VC Particles Research Articles

Interface characteristics and solute segregation behavior on CLAM steel

Characteristics of matrix/matrix interface, i.e. grain boundary, and precipitate/matrix interface on China Low Activation Martensitic (CLAM) steel were analyzed by Atom Probe Tomography (APT), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM), as well as the solute segregation behaviors on those interfaces. It is found that the Cr, Mn, C, Si and P were enriched on the matrix/matrix interface, while the segregation of Fe, W and V were not obvious. The compositions of nano-size carbide were mainly C, Ta and V, and VC particle formed associated with the TaC and they are inter-soluble to form the (V, Ta)C, which exhibited a ball coronary morphology with maximum diameter about 40 nm. The results showed that the Cr, W, P and Si were more easily to segregate at the V-rich side, which might weaken the adhesive strength of interface and consequently result in intergranular fracture.

Super-high-strength and formable medium Mn steel manufactured by warm rolling process

In this paper, we demonstrate the design and manufacture of super-high strength medium Mn steel with good ductility, via the combination of warm rolling and V alloying. This new strategy could produce not only the extensive precipitation of fine VC particles in both ferrite and austenite but also the bimodal size distribution of retained austenite (RA) grains. A substantial increase of yield strength up to 650 MPa was achieved after the intercritical annealing of this V-alloyed medium Mn steel without deteriorating ductility; whereas the calculated precipitation strengthening using the classical Ashby–Orowan theory is no more than 400 MPa. Quantifying possible strengthening mechanisms revealed that dislocation strengthening should make an additional contribution because recovery in austenite could be strongly retarded by the nanosized precipitates. In this V-alloyed steel, the coarse RA grains firstly transformed to martensite during yielding at the much higher stress threshold, followed by a significant work hardening due to dislocations multiplication, twinning-induced-plasticity (TWIP) and transformation-induced-plasticity (TRIP) in the ultrafine RA grains. The best combination of strength and ductility included 1.5 GPa ultimate tensile strength and 28% total elongation. This was achieved due to the sustainable TRIP and TWIP effects in the later straining stage resulting from the ultrafine austenite grains, the latter were reversely transformed from the recrystallized ferrite grains.

Micromechanical behavior of a fine-grained China low activation martensitic (CLAM) steel

Micromechanical behavior of a fine-grained China Low Activation Martensitic (CLAM) steel under nanoindentation was studied in this work. The grain size of the as-prepared 0.1Ti-CLAM steel is ∼5 μm and the average diameter of the spherical precipitates is ∼5 nm. Both elastic modulus and hardness decrease with increasing contact depth of the nanoindenter, following an exponential decreasing function. The abnormally large contact depths should be resulted from defect concentration under the indenter. The effect of nanosized precipitates on hardness is responsible for the pop-ins occurring in the load-depth curves, corresponding to the blockage of nanosized precipitates to the dislocation movement. Nanosized VC and M23C6precipitates with the volume fractions of 0.32% and 1.21% can be identified, respectively. Different strengthening mechanisms originated from the two types of nanosized precipitates. The blockage of dislocations by VC particles leads to an Orowan strengthening whilst dislocations could cut through theM23C6particles because of the large size of the particles. The strengthening effects originated from the VC and M23C6 precipitates lead to the strength increase of ∼448 MPa and ∼254 MPa, respectively.

Design, preparation, microstructure and properties of novel wear-resistant stainless steel-base composites using laser melting deposition

We have designed and fabricated a novel wear-resistant stainless steel-based composites using laser melting deposition (LMD) in current study. The main composition of as-deposited specimens were α-Fe, VC, Cr7C3, and Cr23C6. Based on the microstructure observation and thermodynamics calculations, in situ synthesized VC particles provided sufficient places for the heterogeneous nucleation of α-Fe and chromium carbides during the solidification process of molten pool, which significantly refined the grains. The average microhardness of specimen was in range of 521 ± 9 HV - 603 ± 12 HV. The specimen with 12 wt % V content exhibited the highest wear resistance among the tested specimens as indicated by the lowest average specific wear rate of 5.011 × 10−6 mm3/N m.

