Cr23C6 Carbides Research Articles

Multimodal Discontinuous Rection in Ni-Fe-Cr-Al Alloy

Ni-Fe-Cr-Al alloy has been subjected to a short aging treatment, which increased the mechanical properties and resulted in discontinuous precipitation at the grain boundaries. In this study, analytical electron microscopy was applied to explore the relationship between the occurring grain boundary phenomena and mechanical behavior. Within the discontinuous precipitation zone, three different phases were recognized, namely γ solid solution with lamellar Cr23 C6 carbides and elongated γ’ precipitates at the reaction front. Coherency of the discontinuous precipitates was identified as they have shown a cube-on-cube orientation relationship. Furthermore, the development of multiple discontinuous reactions was discussed.

Deposition of Stellite 6 alloy on steel substrates using wire and arc additive manufacturing

Stellite 6 is a cobalt-based superalloy which has a good wear and corrosion resistance and retains these properties at high temperatures. In this study, wire and arc additive manufacturing (WAAM based on the GMAW) was employed to deposit Stellite 6 wire on low alloy high strength steel (S355) and stainless steel (AISI 420) plates. One of the main interests of this study is to produce WAAM Stellite 6 deposits with quality comparable with laser deposition. The advantages of the WAAM process include the high deposition rate, high productivity, high material usage, and energy efficiency with low cost. However, superalloy deposition generally requires maintaining a low dilution level to avoid jeopardizing the integrity of the deposit. As a result, it is important to manage the excess heat input during the WAAM deposition process through a parametric optimization of WAAM deposition of Stellite 6 on S355 steel substrates. In this study, a WAAM process window is established to guide the process optimization. The optimization method used in this study has been applied in our previous laser cladding work. The generated process window also shows some correlations among the heat input, bead geometry, and dilution. The effects of heat input on the resulting microstructure, elemental distribution, and hardness were discussed. Dilution, microstructure, and hardness were considered for comparison from previous studies of laser cladding deposits. In addition, the obtained optimal conditions were adapted to apply WAAM deposition of Stellite 6 layers on AISI 420 stainless steel substrates. The XRD result shows that WAAM deposits contain Co-Cr-Fe solid solution (FCC), Cr7C3 and Cr3C2 carbides, and Co4W2C intermetallic compound. The results obtained through this study lay a foundation for future research on the wear properties of WAAM Stellite 6 deposits. This can contribute to further development of automated deposition of Stellite 6 using WAAM process for industrial applications.

Decreased Ti/Al ratio modifies the precipitate behavior of Cr23C6 and improves long-term stress-rupture life

The long-term stress-rupture life of Nimonic 80A with varied Ti/Al ratios was investigated at 650 °C/300 MPa. The stress-rupture life increases from about 6479 to 7734 and 11713 h, and the duplex grain size distribution evolves from bimodal morphology to near-homogeneous morphology with the decreased Ti/Al ratio. Although the stress-rupture life increases significantly, the stable γ′ homogeneously distributes in γ matrix with a coherent orientation relationship with γ on {100}γ′/γ and {110}γ′/γ atomic planes. With the decreased Ti/Al ratio, more precipitation of TiC particles leads to the depletion of C element near grain boundary, which suppresses precipitation of Cr23C6. A large amount of plate-like Cr23C6 carbide which precipitates at grain boundaries can easily lead to the intergranular cracking, but the blocky Cr23C6 carbide which shows an orientation relationship on {111}γ atomic planes with the nearby matrix at the twin boundary does not cause grain boundary cracking. Lower Ti/Al ratio benefits longer stress-rupture life based on the near-homogeneous duplex grain size distribution and none precipitation of plate-like Cr23C6.

Tailoring M7C3 carbide via electron work function-guided modification

M7C3 carbide [M: metallic element] is the primary reinforcement in white cast irons. Further improving its properties for maximized benefits is highly desired. However, no clear clues are available to guide the modification. Through atomic force microscopy in-situ analysis and first-principles calculations, we demonstrate that (Fe,Cr)7C3 carbide's strength is governed by its metallic bond component. We theoretically prove that M7C3 carbide is tailorable by partially replacing its metallic elements with substitutes selected based on their work function as an indicator. This study provides an insight into the electronic origin of carbide's mechanical behaviour, helping guide developing high-performance M7C3 carbides.

