A duplex stainless steel and a laser powder bed fusion manufacturing method thereof

Abstract

The application discloses a kind of duplex stainless steel and its laser powder bed fusion manufacturing method, belong to advanced metal additive manufacturing post-processing technical field, to solve the technical problem that the strength and plasticity of duplex stainless steel prepared using prior art cannot simultaneously meet the use requirement.The laser powder bed fusion manufacturing method of the duplex stainless steel includes: step 1, using duplex stainless steel powder to prepare duplex stainless steel print;Step 2, short-time annealing treatment;The print obtained in step 1 is placed in a heat treatment equipment for short-time annealing treatment, the short-time annealing treatment temperature is 1050-1200 DEG C, and the holding time is 1-10 min;Step 3, rapid cooling;Print is rapidly cooled, and the rapid cooling rate is 100-200 DEG C / s, to obtain duplex stainless steel.The tensile strength of the print prepared by the application is 950-1072 MPa, the yield strength is 710-814 MPa, and the elongation after fracture is 18.5-28%, which can realize the synergistic improvement of high density, high strength and good plasticity.

Description

Technical Field

[0001] This invention belongs to the field of advanced metal additive manufacturing post-processing technology, and particularly relates to a method for manufacturing duplex stainless steel and its laser powder bed melting process. Background Technology

[0002] Duplex stainless steel is a typical type of super duplex stainless steel, characterized by high Cr, Mo, and N content. It combines the high strength of the ferrite phase, the good ductility and toughness of the austenitic phase, and excellent resistance to chloride corrosion. It is widely used in marine engineering, petrochemicals, energy equipment, seawater desalination, and complex corrosion-resistant structural components. Laser powder bed melting, as an advanced metal manufacturing process, can achieve near-net-shape forming of complex components, providing a new technological path for the manufacture of complex duplex stainless steel structures.

[0003] However, duplex stainless steel undergoes rapid melting, rapid solidification, and cyclic heating during laser powder bed molding. The resulting microstructure in the printed state typically exhibits ferrite dominance and insufficient austenite formation, accompanied by high residual stress, high-density dislocations, a molten pool substructure, and supersaturated solid solution of nitrogen in the ferrite. While this non-equilibrium microstructure is beneficial for improving printed strength, it leads to insufficient plasticity.

[0004] To address the aforementioned issues, existing technologies typically employ traditional solution treatment or prolonged heat treatment (holding time 30–120 min). However, while this prolonged high-temperature holding treatment can promote austenite recovery and release residual stress, it also causes significant coarsening of the microstructure, leading to substantial recovery of dislocation structures, disappearance of molten pool refinement characteristics, and weakening of the strengthening effect of nano-oxide / nitrides. Ultimately, this results in a significant decrease in the material's yield strength to 620–650 MPa, failing to achieve a synergistic match between strength and plasticity, and increasing the difficulty of subsequent performance stability control. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a method for manufacturing duplex stainless steel and its laser powder bed melting, in order to solve the technical problem that the strength and plasticity of duplex stainless steel prepared by existing technology cannot simultaneously meet the requirements for use.

[0006] The objective of this invention is mainly achieved through the following technical solutions: On one hand, the present invention provides a duplex stainless steel powder, the composition of which, by mass percentage, comprises: C: ≤0.03%, Cr: 24.0%~26.0%, Mo: 3.0%~5.0%, Ni: 6.0%~8.5%, Mn: ≤1.2%, Si: ≤1.0%, N: 0.09%~0.42%, P: ≤0.03%, S: ≤0.01%, Al: 0.01%~0.05%, O: 0.01%~0.03%, W: 0.05%~0.30%, with the balance being Fe and unavoidable impurities.

[0007] In one possible design, the mass ratio of Al to O satisfies the following relationship: R Al / O =Al / O (1) In equation (1), Al and O are the mass percentages of Al and O, respectively, and R... Al / O The value ranges from 0.8 to 3.0.

[0008] In one possible design, R Al / O It ranges from 1.0 to 2.5.

[0009] On the other hand, the present invention also provides a laser powder bed melting manufacturing method for duplex stainless steel, using the aforementioned duplex stainless steel powder; the manufacturing method includes: Step 1: Prepare duplex stainless steel printed parts; Duplex stainless steel powder is provided, and the raw material is placed in a molding chamber and molded under an inert gas protective atmosphere to obtain a printed part; Step 2: Short-time annealing; The printed part obtained in step 1 is placed in a heat treatment device for short-time annealing. The short-time annealing temperature is 1050℃~1200℃ and the holding time is 1min~10min. Step 3: Rapid cooling; The printed parts are rapidly cooled at a rate of 100-200℃ / s to obtain duplex stainless steel.

[0010] Furthermore, in step 1, the inert gas protective atmosphere is argon, nitrogen, or an argon-nitrogen mixture; The oxygen content inside the molding cavity is controlled below 1000 ppm.

[0011] Furthermore, in step 1, during molding, the volumetric energy density (VED) is controlled to be 62–132 J / mm³.

[0012] Furthermore, in step 1, during molding, the volumetric energy density (VED) is controlled to be 100–132 J / mm³.

[0013] Furthermore, in step 2, the short-time annealing temperature is 1080℃~1200℃, and the holding time is 1min~10min.

[0014] Furthermore, in step 3, the rapid cooling rate is 150-200℃ / s to obtain duplex stainless steel.

[0015] In another aspect, the present invention also provides a duplex stainless steel printed part, which is prepared by the above-mentioned laser powder bed melting manufacturing method for duplex stainless steel; The relative density of this duplex stainless steel printed part is greater than or equal to 99.5%; its tensile strength is 950-1072 MPa, its yield strength is 710-814 MPa, and its elongation after fracture is 18.5-28%.

