A heat treatment process for OCr17Ni4Cu4Nb bar

Abstract

This invention discloses a heat treatment process for OCr17Ni4Cu4Nb bars, belonging to the field of metal material heat treatment technology, aiming to solve the problems of difficulty in achieving the required hardness in one go and inaccurate parameter matching in existing processes. The process includes three steps: raw material preparation, quenching, and tempering. Hot-rolled, annealed, and polished OCr17Ni4Cu4Nb bars conforming to GJB2294 standard are selected, heated to 1040℃ and held for 2 hours, followed by water quenching. Within 4 hours after quenching, air-cooled tempering is performed according to the target hardness: HRC40~45 corresponds to 450℃~480℃×2h~4h, HRC35~40 corresponds to 500℃~560℃×2h~4h, and HRC31~35 corresponds to 580℃~660℃×2h~4h. This process expands the tempering temperature range to 450℃~660℃, clarifies the matching relationship between parameters and hardness and mechanical properties, results in a high first-pass yield rate, and ensures that the mechanical properties meet the requirements of tensile strength of 800MPa~1450MPa and elongation of 17%~21%. It is easy to operate, requires no additional equipment, and is suitable for bar processing in aerospace and other fields.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for metallic materials, specifically a heat treatment process for OCr17Ni4Cu4Nb bars. Background Technology

[0002] OCr17Ni4Cu4Nb steel, also known as 17-4PH steel, is a martensitic precipitation hardening stainless steel. It is developed from 18-8 type austenitic stainless steel and has high strength, excellent corrosion resistance, oxidation resistance and weldability. Different comprehensive mechanical properties can be obtained by adjusting the tempering temperature and time. It is widely used in high-end equipment manufacturing fields such as aerospace. In actual production, when using existing enterprise standard processes to heat-treat OCr17Ni4Cu4Nb bars, the hardness after the first quenching and tempering is generally higher than the required value, requiring rework at a higher tempering temperature to achieve qualification. However, when heat-treating according to the GJB2294-1995 standard, the hardness after the first quenching and tempering is lower than the required value, requiring rework at a lower tempering temperature to achieve qualification. Currently, there is insufficient accumulated parameters and experience regarding the quenching and tempering heat treatment of OCr17Ni4Cu4Nb bars, and a lack of precise matching relationships between different tempering temperatures and hardness and properties. This makes it difficult to deliver qualified products for inspection on the first attempt, seriously affecting production efficiency and increasing production costs. Therefore, we propose a heat treatment process for OCr17Ni4Cu4Nb bars. Summary of the Invention

[0003] The purpose of this invention is to provide a heat treatment process for OCr17Ni4Cu4Nb bars, which solves the problems mentioned in the background art.

[0004] To achieve the above objectives, the present invention provides the following technical solution: a heat treatment process for OCr17Ni4Cu4Nb bars, comprising the following process steps: Step 1: Raw material preparation: Select OCr17Ni4Cu4Nb precipitation hardening stainless steel bars conforming to GJB2294 technical standards. The delivery condition should be hot-rolled and annealed / polished. The chemical composition should meet the following requirements: Fe 70.49~78.85%, Cr 15.00~17.00%, Ni 3.00~5.00%, Cu 3.00~5.00%, Nb 0.15~0.45%, C≤0.07%. Step 2, Quenching treatment: Place the bar stock from Step 1 into the heating equipment, heat it to 1040℃, hold it at that temperature for 2 hours, and then use water cooling for rapid cooling. During the cooling process, ensure that the bar stock is heated and cooled evenly to avoid stress concentration. Step 3, Tempering: Within 4 hours after quenching, the bar stock is tempered. After tempering, it is cooled by air cooling. The appropriate tempering temperature and holding time are selected according to the target hardness requirements. Specifically: If the target hardness is HRC40~45, the tempering temperature should be controlled at 450℃~480℃, and the holding time should be 2h~4h; If the target hardness is HRC35~40, the tempering temperature should be controlled at 500℃~560℃, and the holding time should be 2h~4h. If the target hardness is HRC31~35, the tempering temperature should be controlled at 580℃~660℃, and the holding time should be 2h~4h.

[0005] In a preferred embodiment of the present invention, the diameter of the OCr17Ni4Cu4Nb rod in step one is φ30mm~φ50mm and the length is 50mm~100mm.

[0006] In a preferred embodiment of the present invention, the heating device in step two is a high-temperature box furnace. The effective heating area of ​​the high-temperature box furnace is 1200×600×400mm, the maximum temperature is 1250℃, the working temperature range is 750~1200℃, the furnace chamber has a square structure, and resistance wire heating is used.

