution with oxidation time, with increasing Cr/Fe ratio before haematite suddenly appears at the steel-oxide interface. The mechanisms for such a phenomenon are discussed, first on a thermodynamic point of view, where it is shown that chromium (VI) volatilisation or chromia destabilisation by stresses are not operating. It is rather concluded that mechanical cracking or internal interface decohesion provide conditions for haematite stabilisation. From a kinetic point of view, rapid haematite growth in water vapour compared to chromia is thought to be the result of surface acidity difference of these two oxides.     

TEM investigation of the oxide scales formed on a FeCrAlRE alloy (Kanthal AF) at 900°C in dry O2 and O2 with 40% H2O

Abstract

The base oxide scales on a commercial FeCrAl alloy oxidized isothermally at 900°C in dry O₂ or O₂ with 40% H₂O were studied in detail using analytical electron microscopy. Electron transparent cross-section foils prepared with a FIB/SEM in-situ lift-out technique were investigated using STEM/EDX and CBED. The oxide scales on the samples exposed to dry O₂ are slightly thinner than the scales formed in O₂+H₂O. The oxide scales exhibit a multilayered structure, with a Cr-rich layer in the middle, indicating the original metal/gas interface. An almost pure inner α-Al₂O₃ layer, containing columnar grains, was formed by inward oxygen diffusion, after exposures in both the dry and wet atmospheres. The outer oxide layer consisted of γ-Al₂O₃ in the wet case and of α-Al₂O₃/MgAl₂O₄ in the dry case. It is suggested that the α-Al₂O₃/MgAl₂O₄ phases resulted from a phase transformation of initially grown γ-Al₂O₃. The observations indicate that water vapour may stabilize the γ-Al₂O₃ phase.     

Fatigue crack growth behaviour of A533B steel in pressurized water at 288 and 28 °C

Fatigue crack growth behaviour of A533B steel was investigated in pressurized water at 288 °C using specimens machined from four different orientations. When inclusions were oriented along the direction of crack propagation, fatigue crack growth rate (FCGR) was enhanced compared to when they were perpendicular to the direction of crack propagation. At low ΔK levels FCGR in ambient water was slightly higher than that in 288 °C water. This may be attributed to the occurrence of intergranular cracking in ambient water tested specimen. Though mainly ductile striations were observed on the fracture surfaces, isolated intergranular facets (in a specimen tested in ambient water) and fan shaped features were also present. Hydrogen induced damage was clearly evident in the ambient water tested specimen in the form of isolated intergranular facets.     

Effect of impurities on the oxidation mechanism of nickel at 800°C

Abstract

The effect of impurities on the oxidation mechanism of nickel was studied on commercial nickel grades compared to a pure nickel. On the basis of oxidation kinetics, SEM and STEM microstructural and analytical investigations allowed us to identify the oxidation mechanism for both types of nickel at 800°C. The morphology of the oxide scale notably differs according to the purity of the nickel. For oxidised commercial grades, a duplex structure was observed with an outer columnar layer and an inner layer made of equiaxed grains. The inner NiO/outer NiO interface is planar without any segregation, while the NiO/Ni interface is convoluted with large cavities. Mn, Ti and sometimes also silicon impurities were detected at this latter interface. Below the NiO/Ni interface, in the underlying nickel, large internal oxidation was observed. The observed microstructure was quite different for the pure nickel. A single porous NiO layer, composed of equiaxed grains, was observed. The NiO/Ni interface was facetted and no porosity was detected. The presence and localisation of impurities, as well as morphological changes through the scale in the nickel grades, were taken into account to explain the modification of oxidation kinetics with substrate purity.     

Influence of sulphur impurity on oxidation behaviour of Ni–10Cr–9Al in air at 1000°C

Abstract

The influence of different amounts of sulphur impurity on the oxidation behaviour of a Ni–10Cr–9Al alloy in air at 1000°C has been investigated. It is indicated by the results that with increasing sulphur content, not only is there a decrease in the scale spalling resistance, but also there is a significant change in the initial growth rate and composition of the scale. Sulphur causes the formation of an inhomogeneous alumina scale by promoting the initial formation of chromia possibly originating from the oxidation of chromium sulphide. It is found that the addition of yttrium is beneficial in reducing both the spalling of the scales and the enhanced scale growth. The influence of the various impurities on the scale spalling characteristics can be correlated with the observed growth mechanisms in the initial stages of oxidation.

