Carburizing Atmosphere Research Articles

Evaluation of the effects of the addition of Al, Ti, and Zr on microstructure, carburization and creep resistance of the HPNb alloy

The process of thermally cracking hydrocarbons and steam in pyrolysis furnaces to obtain ethylene is essential in the production chain of products relevant to our daily lives, such as fertilizers, polymers, pharmaceuticals, etc. The operating temperature of these furnaces is limited by the occurrence of carburization and/or creep in the metallic alloy used. The objective of this work was to investigate the effect of adding Aluminum (Al) at approximately 3 wt%, Titanium (Ti) at around 0.2 wt%, and Zirconium (Zr) at about 0.2 wt% on the carburization and creep resistance of the HPNb alloy. This material is a high-performance nickel-chromium-iron (Ni–Cr–Fe) system alloy enriched with niobium (Nb). Carburization in the samples was investigated by exposure to 1150 °C for 480 h in a carburizing atmosphere. Creep was evaluated through creep-rupture tests with a constant load, performed under a tension of 17 MPa and a temperature of 1150 °C, in four metallic alloys produced by static pouring with different chemical compositions. The data obtained were analyzed using optical microscopy, SEM, EDS, and EBSD. This study’s main findings indicate that while Al enhances carburization resistance, its impact on creep resistance varies depending on exposure to carburizing conditions. The addition of Ti or Zr did not significantly alter carburization resistance but contributed to improved creep resistance under specific conditions. This study provides insights into optimizing alloy compositions for enhanced performance in high-temperature industrial applications.

Analysis Of the Effect of Vegetal and Coking Coals on the Surface Hardness of Carburized AISI 8620 Steel

In this investigation, the comparative study of the superficial hardness of AISI 8620 steel thermally treated by carburizing, using vegetable and coking coals, was developed. The objective was to propose an alternative to the element used to generate the carburizing atmosphere. The previous stages have been described, such as the preparation of the material and its verification, as well as the dimensions and characteristics, defined by the ASTM E-18 standard. By considering four variables, each one with two levels, an orthogonal array matrix of 16 experiments was generated. The surface hardness measurements were processed through an analysis of variance (ANOVA), where the significance of each variable considered as a factor was obtained. Coking coal can be used as an element for the generation of carburizing atmosphere since a maximum relative error of 3.86 % was obtained between the experimental values and the values in the literature. In contrast, the vegetable coal did not improve the mechanical properties of the steel, reaching an average hardness of 25.95 HRC after heat treatments.

Evaluation of metal dusting resistance of thermal sprayed NiCrAlSiTaY and NiCrAlSiTaY/laser re-melted Al2O3 coatings on Inconel 625

The present study assessed the resistance against MD corrosion in a single-layer, oxide-forming NiCrAlSiTaY coating, and a double-layer NiCrAlSiTaY/laser-remelted (LRed) Al2O3 coating on an Inconel 625 (IN625) alloy. The NiCrAlSiTaY and Al2O3 coatings were applied onto the IN625 substrate using high velocity oxy-fuel spraying (HVOF) and air plasma spraying (APS) techniques, respectively. Coated and non-coated coupons were exposed, for 70 cycles, to a strong carburizing atmosphere of CO-50 % H2 in the temperature range of 550–750 °C to evaluate MD behavior. The results showed that the non-coated IN625 alloy contained a carbide free zone (CFZ), and secondary carbide precipitations of M23C6 and M6C and their subsequent in-situ oxidation were observed in the sub-surface layer. Such a development was indicative of the type III MD characteristic. The results also showed that the single-layer coating successfully protected the alloy, and only a part of the substrate underwent local oxidation. In addition, performing laser remelting (LR) on the Al2O3 coating significantly increased the resistance against the MD of the alloy through the removal of defects such as interconnected porosities, and via alumina phase transformation (γ → α). The lowest weight gain after 70 cycles of MD test was recorded for the double-layer coating coupons.

