A method for heat treating a maraging stainless steel and a maraging stainless steel

Abstract

The application belongs to the field of steel heat treatment, and particularly relates to a heat treatment method of a martensitic age-hardening stainless steel and the martensitic age-hardening stainless steel. The heat treatment method comprises the following steps: solid solution treatment and quenching of the martensitic age-hardening stainless steel material to be treated; placing in a deep cooling environment for heat preservation, and then placing in a room temperature environment for recovery to room temperature to complete a deep cooling-room temperature treatment; continuously performing deep cooling-room temperature treatment; subcritical treatment at 700-880 DEG C and quenching; and aging treatment after the subcritical treatment, with the aging treatment temperature being 10-30 DEG C lower than a standard aging temperature. The method first introduces deep cooling-subcritical treatment as an intermediate method, completely eliminates residual austenite which does not occur martensitic phase change in the material after quenching, and simultaneously controls the size and distribution of reverse austenite in the martensitic age-hardening stainless steel, so that a martensitic age-hardening stainless steel material component with better comprehensive mechanical properties than that obtained by a conventional heat treatment method can be finally obtained.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment of steel, specifically relating to a heat treatment method for martensitic aging stainless steel and martensitic aging stainless steel. Background Technology

[0002] As the global industrial system continues its accelerated transformation towards high-end and precision manufacturing, increasingly stringent requirements are being placed on the comprehensive performance of key structural materials across various sectors. Martensitic aging stainless steel, a precipitation-hardening stainless steel, is a typical representative of this high-strength stainless steel. It successfully achieves a perfect combination of the superior mechanical properties of high-strength steel and high stability under stress corrosion conditions, while maintaining excellent formability, weldability, oxidation resistance, and fatigue durability. Therefore, it has been widely used in important fields such as aerospace and petrochemicals.

[0003] Heat treatment is one of the important factors affecting the comprehensive mechanical properties of martensitic aging stainless steel. Taking PH13-8Mo steel as an example, the traditional heat treatment method for existing martensitic aging stainless steels employs a solution treatment followed by aging treatment. Solution treatment is completed by holding the raw material at 810~1000℃ for 30~90 minutes, forming a low-carbon martensitic matrix, which lays the microstructure foundation for the precipitation of strengthening phases during the subsequent aging process. Then, according to different tensile strength and elongation requirements, different aging temperatures (e.g., 525℃, 540℃, 595℃) are applied to obtain martensitic aging stainless steels with different strength and toughness grades.

[0004] However, modern service environments, such as those for fasteners, aircraft landing gear, reactor components, and critical components in petrochemical equipment, place increasingly stringent demands on the plasticity of materials. While components made from maraging stainless steel obtained using the aforementioned traditional heat treatment methods possess high strength, their plasticity remains insufficient, affecting the stability of maraging stainless steel during long-term service.

[0005] Chinese invention patent CN104099455B, published on March 9, 2016, discloses a non-isothermal heat treatment method for high-strength and toughness precipitation-hardening stainless steel. The method comprises the following steps: 1. Non-isothermal solution treatment: The precipitation-hardening stainless steel is heated from room temperature to 1070℃~1085℃ at a heating rate of 2℃ / min~5℃ / min, then subjected to non-isothermal solution treatment at a cooling rate of 0.5℃ / min~1.5℃ / min for 60 minutes, followed by air cooling or wind cooling to room temperature, resulting in precipitation-hardened stainless steel after non-isothermal solution treatment. 1. Precipitation hardening stainless steel: The precipitation hardening stainless steel obtained in step 1 after non-isothermal solution treatment is subjected to cryogenic treatment at -70℃ for 120 minutes to obtain cryogenically hardened stainless steel; 2. Aging treatment: The cryogenically hardened stainless steel is heated to 500℃~540℃ at a heating rate of 2℃ / min~5℃ / min, and then aged at 500℃~540℃ at a cooling rate of 1.0℃ / min for 120 minutes to complete the high-strength and high-toughness non-isothermal heat treatment of precipitation hardening stainless steel.

[0006] The above heat treatment method uses a solution-deep cryogenic-aging process to treat precipitation-hardening stainless steel. However, due to the failure to finely control the austenite, it still faces the contradiction of strength and toughness synergy. Under the premise of ensuring a certain strength, it is impossible to further increase the plasticity and toughness of the material, and it cannot meet the increasingly demanding performance requirements of aerospace structural components.

