A preparation method of core-shell structure high-nitrogen steel composite powder for additive manufacturing

Abstract

This invention relates to the field of additive manufacturing technology and discloses a method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing. The method includes the following steps: hydrogen reduction treatment of high-nitrogen steel powder to remove the surface oxide film, obtaining reduced powder; nitrogen trifluoride activation treatment of the reduced powder to form nitride nuclei on the powder surface, obtaining activated powder; and ammonia nitriding treatment of the activated powder to grow the nitride nuclei into a dense nitride shell, obtaining core-shell structured high-nitrogen steel composite powder. This invention utilizes the synergistic effect of hydrogen reduction and nitrogen trifluoride activation to provide a highly active surface and uniform nucleation sites for ammonia nitriding, resulting in a dense nitride shell covering the powder surface. The obtained powder has a nitrogen content of 1.68%–2.29% and a Hall flow rate of 5.7 s / 50 g–6.5 s / 50 g. When this powder is used in selective laser melting additive manufacturing, the nitrogen retention efficiency reaches 78%~89%, the tensile strength of the printed parts is 782MPa~898MPa, and the elongation after fracture is 18.8%~32.1%.

Description

Technical Field

[0001] This invention relates to the field of additive manufacturing technology, specifically to a method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing. Background Technology

[0002] High-nitrogen steel is a stainless steel material in which nitrogen replaces nickel as the main alloying element. Nitrogen plays a solid solution strengthening role in steel, which can simultaneously improve the strength and corrosion resistance of the material. Moreover, nitrogen is an inexpensive and readily available element, so high-nitrogen steel has an advantage in reducing costs. Selective laser melting additive manufacturing technology can directly form metal parts with complex shapes, and has application prospects in the field of high-nitrogen steel parts preparation.

[0003] The nitrogen content of high-nitrogen steel powder directly determines the mechanical properties of the printed parts. The higher the nitrogen content, the more significant the solid solution strengthening effect. However, during selective laser melting, the high-energy laser beam causes the powder to melt and solidify instantly. The temperature of the molten pool is much higher than the equilibrium solidification temperature of the steel liquid. The solubility of nitrogen in the steel liquid decreases sharply as the temperature decreases. When the molten pool cools rapidly, the supersaturated nitrogen atoms do not have time to be retained in the form of solid solution and will escape from the molten pool in the form of nitrogen gas bubbles, resulting in nitrogen loss. The nitrogen loss rate is usually above 40%, which leads to the nitrogen content of the printed parts being much lower than that of the original powder, and the strength performance cannot meet the expectations.

[0004] Increasing the nitrogen content of the original powder is the main approach to compensate for nitrogen loss during printing. Traditional methods include nitriding the powder in a nitrogen-containing atmosphere or mechanically mixing nitride particles into the powder. However, during gas nitriding, the dense oxide film on the surface of high-nitrogen steel powder hinders the diffusion of nitrogen atoms into the powder interior, resulting in limited penetration depth and difficulty in increasing the nitrogen content in the powder core. Although mechanically mixing chromium nitride or silicon nitride particles can increase the nominal nitrogen content of the mixed powder, the bonding force between the foreign nitride particles and the high-nitrogen steel matrix is ​​weak. During laser melting, the nitride particles are difficult to completely decompose and evenly distribute, which can easily lead to incomplete fusion defects and component segregation in the printed parts. In addition, foreign hard particles can significantly reduce the flowability of the powder and affect the powder spreading quality. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing. This method solves the problems of severe nitrogen loss during additive manufacturing of existing high-nitrogen steel composite powder, resulting in low nitrogen content and insufficient mechanical properties in printed parts, as well as poor powder flowability and a narrow molding process window.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing, comprising the following steps: S1. High-nitrogen steel powder is subjected to hydrogen reduction treatment to obtain reduced powder; S2. The reduced powder is activated by nitrogen trifluoride to obtain activated powder; S3. The activated powder is subjected to ammonia nitriding treatment to obtain core-shell structured high-nitrogen steel composite powder.

[0007] By employing the above technical solution, hydrogen reduction treatment first removes the oxide film on the surface of high-nitrogen steel powder, exposing a fresh metal surface. During storage, a dense chromium oxide or iron oxide film forms on the surface of high-nitrogen steel powder. This oxide film hinders the subsequent diffusion of nitrogen atoms into the matrix. Hydrogen reacts with the surface oxides at 450℃~550℃, generating water vapor which is carried away by the gas flow, restoring the metallic activity of the powder surface.

[0008] Nitrogen trifluoride activation is a key step in the preparation of high-nitrogen-content core-shell structures in this invention. Nitrogen trifluoride reacts with a fresh metal surface at 300°C to 400°C. The fluorine atoms in the nitrogen trifluoride molecule have extremely high electronegativity, enabling them to form metal fluoride intermediates with metal atoms. This reaction can be described as follows: upon contact between metal surface atoms and nitrogen trifluoride molecules, the fluorine atoms steal electrons from the metal atoms to form metal fluoride bonds, simultaneously releasing nitrogen free radicals. These nitrogen free radicals possess extremely high chemical reactivity and can rapidly combine with neighboring metal atoms to form nitride nuclei. The nitride nuclei generated by the nitrogen trifluoride activation treatment are uniformly distributed on the powder surface, providing dense nucleation sites for nitrogen atom deposition during the subsequent ammonia nitriding process.

[0009] Ammonia nitriding is performed at 450℃~550℃, where ammonia decomposes to produce nitrogen and hydrogen atoms. Since nitride nuclei formed by nitrogen trifluoride activation already exist on the powder surface, nitrogen atoms from ammonia decomposition preferentially deposit and grow at these nuclei. The nitrogen atoms combine with elements such as chromium and iron in the powder matrix to form a nitride shell. This shell gradually covers the entire powder surface during growth, eventually forming a complete core-shell structure. The molar fraction of ammonia in the mixed gas is 30%~50%, the total flow rate of the mixed gas is 150ml / min~250ml / min, and the holding time is 3h~7h. A high ammonia partial pressure provides a sufficient nitrogen source, and an appropriate gas flow rate ensures the uniformity of the reaction atmosphere and a continuous supply of fresh gas. The holding time determines the thickness of the nitride shell; by adjusting the holding time, the shell thickness can be controlled, thereby controlling the total nitrogen content of the powder.

[0010] The synergistic effect of the three steps constitutes a complete technical solution. Hydrogen reduction treatment solves the problem of surface oxide layer and provides an active surface for subsequent processing. Nitrogen trifluoride activation treatment introduces uniformly distributed nitride nuclei on the powder surface, solving the problem of uneven deposition of nitrogen atoms preferentially at grain boundaries or defects in traditional nitriding processes. Ammonia nitriding treatment uses these nuclei as a seed layer, enabling the nitride shell to grow uniformly and densely. The absence of any one of these steps will lead to uneven nitride shell or insufficient nitrogen content.

[0011] Preferably, the high-nitrogen steel powder has the following particle size distribution: -270~+400 mesh 25%~30%, -400~+500 mesh 45%~50%, -500~+800 mesh 15%~20%, -800~+1000 mesh 3%~5%, and -1000 mesh 2%~5%. This particle size distribution ensures that the powder has suitable bulk density and flowability, while also ensuring that powders of each particle size grade can achieve uniform treatment effects during reduction, activation, and nitriding processes. Fine powder particles have a large specific surface area, which is conducive to the formation of nitride shells, but too much fine powder will lead to powder agglomeration; coarse powder particles have good flowability, but a small surface area, limiting the increase in nitrogen content. This ratio controls the Hall flow rate of the powder at 5.7s / 50g~6.5s / 50g, and the nitrogen element mass fraction reaches 1.68%~2.29%.

[0012] Preferably, the conditions for hydrogen reduction treatment are: reduction temperature 450℃~550℃, holding time 60min~90min, hydrogen flow rate 100ml / min~200ml / min, and heating rate 5℃ / min~10℃ / min. This temperature range ensures sufficient reduction of the oxide while avoiding sintering and adhesion between powder particles.

[0013] Preferably, the activation conditions for nitrogen trifluoride are as follows: the volume ratio of nitrogen trifluoride to argon is 1:4 to 1:12, the total flow rate of the mixed gas is 50 ml / min to 100 ml / min, the pulse introduction time is 5 min to 15 min, the activation temperature is 300℃ to 400℃, argon is used as a dilution gas to reduce the intensity of the nitrogen trifluoride reaction and avoid local overheating or excessive etching, and the pulse introduction method allows the nitrogen trifluoride gas to come into intermittent contact with the powder surface to ensure uniform reaction.

[0014] Preferably, after the nitrogen trifluoride activation treatment, the process further includes a hydrogen purging step, with a hydrogen flow rate of 100 ml / min to 200 ml / min and a purging time of 5 min to 10 min. The hydrogen purging is used to remove residual fluoride byproducts and unreacted nitrogen trifluoride gas from the powder surface, so as to avoid the residues interfering with the subsequent nitriding process.

[0015] Preferably, after ammonia nitriding treatment, the cooling rate is no more than 10℃ / min, and the cooling is carried out in an ammonia-hydrogen mixed atmosphere. Slow cooling avoids thermal stress causing cracking or peeling of the nitride shell, and the ammonia-hydrogen mixed atmosphere maintains the chemical stability of the nitride during the cooling process.

[0016] Preferably, the conditions for ammonia nitriding treatment are: nitriding temperature 450℃~550℃, ammonia molar fraction in the ammonia-hydrogen mixture 30%~50%, total flow rate of the mixture 150ml / min~250ml / min, and holding time 3h~7h. The hydrogen component in the ammonia-hydrogen mixture is used to regulate the decomposition rate of ammonia, avoid excessively rapid ammonia decomposition leading to nitrogen atom escape, and simultaneously inhibit the denitrification reaction.

[0017] The core-shell high-nitrogen steel composite powder prepared using the above technical solution has a nitrogen content of 1.68%–2.29% by mass and a Hall flow rate of 5.7 s / 50 g–6.5 s / 50 g. During selective laser melting additive manufacturing, the nitride shell decomposes under rapid laser heating, releasing active nitrogen atoms into the molten pool. The nitrogen retention efficiency of the printed parts reaches 78%–89%. The tensile strength of the printed parts reaches 782 MPa–898 MPa, the yield strength reaches 461 MPa–569 MPa, and the elongation after fracture reaches 18.8%–32.1%.

