Influence of sulfur on the nature and morphology of carbides formed on the surfaces of Fe-Ni-Cr alloys at high temperatures

The influence of sulfur upon surface carbide formation on a range of Fe-Ni-Cr alloys has been investigated at a temperature of 1273 K. The addition of small amounts of H₂S, up to 100 ppm (range of = 2.2×10^–12 to 5.5×10 su–11 bar), to a H₂-CH₄ carburizing atmosphere (a_c=0.8) was found to significantly modify the behavior from that normally observed in the sulfur-free environment. The carburization of these alloys in the H₂-CH₄ atmosphere led to the formation of globular particles of M₇C₃ on the surface of the alloy, but the addition of H₂S affected the type, morphology, and distribution of the surface carbides. Initially, the lower carbon containing M₂₃C₆ was formed, which transformed to M₇C₃ at a rate determined by the concentration of sulfur in the environment. The morphology of the M₇C₃ was modified by the presence of sulfur, and the carbide exhibited a preferred crystallographic orientation in the [001] direction. Particles of manganese sulfide were formed on the commercial alloys at > 2.3 × 10^–11 bar at 1273 K, and these served as nucleation sites for carbides sothat, in contrast to the behavior in sulfur-free conditions, complete surfacecarbide layers were formed.     

Metal dusting

Metal dusting is a catastrophic carburization of steels which leads to disintegration of the material to a mixture of powdery carbon and metal particles leaving pits and grooves. The phenomenon was simulated by carburization of low-and high-alloy steels in CO-H₂-H₂O mixtures at carbon activities > 1 in the temperature range 600–700°C. The occurance of an unstable carbide M₃C (M=Fe, Ni), as an intermediate in metal dusting, was proven—the reaction sequence involves over saturation of the metal matrix with carbon, M₃C formation at the surface, subsequent decomposition of the M₃C layer M₃C3 M+ C, leading to carbon with interspersed metal particles which act as catalysts for additional carbon deposition from the gas atmosphere. With increasing Ni content in Fe-Ni alloys, typical metal dusting is suppressed, but another mode of deterioration was observed, involving graphite growth on the grain boundaries. The high-alloy, chromia-forming alloys showed metal dusting only when chromia formation was suppressed by electropolishing the materials.     

Metal dusting behaviour of several nickel‐ and cobalt‐base alloys in CO?H2?H2O atmosphere

The metal dusting behaviour of total 11 nickel‐ and cobalt‐base alloys at 680 °C in a gas of 68%CO? 31%H₂? 1%H₂O (a_C = 19.0, = 5.4 × 10^?25 atm) was investigated. All samples were electropolished and reacted in a thermal cycling apparatus. On the basis of their reaction kinetics, these alloys can be classified into three groups: the first, with rapid carbon uptake and significant metal wastage, consists of alloys of relatively high iron content (AC 66, 800H and NS‐163); the second, with intermediate rates, consists of some Co‐base alloys (HAYNES 188, HAYNES 25 and ULTIMET) and the third, with very low reaction rates, consists of nickel‐base alloys with high chromium levels (601, HAYNES HR 160, 230, G‐35 and EN 105). An external chromia scale protected group 3 alloys from carburization and dusting. However, this protective scale was damaged and not rehealed for group 1 and group 2 alloys, allowing carbon attack. In all cases, coke deposited on the surface with two typical morphologies: filaments and graphite particle clusters. Subsurface spinel formation in high iron‐content alloys led to rapid dusting due to the significant volume expansion. Alloy carbon permeability was calculated from a simple law of mixtures, and shown to correlate reasonably well with initial dusting rate except for one cobalt‐base alloy in which iron spinel formation was significant.     