Study on size effect of γ-Fe2O3 nanoparticles and gas atmosphere on carburization process

Different sizes of γ-Fe2O3 nanoparticles (4–19 nm) were prepared by thermal decomposition of iron oleate and carburized in three different gas atmosphere of 5%CO/He, 5%CO/10%H2/He and 5%CO/20%H2/He at 350 °C. The carburization process and phase transformation of γ-Fe2O3 nanoparticles were investigated by in-situ XRD, Raman spectroscopy, CO-TPR and TEM. The results show that χ-Fe5C2 and θ-Fe3C phases with a stable ratio formed after carburization. The time to complete carburization was shortened for increasing sizes of γ-Fe2O3 particles under the same carburization atmosphere. While the smaller γ-Fe2O3 particles showed more residual carbon on the surface, which can inhibit the carburization process. The relative content of θ-Fe3C increased with the increase of the size of γ-Fe2O3 nanoparticles. For γ-Fe2O3 nanoparticles with the same sizes, the time to complete carburization in different atmospheres was firstly shortened and then slightly lengthened with the increase of H2 partial pressure, while the relative content of θ-Fe3C increased with the increase of H2 partial pressure. By adjusting the particle size of γ-Fe2O3 and the carburization atmosphere, the mixed phases of χ-Fe5C2 and θ-Fe3C can be optimized.

Highly stable activity of cobalt based catalysts with tungsten carbide-activated carbon support for dry reforming of methane: Role of tungsten carbide

Although tungsten carbide (WC) has similar properties, it is easily oxidized and deactivated during dry methane reforming (DRM). Therefore, it is of great significance to investigate the influence of carburizing atmosphere on the structure and properties of the prepared tungsten carbide material. Here, using ammonium metatungstate as the tungsten source and activated carbon (AC) as the matrix, under different carburizing atmospheres, a tungsten carbide-activated carbon (WC-AC) composite support was prepared through in-situ reduction and carbonization technology. After further loading cobalt (Co) and cerium (Ce), a series of Co-Ce/WC-AC catalysts were prepared. The catalytic performance for dry reforming was investigated. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (H2-TPR) and Thermogravimetric (TG) were used to characterize the crystal phase, microstructure and chemical composition of different samples. The results show that the main component of tungsten carbide prepared in a carburizing atmosphere containing CH4 was bitungsten carbide (W2C); tungsten carbide prepared in a pure hydrogen carburizing atmosphere was composed of W2C and WC. After loading Co and Ce, the catalytic activity and stability of catalysts containing WC are significantly improved compared with those mainly containing W2C. Due to the presence of WC, the interaction between the metal and the carrier was enhanced, thereby improving the dispersibility and oxidation resistance of the active component Co species. At the same time, WC was helpful to transform Co3+ to Co2+ and Ce4+ to Ce3+ on the surface and inner of catalyst, which was conducive to the enrichment of chemically adsorbed oxygen on the catalyst surface and the formation of oxygen vacancies. It promoted the adsorption and activation of CO2, and improved the anti-carbon deposition performance of the catalyst.

Investigation of the metal dusting attack on the temperature range 500−700 °C using X-ray tomography

A test rig consisting of a quartz tube placed in a horizontal furnace made of five independently regulated heating zones allowed the evaluation of the metal dusting attack of a FeNiCr alloy. Fifteen samples were exposed in a temperature gradient from 504° to 704°C, 1.1 bar, to a highly carburizing atmosphere for 1500 h. Below 561 °C the high density of pits led to uniform corrosion whereas localized pits developed at higher temperatures. In depth pit growth rate raised to a maximum at 632 °C, then decreased showing that it depends both on temperature and on the carbon activity gradient.

The Effect of Cu Content and Surface Finish on the Metal Dusting Resistance of Additively Manufactured NiCu Alloys

Due to the inhibiting behavior of Cu, NiCu alloys represent an interesting candidate in carburizing atmospheres. However, manufacturing by conventional casting is limited. It is important to know whether the corrosion behavior of conventionally and additively manufactured parts differ. Samples of binary NiCu alloys and Monel Alloy 400 were generated by laser powder bed fusion (LPBF) and exposed to a carburizing atmosphere (20 vol% CO–20% H2–1% H2O–8% CO2–51% Ar) at 620 °C and 18 bar for 960 h. Powders and printed samples were investigated using several analytic techniques such as EPMA, SEM, and roughness measurement. Grinding of the material after building (P1200 grit surface finish) generally reduced the metal dusting attack. Comparing the different compositions, a much lower attack was found in the case of the binary model alloys, whereas the technical Monel Alloy 400 showed a four orders of magnitude higher mass loss during exposure despite its Cu content of more than 30 wt%.