Difference in Local Deformation and Ductile Fracture Behaviors between Hard VC and Soft Cu Particle Dispersion Ferritic Steels

Difference in Local Deformation and Ductile Fracture Behaviors between Hard VC and Soft Cu Particle Dispersion Ferritic Steels

Super-High-Strength and Formable Medium Mn Steel Manufactured by Warm Rolling Process

In this paper, we demonstrate the design and manufacture of super-high strength medium Mn steel with good ductility, via the combination of warm rolling and V alloying. This new strategy produced not only the extensive precipitation of fine VC particles in both ferrite and austenite but also the bimodal size distribution of retained austenite (RA) grains. A substantial increase of yield strength up to 650 MPa was achieved after the intercritical annealing (IA) of this V-alloyed medium Mn steel without deteriorating ductility; whereas the calculated precipitation strengthening using the classical Ashby-Orowan theory is no more than 400 MPa. Quantifying possible strengthening mechanisms revealed that dislocation strengthening should make an additional contribution because recovery in austenite could be strongly retarded by the nanosized precipitates. In this V-alloyed steel, the coarse RA grains firstly transformed to martensite during yielding at the much higher stress threshold, followed by a significant work hardening during successive deformation due to dislocations multiplication, twinning-induced-plasticity (TWIP) and transformation-induced-plasticity (TRIP) in fine RA grains. The best achieved combination of tensile properties included 1.8 GPa ultimate tensile strength, 1.0 GPa yield strength and 18% total elongation.

Using differential scanning calorimetry to characterize the precipitation and dissolution of V(CN) and VC particles during continuous casting and reheating process

Abstract

On the origin of the improvement of shape memory effect by precipitating VC in Fe–Mn–Si-based shape memory alloys

We studied the role of VC precipitation in improving the shape memory effect (SME) of the as-solution treated Fe–Mn–Si-based shape memory alloys by examining the microstructures developed during aging and deformation using transmission electron microscopy and electron channeling contrast imaging. Our results suggest that VC particles are not the only product of aging. Upon aging at 650 °C, the precipitation of VC particles is accompanied by the formation of profuse dislocations (2.26 ± 0.098 × 1013 m−2). In this case, the SME is not improved compared to the as-solution treated reference state. Upon aging at high temperatures (700–900 °C), a number of stacking faults are formed accompanying the VC precipitation and the SME is significantly improved, where the recovery ratios reach almost twice that of the as-solution treated state (<50%). For these high-temperature aged states, in situ straining experiments reveal that the stacking faults rather than the VC particles play an important role in the stress-induced martensitic transformation, leading to the formation of very thin (<3 nm) martensite plates with a single crystallographic variant within each grain. These martensite plates are in contrast to the very thick (from tens to hundreds of nanometers) and multi-variant martensite plates that prevail in the as-solution treated state. By comparing the characteristics of the martensite plates between the as-solution treated and the high-temperature aged states, the reasons for the improvement of SME by precipitating VC were discussed.

Phase evolution and wear resistance of in situ synthesized V8C7 particles reinforced Fe-based coating by laser cladding

In situ synthesized V8C7 reinforced Fe-based composite coating was successfully prepared on 35CrMo steel by laser cladding, aiming at improving the wear resistance. Constituent phases, microstructure, microhardness and wear resistance of the coating were investigated using XRD, SEM, microhardness tester and friction-wear tester. The results showed that the obtained coating was uniform and dense with no pores or cracks appearing, in addition to satisfied metallurgical bonding to the substrate. The coating was mainly composed of α-Fe, V8C7, Cr7C3, Fe3C and Mo2C phase. Fine V8C7 reinforced particles were in situ synthesized during the melting process. The shape of V8C7 particles changed from sphere to irregular blocks as a distance from the interface, which was due to the crash and binding of V8C7 particles with each other. The microhardness of coating was 775 HV0.3, which was about 4 times greater than the substrate. The wear resistance of the coating was significantly improved as indicated by the higher relative wear resistance.