Aufkohlung von Chrom-Nickel-Eisen-Stählen in der Kohlenstoffpackung

Carburization of chromium nickel steels in a carbon bed Annealing the samples in carbon powder is a simple method for testing the carburization behaviour of heat resistant CrNiFe steels. It is shown that according to the thermodynamic conditions during this test a very thin layer of Cr₂O₃ is formed on the surface of the samples in the temperature range 900° to 1050°C. This layer virtually prevents carburization. Above 1050°C. The oxide layer is transformed to a carbide and carburization of the alloy can take place without restraint. However, the influence of temperature on carburization as described may not apply to service conditions, in particular, carburization may also occur if the oxide layer is thermodynamically stable but porous and fissured. Upon carburization carbon is diffusing into the alloy and reacting with chromium and iron under formation of the carbides M₇C₃ in an outer zone and M₂₃C₆ in an inner zone. The penetration of these zones is also determined by the solubility of carbon in the carburized region. By a high nickel content of the alloy the carbon solubility is diminished and therefore the rate of carburization is retarded. This influence of the nickel content also shows under service conditions.     

High-temperature corrosion and creep of Ni-Cr-Fe alloys in carburizing and oxidizing environments

The carburization of NiCr 32 20 and NiCrSi 60 16 has been studied in CH₄-H₂ mixtures in the temperature range 900–1100°C. The methods included thermogravimetric measurements and studies on reacted specimens by X-ray diffraction, metallographic, and chemical analysis. Upon carburization internal carbides M₇C₃ and M₂₃C₆ are formed (M=mainly Cr); the rate of carburization is determined by carbon diffusion in the Fe-Ni matrix with carbide precipitations. The effect of the alloying elements Ni and Si on the carburization resistance of austenitic alloys is explained. By the same methods the oxidation and carburization in CO-H₂O-H₂ mixtures have been studied. The important role of a stable chromium oxide layer for the carburization resistance was confirmed. Creep tests at 1000°C in a CO-H₂O-H₂ atmosphere where Cr₂O₃ is stable showed carburization occurring through cracks in the oxide layer. At high strain rates premature failure occurs by carburization, which is followed by internal oxidation and formation of cracks, voids, and holes.     

Veränderung der Werkstoffeigenschaften austenitischer CrNiFe-Legierungen durch Aufkohlung

Changes in material properties of austenitic CrNiFe-alloys by carburization In gravimetric experiments austenitic CrNiFe-alloys, especially Incoloy 800, have been carburized in CH₄-H₂ in the temperature range 800–1100°C. Upon carburization internal carbide formation occurs which is controlled by carbon diffusion. Samples in different states of carburization have been studied by metallographic, chemical and X-ray analysis. Upon carburization carbides of the types M₇C₃ and M₂₃C₆ (M = Cr, Fe, Ni) are formed in an austenitic Fe-Ni matrix. Through this process the volume of the material increases and changes of the heat conductivity and the magnetic properties occur. The oxidation resistance of carburized samples was tested at 1000°C in flowing air. Deeply penetrating internal oxidation occurs in weakly carburized samples with M₂₃C₆ precipitates at grain boundaries. Highly carburized samples oxidize under formation of a protective Cr₂O₃-layer, slower than virgin material. The internal carbide formation also changes the mechanical properties which have been tested by tensile and notch impact tests. With increasing carburization of the samples the tendency to brittle fracture increases.     

Oxidation of iron-nickel aluminum alloys in oxygen at 600–800° C

The oxidation behavior of iron-nickel-aluminum alloys containing 0–40 wt.% nickel and 0–30 wt.% aluminum has been investigated at 600 and 800° C. Through the construction of oxide maps it can be shown that three possible oxide morphologies may exist: Alloys containing less than approximately 5 wt.% aluminum form scales consisting of predominantly Fe₂O₃ and spinel; alloys with between 5 and 10 wt.% aluminum form Al₂O₃ scales interspersed by Fe₂O₃ nodules, and alloys with greater than 10 wt.% aluminum form predominantly -Al₂O₃ scales.     

