Precipitation Of Carbide Particles Research Articles

Sintered Fe-Co-Si alloy with excellent magnetic and mechanical properties by coherent precipitation of submicrometer-sized carbide particles

A novel sintered Fe-Co-Si alloy with coherent precipitation of submicrometer-sized carbide particles of Ta3Co3C containing iron and silicon is fabricated by a unique synthesis route through evaporation of an Fe-Co-Si matrix and diffusion of tantalum and carbon by sintering in high vacuum at a high temperature. The carbide particles precipitate homogeneously on the surface, in the grain boundaries, and inside the grains of the matrix. In particular, many particles precipitate homogeneously along the grain boundaries, which can contribute to improving mechanical strength by preventing intergranular fracture in spite of very small volume fraction (0.8 vol%) of the precipitated particles. The strength improvement can be explained by the particle shearing mechanism by the coherent precipitation of carbide particles. The orientation relationships between the carbide particles and Fe-Co-Si matrix are (4̅04)particle//(2̅02)matrix, (04̅0)particle//(02̅0)matrix, and (4̅00)particle//(2̅00)matrix with a very small lattice mismatch (<2%). As a result of the coherent precipitation, the magnetic properties such as low magnetic loss and high magnetic permeability does not deteriorate, and are rather improved mainly by a promoted densification with very few pores and grain growth by sintering in high vacuum at a high temperature. It is also demonstrated that the fabricated alloy has excellent magnetic and mechanical properties compared to conventional sintered or compressed powder materials.

Austenite transformation associated with δ-ferrite phase in a medium-Mn steel after cold-rolling and intercritical annealing

By adjusting the initial δ-ferrite (δ) microstructure based on the cold-rolling, two types of unusual austenite (γ) transformation were investigated in a medium-Mn steel during intercritical annealing. First, δ/α' (martensite) phase interfaces are considered advantageous nucleation sites for γ transformation. When the cold-rolling reduction ratio reaches up to 70% (CR70 sample), δ phase morphology evolves from fibrous structure to small pieces, resulting in more δ/α' (martensite) phase interfaces. The results reveal that an increase in δ/α' phase interfaces promotes γ formation ranging from 15.7% to 17.9% (in volume fraction) for samples subjected to quenching and annealing. Furthermore, the γ nuclei are found to grow into both phases, accompanied by a gradual shrinking of local α' and δ phases due to γ transformation. Secondly, precipitation of carbide particles from δ phase in cold-rolled samples also creates a condition for subsequent γ nucleation within δ grains, and then the dissolution of the carbides accelerates γ development. Besides, the thin piece structure of δ phase in the CR70 sample can transform into γ phase at an elevated degree of austenitization. This study innovatively sheds light on the effect of δ phase on γ transformation in medium-Mn steels.

Effect of heating on the microstructure and mechanical properties of hot rolled high strength bainitic steel rail

ABSTRACT In this paper, the microstructural evolution and mechanical properties of the hot-rolled U25CrNi high-strength bainitic steel rail under different tempering conditions were investigated. The results indicate that both the hot-rolled and tempered microstructures of the tested steel consist of bainitic, martensite and retained austenite. When the tempering temperature rises to 400°C, relatively large volume fraction of residual austenite as well as a smaller amount of carbide precipitation in the tested steel have been detected. When the tempering time is extended to 360 min at the same temperature, the precipitation of carbide particles in the steel sample increases. When the tempering temperature is 500°C, the bainitic ferrite in the tested steel is obviously coarsened, accompanied by the precipitation of a large amount of carbide. In a word, the optimum tempering process for this hot-rolled U25CrNi high-strength bainitic rail is 400°C × 200 min.

Microstructure and flow behaviour of a quenching and partitioning steel at elevated temperatures

ABSTRACT Hot compression tests were carried out on a quench and partitioning steel in the temperature range of 850-1050 °C and strain rates of 10-3- 1 s-1. The single-peak flow curves were appeared at high strain rates and low temperatures and the multi-peaks at the opposite condition. It was found that the mechanism of dynamic recrystallization (DRX) changes by passing from 950 °C due to dynamic precipitation of carbide particles. The microstructural characterizations using optical microscopy and electron back scattered diffraction (EBSD) techniques revealed various mechanisms of DRX in different deformation conditions. EBSD results showed opposite variations of high and low angle grain boundaries with increasing deformation temperature and decreasing strain rate. By using the hyperbolic sine constitutive equation, the value of apparent activation energy at the typical strain of 0.5 was determined as 297.67 kJ/mol.

