Drill pipe joint for oil drilling and method of manufacturing the same

Abstract

The application discloses a drilling pipe joint for oil drilling and a preparation method thereof, and belongs to the technical field of drilling pipe joints. A laser quenching hardened layer is arranged on the thread guide side and the load bearing side of the joint, the thickness of the hardened layer is distributed in a gradient along the axial direction, and the thickness of the hardened layer in a high wear area is 15%-25% thicker than that in other areas; the high wear area is an area starting from the end of the large end axis and accounting for 15%-30% of the total length of the axis. The application at least solves the technical problems that the existing technology causes the large end to be prone to early failure due to the mismatch between the thickness distribution of the hardened layer and the actual wear working condition, and is prone to brittle cracks due to the difficulty in balancing the hardness and toughness of the hardened layer, so that the thickness of the hardened layer is matched with the wear working condition, the hardness and the toughness are balanced, the wear resistance is improved, the risk of brittle cracks is reduced, and the service life of the joint is prolonged.

Description

Technical Field

[0001] This invention belongs to the field of drill pipe joint technology, specifically relating to a drill pipe joint for oil drilling and its preparation method. Background Technology

[0002] In oil drilling downhole tools, drill pipe accounts for more than 80% of the tubing string length and is a key component for transmitting power and delivering drilling fluid. The tubing string needs to withstand combined stresses such as tension, compression, bending, internal pressure, external pressure, and thermal cycling for extended periods in different well sections. Drill pipes are connected by threads to form the tubing string, and the threaded connection is a critical stress-bearing area. Statistics show that more than 80% of tubing string failures occur at threaded connections. The performance of the threads directly affects the drilling rig's efficiency and safety, and thread failure often begins with thread sticking.

[0003] To prevent thread sticking, current API standards recommend copper plating or phosphating the thread surface and applying thread grease to reduce the likelihood of thread sticking failure. However, this approach does not fundamentally solve the problem. Because after heat treatment according to API physical and chemical performance requirements, the surface hardness of existing connector materials is typically only around 32 HRC. In vibrating working environments, if the thread surface hardness is too low and not wear-resistant, fretting wear can easily occur, leading to sticking. Furthermore, while copper plating and phosphating can evenly cover the thread surface, effectively preventing contact between the thread and other metals and air, reducing the chance of corrosion and oxidation, their production processes are relatively polluting and have gradually been phased out due to environmental restrictions. In addition, to improve the wear resistance of the thread surface, the industry has also tried various surface treatment technologies such as nitriding, coating, and sandblasting. Nitriding can significantly improve the surface hardness of the thread, but the hardness difference between the hardened layer and the substrate is too large and lacks a transition, easily causing deformation and cracks. Coating technology has been tried in the industry, but its effectiveness is unclear and its high cost has prevented widespread application.

[0004] In recent years, laser hardening technology has been increasingly applied to the surface hardening of threads in the oil pipe industry due to its advantages such as high efficiency, energy saving, and minimal deformation. Laser hardening itself has also attracted considerable attention in existing technologies due to its high energy density, rapid heating and cooling rates, small heat-affected zone, and the absence of a quenching medium. However, in practical applications, existing laser hardening processes still have the following drawbacks: First, the distribution of the hardened layer thickness does not match the actual wear conditions. Existing thread surface hardening treatments typically use uniform process parameters, resulting in a roughly consistent hardened layer thickness across all thread teeth. However, for drill pipe joints, the larger end region (especially the first few teeth) experiences the greatest stress and wears relatively quickly along the thread axis. A uniform hardened layer thickness leads to insufficient wear resistance in the larger end region, while an excessively deep hardened layer in the smaller end region may waste material toughness. This prevents the optimal match between wear resistance and matrix toughness, resulting in the overall service life of the joint being limited by early failure in the larger end region.

[0005] Secondly, the mechanical properties of the hardened layer and the substrate are poorly matched. Traditional laser hardening processes primarily aim to maximize surface hardness, typically forming a high-hardness martensitic structure directly on the surface. However, wear resistance requires not only hardness but also a certain degree of toughness to resist impact loads (such as impact wear conditions). The hardened layer formed by existing technologies has too large a difference in hardness between itself and the substrate, with no transition. Under external impact or temperature changes, excessive internal stress causes the hardened layer to easily crack. Especially when the drill bit is subjected to high tensile forces, the hard and brittle surface layer easily becomes a crack initiation point, reducing the overall toughness of the hardened layer and posing a risk of brittle fracture.

[0006] Third, the microstructure is too uniform, making it difficult to balance hardness and toughness. Current laser hardening processes primarily produce a single type of martensite microstructure, lacking a gradient microstructure design. This uniform microstructure cannot simultaneously meet the different requirements of the thread guide side (emphasizing wear resistance and friction reduction) and the load-bearing side (emphasizing wear resistance and impact resistance). If the surface hardness is too high but the toughness is insufficient, under complex downhole vibrations and alternating loads, the hardened layer is prone to peeling or microcrack propagation, which reduces the reliability of the threaded connection.

