Effect of H2S on formation and decomposition of Fe3C and Fe5C2 under metal dusting conditions

The high temperature corrosion process metal dusting leads to the formation and decomposition of metastable iron carbides at the surface of iron samples. A small amount of H₂S in the carburising atmosphere causes the adsorption of sulphur onto the sample surface, which decreases the carbon transfer rate and retards or suppresses the start of metal dusting. The extent of retardation of metal dusting depends on temperature, carbon activity and H₂S content. The higher the carbon activity the higher the H₂S content required for suppression of metal dusting. At very high carbon activities a second iron carbide, Fe₅C₂ (Hägg carbide), forms on the cementite surface. The carburisation experiments were conducted at 500°C using CO‐H₂‐H₂O‐H₂S gas mixtures. The microstructural investigations show that both metastable carbides decompose during metal dusting.     

Microprocesses of metal dusting on nickel and Ni-base alloys

Microstructural and microchemical investigations were carried out on nickel and Inconel 600 after exposure to strongly carburizing atmospheres at temperatures of about 600 to 650°C to study their metal dusting behaviour. Contrary to iron and low-alloy steels, where metal dusting proceeds via the formation and disintegration of a metastable carbide M₃C, both nickel and Inconel 600 directly disintegrate. Inside the metal this disintegration proceeds by formation of thin graphite filaments of nearly 10 nm in diameter, the atomic basal planes of which are oriented perpendicular to the surface thus effecting a high reactivity at the growth front. Subsequently, fine particles or larger parts of the metal surface layer are released, which are buried in the deposited graphite layer on the surface. In case of Inconel 600, containing Cr with mass contents of about 15%, the disintegration can be delayed by the formation of a chromium oxide layer, but no safe protection against metal dusting is obtained.     

Effect of sulfur on the stability of cementite

Cementite Fe₃C is an unstable carbide, which should decompose to iron and graphite. It was possible to grow cementite on iron samples in flowing CO-H₂-H₂O-H₂S mixtures at temperatures between 400–700°C. The cementite is stabilized by adsorbed sulfur, preventing graphite deposition, which would initiate metal dusting. Thus, this corrosion phenomenon, occurring in CO-H₂-H₂O and other strongly carburizing atmospheres can be suppressed by the presence of some H₂S. The range of H₂S/H₂ ratios and temperatures in which iron is inert against metal dusting, corresponds to the range where a monolayer of sulfur is adsorbed in iron. It may be supposed that the iron carbide process, i.e. the production of Fe₃C by reduction of ores in methane, is possible only by the presence of some sulfur.     

Catalytic effect of iron on decomposition of carbon monoxide: I. carbon deposition in H2-CO Mixtures

Hematite ore reduced with hydrogen was used as a catalyst in the present investigation of the rate of decomposition of carbon monoxide in H₂-CO mixtures. It was found that the amount of carbon deposited from purified carbon monoxide was directly proportional to the amount of porous iron catalyst present in the system. However, this simple relation did not hold for H₂-CO mixtures. During carbon deposition the porous iron granules dis-integrated and were dispersed evenly in the carbon deposit. The deposit consisted of graphite, cementite and iron, with cementite/iron ratio increasing as more soot accumulated. When most of the iron was converted to cementite, carbon deposition ceased. A small amount of hydrogen enhanced markedly the rate of decomposition of carbon monoxide. Indications are that hydrogen adsorbed on iron catalyzes the decomposition of carbon monoxide, 2CO → C + CO₂, in addition to the occurrence of the second reaction H₂ + CO → C + H₂O.     

Thermodynamics of Hägg carbide (Fe5C2) formation

Gas carburisation experiments on iron samples were conducted in CO‐H₂‐H₂O‐H₂S atmospheres with different carbon activities at 500 °C. Carbon activities well above a_C = 1 (equilibrium iron/graphite) were used in order to form iron carbides such as cementite (Fe₃C) and Hägg carbide (Fe₅C₂). The Gibbs energy of formation ΔG for Fe₅C₂ was determined at 500 °C. This result combined with thermodynamic properties at other temperatures taken from literature strongly supports the existence of the three phase equilibrium α‐Fe+Fe₃C+Fe₅C₂ at about 350°C in the binary Fe‐C system.     

