A method for manufacturing a high-nitrogen nickel-free austenitic stainless steel component

Abstract

The application provides a high-nitrogen nickel-free austenitic stainless steel component preparation method. The method is based on existing national standard ferritic stainless steel and heat-resistant steel S12791 and S13091 powder, and a high-nitrogen nickel-free austenitic stainless steel component is prepared through in-situ addition of MnN laser additive manufacturing, so as to solve the problems of complex traditional preparation process, high cost, long cycle and the like of the existing high-nitrogen nickel-free austenitic stainless steel component, the problem of difficult acquisition of laser additive manufacturing powder raw materials, and the problem of immature development of special high-nitrogen nickel-free austenitic stainless steel components, and belongs to the field of additive manufacturing.

Description

Technical Field

[0001] The preparation method of high-nitrogen nickel-free austenitic stainless steel components specifically involves the composition ratio design of high-nitrogen nickel-free austenitic stainless steel and the laser additive manufacturing process of high-nitrogen nickel-free austenitic stainless steel components, which belongs to the field of additive manufacturing. Background Technology

[0002] Austenitic stainless steel refers to stainless steel that has an austenitic structure at room temperature. Due to its excellent mechanical properties and corrosion resistance, it has become the most widely used structural engineering alloy. Austenitic stainless steel typically uses Ni (approximately 8%–25%) as the austenite-forming and stabilizing element. Its austenite-forming ability can be measured by the chromium-nickel equivalent ratio of Cr. eq / Ni eq Make predictions (Cr) eq =%Cr + 1.5 × %Si + %Mo, Ni eq =%Ni+30×(%C+%N)+0.5×%Mn), where Cr eq / Ni eq A fully austenitic structure can be obtained when the temperature is less than 1.25.

[0003] However, nickel (Ni) is an important strategic resource for my country, but it is costly and has poor biocompatibility. Therefore, finding alternative elements to replace Ni in the production of austenitic stainless steel is becoming increasingly important. According to the nickel equivalent calculation formula, nitrogen's austenite-forming ability is 30 times that of Ni, making it a strong austenite-forming and stabilizing element. Furthermore, nitrogen acts as an interstitial atom, providing solid solution strengthening and grain refinement. Therefore, high-nitrogen, nickel-saving austenitic stainless steel, which replaces nickel with nitrogen, offers advantages such as low cost, high strength, high toughness, high corrosion resistance, and excellent biocompatibility. It is an ideal substitute for traditional Cr-Ni stainless steel and has promising application prospects in defense, energy and chemical industries, transportation, and medical devices.

[0004] High-nitrogen nickel-free austenitic stainless steel contains more than 0.4% nitrogen by mass; however, the equilibrium solubility of nitrogen in the various phases of the steel is low (2.8 wt.% in γ-austenite, 0.04 wt.% in L-liquid, and 0.0128 wt.% in δ-ferrite). Therefore, high-nitrogen nickel-free austenitic stainless steel is currently typically prepared using high-pressure melting and other methods. The conventional high-pressure manufacturing process is complex, and the high strength and hardness of high-nitrogen nickel-free austenitic stainless steel make it difficult to process complex components using traditional methods, thus limiting its development and application.

[0005] Laser additive manufacturing, as an emerging manufacturing technology, can rapidly manufacture complex three-dimensional structural parts using a layer-by-layer additive process, making it one of the most promising research directions for the fabrication of high-nitrogen nickel-free austenitic stainless steel components. However, existing processes struggle to produce high-nitrogen nickel-saving austenitic stainless steel powder for laser additive manufacturing at low cost, and there are currently no commercially available high-nitrogen nickel-saving austenitic stainless steel powders of various standard grades. Therefore, obtaining high-nitrogen stainless steel powder is a prerequisite for laser additive manufacturing of high-nitrogen nickel-free austenitic stainless steel and a key to the successful fabrication of high-nitrogen nickel-free austenitic stainless steel components.

