Preparation process of high-carbon ferrochrome based on plasma spheroidization technology

Abstract

This invention discloses a preparation process for high-carbon ferrochrome based on plasma spheroidization technology, belonging to the field of powder metallurgy. The process involves crushing and ball milling high-carbon ferrochrome blocks to obtain pre-formed powder, adding tannic acid, urea, nickel acetate tetrahydrate, and citric acid for coating, followed by spray drying. After pre-reduction carbonization in an argon-hydrogen atmosphere, the powder is then spheroidized using a radio frequency inductively coupled plasma torch. In-situ nanostructure growth is achieved in a mixed atmosphere of argon, hydrogen, and acetylene. Nickel acetate is reduced to catalytically active elemental nickel nanoparticles under the action of hydrogen and carbon monoxide. The powder is then cooled, graded, and inertly encapsulated to obtain a high-sphericity, low-oxygen-content, high-carbon ferrochrome-based core-shell structured spherical powder. The process flow of this invention is continuous and controllable, effectively improving powder flowability, bulk density, hardness, and corrosion resistance, and is applicable to additive manufacturing, thermal spraying, and powder metallurgy forming.

Description

Technical Field

[0001] This invention belongs to the field of powder metallurgy technology, specifically, it relates to a preparation process of high-carbon ferrochrome based on plasma spheroidization technology. Background Technology

[0002] High-carbon ferrochrome (HC FeCr) is an important ferroalloy, mainly composed of chromium, iron, and carbon. Chromium typically accounts for 50-70% by mass, carbon 4-9% by mass, with the balance being iron and small amounts of impurities. It is the primary chromium source in stainless steel production and is widely used in metallurgy, wear-resistant materials, hardfacing, and surface coatings. Traditional high-carbon ferrochrome production primarily employs electric furnace or submerged arc furnace smelting processes, yielding blocky or granular materials. Subsequently, to meet downstream application demands, it is often crushed and ball-milled into powder form. Traditional powder preparation methods include mechanical crushing, air jet milling, water atomization, or gas atomization. Powders produced by these methods are often irregularly shaped, with a wide particle size distribution, rough surfaces, and numerous satellite spheres and sharp-angled structures. This irregular morphology results in poor powder flowability (Hall flow rate typically >25s / 50g) and low bulk density (<4.0g / cm³). 3 High-carbon ferrochrome powder is prone to dust generation and agglomeration during transportation, storage, and use. Furthermore, mechanical crushing and ball milling processes inevitably introduce oxygen contamination, often resulting in powder oxygen content as high as 0.1–0.5 wt%, severely impacting the mechanical properties and corrosion resistance of subsequent materials. In additive manufacturing (AM) fields, processes such as laser bed fusion (LPBF) and directed energy deposition (DED) place extremely stringent requirements on powders: high sphericity (>90%), good flowability (<20s / 50g), narrow particle size distribution (15–100μm), and low oxygen content (<0.05 wt%). Traditional irregular high-carbon ferrochrome powder struggles to meet these requirements, limiting its adoption in high-end applications.

[0003] In recent years, with the rapid development of additive manufacturing, thermal spraying, and powder metallurgy technologies, the demand for spherical metal powders has increased dramatically. Spherical powders possess excellent powder spreading uniformity, stable melting behavior, and densification capabilities, significantly improving the density, mechanical properties, and surface quality of manufactured parts. Spheroidization of high-carbon ferrochrome powder has become a hot research topic in the industry. Plasma spheroidization, as a highly efficient powder post-treatment method, is widely used for spheroidizing titanium alloys, stainless steel, refractory metals (such as tungsten, molybdenum, and tantalum), and high-temperature alloy powders. This technology utilizes a radio frequency inductively coupled plasma torch (RF-ICP) or DC arc plasma to generate a high-temperature plasma jet (>10000℃), causing irregular powder particles to rapidly melt during flight and naturally spheroidize due to surface tension, followed by rapid cooling and solidification. Compared to gas atomization or electrode induction melting gas atomization (EIGA), plasma spheroidization offers advantages such as process flexibility, suitability for small batches of high-value powders, high sphericity (>95%), fewer satellite spheres, and lower oxygen increment. Literature reports show that the sphericity of 316L stainless steel powder after plasma spheroidization can reach over 98%, and the flowability is improved to below 14s / 50g, significantly improving the tensile strength and elongation of additively manufactured parts.

