A kinetic and morphological study of the coking of some heat-resistant steels

The coking behaviour of a range of austenitic, heat-resistant steels has been examined in the temperature range 700-1000°C. At and below 800°C, catalytic coke in the form of bundles of filaments formed at localized defect sites in the carbide scales. A wide range in weight-gain kinetics resulted from the differing efficacy of the non-catalytic carbide scales in excluding carbon from the catalytically active substrate. At and above 900°C, catalytic coke formation gave way to pyrolytic coke formation and internal carburization became significant. Parabolic kinetics resulted from the fact that internal carburization was rate-determining. Carburizing alloys gained weight an order of magnitude faster than did alloys protected by oxide films. This was a consequence of dissolution of carbon into the alloy directly from the gas stream being much faster than the rate of coke formation on the alloy surface. Oxide-protected alloys all gained weight at a similar rate, the rate being that of coke deposition on coke. Oxide films containing aluminium were more effective in excluding carbon from the alloy than chromium-containing oxides. However, under reducing conditions, preformed oxide films were not beneficial in limiting carburization in the longer term, because they were prone to spalling, cracking and conversion to non-protective carbide.     

Coke deposition on and removal from metals and heat-resistant alloys under steam-cracking conditions

The kinetics of coke formation and its removal from nickel, copper and alloys have been studied and the results correlated with the morphology of the deposits. Coke deposition from hydrogen-propylene-steam was fast on nickel and slower on copper and the alloys. The deposition on nickel was filamentary in appearance, resulting from a heterogeneously catalysed process, whereas the deposit on the alloys and on non-catalytic copper was essentially uniform. On the alloys, protection appeared to result from the production of scales, containing predominantly Cr₃C₂ and Cr₂O₃, on the surface. At the same time, carburization of the alloy was reduced, apparently as a result of the formation of a silicon-enriched layer at the base of the scale. Steam was found to have little effect on coke formation. However, in the absence of gas-phase hydrocarbons, coke gasification by steam was rapid for nickel but several orders of magnitude slower for the alloys. The main effect of steam is to aid in the formation of scales that are protective against coking and carburization, by promoting oxide formation.     

The role of silicon in wetting and pressureless infiltration of SiCp preforms by aluminum alloys

Silicon plays an important role in the production of Al/SiC metal matrix composites. As an alloying element in aluminum, silicon retards the kinetics of the chemical reactions that result in the formation of the unwanted intermetallics Al₄C₃ and Al₄SiC₄. As a thin coating on silicon carbide, silicon becomes an active participant in a thermally activated chemical reaction that enhances wetting of silicon carbide by aluminum alloys. Consequently, Al/SiC composites made with siliconized silicon carbide and silicon rich aluminum alloys show mechanical properties that are significantly different from those of similar composites produced with unsiliconized silicon carbide or with aluminum alloys that do not contain silicon. It is shown that a silicon coating on SiC significantly enhances wetting of SiC particles by aluminum alloys, reduces porosity, does not affect the modulus of elasticity, but decreases the modulus of rupture of Al/SiC metal matrix composites.     

Failure Analysis of Ethylene Cracking Tube

Radiant tubes of an ethane furnace at a petrochemical plant fabricated from an austenitic heat resistant steel casting (HP grade) failed along longitudinal direction after a fraction of anticipated service life. To study the cause of failure, microstructures of as-received and used tubes were investigated by optical and scanning electron microscopy and the microchemical composition of tubes and precipitated carbide were determined by energy dispersive X-ray (EDS). Also, the morphology of deposited coke particles was determined by SEM/EDS. Finally to measure the extent of carbon penetration, hardness testing was performed on the inner and outer surface of tube. The experimental results show that the improper coking and decoking cycles remove the protective oxide layer (Cr₂O₃) that forms on the exposed surfaces and that, with this layer removed, the coke could easily deposit on inner, non-protected surface. The carbon diffusion into the metal was accelerated with deposited coke and caused microstructural degradation and drastically reduced the ductility of material at high temperatures.     

A hot-wire chemical vapor deposition (HWCVD) method for metal oxide and their alloy nanowire arrays

A concept for synthesizing nanowire arrays of transition metal oxides and their alloys using hot wire chemical vapor deposition (HWCVD) is discussed. Here, unlike conventional HWCVD, the hot filaments act as the source of the metal for the synthesis of one dimensional nanostructures. In the present concept, the chemical vapor transport of metal oxides generated by heating the filaments in low amounts of oxygen, onto substrates maintained at lower temperatures leads to the formation of metal oxide nanowires. Experiments performed using tungsten and molybdenum filaments showed that the nucleation density of the resulting metal oxide nanowires could be varied by varying the substrate temperature. Experiments performed using a magnesium source inside the reactor, in addition to tungsten filaments, resulted in the formation of MgWO₄ nanowires. This clearly indicates the possibility of either doping the metal oxide nanowires or alloying during synthesis.     

