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Stress corrosion cracking, similar to intergranular corrosion in austenitic steel welds, primarily affects grain boundaries in the heat-affected zone when exposed to a corrosive environment, leading to sensitization – a reduction in structural integrity, which causes cracking and grain loss. The object of research were samples cut out of «coils» of pipelines of the 3rd power unit of the Chernobyl NPP in the areas of damaged welded joints. Based on radiographic and ultrasonic inspection of pipeline welds, as well as determination in the examined samples of the content of alloying elements, mechanical properties, metallographic, fractographic and micro-X-ray spectral studies, including investigation of fracture surfaces of samples that were cyclically loaded, taking into account fatigue grooves, and with regard to solution of the problem of corrosion cracking of austenitic steels in boiling water reactors, a set of measures was developed: long-term measures – replacement of steels with others not prone to corrosion cracking; short-term measures – application of external weld coating, repair of defective areas, relieving residual stresses; temporary measures – justification of the admissibility of the defective weld for operation. [ABSTRACT FROM AUTHOR]

This paper presents a study on thermally sprayed coatings. Coatings produced by high-velocity oxygen–fuel spraying HVOF and plasma spraying deposited on the A03590 aluminum casting alloy are tested. The subject of this research concerns coatings based on tungsten carbide WC, chromium carbide Cr3C2, composite coatings NiCrSiB + 2.5%Fe + 2.5%Cr, mixtures of tungsten and chromium powders WC-CrC-Ni, mixtures of carbide powders with the Cr3C2-NiCr + the composite 5% NiCrBSi and WC-Co + 5% NiCrBSi. The aim of this research is to find a coating most resistant to the erosive impact of particles contained in the medium centrifuged by industrial rotors. The suitability of the coating is determined by its high level of microhardness. The hardest coatings are selected from the coatings tested and subjected to abrasion tests against a sand particle impact jet and the centrifugation of a medium with corundum particles. It is found that the most favorable anti-erosion properties are demonstrated by a coating composed of a mixture of tungsten carbide and chromium carbide WC-CrC-Ni powders. It is concluded that the greatest resistance of this coating to the erosive impact of the particle jet results from the synergistic enhancement of the most favorable features of both cermets. [ABSTRACT FROM AUTHOR]

Mechanically-alloyed CoCrFeNi high entropy alloys (MA-HEAs) show better mechanical properties than as-cast HEAs. However, the limited amount of powder milled in one-milling cycle creates a challenge in larger-scale applications. The low ball-to-powder ratio (BPR) milling used to increase powder yield resulted in the formation of Cr-rich second phases in the face-centered cubic matrix. To overcome the above issues, in this work, with a low BPR of 5:1, four different processes were analyzed: quicker milling intervals, higher milling speed, and the pre-alloying of Cr and Ni. All parameters mentioned before were used and tested together in the fourth sample. Samples were consolidated by spark plasma sintering technique. Microstructural characterization was made on a Scanning Electron Microscope equipped with EDS and EBSD detectors and a Transmission Electron Microscope. X-ray diffraction was employed to develop the structural evolution of powders sintered and annealed samples. The global hardness was measured using microhardness tests, while the nanohardness tests helped us correlate the evolution of secondary phases with their mechanical properties. The results show that pre-alloying elements with the lowest diffusion coefficient before the main milling process might be an attractive strategy to improve productivity and optimize microstructure without limiting the efficiency of low BPR MA-HEAs. The results described below demonstrate the promising perspective for the implementation of powder metallurgy techniques in a mass scale production as low BPR is essential for numerous applications like aerospace industry, car engines and medicine, where improved productivity of powder mixing is mandatory.

