Aufkohlung von Schleudergußrohren in der petrochemischen Industrie und Erklärung dieses Verhaltens durch Laborversuche mit Chrom-Nickel-Stählen und - Werkstoffen im Temperaturbereich von 800–1150°C

Carburization of centrifugally cast tubes in the petrochemical industries and explication of this behaviour by laboratory tests with chromium nickel steels and chromium nicked materials in the temperature range 800 to 1150°C The carburization behaviour of the tubes used under the conditions of petrochemical cracking processes depends in a first line on the temperature. Up to 1000°C carbon pickup is low, but above 1050°C heavy carbon pickup and increasing carburization depth must be counted with. This temperature dependence is due to the fact that at 1050°C equilibrium is attained between chromium oxide and carbide, so that the oxide is no longer stable and the original protective effect of the oxide layer is lost. It is for this reason that pre-oxidation of the tubes has not only favourable effects: during cooling cracks may form in the oxide layer, so that carburization of a surface layer may set in at temperatures as low as 800°C. Carburization is delayed by high Cr, Ni and Si contents: for uses at 1100°C at least 28% Cr, 32% Ni and 2.5% Si are required.     

Unterschiedliches Aufkohlungsverhalten von Schleudergußrohren aus G-X 40 NiCr 35 25 (W.-Nr. 1.4857) in einem Cracker zur Äthylenherstellung

Carburization of centrifugally cast tubes in the petrochemical industries and explication of this behaviour by laboratory tests with chromium nickel steels and chromium nicked materials in the temperature range 800 to 1150°C The carburization behaviour of the tubes used under the conditions of petrochemical cracking processes depends in a first line on the temperature. Up to 1000°C carbon pickup is low, but above 1050°C heavy carbon pickup and increasing carburization depth must be counted with. This temperature dependence is due to the fact that at 1050°C equilibrium is attained between chromium oxide and carbide, so that the oxide is no longer stable and the original protective effect of the oxide layer is lost. It is for this reason that pre-oxidation of the tubes has not only favourable effects: during cooling cracks may form in the oxide layer, so that carburization of a surface layer may set in at temperatures as low as 800°C. Carburization is delayed by high Cr, Ni and Si contents: for uses at 1100°C at least 28% Cr, 32% Ni and 2.5% Si are required.     

Sub-Scale Depletion and Enrichment Processes During High Temperature Oxidation of the Nickel Base Alloy 625 in the Temperature Range 900�C1000?��C

Numerous chromia-forming austenitic steels and nickel-base alloys contain chromium-rich strengthening precipitates, e.g. chromium-base carbides. During high temperature exposure the formation of the chromia base oxide scale results in chromium depletion in the alloy matrix and consequently in dissolution of the strengthening phase in the sub-surface zone. The present study describes the oxidation induced phase changes in the chromium depletion layer in case of alloy 625, a nickel base alloy in which the strengthening precipitates contain hardly any or only minor amounts of chromium. Specimens of alloy 625 were subjected to oxidation up to 1000 h at 900 and 1000 °C and analyzed in respect to oxide formation and microstructural changes using light optical microscopy, scanning electron microscopy, energy and wavelength dispersive analysis, glow discharge optical emission spectroscopy, and X-ray diffraction. In spite of the fact that the alloy precipitates ??-Ni₃Nb and/or (Ni, Mo)₆C contain only minor amounts of chromium, the oxidation induced chromium depletion results in formation of a wide sub-surface zone in which the precipitate phases are depleted. However, in parallel, substantial niobium diffusion occurs towards the alloy surface resulting in formation of a thin layer of ??-Ni₃Nb phase adjacent to the alloy/oxide interface. By modeling phase equilibria and diffusion processes using Thermo-Calc and DICTRA it could be shown that the phase changes in the sub-scale zone are governed by the influence of alloy matrix chromium concentration on the thermodynamic activities of the other alloying elements, mainly niobium and carbon. The ??-phase depletion/enrichment process is caused by a decreasing niobium activity with decreasing chromium concentration whereas the (Ni,Mo)₆C dissolution finds its cause in the increasing carbon activity with decreasing chromium content.     

