Einflüsse kleiner Zusätze von Niob oder Cer auf Korrosion und Kriechen von Incoloy 800 in Co-H2O-H2-Atmosphären

Effects of small alloying additions of niobium or cerium on the corrosion and creep of Incoloy 800 in CO-H₂O-H₂-atmospheres In oxidizing and carburizing atmospheres at high temperatures Fe-Ni-Cr-alloys are carburized under creep conditions by carbon transfer through cracks in the oxide layer. In creep experiments in CO-H₂O-H₂ atmospheres at 1000 °C with several alloys based on Incoloy 800 the carburization could be related to the strain of the specimens. Alloying additions of Nb in the range 0.2 to 1% caused changes in the creep rate, a decrease for >0.35% and an increase for >0.35% Nb. The creep resistance for the high Nb concentrations could be improved by solution annealing at high temperatures (1200 °C). Niobium strongly decreases the carburization - this effect can be explained by the formation of an internal layer of Al- and Nb-oxides beneath the outer Cr₂O₃ layer. An alloying addition of Ce (0.06%) also has beneficial effects on the creep resistance and carburization resistance of Incoloy 800.     

High Temperature Corrosion of Cast Alloys in Exhaust Environments. II—Cast Stainless Steels

This paper describes in detail the oxidation of two cast stainless steels in synthetic diesel and gasoline exhaust gases. One alloy was ferritic (Fe18Cr1.4Nb2.1Mn0.32C) and one austenitic (Fe20Cr9Ni1.9Nb2.7W0.47C). Polished sections were exposed, mostly for 50 h, at temperatures between 650 and 1,050 °C. The oxidation product was characterized by means of SEM/EDX, AES, and XRD. Inter-dendritic non-Cr carbides initiated thick oxides. The ferritic steel formed a rather thin and adherent oxide scale at all temperatures. It consisted of (Mn, Cr) oxide on top of Cr₂O₃ and, starting at 850 °C, a thin silica film at the metal–oxide interface. Chromium depletion triggered dissolution of carbides providing Cr to the oxide. Water vapor did not accelerate the attack since the outer (Mn, Cr) spinel oxide reduced the Cr evaporation. The austenitic grade was very sensitive to water vapor. Chromium segregation directed pitting to the dendrites up to 950 °C whereas uniform catastrophic oxidation occurred at 1,050 °C.     

The corrosion behaviour of chromium in hydrogen chloride gas and gas mixtures of hydrogen chloride and oxygen at high temperatures

A kinetic investigation of the CrHClO₂ system at 1 atm was carried out varying the oxygen content from 0 to 75 vol% at 400–800°C by the thermogravimetric method in stagnant gases and by measurements of weight loss and weight of sublimates after corrosion tests in flowing gases. In hydrogen chloride gas the corrosion rate is determined by the rates of formation and evaporation of a CrCl₂ scale: the scale was protective, to some extent, up to 600°C, but rapidly evaporated at still higher temperatures. The addition of oxygen led to suppression of corrosion loss up to about 500°C but to catastrophic corrosion at higher temperatures. The scale formed in the gas mixtures consisted mainly of Cr₂O₃ but vigorous vaporization of CrCl₂ and water occurred at the higher temperatures due to oxy-chlorination.     

Einfluß der Aufkohlung auf das Kriechverhalten einer FeNiCr-Hochtemperaturlegierung

Effects of Carburization of the Creep Behaviour of a FeNiCr-High Temperature Alloy Incoloy 800 and Incoloy 800 doped with 1% Nb were carburized at 1000 °C in CH₄–H₂ to 0.83% C (mass content). The undoped alloy shows relatively coarse large M₂₃C₆ carbides at the grain boundaries, the alloy with 1% Nb has mainly fine carbides in the grains. Creep experiments were performed with the carburized and uncarburized specimens at 1000 °C, in which creep rates were attained in the range 10^?9… 10^?7 s^?1 of secondary creep. The stress dependence of the creep rate indicates two creep mechanisms: diffusion creep at low stresses and dislocation creep at high stresses. The diffusion creep is faster for both alloys after carburization. The dislocation creep is retarded by carburization for the undoped alloy. At about equal creep rate ε = 10^?7 s^?1 the carburized specimens have a longer lifetime. The fracture is brittle for Incoloy 800 in the uncarburized and carburized state, characterized by void and crack formation and poor reduction in area. The fracture of the carburized Incoloy 800 with 1% Nb is rather ductile with less void formation. The results indicate that carburization does not deteriorate the creep behaviour of the FeNiCr alloy if the reached carbon content is not too high. An addition of Nb is very favorable for the creep properties after carburization.     