[0007] Chinese invention patent application CN116814919A, published on September 29, 2023, discloses a heat treatment method for improving the strength and toughness of martensitic aging stainless steel, which includes: (1) solution treatment; (2) first cryogenic treatment; (3) pre-aging treatment; (4) second cryogenic treatment; (5) long-term aging treatment; and (6) third cryogenic treatment. This method adopts a double aging process + triple cryogenic heat treatment method, which solves the problem of balancing the precipitation degree of precipitates with the content and morphology of austenite under the existing heat treatment process technology, and finally obtains martensitic stainless steel with ultra-high strength and ultra-high toughness.

[0008] The chemical composition of the martensitic aging stainless steel to which the above heat treatment method is applicable is as follows: C: 0.1-0.3%, Cr: 11-14%, Ni: 5.0-8.0%, Mo: 1.0-4.0%, Co: 6.5-8.5%, V: 0.2-0.45%, Nb: 0.012-0.04%, Cu: 0-2%, Al: 0-1.5%, P: ≤0.02%, S: ≤0.02%, with the balance being Fe.

[0009] For common martensitic aging stainless steels, such as PH13-8Mo steel, the above heat treatment methods can only slightly improve the tensile strength of the material and cannot further improve the plasticity and toughness of the material, thus having limited guidance for industrial production. Summary of the Invention

[0010] The first objective of this invention is to provide a heat treatment method for martensitic aging stainless steel to solve the problem that traditional heat treatment methods cannot further improve plasticity while maintaining high tensile strength.

[0011] The second objective of this invention is to provide a martensitic aging stainless steel obtained by the above-mentioned heat treatment method, so as to solve the problem that the comprehensive mechanical properties of existing martensitic aging stainless steel need to be further improved.

[0012] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0013] A heat treatment method for martensitic aging stainless steel includes the following steps:

[0014] (1) The martensitic aging stainless steel material to be treated is solution treated and then quenched;

[0015] (2) Place the material treated in step (1) in a cryogenic environment for insulation, and then place it in a room temperature environment to restore it to room temperature, thus completing one cryogenic-room temperature treatment; continue to perform the cryogenic-room temperature treatment.

[0016] (3) The material treated in step (2) is subjected to sub-temperature treatment at 700~880℃ and then quenched;

[0017] (4) After sub-temperature treatment, aging treatment is carried out. The aging treatment temperature is 10~30℃ lower than the standard aging temperature, and then quenched.

[0018] This invention is an improved version. First, a uniform supersaturated austenitic matrix is ​​obtained through solution treatment. Then, quenching ensures martensitic transformation while promoting the dispersion and precipitation of nanoscale intermetallic compounds from the supersaturated matrix. After quenching, a cryogenic-sub-temperature treatment is added as an intermediate heat treatment step. First, the cryogenic treatment completely eliminates residual austenite, and then the sub-temperature treatment promotes the formation of reverse-transformed austenite, while simultaneously adjusting the size and distribution of the reverse-transformed austenite. Finally, different aging temperatures are matched to control the comprehensive mechanical properties of the martensitic aged stainless steel.

[0019] This method introduces cryogenic-sub-thermal treatment as an intermediate method for the first time, which completely eliminates the residual austenite in the material that has not undergone martensitic transformation after quenching. At the same time, it controls the size and distribution of reverse-transformed austenite in martensitic aging stainless steel, and finally obtains martensitic aging stainless steel material components with comprehensive mechanical properties that are superior to those of conventional heat treatment methods.

[0020] Preferably, in step (2), the temperature of the cryogenic environment is -40℃ to -196℃, and the holding time is 1 to 9 hours; step (2) involves 2 to 3 cryogenic-room temperature treatments. During cryogenic treatment, the temperature of the cryogenic environment should remain unchanged, and the residual austenite should be completely eliminated by cryogenic treatment.

[0021] More preferably, the temperature of the cryogenic environment is -70℃ to -60℃, and the holding time is 3h to 4h. Using these preferred cryogenic treatment conditions, residual austenite can be quickly and completely eliminated.

[0022] Preferably, in step (3), the sub-temperature treatment temperature is 760±5℃, and the holding time is 1h±10min. The reverse-transformed austenite obtained by sub-temperature treatment is distributed along the martensite lath boundary in a thin film form, forming a semi-continuous "tough coating" structure, which improves the fracture toughness and crack propagation resistance of the material.

[0023] More preferably, in step (4), for martensitic aging stainless steel with a tensile strength of 1500 MPa, the aging temperature is 510℃±5℃; for martensitic aging stainless steel with a tensile strength of 1400 MPa, the aging temperature is 510~525℃; for martensitic aging stainless steel with a tensile strength of 1000 MPa, the aging temperature is 580℃±5℃; and the holding time for aging is 4h±10min. The strength and toughness of the material can be controlled in the above manner to meet different engineering application requirements.

