A heat treatment process for improving the thermal fatigue properties of a special alloy steel

Abstract

The application discloses a heat treatment process for improving the thermal fatigue performance of special alloy steel and belongs to the technical field of metal material heat treatment. In view of the technical defects of the existing special alloy steel under the alternating cold and hot cycle conditions, such as thick martensite structure, continuous segregation of grain boundary carbide, serious accumulation of cyclic thermal stress and low thermal fatigue life, the application adopts a composite heat treatment process of segmented homogeneous preheating, low-temperature supersaturated austenitizing, bainite interval isothermal phase change, stress release gradient tempering and low-temperature lattice stabilization. After the process, the thermal fatigue cycle life of the special alloy steel is improved by more than 135%, the average thermal crack propagation rate is reduced by 55%, the network precipitation rate of the grain boundary carbide is less than 3%, and the residual stress is controlled within ± 25 MPa. The application is suitable for high-end special alloy steel, such as martensitic heat-resistant special steel and high-strength special alloy structural steel, and can be widely applied to the alternating temperature and variable structure members in the fields of aerospace, energy power and precise special equipment.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, and more specifically, relates to a heat treatment process for improving the thermal fatigue properties of special alloy steel. Background Technology

[0002] Special alloy steels, characterized by high purity, precise alloy ratios, and excellent comprehensive mechanical properties, are widely used in harsh operating conditions such as aerospace, energy, military, and high-end equipment. In actual service, special alloy steel components in these fields are often subjected to alternating temperature environments of repeated heating and cooling. Under the action of thermal expansion and contraction cycles, alternating thermal stress is continuously generated inside the material, making it highly susceptible to the initiation of microcracks at grain boundaries, inclusions, and carbide segregation sites. These microcracks gradually propagate into penetrating thermal fatigue cracks, ultimately leading to component failure and fracture, severely impacting the safety and reliability of equipment operation.

[0003] Currently, the conventional heat treatment of special alloy steels in the industry mostly adopts a single quenching followed by a single medium-high temperature tempering process. This process has several technical shortcomings: First, the conventional austenitizing temperature is too high, which easily leads to abnormal growth of austenite grains, generating coarse lath martensite structures, resulting in poor material plasticity and easy dislocation accumulation during thermal cycling. Second, carbides tend to precipitate continuously along grain boundaries in a network, forming brittle grain boundaries, which provides a convenient channel for the rapid propagation of hot cracks. Third, the residual stress inside the workpiece is large after conventional heat treatment. During the alternating hot and cold process, the residual stress and the working thermal stress are superimposed, which will significantly accelerate the initiation of cracks. Fourth, the matrix structure is simple and lacks a tough buffer phase, resulting in weak thermal shock resistance and low thermal fatigue cycle life.

[0004] Existing publicly available heat treatment improvement technologies for special alloy steels mostly focus on enhancing the material's room temperature strength, hardness, and corrosion resistance, with very few addressing targeted microstructural control of the thermal fatigue crack initiation and propagation mechanisms under alternating temperature conditions. Some improvement processes employing isothermal heat treatment, due to inadequate control of the phase transformation range, are prone to generating brittle and undesirable microstructures, failing to meet the long-life and high-reliability service requirements of high-end special components. Therefore, developing a dedicated heat treatment process capable of refining grains, breaking down network carbides, mitigating cyclic thermal stress, and improving thermal fatigue performance has significant engineering application value.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a heat treatment process to improve the thermal fatigue performance of special alloy steel, thereby solving the problems mentioned in the background art.

