Additive manufacturing method of SA508-3 steel

Abstract

The invention discloses an additive manufacturing method of SA508-3 steel, which comprises the following steps: in a nitrogen or inert gas atmosphere, SA508-3 steel powder is prepared into an SA508-3 steel printing piece through a selective laser melting method, and the parameters of selective laser melting are as follows: the laser power is 200-350W, the dot spacing is 40-80 microns, the exposure time is 80-100 microseconds, the laser scanning speed is 500-700mm / s, and the laser scanning speed is 500-700mm / s; the distance between the scanning lines is 90-100 microns; the SA508-3 steel printing piece is subjected to heat treatment, the SA508-3 steel is obtained, and the heat treatment comprises the steps that heat preservation is conducted for 0.3-0.6 h at the temperature of 820-860 DEG C, cooling is conducted to T DEG C, heat preservation is conducted for 0.3-0.6 h at the temperature of T DEG C, cooling is conducted to the room temperature, and T DEG C is 300-420 DEG C. The tensile strength, the yield strength and the ductility of the SA508-3 steel prepared through the additive manufacturing method of the SA508-3 steel at the room temperature are 777 MPa, 609 MPa and 27% respectively; and the tensile strength and the yield strength at 350 DEG C are 761 MPa and 609 MPa respectively, and the ductility is 37%.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of additive manufacturing technology, and particularly relates to an additive manufacturing method of SA508-3 steel. BACKGROUND

[0002] SA508-3 steel belongs to a typical low-carbon Mn-Ni-Mo alloy steel. At present, it has been widely used in nuclear reactor pressure vessels in various countries in the world due to its excellent weldability, hardenability, workability, good low-temperature impact toughness, and resistance to neutron irradiation brittleness and other advantages. With the development of nuclear power and the continuous updating of nuclear reactor equipment, pressure vessels tend to be integrated manufactured, which puts forward higher requirements on their performance and safety. Therefore, it is urgent to explore a homogeneous integrated manufacturing process for SA508-3 steel.

[0003] Selective laser melting (SLM) is an additive manufacturing method that uses a mobile high-energy laser beam as a heat source to instantaneously heat, melt and rapidly cool and solidify powder materials. It not only has the advantages of high material utilization rate and short manufacturing cycle, but also can take into account the accuracy, complexity and functional gradient of the parts, providing a new solution for the preparation of SA508-3 steel complex components for nuclear power. However, the extremely high temperature gradient and cooling rate during the preparation of SA508-3 steel printed parts by selective laser melting method results in a large number of dislocations, vacancies and other non-steady-state structures in the interior of the SA508-3 steel obtained by additive manufacturing, which causes extremely high residual stress in the interior of the SA508-3 steel, which seriously deteriorates the use performance of the SA508-3 steel at high temperature, especially the high-temperature plasticity. Therefore, the mechanical properties of the additive manufactured SA508-3 steel must be regulated by subsequent heat treatment, but due to the special process of the selective laser melting method, the original microstructure of the obtained SA508-3 steel is quite different from that of the SA508-3 steel prepared by traditional methods (such as casting, forging and rolling), so the existing methods cannot meet the performance requirements of the additive manufactured SA508-3 steel. It is urgent to develop a method suitable for additive manufacturing of SA508-3 steel by selective laser melting method based on the phase transformation characteristics of SA508-3 steel during heat treatment and the precipitation characteristics of second phases such as carbides, in order to prepare high-performance SA508-3 steel pressure vessel parts. SUMMARY

[0004] In view of the deficiencies of the prior art, the purpose of the present application is to provide an additive manufacturing method of SA508-3 steel to overcome the problem of poor high-temperature mechanical properties of SA508-3 steel after additive manufacturing by laser selective melting method, which can effectively inhibit the precipitation and coarsening of large-size continuous carbides at the grain boundary, effectively inhibit the crack propagation during deformation, and avoid premature failure due to stress concentration, so that the SA508-3 steel obtained by the additive manufacturing method of the present application has martensite, ferrite, bainite and residual austenite existing simultaneously in the microstructure, thereby ensuring that the SA508-3 steel has excellent high-temperature strength and plasticity.

[0005] The purpose of the present application is achieved by the following technical solutions.

[0006] An additive manufacturing method of SA508-3 steel, comprising the following steps:

[0007] Step 1: preparing a SA508-3 steel printed part by laser selective melting method under a nitrogen or inert gas atmosphere, wherein the parameters of the laser selective melting are as follows: laser power is 200-350 W, point spacing is 40-80 μm, exposure time is 80-100 μs, laser scanning speed is 500-700 mm / s, and scanning line spacing is 90-100 μm.