[0024] Preferably, in step (1), the solution treatment temperature is 810~1000℃ and the holding time is 30~90min. During the solution treatment process, the furnace temperature of the heat treatment furnace should be as accurate as possible, with a furnace temperature error of ±5℃ and a holding time error range of ±3min, so as to maximize the heat treatment stability of martensitic aging stainless steel.

[0025] More preferably, in step (1), the solution treatment temperature is 920~930℃, and the holding time is 40~50min. Under these conditions, a uniform supersaturated austenitic matrix can be obtained efficiently, laying a good foundation for subsequent processing.

[0026] Preferably, the maraging stainless steel is PH13-8Mo. PH13-8Mo is a high-performance, widely used maraging stainless steel with promising applications in key components such as fasteners, aircraft landing gear, reactor parts, and petrochemical equipment.

[0027] Martensitic aging stainless steel obtained using the above heat treatment method.

[0028] The above heat treatment methods can be used to obtain martensitic aging stainless steel with better comprehensive mechanical properties. While basically maintaining the tensile strength, the toughness of the material can be further improved, or the tensile strength of the material can be significantly improved while maintaining the toughness of the material, or the tensile strength and toughness of the material can be improved simultaneously, thereby further improving the stability of martensitic aging stainless steel during long-term service.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] This method combines sub-temperature heat treatment with low-temperature aging to achieve selective enrichment of Ni in reversible austenite, significantly improving its thermal stability. By appropriately increasing the content of reversible austenite, the formation of small-sized thin-film austenite is induced, allowing it to be uniformly distributed at the martensite block interface, thereby optimizing the microstructure of the material. The cryogenic-sub-temperature composite treatment method precisely controls the austenite distribution characteristics and phase stability, further improving the elongation of martensitic aging stainless steel precipitation-hardening stainless steel while maintaining its tensile strength essentially unchanged, thus exhibiting a superior synergistic effect of strength and toughness.

[0031] After treatment using a cryogenic-sub-temperature annealing composite heat treatment method, the retained austenite can be completely eliminated, while the content of the reversible austenite phase with better thermal stability increases, and its size decreases to approximately 0.1 μm. The reversible austenite obtained by sub-temperature treatment is distributed along the martensite lath boundaries in a thin film morphology, forming a semi-continuous "ductile encapsulation layer" structure. When a propagating microcrack enters the ductile film from the brittle martensite matrix, the crack tip is blunted, and stress concentration is effectively alleviated. At the same time, the crack requires additional energy to cross this film layer, thereby improving the fracture toughness and crack propagation resistance of the material. Compared with island-like retained austenite, the thin film distribution of reversible austenite increases its interfacial bonding area and bonding strength with the matrix, effectively suppressing interfacial delamination. In addition, this discontinuous thin film distribution avoids the formation of large-area continuous soft regions in the matrix, so that while introducing a ductile phase, it does not significantly damage the overall strength of the material, achieving a good synergy between the material's strength and toughness. Attached Figure Description

[0032] Figure 1 This is a flowchart of the heat treatment method for martensitic aging stainless steel according to the present invention.

[0033] Figure 2 This is a diagram of the mechanical test specimen in an embodiment of the present invention;

[0034] Figure 3 The graph shows the mechanical properties of martensitic aging stainless steel under different heat treatment conditions in Example 1 of this invention.

[0035] Figure 4The following are microstructures of martensitic aged stainless steel after treatment by different methods in Example 1 of the present invention: (a) inverse pole figure after treatment by conventional method; (b) inverse pole figure after treatment by cryogenic-sub-thermal composite method; (c) phase diagram after treatment by conventional method; (d) phase diagram after treatment by cryogenic-sub-thermal composite method.

[0036] Figure 5 The graph shows the mechanical properties of martensitic aging stainless steel under different heat treatment conditions in Example 2 of this invention.

[0037] Figure 6 Microstructure of martensitic aging stainless steel obtained by the heat treatment method of Example 2 of the present invention: (a) inverse pole figure; (b) phase diagram;

[0038] Figure 7 The graph shows the mechanical properties of martensitic aging stainless steel under different heat treatment conditions in Example 3 of this invention.

[0039] Figure 8 Microstructure of martensitic aging stainless steel obtained by the heat treatment method of Example 3 of the present invention: (a) inverse pole figure; (b) phase diagram;

[0040] Figure 9 The mechanical properties of PH13-8Mo under different heat treatment processes in Comparative Example 1 are shown in the figure.