[0007] To solve the above-mentioned technical problems, the basic concept of the technical solution adopted by the present invention is as follows: A heat treatment process for improving the thermal fatigue properties of special alloy steel includes the following steps: Step (1) Refined pretreatment: Grind, deburr, and ultrasonically degrease the special alloy steel workpiece to remove the surface oxide layer and sharp stress concentration points, ensuring that the surface roughness Ra≤6.3μm; The process parameters for ultrasonic degreasing are: temperature 40~60℃, time 15~30min, use alkaline water-based cleaning agent with pH value of 8.5~10.5, rinse with deionized water more than 3 times after cleaning, and then dry with hot air at 80~100℃ for 10~20min; Step (2) Segmented homogeneous preheating: The workpiece is placed in a sealed protective atmosphere furnace and heated in two stages. The first stage heats the workpiece to 260-320℃ at a rate of 2-5℃ / min and holds it for 40-60min. The second stage heats the workpiece to 410-470℃ at a rate of 3-6℃ / min and holds it for 55-85min. A high-purity inert protective gas is introduced throughout the process. The inert protective gas is high-purity nitrogen or argon with a purity of ≥99.99%. A slight positive pressure of 50-150Pa is maintained in the sealed protective atmosphere furnace throughout the process to prevent external air from seeping in and causing high-temperature oxidation and decarburization of the workpiece. Step (3) Low-temperature supersaturated austenitization: After preheating, the temperature is raised to 930-1010℃ at a constant rate of 4-7℃ / min and held at a constant temperature for 1.4-2.6h to achieve low-temperature supersaturated solid solution of alloying elements; wherein, a low-frequency alternating magnetic field of 0.15-0.7T is applied during the austenitization stage, and the magnetic field frequency is 35-85Hz. Step (4) Bainitic isothermal quenching: After austenitization, the workpiece is quickly transferred to an isothermal cooling medium and cooled to the bainitic transformation range of 330-390℃ at a rate of 10-18℃ / min. It is held isothermally for 30-55 minutes, and then slowly cooled to room temperature at a rate of 2-5℃ / min. The isothermal cooling medium is a neutral salt bath with a constant temperature fluctuation of ≤±2℃. After austenitization, the time for the workpiece to be transferred from the protective atmosphere furnace to the isothermal cooling medium is ≤15s to avoid oxidation of the workpiece surface and precipitation of proeutectoid ferrite at high temperatures. Step (5) Stress-relieving gradient tempering: A three-stage decreasing gradient tempering method is adopted, with air cooling after each tempering stage; First-stage tempering: 540~590℃, held for 1.2~2.4h; Second-stage tempering: 420~480℃, held for 50~95min; Third-stage tempering: 310~370℃, held for 40~70min; The heating rate of each tempering stage is controlled at 3~7℃ / min, and the oxygen content in the furnace air is ≤0.3%; Step (6) Low-temperature lattice stabilization: Place the tempered workpiece in a low-temperature treatment chamber and cool it down to -55 to -90°C at a rate of 4 to 8°C / min. Maintain the temperature for 1.6 to 3.2 hours and allow it to slowly return to room temperature to complete the thermal fatigue modification heat treatment. Dry high-purity nitrogen gas is introduced into the low-temperature treatment chamber, and the natural temperature return rate of the workpiece is controlled at 1 to 3°C / min to prevent secondary thermal stress and frost oxidation on the workpiece surface during the temperature return process.

[0008] Optionally, the special alloy steel is any one of martensitic heat-resistant special steel, precipitation-hardening special alloy steel, high-strength alloy structural steel, and temperature-alternating special alloy steel.

[0009] By adopting the above technical solution, the present invention has the following beneficial effects compared with the prior art. Of course, any product implementing the present invention does not necessarily need to achieve all of the following advantages at the same time: 1. This invention significantly improves the thermal fatigue cycle life of special alloy steel by constructing a multiphase tough matrix of lath martensite and acicular bainite and purifying grain boundaries, making it suitable for the harsh service conditions of long-term alternating temperature changes in aerospace, energy and power and other fields.

[0010] 2. This invention suppresses grain boundary carbide precipitation through low-temperature supersaturated austenitization and breaks up the formed network carbides through three-stage gradient tempering, thereby reducing the average propagation rate of hot cracks and preventing the formation of penetrating coarse hot cracks in the workpiece during service.

