Improved Electrode Induction Gas Atomization Process for Producing Spherical Chromium Powder
Release time:
2025-12-12
Chromium is an inert metal that remains stable against water and moisture at room temperature, exhibiting strong corrosion resistance. It also demonstrates exceptional stability in corrosive media such as alkalis, nitric acid, sulfides, carbonates, and organic acids. Chromium metal coatings exhibit high hardness, excellent wear resistance, strong reflectivity, and good heat resistance [1-3]. Consequently, chromium metal powder finds extensive applications across chemical engineering, metallurgy, energy, electronics, automotive, and aerospace industries [3-4]. For instance, in solar cell production, high-purity chromium powder serves as a reflective layer material to enhance cell efficiency. Continuous technological advancement imposes higher demands on the quality and performance of chromium powder, particularly high-quality spherical chromium powder.
Currently, primary methods for metal powder preparation include gas atomization (e.g., Vacuum Induction Gas Atomization (VIGA), Electrode-less Induction Gas Atomization (EIGA), induction melting water atomization (WA)), plasma methods (e.g., plasma torch atomization (PA), plasma spheroidization (PS), plasma rotary electrode atomization (PREP)), mechanical methods (e.g., mechanical comminution, ball milling), and chemical methods (e.g., electrolysis, redox processes) [5-7]. However, domestic manufacturers of spherical chromium powder remain scarce.
Due to chromium's high melting point of 1907°C, high-quality spherical chromium powder is currently primarily produced via plasma spheroidization. This involves manufacturing chromium powder through electrolysis [1], followed by mechanical grinding of the resulting irregular chromium powder to the desired particle size, and finally obtaining spherical chromium powder through plasma spheroidization [6-8]. Challenges with this technology include: (1) Low production efficiency (less than 5 kg/h) to ensure spheroidization rate; (2) Incomplete domestic plasma spheroidization technology, with the primary method being Canada's TEKNA plasma spheroidization technology [8]. These challenges limit the engineering applications of high-quality spherical chromium powder.
As reported in [9], spherical chromium powder (15–53 μm) exhibits properties including a bulk density of 4.20 g/cm³, a tapped density of 5.41 g/cm³, and a flow time of of 16.50 s/50 g, and sphericity exceeding 98%. Plasma-processed spherical chromium powder exhibits superior properties, significantly surpassing the requirements of GB/T 43110-2023 “Metal Chromium Powder for Additive Manufacturing.” However, research by Chen Luenjiang et al. [3] using an induction-coupled plasma method for spherical chromium powder production indicates that the maximum powder feed rate of this equipment is 45 g/min, resulting in a production efficiency of 2.7 kg/h per plasma spheroidizer. Considering actual production conditions, plasma-based spherical chromium powder production faces challenges such as low efficiency and high costs. The widespread application of spheroidal chromium powder with superior comprehensive properties has not only accelerated the development of powder metallurgy technology but also significantly enhanced the performance of powder metallurgy products [10]. How to efficiently prepare high-end spheroidal chromium powder and overcome the bottlenecks in large-scale engineering applications has become a research hotspot in the field of powder metallurgy.
This study innovatively employs an improved induction melting argon atomization technique, focusing on exploring trial production processes for high-quality spherical chromium powder. It verifies the feasibility and unique advantages of the independently developed gas atomization technology in producing high-melting-point spherical chromium powder. Research in this area remains scarce both domestically and internationally, with no existing reference materials available.
The experiment aimed to uncover new pathways for spherical chromium powder production while striving to reduce manufacturing costs, thereby enhancing the economic viability and efficiency of the preparation process. Additionally, the study conducted an in-depth analysis of multiple key characteristics of the prepared spherical chromium powder, including powder sphericity, flowability, bulk density, and oxygen content control, providing a comprehensive evaluation of powder quality. This series of research not only fills a technical gap but also lays a solid theoretical foundation and provides data support for further research and widespread application of high-end spherical chromium powder in materials science, advanced manufacturing, and other fields, demonstrating significant scientific value and practical application prospects.
