A water-gas combined atomization method for preparing 316L stainless steel powder for injection molding

Abstract

The present invention discloses a method for preparing 316L stainless steel powder for injection molding by water-gas combined atomization. The metal raw materials are dispensed according to the composition of 316L stainless steel, smelted to obtain molten metal, and the molten metal is allowed to flow into the atomization area of the atomizing equipment. The molten metal is broken into fine droplets by the atomizing gas medium and the atomizing water medium. The droplets are cooled, solidified, and dried to obtain 316L stainless steel powder. The atomizing water medium contains a corrosion inhibitor and a dispersant, and the corrosion inhibitor is an organic phosphate. The method of the present invention is relatively simple and controllable, with low production cost, and can effectively reduce the oxygen content of the powder, improve the agglomeration effect of the metal powder, and increase the dispersibility and tap density of the powder. The prepared 316L stainless steel powder has good sphericity, high tap density, and low oxygen content, and is suitable for metal injection molding.

Description

Technical Field

[0001] The present invention relates to the technical field of metal powder preparation, and in particular to a water-gas combined atomization preparation method of 316L stainless steel powder for injection molding. Background Art

[0002] Metal Injection Molding (MIM) is a novel production process that combines the characteristics of powder metallurgy and injection molding. It uses metal powder and a binder as raw materials. The powder and binder are mixed into a uniform feedstock at a certain temperature using an appropriate method. After granulation, the feedstock is injection molded and then debinded and sintered. It is suitable for the preparation of complex structural parts and mass production. MIM has high requirements for raw material powders. Common MIM raw material powders require not only micron-level particle size and near-spherical shape, but also high requirements for particle size distribution and tap density. The tap density reflects the powder's sphericity. Powders with good sphericity require less binder, have good flowability, and can be evenly injected into the mold cavity. The resulting sintered body is more likely to achieve high density and have lower shrinkage, thus reducing the dimensional tolerance of the part. MIM raw material powders are also required to have a low oxygen content. This is because oxides such as Cr2O3 and SiO2 formed on the powder particle surface by oxygen are difficult to reduce and remove during subsequent sintering, affecting sintering densification and the mechanical properties of the MIM product. Therefore, in injection molded stainless steel, oxygen content is also an important powder performance indicator. Low oxygen content is conducive to sintering densification and improving the mechanical properties of sintered materials.

[0003] Currently, the main method for preparing stainless steel powder for injection molding is atomization, which can be divided into gas atomization and water atomization. The stainless steel powder prepared by the gas atomization method has good sphericity, high tap density and low oxygen content, but the powder particle size is relatively large, and the inert gas is used as the atomization medium, which leads to high production costs. The stainless steel powder prepared by the water atomization method has fine particle size, but the water vapor generated during the atomization process easily reacts with the metal powder, resulting in a high oxygen content in the water atomized powder. The powder also usually has an irregular morphology, a rough surface, and a low tap density. Therefore, it is still difficult to prepare high-quality stainless steel powder for injection molding with good sphericity and low oxygen content using conventional water atomization methods. Patent CN105290412A discloses an atomization method and equipment for preparing ultrafine spherical low-oxygen metal powder. This method crushes the molten metal liquid through multiple high-pressure gas and high-pressure water, and finally solidifies it into ultrafine near-spherical low-oxygen metal powder. The prepared powder particle size is 8.067μm, which is finer than traditional gas atomization and has better sphericity than traditional water atomization. The powder oxygen content is 3400ppm. Patent CN202110636016.1 discloses a method for producing low-oxygen stainless steel powder for injection molding. This method adds a strong reducing agent, hydrazine hydrate, to an existing pure water tank to remove dissolved oxygen in the water and inhibit the oxidation reaction between the high-temperature metal melt and water vapor during atomization. Protective agents stearic acid and methylbenzimidazole are added to the powder collecting tank to prevent the reduced metal from oxidizing and agglomerating in the water. The prepared stainless steel powder has an oxygen content of 0.140% and an average particle size of 9.5μm. However, the strong reducing agent hydrazine hydrate used is a hazardous chemical and is toxic if inhaled or in contact with the skin. Summary of the Invention