[0011] 3. This invention utilizes a multi-level stress relief system that eliminates processing stress through segmented homogeneous preheating, releases phase transformation stress through bainitic isothermal quenching and slow cooling, and eliminates residual quenching stress through three-stage gradient tempering. This system effectively controls the residual stress of the workpiece at an extremely low level, resulting in minimal deformation under repeated thermal cycling, thus meeting the dimensional accuracy requirements of precision special equipment components.

[0012] 4. This invention refines the original grains through low-temperature supersaturated austenitization, eliminates metastable structures and repairs lattice defects through low-temperature lattice stabilization, so that the matrix structure does not show obvious coarsening under thermal cycling conditions, carbides do not aggregate and grow, and the structure has excellent resistance to thermal aging and cycling stability.

[0013] 5. This invention has strong versatility and high industrial adaptability. It is compatible with a variety of high-end special alloy steels, has a high process error tolerance, and can be completed using conventional heat treatment furnaces and low-temperature treatment equipment without the need for expensive customized equipment. It is suitable for mass production of high-end components.

[0014] The specific embodiments of the present invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0015] The accompanying drawings described below are merely some embodiments. Those skilled in the art can obtain other drawings based on these drawings without any creative effort. In the drawings: Figure 1 This is a process flow diagram of the present invention.

[0016] It should be noted that these accompanying drawings and textual descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art by referring to specific embodiments. Detailed Implementation

[0017] The invention will now be described in further detail with reference to the accompanying drawings.

[0018] Please see Figure 1 As shown in this embodiment, a heat treatment process for improving the thermal fatigue properties of special alloy steel is provided, including the following steps: Step (1) Refined pretreatment: Grind, deburr, and ultrasonically degrease the special alloy steel workpiece to remove the surface oxide layer and sharp stress concentration points, ensuring that the surface roughness Ra≤6.3μm; The process parameters for ultrasonic degreasing are: temperature 40~60℃, time 15~30min, use alkaline water-based cleaning agent with pH value of 8.5~10.5, rinse with deionized water more than 3 times after cleaning, and then dry with hot air at 80~100℃ for 10~20min; Step (2) Segmented homogeneous preheating: The workpiece is placed in a sealed protective atmosphere furnace and heated in two stages. The first stage heats the workpiece to 260-320℃ at a rate of 2-5℃ / min and holds it for 40-60min. The second stage heats the workpiece to 410-470℃ at a rate of 3-6℃ / min and holds it for 55-85min. A high-purity inert protective gas is introduced throughout the process. The inert protective gas is high-purity nitrogen or argon with a purity of ≥99.99%. A slight positive pressure of 50-150Pa is maintained in the sealed protective atmosphere furnace throughout the process to prevent external air from seeping in and causing high-temperature oxidation and decarburization of the workpiece. Step (3) Low-temperature supersaturated austenitization: After preheating, the temperature is raised to 930-1010℃ at a constant rate of 4-7℃ / min and held at a constant temperature for 1.4-2.6h to achieve low-temperature supersaturated solid solution of alloying elements; wherein, a low-frequency alternating magnetic field of 0.15-0.7T is applied during the austenitization stage, and the magnetic field frequency is 35-85Hz. Step (4) Bainitic isothermal quenching: After austenitization, the workpiece is quickly transferred to an isothermal cooling medium and cooled to the bainitic transformation range of 330-390℃ at a rate of 10-18℃ / min. It is held isothermally for 30-55 minutes, and then slowly cooled to room temperature at a rate of 2-5℃ / min. The isothermal cooling medium is a neutral salt bath with a constant temperature fluctuation of ≤±2℃. After austenitization, the time for the workpiece to be transferred from the protective atmosphere furnace to the isothermal cooling medium is ≤15s to avoid oxidation of the workpiece surface and precipitation of proeutectoid ferrite at high temperatures. Step (5) Stress-relieving gradient tempering: A three-stage decreasing gradient tempering method is adopted, with air cooling after each tempering stage; First-stage tempering: 540~590℃, held for 1.2~2.4h; Second-stage tempering: 420~480℃, held for 50~95min; Third-stage tempering: 310~370℃, held for 40~70min; The heating rate of each tempering stage is controlled at 3~7℃ / min, and the oxygen content in the furnace air is ≤0.3%; Step (6) Low-temperature lattice stabilization: Place the tempered workpiece in a low-temperature treatment chamber and cool it down to -55 to -90°C at a rate of 4 to 8°C / min. Maintain the temperature for 1.6 to 3.2 hours and allow it to slowly return to room temperature to complete the thermal fatigue modification heat treatment. Dry high-purity nitrogen gas is introduced into the low-temperature treatment chamber, and the natural temperature return rate of the workpiece is controlled at 1 to 3°C / min to prevent secondary thermal stress and frost oxidation on the workpiece surface during the temperature return process.