Improvement of the application of an electrochemical method for the determination of transport properties of anα-alumina scale. Part II: Influence of yttrium and palladium on alumina scales developed on aβ-NiAl alloy

An electrochemical method based on analysis of potential-current curves at various appliedP _{O 2} for an alumina scale developed at 1100°C on either undoped or yttrium-palladium doped -NiAl alloys was used to determine transport parameters in the scale. In the case of scales formed on undoped or Y-doped NiAl alloys, thet _i variations, at least in theP _{o 2} range experimentally studied, are very close: the scale consists of an outer domain characterized byt _i0.1, thent _i increases with decreasingP _{o 2}. The ionic conductivity in both cases shows a V-shape and decreases in the presence of Y. The presence of palladium notably modifies the shape of thet _i variation and induces an inversion of the shape of ₁ variation withP _{o 2}. The analysis of these results suggests that at lowP _{O 2} yttrium, localized in the inner part of the scale, decreasest _i, creates defect complexes and intergranular yttrogarnet precipitates, and prevents the outer part of the scale from contamination by nickel, which is thought also to give defect complexes when present. The variation of the oxygen chemical potential inside the scale shows a progressive variation of _o. The calculated oxidation constants are close to the experimental ones, which indicates that the alumina scale growth is ensured by grain-boundary diffusion of ionic species.     

Oxidation Behavior of Nanocrystalline Al Alloys Containing 5 and 10 at.% Ti

The oxidation behavior of mechanically alloyed(MA) Al-Ti alloys containing 5 and 10 at.% Ti wereinvestigated at 500-600°C under 1 atm of oxygen. Ateach temperature, alloys oxidized linearly during the initial stage and later followed the parabolicrate law. During the initial stage, the oxidation ratesof nanocrystalline (50 nm) Al-Ti alloys were fasterthan those of conventional (200 nm) alloys. It is suggested that more grain boundaries innanocrystalline alloys provide more nucleation sites foroxides, so that the oxide scales grew faster as denseprotective layers. During the parabolic stage, the nanocrystalline alloys had greater oxidationresistance than conventional alloys because of the denseprotective layer. Oxide scales on both alloys consistedof a mixture of -Al₂O₃ andTiO₂ in the outer layer and-Al₂O₃ near the alloy as aprotective layer.     

Carburization resistance of nickel-base, heat-resisting alloys

Cast heat-resisting alloys containing 45 or 60 wt% nickel were isothermally carburized in flowing gas mixtures of H₂–5CH₄ (volume percent) at temperatures of 900–1,100 °C, and their performance compared with that of standard commercial grades containing 30–35% nickel. Chromium-rich M₇C₃ and M₂₃C₆ precipitated internally in all materials, to depths which increased according to parabolic kinetics. Comparison of the rate constants with those predicted from Wagner’s diffusion theory showed that carbon diffusion through a chromium-depleted alloy matrix was the rate-controlling process. The 45% nickel alloys carburized more slowly than the 30–35% nickel grades as a result of decreased carbon solubility and diffusivity at the higher nickel level. The 60% nickel alloys also contained aluminum. At higher temperatures, diffusion of aluminum to the surface led to A1₂O₃ scale formation and enhanced carburization resistance. The degree of protection obtained depended on alloy aluminum content. P. Becker—On secondment from Schmidt & Clemens GmbH, Post Fach 1140, 51779 Lindlar, Germany.     

Carburization kinetics of heat-resistant steels

A series of 21 commercial austenitic heat-resistant alloys was carburized in H₂-C₃H₆ atmospheres which were reducing to Cr₂O₃ and SiO₂ and which provided a carbon activity of one. Reactions were carried out at 900T1100°C and the resulting depth of carburization was measured metallographically. All alloys showed parabolic carburization kinetics after an initial brief period, at low temperatures, of more rapid reaction. The apparent activation energies for carburization are discussed in terms of carbon solubilities and diffusion coefficients. It is concluded that Wagner's theory of internal oxidation is quantitatively applicable to simple alloys but not to alloys which contain additions of reactive elements. The effectiveness of additions of niobium, aluminum, titanium, and silicon is demonstrated and is discussed in the context of Wagner's theory.     