Redistribution of Carbon Atoms in Differentially Quenched Rail on Prolonged Operation

The change in structure, phase composition, and defect substructure in the head of differentially quenched rail after the passage of gross traffic amounting to 691.8 million t is investigated over the central axis, at different distances from the top surface, by means of transmission electron microscopy. The results confirm that prolonged rail operation is accompanied by two simultaneous processes that modify the structure and phase composition of the plate-pearlite colonies: cutting of the cementite plates; and solution of the cementite plates. The first process involves cutting of the carbide particles and removal of their fragments, accompanied simply by change in their linear dimensions and morphology. The second process involves the extraction of carbon atoms from the crystal lattice of cementite by dislocations. That permits phase transformation of the metal in the rail, which is associated with marked relaxation of the mean binding energy of the carbon atoms at dislocations (0.6 eV) and at iron atoms in the cementite lattice (0.4 eV). The stages in the transformation of the cementite plates are as follows: the plates are wrapped in slipping dislocations, with subsequent splitting into slightly disoriented fragments; the slipping dislocations from the ferrite lattice penetrate into the cementite lattice; and the cementite dissolves with the formation of nanoparticles. The cementite nanoparticles are present in the ferrite matrix as a result of their transfer in the course of dislocational slip. On the basis of equations from materials physics and X-ray structural data, the content of carbon atoms at structural elements of the rail steel is assessed. It is found that prolonged rail operation is accompanied by significant redistribution of the carbon atoms in the surface layer. In the initial state, most of the carbon atoms are concentrated in cementite particles. After prolonged rail operation, the carbon atoms and cementite particles are located at defects in the steel’s crystalline structure (dislocations, grain and subgrain boundaries). In the surface layer of the steel, carbon atoms are also observed in the crystal lattice based on α iron.     

Microscale investigations of the metal-dusting corrosion mechanism on mild steel

The internal and external products from metal-dusting corrosion of a mild steel specimen have been investigated, with the intention of further exploring the corrosion reaction mechanism. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been employed. A section of a steel tube, which had been subjected to heavily corrosive attack under controlled conditions, was studied. Electron-transparent TEM samples from this specimen were observed in the cross-sectional and plan-view orientations of the corrosion interface. Deposit on the corroded surface contained amorphous carbon, graphitic crystalline carbon, and decomposition products. Supersaturated cementite is an intermediate reaction product and was present at the surface of the exposed alloy. Surface cementite was seen to decompose locally into the graphite, where graphite basal planes were oriented perpendicular to the cementite surface. Iron was transported through the graphite, and the mild steel disintegrated by continuous formation and disintegration of the surface cementite. The observations are discussed with reference to the prevailing model mechanism for metal-dusting corrosion.     

A study on inhomogeneous distribution of temper graphite particles in strip-cast Fe-C-Si alloys

This study examined the relationship between solidification structure and graphitization characteristics of white cast iron strips produced by strip casting. Experimental results showed that there was an unusual distribution of temper graphite particles along the through-thickness direction of the graphitized strips in comparison with gravity-cast chill plate. In particular, the graphite-free zones appeared in the vicinity of the strip surface after the completion of graphitization, especially in the strip with low carbon and silicon content. There were abnormally straight interfaces between matrix and eutectic cementite with a strong preferred [001]_c growth direction caused by the effect of directional solidification found in the near-surface regions of the strips. The interfaces did not form a site for the graphite to nucleate and gave rise to the graphite-free zones close to the strip surface. An increase in carbon and silicon content could significantly increase the number of temper graphite particles and shorten the time for the completion of graphitization, but an inhomogeneous distribution feature of graphite particles was still observed in strips with a higher carbon equivalent value (CE). Furthermore, variations in carbon and silicon content resulted in transitions in carbide morphology and composition, which had a tremendous effect on the graphitization characteristics of the cast iron strips.     

Characterization of phases formed in the iron carbide process by X-ray diffraction, mossbauer, X-ray photoelectron spectroscopy, and raman spectroscopy analyses

Iron carbide was prepared by iron ore reduction and iron cementation using Ar-H₂-CH₄ gas mixture with and without sulfur. Phases formed in the reduction/cementation process were examined by X-ray diffraction (XRD), Mossbauer, and Raman spectroscopy. The sample surface was also analyzed by X-ray photoelectron spectroscopy (XPS). XRD and Mossbauer analyses showed that iron oxide was first reduced to metallic iron, and then, metallic iron was carburized to cementite. Addition of a small amount of H₂S to the reaction gas retarded the cementite formation but made the cementite more stable. XPS analysis showed that the surface of samples converted to iron carbide using sulfurcontaining gas consisted of mainly Fe₃C and a small amount of graphitic carbon. Raman spectra of a sample produced in the iron carbide process showed the G and D bands, which are characteristic for carbon-carbon bonds. The intensity ratio of G/D bands depended on the sulfur content in the reducing/carburizing gas.     

