A nitrogen-containing duplex stainless steel material and a pipe valve manufacturing method

Abstract

This invention discloses a method for preparing nitrogen-containing duplex stainless steel materials and pipe fittings, belonging to the field of powder metallurgy preparation technology. The composite powder consists of three parts: a base metal powder, whose chemical composition by weight percentage is Cr 25.0-30.0%, Ni 6.0-10.0%, Mo 3.0-4.0%, C≤0.01%, P≤0.02%, S≤0.01%, with the balance being Fe; a nitride alloy powder, one or more of ferrochromium nitride, ferromanganese nitride, and ferrovanadium nitride, refined by high-energy ball milling to D50≤800nm, achieving a total nitrogen content of 0.3-0.5%; and nano-copper powder with a particle size D50≤200nm, added at an amount of 0.2-0.6%. The preparation method includes stepwise mixing, forming, catalytic degreasing, and sintering under 1-3MPa pressure. This invention introduces volatile elements by using nitride alloys and nano-copper powder, effectively avoiding the composition control difficulties of traditional smelting methods and achieving near-net-shape forming of complex components. The resulting components have a relative density of ≥99%, a duplex structure ratio of approximately 50:50, a tensile strength of ≥730MPa, a yield strength of ≥510MPa, and an elongation of ≥13%, exhibiting both excellent mechanical properties and corrosion resistance, while reducing manufacturing costs.

Description

Technical Field

[0001] This invention belongs to the field of powder metallurgy preparation technology, and specifically provides a method for preparing nitrogen-containing duplex stainless steel materials and pipe valves. Background Technology

[0002] Duplex stainless steel, due to its two-phase microstructure of ferrite (α) and austenite (γ), combines the excellent toughness of austenitic stainless steel with the high strength and stress corrosion resistance of ferritic stainless steel, making it widely used in marine engineering, petrochemicals, and shipbuilding. With the increasing demands for material performance in modern industry, high-nitrogen duplex stainless steel, prepared by introducing nitrogen (N) into stainless steel, can achieve both excellent strength and toughness while maintaining higher resistance to pitting and crevice corrosion. However, the conventional preparation of high-nitrogen duplex stainless steel faces the following technical challenges: 1. High smelting difficulty and cost. The smelting temperature of high-nitrogen duplex stainless steel reaches above 1500℃, and common alloying elements, such as nitrogen, copper, and manganese, which are volatile, are difficult to maintain stably in the melt under normal pressure. To obtain the target nitrogen content (usually 0.3-0.5%), special processes such as nitrogen-pressurized melting or pressurized electroslag remelting are required, resulting in large equipment investments and significantly increased preparation costs; 2. Poor hot working performance. While high-chromium, high-molybdenum, and high-nitrogen alloys ensure corrosion resistance and strength, they can lead to the precipitation of intermetallic compounds (such as σ and χ phases) along grain boundaries during hot working, significantly increasing matrix brittleness and making subsequent deformation processes such as forging and rolling difficult. For complex precision pipes and valves, extensive machining is often required to form specialized pipes, resulting in low material utilization and long manufacturing cycles. Furthermore, oxygen content control is challenging. Chromium is a highly oxidizable element, easily introducing oxide inclusions during conventional smelting and casting processes, impairing the material's fatigue and corrosion resistance. Traditional methods use carbon deoxidation, but excessively high carbon content affects nitrogen solubility in molten steel (carbon increases nitrogen activity coefficient and reduces nitrogen solubility), placing extremely high demands on the smelting process.

[0003] Emerging fields such as large-scale supercomputing clusters and new energy vehicles have an urgent need for thermal management materials and supporting components, with copper being used in large quantities. With the rapid rise in global copper prices, nitrogen-containing duplex stainless steel, which possesses excellent toughness and corrosion resistance, can be used in non-heat-dissipating core areas such as pipes and valves to reduce costs. Metal powder metallurgy technology can form complex duplex stainless steel pipes and valve components in a single process, requiring little or no subsequent processing and achieving high material utilization. Patent ZL200810119091.5 discloses a method for preparing nitrogen-containing / high-nitrogen stainless steel parts by sintering isostatic pressing, using stainless steel powder with a nitrogen content of 0.4-1.0% and employing pressure sintering in a nitrogen-containing atmosphere.