The influence of yttrium additions on the oxidation resistance of a directionally solidified Ni-Al-Cr3C2 eutectic alloy

Isothermal oxidation of a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy results in development of an external -Al₃O₃-rich scale. However, this scale breaks down after relatively short times at temperature and a less protective Cr₂O₃-rich scale is formed, together with substantial internal oxide in the alloy. In an attempt to maintain the external -Al₂O₃-rich scale and prevent damaging subscale oxidation, modified yttrium-containing directionally solidified alloys have been developed. The oxidation resistance of these alloys at 1000 and 1100°C in flowing air has been investigated and found to be considerably better than that of the corresponding yttrium-free alloy. At both temperatures an external -Al₂O₃-rich scale is produced and is retained for much longer periods than on the yttrium-free alloys during isothermal and thermal cycling oxidation. Some scale breakdown does occur during thermal cycling at 1100°C, but -Al₂O₃ is able to re-form as the surface oxide. However, although external -Al₂O₃-rich scales are retained for long periods on these alloys, some oxide penetration into the alloy beneath these scales does occur where coarse carbide fibers intersect the alloy surface. This is associated with relatively poor scale integrity at these intersections.     

Oxidation resistance of aluminized coatings on a directionally solidified Ni-Al-Cr3C2 eutectic alloy

Although a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy has good high-temperature mechanical properties, it does not have adequate oxidation resistance for prolonged exposure to high surface temperatures. Thus the oxidation behavior of several aluminized coating systems on this alloy in flowing air at temperatures of 900 to 1100°C under isothermal and thermal cycling conditions has been investigated. Attempts to produce an oxidation-resistant system by direct aluminizing have not been successful since removal of carbide fibers results in a porous coating which gives little protection to the alloy. The deposition of a layer of nickel or a Ni-20%Co-10%Cr-4%Al alloy on the eutectic prior to aluminizing gives improved isothermal oxidation resistance for prolonged exposure to high surface temperatures. Thus the eutectic alloy substrate occur during thermal cycling. A more successful system has been produced by depositing a thin layer of platinum on the eutectic alloy prior to aluminizing. Protective -Al₂O₃ scales are formed and maintained during isothermal and thermal cycling oxidation at 900 and 1000°C. Similar scales are developed at 1100° C although these do break down during thermal cycling. However, surface -Al₂O₃ scales are able to re-form rapidly, thereby preventing excessive oxidation of the coating.     

The initial stages of high-temperature corrosion of Fe-Cr-Ni and Cr-Ni alloys in a carburizing atmosphere of low oxygen partial pressure

The initial corrosion reactions on a number of Fe-Cr-Ni and Cr-Ni alloys when exposed in a H₂-CH₄ gas mixture ofa _c =0.8 and 10^–30 bar at1098 K have been investigated. All alloys were studied with surfaces in boththe cold-worked and electropolished conditions. The experiments, in whichexposure times up to 400 rain were used, were carried out in a conventionalcorrosion rig and also in a small reaction chamber contained within an X-rayphotoelectron spectrometer. The use of XPS, SEM, and TEM showed thattransient -Cr₂O₃ layers formed on all alloy surfaces during the heat-up period.On the electropolished surfaces, the oxide layers developed nonuniformly withrespect to microstructure and thickness, whereas homogeneous, compact, andfine-grained -Cr₂O₃ layers formed on the cold-worked specimens. Followingan incubation period, M₇C₃ carbides nucleated on the surfaces; they grew byconsuming the less stable -Cr₂O₃ and also through diffusion of metal fromthe alloy substrate. The mode of nucleation of the M₇C₃ and its rate of growthwere strongly dependent on the character of the transient -Cr₂O₃ layers. Afterremoval of -Cr₂O₃, carbon ingressed into the substrate where M₂₃C₆ wasformed internally. These results are broadly in line with the thermodynamicstability predictions and are discussed in terms of simple models.     

