Method for improving plasticity of additive manufacturing duplex stainless steel

Abstract

The invention discloses a method for improving plasticity of additive manufacturing duplex stainless steel, and belongs to the technical field of metal additive manufacturing. According to the method, in LPBF, by controlling technological parameters and the depth-to-width ratio of a molten pool, a rotating cubic texture and an alpha-fiber texture beneficial for improving the strength and plasticity are obtained, meanwhile, the grain size of ferrite is regulated and controlled, a high-density printing-state sample is prepared, then short-time heat treatment is conducted on the high-density printing-state sample, and by regulating and controlling the heat treatment time and controlling the austenite nucleation and growth process, the high-density printing-state sample is obtained. And a double-peak heterostructure of ferrite and austenite grains is formed in the material, and then the material is cooled, so that the duplex stainless steel with improved plasticity is obtained. According to a sample prepared through the method, the high strength is kept, meanwhile, the elongation rate can be increased by 40%, the strength and plasticity matching of the sample is remarkably superior to the ASTM 240 standard requirement, and the inversion rule limitation of high strength-low plasticity or low strength-high plasticity of traditional duplex stainless steel is effectively broken through.

Description

Technical Field

[0001] This invention belongs to the field of metal additive manufacturing technology, specifically relating to a method for controlling grain orientation and improving the plasticity of additively manufactured duplex stainless steel, which is particularly suitable for controlling the toughness of duplex stainless steel formed by laser powder bed melting (LPBF). Background Technology

[0002] Duplex stainless steel, as an important structural material, possesses a good balance of strength and ductility, and is widely used in industries such as petrochemicals, marine engineering, energy, and construction. However, the different deformation mechanisms of ferrite and austenite result in poor machinability of duplex stainless steel, limiting its application in complex structural components. Laser bed fusion (LPBF), an additive manufacturing technology, enables the three-dimensional forming of complex structural parts through a layer-by-layer stacking of molten pools. Using LPBF to form duplex stainless steel not only improves the material's mechanical properties but also addresses the problem of its poor hot working properties, which limits its application in complex structural parts. However, compared to duplex stainless steel prepared by traditional methods, while the yield strength of LPBF-prepared materials is improved, the elongation still lags significantly, requiring further improvement. To enhance the ductility of LPBF-prepared duplex stainless steel, researchers from various countries have conducted extensive studies on printing process optimization and heat treatment. However, while the strength of LPBF-prepared samples is currently higher than that of as-cast samples of the same composition, the elongation is still inferior, necessitating further optimization.

[0003] In fact, the high stress concentration generated inside the material during deformation leads to premature fracture, which is the main reason for the decrease in plasticity. The activation of slip systems can effectively alleviate stress concentration at grain boundaries, delay crack initiation and propagation, and improve the strength and plasticity of the material. Therefore, increasing the types and number of slip systems activated during deformation can significantly improve the strength and plasticity of the material. Generally, the activation of slip systems is closely related to the initial orientation and texture of the grains in the material. During LPBF forming, grains always grow along the direction of maximum heat flux density gradient at the molten pool boundary. Therefore, the morphology and size of the molten pool largely determine the initial orientation and texture of the grains in the material. In addition, duplex stainless steel prepared by LPBF is almost entirely ferrite phase. After heat treatment, austenite precipitates and grows from the ferrite grain boundaries. Therefore, it is possible to control the austenite grain size during heat treatment to form a bimodal structure with the grain size in the ferrite. At the same time, the ferrite grains can almost maintain their original orientation during this process. Therefore, it is worth further investigation whether the elongation of duplex stainless steel prepared by LPBF can be improved by controlling the morphology and size of the molten pool to form more grains with favorable orientation, and then by establishing a bimodal heterostructure of ferrite and austenite through heat treatment. Summary of the Invention

[0004] To address the common problem of high strength but poor plasticity (elongation typically ≤30%) in duplex stainless steel prepared by traditional laser powder bed melting (LPBF), this invention provides a method to improve the plasticity of additively manufactured duplex stainless steel. By controlling grain orientation and particle size, the duplex stainless steel prepared by LPBF can achieve improved plasticity while maintaining high strength, with an elongation of up to 40%.

