Method for improving hydrogen embrittlement resistance of iron-based superalloy by grain boundary regulation and application thereof

Abstract

The application discloses a method for improving hydrogen embrittlement resistance of iron-based superalloy by grain boundary regulation and application thereof. The application uses iron-based superalloy powder containing trace elements as raw material, and regulates hydrogen embrittlement resistance of the iron-based superalloy by laser powder bed melting process combined with vacuum sealing tube treatment and five-step heat treatment process. The first three steps are high-temperature heat treatment, and the last two steps are low-temperature heat treatment. The hydrogen embrittlement resistant iron-based superalloy obtained by the process has high strength and plasticity matching at room temperature and 600 DEG C, and has small elongation loss after hydrogen charging, and the hydrogen embrittlement resistance is significantly better than that of Inconel 718 nickel-based alloy. The process regulates the grain boundary type and element distribution in the alloy through multi-stage synergistic regulation, not only significantly improves the comprehensive mechanical properties of the material, but also endows the material with excellent hydrogen embrittlement resistance, can effectively meet the strict requirements of liquid hydrogen rocket engine nozzle and key parts of aviation system on high reliability, long service life and extreme environment adaptability, and has wide application prospect.

Description

Technical Field

[0001] This invention relates to the field of additive manufacturing technology for high-temperature alloys, specifically to a method for improving the hydrogen embrittlement resistance of iron-based high-temperature alloys through grain boundary regulation and its application. Background Technology

[0002] With the rapid development of aerospace, liquid hydrogen rocket engines are widely used in aerospace propulsion systems due to their high energy density and clean, pollution-free operation. However, liquid hydrogen rocket engines operate in high-temperature, high-pressure hydrogen environments for extended periods, posing greater challenges to the strength, plasticity, and hydrogen resistance of materials. Austenitic single-phase alloys with their face-centered cubic structure, due to the low diffusion coefficient of hydrogen, are often considered hydrogen-resistant engineering materials. However, the strength of single-phase austenitic alloys is often insufficient for aerospace structural components. Therefore, the materials used in liquid hydrogen rocket engines are often the higher-strength Inconel 718 nickel-based alloys. The problem is that Inconel 718 nickel-based alloys have high hydrogen embrittlement sensitivity, and their performance degrades significantly under high-pressure hydrogen environments. To address these issues, NASA has developed a novel precipitation-strengthened iron-based superalloy. By adding alloying elements, it achieves uniform precipitation of the γ′ phase in the γ matrix, resulting in a strong hydrogen-resistant alloy with a balance of strength and plasticity.

[0003] Currently, aerospace structural components and parts are becoming increasingly complex, integrated, and demanding higher performance. This necessitates continuous exploration of processing technologies for aerospace materials. Additive manufacturing, with its advantages of high degree of freedom, short cycle time, and the ability to achieve integrated forming, has made the demand for additive manufacturing of high-temperature alloys even more urgent. Laser Powder Bed Fusion (LPBF), as an advanced metal additive manufacturing process, is currently highly favored. This process involves depositing metal powder layer by layer and using a high-energy-density laser beam to locally melt and subsequently solidify the powder, forming dense metal layers. These layers are stacked until the desired three-dimensional part is constructed. Compared to traditional casting and forging methods, LPBF technology exhibits a significant advantage in cooling rate, which is beneficial for refining the grain size in the microstructure of the metal material, thereby improving the mechanical properties of the finished product.

[0004] However, the LPBF forming process of precipitation-strengthened iron-based superalloys is accompanied by a strong non-equilibrium solidification process. The characteristics of large temperature gradients, high cooling rates, and cyclic heating easily induce elemental segregation, phase transformation, and residual stress accumulation, resulting in defects such as porosity and microcracks in additively manufactured samples. Current research has found that adding γ′ phase-forming elements such as Al and Ti reduces the alloy's plastic deformation capacity and crack resistance. Due to the obvious directional characteristics in the deposited products, and the increased Ti content in the alloy, along with significant Ti segregation between dendrites and grain boundaries, certain inappropriate heat treatment conditions can lead to the formation of needle-like η phases (Ni3Ti). The presence of η phases severely impairs the alloy's mechanical properties and hydrogen resistance. In addition to elemental segregation, the type of grain boundaries also has a significant impact on the alloy's hydrogen resistance. The type of grain boundary can be measured by the Σ value of the lattice grain boundaries at coincident sites. One type is low CSL grain boundaries with Σ≤29, also known as special grain boundaries, and the other type is random grain boundaries with Σ>29. Existing technologies show that hydrogen-induced cracks readily form at random grain boundaries; however, low-Σ grain boundaries exhibit good resistance to hydrogen-induced cracks.

[0005] The existing "solution + aging" heat treatment process is directly applied to cast / forged high-temperature alloys. In LPBF-formed iron-based γ′ strengthened alloys, the following results are observed: insufficient diffusion of Ti and Al elements between dendrites during the solution treatment stage; uneven distribution of the γ′ phase after aging; residual stress release caused by high-temperature holding leading to microcracks; large performance variation within the same batch; limited improvement in hydrogen embrittlement; and difficulty in meeting the requirements of liquid hydrogen rocket engines for high strength, toughness, and hydrogen resistance. Summary of the Invention

[0006] Objective: To address the problems existing in the prior art, this invention provides a method for improving the hydrogen embrittlement resistance of iron-based superalloys through grain boundary regulation. This method utilizes acoustic resonance technology to mix trace elements, enhancing grain boundary strengthening. A novel five-stage heat treatment chain is proposed, employing laser powder bed melting to synergistically eliminate residual stress, homogenize composition, and precisely regulate multi-scale precipitates. This process solves the problems of microcracks and elemental segregation in additive manufacturing. By increasing grain boundary strength through trace elements, it achieves a high-density, low-Σ grain boundary microstructure. Mechanically, this results in an iron-based superalloy exhibiting a strong ductile coordination effect while maintaining excellent hydrogen embrittlement resistance.