Evaluation of Metal Dusting Behavior in Reformer Tube Made of HP–Nb Alloy on the Outer Surfaces

The metal dusting behavior of Iron–Chromium–Nickel heat resistant HP–Nb steel specimen was investigated at the outer surfaces while the methane gas was passed inside the hole of the specimen. After an exposure of 130 h in a flowing methane (CH₄) gas at 680 °C, different dispersed corrosion products were formed on the outer surface of the specimen near the hole. Conventional metallography and scanning electron microscopy were used to identify the microstructure of the reaction products. Energy dispersive X-ray spectroscopy was used for microchemical analysis. The phases produced on the surface were identified by X-ray diffraction. Some of reaction products found as surface deposits on the outer surfaces of specimen near the hole contained Fe and Cr carbides, Fe, Cr and Ni oxides, scale of Ni, Fe particles and free C. Results revealed that carbon nano-filaments materials could be formed during disintegration of heat resistant HP–Nb steel under metal dusting environment.     

Characterisation of functionally graded and VC/steel surface alloyed composites produced using powder metallurgy

Abstract

In this study, low carbon steel specimens with surface alloyed composites were produced by means of powder metallurgy. Vanadium carbide, graphite (1·2 wt-%) and Fe were used for the surface alloyed composite, while Fe and graphite (0·2 wt-%) were used for the low carbon steel side. The powder mixtures were compacted together in the same mould. On the surface alloyed side the vanadium carbide content was changed from 5 to 25 wt-%. Microstructural investigations including EDX and X-ray, hardness measurement and abrasive wear tests were performed. The results showed that V₈C₇ formed in the alloyed surface and carbon diffusion from the alloyed surface to the parent metal created a functionally graded material. The hardness values decreased towards the parent metal. Wear resistance increased as the vanadium carbide increased in the surface alloyed composite. Thus, a functionally graded steel having a surface composite that is resistant to abrasive wear can be obtained via the powder metallurgy route.     

The effect of additives on the eutectoid transformation of ductile iron

The eutectoid transformation of austenite in cast iron is known to proceed by both the meta-stable γ → α + Fe₃C reaction common in Fe-C alloys of near eutectoid composition, and by the direct γ → α + Graphite reaction, with the graphite phase functioning as a car-bon sink. In addition, the meta-stable cementite constituent of the pearlite can dissolve near the graphite phase (Fe₃C → α + Graphite), producing free ferrite. Isothermal trans-formation studies on a typical ductile iron (nodular cast iron) confirmed that all of these reaction mechanisms are normally operative. The addition of 1.3 pct Mn was found to substantially retard all stages of the transformation by retarding the onset of the eutectoid transformation, decreasing the diffusivity of carbon in ferrite, and stabilizing the cemen-tite. Minor additions of Sb (0.08 pct) or Sn (0.12 pct) were found to inhibit the γ →α + Graphite reaction path, as well as the Fe₃C → α + Graphite dissolution step, but did not significantly affect the meta-stable γ → α + Fe₃C reaction. Scanning Auger microprobe analysis indicated that Sn and Sb adsorb at the nodule/metal interphase boundaries during solidification. This adsorbed layer acts as a barrier to the carbon flow necessary for the direct γ → α + Graphite and Fe₃C → α + Graphite reactions. With the graphite phase dis-abled as a sink for the excess carbon, the metal transforms like a nongraphitic steel. The effects of Mn, Sn, and Sb on the eutectoid transformation of ductile iron were shown to be consistent with their behavior in malleable iron.     

Phase instability of manganese-iron carbides

The stability of carbon saturated manganese-iron alloys was studied by means of simulated decrepitation tests, and it was found that the product must contain a minimum of about 5 wt pct iron to be stable during storage. By means of several experimental techniques it was shown that the structure of the carbide phase present in carbon saturated ferromanganese determines whether the alloy is stable. Below the critical iron content of about 5 wt pct, the carbide phase is Hāgg carbide (MnFe)₅C₂, whereas above about 5 wt pct iron the carbide phase is cementite (MnFe)₃C. The role of iron is to stabilize the cementite phase. Experiments with the synthetic manganese carbides, Mn₅C₂ and Mn₃C, showed that the former reacts readily with water whereas the latter is stable.     