1 Experiment
1.1 Experimental Setup and Principle
Traditional electrode induction melting gas atomization (EIGA) is an advanced ultra-clean metal powder preparation technology. Since it does not use crucibles or refractory materials in the production process, the resulting powder exhibits characteristics such as controllable particle size, high sphericity, and minimal non-metallic inclusions. It has now become a key method for batch production of ultra-clean metal powders used in powder metallurgy and additive manufacturing [10-13]. The working principle is as follows: This process utilizes metal or alloy self-consuming electrode rods. The lower end of the electrode, featuring a 45° conical surface, is melted into a liquid metal stream through high-frequency induction heating. Under the impact of a high-velocity inert gas jet (high-purity argon), the liquid is fragmented into fine droplets. During the condensation process, spherical powders are formed due to surface tension.
Currently, there are no published reports of successful spherical chromium powder production using the EIGA method domestically or internationally. This may be attributed to chromium's extremely high melting point, which poses significant challenges for conventional EIGA equipment when melting chromium rods. To address this issue, an optimized electrode induction gas atomization powder production device was developed through experimentation, featuring comprehensive upgrades to both the feeding mechanism and heating system.
As shown in Figure 1, this improved equipment integrates four core components: a self-regulating feeding mechanism, a high-efficiency induction heating chamber, an atomization chamber, and a powder collection system [12]. The self-regulating feeding mechanism employs intelligent control technology to monitor and respond in real-time to deviations between the actual and preset solution temperatures, dynamically adjusting the descent speed of the chromium rod. This innovative design ensures highly consistent and stable solution temperature throughout the atomization process, laying a solid foundation for producing high-quality spherical chromium powder.
Compared to conventional EIGA equipment, this system features significantly optimized heating capabilities, substantially increasing heat output per unit time. This enhancement stems from increased induction coil turns, boosted power output, and improved system stability. According to the principles of induction heating [14-15], increasing coil turns and boosting heating power directly enhances the heat generated per unit time by the electrode. Once the accumulated heat reaches a critical threshold, the electrode's induction material can be easily melted.
To address the melting challenges of high-purity chromium electrodes and achieve a stable, efficient “drip melting” process, a specially designed 5-turn copper induction coil was developed. This coil features three initial turns with progressively increasing diameters from bottom to top, followed by two turns maintaining the same diameter as the third. This design significantly enhances the preheating, heating, and melting efficiency of high-melting-point alloy rods.
Given the immense heat generated during melting high-melting-point alloy rods, the existing induction heating system underwent targeted modifications to ensure stable coil operation under extreme conditions. This involved enhancing the thermal conductivity of the water-cooled copper coil—despite copper's inherent thermal efficiency—ensuring effective heat dissipation during high-intensity, high-temperature operations and guaranteeing smooth melting processes.
1.2 Experimental Materials
The experiment utilized 99.9% (mass fraction) pure chromium rod stock with dimensions of φ50mm × 600mm. The lower end of the rod was machined into a 45° tapered surface with a surface roughness not exceeding Ra1.6.
1.3 Sample Performance Testing and Characterization
Powder samples prepared using the modified EIGA method were analyzed according to GB/T 5314 sampling standards. Product appearance quality was assessed visually; samples should exhibit a bright gray or light gray color with no visible inclusions. Particle size distribution was measured using a Bettersize 2000 laser particle size analyzer per GB/T 19077. Bulk density was measured using the funnel method per GB/T 1479.1. Tapped density was determined using the BT1001 Intelligent Powder Characteristic Tester per GB/T 5162. Flowability was assessed with a Hall flow meter per GB/T 1482. The sphericity and microstructure of the product were observed using a Gemini SEM 450 scanning electron microscope according to GB/T 15445.6; The oxygen content was tested using an ONH-3000 oxygen-nitrogen-hydrogen analyzer according to GB/T 14265.
2 Results and Analysis
2.1 Feasibility of Spherical Chromium Powder Production via Modified Induction Melting Gas Atomization
When preparing spherical chromium powder using the improved electrode induction melting gas atomization powder production equipment, heating power reaching 73 kW successfully melted the experimental chromium rod. The melting rate at the rod's lower end ensured normal atomization powder production operation, with stable equipment performance. Changes in the lower end of the chromium rod during the induction heating power increase phase are shown in Table 1.
2.2 Effect of Heating Power on Powder Particle Size
Experiments investigated the influence of varying heating powers on the particle size distribution of raw chromium powder (unprocessed powder produced by atomization) at an atomization pressure of 4 MPa, as shown in Figure 2.