This paper reports the findings of a study on the Spark Plasma Sintering of High-Entropy Alloys (HEAs) using atomized alloy powder. The sintering process was conducted within a graphite matrix at a predetermined optimum temperature of 1050 °C. The resulting material exhibited a density close to the theoretical value. Hardness tests and microstructural observations revealed the impact of the graphite tool used during the sintering process, particularly in the near-surface layer. Chemical and phase composition analyses indicated the formation of chromium carbides in the near-surface layer, leading to the depletion of FCC phase grains in chromium and alterations in the shape and size of these grains. [ABSTRACT FROM AUTHOR]

The package of the applied programs with base of the thermodynamic data is used and the theoretical analysis of physical and chemical conditions of complex- saturation carbon steel by silicon and chrome in medium of chlorine in the closed reactionary space is carried out at the lowered pressure of a gas phase. Simultaneous saturation of steel 40 with chrome and silicon in chlorine atmosphere for 6 hours at a temperature of 1323 K and a pressure of 102 Pa of the gas phase is carried out in this work. By means of the x-ray diffraction studies, it is found that, under the assumed conditions of saturation on the steel 40 surface, diffusion layer with thickness of 100-110 µm is formed, which corresponds to the phase of the corresponding chromium carbides Cr23C6 and Cr7C3 doped with silicon, and a solid solution of chromium and silicon in α-iron. The corrosion studies carried out in this work show that chromosilicide coatings applied onto carbon steel 40 are less stable in solutions of 10% hydrochloric and sulphuric acids, and the most stable in solutions of nitric acid. [ABSTRACT FROM AUTHOR]

This paper examines the impact of heat treatments on a ferritic high-chromium cast iron's properties and resistance to erosion and corrosion. For that purpose, four samples were submitted to different temperatures (800, 900, 1000 and 1100 °C) for 2 h and then air-cooled. All samples were then studied in terms of hardness, microstructure, resistance to erosion and resistance to corrosion. Results showed that heat treatments affected those properties through three mechanisms: carbide coalescence, carbide dissolution and ferrite recrystallization. In fact, carbide coalescence slightly increased carbide thickness and volume fraction which in turn improved wear resistance. At around 1000 °C, carbide dissolution replaces coalescence inducing wear resistance loss, while the resistance to corrosion improves due to the added chromium content in the matrix. At higher temperatures, ferrite recrystallization occurs, resulting in the softening of the matrix grains which lowers wear resistance. Nevertheless, the overall hardness value increases due to the smaller grain size. [ABSTRACT FROM AUTHOR]

The treatment of inoculation of white cast iron with carbide precipitations that consist of increasing the number of primary austenite grains is not as well-known as the treatment of inoculation of gray cast iron in which the number of eutectic grains increases. In the studies included in the publication, experiments were carried out using the addition of ferrotitanium as an inoculant for chromium cast iron. The Cellular Automaton Finite Elements (CAFE) module of ProCAST software was used in order to analyze the formation of the primary structure of hypoeutectic chromium cast iron in a casting of various thicknesses. The modeling results were verified using Electron Back-Scattered Diffraction (EBSD) imaging. The obtained results confirmed obtaining a variable number of primary austenite grains in the cross-section of the tested casting, which significantly affects the strength properties of the obtained chrome cast iron casting. [ABSTRACT FROM AUTHOR]

Six alloys (25 wt% Cr, 0.4 C, 1.6 Ti, Co and Ni (bal.)) were prepared by casting for evaluating their oxidation and creep properties at 1200°C. Their microstructures consist of a dendritic matrix with eutectic carbides in the interdendritic spaces. Carbides are TiC for the alloys richer in Co than in Ni, and chromium carbides in the alloys richer in Ni than in Co. Differential thermal analysis shows that all alloys start melting above 1250°C. The oxidation tests in air at 1200°C allow observing a chromia–forming behavior of the alloys rich in nickel better than the cobalt alloys. In contrast, the creep resistance tests at 1200°C demonstrate an undeniable superiority of the alloys rich in cobalt. Good compromise for both types of high temperature behavior appears to be obtained by a Co-base alloy containing nickel as minor element but with significant proportion in nickel (15–30 wt% Ni). [ABSTRACT FROM AUTHOR]

The structure and tensile mechanical properties of sparingly alloyed steel 20Kh3 are studied after quenching and tempering at different temperatures. Comparative analysis of the load-extension curves in individual deformation stages and of the fracture zones after tempering at 250 – 650°C is performed. It is shown that the low ductility margin of steel 20Kh3 after high-temperature tempering is a result of precipitation of special carbides controlling the density and mobility of dislocations both inside grains (Cr7C3) and over grain boundaries (Cr7C3, Cr23C6). [ABSTRACT FROM AUTHOR]