[0007] Therefore, developing a drill pipe joint for oil drilling and its preparation method to improve the existing problems of hardened layers has become a technical problem that urgently needs to be solved by those skilled in the art.

[0008] It should be noted that this part of the present invention only provides background technology related to the present invention, and does not necessarily constitute prior art or known technology. Summary of the Invention

[0009] This invention provides a drill pipe joint for oil drilling and its preparation method, which at least solves the technical problems of insufficient wear resistance and early failure in the large end area of ​​the drill pipe joint due to the mismatch between the thickness distribution of the hardened layer on the thread surface and the actual wear conditions, as well as the technical problems of the drill pipe joint being prone to brittle cracks when subjected to impact loads due to the difficulty in achieving both hardness and toughness of the hardened layer.

[0010] To achieve the above objectives, in a first aspect, the present invention provides a drill pipe joint for oil drilling, comprising a joint body and a threaded structure disposed at the end of the joint body. The threaded structure includes a tooth crest, a tooth root, and a guide side and a bearing side connecting the tooth crest and the tooth root. The surfaces of the guide side and the bearing side are provided with a hardened layer formed after laser quenching. The thickness of the hardened layer is gradient-distributed along the axial direction of the threaded structure. The thickness of the hardened layer located in the high wear region of the threaded structure is 15% to 25% thicker than the thickness of the hardened layer located in other regions of the threaded structure, excluding the high wear region. The high wear region is defined as the region that starts from the axial end near the large end of the threaded structure and extends from the large end to the small end, accounting for 15% to 30% of the total axial length of the threaded structure.

[0011] Preferably, the high-wear region includes 3 to 6 teeth arranged sequentially from the large end of the thread structure towards the small end, starting from the axial end near the large end of the thread structure.

[0012] Preferably, the thickness of the hardened layer in the high wear area is 0.12mm~0.5mm, and the thickness of the hardened layer in other areas of the thread structure other than the high wear area is 0.1mm~0.42mm.

[0013] Preferably, the hardened layer comprises, from the outside to the inside, a surface layer, a subsurface layer, and a transition layer; the surface layer is composed of acicular martensite and retained austenite, with an average grain size of 1 μm to 5 μm; the subsurface layer is composed of first lath martensite, with an average grain size of 8 μm to 12 μm; and the transition layer is composed of second lath martensite, with an average grain size of 20 μm to 50 μm.

[0014] Preferably, the hardened layer has a hardness gradient from the outside to the inside; the hardness of the surface layer and the transition layer is lower than that of the subsurface layer.

[0015] Preferably, the surface layer has a hardness of 50 HRC to 60 HRC; the subsurface layer has a hardness of 55 HRC to 65 HRC; and the transition layer has a hardness of 45 HRC to 55 HRC.

[0016] Preferably, the material of the joint body is alloy steel, and after heat treatment, the microstructure of the guide side and the bearing side of the joint body is tempered sorbite.

[0017] Preferably, the width of the hardened area on one side of the tooth crest is no more than 0.5 mm, and the surface of the tooth root does not have a hardened layer.

[0018] Secondly, the present invention provides a method for preparing a drill pipe joint for oil drilling, comprising the following steps: S1. Remove oil and dirt from the surface of the threaded structure; S2. Secure the drill pipe joint; S3. Use a laser beam to scan and quench the guide side and the load-bearing side of the threaded structure; Among them, the laser power of teeth in high wear areas is 15% to 20% higher than that of teeth in other areas, or the scanning speed of teeth in high wear areas is 5% to 10% lower than that of teeth in other areas.

[0019] S4. Apply rust-preventive oil to the quenched thread structure.

[0020] Preferably, in step S3, for teeth in high-wear areas, the laser power is 500W~2000W and the scanning speed is 10mm / s~60mm / s; for teeth in other areas, the laser power is 300W~1500W and the scanning speed is 10mm / s~60mm / s; the laser beam spot type is a rectangular spot or a flat-topped circular spot. When the light spot type is a flat-topped circular light spot, the diameter of the flat-topped circular light spot is 85% to 95% of the linear length on the bearing side; or, when the light spot type is a rectangular light spot, the width of the rectangular light spot is 85% to 95% of the linear length on the bearing side.

[0021] Preferably, in step S3, the irradiation angle of the laser beam is 30°~70°.

[0022] Preferably, in step S3, argon gas is used for protection, and the argon gas flow rate is 10L / min~20L / min.