[0004] As mentioned above, existing powder metallurgy methods for preparing nitrogen-containing stainless steel have not solved the problem of difficulty in preparing nitrogen-containing stainless steel powder at the front end. At the same time, the densification process relies on pressure sintering in a single nitrogen-containing atmosphere, and the long-term high-temperature sintering process will also lead to coarse grains and reduce the strength and toughness of the material. Summary of the Invention

[0005] To address the above problems, this invention proposes a novel high-nitrogen duplex stainless steel composition suitable for powder injection molding and its preparation method. Through an innovative "step-by-step addition" composition design strategy and a suitable injection molding process, this invention avoids the shortcomings of traditional smelting methods for preparing high-nitrogen duplex stainless steel powder, while achieving low-cost, high-performance, and efficient preparation of complex-shaped high-nitrogen duplex stainless steel pipe and valve components.

[0006] The purpose of this invention is to provide a high-nitrogen duplex stainless steel composition suitable for powder injection molding, and a complete process method for forming complex components using powder of this composition, so as to solve the problems of high preparation cost, high processing difficulty and difficulty in composition control in the prior art.

[0007] In a first aspect, the present invention provides a high-nitrogen duplex stainless steel composite powder for powder injection molding, characterized in that the composite powder is composed of the following three components: (1) The basic metal powder, whose chemical composition by weight percentage is: Cr: 25.0-30.0%; Ni: 6.0-10.0%; Mo: 3.0-4.0%; C: ≤0.01%; P: ≤0.02%; S: ≤0.01%; the balance is Fe and unavoidable impurities; (2) The nitride alloy powder is one or more of ferrochromium nitride, ferromanganese nitride and ferrovanadium nitride. The amount added is based on the mass fraction of nitrogen element, so that the total nitrogen content in the composite powder reaches 0.3-0.5%; the nitride alloy powder is refined by high-energy ball milling, and the particle size D50≤800nm; (3) Nano copper powder, prepared by liquid phase reduction method, with a particle size D50≤200nm, and its addition amount is 0.2-0.6% of the total mass of composite powder.

[0008] Furthermore, the basic metal powder is prepared by water atomization, and the powder particle size distribution is D10: 2-5μm, D50: 10-15μm, D90: 25-30μm.

[0009] Furthermore, the high-energy ball milling refining process of the nitride alloy powder is as follows: using cemented carbide balls as the ball milling medium, with a ball-to-material ratio (mass ratio) of 8:1 to 12:1; filling the ball milling jar with high-purity nitrogen as a protective atmosphere; using a mixed solution of ammonia and anhydrous ethanol as the ball milling medium, wherein the volume percentage of ammonia is 5-10%; the ball milling speed is 200-300 rpm, and the ball milling time is 20-40 h.

[0010] Furthermore, the liquid-phase reduction method for preparing the nano-copper powder is as follows: using copper sulfate as the copper source, ammonia as the complexing agent, and hydrazine hydrate or ascorbic acid as the reducing agent, the reaction is carried out at a temperature of 50-70℃ for 1-3 hours in the presence of a surfactant (such as polyvinylpyrrolidone), and then obtained by centrifugation, washing, and vacuum drying.

[0011] Secondly, the present invention provides a method for preparing complex high-nitrogen duplex stainless steel components using the above-mentioned composite powder, characterized by comprising the following steps: (1) Preparation of feedstock by mixing: The base metal powder, alloy nitride powder, and nano copper powder are mixed with the binder to obtain the feedstock for injection molding; the mixing process adopts a step-by-step feeding method: the binder is added first, and after melting at the mixing temperature, the alloy nitride powder and nano copper powder are added and mixed evenly, and then the base metal powder is added and mixed until uniform; the amount of binder added is 8-15% of the total mass of the composite powder; (2) Injection molding: The feed material obtained in step (1) is injected into the mold cavity to obtain a green blank of a complex component; (3) Degreasing: The green body obtained in step (2) is subjected to catalytic degreasing to remove most of the binder and obtain a degreased green body; (4) Sintering: The degreased blank obtained in step (3) is placed in a gas pressure sintering furnace and sintered under a nitrogen atmosphere. The sintering pressure is 1-3 MPa, the sintering temperature is 1250-1400 ℃, and the holding time is 0.5-4 h to obtain a high-density sintered blank. Further, the binder mentioned in step (1) is a plastic-based binder system, including 80-89.5% of the main skeleton polymer polyoxymethylene, 3-10% of the secondary skeleton polymer polyethylene or polypropylene, 1-5% of the interface modifier ethylene-vinyl acetate copolymer, 0.5-5% of the interface modifier stearic acid / zinc stearate and 1-2% lubricating oil; the mixing temperature is 170-190℃ and the mixing time is 1.5-3h.