[0016] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: (1) By adding Al and O and controlling their ratio, the present invention can form fine and dispersed Al2O3 and MnAl2O4 composite oxides during rapid solidification. These nano-oxides remain in the structure after short-time annealing, playing a role in heterogeneous nucleation and structure refinement, while providing dispersion strengthening effect. The addition of W further improves the material's resistance to pitting corrosion and crevice corrosion in chlorine-containing environments. In addition, by synergistically controlling Ni and N through the Ni equivalent formula, it can be ensured that the material retains an appropriate amount of austenite nucleation potential after rapid solidification of laser powder bed melting, while promoting rapid austenite nucleation and growth during short-time annealing, so that the austenite volume fraction in the final dual-phase structure is stabilized in the range of 21.5% to 43%, achieving a match between the high strength bearing capacity of ferrite and the coordinated plastic deformation of austenite.

[0017] (2) This invention effectively avoids the problems of grain coarsening, excessive recovery of dislocation structure, and loss of molten pool refinement characteristics caused by traditional long-term heat preservation (30-120 min) through short-time annealing (1-10 min). The short-time treatment retains some of the original high-density dislocations, fine molten pool substructures, and diffuse distribution of endogenous nano-oxides (such as Al2O3 and MnAl2O4) in the printed state. These strengthening structures continue to play the role of dislocation strengthening and dispersion strengthening. Furthermore, rapid cooling (100℃ / s~200℃ / s) ensures that the dual-phase structure formed at high temperature is quickly frozen, inhibiting the formation of harmful precipitates (such as chromium-rich carbides and nitrides) and avoiding the deterioration of microstructure properties. Through this mechanism, while promoting the rapid formation of austenite and improving the plasticity of the material, the contribution of the non-equilibrium strengthening structure to the strength is preserved. Ultimately, an excellent strength-plasticity match of yield strength of 710MPa~814MPa, tensile strength of 950MPa~1072MPa and elongation after fracture of 18.5%~28% is achieved, which is significantly better than the performance level of traditional long-term heat treatment process (yield strength 620~650MPa) and printed state (elongation after fracture 3.5%).

[0018] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0019] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0020] Figure 1 Here is a powder scanning electron microscope image of Example 1; Figure 2 Metallographic microstructure of the untreated front side of Example 1; Figure 3 Metallographic microstructure of the front side after heat treatment in Example 2; Figure 4 Metallographic microstructure of the heat-treated side surface of Example 2; Figure 5 Image of the finished product from additive manufacturing; Figure 6 This is a photograph of the cracked finished product in Comparative Example 4. Detailed Implementation

[0021] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0022] On the one hand, the present invention also provides a laser powder bed melting manufacturing method for duplex stainless steel, using the aforementioned duplex stainless steel powder; the manufacturing method includes the following steps: Step 1: Prepare duplex stainless steel printed parts; The raw material was prepared according to the above-mentioned formula for duplex stainless steel powder, and the raw material was placed in a molding chamber and molded under an inert gas protective atmosphere.

[0023] In step 1 above, the forming process adopts a 67°-70° (e.g., 67°, 68°, 70°) interlayer rotation scanning strategy. For example, when the 67° interlayer rotation scanning strategy is adopted, the laser scanning direction between adjacent forming layers is rotated by 67°, so that the melt channels of adjacent layers are staggered and overlapping, the formed structure is mainly ferrite, and the arc-shaped melt pool boundary is retained.

[0024] Compared with existing technologies, this scanning strategy can weaken the continuous superposition of heat accumulation and shrinkage stress in a single direction, improve the continuity of interlayer fusion, and reduce the risk of thermal stress cracking in high-N, Al-containing duplex stainless steel systems.

[0025] In step 1 above, the inert gas protective atmosphere is argon, nitrogen or argon-nitrogen mixture; the oxygen content in the molding cavity is controlled below 1000 ppm (for example, oxygen content of 950 ppm, 900 ppm, 860 ppm, 800 ppm).

[0026] In step 1 above, the volumetric energy density (VED) of the laser powder bed melting molding is used as the final control index for the molding heat input. The volumetric energy density (VED) is controlled to be 62-132 J / mm³ (e.g., 62 J / mm³, 74.07 J / mm³, 83.95 J / mm³, 93.83 J / mm³, 108.23 J / mm³, 113.58 J / mm³, 132 J / mm³).

[0027] Furthermore, the aforementioned volumetric energy density VED is controlled to be 100-132 J / mm³ (100 J / mm³, 108.23 J / mm³, 113.58 J / mm³, 132 J / mm³).

[0028] The formula for calculating the volumetric energy density VED is as follows: VED=P / (v·h·t)(3) In equation (3), VED is the volumetric energy density, in J / mm³.

[0029] P represents laser power, measured in W. v represents the scanning speed, measured in mm / s; h represents the scanning interval, in mm; t represents the thickness of the powder layer, in mm.

[0030] In step 2 above, the forming process window for duplex stainless steel is as follows: laser power P is 170 W to 230 W (e.g., 170 W, 190 W, 210 W, 230 W), scanning speed v is 650 mm / s to 1000 mm / s (e.g., 650 mm / s, 750 mm / s, 850 mm / s, 950 mm / s, 1000 mm / s), scanning spacing h is 0.08 mm to 0.10 mm (e.g., 0.08 mm, 0.085 mm, 0.09 mm, 0.1 mm), and powder layer thickness t is 0.025 mm to 0.035 mm (e.g., 0.025 mm, 0.028 mm, 0.030 mm, 0.032 mm, 0.035 mm).