[0006] Adding nitrides as a nitrogen source to existing national standard grades of nickel-free stainless steel (ferritic stainless steel, martensitic stainless steel, and heat-resistant steel) is an effective method for preparing high-nitrogen nickel-free austenitic stainless steel. However, the content of elements such as C, Si, Mn, P, S, Cr, Mo, Al, and Ti in stainless steel has a significant impact on the formation of austenite phase, chromium-nickel equivalent, nitrogen solubility, and carbides. Therefore, it is crucial to select appropriate grades of nickel-free stainless steel from among many available options, as well as to choose the right nitride and determine its addition ratio. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing high-nitrogen nickel-free austenitic stainless steel components, in order to solve the problems of complex, costly, and long-cycle traditional preparation processes for high-nitrogen nickel-free austenitic stainless steel components, the difficulty in obtaining laser additive manufacturing powder raw materials, and the immature development of special high-nitrogen nickel-free austenitic stainless steel components.

[0008] To address the aforementioned issues, a method for preparing high-nitrogen nickel-free austenitic stainless steel components is proposed. This method is based on existing national standard ferritic stainless steel and heat-resistant steel S12791 and S13091 powders, and prepares high-nitrogen nickel-free austenitic stainless steel components through in-situ addition of MnN laser additive manufacturing.

[0009] The specific weight ratio is as follows: 8%~13% MnN is added to S12791; 9%~13% MnN is added to S13091; the designed MnN addition ratio can ensure the production of high-nitrogen nickel-free austenitic stainless steel (Cr). eq / Ni eq <1.25); The specific addition ratio of MnN is adjusted according to the N content, mechanical properties, corrosion resistance, and other requirements of the formed component. Among them, the yield strength The pitting equivalent PREN = 1[Cr] + 3.3[Mo] + 30[N] can be calculated based on the mechanical properties, corrosion resistance requirements, yield strength, and pitting equivalent of the formed component. The optimal MnN addition ratio can be obtained.

[0010] Laser additive manufacturing processes can be either powder-spreading or powder-feeding processes. Powder-spreading mainly refers to selective laser melting, while powder-feeding mainly refers to laser melting deposition.

[0011] The particle size range of S12791, S13091, and MnN powders is between 20 and 100 μm; the laser selective melting process selects powders with a particle size of 20 to 50 μm, and the laser melting deposition process selects powders with a particle size of 50 to 100 μm.

[0012] The specific principle is as follows:

[0013] 1) Reasons for selecting base powder: S12791 and S13091 are nickel-free, have a moderate Cr content (austenitic stainless steel usually contains more than 18% Cr to ensure the corrosion resistance of stainless steel), and have a low carbon content. In laser additive manufacturing, excessive carbon content can easily form carbides, which is not conducive to corrosion resistance. The low carbon content ensures that no carbides are formed. They also contain no Al or Ti elements. In laser additive manufacturing, N can easily form AlN, TiN and other nitrides with Al and Ti. On the one hand, this reduces the solid solution N content, which is not conducive to the formation of austenitic phase. On the other hand, it is not conducive to corrosion resistance. The absence of Al and Ti elements ensures that no nitrides are formed. Among ferritic stainless steel, martensitic stainless steel and heat-resistant steel, only S12791 and S13091 are suitable if they meet the above conditions.

[0014] 2) The effects of adding MnN include: First, based on the in-situ melting, decomposition, diffusion, and mass transfer of MnN, it serves as a N source to achieve in-situ N and Mn increase in the molten pool (MnN → [Mn] + [N]); Second, as an austenite phase formation and stabilizing element, increasing the Mn content is more conducive to ensuring austenite phase formation. Simultaneously, increasing the proportion of austenite phase is more conducive to increasing the N content in the molten pool (the solubility of N in the L liquid phase, δ ferrite phase, and γ austenite phase during high-temperature molten pool solidification is 0.04 wt.%, 0.0128 wt.%, and 2.8 wt.%, respectively, with γ austenite phase having the highest solubility), conversely, increasing the N content in the molten pool promotes austenite phase formation; Third, as an element that reduces the N activity coefficient and increases the N solubility in stainless steel, increasing the Mn content is more conducive to increasing the N content in the molten pool and... Fourth, the addition of MnN can promote the transformation of the solidification mode of the molten pool from type F (L→L+δ→δ→δ+γ), type FA (L→L+δ→L+δ+γ→δ+γ), type AF (L→L+γ→L+γ+δ→δ+γ) to type A (L→L+γ→γ), avoiding "ferrite traps" (N has extremely low solubility in the ferrite phase, causing N precipitation loss), reducing N loss and nitrogen porosity defects (N element precipitates in the form of N2, which easily forms nitrogen porosity defects).

