Nano-sized Carbide Research Articles

Effects of Low-Temperature Tempering on Microstructure and Properties of the Laser-Cladded AISI 420 Martensitic Stainless Steel Coating

Post-treatment is crucial to improve the comprehensive performance of laser-cladded martensitic stainless steel coatings. In this work, a low-temperature tempering treatment (210 °C), for the first time, was performed on the laser-cladded AISI 420 martensitic stainless steel coating. The microstructure and properties of the pre- and post-tempering specimens were carefully investigated by XRD, SEM, TEM, a micro-hardness tester, a universal material testing machine and an electrochemical workstation. The results show that the as-cladded AISI 420 stainless steel coating mainly consisted of martensite, austenite, Fe3C and M23C6 carbides. The phase constituent of the coating remained the same, however, the martensite decomposed into finer tempered martensite with the precipitation of numerous nano-sized Fe3C carbides and reverted austenite in the as-tempered specimen. Moreover, a slight reduction was found in the micro-hardness and tensile strength, while a significant increase in elongation was achieved after tempering. The fractography showed a transition from brittle fracture to ductile fracture accordingly. The as-tempered coating exhibited a striking combination of mechanical properties and corrosion resistance. This work can provide a potential strategy to enhance the overall properties of the laser-deposited Fe-based coating for industrial applications.

Atom Probe Compositional Analysis of Interphase Precipitated Nano-Sized Alloy Carbide in Multiple Microalloyed Low-Carbon Steels.

The composition of nano-sized alloy carbides formed by interphase precipitation in V-Nb and V-Ti multiple microalloyed low-carbon steels is analyzed by using three-dimensional atom probe. Carbide-forming alloying elements including V, Nb, and Ti, are simultaneously precipitated from the early stage of isothermal treatment, whose atoms are uniformly distributed in the carbide particles, even after prolonged holding. Cluster analysis by the maximum separation method, with parameters optimized using different methods, is carried out to extract alloy carbides from matrix. The composition of alloy carbides evaluated by site fraction of substitutional carbide-forming alloying elements indicates that at the early stage of their formation, Nb and Ti are more strongly enriched than V.

EVALUATION OF THE MECHANICAL PROPERTIES OF DIFFUSION LAYER IN THE PROCESS OF MICROARC STEEL VANADATION

Traditional processes of thermochemical treatment of steel have a longer duration, so there are proposed the new methods of intensification of diffusion saturation with high-energy impacts on the material surface. In the process of micro-arc alloying the steel product is immersed in a container filled with powder of coal, and is heated by passing electric current. In a powder environment, microdischarges are formed, which are concentrated around the product and create an area of gas discharge with the formation of a carbonaceous gas environment, which enables carburizing of steel. The application of coating containing diffusant allows forming coating of a carbide type due to simultaneous carbon diffusion into alloying elements. The influence of micro-arc surface alloying of steel with vanadium on mechanical properties of diffusion coatings is studied, and the primary mechanism of steel hardening at microarc alloying is revealed. Cylindrical samples of 20 steel were used; the source diffusant was a powder of ferrovanadium. Current density on the sample surface was 0.3 A/cm2, total duration of the process was 3 min. The mechanical properties of coatings were evaluated by means of indentation using pyramidal indenter, at loads of 2.5 mN, 20 mN and 100mN. The diffusion layer with thickness of 170 – 180 μm consists of a base with hardness of 8 – 9 GPa, containing mild etching inclusions of up to 5 μm with microhardness of 21 – 25 GPa. The base of the layer represents an α-solid solution of vanadium in iron, and inclusions are carbides of VC0.863 type. By atomic force microscopy it was established, that the surface relief is defined by single, relatively large carbide particles with a size of up to 3 μm, and by plural nano-sized carbide particles, which act as the strengthening phase, providing high microhardness of the coating. By method of indentation of the hardened layer cross section using different loads hardening effect of the carbide particles is proven. Estimation of possible mechanisms of hardening have shown that the greatest contribution to diffusion layer hardening is made by dispersion component significantly increasing the yield stress of α-solid solution of iron in comparison with the initial state, which is 38 times greater than the contribution of solid-solution hardening.

Synthesis and Ceramic Conversion of a New Organodecaborane Preceramic Polymer with High-Ceramic-Yield.