As shown in Figure 2, when the heating power was 73–74 kW, the D50 of the total powder decreased with increasing heating power. This occurs because an appropriate degree of droplet superheat promotes the formation of fine particles [16]. At 74 kW, the D50 of the total powder was minimal, indicating the finest powder at this power level. When the heating power ranges from 74 to 76 kW, the D50 of the powder increases with increasing heating power. This occurs because, beyond a certain superheat value, the gas-to-material ratio relatively decreases under constant process parameters. That is, at the same gas flow rate, the mass of droplets required for atomization increases, leading to larger powder particle sizes. The specific analysis of this phenomenon is detailed in Equation (1).
Where: kd is a constant ranging from 4×10⁻⁶ to 5×10⁻⁶; ηm (m²/s) and ηg (m²/s) are the viscosities of molten metal and atomized gas; M (kg/s) and G (kg/s) represent the mass flow rates of molten metal and gas, respectively; the Weber number can be calculated as We = U²ρmd₀/γ_m, where ρ_m (g/cm³) and γ_m (N/m) denote the density and surface tension of molten metal, respectively; d₀ is the diameter of the atomizing disc nozzle; U is the relative velocity between gas and liquid [17]. According to the average powder particle size calculation formula (1), as heating power increases, the solution temperature of the melted induction rod material rises, and the viscosity ηm of the molten metal decreases, being proportional to the average powder particle size D50. When the molten metal viscosity decreases, the mass flow rate of the molten metal M increases, being inversely proportional to the average powder particle size D50. Therefore, as heating power increases, the average powder particle size first decreases and then increases.
Considering uncertainties in both experimental and production processes, the atomization temperature set by the control system should be selected within a defined range. Based on the experimental data in Figure 3, the atomization temperature should be chosen at a heating power of 74–74.5 kW, corresponding to an excess temperature of 230–300°C for the electrode induction gas atomization preparation of spherical chromium powder.
2.3 Effect of Atomization Pressure on Powder Particle Size and Yield
Atomization pressure is a critical process parameter in electrode induction melting gas atomization technology [18-19]. Experiments investigated the influence of varying atomization pressures on powder particle size and yield at a heating power of 74 kW, as shown in Figure 3. As shown in Figure 3, the D50 of the total powder decreases significantly with increasing atomization pressure. A smaller D50 indicates finer powder. When the atomization pressure exceeds 5 MPa, the decrease in D50 becomes relatively stable, indicating minimal variation in the yield of fine powder. The yield of powder particles in the 15–53 μm size range increases markedly with higher atomization pressure. At 5 MPa, the yield reaches its maximum; further increases in atomization pressure result in a relatively stable decline in this yield. The D50 particle size distribution of powder produced at atomization pressures of 5.5 and 6.0 MPa was smaller than that at 5.0 MPa, but the yield of the 15–53 μm fraction was lower than at 5.0 MPa, primarily due to the finer particle size. The yield of 0–15 μm particles increases at atomization pressures of 5.5 and 6.0 MPa, while the yield of 15–53 μm particles relatively decreases.
Research indicates that an appropriate atomization pressure is beneficial for reducing the D50 of the bulk powder and increasing the yield of fine powder (15–53 μm). Therefore, at an atomization pressure of 5 MPa, the D50 of the bulk powder is more reasonable, the yield of fine powder (15–53 μm) reaches its maximum, and atomization gas consumption is reduced compared to pressures of 5.5 and 6.0 MPa. Powder average particle size calculation formula (1) indicates that, beyond material properties and equipment parameters, atomized droplet size is primarily influenced by M, G, and U. In supersonic gas atomization, the gas/liquid relative velocity U is assumed to approximate the atomizing gas velocity vg. Both larger G and smaller M values reduce atomized droplet size, but the former consumes more atomizing gas and increases production costs, while the latter significantly lowers productivity.