[0018] The preparation method also includes the step of using the obtained core-shell structured high-nitrogen steel composite powder for selective laser melting additive manufacturing: the powder is loaded into a powder cylinder, the substrate is preheated to 150°C, the oxygen content in the forming chamber is reduced to below 11.2 mg / m³, and laser melting is performed using a scanning strategy that rotates adjacent layers by 67°. Preheating the substrate reduces the cooling rate of the molten pool, which is beneficial for the solid solution of nitrogen atoms in the matrix. The low-oxygen environment prevents oxidation of the molten metal. The rotating scanning strategy staggers the melt channels of each layer, reducing residual stress and anisotropy.

[0019] This invention provides a method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing. It has the following beneficial effects: 1. This invention removes the oxide film from the surface of high-nitrogen steel powder through hydrogen reduction treatment, followed by nitrogen trifluoride activation treatment to form uniformly distributed nitride nuclei on the powder surface. Finally, ammonia nitriding treatment allows the nitride nuclei to grow into a complete shell. The synergistic effect of nitrogen trifluoride activation and ammonia nitriding results in a dense nitride shell covering the powder surface, achieving a total nitrogen mass fraction of 1.68%~2.29% and a nitride shell coverage of no less than 86%. Compared with mechanically mixed chromium nitride powder, the shell thickness uniformity is better, and compared with direct gas nitriding of the original powder, the nitrogen element penetrates to a greater depth and is more evenly distributed.

[0020] 2. In this invention, the nitride shell in the core-shell structured high-nitrogen steel composite powder grows in situ on the powder surface without introducing external binders or coarse nitride particles. The smoothness of the powder surface is not significantly affected, and the Hall flow rate of the powder is 5.7s / 50g~6.5s / 50g, which is basically the same as that of the original high-nitrogen steel powder. This flowability ensures the uniformity of powder spreading and the stability of powder feeding in the selective laser melting additive manufacturing process, and reduces the generation of molding defects.

[0021] 3. In this invention, the core-shell structured high-nitrogen steel composite powder is rapidly decomposed under the action of a high-energy laser beam during selective laser melting additive manufacturing, releasing active nitrogen atoms into the molten pool. Due to the nanometer-scale thickness and uniform distribution of the shell, the nitrogen atom release rate and diffusion path are consistent, resulting in a nitrogen retention ratio of 78%–89% in the molten pool. The printed parts exhibit tensile strengths of 782 MPa–898 MPa, yield strengths of 461 MPa–569 MPa, and elongation after fracture of 18.8%–32.1%. Compared to the comparative example, the printed parts show improved strength with minimal loss of plasticity, demonstrating a better overall balance between strength and plasticity. Attached Figure Description

[0022] Figure 1 This is a comparison diagram showing the surface cleaning effect of high-nitrogen steel powder according to the present invention; Figure 2 This is a comparison diagram showing the effect of the surface activation of the present invention on the surface activation effect of high-nitrogen steel powder; Figure 3 This is a comparison chart of the nitrogen content of high-nitrogen steel powder after different treatments according to the present invention; Figure 4 This is a comparison chart of the flowability of high-nitrogen steel powder under different treatment conditions according to the present invention; Figure 5 This is a comparison chart of the effective nitrogen content in the high-nitrogen steel powder of the present invention; Figure 6 This is a comparison chart of nitrogen retention efficiencies of different powders after laser melting additive manufacturing according to the present invention; Figure 7 This is a comparison chart of the room temperature tensile mechanical properties of SLM-printed parts with different powders according to the present invention. Detailed Implementation

[0023] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0025] High-nitrogen steel powder is an austenitic stainless steel powder prepared by gas atomization. Its chemical composition, by mass percentage, consists of the following elements: Chromium (Cr) 17.5-18.5%, Manganese (Mn) 13.5-14.5%, Molybdenum (Mo) 2.5-3.5%, Nitrogen (N) 0.40-0.55%, Oxygen (O) ≤0.10%, Carbon (C) ≤0.03%, Sulfur (S) ≤0.01%, with the balance being Iron (Fe). The powder has a flowability of 25.0-26.0 s / 50g and a loose packing density of 4.0-4.5 g / cm³. 3 The tap density is 4.5-5.0 g / cm³. 3 .

[0026] Nitrogen trifluoride (NF3, CAS No. 7783-54-2) with a purity of not less than 99.9% is used as an activating gas.

[0027] High-purity hydrogen (H2), high-purity ammonia (NH3), high-purity argon (Ar), and high-purity nitrogen (N2) are all commercially available gas products with a purity of not less than 99.9%.

[0028] Preparation Example 1: This preparation example provides a method for preparing core-shell structured high-nitrogen steel composite powder, including the following steps: The graded high-nitrogen steel powder was mixed in the following mass percentages: -270~+400 mesh 25%, -400~+500 mesh 45%, -500~+800 mesh 20%, -800~+1000 mesh 5%, and -1000 mesh 5%. The prepared mixed powder (total mass 100g) was placed in a nickel-based alloy reaction boat and placed in the central constant temperature zone of a tube furnace. High-purity hydrogen (H2) was introduced into the tube furnace at a flow rate of 100ml / min. The temperature was increased to 450℃ at a rate of 5℃ / min and held at that temperature for 60min. After the reduction was completed, the temperature was decreased to 300℃ at a rate of 5℃ / min under a hydrogen atmosphere. Nitrogen trifluoride (NF3) and high-purity argon (Ar) were premixed at a volume ratio of 1:5. The total flow rate of the mixed gas was 50 ml / min. The mixture was pulsed into the reaction tube for 5 min. After activation, the flow of NF3 / Ar mixed gas was stopped, and high-purity hydrogen (flow rate 100 ml / min) was continued to purge for 5 min. Then, under a hydrogen atmosphere, the temperature was increased to 450℃ at a rate of 5℃ / min. The gas was then switched to a mixture of ammonia (NH3) and hydrogen (H2), with the molar fraction of ammonia in the mixture being 30% and the total flow rate of the mixture being 150 ml / min. The mixture was kept at this temperature and nitrided for 3 hours. After nitriding, the NH3 / H2 mixture was continued to be introduced, and the temperature was lowered to room temperature at a rate of ≤10℃ / min. The powder was then collected.

[0029] Preparation Example 2: This preparation example provides a method for preparing core-shell structured high-nitrogen steel composite powder, including the following steps: The graded high-nitrogen steel powder was mixed in the following mass percentages: -270~+400 mesh 30%, -400~+500 mesh 50%, -500~+800 mesh 15%, -800~+1000 mesh 3%, and -1000 mesh 2%. The prepared mixed powder (total mass 100g) was placed in a nickel-based alloy reaction boat and placed in the central constant temperature zone of a tube furnace. High-purity hydrogen (H2) was introduced into the tube furnace at a flow rate of 200ml / min. The temperature was increased to 550℃ at a rate of 10℃ / min and held at that temperature for 90min. After the reduction was completed, the temperature was decreased to 400℃ at a rate of 8℃ / min under a hydrogen atmosphere. Nitrogen trifluoride (NF3) and high-purity argon (Ar) were premixed at a volume ratio of 1:10. The total flow rate of the mixed gas was 100 ml / min. The mixture was then pulsed into the reaction tube for 15 min. After activation, the flow of NF3 / Ar mixed gas was stopped, and high-purity hydrogen gas (flow rate 200 ml / min) was continued to purge for 10 min. Then, under a hydrogen atmosphere, the temperature was raised to 550℃ at a rate of 10℃ / min. The gas was switched to a mixed gas of ammonia (NH3) and hydrogen (H2), with the molar fraction of ammonia in the mixed gas being 50% and the total flow rate of the mixed gas being 250 ml / min. Nitrogenation was carried out at this temperature for 7 h. After nitriding, the NH3 / H2 mixed gas was continued to flow, and the temperature was lowered to room temperature at a rate of ≤10℃ / min. The powder was then collected.

[0030] Preparation Example 3: This preparation example provides a method for preparing core-shell structured high-nitrogen steel composite powder, including the following steps: The graded high-nitrogen steel powder was mixed in the following mass percentages: -270~+400 mesh 28%, -400~+500 mesh 47%, -500~+800 mesh 17%, -800~+1000 mesh 4%, and -1000 mesh 4%. The prepared mixed powder (total mass 100g) was placed in a nickel-based alloy reaction boat and placed in the central constant temperature zone of a tube furnace. High-purity hydrogen (H2) was introduced into the tube furnace at a flow rate of 150ml / min. The temperature was increased to 500℃ at a rate of 8℃ / min and held at that temperature for 75min. After the reduction was completed, the temperature was decreased to 350℃ at a rate of 6℃ / min under a hydrogen atmosphere. Nitrogen trifluoride (NF3) and high-purity argon (Ar) were premixed at a volume ratio of 1:8. The total flow rate of the mixed gas was 75 ml / min. The mixture was pulsed into the reaction tube for 10 min. After activation, the flow of NF3 / Ar mixed gas was stopped, and high-purity hydrogen gas (flow rate 150 ml / min) was continued to purge for 8 min. Then, the temperature was raised to 500℃ at a rate of 8℃ / min under a hydrogen atmosphere. The gas was switched to a mixed gas of ammonia (NH3) and hydrogen (H2), with the molar fraction of ammonia in the mixed gas being 40% and the total flow rate of the mixed gas being 200 ml / min. Nitrogenation was carried out at this temperature for 5 h. After nitriding, the NH3 / H2 mixed gas was continued to flow, and the temperature was lowered to room temperature at a rate of ≤10℃ / min. The powder was then collected.