[0007] Technical solution: The present invention describes a method for improving the hydrogen embrittlement resistance of iron-based superalloys through grain boundary regulation. The method uses iron-based superalloy powder containing trace elements as the raw material for mixing, and uses a laser powder bed melting process combined with vacuum tube sealing treatment and a five-step heat treatment process to synergistically regulate the hydrogen embrittlement resistance of the iron-based superalloy. The first three steps are high-temperature heat treatment, and the last two steps are low-temperature heat treatment.

[0008] Further, the iron-based alloy powder comprises the following components: Cr: 15-15.5wt%, Mo: 1.5-2wt%, V: 0.4-0.6wt%, Ti: 2-2.5wt%, Al: 0.25-0.3wt%, Co: 3.6-3.8wt%, W: 1.5-1.8wt%, Ni: 34-35wt%, Fe: 39-41wt%.

[0009] Furthermore, the iron-based high-temperature alloy powder containing trace elements is mixed using acoustic resonance, with an acceleration of 70-90 m / s² and a time of 20-40 min.

[0010] Furthermore, the vacuum sealing process involves packaging the sample with tantalum sheets and then vacuum-sealing it in a glass tube. This vacuum sealing process isolates the environmental hydrogen source (such as hydrogen produced by the decomposition of water vapor in the atmosphere) during subsequent heat treatment, preventing secondary hydrogen absorption during the heat treatment process.

[0011] Furthermore, the laser powder bed melting process parameters are as follows: laser power of 200-400W, scanning speed of 700-1300mm / s, layer thickness of 0.03-0.05mm, and rotation angle between adjacent layers of 66-68°.

[0012] Preferably, the laser powder bed melting process parameters are: laser power of 300W-350W, scanning speed of 800-1200mm / s, layer thickness of 0.04mm, and rotation angle between adjacent layers of 67°.

[0013] Furthermore, the first step of high-temperature heat treatment is carried out at a temperature of 1050-1150℃ for 3-5 hours.

[0014] Furthermore, the second step of high-temperature heat treatment is carried out at a temperature of 1150-1250℃ for 5-12 hours.

[0015] Furthermore, the third step of high-temperature heat treatment is carried out at a temperature of 1050-1150℃ for 2-5 hours.

[0016] The fourth and fifth steps are low-temperature two-stage heat treatment. The first-stage heat treatment temperature is 630-670℃ and the heat treatment time is 13-17h; the second-stage heat treatment temperature is 610-630℃ and the heat treatment time is 13-17h.

[0017] Preferably, in the low-temperature insulation process, the first insulation temperature is 600℃ and the insulation time is 12-16h; the second insulation temperature is 600℃ and the insulation time is 12-16h.

[0018] Furthermore, each of the first three high-temperature insulation processes is followed by a cooling step, with a cooling rate of 4-6°C / min.

[0019] Preferably, the cooling rate is 5°C / min.

[0020] Furthermore, the cooling method after the third step of high-temperature heat treatment is either furnace cooling or air cooling.

[0021] The cooling method after the third step of high-temperature heat treatment is furnace cooling, which can obtain alloys with higher strength and alloys with higher plasticity.

[0022] Preferably, the method for improving the hydrogen embrittlement resistance of iron-based superalloys through grain boundary regulation is as follows:

[0023] (1) Trace elements and iron-based high-temperature alloy powder were mixed using an acoustic resonance device, and a deposited sample was prepared using a laser powder bed melting process.

[0024] (2) The deposited sample was packaged in tantalum sheets and then vacuum sealed.

[0025] (3) The sedimented sample after sealing in step (2) is subjected to the first step of high temperature heat treatment (stress relief annealing).

[0026] (4) The sample after the heat treatment in step (3) is subjected to a second high-temperature heat treatment (diffusion annealing).

[0027] (5) The diffusion annealed sample obtained in step (4) is subjected to a third step of high-temperature heat treatment (solution treatment).

[0028] (6) The sample obtained after heat treatment in step (5) is subjected to a two-step low-temperature insulation process (two-stage aging treatment).

[0029] The present invention relates to a method for improving the hydrogen embrittlement resistance of iron-based superalloys through grain boundary regulation, as described in this invention, which produces hydrogen embrittlement resistant iron-based superalloys.

[0030] The hydrogen-resistant iron-based superalloy described in this invention is used in liquid hydrogen rocket engine nozzles or aerospace system components.

[0031] The present invention describes a preparation process for an anti-hydrogen embrittlement iron-based superalloy, which uses iron-based superalloy powder as raw material and is prepared by laser powder bed melting process. Through the design of heat treatment process, a multi-scale microstructure with micron-sized grains, nano-sized second phase and high-density low CSL grain boundaries is obtained.

[0032] The material and process design of this invention is reasonable and reduces the requirements for production process. The iron-based high-temperature alloy sample prepared by laser powder bed melting has a good strength-plasticity coordination effect after the designed heat treatment process, and also has excellent hydrogen resistance. This provides the necessary conditions for its application in high temperature and high pressure hydrogen environment.

[0033] Design Principle: This invention presents a preparation process for a hydrogen-resistant iron-based superalloy. In terms of material design, the strengthening effects of iron-based superalloys are mainly dislocation strengthening, grain boundary strengthening, precipitation strengthening, and solid solution strengthening. Fe and Ni are the main matrix elements, while Al and Cr form an oxide film that contributes to high-temperature corrosion resistance. Ti is the main element in the second phase γ' and Ni3(Al,Ti). The addition of trace element B, compared to Ti, increases the segregation tendency and effectively suppresses the formation of the acicular η phase. Furthermore, both trace element B and γ' in the material's microstructure have a strong attraction to hydrogen, acting as hydrogen traps to prevent the continuous diffusion of hydrogen within the alloy and improve its hydrogen resistance. The heat treatment process design proposes a five-stage heat treatment holding regime using vacuum sealing, with quantitative control of the cooling rate for each stage. Vacuum sealing aims to ensure the surface quality of the alloy sample and prevent oxidation of boron (B) in the alloy. The first high-temperature holding heat treatment aims to eliminate residual stress generated during additive manufacturing. The second holding heat treatment uses an even higher temperature to eliminate elemental segregation within the microstructure. The third holding heat treatment forms a saturated solid solution, with controlled cooling rate to regulate the size, shape, and distribution of precipitates. The fourth and fifth low-temperature holding heat treatments are performed simultaneously to further promote precipitate growth. Using this designed heat treatment process, a multi-scale microstructure with high density, micron-sized grains, nano-sized second phases, and high-density annealed twins was obtained.