[0041] Figure 10 The mechanical properties of PH13-8Mo under different heat treatment processes in Comparative Example 2 are shown in the figure. Detailed Implementation

[0042] (I) The heat treatment method for martensitic aging stainless steel of the present invention and the martensitic aging stainless steel

[0043] Traditional heat treatment methods for martensitic aging stainless steel employ a solid-quenching-aging process. Common standard aging temperatures, such as 525℃, 540℃, and 595℃, are used depending on the required tensile strength and elongation. After solution treatment, martensitic aging stainless steel forms a low-carbon martensite matrix, laying the microstructural foundation for the precipitation of strengthening phases during the subsequent aging process. This alloy allows for flexible control of strength and toughness by adjusting aging parameters. At 525℃ aging, the tensile strength can reach over 1500 MPa, and the hardness exceeds 45 HRC, exhibiting excellent mechanical properties. This performance control strategy based on the synergistic optimization of composition and heat treatment makes martensitic aging stainless steel an important representative of high-strength and high-toughness stainless steel systems.

[0044] To meet the long-term service stability requirements of modern high-precision structural components, this invention further enhances their strength and toughness through optimized heat treatment methods. Based on the traditional solution-aging heat treatment method, this invention introduces cyclic deep cryogenic treatment after solution treatment to eliminate residual austenite in the martensitic aging stainless steel after quenching to the greatest extent possible. Furthermore, by combining sub-temperature heat treatment and quenching processes, dispersed reverse-transformed austenite is introduced, optimizing the microstructure of the material, thereby enabling the martensitic aging stainless steel to exhibit a superior synergistic effect of strength and toughness.

[0045] The above-mentioned heat treatment method for martensitic aging stainless steel is shown in the flowchart below. Figure 1 As shown, the following steps are taken:

[0046] (1) Solution treatment

[0047] The martensitic aging stainless steel raw material to be treated is placed in an environment with a temperature of 810~1000℃ for solution treatment, and the holding time is 30~90min, followed by quenching.

[0048] The processing temperature and holding time in this step are consistent with conventional heat treatment methods, with a holding temperature error range of ±5℃ and a holding time error range of ±3min. After solution treatment, quenching is performed, and the ratio of the volume of the quenching cooling medium to the volume of the material being treated is controlled to be above 100:1, for example, it can be 100~500:1.

[0049] The solution treatment is further preferably performed at 920~930℃, with a holding time of 40~50min. The furnace temperature of the heat treatment furnace should be as precise as possible, with a temperature error of ±5℃ and a holding time error of ±3min, to maximize the heat treatment stability of martensitic aging stainless steel.

[0050] By controlling the volume ratio of the quenching cooling medium to the material being treated within the above range, the temperature rise of the medium caused by the heat released by the sample can be effectively suppressed, thereby avoiding the influence of the quenching structure transformation process due to local boiling of the medium or a decrease in cooling capacity, and ensuring that the heat treatment results have good uniformity and reproducibility.

[0051] (2) Cyclic cryogenic treatment

[0052] The quenched martensitic aging stainless steel material is placed in an environment of -40℃ to -196℃ for 1 to 9 hours. After the treatment, the material is placed in a room temperature environment. After the temperature returns to room temperature, the material is placed in a cryogenic environment for the same amount of time. Then the material is placed in a room temperature environment to return to room temperature. The cryogenic cycle is repeated 1 to 6 times.

[0053] In this step, cyclic cryogenic treatment is performed at temperatures ranging from -40℃ to -196℃, with each holding time lasting 1 to 9 hours. This two-stage cryogenic treatment method decomposes the transformation process from retained austenite to martensite into multiple progressive stages, effectively dispersing and mitigating local lattice distortion caused by the intense phase transformation during a single cryogenic treatment. Simultaneously, cyclic thermal stress provides a continuous driving force for dislocation reorganization, promoting dynamic recovery in high dislocation density regions and forming lower-energy dislocation cellular structures, thereby directly reducing the micro-orientation gradient. The significant decrease in the KAM value (nuclear average orientation difference) indicates that the micro-strain and long-range internal stress within the material have been effectively relaxed. This structural optimization helps improve the material's fracture toughness, fatigue crack initiation resistance, and high-cycle fatigue performance. Therefore, cyclic cryogenic treatment can provide a matrix structure with an ideal low-strain state for subsequent aging, laying the microstructural foundation for achieving a synergistic improvement in strength and plasticity.

[0054] In this step, the cryogenic environment should be kept as constant as possible during the cryogenic treatment. Preferably, the cryogenic treatment temperature is -90℃ to -60℃, and the holding time is greater than 3 hours. More preferably, the cryogenic treatment temperature is -70℃ to -60℃, and the holding time is 3 to 4 hours.

[0055] The time after returning to room temperature is 0.5h-1h.

[0056] (3) Sub-temperature treatment

[0057] The martensitic aging stainless steel material after cryogenic treatment is subjected to sub-temperature treatment at 700~880℃ for 30-480 minutes, followed by quenching.