[0004] However, applying plasma spheroidization technology directly to high-carbon ferrochrome powder presents a series of unique challenges. First, high-carbon ferrochrome has a high melting point (approximately 1500–1650 °C) and a high carbon content, making it prone to carbon volatilization or oxidation in a high-temperature plasma environment. This leads to component segregation and carbon loss, affecting the chemical stability of the powder. Second, chromium is highly reactive and readily reacts with residual oxygen to form Cr₂O. 3 For oxides, the oxygen content of powder is difficult to control below 0.05 wt%. Existing research shows that during conventional plasma spheroidization, an oxide layer often forms on the surface of high-carbon materials, reducing the powder's corrosion resistance and hardness. Furthermore, plasma loading capacity is limited; when the powder feed rate is too high, the particles are not sufficiently heated, leading to a decrease in spheroidization rate (<90%); while too low a rate results in low production efficiency. Parameters such as chamber pressure, carrier gas flow rate, and jacket gas design need to be precisely matched; otherwise, hollow powder or wall adhesion may occur. While there have been many optimizations for plasma spheroidization of refractory metals, such as preheating sections, stepped cooling channels, and spiral airflow trajectory designs, there are few reports on high-carbon ferrochrome, which possesses both high melting point and high reactivity. The process window is narrow, making stable large-scale production difficult.

[0005] Furthermore, high-carbon ferrochrome powder places even higher demands on surface properties for wear-resistant and corrosion-resistant applications. While simple spheroidization improves flowability, the hardness (approximately 800–1000 HV) and corrosion resistance of the matrix material itself remain limited, failing to meet the requirements of extreme working conditions (such as high-temperature oxidation, acidic media corrosion, and abrasive wear). Therefore, the industry has attempted to introduce reinforcing phases, such as carbides, nitrides, or nanostructured layers, through surface modification. Carbon nanotubes (CNTs) are considered ideal surface strengthening materials due to their excellent mechanical properties (Young's modulus > 1 TPa, tensile strength > 100 GPa), thermal conductivity, and chemical stability. CNTs coating metal powder can form a core-shell structure, significantly improving the powder's wear resistance, corrosion resistance, and thermal conductivity, while also improving interfacial bonding, exhibiting excellent performance in thermal spray coatings or additive manufacturing parts. Existing methods for CNTs coating metal powder mainly include chemical vapor deposition (CVD), electrodeposition, sol-gel, and mechanical mixing methods. CVD can catalytically grow CNTs on metal surfaces, but it requires a separate high-temperature furnace (800–1100℃), making the process complex and energy-intensive. Furthermore, it is sensitive to substrate shape and difficult to uniformly coat micron-sized spherical powders. Electrodeposition or sol-gel methods suffer from weak interfacial bonding, uneven CNT distribution, and easy agglomeration; subsequent heat treatment may also lead to substrate oxidation. While mechanical mixing is simple, CNTs are only physically adsorbed onto the metal powder, making them prone to detachment in practical applications and limiting the strengthening effect.

[0006] In the field of plasma, some studies have attempted to generate carbon layers or nanostructures in situ using the high-temperature environment of plasma. For example, there are reports of generating carbon-coated microspheres by laser heating nickel powder in ethanol medium, or introducing a carbon source to form a surface carbonized layer when plasma spheroidizing stainless steel powder. However, these methods are mostly for low-carbon or single metals, and the carbon structure is mostly amorphous carbon or graphite layer, making it difficult to form a carbon nanotube network. For carbon-containing matrices such as high-carbon ferrochrome, existing technologies have not yet achieved effective coupling between plasma spheroidization and in-situ catalytic growth of CNTs. The main difficulties are: (1) lack of uniformly dispersed catalyst nanoparticles (such as Fe, Ni, Co); (2) unstable carbon source supply, making it difficult to balance the solid-phase matrix carbon and the gas-phase carbon source; (3) rapid cooling process (10 4 ~10 6 At ℃ / s, the growth time of CNTs is extremely short (millisecond level), and nucleation and growth are difficult to control; (4) the process monitoring is insufficient, and it is impossible to provide real-time feedback to adjust the carbon structure quality (such as G / D peak ratio, radial breathing mode).