[0012] Furthermore, the injection molding process parameters in step (2) are: injection temperature 180-210℃, mold temperature 120-140℃, injection pressure 100-140 MPa, and holding time 5-15s.

[0013] Furthermore, in step (3), catalytic degreasing is carried out in an oxalic acid or nitric acid atmosphere, with a degreasing temperature of 110-130℃ and a degreasing time of 3-6h.

[0014] Furthermore, in step (4), the sintering process adopts a segmented heating system: first, the temperature is raised to 600-800℃ at a rate of 5-10℃ / min, and held for 30-60min to remove residual binder; then, the temperature is raised to the sintering temperature at a rate of 3-8℃ / min, while nitrogen is introduced to make the pressure inside the furnace reach 1-3 MPa, and the furnace is held for sintering; after sintering, the furnace is cooled to room temperature.

[0015] Thirdly, the present invention provides a complex nano-high nitrogen duplex stainless steel component obtained by the above preparation method, characterized in that the component has a relative density ≥99%, the volume ratio of ferrite to austenite in the duplex structure of the heat-treated part is close to 50:50, the tensile strength ≥730 MPa, the yield strength ≥510 MPa, and the elongation ≥13%.

[0016] Compared with the prior art, the technical solution of the present invention has the following beneficial effects: 1. Innovative step-by-step composition design completely avoids the volatilization problem of traditional smelting methods. This invention breaks down the alloy composition into three parts: "basic powder + nitride alloy + nano copper powder". The basic powder does not contain volatile elements such as nitrogen, copper, and manganese, and can be directly prepared using a low-cost water atomization method (the water-atomized powder is irregular in shape, which is beneficial to the strength of the green blank); nitrogen is introduced in the form of nitride alloy, avoiding the volatilization of nitrogen in the high-temperature melt; copper is introduced in the form of nano copper powder, avoiding the volatilization and segregation of copper during the smelting process. This design fundamentally solves the common industry problem of difficulty in controlling volatile elements in the preparation of high-nitrogen duplex stainless steel.

[0017] 2. The design of nano-nitride alloys achieves synergistic effects of uniform nitrogen enrichment and sintering densification. Nitride alloys are ball-milled to the nanoscale (D50≤500nm) using high-energy methods. During the mixing process, the shear dispersion effect of the binder allows them to uniformly adhere to the surface of the base powder particles. During sintering, the nanoscale nitride alloys possess high surface energy and diffusion activity, enabling them to decompose and release nitrogen atoms at lower temperatures, achieving uniform nitriding of the matrix. Simultaneously, the chromium, manganese, vanadium, and other elements released during the decomposition of the nitride alloys further strengthen the matrix. Compared to directly using micron-sized nitrides, the nanoscale design significantly improves the uniformity of nitrogen distribution and utilization rate.

[0018] 3. Low-temperature liquid-phase sintering promoting effect of nano-copper powder. Nano-copper powder (D50≤200nm) has a low melting point (the melting point is further reduced at the nanoscale). During the sintering heating process, it can form a transient liquid phase at a relatively low temperature (about 900-1000℃), effectively wetting the surface of solid particles and promoting particle rearrangement and densification. At the same time, copper, as an austenite-forming element, can adjust the ratio of the two phases; the solid solution of trace amounts of copper can also improve the corrosion resistance of the matrix in reducing media and play a strengthening role.