[0023] The beneficial effects of this invention are as follows: This invention effectively solves the problem of mismatch between hardened layer thickness and actual wear conditions in existing technologies by designing a hardened layer thickness that is gradient-distributed along the thread axis. Since the large end region of the drill pipe joint experiences the greatest stress and wears the fastest, this invention specifically sets the hardened layer thickness in this region to be 15%~25% thicker than in other regions. This targeted reinforcement design significantly improves the wear resistance of the large end region, preventing the entire joint from being scrapped due to premature failure of the large end, thereby extending the service life of the drill pipe joint. Simultaneously, the remaining regions maintain a relatively shallow hardened layer, avoiding the impact of an excessively deep hardened layer on the matrix toughness, achieving an optimal match between wear resistance and matrix toughness, resulting in a more balanced overall joint performance.

[0024] Furthermore, this invention, by controlling the laser quenching process, creates a unique microstructure and hardness gradient within the hardened layer, with the peak hardness located in the subsurface layer. This structure gives the surface layer good toughness to buffer contact stress, while the subsurface layer provides the highest hardness to support wear resistance, effectively reducing surface stress concentration and significantly improving the quenched layer's resistance to impact loads, thus reducing the risk of brittle cracking when the drill bit is subjected to large tensile forces. The microstructure gradually transitions from fine-grained martensite to coarse lath martensite and then to tempered sorbite from the surface to the matrix. This smooth transition reduces internal stress at the interface, further enhancing the overall strength and toughness of the joint and ensuring reliability under complex downhole conditions. The gradient microstructure design of this invention, through surface toughness buffering and contact stress homogenization, significantly reduces the tendency of the quenched surface to generate hard flaking debris during wear, improving the wear resistance of the joint and demonstrating significant engineering application value.

[0025] Finally, the laser quenching process used in this invention requires no quenching medium, has a small heat-affected zone, and results in minimal workpiece deformation. Furthermore, differentiated treatment of different areas can be achieved by precisely controlling the laser power and scanning speed, facilitating automated production. Compared to traditional copper plating or phosphating, this invention not only significantly improves the hardness and wear resistance of the thread surface but also avoids environmental pollution, meeting the requirements of green manufacturing. Simultaneously, by controlling the hardening range of the tooth crest and root, the safety hazard of cracking at the tooth root due to the presence of hard and brittle phases is avoided, ensuring the safe and reliable operation of the drill pipe joint under complex downhole conditions. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the structure of the laser-quenched region for external threads provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of the laser-quenched internal thread region provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the thread profile provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of the structure of laser quenching on the external thread guide side provided in an embodiment of the present invention; Figure 5 This is a schematic diagram of the structure of laser quenching on the bearing side of the external thread provided in an embodiment of the present invention; Figure 6A partially enlarged structural diagram of laser quenching on the external thread guide side and the load-bearing side provided in an embodiment of the present invention; Figure 7 This is a schematic diagram of the structure of laser quenching on the guide side of the internal thread provided in an embodiment of the present invention; Figure 8 This is a schematic diagram of the structure of laser quenching on the bearing side of the internal thread provided in an embodiment of the present invention; Figure 9 Electron microscope images of the surface layer of the thread structure after laser quenching of the external thread, provided in an embodiment of the present invention; Figure 10 Electron microscope images of the subsurface layer of the thread structure after laser quenching of the external thread, provided in an embodiment of the present invention; Figure 11 Electron microscope images of the surface layer, subsurface layer, and transition layer of the thread structure after laser quenching of the external thread, provided in an embodiment of the present invention; Figure 12 A hardness comparison diagram of the bearing side and the guide side of the internal thread after laser quenching, provided for an embodiment of the present invention.

[0028] Explanation of reference numerals in the attached figures: 1. External thread; 2. Quenched thread area of ​​external thread; 3. Thread axis; 4. Internal thread; 5. Quenched thread area of ​​internal thread; 6. Thread bearing flank; 7. Thread crest; 8. Thread root arc; 9. Thread guide flank; 10. Thread taper line; 11. External thread laser; 12. Quenching laser beam; 13. Internal thread laser. Detailed Implementation

[0029] In this invention, unless otherwise stated, directional terms such as "up," "down," "left," and "right" are generally understood in conjunction with the accompanying drawings and the directions shown in actual applications.

[0030] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0031] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "beneath" of the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0032] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. The terms "optional" and "discretionary" mean that they may or may not be included (or may or may not be present).

[0033] To achieve the above objectives, in a first aspect, the present invention provides a drill pipe joint for oil drilling, comprising a joint body and a threaded structure disposed at the end of the joint body. The threaded structure includes a tooth crest, a tooth root, and a guide side and a bearing side connecting the tooth crest and the tooth root. The surfaces of the guide side and the bearing side are provided with a hardened layer formed after laser quenching. The thickness of the hardened layer is gradient-distributed along the axial direction of the threaded structure. The thickness of the hardened layer located in the high wear area of ​​the threaded structure is 15% to 25% thicker than the thickness of the hardened layer located in other areas of the threaded structure, except for the high wear area. The thickness can be 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, and any value between them.