[0058] In this step, the heat treatment process of holding at 700~880℃ for 30-480min is defined as sub-temperature treatment. Sub-temperature treatment can induce the formation of a large amount of Ni-enriched reverse austenite in martensitic aging stainless steel (martensitic precipitation hardening stainless steel), which can effectively improve its plasticity without affecting or even improving its strength.

[0059] During sub-critical heat treatment, the temperature error should be controlled within ±5℃, and the holding time error should be controlled within ±10min. Different holding times can be selected according to different strength and plasticity requirements. During sub-critical heat treatment, the elongation of the material first increases and then decreases with increasing holding time, while the tensile strength also shows a trend of first decreasing and then slightly increasing. Generally, holding at 760±5℃ for 1h±10min yields martensitic aging stainless steel with good overall mechanical properties.

[0060] (4) Timeliness processing

[0061] The martensitic aging stainless steel material after sub-temperature treatment is subjected to aging treatment at a temperature 10~30℃ lower than the standard temperature, preferably 15~30℃ lower than the standard temperature, and the holding time is 4h±10min.

[0062] Depending on the application environment and strength requirements, the commonly used tensile strength grades for martensitic aging stainless steel are above 1500MPa, above 1400MPa, and above 1000MPa, with corresponding standard aging temperatures of 525℃, 540℃, and 595℃, respectively. The aging treatment holding time is 4 hours for all of them. After sub-temperature treatment, aging treatment should be carried out at 510℃±5℃, 525℃±5℃, and 580℃±5℃.

[0063] The preferred embodiments described above are illustrated below with specific examples. In the following examples and comparative examples, during the cryogenic treatment stage, the temperature was maintained at room temperature for 0.5 hours.

[0064] Example 1

[0065] This embodiment describes the heat treatment method for maraging stainless steel with a tensile strength of 1400 MPa, using the following steps:

[0066] S1. According to the standard specifications, sample blanks are oriented and cut from the PH13-8Mo plate to be tested. (See [reference]) Figure 2 The blank is precision machined to the specified geometric dimensions. The sample is then mechanically polished using wet sandpaper with mesh sizes of 240#, 800#, 1500#, and 2500# to ensure the surface quality of the sample. Before testing, the original width and thickness of the parallel length segment of the sample are accurately measured using precision measuring tools, its initial cross-sectional area (S0) is calculated, and the original gauge length (L0) is marked on the sample surface.

[0067] S2. Technical optimization was carried out on martensitic aging stainless steel with a tensile strength of 1400MPa. Based on traditional heat treatment methods, a cryogenic-sub-temperature composite heat treatment method was developed, as shown in Table 1.

[0068] Table 1 Composite Heat Treatment Methods

[0069]

[0070] In step S2, the quenching method for solution treatment is oil quenching, the quenching method for sub-temperature treatment is oil quenching, and the quenching method for aging treatment is air cooling.

[0071] S3. Mount the heat-treated mechanical sample into the fixture of the universal testing machine. Ensure the tensile axis of the sample coincides with the force axis of the testing machine using a precision alignment device. At the start of the test, securely mount the extensometer to the gauge length of the sample to accurately measure micro-deformation. Set the loading regime in the control system: typically, strain rate or stress rate control is used before the expected yield strength is reached, switching to a constant beam displacement rate after yielding. The testing machine applies a monotonically increasing tensile load according to the set program, and the data acquisition system synchronously records the load-displacement data until the sample fractures. After fracture, remove the sample and measure its post-fracture gauge length to calculate the post-fracture elongation. Based on the collected raw data, the engineering stress-strain curve can be calculated, and the key mechanical property parameters of the material can be evaluated accordingly.

[0072] The mechanical property test results of the heat treatment method and the traditional heat treatment method in this embodiment are shown in Table 2. The mechanical curves are shown in [reference needed]. Figure 3 .

[0073] Table 2 Composite Heat Treatment Methods

[0074]

[0075] As can be seen from the results in Table 2, the tensile strength of the heat treatment method in the embodiment is basically maintained, while the elongation after fracture is increased from about 11% to 15%.

[0076] The martensitic aging stainless steel of this embodiment is obtained by the above-described heat treatment method. Its microstructure and phase distribution are as follows: Figure 4 As shown.

