Cr23C6 Carbides Research Articles

Agglomeration model of (Fe,Cr)7C3 carbide in hypereutectic Fe-Cr-C alloy

The microstructure, growth morphology and crystallographic characteristic of the primary (Fe, Cr)7C3 carbide in hypereutectic Fe-Cr-C alloy are investigated. The primary (Fe, Cr)7C3 carbide is irregular polygonal shape with several hollows in the center and gaps on the edge. In the initial growth stage, the crystallographic orientations of neighboring (Fe, Cr)7C3 carbides are the same and some protrusions are emerged. The primary (Fe, Cr)7C3 carbide consists of several parts, which are divided by the low-angle boundaries. Analysis suggests that the primary (Fe, Cr)7C3 carbide exhibits agglomeration phenomenon which results in the low-angle boundary, as well as the hollows and gaps.

Plasma Transferred Arc Surface Alloying of Cr-Ni-Mo Powders on Compacted Graphite Iron

A Cr-Ni-Mo overlayer was deposited on the surface of compacted graphite iron (CGI) by the plasma transferred arc (PTA) alloying technique. The microstructure of Cr-Ni-Mo overlayer was characterized by optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), and X-ray diffractometer (XRD). Results show that the cross-section consists of four regions: alloying zone (AZ), molten zone (MZ), heat affected zone (HAZ), and the substrate (SUB). The microstructure of AZ mainly consists of cellular γ-(Fe, Ni) solid solution, residual austenite and a network of eutectic Cr7C3 carbide while the MZ area has a typical feature of white cast iron (M3C-type cementite). The martensite/ledeburite double shells are observed in the HAZ. With decreasing the concentration of Cr-Ni-Mo alloys, the fracture mode changes from ductile in the AZ to brittle in the MZ. The maximum hardness of the AZ (450 HV0.2) is lower than that of the MZ (800 HV0.2). The eutectic M3C and M7C3 carbides increase the microhardness, while the austenite decreases that of the AZ.

Microstructure and wear behaviour of plasma transferred arc (PTA)-deposited FeCrC composite coatings on AISI 5115 steel

Plasma transferred arc (PTA) cladding was used to produce composite coatings on the surface of AISI 5115 steel using five different energy inputs and ferrochromium (FeCrC) powder. The effects of different energy inputs and of ferrochromium powder on the microstructure, hardness, and wear behaviour were investigated. The dry-sliding wear properties of the samples were determined using the ball-on-disk wear test method. The alloyed surfaces exhibited increases in hardness and wear resistance and a decrease in the coefficient of friction. The increase in hardness was explained by the presence of hard phases, such as the martensitic phase, Cr7C3, C3Cr7, Fe23C6 and iron carbides, in the microstructure. The average coefficient of friction value of the substrate was determined to be 0.83, whereas these values for the composite layers varied between 0.62 and 0.73. The lowest wear rate (2.88×10−6mm3/N/m) was observed for the 100A PTA cladding sample. The industrial coating process was applied to an AISI 5115 steel rolling cylinder using the optimum parameters (100A), which were determined after experimental studies. The PTA cladding process on the rolling cylinder improved the useful life by approximately 2.5-fold compared to the actual rolling cylinder.

Solidification microstructure formation in HK40 and HH40 alloys

The microstructure formation processes in HK40 and HH40 alloys were investigated through JmatPro calculations and quenching performed during directional solidification. The phase transition routes of HK40 and HH40 alloys were determined as L → L + γ → L + γ + M7C3 → γ + M7C3 → γ + M7C3 + M23C6→ γ + M23C6 and L → L + δ → L + δ + γ→ L + δ + γ + M23C6 δ + γ + M23C6, respectively. The solidification mode was determined to be the austenitic mode (A mode) in HK40 alloy and the ferritic–austenitic solidification mode (FA mode) in HH40 alloy. In HK40 alloy, eutectic carbides directly precipitate in a liquid and coarsen during cooling. The primary γ dendrites grow at the 60° angle to each other. On the other hand, in HH40 alloy, residual δ forms because of the incomplete transformation from δ to γ. Cr23C6 carbide is produced in solid delta ferrite δ but not directly in liquid HH40 alloy. Because of carbide formation in the solid phase and no rapid growth of the dendrite in a non-preferential direction, HH40 alloy is more resistant to cast defect formation than HK40 alloy.