Boron carbide is one of the hardest materials known, with diamond-like mechanical properties and excellent chemical stability. It is wildly used in military defense area, nuclear industry, aerospace technology, etc. Precursor-derived ceramics have made it easier to produce pure boron carbide in processed forms and expand its applications. The challenge of this method is the synthesis of precursor polymer with high-ceramic-yield. The aim of the present work is to develop a new poly(6-norbornenyldecaborane-co-decaborane) [P(ND-co-D)] copolymer, which was successfully synthesized via ring-opening metathesis polymerization of 6-norbornenyldecaborane and tandem hydroboration with decaborane. The obtained light-yellow powder displayed good solubility, and was fully characterized by NMR, FT-IR and GPC analysis. Thermogravimetric analysis demonstrated that the char yield was up to 79%. The polymer-to-ceramic transformation process and pyrolysis mechanism has shown that the rearrangement of carbon chains of P(ND-co-D) mainly occurred in the temperature range of 350 °C~470 °C. Furthermore, the crystallization behavior and microstructures of derived ceramics were studied by XRD and SEM. Nano-sized boron carbide powders were prepared by pyrolysis of P(ND-co-D) under argon at 1400 °C for 2 h, while the structure and morphologies of the obtained rhombohedral B4C were investigated.

Novel laser rapidly solidified medium-entropy high speed steel coatings with enhanced hot wear resistance

The microstructure and hot wear resistance of a laser cladded novel medium-entropy high speed steel (ME-HSS) coating, by increasing the content of alloying elements Mo, V, Co, Ni and Al in a commercialized W6Mo5Cr4V2 (M2) HSS, are studied here. The results show that the newly designed ME-HSS coating has less amount of undesired retained austenite, and enhanced secondary hardening effect with more nano-size M2C carbide precipitates after tempering, in comparison with the M2 coating. In particular, the tempered ME-HSS coating exhibits excellent oxidative wear resistance at 500 °C because of the high content of anti-oxidation elements like Co, Ni and Al, leading to the formation of a more continuous and compact oxidation film on its worn surface.

Characterization of Fe49Cr18Mo7B16C4Nb6 high-entropy hardfacing layers produced by gas tungsten arc welding (GTAW) process

An iron based glassy alloy with the nominal stoichiometry of Fe49Cr18Mo7B16C4Nb6 was deposited on a carbon steel substrate by gas tungsten arc welding (GTAW) process. Effects of the number of weld layers and weld heat input were studied. Optical microscope (OM), scanning electron microscope (FE_SEM) and X-ray diffraction analysis (XRD) were used to characterize the microstructure and phase distribution. Hardness and wear behavior of the deposited layers were also investigated. The results showed that a multi-phase structure consisting of hard boride (Mo2FeB2) and nano-sized carbide (NbC) phases were formed in a nanostructured α-Fe matrix. The ratio of hard phases and subsequently hardness of layers were increased by increasing the number of layers or by decreasing the heat input, leading to hardness values in excess of 1000 HV. The wear resistance of the hardfaced layers was found to be considerably higher than that of the carbon steel substrate while the dominant wear mechanisms were cutting and ploughing accompanied by delamination and tearing in some regions.

Effect of solid carburization on the surface microstructure and mechanical properties of the equiatomic CoCrFeNi high-entropy alloy

The surface modification of the equiatomic CoCrFeNi high-entropy alloy has been studied via solid carburization at 920 °C for 10 h. Microstructure, hardness and wear resistance were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), microhardness and nanoindentation tester. Microstructure and phase analysis indicated that two types of nano-size carbide precipitates (M7C3 and M23C6) formed in the surface. The orientation relationship between the M7C3 precipitate and FCC matrix were (1(_)100)C1//(11(_)1)M and [0001]C1//[1(_)12]M. The M23C6 precipitate possessed a cube-cube orientation relationship with the FCC matrix. Vickers hardness and nanoindentation testing results indicated that the carburized surface performed significant improvement of about 100% in hardness compared with the matrix. The strengthening mechanisms including solid-solution strengthening and precipitate strengthening were studied. Moreover, nano-scratch experiments have been conducted on the cross section of the carburized sample under a constant load mode to investigate the friction and wear behavior. The surface profile and depth of scratched tracks were examined using scanning probe microscopy (SPM) technology. The wear and deformation mechanisms at different distances from the surface have been discussed. The calculated values of the wear volume, wear rate, and wear resistance coefficient reveal that the carbide precipitates can improve the wear resistance of the carburized surface.