[0034] The core objective of the above technical solution is to address the technical problem in existing technologies where the distribution of the hardened layer thickness on the thread surface does not match the actual wear conditions, leading to insufficient wear resistance of drill pipe joints and premature failure in the large-end region. During oil drilling, the threaded connection of the drill pipe joint is a critical stress-bearing part of the tubing string. Statistics show that over 80% of tubing string failures occur at the threaded connection, and thread failure often begins with thread sticking. The drill pipe joint described in this invention typically employs a tapered thread structure, where the large end of the thread structure refers to the end with the larger thread diameter, usually near the joint shoulder, while the small end refers to the end with the smaller thread diameter, usually far from the joint shoulder. During the tightening process, this tapered structure inevitably results in extremely uneven load distribution between the thread teeth.

[0035] Current technologies typically employ uniform process parameters for laser hardening of all thread teeth, resulting in a generally consistent hardened layer thickness across all threads. However, for drill pipe joints, due to taper fit and machining errors along the thread axis, according to thread connection mechanics theory, under axial tensile force, both internal and external threads undergo elastic deformation. This causes the load to concentrate primarily on the first few threads at the larger end of the engagement region. These first few threads in the larger end region bear the majority of the axial load and torque, experiencing the greatest stress and relatively faster wear, while the smaller end region experiences less stress. Furthermore, a uniform hardened layer thickness leads to insufficient wear resistance in the larger end region, while an excessively deep hardened layer in the smaller end region may waste material toughness, failing to achieve a balance between wear resistance and matrix toughness.

[0036] This invention sets a gradient distribution of hardened layer thickness, making the hardened layer in high-wear areas 15% to 25% thicker than in other areas. This range helps balance the strengthening effect with the risk of interfacial stress, achieving a match between wear life and structural integrity. If the thickness difference is less than 15%, the strengthening effect in the large-end area is not significant, and the hardened layer thickness is insufficient to resist fretting wear under high contact stress, failing to effectively delay early wear failure in the large-end area, resulting in limited improvement in joint service life. If the thickness difference is greater than 25%, excessive hardness abruptness and stress concentration will occur at the gradient interface. Due to the differences in thermal expansion coefficients and yield strength between the hardened layer and the substrate, as well as between hardened layers of different thicknesses, excessive thickness difference will cause the interface to become a new stress concentration point. Under alternating loads, the hardened layer may peel off or crack at the interface.

[0037] The high-wear area is the region that starts from the end of the axis near the large end of the thread structure and extends from the large end to the small end, accounting for 15% to 30% of the total length of the thread structure axis. It can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, and any value between them.

[0038] This invention defines the high-wear area as the region accounting for 15% to 30% of the total length of the threaded structure's axis. This proportion corresponds to the number of thread teeth. For conventional API standard drill pipe joint threads, the effective number of engaging teeth is typically between 10 and 25. The 15% to 30% axis length proportion roughly corresponds to the length occupied by the first 3 to 6 thread teeth at the large end. Furthermore, the mechanical basis for defining this area is that when a threaded connection is subjected to axial tensile force and torque, due to the difference in elastic deformation between the internal and external threaded components, the load is distributed extremely unevenly along the thread axis. Most of the load is concentrated on the first few threads at the large end of the engaged end. Usually, the first few threads at the large end bear the highest proportion of the load and are the most severely worn area.

[0039] If the high wear area is less than 15%, the reinforced area is too short in space, causing the transition zone of the hardened layer thickness gradient to be too close to the large end face. This makes it unable to cover the main high-stress wear area, resulting in the reinforced area being insufficient to bear the main load. The first few teeth at the large end will still wear rapidly, failing to demonstrate the advantages of gradient reinforcement. If it is greater than 30%, the reinforced area will extend into the low-stress area. At this point, the load borne by subsequent thread teeth will be significantly reduced. Continuing to increase the thickness or range of the hardened layer will have a diminishing marginal effect on improving the overall wear resistance. This not only wastes process costs, but an excessively thick hardened layer may also reduce the supporting effect of the tough matrix, reduce the joint's ability to resist vibration and impact loads, and increase the risk of brittle fracture.

[0040] In summary, the gradient distribution design of this invention matches the hardened layer thickness with the actual wear conditions, significantly improving the wear resistance of the large end area and preventing the entire joint from being scrapped due to early failure of the large end, thereby extending the service life of the drill pipe joint. At the same time, the remaining areas maintain a relatively shallow hardened layer, avoiding the impact on the toughness of the matrix due to an excessively deep hardened layer, achieving a match between wear resistance and matrix toughness, making the overall performance of the joint more balanced, and effectively solving the problem of early failure of drill pipe joints under complex downhole conditions.