High-strength economically alloyed corrosion-resistant steels with the structure of nitrogen martensite

The use of nitrogen as the main alloying element allowing one both to increase the corrosion resistance and mechanical properties of steels and to improve their processability is a new trend in physical metallurgy of high-strength corrosion resistant steels. The principles of alloying, which are developed for high-nitrogen steel in IMET RAS, ensure the formation of the structure, which contains predetermined amounts of martensite (70-80%) and austenite (20-30%) and is free from δ-ferrite, σ-phase, and Cr23C6 carbide. These principles were used as the base for the creation of new high-strength corrosion-resistant weldable and deformable 0Kh16AN5B, 06Kh16AN4FD, 08Kh14AN4MDB, 09Kh16AN3MF, 27Kh15AN3MD2, 40Kh13AN3M2, and 19Kh14AMB steels, which are operative at temperatures ranging from - 70 to 400°C. The developed nitrogen-containing steels compared with similar carbon steels are characterized by a higher resistance to pitting and crevice corrosion and are resistant to stress corrosion cracking. The new steels successfully passed trial tests as heavy duty articles.

Microstructure and Nano-Hardness of 10 MeV Cl-Ion Irradiated T91 Steel**supported by National Natural Science Foundation of China (Nos. 11374299, 11375230, 11274309)

Hardening and elemental segregation of T91 martenstic steel irradiated by 10 MeV Cl ions to doses from 0.06 dpa to 0.83 dpa were investigated with the nanoindentation technique and transmission electron microscopy (TEM). The results demonstrated that the irradiation hardening was closely related with irradiation dose. By increasing the dose, the hardness increased rapidly at first from the initial value of 3.15 GPa before irradiation, and then tended to saturate at a value of 3.58 GPa at the highest dose of 0.83 dpa. Combined with TEM observation, the mechanism of hardening was preliminary attributed to the formation of M(Fe,Cr)23C6 carbides induced by the high energy Cl-ion irradiation.

Characterization of phases and defects in chromium carbide overlays deposited by SAW process

Owing to their enhanced wear resistance, chromium carbide overlay (CCO) plates deposited by submerged arc welding (SAW) process have been widely used in various branches of the mining industry. Defects in the deposited CCO directly determine the wear performance of this material. Several characterization techniques, including optical microscope (OM), optical emission spectroscopy (OES), and field emission scanning electron microscope (FESEM) equipped with an energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD), have been used to identify the most common defects and phase distribution in the SAW-deposited CCO. The main phases are consist of primary carbide, secondary Cr7C3 and Cr23C6 carbides, austenite, and a small amount of ferrite. Defects in CCO, such as cracks, porosity, and clusters of particles known in the industry as “rice crispies” are observed near the top surface of the CCO. The undissolved Cr7C3 particles with surrounding porosity seem to have provided the preferential nucleation sites for the initial micro-cracks. These initial cracks are observed to propagate along the interface between the austenite and Cr7C3 carbides.

Improving the high-temperature creep strength of 15Cr ferritic creep-resistant steels at temperatures of 923–1023 K

The precipitation strengthening of creep-resistant 15Cr ferritic steels at temperatures of 923–1023K was improved by nickel addition. The addition of nickel, an austenite-stabilizing element, enhanced the formation of martensitic grains in the ferritic grain boundaries and changed the precipitation behavior during creep at elevated temperatures. The precipitates produced in the creep-ruptured 15Cr steels were intermetallic compounds (Laves phase and χ-phase), carbide (Cr23C6), and Z phase (Cr(V,Nb)N). An increase in the amounts of intermetallic compounds and the preferential precipitation of Cr23C6 carbide on the grain boundaries between the ferritic and martensitic grains were accelerated by the nickel addition. The 15Cr steel after 10,000h at 973 and 1023K demonstrated approximately double the creep rupture strength of 9Cr steel (ASME Grade T92) with a tempered martensitic microstructure. At temperatures of up to 1023K, the 15Cr steel had over 10 times the creep rupture lifetime of T92 steel, primarily because of increased precipitation of intermetallic compounds and Cr23C6 carbide enhanced by nickel addition.

