An easily sintered high-hardness austenitic stainless steel powder for injection molding and its use

Abstract

This invention discloses an easily sinterable, high-hardness austenitic stainless steel powder for injection molding and its applications. The austenitic stainless steel powder, by mass percentage, has the following composition: Cr 20-23%, Ni 14-16%, Mo 4.4-6.8%, Cu 0.01-3%, B 0.01-0.5%, C 0.01-0.2%, Mn≦1%, Si≦1%, N≦0.1%, with the balance being Fe and unavoidable impurities. The austenitic stainless steel powder is mixed with a binder and a certain amount of carbon powder, then injection molded, followed by debinding and sintering. The alloy sintering temperature is significantly lower than that of 316L, and the sintering difficulty is reduced. The density of the obtained stainless steel parts can exceed 7.9 g / cm³. 3 The hardness exceeds 200 Hv, and a distinct dense layer forms on the sample surface. Salt spray and electrochemical tests show that the alloy has better corrosion resistance than 316L alloy.

Description

Technical Field

[0001] This invention falls under the category of injection molding and relates to a novel austenitic stainless steel, specifically involving an easily sinterable, high-hardness austenitic stainless steel powder for injection molding and its applications. Background Technology

[0002] Austenitic stainless steel, with its excellent plasticity, corrosion resistance, and biocompatibility, is widely used in aerospace, marine, petroleum, chemical, pharmaceutical, transportation, electronics, and daily-use fields. This type of stainless steel generally contains high levels of nickel and chromium to ensure its austenitic matrix and corrosion resistance. Among austenitic stainless steels, 316L is one of the most widely used and is currently the most mature system in injection molding. Injection molding can produce precision parts that are difficult to manufacture using traditional methods. Furthermore, the material prepared by this method has fine grains, a uniform microstructure, and good mechanical properties. However, 316L austenitic stainless steel has a relatively low hardness, generally between 140-160 HV, which greatly limits its application in wear-resistant and high-stress environments.

[0003] Methods to increase the hardness of austenitic stainless steel include work hardening, surface treatment, alloying, and second-phase particle strengthening. However, these methods may not be suitable for injection-molded products, or may lead to decreased alloy density, increased ferrite or martensite content (resulting in magnetic properties that limit applications), reduced corrosion resistance, impaired polishing performance, and high manufacturing costs. 316L stainless steel typically contains 2% molybdenum; a small amount of molybdenum only causes solid solution strengthening. High-molybdenum austenitic stainless steel typically contains 4-7% molybdenum, which can combine with carbon in the steel to precipitate fine carbides, strengthening the stainless steel. This type of stainless steel has already found applications in ingot casting and machining. However, research in powder metallurgy, especially injection molding, is limited. The reason for this is that molybdenum-containing stainless steel has a strong tendency for grain boundary precipitation. The second phase precipitated at the grain boundaries significantly reduces the alloy's corrosion resistance, thus requiring nitrogen sintering to suppress the precipitation of the second phase near the grain boundaries. However, nitrogen has very low solubility in austenitic stainless steel, making nitrogen content control difficult, thus affecting process stability and alloy performance.

[0004] This invention designs a pre-alloyed austenitic stainless steel powder containing molybdenum and copper, prepared by gas atomization. This method provides lower particle size and lower oxygen content, suppressing oxide precipitation. Molybdenum can form a small amount of carbide and intermediate compound reinforcing phases, which can be uniformly distributed in the stainless steel within the range described in this invention. This improves the hardness of the sample without significantly reducing the corrosion resistance of the alloy. Summary of the Invention

[0005] In the existing technology, the hardness of 316L stainless steel is between 140 and 160 Hv, which cannot meet the requirements of high-hardness (above 200 Hv) austenitic parts. Traditional surface treatment and work hardening methods are difficult to meet the requirements. Furthermore, general high-molybdenum austenitic stainless steel has the problem of difficulty in controlling the nitrogen content during sintering. The first objective of this invention is to provide an easily sinterable high-hardness austenitic stainless steel powder for injection molding with high polishing effect.