[0019] 4. Gas-pressure sintering achieves a balance between efficient densification and stable composition control. Using nitrogen gas pressure sintering at 1-3 MPa serves two purposes: firstly, according to Syved's law, the higher nitrogen partial pressure suppresses nitrogen escape during sintering and can even supplement a small amount of nitrogen, ensuring the stable maintenance of the target nitrogen content; secondly, the high-pressure atmosphere helps overcome the gas pressure within closed pores, promoting pore shrinkage and closure, resulting in high density (≥99%). Pressure sintering also suppresses the volatilization of elements such as manganese and chromium at high temperatures, maintaining the accuracy of the chemical composition. Attached Figure Description

[0020] Figure 1 The image shown is a scanning electron microscope (SEM) image of the ferrochromium nitride powder used in Example 1 of this invention after high-energy ball milling, showing that the powder particle size reaches the nanoscale.

[0021] Figure 2 The macroscopic morphology of the high-nitrogen duplex stainless steel tensile part prepared in Example 1 of the present invention.

[0022] Figure 3 The image shows the metallographic structure of the high-nitrogen duplex stainless steel sintered billet prepared in Example 1 of this invention, displaying the duplex structure characteristics of ferrite (dark) and austenite (light). Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below with reference to specific embodiments. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0024] This invention discloses a nitrogen-containing duplex stainless steel, which comprises a base metal powder, a nitride alloy powder, and nano-copper powder.

[0025] The base metal powder contains the following elements by weight percentage: Cr: 25.0-30.0 WT; Ni: 6.0-10.0 WT; Mo: 3.0-4.0 WT; C: ≤0.01 WT; P: ≤0.02 WT; S: ≤0.01 WT; with the balance being Fe and unavoidable impurities. The addition amount is based on the mass percentage of nitrogen, ensuring a total nitrogen content of 0.3-0.5 WT in the nitride alloy powder. The amount of nano copper powder added is 0.2-0.6 WT of the total mass of nitrogen-containing stainless steel.

[0026] Furthermore, the nitrogen-containing duplex stainless steel is characterized in that the base metal powder is prepared by water atomization, and the powder particle size distribution is D10: 2-5μm, D50: 10-15μm, D90: 25-30μm. The nitride alloy powder is one or more of ferrochromium nitride, ferromanganese nitride, and ferrovanadium nitride, and the nitride alloy powder is refined by high-energy ball milling, with a particle size D50 ≤ 800nm; the nano copper powder is prepared by liquid-phase reduction, with a particle size D50 ≤ 200nm.

[0027] This invention also discloses a method for preparing complex structures using the above-mentioned nitrogen-containing duplex stainless steel, specifically including the following steps: S1) Mixing to prepare feedstock: The base metal powder, nitride alloy powder, nano copper powder, and binder are mixed to obtain a feedstock for injection molding; the mixing process adopts a step-by-step feeding method: the binder is added first, melted at the mixing temperature, then the nitride alloy powder and nano copper powder are added, mixed evenly, and then the base metal powder is added, and mixing continues until uniform; the amount of binder added is 8-15% of the total mass of the composite powder; S2) Injection molding: The feed material obtained in step S1) is injected into the mold cavity to obtain a green blank of a complex component; S3) Degreasing: The green body obtained in step S2) is subjected to catalytic degreasing to remove most of the binder and obtain a degreased green body; S4) Sintering: The degreased blank obtained in step S3) is placed in a pressure sintering furnace and sintered under a nitrogen atmosphere. The sintering pressure is 1-3 MPa, the sintering temperature is 1250-1400 ℃, and the holding time is 0.5-4 h to obtain a complex structure of high-density nitrogen-containing biaxial stainless steel. Furthermore, the binder in S1) is a plastic-based binder system, comprising 80-89.5% of the main skeleton polymer polyoxymethylene, 3-10% of the secondary skeleton polymer polyethylene or polypropylene, 1-5% of the interface modifier ethylene-vinyl acetate copolymer, 0.5-5% of the interface modifier stearic acid / zinc stearate, and 1-2% lubricating oil; the mixing temperature is 170-190℃, and the mixing time is 1.5-3h.