[0007] In summary, although plasma spheroidization technology and CNT surface modification have been extensively studied, there remains a significant technological gap in the integrated preparation of high-carbon ferrochrome spherical powders, particularly in the process of simultaneously depositing a carbon nanotube network shell during spheroidization. Existing methods either achieve spheroidization but lack sufficient surface properties, or the surface modification steps are cumbersome, costly, have weak interfaces, and are difficult to industrialize. This leads to a shortage of high-performance high-carbon ferrochrome-based core-shell structured powders, restricting their application in high-end additive manufacturing, wear-resistant coatings, and special metallurgy. Therefore, there is an urgent need to develop an efficient, green, and controllable preparation method that, through precise control of process parameters, can simultaneously achieve high sphericity, low oxygen content, and surface carbon nanotube network construction of high-carbon ferrochrome powders to meet the pressing performance requirements of downstream high-end applications. Summary of the Invention

[0008] To address the shortcomings of existing high-carbon ferrochrome powders, such as irregular particle morphology, poor flowability, high oxygen content, easy component segregation and surface oxidation during plasma spheroidization, and the complex processes, poor coating uniformity, weak interfacial bonding, and difficulty in achieving continuous integrated preparation of existing carbon nanotube surface modification methods, this invention provides a high-carbon ferrochrome preparation process based on plasma spheroidization technology to achieve stable preparation of high-carbon ferrochrome powder with high sphericity, low oxygen content, high surface activity, and excellent comprehensive properties.

[0009] The present invention adopts the following technical solution: a preparation process of high carbon ferrochrome based on plasma spheroidization technology, which includes the following steps by mass: (1) Raw material pre-processing: high carbon ferrochrome blocks containing 4.5-8.5% carbon by mass, 52-68% chromium by mass, and the balance being iron and unavoidable impurities are crushed and then subjected to planetary ball milling to obtain pre-processed powder A with a particle size of 15-120 micrometers; the ball milling uses zirconia balls with a diameter of 3-8 mm, the ball-to-material mass ratio is 6:1-12:1, the ball milling time is 3-10 hours, and high-purity argon gas is continuously introduced during the ball milling process at a flow rate of 1-4 liters / minute, and the oxygen content is controlled below 0.2% by volume; (2) Composite precursor coating: Add 3-9 parts by weight of tannic acid (CAS No. 1401-55-4), 1-4 parts by weight of urea (CAS No. 57-13-6), 0.05-0.8 parts by weight of nickel acetate tetrahydrate (CAS No. 6018-89-9), and 0.1-1.5 parts by weight of citric acid (CAS No. 77-92-9) to 100 parts by weight of pre-prepared powder A. Then add a mixed solvent of deionized water and anhydrous ethanol (CAS No. 64-17-5) in a volume ratio of 1:0.5 to 1:1.5 to make the solid-liquid mass ratio 1:0.4 to 1:0.9. First, ultrasonically disperse for 0.5-2 hours, then mechanically stir for 1-4 hours. After completion, spray drying is carried out. The spray inlet temperature is 140-260℃ and the outlet temperature is 80-120℃ to obtain composite powder B with a tannic acid complex layer on the surface; (3) Pre-reduction carbonization: Composite powder B is heated to 400-580℃ at 3-9℃ / min in a mixed atmosphere of argon and hydrogen (hydrogen gas fraction 8-25%) and kept at the temperature for 1-4 hours to obtain pre-reduction carbonized powder C; (4) Radio frequency plasma spheroidization and in-situ nanostructure growth: Pre-reduction carbonized powder C is axially fed into a radio frequency inductively coupled plasma torch at a powder feeding rate of 8-60 g / min. The working gas is a mixture of argon, hydrogen and acetylene (CAS No. 74-86-2). The gas mixture has an argon gas fraction of 75-94%, a hydrogen gas fraction of 5-20%, and an acetylene volume fraction of 1-5%. The total flow rate is 70-250 standard liters / minute, the torch power is 40-180 kilowatts, the cavity pressure is 8-60 kPa, and the carrier gas is pure argon gas with a flow rate of 8-40 standard liters / minute. The average residence time of the powder in the high-temperature zone is 3-40 milliseconds. After rapid cooling, high-carbon chromium iron-based core-shell nanostructured spherical alloy powder D is obtained. (5) Classification and inert packaging: Powder D is classified and sieved by airflow. Powder with a particle size of 20-100 micrometers is retained as the target product and sealed in an inert atmosphere with a dry argon gas fraction of ≥99.99%. The meaning of deionized water and anhydrous ethanol (solid-liquid mass ratio 1:0.4-1:0.9) is as follows: Add a mixed solvent of deionized water and anhydrous ethanol with a volume ratio of 1:0.5-1:1.5 to make the solid-liquid mass ratio 1:0.4-1:0.9.