[0031] Preferably, in step 1 above, the laser power P is 190 W to 230 W, the scanning speed v is 750 mm / s to 1000 mm / s, the scanning spacing h is 0.09 mm to 0.10 mm, and the powder layer thickness t is 0.028 mm to 0.035 mm.

[0032] It should be noted that the scanning spacing h and powder layer thickness t mentioned above are not independent fixed parameters, but can be dynamically adjusted to match the actual laser energy input. Specifically, using the laser line energy E=P / v to characterize the energy input per unit scan length, the scanning spacing h and powder layer thickness t satisfy the following relationship: h·t=K·E(4) Where K is the process matching coefficient, mm³ / J; preferably controlled at 0.009~0.012 mm³ / J; h represents the scanning interval, in mm; t represents the thickness of the powder layer, in mm; E represents the laser line energy, measured in J / mm.

[0033] Compared with the prior art, the laser powder bed fusion additive manufacturing method of the present invention is no longer limited to fixed scanning spacing h and powder layer thickness t, but can be dynamically matched and adjusted according to specific forming conditions: when the laser line energy E is high, the scanning spacing h and powder layer thickness t are increased accordingly to expand the forming volume corresponding to a single molten pool, reduce local overheating, and reduce N element escape, Cr and Mn burn-off, spatter enhancement, and thermal stress cracking; when the laser line energy E is low, the scanning spacing h and powder layer thickness t are decreased accordingly to ensure sufficient overlap of the molten pool and interlayer metallurgical bonding.

[0034] In step 1 above, to ensure the continuity of the molten pool overlap and the stability of interlayer fusion, the scanning spacing h and the powder layer thickness t satisfy the following relationship: h / t=λ(5) Wherein, λ is the matching coefficient between scanning spacing and powder layer thickness; h represents the scanning interval, in mm; t represents the thickness of the powder layer, in mm.

[0035] In step 1 above, preferably, λ is 2.5 to 3.5.

[0036] Preferably, λ is 3.0.

[0037] Substituting equation (5) h=λt into equation (4) h·t=K·E, we obtain the coupling relationship between the powder layer thickness and the scanning spacing: t=(K·E / λ)^(1 / 2)(6) h=(λ·K·E)^(1 / 2)(7) Through the above coupling relationship, the scanning spacing and powder layer thickness can be dynamically matched according to the actual laser power and scanning speed. Combined with the VED window, the volumetric energy density VED can be kept in the range of 62 J / mm³ to 132 J / mm³. Combined with the 67°-70° interlayer rotation scanning strategy, the molten pool overlap, thermal stress accumulation, nitrogen element burn-off, Al-O compound formation and element migration behavior can be synergistically controlled. This reduces the risk of element burn-off (Cr and Mn), molten pool disturbance and thermal stress cracking in high-N systems, thereby achieving stable forming of duplex stainless steel and subsequent duplex microstructure reconstruction.

[0038] In step 1, the prepared printed part (i.e. the printed part before short-time annealing) has a non-equilibrium structure with a ferrite content of more than 90%, that is, a non-equilibrium structure dominated by printed ferrite, and contains supersaturated N elements, residual stress, high-density dislocations and fine substructures formed by rapid solidification of the molten pool.

[0039] Step 2: Short-time annealing treatment; The printed part obtained in step 1 is placed in a heat treatment device for short-time annealing. The short-time annealing temperature is 1050℃~1200℃ (1050℃, 1100℃, 1150℃, 1180℃, 1200℃), and the holding time is 1min~10min (1min, 2min, 4min, 5min, 6min, 8min, 10min).

[0040] Compared with existing technologies, short-time annealing promotes the release of supersaturated nitrogen (N) in printed duplex stainless steel, rapid austenite nucleation and growth, and duplex microstructure reconstruction. The reconstruction mechanism is as follows: rapid solidification of the laser powder bed melt causes N to become supersaturated and dissolved in ferrite, forming high-energy sites at grain boundaries, subgrain boundaries, dislocations, and molten pool boundaries. During short-time annealing, N rapidly diffuses and accumulates in these high-energy sites, lowering the austenite nucleation barrier and promoting rapid austenite formation. This transforms the ferrite-dominated non-equilibrium microstructure of the printed state into a duplex microstructure where ferrite and austenite coexist.

[0041] Preferably, in step 2 above, the short-time annealing temperature is 1080℃~1200℃, and the holding time is 4min~10min.

[0042] Step 3: Rapid cooling; After short-time annealing, the printed parts are rapidly cooled at a rate of 100-200℃ / s (100℃ / s, 130℃ / s, 150℃ / s, 180℃ / s, 200℃ / s) to obtain short-time annealed duplex stainless steel.

[0043] In step 3 above, the rapid cooling is one of water quenching, air cooling or forced air cooling, used to rapidly freeze the high-temperature biphase structure to room temperature, inhibit the formation of harmful precipitates, and avoid the deterioration of the structure properties.

[0044] After short-time annealing and rapid cooling, the duplex stainless steel forms a dual-phase microstructure in which ferrite and austenite coexist, with austenite dispersed within the ferrite matrix. This dual-phase microstructure achieves a balance between the high strength load-bearing capacity of ferrite and the coordinated plastic deformation of austenite, thus maintaining good plasticity while improving strength.

[0045] This invention does not employ the traditional long-term solution treatment and heat preservation method. Instead, it utilizes the ferrite-dominant non-equilibrium structure (>90%), supersaturated solid solution nitrogen element, and high-energy sites such as high-density dislocations, subgrain boundaries, ferrite grain boundaries, and molten pool boundaries retained in the printed duplex stainless steel to promote rapid austenite nucleation and growth in a short period of 1 to 10 minutes.

