Vanadium Carbide Precipitation Research Articles

Influence of vanadium-carbide precipitation on the deformation behavior of Fe-30Mn-10Co-10Cr-2.5 V-1.5C multi-principal element alloy

Multi-principal element alloys represent a compelling avenue for the development of innovative alloys that have the potential to surpass conventional alloys in various applications, owing to the extensive compositional space they offer. For structural application though, this potential has not yet been realized, mostly because of hardly superior mechanical properties and elevated alloying costs. To overcome this hurdle, efforts have been made to increase mechanical properties using individual strengthening mechanisms, such as grain refinement, prior plastic deformation, precipitation hardening, and solid solution strengthening. In this study, we aim to combine multiple of these mechanisms into a singular alloy by adding V and C to the Fe30Mn10Co10Cr (at.-%) alloy. Using a custom calphad database, thermodynamic calculations were carried out to determine a suitable chemical composition which was subsequently cast and thermo-mechanically processed. Aging treatments above 700 °C for 4–24 h led to vanadium carbide precipitation, increasing the yield strength by up to 450 MPa to nearly 1GPa. Extensive aging times were necessary to facilitate precipitation, thereby indicating slow diffusion processes. A transition from twinning-induced to transformation-induced plasticity by vanadium carbide precipitation was observed and attributed to the reduction of C-content and stacking fault energy in the matrix as well as twin-precipitate interactions.

A strong and ductile ferrite-based lightweight steel manufactured via high-temperature warm rolling process

A trade-off between strength and ductility is unavoidable in ferrite-based lightweight steels due to the presence of δ-ferrite, and improving the properties of such steels requires precise control of the retained austenite and the complete mobilization of multiple strengthening mechanisms. In this study, we obtain a ferrite-based lightweight steel with the chemical composition Fe-0.3C-2.8Mn-3.5Al-0.5Cr-0.2V by adopting a higher temperature warm rolling process, endowing the best comprehensive properties. The results demonstrate that multi-scale heterogeneous microstructures are built without additional recrystallization annealing under the higher temperature (>700 °C) warm rolling process. Meanwhile, a large amount of retained austenite, higher dislocation density and strain-induced vanadium carbide precipitation were obtained through the warm rolling process. As warm rolling refines austenite grains, Mn and Cr are further diffused and enriched, enhancing the chemical stability of the retained austenite. After low-temperature tempering, this warm-rolled steel shows excellent comprehensive properties, including a tensile strength of 1140 MPa and elongation of 30%. The enhanced strength-ductility is attributed to the higher stability of retained austenite, stable maintenance of dislocation density and the pinning effect of precipitation relative to dislocation lines. Furthermore, a stronger discontinuous yielding behavior of the warm-rolled and tempered steel was obtained, enhancing the overall strain.

Phase-transformation and precipitation kinetics in vanadium micro-alloyed steels by in-situ, simultaneous neutron diffraction and SANS

In-situ Neutron Diffraction and Small-Angle Neutron Scattering (SANS) are employed for the first time simultaneously in order to reveal the interaction between the austenite to ferrite phase transformation and the precipitation kinetics during isothermal annealing at 650 and at 700 °C in three steels with different vanadium (V) and carbon (C) concentrations. Austenite-to-ferrite phase transformation is observed in all three steels at both temperatures. The phase transformation is completed during a 10 h annealing treatment in all cases. The phase transformation is faster at 650 than at 700 °C for all alloys. Additions of vanadium and carbon to the steel composition cause a retardation of the phase transformation. The effect of each element is explained through its contribution to the Gibbs free energy dissipation. The austenite-to-ferrite phase transformation is found to initiate the vanadium carbide precipitation. Larger and fewer precipitates are detected at 700 than at 650 °C in all three steels, and a larger number density of precipitates is detected in the steel with higher concentrations of vanadium and carbon. After 10 h of annealing, the precipitated phase does not reach the equilibrium fraction as calculated by ThermoCalc. The external magnetic field applied during the experiments, necessary for the SANS measurements, causes a delay in the onset and time evolution of the austenite-to-ferrite phase transformation and consequently on the precipitation kinetics.

Re-Melting Behaviour and Wear Resistance of Vanadium Carbide Precipitating Cr27.5Co14Fe22Mo22Ni11.65V2.85 High Entropy Alloy.