[0010] Preferably, in step (1), 0.2 to 2.5 parts by weight of alumina nanopowder (CAS No. 1344-28-1) with an average particle size of 20 to 80 nanometers and 0.05 to 0.8 parts by weight of natural flake graphite (carbon content ≥99.5%, CAS No. 7782-42-5) are added to the high-carbon ferrochrome block before ball milling. The oxygen content is controlled to be below 0.1% by volume during ball milling so that alumina and graphite can synergistically improve droplet wettability and carbon nanotube nucleation density during subsequent plasma ball milling.

[0011] Preferably, in step (2), after the mixed solvent is added, tris(hydroxymethyl)aminomethane (CAS No. 77-86-1) is added first to adjust the pH to 8.0-9.0, and then ultrasonic dispersion and mechanical stirring are performed to allow tannic acid to coordinate and complex on the powder surface to form a tannic acid complex coating layer with a thickness of 50-300 nanometers; during spray drying, the atomizing disc speed is 8000-18000 rpm and the feed rate is 10-50 ml / min.

[0012] Preferably, in step (3) of the pre-reduction carbonization process, 2-15% carbon monoxide (CAS No. 630-08-0) by volume is added to an argon-hydrogen mixed atmosphere, the total pressure inside the furnace is controlled at 15-80 kPa, and the holding time is 1.5-3.5 hours, so that the sp2% carbon content in the carbon layer on the surface of the pre-reduction carbonized powder C is reduced. 2 The proportion of hybrid carbon atoms reaches 65-92%.

[0013] Preferably, in step (4), the powder inlet channel is provided with a resistance heating preheating section with a length of 100-400 mm and a preheating temperature of 300-800℃, so that the temperature of the pre-reduced carbonized powder C before entering the high-temperature zone reaches 200-700℃; the plasma torch sheath gas channel is provided with a tangential inlet, a tangential angle of 15-50°, and a sheath gas flow rate of 30-120 standard liters / minute, forming a spiral airflow to extend the particle trajectory, and the standard deviation of the plasma exposure time of a single particle is controlled within 2-15 milliseconds.

[0014] Preferably, in step (4), a stepped cooling channel with a length of 0.3 to 1.8 meters is provided from the plasma torch outlet to the cooling collection chamber, comprising: a first rapid cooling zone using radial multi-hole injection of helium (CAS No. 7440-59-7) at a flow rate of 15 to 180 standard liters / minute; a second slow cooling zone using axial injection of nitrogen (CAS No. 7727-37-9) at a flow rate of 30 to 250 standard liters / minute; and a third fine cooling zone using circulating water jacket cooling at a water temperature of 8 to 20°C, with the overall cooling rate controlled at 10. 4 ~10 6 ℃ / second.

[0015] Preferably, in step (4), a temperature-controlled rotating collection plate is set at the bottom of the collection chamber, with a rotation speed of 100 to 1500 rpm and an angle of 15 to 70° between the plate surface and the plasma axis. The collection chamber is maintained at a positive pressure of 20 to 100 kPa and an oxygen content of ≤0.03% by mass, so as to reduce the adhesion of molten droplets to the wall and inhibit excessive oxidation of the surface.

[0016] Preferably, in step (5), before grading, a cyclone separator and an electrostatic precipitator are connected in series to remove fine powder with a particle size <10 micrometers. The fine powder is returned to step (2) for re-coating via an independent pipeline. Coarse particles with a particle size >130 micrometers are returned to step (1) for re-ball milling to achieve closed-loop circulation particle size control.

[0017] Compared to existing technologies, this invention has the following significant advantages and innovations: By integrating raw material pre-processing, precursor coating, pre-reduction carbonization, radio frequency plasma spheroidization, in-situ carbon nanotube network growth, and hierarchical encapsulation into a single, coupled design, this invention not only significantly improves the sphericity of high-carbon ferrochrome powder, reduces oxygen content, and improves flowability and bulk density, but also simultaneously constructs a firmly bonded and uniformly distributed surface core-shell nanostructure during spheroidization, thereby effectively enhancing the powder's hardness, wear resistance, and corrosion resistance. Furthermore, by synergistically controlling the nickel-based catalytic precursor, carbon source system, and plasma thermal and cooling fields, this invention solves the problems of dispersed steps, weak interfacial bonding, and difficulty in continuous preparation in existing surface modification processes. This results in powders with both excellent processing and service performance, making them particularly suitable for special metallurgy, high-value-added wear- and corrosion-resistant component forming, localized wear-resistant repair layers for aerospace parts, thermal spray coating materials, and high-performance spherical powder raw materials for additive manufacturing. Attached Figure Description