[0046] Specifically, during laser powder bed fusion molding, duplex stainless steel undergoes rapid melting, rapid solidification, and cyclic thermal effects. In the printed microstructure, nitrogen (N) is supersaturated and dissolved in ferrite, accumulating at the aforementioned high-energy sites. During short-time annealing, N rapidly diffuses and segregates towards these high-energy sites, significantly lowering the austenite nucleation barrier. This promotes rapid austenite formation within a short time, transforming the ferrite-dominated microstructure of the printed state into a duplex microstructure where ferrite and austenite coexist.

[0047] Meanwhile, short-time annealing avoids the problems caused by traditional long-time holding (30–120 min), such as grain coarsening, extensive recovery of dislocation structures, loss of melt pool refinement features, and weakening of the strengthening effect of nano-oxide / nitride. Therefore, this invention, through the synergistic control of short-time annealing temperature and holding time, enables the supersaturated nitrogen element dissolved in ferrite to diffuse and enrich towards the austenite nucleation site, and further distributes it into the austenite phase during austenite formation, thereby achieving residual stress release, nitrogen element redistribution, rapid austenite formation, and reconstruction of the ferrite and austenite dual-phase microstructure.

[0048] In contrast, while the printed microstructure (ferrite content > 90%) without the short-time annealing treatment of this invention is beneficial for the formation of printed strength, it limits austenite recovery and the ability of biphase coordinated deformation, resulting in insufficient printed plasticity (elongation after fracture of only 3.5%). Traditional long-term solution treatment, although achieving a higher austenite content and better plasticity, results in significant microstructure recovery and weakened strengthening effect, leading to a significant decrease in yield strength to 620–650 MPa. This invention, through short-time annealing, optimizes the biphase ratio while maintaining the non-equilibrium strengthening structure, achieving an excellent strength-plasticity balance of 710–814 MPa and an elongation after fracture of 18.5–28%.

[0049] It should be noted that in step 1 of the present invention, the components of the above-mentioned duplex stainless steel powder, by mass percentage, include: C: ≤0.03%, Cr: 24.0%~26.0%, Mo: 3.0%~5.0%, Ni: 6.0%~8.5%, Mn: ≤1.2%, Si: ≤1.0%, N: 0.09%~0.42%, P: ≤0.03%, S: ≤0.01%, Al: 0.01%~0.05%, O: 0.01%~0.03%, W: 0.05%~0.30%, with the balance being Fe and unavoidable impurities.

[0050] It should be noted that this composition design is the material basis for subsequent laser powder bed melting and short-time annealing, which determines the ability to form non-equilibrium structures in the printing state (especially the solid solution ability of supersaturated N elements) and the nucleation and growth behavior of austenite during short-time annealing.

[0051] The functions of each component in the above duplex stainless steel powder are as follows: C: C can improve the strength of steel and promote austenite stability, but excessive content can easily form chromium-rich carbides, reducing grain boundary corrosion resistance. Therefore, it should be controlled at ≤0.03% to inhibit carbide precipitation and maintain corrosion resistance.

[0052] Cr: Cr is the core element for forming a stable passivation film and improving pitting corrosion resistance, while also promoting ferrite formation. Controlling Cr at 24.0%–26.0% is beneficial for ensuring corrosion resistance and avoiding excessive ferrite and an increased tendency for the formation of harmful precipitates.

[0053] Mo: Mo can significantly improve the resistance of materials to pitting and crevice corrosion in chlorine-containing environments, and also produces a solid solution strengthening effect. Controlling the Mo content to 3.0%–5.0% can balance corrosion resistance, strength, and structural stability.

[0054] Ni: As an austenite stabilizing element, Ni can promote austenite formation and improve plasticity and toughness. Controlling Ni at 6.0% to 8.5% is beneficial for adjusting the ratio of ferrite to austenite in additive manufacturing and subsequent heat treatment processes.

[0055] Mn: Mn has the functions of deoxidation, increasing nitrogen solubility, and assisting in the stabilization of austenite. Controlling Mn to ≤1.2% can improve the metallurgical reaction in the molten pool, while reducing the risk of MnS inclusion formation and increased local corrosion susceptibility.

[0056] Si: Si mainly plays a deoxidizing role, which helps to improve melt cleanliness and molding stability. Controlling Si to ≤1.0% can achieve the deoxidizing effect while avoiding excessive ferrite formation tendency and increased risk of brittle phase precipitation.

[0057] Nitrogen (N) is a strong austenite-forming element that promotes austenite transformation, improving strength and pitting resistance. Maintaining N levels between 0.09% and 0.42% enhances solid solution strengthening and corrosion resistance while reducing nitride precipitation, porosity, and hot cracking tendency.

[0058] P and S are harmful impurity elements. Excessive content can reduce grain boundary bonding and corrosion resistance, and increase the tendency for hot cracking and inclusion formation. Therefore, P ≤ 0.03% and S ≤ 0.01% are controlled respectively to improve material purity and molding stability.

[0059] Al: Al has a strong deoxidizing effect and a high chemical affinity for O. During the rapid melting and solidification process in additive manufacturing, Al preferentially combines with O in the molten pool to form Al2O3, and can also form MnAl2O4 composite oxides with Mn and O. Controlling the Al content to 0.01%–0.05% is beneficial for the oxides to be distributed in a fine and dispersed form, exerting heterogeneous nucleation, microstructure refinement, and dispersion strengthening effects, while avoiding excessive Al content which can lead to coarse oxide inclusions, excessive ferrite stabilization, and decreased plasticity.