[0006] A second objective of this invention is to provide an application of an easily sinterable, high-hardness austenitic stainless steel powder for injection molding. This austenitic stainless steel powder for injection molding is used to prepare austenitic stainless steel parts.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] This invention discloses an easily sinterable, high-hardness austenitic stainless steel powder for injection molding. The austenitic stainless steel powder, by mass percentage, has the following composition: Cr 20-23%, Ni 14-16%, Mo 4.4-6.8%, Cu 0.01-3%, B 0.01-0.5%, C 0.01-0.2%, Mn≦1%, Si≦1%, N≦0.1%, with the balance being Fe and unavoidable impurities.

[0009] The austenitic stainless steel powder provided by this invention incorporates a high content of molybdenum, which can combine with carbon in the steel to precipitate fine carbides that strengthen the stainless steel. The addition of an appropriate amount of copper, whose melting point is only 1083 degrees Celsius, far below the sintering temperature, significantly lowers the liquidus line of the stainless steel, reducing the difficulty of sintering the material. Simultaneously, copper also has a certain solid solution strengthening effect, and the released copper ions have a certain antibacterial effect, which is beneficial to improving the biocompatibility of the alloy. Furthermore, the addition of a small amount of boron also lowers the sintering temperature of the material, and the addition of a small amount of boron is beneficial to the densification of the material. With the synergistic effect of the above components, the austenitic stainless steel powder can be sintered into a dense state at a lower sintering temperature, with a uniform distribution of the second phase, while avoiding coarseness of the second phase.

[0010] Of course, the amount of alloying components added needs to be controlled within the scope of this invention. For example, excessive boron content will lead to the precipitation of borates between grains, reducing the mechanical properties and corrosion resistance of the material. Appropriate carbon content can combine with elements such as chromium and molybdenum in the alloy to precipitate a large number of nanoscale carbides in the grains, avoiding the precipitation of coarse molybdenum-containing second phases at the grain boundaries. However, excessive carbon (>0.3%) will cause the carbides to grow significantly and take away chromium from the stainless steel, thereby reducing the corrosion resistance of the alloy.

[0011] In a preferred embodiment, the austenitic stainless steel powder is prepared by water atomization or gas atomization, preferably by gas atomization.

[0012] In a preferred embodiment, the mass fraction of oxygen in the austenitic stainless steel powder is 0.005–0.4%.

[0013] The austenitic stainless steel powder in this invention is prepared by gas atomization, which has a smaller particle size and a lower oxygen content, thus helping to suppress the precipitation of oxides.

[0014] In a preferred embodiment, the particle size of the austenitic stainless steel powder is: D 10 =3~12μm, D 50 =9~18μm, D 90 =15~30μm.

[0015] The present invention also provides an application of austenitic stainless steel powder for injection molding that is easy to sinter and has high hardness, wherein the austenitic stainless steel powder is used as a raw material to prepare austenitic stainless steel parts.

[0016] In a preferred embodiment, the austenitic stainless steel part is prepared by: mixing austenitic stainless steel powder, carbon powder, and binder to obtain a feedstock; injection molding the feedstock to obtain a rough blank; sequentially subjecting the rough blank to catalytic degreasing to obtain a degreased blank; sintering the degreased blank to obtain a sintered blank; and heat-treating the sintered blank to obtain the austenitic stainless steel part.

[0017] In a further preferred embodiment, the amount of carbon powder added is 100-2000 ppm of the mass of the austenitic stainless steel powder, preferably 500-2000 ppm, and even more preferably 750-1500 ppm.

[0018] The inventors discovered that by adding a small amount of carbon powder to the feedstock, the loss of carbon due to oxidation during the preparation of austenitic stainless steel powder can be compensated. This ensures that the carbon content is appropriate during the reaction and combines with elements such as chromium and molybdenum in the alloy, resulting in the precipitation of a large number of nanoscale carbides within the crystal. Therefore, the amount of carbon added should not be excessive. Too much carbon will result in too much liquid phase during sintering, causing deformation of the sintered parts and leading to the growth of the second phase, which reduces corrosion resistance.

[0019] In a further preferred embodiment, the adhesive is selected from at least one of polyoxymethylene, EVA, paraffin wax, beeswax, and polyethylene. The polyethylene is low-density polyethylene.

[0020] In a further preferred embodiment, the amount of the binder added is 8.8-9.5% of the mass of the austenitic stainless steel powder.

