Heat treatment process for improving comprehensive mechanical properties of KD-grade sucker rod steel

Abstract

The invention discloses a heat treatment process for improving comprehensive mechanical properties of KD-grade sucker rod steel. By controlling the austenitizing temperature, the cooling mode and the tempering process parameters, the material can obtain a uniform and refined granular bainite structure and an M + A island structure which is small in size and uniform in distribution, and the comprehensive performance of the steel is remarkably improved. Through the room-temperature furnace entering and tempering process, the tensile strength of the steel reaches 865 MPa, the yield strength reaches 665 MPa, the hardness is 283 HV30, and the impact absorption energy reaches up to 56 J. Compared with the prior art, the impact toughness is remarkably improved, and meanwhile high strength and hardness are kept. The process is simple and easy to implement, suitable for industrial popularization and widely applied to the field of manufacturing of high-strength sucker rods for oil exploitation and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of heat treatment of metal materials, and particularly relates to a heat treatment process for improving the comprehensive mechanical properties of steel for KD-class sucker rods. Background Art

[0002] As a key material in the field of oil exploitation, the performance of steel for KD-class sucker rods directly affects the service life and safety of sucker rods. When the sucker rod works, it needs to bear complex tensile, compressive fatigue loads and environmental corrosion. The harsh service conditions pose strict requirements on its mechanical properties and microstructure. If the comprehensive performance of the sucker rod is insufficient, failure accidents such as early piercing or fracture are likely to occur, which will not only affect the exploitation progress and cause huge economic losses, but also lead to the abandonment of the well drilling and bring serious consequences. Therefore, optimizing the strength, toughness and hardness of steel for sucker rods has important engineering value and economic significance.

[0003] At present, the domestic production of steel for KD-class sucker rods mostly adopts the process of hot rolling + normalizing and tempering. By adding alloying elements such as Cr and Mo to the steel, granular bainite structure can be formed at a lower cooling rate, and the mechanical properties of the material can be further optimized through the normalizing and tempering process. However, the influence mechanism of different heat treatment process parameters on the properties of steel still lacks in-depth research. In particular, there are still technical bottlenecks in how to achieve the best matching of strength and toughness by reasonably controlling the austenitizing temperature, cooling rate and tempering conditions. In addition, research shows that the distribution morphology of M+A islands in bainite structure has a significant impact on impact toughness. However, in the existing process, the uniformity and morphology control of M+A islands are insufficient, which easily leads to a decrease in impact performance. Therefore, exploring a new heat treatment process to improve the comprehensive mechanical properties of steel for KD-class sucker rods has become an urgent technical problem to be solved. Summary of the Invention

[0004] The purpose of the present invention is to provide a heat treatment process for improving the comprehensive mechanical properties of steel for KD-class sucker rods. By controlling the austenitizing and tempering process parameters, the strength-toughness matching of the steel is improved to meet the requirements of high strength, high toughness and high durability of steel for sucker rods under harsh working conditions such as oil exploitation.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A heat treatment process for improving the comprehensive mechanical properties of steel for KD-class sucker rods of the present invention includes the following steps:

[0007] (1) Austenitizing treatment: Put it into a high-temperature box furnace, heat and hold at 865±10°C for 25 - 35 minutes to completely transform the internal structure into uniform austenite; the high-temperature austenitizing treatment ensures the formation of high-quality granular bainite structure during the subsequent cooling process;

[0008] (2) Cooling treatment: After the austenitizing heat preservation is completed, it is rapidly cooled to room temperature in the air;

[0009] (3) Tempering treatment: After cooling, it is placed in a tempering furnace and kept at 580 ± 10 °C for 40 - 50 minutes. The tempering treatment optimizes the bainite structure, making the M + A islands refined and evenly distributed; after tempering is completed, it is cooled to room temperature in the air.

[0010] Furthermore, it also includes (4) Performance detection: Performance detection is carried out on the heat-treated specimen, including tensile strength Rm, yield strength Rp0.2, hardness HV30, and impact energy absorption KV2; the detection is based on relevant national standards to ensure the reliability and consistency of the data.