Caburisation of heat-resistant steels

The effects of alloying additions of silicon, molybdenum, reactive elements (Ti, Y, Ta, Hf, Nb, Zr), and aluminium, in cast, heatresistant steels on their carburisation resistance have been investigated at temperatures of 900 to 1150 °C. Under strongly reducing conditions, where neither silicon nor chromium oxidise, it is found that the alloying additions slow carburisation rates significantly. In the case of molybdenum and reactive elements, the beneficial effect is attributed to diffusional blocking of carbon flux due to the formation of carbide precipitates of these elements. Silicon operates by altering the solubility and diffusivity of carbon in the matrix. Aluminium appeared to exclude carbon almost completely from the alloy, probably by forming a tenacious external oxide scale. Comparison between a wrought and a cast alloy showed that the cast alloy was more resistant to carburisation. Under conditions oxidising to silicon and chromium, but not to iron and nickel, carburisation rates are slower. The most important factor is then alloy silicon content, which controls the formation of an external SiO₂ layer.     

Metal dusting of nickel-base alloys

Metal dusting, the disintegration of metallic materials into fine metal particles and graphite was studied on nickel, Fe Ni alloys and commercial Ni-base alloys in CO H₂ H₂O mixtures at temperatures between 450–750°C. At carbon activities a_c > 1 all metals can be destroyed into which carbon ingress is possible, high nickel alloys directly by graphite growth into and in the material, steels via the intermediate formation of instable carbide M₃C. Protection is possible only by preventing carbon ingress. Chromium oxide formation is the best way of protection which is favoured by a high chromium concentration of the alloy and by a surface treatment which generates fast diffusion paths for the supply of chromium to the surface. The metal dusting behaviour of Alloy 600 is described in detail. A ranking of the metal dusting resistance of different commercial nickel-base alloys was obtained by exposures at 650°C and 750°C.     

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.     

Oxide scale formation and subsurface phase transformations during long‐term steam exposure of the cobalt base alloy 25

High chromium nickel and cobalt base alloys are presently being considered as construction materials for various components in high efficiency steam turbines with envisaged operating temperatures around 700 °C. In the present study, the steam oxidation behavior of the cobalt base alloy 25 in the temperature range 650–800 °C was investigated whereby exposures up to 10 000 were carried out. Post exposure analyses of the oxidation products and alloy microstructures included optical microscopy, scanning and transmission electron microscopy, X‐ray diffraction analysis and secondary neutrals mass spectrometry. The experiments showed in all cases formation of oxide scales mainly consisting of chromia with minor amounts of outer Cr/Mn spinel and internal silica. The oxidation induced chromium depletion resulted in a number of microstructural changes in the subsurface depletion layer. First, the intermetallic phase Co₃W became enriched at the scale alloy interface. Additionally, the chromium rich M₂₃C₆ and the tungsten rich M₆C dissolved in the depletion layer. The mechanisms for occurrence of these effects are discussed on the basis of phase equilibria in the binary Co–W and the ternary Co–Cr–W system.     

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.     

Metal Dusting of Alumina-Forming Creep-Resistant Austenitic Stainless Steels

Three developmental alumina-forming austenitic stainless steels were exposed to metal dusting conditions at 650?°C in a gas of 50%CO–49%H₂–1%H₂O (a _C: 36.7 and $ p_{{{\text{O}}_{2} }} $ : 2.83?×?10^?26?atm) under thermal cycling conditions. Metal wastage measurement showed initially slow kinetics followed by a fast weight loss. This observation is attributed to the formation of protective chromia/alumina oxide scales in the early stage of the reaction, followed by local oxide failure/spallation during cyclic reaction. Metal dusting initiated from these local defects, and pitting-type attack was observed after 131 cycles of reaction. After 352 cycles, severe dusting had developed, forming heavy and distinctive “tentacles” of superficial coke. This carbon deposit was composed of fine carbon filaments. Examination by TEM of the coke-metal reaction front showed direct surface metal disintegration, indicating that the dusting follows the classical mechanism for austenitic materials. Etching with aqua regia revealed a carburised zone formed in the alloy underneath the coke layer. Analysis by TEM of this zone revealed the formation of ultra-fine, needle-shaped chromium carbide precipitates within a chromium depleted austenite matrix.     