[0021] In a further preferred embodiment, during injection molding, the mold temperature is 90-100℃, the injection molding temperature is 175-185℃, and the injection molding pressure is 80-90MPa.

[0022] In a further preferred embodiment, the catalytic degreasing is carried out in a fuming nitric acid atmosphere, the catalytic degreasing temperature is 110-120℃, and the catalytic degreasing time is 4-6 hours.

[0023] In a further preferred embodiment, the sintering temperature is 1290-1340℃ and the sintering time is 1-3h.

[0024] More preferably, the sintering process is divided into four stages. The first stage is a negative pressure degreasing stage, which involves heating to 250-350℃ at a rate of 3-5℃ / min and holding for 30-120 min, then heating to 500-700℃ at a rate of 3-5℃ / min and holding for 30-240 min. During the negative pressure degreasing stage, the gas pressure is controlled at 0.001-0.1 Pa, and the flow rate of the protective gas is 20-30 L / min. The second stage is a vacuum internal firing stage, which involves heating to 1000℃ at a rate of 3-5℃ / min. The temperature is held at 1100℃ for 30-90 minutes. During the vacuum firing stage, the gas pressure is controlled at 0.001-0.1 Pa, and no protective gas is introduced. The third stage is the partial pressure sintering stage: the temperature is increased to 1290-1340℃ at a heating rate of 3-5℃ / min, and held for 120-240 minutes. During the partial pressure sintering stage, the partial pressure of the protective gas is controlled at 10-25 kPa, and the flow rate of the protective atmosphere is 20-30 L / min. The fourth stage is the cooling stage, where the temperature is reduced to 850℃ at a cooling rate of 5-15℃ / min, and then cooled with the furnace.

[0025] In this invention, partial degreasing is first performed using catalytic degreasing, and then the resulting degreased blank is placed in a vacuum sintering furnace. Since the catalytic degreasing process cannot completely degrease the binder, it is necessary to perform in-scaling degreasing in the early stage of atmosphere sintering, and then continue to raise the temperature for sintering. Then, the residual air after degreasing is further discharged through the vacuum inner firing stage, and preliminary sintering is performed. Finally, dense austenitic stainless steel parts are obtained by partial pressure sintering. In this invention, by controlling the composition and in conjunction with the segmented sintering process of this invention, dense sintering can be achieved at a lower temperature, thus further avoiding grain growth.

[0026] The product obtained after sintering using the above-described process according to the present invention has a density exceeding 7.9 g / cm³. 3 It is nearly fully dense, and a distinct dense layer is formed on the sample surface, which helps in the polishing of stainless steel parts.

[0027] More preferably, the protective gas is argon.

[0028] In a further preferred embodiment, the heat treatment process is as follows: first, a solution treatment is performed, followed by an aging treatment. The solution treatment temperature is 1100-1250℃, and the solution treatment time is 0.5-2h. After the solution treatment is completed, the sample is water-cooled. The aging treatment temperature is 870-970℃, and the aging treatment time is 0.5h-4h. After the aging treatment is completed, the sample is water-cooled.

[0029] Finally, this invention involves a solution treatment at a higher temperature, which allows the molybdenum-containing second phase formed during sintering to dissolve into the matrix and allows other elements to diffuse evenly, thereby reducing segregation. Then, an aging treatment is performed at a relatively lower temperature. At the aging temperature of this invention, carbides can easily precipitate without growing and can be evenly dispersed, while the molybdenum-containing intermediate phase is difficult to precipitate in large quantities, thus achieving a better strengthening effect.

[0030] Principles and advantages

[0031] This invention provides an austenitic stainless steel powder with a high content of molybdenum. This molybdenum can combine with carbon in the steel to precipitate fine carbides that strengthen the stainless steel. An appropriate amount of copper is also added. Copper has a melting point of only 1083 degrees Celsius, far below the sintering temperature, which significantly lowers the liquidus line of the stainless steel, reducing the difficulty of sintering the material. Copper also has a certain solid solution strengthening effect, and the released copper ions have a certain antibacterial effect, which is beneficial to improving the biocompatibility of the alloy. In addition, the addition of a small amount of boron also lowers the sintering temperature of the material, and the addition of a small amount of boron is beneficial to the densification of the material and the refinement of the grains. With the synergy of the above components, the austenitic stainless steel powder can be sintered densely at a lower sintering temperature, with a uniform distribution of the second phase, and coarseness of the second phase is avoided.