[0011] Furthermore, the mass percentage content of the chemical composition of the steel for KD grade sucker rods is: C 0.23%, Si 0.28%, Mn 0.79%, P ≤ 0.015%, S ≤ 0.007%, Cr 0.91%, Ni 1.13%, Mo 0.23%, V 0.06%, and the rest is Fe and inevitable trace impurities, and the total mass fraction is 100%.

[0012] Furthermore, in the step (1), it is heated and kept at 865 °C for 30 minutes.

[0013] Furthermore, in the step (2), it is kept at 580 °C for 45 minutes.

[0014] Furthermore, a round bar of steel for KD grade sucker rods with a diameter of 22 mm is selected and cut into small segments with a length of 120 mm according to requirements to ensure uniform specimen size, which is convenient for subsequent heat treatment operations and performance tests.

[0015] Furthermore, after heat treatment, performance detection is carried out: tensile strength Rm 865 MPa, yield strength Rp0.2 665 MPa, hardness 283 HV30, impact energy absorption KV2 56 J.

[0016] Furthermore, after heat treatment, it is observed with an optical microscope, and the structure is granular bainite. The structure is refined and uniform, the number of formed "M + A" islands is large, the size is small, and the shape is mostly granular.

[0017] By controlling the key parameters, the comprehensive mechanical properties of the steel for KD-class sucker rods are significantly improved. The specimen is heated and held at 865 °C for 30 minutes to form a uniform austenite structure, and then rapidly cooled to room temperature. The cooled specimen is placed in a tempering furnace at 580 °C, held for 45 minutes and then air-cooled to room temperature. This process refines the bainite structure and optimizes the distribution state of M+A islands, enabling the material to obtain an ideal strength-ductility matching. The present invention significantly improves the comprehensive properties of the steel through the tempering process with furnace charging at room temperature. The present invention is novel, can improve the problem of low impact performance in the prior art, and at the same time maintain high strength and hardness, and is suitable for industrial application.

[0018] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0019] (1) By optimizing the heat treatment process, especially controlling the austenitizing temperature and cooling conditions, the present invention enables the best matching of the tensile strength, yield strength, hardness and impact toughness of the steel for KD-class sucker rods. The experimental results show that after adopting the process of room temperature cooling + tempering at 580 °C, the tensile strength of the steel can reach 865 MPa, and the impact energy absorption is increased to 56 J, greatly improving the comprehensive properties of the material. (2) The present invention precisely controls the distribution and morphology of M+A islands, ensuring that their sizes are small, shapes are uniform and tend to be spherical, avoiding the stress concentration problems caused by uneven distribution and large blocky morphology of M+A islands in the traditional process, thus significantly improving the impact toughness of the steel and avoiding the risk of brittle fracture in low-temperature environments. (3) The heat treatment process adopted by the present invention has simple procedures, convenient operation, and is easy to realize industrial production with existing heat treatment equipment. This process can optimize the toughness of the material while maintaining high strength and hardness, and is widely applicable to the fields of oil extraction and other engineering fields with high strength requirements, having good economic benefits and application prospects. Specific embodiments

[0020] Example 1: A heat treatment process for improving the comprehensive mechanical properties of the steel for KD-class sucker rods according to the present invention is carried out according to the following steps:

[0021] Select a round bar of the steel for KD-class sucker rods with a diameter of 22 mm, cut it into small sections with a length of 120 mm according to requirements, ensure the uniformity of the specimen size, which is convenient for subsequent heat treatment operations and performance tests. The mass percentage content of its chemical components is: C 0.23%, Si 0.28%, Mn 0.79%, P ≤ 0.015%, S ≤ 0.007%, Cr 0.91%, Ni 1.13%, Mo 0.23%, V 0.06%, and the rest are Fe and inevitable trace impurities, and the total mass fraction is 100%;

[0022] Place the specimen in a high-temperature box furnace, heat it at 865 °C for 30 minutes to ensure that the structure inside the specimen is completely transformed into uniform austenite. The high-temperature austenitization treatment ensures the formation of high-quality granular bainite structure during the subsequent cooling process;

[0023] After the austenitization insulation is completed, quickly cool the specimen in air to room temperature;

[0024] Place the cooled specimen in a tempering furnace and keep it at 580 °C for 45 minutes. The tempering process optimizes the bainite structure, making the M+A islands refined and evenly distributed. After tempering is completed, the specimen is cooled in air to room temperature.