Phase instability of the topmost surface layer of Inconel-718 superalloy in low-temperature gas carburization atmosphere, revealed by means of (HR)(S)TEM

The surface microstructure modifications induced by the industrial treatments with the effect of enhancement of the surface engineering parameters in metallic alloys (hardness, wear, corrosion resistance) are utterly important. The recently invented, industrially successful surface hardening by carburization of 316L stainless steel (SS) in gas atmosphere at low-temperature (LT) [1] was extended with similar results to the Ni-Fe base Inconel-718 superalloy (IN-718) [3] and to several other alloys. The accepted model of surface microstructure modification due to processing is not based on full data about the modified topmost layer microstructure. It is missing the visualization of the new microstructure by means of (HR)(S)TEM and analysis. The effect of LT gas carburization is currently explained by a model of surface grains lattice dilatation due to the so called colossal carbon supersaturation (LTCSS) [2] that is not conform to the classical phase diagrams and is claimed to be valid in all alloys including IN-718 [4]. Our work reports new findings that reveal the up to now unknown nanostructure of the topmost carburized (at 570ºC) layer ~5 µm thick of IN-718 and cast doubts on the general validity of the LTCSS. The (HR)(S)TEM and nanoscale analysis were used to reveal the nanostructure of samples prepared in a special way by ion beam milling. We got no evidence of a classical crystalline microstructure of the carburized IN-718 upper-most layer that can match the requests of the LTCSS model. Instead, we put into evidence the nano-structure of the topmost ~ 5 µm thick layer of the carburized surface showing that it consists both (i) in nanocrystalline austenite and carbides and (ii) in an amorphous phase that contains the main alloy elements, with or without carbon (Fig-1). Segregated Ni nanoclusters (Fig-2) are occurring at large areas, apparently not including other main alloy elements. A fragmentation mechanism of the topmost surface layer (consisting initially of µm-sized grains) can be figured out by starting from the observed segregation in the initial alloy grains of the main alloy elements, agglomerated both inside grains close to grain boundaries (GBs) and at the GBs (Fig-3) and by the segregation of carbon both inside grains along GBs (Fig-4) and outside grains at GBs. Those chemical segregates can be considered as precursors in the process of fragmentation and are proving the phase instability of the surface alloy grains in the LT carburizing gas atmosphere, thereby revealing the chemical reactivity of the IN-718 surface grains with the LT carburization gas mixture. The surface reactivity can be valorized industrially through developing new research for finding novel compositions of LT gaseous mixtures able to modify the engineering properties of the alloys surfaces.

Damage characterization of belt conveyor made of the 330Nb alloy after service in a carburizing atmosphere in a continuous heat treatment furnace