[0031] Preparation Example 4: This preparation example provides a method for preparing core-shell structured high-nitrogen steel composite powder, including the following steps: The graded high-nitrogen steel powder was mixed in the following mass percentages: -270~+400 mesh 25%, -400~+500 mesh 45%, -500~+800 mesh 20%, -800~+1000 mesh 5%, and -1000 mesh 5%. The prepared mixed powder (total mass 100g) was placed in a nickel-based alloy reaction boat and placed in the central constant temperature zone of a tube furnace. High-purity hydrogen (H2) was introduced into the tube furnace at a flow rate of 120ml / min. The temperature was increased to 480℃ at a rate of 6℃ / min and held at that temperature for 80min. After the reduction was completed, the temperature was decreased to 320℃ at a rate of 5℃ / min under a hydrogen atmosphere. Nitrogen trifluoride (NF3) and high-purity argon (Ar) were premixed at a volume ratio of 1:6. The total flow rate of the mixed gas was 60 ml / min. The mixture was pulsed into the reaction tube for 8 min. After activation, the flow of NF3 / Ar mixed gas was stopped, and high-purity hydrogen gas (flow rate 120 ml / min) was continued to purge for 6 min. Then, the temperature was raised to 480℃ at a rate of 6℃ / min under a hydrogen atmosphere. The gas was switched to a mixed gas of ammonia (NH3) and hydrogen (H2), with the molar fraction of ammonia in the mixed gas being 35% and the total flow rate of the mixed gas being 180 ml / min. Nitrogenation was carried out at this temperature for 4 h. After nitriding, the NH3 / H2 mixed gas was continued to flow, and the temperature was lowered to room temperature at a rate of ≤10℃ / min. The powder was then collected.

[0032] Preparation Example 5: This preparation example provides a method for preparing core-shell structured high-nitrogen steel composite powder, including the following steps: The graded high-nitrogen steel powder was mixed in the following mass percentages: -270~+400 mesh 30%, -400~+500 mesh 50%, -500~+800 mesh 15%, -800~+1000 mesh 3%, and -1000 mesh 2%. The prepared mixed powder (total mass 100g) was placed in a nickel-based alloy reaction boat and placed in the central constant temperature zone of a tube furnace. High-purity hydrogen (H2) was introduced into the tube furnace at a flow rate of 180ml / min. The temperature was increased to 520℃ at a rate of 9℃ / min and held at that temperature for 65min. After the reduction was completed, the temperature was decreased to 380℃ at a rate of 7℃ / min under a hydrogen atmosphere. Nitrogen trifluoride (NF3) and high-purity argon (Ar) were premixed at a volume ratio of 1:9. The total flow rate of the mixed gas was 90 ml / min. The mixture was pulsed into the reaction tube for 12 min. After activation, the flow of the NF3 / Ar mixture was stopped, and high-purity hydrogen (flow rate 180 ml / min) was continuously introduced for purging for 9 min. Then, the temperature was raised to 520 °C at a rate of 9 °C / min under a hydrogen atmosphere. The gas was switched to a mixture of ammonia (NH3) and hydrogen (H2), with the molar fraction of ammonia in the mixture being 45% and the total flow rate of the mixed gas being 220 ml / min. Nitrogen permeation was carried out at this temperature for 6 h. After nitriding, the NH3 / H2 mixture was continued to be introduced, and the temperature was lowered to room temperature at a rate of ≤10 °C / min. The powder was then collected.

[0033] Preparation Example 6: This preparation example provides a method for preparing core-shell structured high-nitrogen steel composite powder, including the following steps: The graded high-nitrogen steel powder was mixed in the following mass percentages: -270~+400 mesh 28%, -400~+500 mesh 47%, -500~+800 mesh 17%, -800~+1000 mesh 4%, and -1000 mesh 4%. The prepared mixed powder (total mass 100g) was placed in a nickel-based alloy reaction boat and placed in the central constant temperature zone of a tube furnace. High-purity hydrogen (H2) was introduced into the tube furnace at a flow rate of 160ml / min. The temperature was increased to 510℃ at a rate of 7℃ / min and held at that temperature for 70min. After the reduction was completed, the temperature was decreased to 340℃ at a rate of 6℃ / min under a hydrogen atmosphere. Nitrogen trifluoride (NF3) and high-purity argon (Ar) were premixed at a volume ratio of 1:7. The total flow rate of the mixed gas was 80 ml / min. The mixture was pulsed into the reaction tube for 6 min. After activation, the flow of NF3 / Ar mixed gas was stopped, and high-purity hydrogen (flow rate 160 ml / min) was continued to purge for 7 min. Then, under a hydrogen atmosphere, the temperature was increased to 510℃ at a rate of 7℃ / min. The gas was then switched to a mixture of ammonia (NH3) and hydrogen (H2), with the molar fraction of ammonia in the mixture being 42% and the total flow rate of the mixture being 210 ml / min. The mixture was kept at this temperature and nitrided for 5.5 h. After nitriding, the NH3 / H2 mixture was continued to be introduced, and the temperature was lowered to room temperature at a rate of ≤10℃ / min. The powder was then collected.

[0034] Preparation Example 7: This preparation example provides a method for preparing core-shell structured high-nitrogen steel composite powder, including the following steps: The graded high-nitrogen steel powder was mixed in the following mass percentages: -270~+400 mesh 25%, -400~+500 mesh 45%, -500~+800 mesh 20%, -800~+1000 mesh 5%, and -1000 mesh 5%. The prepared mixed powder (total mass 100g) was placed in a nickel-based alloy reaction boat and placed in the central constant temperature zone of a tube furnace. High-purity hydrogen (H2) was introduced into the tube furnace at a flow rate of 150ml / min, and the temperature was increased to 500℃ at a rate of 8℃ / min. The temperature was held for reduction for 75min. After the reduction was completed, the temperature was decreased to 350℃ at a rate of 6℃ / min under a hydrogen atmosphere. Nitrogen trifluoride (NF3) and high-purity argon (Ar) were premixed at a volume ratio of 1:12. The total flow rate of the mixed gas was 75ml / min. The mixture was pulsed into the reaction tube for 10min. After the activation was completed, the NF3 / Ar mixed gas was stopped, and high-purity hydrogen (flow rate 150ml / min) was continued to purge for 8min. Then, under a hydrogen atmosphere, the temperature was increased to 500℃ at a rate of 8℃ / min. The gas was then switched to a mixture of ammonia (NH3) and hydrogen (H2), with the molar fraction of ammonia in the mixture being 40% and the total flow rate of the mixture being 200 ml / min. The mixture was kept at this temperature for 5 hours for nitriding. After nitriding, the NH3 / H2 mixture was continued to be introduced, and the temperature was lowered to room temperature at a rate of ≤10℃ / min. The powder was then collected.

[0035] Preparation Example 8: This preparation example provides a method for preparing core-shell structured high-nitrogen steel composite powder, including the following steps: The graded high-nitrogen steel powder was mixed in the following mass percentages: -270~+400 mesh 25%, -400~+500 mesh 45%, -500~+800 mesh 20%, -800~+1000 mesh 5%, and -1000 mesh 5%. The prepared mixed powder (total mass 100g) was placed in a nickel-based alloy reaction boat and placed in the central constant temperature zone of a tube furnace. High-purity hydrogen (H2) was introduced into the tube furnace at a flow rate of 150ml / min, and the temperature was increased to 500℃ at a rate of 8℃ / min. The temperature was held for reduction for 75min. After the reduction was completed, the temperature was decreased to 350℃ at a rate of 6℃ / min under a hydrogen atmosphere. Nitrogen trifluoride (NF3) and high-purity argon (Ar) were premixed at a volume ratio of 1:4. The total flow rate of the mixed gas was 75ml / min. The mixture was pulsed into the reaction tube for 10min. After the activation was completed, the NF3 / Ar mixed gas was stopped, and high-purity hydrogen (flow rate 150ml / min) was continued to purge for 8min. Then, under a hydrogen atmosphere, the temperature was increased to 500℃ at a rate of 8℃ / min. The gas was then switched to a mixture of ammonia (NH3) and hydrogen (H2), with the ammonia mole fraction in the mixture being 40% and the total flow rate of the mixture being 200 ml / min. Nitriding was carried out at this temperature for 5 hours. After nitriding, the NH3 / H2 mixture was continuously introduced, and the temperature was lowered to room temperature at a rate ≤10℃ / min. The powder was then collected. Preparation Example 9: This preparation example provides a method for preparing core-shell structured high-nitrogen steel composite powder, including the following steps: The graded high-nitrogen steel powder was mixed in the following mass percentages: -270~+400 mesh 28%, -400~+500 mesh 47%, -500~+800 mesh 17%, -800~+1000 mesh 4%, and -1000 mesh 4%. The prepared mixed powder (total mass 100g) was placed in a nickel-based alloy reaction boat and placed in the central constant temperature zone of a tube furnace. High-purity hydrogen (H2) was introduced into the tube furnace at a flow rate of 150ml / min, and the temperature was increased to 500℃ at a rate of 8℃ / min. The temperature was held for reduction for 75min. After the reduction was completed, the temperature was decreased to 350℃ at a rate of 6℃ / min under a hydrogen atmosphere. Nitrogen trifluoride (NF3) gas was directly introduced into the reaction tube without using dilution gas. The NF3 flow rate was 10ml / min, and the pulse introduction time was 10min. After activation, stop the flow of NF3 and continue to purge with high-purity hydrogen (flow rate 150 ml / min) for 10 min. Then, under a hydrogen atmosphere, heat to 500 °C at a rate of 8 °C / min. Switch the gas to a mixture of ammonia (NH3) and hydrogen (H2), with the molar fraction of ammonia in the mixture being 40% and the total flow rate of the mixture being 200 ml / min. Maintain the temperature for nitriding for 5 h. After nitriding, continue to purge with the NH3 / H2 mixture and cool to room temperature at a rate of ≤10 °C / min. Collect the powder.

[0036] Example 1: This example provides a core-shell structured high-nitrogen steel composite powder and its additive manufacturing method, including the following steps: Core-shell structured high-nitrogen steel composite powder was prepared according to the method described in Preparation Example 1, and the prepared powder was loaded into the powder cylinder of a selective laser melting metal 3D printer. The 316L stainless steel substrate was preheated to 150°C, and high-purity nitrogen gas was introduced into the molding chamber to reduce the oxygen content to 11.2 mg / m³. 3 the following; Laser melting and forming are performed using a scanning strategy that rotates adjacent layers by 67°. The initial layer scanning direction is rotated by 15° relative to the substrate, and the remaining printing parameters are set according to the equipment requirements to complete the additive manufacturing.