[0015] 3) MnN powder is moderately priced and can supplement Mn elements while serving as a N source. Mn can promote the formation of austenite phase and increase the solubility of N. It also has low carbon content and no Al or Ti elements.

[0016] Compared with existing technologies, this invention uses existing national standard ferritic stainless steel powders S12791 and S13091 as a basis to prepare high-nitrogen nickel-free austenitic stainless steel components through in-situ MnN addition laser additive manufacturing. This method not only obtains the forming powder raw material for laser additive manufacturing of high-nitrogen nickel-free austenitic stainless steel components at low cost, but also ensures the relevant performance requirements of the designed material composition (stainless steel listed in national standards has good comprehensive performance). Compared with developing high-nitrogen nickel-free austenitic stainless steel separately, this method reduces development costs and saves development time. The formed components, due to the increased N content and nickel-free nature, have low cost, good mechanical properties, good corrosion resistance, and biocompatibility, showing promising application prospects in defense, energy and chemical industries, transportation, and medical devices. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the microstructure characteristics of the molten pool boundary in the longitudinal cross-section of the formed sample;

[0018] Figure 2 and Figure 3 Elemental distribution map of the middle part of the sedimentary layer;

[0019] Figure 4 This is a distribution map of N and Mn elements in the middle of the sedimentary layer;

[0020] Figure 5 This is an EDS energy spectrum of the middle part of the sedimentary layer. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. Example

[0022] This embodiment mainly improves the preparation method of high-nitrogen nickel-free austenitic stainless steel components. The improved method is as follows: based on existing national standard ferritic stainless steel and heat-resistant steel S12791 powder, high-nitrogen nickel-free austenitic stainless steel components are prepared by in-situ addition of 8%~13% MnN through laser additive manufacturing. The grades and chemical compositions of austenitic-ferritic stainless steel are shown in Table 1 below. The designed MnN addition ratio can ensure the preparation of high-nitrogen nickel-free austenitic stainless steel (Cr eq / Ni eq <1.25); The specific addition ratio of MnN is adjusted according to the N content, mechanical properties, corrosion resistance, and other requirements of the formed component. Among them, the yield strength The pitting equivalent PREN = 1[Cr] + 3.3[Mo] + 30[N]. Based on the mechanical properties, corrosion resistance requirements, yield strength, and pitting equivalent calculation formula of the formed component, the optimal addition ratio of MnN can be calculated. The chemical composition of MnN is shown in Table 2 below.

[0023] Table 1. Grades and Chemical Compositions of Austenitic-Ferritic Stainless Steel

[0024]

[0025] Table 2 Chemical composition of MnN

[0026]

[0027] Methods and rationale for determining the MnN addition ratio: The method for determining the MnN addition ratio needs to comprehensively consider the requirements of chromium-nickel equivalent (to ensure austenitic phase formation), nitrogen content (greater than the requirements of high-nitrogen austenitic stainless steel, with a mass fraction exceeding 0.4%), mechanical properties (measured by yield strength), and corrosion resistance (measured by pitting corrosion equivalent). However, due to the unavoidable loss of dissolved nitrogen during the thermal process of laser additive manufacturing (the solubility of N during melting exceeds its saturation solubility), the addition of MnN must ensure a 10% N margin. Calculations show that the nitrogen content mass fraction of the designed powder needs to reach 0.45%. However, excessively high N content in the powder will strengthen the tendency for supersaturated nitrogen precipitation, easily leading to nitrogen porosity defects. Therefore, the N content mass fraction in the powder needs to be lower than 1.1%. The addition ratio, chromium-nickel equivalent, and powder composition mass fraction are determined based on the above, as shown in Table 3. Laser additive manufacturing processes can be either powder-spreading or powder-feeding processes. Powder-spreading processes are represented by selective laser melting (SLM), while powder-feeding processes are represented by laser melting deposition (LMD). When the formed component is small in size, has high surface precision requirements, and a complex structure, selective laser melting is preferred; otherwise, laser melting deposition (LMD) is preferred. The powder particle size range is 20~100um. Selective laser melting (SLM) prefers powder with a particle size of 20~50um, while laser melting deposition (LMD) prefers powder with a particle size of 50~100um.