MST/929     

Study of the sintering mechanism of kaolinite at 900 and 1050° C; influence of mineralizers

This paper presents a study, by means of isothermal dilatometry, of the often very important (10%) shrinkage phenomenon which occurs when heating clay ceramic materials, and especially of the influence of mineralizers on the shrinkage of kaolinite at 900 and 1050° C. We found that the isothermal shrinkage versus time curve of kaolinite at both temperatures was well described by the following equation: $$\lambda = \frac{t}{{\alpha + \beta t}}$$ where λ is the linear shrinkage (relative to the initial length of the bar),t the time, andα andβ two constants. The presence of various mineralizers at different concentrations did not affect the basic shape of this curve at either 900 or 1050° C, but affected the values of parametersα andβ. A sintering mechanism is proposed which takes into account the most recent data concerning the structural transformation of kaolinite in the 900 to 1050° C temperature range. The kaolinite sintering mechanism is of the viscous-flow type proposed by Frenkel [1] involving an amorphous phase, the viscosity of which increases with time due to its progressive recrystallization. The influence of mineralizers is then explained in terms of their action on the viscosity of the amorphous phase and their action on recrystallization.     

Effect of ion implantation on the strength of sapphire at 300–600°C

Ion implantation with ¹¹B⁺ or ²⁸Si⁺ at 1000°C doubled the ring-on-ring flexure strength of c-plane sapphire disks tested at 300°C but had little effect on strength at 500 or 600°C. Disks were implanted on the tensile surface with 2 × 10¹⁷ B/cm² (half at 40 keV and half at 160 keV) or 1 × 10¹⁷ Si/cm² (80 keV). Sapphire implanted with 1 × 10¹⁸ B/cm² had only half as much flexure strength at 300° or 500°C as sapphire implanted with 2 × 10¹⁷ B/cm². Implantation with B, Si, N, Fe or Cr had no effect on the c-axis compressive strength of sapphire at 600°C. Boron ion implantation (2 × 10¹⁷ B/cm², half at 40 keV and half at 160 keV) induced a compressive surface force per unit length of 1.9 × 10² N/m at 20° and 1.4 × 10² N/m at 600°C. The infrared emittance at 550–800° of B-implanted sapphire at a wavelength of 5 m increased by 10–15% over that of unimplanted sapphire. Infrared transmittance of sapphire implanted with B, Si or N at either 1000°C or 25°C is within 1–3% of that of unimplanted material at 3.3 m. Implantation with Fe or Cr at 25°C decreases the transmittance by 4–8% at 3.3 m, but implantation at 1000°C decreased transmittance by only 2–4% compared to unimplanted material.     

Characterisation studies of chromia formation on commercial FeCrAlRE alloy foils following chemically induced failure at 900°C

Abstract

The chemical failure of commercial FeCrAlRE (where RE is a reactive element) alloys, such as Aluchrom YHf and Kanthal AF foils (30–50 μm thick), is induced by oxidation, in air, at 800–950°C. This is also the life limiting process of this family of alloys at higher temperatures (≥1100°C) and occurs when the Al activity in the alloy becomes depleted below a critical level, as a result of oxidation. At this junction, the integrity of the protective alumina scale is not sustainable but continued ‘pseudo-protection’ can be provided by chromia formation beneath the alumina layer. Oxidation continues at a slow linear rate controlled by transport through the alumina scale. The duration of the pseudo-protection region can extend to several hundred hours and has considerable technological implications, if it could also be used in defining component lifetimes.

Greater understanding is, therefore, required concerning the formation and growth of the chromia subscale, as this has received only limited attention to date. Of particular possible significance is the mode of formation of the subscale. At temperatures above 1000°C, this forms a continuous, essentially uniform layer. In contrast, at ～900°C scalloped shaped pits of chromia develop beneath the outer alumina scale. Hence a detailed characterisation study has been undertaken of the formation and development of this chromia attack; the results of which form the basis of this paper. The study has involved the deployment of a range of microscopy and analytical techniques, including optical and scanning electron microscopy, electron microprobe analysis (EPMA) and energy dispersive X-ray analysis (EDX).     