Microstructural characterization of a V2C and V8C7 ceramic-reinforced Fe substrate surface compound layer by EBSD and TEM

Microstructural characterization of a vanadium carbide (V2C and V8C7) surface compound layer was performed by electron backscatter diffraction (EBSD), scanning electron microscopy and transmission electron microscopy. The phase composition and phase distribution of the compound layer and the morphologies, grain sizes and crystallographic orientations of the vanadium carbides were investigated through elemental distribution and crystallographic analyses. The results indicated that the compound layer was composed of four distinct zones consisting of a compact and uniform V2C layer (thickness of 3.2 μm) over the topmost surface followed by a nonuniform V + V2C (2.4 μm) zone, a V8C7 layer (10.7 μm) and a relatively deep V2C + V8C7+α-Fe + Fe3C zone (23.2 μm). The V2C layer was found in the V-rich region, and the EBSD results revealed that this phase grew mainly along the {0001} and {11–20} orientations. However, the V8C7 phase appeared to be nearly random, which indicates that the growth of this phase had no specific orientation. The V2C and V8C7 phases exhibited low-angle grain boundaries, and their misorientation angle distributions were mainly concentrated within the range of 0–5°. Furthermore, nanoindentation tests demonstrated that the hardnesses of the V2C and V8C7 layers were 22.16 ± 0.52 and 32.42 ± 0.61 GPa, respectively. Examination of the vanadium carbide powders obtained by chemical extraction indicated that the V2C grains with a hexagonal structure exhibited equiaxial and short rod-like morphologies, and the V8C7 particles with a cubic structure possessed short rod-like and spherical morphologies.

Effects of austenitizing temperature on microstructure and mechanical property of a 4-GPa-grade PM high-speed steel

Effects of austenitizing temperature on the microstructure and mechanical property of a high-strength PM high-speed steel were systematically investigated using a series of heat treatments in the temperature range of 1070–1180 °C. Results showed that the optimal austenitizing temperature to obtain the desired combination of high compressive strength of 3838 MPa and high ductility of 29.7%, the optimal austenitizing temperature was 1180 °C. A maximum strength of 3870 MPa and a fracture strain of 22.7% was obtained by austenitized at 1120 °C. It is evident that the size distribution and carbide contents of both VC and Fe3W3C can be significantly changed by heat treatment and triple tempering resulted in slight decline in the amount of cracked carbides. Fractography showed that more broken particles of Fe3W3C carbide was observed than that of the VC particles in fractured specimens.

Corrosion Behavior of WC-Co Coatings with VC Additions Applied by Thermal Sprayed HVOF

The addition of nanostructured VC particles in coatings WC-Co improves the mechanical properties of cermet processed by the thermal spray technique HVOF. These coatings are used to protect components from wear, high temperature and corrosion. In the other hand these coatings are an alternative to replace chrome plating in aerospace industry. The objective was study the corrosion resistance of bimodal coatings WC-Co-VC applied by thermal spray HVOF with different concentration of VC on AISI 304. Bimodal mixtures were obtained from commercial micro and nanostructured powders. Then it was deposited on AISI 304 using a HVOF hybrid gun and robotized arm. The electrochemical characterization was performed by polarization resistance and potentiodynamic polarization curves. It was determined that the anodic polarization curves the coatings present a tendency to passivation; however, they were more pronounced in sodium hydroxide. Addition of nanostructured VC particles to the cermet WC-12Co didn’t affect its corrosion resistance.