[0004] To address the shortcomings of the existing technology, the present invention provides a method for preparing 316L stainless steel powder for injection molding using a water-air combined atomization process. This method uses water containing a small amount of a corrosion inhibitor and a dispersant as the atomizing medium to prepare 316L stainless steel powder suitable for injection molding. This method addresses the technical issues associated with existing atomization technologies, such as high production costs, low tap density, and high oxygen content, which hinder the production of high-density MIM sintered parts.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] The present invention discloses a water-gas combined atomization preparation method for 316L stainless steel powder for injection molding. The method comprises the following steps: dispensing metal raw materials according to the composition of the 316L stainless steel, smelting to obtain molten metal, allowing the molten metal to flow into an atomization area of an atomizing device, and crushing the molten metal into fine droplets by an atomizing gas medium and an atomizing water medium. The droplets are cooled, solidified, and dried to obtain the 316L stainless steel powder. The atomizing water medium contains a corrosion inhibitor and a dispersant, and the corrosion inhibitor is an organic phosphate.

[0007] The water-gas combined atomization preparation method provided in the present invention contains an organic phosphate corrosion inhibitor and a dispersant in the atomized water medium. The organic phosphate corrosion inhibitor is an adsorption film-type corrosion inhibitor that can form an adsorption film on the metal surface, effectively reducing the direct contact reaction between water and molten metal and reducing the oxygen content of the powder. The polymer dispersant has a good wetting and dispersing effect due to the steric hindrance effect, which can improve the dispersibility of the powder and increase the tap density of the powder. On the other hand, the dispersant can produce a synergistic effect with the corrosion inhibitor. The dispersant increases the wettability of the metal surface and can further promote the adsorption of the corrosion inhibitor on the metal surface, thereby improving the corrosion inhibition efficiency. The final powder obtained is nearly spherical, has a high tap density and has a low oxygen content, which can meet the requirements of metal injection molding for stainless steel metal powder.

[0008] In a preferred embodiment, the 316L stainless steel has the following composition by mass percentage: Cr: 18%, Ni: 14%, Mo: 3%, and Fe as the balance.

[0009] Preferably, the smelting temperature is 1500-1700°C.

[0010] In a preferred embodiment, a deoxidizer is used to deoxidize and remove slag during the smelting process to obtain the molten metal, and the deoxidizer is a silicon-calcium alloy deoxidizer.

[0011] During the actual operation, the molten metal liquid is poured into the atomizing ladle of the atomizing equipment, and the molten metal liquid flows through the atomizing area through the leakage hole at the bottom of the atomizing ladle.

[0012] In a preferred embodiment, the upper part of the atomization area is an annular seam type gas atomization nozzle, and the lower part is two groups of V-shaped water atomization nozzles. The spray vertex angle of the annular seam type gas atomization nozzle is 30 to 40 degrees, and any one group of V-shaped water atomization nozzles is symmetrically arranged by two water atomization nozzles, and the angle between the two water atomization nozzles is 30 degrees.

[0013] Further preferably, the water sprayed from the water atomizing nozzle is distributed in a fan shape, and the fan angle is 25° to 30°.

[0014] In a preferred embodiment, the atomizing gas medium is nitrogen, the pressure of the atomizing gas medium is 0.3-0.5 MPa, and the gas flow rate is 1-2 m 3 / min.

[0015] In a preferred solution, the water pressure of the atomized water medium is 100-120 MPa, and the water flow rate of the nozzle is 60-80 L / min.

[0016] The inventors have found that under the coordination of the above-mentioned gas atomization parameters and water atomization parameters, the final atomization effect is optimal.

[0017] In a preferred embodiment, the mass fraction of the organic phosphate in the atomized water medium is 0.1% to 1%.

[0018] The inventors found that the final atomization effect is optimal when the mass fraction of the organophosphate is controlled within the above range. If the amount of organophosphate corrosion inhibitor added is too small, the adsorption film formed on the metal surface will not cover the entire surface and the protective effect will be weak. If the amount of organophosphate corrosion inhibitor added is too large, exceeding the above range, there will be no significant improvement in the atomization effect.