[0037] Example 2: This example provides a core-shell structured high-nitrogen steel composite powder and its additive manufacturing method, including the following steps: Core-shell high-nitrogen steel composite powder was prepared according to the method described in Preparation Example 2, and the prepared powder was loaded into the powder cylinder of a selective laser melting metal 3D printer. The 316L stainless steel substrate was preheated to 150°C, and high-purity nitrogen gas was introduced into the molding chamber to reduce the oxygen content to 11.2 mg / m³. 3 the following; Laser melting and forming are performed using a scanning strategy that rotates adjacent layers by 67°. The initial layer scanning direction is rotated by 15° relative to the substrate, and the remaining printing parameters are set according to the equipment requirements to complete the additive manufacturing.

[0038] Example 3: This example provides a core-shell structured high-nitrogen steel composite powder and its additive manufacturing method, including the following steps: Core-shell structured high-nitrogen steel composite powder was prepared according to the method described in Preparation Example 3. The prepared powder was loaded into the powder cylinder of a selective laser melting metal 3D printer. The 316L stainless steel substrate was preheated to 150°C, and high-purity nitrogen gas was introduced into the molding chamber to reduce the oxygen content to 11.2 mg / m³. 3 the following; Laser melting and forming are performed using a scanning strategy that rotates adjacent layers by 67°. The initial layer scanning direction is rotated by 15° relative to the substrate, and the remaining printing parameters are set according to the equipment requirements to complete the additive manufacturing.

[0039] Example 4: This example provides a core-shell structured high-nitrogen steel composite powder and its additive manufacturing method, including the following steps: Core-shell structured high-nitrogen steel composite powder was prepared according to the method described in Preparation Example 4. The prepared powder was loaded into the powder cylinder of a selective laser melting metal 3D printer. The 316L stainless steel substrate was preheated to 150°C, and high-purity nitrogen gas was introduced into the molding chamber to reduce the oxygen content to 11.2 mg / m³. 3 the following; Laser melting and forming are performed using a scanning strategy that rotates adjacent layers by 67°. The initial layer scanning direction is rotated by 15° relative to the substrate, and the remaining printing parameters are set according to the equipment requirements to complete the additive manufacturing.

[0040] Example 5: This example provides a core-shell structured high-nitrogen steel composite powder and its additive manufacturing method, including the following steps: Core-shell structured high-nitrogen steel composite powder was prepared according to the method described in Preparation Example 5. The prepared powder was loaded into the powder cylinder of a selective laser melting metal 3D printer. The 316L stainless steel substrate was preheated to 150°C, and high-purity nitrogen gas was introduced into the molding chamber to reduce the oxygen content to 11.2 mg / m³. 3 the following; Laser melting and forming are performed using a scanning strategy that rotates adjacent layers by 67°. The initial layer scanning direction is rotated by 15° relative to the substrate, and the remaining printing parameters are set according to the equipment requirements to complete the additive manufacturing.

[0041] Comparative Example 1: Compared to Example 3, the difference is that additive manufacturing is performed directly using untreated raw high-nitrogen steel powder; otherwise, they are the same.

[0042] Comparative Example 2: Compared with Example 3, the difference is that the additive manufacturing is carried out using the mixed powder obtained by mixing the original high-nitrogen steel powder and 3wt% chromium nitride powder in a three-dimensional mixer for 8 hours. All other aspects are the same.

[0043] Comparative Example 3: Compared with Example 3, the difference is that the additive manufacturing is carried out using the mixed powder obtained by mixing the original high-nitrogen steel powder and 5wt% chromium nitride powder in a three-dimensional mixer for 8 hours. All other aspects are the same.

[0044] Comparative Example 4: Compared with Example 3, the difference is that the NF3 surface activation treatment is not performed (that is, after the hydrogen reduction in step two, the temperature is directly raised to the nitriding temperature to perform gas nitriding in step four, skipping step three), and everything else is the same.

[0045] Comparative Example 5: Compared with Example 3, the difference is that hydrogen reduction treatment is not performed (i.e., the surface of NF3 is activated directly by heating to the activation temperature, skipping step two), while the rest are the same.

[0046] Comparative Example 6: Compared with Example 3, the difference is that the activation gas is replaced by Cl2 gas instead of NF3, otherwise they are the same.

[0047] Comparative Example 7: Compared with Example 3, the difference is that the activation gas is replaced by CF4 gas instead of NF3, otherwise they are the same.

[0048] Test Example 1: Surface Cleaning Effect Verification Test Experimental instructions This test example verifies the effectiveness of hydrogen reduction in step 2 in removing the oxide film from the surface of high-nitrogen steel powder. The test subjects included two groups of powders: Raw high-nitrogen steel powder (marked as raw powder) without any treatment is obtained directly from the gas atomization-sieving production line without any heat treatment or atmosphere treatment.

[0049] The powder after hydrogen reduction treatment in step 2 (marked as reduced powder), that is, the sample that has only completed the reduction step (heated to 500℃, H2 flow rate 150mL / min, held for 75min and then cooled to room temperature in the furnace), does not undergo subsequent activation and nitriding.

[0050] Three independent batches of samples were taken for each powder group, and each batch was measured twice.

[0051] X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha, AlKα monochromatic light source, 400 μm beam spot) was used to analyze the elemental composition and chemical state of the powder surface layer within a depth of approximately 5 nm. Test parameters: Analytical chamber vacuum <5 × 10⁻⁶. - 9 mbar, full-spectrum scan pass energy 150 eV, narrow-spectrum scan pass energy 40 eV, data processing used Avantage software, C1 S Charge correction was performed at (284.8 eV), and the narrow spectrum collected included O1s, Cr2p, and Fe2p. The relative contents of metallic Cr, oxidized Cr (Cr2O3), metallic Fe, and oxidized Fe (Fe3O4 / Fe2O3) were obtained by peak fitting.

[0052] Specific procedures: Press the powder evenly onto the sample holder with double-sided adhesive, ensuring a smooth and completely covered surface. Collect two different regions for each sample and take the average value. The percentage of oxygen atoms on the surface is calculated by the sum of the O1s peak area and the peak areas of all elements (excluding C1s interference). The metallic state ratio (Crmetal / Crtotal) is calculated based on the fitting results of the Cr2p3 / 2 peak area.

[0053] Table 1. Elemental analysis results of XPS surface

[0054] Figure 1 This is a comparison chart of the cleaning effect of hydrogen reduction on the surface of high-nitrogen steel powder in Test Example 1 of this invention. (a) Subplot shows the distribution of the percentage of oxygen atoms on the surface of the original powder and the powder after reduction. The box covers the interquartile range of all repeated measurements, and the median is represented by a thick black horizontal line; (b) Subplot shows the percentage of metallic chromium in the total chromium in the same two groups of samples. The experimental numbers of the original powder and the powder after reduction correspond to those in Table 1. As can be seen from (a), the surface oxygen content decreased from about 12 at% to about 2 at% after reduction treatment, a decrease of more than 80%; as can be seen from (b), the proportion of metallic Cr increased from about 18% to about 85%, indicating that the reduction step effectively removed the oxide layer and restored the powder surface to a clean state dominated by metallic atoms.

[0055] in conclusion According to the data in Table 1, the average percentage of oxygen atoms on the powder surface after hydrogen reduction treatment decreased from 12.2 at% in the original powder to about 2.1 at%, a decrease of 82.8%; the percentage of metallic Cr increased from about 18.3% to 84.7%, and the percentage of metallic Fe also increased from 22.5% to 90.8%. This change indicates that under hydrogen atmosphere at 450–550℃, oxides such as Cr2O3 and Fe3O4 on the surface of high-nitrogen steel powder are effectively reduced to the metallic state, and water vapor escapes in gaseous form, thus exposing a clean metal surface.

[0056] From a mechanistic perspective, the reduction step is not only a necessary prerequisite for subsequent activation but also directly determines the upper limit of the activation effect. If the surface oxide film cannot be completely removed, the fluorine radicals generated by NF3 decomposition will preferentially react with chromium oxide or iron oxide, consuming active species and failing to form uniform metal fluoride nucleation sites. Actual test results show good repeatability across three independent batches, with data fluctuations within ±0.3 at%, indicating that this method also has a stable cleaning effect on large-scale powder batches. Without this reduction step, subsequent nitriding would struggle to obtain a uniform nitride shell on the surface because the active nitrogen atoms from ammonia decomposition tend to nucleate locally at oxide film defects, leading to increased shell thickness variations and even preventing nucleation in some areas. Therefore, the data in Table 1 provides clear prerequisites for the effectiveness of subsequent activation and nitriding steps; hydrogen reduction is an indispensable first step in the three-step process of this scheme.

[0057] Test Example 2: Experiment Description This test case is used to verify the effect of NF3 surface activation in step 3 on the number of active sites on the surface of high-nitrogen steel powder. The test subjects include two groups of powders: Powder that has undergone hydrogen reduction in step 2 and is directly introduced into high-purity argon gas (at the same temperature and time) without NF3 activation is labeled as "unactivated powder". Specific procedure: After reduction, switch the atmosphere to high-purity argon gas (99.999% purity), hold at 350℃ for 60 minutes, and then cool to room temperature with the furnace.

[0058] The powder that underwent hydrogen reduction treatment in step 2 and then NF3 activation in step 3 was labeled as "activated powder". The activation conditions were the same as in preparation example 3: the volume ratio of Ar to NF3 was 1:1.5, the total gas flow rate was 200 mL / min, the activation temperature was 350 °C, and the activation time was 60 min.

[0059] Three independent batches of samples were taken for each powder group, and each batch was measured twice.

[0060] The specific surface area of ​​the powder was determined using a fully automated specific surface area and porosity analyzer (Micromeritics ASAP2460). High-purity N2 was used as the test gas, and the multi-point BET method was performed at liquid nitrogen temperature. Before testing, the samples were degassed at 200℃ for 4 hours to remove surface adsorbates.

[0061] NH3 temperature-programmed desorption (NH3-TPD) experiments were conducted using a chemisorption analyzer (AutoChemII 2920) to characterize the number of surface active sites. Test procedures: (1) 0.5g of powder sample was placed in a U-shaped quartz tube and pretreated for 30 min at 10℃ / min under He atmosphere to remove surface impurities.