[0032] When the aforementioned austenitic stainless steel powder is used to prepare austenitic stainless steel parts, the sintered billet, after heat treatment, exhibits fine carbide precipitation in the alloy, significantly increasing its hardness to 200-290 HV. Molybdenum is a strong strengthening element, combining with carbon to form carbides, thus significantly improving the alloy's strength. However, molybdenum-containing second phases may aggregate and precipitate at grain boundaries, reducing the alloy's hardness and corrosion resistance.

[0033] This invention achieves a better strengthening effect by adjusting the carbon content of the powder and the heat treatment process to make the second phase evenly distributed, without reducing the corrosion resistance.

[0034] Furthermore, by incorporating elements such as copper into the composition, this invention facilitates sintering of the alloy, enabling it to approach full density at 1340 degrees Celsius, thus significantly reducing the sintering temperature. Attached Figure Description

[0035] Figure 1This describes the precipitate morphology of the modified 316L austenitic stainless steel sample after heat treatment according to the present invention. Figure 1 The precipitate morphology of heat-treated modified 316L austenitic stainless steel samples with added (a) 0 ppm; (b) 500 ppm; (c) 750 ppm; (d) 1000 ppm; (e) 1500 ppm; (f) 2000 ppm; (g) 3000 ppm; (h) 5000 ppm carbon powder were determined.

[0036] It can be seen in low-carbon alloys ( Figure 1 In cases a and 1b), a distinct high-molybdenum second phase is present at the grain boundaries, while carbides within the grains are extremely rare. However, when the carbon content increases to 750 ppm, the high-molybdenum phase at the grain boundaries significantly decreases and transforms from a coarse network to a needle-like or spherical structure. Simultaneously, granular carbides are dispersed within the grains. However, adding carbon exceeding 3000 ppm easily causes overheating of the stainless steel, thus requiring a reduction in sintering temperature, resulting in numerous pores. Figure 1 g and 1h).

[0037] Figure 2 TEM images of modified 316L austenitic stainless steel samples. TEM images of modified 316L austenitic stainless steel samples: (a) Twins in heat-treated 316L austenitic stainless steel with 0 ppm carbon powder addition; (b, c) Nanotwins and their SAED patterns in 316L austenitic stainless steel with 0 ppm carbon powder addition, respectively; (d) Carbide precipitation in 316L austenitic stainless steel with 0 ppm carbon powder addition; (e, f) Carbides and their SAED patterns in 316L austenitic stainless steel with 750 ppm carbon powder addition, respectively; (g)

[0038] Grain boundary carbides in 316L austenitic stainless steel modified with 750ppm carbon powder; (h, i) represent carbide twins and their SAED patterns in 316L austenitic stainless steel modified with 750ppm carbon powder, respectively; (j) represent regular blocky carbides in 316L austenitic stainless steel modified with 1000ppm carbon powder, respectively; (k, l) represent large-particle carbides and their SAED patterns in 316L austenitic stainless steel modified with 2000ppm carbon powder, respectively.

[0039] Stainless steel contains a large number of microtwin structures. Figure 2 (a) and (2b) This structure is highly favorable for plasticity. In low-carbon samples, such as those with an addition of 500-1000 ppm, the steel contains a large number of irregularly shaped carbide particles with a size of 50-150 nm. As the amount of carbon powder added increases, the carbides gradually transform into rhomboid or rectangular morphologies (e.g., ...). Figure 2 j). They also have a coherent relationship with the matrix (see j). Figure 2f) This microstructure has a good strengthening effect on stainless steel. In high-carbon samples... Figure 2 Large carbides were found in the K matrix, which are incompatible with the matrix. Therefore, high carbon addition is detrimental to the properties of stainless steel.

[0040] Figure 3 These are the polarization curves of the modified 316L austenitic stainless steel samples of this invention. The polarization curves are for sintered 316L stainless steel and carbon-modified 316L austenitic stainless steel samples.

[0041] Figure 4 The impedance reaction of stainless steel samples with different concentrations of carbon added after heat treatment according to this invention.