[0041] More preferably, the high-wear region includes 3 to 6 teeth arranged sequentially from the large end of the thread structure to the small end, starting from the axial end near the large end of the thread structure. These teeth can be 3, 4, 5, or 6.

[0042] This invention is based on the stress distribution of drill pipe joint threads, controlling the high-wear area to include 3 to 6 threads. After the drill pipe joint is screwed on, the load is not evenly distributed across all thread threads, but is mainly concentrated on the first few threads at the large end. Studies have shown that the first 3 threads at the large end experience the greatest stress and wear relatively quickly. Controlling the high-wear area to include 3 to 6 threads ensures coverage of the critical area with the greatest stress. If there are fewer than 3 threads, the reinforced area is too small and may not be able to effectively distribute the load, leading to excessive stress concentration and accelerating failure. If there are more than 6 threads, the reinforced area will extend to areas with lower stress, wasting process resources and potentially affecting the thread fit accuracy due to an excessively large hardened area.

[0043] It should be noted that the definition of "high wear area as the region accounting for 15% to 30% of the total axial length of the thread structure" in this invention aims to have broader applicability, independent of specific thread count or pitch specifications, and can cover drill pipe joints of different models and even new thread structures. The definition of "high wear area containing 3 to 6 sequentially arranged teeth" has the advantage of facilitating process control and inspection in actual production. During laser hardening, operators or control systems can more easily determine the power switching critical point by counting the number of thread teeth, eliminating the need for complex length measurements and conversions, thus improving process operability and repeatability.

[0044] Therefore, in the preferred embodiment of the present invention, the two work together to ensure that the high-wear area covers the critical parts with the greatest stress, while avoiding performance imbalance caused by an excessively large or small reinforced area. If a special thread specification is encountered, those skilled in the art can adjust the specific number of teeth within a range of 15% to 30% of the total axial length of the thread structure, or conversely, adjust the proportion of the area within the total axial length within a range of 3 to 6 teeth, based on stress distribution test results; both fall within the technical scope disclosed in this invention.

[0045] Preferably, the thickness of the hardened layer in the high-wear area is 0.12mm to 0.5mm, and can be any value between 0.12mm, 0.15mm, 0.20mm, 0.25mm, 0.30mm, 0.35mm, 0.40mm, 0.45mm, and 0.50mm. The thickness of the hardened layer in other areas of the thread structure besides the high-wear area is 0.1mm to 0.42mm, and can be any value between 0.10mm, 0.15mm, 0.20mm, 0.25mm, 0.30mm, 0.35mm, 0.40mm, and 0.42mm.

[0046] This invention controls the specific numerical range of the hardened layer thickness, ensuring that the values ​​match the aforementioned thickness gradient requirement of 15% to 25%. The hardened layer thickness is positively correlated with laser energy density and negatively correlated with velocity; it can be controlled within this range by adjusting the laser power and scanning speed.

[0047] If the hardened layer thickness in the high-wear area is too small, on the one hand, it cannot form an effective 15%~25% thickness increment based on the baseline of other areas, resulting in insufficient gradient strengthening effect. The wear resistance reserve in the high-wear area cannot be significantly higher than that in other areas, and the advantages of gradient strengthening cannot be reflected. On the other hand, if the hardened layer is too shallow, it is easily worn through during the wear process under high contact stress, losing its protective function and causing the matrix to be directly exposed to the wear environment, accelerating failure. If the hardened layer thickness in the high-wear area is too large, the hardened layer will be too deep, which may lead to excessive heat input into the matrix, causing large thermal deformation, affecting thread accuracy. Moreover, an excessively deep hardened layer will reduce the overall toughness of the joint and increase the risk of brittle fracture. The residual tensile stress inside the hardened layer increases significantly. When the drill bit is subjected to large tensile forces, the excessively thick hardened and brittle layer is prone to become a crack initiation, and the crack is very likely to propagate to the stress concentration at the thread root, causing joint fracture. By controlling the hardened layer thickness in the high-wear area between 0.12mm and 0.5mm, it is helpful to ensure sufficient wear resistance reserve, ensure that the service life of the high-wear area is coordinated with other areas, and avoid toughness loss due to excessive depth.