[0007] In a preferred embodiment of the present invention, the tempering treatment in step three uses a medium-temperature box furnace or a pit furnace; wherein, the effective heating zone of the medium-temperature box furnace is 1200×600×400mm, the maximum temperature is 1000℃, and the working temperature is 600~950℃; the effective heating zone of the pit furnace is φ500×650mm or φ800×1000mm, and the corresponding working temperature ranges are 400~800℃ or 100~650℃, respectively.

[0008] In a preferred embodiment of the present invention, during the tempering treatment in step three, when the tempering temperature is in the range of 560℃ to 660℃, the bar stock does not exhibit temper brittleness, and its impact performance improves with increasing tempering temperature.

[0009] In a preferred embodiment of the present invention, the cooling rate of water cooling in step two is not less than 10°C / s to ensure that no compounds precipitate in the austenite body and to ensure that the tempering precipitation strengthening effect is fully exerted.

[0010] As a preferred embodiment of the present invention, the mechanical properties of the tempered bar stock meet the following requirements: tensile strength 800MPa~1450MPa, yield strength 700MPa~1250MPa, elongation 17%~21%, reduction of area 55%~70%, and impact toughness 45J~190J.

[0011] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention achieves precise matching of the hardness of OCr17Ni4Cu4Nb bars by accurately controlling the quenching temperature, holding time, and cooling method, combined with the tempering temperature and holding time range corresponding to different target hardnesses. It can meet various hardness requirements in actual production, such as HRC25~30, HRC31~35, and HRC31~38, thereby improving the first-pass yield rate and effectively reducing rework costs. This invention broadens the research range of tempering temperature to 450℃~660℃, breaks through the temperature limitations of existing standards and enterprise standards, and clarifies the matching relationship between different tempering temperatures and hardness and mechanical properties. As the tempering temperature increases, the tensile strength and yield strength of the bar stock decrease, while the elongation, reduction of area and impact toughness gradually increase. Process parameters can be flexibly selected according to the mechanical performance requirements of the actual application scenario. The process parameters of this invention are clear and the operation is simple. The heating equipment and cooling methods used are all existing mature equipment and processes, which do not require additional investment in new equipment. This makes it easy to promote and apply in production, effectively improve production efficiency, and ensure product quality stability. Attached Figure Description

[0012] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a flow chart of the heat treatment process for OCr17Ni4Cu4Nb bars according to the present invention. Detailed Implementation

[0013] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments. Example 1: Target hardness HRC40-45

[0014] Raw material selection: OCr17Ni4Cu4Nb bar stock of specification 1 (diameter φ30mm, length 50mm). After spectral analysis, its chemical composition is Fe 75.23%, Cr 16.15%, Ni 4.02%, Cu 3.87%, Nb 0.31%, C 0.05%, which meets the standard requirements. Ultrasonic testing showed no internal defects, and the dimensional tolerance is φ30.0mm×50.1mm, which meets the processing requirements.

[0015] Quenching Treatment: Place the cleaned bars evenly on the rack of the high-temperature box furnace, ensuring a minimum spacing of 20mm between bars to prevent interference during heating. Close the furnace door and set the heating program: raise the temperature from room temperature to 1040℃ at a rate of 15℃ / min, and hold at the set temperature for 2 hours. After holding, quickly open the furnace door, remove the bars, and immerse them in the water cooling system at a rate of 12℃ / s. During cooling, use a mechanical agitator to circulate the water, ensuring uniform cooling of all parts of the bars and preventing stress concentration. Cool for 15 minutes, until the bar temperature drops to room temperature.

[0016] Tempering: Within 3 hours after quenching, place the bar stock into a medium-temperature box furnace preheated to 460℃ and hold for 3 hours. Record the furnace temperature every 30 minutes during the holding period to ensure the temperature remains stable within the range of 460±3℃. After tempering, open the furnace door, remove the bar stock, and allow it to cool naturally in the air. During the cooling process, avoid contact with humid environments or collisions with hard objects.

[0017] Performance testing and microscopic tissue observation: Hardness testing: Three test points were selected at each end and the middle of the bar using a Rockwell hardness tester. The measurement results were HRC42.1, HRC42.7, HRC42.4, HRC42.3, HRC42.6, and HRC42.5, with an average value of HRC42.5. This is within the target hardness range of HRC40 to 45, indicating good hardness uniformity.

[0018] Mechanical property testing: Standard tensile and impact specimens were cut and tested by a universal testing machine. The tensile strength was 1380 MPa, the yield strength was 1150 MPa, the elongation was 17.5%, and the reduction of area was 58%. The impact toughness was 52 J. All mechanical properties met the technical requirements.