[0060] O: In this invention, O serves as a crucial element for regulating the formation and distribution of oxides. Appropriate amounts of O can react with elements such as Al and Mn to form stable, fine oxide particles, and exert a certain pinning effect on grain and phase boundary migration during subsequent heat treatment. Controlling the O content to 0.01%–0.03% balances oxide dispersion strengthening and compact forming, while avoiding excessive O content that leads to increased oxide film, coarse inclusions, and porosity.

[0061] Preferably, the mass ratio of Al to O satisfies the following relationship: R Al / O =Al / O (1) In equation (1), Al and O are the mass percentages of Al and O, respectively, and R... Al / O The value ranges from 0.8 to 3.0.

[0062] Preferably, R Al / O It ranges from 1.0 to 2.5.

[0063] It should be noted that when R Al / O When the ratio is too low, Al is insufficient to effectively fix O, easily leading to the formation of uncontrolled Cr-rich or Mn-rich oxides; when R... Al / O When the ratio is too high, excess Al will lead to the formation of coarse Al-enriched oxides and enhance the stability of ferrite. Therefore, by controlling the Al content, O content and their ratio, the quantity, size and distribution of oxides can be synergistically regulated.

[0064] W: W can improve the pitting and crevice corrosion resistance of duplex stainless steel in chlorine-containing environments and has a certain solid solution strengthening effect. Controlling W at 0.05% to 0.30% is beneficial to further improve corrosion resistance and strength, while avoiding an excessive tendency for intermetallic compound precipitation.

[0065] Among the above components, Cr and Mo are the main elements that ensure the pitting corrosion resistance of duplex stainless steel, and Cr is also a ferrite-forming element.

[0066] In the above components, the present invention synergistically controls Ni and N through Ni equivalent to satisfy the following: 7.5≤Ni+21.75N≤15(2; Used for austenite ratio control, where Ni and N are both mass percentage contents.

[0067] The composition of the above duplex stainless steel powder, by mass percentage, includes: C: 0.005-0.03%, Cr: 24.5%-26.0%, Mo: 3.3%-5.0%, Ni: 6.5%-8.5%, Mn: ≤1.0%, Si: ≤1.0%, N: 0.12%-0.42%, P: ≤0.03%, S: ≤0.01%, Al: 0.01%-0.05%, O: 0.01%-0.03%, W: 0.05%-0.30%, with the balance being Fe and unavoidable impurities.

[0068] For example, the content of C can be: 0.005%, 0.007%, 0.01%, 0.015%, 0.018%, 0.021%, 0.03%.

[0069] The Cr content can be: 24.5%, 25%, 25.7%, 26.0%.

[0070] The Mo content can be: 3.3%, 3.8%, 4.2%, 4.5%, 4.8%, 5.0%.

[0071] The Ni content can be: 6.5%, 6.8%, 7.2%, 7.6%, 7.8%, 8.0%, 8.5%.

[0072] The Mn content can be: 0.2%, 0.3%, 0.4%, 0.6%, 0.8%, 1.0%.

[0073] The Si content can be: 0.1%, 0.2%, 0.5%, 0.7%, 1.0%.

[0074] The nitrogen content can be: 0.12%, 0.22%, 0.32%, 0.42%.

[0075] The content of O can be: 0.01%, 0.02%, 0.025%, 0.03%.

[0076] The content of W can be: 0.05%, 0.08%, 0.12%, 0.22%, 0.28%, 0.30%.

[0077] In summary, this invention, by adding Al and O and controlling their ratio, enables the formation of fine, dispersed Al2O3 and MnAl2O4 composite oxides during rapid solidification. These nano-oxides remain in the microstructure after short-time annealing, playing a role in heterogeneous nucleation and microstructure refinement, while also providing dispersion strengthening. The addition of W further improves the material's resistance to pitting and crevice corrosion in chlorine-containing environments. Furthermore, by synergistically controlling Ni and N using the Ni equivalent formula, it is ensured that the material retains an appropriate amount of austenite nucleation potential after rapid solidification via laser powder bed melting, while simultaneously promoting rapid austenite nucleation and growth during short-time annealing. This results in the austenite volume fraction in the final dual-phase microstructure remaining stable within the range of 21.5% to 43%, achieving a balance between the high strength load-bearing capacity of ferrite and the coordinated plastic deformation of austenite.

[0078] Compared with the prior art, the preparation method of the present invention has the following beneficial effects: (1) The present invention utilizes the non-equilibrium structure, high-density dislocations, residual stress and supersaturated N elements retained in the printed material through short annealing treatment of 1 min to 10 min to promote the rapid nucleation and growth of austenite, thereby avoiding the coarsening of the structure and the disappearance of the strengthening structure caused by traditional long-term heat preservation treatment.

[0079] (2) The present invention can retain some of the refined microstructure, dislocation structure and nano-oxide distribution characteristics of the melt pool while releasing residual stress and improving the proportion of dual-phase microstructure, so that the printed parts have both high strength and good plasticity.

[0080] (3) Unlike traditional solid solution long-term heat treatment, the present invention can shorten the post-treatment cycle, reduce the risk of grain coarsening and harmful precipitation caused by high temperature stay, and achieve synergistic matching of density, mechanical properties and corrosion resistance of laser powder bed melt-formed duplex stainless steel.

[0081] (4) The present invention adopts a short-time annealing post-treatment method, which promotes the rapid formation of austenite and release of residual stress while retaining the fine structure of the printed state, the dislocation substructure and the strengthening effect of endogenous nano oxides, thereby achieving a synergistic match of strength, plasticity and corrosion resistance: wherein, the relative density is greater than or equal to 99.5%; the tensile strength is 950-1072 MPa, the yield strength is 710-814 MPa, and the elongation after fracture is 18.5-28%.