[0008] In step 1, the SA508-3 steel powder is laid on a substrate with a laying thickness of 30-50 μm, and the substrate temperature is 80-120℃.

[0009] In step 1, the phase angle of laser selective melting is 67-90°.

[0010] In step 1, the SA508-3 steel powder is spherical powder with a particle size of 15-53 μm.

[0011] Step 2: heat treating the SA508-3 steel printed part to obtain SA508-3 steel, wherein the heat treatment comprises: holding at 820-860℃ for 0.3-0.6 h, cooling to T℃, then holding at T℃ for 0.3-0.6 h, and cooling to room temperature, wherein T℃ = 300-420℃.

[0012] In step 2, the cooling speed of cooling to room temperature is 150-250℃ / s, and the cooling to room temperature is by water cooling, oil cooling or gas cooling.

[0013] In step 2, the rate of heating from room temperature to 820-860℃ is 10-20℃ / min.

[0014] In step 2, the method of decreasing from 820-860℃ to T℃ is furnace cooling.

[0015] The above-mentioned additive manufacturing method for SA508-3 steel is used to improve the elongation of SA508-3 steel.

[0016] In the above technical solution, the SA508-3 steel produced by the additive manufacturing method has a tensile strength, yield strength and elongation of 777MPa, 609MPa and 27% at room temperature, respectively; and a tensile strength and yield strength of 761MPa and 609MPa at 350℃, respectively, with an elongation of 37%.

[0017] Compared to traditional methods, the SA508-3 steel obtained by the additive manufacturing method of this invention has a fine internal grain size of 3-4 μm, and the proportion of low-angle grain boundaries (LAGB) reaches 60.4%. Due to the high proportion of LAGB, crack propagation during deformation can be effectively hindered. When a crack propagates in SA508-3 steel and encounters a LAGB, the irregular arrangement of atoms at the LAGB will change the direction of crack propagation, increase the path length of crack propagation, consume more energy, and make the stress distribution more uniform during tensile process, alleviate local stress concentration, and thus ensure the good plasticity of SA508-3 steel at room temperature and high temperature. Attached Figure Description

[0018] Figure 1 The grain state of the SA508-3 steel prepared in Example 1;

[0019] Figure 2 The grain state of SA508-3 steel prepared in Comparative Example 1 is shown. Detailed Implementation

[0020] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0021] Example 1

[0022] An additive manufacturing method for SA508-3 steel includes the following steps:

[0023] Step 1: Under a nitrogen atmosphere, SA508-3 steel powder (SA508-3 steel powder is spherical powder with a particle size of 15-53 μm) is prepared. (SA508-3 steel powder reference: Yiru L, Ran D, Ying H, et al. Preparation of high-performance SA508 Grade 3 steel by laser powder bed fusion: role of high cooling rate on microstructure and mechanical properties[J]. Journal of Materials) Science, 2023. DOI: 10.1007 / s10853-023-08381-y. SA508-3 steel printed parts were prepared by selective laser melting (SLM). Specifically, SA508-3 steel powder was laid on a substrate of a selective laser melting (SLM) device (manufacturer: Tianjin Leiming Laser Technology Co., Ltd., model: LM-150A) with a thickness of 45 μm. The substrate temperature was 80℃. The parameters of SLM were: a phase angle of 67° (interlayer rotation 67°), a laser power of 350W, a dot pitch of 60 μm, an exposure time of 90 μs, a laser scanning speed of 625 mm / s, a scanning line spacing of 100 μm, and a strip-type bidirectional scanning method.

[0024] Step 2: Place the SA508-3 steel printed part in a muffle furnace for heat treatment to obtain SA508-3 steel. The heat treatment includes: heating from room temperature to 840℃ at a rate of 20℃ / min and holding at 840℃ for 0.5h, cooling in air with the furnace to 350℃, holding at 350℃ for 0.5h, and water cooling to room temperature (cooling rate is 200℃ / s).

[0025] The SA508-3 steel prepared in Example 1 was ground, polished, and subjected to metallographic etching (metallographic etching: placed in the etching solution for 15 seconds; the etching solution was a mixture of nitric acid (the concentration of HNO3 in the nitric acid was 68 wt%) and alcohol, with a volume ratio of nitric acid to alcohol of 4:96). Microstructural observation was then performed. Figure 1As shown, the internal grain size of the SA508-3 steel prepared in Example 1 is maintained between 3 and 4 μm. The proportion of low-angle grain boundaries (LAGB) in the SA508-3 steel prepared in Example 1 reaches 60.4%, and the proportion of high-angle grain boundaries is 39.6%. The high proportion of low-angle grain boundaries can effectively hinder crack propagation during deformation. When a crack propagates in SA508-3 steel and encounters a low-angle grain boundary, the irregular arrangement of atoms at the low-angle grain boundary will change the direction of crack propagation, increase the path length of crack propagation, consume more energy, and make the stress distribution more uniform during tensile process, alleviate local stress concentration, and thus improve the toughness of SA508-3 steel.