[0042] Impedance response of sintered 316L stainless steel and heat-treated 316L modified austenitic stainless steel samples with different concentrations of carbon powder: (a) Nyquist plot; (b, c) Bode plot; (d) Equivalent circuit used in ZSimpWin software.

[0043] Figure 3 The results show that, except for modified austenitic stainless steel without added carbon powder, the self-corrosion density of modified austenitic stainless steel with 500-1500 ppm of carbon is lower than that of 316L stainless steel.

[0044] Figure 4 The results showed that the Rct values ​​(charge transfer resistance) of modified stainless steel samples with added carbon powder (500-3000 ppm) were all greater than those of 316L stainless steel. This indicates that the corrosion resistance of modified austenitic stainless steel with added carbon powder is superior to that of 316L stainless steel. Detailed Implementation

[0045] Example 1

[0046] This invention uses gas atomization to prepare modified austenitic stainless steel powder. The austenitic stainless steel consists of 21.57% Cr, 14.83% Ni, 6.01% Mo, 2.12% Cu, 0.31% B, 0.026% C, with the balance being iron and unavoidable impurities, of which the oxygen impurity content is 0.078%.

[0047] The specific process of applying the above-mentioned austenitic stainless steel powder to prepare 316L austenitic stainless steel parts is as follows: Carbon powder with an average particle size of 7.44 micrometers and modified 316L austenitic stainless steel powder with an average particle size of 12.09 micrometers, binder (polyoxymethylene), are mixed, crushed, and then injection molded. The amount of carbon powder added is 500 ppm of the modified 316L austenitic stainless steel powder mass, and the binder is 9.05%. The injection molding process is as follows: mold temperature 95℃, maximum injection temperature 170.45℃, and injection pressure 88.72 MPa. A two-step degreasing method is used to remove the binder. The injection blank first undergoes catalytic degreasing in a fuming nitric acid atmosphere at a temperature of 110℃ for 4 hours. Catalytic degreasing cannot completely remove the binder; therefore, thermal degreasing is required in the early stage of atmosphere sintering. The sintering process mainly consists of four parts: negative pressure degreasing (i.e., thermal degreasing), vacuum internal sintering, partial pressure sintering, and forced cooling. Throughout the experiment, only the partial pressure sintering stage showed variation due to differences in the highest sintering temperature; the other stages were the same. Negative pressure degreasing stage: The temperature was increased to 300℃ at 4℃ / min and held for 60 min, then increased to 600℃ at 4℃ / min and held for 120 min, with the gas pressure at 0 and the gas flow rate at 25 L / min. Vacuum firing stage: The temperature was increased to 1050℃ at 4℃ / min and held for 60 min, with both gas pressure and flow rate at 0. Partial pressure sintering stage: The temperature was increased to 1335℃ at 4℃ / min and held for 180 min, then decreased to 850℃ at 10℃ / min after sintering. Throughout the partial pressure sintering stage, the gas pressure was maintained at 15 kPa and the gas flow rate at 25 L / min. Argon was used as the protective gas throughout the process. The density of the obtained stainless steel parts exceeded 7.9 g / cm³. 3 A distinct dense layer forms on the sample surface, which is beneficial for polishing stainless steel parts. The alloy requires heat treatment, the process being: solution treatment at 1250 degrees Celsius for 0.5 hours, followed by water cooling; aging at 950 degrees Celsius for 0.5 hours, followed by water cooling.

[0048] The alloy sample with 500 ppm carbon added achieved a hardness of 184.6 ± 5 Hv after heat treatment, as shown by electron microscopy analysis. Figure 1 b) Numerous blocky precipitates were found at the grain boundaries of the alloy, while some grain boundaries consisted of continuous short rod-shaped precipitates. No obvious precipitates were found within the grains. Energy dispersive spectroscopy (EDS) analysis confirmed that these precipitates were all M... 23 C6 type carbides improve hardness to some extent. Salt spray and electrochemical experiments were conducted under the same conditions, and the results showed a charge transfer resistance of 184 kΩcm. 2 The self-corrosion density of the alloy with 500 ppm carbon is 0.015 μA / cm³. 2 The charge transfer resistance is 6740 kΩcm. 2In electrochemical corrosion experiments, the higher the self-corrosion current density, the lower the corrosion resistance of the material; the higher the charge transfer resistance value, the better the corrosion resistance. This demonstrates that the modified 316L stainless steel with 500 ppm carbon addition exhibits superior corrosion resistance compared to 316L alloy.