This study proved the influence of an inoculation substance on the primary structure of chromium-cast iron. The inoculation procedure has developed very well in the field of grey cast iron production and mainly concerns the crystallisation of graphite eutectic grains in this material. However, in chromium cast iron, the inoculation problem is not well-recognised due to the formation of chromium carbides in white cast iron. One can easily increase the number of carbides in the cast iron's structure, but this procedure will not always bring the expected benefits in terms of increasing the overall mechanical properties. In the research included in this publication, an experiment was carried out with the use of ferrotitanium as an inoculant for chromium-cast iron. As a result of using the EBSD (electron backscatter diffraction analysis) imaging method, it was proven that the Fe–Ti interaction significantly influenced the primary structure of chromium cast iron that was formed by austenite grains. The paper presents the growth laws of primary austenite grains in modified hypoeutectic chromium cast iron depending on the degree of supercooling, ΔT, and the amount of the Fe–Ti inoculant M. The results of the research made it possible to predict the structure of hypoeutectic chromium cast iron after Fe–Ti inoculation treatment. The article proves that the use of the inoculant can change the primary structure of chromium cast iron, increasing its impact strength by more than three times. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive approach to optimising the structure and properties of chromium cast iron that is intended for use in the production of castings that operate under abrasive-wear conditions. In the study, chromium cast iron was inoculated to reduce the grain size in the solidification structure. The finer-grained structure of the casting has a positive effect on its mechanical properties. A number of inoculants have been used that allow the elimination of many types of casting defects: hot cracks and porosities that often occur during the production of chromium cast iron castings. Another advantage of the developed inoculation procedure is the resulting increase in the toughness of chromium cast iron. It should be emphasised that this cast iron does not have a high impact strength in its as-cast condition due to the formation of chromium carbides in the structure. This work also proposes a specially designed heat treatment for inoculated cast iron. The parameters of the applied heat treatment were determined on the basis of dilatometric tests. The visible deviation on a dilatogram at a temperature of about 600 °C is the result of a partial martensitic transformation in the area of grain boundaries. Therefore, the increase in abrasion resistance chromium cast iron is mainly due to the appearance of martensite. The microstructure of the investigated cast iron is particularly desirable in the case of alloys that work with lubrication. The microcavities that are formed by the abrasion of the softer phase constitute natural grease, which reduces abrasive wear. Under the influence of heat treatment, only a part of austenite located near the carbides is destabilized and transformed into martensite. Therefore, this phase of composition formation provided much greater resistance to abrasive wear and hardness. [ABSTRACT FROM AUTHOR]

The paper considers the development of a technological scheme for preparing metal matrix nanocomposites based on the interaction between nanodiamond reinforcing particles and a chromium matrix when being heated, forming chromium carbide nanoparticles. These carbides are in situ synthesized ceramic reinforcing nanoparticles. The first stage of preparing composites is to obtain composites with the chromium matrix and nanodiamond reinforcing particles. For this purpose, mechanical alloying is used, i.e., processing in planetary mills. The size of a primary nanodiamond particle is 5 nm, but they are combined in agglomerates that are hundreds of micrometers in size. The time of processing in the planetary mill defines the crushing degree of the agglomerates. In this study, processing was carried out for 0.5 h, 2 h, and 4 h. The second stage for obtaining composites with reinforcing particles of chromium carbides is thermal processing. Explorations using the method of differential scanning calorimetry showed that reducing the size of nanodiamond reinforcing particles (by prolonging the time of processing in the planetary mill) leads to a decrease in the initial temperature of the reaction for developing carbides. The worked-out technique for obtaining composites was patented in the Russian Federation (the patent for invention 2772480). [ABSTRACT FROM AUTHOR]

New methods of diffusion metallization of AISI W1-7 steel have been developed. The paper proposes a comparative analysis of the properties and characteristics of AISI W1-7 steel after three methods of chemical heat treatment: diffusion nitriding, nitrogen titration by physical deposition from the gas phase, and diffusion chromium plating with subsequent titanium alloys. The results are presented as a comprehensive analysis of coatings: metallographic, micro-X-ray spectral, X-ray phase, durometric, heat resistance, and wear resistance. It is established that multilayer protective coatings are formed as a result of treatments. It is shown that the coatings consist of carbide and nitride zones, intermetallic, and an Al2O3 layer outside the coating. The coatings have been found to contain barrier layers that prevent aluminum from penetrating the substrate. The maximum microhardness is typical for layers based on titanium carbide—30.3–35.5 GPa and titanium nitride—22.0–22.6 GPa, heat-resistant steels AISI W1-7 at a temperature of 900 °C in 4.2–8.5 times and wear resistance under sliding friction without lubrication up to 5.4 times compared to samples without treatment. [ABSTRACT FROM AUTHOR]