[0028] Furthermore, the injection molding process parameters in S2) are: injection temperature 180-210℃, mold temperature 120-140℃, injection pressure 100-140 MPa, and holding time 5-15s.

[0029] Furthermore, in step S3), the catalytic degreasing is carried out in an oxalic acid or nitric acid atmosphere, the degreasing temperature is 110-130℃, and the degreasing time is 3-6h.

[0030] Furthermore, the sintering process in S4) adopts a segmented heating regime: first, the temperature is raised to 600-800℃ at a rate of 5-10℃ / min and held for 30-60min to remove residual binder; then, the temperature is raised to the sintering temperature at a rate of 3-8℃ / min, while nitrogen is introduced to make the pressure inside the furnace reach 1-3 MPa, and the furnace is held for sintering; after sintering, the furnace is cooled to room temperature.

[0031] Furthermore, the high-density nitrogen-containing biaxial stainless steel complex structural components prepared by the above method have a relative density ≥99%, a volume ratio of ferrite to austenite in the duplex microstructure after heat treatment of 45:55 to 55:50, a tensile strength ≥730 MPa, a yield strength ≥510 MPa, and an elongation ≥13%.

[0032] Example 1

[0033] (I) Preparation of nano-nitride alloys Ferrochromium nitride raw material powder (N content 8.5wt%, Cr content 62wt%, Fe content balance, D50≈15μm) was placed in a high-energy ball mill jar, using cemented carbide balls (a mixture of 5mm and 10mm diameter balls) at a ball-to-material mass ratio of 10:1. The ball mill jar was filled with high-purity nitrogen (99.999% purity) as a protective atmosphere. The milling media consisted of a mixed solution of ammonia (8% by volume) and anhydrous ethanol (92% by volume), added in an amount sufficient to achieve a slurry solid content of approximately 30 vol%. The ball milling speed was 250 rpm, and the milling time was 30 h. After milling, the slurry was dried in a vacuum drying oven at 60℃ for 24 h to obtain ferrochromium nitride nanoparticles. Laser particle size analysis showed a D50 of 680 nm (meeting the requirement of ≤800 nm). Figure 1 As shown, the particle size of the ball-milled ferrochromium nitride powder is significantly refined to the nanoscale, and the particle distribution is relatively uniform, which is beneficial to the uniform diffusion of nitrogen during the subsequent sintering process.

[0034] (II) Preparation of Nano Copper Powder Prepare 500 mL of a 0.2 mol / L copper sulfate solution, adjust the pH to 10-11 with ammonia to form a copper-ammonia complex, and add 5 g of polyvinylpyrrolidone (PVP, molecular weight 10000) as a surfactant, stirring to dissolve. Heat the solution to 60 °C, and slowly add 0.5 mol / L ascorbic acid solution (reducing agent) dropwise with stirring. After the addition is complete, continue the reaction for 2 h. The reaction product is centrifuged (8000 rpm, 10 min), washed three times successively with deionized water and anhydrous ethanol, and finally dried under vacuum at 50 °C for 24 h to obtain nano-copper powder. Scanning electron microscopy and particle size analysis showed a D50 of 120 nm.

[0035] (III) Preparation of basic metal powders The master alloy was prepared according to the following weight percentages: Cr 27.0%, Ni 8.0%, Mo 3.5%, C 0.008% (≤0.01%), P 0.01%, S 0.005%, and Fe balance. Metal powder was prepared using a vacuum induction melting and water atomization method: the master alloy was melted in a vacuum induction furnace (vacuum degree ≤5Pa), held at this temperature for 10 minutes under argon protection, and then flowed out through a nozzle (diameter φ5mm) and atomized using high-pressure water jet at a water pressure of 80MPa. The atomized powder was dehydrated, vacuum dried, and then mechanically vibrated and sieved to obtain powder with a particle size ≤45μm, where D10=3.8μm, D50=13.2μm, and D90=28.5μm, exhibiting an irregular morphology.