[0019] Microstructure observation: Metallographic microscopy revealed that the bar stock's microstructure consisted of tempered martensite with a small amount of retained austenite. The microstructure was uniform and fine, with no obvious grain growth. No precipitated compounds were found in the austenite, indicating that the quenching cooling rate was reasonably controlled, laying a good foundation for tempering precipitation strengthening. Example 2: Target hardness HRC35-40

[0020] Raw material selection: OCr17Ni4Cu4Nb bar stock of specification two (diameter φ40mm, length 80mm). The chemical composition test results are Fe74.89%, Cr15.87%, Ni3.95%, Cu3.76%, Nb0.28%, C0.04%, which meets the standard; ultrasonic flaw detection is qualified, and the size is φ40.1mm×80.0mm, which meets the requirements.

[0021] Quenching treatment: The bar stock is placed in a high-temperature box furnace, using a layered arrangement to ensure smooth heat circulation within the furnace. The heating program is set as follows: heating rate 12℃ / min, reaching 1040℃ and holding for 2 hours. After holding, the bar stock is quickly transferred to a water-cooling system, with a cooling rate maintained at 11℃ / s for 20 minutes, until the bar stock temperature drops to room temperature. The temperature change of the bar stock is monitored in real time during the cooling process to ensure uniform cooling.

[0022] Tempering: Within 2.5 hours after quenching, place the bar stock into a pit furnace (effective heating zone φ500×650mm) preheated to 530℃ and hold for 3.5 hours. The pit furnace uses a vertical suspension method to place the bar stock, ensuring more uniform heating. During the holding period, regularly check the furnace temperature to ensure it remains stable at 530±3℃. After tempering, remove the bar stock and air cool to room temperature.

[0023] Performance testing and microscopic tissue observation: Hardness testing: Six test points were selected at different parts of the bar stock, and the measured hardness values ​​were HRC37.2, HRC37.9, HRC37.6, HRC38.0, HRC37.7 and HRC37.8, with an average value of HRC37.8, which meets the requirement of target hardness of HRC35 to 40 and the hardness distribution is uniform.

[0024] Mechanical property testing: Tensile test results showed that the tensile strength was 1120MPa, the yield strength was 980MPa, the elongation was 19.2%, the reduction of area was 63%, and the impact toughness was 105J. All mechanical property indicators met the technical standards, and the plasticity and toughness were improved compared with Example 1.

[0025] Microstructure observation: The microstructure consists of tempered martensite and diffusely distributed precipitates. The precipitates are uniformly precipitated, effectively strengthening the structure, and there are no abnormal defects in the microstructure. Example 3: Target hardness HRC31~35

[0026] Raw material selection: OCr17Ni4Cu4Nb bar stock of specification three (diameter φ50mm, length 100mm), with a chemical composition of Fe73.98%, Cr15.92%, Ni4.15%, Cu3.91%, Nb0.33%, and C0.06%, which meets the standard requirements; ultrasonic testing showed no internal defects, and the dimensions are φ50.2mm×100.1mm, which meets the processing requirements.

[0027] Quenching treatment: Place the bar stock into a high-temperature box furnace. The rack is made of high-temperature resistant alloy material, and the distance between the bar stock and the furnace wall should be no less than 30mm. Set the heating program: heating rate 10℃ / min, slowly rise to 1040℃, hold for 2 hours to ensure sufficient internal heating of the large-sized bar stock. After holding, quickly immerse the bar stock in a water cooling system, controlling the cooling rate at 10.5℃ / s, cooling time 25 minutes, until room temperature. During the cooling process, use multiple sets of nozzles to spray water simultaneously to ensure uniform cooling of the bar stock surface and avoid uneven cooling due to its large size.

[0028] Tempering treatment: Within 3.5 hours after quenching, place the bar stock into a medium-temperature box furnace preheated to 620℃ and hold for 2.5 hours. During the holding period, strictly control the temperature fluctuation inside the furnace to ensure it remains within the range of 620±3℃. After tempering, remove the bar stock and air-cool it to room temperature. During the air-cooling process, cover the bar stock to prevent dust adhesion and surface oxidation.

[0029] Performance testing and microscopic tissue observation: Hardness testing: Two testing points were selected at each end, the middle, and 1 / 4 of the length of the bar stock, for a total of eight testing points. The measured hardness values ​​were HRC33.0, HRC33.3, HRC33.1, HRC33.4, HRC33.2, HRC33.5, HRC33.1, and HRC33.2, respectively. The average value was HRC33.2, which is within the target hardness range of HRC31 to 35, indicating excellent hardness uniformity.