Effect of Ti on the microstructure evolution and wear behavior of VN alloy/Co-based composite coatings by laser cladding

VN alloy/Co-based composite coatings modified by Ti was prepared on a mild steel using laser cladding. The effect of Ti addition on the microstructure and wear resistance of VN alloy/Co-based coatings was investigated using optical microscopy, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, micro hardness tester and wear tester. The results showed that TiN and VC phases were detected in the VN alloy/Co-based composite coatings after adding Ti, in addition to γ-Co, Cr23C6 and σ-FeV phases. Massive short rod-like dendrites, equiaxed grains and fine grains appeared in the composite coatings after adding Ti, and the cross-slip and jog of the dislocation disappeared. The microhardness of the composte coatings was increased significantly with the increase of additive Ti, and was improved by 21.6%, for 9.6wt.% Ti addition. The wear resitance of the composite coatings was firstly increased and thereafter decreased with the increase of additive Ti, and was improved by 25.9%, for 4.8wt.% Ti addition. The abrasive mechanism of the composite coatings after adding Ti was the abrasive wear. Thus, it can be concluded that adding Ti effectively promotes the formation of TiN and VC particles to improve the wear resistance of the composite coatings.

Grain Size and Adhesion Strength of the V&lt;sub&gt;8&lt;/sub&gt;C&lt;sub&gt;7&lt;/sub&gt; Coatings Produced &lt;i&gt;In Situ&lt;/i&gt;

The vanadium carbide (V8C7) coating was fabricated by in situ reaction which used gray cast iron and vanadium plate as raw materials providing carbon and vanadium sources, respectively. The microstructure, phases, and adhesion strength of V8C7 coating were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD) and a single scratch test. The XRD results showed that the coating consisted of V8C7 and α-Fe, with the peaks of (222), (400), (440), (622) and (444) of V8C7 phase were confirmed. Moreover, the average diameter (D) of the V8C7 particles in the range of 432~582nm was calculated on the basis of the Scherrer and Halder-Wagner equations. The critical load of interface between V8C7 and substrate was 98.3 N, which implied that the interface of V8C7 coating /substrate had excellent adhesion strength.

Development of Steel Microalloying System for Cold-Resistant Coiled Product Designated for Pipes Under CRC Conditions

are provided for a study of the effect of microalloying steel with niobium, titanium,, and vanadium on the level of mechanical properties and cold resistance of coiled rolled product for pipe purposes produced under conditions of the OMK-Stal casting and rolling complex (CRC). It is shown that with a standard nitrogen content in steel (less than 0.008%) under CRC conditions introduction into pipe steel of microadditions of vanadium does not provide strengthening for coiled rolled product by a precipitation hardening mechanism. Precipitation of vanadium carbide phase is difficult as a result accelerated strip cooling on the run-out table before winding strip on a coil and use of relatively low coiling temperatures under CRC conditions. Separation of VC particles after tempering at 650°C confirms retention of vanadium in solid solution. The presence of fine VN particles in steel with an increased nitrogen content may be explained by the fact the vanadium nitride phase may precipitate at higher temperatures compared with carbide. Research work has made it possible to assimilate production of cold-resistant coiled rolled product more than 10 mm thick of strength class up to K60 under CRC conditions without microalloying steel with vanadium.

Study on VC formation mechanism and the microstructure of Fe-VC composite

Iron matrix composite reinforced with VC reinforcement was produced through the synthesis reaction from FeV and C. VC formation mechanism was investigated using differential thermal analysis and X-ray diffraction analysis. The results show that FeV firstly decomposes into α-Fe and V, then V reacts with C to generate VC. The microstructure of Fe-VC composite was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the results indicate that the VC particles are distributed uniformly in the pearlite matrix, and the interface between VC reinforcement and pearlite matrix is clean. The abrasive wear test of Fe-VC composite was investigated with a MM-200 wear-test apparatus. The results show that as the load increases, the wear surface of Fe-VC composite is changed from the intermittent shallow grooves to long continuous deep grooves.