[0019] Optionally, the special alloy steel is any one of martensitic heat-resistant special steel, precipitation-hardening special alloy steel, high-strength alloy structural steel, and temperature-alternating special alloy steel.

[0020] Example 1: Heat treatment of martensitic heat-resistant special alloy steel Refined pretreatment: The martensitic heat-resistant special alloy steel workpiece is mechanically ground to remove burrs and oxide layers, and then ultrasonically degreased and cleaned. The ultrasonic degreasing process parameters are: temperature 50℃, time 22min, using an alkaline water-based cleaning agent with a pH value of 9.5. After cleaning, it is rinsed 4 times with deionized water and then dried with hot air at 90℃ for 15min. The surface roughness Ra of the workpiece is guaranteed to be ≤6.3μm. Segmented homogeneous preheating: The workpiece is placed in a sealed protective atmosphere furnace, and high-purity nitrogen with a purity of ≥99.99% is introduced as a protective gas; the first stage heats up to 290℃ at a rate of 4℃ / min and holds for 50min; the second stage heats up to 440℃ at a rate of 5℃ / min and holds for 70min. Low-temperature supersaturated austenitization: After preheating, the temperature is uniformly increased to 970℃ at a rate of 5℃ / min, and held at a constant temperature for 2.0h, while a low-frequency alternating magnetic field of 0.4T and 60Hz is applied. Bainitic isothermal quenching: After austenitization, the workpiece is quickly transferred to a neutral salt bath and cooled to 360°C at a rate of 14°C / min, and held isothermally for 42 min; then it is slowly cooled to room temperature at a rate of 3°C / min, and the temperature fluctuation of the salt bath is controlled within ±2°C. Stress-relieving gradient tempering: A three-stage decreasing gradient tempering method is adopted, with the heating rate of each tempering stage controlled at 5℃ / min. The oxygen content in the furnace air is ≤0.3%, and air cooling is performed after each tempering stage. First-stage tempering: 565℃ for 1.8h; Second-stage tempering: 455℃ for 75min; Third-stage tempering: 340℃ for 55min. Low-temperature lattice stabilization: The tempered workpiece is placed in a low-temperature treatment chamber and cooled to -72°C at a rate of 6°C / min. It is then kept at this temperature for 2.4 hours and then allowed to slowly return to room temperature to complete the heat treatment.

[0021] Example 2: Heat treatment of precipitation-hardening special alloy steel Refined pretreatment: Precipitation-hardened special alloy steel workpieces are finely polished and ultrasonically cleaned to remove surface oxide inclusions and oil stains; the ultrasonic degreasing process parameters are: temperature 42℃, time 28min, using an alkaline water-based cleaning agent with a pH value of 8.8, after cleaning, rinsed 3 times with deionized water, and then dried with hot air at 85℃ for 18min; ensuring that the surface roughness Ra≤6.3μm; Segmented homogeneous preheating: The workpiece is placed in a sealed protective atmosphere furnace, and high-purity argon gas with a purity of ≥99.99% is introduced as a protective gas; the first stage heats up to 270℃ at a rate of 3℃ / min and holds for 58min; the second stage heats up to 420℃ at a rate of 4℃ / min and holds for 82min. Low-temperature supersaturated austenitization: After preheating, the temperature is uniformly increased to 940℃ at a rate of 4℃ / min, and held at a constant temperature for 2.5h without applying an alternating magnetic field; Bainitic isothermal quenching: After austenitization, the workpiece is quickly transferred to a neutral salt bath and cooled to 340℃ at a rate of 12℃ / min, and held isothermally for 50 min; then it is slowly cooled to room temperature at a rate of 2.5℃ / min, and the temperature fluctuation of the salt bath is controlled within ±2℃. Stress-relieving gradient tempering: A three-stage decreasing gradient tempering method is adopted, with the heating rate of each tempering stage controlled at 4℃ / min, and the oxygen content in the furnace air ≤0.3%. After each tempering stage, air cooling is performed; First-stage tempering: 545℃ for 2.2h; Second-stage tempering: 430℃ for 90min; Third-stage tempering: 320℃ for 65min. Low-temperature lattice stabilization: The tempered workpiece is placed in a low-temperature treatment chamber and cooled to -60°C at a rate of 5°C / min. It is then kept at this temperature for 3.0 hours and then allowed to slowly return to room temperature to complete the heat treatment.