Oxide-Layer Formation and Stability on a Nickel-Base Alloy in Impure Helium at High Temperature

The corrosion behavior in impure helium of Haynes 230, a nickel base alloy candidate for heat exchangers in Very High Temperature Reactors (VHTR), has been investigated. The study focused on the formation and the subsequent destruction of the surface oxide layer at 900 °C and 980 °C. In-situ gas-phase analysis coupled to post-exposure surface analyses showed that a chromium-rich surface oxide formed on Haynes 230 at 900 °C but was unstable above a critical temperature T _A : the chromium-rich oxide reacted with carbon in solution in the alloy to produce chromium and CO(g). The effect of carbon monoxide partial pressure in the gas phase as well as the influence of chromium and carbon activities in the alloy on T _A are discussed taking thermodynamics and kinetics aspects into account.     

Influence de l'adherence de la calamine sur la concentration du carbone au voisinage de la surface des aciers

Influence of scale adhesion on the carbon concentration in the neighbourhood of the steel surface The oxidation of carbon-iron alloys can theoretically proceed with decarburization or carburization depending on the temperature and pertial pressure of carbon monoxide and carbon dioxide in the furnace atmosphere. In the usual conditions only decarburization is observed. Nevertheless a superficial carbon enrichment can temporarily appear despite these theoretical previsions. The latter effect is very important between 730 and 800°C. It disappears above 950°C. The enrichment exists only where the scale is adherent. Ferrous ions cross the oxide but the carbon does not. Furthermore the carbon solubility in wustite is less that 0.5 ppm. The presence of silicon, manganese or nickel includes spalling and consequently the decarburization of subscale metal. With silicon alloys spalling occurs periodically during soaking and the external scale layers consist of Fe₃O₄ and Fe₂O₃. With manganese alloys the scale is always non adherent. Separation proceeds either at high temperature or during cooling. In the firs case, over-oxidation of the scale is induced. With nickel alloys the nickel enriched zone, which lies between the compact scale and the metal, adheres very tightly. The porosity of this zone allows rapid decarburization.     

Effect of H2S on High Temperature Corrosion of Fe–Ni–Cr Alloys in Carburizing/Oxidizing Environments

This investigation involves the corrosion behavior of two Fe–Ni–Cr alloys containing different Si content at 1050?°C in carburizing-oxidizing environments (typical of ethylene pyrolysis) with varied concentration of H₂S. High-Si containing alloy could form thinner but less uniform oxide scale than low-Si alloy after pre-oxidation due to the barrier effect of continuous SiO₂ at interface of scale/substrate. Pre-oxidized alloy showed a better resistance to carburization/sulfidation attacks than the bare alloy in absence of pre-oxidation. It was found that carburization and sulfidation of the Fe–Ni–Cr alloys could be prevented in the environment with a ratio of $ P_{{{\text{H}}_{ 2} {\text{S}}}} /P_{{{\text{H}}_{ 2} }} $ at 1.7?×?10^?5. When the sulfur partial pressure was lower than this value, oxides were found to be converted to porous and non-protective carbides. When the sulfur potentials were increased, manganese or chromium sulfide on outer layer and internal sulfide stringers mixed with silicon oxide in substrate could be formed. Under high sulfur partial pressures, spallation of outer sulfide or oxide scale was observed on high-Si alloy due to less stability of oxide layer formed at surface which was converted to sulfide faster than on low-Si alloy.     