[0005] To achieve the above object, the present invention adopts the following technical solutions: A method for improving the plasticity of additively manufactured duplex stainless steel includes the following steps: Step 1: Using laser powder bed melting technology, a higher proportion of rotational cubic texture and α-fiber texture is obtained by adjusting process parameters and the aspect ratio of the molten pool, while simultaneously controlling the ferrite grain size to prepare a high-density printed sample. Step 2: The prepared high-density printed sample is subjected to short-time heat treatment to control the austenite nucleation and growth rate and the ferrite recrystallization process, optimize the austenite grain size, thereby achieving a bimodal structure of ferrite and austenite grains, and then cooled to obtain duplex stainless steel with improved plasticity.

[0006] Furthermore, the process parameters of the laser powder bed melting technology described in step 1 are as follows: laser power 225-300 W, scanning speed 300-750 mm / s, laser line energy density between 1.67-4 J / mm, and the aspect ratio (d / w) of the prepared molten pool between 0.5-0.6.

[0007] Furthermore, in step 1, by adjusting the scanning spacing H, the density of the obtained high-density printed sample is higher than 99%, and the average size of the ferrite grains is 4-8 μm, which prepares for the subsequent bimodal heterostructure of ferrite and austenite grains.

[0008] Furthermore, the temperature of the short-time heat treatment in step 2 is 1000℃, and the treatment time is 5-30 minutes, in order to regulate the austenite nucleation and growth rate, as well as the ferrite recrystallization process, and slow down the degree of ferrite grain refinement.

[0009] Furthermore, the cooling described in step 2 involves rapidly water cooling the sample after short-term heat treatment in water at 0°C at a rate of 100-200°C / s to quickly terminate the recrystallization of ferrite and the nucleation and growth of austenite.

[0010] Furthermore, the average size of the ferrite grains in the obtained duplex stainless steel is 4-8 μm, and the average size of the austenite grains is 1-3 μm, with an austenite content between 30-40%, forming a bimodal heterogeneous structure of ferrite and austenite grains. Simultaneously, because grain refinement is suppressed, the rotational cubic texture and α-fiber texture in the ferrite are effectively preserved, which is beneficial for improving the strength and plasticity of the prepared samples.

[0011] The significant advantages of this invention are: (1) This invention combines LPBF to prepare all-ferrite, and through subsequent heat treatment, austenite nucleates and grows from ferrite through solid-state phase transformation, thus maximizing the preservation of ferrite grain orientation. By controlling the aspect ratio of the molten pool in LPBF, rotational cubic texture and α-fiber texture that are beneficial to improving plasticity are obtained, and the ferrite grain size is simultaneously controlled to obtain a highly dense printed sample. Then, a short-time heat treatment is performed on it. By controlling the heat treatment time, the process of austenite nucleation and growth is controlled, and a bimodal heterostructure of ferrite and austenite grains is formed in the material, thereby obtaining duplex stainless steel with improved plasticity.

[0012] (2) The sample prepared by the present invention can increase the elongation to 40% while maintaining high strength. Its strength-plasticity matching is significantly better than the requirements of ASTM 240 standard, effectively breaking through the inverted rule of traditional duplex stainless steel of "high strength-low plasticity" or "low strength-high plasticity". Attached Figure Description

[0013] Figure 1 This is a graph showing the relationship between the laser powder bed melting process parameters and the aspect ratio (a) and energy density (b) of the molten pool in this invention. The graph shows the aspect ratio of the formed molten pool and the corresponding laser energy density under different combinations of laser power and scanning speed.