[0018] Figure 1 This is a scanning electron microscope image of the pre-prepared powder A prepared in Example 1;

[0019] Figure 2 Here is a scanning electron microscope image of composite powder B prepared in Example 1;

[0020] Figure 3 This is the full X-ray photoelectron spectroscopy (XPS) spectrum of the pre-reduced carbonized powder C prepared in Example 1;

[0021] Figure 4 This is a high-resolution XPS peak fitting diagram of the C1s surface of the pre-reduced carbonized powder C prepared in Example 1;

[0022] Figure 5 This is the Raman spectrum of the pre-reduced carbonized powder C prepared in Example 1;

[0023] Figure 6 This is a transmission electron microscope (TEM) image of the carbon nanotube network layer on the surface of alloy powder D obtained in Example 1. Detailed Implementation

[0024] The present invention will be described in detail below through specific embodiments. However, the uses and purposes of these illustrative embodiments are only for illustrating the invention and do not constitute any limitation on the actual scope of protection of the invention, nor are they intended to limit the scope of protection of the invention to these embodiments. For parameter ranges not mentioned, intermediate values ​​are selected. Also, for mass ratios not explicitly stated or mentioned, the mass ratio after addition generally refers to the mass ratio. Furthermore, in this invention, the unit of mass is grams (g). The sphericity test is performed using a Malvern Morphologi 4 particle morphology analyzer, and the sphericity index is defined as: 4πA / P. 2 Where A is the projected area and P is the perimeter, and particles >0.9 are classified as spherical. The ball milling is performed under a protective atmosphere.

[0025] Example 1.