[0026] Meanwhile, the microstructure of the SA508-3 steel prepared in Example 1 consists of martensite, ferrite, bainite and retained austenite. During plastic deformation, ferrite has more movable dislocations and dislocation slip occurs preferentially. The resulting stress concentration can be transferred to the martensite through higher-density grain boundaries, thus enhancing deformation compatibility.

[0027] Tensile tests were conducted at room temperature. The SA508-3 steel prepared in Example 1 exhibited tensile strength, yield strength, and elongation of 777 MPa, 609 MPa, and 27%, respectively. Tensile tests were also conducted at 350°C. At 350°C, the SA508-3 steel prepared in Example 1 showed tensile strength and yield strength of 761 MPa and 609 MPa, respectively, with an elongation of 37%. Therefore, the additive manufacturing method of Example 1 achieved a better balance between strength and plasticity in SA508-3 steel, resulting in superior overall performance compared to SA508-3 steel prepared by forging (the traditional method). This meets the current performance requirements for SA508-3 steel in nuclear reactor pressure vessels. Furthermore, the effective increase in elongation ensures that SA508-3 steel can better adapt to stress distribution under external forces, reducing localized stress concentration and lowering the risk of failure.

[0028] Comparative Example 1

[0029] An additive manufacturing method for SA508-3 steel is basically the same as that in Example 1, except that the heat treatment is different. The heat treatment in Comparative Example 1 includes: heating from room temperature to 940°C at a rate of 20°C / min and holding at 940°C for 2 hours, and then water cooling to room temperature (cooling rate of 200°C / s) to obtain SA508-3 steel.

[0030] The SA508-3 steel prepared in Comparative Example 1 was ground, polished, and etched (using the same etching method as in Example 1) for microstructure observation. Figure 2As shown, the grain size of the SA508-3 steel prepared in Comparative Example 1 is significantly coarser than that of the SA508-3 steel in Example 1, where the grain size remains between 8 and 9 μm. The proportion of low-angle grain boundaries in the SA508-3 steel prepared in Comparative Example 1 is reduced to 48.7%, while the proportion of high-angle grain boundaries is increased to 51.3%. This reduces the resistance to crack propagation, making the SA508-3 steel prepared in Comparative Example 1 more prone to brittle fracture and exhibiting decreased toughness.

[0031] Meanwhile, the microstructure of the SA508-3 steel prepared in Comparative Example 1 was mainly lath martensite. Although lath martensite can improve the strength of the material, it will seriously deteriorate the plasticity. During the high-temperature holding process, large-sized carbides will continuously precipitate at the grain boundaries, which will seriously reduce the strength (tensile strength and yield strength) of SA508-3 steel at high temperature.

[0032] Tensile tests were conducted at room temperature. The SA508-3 steel prepared in Comparative Example 1 exhibited tensile strength, yield strength, and elongation of 653 MPa, 569 MPa, and 13%, respectively. Tensile tests were also conducted at 350°C. At this temperature, the SA508-3 steel showed tensile strength and yield strength of 610 MPa and 452 MPa, respectively, with an elongation of 19%. Therefore, the conventional heat treatment process used in Comparative Example 1 is not suitable for additive manufacturing of SA508-3 steel, as it would severely degrade its overall mechanical properties, failing to meet the current performance requirements of SA508-3 steel for nuclear reactor pressure vessels.

[0033] Comparative Example 2

[0034] 316L stainless steel was prepared using the additive manufacturing method for SA508-3 steel described in Example 1. The method was essentially the same as in Example 1, except that "SA508-3 steel powder" was replaced with "316L stainless steel spherical powder." The particle size of the 316L stainless steel spherical powder was 15–53 μm.

[0035] The 316L stainless steel prepared in Comparative Example 2 was subjected to tensile testing at room temperature. The tensile strength, yield strength, and elongation of the 316L stainless steel prepared in Comparative Example 2 at room temperature were 545 MPa, 485 MPa, and 23%, respectively. When tensile testing was conducted at 350℃, the tensile strength and yield strength of the 316L stainless steel prepared in Comparative Example 2 at 350℃ were 455 MPa and 365 MPa, respectively, and the elongation was 35%, indicating poor overall mechanical properties.