Effect of the Laves phase and carbide on the creep strength of Fe–C–15Cr–W alloys

To clarify the effects of the Fe2W Laves phase and Cr23C6 carbide on the creep strength of a new ferritic heat-resistant steel with the ferrite parent phase strengthened by intermetallic compounds and carbonitrides, the creep strength at 973 K and microstructure of the ternary Fe–15Cr-(6, 9)W alloys and the quaternary Fe-0.05C–15Cr-(6, 9)W alloys (mass%) were investigated. Increasing the amount of Laves phase precipitates by increasing the tungsten content was effective in improving the creep strength due to the film-like Laves phase precipitates at the grain boundaries (GBs) of ferrite phase. Laves phase precipitates within the grains of ferrite phase became more uniform and finely distributed with the addition of carbon, but that did not improve the creep strength. Voids were formed around the Cr23C6 carbide precipitates at the GBs, and precipitate-free zones were observed around the GBs. We have concluded that these microstructures of the GBs caused creep rupture. In order to improve the creep strength of new precipitation-strengthened ferritic steel, reducing of the carbon content to suppress GB precipitation of Cr23C6 carbide, and increasing of the tungsten content to enhance the precipitation of Laves phase at the GBs are necessary.

Preparation and Microstructural Characterization of a High-Cr White Cast Iron Reinforced with WC Particles.

High-chromium white cast iron (WCI) specimens locally reinforced with WC–metal matrix composites were produced via an ex situ technique: powder mixtures of WC and Fe cold-pressed in a pre-form were inserted in the mold cavity before pouring the base metal. The microstructure of the resulting reinforcement is a matrix of martensite (α’) and austenite (γ) with WC particles evenly distributed and (Fe,W,Cr)6C carbides that are formed from the reaction between the molten metal and the inserted pre-form. The (Fe,W,Cr)6C precipitation leads to the hypoeutectic solidification of the matrix and the final microstructure consists of martensite, formed from primary austenite during cooling and eutectic constituent with (Fe,Cr)7C3 and (Fe,W,Cr)6C carbides. The presence of a reaction zone with 200 µm of thickness, between the base metal and the composite should guarantee a strong bonding between these two zones.

Corrosion state of oil cracking reactors as the main factor of fire and explosion safety during their operation

The corrosion and mechanical properties of the oil catalytic cracking reactor vessel made of stainless steel Y1-T (1X18N9T) after its operation for more than 60 years have been determined. The chemical composition of the reactor metal meets the requirements of GOST and involves the isolation of non-metallic inclusions of manganese sulfide in its structure, which cause the development of pitting, and chromium carbides, which are responsible for the propensity to intergranular corrosion. After long-term operation, the mechanical characteristics of the metal (tensile strength σb, yield strength σt, elongation δ, relative contraction Ψ), determined using cylindrical samples at a temperature of 20 °C (GOST 1497-84) and at an operating temperature of 465 °C (GOST 9651), on the breaking machine TC 110M-50, correspond to the standard values. Metallographic studies carried out using an optical microscope “Axio Observer Z1m « at magnifications of X25, X75, x125, X200, X250, X400, X500, x1000, X2000 showed that long – term operation of the metal at a temperature of 375…425 °C with periodic overheating up to 550…650 °C leads to the release of excess phases, namely, chromium carbides and titanium carbonitrides. The metal grain boundaries are enriched with Cr23C6 carbides, which cause a loss of resistance against intergranular corrosion. Corrosion pitting and weakened metal grain boundaries are promoters of the occurrence of corrosion cracks. The most dangerous areas of large-size equipment are welded joints and zones of thermal influence. A mandatory condition for fire and explosionproof operation of the refinery is periodic diagnostics of the corrosion state of the equipment in order to identify and timely eliminate potentially dangerous areas, the probability of which breaching with subsequent spillage and fire of oil and petroleum products is the highest.