Optimal Material Selection for Ship Body Based on Fabricated Zirconium Dioxide/ Silicon Carbide Filled Aluminium Hybrid Metal Alloy Composites Using Novel Fuzzy Based Preference Selection Index

Fabrication and selection of optimal material with diverse properties for ship body is often a troublesome and difficult task, as inadequate selection of materials may result in improper functioning and failure of the components at any stage during its functioning. To this end, this paper presents a novel hybrid fuzzy Preference Selection Method (f-PSI) method for selection of optimal alternative ship body material based on physical, mechanical and corrosion characteristics of a novel hybrid aluminium metal alloy composites. Four hybrids aluminium alloy (AL7075) composites are fabricated through stir casting method by reinforcing varying quantity of zirconium dioxide (2%, 4% and 6%) and silicon carbide (3%, 6% and 9%). The physical characterization results show that void contents and density continuously increases with increase in the wt.% of the reinforcement. The mechanical characterization and fractography analysis results confirms upturn in hardness, ultimate tensile strength, flexural strength and impact strength up to 10% wt. of reinforcement and decreases for 15% wt. of reinforcement. The corrosion characterization and microstructural analysis show that corrosion rate decreases considerably up to 10% wt. of reinforcement and increase after that due to the precipitation of carbide particles along the grain boundary region of hybrid metal alloy composite. For validation and robustness of the proposed method, sensitivity analysis is performed, and comparison is performed with already available fuzzy TOPSIS and fuzzy VIKOR methods. Seven evaluation criteria such as void content, density, micro-hardness, ultimate tensile strength, flexural strength, impact strength and corrosion rate are considered in the study to choose the best material for ship body. From the analysis of results, it was found that A-2 (having 4 wt.% ZrO2 and 6 wt.% SiC as reinforcement) alternative material possesses the best combination of all the properties for the given ship body application.

Verification of Johnson-Cook parameters of ferritic stainless steel by drilling process: experimental and finite element simulations

Ferritic stainless steels contain 16%–30% chromium and about 0.05%–0.25% carbon in their compositions. As the carbon and chromium content is high, precipitation of carbide particles provides high strength but also causes brittleness. Ferritic stainless steels are of moderate formability with an excellent corrosion resistance and relatively inexpensive. In this study, the predetermined JC model parameters of the AISI 430 ferritic steels have been verified by comparing the experimental drilling process and drilling simulations. Firstly, the drilling experiments of 430 steel with the uncoated tungsten carbide drill have been performed at three different feed rates and cutting speeds. Then, the pre-determined JC parameters have been adapted into a simulation software running with the finite element method and after the drilling simulations have been operated under the same drilling conditions. By comparing the experimental and numerical data based on the thrust force (Fz), a deviation of 4.83% has been calculated and the validity of the material's JC parameters has been demonstrated.

Features of Phase Transformations of Low-activation 12%-Chromium Ferritic-Martensitic Steel Ek-181

An investigation of the features of structural-phase transformations during heating and cooling of the lowactivation 12%-chromium ferritic-martensitic steel EK-181 (Fe–12Cr–2W–V–Ta–B) is performed in the range of temperatures from 30 to 1100°C by the methods of high-temperature X-ray diffraction analysis (XRD) in situ. The critical points of an (α →γ) - transformation (Ас1 ≈ 850°С, Ас3 ≈ 950°С) in the course of steel heating and the temperature interval (910–890°C) of the diffusion-induced transformation of the austenite →α-ferrite type under cooling are determined. It is shown that the position of these points changes depending on the method (XRD or differential scanning calorimetry) of their identification, which is attributed to the differences in their heating-cooling rates. During heating in the range of temperatures of 830–875°C, a significant increase in the volume fraction of М23С6 carbides is observed. Further temperature increase up to 1000–1100°C leads to their complete dissolution. In the course of cooling of the steel specimens from 1100 to 30°C, there is no carbide particle precipitation.