[0014] Figure 2 The figures show the morphology of the molten pools prepared in Examples 1(a) and 2(b). The figures show that the molten pools prepared in Examples 1 and 2 are both semi-elliptical in size, typical of transition mode molten pools, and the molten pool size in Example 1 is larger than that in Example 2.

[0015] Figure 3 The phase diagrams and inverse pole figure (IPF) diagrams of the duplex stainless steel samples prepared in Examples 1(a) and 2(b) are shown. The figures show that the samples prepared in Examples 1 and 2 are composed of ferrite and austenite, and both exhibit a bimodal grain structure with small grains enclosing large grains. The average grain sizes of ferrite and austenite are 5.2~5.8 μm and 2.8~3.0 μm, respectively.

[0016] Figure 4The figures show the texture statistics of the duplex stainless steel samples from Examples 1(a) and 2(b). The figures show that the sample prepared in Example 1 exhibits a higher proportion of transcubic and α-fiber textures. Detailed Implementation

[0017] A method for improving the plasticity of additively manufactured duplex stainless steel includes the following steps: Step 1: The laser powder bed melting technology is used for preparation. The process parameters are: laser power 225-300 W, scanning speed 300-750 mm / s, laser line energy density between 1.67-4 J / mm, and melt pool aspect ratio of 0.5-0.6. By adjusting the scanning interval, a high-density printed sample with a density higher than 99% and an average ferrite grain size of 4-8 μm is obtained. Step 2: The prepared high-density printed sample is heat-treated at 1000℃ for 5-30 minutes, and then the heat-treated sample is rapidly water-cooled in 0℃ water at a rate of 100-200℃ / s to obtain duplex stainless steel with improved plasticity. The average size of the ferrite grains in the obtained duplex stainless steel is 4-8 μm, the average size of the austenite grains is 1-3 μm, and the austenite content is between 30-40%, forming a bimodal heterogeneous structure of ferrite and austenite grains.

[0018] like Figure 1 As shown, this invention establishes a mapping relationship between the molten pool aspect ratio (d / w) and the energy density process parameters by adjusting the laser power P and the scanning speed V, in order to obtain a higher proportion of rotated cube texture (to increase the initiation ratio of the {112}〈111〉 slip system) and α-fiber texture (to improve work hardening capability).

[0019] To make the content of this invention easier to understand, the technical solution of this invention will be further described below with reference to specific embodiments, but this invention is not limited thereto.

[0020] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0021] Example 1: 1) Using 2205 duplex stainless steel powder as material, a printed sample was prepared by laser powder bed melting technology. The laser power, scanning speed and scanning distance were 250 W, 500 mm / s and 55 μm, respectively. The powder thickness was 30 μm, the linear energy density was 1.5 J / mm, the aspect ratio of the resulting molten pool was 0.51, and the density of the printed sample was 99.2%.

[0022] 2) Place the sample in a heat treatment furnace and hold it at 1000℃ for 30 minutes. Then, rapidly cool it in water at 0℃ at a rate of 100-200℃ / s to obtain duplex stainless steel with improved plasticity.

[0023] The average grain sizes of ferrite and austenite in the obtained duplex stainless steel sample were 5.8 μm and 2.8 μm, respectively, and the austenite content was 33%.

[0024] Example 2: 1) Using 2205 duplex stainless steel powder as material, a printed sample was prepared by laser powder bed melting technology. The laser power, scanning speed and scanning distance were 200 W, 1000 mm / s and 50 μm, respectively. The powder thickness was 30 μm, the linear energy density was 0.25 J / mm, the aspect ratio of the resulting molten pool was 0.31, and the density of the printed sample was 99.1%.

[0025] 2) Place the sample in a heat treatment furnace and hold it at 1000℃ for 30 minutes. Then, rapidly cool it in water at 0℃ at a rate of 100-200℃ / s to obtain duplex stainless steel with improved plasticity.

[0026] The average grain sizes of ferrite and austenite in the obtained duplex stainless steel sample were 5.2 μm and 3.0 μm, respectively, and the austenite content was 38%.