A novel method to synthesize submicrometer vanadium carbide by temperature programmed reaction from vanadium pentoxide and phenolic resin

Submicrometer vanadium carbide (V8C7) can be synthesized from V2O5 encapsulated in phenol resin. The precursor powders were produced by coating V2O5 powders with phenolic resin shells. Then the precursor powders were roasted at various temperatures from 1173K to 1473K for 2h in a flowing Ar atmosphere. The decomposed carbon from phenolic resin could be used to reduce V2O5. The samples obtained with different conditions were characterized by X-ray diffraction (XRD), Field-emission scanning electron microscope (FE-SEM), Transmission Electron Microscope (TEM), and Energy Disperse Spectroscopy (EDS). The results show that phase evolution sequences from V2O5 are V2O5→VO2→V2O3→V6C5→V8C7. The single phase of V8C7 can be prepared as the molar ratio of V2O5 to phenol resin is 1:1.2 at the temperature higher than 1373K. The prepared V8C7 particles are about 300nm in size.

Determination of processing maps for the warm working of vanadium microalloyed eutectoid steels

Microalloying of an eutectoid steel with V may facilitate formation of dispersed nano-scaled VC particles in the microstructure during thermomechanical processing or subsequent heat treatment. In this research, the constitutive flow behavior of vanadium microalloyed eutectoid steel has been investigated in the temperature range 620–770°C at strain rates in the range 0.01–10s−1. Microstructural characterization of the deformed specimens was conducted using SEM and EBSD techniques. In this context, various deformation mechanisms occurring during warm deformation have been characterized and delineated through construction of a processing map by establishing a power dissipation map and an instability map for the steel and superimposing them. The results show that the pearlitic microstructure exhibits several deformation mechanisms within these warm working conditions. Dynamic spheroidization of cementite lamella takes place in the range 660–720°C and 0.01–0.1s−1 with a power dissipation efficiency of 27–33%, characterizing a safe window of processing this steel. The presence of vanadium carbides at grain boundaries strengthened the pearlitic microstructure and retarded the occurrence of some deformation defects during low temperature, high speed deformation in the range 620–720°C and 1–10s−1.

Formation of VC-composite surface layer on high C–Cr bearing tool steel by laser surface cladding

YAG fiber laser was used to clad high C–Cr bearing tool steel with VC particles. The cladding process was carried out at laser powers of 1000, 1500, and 2000W, and a fixed travelling speed of 4mm/s. Argon gas was used as a shielding during and after laser cladding at flow rate of 15l/min. VC-surface composite layers consists of VC dendrites/particles dispersed in a matrix of martensite lathes, acicular carbides and some retained austenite were formed through all the laser processing conditions. The depth of the treated zones is in direct proportion to the laser power. On the other hand, the laser power has no great effect on the treated zone width. Some of the VC particles are melted and then re-solidified in the form of fine dendrites during the laser processing. The amount of these melted and re-solidified VC particles were increased by increasing the laser power. The formed composite layers show good metallurgical bonding with the substrate without cracking, except at laser power of 1000W. The hardness values of the build-up zone in the three different power cases are almost three times as that of the base metal. Decreasing laser power was found to increase the hardness at/near the free surface of treated-zone. The wear resistance of the laser cladded surface was remarkably improved.

Preparation and mechanisms of cemented carbides with ultrahigh fracture strength

WC–Co cemented carbides were prepared by liquid-state sintering of in situ synthesized composite powders with a constant Co content but different carbon concentrations, and with different size scales of VC particles as grain-growth inhibitor. With an optimized carbon addition and doping with microscale VC particles, an ultrahigh fracture strength with a mean value above 5000 MPa was achieved for cemented carbides. By detailed crystallographic analysis of the configuration and interactions of the WC, Co and VC phases, the effects of VC particle size on the microstructure and mechanical properties of cemented carbides are identified. The mechanisms by which the fracture strength depends on the VC particle size contain the effects on the changes in Co binder distribution, atomic matching at the phase boundary and WC grain size. The dominant factors for ultrahigh fracture strength of cemented carbides are proposed.