Structural Aspect of Isothermal Bainitic Transformation in High-Carbon Manganese–Silicon Steel

Transmission and scanning electron microscopy and X-ray diffraction analysis were used to compare the structural states of high-carbon manganese–silicon steel which underwent low-temperature isothermal bainitic and martensitic transformations. It was shown that bainite consisting of thin α-phase plates and residual austenite interlayers forms during isothermal holding at 300°С. As the isothermal holding time increases, fine carbide particles precipitate within the bainite α-phase.

Nanoscale Solute Partitioning and Carbide Precipitation in a Multiphase TRIP Steel Analyzed by Atom Probe Tomography

Nanoscale solute partitioning across multiple constituent phases in a 980-grade quenched and partitioned (Q&P) steel was analyzed using atom probe tomography (APT). The Q&P process was used to increase the C content in the retained austenite phase thereby improving its stability under plastic straining. Significant carbon enrichment of austenite was measured with decreased levels of C in martensite and almost depleted C content in ferrite, supporting the C partitioning mechanism in the literature. The APT analysis of retained austenite surrounded by martensite demonstrated a higher amount of C content compared with retained austenite surrounded by the ferrite phase. Lath and discrete carbide particle precipitation was also observed inside martensite colonies, tying up C and reducing the total amount of C available for austenite stabilization. In addition, the partitioning of Mn and other minor elements was quantitatively investigated by correlating APT and SEM-EBSD. These techniques provide a robust methodology for analyzing nanoscale compositional partitioning in multiphase steels, TRIP steels in particular, which can be used to better explain their microstructure-mechanical property relationships.

Formation of nano-sized M2C carbides in Si-free GH3535 alloy

GH3535 alloy is one of the most promising structural materials for molten salt reactors (MSRs). Its microstructure is characterized by equiaxed grains and coarser primary M6C carbide strings. In this study, stable nano-sized M2C carbides were obtained in GH3535 alloy by the removal of Si and thermal exposure at 650 °C. Nano-sized M2C carbide particles precipitate preferentially at grain boundaries during the initial stage of thermal exposure and then spread all over the grain interior in two forms, namely, arrays along the {1 1 1} planes and randomly distributed particles. The precipitate-free zones (PFZs) and the precipitate-enriched zones (PEZs) of the M2C carbides were found to coexist in the vicinity of the grain boundaries. All M2C carbides possess one certain orientation relationship (OR) with the matrix. Based on microstructural characterizations, the formation process of M2C carbides with different morphologies was discussed. The results suggested that the more-stable morphology and OR of M2C carbides in the Si-free alloy provide higher hardness and better post-irradiation properties, as reported previously. Our results indicate the preferential application of Si-free GH3535 alloy for the low-temperature components in MSRs.

Formation of internal stress fields in rails during long-term operation

The structure and the internal stress fields in R65 rails withdrawn from operation because of side wear after long-term operation are studied and estimated. A high scalar dislocation density (higher by a factor of 1.5–2), the fragmentation of cementite lamellae, and the precipitation of carbide particles are detected in the layers adjacent to the roll surface. The stresses at the boundaries of the particles with the ferrite matrix can exceed the ultimate strength of the steel.

Deposition of a Silicon Carbide Reinforced Metal Matrix Composite (P25) Layer Using CO2 Laser

The objective of this research was to deposit a silicon carbide (SiC) reinforced layer of P25 (iron-based matrix material) on substrate material surface using CO2 laser. Two experiments using CO2 laser were carried out in this research. In the first experiment set, a gravity feed system was used with one powder feed containing different percentages of SiC particles and iron-based powder. In the second experiment set, preplaced powder was placed on substrate material surface. According to the experimental results, only few SiC particles were found in the clad matrix in the first experiment, and no SiC particles were found in the second experiment. A high microhardness value was noted in the SiC clad (above 1000 HV) in the first experiment compared to the second experiment with hardness values ranging from 200 HV to 700 HV. This was due to the high precipitation of carbide particles in the clad material during the first experiment. A comparison of the two different experiments signifies that the first one was the best because a more uniform layer with less porosity was produced.