[0026] The preparation process of high-carbon ferrochrome based on plasma spheroidization technology in this embodiment includes the following steps: (1) Raw material preparation: Select high-carbon ferrochrome blocks with a carbon mass fraction of 6.5%, a chromium mass fraction of 60%, and the remainder being iron and unavoidable impurities. Add 1.35 parts by mass of alumina nanopowder (CAS No. 1344-28-1) with an average particle size of 50 nanometers and 0.4 parts by mass of natural flake graphite (carbon content 99.8%, CAS No. 7782-42-5). After crushing the mixture, place it in a planetary ball mill. The ball milling media are zirconia balls with a diameter of 5 mm and a ball-to-material mass ratio of 9:1. The ball milling time is 6.5 hours. During the ball milling process, high-purity argon gas (CAS No. 7440-37-1) is continuously introduced at a flow rate of 2.5 liters / minute. The oxygen content is controlled at a volume fraction of 0.08% by online monitoring with an oxygen analyzer. After ball milling, a pre-prepared powder A with a particle size distribution of 15-120 micrometers is obtained, such as Figure 1As shown. (2) Composite precursor coating: Add 6 parts by mass of tannic acid, 2.5 parts by mass of urea (CAS No. 57-13-6), 0.4 parts by mass of nickel acetate tetrahydrate (CAS No. 6018-89-9), and 0.8 parts by mass of citric acid (CAS No. 77-92-9) to 100 parts by mass (e.g., 1000g) of pre-prepared powder A. Add a mixed solvent of deionized water and anhydrous ethanol (CAS No. 64-17-5) in a volume ratio of 1:1 to make the solid-liquid mass ratio 1:0.65. After adding the mixed solvent, first add tris(hydroxymethyl)aminomethane (CAS No. 77-86-1) to adjust the pH to 8.5. First, perform ultrasonic dispersion for 1.25 hours, and then mechanically stir for 2.5 hours to allow tannic acid to coordinate and complex on the powder surface to form a tannic acid complex coating layer with a thickness of about 180 nanometers. After stirring, spray drying was performed using a high-speed centrifugal spray dryer with an atomizing disc speed of 13,000 rpm, a feed rate of 30 ml / min, a spray inlet temperature of 170℃, and an outlet temperature of 100℃, resulting in composite powder B with a tannic acid complex layer on the surface. Figure 2 As shown. The fine powder recovery system returns fine powder with a particle size <10 micrometers to this place for re-coating via an independent pipeline. (3) Pre-reduction carbonization: The composite powder B is placed in a tube furnace and treated in a mixed atmosphere of argon, hydrogen (CAS No. 1333-74-0) and carbon monoxide (CAS No. 630-08-0), wherein the hydrogen gas fraction is 16%, the carbon monoxide volume fraction is 8%, and the remainder is argon. The total pressure in the furnace is controlled at 47.5 kPa. The temperature is increased to 490℃ at 6℃ / min and held for 2.5 hours. During this process, nickel acetate tetrahydrate decomposes and forms nickel-based nanoparticles in situ. The surface coating layer undergoes pre-reduction and carbonization transformation to obtain pre-reduction carbonized powder C, whose XPS full spectrum is shown in the figure. Figure 3 As shown, the high-resolution XPS peak fitting diagram of its surface C1s is as follows: Figure 4 As shown, the Raman spectrum is as follows Figure 5 As shown. Detection revealed that sp in the carbon layer on the surface of powder C... 2The proportion of hybrid carbon atoms is 78%. (4) Radio frequency plasma spheroidization and in-situ nanostructure growth: The pre-reduced carbonized powder C is axially fed into the radio frequency induction coupling plasma torch through the powder feeder at a powder feed rate of 34 g / min. The powder feed channel is equipped with a resistance heating preheating section with a length of 250 mm and a preheating temperature of 550℃, so that the powder temperature reaches 450℃ before entering the torch. The working gas is a mixture of argon, hydrogen and acetylene, of which argon gas fraction is 85%, hydrogen gas fraction is 12%, acetylene volume fraction is 3%, and the total flow rate is 160 standard liters / min. The torch power is 110 kW and the cavity pressure is 34 kPa. The plasma torch jacket gas channel is equipped with a tangential inlet with a tangential angle of 32° and a jacket gas flow rate of 75 standard liters / min, forming a spiral airflow. The carrier gas is pure argon with a flow rate of 24 standard liters / min. The average residence time of the powder in the high temperature zone is 15 ms (standard deviation 4.5 ms). Next, a carbon nanotube network layer with a diameter of approximately 20 nanometers and a length of approximately 250 nanometers was deposited. The cooling system consisted of a 1-meter-long stepped cooling channel: the first rapid cooling zone used radial porous injection of helium (CAS No. 7440-59-7) at a flow rate of 95 standard liters / minute; the second slow cooling zone used axial injection of nitrogen (CAS No. 7727-37-9) at a flow rate of 140 standard liters / minute; and the third fine cooling zone used a circulating water jacket cooling system at a water temperature of 14°C. The overall cooling rate was approximately 10... 5 ℃ / second. A temperature-controlled rotating collection plate is set at the bottom of the collection chamber, with a rotation speed of 800 rpm and an angle of 42° between the plate surface and the plasma axis. The chamber is maintained at a positive pressure of 60 kPa for argon and an oxygen content of 0.015%. The process control adopts online Raman spectroscopy and infrared thermal imaging for joint monitoring. The Raman laser wavelength is 532 nm. The G / D peak intensity ratio and radial breathing mode peak are monitored in real time, and the torch power and acetylene flow rate are adjusted to keep the G / D ratio stable at about 0.75. (5) Grading and inert packaging: The collected powder D (such as Figure 6 (As shown) Cyclone separation and sieving are performed. Coarse particles with a particle size > 130 micrometers are returned to step (1) for re-ball milling, and powder with a particle size in the range of 20 to 100 micrometers is retained as the target product. The powder is sealed and packaged in an inert atmosphere glove box with a dry argon gas integral of 99.999%.

[0027] Example 2-12 is based on Example 1, with some process parameters adjusted, while the remaining steps remain the same as in Example 1. Specific parameter adjustments are detailed in Table 1-4 below.

[0028] Table 1: Parameters for Step (1) Raw Material Preparation and Step (2) Composite Precursor Coating in Examples 1-12

[0029]

[0030] Table 2: Pre-reduction carbonization parameters for step (3) of Examples 1-12

[0031]

[0032] Table 3: Example 1-12 Step (4) Plasma Spheroidization and Growth Parameters Table-1 (Input and Power)

[0033]

[0034] Table 4: Cooling and Collection Parameters for Step (4) of Examples 1-12 - 2

[0035]

[0036] Comparative Examples 1-12 were created based on Example 1 with specific parameter adjustments or missing raw materials to verify the effects of each technical feature. Specific differences are detailed in Table 5 below.