Enhancing the interface bonding in carbon nanotubes reinforced Al matrix composites by the in situ formation of TiAl3 and TiC

Achieving effective load transfer at the interface between carbon nanotubes (CNTs) and aluminum (Al) is a crucial issue for fabricating high-performance CNTs reinforced Al matrix (CNT/Al) composites. In this work, CNT/Al composites with different Ti additions and the compared materials were prepared by powder metallurgy. Micro-sized Ti particles in which CNTs are well-dispersed firstly circumvent the difficulty of CNT dispersion, and subsequently act as nucleation site for sandwiched TiAl3 layers that lock the dispersed CNTs in place and improve the CNT-Al interface bonding. Additionally, Ti addition not only allows modification of the dispersed CNTs but also enhances the strength of the composites by enhancing the load-bearing capacity of the CNTs through in situ formation of nano-sized titanium carbide (TiC). This work provides a new approach to improve the load transfer efficiency of CNTs by strengthening the interface bonding for fabricating high strength CNT/Al composites.

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.

Production and Characterization of WC-Reinforced Co-Based Superalloy Matrix Composites

Cobalt-based superalloy matrix composite materials were produced through the powder metallurgy technique using element powders at high purity and nano-sized wolfram carbide (WC) reinforcement in this study. An alloy that had the same chemical composition as the Stellite 6 alloy but not containing carbon was selected as the matrix alloy. The powder mixtures obtained as a result of mixing WC reinforcing member and element powders at the determined ratio were shaped by applying 300 MPa of pressure. The green components were sintered under argon atmosphere at 1240 °C for 120 minutes. The densities of the sintered components were determined by the Archimedes’ principle. Microstructural characterization was performed via X-ray diffraction analysis, scanning electron microscope examinations, and energy-dispersive spectrometry. Hardness measurements and tensile tests were performed for determining mechanical characteristics. The relative density values of the sintered components increased by increasing the WC reinforcement ratio and they could almost reach the theoretical density. It was determined from the microstructural examinations that the composite materials consisted of fine and equiaxed grains and coarse carbides demonstrating a homogeneous dispersion along the microstructure at the grain boundaries. As it was the case in the density values, the hardness and strength values of the composites increased by increasing the WC ratio.

In situ neutron diffraction in quantifying deformation behaviors of nano-sized carbide strengthened UFG ferritic steel

The microstructures and mechanical properties of a low-alloy medium-carbon steel with a duplex microstructure composed of nanoscale spheroidized carbides in ultrafine-grained (UFG) ferritic steel are examined. The average grain size of the studied steel is ~ 430 nm, and these grains coexist with numerous carbides. Neutron diffraction reveals that the intensity of (011) and (022) peaks for the UFG sample is significantly enhanced, suggesting that the (011)//RD texture is a result of the warm rolling process. The lattice parameter of UFG steel is smaller than that of a martensitic steel (M steel) counterpart, indicating a lower carbon concentration in the lattice. The estimated dislocation densities for M steel and UFG steel are 2.59 × 1014 cm−2 and 1.76 × 1012 cm−2, respectively. The UFG steel reveals a nearly isotropic lattice strain response under initial tension from 0 to 450 MPa, where the lattice strains of the (110), (002), and (112) planes are identical. The increase of lattice strain of the (110) plane becomes smaller than that of the (002) and (112) planes as the stress exceeds 450 MPa, suggesting that the nanosized carbides contribute to the hardening ability by promoting the accumulation of geometrically necessary dislocations around the particles, and the (110) lattice becomes harder compared to the other two planes.

Enhancing thermal and mechanical response of aluminum using nanolength scale TiC ceramic reinforcement

In the present work, nano-sized titanium carbide (0.5, 1.0 and 1.5 vol%) reinforced aluminum (Al) metal matrix composites were synthesized by powder metallurgy incorporating microwave sintering and hot extrusion. Microstructural, mechanical and thermal properties of hot extruded unreinforced aluminum and titanium carbide (TiC) reinforced aluminum composites are presented in this paper. X-ray diffraction (XRD) patterns and scanning electron microcopy (SEM) images show the homogeneous distribution of TiC nanoparticles in the Al matrix. The tensile and compressive strengths of Al composites increased with the increase in TiC content, while the ductility decreased. The CTE of Al composite decreased with the progressive addition of hard TiC nanoparticles. Overall, hot extruded Al 1.5 vol% TiC nanocomposite exhibited the best combination of tensile, compressive, hardness and Young's modulus of 186 ± 3 MPa, 416 ± 4 MPa, 9.75 ± 0.5 GPa and ~103 GPa, respectively. High tensile strength and good thermal stability exhibited by Al-TiC nanocomposites developed in this study show the potential for a variety of weight-critical engineering applications.