[0030] Mechanical property testing: tensile strength 950 MPa, yield strength 820 MPa; elongation 20.5%, reduction of area 68%; impact toughness 160 J. In this embodiment, the elongation, reduction of area, and impact toughness of the bar stock are all superior to those of Embodiments 1 and 2, and there is no temper brittleness, which meets the requirements for use in high-toughness scenarios.

[0031] Microstructure observation: The microstructure consists of uniform tempered martensite with a large amount of dispersed precipitate phase. As the tempering temperature increases, the precipitate phase precipitates more fully and the grains become finer, which significantly improves the toughness of the material, consistent with the mechanical property test results.

[0032] In summary, this process has clear parameters, is easy to operate, requires no additional specialized equipment, and utilizes existing mature equipment such as high-temperature box furnaces, medium-temperature box furnaces, and pit furnaces, reducing production input costs and facilitating large-scale application in actual production. Secondly, it boasts a high first-pass yield rate, effectively reducing rework caused by substandard hardness, lowering production costs, and improving production efficiency. Thirdly, the process parameters can be flexibly adjusted according to the actual needs of different fields such as aerospace and machinery manufacturing to meet varying hardness and mechanical property requirements, making it widely applicable.

[0033] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0034] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A heat treatment process for OCr17Ni4Cu4Nb bars, characterized in that: The process includes the following steps: Step 1: Raw material preparation: Select OCr17Ni4Cu4Nb precipitation hardening stainless steel bars conforming to GJB2294 technical standards. The delivery condition should be hot-rolled and annealed / polished. The chemical composition should meet the following requirements: Fe 70.49~78.85%, Cr 15.00~17.00%, Ni 3.00~5.00%, Cu 3.00~5.00%, Nb 0.15~0.45%, C≤0.07%. Step 2, Quenching treatment: Place the bar stock from Step 1 into the heating equipment, heat it to 1040℃, hold it at that temperature for 2 hours, and then use water cooling for rapid cooling. During the cooling process, ensure that the bar stock is heated and cooled evenly to avoid stress concentration. Step 3, Tempering: Within 4 hours after quenching, the bar stock is tempered. After tempering, it is cooled by air cooling. The appropriate tempering temperature and holding time are selected according to the target hardness requirements. Specifically: If the target hardness is HRC40~45, the tempering temperature should be controlled at 450℃~480℃, and the holding time should be 2h~4h; If the target hardness is HRC35~40, the tempering temperature should be controlled at 500℃~560℃, and the holding time should be 2h~4h. If the target hardness is HRC31~35, the tempering temperature should be controlled at 580℃~660℃, and the holding time should be 2h~4h.

2. The heat treatment process for OCr17Ni4Cu4Nb bar stock according to claim 1, characterized in that: The diameter of the OCr17Ni4Cu4Nb rods mentioned in step one is φ30mm~φ50mm, and the length is 50mm~100mm.

3. The heat treatment process for OCr17Ni4Cu4Nb rods according to claim 1, characterized in that: The heating equipment mentioned in step two is a high-temperature box furnace. The effective heating area of ​​the high-temperature box furnace is 1200×600×400mm, the maximum temperature is 1250℃, the working temperature range is 750~1200℃, the furnace chamber has a square structure, and resistance wire heating is used.

4. The heat treatment process for OCr17Ni4Cu4Nb bar stock according to claim 1, characterized in that: The tempering process described in step three uses a medium-temperature box furnace or a pit furnace. The medium-temperature box furnace has an effective heating zone of 1200×600×400mm, a maximum temperature of 1000℃, and an operating temperature of 600~950℃. The pit furnace has an effective heating zone of φ500×650mm or φ800×1000mm, with corresponding operating temperature ranges of 400~800℃ or 100~650℃, respectively.

5. The heat treatment process for OCr17Ni4Cu4Nb rods according to claim 1, characterized in that: During the tempering process in step three, when the tempering temperature is in the range of 560℃ to 660℃, the bar stock does not exhibit temper brittleness, and its impact performance improves with increasing tempering temperature.

6. The heat treatment process for OCr17Ni4Cu4Nb bar stock according to claim 1, characterized in that: In step two, the cooling rate of water cooling should not be less than 10℃ / s to ensure that no compounds precipitate in the austenite and to ensure that the tempering precipitation strengthening effect is fully utilized.

7. The heat treatment process for OCr17Ni4Cu4Nb bar stock according to claim 1, characterized in that: The mechanical properties of the tempered bars meet the following requirements: tensile strength 800MPa~1450MPa, yield strength 700MPa~1250MPa, elongation 17%~21%, reduction of area 55%~70%, and impact toughness 45J~190J.

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