[0034] This invention employs appropriate additive forming parameters, followed by a designed heat treatment process, successfully solving the problem of high strength but low plasticity after heat treatment in additively manufactured high-Al, Ti content iron-based superalloys. The iron-based superalloy prepared by this invention, after mechanical property and hydrogen resistance tests, showed that at room temperature, the tensile strength of the alloy before hydrogen charging was 1020 MPa with an elongation of 44.1%, while the tensile strength of the alloy after hydrogen charging was 980 MPa with an elongation of 31.2%. At room temperature (600℃), the tensile strength of the alloy before hydrogen charging was 710 MPa with an elongation of 19.8%, while the tensile strength of the alloy after hydrogen charging was 704 MPa with an elongation of 15.1%, achieving a synergistic effect of high strength, plasticity, and hydrogen resistance in the iron-based superalloy.

[0035] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0036] (1) The present invention uses acoustic resonance technology to achieve effective mixing of trace element B with iron-based high-temperature alloy powder, vacuum tube sealing treatment and five-step heat treatment process, and improves the strengthening effect of the alloy through grain boundary regulation. At the same time, it can also be used as a H trap to further improve the hydrogen resistance of the alloy.

[0037] (1) This invention proposes a novel five-stage heat treatment process chain for vacuum sealing tubes, aiming to eliminate the high residual stress, elemental segregation, and microstructure inhomogeneity after the laser powder bed melting process. This significantly improves the hydrogen embrittlement resistance of iron-based superalloys.

[0038] (2) The process has a clear process window and controllable parameters, which is easy to integrate with the additive manufacturing process and the preparation process is simple and controllable. After the designed heat treatment, the sample has both good mechanical properties and excellent resistance to hydrogen embrittlement, which is convenient for large-scale industrial application. It provides a new method for developing additive manufacturing of hydrogen-resistant iron-based high-temperature alloys and their mechanical property control.

[0039] (3) The hydrogen embrittlement resistant iron-based high-temperature alloy described in this invention significantly improves the hydrogen embrittlement resistance, mechanical strength and microstructure stability of the material under the alternating low temperature and high temperature environment of liquid hydrogen through the optimized preparation process. It can effectively meet the stringent requirements of high reliability, long life and extreme environmental adaptability of liquid hydrogen rocket engine nozzles and key components of aviation systems, and has broad application prospects. Attached Figure Description

[0040] Figure 1 SEM images of the iron-based high-temperature alloy powder used in the embodiments and comparative examples of this invention;

[0041] Figure 2 The images show LPBF-formed rectangular specimens from embodiments and comparative examples of the present invention.

[0042] Figure 3 These are vacuum-sealed tube samples from Example 1 and Comparative Examples 1, 2, and 4 of the present invention.

[0043] Figure 4 SEM images of the microstructure of the iron-based superalloy after heat treatment in Example 1 (4a) and Comparative Example 1 (4b) of the present invention, and CSL grain boundary distribution diagrams of Example 1 (4c) and Comparative Example 1 (4d);

[0044] Figure 5 TEM images of the nanoscale precipitates in the iron-based superalloys in Example 1(a) and Comparative Example 1(b) of the present invention;

[0045] Figure 6 Size distribution diagram of nanoscale precipitates in iron-based superalloys in Example 1 of this invention;

[0046] Figure 7 This is a diagram showing the dimensions of the tensile specimens in the embodiments and comparative examples of the present invention.

[0047] Figure 8 The figures are the engineering stress-strain curves before and after hydrogen charging at room temperature in Example 1 of this invention.

[0048] Figure 9 The figures are the engineering stress-strain curves before and after hydrogen charging at 600°C in Example 1 of this invention.

[0049] Figure 10 This is a CSL grain boundary distribution diagram for Comparative Example 3. Detailed Implementation

[0050] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0051] The materials and reagents used in the following examples are, unless otherwise specified.

[0052] Special notes are available through commercial channels.

[0053] Among them, the iron-based high-temperature alloy powder is designated as SLFM-FeNiCrMoV-20241015; manufacturer: Xi'an Sailong Additive Manufacturing Co., Ltd.; the nickel-based alloy 718 is also manufactured by Xi'an Sailong Additive Manufacturing Co., Ltd.; the laser powder bed melting process equipment is the Huashu FS271M selective laser melting equipment. The acoustic resonance equipment for additive manufacturing composite material powder preparation is manufactured by Shenzhen Huasheng Reinforcement Technology Co., Ltd.

[0054] Example 1: Preparation of hydrogen-embrittlement-resistant iron-based superalloys

[0055] (1) Preparation of iron-based high-temperature alloy powder: The powder was prepared using a plasma rotating electrode atomization powder preparation device. The powder composition was Cr: 14.5wt%, Mo: 1.91wt%, V: 0.41wt%, Ti: 2.29wt%, Al: 0.29wt%, Co: 3.78wt%, W: 1.71wt%, B: 0.004wt%, Ni: 34.98wt%, Fe: 40.126wt%. Trace elements were mixed with the iron-based high-temperature alloy powder using an acoustic resonance device. The process parameters were: acceleration 80 m / s², time 30 min. Laser powder bed melting was performed on a Huashu FS271M selective laser melting device. The process parameters were: laser power 320 W, scanning speed 1200 mm / s, layer thickness 0.04 mm, serpentine scanning strategy with interlayer rotation of 67°, to prepare a deposited sample.