Effects of Ti and Ta addition on microstructure stability and tensile properties of reduced activation ferritic/martensitic steel for nuclear fusion reactors

The effects of Ti and Ta addition on microstructure stability and tensile properties of a reduced activation ferritic/martensitic (RAFM) steel have been investigated. Ti addition of 0.06 wt% to conventional RAFM reference base steel (Fe-9.3Cr-0.93W-0.22V-0.094Ta-0.1C) was intended to promote the precipitation of nano-sized (Ti,W) carbides with a high resistance to coarsening. In addition, the Ti addition was substituted for 0.094 wt% Ta. The Ti-added RAFM steel (Ti-RAFM) exhibited a higher yield strength (ΔYS = 32 MPa) at 600 °C than the reference base steel due to additional precipitation hardening by (Ti,W)-rich MX with an average size of 6.1 nm and the area fraction of 2.39%. However, after thermal exposure at 600 °C for 1000 h, this Ti-RAFM was more susceptible to degradation than the reference base steel; the block width increased by 77.6% in Ti-RAFM after thermal exposure while the reference base steel showed only 9.1% increase. In order to suppress diffusion rate during thermal exposure, the large-sized Ta element with low activation was added to Ti-RAFM. The Ta-added Ti-RAFM steel exhibited good properties with outstanding microstructure stability. Quantitative comparison in microstructures was discussed with a consideration of Ti and Ta addition.

Research of nanoscale titanium carbides influence on the structure and properties of weld metal in automatic submerged arc welding

The influence of nanoscale refractory titanium carbide particles on the structure and properties of weld metal in automatic submerged arc welding is considered. Composite granules based on nickel were used to introduce the compound into the composition of the weld pool. Two schemes for introducing granules into the weld pool were tested, characterized by different temperature conditions: to the head part of the welding pool with the help of “ligature” and to the tail section with the help of additional filler wire. The prospects of introducing nano-sized titanium carbide into the tail part of the weld pool as part of a flux-cored wire are shown. With this method, the structure of the weld metal is observed to modify: the average size of the primary crystals of the weld metal is reduced by almost 50%. At the same time, the value of the toughness of the weld metal increases: the average value of this index increases by 36%. When titanium carbide is introduced as part of the “ligature” into the head of the weld pool, despite the effect of modifying (reducing the width of the primary crystals by 30%), the average value of the toughness of the weld metal decreases.

Polyoxometalate and Resin-Derived P-Doped Mo2 C@N-Doped Carbon as a Highly Efficient Hydrogen-Evolution Reaction Catalyst at All pH Values.

A new type of P-doped Mo2 C coated by N-doped carbon (P-Mo2 C@NC) has been successfully prepared by calcining a mixture of H3 [PMo12 O40 ] polyoxometalates (POMs) and urea-formaldehyde resin under an N2 atmosphere. Urea-formaldehyde resin not only serves as the carbon source to ensure carbonization but also facilitates the uniform distribution of POM precursors, which efficiently avoid the aggregation of Mo2 C particles at high temperatures. TEM analysis revealed that the average diameter of the Mo2 C particles was about 10 nm, which is coated by a few-layer N-doped carbon sheet. The as-prepared P-Mo2 C@NC displayed excellent hydrogen-evolution reaction (HER) performance and long-term stability in all pH environments. To reach a current density of 10 mA cm-2 , only 109, 159, and 83 mV were needed for P-Mo2 C@NC in 0.5 m H2 SO4 (pH 0), 0.1 m phosphate buffer (pH 7), and 1 m KOH (pH 14), respectively. This could provide a high-yield and low-cost method to prepare uniform nanosized molybdenum carbides with highly efficient and stable HER performance.

Influence of tempering treatment on microstructure and pitting corrosion of 13 wt.% Cr martensitic stainless steel

Tempering of 13 wt.% Cr martensitic stainless steel resulted in nano-sized M3C carbides at 300 °C, nano-sized Cr-rich M23C6 carbides at 550 °C and sub-micron sized Cr-rich M23C6 carbides at 700 °C. Austenitization resulted in lath martensite with undissolved M23C6 carbides. Pitting resistance for tempered condition was lower than the austenitized condition with least resistance at 550 °C. The observation was attributed to the presence of a Fe-rich surface film and massive carbide precipitation with a Cr depletion zone of 7–9 nm at carbide interface for 550 °C tempered condition as opposed to a Cr-enriched passive film for the austenitized condition.

Structure and stability of hcp iron carbide precipitates: A first-principles study.

Hexagonal close-packed (hcp) iron carbides play an important role in steel processing and in steel products. The recent discovery of novel ultrafine (2–5 nm) iron carbide (ε'-Fe2+xC) precipitates in TRIP steel sheds a new light on the hcp family of carbides. Here we present a first-principles study on the relative stability, and the electronic, magnetic properties of the ε'-Fe2C phases. Different stackings of Fe-sheets and orderings of C atoms were investigated and compared with experimental data and with Jack’s model. We find very favorable formation enthalpies for these new members of the hcp family, and we present a first-principles-refined model for the crystal structure of the ultrafine Fe(C) precipitates. These findings are useful for the characterization of nano-sized iron carbide precipitates, for understanding their role in the microstructure of steels, and for the design of novel steels having even more desirable properties.