[0048] If the hardened layer thickness in other areas is too small, the hardened layer will be too shallow. Even under low stress conditions, it will be easily worn through during wear, losing its protective function and causing the matrix to be directly exposed to the wear environment, accelerating failure. Furthermore, it cannot provide a reasonable benchmark for the gradient thickness in high-wear areas. If the hardened layer thickness in other areas is too large, although it improves the wear resistance of those areas, it extends the reinforced area into the low-stress zone. At this point, the load borne by subsequent thread teeth is significantly reduced, and further increasing the hardened layer thickness has a diminishing marginal effect on improving overall wear resistance. This not only wastes process costs but may also affect the overall toughness of the joint due to an excessively deep hardened layer. An excessively deep hardened layer reduces the supporting effect of the tough matrix, lowering the joint's ability to resist vibration and impact loads and increasing the risk of brittle fracture. By controlling the hardened layer thickness in other areas between 0.1mm and 0.42mm, a gradient difference with the high-wear area can be maintained, while ensuring that non-high-wear areas have sufficient toughness to withstand vibration and impact loads.

[0049] In summary, this thickness design necessitates matching the hardened layer to the wear load, which helps achieve ideal hardened layer thickness control. The thickness coordination between high-wear areas and other areas satisfies the wear resistance requirements under high loads at the large end while ensuring toughness reserves at the small end and in the middle region, thus optimizing the overall joint performance.

[0050] Preferably, the hardened layer comprises, from the outside to the inside, a surface layer, a subsurface layer, and a transition layer; the surface layer is composed of acicular martensite and retained austenite, with an average grain size of 1 μm to 5 μm, which can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, or any value between them; the subsurface layer is composed of first lath martensite, with an average grain size of 8 μm to 12 μm, which can be 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, or any value between them; the transition layer is composed of second lath martensite, with an average grain size of 20 μm to 50 μm, which can be 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, or any value between them.

[0051] This invention, through the gradient design of the microstructure of the hardened layer, helps to improve the toughness of the hardened layer and reduce the possibility of brittle cracks when the drill bit is subjected to large tensile forces. This specific three-layer microstructure distribution is determined by the temperature field gradient and cooling rate difference from the surface to the matrix during laser hardening. After laser hardening, the heat output of the joint material is concentrated on the surface, and the surface layer directly receives laser irradiation, resulting in the fastest heating and cooling rate, large undercooling, and high nucleation rate, thus forming fine needle-like martensite. At the same time, due to the local decarburization phenomenon on the thread surface at high temperature, the surface carbon content decreases while the alloying elements are relatively enriched, leading to a lower martensite transformation completion temperature, so that the surface layer retains an appropriate amount of retained austenite. The secondary surface layer is located below the surface, receiving slightly less heat than the surface but more heat than the transition layer, and cooling at a slightly slower rate than the surface, which is conducive to the formation of lath martensite; and the carbon content in this region does not decrease as significantly as on the surface, being in the optimal hardening range, thus becoming the peak hardness region. The transition layer is close to the matrix, where more heat accumulates and the cooling rate is relatively slow, giving the grains enough time to grow, thus forming a second lath martensite with relatively coarse grains.

[0052] Understandably, the surface layer's microstructure consists of acicular martensite and retained austenite, with a grain size controlled between 1 μm and 5 μm. This is because the instantaneous laser heating and cooling process generates fine-grained martensite and a large number of dislocations, introducing compressive stress on the thread surface, improving thread fatigue performance and toughness. The fine-grained structure effectively hinders crack propagation, improves the surface's resistance to contact fatigue, and an appropriate amount of retained austenite can alleviate stress concentration through phase transformation-induced plasticity. If the surface layer grain size is greater than 5 μm, it indicates insufficient cooling rate or excessive heating temperature, which weakens the fine-grained strengthening effect, reduces the hindering effect of grain boundaries on dislocation movement, decreases surface toughness, and may lead to excessively high surface hardness and increased brittleness, making it prone to microcracks. If the surface layer grain size is less than 1 μm, the excessively rapid cooling rate may result in excessively high retained austenite content, leading to insufficient surface hardness, decreased wear resistance, and poor microstructural stability.

[0053] Understandably, the subsurface layer's microstructure consists of first-layer lath martensite with a grain size controlled between 8μm and 12μm. This layer is where the hardness peaks, corresponding to the hardest part of the hardened layer. Lath martensite possesses a high hardness and good toughness combination, enabling it to undertake the main wear-resistant function. The subsurface layer has the highest hardness because the laser center temperature is high, and the surface of the quenched area undergoes localized decarburization at high temperatures, resulting in a relatively higher alloy element content. However, the fine-grained martensite on the surface has a low carbon content and lower hardness. If the subsurface layer grain size is less than 8μm, excessively rapid cooling may lead to excessive residual stress, increasing the risk of cracking. If the subsurface layer grain size is greater than 12μm, the martensite lath bundles become coarser, the hardness decreases significantly, and the wear resistance is insufficient, failing to effectively support the surface layer against wear loads.