Air Oxidation of Ni–Ti Alloys at 650–850°C

The oxidation of pure Ni and three Ni–Ti alloys containing 5, 10, and 15 wt.% Ti over the temperature range 650–850°C in air was studied to examine the effect of titanium on the oxidation resistance of pure nickel. Ni–5Ti is a single-phase solid solution, while the other two alloys consisted of nickel solid solution (-Ni) and TiNi₃. The oxidation of Ni–Ti alloys at 650°C follows an approximately parabolic rate law and produces a decrease in the oxidation rate of pure Ni by forming an almost pure TiO₂ scale. At higher temperatures, Ni–Ti alloys also follow an approximately parabolic oxidation, and their oxidation rates are close to or faster than those of pure Ni. Duplex scales containing NiO, NiTiO₃ and TiO₂ formed. Some internal oxides of titanium formed, especially at 850°C. In addition, the two-phase structure of Ni–10Ti and Ni–15Ti was transformed into a single-phase structure beneath the scales.     

Compositional changes due to the removal of one constituent in an alloy by a surface reaction

The depletion of an alloy by the selective removal of one element is considered when various rate laws apply. Duhamel's technique has been employed to obtain analytical solutions for the change in alloy composition with time. The effect of the depletion of the matrix by selective oxidation on the subsequent oxidation behavior of the alloy is discussed in terms of operating temperature of the alloy.Notation C concentration of solute (g cm^–3) - C _m bulk concentration of solute (g cm^–3) - C(x, t) concentration of solute at distancex from surface after timet(g cm^–3) - t time - x distance - W _o increase in weight due to takeup of oxygen - W _m change in weight due to metal entering the oxide - M _o molecular weight of anion - M _m molecular weight of cation - Z valency of cation - 2M _m/M _o - k _l linear rate constant (g cm^–2 sec^–1) - k _p andk _p parabolic rate constants (g² cm^–4 sec^–1) - k _e andk _e logarithmic rate constants - D diffusion coefficients of solute in metal     

The morphology of oxidation of alumina-forming iron-base alloys containing chromium and aluminum

A series of iron-base alloys containing different amounts of Cr and Al have been oxidized at atm and at atm. The morphologies of the A1₂O₃ scales depend on the heating rate of the specimens, the Cr content of the alloys, and the of the environment. When the specimens are slowly introduced in the hot zone and oxidized in air, all the alloys form Al₂O₃ scales with a convoluted morphology. The extent of the convolutions decreases with increase in the Cr content. When the specimens are rapidly heated and oxidized in air, the low Cr-containing alloys form internal oxides and a surface scale of Al₂O₃ which shows no convolutions. In the higher Cr containing alloys, the internal oxidation is limited. In the low .     

Carburization of high temperature PM-materials

Carburization of powder metallurgically processed materials for high temperature use is interesting for several reasons: Production of carbide powders e.g. for hard metals, formation of carbide layers on bulk materials to improve corrosion and/or wear resistance and degradation of material properties like strength and ductility by metal-carbon reactions. Molybdenum, TZM (Mo-0.5Ti-0.07Zr-0.05C) and tungsten are finding wide application as construction material in high temperature furnaces which are operated under high vacuum or inert/protective gas conditions. If grafite is used in the same system reactions of the refractory metal components with carbon containing species have to be considered. Therefore a variety of examinations was performed on the carburization characteristics of molybdenum and tungsten (and alloys), especially at temperatures above 1200°C where carburization rates become technically relevant. High temperature oxidation resistant alloys like steels or Ni- and Co-based superalloys can withstand severe carburization as long as their surfaces are covered with tight, protective chromia scales. In case of porous or cracked scales or conditions where chromia is not stable anymore a front of carburization proceeds through the materials – frequently along the grain boundaries. The PM-materials Ducropur (pure chromium) and PM 2000 (Fe-19Cr-5.5Al-0.5Ti-0.5Y₂O₃) show distinctly lower carburization rates: Ducropur forms tight chromium carbide layers, whereas PM 2000 is nearly unaffected up to 1100°C because of its tight and stable alumina scale.     

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.