Bipolar electrochemistry produces a linear potential gradient between two feeder electrodes, providing access to the full spectrum of anodic‐to‐cathodic electrochemical behaviour. A type 420 martensitic stainless steel has been used to investigate microstructure evolution and corrosion behaviour with application of different tempering heat treatments. Tempering treatments at 250°C, 400°C and 700°C revealed the occurrence of pitting corrosion, with treatments at 550°C resulting in general and intergranular corrosion. Cr23C6 was present in all tempering conditions, with Cr7C3 and CrC only observed for tempering at 550°C. The 250°C tempering treatment had the highest corrosion resistance with a hardness value much higher than 500 HV. [ABSTRACT FROM AUTHOR]

The paper considers the development of a technological scheme for preparing metal matrix nanocomposites based on the interaction between nanodiamond reinforcing particles and a chromium matrix when being heated, forming chromium carbide nanoparticles. These carbides are in situ synthesized ceramic reinforcing nanoparticles. The first stage of preparing composites is to obtain composites with the chromium matrix and nanodiamond reinforcing particles. For this purpose, mechanical alloying is used, i.e., processing in planetary mills. The size of a primary nanodiamond particle is 5 nm, but they are combined in agglomerates that are hundreds of micrometers in size. The time of processing in the planetary mill defines the crushing degree of the agglomerates. In this study, processing was carried out for 0.5 h, 2 h, and 4 h. The second stage for obtaining composites with reinforcing particles of chromium carbides is thermal processing. Explorations using the method of differential scanning calorimetry showed that reducing the size of nanodiamond reinforcing particles (by prolonging the time of processing in the planetary mill) leads to a decrease in the initial temperature of the reaction for developing carbides. The worked-out technique for obtaining composites was patented in the Russian Federation (the patent for invention 2772480).

The use of chromium carbides in flux-cored wire for applying abrasion-resistant coatings is considered. It is found that such use of chromium carbides does not result in reinforcement of the resulting coatings by intact carbide particles. However, the replacement of ferrochrome by chromium carbides changes the structure of the wire and coatings, with increase in the hardness. [ABSTRACT FROM AUTHOR]

The use of chromium carbides to harden surfaced metal that is subject to abrasive wear is considered. With correct hardening of the applied layer, carbide formation extends the working life. [ABSTRACT FROM AUTHOR]

This paper studies the influence of variables such as holding temperatures and times during austempering of High Chromium White Cast Iron (HCWCI), with the following chemical composition: Cr 25 %, C 3 %, Si 0.47 %, Mn 0.74 % and Mo 1.02 %. The aim of the austempering was to modify the percentage of retained austenite and its correlation to abrasive wear resistance under different conditions.Microhardness tests, SEM-EDS and XRD were performed to determine mechanical properties, chemical composition, and type of carbides and microstructures present, respectively. The tests complied with the ASTM G-65 standard. Results showed that the best performance against abrasion was achieved for austempering at 450 ºC with holding time of 6 hours.

In order to save chromium resources, technology of flux-cored wire surfacing is of great practical interest. In this case, Cr2O3 chromium oxide and carbon as a reducing agent are used as fillers. The thermodynamic probability assessment of 16 reactions between them under standard conditions, as well as for certain reactions under different standard conditions, was carried out using tabulated thermodynamic data of reactants in the temperature range of 1500–3500 K. The following were considered as standard states for reactants: Cr(ref) (reference state, melting point 2130 K, boiling point 2952 K), Cr(liq), Cr(gas), Cr2O3(cr, liq), Cr2O3(gas), C(ref), and as possible reaction products and standard states for them CO(gas), CO2(gas), Cr23C6(сr), Cr7C3(cr), Cr3C2(cr). The reaction probability was estimated using standard Gibbs energy and was calculated using the Van Goff isotherm equation. The chromium dissolution in metal of surfacing bath or probable partial pressures of CO and CO2 in gas phase was considered and then calculated from the equilibrium of carbon gasification reaction. The carbon presence in flux-cored wire with chromium oxide Cr2O3 as a reducing agent will necessarily lead to the occurrence of reduction reactions with chromium carbide generation, and possibly chromium itself. The generation of Cr7C3(сr) carbide is likely. With a longer life span of chromium oxide and carbon at a temperature above 2500 K, chromium generation as a component of the surfacing bath is more thermodynamically probable than carbide generation. Chromium oxide has the highest reactivity in Cr2O3(liq) state. Direct reduction is preferential. The generation of CO(gas) as a carbon oxidation product is more probable. The chromium dissolution in metal increases the thermodynamic reaction probability with its generation, as well as further reduces the reaction probability in which chromium is the starting material. [ABSTRACT FROM AUTHOR]