[0056] (2) The deposited sample was packaged with tantalum sheets and then vacuum sealed.

[0057] (3) The sedimented sample after sealing in step (2) is subjected to the first step of high temperature heat treatment (stress relief annealing), which is heated to 1050 ℃ at 5 ℃ / min and held for 3 h, then cooled to 500 ℃ at 5 ℃ / min and then air-cooled to room temperature.

[0058] (4) After the heat treatment in step (3), the sample is subjected to a second high-temperature heat treatment (diffusion annealing), which is heated to 1150 ℃ at 5℃ / min and held for 5 h, and then cooled to 500 ℃ at a cooling rate of 5℃ / min to obtain the diffusion annealed sample.

[0059] (5) The diffusion annealed sample obtained in step (4) is subjected to a third high-temperature heat treatment (solution treatment), which is heated to 1050℃ at a rate of 5℃ / min and held for 2 hours, and then cooled to 500℃ at a rate of 5℃ / min and cooled to room temperature in the furnace.

[0060] (6) The sample obtained after heat treatment in step (5) is subjected to a two-step low-temperature holding process (double-stage aging treatment). The first-stage holding temperature is 600℃ and the holding time is 12h. The sample is then furnace cooled to 600℃ at a cooling rate of 5℃ / min. Then, the second-stage holding temperature is 600℃ and the holding time is 12h to obtain a hydrogen-resistant iron-based high-temperature alloy.

[0061] See Figure 1 The trace element B prepared in this invention, when mixed with iron-based high-temperature alloy powder, has a particle size range of 15-53 μm, a gray surface, and good sphericity. Due to the low content of trace element, no obvious adhesion was found on the surface of the spheres.

[0062] In this embodiment, see Figure 2 and Figure 3 The deposited samples were prepared using laser powder bed melting technology. After vacuum sealing, the microstructure of the iron-based superalloy obtained using the above heat treatment method did not show acicular η phases, but rather a dense structure composed of micron-sized grains and high-density twins. (See also...) Figure 4 (a) Nanoscale precipitates, spherical in shape; see also Figure 5 (a) Count 100 spheres; see also Figure 6 The average size of the γ′ phase is 18 nm. (See also...) Figure 4 (c) The EBSD test CSL grain boundary type map shows that large-angle grain boundaries account for 96%, and twin boundaries (∑3) account for nearly 80%.

[0063] The resulting hydrogen-embrittlement-resistant iron-based superalloy will be used... Figure 7The tensile specimens (gauge length 25 mm) shown were tested for mechanical properties at room temperature and 600 °C under a strain rate of 1 × 10⁻³ s⁻¹. Hydrogen resistance was tested using the same tensile method, except that the iron-based superalloy specimens were pre-charged with hydrogen at 450 °C and 30 MPa for 1 h before testing. The results are shown in Table 1.

[0064] Table 1 Mechanical properties and hydrogen resistance of hydrogen-resistant iron-based superalloys in Example 1

[0065]

[0066] Based on the analysis in Table 1, see [link / reference]. Figure 8 At room temperature, the alloy's tensile strength before hydrogen charging was 1195 MPa, and its elongation was 14.74%. After pre-charging with hydrogen at 450℃ and 30 MPa for 1 hour, the tensile strength was 1190 MPa, a decrease of only 5 MPa, which is within the error range of conventional mechanical testing and can be considered as no substantial loss of strength. The elongation decreased from 14.74% to 12.10%, an absolute decrease of 2.64%, corresponding to a plasticity loss rate of 17.9%. These results indicate that the alloy has excellent resistance to hydrogen embrittlement at room temperature.

[0067] See Figure 9 At 600℃, the tensile strength of the alloy before hydrogen charging was 982 MPa, and the elongation was 15.9%. After hydrogen charging, the tensile strength was 975 MPa, a decrease of only 7 MPa, indicating that even at high temperatures, the effect of hydrogen on the alloy's strength remains very weak. The elongation decreased from 15.9% to 12.5%, a decrease of 3.4% in absolute value, and the plasticity loss rate reached 21.4%. Although the plasticity loss rate at high temperature was slightly higher than at room temperature (an increase of 3.5%), reflecting that the increased temperature promoted the diffusion of hydrogen and its accumulation in weak regions such as grain boundaries, thus leading to an increase in hydrogen embrittlement sensitivity, the overall elongation remained at a high level, and the strength was significantly better than most traditional iron-based superalloys, fully demonstrating that the alloy still has good resistance to hydrogen embrittlement in high-temperature environments.

[0068] In this embodiment, alloy powder containing trace element B is used. A laser powder bed melting (LPBF) process combined with vacuum sealing and a five-step heat treatment process is employed to synergistically regulate the microstructure and hydrogen embrittlement resistance of the iron-based superalloy. Both trace element B and the dispersed γ' precipitates in the alloy possess strong hydrogen-trapping capabilities, acting as effective hydrogen traps to inhibit long-range hydrogen diffusion in the matrix and its enrichment in sensitive regions such as grain boundaries and dislocations, thereby significantly improving the material's resistance to hydrogen embrittlement. Simultaneously, the vacuum sealing process not only ensures the surface quality of the sample but, more importantly, prevents the oxidation loss of active elements such as B during subsequent heat treatment, ensuring their effective function.

[0069] The five-step heat treatment process further optimized the alloy's microstructure: the first step, high-temperature holding, aimed to eliminate residual stress introduced during LPBF (Laser Bed Fusion) forming; the second step, even higher-temperature holding, alleviated solidification segregation and achieved compositional homogenization; the third step, high-temperature solution treatment, formed a saturated solid solution, and precisely controlled the cooling rate to regulate the size, quantity, and spatial distribution of the γ' precipitate; the subsequent two steps of low-temperature aging heat treatment further optimized the dispersion of the precipitate and grain boundary characteristics, strengthening the matrix and improving the grain boundaries' resistance to hydrogen-induced cracking. The combined effects of these multi-stage heat treatments, vacuum sealing, and trace element B achieved a synergistic improvement in strength and resistance to hydrogen embrittlement.