[0077] Depend on Figure 4 It is known that the reversible austenite is distributed in a needle-like form along the martensite lath boundaries and exhibits excellent phase stability. Nickel, as a strong austenite stabilizing element, significantly reduces the phase transformation driving force of the reversible austenite to martensite transformation, thus maintaining its stability under service conditions. The reversible austenite is dispersed between the martensite laths in a needle-like or film-like morphology, which can effectively divide the martensite lath bundles, producing fine grain strengthening and interface strengthening effects. At the same time, its discontinuous distribution characteristics prevent the formation of through channels by the soft phase, thus maintaining the strength of the matrix. The induced reversible austenite, as a functional phase, is distributed along the martensite lath boundaries in a film-like morphology, forming a semi-continuous "ductile encapsulation layer" structure. When the propagating microcrack enters the ductile film from the brittle martensite matrix, the crack tip is blunted, and the stress concentration is effectively relieved. At the same time, the crack needs to consume additional energy to cross this film layer, thereby improving the fracture toughness and crack propagation resistance of the material.

[0078] Compared to isolated residual austenite, the thin-film distribution of reverse-transformed austenite increases the interfacial bonding area and bonding strength with the matrix, effectively suppressing interfacial delamination. Furthermore, this discontinuous thin-film distribution avoids the formation of large-area continuous soft regions in the matrix, ensuring that the introduction of a ductile phase does not significantly impair the overall strength of the material, achieving a good synergy between strength and toughness.

[0079] Example 2

[0080] This embodiment describes the heat treatment method for maraging stainless steel with a tensile strength of 1000 MPa, using the following steps:

[0081] S1. According to the standard specifications, sample blanks are oriented and cut from the PH13-8Mo plate to be tested. (See [reference]) Figure 2 The blank is precision machined to the specified geometric dimensions. The sample is then mechanically polished using wet sandpaper with mesh sizes of 240#, 800#, 1500#, and 2500# to ensure the surface quality of the sample. Before testing, the original width and thickness of the parallel length segment of the sample are accurately measured using precision measuring tools, its initial cross-sectional area (S0) is calculated, and the original gauge length (L0) is marked on the sample surface.

[0082] S2. Technical optimization was carried out on martensitic aging stainless steel with a tensile strength greater than 1000MPa. Based on traditional heat treatment methods, a cryogenic-sub-temperature composite heat treatment method was developed, as shown in Table 3.

[0083] Table 3 Composite Heat Treatment Methods

[0084]

[0085] In step S2, the quenching method for solution treatment is oil quenching, the quenching method for sub-temperature treatment is oil quenching, and the quenching method for aging treatment is air cooling.

[0086] S3. The heat-treated mechanical sample is mounted in the fixture of the universal testing machine, ensuring that the tensile axis of the sample coincides with the force axis of the testing machine using a precision alignment device. At the start of the test, the extensometer is securely mounted in the gauge length of the sample to accurately measure micro-deformation. The loading regime is set in the control system: typically, strain rate or stress rate control is used before the expected yield strength is reached, switching to a constant beam displacement rate after yielding. The testing machine applies a monotonically increasing tensile load according to the set program, and the data acquisition system synchronously records the load-displacement data until the sample fractures. After fracture, the sample is removed, and its post-fracture gauge length is measured to calculate the post-fracture elongation. Based on the collected raw data, the engineering stress-strain curve can be calculated, and the key mechanical property parameters of the material can be evaluated accordingly.

[0087] The mechanical property test results of the heat treatment method and the traditional heat treatment method in this embodiment are shown in Table 4. The mechanical curves are shown in [reference needed]. Figure 5 .

[0088] Table 4 Composite Heat Treatment Methods

[0089]

[0090] As shown in Table 4, for 1000MPa martensitic aging stainless steel, the elongation after fracture of the heat treatment method in the embodiment is the same as that of the conventional heat treatment, and the tensile strength is increased from 1086MPa to 1189MPa. That is, while maintaining a high elongation after fracture, the tensile strength is further improved.

[0091] The martensitic aging stainless steel of this embodiment is obtained by the above-described heat treatment method, and the corresponding PH13-8Mo microstructure is as follows: Figure 6 As shown, (a) and (b) are the inverse pole diagram and phase diagram, respectively.

[0092] Depend on Figure 6 It can be seen that austenite is dispersed around the martensite block, forming a continuous tough coating layer, which can hinder crack propagation and thus achieve a synergistic improvement in the strength and toughness of the material.

[0093] Example 3

[0094] This embodiment describes the heat treatment method for maraging stainless steel with a tensile strength of 1500 MPa, using the following steps:

[0095] S1. According to the standard specifications, sample blanks are oriented and cut from the PH13-8Mo plate to be tested. (See [reference]) Figure 2 The blank is precision machined to the specified geometric dimensions. The sample is then mechanically polished using wet sandpaper with mesh sizes of 240#, 800#, 1500#, and 2500# to ensure the surface quality of the sample. Before testing, the original width and thickness of the parallel length segment of the sample are accurately measured using precision measuring tools, its initial cross-sectional area (S0) is calculated, and the original gauge length (L0) is marked on the sample surface.