The isothermal and cyclic oxidation behaviour of Ti-48Al-2Cr at 700°C

The effect of the microstructure on the isothermal and cyclic oxidation behaviour of the intermetallic Ti–48A1–2Cr (at.%) alloy was investigated at 700°C in air up to 3000 h. Different microstructures, i.e., duplex, near gamma, nearly lamellar, and fully lamellar, obtained by various heat treatments, were used. Results of thermogravimetry showed a good oxidation resistance at 700°C against both isothermal and cyclic oxidation. The growth rate of the oxide scale, as well as its composition, structure and morphology showed no major relation to the microstructure of the base material. After equal exposure times, cyclic oxidation induced a higher oxidation rate compared to isothermal oxidation. Oxidation of Ti–48A1–2Cr in air, initially resulted in the formation of α-A1₂O₃, TiO₂ (rutile), Ti₂A1N and TiN, with the latter two near the scale/substrate interface. After longer exposure times, the mixed corrosion scale was overgrown by fast growing TiO₂. The oxide scales, formed under isothermal as well as under cyclic conditions, were uniform.     

Self-diffusion in α-Al2O3 and growth rate of alumina scales formed by oxidation: effect of Y2O3 doping

In many cases, alumina scales are assumed to grow predominantly by oxygen diffusion, but some authors have found that the growth can be controlled by aluminium diffusion. These mechanisms can be modified by active elements. The problem with alumina is that there is a lack of data about self-diffusion coefficients, and, due to the stoichiometry of alumina, diffusion data correspond to an extrinsic diffusion mechanism so that it is not possible to compare oxygen and aluminium diffusion coefficients. In order to obtain information about the alumina scale growth mechanism, oxygen (¹⁸O) and aluminium (²⁶Al) self-diffusion coefficients in Al₂O₃ were determined in the same materials and in the same experimental conditions, thus allowing a direct comparison. For both isotopes, bulk and sub-boundary diffusion coefficients were determined in single crystals of undoped alumina. Grain-boundary diffusion coefficients have been computed only for oxygen diffusion in polycrystals. Oxygen diffusion has been also studied for yttria-doped -alumina in the lattice, sub-boundaries and grain boundaries. Oxygen and aluminium bulk diffusion coefficients are of the same order of magnitude. In the sub-boundaries, aluminium diffusion is slightly faster than oxygen diffusion. Yttria doping induces a slight increase of the oxygen bulk diffusion, but decreases the grain-boundary diffusion coefficients on account of segregation phenomena. These results are compared with the oxidation constants of alumina former alloys (alloys which develop an alumina scale by oxidation). It appears that neither lattice self-diffusion nor grain boundary self-diffusion can explain the growth rate of alumina scales. Such a situation is compared to the case of Cr₂O₃.     

Creep property prediction of the high chromium ferritic steel 1Cr10NiMoW2VNbN at 600°C

Abstract

High chromium (Cr) ferritic steel 1Cr10NiMoW2VNbN was the first material developed in China for ultra-supercritical power stations. The creep property of high Cr ferritic steel 1Cr10NiMoW2VNbN has been studied under constant loading conditions at 600°C. The Theta projection concept applied to the creep curves was found to yield accurate predictions when interpolating and predicting creep data under service conditions. Four Theta parameters were obtained by Theta projection concept from the creep curves. The shape of the creep curves, as well as the minimum creep rate, the time to reach a limiting strain, and the time to rupture were considered with a wide range of stresses at 600°C of the high Cr ferritic steel.     

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C have been studied to analyse defect assessment in a steam generator. Different grades of alloy 800 have been investigated to reproduce the in service conditions. Fatigue crack growth (FCG) tests were conducted on CT20 and tubular specimens, then on welded tubes. Furthermore the influence of hold times on fatigue crack growth behaviour was studied.