[0036] (iv) Feed preparation Based on a total mass of 1000g of composite powder, take 950g of the basic metal powder prepared in step (III), 5g (0.5wt%) of the chromium iron nitride nanopowder prepared in step (I) (the amount to be added is calculated based on the N content as follows: target N content 0.4%, N 4g is required; chromium iron nitride contains N 8.5%, so 47.1g needs to be added), and 5g (0.5wt%) of the nano copper powder prepared in step (II).

[0037] The binder formulation is as follows: 86 wt% polyoxymethylene (POM), 6 wt% high-density polyethylene (HDPE), 3 wt% ethylene-vinyl acetate copolymer (EVA), 3 wt% stearic acid (SA), and 2 wt% lubricating oil (synthetic ester). The total amount of binder added is 10 wt% (i.e., 100 g) of the composite powder mass.

[0038] The mixing process is carried out in an internal mixer at a mixing temperature of 180℃. First, the binder is added to the mixing chamber and, after complete melting (approximately 5 minutes), nano-chromium ferronitride powder and nano-copper powder are added and mixed for 20 minutes to ensure uniform dispersion of the nanoparticles in the binder. Then, the base metal powder is added in batches, and mixing continues for 40 minutes until the feedstock is uniformly mixed. Finally, the feedstock is extruded and granulated to obtain granules for injection molding.

[0039] (v) Injection molding The feed material obtained in step (iv) is added to the injection molding machine. The injection process parameters are: injection temperature 190℃, mold temperature 130℃, injection pressure 130MPa, holding time 10s, and cooling time 20s. Injection yields standard tensile specimen blanks and green blanks for complex structure test pieces. For example... Figure 2 As shown, the resulting stretched part is fully formed, with clear edge contours and no obvious defects such as cracks, collapses or missing materials.

[0040] (vi) Defatting The green body obtained in step (v) was placed in a catalytic degreasing furnace and degreased under an oxalic acid atmosphere. The degreasing temperature was 120℃, the oxalic acid flow rate was 1.5L / min, and the degreasing time was 5h. The weight loss of the green body after degreasing was approximately 7.8%.

[0041] (vii) Sintering The degreased blank obtained in step (vi) was placed in a gas pressure sintering furnace and a segmented sintering process was adopted: in the first stage, the temperature was raised to 650°C at 8°C / min and held for 40 min, and high-purity argon gas (flow rate 100 L / min) was introduced. The pressure inside the furnace was kept slightly positive to further remove residual binder; in the second stage, the temperature was raised to 1380°C at 5°C / min, and the argon gas was turned off and high-purity nitrogen gas was introduced to make the pressure inside the furnace reach 2.5 MPa and held for 3 h; after the holding period, the furnace was cooled to room temperature.

[0042] (viii) Heat treatment The sintered billet obtained in step (vii) is placed in a vacuum heat treatment furnace, heated to 1100°C under nitrogen protection, held for 1.5 hours, and then quickly removed for water quenching.

[0043] (ix) Performance Testing The obtained samples underwent performance testing: the relative density, measured using the Archimedes displacement method, was 99.4%; metallographic observation showed that ferrite and austenite were evenly distributed, with a phase ratio of approximately 48:52; Figure 3 As shown, after sintering and heat treatment, the material forms a typical dual-phase structure, with the dark area being the ferrite phase and the light area being the austenite phase. The two phases are evenly distributed, and no obvious coarse precipitates or abnormal grains were observed. The tensile properties test results are a tensile strength of 856 MPa, a yield strength of 612 MPa, and an elongation of 13.2%; the Vickers hardness test result is 295 HV.

[0044] Example 2

[0045] This embodiment is basically the same as Embodiment 1, except that the composition of the base metal powder and the type of nitride alloy are adjusted.

[0046] Basic metal powder composition: Cr 25.5%, Ni 9.0%, Mo 3.2%, C 0.009%, P 0.012%, S 0.006%, Fe balance.

[0047] The nitride alloy uses ferromanganese nitride (7.2% N, 75% Mn, balance Fe), which is processed by high-energy ball milling (the ball milling process is the same as in Example 1) to obtain nanoparticles with D50=720nm. The addition amount is calculated based on the target N content of 0.45% (4.5g of N and 62.5g of ferromanganese nitride are required).