[0070] In summary, the iron-based superalloy prepared in this embodiment through a synergistic process combining trace element B modification, laser powder bed melting, vacuum sealing, and five-step heat treatment exhibits high strength (>1190 MPa at room temperature, >975 MPa at 600℃) and excellent resistance to hydrogen embrittlement (plasticity loss ≤21.4%) at both room temperature and 600℃, significantly superior to conventional alloys of the same type. This performance advantage mainly stems from the synergistic optimization of the grain boundary structure: the segregation of trace element B at the grain boundaries not only strengthens the grain boundary bonding force but also forms hydrogen trap centers with high binding energy; while the two-stage low-temperature aging treatment in the five-step heat treatment, by regulating the distribution characteristics of the γ' phase at the grain boundaries, constructs a three-in-one hydrogen trapping network of "γ' phase-grain boundary-B element", effectively hindering the diffusion and enrichment of hydrogen atoms to the grain boundaries and suppressing intergranular hydrogen embrittlement cracking. This synergistic strategy significantly improves the hydrogen embrittlement resistance of iron-based superalloys through grain boundary regulation, while also taking into account the balance between high-temperature strength and plasticity, fully verifying its effectiveness and advanced nature in improving the overall service performance of iron-based superalloys.

[0071] Comparative Example 1: Comparison of Cooling Methods

[0072] The steps are the same as in Example 1, except that after step (5) high temperature heat treatment (solution treatment) is completed, the temperature is cooled to 500°C at a rate of 5°C / min and then cooled to room temperature in the furnace. Instead, air cooling is used to cool to room temperature.

[0073] In this implementation example, see Figure 4 (b) The microstructure is similar to that in Example 1, also exhibiting micron-sized grains and high-density annealing twins. See [reference needed]. Figure 5 (b) The resulting hydrogen embrittlement resistant superalloy exhibits nanoscale precipitates, smaller in size than the hydrogen embrittlement resistant superalloy obtained in Example 1, but with larger spacing, and a spherical shape. See also Figure 4 (d)

[0074] In EBSD testing of CSL grain boundary types, large-angle grain boundaries accounted for approximately 93%, and twin boundaries (∑3) accounted for approximately 75%.

[0075] The obtained hydrogen-resistant iron-based superalloy was subjected to tensile testing using the specimens shown in Figure 6 (gauge length 25 mm) at a strain rate of 1×10⁻³ s⁻¹, and the mechanical properties were measured at room temperature and 600 ℃, respectively. The hydrogen resistance test was conducted using the same tensile method, except that the iron-based superalloy specimens were pre-charged with hydrogen at 450 ℃ and 30 MPa for 1 h before testing. The results are shown in Table 2.

[0076] Table 2 Mechanical properties and hydrogen resistance of hydrogen-resistant iron-based superalloys in Comparative Example 1

[0077]

[0078] Analysis of Table 2 shows that, under room temperature conditions, the tensile strength of the alloy before hydrogen charging is 1020 MPa, and the elongation is as high as 44.1%, exhibiting excellent plasticity. After pre-charging with hydrogen, the tensile strength slightly decreases to 980 MPa (a decrease of 40 MPa). This change is within the normal performance fluctuation range of high-strength materials, and it can be considered that hydrogen has no significant effect on strength. However, the elongation decreases from 44.1% to 31.2%, an absolute decrease of 12.9%, corresponding to a plasticity loss rate of 29.3%, indicating that hydrogen has a significant weakening effect on the plasticity of the alloy, although it still retains high ductility.

[0079] At 600°C, the alloy exhibited a tensile strength of 710 MPa and an elongation of 19.8% before hydrogen charging; after hydrogen charging, the tensile strength decreased to 704 MPa and the elongation to 15.1%. The tensile strength decreased by only 6 MPa (approximately 0.8%), while the plasticity loss rate was 23.7%, slightly lower than the 29.3% at room temperature. This indicates that although high temperature accelerates hydrogen diffusion, it simultaneously enhances dynamic recovery and dislocation climb capabilities, helping to relax local stress concentrations and promote crack tip passivation, thus partially suppressing the hydrogen-induced embrittlement effect. Therefore, this alloy still exhibits relatively good plasticity retention in high-temperature hydrogen-containing environments. It is noteworthy that although the plasticity loss rate at 600°C is slightly lower, its absolute elongation (15.1%) and tensile strength (704 MPa) are significantly lower than those of Example 1 (elongation 12.5% ​​vs. 15.1%, but tensile strength 975 MPa vs. 704 MPa), indicating that its overall high-temperature mechanical properties, especially strength and resistance to hydrogen embrittlement, are weaker than those of Example 1.

[0080] This comparative example uses air cooling after the third heat treatment step. Compared to the furnace cooling combined with two-stage low-temperature aging in Example 1, air cooling, while suppressing the precipitation of harmful phases and refining the grains through rapid cooling, imparts high room-temperature plasticity (44.1%) to the alloy. However, omitting or shortening the subsequent aging process results in insufficient precipitation and poor distribution of the γ' strengthening phase, causing a significant decrease in room-temperature strength (1020 MPa) compared to Example 1 (1195 MPa). Furthermore, the hydrogen trap density is insufficient, and the room-temperature plasticity loss rate (29.3%) is significantly increased. More seriously, the strengthening phase in this microstructure lacks stability at high temperatures, causing the strength at 600 °C to plummet to 710 MPa, far below the level of over 975 MPa in Example 1, indicating a significant disadvantage in overall high-temperature performance.