[0049] Example 2

[0050] This invention uses gas atomization to prepare modified austenitic stainless steel powder. The austenitic stainless steel consists of 22.51% Cr, 15.45% Ni, 4.41% Mo, 0.56% Cu, 0.34% B, 0.0042% C, with the balance being iron and unavoidable impurities, of which the oxygen impurity content is 0.117%.

[0051] The above-mentioned austenitic stainless steel powder was used to prepare 316L austenitic stainless steel parts, wherein the amount of carbon powder added was 1500 ppm of the mass of the modified 316L austenitic stainless steel powder, and the sintering temperature during partial pressure sintering was 1320℃; other process parameters were the same as in Example 1.

[0052] The density of the obtained stainless steel parts can exceed 7.9 g / cm³. 3 A distinct dense layer forms on the sample surface, which facilitates the polishing of stainless steel parts. The stainless steel parts are then subjected to heat treatment: solution treatment at 1200°C for 0.5 hours, followed by water cooling; aging at 950°C for 0.5 hours, followed by water cooling. The alloy sample with 1500ppm carbon added achieves a hardness of 213.2±6.85 Hv after heat treatment. Electron microscopy analysis (…) Figure 1 e) Numerous needle-like carbides and dispersed fine granular carbides were observed in the alloy. The grain boundary carbides in the sample were relatively small. Energy dispersive spectroscopy analysis confirmed that these precipitates were all M... 23 C6-type carbides improve hardness to some extent. Salt spray and electrochemical experiments conducted under the same conditions showed that the self-corrosion density of the alloy with 1500 ppm carbon addition was 0.054 μA / cm³. 2 The charge transfer resistance is 541 kΩcm. 2 This demonstrates that the modified 316L stainless steel with 1500ppm carbon addition exhibits superior corrosion resistance compared to 316L alloy.

[0053] Example 3

[0054] This invention uses gas atomization to prepare modified austenitic stainless steel powder. The austenitic stainless steel is composed of 22.47% Cr, 14.12% Ni, 5.39% Mo, 1.77% Cu, 0.3% B, 0.06% C, with the balance being iron and unavoidable impurities, of which the oxygen impurity content is 0.13%.

[0055] The above-mentioned austenitic stainless steel powder was used to prepare 316L austenitic stainless steel parts, wherein the amount of carbon powder added was 2000 ppm of the mass of the modified 316L austenitic stainless steel powder, and the sintering temperature during partial pressure sintering was 1300℃; other process parameters were the same as in Example 1.

[0056] The density of the obtained stainless steel parts can exceed 7.9 g / cm³. 3 A distinct dense layer forms on the sample surface, which facilitates the polishing of stainless steel parts. The stainless steel parts are then subjected to heat treatment: solution treatment at 1200°C for 0.5 hours, followed by water cooling; aging at 950°C for 0.5 hours, followed by water cooling. The alloy sample with 2000ppm carbon added achieves a hardness of 214.23±6.12 Hv after heat treatment. Electron microscopy analysis (…) Figure 1 f) Numerous fine granular carbides were observed in the alloy, with fewer grain boundary carbides. Energy dispersive spectroscopy (EDS) analysis confirmed that these precipitates were all M... 23 C6 type carbides improve hardness to some extent. However, under TEM (... Figure 2 k) Coarse carbides were observed, which may be detrimental to corrosion resistance. Salt spray and electrochemical experiments conducted under the same conditions showed that the self-corrosion density of the alloy with 2000 ppm carbon addition was 0.184 μA / cm³. 2 Slightly higher than 316L (0.147μA / cm). 2 The charge transfer resistance is 681 kΩcm. 2 Higher than 316L stainless steel (184kΩcm) 2 This demonstrates that the corrosion resistance of modified 316L stainless steel with 2000ppm carbon addition is no weaker than that of 316L alloy.