Änderungen der Struktur und der chemischen Zusammensetzung von Rohrwerkstoffen bei der Hochtemperaturpyrolyse

Changes of structure and chemical composition of tubing materials for high temperature pyrolysis The changes of structure and chemical composition of the surface layers of radiation tubes for hydrocarbon pyrolysis furnaces made of Cr25Ni35Nb steel were investigated after longterm service. It was found that carburization of the tube wall occurs where the primarily formed M₂O₃ oxide film with high chromium content is damaged by local spalling. The M₂O₃ oxide film spalling or deterioration is usually attributed to reduction by carbon from the pyrolysis medium. Our own investigations showed M₂O₃ oxide film deterioration to be caused by residual stresses generated during the growth of the zone beneath the oxide film, which is internally oxidised and affected by intercrystal line corrosion. During further oxidation negative effects including the dangerous carburization and furnace life is reduced by the formation of M₃O₄ oxide. This oxide contains less chromium and deteriorates the protection. The regeneration of the M₂O₃ oxide film having a high chromium content is not possible because of the lower chromium, silicon and manganese depletion in the surface-near zone, which prevents M₃O₄ oxide formation.     

Thermodynamics,mechanisms and kinetics of metal dusting

A survey is given on recent research on “metal dusting” i.e. a catastrophic carburization or rather graphitization of metals and alloys occuring in carbonaceous atmospheres at carbon activities a_C>1. The thermodynamics are explained, the mechanisms for iron, low and high alloy steels, nickel and Ni-base alloys are described and the kinetics derived for iron and low alloy steels. Protection against metal dusting is possible by the presence of sulfur in the atmosphere, since adsorbed sulfur retards carbon transfer and hems graphite nucleation. Also dense oxide layers are protective, the preconditions for the formation of Cr-rich protectivee layers on steels and Ni-base alloys are shortly presenteed.     

Metal dusting

This introductory review paper summarizes shortly the research on metal dusting, conducted in the MPI for Iron Research during the last dozen years. Metal dusting is a disintegration of metals and alloys to a dust of graphite and metal particles, occurring in carburizing atmospheres at a_C > 1 and caused by the tendency to graphite formation. The cause of destruction is inward growth of graphite planes into the metal phase, or in the case of iron and low alloy steels into cementite formed as an intermediate. The kinetics of metal dusting on iron and steels was elucidated concerning dependencies on time, temperature and partial pressures. High alloy steels and Ni‐base alloys are attacked through defects in the oxide scale which leads to pitting and outgrowth of coke protrusions, after initial internal formation of stable carbides M₂₃C₆, M₇C₃ and MC. A dense oxide layer prevents metal dusting, but formation of a protective Cr‐rich scale must be favored by a fine‐grain microstructure and/or surface deformation, providing fast diffusion paths for Cr. Additional protection is possible by sulfur from the atmosphere, since sulfur adsorbs on metal surfaces and suppresses carburization. Sulfur also interrupts the metal dusting mechanism on iron and steels, causing slow cementite growth. Under conditions where no sulfur addition is possible, the use of high Cr Nickelbase‐alloys is recommended, they are largely protected by an oxide scale and if metal dusting takes place, its rate is much slower than on steels.     

Carbide precipitation in austenitic stainless steel carburized at low temperature

Low-temperature gas-phase carburization can significantly improve the surface mechanical properties and corrosion resistance of austenitic stainless steel by generating a single-phase “case” with concentrations of interstitially dissolved carbon exceeding the equilibrium solubility limit by orders of magnitude. Upon prolonged treatment, however, carbides (mostly χ, M₅C₂) can precipitate and degrade the properties. High-resolution and spatially resolved analytical transmission electron microscopy revealed the precise carbide–austenite orientation relationship, a highly coherent interface, and that precipitation only occurs when (i) the carbon-induced lattice expansion of the austenite has reached a level that substantially reduces volume-misfit stress and (ii) diffusional transport of nickel, chromium, and iron – enhanced by structural defects – can locally reduce the nickel concentration to the solubility limit of nickel in χ-carbide.     