High entropy alloys (HEAs) are among of the most promising new metal material groups. The achievable properties can exceed those of common alloys in different ways. Due to the mixture of five or more alloying elements, the variety of high entropy alloys is fairly huge. The presented work will focus on some first insights on the weldability and the wear behavior of vanadium carbide precipitation Cr27.5Co14Fe22Mo22Ni11.65V2.85 HEA. The weldability should always be addressed in an early stage of any alloy design to avoid welding-related problems afterwards. The cast Cr27.5Co14Fe22Mo22Ni11.65V2.85 HEA has been remelted using a TIG welding process and the resulting microstructure has been examined. The changes in the microstructure due to the remelting process showed little influence of the welding process and no welding-related problems like hot cracks have been observed. It will be shown that vanadium carbides or vanadium-rich phases precipitate after casting and remelting in a two phased HEA matrix. The hardness of the as cast alloy is 324HV0.2 and after remelting the hardness rises to 339HV0.2. The wear behavior can be considered as comparable to a Stellite 6 cobalt base alloy as determined in an ASTM G75 test. Overall, the basic HEA design is promising due to the precipitation of vanadium carbides and should be further investigated.

Interaction of precipitation with austenite-to-ferrite phase transformation in vanadium micro-alloyed steels

The precipitation kinetics of vanadium carbides and its interaction with the austenite-to-ferrite phase transformation is studied in two micro-alloyed steels that differ in vanadium and carbon concentrations by a factor of two, but have the same vanadium-to-carbon atomic ratio of 1:1. Dilatometry is used for heat-treating the specimens and studying the phase transformation kinetics during annealing at isothermal holding temperatures of 900, 750 and 650 °C for up to 10 h. Small-Angle Neutron Scattering (SANS) and Atom Probe Tomography (APT) measurements are performed to study the vanadium carbide precipitation kinetics. Vanadium carbide precipitation is not observed after annealing for 10 h at 900 and 750 °C, which is contrary to predictions from thermodynamic equilibrium calculations. Vanadium carbide precipitation is only observed during or after the austenite-to-ferrite phase transformation at 650 °C. The precipitate volume fraction and mean radius continuously increase as holding time increases, while the precipitate number density starts to decrease after 20 min, which corresponds to the time at which the austenite-to-ferrite phase transformation is finished. This indicates that nucleation and growth are dominant during the first 20 min, while later precipitate growth with soft impingement (overlapping diffusion fields) and coarsening take place. APT shows gradual changes in the precipitate chemical composition during annealing at 650 °C, which finally reaches a 1:1 atomic ratio of vanadium-to-carbon in the core of the precipitates after 10 h.

Precipitation process of vanadium carbide in M23C6 by atomic-scale configuration analysis

The effect of vanadium (V) on the nucleation of M23C6 and vanadium carbide (VC) precipitation in Fe–Cr–V alloy was studied by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy and thermodynamic calculations. The results revealed that the lattice constant of M23C6 decreased when V atoms diffused into M23C6, which resulted in a reduction in the strain energy and increase in the driving force for the nucleation of M23C6. The V-rich zone was also formed in M23C6 as the V content increased, thereby resulting in the precipitation of a new VC particle adjacent to M23C6 as well as a new orientation relationship (OR), specifically $$ (1\bar{1}1)_{\text{VC}} //(2\bar{4}2)_{{{\text{M}}_{23} {\text{C}}_{6} }} $$ and $$ [011]_{\text{VC}} //[111]_{{{\text{M}}_{ 2 3} {\text{C}}_{ 6} }} $$ . Based on the OR, the formation process of VC in M23C6 was discussed by atomic-scale structure analysis.

Influence of Vanadium Microalloying on Deformation-Induced Pearlite Transformation of Eutectoid Steel

In order to investigate the influence of vanadium microalloying on deformation-induced pearlite transformation (DIPT) of eutectoid steel, thermomechanical simulation tests were carried out in this study. The following four compositions of vanadium microalloying were applied in the tests: vanadium free in Steel A, vanadium content of 0.1 mass% in Steel B, vanadium content of 0.27 mass% in Steel C, and vanadium content of 0.1 mass% with the addition of 0.02 mass% N in Steel D. The dissolution of vanadium and precipitation of vanadium carbides, nitrides, or carbonitrides and the effect of vanadium microalloying on the fraction and morphology of deformation-induced pearlite for different magnitudes of strain were examined, and the mechanism of the effect was elucidated. The results revealed that DIPT could be significantly improved by vanadium microalloying with the addition of N but decreased and postponed without the addition of N because vanadium nitrides or carbonitrides were precipitated in austenite under a small strain and facilitated the nucleation of pearlite both along the boundary of austenite grain (AG pearlite) and intragranular (IG pearlite). Moreover, transformation kinetics of DIPT was fitted and compared. The results further revealed that the rate of DIPT in vanadium-microalloyed steel with the addition of N was twice as fast as that in the vanadium-free steel. In order to ensure the complete spheroidization of lamellar cementites in vanadium-microalloyed steel, a comparison of the morphology of cementites revealed that a greater magnitude of strain was required.