Effects of 7·5 vol.-% water vapour on oxidation of nickel based alloy between 900 and 1100°C

Abstract

A Ni based SY 625 alloy was oxidised at 900, 1000 and 1100°C under dry and wet conditions. Water vapour has little effect on the oxidation rate and scale composition. At 900 and 1000°C, the outer scale is composed of Cr₂O₃, and a continuous NbNi₄–Ni₃Mo subscale is found at the oxide/alloy interface. At 1100°C, the scale is composed of an outer chromia scale and an internal CrNbO₄ subscale. Nevertheless, the oxide scale morphology differs between dry and wet conditions. Under dry conditions, the oxide scale appears to be compact, and chromia pegs are observed at the internal interface. The oxide scales formed under wet conditions show that porosities spread inside the scale, and the chromia grain size is smaller. At 1100°C, some scale spallation is observed under dry and wet conditions probably due to the molybdenum oxidation, leading to MoO₃ evaporation and void accumulation at the internal interface.     

High Temperature Oxidation of the Austenitic (35Fe27Cr31Ni) Alloy Sanicro 28 in O2 + H2O Environment

The present study investigates the high temperature oxidation of alloy Sanicro 28 (35Fe27Cr31Ni) in 5% O₂ and in 5% O₂ + 40% H₂O. Polished steel coupons were isothermally exposed in a tube furnace at 600, 700 and 800 °C for up to 168 h. The samples were investigated by gravimetry, grazing angle X-ray diffraction (XRD), Auger electron spectroscopy (AES), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy/energy dispersive X-rays (STEM/EDX). The results show that the material forms a protective scale in both environments. The scale is duplex. The inner part of the scale consists of corundum type chromium-rich (Cr _x Fe_1?x )₂O₃, and the outer layer consists of spinel type oxide. Chromia is lost from the protective oxide by vaporization of CrO₂(OH)₂ in O₂ + H₂O environment. The capacity of Sanicro 28 to suffer chromia vaporization without forming a rapidly growing iron-rich oxide is attributed to its high Cr/Fe ratio. The spinel formed at the oxide/gas interface could in addition be beneficial for oxidation behavior in wet oxygen because it may slow down chromia evaporation.     

Improving the oxidation resistance of γ‐titanium aluminides by halogen treatment

Intermetallic alloys based on TiAl are candidates for several structural high temperature applications but their oxidation resistance is limited to temperatures below 800 °C. In this paper the results of high temperature oxidation and creep tests will be presented and discussed. The treatment with halogens improves the oxidation resistance of these alloys up to 1050 °C. A thin protective Al₂O₃‐layer is formed after treatment with halogens instead of the mixed TiO₂/Al₂O₃/TiN scale typically grown on these alloys. This alumina layer protects the component under isothermal and thermocyclic conditions. The protective effect is stable up to at least 8760 h. Creep tests of halogen treated materials at high temperatures showed no effect on the creep behaviour. Automotive turbocharger rotors were exposed at 1050 °C in air with and without fluorine‐treatment for demonstration of real parts.     

The isothermal and cyclic high temperature oxidation behaviour of Ti–48Al–2Mn–2Nb compared with Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr

The isothermal and cyclic oxidation behaviour of Ti–48Al–2Mn–2Nb (at%) were studied at high temperatures in air in comparison with the intermetallic alloys Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr. Tests were performed in air between 800 and 900°C. At 800°C Ti–48Al–2Mn–2Nb showed an excellent oxidation resistance under isothermal and cyclic conditions, comparable with Ti–48Al–2Cr–2Nb, and superior to Ti–48Al–2Cr. At 900°C the isothermal oxidation rate of Ti–48Al–2Mn–2Nb was similar as found for Ti–48Al–2Cr–2Nb, but much lower as that of Ti–48Al–2Cr. Upon cooling the oxide scale formed on Ti–48Al–2Mn–2Nb was prone to spallation. During the cyclic oxidation at 900°C, a steady state condition is reached for both niobium bearing materials, with a net linear mass loss rate, due to spallation and (re-)growth of the oxide scale. The linear mass loss rate for the Ti–48Al–2Mn–2Nb was higher than that of Ti–48Al–2Cr–2Nb, indicative of a higher susceptibility for spallation. During the initial stage of oxidation of all tested materials a complex multi-phased and multi-layered scale was formed consisting of α-Al₂O₃, TiO₂ (rutile), TiN and Ti₂AlN. After longer exposure times the outer scale was dominated by TiO₂. In case of the niobium containing materials no loss of protectivity of the oxide scale was found during the growth of the outer TiO₂ layer (under isothermal conditions). Two-stage oxidation experiments with isotope tracers were performed to study the oxidation mechanism in more detail.     

The high temperature oxidation of 11% chromium steel: Part II – Influence of flow rate

The oxidation of type X20 CrMoV 11 1 steel at 600°C in the presence of dry O₂ and O₂ + 10 or 40% H₂O was investigated. The flow rate was varied between 0.25 to 10.0 cm/s. Exposure time was 168 hours. The oxidized samples were investigated gravimetrically and by a number of surface analytical techniques including grazing angle SEM/EDX, GDOES and XRD. Oxidation is strongly influenced by pH₂O and flow rate. In O₂ + H₂O environment at 600°C, the protective Cr‐rich α‐(Cr,Fe)₂O₃ oxide loses chromium by vaporization of CrO₂(OH)₂. When chromium loss is limited (e. g., in 10% H₂O and in 40% H₂O at low flow rates) the supply of chromium from the alloy compensates for chromium vaporization and the oxide retains its protective properties, resulting in slow oxidation. In 40/60 H₂O/O₂ and high flow rates chromium evaporation becomes so rapid that the protective properties of the oxide are lost and a thick duplex (Fe₂O₃/Fe₂CrO₄) scale develops.     

Corrosion of Alloy 600 in a carburization furnace

The failure case of a shielding tube was investigated which had been used in a carburization furnace for protection of an oxygen sensor. The tube made of Alloy 600 showed all kinds of attack by carbonaceous gases: (1) at the hot end within the CO‐H₂‐H₂O atmosphere at a_C ≤ 1, carburization leading to internal carbide formation, followed by internal oxidation of the carbides, (2) graphitisation, i.e. internal graphite formation in a region of lower temperature and a_C > 1 and (3) metal dusting, i.e. disintegration to a dust of graphite and metal particles and pitting. The phenomena are described and discussed. The Alloy 600 is not sufficiently corrosion resistant at high temperatures, due to its low Cr‐content (≤ 16% Cr) a protective scale is not formed for sure. Better corrosion resistance for this alloy may be obtained by surface working or a preoxidation under appropriate conditions.     

A case of nitridation,carburization and oxidation on a stainless steel

A case of corrosion was studied on stainless steel tubes, exposed to a nitriding, carburizing and oxidizing environment (mainly NH₃ and CO₂) at 390–450°C. Due to the high nitriding potential prior formation of internally nitrided layers occurs, at higher temperatures (> about 425°C) under precipitation of CrN in the layer and at lower temperatures under formation of the γ_N‐phase, i.e. austenite with high N‐content and expanded lattice. The latter process causes more severe corrosion, due to the high expansion, the stresses in the nitrided layers lead to bursting and repeated spalling of the scales. Carburization and oxidation are less important. The carburization is slower than nitridation, Fe₃C formation is observed and carbon deposition. Also the oxidation by CO₂ is slow and converts the nitrides and carbides formed before, to unprotective oxide flakes.     

Calculation of Reactive-evaporation Rates of Chromia

A methodology is developed to calculate Cr-evaporation rates from Cr₂O₃ with a flat planar geometry. Variables include temperature, total pressure, gas velocity, and gas composition. The methodology was applied to solid-oxide, fuel-cell conditions for metallic interconnects and to advanced-steam turbines conditions. The high velocities and pressures of the advanced steam turbine led to evaporation predictions as high as 5.18 × 10⁻⁸ kg/m²/s of CrO₂(OH)₂(g) at 760 °C and 34.5 MPa. This is equivalent to 0.080 mm per year of solid Cr loss. Chromium evaporation is expected to be an important oxidation mechanism with the types of nickel-base alloys proposed for use above 650 °C in advanced-steam boilers and turbines. It is shown that laboratory experiments, with much lower steam velocities and usually much lower total pressure than found in advanced steam turbines, would best reproduce chromium-evaporation behavior with atmospheres that approach either O₂ + H₂O or air + H₂O with 57% H₂O.     