Analysis of the Drawing Process of Small-Sized Seam Tubes

This article is focused on an analysis of factors negatively affecting the tube production process of tubes made from austenitic stainless steel with a very small diameter of ϕ 0.34 mm. The analysis was concentrated on factors that affect the drawing process stability of the seam tubes where the desired final dimensions—a diameter of ϕ 0.34 mm and a wall thickness of 0.057 mm—are limiting factors. Seam tubes made from steel 1.4306 and 1.4301, from producers KT and EW with a longitudinal weld line made by tungsten inert gas (TIG) welding, were used as blanks for constituent drawing operations. It is desirable to provide sufficient inert gas flow and cooling during the formation of a weld joint in a protective atmosphere chamber. A significant temperature gradient prevents the formation of undesirable Cr23C6 carbides in the heat-affected zone (HAZ) which negatively affects the plasticity and formability of the steel and is the cause of technological fractures.

Effect of Nb additive on wear resistance and tensile properties of the hypereutectic Fe-Cr-C hardfacing alloy

The effects of Nb additive on wear resistance and tensile properties of the hypereutectic Fe-Cr-C hardfacing (harden-surface-welding) alloys were investigated. The microstructures and phase structures of the alloys with and without Nb additive were compared by means of optical microscopy (OM) and X-ray diffractometer (XRD). The wear behaviors were measured by reciprocating wear tests via CSM Tribometer. The tensile properties were measured by universal testing machine. In order to investigate the (Fe,Cr)7C3 carbides refinement by Nb additive, the formation thermodynamics of NbC and the misfit between NbC and (Fe,Cr)7C3 were calculated. The results show that the alloy with Nb additive contains NbC as well as (Fe,Cr)7C3 and γ-Fe. The size of the primary (Fe,Cr)7C3 carbides in the alloy with Nb additive is obviously smaller than that without Nb additive. The mass loss of the alloy with Nb additive is less than that without Nb additive. The worn surfaces also indicate that the amount of the cracks on the alloy surface with Nb additive is less than that without Nb additive. The tensile strength parallel to the rod-like primary (Fe,Cr)7C3 carbides is larger than that vertical to the primary (Fe,Cr)7C3 carbides. The tensile strength parallel to the primary (Fe,Cr)7C3 carbides of the alloy with Nb additive is larger than that without Nb additive, while the tensile strength vertical to the primary (Fe,Cr)7C3 carbides of the alloy is lower than that without Nb additive. The phase fraction vs. temperature curves of the alloy shows that the alloy with Nb additive preferentially forms NbC and then primary (Fe,Cr)7C3 carbides. The lattice misfit between NbC and (Fe,Cr)7C3 is 2.16%, which proves NbC can be as the heterogeneous nucleus to refine primary (Fe,Cr)7C3 carbides in the Fe-Cr-C hardfacing (harden-surface-welding) alloy.

Microstructural evolution of laser-clad 75Cr3C2+25(80Ni20Cr) powder on Inconel 718 superalloy

Abstract Nickel-based superalloys such as Inconel 718 are widely used in the aerospace industry. The surface properties of Inconel 718 can be improved by the addition of chromium carbide–nickel chromium powder. In this study, 75Cr3C2+25(80Ni20Cr) powder was clad on annealed Inconel 718 superalloy by using a pulsed Nd:YAG laser. The aim of this research is to investigate the effect of laser process parameters such as pulse width, laser frequency and scanning speed on the microstructure and microhardness of the clad samples. It is found that the increasing pulse width and laser frequency or decreasing scanning speed can increase the heat input and the resultant dilution ratio. The clad zone of the samples with a high dilution ratio consists of eutectic structure (γ and Cr7C3). However, the clad zone of specimens with a low dilution ratio is composed of a hypereutectic structure of Cr7C3+(γ+Cr7C3). This microstructure leads to the significant increase in the hardness value in comparison with the specimens with a high dilution ratio, due to the presence of proeutectic Cr7C3 carbides in the hypereutectic structure. The maximum hardness value of the clad zone is found to increase almost 2.5 times that for the substrate in the annealed condition. However, the austenitic and Laves phases are formed in the partially mixed zone. This zone is rather narrow, approximately 30 μm. The grain coarsening occurs in the heat-affected zone of all the deposited specimens. The hardness values of the partially mixed and heat-affected zones are lower than that for the clad zone.