[0025] Perform performance testing on the heat-treated specimen. The tensile strength Rm is 865 MPa, the yield strength Rp0.2 is 665 MPa, the hardness is 283 HV30, and the impact energy absorption KV2 is 56 J. Observed with a metallographic microscope, the structure is granular bainite, the structure is refined and uniform, the number of formed "M+A" islands is large, the size is small, and the shape is mostly granular.

[0026] Comparative Example 1: A heat treatment process for improving the comprehensive mechanical properties of KD grade sucker rod steel of the present invention, and is carried out according to the following steps:

[0027] Select a round bar of KD grade sucker rod steel with a diameter of 22 mm, cut it into small sections with a length of 120 mm according to requirements to ensure uniform specimen size for subsequent heat treatment operations and performance tests. The mass percentage content of its chemical composition is: C 0.23%, Si 0.28%, Mn 0.79%, P≤0.015%, S≤0.007%, Cr 0.91%, Ni 1.13%, Mo 0.23%, V 0.06%, and the rest is Fe and inevitable trace impurities, and the total mass fraction is 100%;

[0028] Place the specimen in a high-temperature box furnace, heat it at 865 °C for 30 minutes to ensure that the structure inside the specimen is completely transformed into uniform austenite. The high-temperature austenitization treatment ensures the formation of high-quality granular bainite structure during the subsequent cooling process;

[0029] After the austenitization insulation is completed, cool the specimen in air to 300 °C;

[0030] Quickly place the specimen cooled to 300 °C in a tempering furnace and keep it at 580 °C for 45 minutes. The tempering process optimizes the bainite structure, making the M+A islands refined and evenly distributed. After tempering is completed, the specimen is cooled in air to room temperature.

[0031] The properties of the heat-treated specimens were tested. The tensile strength Rm was 851 MPa, the yield strength Rp0.2 was 651 MPa, the hardness was 263 HV30, and the impact energy absorbed KV2 was 35.8 J. The microstructure was granular bainite + a very small amount of ferrite. The distribution of "M+A" islands on the bainite was uneven, with different sizes and shapes.

[0032] Comparative Example 2: A heat treatment process for improving the comprehensive mechanical properties of KD-grade sucker rod steel according to the present invention was carried out in the following steps:

[0033] A round bar of KD-grade sucker rod steel with a diameter of 22 mm was selected and cut into small sections with a length of 120 mm according to requirements to ensure uniform specimen size for subsequent heat treatment operations and performance tests. The mass percentage of its chemical composition was: C 0.23%, Si 0.28%, Mn 0.79%, P≤0.015%, S≤0.007%, Cr 0.91%, Ni 1.13%, Mo 0.23%, V 0.06%, and the rest was Fe and unavoidable trace impurities, with a total mass fraction of 100%;

[0034] The specimens were placed in a high-temperature box furnace and heated and held at 865 °C for 30 minutes to completely transform the structure in the specimens into uniform austenite. The high-temperature treatment of austenitization ensured the formation of high-quality granular bainite structure during the subsequent cooling process;

[0035] After the austenitization holding was completed, the specimens were air-cooled to 500 °C;

[0036] The specimens cooled to 500 °C were quickly placed in a tempering furnace and held at 580 °C for 45 minutes. The tempering process optimized the bainite structure, refined the M+A islands and made their distribution uniform. After tempering was completed, the specimens were air-cooled to room temperature.

[0037] The properties of the heat-treated specimens were tested. The tensile strength Rm was 875 MPa, the yield strength Rp0.2 was 632 MPa, the hardness was 297 HV30, and the impact energy absorbed KV2 was 28.6 J. The microstructure was mainly bainite + ferrite, the amount of ferrite increased significantly, the M+A islands in the bainite were large in size, small in number, and had distinct edges and corners.