[0036] The 316L stainless steel additively manufactured using existing methods (Chen Y, Wang X, Li D, et al. Experimental characterization and strengthening mechanism of process-structure-property of selective lasermelted 316L[J]. Materials Characterization, 2023.DOI:10.1016 / j.matchar.2023.112753.) exhibits tensile strength, yield strength, and elongation of 525 MPa, 334 MPa, and 79% at room temperature. The 316L stainless steel prepared in Comparative Example 2 shows a decrease in performance compared to existing technologies. Therefore, the additive manufacturing method of this invention is only applicable to SA508-3 steel. Applying the additive manufacturing method of this invention to the preparation of other steels cannot effectively improve their performance.

[0037] Comparative Example 3

[0038] An additive manufacturing method for controlling defects in steel used in nuclear reactor pressure vessels is an embodiment 1 of the invention patent with publication number CN113798513A.

[0039] The SA508-3 steel prepared in Comparative Example 3 has a tensile strength of 1227 MPa and an elongation of 22.5% at room temperature; in a tensile test at 350°C, the tensile strength is 962 MPa and the elongation is 18%. Although Comparative Example 3 exhibits high tensile strength at both room temperature and 350°C, its elongation at 350°C is poor. Under impact loads or stress concentration during use, the SA508-3 steel prepared in Comparative Example 3 is prone to sudden brittle fracture without warning, leading to component failure and potentially causing serious safety accidents. The SA508-3 steel obtained using the additive manufacturing method of Comparative Example 3 shows a decrease in both tensile strength and elongation at 350°C compared to the data at room temperature, while the SA508-3 steel obtained using the additive manufacturing method of Example 1 shows an increase in elongation at 350°C compared to the elongation at room temperature.

[0040] As can be seen from the above results, the additive manufacturing method for SA508-3 steel provided by the present invention can achieve a mixed distribution of martensite, ferrite, bainite and retained austenite, with dispersed carbides uniformly distributed inside the grains of SA508-3 steel and a reduced number of carbides at grain boundaries. This results in a good match between the high-temperature strength and plasticity of SA508-3 steel, and its comprehensive performance is superior to that of SA508-3 steel prepared by forging (traditional method). At the same time, it meets the current performance requirements of SA508-3 steel for nuclear reactor pressure vessels.

[0041] The present invention has been described above by way of example. It should be noted that any simple modifications, alterations or other equivalent substitutions that can be made by those skilled in the art without creative effort without departing from the core of the present invention fall within the protection scope of the present invention.

Claims

1. An additive manufacturing method for SA508-3 steel, characterized in that, Includes the following steps: Step 1: Under a nitrogen or inert gas atmosphere, SA508-3 steel powder is used to prepare SA508-3 steel printed parts by selective laser melting. The parameters of selective laser melting are: laser power of 200-350W, dot pitch of 40-80μm, exposure time of 80-100μs, laser scanning speed of 500-700mm / s, and scanning line spacing of 90-100μm. Step 2: Heat-treat the SA508-3 steel printed part to obtain SA508-3 steel. The heat treatment includes: holding at 820-860℃ for 0.3-0.6h, cooling to T℃, holding at T℃ for another 0.3-0.6h, and cooling to room temperature, wherein T℃ = 300-420℃.

2. The additive manufacturing method according to claim 1, characterized in that, In step 1, the SA508-3 steel powder is laid on the substrate with a thickness of 30-50 μm and the substrate temperature is 80-120°C.

3. The additive manufacturing method according to claim 1, characterized in that, In step 1, the phase angle of the laser selective melting is 67–90°.

4. The additive manufacturing method according to claim 1, characterized in that, In step 1, the SA508-3 steel powder is spherical powder with a particle size of 15-53 μm.

5. The additive manufacturing method according to claim 1, characterized in that, In step 2, the cooling rate to room temperature is 150-250°C / s, and the cooling method to room temperature is water cooling, oil cooling, or air cooling.

6. The additive manufacturing method according to claim 1, characterized in that, In step 2, the rate of temperature increase from room temperature to 820–860°C is 10–20°C / min.

7. The additive manufacturing method according to claim 1, characterized in that, In step 2, the temperature is reduced from 820-860℃ to T℃ by furnace cooling.

8. Use of the additive manufacturing method for SA508-3 steel as described in any one of claims 1 to 7 in improving the elongation of SA508-3 steel.

9. The application according to claim 8, characterized in that, The SA508-3 steel produced by the additive manufacturing method has a tensile strength, yield strength, and elongation of 777 MPa, 609 MPa, and 27% at room temperature, respectively.

10. The application according to claim 8, characterized in that, The SA508-3 steel produced by the additive manufacturing method has a tensile strength of 761 MPa and a yield strength of 609 MPa at 350℃, and an elongation of 37%.

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