Thermal stability of lamellar structure of PST crystal and lamellar boundary design for creep resistant TiAl alloy

The stability of lamellar structure is crucial for the creep resistance of TiAl alloys, but degradation of the lamellar structure is unavoidable at high temperatures. The degradation of the lamellar structure in PST crystals of Ti-48mol.%Al was studied during high temperature exposure (annealing and creep testing) to examine how to make a stable lamellar structure with high creep deformation resistance. Since the six orientation variants of γ lamellae are nucleated independently of the adjoining lamellae, pseudo twin and 120° rotational fault boundaries are most frequently observed at the initial stage of lamellar formation. The preferential removal of high energy (pseudo twin and 120° rotational fault) boundaries during the evolution of lamellar structure results in the highly probable appearance of a true twin boundary at a later stage of lamellar evolution. The coarsening of lamellar spacing and the spheroidization of the lamellae are the major degradation events occurring during creep deformation, and the migration of the lamellar boundaries brings both of them about. The lamellar structures of TiAl alloy contain four types of lamellar boundaries. The stability of the four types of boundaries decreases in the following order: γ/α₂ > true twin > pseudo twin > or=120° rotational fault boundaries. The γ/α₂ boundary has the highest stability (lowest mobility), and the high density of γ/α₂ boundaries is proposed to make a stable lamellar structure with good creep resistance. A material having the high density of γ/α₂ boundaries was produced through the heat treatment of a PST crystal in the α+γ two-phase regime. The excellent creep properties of the material were proven through creep tests of hard oriented PST crystals made of the material. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

Thermal degradation of alloy CCA617 in pure steam at 750 and 850°C

Oxidation and oxidation-induced microstructure evolution of CCA617 alloy was studied in pure steam at 750 and 850°C. Results showed that the alloy was oxidised approximately following a parabolic law. Temperature increase greatly promoted the external and internal oxidation. A single (Cr, Mn)₂O₃ layer with distribution of minor TiO₂ and MnCr₂O₄ formed at 750°C whereas a duplex oxide scale formed at 850°C consisting of a thin MnTiO₃ outer layer and a thicker (Cr, Mn)₂O₃ inner layer. TiO₂ and Al₂O₃ were formed internally especially along grain boundaries. Intragranular Mo-rich M₂₃C₆ type carbides were also observed. The carbides dissolved into the matrix as oxidation progressed. Based on detailed compositional and microstructural characterisation of the oxidised alloy, the mechanisms of external and internal oxidation as well as dissolution of the intragranular carbides are presented.     

Evaporation of Cr2O3 in Atmospheres Containing H2O

Stainless steels in atmospheres containing H₂O form a Cr₂O₃ scale in the early stage of oxidation. However, the Cr₂O₃ scale gradually degrades with time. In order to determine the effect of H₂O on the deterioration of a Cr₂O₃ scale, the evaporation behavior of Cr₂O₃ in N₂–O₂–H₂O atmospheres was investigated. The rate of mass loss in an N₂–O₂–H₂O atmosphere was found to be one order of magnitude higher than the rates in N₂–O₂ and N₂–H₂O atmospheres, indicating that deterioration of the Cr₂O₃ scale is likely to occur in mixed atmospheres of oxygen and water vapor. Volatilization of Cr₂O₃ is probably based on the following reactions: 1/2Cr₂O₃(s)+3/4O₂(g)+H₂O(g)=CrO₂(OH)₂(g). However, it is also speculated that the reaction, Cr₂O₃(s)+2/3O₂(g)=2CrO₃(g), affects the evaporation of Cr₂O₃ at temperatures higher than 1323 K. The evaporation rate of Cr₂O₃ is roughly comparable to the growth rate of the Cr₂O₃ scale. Therefore, a Cr₂O₃ scale can be degraded by the evaporation of Cr₂O₃.     