Bake-partitioning in a press-hardening steel

Bake-partitioning, a process integrating paint-baking of car-body assembling to induce carbon partitioning into austenite, has been proposed for enhancing the ductility of press-hardening steels. Carbon is estimated to diffuse a maximum distance of 2.7 nm in austenite at 170 °C for 20 min. Austenite films with a thickness of approximately 5 nm are therefore desired for sufficient bake-partitioning. This was realised by the ultra-fine prior austenite grain due to 7 wt% of manganese addition combined with vanadium carbide precipitation. The novel alloy exhibits 1800 MPa ultimate tensile strength combined with 16% elongation, which has never been achieved in press-hardening steels.

Effect of hydrogen on the fatigue crack growth rate of quenched and tempered CrMo and CrMoV steels

In order to select the most appropriate steel to deal with pressurized hydrogen over long periods of time, the fatigue crack propagation rate of quenched and tempered CrMo and CrMoV steel grades was assessed by means of tests performed on thermally pre-charged specimens in a hydrogen reactor at 195 bar and 450 °C during 21 h. Cylindrical samples were used to measure the hydrogen content and their desorption kinetics at room temperature and compact tensile specimens to determine the fatigue crack growth rate. Under the aforementioned pre-charging conditions, significant amounts of hydrogen were introduced, being much larger in the CrMoV steel grades, which also have much lower apparent diffusion coefficients, as precipitation of fine vanadium carbides during tempering provides strong hydrogen traps. Moreover, the fatigue crack growth rate increased significantly due to the presence of internal hydrogen in the CrMo grades for test frequencies lower than 10 Hz in comparison with tests performed in air. Furthermore, the presence of vanadium carbides in the CrMoV steel significantly improved fatigue crack growth performance, the effective hydrogen diffusion distance per cycle and the hydrogen concentration in the process zone ahead of the advancing crack being considerably reduced.

Origin of hydrogen trapping site in vanadium carbide precipitation strengthening steel

We successfully achieved atomic scale visualization of hydrogen atoms at trapping sites associated with the nano-sized precipitates of titanium carbide and vanadium carbide in ferritic steels using a modified three-dimensional atom probe with a deuterium charge cell. We proposed that the hydrogen trapping sites of fine carbide precipitates were at the (001) broad interface between the precipitate and the ferrite matrix. In this study, the precipitate size dependence in the trapping site and its energy was systematically investigated in vanadium carbide precipitation strengthening steels with various aging times. Hydrogen thermal desorption spectrometry analysis showed that the hydrogen trapping energy in the peak- and over-aged steels was larger than that in the under-aged steel. Atom probe tomography analysis showed the {001} platelets of vanadium carbide were covered by charged deuterium atoms in the peak-aging steel with large trapping energy, whereas no deuterium atoms around the {001} platelets were observed in the under-aged steel with small trapping energy. High-resolution transmission electron microscopy observation showed that misfit dislocations hardly appeared on the (001) surface of the precipitates in the two steels with large and small trapping energies. In contrast, the vanadium–carbon atomic ratios of vanadium carbide precipitates were definitely different between the two steels with large and small trapping energies. The precipitates in the under-aged steel with small trapping energy showed a chemical composition similar to VC, whereas the precipitates in the peak- and over-added steels with large trapping energy showed a chemical composition similar to V4C3. These results suggested that the origin of the hydrogen trapping site with large trapping energy is not the misfit dislocation core but the carbon vacancy on the (001) broad surface of V4C3 precipitates.

Dynamic energy absorption of ultrafine-grained TWIP steel under axial impact loading

This research focuses on studying the dynamic energy absorption property of a micro-alloyed TWIP steel, which was proposed to act as a connection part between car front bumper and chassis in middle-class cars for vehicle safety. The studied TWIP steel was designed based on stacking fault energy of 25mJ/m2. The as-cast steel was deformed in hot and cold rolling to 2 mm thick sheets. Subsequently, recrystallization annealing was applied to the heavily cold-worked steel at different temperatures to obtain different ultrafine grain structures. The mechanical properties were determined using tensile tests. It was observed that at 900°C, the optimal temperature for strengthening by vanadium carbide precipitation, it was too low to complete recrystallization. However, at 1000°C, an ultrafine-grained structure was formed with high yield, tensile strengths and elongation of about 700MPa, 1100MPa and 30% respectively. Accordingly, TWIP steel was used for crash analysis simulation using ANSYS Workbench R14.5. Thin-walled square columns of that steel were employed for energy absorbance during a collision regarding progressive plastic deformation. The crashworthiness criteria were studied under different impact conditions with thicknesses of 0.25, 0.5, 0.75 and 1 mm. Simulation results showed high initial peak force during the impact. Hence, a trigger mechanism of an external tapered plunger was proposed to reduce it. This combination of new material and innovative design promises enhancement car safety.