[0037] Table 5: Differences in experimental conditions between comparative examples 1-12

[0038]

[0039] Performance tests were conducted on the powders prepared in Examples 1-12 and Comparative Examples 1-12, including the following tests: Sphericity and particle size: The sphericity (>0.9 defined as spherical particles) was measured using a Malvern Morphologi 4 particle morphology analyzer, and the percentage of spheric particles was calculated. Flowability: The flow rate (s / 50g) of 50g of powder was measured using a Hall flowmeter according to GB / T1482-2010 standard. Loose packing density: The loose packing density (g / cm³) was measured according to GB / T1479.1-2011 standard. 3 Microstructure (CNT quality): A micro Raman spectroscopy system with a laser wavelength of 532 nm was used to measure the intensity ratio of the G peak to the D peak (Id / Ig, characterizing the degree of defect; lower is better, the G / D ratio corresponding to the claim is Ig / Id; higher ratios indicate better graphitization; the Ig / Id value was recorded here). Oxygen content: The oxygen content (wt%) of the powder was measured using a LECOON H836 oxygen, nitrogen, and hydrogen analyzer. Microhardness: After the powder was embedded and polished, the cross-sectional hardness was measured using a Vickers hardness tester (HV0.1). Corrosion resistance: Electrochemical polarization curves were tested in a 3.5 wt% NaCl solution, and the self-corrosion current density (Icorr, µA / cm²) was recorded. 2 The test results are shown in Tables 6 and 7.

[0040] Table 6: Physical Performance Test Results of Examples 1-12 and Comparative Examples 1-12

[0041]

[0042] Table 7: Structural properties and corrosion resistance test results of Examples 1-12 and Comparative Examples 1-12

[0043]

[0044] In summary, this invention is not a simple superposition of multiple components or conventional parameters, but rather achieves deep coupling of multiple physicochemical fields through rigorous microscopic interface strong complexation coordination chemistry design, heterogeneous nucleation regulation, synergistic supply of solid-gas dual-phase carbon sources, and highly challenging millisecond-level plasma thermodynamics and multi-stage rapid cooling flow field control. This process cleverly utilizes the kinetic window of the plasma tail flame's extremely cold cooling zone, not only completely overcoming the technical challenges of easy decarburization, deep oxidation, and easy peeling of the surface modification layer of high-carbon ferrochrome powder during ultra-high temperature spheroidization, but also successfully achieving integrated continuous preparation of high-sphericity alloy powder and surface CNT deposition, possessing extremely outstanding substantive characteristics and significant industrial application value.

[0045] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered to fall within the scope of protection defined by the claims submitted herein.

Claims

1. A preparation process for high-carbon ferrochrome based on plasma spheroidization technology, characterized in that, The steps, by mass, are as follows: (1) High-carbon ferrochrome blocks containing 4.5-8.5% carbon, 52-68% chromium, and the remainder iron are crushed and ball-milled to obtain pre-made powder A; (2) 3-9 parts by mass of tannic acid, 1-4 parts by mass of urea, 0.05-0.8 parts by mass of nickel acetate tetrahydrate and 0.1-1.5 parts by mass of citric acid are added to 100 parts by mass of pre-made powder A, and then a mixed solvent of deionized water and anhydrous ethanol in a volume ratio of 1:0.5-1:1.5 is added to make the solid-liquid mass ratio 1:0.4-1:0.

9. The mixture is first ultrasonically dispersed for 0.5-2 hours, then mechanically stirred for 1-4 hours. After stirring, the mixture is spray-dried to obtain composite powder B; (3) Composite powder B is heated to 40°C at a rate of 3-9°C / min under a mixed atmosphere of argon and hydrogen. 00~580℃, heat preservation for 1~4 hours, to obtain pre-reduced carbonized powder C; (4) The pre-reduced carbonized powder C is axially fed into the radio frequency inductively coupled plasma torch at a feed rate of 8~60 g / min. The working gas is a mixture of argon, hydrogen and acetylene, with a total flow rate of 70~250 standard liters / min, torch power of 40~180 kW, cavity pressure of 8~60 kPa, and carrier gas is pure argon with a flow rate of 8~40 standard liters / min, so that the powder has an average residence time of 3~40 milliseconds in the high temperature zone. After cooling, high carbon chromium iron-based core-shell nanostructure spherical alloy powder D is obtained; (5) Grading and inert packaging: The alloy powder D is graded and sieved by airflow, and the powder with a particle size of 20~100 micrometers is retained as the target product and sealed in an inert atmosphere with a dry argon gas integral of ≥99.99%.