[0096] S2. Technical optimization was carried out on martensitic aging stainless steel with a tensile strength of 1500MPa. Based on traditional heat treatment methods, a cryogenic-sub-temperature composite heat treatment method was developed, as shown in Table 5.

[0097] Table 5 Composite Heat Treatment Methods

[0098]

[0099] In step S2, the quenching method for solution treatment is oil quenching, the quenching method for sub-temperature treatment is oil quenching, and the quenching method for aging treatment is air cooling.

[0100] S3. The heat-treated mechanical sample is mounted in the fixture of the universal testing machine, ensuring that the tensile axis of the sample coincides with the force axis of the testing machine using a precision alignment device. At the start of the test, the extensometer is securely mounted in the gauge length of the sample to accurately measure micro-deformation. The loading regime is set in the control system: typically, strain rate or stress rate control is used before the expected yield strength is reached, switching to a constant beam displacement rate after yielding. The testing machine applies a monotonically increasing tensile load according to the set program, and the data acquisition system synchronously records the load-displacement data until the sample fractures. After fracture, the sample is removed, and its post-fracture gauge length is measured to calculate the post-fracture elongation. Based on the collected raw data, the engineering stress-strain curve can be calculated, and the key mechanical property parameters of the material can be evaluated accordingly.

[0101] The mechanical property test results of the heat treatment method and the traditional heat treatment method in this embodiment are shown in Table 6. The mechanical curves are shown in [reference needed]. Figure 7 .

[0102] Table 6 Composite Heat Treatment Methods

[0103]

[0104] As shown in Table 6, for 1500MPa martensitic aging stainless steel, the heat treatment method of the embodiment improves the tensile strength and elongation after fracture to a certain extent compared with the traditional heat treatment method.

[0105] The martensitic aging stainless steel of this embodiment is obtained by the above-described heat treatment method. S+C+I+A 510℃ Microstructure of the processed material as shown in the figure. Figure 8 As shown.

[0106] Figure 8After undergoing a cryogenic-sub-temperature annealing composite process, the amount of austenite further increases, the overall integral number rises, and the average size decreases to approximately 0.1 μm. This small-sized austenite is distributed along the martensite lath boundaries in a thin-film morphology, forming a semi-continuous "ductile encapsulation layer" structure. When a propagating microcrack enters the ductile film from the brittle martensite matrix, the crack tip is blunted, effectively alleviating stress concentration. Simultaneously, the crack traversing this film layer requires additional energy, thereby enhancing the material's fracture toughness and crack propagation resistance. Compared to bulk austenite, the thin-film morphology increases the interfacial bonding area and bonding strength with the matrix, effectively suppressing interfacial delamination. Furthermore, this discontinuous thin-film distribution avoids the formation of large-area continuous soft regions in the matrix, ensuring that the introduction of a ductile phase does not significantly impair the overall strength of the material, achieving a good synergy between strength and toughness.

[0107] (ii) Comparative Example

[0108] Comparative Example 1

[0109] To evaluate the effect of sub-temperature-cryogenic composite heat treatment on the macroscopic mechanical properties of PH13-8Mo, samples treated with different processes were prepared and then subjected to room temperature tensile tests under the same conditions. The results are as follows: Figure 9 As shown. Different processing techniques are named S+A respectively. 525℃ S+A 540℃ S+I+A 525℃ S+C+I+A 525℃ The specific heat treatment process is shown in Table 7.

[0110] Table 7 Different heat treatment processes

[0111]

[0112] The aging treatment at 525℃ (S+A) shown in the figure 525℃ Although PH13-8Mo exhibits high tensile strength after treatment, its elongation is poor. This is further addressed by introducing a sub-temperature treatment process (S+I+A). 525℃ After treatment, its strength decreased by only 160 MPa, while its elongation after fracture increased by approximately 75%. This is in contrast to the traditional industrial solution treatment at 927℃ followed by aging at 540℃ (S+A). 540℃ Introducing sub-temperature treatment before quenching and controlling its holding time can effectively improve the ductility of the material, increasing the elongation after fracture by approximately 40%. Furthermore, employing a cryogenic-sub-temperature composite process (S+C+I+A) can further enhance the material's ductility. 525℃ After treatment, the ductility of PH13-8Mo was further improved, with the elongation after fracture increasing by about 50%, while the tensile strength remained at a similar level to that after solution-aging treatment, achieving a good synergy between strength and plasticity.