In this paper, the failure of the belt conveyor made of 330 Nb heat-resistant steel, working in a furnace for continuous heat treatment in a carburizing and oxidizing atmosphere is investigated. On the basis of microscopic tests and X-ray phase analysis, changes in the microstructure in the cross-section of the conveyor wire after 731, 1642 and 2138 cycles carburizing and oxidizing in the temperature range 30–850 °C in 160 min were determined. Moreover, tensile tests were carried out to investigate the effect of the microstructure on the mechanical properties of heat-resistant steel. The failure was due to a cross crack of wires of the conveyor belt mesh. The results show that the on the surface of the alloy, Cr2O3 chromium oxides and FeCr2O4, Mn1.5Cr1.5O4 oxides are formed. On the border of the alloy and chromium oxides, the airtight SiO2 layer is created. As a result of the actions of external forces, transverse and longitudinal cracks are formed in the oxide layer. These cracks are the ways of carbon and oxygen diffusion. In addition, in the oxidation and carburization process, degradation of the SiO2 layer is promoted by the presence of Na, which is applied in washing agents used for cleaning products before placing them on the conveyor. Microstructure of the unused wire of the conveyor belt consisted of G and H phases and NbC in austenitic matrix. Under the operating conditions of the conveyor belt, phase H dissolves in the matrix with the simultaneous increase of the G phase, and also the G phase is formed around the NbC carbides. High temperature carburizing leads to the precipitation of massive carbides M23C6 with simultaneous significant depletion of chromium into an austenitic matrix to about 13%at. Quantitative microstructural analysis showed that the there was an increase in the size and participation, of hard phases M23C6, G, NbC in the alloy microstructure. The volume fraction of precipitation increased from 3 to 24%vol. The resulting structure is characterized by low strength. The ductility of the alloy has decreased significantly.

What causes the colossal C supersaturation of δ-ferrite in stainless steel during low-temperature carburization?

This study shows for the first time, with the help of thermodynamic calculations, that the colossal carbon supersaturation in delta-ferrite during low temperature carburization of stainless steels is not opposed to the concept of metastable carbon paraequilibration of delta-ferrite with the carburizing atmosphere. Rather, it is a consequence of triggering spinodal decomposition during the process of establishment of carbon paraequilibrium. This kind of paraequilibrium between spinodally decomposed ferrite and carburization atmosphere has not been recognized before. This renewed understanding explains the origin of colossal C supersaturation and also the strong variation in C solubility within a ferrite grain.

Corrosion of Fe-Ni-Cr alloys with various aluminum additions in a carburizing-oxidizing atmosphere at 900 °C

The corrosion behavior of Fe-19Ni-13/21Cr-xAl (x = 0, 2, 6 at.%) alloys in a carburizing-oxidizing atmosphere was compared with that in a purely carburizing atmosphere at 900 °C. The presence of oxygen did not improve the corrosion resistance of the two alloys Fe-19Ni-13Cr-xAl (x = 0, 2 at.%), while oxygen reduced the carbon attack significantly for the other alloys. The extent of the carbon attack decreased with the increase of the aluminum content, except for the addition of 2 at.% Al to Fe-19Ni-13Cr, which did not reduce the carbon attack. Addition of sufficient aluminum promoted the formation of external Cr2O3.

Reviews on Modeling of Diffusion-Induced Mass Transportation in Functionally Graded Cemented Carbides

Graded sintering is the fundamental process of fabricating functionally gradient cemented carbide (FGCC). The diffusion-induced mass transport in cemented carbide can result in the formation of gradient microstructure and thusly lead to gradual changes in micro property. So far, several types of FGCC have been developed, and the factors that can influence the gradient formation are complex. Section 2 introduces the process of forming diffusion-controlled near-surface layer in WC-Ti (C,N)-Co hardmetal as well as the kinetic modeling work that reveals the key factors for the layer formation. Section 3 reviews the dual properties carbide produced under carburization atmosphere, for which the carbon content is a main factor of the gradient thickness. There are two models describing this process, representing different mass-transport mechanism of the so-called liquid phase migration (LPM) process. In section 4, previous and new results of modeling LPM in different dimensions and scales are presented, and the diffusion-controlled nature of LPM are discussed.

Microstructure of Al‐Si Slurry Coatings on Austenitic High‐Temperature Creep Resisting Cast Steel

This paper presents the results of microstructural examinations on slurry aluminide coatings using scanning electron microscopy, X‐ray microanalysis, and X‐ray diffraction. Aluminide coatings were produced in air atmosphere on austenitic high‐temperature creep resisting cast steel. The function of aluminide coatings is the protection of the equipment components against the high‐temperature corrosion in a carburising atmosphere under thermal shock conditions. The obtained coatings had a multilayered structure composed of intermetallic compounds. The composition of newly developed slurry was powders of aluminium and silicon; NaCl, KCl, and NaF halide salts; and a water solution of a soluble glass as an inorganic binder. The application of the inorganic binder in the slurry allowed to produce the coatings in one single step without additional annealing at an intermediate temperature as it is when applied organic binder. The coatings were formed on both: the ground surface and on the raw cast surface. The main technological parameters were temperature (732–1068°C) and time of annealing (3.3–11.7 h) and the Al/Si ratio (4–14) in the slurry. The rotatable design was used to evaluate the effect of the production parameters on the coatings thickness. The correlation between the technological parameters and the coating structure was determined.