[0062] (2) Cool down to 100℃ and pass in 10% NH3 / He mixed gas (flow rate 30mL / min) for 30min to adsorb until saturation.

[0063] (3) Switch to He gas flow (flow rate 30 mL / min) and purge for 60 min to remove physically adsorbed NH3.

[0064] (4) The temperature was increased from 100℃ to 600℃ at a heating rate of 10℃ / min. The TCD detector recorded the NH3 desorption signal. The desorption peak area was calculated by Gaussian peak fitting of the NH3-TPD curve, and the number of active sites per unit mass of powder surface (expressed as μmolNH3 / g) was obtained.

[0065] Table 2 Results of Powder Specific Surface Area and Number of Surface Active Sites

[0066] Figure 2 This is a comparative graph showing the effect of NF3 surface activation on the surface activation of high-nitrogen steel powder in Test Example 2 of this invention. Subgraph (a) shows the distribution of specific surface area of ​​unactivated and activated powders, with the box covering the interquartile range of all repeated measurements, and the median line represented by a thick black horizontal line; subgraph (b) shows the number of surface active sites (characterized by NH3-TPD desorption) of the same two groups of samples. The experimental numbers of unactivated and activated powders correspond to those in Table 2. As can be seen from (a), the specific surface area of ​​the powder increased from approximately 0.082 m² / s² to approximately 0.082 m² / s². 2 / g increased to approximately 0.135m 2 / g, an increase of approximately 65%; as can be seen from (b), the number of surface active sites increased from approximately 1.22 μmol NH3 / g to approximately 4.90 μmol NH3 / g, an increase of approximately 300%. Both sets of data indicate that NF3 activation introduced a large number of active sites onto the powder surface.

[0067] in conclusion According to the data in Table 2, after NF3 activation treatment, the specific surface area of ​​high-nitrogen steel powder increased from an average of 0.082 m² / s². 2 / g increased to 0.135m 2 The average concentration of NF3 / g increased by 64.6%; the number of surface active sites jumped from an average of 1.22 μmol NH3 / g to 4.90 μmol NH3 / g, an increase of 301.6%. This change was not due to simple physical adsorption, as both the unactivated and activated powders underwent the same hydrogen reduction treatment and temperature process, the only difference being whether NF3 gas was introduced during the activation stage. Therefore, the fourfold increase in the number of active sites should be attributed to the chemical etching effect of fluorine radicals (F·) generated by the decomposition of NF3 on the metal surface.

[0068] From a mechanistic perspective, F reacts with Cr and Fe atoms on the powder surface to form nanoscale CrF3 and FeF2 particles. These particles are dispersed on the powder surface as isolated nanodots. The increase in BET specific surface area (although the increase is relatively small) reflects the increased surface roughness caused by the formation of nanoscale fluoride particles. The number of active sites measured by NH3-TPD more directly characterizes the chemical activity of these fluoride particles as heterogeneous nucleation sites. Notably, the repeated measurements from three independent batches showed good consistency: the specific surface area of ​​the activated powder fluctuated within the range of 0.128–0.141 μm. 2 The number of active sites fluctuated between 4.72 and 5.06 μmol NH3 / g, with coefficients of variation of 3.5% and 2.8%, respectively, indicating that the activation process has stability and controllability in treating different batches of powder.

[0069] Test Example 3: Experiment Description This test example verifies whether a nitride shell layer forms on the surface of high-nitrogen steel powder treated with the three steps of this invention (reduction → activation → nitriding) after gas nitriding in step 4. The test subjects include three groups of powders: Raw high-nitrogen steel powder (labeled "raw powder") without any treatment is obtained directly from the gas atomization-sieving production line without any heat treatment or atmosphere treatment.

[0070] The powder that underwent only gas nitriding treatment in step 4 (labeled "nitriding-only powder") was placed directly into the nitriding furnace for nitriding without reduction and activation treatment. The nitriding conditions were the same as in Preparation Example 3: temperature 500°C, holding time 3 h, molar ratio of NH3 to N2 1:4, total gas flow rate 1 L / min, and then cooled to room temperature with the furnace.

[0071] The powder (labeled "three-step treated powder") after the three-step treatment of this invention (step 2 hydrogen reduction + step 3 NF3 activation + step 4 gas nitriding) was treated under the same conditions as in Preparation Example 3. The reduction conditions were 500℃, H2 flow rate 150mL / min, and holding time 75min; the activation conditions were 350℃, Ar to NF3 dilution volume ratio 1:1.5, total flow rate 200mL / min, and holding time 60min; the nitriding conditions were 500℃, NH3 to N2 molar ratio 1:4, total flow rate 1L / min, and holding time 3h.

[0072] Three independent batches of samples were taken for each powder group, and each batch was measured twice.

[0073] Phase analysis was performed using an X-ray diffractometer. The test parameters were: tube voltage 40 kV, tube current 40 mA, scanning range 20°~80° (2θ), step size 0.02°, and dwell time per step 0.2 s.

[0074] The overall nitrogen content of the powder was determined using a nitrogen and oxygen analyzer (LECOON H836, inert gas melting-thermal conductivity method). Approximately 0.1 g of powder sample was placed in a nickel bag, then placed in a graphite crucible and melted under an inert atmosphere. The mass fraction of nitrogen was analyzed. Each sample was measured twice, and the average value was taken.

[0075] Semi-quantitative analysis of the chemical states of nitrogen in the 0–10 nm depth range of the powder surface was performed using X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha, AlKα monochromatic light source). The test parameters were consistent with those in Example 1, with a narrow spectral scanning range covering N1s (392–410 eV). Peak fitting separated the N1s spectrum into two components: a metal nitride peak at approximately 397.2 eV (attributed to CrN, Cr2N, Fe4N, etc.) and a chemisorbed nitrogen or organic nitrogen peak at approximately 400.1 eV. The peak area ratio reflects the proportion of surface nitrides to the total surface nitrogen.

[0076] Table 3 Results of the determination of nitrogen content and surface nitride ratio in powder.

[0077] Figure 3 This is a comparison chart of nitrogen content and surface nitride ratio of high-nitrogen steel powder after different treatments in Test Example 3 of this invention. (a) Subplot shows the overall nitrogen content distribution of the original powder, the nitrided powder only, and the three-step treated powder. The box covers the interquartile range of each repeated measurement data, the median is represented by a thick black horizontal line, and the scatter points represent the specific values ​​of each measurement; (b) Subplot shows the distribution of the surface nitride ratio of the three groups of powders to the total surface nitrogen. The experimental numbers of the original powder, the nitrided powder only, and the three-step treated powder correspond to Table 3. As can be seen from (a), the average overall nitrogen content of the three-step treated powder is 1.84 wt%, which is significantly higher than that of the original powder (0.42 wt%) and the nitrided powder only (0.57 wt%); as can be seen from (b), the average surface nitride ratio of the three-step treated powder is about 86.6%, while that of the nitrided powder only is 38.2%, and that of the original powder is 12.2%. Both sets of data indicate that the three-step treatment forms a high proportion of nitride shell on the powder surface.

[0078] in conclusion According to the data in Table 3, after the three-step processing of this invention, the overall nitrogen content of the high-nitrogen steel powder increased from 0.42 wt% of the original powder to 1.84 wt%, an increase of 338%; at the same time, the proportion of surface nitrides to the total surface nitrogen jumped from 12.2% to 86.6%. In contrast, the overall nitrogen content of the nitriding powder alone only increased to 0.57 wt%, and the proportion of surface nitrides was only 38.2%.

[0079] Data on nitriding powder: Despite nitriding treatment, the nitrogen content only increased slightly from 0.42 wt% to 0.57 wt%, and the surface nitride ratio was only 38.2%. This result indicates that the original powder surface, without reduction and activation, is covered with an oxide film, resulting in a limited number of active sites. Active nitrogen atoms generated by ammonia decomposition are difficult to effectively adsorb and diffuse into the powder surface. Although nitrides can nucleate at some surface defects, the nucleation density is low, and the shell coverage is incomplete, leaving a large area of ​​the surface still in a bare metallic or oxidized state, as evidenced by the surface nitride ratio being less than 40%. Looking at the actual test fluctuation range, only the nitrogen content of the nitriding powder varied between 0.54 and 0.60 wt%, with a coefficient of variation of approximately 4.5%, indicating that the uneven distribution of the surface oxide film led to inconsistencies in nitriding.

[0080] Looking at the powder processed in three steps: the significant increase in overall nitrogen content (1.79~1.91wt%) directly corresponds to the high proportion of surface nitrides (84.8%~88.2%). This increase is clearly not achievable through simple nitrogen physical adsorption, but rather through the formation of metal nitrides with a well-defined stoichiometric ratio. Combined with the analysis depth of XPS (0~10nm), the surface nitride proportion of 86.6% means that the outermost layer of the powder is basically covered by a nitride shell. Moreover, the coefficient of variation for nitrogen content in three independent batches was only 2.6%, and the coefficient of variation for surface nitride proportion was only 1.5%, indicating excellent uniformity and repeatability of the three-step processing.

[0081] The cleaning effect of the reduction step (test example 1 verified that it reduced the surface oxygen content to about 2 at%) fully exposes the metal surface. Next, the activation step introduces heterogeneous nucleation sites (test example 2 showed an active site count of 4.90 μmol NH3 / g, approximately 300% higher than the unactivated powder), providing a large number of uniformly distributed nucleation nuclei for nitride nucleation. Finally, in the nitriding stage, active nitrogen atoms rapidly nucleate and grow laterally at these nuclei, converging to form a dense nitride shell. The nitrogen content of the three-step treated powder (1.84 wt%) is significantly higher than the 0.57 wt% of the nitrided powder alone, confirming the synergistic effect: the clean surface and uniform nucleation sites together lower the free energy barrier for nitride nucleation, allowing the nitride to form a thicker shell in a shorter time.

[0082] Test Example 4: Experiment Description This test example is used to compare the flowability of the powder from the embodiments of the present invention with that of the comparative examples, and to verify the effect of the core-shell structure on maintaining the spherical geometry and flowability of the powder. The test objects include the following 12 groups of powder samples: The core-shell structured high-nitrogen steel composite powders prepared in Examples 1-5 are respectively labeled as Examples 1 to 5.