[0057] Comparative Example 1:

[0058] This invention uses gas atomization to prepare 316L stainless steel powder. The austenitic stainless steel is composed of 17.44% Cr, 12.07% Ni, 2.42% Mo, and 0.006C, with the balance being iron and unavoidable impurities, of which the oxygen impurity content is 0.067%.

[0059] The above-mentioned stainless steel powder was used to prepare 316L stainless steel parts without adding carbon. The sintering process was the same as in Example 1, except that the sintering temperature was 1400℃. The density of the obtained stainless steel parts could exceed 7.9 g / cm³. 3A distinct dense layer forms on the sample surface, which facilitates the polishing of stainless steel parts. However, at the temperatures of 1290–1340°C used in the examples, it is difficult to achieve high density in 316L stainless steel, resulting in a large number of internal pores. Due to its low molybdenum content, the sintered state of 316L is a fully austenitic structure, and even heat treatment cannot improve its strength and hardness; its hardness is only 150.37 ± 8.15 Hv. Salt spray and electrochemical experiments conducted under the same conditions showed that the self-corrosion density of 316L stainless steel is 0.147 μA / cm³. 2 This is higher than the values ​​in all examples except Example 3, while the charge transfer resistance is 184 kΩcm. 2 This is lower than all embodiments.

[0060] Comparative Example 2

[0061] This invention uses gas atomization to prepare modified austenitic stainless steel powder. The austenitic stainless steel is composed of 21.57% Cr, 14.83% Ni, 6.01% Mo, 2.12% Cu, 0.31% B, 0.026% C, with the balance being iron and unavoidable impurities, of which the oxygen impurity content is 0.078%.

[0062] The above-mentioned austenitic stainless steel powder was used to prepare 316L austenitic stainless steel parts, wherein the amount of carbon powder added was 3000 ppm of the mass of the modified 316L austenitic stainless steel powder. The sintering sample preparation process was the same as in Example 1, except that the sintering temperature was 1270℃. Since stainless steel undergoes significant overheating above 1280℃, resulting in a large amount of liquid phase and sample deformation, the sintering temperature had to be lowered. The obtained stainless steel parts had a low density and significant porosity (e.g., ...). Figure 1 As shown in g), the surface lacks a dense layer, which is detrimental to sample polishing and results in a loss of high polishing effect. The alloy requires heat treatment, the process being: solution treatment at 1150 degrees Celsius for 0.5 hours, followed by water cooling; aging at 950 degrees Celsius for 0.5 hours, followed by water cooling. After heat treatment, the alloy sample with 3000 ppm carbon addition achieved a hardness of 217.00 ± 4.95 Hv. A large number of carbides were observed in the alloy, and although there were many pores, they still improved the sample hardness to some extent. Salt spray and electrochemical experiments were conducted under the same conditions, and the results showed that the self-corrosion density of the alloy with 3000 ppm carbon addition was 0.165 μA / cm², and the charge transfer resistance was 641 kΩcm. 2 Its corrosion resistance is comparable to that of 316L.

[0063] Comparative Example 3

[0064] This invention uses gas atomization to prepare modified austenitic stainless steel powder. The austenitic stainless steel is composed of 20.97% Cr, 15.02% Ni, 5.66% Mo, 1.97% Cu, and 0.076% C, and is free of boron. The balance is iron and unavoidable impurities, of which the oxygen impurity content is 0.078%.

[0065] The aforementioned austenitic stainless steel powder was used to prepare 316L austenitic stainless steel parts, with carbon powder added at 1000 ppm of the modified 316L austenitic stainless steel powder mass. The sintering sample preparation process was consistent with Example 1, except for the sintering temperature of 1340℃; higher temperatures would result in significant over-burning. The density of the obtained stainless steel parts was 7.85 g / cm³. 3 The sample contains a certain amount of porosity and lacks a distinct dense layer on the surface, which is detrimental to polishing. The alloy requires heat treatment: solution treatment at 1150°C for 0.5 hours followed by water cooling; aging at 950°C for 0.5 hours followed by water cooling. After heat treatment, the sample achieves a hardness of 181.87 ± 7.10 Hv. A large amount of carbides can be observed in the alloy, and despite the porosity, this contributes to improved hardness. Salt spray and electrochemical experiments were conducted under the same conditions. The results showed that the carbon-added stainless steel alloy exhibited a self-corrosion density of 0.161 μA / cm² and a charge transfer resistance of 163 kΩ / cm². 2 Its corrosion resistance is lower than that of the example. Therefore, the absence of boron is detrimental to the performance of stainless steel.