Fatigue Behaviour of Carbide Precipitation Hardened Austenitic Steels

The paper presents the results of investigations of the microstructure and fatigue behaviour of two newly invented Cr-Ni and Cr-Ni-Mn austenitic steels of 13/13 and 12/8/8 type strengthened through carbide particle precipitation. The specimens of the investigated steels were subjected to tests after heat treatment, i.e. solution heat treatment (1200°C/0.5 h/water) and aged at a temperature of 700°C for 12 h, with cooling in air. The heat treated specimens were then subjected to low-cycle fatigue tests (LCF), carried out at room temperature and at an increased temperature of 600°C. Diagrams of fatigue characteristics of the investigated steels at room temperature as well as at elevated temperature have been worked up. It has been found that during low-cycle fatigue tests, at both temperatures, the investigated austenitic steels indicated a fatigue softening effect. The results of LCF at room temperature showed that the fatigue durability (Nt) of both austenitic steels is located in the range 0.8÷1.3×103 cycles. The results of low-cycle fatigue tests at an increased temperature 600°C indicated that the fatigue durability of the investigated steel was lower, and is located in the range Nt = 0.5÷0.6×103 cycles. It has been pointed out that the investigated austenitic steels are characterized by a stability of structure in conditions of cyclic fatigue.

Precipitation behavior of grain boundary M23C6 and its effect on tensile properties of Ni–Cr–W based superalloy

Precipitation behavior of grain boundary (GB) M23C6 and its effect on tensile properties at elevated temperature were investigated systematically in a Ni–Cr–W based superalloy. The results show that the M23C6 precipitation behavior is influenced obviously by grain boundary character (GBC) and interfacial energy. The Σ≤9 GBs and low angle GBs have low interfacial energy, and no M23C6 carbide precipitates at these GBs. Plenty of M23C6 carbide particles precipitate at the large angle GBs with high interfacial energy. The coherent orientation relationship between M23C6 and the matrix plays an important role on the precipitation morphology of M23C6. M23C6 carbides with four typical morphologies distribute at the large angle GBs, including lamellar carbide which grows into the matrix near one side or both sides of the GBs, rod-like carbide and small lamellar carbide both of which grow along GBs. Moreover, the decrease of both tensile and yield strength of the aged alloy is mainly caused by the lamellar M23C6 carbide breaking. The tensile properties vary irregularly with increasing aging time.

The improvement of ductility in nano/ultrafine grained low carbon steels via high temperature short time annealing

A new method has been applied to fabricate bimodal grain structure in ultrafine-grained low carbon steels. This method is based on the plane strain compression of a martensitic starting structure and subsequent annealing at elevated temperature for short time. The annealing process was carried out at the temperature of 500 and 600°C for 1.5ks and 120s, respectively. The final microstructure after recrystallization is composed of ultrafine and coarse grains along with the precipitation of carbide particles (cementite) within the coarse grains. This has been considered to be the reason for the observed superior balance of ductility and strength.

Influence of heat treatment regimes on microstructure and creep properties of FGH95 nickel base superalloy

Abstract By means of creep curves measurement and microstructure observation, an investigation has been made into the influence of heat treatment regimes on microstructure and creep properties of FGH95 nickel base superalloy. Results show that, after the alloy is solution treated and cooled in oil bath, coarser γ′ precipitates are distributed in the previous particle boundaries (PPB) regions in which appears the γ′-free phase zone. No coarser γ′ phase is detected in the alloy cooled in molten salt, the grain size of which increases slightly and fine γ′ phase is dispersedly distributed in the grains, in addition, some (Nb, Ti)C particles are precipitated along the boundaries. In the ranges of the experimental stresses and temperatures, the alloy cooled in molten salt possesses a better creep resistance and longer creep life, the creep activation energy of the alloy is measured to be Q = 590.3 kJ/mol. During creep, the deformed mechanism of the alloy cooled in oil bath is that the slipping dislocations with double orientations are activated in the matrix, and some finer carbide particles are precipitated along the slipping traces. Moreover, the wider PPB regions are broken into finer grains due to the severe plastic deformation. However, not only dislocations tangle but also stacking fault appears in the alloy cooled in molten salt. The fact that the carbide particles precipitate along the boundary may effectively restrain the movement of dislocation slipping, which is one important factor for the alloy possessing better creep resistance and longer creep life.