[0022] Example 3: Heat Treatment of High-Strength Alloy Structural Special Steel Refined pretreatment: The high-strength alloy structural special steel workpiece is ground and deburred, then ultrasonically degreased and cleaned to remove the surface oxide layer; the ultrasonic degreasing process parameters are: temperature 58℃, time 16min, using an alkaline water-based cleaning agent with a pH value of 10.2, after cleaning, rinsed 5 times with deionized water, and then dried with hot air at 95℃ for 12min; ensuring that the surface roughness Ra≤6.3μm; Segmented homogeneous preheating: The workpiece is placed in a sealed protective atmosphere furnace, and high-purity nitrogen with a purity of ≥99.99% is introduced as a protective gas; the first stage heats up to 310℃ at a rate of 5℃ / min and holds for 45min; the second stage heats up to 460℃ at a rate of 6℃ / min and holds for 60min. Low-temperature supersaturated austenitization: After preheating, the temperature is uniformly increased to 1000℃ at a rate of 7℃ / min, and held at a constant temperature for 1.5h, while a low-frequency alternating magnetic field of 0.6T and 75Hz is applied. Bainitic isothermal quenching: After austenitization, the workpiece is quickly transferred to a neutral salt bath and cooled to 380℃ at a rate of 17℃ / min, and held isothermally for 33min; then it is slowly cooled to room temperature at a rate of 4.5℃ / min, and the temperature fluctuation of the salt bath is controlled within ±2℃. Stress-relieving gradient tempering: A three-stage decreasing gradient tempering method is adopted, with the heating rate of each tempering stage controlled at 6℃ / min. The oxygen content in the furnace air is ≤0.3%, and air cooling is performed after each tempering stage. First-stage tempering: 580℃ for 1.4h; Second-stage tempering: 470℃ for 62min; Third-stage tempering: 360℃ for 45min. Low-temperature lattice stabilization: The tempered workpiece is placed in a low-temperature treatment chamber and cooled to -85°C at a rate of 7°C / min. It is then kept at this temperature for 1.8 hours and then allowed to slowly return to room temperature to complete the heat treatment.

[0023] It should be noted that all electrical devices involved in this application can be powered by batteries or external power sources.

[0024] This invention is not limited to the embodiments described above. Anyone should understand that structural changes made under the guidance of this invention, and any technical solutions that are the same as or similar to this invention, fall within the protection scope of this invention. Technical aspects, shapes, and structures not described in detail in this invention are all publicly known technologies.