Cr7C3: A potential antioxidant for low carbon MgO–C refractories

Magnesia carbon (MgO–C) refractory, one of the most commonly used refractories in the steelmaking system, relies on graphite to improve the thermal shock resistance and slag corrosion resistance. The oxidation of graphite carbon in a MgO–C brick usually leads to the destruction of the carbon network in the brick, which causes the structure of the brick to become loose and easily eroded. At present, metal powders, carbides, and borides are used as antioxidants to prevent the oxidation of carbon in MgO–C bricks. The metal carbide Cr7C3 can be prepared from aluminum chromium slag through a simple synthetic process and at a low cost. In this work, we investigated the oxidation resistance of low carbon MgO–C refractories with different amounts of Cr7C3 powder (1, 2, 3, and 4 wt%). The refractories with 3 wt% Cr7C3 powder showed optimal resistance to oxidation. The microstructure indicated that oxygen reacts with Cr7C3 preferentially over carbon to form chromium oxide and magnesium chromium spinel, blocking the pores and hindering oxygen diffusion. Carbon arising from the reduction of carbon monoxide by Cr7C3 can act as a supplementary carbon source. The better oxidation resistance also contributed to the improvements in slag corrosion and thermal shock resistance of the refractories.

Effect of Temper on Deformation and Mechanical Behaviors of Laser Melt-Deposited 12CrNi2 Low-Alloy Steel

The effect of tempering (673 K for 30 min) on the microstructure and tensile properties of laser melt-deposited (LMD) 12CrNi2 low-alloy steel is studied. The results show that LMD 12CrNi2 mainly consists of the ferrite and some Cr23C6 carbides. Characterization of the Schmid factor near the fracture surface shows that after tempering, the slip system with the largest Schmid factor is still {123} , but the Schmid factor of {100} and {112} slip systems increases. Tempering increases the elongation from 6.97 to 11.49% and the tensile strength from 849 to 951 MPa. Tempering increases the average distribution of misorientation between crystals after tensile test, which improves the average dislocation density and promotes strengthening.

The Effect of Precipitation Hardening on the Properties Hadfield Steel

The precipitation hardening in chromium containing Hadfield steel is considered as the most traditional mechanism for promoting the technological and mechanical properties of such high alloy steel. Two methodologies of precipitation hardening were suggested to attain the strengthening mechanical and technical properties of this steel. Different precipitates of chromium carbides were stimulated through either pre and post solution hardening treatment. Thereby, the results of the two mechanisms were compared in terms of precipitation morphology, compositions, and mechanical properties. It was found that the deteriorative of toughness was significantly observed at the post solution treatment. Fine chromium carbides Cr7C3, and Cr23C6 were observed throughout the austenite matrix at 650oC, and 700oC as the temperature of the pre-solution treatment technique. It was found that the morphology of chromium carbides has the most significant effect on promoting the strain-hardening property of Hadfield steel studied. When conducting the age-hardening regime followed the solution treatment observed the best toughness of Hadfield steel.

Relationship between carbon segregation and the carbide precipitation along grain boundary based on the structural unit model