Enhanced Oxidation Resistance of Mo–Si–B–Ti Alloys by Pack Cementation

As new high-temperature structural materials, Mo–Si–B alloys satisfy several requirements such as oxidation and creep resistance. Recently, novel Ti-rich Mo–Si–B alloys have shown an increased creep resistance compared to Ti-free alloys. However, due to the formation of a duplex SiO₂–TiO₂ oxide layer, which allows for fast ingress of oxygen, the oxidation resistance is poor. To improve the oxidation resistance, a borosilicate-based coating was applied to a Mo–12.5Si–8.5B–27.5Ti (in at.%) alloy. After co-deposition of Si and B by pack cementation at 1000 °C in Ar, a conditioning anneal at 1400 °C is used to develop an outer borosilicate layer followed by an inner MoSi₂ and Mo₅Si₃ layer. During both isothermal and cyclic oxidation after an initial mass loss during the first hours of exposure, a steady state is reached for times up to 1000 h at temperatures ranging from 800 to 1200 °C, demonstrating a significantly enhanced oxidation resistance.     

Microstructures and mechanical properties of Fe–Al–Ta alloys with strengthening Laves phase

The addition of Ta to Fe–Al alloys results in the formation of a stable Ta(Fe,Al)2 Laves phase with hexagonal C14 structure in the Fe–Al phase at temperatures of 800, 1000 and 1150 °C. It was found that the solubility of Ta in Fe–Al is generally low and the solubility of Ta varies with Al content. Respective isothermal sections of the Fe–Al–Ta system have been established. Particular attention has been given to precipitation in the Fe₃Al phase with a small addition of Ta. At intermediate temperatures, 600–750 °C, an additional Heusler-type phase with L2₁-structure precipitates, which transforms at longer times and high temperatures to the stable C14 Laves phase. The yield stress in compression and the creep behaviour of the Fe–Al–Ta alloys with various microstructures were studied. Due to the presence of the L2₁-Heusler phase, the yield stress and the creep resistance at temperatures below 700 °C was increased considerably.     

Creep of Copper at Intermediate Temperatures

Activation energies for creep of copper at intermediate temperatures, where crystal recovery was negligible, were determined by the simple technique of rapidly alternating the test temperature between T₁ and T₂ (T₂= T₁ + about 10°K) throughout a constant stress creep test. The activation energy for creep ΔH was found to be 37,000 ± 3,000 cal per mol, independent of stress and strain. The same creep laws as have been previously established for high temperature creep were found to be valid for creep at intermediate temperatures. But the ΔH was found to be lower than that for self-diffusion in the intermediate temperature range whereas it is known to be equal to that for self-diffusion at high temperatures.     

Characterisation of creep-weak zones (white bands) in grade 91 weld metal

Abstract

A microstructural study of creep failure in grade 91 weld metal has revealed two primary modes of creep failure. In addition to creep fractures along columnar grain boundaries (typical of weld metal creep failure), creep fractures were also found along creep-weak 'white bands' which had formed at the inter-bead boundaries. The white-band regions consisted of material where the M₂₃C₆ carbides had dissolved during creep testing; the loss of carbides had allowed recrystallisation of the martensitic structure to ferrite and consequently this material was much softer than the bulk weld metal. The element mapping over the weld metal by laser-induced breakdown spectroscopy demonstrated that there was significant inhomogeneity in the distribution of certain elements, most significantly, chromium. This inhomogeneity resulted in strong activity gradients in carbon (even though the carbon concentration was homogeneous following welding) resulting in carbon loss from the alloy-depleted regions, the associated dissolution of carbides and the recrystallisation that accompanied this, and thus the poor mechanical properties which resulted in creep failure.     

Creep behaviour of cemented carbides — Influence of binder content,binder composition and WC grain size

The high-temperature creep behaviour of cemented carbides was evaluated for a wide variety of binder contents, binder compositions and WC grain sizes at temperatures ranging from 700 °C to 950 °C. The creep behaviour was characterised using compressive high-temperature experiments. The results show that the above mentioned microstructural parameters as well as the binder composition have a significant influence on the samples' plastic deformation. Based on these findings, a structured creep behaviour control map may be established for future materials development, aiding in the design of new high performing cemented carbides for challenging technical applications.