[0019] In a preferred embodiment, the structural formula of the organic phosphate is shown in Formula 1:

[0020]

[0021] Wherein, R is an alkyl group or an aromatic group, R1 is one of H, an alkyl group, and an aromatic group, and R2 is one of H, an alkyl group, and an aromatic group.

[0022] In a preferred embodiment, the organic phosphate is cyclohexane hexanol phosphate. The inventors have found that the atomization effect is optimal when cyclohexane hexanol phosphate is selected.

[0023] In a preferred embodiment, the dispersant is polyvinyl alcohol (PVA).

[0024] In a preferred embodiment, the mass ratio of the dispersant to the corrosion inhibitor is 1:5-6.

[0025] When the concentration ratio of corrosion inhibitor and dispersant is appropriate, the protective film formed on the metal surface is denser due to the synergistic effect of the two. When the concentration of one of them is not appropriate, the protective effect of both is reduced due to competitive adsorption.

[0026] Preferably, the mixture is dried in a vacuum dryer.

[0027] The beneficial effects of the present invention are as follows:

[0028] The present invention adopts a water-gas combined atomization method to prepare 316L stainless steel powder for injection molding. This method combines the advantages of gas atomization and water atomization. Compared with the traditional gas atomization method, the crushing effect of high-pressure water is added, and the powder particle size is finer. Compared with the traditional water atomization method, since gas is involved in the atomization in the early stage, the amount of atomized water used is greatly reduced, and the solidification of the powder is delayed. Therefore, the sphericity is better, the tap density is increased, and the oxygen content of the powder is also reduced.

[0029] The present invention performs atomized water treatment in the atomization step, and adds an organic phosphate corrosion inhibitor and a dispersant to the atomized water. The organic phosphate corrosion inhibitor is an adsorption film type corrosion inhibitor. The phosphate molecule is mainly composed of two parts: a phosphate group and a hydrocarbon group. The phosphate group is a hydrophilic group containing two heteroatoms, P and O, and can be adsorbed on the metal surface as an adsorption center; while the hydrocarbon group is a hydrophobic group. When the phosphate molecule is adsorbed on the metal surface, the hydrocarbon group can promote the formation of the adsorption film on the metal surface through the van der Waals force interaction between molecules and can improve the orderliness and density of the adsorption film. Therefore, the organic phosphate can form a layer of adsorption film on the metal surface during the atomization process, effectively reducing the direct contact reaction between water and metal liquid and reducing the oxygen content of the powder. Due to the steric hindrance effect, the polymer dispersant has a good wetting and dispersing effect, which can improve the dispersibility of the powder and increase the tap density of the powder. On the other hand, the dispersant can produce a synergistic effect with the corrosion inhibitor. The dispersant increases the wettability of the metal surface, which can further promote the adsorption of the corrosion inhibitor on the metal surface and improve the corrosion inhibition efficiency.

[0030] The tap density of the 316L stainless steel powder prepared by the present invention is 4.5-4.6 g / cm 3 , the oxygen content is below 3000ppm, the powder is nearly spherical, has high tap density and low oxygen content, which can meet the needs of metal injection molding for stainless steel metal powder. DETAILED DESCRIPTION

[0031] The specific embodiments of the present invention are further described below, but the present invention is not limited to these embodiments. Any improvements made without departing from the spirit of the present invention shall fall within the scope of protection of the present invention.

[0032] Example 1

[0033] This embodiment provides a method for preparing 316L stainless steel powder for injection molding by water-gas combined atomization, comprising the following steps:

[0034] Weigh 316L stainless steel ingredients by weight percentage: Cr: 18%, Ni: 14%, Mo: 3%, Fe: balance, total weight 100kg, add to the medium frequency melting furnace for smelting, and use silicon calcium alloy deoxidizer for deoxidation and slag removal. When the molten metal temperature reaches 1650℃, pour the molten metal into the atomizing ladle of the atomizing equipment. The molten metal flows through the hole at the bottom of the ladle through the atomizing nozzle. The annular seam type gas atomizing nozzle has a spray top angle of 30°, and nitrogen is selected as the inert atomizing gas. The atomizing gas pressure is controlled at 0.5MPa and the gas flow rate is 2m 3 / min, the angle between the two V-shaped water atomizing nozzles is 30°, the fan angle of the V-shaped nozzle is 25°, the atomizing water pressure is controlled to be 120MPa, the water flow rate is 80L / min, and a 1% organic phosphate corrosion inhibitor and a 0.2% dispersant are added to the atomizing medium water. The liquid metal is broken into a large number of fine droplets by the two media of atomizing gas and atomizing water. The droplets are then cooled and solidified into powder in the atomizing collection barrel, and then dehydrated and vacuum dried. The obtained 316L stainless steel powder obtained by injection molding using water and gas combined atomization is nearly spherical, has a low oxygen content of 2610ppm, and a tap density of 4.67g / cm 3 .

[0035] Example 2

[0036] The difference between this embodiment and embodiment 1 is that in this embodiment 2, a water-gas combined atomization process for 316L stainless steel powder for injection molding is adjusted, including the following steps:

[0037] In terms of weight percentage, the metal raw material composition is Cr: 17%, Ni: 14%, Mo: 2%, Fe: balance. Weigh 100kg and put it into the medium frequency melting furnace for smelting, and add silicon calcium alloy deoxidizer for deoxidation and slag removal. When the metal liquid temperature reaches 1650℃, pour the molten metal into the atomizing ladle of the atomizing equipment. When the molten metal liquid passes through the hole at the bottom of the ladle and flows through the atomizing nozzle, control the atomizing gas pressure to 0.3MPa and the gas flow rate to 1m 3 / min, the atomizing water pressure is 100MPa, the water flow rate is 60L / min, and a 1% corrosion inhibitor and 0.2% dispersant are added to the atomizing medium water. The liquid metal is broken into a large number of fine droplets by the two media of atomizing gas and atomizing water. The droplets are then cooled and solidified into powder in the atomizing collection barrel, and then dehydrated and vacuum dried. The obtained injection molding water-gas combined atomization 316L stainless steel powder is nearly spherical, with an oxygen content of 2770ppm and a tap density of 4.61g / cm 3 .

[0038] Example 3

[0039] The difference between Example 3 and Example 1 is that in Example 3, the ratio of adding a corrosion inhibitor and a dispersant to atomized water in a water-gas combined atomization process for 316L stainless steel powder for injection molding is adjusted, including the following steps:

[0040] The raw metal composition is Cr: 17%, Ni: 14%, Mo: 2%, Fe: balance. Weigh 100kg and put it into the medium frequency melting furnace for smelting, and add silicon calcium alloy deoxidizer for deoxidation and slag removal. When the molten metal temperature reaches 1650℃, pour the molten metal into the atomizing ladle of the atomizing equipment. When the molten metal passes through the hole at the bottom of the ladle and flows through the atomizing nozzle, control the atomizing gas pressure to 0.5MPa and the gas flow rate to 2m 3 / min, the atomizing water pressure is 120MPa, the water flow rate is 80L / min, and a corrosion inhibitor of 0.1% and a dispersant of 0.02% are added to the atomizing medium water. The liquid metal is broken into a large number of fine droplets by the two media of atomizing gas and atomizing water. The droplets are then cooled and solidified into powder in the atomizing collection barrel, and then dehydrated and vacuum dried. The obtained injection molding water-gas combined atomization 316L stainless steel powder is nearly spherical, with an oxygen content of 2950ppm and a tap density of 4.54g / cm 3 .

[0041] Comparative Example 1

[0042] The difference between this comparative example and Example 1 is that the conventional water atomization powder making process is used to prepare 316L stainless steel powder in this comparative example, including the following steps:

[0043] In terms of weight percentage, the metal raw material composition is Cr: 17%, Ni: 14%, Mo: 2%, Fe: balance. Weigh 100kg, put it into a medium frequency melting furnace for smelting, and add silicon calcium alloy deoxidizer for deoxidation and slag removal. When the metal liquid temperature reaches 1650℃, pour the molten metal liquid into the atomizing ladle of the atomizing equipment. The molten metal liquid is impacted by the atomized water into a large number of fine droplets, cooled and solidified into powder, and then dehydrated and vacuum dried. The obtained water-atomized 316L stainless steel powder has an oxygen content of 5190ppm and a tap density of 4.26g / cm 3 .