Thermodynamics of the solid phases in the system Fe−Mn−C

Phase relationships in the Fe−Mn−C system in the temperature range 600 to 1100°C have been studied using metallographic and X-ray methods and the electron microprobe. Isothermal sections of the phase diagram of the system are reported based on the present results and those of earlier investigators. The fcc λ-phase (austenite) containing carbon is stable at all values ofy _Mn=x _Mn/(x _Mn+x _Fe) in the range 890 to 1100°C and in a more restricted composition range at lower temperatures. Its composition under conditions of equilibrium with the carbides (Fe, Mn)₃C, (Fe, Mn)₂₃C₆, ε, and liquid are shown for several temperatures. The free energy of formation of the cementite phase, (Fe, Mn)₃C, at 1000°C, from γ-Fe, γ-Mn (undercooled) and graphite is ΔG ₁₂₇₃=−35,790y _Mn−2760y _Fe+3RT (y _Mn lny _Mn+y _Fe lny _Fe). The data show that the alloyed cementite is essentially and ideal mixture of Fe₃C and Mn₃C,i.e., the metal atoms are distributed at random on the metal sites in the lattice. ROBERT BENZ, formerly of the Research Staff, Massachusetts Institute of Technology, Cambridge, Massachusetts     

On the iron-carbon phase diagram

Experimental results that are obtained by electron microscopy, X-ray diffraction, and carbide analysis and indicate the precipitation of carbon atoms clusters in a hypereutectoid steel during its annealing above the eutectoid temperature are presented. These results are compared to the reported data in order to construct a new Fe-C phase diagram, where cementite forms below the eutectoid temperature due to the tendency of the Fe-C system toward ordering and carbon unbound to iron precipitates above this temperature in the form of clusters or graphite particles due to the tendency of this system toward phase separation.     

The Effect of Carbon on the Transition from Graphite to Cementite Eutectic in Cast Iron

In this work, an analytical solution is proposed to explain the influence of carbon on the transition from graphite to cementite eutectic in cast iron. The outcome from this work indicates that this transition can be related to (a) the graphite nucleation potential (directly characterized by the cell count, N and indirectly by the nucleation coefficients N _s and b), (b) the eutectic graphite growth rate coefficient, μ, (c) the temperature range, ?T _sc = T _s  ? T _c (where T _s and T _c are the equilibrium temperature for graphite eutectic and the formation temperature for cementite eutectic, respectively), and (d) the liquid volume fraction, f, after pre-eutectic austenite solidification. In addition, the absolute and the relative chilling tendencies, CT and CT_r, respectively, as well as the critical cooling rate, Q _cr, and the chill width, w, can be predicted from this work. The analytical model was experimentally verified for castings with various carbon contents. It was found that the carbon content increases the eutectic cell count, N while reducing the maximum degree of undercooling at the onset of graphite eutectic solidification, ?T _m. From this work it is evident that the main role of carbon on the transition from graphite to cementite eutectic is through its effect on increasing the growth coefficient and hence, the graphite eutectic growth rate, u. Moreover, at increasing carbon contents the absolute and the relative chilling tendencies including the chill width, all are significantly reduced. Finally, the equations derived using theoretical arguments for the chill width are rather similar to expressions based on a statistical analysis of the experimental outcome.     

Room Temperature Case Carburizing of a Ni–Cr–Mo Steel Through Shot Peening/Blasting Techniques

Controlled shot-peening/blasting is an operation which is used largely in the manufacturing industry. An attempt is made to create a case-hardened surface by the shot peening technique on a Ni–Cr–Mo steel. Steel shots with activated carbon powders were introduced at the surface of the samples for different periods of time, i.e. for 5, 10, 15 and 20 min. The microstructure, chemical composition, and hardness were investigated by optical microscopy, SEM, OES, XRF, XRD and micro-Vickers hardness tester, respectively. An effective diffusion of carbon atoms was found in the samples subjected to shot peening for 10 min. and above. However, the hardness values were found to be non-uniform and showed a maximum hardness of 300 HV_0.3 at activated spots. The microstructural studies revealed formation lamellar-shaped cells consisting of nano grains at the shot peened surface. A close observation of these lamellae shows α-ferrite and cementite (Fe₃C) formed by diffusion of carbon into iron at room temperature. XRD results confirmed the formation of Cr₂₃C₆ and Fe₃C by shot peening with activated carbon.     