High Temperature Aging and Corrosion Study on Alloy 617 and Alloy 230

Abstract The very high temperature gas-cooled reactor (VHTR), with dual capacities of highly efficient electricity generation and thermochemical production of hydrogen, is considered as one of the most promising Gen-IV nuclear systems. The primary candidate materials for construction of the intermediate heat exchanger (IHX) for the VHTR are alloy 617 and alloy 230. To have a better understanding of the degradation process during high temperature long-term service and to provide practical data for the engineering design of the IHX, aging experiments were performed on alloy 617 and alloy 230 at 900°C and 1000°C. Mechanical properties (hardness and tensile strength) and microstructure were analyzed on post-aging samples after different aging periods (up to 3000 h). Both alloys attained increased hardness during the early stages of aging and dramatically soften after extended aging times. Microstructural analysis including transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and electron backscatter diffraction was carried out to investigate the microstructure evolution during aging. A carbide particle precipitation, growth, and maturing process was observed for both alloys, which corresponds to the changes of the materials’ mechanical properties. Few changes in grain boundary character distribution and grain size distribution were observed after aging. In addition, high temperature corrosion studies were performed at 900°C and 1000°C for both alloys. Alloy 230 exhibits much better corrosion resistance at elevated temperature compared with alloy 617.

Microstructure and High Temperature Strength in Fe&lt;sub&gt;3&lt;/sub&gt;Al Base Alloys Containing Fine Carbide Particles

The precipitation of carbide particles and high temperature strength in a Fe3Al base alloy with C, Cr and Mo were investigated. The -Fe3AlC (E21) and M2C (B81) phases precipitated in the temperature range between 1000 °C and 600 °C in periods up to 10 h. The -Fe3AlC phase tended to precipitate in a film-like morphology on grain boundaries by annealing at relatively high temperatures. The M2C phase was dispersed in a fine needle shape within grains by annealing at 700 °C. The film-like  precipitates covering grain boundaries as well as finely dispersed M2C particles was suggested to contribute to strengthening at 600 °C.

Precipitate Redistribution during Creep of Alloy 617

Nickel-based superalloys are being considered for applications within advanced nuclear power generation systems due to their high-temperature strength and corrosion resistance. Alloy 617, a candidate for use in heat exchangers, derives its strength from both solid solution strengthening and the precipitation of carbide particles. However, during creep, carbides that are supposed to retard grain boundary motion are found to dissolve and reprecipitate on boundaries in tension. To quantify the redistribution, we have used electron backscatter diffraction (EBSD) and energy-dispersive spectroscopy (EDS) to analyze the microstructure of 617 after creep testing at 900 °C and 1000 °C. The data were analyzed with respect to the location of the carbides (e.g., intergranular vs intragranular), grain boundary character, and precipitate type (i.e., Cr rich or Mo rich). We find that grain boundary character is the most important factor in carbide distribution; some evidence of preferential distribution to boundaries in tension is also observed at higher applied stresses. Finally, the results suggest that the observed redistribution is due to the migration of carbides to the boundaries and not the migration of boundaries to the precipitates.

Nanocrystalline structure formation in EK-181 steel surface layers on ultrasonic treatment

The microstructure and the phase composition of surface layers of ferritic-martensitic EK-181 steel (Fe-12Cr-2W-V-Ta-B-C) subjected to various combinations of ultrasonic and heat treatment were examined. It is shown that a nanocrystalline a-phase structure with grain boundaries on which vanadium carbide particles precipitate is formed only on intermediate ultrasonic treatment between quenching and ageing. The intermediate ultrasonic treatment provides the greatest increase in the yield limit of the test specimens under uniaxial tension. A correlation is found between the microstructure, the mechanical characteristics and the fracture mode.