[0044] Comparative Example 2

[0045] The other conditions were the same as those in Example 1, except that hexamethylenetetramine (an organic amine corrosion inhibitor) was added to the atomized water. The oxygen content of the water-atomized 316L stainless steel powder was 3140 ppm, and the tap density was 4.53 g / cm 3 .

[0046] Comparative Example 3

[0047] The other conditions were the same as those in Example 1, except that no dispersant was added to the atomizing medium water. The oxygen content of the water-atomized 316L stainless steel powder was 3620 ppm and the tap density was 4.46 g / cm 3 .

[0048] Comparative Example 4

[0049] Other conditions were the same as in Example 1, except that 1% of an organic phosphate corrosion inhibitor and 1% of a dispersant were added to the atomizing medium water. The water-atomized 316L stainless steel powder obtained had an oxygen content of 3470 ppm and a tap density of 4.48 g / cm 3 .

Claims

1. A method for preparing 316L stainless steel powder for injection molding by water-gas combined atomization, characterized in that: Metal raw materials are prepared according to the composition of 316L stainless steel, smelted to obtain molten metal, and the molten metal is allowed to flow into the atomization area of an atomizing device. The molten metal is broken into fine droplets by an atomizing gas medium and an atomizing water medium. The droplets are cooled, solidified, and dried to obtain 316L stainless steel powder; the atomizing water medium contains a corrosion inhibitor and a dispersant, and the corrosion inhibitor is an organic phosphate. In the atomized water medium, the mass fraction of the organic phosphate is 0.1% to 1%; The structural formula of the organic phosphate is shown in Formula 1: Formula 1 Wherein, R is an alkyl group or an aromatic group, R1 is one of H, an alkyl group, and an aromatic group, and R2 is one of H, an alkyl group, and an aromatic group; The mass ratio of the dispersant to the corrosion inhibitor is 1:5-6.

2. The method for preparing 316L stainless steel powder for injection molding by water-gas combined atomization according to claim 1, characterized in that: The 316L stainless steel has the following composition by mass percentage: Cr: 18%, Ni: 14%, Mo: 3%, and Fe as the balance.

3. The method for preparing 316L stainless steel powder for injection molding by water-gas combined atomization according to claim 1, characterized in that: The smelting temperature is 1500-1700°C; During the smelting process, a deoxidizer is used to deoxidize and remove slag to obtain molten metal, and the deoxidizer is a silicon-calcium alloy deoxidizer.

4. The method for preparing 316L stainless steel powder for injection molding by water-gas combined atomization according to claim 1, characterized in that: The upper part of the atomization area is an annular seam gas atomization nozzle, and the lower part is two groups of V-shaped water atomization nozzles. The spray vertex angle of the annular seam gas atomization nozzle is 30~40°. Any group of V-shaped water atomization nozzles is symmetrically arranged with two water atomization nozzles, and the angle between the two water atomization nozzles is 30°.

5. The method for preparing 316L stainless steel powder for injection molding by water-gas combined atomization according to claim 4, characterized in that: The water sprayed from the water atomizing nozzle is distributed in a fan shape, and the fan angle is 25° to 30°.

6. The method for preparing 316L stainless steel powder for injection molding by water-gas combined atomization according to claim 1 or 4, characterized in that: The atomizing gas medium is nitrogen, the pressure of the atomizing gas medium is 0.3~0.5MPa, and the gas flow rate is 1~2m 3 / min; The water pressure of the atomized water medium is 100-120 MPa, and the water flow rate of the nozzle is 60-80 L / min.

7. The method for preparing 316L stainless steel powder for injection molding by water-gas combined atomization according to claim 1, characterized in that: The organic phosphate is cyclohexane hexanol phosphate.

8. The method for preparing 316L stainless steel powder for injection molding by water-gas combined atomization according to claim 1 or 4, characterized in that: The dispersant is polyvinyl alcohol.

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