[0113] In the cryogenic-sub-thermal composite process, after cyclic cryogenic treatment (-70°C / 4h), a large amount of retained austenite transforms into martensite. After cyclic cooling, the content of retained austenite is reduced to the theoretical limit. Cyclic cryogenic treatment effectively eliminates the non-uniform micro-stress generated during quenching through repeated thermal stress. The homogenized stress promotes the more synchronous and thorough transformation of retained austenite in different regions. Based on this, PH13-8Mo is subjected to sub-thermal treatment, holding it in the martensite and austenite dual-phase region for 1h, which promotes the segregation of elements such as Ni and Mo in the martensite, reduces the austenite nucleation barrier, and thus forms relatively stable reversible austenite. The small-sized, continuously distributed reversible austenite film can form a tough coating layer around the martensite block, improving the fracture toughness and crack propagation resistance of the material. Therefore, after cryogenic-sub-thermal composite treatment, PH13-8Mo has a tensile strength close to that after aging at 540°C, while its elongation after fracture can be significantly improved.

[0114] Comparative Example 2

[0115] This comparative study investigated the optimal heat treatment process for PH13-8Mo with tensile strength Rm = 1410-1560 MPa. The heat treatment process was formulated as shown in Table 8 below. The mechanical properties of PH13-8Mo after heat treatment are as follows: Figure 10 As shown.

[0116] Table 8 Composite Heat Treatment Process

[0117]

[0118] according to Figure 10 The mechanical properties shown are obtained after S+C+I 60min +A 580℃ PH13-8Mo processed by this method is compared with the conventional heat treatment process S+A 595℃ In comparison, the tensile strength increased by approximately 180 MPa, while the elongation after fracture remained essentially unchanged, demonstrating superior tensile strength and ductility.

[0119] Through the above technical solution, this invention can effectively adjust the size, content, and distribution of reverse-transformed austenite, inducing a small-sized, continuously distributed reverse-transformed austenite film to form a "tough coating layer" around the martensite block, thereby improving the material's fracture toughness and crack propagation resistance. Martensitic aging stainless steel treated by the cryogenic-sub-thermal composite method can further improve its toughness while basically maintaining its tensile strength, or significantly improve its tensile strength while maintaining its toughness, or achieve a simultaneous improvement in both tensile strength and toughness, thus further enhancing the comprehensive mechanical properties of martensitic aging stainless steel.

[0120] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A heat treatment method for martensitic aging stainless steel, characterized in that, Includes the following steps: (1) The martensitic aging stainless steel material to be treated is solution treated and then quenched; (2) Place the material treated in step (1) in a cryogenic environment for insulation, and then place it in a room temperature environment to restore it to room temperature, thus completing one cryogenic-room temperature treatment; continue to perform the cryogenic-room temperature treatment. (3) The material treated in step (2) is subjected to sub-temperature treatment at 700~880℃ and then quenched; (4) After sub-temperature treatment, aging treatment is carried out. The aging treatment temperature is 10~30℃ lower than the standard aging temperature, and then quenched.

2. The heat treatment method for martensitic aging stainless steel as described in claim 1, characterized in that, In step (2), the temperature of the cryogenic environment is -40℃ to -196℃, and the heat preservation time is 1 to 9 hours; step (2) involves 2 to 3 cryogenic-room temperature treatments.

3. The heat treatment method for martensitic aging stainless steel as described in claim 2, characterized in that, The temperature of the cryogenic environment is -70℃ to -60℃, and the insulation time is 3h to 4h.

4. The heat treatment method for martensitic aging stainless steel as described in claim 1, characterized in that, In step (3), the sub-temperature treatment temperature is 760±5℃ and the holding time is 1h±10min.

5. The heat treatment method for martensitic aging stainless steel as described in claim 1 or 4, characterized in that, In step (4), for martensitic aging stainless steel with a tensile strength of 1500 MPa, the aging temperature is 510℃±5℃; for martensitic aging stainless steel with a tensile strength of 1400 MPa, the aging temperature is 510~525℃; for martensitic aging stainless steel with a tensile strength of 1000 MPa, the aging temperature is 580℃±5℃; and the holding time for aging is 4h±10min.

6. The heat treatment method for martensitic aging stainless steel as described in claim 1, characterized in that, In step (1), the solution treatment temperature is 810~1000℃ and the holding time is 30~90min.

7. The heat treatment method for martensitic aging stainless steel as described in claim 6, characterized in that, In step (1), the solution treatment temperature is 920~930℃ and the holding time is 40~50min.

8. The heat treatment method for martensitic aging stainless steel as described in claim 1, characterized in that, The martensitic aging stainless steel is PH13-8Mo.

9. A martensitic aging stainless steel obtained by the heat treatment method according to any one of claims 1-8.

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