[0054] Understandably, the transition layer consists of second-layer lath martensite with grain sizes controlled between 20 μm and 50 μm, arranged along the direction of thermal diffusion. This layer serves as a transition between the hardened layer and the matrix, with gradually coarsening grains and decreasing hardness, forming a downward hardness gradient from the subsurface to the matrix. This layer effectively alleviates the stress abrupt change between the hardened layer and the matrix, preventing deformation and cracking due to excessive hardness differences and lack of transition. If the transition layer grain size is less than 20 μm, the gradient transition is not smooth, and stress concentration at the interface between the hardened layer and the matrix cannot be effectively released, failing to effectively buffer stress. If the transition layer grain size is greater than 50 μm, the transition layer is too thick or the microstructure is too coarse, resulting in an excessively large overall thickness of the hardened layer. This reduces the supporting effect of the tough matrix, lowers the joint's resistance to vibration and impact loads, and increases the risk of brittle fracture.

[0055] In summary, this microstructure, consisting of a surface layer, a subsurface layer, and a transition layer from the outside in, corresponds to fine-grained martensite and a small amount of retained austenite, lath martensite, and coarse lath martensite, respectively, before finally transitioning to the tempered sorbite in the matrix, achieving a smooth transition in microstructure and properties. Combined with the resulting hardness gradient distribution, this effectively reduces surface stress concentration. The surface layer possesses good toughness to buffer contact stress, the subsurface layer provides the highest hardness to support wear resistance, and the transition layer achieves a smooth transition to the matrix, significantly improving the hardened layer's resistance to impact loads and ensuring reliability under complex downhole conditions.

[0056] Preferably, the hardened layer has a hardness gradient from the outside to the inside; the hardness of both the surface layer and the transition layer is lower than that of the subsurface layer. More preferably, the hardness of the subsurface layer is greater than that of the surface layer, and the hardness of the surface layer is greater than that of the transition layer.

[0057] This invention, based on the thermodynamic behavior during laser quenching and the mechanical requirements of thread service conditions, rationally controls the hardness gradient distribution of the hardened layer. After laser quenching, the heat output of the laser is concentrated on the surface, resulting in different microstructures from the thread surface to the substrate due to the different amounts of heat received, thus forming a specific hardness distribution.

[0058] Understandably, the subsurface layer, as the peak hardness region, has a higher hardness than the surface layer and transition layer. This is because the high temperature at the laser center causes localized decarburization on the surface of the quenching area, leading to a decrease in surface carbon content. However, the carbon content of the subsurface layer does not decrease as significantly as that of the surface, remaining within the optimal hardening range. Furthermore, its suitable cooling rate allows for the formation of high-hardness lath martensite. The subsurface layer has the highest hardness, providing strong hardness support for the surface layer while ensuring the overall wear resistance of the thread surface, preventing plastic deformation of the surface layer under high contact stress. If the hardness of the subsurface layer is not higher than that of the surface layer, it indicates that the quenching process parameters are out of control or the carbon content distribution of the material is abnormal. This prevents the formation of effective peak hardness support, resulting in a decrease in the overall load-bearing capacity of the hardened layer, insufficient wear resistance, and inability to resist fretting wear during drilling.

[0059] Understandably, the surface layer hardness is controlled to be lower than that of the subsurface layer. This is partly because surface decarburization naturally reduces hardness, and partly because it helps improve toughness. The surface layer directly bears contact stress and frictional wear. If the surface layer hardness is too high, approaching or exceeding that of the subsurface layer, surface brittleness increases, residual tensile stress levels become too high, and microcracks are easily generated. Especially when the drill bit is subjected to large tensile or impact loads, the hard and brittle surface layer can easily become a crack initiation, leading to spalling or fracture. Therefore, the surface layer maintains a relatively low hardness (but still higher than the transition layer), ensuring basic wear resistance while possessing a certain degree of plasticity to buffer contact stress and improve the toughness of the quenched layer. At the same time, the surface layer hardness is higher than that of the transition layer because the surface layer still needs to bear the main frictional wear task. If the surface layer hardness is too low, the surface wear resistance is insufficient, failing to effectively protect the high-hardness area of ​​the subsurface, leading to accelerated wear rate and shortened joint service life.

[0060] Understandably, the transition layer, acting as a buffer zone between the hardened layer and the substrate, has the lowest hardness, effectively mitigating stress abrupt changes between them. The substrate is tempered sorbite, with low hardness and good toughness. If the transition layer's hardness is too high, approaching that of the subsurface layer, the hardness difference at the interface between the hardened layer and the substrate becomes too large, leading to stress concentration that cannot be effectively released. Under alternating loads, cracks are prone to form at the interface and propagate into the substrate, causing the entire hardened layer to peel off. Therefore, the transition layer has the lowest hardness, achieving a smooth transition to the substrate and ensuring the overall strength and toughness of the joint.