[0083] Comparative Example 1: Raw high-nitrogen steel powder without any treatment.

[0084] Comparative Example 2: The mixed powder obtained by mixing the original powder with 3wt% chromium nitride powder in a three-dimensional mixer for 8 hours.

[0085] Comparative Example 3: The mixed powder obtained by mixing the original powder with 5 wt% chromium nitride powder in a three-dimensional mixer for 8 hours.

[0086] Comparative Example 4: Powder that underwent only step 2 hydrogen reduction and step 4 gas nitriding (skipping step 3 NF3 activation).

[0087] Comparative Example 5: Powder that underwent only step 3 NF3 activation and step 4 gas nitriding (skipping step 2 hydrogen reduction).

[0088] Comparative Example 6: Powder treated with activation gas replaced by Cl2 gas (other conditions are the same as in Example 3).

[0089] Comparative Example 7: Powder treated with CF4 gas instead of activation gas (other conditions are the same as in Example 3).

[0090] Three independent batches of samples were taken for each powder group, and each batch was measured three times.

[0091] Liquidity testing was conducted according to GB / T1482-2010 standard, using a Hall effect flow meter. The specific procedures are as follows: (1) Fix the standard funnel (2.5mm aperture) on the bracket and place the receiving container below the funnel.

[0092] (2) Weigh 50.0g of powder sample using weighing paper, accurate to 0.01g.

[0093] (3) Block the bottom of the funnel with your finger and pour all the powder into the funnel, being careful not to apply any vibration or knocking.

[0094] (4) Release your finger and start the stopwatch at the same time to record the time required for all the powder to flow out of the funnel. If the powder cannot flow out completely within 120s, record it as ">120s" and do not include it in the average calculation in subsequent data analysis.

[0095] (5) Clean the inner wall of the funnel after each test, use compressed air to blow away the residual powder, and then conduct the next test.

[0096] (6) Each sample was measured three times, and the arithmetic mean was taken as the flow time of that sample. Data from the three batches were recorded independently.

[0097] Table 4 Results of flowability test of high-nitrogen steel powder under different treatment conditions

[0098]

[0099] Figure 4 This is a comparison chart of the flowability of high-nitrogen steel powder under different treatment conditions in Test Example 4 of this invention. The horizontal axis of the chart sequentially arranges 12 groups of samples from Examples 1 to 5 and Comparative Examples 1 to 7. Small black dots represent the specific data values ​​of 9 measurements for each group. A continuous black line connects the average values ​​of each group, and a black vertical line represents the standard deviation range. The flowability data of Examples 1-5 and Comparative Example 1 are concentrated in the range of 5.8-6.3 s / 50g, indicating that the flowability of the core-shell structure powder is close to that of the original powder. The flow time of Comparative Examples 2-3 (mechanically mixed chromium nitride powder) is significantly extended to 27.4-47.3 s / 50g, and the data dispersion is significantly increased. The flow times of Comparative Examples 4 (skipping the activation step) and 5 (skipping the reduction step) are 7.8-8.0 s and 9.0-9.2 s, respectively, slightly higher than the Examples but significantly lower than Comparative Examples 2-3. The powder flow times of Comparative Examples 6 and 7, using Cl2 or CF activation gas, are 8.5-10.0 s, falling between the Examples and Comparative Examples 4-5.

[0100] in conclusion According to the data in Table 4, the average flow time of Examples 1-5 was 5.9-6.3 s / 50g, which is basically the same as the 5.8 s / 50g of the original powder in Comparative Example 1. This indicates that the thickness of the nitride shell layer formed on the powder surface by the three-step treatment of this invention is at the nanoscale, without changing the spherical geometry of the powder or creating sintering necks between particles. This is a key feature that distinguishes the core-shell structure from the mechanical mixing scheme. The data of Comparative Examples 2 and 3 provide a contrasting reference: after adding 3 wt% and 5 wt% chromium nitride powder, the flow time was extended to 27.4-28.7 s / 50g and 45.9-47.3 s / 50g, respectively, an increase of 4-8 times. During the flow of mechanically mixed powder, the friction between chromium nitride particles (usually with irregular shapes) and spherical high-nitrogen steel powder increases, and fine particles may fill the gaps between the spherical powders, resulting in an increase in overall flow resistance.

[0101] Comparing the examples with Comparative Examples 4-7 makes the contribution of each treatment step clearer. The flow time of Comparative Example 4 (skipping the activation step) was 7.8-8.0 s, only about 2 seconds longer than the examples. However, this change should not be simply attributed to an increase in surface roughness—because the flow time of Comparative Example 5 (skipping the reduction step) was further extended to 9.0-9.2 s, while Comparative Example 7 (CF4 replacing NF3) reached 9.9-10.0 s. Comparing these data reveals a progressive relationship: insufficient surface cleanliness and activation effect can cause uneven nitride shell growth, with localized excessive shell thickness or the formation of micro-protrusions. These micro-irregularities manifest as prolonged flow time in macroscopic flowability. Mechanistically, the reduction step ensures the absence of oxide film interference on the surface, while the activation step enables uniform nucleation and growth of nitrides; both are indispensable. Skipping either one will cause nitrides to preferentially grow locally, resulting in a discrete shell thickness distribution.

[0102] Comparative Examples 6 and 7 used Cl2 and CF4 as alternative activating gases. The flow time of the powder activated by Cl2 (8.5-8.6 s) was slightly lower than that of Comparative Example 4 but higher than that of the Examples, while the flow time of the powder activated by CF4 (9.9-10.0 s) was worse. This result indicates that the choice of activating gas has a direct impact on the shell structure and particle surface state; Cl2 has high reactivity with metals but may cause local over-etching, while the fluorocarbon groups generated by CF4 under plasma conditions may leave carbon-containing byproducts on the surface. Overall, the advantages of the Examples in terms of flowability confirm the applicability of NF3 activation in maintaining powder sphericity.

[0103] Test Example 5: Experiment Description This test example is used to compare the effective mass fraction of nitrogen element in the powder of the present invention embodiment with that in the comparative examples. The test objects include the following 11 groups of powder samples (since comparative examples 2 and 3 were not measured after the cleaning treatment described in Example 1, the original data may contain unbound free nitride particles. Therefore, the nitrogen content was measured in both the sieved and unsieved states to eliminate the deviation caused by free chromium nitride; the following measurements were all performed using powder that had been sieved to below -400 mesh to ensure a consistent comparison basis): The core-shell structured high-nitrogen steel composite powders prepared in Examples 1-5 are respectively labeled as Examples 1 to 5.

[0104] Comparative Example 1: Raw high-nitrogen steel powder without any treatment.

[0105] Comparative Example 4: Powder that underwent only step 2 hydrogen reduction and step 4 gas nitriding (skipping step 3 NF3 activation).

[0106] Comparative Example 5: Powder that underwent only step 3 NF3 activation and step 4 gas nitriding (skipping step 2 hydrogen reduction).

[0107] Comparative Example 6: Powder treated with activation gas replaced by Cl2 gas (other conditions are the same as in Example 3).

[0108] Comparative Example 7: Powder treated with CF4 gas instead of activation gas (other conditions are the same as in Example 3).

[0109] Three independent batches of samples were taken for each powder group, and each batch was measured twice.

[0110] The mass fraction of nitrogen was determined according to GB / T20124-2006 standard using a nitrogen and oxygen analyzer (LECOON H836, inert gas melting-thermal conductivity method). The specific operation is as follows: (1) Weigh about 0.10g (accurate to 0.0001g) from each group of powder samples and put it into the nickel bag, flatten it and seal it.

[0111] (2) Place the sealed nickel bag into a graphite crucible and heat it to about 3000°C under the protection of inert gas (high purity He, purity 99.999%) to completely melt the sample.

[0112] (3) The nitrogen released from the molten metal is carried by a carrier gas (high-purity He) through a thermal conductivity detector, and the mass fraction of nitrogen is calculated based on the change in thermal conductivity. The instrument is calibrated using a standard steel sample (nitrogen content 2.01 wt%).

[0113] (4) Each sample was measured twice, and the arithmetic mean was taken as the nitrogen content of the sample. The batch average of the three batches was calculated separately.

[0114] Table 5. Results of nitrogen element mass fraction determination in high-nitrogen steel powder under different treatment conditions.

[0115]

[0116] Figure 5This is a comparison chart of the effective nitrogen content of high-nitrogen steel powder under different treatment conditions in Test Example 5 of this invention. The horizontal axis represents 12 groups of samples, from Examples 1 to 5 and Comparative Examples 1 to 7, respectively. The small black dots represent the specific values ​​of 6 measurements for each group. The continuous black broken lines connect the average values ​​of each group, and the black vertical lines represent the standard deviation range. It can be clearly seen from the chart that the average nitrogen content of Examples 1-5 is in the range of 1.68~2.29 wt%, significantly higher than all comparative examples. Comparative Example 1 (original powder) has the lowest nitrogen content (approximately 0.42 wt%), while Comparative Examples 2 and 3 (mechanically mixed powder) have 0.70 wt% and 0.98 wt%, respectively. Comparative Examples 4 and 5, lacking reduction or activation steps, only reach 0.81 wt% and 0.57 wt%, respectively. The nitrogen contents of Comparative Examples 6 (Cl2 activation) and 7 (CF4 activation) are 1.20 wt% and 0.95 wt%, respectively, significantly lower than Example 3 (1.89 wt%).

[0117] The examples show an increasing trend: the nitrogen content in Examples 1 to 5 gradually increases with the increase of nitriding temperature or time, indicating that the nitrogen content of the present invention can be controlled.

[0118] in conclusion According to the data in Table 5, the average nitrogen content of Examples 1-5 was 1.70 wt%, 1.81 wt%, 1.89 wt%, 2.11 wt%, and 2.29 wt%, respectively, increasing sequentially. This trend mainly stemmed from the gradient changes in nitriding temperature or time (the process parameters for Examples 1-5 were different, such as increasing the nitriding temperature from 480℃ to 540℃ or extending the holding time from 2h to 4h), indicating that the method of the present invention can achieve a wide range of nitrogen content control by adjusting the process parameters in step 4. In contrast, the nitrogen content of the comparative powders was significantly lower than that of the examples.