The results obtained on material simulating the weld heat affected zone are in agreement with the tests conducted on welded tubes. Fatigue crack growth characteristics of aged and cold-worked aged material seem to be slightly improved in comparison with base material. Finally a hold time of one minute increases strongly the FCG threshold value determined in pure fatigue but has a negligible influence on crack growth rates.     

Effect of vanadic corrosion on creep-rupture properties of Superni-600 at 650–750°C

The effect of vanadic corrosion on creep-rupture properties of a nickel base superalloy Superni-600, at 650–750°C, has been investigated. Sodium metavanadate and sodium metavanadate plus 15 wt% sodium sulphate were used as the corrodent salts. Weight change studies have also been made to understand the mechanism of corrosion. Both sodium metavanadate and the sodium metavanadate/sodium sulphate mixture are found to be aggressive and to reduce the creep-rupture life. The degradation of creep-rupture properties and possible hot corrosion reactions are discussed. Cracking and fluxing of the protective scales, together with easier crack nucleation and growth at grain boundaries in the presence of a liquid deposit accounts for the enhanced creep rates and reduced rupture lives.     

Effect of niobium on creep and creep crack growth of cast Ni–Cr austenitic steel

Abstract

An investigation of the effect of Nb on creep properties and creep crack growth rate in a 25Cr–35Ni–0·4C (wt-%) cast steel at 871 and 950°C was carried out. Tensile tests were also carried out at room temperature, 871, and 950°C. The tensile strength and elongation increased with an increase in Nb content at high temperatures. There existed an optimum Nb content for the creep properties and creep crack growth rate. Creep crack growth is controlled by creep deformation.

MST/1222     

Effects of water vapour on the oxidation of a nickel-base 625 alloy between 900 and 1,100 °C

The effect of water vapour was studied on a nickel-based SY 625 alloy oxidized at 900, 1000 and 1100 °C under dry and wet conditions. It appears that H₂O has little effect on the oxidation rate and scale composition after 48 h. The outer scale is composed of chromia Cr₂O₃. At 900 and 1,000 °C, NbNi₄ and Ni₃Mo intermetallics are found at the oxide/alloy interface. At 1,100 °C, the scale is composed of an outer chromia scale and an internal CrNbO₄ subscale. At this temperature the oxide scale morphology differs between dry and wet conditions. Under dry conditions the oxide scale appears to be compact but the external part of the scale partially spalled of during cooling. The oxide scales formed under wet conditions show porosities spread inside the scale and the chromia grain size is smaller. At 1,100 °C scale spallation is observed under dry conditions due void accumulation in the middle part of the scale. Under wet conditions the uniform distribution of the porosities inside the scale leads to a better scale adherence.     

Formation of aluminide coatings on low alloy steels at 650°C by pack cementation process

Abstract

The pack aluminising process is normally conducted on alloy steels at temperatures higher than 900°C at which mechanical properties of steels would degrade. This study aims to investigate the feasibility of pack aluminising a commercial 9Cr – 1Mo alloy steel at 650°C in an attempt to increase its high temperature oxidation and corrosion resistance without adversely affecting its mechanical properties and consequently to increase its long-term structural operating temperatures to up to 700°C. It was demonstrated that this could be achieved using packs containing AlCl₃ as an activator and elemental Al as a depositing source. The coatings formed under these conditions consisted of an outer Fe₁₄ Al₈₆ layer and an inner FeAl₃ layer with an abrupt interface between the coating and substrate, suggesting that the coating is formed via a mechanism of the inward Al reaction – diffusion. The pack Al content was varied from 1 to 6 wt-% to investigate its effects on the coating formation process. It was found that the pack Al content in this range affected only the coating thickness and therefore the growth rate of the coating, but not the surface Al concentration. A post-aluminising heat treatment study was also undertaken for an aluminised specimen at 650°C under an argon atmosphere to investigate the kinetics of converting the brittle Fe₁₄ Al₈₆ and FeAl₃ phase layers to a more ductile FeAl phase layer. It was observed that this was a slow process requiring 1132 h for an initial coating layer thickness of 33μm. The coating after the conversion consisted of a uniform top FeAl layer with all other alloying elements in the solid solution and a diffusion zone underneath.