Effect of carbon content on the oxidation behaviour of FeCrAlY alloys in the temperature range 1200–1300°C

Abstract

Two FeCrAlY alloys with different carbon contents (90 and 500 ppm respectively) were investigated in respect to their oxidation behaviour at 1200 and 1300°C in air. Oxidation tests, with exposure times ranging from a few hundred to several thousands of hours, revealed that the growth rate of the protective alumina scale was hardly affected by the alloy C-content. However, the time to occurrence of breakaway oxidation for the specimens (1 mm thickness) was substantially shorter for the high-than for the low-C alloy. This was primarily caused by poorer oxide scale adherence but additionally by a higher critical Al-content for occurrence of breakaway of the high-C alloy compared to the low-C alloy.

Extensive microstructural studies revealed formation of Cr-carbides at the grain boundaries in both alloys. The high-C alloy additionally showed carbide formation at the scale/metal interface, thus deteriorating scale adhesion. Furthermore, inter- and intra-granular carbide precipitation is considered to induce strengthening of the metal, thus hindering relaxation of the thermally-induced oxide stresses by substrate creep. In a series of experiments with variations in the cooling rates, it was verified that carbide formation very likely occurs during specimen cooling.     

Interfacial reactivity of aluminium/fibre systems during heat treatments

The interfacial reactivity of specimens composed of aluminium coated on SiC-based fibres, carbon fibres and protected carbon fibres, was investigated. The woven fibres were coated with aluminium by physical vapour deposition and the obtained materials were heat treated in a furnace which was connected to a mass spectrometer. It was shown that reactions occur between CO and CO₂ gases, which are released by the fibres, and aluminium, when the temperature is above 650°C. These gases react during their passage through the aluminium layer and form aluminium carbide. Aluminium carbide is also produced by reactions between the solid-species constituents of the fibres and the metal. The amount of aluminium carbide formed at the fibre/metal interface during heat treatment was determined by hydrolysis. It was thus possible to ascertain that the aluminium carbide is mainly formed by the latter reactions. The efficiency of various protective coatings against the formation of aluminium carbide was also investigated.     

The coking kinetics of heat resistant austenitic steels in hydrogen-propylene atmospheres

The kinetics of coke formation on several commercial Fe-Ni-Cr alloys used in the construction of ethylene steam crackers, nickel and copper were investigated over the temperature range 450 to 1000‡ C in hydrogen-propylene atmospheres using a tubular microbalance reactor. Between 900 and 1000‡ C steady state coking was controlled by gas-phase pyrolysis and similar coking rates were observed for all materials. At 900‡ C alloy carburization led to lengthy periods of parabolic kinetics before the onset of steady state. Below 900‡ C alloy coking rates were between those of copper and nickel. Major differences in coking rates of alloys were only observed below 800‡ C and arise from variations in the effectiveness of chromium carbide scales in excluding the hydrocarbon atmosphere from contact with the underlying nickel-rich and iron-rich alloy.     

Processing and Performance Characteristics of Aluminum-Nano Boron Carbide Metal Matrix Nanocomposites

In this research work, ultrasonic cavitation-assisted casting process was used to fabricate the aluminum alloy-nano boron carbide metal matrix nanocomposites. The optical microscopy results revealed the refined matrix grains in the microstructure of the aluminum alloy-nano boron carbide composites. Boron carbide nanoparticles were uniformly distributed in the aluminum alloy matrix, which can be confirmed by scanning electron microscopic images. The aluminum alloy-nano boron carbide composites show increased dislocation density, compared to the monolithic alloys, which was observed from the transmission electron microscopic images. The addition of nano-boron carbide in aluminum alloy matrix significantly improved its hardness and tensile strength, while the good ductility and impact resistance of the aluminum alloy was almost retained. The results of the dry sliding wear pin-on-disc tests showed improved wear resistance properties of aluminum alloy-nano boron carbide composites compared to the monolithic aluminum alloys.     

Carbon fibres coated with titanium carbide using the liquid metal transfer agent technique

Protective coatings of titanium carbide were applied to PAN type carbon fibres by a liquid metal transfer agent (LMTA) technique using tin as a transfer agent. The effect of the coating on the strength of the fibres was evaluated by performing single fibre tensile tests. The coatings were examined metallographically, by X-ray diffractometry, and by scanning electron microscopy. Carbide coating thicknesses obtained ranged from approximately 0.05 to 0.5 μm and the coatings were found to be uniform and adherent to the fibres. It was found that wetting of the fibres by the tin alloy is associated with the spontaneous formation of a carbide layer with a thickness dependent upon the melt temperature, after which the carbide layer was found to grow parabolically with time and with an apparent activation energy of 187 kJ mol⁻¹. The strength of the carbon fibres decreased with increasing coating thickness according to a Griffith relation.     