[0028] Table 3. Powder ratio, chromium-nickel equivalent, and chemical composition of high-nitrogen nickel-free austenitic stainless steel for laser additive manufacturing.

[0029]

[0030] Example 2

[0031] This embodiment represents an improvement to the preparation method of high-nitrogen nickel-free austenitic stainless steel components. The improved method is as follows: based on existing national standard ferritic stainless steel and heat-resistant steel S13091 powder, high-nitrogen nickel-free austenitic stainless steel components are prepared by in-situ addition of 9%~13% MnN using laser additive manufacturing. The austenitic-ferritic stainless steel grades and chemical compositions are shown in Table 1. The designed MnN addition ratio can ensure the production of high-nitrogen nickel-free austenitic stainless steel (Cr eq / Ni eq <1.25); The specific addition ratio of MnN is adjusted according to the N content, mechanical properties, corrosion resistance, and other requirements of the formed component. Among them, the yield strength The pitting equivalent PREN = 1[Cr] + 3.3[Mo] + 30[N]. Based on the mechanical properties, corrosion resistance requirements, yield strength, and pitting equivalent calculation formula of the formed component, the optimal addition ratio of MnN can be calculated. The chemical composition of MnN is shown in Table 2.

[0032] Methods and rationale for determining the MnN addition ratio: The method for determining the MnN addition ratio needs to comprehensively consider the requirements of chromium-nickel equivalent (to ensure austenitic phase formation), nitrogen content (greater than the requirements of high-nitrogen austenitic stainless steel, with a mass fraction exceeding 0.4%), mechanical properties (measured by yield strength), and corrosion resistance (measured by pitting corrosion equivalent). However, due to the unavoidable loss of dissolved nitrogen during the thermal process of laser additive manufacturing (the solubility of N during melting exceeds its saturation solubility), the addition of MnN must ensure a 10% N margin. Calculations show that the nitrogen content mass fraction of the designed powder needs to reach 0.45%. However, excessively high N content in the powder will strengthen the tendency for supersaturated nitrogen precipitation, easily leading to nitrogen porosity defects. Therefore, the N content mass fraction in the powder needs to be lower than 1.1%. The addition ratio, chromium-nickel equivalent, and powder composition mass fraction are determined based on the above, as shown in Table 3.

[0033] Laser additive manufacturing processes can be either powder-spreading or powder-feeding processes. Powder-spreading processes are represented by selective laser melting (SLM), while powder-feeding processes are represented by laser melting deposition (LMD). When the formed component is small in size, has high surface precision requirements, and a complex structure, selective laser melting is preferred; otherwise, laser melting deposition (LMD) is preferred. The powder particle size range is 20~100um. Selective laser melting (SLM) prefers powder with a particle size of 20~50um, while laser melting deposition (LMD) prefers powder with a particle size of 50~100um.

[0034] The above embodiments are based on existing national standard ferritic stainless steel powders S12791 and S13091. High-nitrogen nickel-free austenitic stainless steel components are prepared by in-situ MnN addition laser additive manufacturing. This method not only obtains the forming powder raw material for laser additive manufacturing of high-nitrogen nickel-free austenitic stainless steel components at low cost, but also ensures the relevant performance requirements of the designed material composition (stainless steel listed in national standards has good comprehensive performance). Compared with developing high-nitrogen nickel-free austenitic stainless steel separately, this reduces development costs and saves development time. Due to the increased N content and nickel-free nature, the formed components have low cost, good mechanical properties, good corrosion resistance, and biocompatibility, showing promising application prospects in defense, energy and chemical industries, transportation, and medical devices.