[0082] Furthermore, the present invention also provides a duplex stainless steel printed part, which is prepared by the above-described preparation method.

[0083] The following specific embodiments and comparative examples demonstrate the advantages of synergistic control between short-time annealing temperature, holding time, and rapid cooling in this invention.

[0084] An embodiment of the present invention provides a short-time annealing post-treatment method suitable for laser powder bed fusion forming of duplex stainless steel, the chemical composition of which is shown in Table 1.

[0085] The preparation methods of Examples 1-10 include: Step 1: Preparation of dual-phase stainless steel printed parts by laser powder bed fusion molding; Step 1: Prepare duplex stainless steel printed parts; The raw material was prepared according to the above-mentioned formula for duplex stainless steel powder, and the raw material was placed in a molding chamber and molded under an inert gas protective atmosphere.

[0086] In step 1 above, the forming process adopts a 67°-70° (e.g., 67°, 68°, 70°) interlayer rotation scanning strategy; for example, when the 67° interlayer rotation scanning strategy is adopted, the laser scanning direction between adjacent forming layers is rotated by 67°, so that the melt channels of adjacent layers are staggered and overlapping, the formed structure is mainly ferrite, and the arc-shaped melt pool boundary is retained.

[0087] In step 1 above, the inert gas protective atmosphere is argon, nitrogen or argon-nitrogen mixture; the oxygen content in the molding cavity is controlled below 1000 ppm (for example, oxygen content of 950 ppm, 900 ppm, 860 ppm, 800 ppm).

[0088] In step 1 above, the volumetric energy density (VED) of the laser powder bed melting molding is used as the final control index for the molding heat input. The volumetric energy density (VED) is controlled to be 62-132 J / mm³ (e.g., 62 J / mm³, 74.07 J / mm³, 83.95 J / mm³, 93.83 J / mm³, 108.23 J / mm³, 113.58 J / mm³, 132 J / mm³).

[0089] Furthermore, the aforementioned volumetric energy density VED is controlled to be 100-132 J / mm³ (100 J / mm³, 108.23 J / mm³, 113.58 J / mm³, 132 J / mm³).

[0090] The formula for calculating the volumetric energy density VED is as follows: VED=P / (v·h·t)(3) In equation (3), VED is the volumetric energy density, in J / mm³.

[0091] P represents laser power, measured in W. v represents the scanning speed, measured in mm / s; h represents the scanning interval, in mm; t represents the thickness of the powder layer, in mm.

[0092] In step 2 above, the forming process window for duplex stainless steel is as follows: laser power P is 170 W to 230 W (e.g., 170 W, 190 W, 210 W, 230 W), scanning speed v is 650 mm / s to 1000 mm / s (e.g., 650 mm / s, 750 mm / s, 850 mm / s, 950 mm / s, 1000 mm / s), scanning spacing h is 0.08 mm to 0.10 mm (e.g., 0.08 mm, 0.085 mm, 0.09 mm, 0.1 mm), and powder layer thickness t is 0.025 mm to 0.035 mm (e.g., 0.025 mm, 0.028 mm, 0.030 mm, 0.032 mm, 0.035 mm).

[0093] Preferably, in step 1 above, the laser power P is 190 W to 230 W, the scanning speed v is 750 mm / s to 1000 mm / s, the scanning spacing h is 0.09 mm to 0.10 mm, and the powder layer thickness t is 0.028 mm to 0.035 mm.

[0094] It should be noted that the scanning spacing h and powder layer thickness t mentioned above are not independent fixed parameters, but can be dynamically adjusted to match the actual laser energy input. Specifically, using the laser line energy E=P / v to characterize the energy input per unit scan length, the scanning spacing h and powder layer thickness t satisfy the following relationship: h·t=K·E(4) Where K is the process matching coefficient, mm³ / J; preferably controlled at 0.009~0.012 mm³ / J; h represents the scanning interval, in mm; t represents the thickness of the powder layer, in mm; E represents the laser line energy, measured in J / mm.

[0095] In step 1 above, to ensure the continuity of the molten pool overlap and the stability of interlayer fusion, the scanning spacing h and the powder layer thickness t satisfy the following relationship: h / t=λ(5) Wherein, λ is the matching coefficient between scanning spacing and powder layer thickness; h represents the scanning interval, in mm; t represents the thickness of the powder layer, in mm.

[0096] In step 1 above, preferably, λ is 2.5 to 3.5.

[0097] Preferably, λ is 3.0.

[0098] Substituting equation (5) h=λt into equation (4) h·t=K·E, we obtain the coupling relationship between the powder layer thickness and the scanning spacing: t=(K·E / λ)^(1 / 2)(6) h=(λ·K·E)^(1 / 2)(7) Through the above coupling relationship, the scanning spacing and powder layer thickness can be dynamically matched according to the actual laser power and scanning speed. Combined with the VED window, the volumetric energy density VED can be kept in the range of 62 J / mm³ to 132 J / mm³. Combined with the 67°-70° interlayer rotation scanning strategy, the molten pool overlap, thermal stress accumulation, nitrogen element burn-off, Al-O compound formation and element migration behavior can be synergistically controlled. This reduces the risk of element burn-off (Cr and Mn), molten pool disturbance and thermal stress cracking in high-N systems, thereby achieving stable forming of duplex stainless steel and subsequent duplex microstructure reconstruction.

[0099] In step 1, the prepared printed part (i.e. the printed part before short-time annealing) has a non-equilibrium structure with a ferrite content of more than 90%, that is, a non-equilibrium structure dominated by printed ferrite, and contains supersaturated N elements, residual stress, high-density dislocations and fine substructures formed by rapid solidification of the molten pool.