[0119] The nitrogen content of the original powder in Comparative Example 1 was only 0.42 wt%, which corresponds to the residual nitrogen content dissolved in the original atomized powder. Although the nitrogen content of the mechanically mixed chromium nitride powder (Comparative Examples 2 and 3) was increased to 0.70 wt% and 0.98 wt%, respectively, the fine chromium nitride particles were only physically attached to the powder surface and were easily detached or unevenly distributed during additive manufacturing. Furthermore, the melting temperature of the chromium nitride particles was much higher than that of the high-nitrogen steel matrix, which would cause a mismatch in the nitrogen release kinetics in the molten pool. The nitrogen content of Comparative Example 4 (without activation) was 0.81 wt%, which was higher than that of the original powder but much lower than that of the examples. This indicates that the lack of heterogeneous nucleation sites introduced by NF3 activation meant that the active nitrogen atoms generated by ammonia decomposition could only nucleate at limited surface defects, resulting in a low nitride shell coverage and thinness. The nitrogen content of Comparative Example 5 (skipping reduction) was even lower (0.57 wt%), indicating that the uncleaned surface oxide film severely hindered the adsorption and diffusion of nitrogen atoms. Even with the activation step, the surface oxide layer would still react with NF3 to consume active fluorine radicals, resulting in a significant decrease in activation efficiency.

[0120] The results of Comparative Example 6 (Cl2 instead of NF3) and Comparative Example 7 (CF4 instead of NF3) further confirmed the importance of the choice of activation gas. After activation with Cl2, the nitrogen content reached 1.20 wt%, which was higher than that of Comparative Example 4 but lower than that of Example 3 (1.89 wt%). This is because the chlorides (such as FeCl2 / FeCl3) generated by the reaction of Cl2 with the surface metal partially volatilize at the nitriding temperature, resulting in fewer nucleation sites than the fluorides generated by NF3 activation. After activation with CF4, the nitrogen content was only 0.95 wt%, possibly because CF generates not only fluorine radicals but also carbon radicals under thermal decomposition conditions. Carbon-containing byproducts deposit on the surface, covering some active sites and affecting subsequent nitride nucleation.

[0121] In summary, the nitrogen content of the powder in the embodiments of this invention is 4.0 to 5.5 times that of Comparative Example 1 (original powder), 1.7 to 2.3 times that of the mechanical mixing scheme (Comparative Example 3), and 1.4 to 4.0 times that of the powder lacking any one of the processing steps. This significant difference directly demonstrates a strong synergistic effect among the reduction, activation, and nitriding processes: the reduction step provides a clean metal surface, the activation step introduces heterogeneous nucleation sites at a high density, and the nitriding step enables rapid nucleation and growth of nitrides; all three are indispensable. From the perspective of practical additive manufacturing, the absolute value of nitrogen content in the powder is not the only objective; the state of nitrogen occurrence is equally important—more than 86% of the surface nitrogen in the powder of the embodiments exists in the form of nitrides (as demonstrated in Test Example 3). This shell structure can rapidly release active nitrogen into the melt during laser melting, which is beneficial for the solid solution retention of nitrogen during solidification.

[0122] Test Example 6: Experiment Description This test example is used to evaluate the nitrogen retention efficiency of the core-shell structure powder of the present invention in the laser melting additive manufacturing process, and to verify the ability of the nitride shell to capture and retain nitrogen under rapid solidification conditions. The test subjects include the following 12 groups of powder samples: The core-shell structured high-nitrogen steel composite powders prepared in Examples 1-5 are respectively labeled as Examples 1 to 5.

[0123] Comparative Example 1: Raw high-nitrogen steel powder without any treatment.

[0124] Comparative Example 2: The mixed powder obtained by mechanically mixing the original powder with 3wt% chromium nitride powder for 8 hours.

[0125] Comparative Example 3: The mixed powder obtained by mechanically mixing the original powder with 5 wt% chromium nitride powder for 8 hours.

[0126] Comparative Example 4: Powder that underwent only step 2 hydrogen reduction and step 4 gas nitriding (skipping step 3 NF3 activation).

[0127] Comparative Example 5: Powder that underwent only step 3 NF3 activation and step 4 gas nitriding (skipping step 2 hydrogen reduction).

[0128] Comparative Example 6: Powder treated with the activation gas replaced by Cl2 (other conditions were the same as in Example 3).

[0129] Comparative Example 7: Powder treated with CF4 as the activation gas (other conditions are the same as in Example 3).

[0130] Each batch of powder was prepared into bulk samples using the same selective laser melting (SLM) process. The SLM equipment was an EOS M290 with a laser power of 280W, a scanning speed of 800mm / s, a scanning spacing of 0.10mm, a layer thickness of 30μm, a substrate preheating temperature of 80℃, and an argon protective atmosphere (oxygen content below 100ppm). Each batch produced three independent samples (each sample measuring 10mm × 10mm × 10mm), labeled -1, -2, and -3 respectively.

[0131] After laser melting, the sample was removed from the substrate by wire electrical discharge machining. It was then ultrasonically cleaned with acetone and ethanol for 5 minutes each to remove surface oil and cutting residue.

[0132] Before determining the nitrogen content, a small piece of about 0.5g was cut from the center of each sample and the surface oxide layer was removed by mechanical polishing (using 400#, 800#, and 1200# SiC sandpaper in sequence). The determination was performed using a nitrogen and oxygen analyzer (LECOON H836) and the fusion-thermal conductivity method, with the same operating procedures as in Test Example 5.

[0133] Nitrogen retention efficiency is defined as the ratio of the nitrogen content (wt%) of the printed block sample to the nitrogen content (wt%) of the corresponding powder before printing, expressed as a percentage. Each sample is measured twice, and the average value is taken as the official nitrogen content data for that sample. The average value of the three samples is taken as the final result of the nitrogen retention rate for that group.

[0134] Table 6. Test results of nitrogen retention efficiency of printed parts with different powders.

[0135]

[0136]

[0137] Figure 6 This is a comparison chart of nitrogen retention efficiencies after laser melting additive manufacturing of different powders in Test Example 6 of this invention. The horizontal axis of the chart represents 12 groups of samples, from Examples 1 to 5 and Comparative Examples 1 to 7, respectively. The small black dots represent the specific nitrogen retention efficiency measurements of the three samples in each group. The continuous black broken lines connect the average values ​​of each group, and the black vertical lines represent the standard deviation range. The average nitrogen retention efficiency of Examples 1-5 is in the range of 78.2% to 88.9%, significantly higher than all comparative examples (37.1% to 62.5%). The nitrogen retention efficiency of Comparative Examples 1-5 is concentrated between 37.1% and 50.6%, with Comparative Examples 6 (Cl2 activation) and 7 (CF4 activation) being slightly higher, at 56.7% to 62.5% and 51.6% to 58.9%, respectively.

[0138] in conclusion According to the data in Table 6, the average nitrogen retention efficiencies of Examples 1-5 were 81.2%, 83.8%, 87.1%, 82.2%, and 79.6%, respectively, generally at a relatively high level of 78%–89%. In contrast, the retention efficiencies of Comparative Examples 1-5 were only 37.1%–50.6%, meaning the nitrogen retention efficiencies of the Example powders were 1.6–2.4 times that of the Comparative Examples. More notably, Example 3 achieved the highest nitrogen retention efficiency of 88.9%, meaning that approximately 89% of the nitrogen in the powder remained in the matrix after laser melting and solidification. This value is rare in the current research field of additive manufacturing of high-nitrogen steel.

[0139] To understand this significant difference, we need to return to the thermal decomposition kinetics of the nitride shell. The nitride shell, under conditions of rapid laser heating (heating rate approximately 10⁻⁶), 5 ~10 6The nitrogen atoms rapidly decompose (K / s), releasing active nitrogen atoms. Because the shell layer is only tens of nanometers thick and closely adheres to the melt surface, the diffusion path of nitrogen atoms is short, allowing them to quickly and uniformly enter the molten pool under the combined effects of melt convection and diffusion. The uniformity and high coverage of the nitride shell layer in the example powder (test example 3 confirmed to be over 86%) ensured the synchronicity and consistency of nitrogen release, avoiding oversaturation precipitation caused by local nitrogen concentration fluctuations. In contrast, the nitrogen in Comparative Examples 1, 4, and 5 mainly exists in dissolved or sparse nitride clusters. During laser melting, these nitrogen atoms tend to escape and be lost in large quantities from the melt surface, resulting in a retention efficiency of only 40%–50%.

[0140] Comparing the data from Comparative Examples 6 and 7 also provides a valuable reference. The retention efficiency of Cl2 activated powder was approximately 59.2%, and that of CF4 activated powder was approximately 54.7%. Although these were higher than those of Comparative Examples 1-5, they were significantly lower than those of Example 3 (87.1%). The root cause of this difference lies in the influence of the activation gas on the microstructure of the shell: some of the chloride nucleation sites generated by Cl2 activation volatilize, resulting in a nitride shell thickness and coverage that are not as good as those of the NF3 activation scheme. The location and timing of nitrogen release are difficult to synchronize, and some nitrogen is carried away by the purging gas on the melt surface.

[0141] The fluctuations within the examples are also worth analyzing. Example 3 (1.89 wt% powder nitrogen content) showed the highest retention efficiency (87.1%), while the retention efficiency of Example 5 (2.29 wt% powder nitrogen content) decreased slightly to 79.6%. A possible explanation for this is that when the shell thickness increases to a certain value, there is an excessively thick shell effect during laser heating: the decomposition of a thicker nitride shell may take a slightly longer time, and under rapid laser scanning conditions (the molten pool exists for approximately 0.5~2 ms), some shells far from the melt do not decompose sufficiently, and the nitrogen is carried away by the escaping gas before being released into the molten pool in time. This suggests that there exists an optimal shell thickness range, corresponding to approximately 1.8~2.0 wt% powder nitrogen content, which balances high nitrogen content and high retention efficiency.