Claims

1. An application of an easily sinterable, high-hardness austenitic stainless steel powder for injection molding, characterized in that: The austenitic stainless steel powder, by mass percentage, has the following composition: Cr 20~23%, Ni 14~16%, Mo 4.4~6.8%, Cu 0.01~3%, B 0.01~0.5%, C 0.01~0.2%, Mn≦1%, Si≦1%, N≦0.1%, with the balance being Fe and unavoidable impurities; the austenitic stainless steel powder is used as a raw material to prepare austenitic stainless steel parts; The preparation method of the austenitic stainless steel parts is as follows: austenitic stainless steel powder, carbon powder and binder are mixed to obtain feedstock, the feedstock is injection molded to obtain a rough blank, the rough blank is catalytically degreased to obtain a degreased blank, the degreased blank is sintered to obtain a sintered blank, and the sintered blank is heat-treated to obtain the austenitic stainless steel parts. The sintering temperature is 1290-1340 ° C, the sintering time is 1-3h; The heat treatment process is as follows: first, a solution treatment is performed, followed by an aging treatment. The solution treatment temperature is 1100-1250℃, and the solution treatment time is 0.5-2h. After the solution treatment is completed, the sample is cooled with water. The aging treatment temperature is 870-970℃, and the aging treatment time is 0.5h-4h. After the aging treatment is completed, the sample is cooled with water.

2. The application of the easily sinterable, high-hardness austenitic stainless steel powder for injection molding according to claim 1, characterized in that: The amount of carbon powder added is 100-2000 ppm of the mass of the austenitic stainless steel powder.

3. The application of the easily sinterable, high-hardness austenitic stainless steel powder for injection molding according to claim 1, characterized in that: The adhesive is selected from at least one of polyoxymethylene, EVA, paraffin wax, beeswax, and polyethylene; The amount of binder added is 8.8-9.5% of the mass of the austenitic stainless steel powder.

4. The application of the easily sinterable, high-hardness austenitic stainless steel powder for injection molding according to claim 1, characterized in that: During injection molding, the mold temperature is 90-100℃, the injection molding temperature is 175-185℃, and the injection molding pressure is 80-90 MPa. The catalytic degreasing is carried out in a fuming nitric acid atmosphere, at a temperature of 110-120 °C, for a time of 4-6 h.

5. The application of the easily sinterable, high-hardness austenitic stainless steel powder for injection molding according to claim 1, characterized in that: The sintering process is divided into four stages. The first stage is the negative pressure degreasing stage, which involves heating at a rate of 3-5℃ / min to 250-350℃ and holding for 30-120 min, then heating at a rate of 3-5℃ / min to 500-700℃ and holding for 30-240 min. During the negative pressure degreasing stage, the gas pressure is controlled at 0.001-0.1 Pa, and the flow rate of the protective gas is 20-30 L / min. The second stage is the vacuum internal firing stage, which involves heating at a rate of 3-5℃ / min to 1000-1100℃ and holding for 30-90 min. During the vacuum internal firing stage, the gas pressure is controlled at 0.001-0.1 Pa, and no protective gas is introduced. The third stage is the partial pressure sintering stage, which involves heating at a rate of 3-5℃ / min to 1290~1340℃. ° C, hold at temperature for 120-240 min. During the partial pressure sintering stage, control the partial pressure of the protective gas to be 10-25 kPa and the flow rate of the protective atmosphere to be 20-30 L / min. The fourth stage is the cooling stage, where the temperature is reduced to 850℃ at a rate of 5-15℃ / min, and then cooled with the furnace.

6. The application of the easily sinterable, high-hardness austenitic stainless steel powder for injection molding according to claim 1, characterized in that: The austenitic stainless steel powder is prepared by water atomization or gas atomization. The mass fraction of oxygen in the austenitic stainless steel powder is 0.005~0.4%; The particle size of the austenitic stainless steel powder is: D 10 =3~12 μm, D 50 =9~18μm, D 90 =15~30 μm.

Images