[0035] Experimental Example

[0036] Experiment on the preparation of high-nitrogen nickel-free austenitic stainless steel components:

[0037] I. Materials and Experiments

[0038] With 008Cr27Mo b (Code: S12791) Based on austenitic-ferritic stainless steel, high-nitrogen nickel-free austenitic stainless steel was prepared by in-situ addition of 10% by mass of MnN powder via laser melting deposition. Single-pass, single-layer forming experiments were conducted with a laser power of 1000W, a scanning speed of 10mm / s, and a powder feed rate of 8g / min. The formed samples showed suitable melt width and depth, with no obvious defects.

[0039] II. Microstructural Features

[0040] A schematic diagram of the microstructure characteristics of the molten pool boundary in the longitudinal cross-section of the formed specimen is shown below. Figure 1 As shown in the figure, the sedimentary layer is dominated by austenite phase, and its microstructure consists of columnar dendrites and equiaxed grains. There are few pores and cracks, and the microstructure characteristics are relatively good.

[0041] III. Nitrogen Content and Distribution

[0042] EDS energy dispersive spectroscopy was performed on the middle part of the sediment layer. Figure 2 , Figure 3 It can be seen that, with 008Cr27Mo b (Code: S12791) Based on austenitic-ferritic stainless steel, a high-nitrogen nickel-free austenitic stainless steel deposition layer was prepared by in-situ addition of 10% by mass of MnN powder via laser melting deposition. The deposition layer exhibited uniform elemental distribution and no compositional segregation. Figure 4The EDS energy dispersive spectroscopy (EDS) spectrum of the middle layer shows that the N content is 0.65% by mass and the Mn content is 9.0% by mass, meeting the composition requirements for high-nitrogen nickel-free austenitic stainless steel components. The N and Mn contents in the deposited layer are somewhat reduced compared to the designed composition. This is due to the precipitation of supersaturated dissolved N and the evaporation loss of Mn during laser melting deposition. However, the allowance design ensures that high-nitrogen nickel-free austenitic stainless steel can still be produced even after the loss of N and Mn elements.

[0043] Therefore, with 008Cr27Mo b Based on austenitic-ferritic stainless steel (code: S12791), high-nitrogen nickel-free austenitic stainless steel was prepared by in-situ addition of 10% by mass of MnN powder via laser melting deposition. The nitrogen content and microstructure of the deposited samples met the requirements for high-nitrogen nickel-free austenitic stainless steel. High-nitrogen nickel-free austenitic stainless steel components can be successfully prepared by selecting appropriate process parameters such as overlap ratio, scanning path, and Z-axis lift in multiple passes and layers. Furthermore, appropriately increasing or decreasing the MnN addition ratio (10% by mass) can also prepare high-nitrogen nickel-free austenitic stainless steel via laser melting deposition. Theoretical calculations show that an addition ratio of 8-13% can effectively balance nitrogen content, microstructure, and defects. In addition, 008Cr30Mo2... b (Code: S13091) and 008Cr27Mo b (Code: S12791) Both are austenitic-ferritic stainless steels with similar chemical compositions, such as 008Cr30Mo2. b (Code: S13091) Based on austenitic-ferritic stainless steel, high-nitrogen nickel-free austenitic stainless steel can also be successfully prepared by adding 9~13% MnN in situ.

[0044] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing high-nitrogen nickel-free austenitic stainless steel components, characterized in that: This method is based on existing national standard ferritic stainless steel and heat-resistant steel S12791 and S13091 powders, and prepares high-nitrogen nickel-free austenitic stainless steel components by in-situ addition of MnN laser additive manufacturing. The specific weight ratio is 8%~13% MnN added to S12791; and 9%~13% MnN added to S13091. Laser additive manufacturing processes can be either powder-spreading or powder-feeding processes. Powder-spreading is a selective laser melting process, while powder-feeding is a laser melting deposition process. The particle size range of S12791, S13091, and MnN powders is between 20 and 100 μm; the laser selective melting process selects powders with a particle size of 20 to 50 μm, and the laser melting deposition process selects powders with a particle size of 50 to 100 μm.

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