Claims

1. A heat treatment process for improving the thermal fatigue properties of special alloy steel, characterized in that, Includes the following steps: Step (1) Refined pretreatment: Grind, deburr, and ultrasonically degrease the special alloy steel workpiece to remove the surface oxide layer and sharp stress concentration points, and ensure that the surface roughness Ra≤6.3μm; Step (2) Segmented homogeneous preheating: Place the workpiece in a sealed protective atmosphere furnace and use a two-stage heating method. In the first stage, the temperature is increased to 260-320℃ at 2-5℃ / min and held for 40-60min. In the second stage, the temperature is increased to 410-470℃ at 3-6℃ / min and held for 55-85min. High-purity inert protective gas is introduced throughout the process. Step (3) Low-temperature supersaturated austenitization: After preheating, the temperature is raised to 930-1010℃ at a uniform rate of 4-7℃ / min, and held at a constant temperature for 1.4-2.6h to achieve low-temperature supersaturated solid solution of alloying elements; Step (4) Bainite range isothermal quenching: After austenitization, quickly transfer to an isothermal cooling medium and cool to the bainite phase transformation range of 330-390℃ at 10-18℃ / min. Hold isothermally for 30-55min, and then slowly cool to room temperature at 2-5℃ / min. Step (5) Stress-relieving gradient tempering: Three-stage decreasing gradient tempering is adopted, and air cooling is performed after each tempering stage; First stage tempering: 540~590℃ for 1.2~2.4h; Second stage tempering: 420~480℃ for 50~95min; Third stage tempering: 310~370℃ for 40~70min; Step (6) Low-temperature lattice stabilization: Place the tempered workpiece in a low-temperature treatment chamber and cool it down to -55 to -90℃ at a rate of 4 to 8℃ / min. Maintain the temperature for 1.6 to 3.2 hours and allow it to slowly return to room temperature to complete the thermal fatigue modification heat treatment.

2. The heat treatment process for improving the thermal fatigue properties of special alloy steel according to claim 1, characterized in that, The special alloy steel is any one of martensitic heat-resistant special alloy steel, precipitation-hardening special alloy steel, high-strength alloy structural steel, and temperature-alternating special alloy steel.

3. The heat treatment process for improving the thermal fatigue properties of special alloy steel according to claim 1, characterized in that, The inert protective gas mentioned in step (2) is high-purity nitrogen or argon, with a gas purity ≥ 99.99%.

4. The heat treatment process for improving the thermal fatigue properties of special alloy steel according to claim 1, characterized in that, Step (3) During the austenitization stage, a low-frequency alternating magnetic field of 0.15 to 0.7T is applied, with a magnetic field frequency of 35 to 85Hz.

5. The heat treatment process for improving the thermal fatigue properties of special alloy steel according to claim 1, characterized in that, The isothermal cooling medium mentioned in step (4) is a neutral salt bath, and the temperature fluctuation of the salt bath is ≤±2℃.

6. The heat treatment process for improving the thermal fatigue properties of special alloy steel according to claim 1, characterized in that, In step (5), the heating rate of each stage of tempering is controlled at 3-7℃ / min, and the oxygen content in the furnace air is ≤0.3%.

7. The heat treatment process for improving the thermal fatigue properties of special alloy steel according to claim 1, characterized in that, The process parameters for ultrasonic degreasing in step (1) are: temperature 40-60℃, time 15-30min, use alkaline water-based cleaning agent with pH value of 8.5-10.5, rinse with deionized water more than 3 times after cleaning, and then dry with hot air at 80-100℃ for 10-20min.

8. The heat treatment process for improving the thermal fatigue properties of special alloy steel according to claim 1, characterized in that, In step (2), the sealed protective atmosphere furnace is kept under a slight positive pressure of 50-150 Pa throughout the process to prevent outside air from seeping in and causing high-temperature oxidation and decarburization of the workpiece.

9. The heat treatment process for improving the thermal fatigue properties of special alloy steel according to claim 1, characterized in that, After austenitization in step (4), the time for the workpiece to be transferred from the protective atmosphere furnace to the isothermal cooling medium is ≤15s to avoid oxidation of the workpiece surface and precipitation of proeutectoid ferrite at high temperature.

10. The heat treatment process for improving the thermal fatigue properties of special alloy steel according to claim 1, characterized in that, In step (6), dry high-purity nitrogen gas is introduced into the low-temperature treatment chamber, and the natural temperature recovery rate of the workpiece is controlled at 1 to 3℃ / min to prevent secondary thermal stress and frost oxidation on the workpiece surface during the temperature recovery process.

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