[0048] The amount of nano copper powder added is 0.4% (4g).

[0049] The sintering temperature was adjusted to 1360℃, held for 4 hours, and the nitrogen pressure was 2.5 MPa.

[0050] The properties of the obtained sample are: relative density 99.3%, tensile strength 832 MPa, yield strength 587 MPa, elongation 14.5%, and hardness 288 HV.

[0051] Example 3

[0052] This embodiment is basically the same as Embodiment 1, except that the nitride alloy is added in a composite manner.

[0053] Basic metal powder composition: Cr 28.5%, Ni 7.0%, Mo 3.8%, C 0.006%, P 0.008%, S 0.004%, Fe balance.

[0054] The nitride alloy was prepared by mixing ferrochromium nitride (8.5% N content) and ferrovanadium nitride (11% N content, 65% V content, balance Fe) at a mass ratio of 2:1, and obtaining nanocomposite powder with D50=750nm through high-energy ball milling. The total amount added was calculated based on a target N content of 0.35%.

[0055] The amount of nano copper powder added is 0.6% (6g).

[0056] The sintering temperature was adjusted to 1400℃, held for 2.5 hours, and the nitrogen pressure was 2.0 MPa.

[0057] The properties of the obtained sample are: relative density 99.3%, tensile strength 885 MPa, yield strength 645 MPa, elongation 15.8%, and hardness 315 HV.

[0058] Comparative Example 1 The same base metal powder and nano copper powder as in Example 1 were used, but without the addition of nano nitride alloys. Instead, high-pressure nitrogen gas (3 MPa) was directly introduced during the sintering stage for nitriding. The remaining processes were the same as in Example 1.

[0059] The obtained sample exhibits the following properties: a relative density of 98.7%, but nitrogen content analysis revealed a nitrogen content of only 0.13% in the core and 0.23% in the surface, indicating a significant nitrogen distribution gradient. Metallographic analysis showed a high ferrite content (approximately 65%) in the core and a high austenite content (approximately 60%) in the surface, indicating an uneven microstructure. Tensile properties included a tensile strength of 684 MPa, a yield strength of 442 MPa, and an elongation of 7.8%. This demonstrates that atmospheric nitriding alone is insufficient to achieve uniform nitrogen enrichment in thick-section components.

[0060] Comparative Example 2 The same base metal powder and nano-nitride alloy as in Example 1 were used, but without the addition of nano-copper powder. The remaining processes were the same as in Example 1.

[0061] The obtained sample exhibited the following properties: a relative density of 97.5%, lower than that of Example 1 (98.9%), indicating that the liquid-phase sintering promoting effect of the nano-copper powder significantly influences densification. Tensile properties included a tensile strength of 765 MPa, a yield strength of 523 MPa, an elongation of 8.2%, and a hardness of 265 HV. These properties are lower than those of Example 1, suggesting that the nano-copper powder not only promotes densification but also has a strengthening effect.

[0062] Table 1. Ball milling process parameters and powder characteristics of alloy nitride in Examples 1-3 Table 2 Sintering process parameters for Examples 1-3 Table 3 Comparison of mechanical properties of Examples 1-3 and Comparative Examples 1-3 As can be seen from the above embodiments and comparative examples, this invention, through an innovative three-component design of "basic powder + nano-nitride alloy + nano-copper powder," combined with powder injection molding and gas pressure sintering processes, successfully prepared complex high-nitrogen duplex stainless steel components with uniform microstructure and excellent performance. Compared with the traditional smelting-forging process, the process route of this invention effectively avoids the loss of volatile elements, achieves near-net-shape forming of complex components, and significantly reduces manufacturing costs. Compared with the control examples lacking nano-nitride alloy or nano-copper powder, the solution of this invention has significant advantages in terms of densification, microstructure uniformity, and mechanical properties.

[0063] This invention has been described by way of examples, but does not constitute a limitation thereof. Other variations of the disclosed embodiments, which are readily apparent to those skilled in the art, should fall within the scope of the claims of this invention, with reference to the description of this invention.