[0038] It can be seen from the above examples and comparative examples that:

[0039] (1) Compared with Comparative Process 1 (tempering by entering the furnace at 300 °C) and Comparative Process 2 (tempering by entering the furnace at 500 °C), the room-temperature cooling + 580 °C tempering process of the present invention significantly improved the impact energy absorbed by the specimens, from 28.6 J and 35.8 J to 56 J, significantly enhancing the toughness and impact resistance of the material.

[0040] (2) The room-temperature cooling process of the embodiment makes the size of M+A islands in the material small and uniformly distributed, and the morphology tends to be spherical, avoiding the stress concentration phenomenon. Thus, while improving the impact toughness, a relatively high tensile strength and hardness are maintained. In contrast, the tempering process at 500 °C results in an increase in the size of M+A islands and an irregular morphology, and the impact toughness decreases significantly.

[0041] (3) The room-temperature cooling process of the embodiment not only significantly improves the impact toughness but also maintains a high strength (tensile strength of 865 MPa and yield strength of 665 MPa), achieving the best balance between strength and toughness, while Comparative Processes 1 and 2 cannot optimize these two key mechanical properties simultaneously.

[0042] The embodiments described above are only descriptions of the preferred modes of the present invention and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A heat treatment process for improving the comprehensive mechanical properties of steel rods for KD-level pumping units, characterized in that, It includes the following steps: (1) Austenitizing treatment: Put it into a high-temperature box furnace and heat it at 865 ± 10 °C for 25 - 35 minutes to completely transform the structure inside into uniform austenite; The high-temperature austenitizing treatment ensures the formation of high-quality granular bainite structure during the subsequent cooling process; (2) Cooling treatment: After the austenitizing heat preservation is completed, quickly cool it to room temperature in the air; (3) Tempering treatment: After cooling, place it in a tempering furnace and keep it at 580 ± 10 °C for 40 - 50 minutes. The tempering treatment optimizes the bainite structure, making the M + A islands refined and evenly distributed; After tempering is completed, cool it to room temperature in the air.

2. The heat treatment process for improving the comprehensive mechanical properties of KD-level sucker rod steel according to claim 1, characterized in that, It also includes (4) Performance testing: Perform performance testing on the heat-treated specimen, including tensile strength Rm, yield strength Rp0.2, hardness HV30, and impact energy absorption KV2; The testing is based on relevant national standards to ensure the reliability and consistency of the data.

3. The heat treatment process for improving the comprehensive mechanical properties of KD-level sucker rod steel according to claim 1, characterized in that, The mass percentage content of the chemical composition of the steel for KD grade sucker rod is: C 0.23%, Si 0.28%, Mn 0.79%, P ≤ 0.015%, S ≤ 0.007%, Cr 0.91%, Ni 1.13%, Mo 0.23%, V 0.06%, and the rest is Fe and unavoidable trace impurities, with the total mass fraction being 100%.

4. The heat treatment process for improving the comprehensive mechanical properties of steel for KD-class sucker rods according to claim 1, characterized in that, In the step (1), heat it at 865 °C for 30 minutes.

5. The heat treatment process for improving the comprehensive mechanical properties of steel for KD-class sucker rods according to claim 1, characterized in that, In the step (2), keep it at 580 °C for 45 minutes.

6. The heat treatment process for improving the comprehensive mechanical properties of KD grade sucker rod steel according to claim 1, characterized in that, Select a round bar of steel for KD grade sucker rod with a diameter of 22 mm and cut it into small segments with a length of 120 mm according to requirements to ensure uniform specimen size, which is convenient for subsequent heat treatment operations and performance tests.

7. The heat treatment process for improving the comprehensive mechanical properties of steel for KD-level sucker rods according to claim 3, characterized in that, Perform performance testing after heat treatment: Tensile strength Rm 865 MPa, yield strength Rp0.2 665 MPa, hardness 283 HV30, impact energy absorption KV2 56 J.

8. The heat treatment process for improving the comprehensive mechanical properties of KD-grade sucker rod steel according to claim 3 or 7, characterized in that, Observe with a metallographic microscope after heat treatment. The structure is granular bainite, with refined and uniform structure. The number of formed "M + A" islands is large, the size is small, and the shape is mostly granular.