Influence of the WC grain size on the properties of PVD/HVOF duplex coatings

Duplex coatings are an innovative approach to further improve cemented carbide surfaces in terms of tribo-mechanical properties by depositing a thin Physical Vapor Deposition (PVD) layer on top of the High Velocity Oxygen Fuel (HVOF) sprayed coating. Within this investigation, AISI M3 steel substrates were primarily coated with three different WC-12Co feedstock powders by means of HVOF spraying. The agglomerate size of all powders was constant (−45+15μm), the WC grain size was varied from conventional (carbide size 2.5μm) over fine (carbide size 1μm) to nanosized (carbide size 0.1μm) carbides. Subsequently, the samples were polished prior to the deposition of a CrAlN PVD coating by means of magnetron sputtering. The aim is to evaluate the influence of the WC grain size on the CrAlN/WC-12Co duplex coating properties.It was shown that a smaller carbide grain size influences the microstructure of the CrAlN coating and causes a finer crystalline structure. Furthermore, higher compressive residual stresses occur in the duplex interlayer and the top layer when using nanosized carbides. Moreover, nanosized carbides in the HVOF coating are favorable in terms of layer adhesion, which was analyzed by means of HRC indent test and scratch test. Further enhancements are the increase in hardness and the reduction in roughness which was directly influenced by the WC grain size. Considering the obtained results, it is to conclude that a reduction of the interlayer WC grain size improves the performance of CrAlN/WC-12Co duplex coatings.

Microstructural characterization of TIG surface treating in Co-Cr-Mo-C alloy

The aim of the present study is to investigate the effect of surface melting using tungsten inert gas (TIG) welding process and rapid solidification on the microstructure and hardness of Ni-free biomedical Co–28Cr–5Mo-0.3%C alloy. The microstructural studies revealed that surface melting via TIG welding process results in uniform distribution of the nano-sized M23C6 carbides in the range of 10–100nm at γ-fcc/ε-hcp interface. It was found that the volume fraction of the athermal martensite plays a key role on the size of the finely dispersed nano-sized M23C6 carbides at γ-fcc/ε-hcp interface. The hardness of TIG-treated surface was found to be greater than as-cast alloy which could be attributed to the dispersion of the nano-sized carbides at γ-fcc/ε-hcp interface as well as higher volume fraction of athermal martensite.

Precipitation of Nano-Sized Carbides in a Ti-Mo Bearing Steel at a Low Transformation Temperature

While the role of Ti and Mo elements on precipitation strengthening in ferrite grains formed during austenite/ferrite transformation is very clear, some uncertainty still presents concerning influence of microalloying elements on bainite transformation. Therefore, the present study focuses on the precipitation behavior occurred in a Ti-Mo bearing steel during bainitic phase transformation under different heat treatment conditions, and the correlation of the precipitation behavior with hardness distribution. Through the present work, it is expected to achieve a better understanding of low-temperature precipitation behavior to assist metallurgists to find out the reason for maintaining a high hardness by longtime isothermal holding, which can provide insight to design a better quality steel product. Vickers hardness was measured from the 1C-2Ti-2Mo, 1C-2Ti-2Mo and 0.5C-1Ti-2Mo steels treated by isothermal holding at 550 oC for 5 to 60 min. The average Vickers hardness was in the range of 245 - 276, 290 - 335 and 220 - 245, respectively. Therefore, higher hardness can be obtained if the steel containing higher carbon and microalloying elements can form precipitations in the ferrite matrix. On the other hand, increasing Vickers hardness with isothermal holding times indicates a good thermal stability character of complex carbides. The excellent thermal stability can be attributed to the addition of Mo element, which can inhibit the growth of carbides during longtime isothermal holding. Furthermore, the addition of Mo in the steel can avoid annihilation of dislocations during longtime aging. By taking advantages of these two effects, high strength can be achieved for high-strength low-alloy steels containing Mo element. Transmission electron microscopy image showed nano-sized carbides nucleated at dislocations, instead of interfacial precipitations within ferrite grain matrix, because the interface precipitation morphology only occurred accompanying the austenite decomposition reaction. However, the bainitic phase transformation was of a displacive transformation character, thus the complex carbides could not form during the bainitic phase transformation due to a very fast transformation velocity.