Using an improved electrode induction gas atomization powder production process: with a heating power of 74 kW and atomization pressure of 5 MPa, the production efficiency reached approximately 19.8 kg/h. The prepared spherical chromium powder exhibited a light gray appearance with no visually detectable inclusions. After sieving with 15, 53, and 150 μm screens, the yield of 15–53 μm spherical chromium powder was 40.8%, resulting in a production efficiency of approximately 8.07 kg/h for this particle size range. Compared to the production efficiency of 2.7 kg/h achieved by Chen Luenjiang et al. [3] using an induction-coupled plasma method, the modified EIGA method for producing 15–53 μm spherical chromium powder demonstrates nearly threefold higher efficiency. Moreover, the plasma method for spheroidizing irregular chromium powder requires time-consuming and inefficient grinding to achieve the desired particle size. Comprehensive analysis indicates that the modified EIGA method significantly enhances production efficiency for spherical chromium powder compared to the plasma method.
2.4 Microstructural Characterization of Spherical Chromium Powder
The microstructure of 15–53 μm spherical chromium powder produced by the improved EIGA method is shown in Figure 4. As depicted, the majority of particles exhibit spherical or near-spherical shapes with no non-metallic inclusions, achieving a spheroidization rate exceeding 98%. This indicates that the spherical chromium powder produced by the improved EIGA method possesses high purity and excellent sphericity.
2.5 Powder Properties of Spherical Chromium Powder
The particle size distribution of spherical chromium powder prepared by improved electrode induction gas atomization is shown in Figure 5, representing the particle size distribution of the raw powder (0–250 μm) and the sieved powder (0–15, 15–53, 53–150 μm). As shown in Figure 5, the powder particle size exhibits a normal distribution.
The flowability of spherical chromium powder was measured using a Hall flowmeter. Its properties were compared with those of spherical chromium powder from Reference [9], as shown in Table 2. Table 2 indicates that the performance of spherical chromium powder produced by the improved electrode induction gas atomization method is comparable to or exceeds that of powder produced by the plasma method.
3 Conclusions
(1) Feasibility Validation: The improved EIGA method was effectively validated for spherical chromium powder production, establishing optimal process parameters:
Stable spherical chromium powder production was achieved at atomization temperatures of 2130–2200°C (corresponding to superheat of 230–300°C), heating power of 74 kW, and atomization pressure of 5 MPa.
(2) With increasing heating power, the average particle size of the chromium powder first decreased and then increased, revealing the complex effect of heating power on particle size control. Increasing atomization pressure significantly reduces particle size, with the reduction trend eventually stabilizing, indicating that high-pressure atomization facilitates obtaining finer spherical powders with more uniform size distribution. Under optimized conditions, the experiment successfully achieved a 40.8% yield of spherical chromium powder in the 15–53 μm size range, while attaining a production efficiency of approximately 8.07 kg/h, demonstrating promising industrial application prospects.
(3) Spherical chromium powder produced via the modified EIGA method exhibits not only high purity and absence of non-metallic inclusions but also exceptional sphericity exceeding 98%, demonstrating significant advantages in preparing high-quality spherical powders.
(4) Compared to spherical chromium powder produced by existing plasma methods, the modified EIGA-processed powder exhibits comparable performance and production efficiency, even demonstrating superiority in certain aspects. This provides a new, highly efficient, and high-quality option for spherical chromium powder production.
In summary, this research not only enriches the theoretical framework of electrode-induced gas atomization powder production technology but also provides crucial reference and technical support for the efficient, high-quality production of spherical chromium powder.
References: Powder Metallurgy Industry, DOI: 10.13228/j.boyuan.issn1006-6543.20240098; Process for Spherical Chromium Powder Production via Modified Electrode Induction Gas Atomization; Chen Xi, Tan Jianjun, Zhuo Yijiao, Du Wendong, Huo Hao, Hu Peng
Stardust Technology, as a national high-tech enterprise, produces spherical chromium powder using radio frequency plasma spheroidization technology, showcasing outstanding core advantages. The product boasts a purity ≥99.95%, oxygen content ≤500ppm, sphericity exceeding 95%, a smooth surface free of satellite particles, and uniform particle size distribution. It supports multiple specifications ranging from 5-150μm and accommodates customized requirements. It exhibits excellent flowability, high bulk and tapped densities, and is compatible with diverse processes including laser/electron beam additive manufacturing, hot isostatic pressing, laser cladding, and vacuum coating. Widely applied in cemented carbides, diamond tools, welding materials, and high-temperature alloys, our products leverage over three decades of powder material R&D expertise and comprehensive integrated solutions to ensure consistent quality for all customer requirements. For further product details, please contact manager Cathie Zheng at +86 13318326187.
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