[0081] In summary, although both the comparative example and Example 1 achieved similar microstructure characteristics such as micron-sized grains, a uniform second phase, and high-density, low-Σ grain boundaries, the key difference in the heat treatment cooling methods led to drastically different strengthening and toughening mechanisms: while the air-cooling process achieved "high plasticity," it sacrificed strength and hydrogen resistance due to insufficient precipitation strengthening; whereas Example 1, through furnace cooling combined with two-stage low-temperature aging, achieved synergistic optimization of the precipitated phase and grain boundaries, significantly improving strength and hydrogen embrittlement resistance while maintaining high plasticity. Therefore, differentiated heat treatment strategies can be adopted for different service requirements: if formability and high room-temperature plasticity are prioritized, allowing for appropriate sacrifice of strength and hydrogen resistance, an air-cooling process is used after the third heat treatment; if the service environment involves high-pressure hydrogen or high-temperature, high-stress conditions, requiring a balance between high strength and excellent hydrogen embrittlement resistance, then a furnace cooling process is used after the third heat treatment to fully promote the orderly precipitation of the precipitated phase and grain boundary optimization, achieving a synergistic improvement in overall performance.

[0082] Comparative Example 2: The role of trace element (B)

[0083] The steps are the same as in Example 1, except that no trace element boron (B) is added to the iron-based high-temperature alloy powder, and the Fe content is 40.126 wt%.

[0084] The CSL grain boundary type of this comparative sample was tested in EBSD, with large-angle grain boundaries accounting for approximately 91% and twin boundaries (∑3) accounting for approximately 68%. The obtained iron-based superalloy was subjected to tensile testing using the specimens shown in Figure 6 (gauge length 25 mm) at a strain rate of 1 × 10⁻³ s⁻¹, and the mechanical properties were measured at room temperature and 600 ℃. The hydrogen resistance test followed the same tensile method, except that the iron-based superalloy specimens were pre-charged with hydrogen at 450 ℃ and 30 MPa for 1 h before testing. The results are shown in Table 4.

[0085] Table 3 Mechanical properties and hydrogen resistance of iron-based superalloys in Comparative Example 2

[0086]

[0087] Based on the analysis in Table 3, under room temperature conditions, the tensile strength of Comparative Example 3 (without element B) before hydrogen charging was 1083 MPa, significantly lower than the 1195 MPa of Example 1; its elongation was 14.9%, slightly higher than the 14.74% of Example 1. However, after pre-charging with hydrogen, the elongation of Comparative Example 3 decreased to 11.2%, and the plasticity loss rate reached 24.83%, significantly higher than the 17.9% of Example 1. This indicates that although the initial plasticity of both is similar, Comparative Example 3, lacking element B, exhibits more severe plasticity degradation and weaker resistance to hydrogen embrittlement under hydrogen conditions.

[0088] Under high-temperature conditions, Comparative Example 3 exhibited an elongation of 16.1% at 600°C, slightly higher than its room-temperature value, but this plummeted to 10.3% after pre-filling with hydrogen, resulting in a plasticity loss rate as high as 36.02%. In contrast, Example 1 maintained its original elongation of 15.9% at a higher temperature (650°C), and after hydrogen filling, it reached 12.5%, with a plasticity loss rate of only 21.4%. This demonstrates that the addition of element B not only improved the high-temperature strength of the alloy (982 MPa vs 857 MPa) but also significantly enhanced its plasticity retention in a high-temperature hydrogen environment.

[0089] Comparing the data from Example 1 and Comparative Example 3 reveals that the absence of trace element B is the key factor leading to the aforementioned differences. B effectively suppresses the formation of the η phase at grain boundaries and strengthens grain boundary bonding by segregating at grain boundaries. Comparative Example 3, lacking B, suffers from ineffective grain boundary regulation, insufficient intrinsic plasticity reserve (low elongation at both room temperature and high temperature), and hydrogen atoms are more prone to segregation at weakened grain boundaries, reducing grain boundary bonding energy and significantly increasing susceptibility to hydrogen-induced intergranular cracking. In summary, Comparative Example 3, lacking B, exhibits lower tensile strength than the examples and shows worse resistance to hydrogen embrittlement at both room temperature and high temperature. Therefore, the introduction of B plays a crucial role in improving the overall strength, plasticity, and resistance to hydrogen embrittlement of iron-based superalloys.

[0090] Comparative Example 3: The Role of Vacuum Sealing

[0091] The steps are the same as in Example 1, except that no vacuum sealing treatment was performed before heat treatment.

[0092] The obtained iron-based superalloy was subjected to tensile testing using the specimens shown in Figure 6 (gauge length 25 mm) at a strain rate of 1×10⁻³ s⁻¹, and the mechanical properties were measured at room temperature and 600 ℃, respectively. The hydrogen resistance test was conducted using the same tensile method, except that the iron-based superalloy specimens were pre-charged with hydrogen at 450 ℃ and 30 MPa for 1 h before testing. The results are shown in Table 5.

[0093] Table 4 Mechanical properties and hydrogen resistance of iron-based superalloys in Comparative Example 3

[0094]

[0095] Based on the analysis in Table 4, compared with Example 1 (Table 1), this comparative example lacks vacuum sealing treatment, resulting in surface oxidation and loss of alloy elements during high-temperature heat treatment, which weakens the solid solution strengthening effect and significantly reduces the room temperature tensile strength to 907 MPa (a decrease of approximately 24.1% compared to 1195 MPa in Example 1).

[0096] As attached Figure 10 In this comparative sample, the CSL grain boundary type was tested in EBSD. Large-angle grain boundaries accounted for approximately 92%, and twin boundaries (∑3) accounted for approximately 66%. A high proportion of ∑3 grain boundaries is generally considered beneficial for improving crack propagation resistance, but in this comparative sample, it did not effectively suppress hydrogen embrittlement; instead, it resulted in significant plasticity loss. This further illustrates that even with a high proportion of low-ΣCSL grain boundaries, if the grain boundaries are contaminated by oxides or impurities (due to the lack of vacuum sealing), their inherent advantage in resisting hydrogen embrittlement will be significantly weakened or even rendered ineffective.