Claims

1. A nitrogen-containing duplex stainless steel, characterized in that, The nitrogen-containing duplex stainless steel comprises base metal powder, nitride alloy powder, and nano copper powder. The basic metal powder contains the following elements by weight percentage: Cr: 25.0-30.0 WT % Ni: 6.0-10.0 WT % % Mo: 3.0-4.0 WT % % C: ≤0.01 WT %; P: ≤0.02 WT %; S: ≤0.01 WT; balance is Fe and unavoidable impurities; its addition amount is based on the mass percentage of nitrogen element, so that the total nitrogen content in the nitride alloy powder reaches 0.3-0.5 WT; The amount of nano copper powder added is 0.2-0.6 WT of the total mass of nitrogen-containing stainless steel.

2. The nitrogen-containing duplex stainless steel according to claim 1, characterized in that, The base metal powder is prepared by water atomization, and the powder particle size distribution is D10: 2-5μm, D50: 10-15μm, D90: 25-30μm. The nitride alloy powder is one or more of ferrochromium nitride, ferromanganese nitride, and ferrovanadium nitride. The nitride alloy powder is refined by high-energy ball milling, and the particle size D50 ≤ 800nm. The nano copper powder is prepared by liquid-phase reduction, and the particle size D50 ≤ 200nm.

3. A method for preparing complex structures using nitrogen-containing duplex stainless steel as described in claim 1 or 2, characterized in that, Specifically, the following steps are included: S1) Preparation of feedstock by mixing: The base metal powder, alloy nitride powder, and nano copper powder are mixed with a binder to obtain the feedstock for injection molding; the mixing process adopts a step-by-step feeding method: first, the binder is added and melted at the mixing temperature, then the alloy nitride powder and nano copper powder are added and mixed evenly, then the base metal powder is added and the mixing continues until uniform; the amount of binder added is 8-15% of the total mass of the composite powder; S2) Injection molding: The feed material obtained in step S1) is injected into the mold cavity to obtain a green blank of a complex component; S3) Degreasing: The green body obtained in step S2) is subjected to catalytic degreasing to remove most of the binder and obtain a degreased green body; S4) Sintering: The degreased blank obtained in step S3) is placed in a pressure sintering furnace and sintered under a nitrogen atmosphere. The sintering pressure is 1-3 MPa, the sintering temperature is 1250-1400 ℃, and the holding time is 0.5-4 h to obtain a complex structure of high-density nitrogen-containing biaxial stainless steel.

4. The method according to claim 3, characterized in that, The binder in S1) is a plastic-based binder system, comprising 80-89.5% of the main skeleton polymer polyoxymethylene, 3-10% of the secondary skeleton polymer polyethylene or polypropylene, 1-5% of the interface modifier ethylene-vinyl acetate copolymer, 0.5-5% of the interface modifier stearic acid / zinc stearate, and 1-2% lubricating oil; the mixing temperature is 170-190℃, and the mixing time is 1.5-3h.

5. The method according to claim 3, characterized in that, The injection molding process parameters in S2) are: injection temperature 180-210℃, mold temperature 120-140℃, injection pressure 100-140 MPa, and holding time 5-15s.

6. The method according to claim 3, characterized in that, The catalytic degreasing in S3) is carried out in an oxalic acid or nitric acid atmosphere, at a degreasing temperature of 110-130℃, and for a degreasing time of 3-6 hours.

7. The method according to claim 3, characterized in that, The sintering process in S4) adopts a segmented heating regime: first, the temperature is raised to 600-800℃ at a rate of 5-10℃ / min and held for 30-60min to remove residual binder; then, the temperature is raised to the sintering temperature at a rate of 3-8℃ / min, while nitrogen is introduced to make the pressure inside the furnace reach 1-3 MPa, and the furnace is held for sintering. After sintering, the furnace was cooled to room temperature.

8. The method according to any one of claims 3-7, characterized in that, The high-density nitrogen-containing biaxial stainless steel complex structural components prepared by the method have a relative density ≥99%, and after heat treatment, the volume ratio of ferrite to austenite in the duplex microstructure is 45:55 to 55:50, with a tensile strength ≥730 MPa, a yield strength ≥510 MPa, and an elongation ≥13%.

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