Effect of two-step intercritical annealing on microstructure and mechanical properties of hot-rolled medium manganese TRIP steel containing δ-ferrite

The microstructure-properties relationship, work-hardening behavior and retained austenite stability have been systematically investigated in a hot-rolled medium manganese transformation-induced-plasticity (TRIP) steel containing δ-ferrite subjected to one-step and two-step intercritical annealing. The steel exhibited tensile strength of 752MPa and total elongation of 52.7% for one-step intercritical annealing at 740°C, tensile strength of 954MPa and total elongation of 39.2% in the case of intercritical quenching at 800°C and annealing at 740°C. The austenite obtained by two-step annealing mostly consists of refined lath structures and increased fraction of block-type particles existing at various kinds of sites, which is highly distinguished from those characterized by long lath morphology and small amounts of granular shape in one-step annealed samples. In spite of a higher C and Mn content in austenite and finer austenite laths, two-step annealing can lead to an active and continuous TRIP effect provided by a mixed blocky and lath-type austenitic structure with lower stability, contributing to a higher UTS. In contrast, one-step annealing gave rise to a less active but sustained TRIP effect given by the dominant lath-like austenite having higher stability, leading to a very high elongation. The further precipitation of vanadium carbides and the presence of both dislocation substructure and fine equiaxed grain in ferrite matrix facilitate the increase of yield strength after double annealing.

Hot forging behaviour of medium carbon and microalloyed steel: a comparative study

The present work aims at studying and comparing the effect of forging variables on mechanical properties of two grades of steel, namely, medium carbon steel and medium carbon steel micro-alloyed with vanadium. Forging was done in flat dies of 150 ton hydraulic press at a temperature of 1000 ± 50°C followed by cooling in normal air, forced air and quenching oil mediums. Microstructures and mechanical properties were evaluated by optical microscopy, charpy impact, vickers hardness, yield and tensile tests before and after forging up to 40% reduction. Comparison of results shows that micro-alloyed steels had higher strength and hardness than medium carbon steel for all cooling conditions. Oil quenching leads to the formation of relatively fine ferrite and pearlite in the medium carbon steel whereas, martensite, bainite with precipitation of vanadium carbides and carbonitrides were present in the matrix of ferrite-pearlite in microalloyed steel.

Hot forging behaviour of medium carbon and microalloyed steel: a comparative study

The present work aims at studying and comparing the effect of forging variables on mechanical properties of two grades of steel, namely, medium carbon steel and medium carbon steel micro-alloyed with vanadium. Forging was done in flat dies of 150 ton hydraulic press at a temperature of 1000 ± 50°C followed by cooling in normal air, forced air and quenching oil mediums. Microstructures and mechanical properties were evaluated by optical microscopy, charpy impact, vickers hardness, yield and tensile tests before and after forging up to 40% reduction. Comparison of results shows that micro-alloyed steels had higher strength and hardness than medium carbon steel for all cooling conditions. Oil quenching leads to the formation of relatively fine ferrite and pearlite in the medium carbon steel whereas, martensite, bainite with precipitation of vanadium carbides and carbonitrides were present in the matrix of ferrite-pearlite in microalloyed steel.

Microstructural evolution, coarsening behavior of vanadium carbide and mechanical properties in the simulated heat-affected zone of modified medium manganese steel

The microstructural evolution, coarsening behavior of vanadium carbide (VC) and the mechanical properties of the simulated heat-affected zone (HAZ) of modified medium manganese steel (MMMS) were investigated as a function of peak temperature (Tp). Cementite precipitated at the grain boundary at 650°C–850°C, and the formation of a carbide network was not observed even at a higher Tp. The nucleation of VC precipitates occurred at a Tp of 650°C, and with increasing Tp, they grew at a rate that is greater than the theoretical value calculated according to the volume diffusion-controlled mechanism. However, coarse VC particles disappeared when the Tp reached 1250°C, and VC particles less than 2.0nm precipitated during cooling. The lattice parameter of austenite in the simulated HAZ first decreased with VC precipitation, then increased with VC dissolution and finally decreased again at 1250°C due to VC regeneration. Meanwhile, the formation of the VC nanoparticles increased the microhardness and tensile strength of the simulated HAZ.