Controlling cobalt capping in sintering process of cermets

Critical sintering parameters for the formation of cobalt capping on cermets were evaluated experimentally. The results were compared to existing theories regarding cobalt capping formation on cemented carbides. Four different cermets grades were selected. The influence of the carbon activity, the sintering atmosphere (vacuum, flow rate, gas mixture, pressure) and the cooling rate was investigated. The results are in accordance with the observations that the Co surface top-layer forms during a specific short temperature interval during the cooling step for each grade, but it is affected by the preceding conditions; e.g. enhanced by a preceding Co-enrichment in the surface zone in a decarburizing atmosphere. The formation can be inhibited by using a carburizing atmosphere, such as carbon monoxide gas, or a sinter-HIP (hot isostatic pressure) step during cooling. A phenomenological model for formation of Co-capping in cermets considering melting point of alloys, cobalt migration, carbon activities, vapor pressure of reactive elements as well as processing parameters (sintering atmosphere, cooling rate etc.) is discussed.

Evolution of PVD Al oxide coatings in carburizing atmospheres at high temperature

Abstract Oxide thin films have been investigated as protection against corrosion of Fe-based materials in highly carburizing atmospheres at high temperature. The present study describes the structural evolution of an Al oxide/Cr thin film coating deposited on 304 L steel after 50 h of exposure to a CH4 + H2 + residual O2 atmosphere at 1073 K. Comparisons are then made to the structural evolution observed after 20 h of exposure from a previous study [1]. The coatings were produced by reactive magnetron sputtering at low temperature. The resulting thin films had very fine columnar structure, good adhesion to the substrate, amorphous Al oxide and tensile residual stresses. The response of the films to the carburizing atmosphere was evaluated via thermogravimetric analysis (TGA). The exposed samples were analyzed by X-ray diffraction, scanning electron microscopy equipped with energy dispersive analysis, and optical microscopy. Both, the TGA and structural analyses indicate that the presence of the films prevents the massive formation of internal and external corrosion products observed in uncoated 304 L samples exposed in the same conditions. However, the films underwent some structural modifications, which resulted in abundant interdiffusion through the film changing its as-deposited characteristics.

Activation of Mo/HZSM-5 for methane aromatization

The effect of Mo/HZSM-5 pretreatment at 973 K in inert (He), oxidizing (artificial air), and carburizing (CH4/He mixture) atmospheres on its performance in non-oxidative methane dehydroaromatization (MDA) was investigated. The effect of post-synthesis silylation on deactivation of external acid sites was also studied. Precarburization resulted in increased aromatic selectivity and improved catalyst stability. The benzene selectivity was the highest for the silylated Mo/HZSM-5 catalyst (benzene + naphthalene selectivity after 1 h on stream was close to 100%). The deactivation of precarburized zeolites was less pronounced than that of zeolites heated in air or He. During heating in air or He, larger fractions of the molybdenum oxide species diffused into the micropores than during heating in methane. Carburization of the molybdenum oxide species in the micropores during MDA resulted in the formation of molybdenum carbide particles, and these contributed to pore blocking, making the Brønsted acid sites inaccessible. The formation of molybdenum carbides during heating in methane resulted in a less mobile Mo phase. It is argued that the presence of molybdenum carbide particles in the micropores contributes to rapid catalyst deactivation, in addition to the formation of hard coke on the external surface.