Abstract The present work was undertaken to determine the effect of nickel on the microstructure in rapidly solidified ∼ Fe-25Cr-xNi-5Mo-0.8C alloys. The alloys were synthesised via arc melting and rapidly solidified using the suction casting technique. Phase composition was investigated using synchrotron x-ray diffraction and the solidification range was determined by means of differential scanning calorimetry. The microstructure in as-cast state was analysed using scanning electron microscopy. Properties were investigated by hardness measurements and scratch tests using the nanoindentation technique. The obtained results were discussed with respect to thermodynamic calculations, carried out using the CALPHAD approach. It was observed that with increasing nickel contents the average dendrite size, mean free path of the matrix and the spacing between the lamellar carbides, as well as the regularity and thickness of the carbides decreased. With decreasing mean free path of the matrix a decrease of average scratch depth occurred. This corresponds to abrasive wear, i.e. in alloys having smaller average dendrite size, abrasive particles will more frequently encounter eutectic areas with greater hardness than the matrix. As a result, the material exhibits greater abrasive wear resistance. The results presented in this work show that there is a possibility to enhance abrasive wear resistance of hypoeutectic iron-based alloys by modifying their chemical composition and, in turn, reducing the mean free path of the matrix. Graphical abstract Image 1 Highlights • Effect of nickel on microstructure in ∼Fe-25Cr-xNi-5Mo-0.8C alloys were studied. • Nickel reduces solidification range and mean free path of the matrix. • Alloys with reduced dendrite sizes exhibit enhanced abrasive wear resistance. [ABSTRACT FROM AUTHOR]

Bipolar electrochemistry has been applied to Type 420 ferritic stainless steel in order to determine the full spectrum of anodic-to-cathodic polarisation behaviour. The occurrence of crevice corrosion, pitting corrosion in combination with general corrosion, pitting corrosion only, general corrosion only, followed by a cathodic region has been observed. Instances of pitting corrosion initiated near chromium-rich carbides with Cr23C6, Cr3C2, and Cr7C3 identified as pit nucleation sites. The observed pit growth kinetics were independent of the electrochemical over-potential. Characterisation of the pit size distributions supports the presence of a critical dissolved volume for the transition of metastable to stable pit growth and pit coalescence.

High-Chromium White Cast Iron is a material highly used in mining and drilling shafts for oil extraction, due to its high wear resistance. However, because of the austenitic matrix found in the as-cast state, an adequate heat treatment cycle is necessary. This paper studies the effects of different cooling media after a destabilization treatment on the microstructure, hardening and abrasion resistance behaviors of a hypoeutectic high chromium white cast iron. The results show that although air cooling followed by immersion in CO2 can effectively reduce the retained austenite, this is not enough to transform completely the retained austenite into martensite. The low retained austenite percentages improve bulk hardness, but they decrease the abrasion resistance of the high chromium cast iron. The best combination of hardness and wear resistance was found in the samples cooled in air, due to the percentage of retained austenite and a moderate precipitation of chromium carbide.