[0027] Comparative Example 1: 1) Using 2205 duplex stainless steel powder as material, a printed sample was prepared by laser powder bed melting technology. The laser power, scanning speed and scanning distance were 150W, 700 mm / s and 70 μm, respectively. The powder thickness was 30 μm, the laser energy density was 0.21 J / mm, the aspect ratio of the resulting molten pool was 0.21, and the density of the printed sample was 99.45%.

[0028] 2) Place the sample in a heat treatment furnace and keep it at 1000℃ for 30 minutes, then cool it with water at room temperature to obtain duplex stainless steel.

[0029] The average grain sizes of ferrite and austenite in the obtained duplex stainless steel sample were 4.06 μm and 2.05 μm, respectively, and the austenite content was 41.5%.

[0030] Comparative Example 2: 1) Using 2205 duplex stainless steel powder as material, a printed sample was prepared by laser powder bed melting technology. The laser power, scanning speed and scanning spacing were 200W, 700 mm / s and 60 μm, respectively. The powder thickness was 30 μm, the laser energy density was 0.29 J / mm, the aspect ratio of the resulting molten pool was 0.36, and the density of the printed sample was 99.22%.

[0031] 2) Place the sample in a heat treatment furnace, keep it at 1000℃ for 10 minutes, and then cool it with water at room temperature to obtain duplex stainless steel.

[0032] The average grain size of austenite in the obtained duplex stainless steel sample was 1-2 μm, and the austenite content was 45%.

[0033] The mechanical properties of the duplex stainless steel samples obtained in the examples and comparative examples are compared in Table 1.

[0034] Table 1 Comparison of Mechanical Properties

[0035] Table 1 shows that, compared with Example 2, the sample obtained in Example 1 exhibits significantly better strength and plasticity. This is mainly due to the following two reasons: First, the ferrite grains in the sample obtained in Example 1 are larger than those in Example 2, while the austenite grain size is relatively smaller. Furthermore, the phenomenon of larger ferrite particles being surrounded by austenite is very obvious, exhibiting a strong bimodal grain structure, which is beneficial for simultaneously improving the sample's strength and plasticity. Second, the sample obtained in Example 1 has an α-fiber texture as high as 22.2%, which further enhances the sample's strength. Meanwhile, compared with Comparative Examples 1 and 2, Example 1 shows higher strength and elongation, which further confirms that the method proposed in this invention can improve the material's plasticity while maintaining high strength.

[0036] The above embodiments are preferred implementations of the present invention. In addition, the present invention can be implemented in other ways. Any obvious substitutions without departing from the concept of the present technical solution are within the protection scope of the present invention.

Claims

1. A method for improving the plasticity of additively manufactured duplex stainless steel, characterized in that, The steps include: Step 1: Prepare a high-density printed sample using laser powder bed melting technology; Step 2: After a short heat treatment, the prepared high-density printed sample is cooled to obtain duplex stainless steel with improved plasticity.

2. The method according to claim 1, characterized in that: The process parameters of the laser powder bed melting technology described in step 1 are: laser power 225-300 W, scanning speed 300-750 mm / s, laser line energy density between 1.67-4 J / mm, and the aspect ratio of the prepared molten pool between 0.5-0.

6.

3. The method according to claim 1, characterized in that: The high-density printed sample obtained in step 1 has a density of over 99% and the average size of the ferrite grains is 4-8 μm.

4. The method according to claim 1, characterized in that: The short-time heat treatment in step 2 is performed at a temperature of 1000℃ for 5-30 minutes.

5. The method according to claim 1, characterized in that: The cooling described in step 2 involves rapidly water cooling the sample after short-term heat treatment in water at 0°C at a rate of 100-200°C / s.

6. The method according to claim 1, characterized in that: The average size of the ferrite grains in the obtained duplex stainless steel is 4-8 μm, the average size of the austenite grains is 1-3 μm, and the austenite content is between 30-40%.

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