Intragranular embrittlement is key to the weakening of properties of ultra-supercritical applied materials as HR3C heat-resistant steel in service and is considered to be related to the structure of grain boundary and the precipitation of carbides. In this work, the structural unit model was applied to study the structure of grain boundary and carbide formation with molecular dynamics simulation, and then, the precipitation tendency of carbides was analyzed through the structural similarity between grain boundary and carbide by using radial distribution function of the metal atom. At the same time, an electron backscatter diffraction study on serviced and treated HR3C heat-resistant steel was used for experimental exploration. Results show that the structural similarity between grain boundary and carbide depends on the grain boundary angle, wherein the structural similarity between grain boundary and carbide is higher for the grain boundary angle of 20–50°, while it is lower at the grain boundary angle close to 60° or smaller than 20°. The structure of Cr23C6 and Cr7C3 carbides is more similar to the local structure of grain boundary than that of Cr3C2 carbide. The grain boundary angle study on HR3C heat-resistant steel shows that the high content of 20–50° grain boundary indeed increases the precipitation of carbides, which is proposed to reduce the proportion of 20–50° grain boundaries by a proper procedure of cold deformation + heat treatment as 20% + 900 °C/6 h.

Effect of the Rare Earth Oxide CeO2 on the Microstructure and Properties of the Nano-WC-Reinforced Ni-Based Composite Coating

In this study, the addition of the rare earth oxide CeO2 was investigated to alter the microstructural properties of the nano-WC-reinforced Ni-based composite coatings. The reinforced composite was prepared on the 42CrMo steel surface using a semiconductor laser. The morphology and microstructure of coatings were analyzed using a scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Further, the digital microhardness tester and high-temperature friction and wear tester were used to observe the mechanical properties. The results indicated that the addition of CeO2 eliminated the cracks from the surface of the coatings and effectively reduced the number of pores. The phases were mainly observed as γ-Ni(Fe) in a solid solution, and some residual WC and W2C phases were observed. In addition, Fe3C, Cr23C6, M6C (M = W, Fe, and Ni), SiC and Cr7C3 composite carbides, Si2W and NiW tungsten compounds, and CeFe2- and CeNi2-containing Ce complex compounds were formed on the coating. The rare earth oxide CeO2 composite-modified coating mainly comprised dendrites, crystal cells, strips, and massive microstructures. The reinforced phases of the modified coating presented uniform dispersion distribution with the addition of 1% CeO2, and the structures were significantly refined. The maximum microhardness of the modified coating was approximately 1560 HV0.2, which was approximately 20% higher than that of the unmodified composite coating. The minimum wear loss of the modified coating was 6.1 mg and the minimum frictional coefficient was approximately 0.23, which were better than those of the unmodified coating. The wear mechanism of the nano-WC-reinforced Ni-based coating was primarily adhesive, whereas that of the CeO2 composite modified coating was mainly abrasive particle wear, which accompanied adhesive wear.

In situ TEM observation of the microstructural characteristics and mechanical properties of vacuum hot-press sintered Co–30Cr–6Mo alloys

In this research, Co–30Cr–6Mo alloys were fabricated by vacuum hot-press sintering technique. The experiments utilized various hot-press sintering temperatures (1100 °C, 1150 °C, 1200 °C and 1250 °C) and hot-press pressures (20, 35 and 50 MPa) to find the optimal parameters for Co–30Cr–6Mo alloys, while simultaneously investigating the differences in microstructural characteristics and properties. The experimental results showed that the optimal hot-press sintering parameters of this sample were 1150 °C at 35 MPa for 1 h. Meanwhile, Co–30Cr–6Mo alloys possessed the optimal mechanical and electrical properties. Among them, the sintering density reached 8.13 ± 0.01 g cm−3, the hardness and TRS increased to 78.9 ± 0.2 HRA and 2672.2 ± 25.8 MPa, respectively, and the electrical conductivity was 3.11 ± 0.95 × 104 S m−1. TEM observation also identified the stacking faults, FCC and HCP structures of the Co phase, and the partial Cr23C6 carbides precipitated in the Co–30Cr–6Mo alloys, respectively. Consequently, the sintering characteristic of Co–30Cr–6Mo was effectively improved, and the mechanical properties were enhanced at a relatively lower sintering temperature by the vacuum hot-press sintering process.