The paper presents the results of the characterization of alloy from Fe-Cr-C (carbon content 0.79 wt.%) system including the microstructure, phase analysis, morphology and hardness in as cast state. The chemical composition was designed to create chromium-rich ferritic matrix with high volume fraction of carbides in form of interdendritic eutectics. The research was carried out on the cross section of the ingot, synthesized in an arc furnace under high purity argon atmosphere and crystallized on water-cooled copper mould. Microstructural characterization was carried out by light microscopy and scanning electron microscopy (SEM). Phase identification was performed by X-Ray diffraction (XRD). The microstructure of the investigated alloy is composed of primary and secondary dendrites Fe-Cr solid solution and complex M23C6 and M7C3 carbides in interdendritic areas. Segregation of Cr and C during crystallization causes precipitation of M7C3 carbides. The average hardness of the alloy is 205±12 HV10.

Nickel based alloys, especially nickel based superalloys have gained the advantage over other alloys in the field of high temperature applications, and thus become irreplaceable at high temperature creep and aggressive corrosion environments, such as jet engines and steam turbines. However, the wear resistance of these alloys is insufficient at high temperatures. This work describes a microstructure of a new cast alloy. The microstructure consists of γ matrix strengthened by γ’ fine precipitates (dendrites) improving the high temperature strength and of Chromium Cr7C3 primary carbides (in interdendritic eutectics) which are designed to improve wear resistance as well as the high temperature strength.

Abstract Synthetic diamond has excellent mechanical, thermal, and electrical properties, which makes it an ideal material in a wide range applications from abrasive grinding tools to modern electronic devices. Hence, understanding the wettability of metals on the synthetic diamond is of great importance for the development of diamond-related materials and devices. In this study, the wettability and spreading kinetics of binary Sn Cr alloy on chemical vapour deposed (CVD) polycrystalline diamond compacts were investigated using a sessile drop method. In situ observation of contact angle at elevating temperatures indicated trace addition of Cr dramatically improved the wettability of Sn on CVD diamond, and the Sn Cr alloy started to wet CVD diamond at approximately 750 °C. Isothermal spreading kinetic analysis revealed that the spreading of Sn Cr alloy on CVD diamond was controlled by the kinetics of chemical reaction at advancing triple line. Microstructure characterization indicated that the formation of nano-sized scallop-like Cr 7 C 3 grains was responsible for the improved wettability of Sn Cr alloy on CVD diamond substrate. The wetting temperature was found to play a determinant role in the interfacial carbide formation, and hence the reactive wetting of Sn Cr alloy on CVD diamond at temperatures from 700 to 900 °C. Graphical abstract Unlabelled Image Highlights • Small amount of Cr initiates the reactive wetting of Sn on CVD diamond at 750 °C. • Wettability and spreading kinetics of Sn Cr alloys are very sensitive to temperature. • Spreading of Sn Cr alloy is controlled by the chemical reaction kinetics during isothermal wetting. • Formation of nano-sized Cr 7 C 3 carbide improves the wettability of Sn Cr alloy on CVD diamond. [ABSTRACT FROM AUTHOR]

Abstract Traditional arc welding was applied to create Fe-based hardfacing deposits onto carbon steel substrates used on the edges of excavation equipment in a quarry plant, in order to enhance their service lifetime. Three electrodes of different chemical composition were used to deposit 5–6 mm-thick protective overlayers. Prior to laboratory characterisation of the surface-hardened samples, the Thermocalc© software was used to predict microstructure features that could be possibly present in the weld metal areas after solidification, taking into account a 10% dissolution of the base metal in the molten pool. The calculation results were compared to the experimental findings from optical and scanning electron microscopy observations, X-ray diffraction and microhardness measurements. The thermodynamic calculations provided an accurate prediction of the phases obtained, whilst microhardness values and their distribution were found to be strongly depended on the nature of the carbides, their interaction with the liquid metallic matrix and their dispersion within the final solid. Highlights • Composite, hypereutectic and hypoeutectic Fe-hardfacings were deposited via welding. • Experimental findings were in accordance to thermodynamic predictions. • Liquid metal attack causes dissolution, cracking, fragmentation of carbide feedstock. • Microhardness distribution of the deposits depends on the carbides dispersion density. [ABSTRACT FROM AUTHOR]