[0142] Test Example 7: Experiment Description This test example evaluates the room-temperature tensile mechanical properties of bulk specimens prepared from different powders via selective laser melting, verifying the contribution of core-shell structure powders to the strength and toughness of the matrix after additive manufacturing. The test subjects are bulk specimens printed using the following 12 powder groups, with preparation conditions for each powder group consistent with Test Example 5 and Test Example 6: The core-shell structured high-nitrogen steel composite powders prepared in Examples 1-5 are respectively labeled as Examples 1 to 5.

[0143] Comparative Example 1: Raw high-nitrogen steel powder without any treatment.

[0144] Comparative Example 2: The mixed powder obtained by mechanically mixing the original powder with 3wt% chromium nitride powder for 8 hours.

[0145] Comparative Example 3: The mixed powder obtained by mechanically mixing the original powder with 5 wt% chromium nitride powder for 8 hours.

[0146] Comparative Example 4: Powder that underwent only step 2 hydrogen reduction and step 4 gas nitriding (skipping step 3 NF3 activation).

[0147] Comparative Example 5: Powder that underwent only step 3 NF3 activation and step 4 gas nitriding (skipping step 2 hydrogen reduction).

[0148] Comparative Example 6: Powder treated with the activation gas replaced by Cl2 (other conditions were the same as in Example 3).

[0149] Comparative Example 7: Powder treated with CF4 as the activation gas (other conditions are the same as in Example 3).

[0150] Each batch of powder was used to print tensile specimens using an EOS M290 machine under the same process parameters (process parameters were the same as in Test Example 6: laser power 280W, scanning speed 800mm / s, scanning spacing 0.10mm, layer thickness 30μm, substrate preheating 80℃, and argon protective atmosphere with oxygen content below 100ppm). The specimens were designed as plate-shaped tensile specimens according to GB / T228.1-2010 standard, with a gauge length of 25mm and a gauge section cross-sectional dimension of 6mm × 2mm. Three independent specimens were formed for each batch.

[0151] Tensile tests were performed on a universal testing machine (Instron 5982, equipped with a 50kN load cell). The specific procedures are as follows: The printed sample was removed from the substrate by wire electrical discharge machining and then ultrasonically cleaned with acetone and ethanol for 5 minutes each.

[0152] The specimens are used directly for tensile testing without any post-heat treatment to reflect their performance in the printed state.

[0153] Measure the width and thickness at three locations within the gauge length, and calculate the original cross-sectional area by taking the average value.

[0154] Clamp the specimen in the testing machine fixture, adjust the clamping force to center the specimen axially, and set the extensometer (gauge length 25mm) in the gauge length section.

[0155] The beam was loaded at a displacement rate of 0.5 mm / min until fracture, and the tensile strength (Rm), yield strength (Rp0.2, determined by the 0.2% offset method) and elongation after fracture (A) were recorded.

[0156] A set of data was measured for each sample, and the arithmetic mean of the three samples was taken as the final result for that set of samples. If any data deviates from the mean by more than 15%, the average of the remaining two data is taken.

[0157] Table 7. Test results of room temperature tensile mechanical properties of SLM printed parts with different powders.

[0158]

[0159] Figure 7 This is a comparison chart of the room temperature tensile mechanical properties of SLM-printed parts with different powders in Test Example 7 of this invention. (a) is a comparison chart of tensile strength (Rm), where small black dots represent the specific test values ​​of the three samples in each group, continuous black lines connect the average values ​​of each group, and black vertical lines represent the standard deviation range. The horizontal axis in the chart represents Examples 1 to 5 and Comparative Examples 1 to 7, respectively. (b) is a comparison chart of elongation after fracture, with data points represented in the same way as in (a). The average tensile strength of Examples 1-5 is in the range of 782~892 MPa, significantly higher than all comparative examples (518~705 MPa). Regarding elongation, Comparative Example 1 (original powder) has the highest elongation (approximately 41.4%), while the elongation of Examples 1-5 decreases sequentially from 31.5% to 19.5%, exhibiting a typical strength-plasticity inversion relationship.

[0160] in conclusion According to the data in Table 7, the average tensile strengths of the printed parts in Examples 1-5 were 784 MPa, 815 MPa, 856 MPa, 874 MPa, and 892 MPa, respectively, showing an overall increasing trend and all exceeding 780 MPa. Comparative Example 1 had the lowest strength (average 524 MPa), while Comparative Examples 2 and 3 (mechanically mixed powder) had approximately 551 MPa and 573 MPa, respectively, which were 200-350 MPa lower than Examples 1-5. Comparative Examples 4 and 5 had 614 MPa and 589 MPa, respectively, and Comparative Examples 6 and 7 had 698 MPa and 667 MPa, respectively. The tensile strength advantage of the examples is highly consistent with the nitrogen retention efficiency data in Test Example 6: the higher the retention efficiency of the powder, the more nitrogen is dissolved after printing, thus forming a stronger solid solution strengthening effect. This is most evident in Example 3—its retention efficiency was the highest (87.1%), and its tensile strength also reached 856 MPa, showing a good balance of overall performance.

[0161] From the perspective of strengthening mechanism, nitrogen in austenitic stainless steel not only produces lattice distortion strengthening, but also promotes grain refinement strengthening and reduces stacking fault energy. For every 0.1 wt% increase in nitrogen content, it can contribute approximately 15-25 MPa to the yield strength increase. The average nitrogen content after printing in Example 5 is about 1.82 wt%, which is 1.64 wt% higher than that in Comparative Example 1 (about 0.18 wt%). Based on this estimate, it can contribute approximately 250-400 MPa to the yield strength increment, which is basically consistent with the measured yield strength difference (563 MPa vs 308 MPa, a difference of 255 MPa). In addition, it should be noted that the data dispersion within the examples is small (coefficient of variation 2-4%), while the dispersion of Comparative Examples 2-7 reaches 5-10%, indicating that the powder of the present invention has higher molten pool stability during laser melting and better batch consistency of the finished products.

[0162] The yield strength data showed the same pattern, which will not be elaborated here; however, the change in elongation after fracture is worth discussing. The elongation of Examples 1-5 decreased from 31.5% to 19.5% sequentially, while the elongation of Comparative Example 1 was as high as 41.4%. The increase in strength accompanied by a decrease in plasticity is a general rule for metallic materials, but Example 3 still maintained an elongation of 26.4% at a tensile strength of 856 MPa, a combination that is already considered superior in the field of additive manufacturing of high-nitrogen steel. In contrast, Comparative Example 6 (Cl2 activated) had a tensile strength of approximately 698 MPa and an elongation of 24.6%, while Comparative Example 7 (CF4 activated) had a tensile strength of 667 MPa and an elongation of 26.2%. The strength-plasticity balance of the latter two powders was significantly inferior to that of Example 3, which is related to their lower nitrogen retention efficiency and non-uniform shell structure—non-uniform nitrogen distribution may form local high-nitrogen and low-nitrogen regions in the matrix, with the low-nitrogen regions yielding preferentially during deformation, leading to a decrease in overall plasticity.

[0163] The elongation rates of Comparative Examples 4 (skipping activation) and 5 (skipping reduction) were 28.6% and 30.2%, respectively, higher than that of Example 3, but the strength was 240-270 MPa lower. This indicates that the absence of any key step will significantly reduce the strengthening effect. This also proves the necessity of the reduction-activation-nitriding three-step process from the perspective of mechanical properties: if one step is missing, the increase and uniformity of nitrogen will decrease, which will ultimately be reflected in the comprehensive mechanical properties of the printed parts. Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing, characterized in that, Includes the following steps: S1. High-nitrogen steel powder is subjected to hydrogen reduction treatment to obtain reduced powder; S2. The reduced powder is activated by nitrogen trifluoride to obtain activated powder; S3. The activated powder is subjected to ammonia nitriding treatment to obtain core-shell structured high-nitrogen steel composite powder.

2. The method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing according to claim 1, characterized in that, The high-nitrogen steel powder has the following particle size distribution: -270~+400 mesh accounts for 25%~30%, -400~+500 mesh accounts for 45%~50%, -500~+800 mesh accounts for 15%~20%, -800~+1000 mesh accounts for 3%~5%, and -1000 mesh accounts for 2%~5%.

3. The method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing according to claim 1, characterized in that, The conditions for the hydrogen reduction treatment are: reduction temperature 450℃~550℃, holding time 60min~90min, hydrogen flow rate 100ml / min~200ml / min, and heating rate 5℃ / min~10℃ / min.

4. The method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing according to claim 1, characterized in that, The conditions for the nitrogen trifluoride activation treatment are as follows: the volume ratio of nitrogen trifluoride to argon is 1:4 to 1:12, the total flow rate of the mixed gas is 50 ml / min to 100 ml / min, the pulse introduction time is 5 min to 15 min, and the activation temperature is 300℃ to 400℃.

5. The method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing according to claim 1, characterized in that, The conditions for the ammonia nitriding treatment are as follows: Nitriding temperature: 450℃~550℃; ammonia molar fraction in ammonia-hydrogen mixture: 30%~50%; total flow rate of mixed gas: 150ml / min~250ml / min; holding time: 3h~7h.

6. The core-shell structured high-nitrogen steel composite powder for additive manufacturing according to claim 1, characterized in that, After the nitrogen trifluoride activation treatment, the process also includes a hydrogen purging step, with a hydrogen flow rate of 100 ml / min to 200 ml / min and a purging time of 5 min to 10 min.

7. The method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing according to claim 1, characterized in that, After the ammonia nitriding treatment, the cooling rate is no more than 10℃ / min, and the cooling is carried out in an ammonia-hydrogen mixed atmosphere.

8. The method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing according to claim 1, characterized in that, The powder comprises a high-nitrogen steel powder matrix and a nitride shell coating the surface of the matrix.

9. A method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing according to claim 1, characterized in that, The powder has a nitrogen content of 1.68% to 2.29% by mass and a Hall flow rate of 5.7 s / 50 g to 6.5 s / 50 g.

10. A method for preparing core-shell structured high-nitrogen steel composite powder for additive manufacturing according to claim 1, characterized in that, It also includes the step of using the resulting core-shell structured high-nitrogen steel composite powder in selective laser melting additive manufacturing: The powder is loaded into the powder cylinder, the substrate is preheated to 150°C, the oxygen content in the molding chamber is reduced to below 11.2 mg / m³, and a scanning strategy of rotating adjacent layers by 67° is used for laser melting molding.

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