X‐ray diffraction (XRD)‐studies on the temperature dependent interface reactions on hafnium,zirconium, and nickel coated monocrystalline diamonds used in grinding segments for stone and concrete machining

Diamond impregnated metal matrix composites are the state of the art solution for the machining of mineral materials. The type of interface reactions between the metal matrix and diamond surface has an essential influence on the tool performance and durability. To improve the diamond retention, the diamonds can be coated by physical vapour deposition with metallic materials, which enforce interface reactions. Hence, this paper focuses on the investigation of the interfacial area on metal‐coated monocrystalline diamonds. Hafnium and zirconium, both known as carbide forming elements, are used as coating materials. The third coating, which is used to determine its catalytic influences when applied as a physical vapour deposition (PVD)‐layer, is nickel. Additionally, the coated diamond samples were heat‐treated to investigate the starting point of the formation of new phases. X‐ray diffraction‐analyses revealed the assumed carbide formation on hafnium and zirconium coated samples. The formation temperature was identified between 800 °C and 1000 °C for hafnium and zirconium coatings.     

Special features of structure formation in polycarbide based hard alloys

The paper addresses structure formation in TiC-(VC, NbC, WC) based hard alloys with a nickel-chromium binder, which were prepared by the conventional process and by a process that excludes the additional preliminary operation of carbide synthesis. The alloys are studied by metallographic methods, X-ray phase analysis, and X-ray microspectral analysis. The influence of preparation methods and chemical composition of the alloys on their microstructure formation is clarified.     

A Novel Processing Technique for Tow Fiber Reinforced Metal Matrix Composites with Tailored Interfaces

A novel metal matrix composite processing technique based upon plasma-assisted physical vapor deposition (PVD) of metais/alloys has been developed wherein, starting with lower cost tow fibers, the tows are spread to separate the individual filaments and then each filament coated uniformly with the matrix by plasma sputtering inside a hollow cathode magnetron. The coated fibers are then densified into a composite. This technique allows the synthesis of metal matrix composites with high fiber volume fractions. The resulting composite exhibits a uniform microstructure with complete infiltration of the matrix. Further, the plasma-assisted deposition process spares the tow filaments from severe strength degradation normally associated with melt-infiltration processing. Also, for chemically or thermo-mechanically incompatible fiber-matrix systems, the technique allows the incorporation of an interfacial layer (engineered for greater chemical inertness or lower residual stresses) prior to the deposition of the matrix. This technique has been applied here to synthesize composites based on Nextel 610 tows and Superalloy matrices (Thermo-span, Haynes 214 or Inconel 718) with a Mo chemical barrier layer.     

高温碳氢气氛下Cr5Mo钢的尘化腐蚀状况

金属渗碳腐蚀(即尘化)是高温碳氢环境下常发生的灾难性腐蚀。Cr5Mo钢的工程应用量大面广,过去对其渗碳腐蚀研究不够。为此,研究了炉管材料Cr5Mo钢在600℃,50%CO-H2-3%H2O气氛下的尘化腐蚀行为,采用X射线衍射分析了腐蚀试样的物相组成,采用扫描电镜对试样进行了微观形貌分析。结果表明:Cr5Mo钢在试验条件下呈现均匀腐蚀,材料自表面向内依次析出Fe5C2和Fe3C脆性腐蚀产物,经560h尘化腐蚀后的试样平均腐蚀深度约为200μm,而基体材料性质无明显改变。因此Cr5Mo钢在尘化过程中出现的腐蚀减薄是由脆性碳化物层的析出引起的。     

Formation of carbides on rubbing metal surfaces and its influence on wear resistance

Data were obtained showing that lubricated friction between alloys containing carbide-forming elements may lead to the formation of etch-resistant surface carbide layers. Metallographie and X-ray diffraction analyses established that the necessary condition for the formation of surface layers of this kind is local seizing of the rubbing metal surfaces. The formation of these layers leads to intense wear of at least one of the friction pair components.     