Corrosion of a T91 steel tube, used in subcritical conditions in an oil power plant for 157,000 h, was characterized mainly through SEM, TEM, EDX and DRX analyses. Severe oxidation and carburization took place in both the outer (boiler) and inner (steam) wall sides. The nature and morphology of the oxide scale multilayer structure (hematite, magnetite and spinel) depended on the environment exposure. Specific attention was given to the internal oxidation zone at the oxide/metal interface. Diffusion of chromium during the oxidation process was determined and was proposed to be responsible for the continuous advancement of oxidation to the core of material, which eventually gave rise to the spinel. Finally, in the bulk material, coarsening of the secondary carbides (M23C6) was the main form of microstructural evolution. [ABSTRACT FROM AUTHOR]

Tantalum carbide (TaC) is a classical strengthening precipitate phase, like chromium carbides, in equi-axed cast Cr-rich cobalt-based alloys. When the former carbides are of a eutectic nature and in quantity high enough, they may be of great interest for applications at elevated temperature, due to their favorable script-like morphology and their high stability at high temperature. Unfortunately, they cannot be so easily obtained in Cr-rich nickel-based alloys of similar composition. This can be resolved by adding cobalt in substitution to nickel, but such chemical modification may change the high temperature oxidation behavior in the wrong direction. In this work, several alloys, all containing 25% Cr, 0.4% C, and 6% Ta (wt%), were fabricated and studied at 1127 °C. It appears that the 27 wt% Co content is high enough to allow obtaining primary carbides, which are essentially TaC. The alloys rich enough in Co containing only tantalum carbides are clearly disadvantaged in terms of oxidation resistance at high temperature by the high Co content and by the absence of interdendritic chromium carbides known to act as useful Cr reservoirs. Among the six studied alloys, these are the three ones for which Co/Ni > 1 that demonstrated starts of catastrophic oxidation before 24 h of exposure to air. [ABSTRACT FROM AUTHOR]

A series of six alloys from Ni‐25Cr‐0.4C‐6Ta to Co‐25Cr‐0.4C‐6Ta with a progressive addition of Co in substitution to Ni were elaborated. They were tested for 24 h at 1237 °C in laboratory air. The corrosion products formed around the samples and the deterioration of the subsurface were analyzed to characterize the high temperature oxidation behavior of these alloys. Three of the studied alloys, the nickel‐richest ones, well behaved until 24 h at this temperature. Their external oxide scales were essentially composed of chromia. In contrast, locally, the three cobalt‐richest alloys were starting to oxidize catastrophically. The significant worsening of the oxidation behavior from the 27 wt% Co‐containing alloy to the 41 wt% Co‐containing one suggests that a critical Co content exists between these two values. [ABSTRACT FROM AUTHOR]

Six alloys, based on Ni and/or Co with Co/(Ni + Co) ratio varying from 0 to 1, and containing 25 wt%Cr, 0.4 wt%C and 1.6 wt%Ti, were synthesized by conventional foundry way. Their as-cast microstructures were controlled by electron microscopy. Parts of these alloys, properly prepared on surface, were then exposed to atmospheric air during 20 h at 1400 K. The obtained surface states and subsurfaces were characterized by optical observations and cross-sectional examinations by electron microscopy. The nickel-rich alloys contained more chromium carbides than titanium carbides, while this was the opposite for the cobalt-rich alloys. This confirms the results of a recent study. It was furthermore found here that the alloys with many chromium carbides and few TiC were much better in oxidation at high temperature than the ones with many TiC and few chromium carbides. By considering the apparent involvement of titanium in the oxidation process, it seems that the major influent parameter is directly the base element, independently on the nature of the obtained primary carbides. [ABSTRACT FROM AUTHOR]

Chromium carbides are coated over base metal (Fe) to increase wear and corrosion resistance. The electronic structure and bonding properties for chromium carbide bulk phases (Cr _3 C _2 , Cr _7 C _3 and Cr _23 C _6 ) and Fe-substituted chromium carbides is investigated using Density Functional Theory (DFT). The bonding in these carbides has been interpreted in the form of partial density of states, electron density distribution and Mulliken population method. Cr _3 C _2 exhibits the strongest covalent character while Cr _7 C _3 displays the highest metallicity. Cr _3 C _2 showed the highest stability among the chromium carbide phases. In the Fe-substituted chromium carbides (Cr _2 FeC _2 ), the site preference of Fe in Cr _3 C _2 system has been reported. In Fe-substituted chromium carbides also show both of metallic and covalent character and Cr _2 Fe3C _2 is found to be the most stable Fe-substituted chromium carbide system.