Thermodynamics of the Fcc Fe−Ni−C and Ni−C alloys

The activity of carbon in the fcc solid solution of the Fe?Ni?C system has been measured at 800°, 1000°, and 1200°C by comparison with observed values in the Fe?C binary by equilibration with methane-hydrogen mixtures. Defining the lattice ratioz _C≡n _C/(n _Fe+n _Ni?n _C), the activity coefficient Ψ_C≡a _C/z _C has been determined as a function of temperature and composition. At infinite dilution log Ψ_C goes through a maximum at about 70 pct Ni in agreement with Smith. The partial molar free energy of carbon in the dilute solution referred to graphite is not a linear function of the base alloy composition, but has a large deviation with maximum at about 60 pct Ni. Similar maxima occur in both ΔH _C ^° and ΔS _C ^° . Linear equations are derived for the activity coefficient of carbon in three composition ranges of Fe?Ni?C alloys; a simplified equation applicable to nickel steels is included. The solubility of graphite in nickel has been determined. The marked deviation from linearity is ascribed to the existence of iron atoms in two electronic states, γ₁ and γ₂ which differ in energy and are antiferromagnetic and ferromagnetic, respectively.     

Reduction Behaviors of Pellets Under Different Reducing Potentials

Owing to the change of gas composition in top gas recycling-oxygen blast furnaces compared with traditional blast furnace, many attentions are attracted to the research on iron oxide reduction again. In order to study the influence of H₂ and CO on the reduction behavior of pellets, experiments were conducted with H₂-N₂, CO-N₂ or H₂-CO gas mixtures at 1173 K by measuring the mass loss, respectively. It was found that the reduction degree increased with increasing the ratio of H₂ or CO in the gas mixture, but the reduction with hydrogen was faster than that with carbon monoxide. The reduction degree could reach 96. 72% after 65 min for the reduction with 50% H₂ + 50% N₂, while it is only 53. 37% for the reduction with 50% CO+ 50% N₂. The addition of hydrogen to carbon monoxide will accelerate the reduction because the hydrogen molecules are more easily chemisorbed and reacted with iron oxide than carbon monoxide. A scanning electron microscope was used to characterize the structures of reduced samples. Dense structure of iron was obtained in the reduction with hydrogen while the structure of iron showed many small fragments for the reduction with carbon monoxide. At the later stage of reduction with the gas mixtures containing carbon monoxide, the reduction curves showed a descending trend because the rate of carbon deposition caused by the thermal decomposition of carbon monoxide was faster than the rate of oxygen loss. Compared with the reduction with CO-N₂ and H₂-CO gas mixtures, H₂ gas could enhance the carbon deposition while N₂ gas would reduce this phenomenon. The results of X-ray diffraction and chemical analysis demonstrated that the carbons are mainly in the form of cementite (Fc₃C) and graphite in reduced sample.     

The Gas Carburization of Linear Cellular Alloys as a Novel Alloy Development Tool

Investigations of the production of thin-walled steel alloys through the gas carburization of structures made from reduced and sintered metal oxide powders were performed. Extrusions with low-alloy steel composition were produced successfully without the occurrence of metal dusting, yielding a novel technique for the production of thin-walled steel structures. Thin strip geometries (~200 to 300 μm final thickness) of samples with the composition of 4140 steel, without carbon, were produced through the extrusion of a paste of metal-oxide powders. Full reduction and sintering in a 10 pct H₂/90 pct Ar atmosphere yielded a metal part containing all necessary alloying elements except carbon. Gas carburization in a controlled CO/CO₂ atmosphere was then used to introduce carbon through the thickness of the structure while carburization parameters were controlled such that metal dusting was not observed. It has been shown in this study, through heat treatment and microstructural investigations, that structures with 4140 composition displaying microstructures and mechanical properties comparable with conventionally made steels can be reached in approximately 30 minutes during gas carburization. The research shows that carbon contents above the eutectoid composition can be reached in less than 30 minutes. As a result, a novel alloy development tool has been introduced.