[0100] Step 2: Short-time annealing; Step 3: Rapid cooling to obtain duplex stainless steel in a short-time annealed state.

[0101] In step 1, the duplex stainless steel alloy is prepared as a pre-alloyed material suitable for laser powder bed melting and printing, and the printed parts are prepared by laser powder bed melting process so that its chemical composition meets the range shown in Table 1.

[0102] In step 2, the printed part is placed in a heat treatment device for short-time annealing. The short-time annealing temperature and holding time are set according to the parameters shown in Table 2. By controlling the short-time annealing temperature to 1050℃~1200℃ and the holding time to 1min~10min, the release of supersaturated N elements and the recovery of austenite in the printed state are completed in a short time, while avoiding the coarsening of the microstructure caused by traditional long-term heat treatment.

[0103] In step 3, after the short-time annealing process is completed, the printed parts are rapidly cooled.

[0104] In step 4, short-time annealed additive manufacturing duplex stainless steel is obtained.

[0105] Table 2 shows the specific post-treatment process parameters for Examples 1-10 and Comparative Examples 1-5, Table 3 shows the phase ratios after heat treatment, and Table 4 shows the tensile properties after heat treatment. As can be seen from Tables 3 and 4, Examples 1-10 were all processed within the short annealing window defined in this invention. After heat treatment, a dual-phase structure of ferrite and austenite was formed, with the austenite volume fraction stably distributed within a reasonable range, and the tensile properties exhibited a good balance of strength and plasticity.

[0106] Comparative Example 1 is an untreated comparative example, and its chemical composition is similar to that of Example 4. The difference is that Comparative Example 1 is in the laser powder bed molten printing state and has not undergone short-time annealing treatment.

[0107] Comparative Example 2 is a conventional long-term solution treatment comparative example, and its chemical composition is similar to that of Example 5. The difference is that Comparative Example 2 adopts conventional long-term solution treatment, with a treatment regime of 1050℃×30min, followed by water quenching.

[0108] Comparative Example 3 is a long-term heat preservation comparative example. Its chemical composition is similar to that of Example 7. The difference is that Comparative Example 3 adopts a long-term heat preservation treatment with a treatment regime of 1100℃×60min, followed by water quenching.

[0109] Comparative Example 4 is a high-temperature heat preservation comparative example with an excessively long time. Its chemical composition is similar to that of Example 9. The difference is that Comparative Example 4 adopts a high-temperature heat preservation treatment with an excessively long time, the treatment regime is 1150℃×120min, followed by water quenching.

[0110] Comparative Example 5 is a low-temperature short-time treatment comparative example. Its chemical composition is similar to that of Example 3. The difference is that Comparative Example 5 adopts a low-temperature short-time treatment with a treatment regime of 950°C for 10 min, followed by water quenching.

[0111] Table 1. Main chemical components (wt%) of Examples 1-10

[0112] Table 2 Specific process parameters for Examples 1-10

[0113] Table 3 Phase ratios after heat treatment

[0114] Table 4 Mechanical Properties

[0115] As shown in Tables 1-4, in Examples 1-10, the short-time annealing temperature and holding time are within the range defined by this invention, which can promote rapid austenite recovery in a short time and retain some of the fine molten pool structure, dislocation structure, and nano-oxide distribution characteristics in the laser powder bed molten printing state. After short-time annealing, the yield strength, tensile strength, and elongation after fracture of the embodiments of this invention are well matched, indicating that the method of this invention can achieve a synergistic match of compactness, mechanical properties, and corrosion resistance in additive manufacturing of duplex stainless steel.

[0116] It should be noted that the powder scanning electron microscope image of Example 1 is as follows: Figure 1 As shown; Metallographic microstructure of the untreated front side of Example 1. Figure 2 As shown; Metallographic microstructure of the front side after heat treatment in Example 2. Figure 3 As shown; the metallographic microstructure of the heat-treated side surface of Example 2 is as follows. Figure 4 As shown; the finished product drawing of additive manufacturing is as follows. Figure 5 As shown; Comparative Example 4 shows a photograph of the cracked finished product. Figure 6 As shown. Among them, from Figure 1 As can be seen, the duplex stainless steel powder used in this invention has good sphericity, meeting the powder spreading requirements for laser powder bed melting; for example Figure 2 As shown, the matrix in the printed state is mainly ferrite with only a small amount of austenite, which confirms the phase ratio results in Table 3. Figure 3 and Figure 4 The results show that after short-time annealing, a uniform ferrite-austenite dual-phase structure was formed on both the front and side surfaces, and no obvious grain coarsening occurred. Figure 5 The results show that the finished product prepared by this method is well-formed and has no obvious deformation or cracking defects; Figure 6 The results show that prolonged high-temperature insulation can lead to cracking during the stress release process of the component, which confirms the rationality of the short-time annealing process of the present invention.

[0117] In Comparative Example 1, without short-time annealing, the ferrite-dominated microstructure in the printed state resulted in high residual stress, supersaturated nitrogen distribution within the ferrite, and insufficient austenite recovery. Although the material exhibited high strength, its elongation after fracture was significantly low, demonstrating high strength but low plasticity. Specifically, the austenite volume fraction was only 6%. While the yield strength reached 890 MPa and the tensile strength reached 1120 MPa, the elongation after fracture was only 3.5%, and its plasticity fell far short of the requirements for practical application.

[0118] In Comparative Example 2, this process can promote austenite recovery and release some residual stress, but the long holding time leads to significant recovery of dislocation structures, weakening of some melt pool refinement characteristics, and lower strength than the short-time annealing example. Specifically, after the traditional 30-minute long-time solution treatment, the austenite volume fraction reaches 41.5%, and the plasticity increases to 30%. However, due to the full recovery of the microstructure caused by the long holding time, the strengthening effect is greatly weakened, with a yield strength of only 650 MPa and a tensile strength of only 910 MPa. The strength level is significantly lower than all embodiments of the present invention, and the strength-plasticity match is poor.