[0097] Regarding hydrogen resistance, although the initial elongation of this comparative example (38.2%) was higher than that of Example 1 (14.74%), the room temperature plasticity loss rate after pre-filling with hydrogen was as high as 34.82%, significantly higher than the 17.9% of Example 1; the plasticity loss rate at 600 °C (30.81%) was also higher than the 21.4% of Example 1 at 650 °C. This indicates that the lack of vacuum sealing treatment significantly deteriorated the alloy's resistance to hydrogen embrittlement.

[0098] The absence of vacuum sealing leads to a deterioration of the grain boundary state, making it easier for hydrogen atoms to accumulate at high-angle grain boundaries, thus inducing grain boundary embrittlement. Although the tensile strength decreased only slightly after pre-filling with hydrogen (only 1.8% at room temperature and about 3.1% at high temperature), the plasticity deteriorated significantly, indicating that the hydrogen embrittlement mechanism is mainly manifested as an enhancement of the brittle fracture mode at grain boundaries. In contrast, Example 1 optimized the grain boundary structure through vacuum sealing, reducing the proportion of high-angle grain boundaries and increasing the proportion of special coincident site lattice (CSL) grain boundaries, thereby effectively suppressing hydrogen-induced grain boundary embrittlement and achieving a balance between high strength and excellent resistance to hydrogen embrittlement.

[0099] Comparative Example 4: Verification of the necessity of the second-step high-temperature heat treatment (diffusion annealing)

[0100] The steps are the same as in Example 1, except that the second step of high-temperature heat treatment (diffusion annealing) is not performed.

[0101] The obtained iron-based superalloy was subjected to tensile testing using the specimens shown in Figure 6 (gauge length 25 mm) at a strain rate of 1×10⁻³ s⁻¹, and the mechanical properties were measured at room temperature and 600 ℃, respectively. The hydrogen resistance test was conducted using the same tensile method, except that the iron-based superalloy specimens were pre-charged with hydrogen at 450 ℃ and 30 MPa for 1 h before testing. The results are shown in Table 6.

[0102] Table 5 Mechanical properties and hydrogen resistance of iron-based superalloys in Comparative Example 4

[0103]

[0104] Based on the analysis in Table 6, compared with Example 1, the lack of diffusion annealing significantly reduced the solid solution strengthening effect of the alloy. This step is usually used to promote the homogenization of alloying elements, eliminate casting segregation, control the distribution of precipitates, and optimize grain boundary conditions. Without this treatment, the tensile strength of the alloy at room temperature is 1050 MPa, significantly lower than the 1195 MPa of Example 1; the elongation is 19.2%, higher than the 14.74% of Example 1, but rapidly drops to 14.1% after pre-charging with hydrogen; the plasticity loss rate reaches 26.6%, significantly higher than the 17.9% of Example 1. This indicates that although the initial plasticity is improved, its plastic stability in a hydrogen environment is poor, and its resistance to hydrogen embrittlement is significantly weakened.

[0105] At a high temperature of 600℃, the tensile strength of Comparative Example 5 was 920 MPa, lower than that of Example 1 at a higher temperature (650℃) of 982 MPa; its elongation decreased from 17.1% to 11.6%, and its plasticity loss rate was 32.2%, much higher than that of Example 1 (21.4%). The high-temperature hydrogen resistance of Comparative Example 5 was still inferior to that of Example 1, indicating that its microstructure failed to achieve sufficient homogenization and strengthening phase optimization through diffusion annealing, resulting in greater sensitivity to hydrogen-induced damage at high temperatures.

[0106] The main function of diffusion annealing is to promote the full diffusion of alloying elements through long-term high-temperature holding, eliminate micro-segregation of chemical composition, form a uniform single-phase austenitic structure, and optimize the grain boundary structure. The absence of this step will lead to: (1) regions with uneven composition become preferential enrichment sites for hydrogen atoms, forming local high hydrogen concentration areas and promoting the initiation of hydrogen-induced cracks; (2) segregated or precipitated phases remaining at the grain boundaries act as hydrogen traps, accelerating hydrogen-induced intergranular embrittlement; (3) the unevenness of the structure leads to uneven stress distribution, making it more prone to local plastic instability under the combined action of hydrogen and mechanical load.

[0107] Comparative Example 5: Inconel 718 Nickel-Based Alloy

[0108] To further highlight the principles and advantages of this invention, a comparison of the mechanical properties and hydrogen resistance of Inconel 718 nickel-based alloy formed by laser powder bed melting is presented. The specific steps are as follows:

[0109] (1) Inconel 718 nickel-based alloy powder with a particle size of 15-53 μm was used for laser powder bed melting on a Farsoon FS271M selective laser melting equipment. Process parameters: 900 mm / s, layer thickness of 0.04 mm, serpentine scanning strategy, and rotation angle between adjacent layers of 67° to obtain deposited samples.

[0110] (2) The deposited nickel-based alloy sample was subjected to a third step of high-temperature heat treatment (solution treatment) at 1050℃. The temperature was increased at a rate of 5℃ / min and held for 1.5h. Then, it was quenched in water and cooled to room temperature to obtain a solution sample.

[0111] (3) The solid solution sample was aged at 780℃, heated at a rate of 5℃ / min, held for 8h, and then cooled to 500℃ at a rate of 5℃ / min. The sample was then cooled to room temperature in the furnace to obtain the nickel-based alloy 718 sample.

[0112] The obtained nickel-based alloy 718 was tested using tensile specimens (gauge length 25 mm) as shown in Figure 6 at a strain rate of 1×10⁻³ s⁻¹, and the mechanical properties were measured at room temperature and 600 ℃, respectively. The hydrogen resistance test was conducted using the same tensile method, except that the iron-based superalloy specimens were pre-charged with hydrogen at 450 ℃ and 30 MPa for 1 h before testing. The results are shown in Table 3.