High-Temperature Synthesis of Cast Materials Based on the MAX Phase Cr2AlC Using CaCrO4 + Al + C Mixtures

Experimental data are presented on the high-temperature synthesis of cast composite materials in the Cr–Al–C system with different relative amounts of the MAX phase Cr2AlC and chromium carbides and aluminides. The experiments were carried out in multipurpose self-propagating high-temperature synthesis (SHS) reactors at an argon pressure p = 5 MPa. The starting mixtures consisted of calcium chromate (CaCrO4), aluminum (ASD-1), and carbon powders. It has been shown that varying the percentage of carbon in the starting mixture may have a significant effect on the synthesis process and the phase composition and microstructure of the final products. It has been found that, in the case of the stoichiometric starting mixture composition, the synthesis yields cast composite materials consisting predominantly of the MAX phase Cr2AlC and containing the lower chromium carbide Cr7C3 and the chromium aluminide Cr5Al8. The addition of excess (superstoichiometric) carbon to the starting mixture leads to an increase in the percentage of the MAX phase Cr2AlC in the synthesis product, disappearance of the chromium aluminide Cr5Al8, and the formation of the higher chromium carbide Cr3C2 instead of the lower carbide. The final synthesis products have been characterized by X-ray diffraction and local microstructural analysis. The structure and composition of the synthesis products obtained under various conditions have been determined.

Microstructures and mechanical properties of FeCoCrNi-Mo High entropy alloys prepared by spark plasma sintering and vacuum hot-pressed sintering

Fully dense FeCoCrNi-Mo high entropy alloy (HEA) was successfully prepared by two different sintering methods: spark plasma sintering (SPS) and vacuum hot-pressed sintering (VHP). The microstructures and mechanical properties of HEA prepared by both methods were systematically investigated and compared. The HEA powders were uniformly distributed at approximately 10 μm after 20 h of ball milling. The SPS sample maintained a single FCC structure and Multiple microstructures of σ phase, Mo2C and Cr23C6 carbides precipitated in the VHP samples. As a result of the precipitation of these hard and brittle phases, the VHP samples exhibited superior hardness and wear resistance but a lower flexural strength and toughness relative to the SPS samples. Considering the different microstructures and mechanical properties between SPS and VHP samples, the sintering method should be selected according to the specific working conditions, and the SPS process has potential in realizing rapid low-temperature sintering for HEAs compared with other sintering methods.

Carbide effects on tensile deformation behavior of [001] symmetric tilt grain boundaries in bcc Fe

The mechanical performance of carbides on the grain boundary (GB) of ferritic steels is always concerned. In this paper, four symmetric tilt GBs in body-centered cubic Fe were studied under tensile tests at 0 and 300 K through atomistic simulations. The GBs contained various sized Cr23C6 precipitates. Stress–strain curves were obtained through the molecular dynamics simulations and the tensile strength varied with GB type. Centro-symmetry parameter, common neighbor and dislocation analyses were performed for defect characterization. GBs with precipitates showed lower tensile strength than pure GBs. As the number and size of carbides increased, the tensile strength decreased. Cavities were easily formed near dislocation loops with a Burgers vector parallel with loading direction during tensile deformation of pure GBs, while ruptures occurred around the precipitate in GBs with carbides. The simulations indicate that Cr23C6 carbides in Fe are a brittle phase where the dislocations and cracks nucleate.

Microstructural study of the NbC to G-phase transformation in HP-Nb alloys

The microstructure of a centrifugally cast HP alloy was studied in its as-received state and after ageing at 900 °C. A multi-scale approach combining X Ray Diffraction (XRD), advanced electron microscopy modes (scanning and transmission electron microscopies (SEM, TEM), together with focused ion beam/SEM nanotomography (FIB-nt)) has been carried out to characterize the evolution of niobium carbides during ageing. After thermal treatment, the carbides exhibit a complex microstructure, consisting of a core of untransformed NbC, an intermediate layer of G-phase (Ni16Nb6Si7) with embedded nanometric titanium carbide precipitates, and an outer shell of alternating chromium carbides Cr23C6 and G-phase. A simple diffusion model was used to explain the faster external growth of G-phase compared to the internal NbC dissolution, and to determine a diffusion coefficient of niobium in the G-phase at 900 °C.