Optimization of processing conditions in plasma activated nitrogen–hydrocarbon carburizing

Problems with intergranular oxidation, energy efficiency and carbon footprint of the conventional endothermic atmosphere (CO–H2–N2) carburizing are forcing heat treating and manufacturing companies to move toward increasingly capital- and operating-cost expensive, low-pressure/vacuum carburizing methods. To offer a less-expensive alternative, a new activated carburizing method has recently been developed, bridging the endothermic atmosphere and vacuum processes, where a plasma-activated, oxygen-free, non-equilibrium nitrogen–hydrocarbons gas blend is utilized. The optimization of activated carburizing for industrial application presented here involved improvement of multi-dimensional case uniformity. As a part of this optimization, a computational fluid dynamics study was conducted for examination of gas flow field inside heat treating furnace and trays holding steel parts treated. To maximize carburizing rate and mitigate soot in non-equilibrium nitrogen–hydrocarbon atmospheres, effects of different gas mixture combinations were investigated. A mixture of N2–0.4% C3H8–1% CH4 mixture has been found to provide a well carburized case with minimized soot in the atmosphere and reduced coke deposition on the metal surface. The soot formation mechanisms for non-equilibrium carburizing atmospheres were also discussed.

Effect of Carburizing Atmosphere Proportion on Low Temperature Plasma Carburizing of Austenitic Stainless Steel

In this study, in order to explore a suitable method to obtain a better wear resistance and corrosion resistance of austenitic stainless steel, low temperature plasma carburizing technology has been studied. Research on the properties of austenitic stainless steel under different carburizing atmosphere proportion, with hardness, wear resistance and corrosion resistance as the properties characterization. The results shows that C3H8:H2=1:40 have better properties with the hardness of 950 HV0.05, the friction coefficient of about 0.25, which showed a better wear resistance. And also the corrosion rate of about 20.3g/m2·h showed a better corrosion resistance.

Investigation of chemical characteristics of primary helium gas coolant of HTTR (high temperature engineering test reactor)

The technical basis of helium gas purification control for HTGRs was established by verifying the design of the Primary Helium Purification System (PHPS) of the HTTR by showing that the measured concentrations of impurities of the primary helium coolant were restricted below the criteria of control to protect the graphite oxidation, and that the carburization atmosphere was maintained to keep intact of metallic high temperature components, in the 30-day continuous operation and the 50-day long-term high temperature operation. The analytical model, which was newly established by improving the conventional method that predicted the impurity concentrations conservatively higher than the measured values, predicted the composition of the impurities such as H2, CO, H2O and CO2, which is determined by the temperature dependency of release of impurities during the rated power operation adequately. In contrast, it was revealed that the measured concentration of H2O remarkably decreased while the concentration of CO increased in the primary helium coolant in the long-term high temperature operation.

Long term out-of-pile thermocouple tests in conditions representative for nuclear gas-cooled high temperature reactors

During irradiation tests at high temperature failure of commercial Inconel 600 sheathed thermocouples is commonly encountered. As instrumentation, in particular thermocouples are considered safety-relevant both for irradiation tests and for commercial reactors, JRC and THERMOCOAX joined forces to solve this issue by performing out-of-pile tests with thermocouples mimicking the environment encountered by high temperature reactor (HTR) in-core instrumentation. The objective was to screen innovative sheathed thermocouples which would consecutively be tested under irradiation. Two such screening tests have been performed in high temperature environment (i.e. temperature in the range 1100–1150°C) with purposely contaminated helium atmosphere (mainly CH4, CO, CO2, O2 impurities) representative for high temperature reactor carburizing atmospheres. The first set of thermocouples embedded in graphite (mainly conventional N type thermocouples and thermocouples with innovative sheaths) was tested in a dedicated furnace at THERMOCOAX lab with helium flushing. The second out-of-pile test at JRC with a partly different set of thermocouples replicated the original test for comparison.Performance indicators such as thermal drift, insulation resistance and loop resistance were monitored. Through these long-term screening tests the effect of several parameters were investigated: niobium sleeves, bending, diameter, sheath composition as well as the chemical environment. SEM examinations were performed to analyze local damage (bending zone, sheath).The present paper describes the two tests, sums up data collected during these tests in terms of thermocouple behavior and describes further instrumentation testing work with fixed point mini cells for qualification under irradiation.