[0113] Table 6 Mechanical properties and hydrogen resistance of hydrogen-resistant Inconel 718 nickel-based alloy (Comparative Example 5)

[0114]

[0115] Based on the analysis in Table 6, the commercial Inconel 718 nickel-based alloy used in Comparative Example 5 had a tensile strength of 1221 MPa and an elongation of 24.4% before hydrogen charging at room temperature. After pre-hydrogen charging, the strength increased slightly to 1237 MPa, which can be considered as no substantial change; however, the elongation decreased significantly to 13.0%, and the plasticity loss rate was as high as 46.7%. At 600℃, the tensile strength before hydrogen charging was 1056 MPa and the elongation was 15.9%; after hydrogen charging, the strength decreased slightly to 1045 MPa, which was still within the test error range, indicating that hydrogen has a limited impact on the stability of the γ′ / γ″ strengthening phase; however, the elongation plummeted to 8.75%, and the plasticity loss rate also reached 45.0%. The above results fully demonstrate that although the Inconel 718 nickel-based alloy can maintain its strength well in a hydrogen environment, its plasticity is extremely sensitive to hydrogen, exhibiting severe hydrogen-induced embrittlement behavior.

[0116] In contrast, the iron-based superalloy in Example 1 of this invention exhibits superior resistance to hydrogen embrittlement under the same testing conditions: its plasticity loss rates at room temperature and 600°C are only about 17.9% and 21.4%, respectively, significantly lower than those of the 718 alloy (46.7% and 45.0%). This indicates that the iron-based alloy not only possesses a strength level comparable to the 718 alloy (as shown in the furnace-cooled sample), but also exhibits superior plasticity retention in a hydrogen environment, demonstrating significantly better resistance to hydrogen embrittlement than the commercially available Inconel 718 nickel-based alloy.

[0117] In summary, the comparison results between the embodiments and comparative examples fully demonstrate that the core innovation of this invention lies in the synergistic effect of the introduction of trace element B, vacuum sealing treatment, and the novel five-step heat treatment process. Specifically, the precise mixing of trace element B through acoustic resonance technology not only effectively strengthens the alloy matrix but also acts as a highly efficient hydrogen trap, significantly improving the material's resistance to hydrogen embrittlement. In the heat treatment stage, the designed five-step heat treatment process chain optimizes the internal microstructure of the alloy through precise control of multiple temperature zones and time periods, greatly increasing the proportion of low-Σ grain boundaries, thereby synergistically enhancing the material's strength, plasticity, and hydrogen resistance. Vacuum sealing treatment ensures the stability and uniform distribution of trace elements during high-temperature processes, avoiding oxidation and volatilization losses, laying the foundation for subsequent heat treatment and performance improvement. The organic combination of these three elements constitutes an inseparable technical whole of this invention, successfully meeting the stringent requirements of high reliability, long lifespan, and excellent hydrogen resistance for liquid hydrogen rocket engine nozzles and key aerospace components under extreme service environments, demonstrating outstanding technological advancement and broad application prospects.

Claims

1. A method for improving the hydrogen embrittlement resistance of iron-based superalloys through grain boundary regulation, characterized in that, Using iron-based superalloy powder containing trace elements as the raw material for mixing, the hydrogen embrittlement resistance of the iron-based superalloy is synergistically controlled by laser powder bed melting process combined with vacuum tube sealing treatment and five-step heat treatment process; the first three steps are high-temperature heat treatment and the last two steps are low-temperature heat treatment.

2. The method for improving the hydrogen embrittlement resistance of iron-based superalloys by grain boundary regulation according to claim 1, characterized in that, The iron-based alloy powder comprises the following components by weight percentage: Cr: 15-15.5wt%, Mo: 1.5-2wt%, V: 0.4-0.6wt%, Ti: 2-2.5wt%, Al: 0.25-0.3wt%, Co: 3.6-3.8wt%, W: 1.5-1.8wt%, B: 0.002-0.005wt%, Ni: 34-35wt%, Fe: 39-41wt%.

3. The method for improving the hydrogen embrittlement resistance of iron-based superalloys by grain boundary regulation according to claim 1, wherein the iron-based superalloy powder containing trace elements is mixed with acoustic resonance powder at an acceleration of 70-90 m / s² for 20-40 min.

4. The method for improving the hydrogen embrittlement resistance of iron-based superalloys by grain boundary regulation according to claim 1, characterized in that, The laser powder bed melting process parameters are as follows: laser power of 300-350W, scanning speed of 800-1200mm / s, layer thickness of 0.03-0.05mm, and rotation angle between adjacent layers of 66-68°.

5. The method for improving the hydrogen embrittlement resistance of iron-based superalloys by grain boundary regulation according to claim 1, characterized in that, The first step of high-temperature heat treatment is carried out at a temperature of 1050-1150℃ for 3-5 hours.

6. The method for improving the hydrogen embrittlement resistance of iron-based superalloys by grain boundary regulation according to claim 1, characterized in that, The second step is high-temperature heat treatment, with a temperature of 1150-1250℃ and a duration of 5-12 hours.

7. The method for improving the hydrogen embrittlement resistance of iron-based superalloys by grain boundary regulation according to claim 1, wherein the third step of high-temperature heat treatment is performed at a temperature of 1050-1150℃ for 2-5 hours.

8. The method for improving the hydrogen embrittlement resistance of iron-based superalloys by grain boundary regulation according to claim 1, characterized in that, The fourth and fifth steps are low-temperature two-stage heat treatment. The first-stage heat treatment temperature is 630-670℃ and the heat treatment time is 13-17h; the second-stage heat treatment temperature is 610-630℃ and the heat treatment time is 13-17h.

9. A hydrogen-embrittlement-resistant iron-based superalloy prepared by the method of improving the hydrogen embrittlement resistance of iron-based superalloys through grain boundary regulation as described in claim 1.

10. The application of the hydrogen-resistant iron-based superalloy of claim 9 in liquid hydrogen rocket engine nozzles or aerospace system components.

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