Improvement of Wear Resistance of Stainless Steel AISI 304L by Diode Laser Surface Alloying with Chromium Carbide

Metal matrix composite layers were fabricated on AISI 304L substrate by diode laser surface alloying with direct injection of chromium carbide Cr3C2 powder into the molten-pool. The influence of laser alloying parameters on the quality of the alloyed layers were investigated. The alloyed layers were examined by optical metallography and scanning electron microscopy. Comparative erosion tests between the AISI 304L substrate and the alloyed layers have been performed following the ASTM G 76 standard test method. The uniform laser beam intensity profile of the laser used ensures to produce fully dense alloying layers with homogenous distribution of Cr3C2 particle throughout the matrix alloy. Distribution and dissolution of Cr3C2 particles are strongly dependent on the laser power level. The alloyed layers exhited noticeable increased erosion resistance in comparison to AISI 304L substrate for both 30° and 90° impact angles.

High-Temperature Corrosion of UNS N10003 in Molten Li2BeF4(FLiBe) Salt

Corrosion testing of UNS N10003 in molten fluoride salt was performed in purified molten 27LiF-BeF2 (66–34 mol%) (FLiBe) salt at 700°C for 1,000 h, in pure nickel and graphite capsules. In the nickel capsule tests, the near-surface region of the alloy exhibited an approximately 200 nm porous structure, an approximately 3.5 μm chromium-depleted region, and MoSi2 precipitates. In the tests performed in graphite capsules, the alloy samples gained weight because of the formation of a variety of Cr3C2, Cr7C3, Mo2C, and Cr23C6 carbide phases on the surface and in the subsurface regions of the alloy. A Cr-depleted region was observed in the near-surface region where Mo thermally diffused toward either the surface or the grain boundary, which induced an approximately 1.4 μm Ni3Fe alloy layer in this region. The carbide-containing layer extended to approximately 7 μm underneath the Ni3Fe layer. The presence of graphite dramatically changes the mechanisms of corrosion attack in UNS N10003 in molten FLiBe salt. In t...

Creep ruptures of AISI 347 austenitic stainless steel foils at elevated temperature of 750 ℃

The creep rupture properties of AISI 347 austenitic stainless steel foil used in compact recuperators have been evaluated at 750 ℃ in the stress range of 54–221 MPa to establish baseline behavior for its extended use. The creep curve of the foil shows that the primary creep stage is brief and creep life is dominated by tertiary creep deformation with rupture lives in the range 3–433 h. Power law relationship was obtained between the minimum creep rate and the applied stress with stress exponent value of n = 4.25. The creep damage tolerance parameter for specimen tested at 750 ℃ and 54 MPa indicates that creep fracture takes place by precipitate coarsening mechanism. Nucleation of voids mainly occurs at second-phase particles (Cr23C6 carbides). The improvement in strength is attributed to the precipitation of fine niobium carbides in the matrix which prevents dislocation movement of the microstructure.

The Effect of Heat Temperature on the Structure of Plasma Coating of the Ni-Cr-Si-B System

The powder of the Ni-Cr-Si-B system was applied on tubes of low carbon steel (0.2 % C) with the help of air-plasma spraying. The effect of heating temperature on the microstructure of obtained coatings was analyzed in this paper. The coatings were heated up to 500, 600, 700, 800, 900, 1000, and 1100 °C and cooled in the air. It was shown by the method of optical and scanning electron microscopy (SEM) and the X-ray diffraction analysis (XRD) that the same phases (γ-Ni, Ni3B, Ni3Si and Cr2B) were present in the coatings before and after heating. Chromium carbide Cr7C3 was observed in the coating structure after heating to above 1000 °C. Coatings which were heated to 1000 °C consisted of grains of the solid nickel base solution, micro volumes of the Ni+Ni3Si eutectic with inclusions of Ni3B and particles of Cr7C3. The coatings which were crystallized from a liquid state (heated up to 1100 °C) consisted of large areas of the solid nickel base solution and particles of chromium carbide Cr7C3. The space between nickel areas is filled by the Ni3Si+Ni3B eutectic.