2. The preparation process of high-carbon ferrochrome based on plasma spheroidization technology according to claim 1, characterized in that: In step (1), 0.2 to 2.5 parts by mass of alumina nanopowder with an average particle size of 20 to 80 nanometers and 0.05 to 0.8 parts by mass of natural flake graphite are added to the high-carbon ferrochrome block before ball milling. The oxygen content is controlled below 0.1% by volume during ball milling so that alumina and graphite can synergistically improve droplet wettability and carbon nanotube nucleation density during subsequent plasma spheroidization. In step (1), zirconia balls with a diameter of 3 to 8 mm are used for ball milling, with a ball-to-material mass ratio of 6:1 to 12:1 and a ball milling time of 3 to 10 hours. High-purity argon gas is continuously introduced during ball milling at a flow rate of 1 to 4 liters / minute, and the oxygen content is controlled below 0.2% by volume. In step (2), the spray inlet temperature is 180 to 260°C and the outlet temperature is 80 to 120°C.

3. The preparation process of high-carbon ferrochrome based on plasma spheroidization technology according to claim 1 or 2, characterized in that: In step (2), after the mixed solvent is added, tris(hydroxymethyl)aminomethane is added to adjust the pH to 8.0-9.0, and then ultrasonic dispersion and mechanical stirring are performed to coordinate and complex tannic acid on the powder surface to form a tannic acid complex coating layer with a thickness of 50-300 nanometers; during spray drying, the atomizing disc speed is 8000-18000 rpm and the feed rate is 10-50 ml / min.

4. The preparation process of high-carbon ferrochrome based on plasma spheroidization technology according to claim 1, characterized in that: In step (3) of the pre-reduction carbonization process, 2-15% carbon monoxide by volume is added to an argon-hydrogen mixed atmosphere, the total pressure in the furnace is controlled at 15-80 kPa, and the holding time is 1.5-3.5 hours.

5. The preparation process of high-carbon ferrochrome based on plasma spheroidization technology according to claim 1, characterized in that: In step (4), the powder inlet channel is equipped with a resistance heating preheating section with a length of 100-400 mm and a preheating temperature of 300-800℃, so that the temperature of the pre-reduced carbonized powder C before entering the high temperature zone reaches 200-700℃; the plasma torch sheath gas channel is equipped with a tangential inlet with a tangential angle of 15-50° and a sheath gas flow rate of 30-120 standard liters / minute, forming a spiral airflow to extend the particle trajectory, and the standard deviation of the plasma exposure time of a single particle is controlled within 2-15 milliseconds.

6. The preparation process of high-carbon ferrochrome based on plasma spheroidization technology according to claim 1, characterized in that: In step (4), a stepped cooling channel with a length of 0.3 to 1.8 meters is set from the plasma torch outlet to the cooling collection chamber, which includes the following steps: the first rapid cooling zone uses helium radial multi-hole injection with a flow rate of 15 to 180 standard liters / minute; the second slow cooling zone uses nitrogen axial injection with a flow rate of 30 to 250 standard liters / minute; and the third fine cooling zone uses circulating water jacket cooling with a water temperature of 8 to 20°C, and the overall cooling rate is controlled at 10. 4 ~10 6 ℃ / second.

7. The preparation process of high-carbon ferrochrome based on plasma spheroidization technology according to claim 1, characterized in that: In step (4), a temperature-controlled rotating collection plate is set at the bottom of the collection chamber, with a rotation speed of 100 to 1500 rpm and an angle of 15 to 70° between the plate surface and the plasma axis. The collection chamber is maintained at a positive pressure of 20 to 100 kPa and an oxygen content of ≤0.03% by mass to reduce the adhesion of molten droplets to the wall and inhibit excessive surface oxidation.

8. The preparation process of high-carbon ferrochrome based on plasma spheroidization technology according to claim 1, characterized in that: In step (5), before grading, a cyclone separator and an electrostatic precipitator are connected in series to remove fine powder with a particle size of <10 micrometers. The fine powder is returned to step (2) for re-coating via an independent pipeline. Coarse particles with a particle size of >130 micrometers are returned to step (1) for ball milling to achieve closed-loop circulation particle size control.

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