Colossal carbon supersaturation in austenitic stainless steels carburized at low temperature

A novel, low-temperature (470 °C) gas-phase carburization treatment, developed by the Swagelok Company, increases the surface hardness of 316 austenitic stainless steels from ≈200 to ≈1000 HV25 and improves the corrosion resistance. While normally the precipitation of carbides restricts the carbon concentration in the austenite of 316 steels to <0.015 at%, the Swagelok treatment generates a colossal supersaturation of up to 12 at% carbon in solid solution. Only upon extended treatment, does carbide precipitation eventually occur, but the colossal carbon supersaturation of the austenite is maintained. Unusual for austenitic stainless steels, the precipitates are Hägg carbide (M₅C₂).     

High temperature corrosion of alloy Haynes 556 in carburizing/oxidizing environments

The iron based alloy Haynes 556 has been recently used in hydrocarbon and carbonaceous environments due to its excellent resistance to carburization. This alloy outperforms stainless steels and some of the best commercial carburization‐resistant nickel‐based alloys. This paper is concerned with the behavior of alloy Haynes 556 in high temperature carburizing environments containing trace amounts of oxygen. Thermal cyclic exposures were conducted in 2% and 10% CH₄/H₂ gas mixtures at 800, 900, 1000, and 1100°C for 10 cycles, 50 h each. Carbon activities, oxygen partial pressures, and stabilities of oxides and carbides were used to identify the role played by the reaction products in providing protection. Thermodynamic analyses, weight changes, and microstructural characterization were correlated with environmental parameters and alloy composition to elucidate the causes of its marked resistance to carburization. The results indicate protective character in both gas mixtures under all exposure conditions except the most aggressive, namely 1100°C in 10% CH₄/H₂ gas mixture. Below 1000°C, the formation of Cr, Al, and Si oxides along with Cr carbides provides the primary means of protection. Catastrophic failure at 1100°C was manifested by extensive fracture and crack development within the outer substrate surface in the 10% CH₄/H₂ gas mixture resulting in a dramatic increase in weight gain. This has been attributed to the increased carbon pick‐up, coupled with the loss of the protective outer scales.     

Stability of aisi alloy steels

Typical AISI alloy steels which are used extensively in the automotive industry were investigated. These steels were studied in order to provide a comparison with the steels now used for piping and elevated temperature service. The results indicate that the Cr-Mo steels now employed for piping are satisfactory in comparison with the AISI alloy steels. The steels were exposed at 900° and 1050° and, in several instances, 1200°F. They were either normalized or annealed before exposure and, in general, two carbon levels were investigated. The microstructure was observed after 34,000 hr exposure. Tensile and creep rupture properties were determined after 10,000 hr exposure. Graphite was observed in the Ni-Mo steels, but not in the chromium bearing steels, which were the most structurally stable of the AISI steels investigated. A slight decrease in tensile strength in the various steels was observed after exposure. The creep rupture strength of the nickel steels was similar to plain carbon steels, but the steels containing molybdenum or vanadium had a higher creep rupture strength.     

Effect of carbide volume fraction on the oxidation of austenitic Fe‐Cr‐C alloys

A series of Fe‐15Cr‐(2‐3)Mo alloys (compositions in weight percent) was produced with different carbon concentrations, to control the distribution of chromium between matrix metal and M₂₃C₆ precipitates. The alloys were oxidized in the austenitic state at 850°C in pure oxygen, with and without a pre‐oxidation treatment at low oxygen potential, where no iron oxide could form. Protective, chromia‐rich scaling took place if the chromium concentration at the metal‐scale interface was high enough. This concentration was controlled by the original alloy matrix chromium concentration, and whether or not a high diffusivity ferrite zone developed at the surface by decarburization. Ferrite zone formation was assisted by pre‐oxidation at low oxygen potentials. The value of the carbides as suppliers of additional chromium was demonstrated by comparison with the oxidation performance of carbide‐free alloys of corresponding matrix chromium levels. However, because dissolution of the coarse carbides could be slow, alloys with high volume fractions of large carbides were unsuccessful.