[0119] In Comparative Example 3, austenite is further formed under this process, but the degree of microstructure recovery is deeper, the substructure of the molten pool and the strengthening effect of dislocation are significantly weakened, the material strength decreases, and the strength-plasticity matching is not as good as that of the embodiment of the present invention. Specifically, after a long holding time of 60 minutes, the austenite ratio is further increased to 45%, and the plasticity reaches 31.5%, but the degree of microstructure recovery is deeper, the strengthening effect of dislocation and substructure is basically eliminated, and the yield strength and tensile strength are further reduced to 620MPa and 875MPa, respectively. The overall performance is not as good as that of the embodiment of the present invention.

[0120] In Comparative Example 4, due to the excessively long high-temperature holding time, the ferrite and austenite structures coarsened, the non-equilibrium strengthening structure essentially disappeared, and the material's yield strength and tensile strength decreased significantly. Specifically, after 120 minutes of high-temperature holding, the microstructure became significantly coarsened, the non-equilibrium strengthening structure completely disappeared, the yield strength dropped to 580 MPa, and the tensile strength dropped to 825 MPa, resulting in severe strength loss and performance that did not meet design requirements.

[0121] In Comparative Example 5, because the heat treatment temperature was lower than the range specified in this invention, the austenite recovery motive force was insufficient, the release of supersaturated N elements and the reconstruction of the two-phase structure were inadequate, and the material still exhibited low plasticity. Specifically, the heat treatment temperature was lower than the specified range, the austenite recovery motive force was insufficient, the austenite volume fraction was only 12%, and the elongation after fracture was only 8%, which still showed obvious low plasticity characteristics and could not meet the application requirements.

[0122] The comparative examples 1-5 above show that, without post-treatment, laser powder bed fusion-formed duplex stainless steel suffers from insufficient austenite and low plasticity. Traditional long-term solution treatment or long-term heat treatment can improve the phase ratio, but it easily leads to excessive microstructure recovery and weakened strengthening effect. Excessive high-temperature heat treatment further coarsens the microstructure; while low-temperature short-time treatment is insufficient to fully activate austenite recovery. This invention, through short-time annealing at 1050℃-1200℃ for 1min-10min, promotes rapid austenite formation while retaining some non-equilibrium strengthening structures, thus achieving a better strength-plasticity balance.

[0123] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing duplex stainless steel using laser powder bed melting, characterized in that, Includes the following steps: Step 1: Prepare duplex stainless steel printed parts using duplex stainless steel powder; A duplex stainless steel powder raw material is provided, and the raw material is placed in a molding chamber and molded under an inert gas protective atmosphere to obtain a duplex stainless steel printed part; Step 2: Short-time annealing; The printed part obtained in step 1 is placed in a heat treatment device for short-time annealing. The short-time annealing temperature is 1050℃~1200℃ and the holding time is 1min~10min. Step 3: Rapid cooling; The printed parts are rapidly cooled at a rate of 100-200℃ / s to obtain duplex stainless steel.

2. The laser powder bed melting method for manufacturing duplex stainless steel according to claim 1, characterized in that, In step 1, the inert gas protective atmosphere is argon, nitrogen, or an argon-nitrogen mixture.

3. The laser powder bed melting method for manufacturing duplex stainless steel according to claim 1, characterized in that, In step 1, the oxygen content in the molding cavity is controlled to be below 1000 ppm.

4. The laser powder bed melting method for manufacturing duplex stainless steel according to claim 1, characterized in that, In step 1, during molding, the volumetric energy density (VED) is controlled to be 62–132 J / mm³.

5. The laser powder bed melting method for manufacturing duplex stainless steel according to claim 4, characterized in that, In step 1, during molding, the volumetric energy density (VED) is controlled to be 100–132 J / mm³.

6. The laser powder bed melting method for manufacturing duplex stainless steel according to claim 1, characterized in that, In step 2, the short-time annealing temperature is 1080℃~1200℃, and the holding time is 1min~10min.

7. The laser powder bed melting method for manufacturing duplex stainless steel according to claim 1, characterized in that, In step 3, the rapid cooling rate is 150-200℃ / s to obtain duplex stainless steel.

8. The laser powder bed melting method for manufacturing duplex stainless steel according to claim 1, characterized in that, In step 1, the composition of the duplex stainless steel powder, by mass percentage, includes: C: ≤0.03%, Cr: 24.0%~26.0%, Mo: 3.0%~5.0%, Ni: 6.0%~8.5%, Mn: ≤1.2%, Si: ≤1.0%, N: 0.09%~0.42%, P: ≤0.03%, S: ≤0.01%, Al: 0.01%~0.05%, O: 0.01%~0.03%, W: 0.05%~0.30%, with the balance being Fe and unavoidable impurities.

9. The laser powder bed melting method for manufacturing duplex stainless steel according to claim 8, characterized in that, The mass ratio of Al to O satisfies the following relationship: R Al / O =Al / O(1) In equation (1), Al and O are the mass percentages of Al and O, respectively, and R... Al / O The value ranges from 0.8 to 3.

0.

10. A duplex stainless steel printed part, characterized in that, The duplex stainless steel was prepared using the laser powder bed melting manufacturing method according to any one of claims 1 to 9; The relative density of the duplex stainless steel printed part is greater than or equal to 99.5%; its tensile strength is 950-1072 MPa, its yield strength is 710-814 MPa, and its elongation after fracture is 18.5-28%.

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