Microstructural Comparison of Spray-Formed and Conventionally Cast 2.5C–19Cr High-Chromium White Iron

A billet of hypoeutectic high-chromium white iron (2.5% C, 19% Cr) was spray formed using gas-to- metal ratios of *0.9, *1.0, and *1.1. The as-sprayed material was close to full density and contained fine (Fe,Cr)7C3 carbides (27-30 vol%) uniformly distributed in a matrix consisting of pearlite and ferrite. Length of the carbides rarely exceeded 30 lm. This was in stark contrast to a conventionally cast starting material, which contained coarse (Fe,Cr)7C3 carbides (*22 vol%) non-uniformly distributed in a matrix consisting of austenite and marten- site. Length of the carbides in the cast material occasion- ally exceeded 100 lm. Varying gas-to-metal ratio between 0.9 and 1.1 did not result in any significant changes in carbide morphology, although slightly coarser carbide morphology was produced with the gas-to-metal ratio of 0.9. Regardless of the gas-to-metal ratio, the finest carbide morphology was found in the surface region of the spray- formed billet.

Investigation on microstructure and wear characteristic of laser cladding Fe-based alloy on wheel/rail materials

The aim of this study is to investigate the influence of laser cladding Fe-based alloy on microstructure and wear characteristic of wheel/rail materials using a rolling–sliding wear testing apparatus. Especially, the effect of lanthanum oxide on the properties of laser cladding coating was explored in detail. The results show that the laser cladding Fe-based alloy coating consists of dendrites and eutectic and is composed of (Fe, Ni) solid solution and Cr7C3 carbide. The hardness and wear-resistance of wheel/rail materials are enhanced using laser cladding. The wear and surface damage of wheel/rail rollers dramatically alleviate owing to the refinement of the microstructure when adding lanthanum oxide into laser cladding coating. The wear mechanism of wheel/rail rollers turns from severe spalling and delamination wear (untreated specimens) to slight peeling (with 1.2% La2O3). Furthermore, the plastic deformation of wheel/rail rollers undergoing laser cladding is significantly thinner than that of untreated rollers. In summary, laser cladding Fe-based alloy coating can be used to prolong the wear life of wheel and rail materials.

Research on the Microstructure and Microhardness of 9Cr18 Semi-Solid Billet

Formed in the semi-solid state, materials can obtain unconventional microstructures and properties compared with traditional method. In this paper, semi-solid billet of 9Cr18 steel was obtained through a wavelike sloping plate. Microstructure analysis of the semi-solid billet was conducted through scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). X-ray diffraction (XRD) test and microhardness test were also used to analyze the properties. The results showed that the structure of 9Cr18 semi-solid billet contained globular primary austenite and secondary austenite-Cr7C3 eutectic. Globular primary austenite grains were first formed during partial solidification in the sloping plate, and then the rest liquid metal formed secondary austenite and Cr7C3 eutectic structure surrounding the primary grains. Cr atoms had a concentration in the rest liquid side, which along with C atoms contributed to the formation of the Cr7C3 carbide. Hardness in the primary solid grain area and the eutectic area was about 330 HV and 650 HV, respectively. These specific properties were important for subsequent thixoforming of the functional graded materials.

INVESTIGATION OF CHANGES IN THE COMPOSITION, STRUCTURE AND DISPERSION OF CHROMIUM CARBONITRIDE DURING STORAGE AND HEATING IN GAS MEDIA

The changes in the composition, structure and dispersion of chromium carbonitride during storage and heating in gas media are investigated. It is found that during annealing in argon and nitrogen at a temperature of 1273 – 1373 K carbonitride becomes carbide Cr3C2 , and during annealing in hydrogen – carbide Cr7C3 . The transformation of carbonitride into carbide Cr3C2 is accompanied by enlargement of the nanopowder that takes place in accordance with the mechanism of solid-state coalescence. Carbonitride interaction with atmospheric gases (oxygen and moisture) occurs in accordance with the adsorption-diffusion mechanism and it is accompanied by a significant increase in oxidation in the first 24 hours. The temperature at the beginning of oxidation in the air depends on the nanoscale and due to the change in particle size from 22 to 53 nm increases from 542 to 568 K. The size dependences for oxidation and the oxidation onset temperature are obtained.