Evaluation of Solid-Solution Hardening of Fe-27 at. pct Al by Vanadium and Comparison to Precipitation Strengthening by Vanadium Carbides

The compressive yield stress of Fe-27Al-xV(-C) (x = 0 to 4 at. pct) at 1073 K (800 °C) has been determined. The increase of the yield stress of Fe-Al by increasing vanadium content is explained by solid-solution hardening. The experimentally observed values of the yield stress at 1073 K (800 °C) are compared with the strengthening given by theories evaluating the interaction between solute atoms and dislocations. The experimental results fit well the increase of the yield stress by the interaction of the solute atoms with screw dislocations. Further increase in yield strength in similar alloys due to vanadium carbides is documented. Precipitated carbides were identified by transmission electron microscopy and Kikuchi patterns. Although precipitation of vanadium carbides increases the compressive yield stress, they also could result in premature failure in tension as their highly anisotropic shape may facilitate crack nucleation.

Impact–abrasion wear characteristics of in-situ VC-reinforced austenitic steel matrix composite

In this investigation, in-situ precipitation of vanadium carbides was employed to reinforce Fe–13Mn and Fe–13Mn–3W alloys by means of conventional melting and casting route. Microstructures were characterized by optical and scanning electron microscopy techniques.Mechanical properties of the materials were determined by hardness, impact toughness and tension tests. It was observed that tungsten improved the strength of the matrix and the reinforcements as well as tensile properties and work hardening rate of the VC-reinforced composite. Ball mill abrasion test was utilized to simulate impact–abrasion wear condition using two types of abrasive minerals. The results showed that the degree of benefit to be gained by the use of in-situ VC-reinforced composite materials depends strongly on crush strength of the abrasives. It was found that the studied particle-reinforced composite materials were only advantageous when the abrasives were relatively soft, providing low-stress abrasion condition.

Hydrogen-Trapping Mechanisms in Nanostructured Steels

Nanoprecipitation-hardened martensitic bearing steels (100Cr6) and carbide-free nanobainitic steels (superbainite) are examined. The nature of the hydrogen traps present in both is determined via the melt extraction and thermal desorption analysis techniques. It is demonstrated that 100Cr6 can admit large amounts of hydrogen, which is loosely bound to dislocations around room temperature; however, with the precipitation of fine coherent vanadium carbide traps, hydrogen can be immobilized. In the case of carbide-free nanostructured bainite, retained austenite/bainite interfaces act as hydrogen traps, while concomitantly retained austenite limits hydrogen absorption. In nanostructured steels where active hydrogen traps are present, it is shown that the total hydrogen absorbed is proportional to the trapped hydrogen, indicating that melt extraction may be employed to quantify trapping capacity.

Crystallographic Analysis of Proeutectoid Ferrite/Austenite Interface and Interphase Precipitation of Vanadium Carbide in Medium-Carbon Steel

To clarify the mechanism of interphase precipitation of vanadium carbide (VC) in a medium-carbon steel, orientation relationships (ORs) and plane orientations of ferrite/austenite interfaces were investigated. It was found that a large part of grain boundary ferrite holds near-K-S OR with at least one side of austenite adjacent to grain boundary regardless of V addition. By the V addition, a fraction of grain boundary ferrite holding near the K-S OR with both sides of austenite is decreased remarkably. Furthermore, only non-K-S ferrite/austenite interfaces migrate dominantly in the V-added alloy in contrast to the V-free alloy. Ferrite/austenite interface orientations are not fixed crystallographically but are randomly distributed in terms of ferrite and austenite orientations. Those results do not agree with the ledge mechanism originally proposed by Honeycombe. Thus, it is proposed that the ledge mechanism is extended to the non-K-S interface, which partially consists of coherent and less-mobile interfaces.

Kinetics and crystal geometry of precipitation of vanadium carbides at the interphase boundary upon pearlitic transformation of steel

In high-carbon vanadium steel, the temperature range (600°C and higher) and time (∼0.1 s) of the formation of vanadium carbide particles 5–10 nm in size at the interphase boundary upon pearlitic transformation have been established. Their distribution in ferrite has been subjected to crystallographic analysis with allowance for the habit of cementite plates. Based on the results of this analysis, a model has been suggested for the volume distribution of vanadium carbide nanoparticles in ferrite interlayers between cementite plates of the pearlitic colony in the form of successive (periodic) corrugated spherical surfaces with a period of ∼20 nm. This particle distribution is determined by the shape of the moving pearlite-austenite interphase boundary and by the direction of growth of the pearlitic colony.