Fabrication and characterization of magnesium–fluorapatite nanocomposite for biomedical applications

Recent studies indicate that there is a high demand for magnesium alloys with adjustable corrosion rates, suitable mechanical properties, and the ability for precipitation of a bone-like apatite layer on the surface of magnesium alloys in the body. An approach to this challenge might be the application of metal matrix composites based on magnesium alloys. The aim of this work was to fabricate and characterize a nanocomposite made of AZ91 magnesium alloy as the matrix and fluorapatite nano particles as reinforcement. A magnesium–fluorapatite nanocomposite was made via a blending–pressing–sintering method. Mechanical, metallurgical and in vitro corrosion measurements were performed for characterization of both the initial materials and the composite structure. The results showed that the addition of fluorapatite nano particle reinforcements to magnesium alloys can improve the mechanical properties, reduce the corrosion rate, and accelerate the formation of an apatite layer on the surface, which provides improved protection for the AZ91 matrix. It is suggested that the formation of an apatite layer on the surface of magnesium alloys can contribute to the improved osteoconductivity of magnesium alloys for biomedical applications.     

Effect of dilution on GTAW Colmonoy 6 (AWS NiCr–C) hardface deposit made on 316LN stainless steel

Abstract

Nickel based Colmonoy 6 (conforming to AWS NiCr–C) hardfacing alloy finds application in hardfacing of various components made of austenitic stainless steel (SS) used in fast reactors. Owing to considerable difference in melting points of the SS and Colmonoy 6 alloys, significant dilution from substrate occurs during hardfacing using gas tungsten arc welding process. Dilution has a significant effect on microstructure, hardness and wear resistance of the deposit. To overcome the adverse effects of dilution on the hardness and, hence, the wear resistance of the deposit, often, the minimum thickness specified for the deposit on hardfaced components is high, which in turn increases the susceptibility of the deposit to cracking during deposition. In the present investigation, microstructure of different layers of multilayer Colmonoy 6 deposits on 316LN SS is characterised by optical and scanning electron microscopy, and the correlation between hardness and microstructure of the individual layers with dilution from the base metal has been established. The dilution from the base material is the highest in the first layer, and it progressively decreases in the subsequent layers. With progressive decrease in dilution, the precipitate fraction increases from about 16 to 20% from the first to the fifth deposit layers. This is accompanied by hardness increase from about 480 to 800 HV. The precipitates in the deposit consist of both borides and carbides, with the boride content varying more with dilution than the carbide content. The boride fraction increased from 5 to 8% with a decrease in dilution; however, layer to layer variation in carbide fraction was only marginal at about 11–12%. High dilution from the base material suppresses the formation of borides in the deposit and is responsible for low hardness of the deposit diluted with the austenitic SS compared to those of the undiluted deposit.     

Failure of a Secondary Superheater Tube in a 140-MW Thermal Power Plant

This investigation was conducted to determine the cause of failure of a welded joint of a secondary superheater tube from a 140-MW thermal power plant. Chemical analyses along with detailed optical microscopic examination of a secondary superheater tube were carried out to predict the probable cause of failure. Microstructure of the secondary superheater tube welded between austenitic stainless steel to low alloy steel revealed presence of a thin layer of metal carbide along the weld interface which eventually led to intergranular cracking at austenite grain boundaries. It was concluded that the formation of brittle carbide layer was due to migration of carbon at elevated temperature led to failure of the tube.     

Phase composition of the surface layers of high-speed cutting steel after electrospark alloying and laser treatment

Due to the metallurgical processes occurring under the action of laser irradiation, which are accompanied by convectional mass transfer, phase formation, and the interaction of alloying elements with the base material and elements of the environment, tungsten carbide fully dissolves in the steel melt in the case where alloying was carried out with alloys based on this carbide. Moreover, the refractory component of tungsten-free electrode materials stayed completely unaltered. by partially losing a non-metal. These transformations occur under nonequilibrium conditions. To improve the tribotechnical properties and durability of a high-speed cutting steel upon cutting, it is reasonable to alloy its surface layer with elements and compounds which form stable oxides strongly bound to the base. Frantsevich Institute of Problems of Materials Technology, the Ukrainian Academy of Sciences, Kiev. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 36, No. 1, pp. 83–86, January–February, 2000.     

Graphite and boron carbide composites made by hot-pressing

Composites consisting of graphite and boron carbide were made by hot-pressing mixed powders of coke carbon and boron carbide. The change of relative density, mechanical strength and electrical resistivity of the composites and the X-ray parameters of coke carbon were investigated with increase of boron carbide content and hot-pressing temperature. From these experiments, it was found that boron carbide powder has a remarkable effect on sintering and graphitization of coke carbon powder above the hot-pressing temperature of 2000° C. At 2200° C, electrical resistivity of the composite and d(002) spacing of coke carbon once showed minimum values at about 5 to 10 wt% boron carbide and then increased. The strength of the composite increased with increase of boron carbide content. It was considered that some boron from boron carbide began to diffuse substitutionally into the graphite structure above 2000° C and densification and graphitization were promoted with the diffusion of boron. Improvements could be made to the mechanical strength, density, oxidation resistance and manufacturing methods by comparing with the properties and processes of conventional graphites.