Molecular dynamics investigation of the interaction of dislocations with carbides in BCC Fe

Different types of carbides are present in many steels used as structural materials. To safely use steel in demanding environments, like nuclear power plants, it is important to know how defects will affect the mechanical properties of the material. In this study, the effect of carbide precipitates on the edge dislocation movement is investigated. Three different types of carbides were investigated by means of molecular dynamics, with a Tersoff-like bond order interatomic potential by Henriksson et al. The obstacles were 4nm in diameter and were of Fe3C- (cementite-), Fe23C6- and Cr23C6-type. The critical unpinning stress was calculated for each type at different temperatures, to get the temperature-dependent obstacle strength. The results showed a decreasing critical stress with increasing temperature, consistent with previous studies. The critical unpinning stress was seen to be dependent on the type of carbide, but the differences were small. A difference was also observed between the obstacles with the same structure, but with different composition. This study shows the relation between the existing Cr23C6 carbide and the experimentally non-existing Fe23C6 carbide, which needs to be used as a model system for investigations with interatomic potentials not able to describe the interaction of Cr in the Fe–C-system. We found the difference to be a between 7% and 10% higher critical unpinning stress for the chromium carbide, than for the iron carbide of the same type.

Study on the (Fe,Cr)<sub>7</sub>C<sub>3</sub> Particles Reinforced Iron-Based Composite Coating

An iron based composite coating reinforced by (Fe,Cr)7C3particles, with a thickness of 4.0 mm, was fabricated on the ductile cast iron substrate by means of a centrifugal casting and in situ techniques. The microstructure and properties of the composite coatings were investigated by optical microscope, X-ray diffraction, scanning electron micrograph, energy dispersive spectrum, microhardness tester and pin-on-disc wear tester. Results show that the composite coatings consists of primary (Fe,Cr)7C3carbides as the reinforcing phase and interdendritic (Fe,Cr)7C3/γ eutectics accompanied by negligible graphite nodules as the matrix. The reinforcements are metallurgical bonded to the ductile cast iron substrate. Compared with unreinforced ductile cast iron, the composite coatings offer virtually a unique wear resistant behaviour under the condition of dry sliding with loads.

Correlation of ferrite formation to creep properties of Cr-Mo steel welds

A factor of four decreases from 16.0 × 10−4 to 4.1 × 10−4 %/h in steady state creep rate was observed in the fine-grained heat-affected zone (FGHAZ) of a Cr-Mo steel weld, by reducing the pre-weld tempering temperature from 760 to 650 °C. The current study used electron backscatter diffraction and synchrotron x-ray diffraction techniques to characterize the microstructure in the FGHAZ of the two tempering temperature conditions. The results showed carbide-free ferrite that formed in the FGHAZ of weldments that were pre-weld tempered at 760 °C, contributed to void formation resulting in lower creep strength. It is proposed that the formation of ferrite in the FGHAZ is due to the incomplete dissolution of Cr23C6 carbide at heating process during welding, which results in Cr enrichment adjacent to the undissolved Cr23C6 carbide. Dictra simulation confirmed ferrite formation at the carbide/austenite boundaries during Cr23C6 carbide dissolution.

Physical, Mechanical, and Dry Sliding Wear Properties of Fe-Cr-W-C Hardfacing Alloys Under Different Tungsten Addition

In this study, the effects of tungsten on microstructure and wear performance of Fe-Cr-C claddings were evaluated. In this regard, tungsten inert gas surfacing process was employed to deposit Fe-Cr-C and Fe-Cr-C-W hardfacing alloys on plain carbon steel substrate using preplaced powders. Phase composition, microstructure, and wear behavior of clad layers were investigated using X-ray diffraction analysis, optical and scanning electron microscopy, and reciprocating wear tests, respectively. The claddings were well bonded to the substrate and showed a uniform microstructure. Cr7C3 and WC carbides were detected in the deposited layers. Further investigations indicated that the hardness and wear resistance can be improved by adding tungsten into Fe-Cr-C hardfacing alloys.