[0061] In summary, this hardness gradient distribution matches the aforementioned microstructure gradient (surface acicular martensite, subsurface lath martensite, and transition layer coarse lath martensite). The subsurface layer provides the highest hardness to support wear resistance, the surface layer has good toughness to buffer contact stress, and the transition layer achieves a smooth transition to the matrix. This hardness distribution effectively reduces surface stress concentration, significantly improves the hardened layer's resistance to impact loads, reduces the possibility of brittle cracks when the drill bit is subjected to large tensile forces, and ensures reliability under complex downhole conditions.

[0062] Preferably, the surface layer has a hardness of 50HRC to 60HRC, which can be any value between 50HRC, 51HRC, 52HRC, 53HRC, 54HRC, 55HRC, 56HRC, 57HRC, 58HRC, 59HRC, 60HRC, and the values ​​therein; the subsurface layer has a hardness of 55HRC to 65HRC, which can be any value between 55HRC, 56HRC, 57HRC, 58HRC, 59HRC, 60HRC, 61HRC, 62HRC, 63HRC, 64HRC, 65HRC, and the values ​​therein; and the transition layer has a hardness of 45HRC to 55HRC, which can be any value between 45HRC, 46HRC, 47HRC, 48HRC, 49HRC, 50HRC, 51HRC, 52HRC, 53HRC, 54HRC, 55HRC, and the values ​​therein.

[0063] This invention controls the specific numerical range of the hardened layer's hardness, ensuring that the three values ​​match the aforementioned hardness gradient requirement: the subsurface layer has the highest hardness, the surface layer the lowest, and the transition layer the lowest. The hardened layer's hardness is related to the laser energy density and cooling rate, and can be controlled within this range by adjusting the laser power, scanning speed, and spot type.

[0064] If the hardness of the subsurface layer is too low, its wear resistance will be insufficient, making it unable to withstand the main wear loads and shortening the service life of the joint. If the hardness of the subsurface layer is too high, the brittleness of the structure will increase, and the residual stress will be too high. When the drill bit is subjected to large tensile forces, it is easy to become a crack initiation and propagate into the matrix, increasing the risk of brittle fracture. By controlling the hardness of the subsurface layer between 55HRC and 65HRC, it is helpful to ensure the highest hardness support while avoiding the loss of toughness due to excessive hardness.

[0065] If the surface layer hardness is too low, the wear resistance will be insufficient, and it will not be able to effectively protect the high-hardness area of ​​the subsurface; if the surface layer hardness is too high, the gradient buffering advantage will be lost, the surface brittleness will increase, stress concentration will not be effectively relieved, and microcracks will easily occur. By controlling the surface layer hardness between 50HRC and 60HRC, basic wear resistance is ensured, while also having a certain degree of plasticity to buffer contact stress.

[0066] If the transition layer has too low a hardness, it will provide insufficient support and will not be able to effectively support the surface hardened layer. If the transition layer has too high a hardness, the hardness difference between the hardened layer and the substrate interface will be too large, causing stress concentration that cannot be effectively released. This will easily lead to cracks at the interface that propagate into the substrate, resulting in the entire hardened layer peeling off. By controlling the hardness of the transition layer to between 45 HRC and 55 HRC, a smooth transition to the substrate can be achieved, ensuring the overall strength and toughness of the joint.

[0067] In summary, this hardness design achieves optimal engineering matching of the hardness of the surface layer, subsurface layer, and transition layer. The subsurface layer provides the highest hardness to support wear resistance, while the surface layer and transition layer provide toughness buffering and stress transition, effectively reducing surface stress concentration and significantly improving the ability of the hardened layer to resist impact loads.

[0068] This invention achieves the aforementioned specific gradient microstructure through precise control of the surface thermal cycling process. Firstly, during quenching, the material surface cools fastest, exhibiting high undercooling and nucleation rate, thus forming fine acicular martensite (1μm~5μm). As the thickness increases, the cooling rate gradually decreases, and the grain growth time increases, sequentially forming coarser lath martensite (8μm~12μm) and second lath martensite (20μm~50μm). Existing techniques often suffer from improper energy density control (e.g., using a Gaussian spot leading to excessively high local energy), easily causing surface grain coarsening or melting, making it impossible to obtain such a fine and clearly gradient microstructure. This invention, by employing a rectangular or flat-topped circular spot of specific size combined with a specific power and speed window, ensures the uniformity of energy input and the controllability of the thermal cycle, thereby stably obtaining this grain gradient.

[0069] Furthermore, during the heating process, slight, controllable decarburization occurs on the threaded surface, resulting in a slightly lower carbon content on the surface compared to the subsurface. Simultaneously, a suitable amount of retained austenite is retained in the fine-grained martensite on the surface to reduce brittleness. Meanwhile, the subsurface is in the optimal hardening range, with no significant reduction in carbon content and a suitable cooling rate, thus forming a peak hardness. Existing technologies typically pursue maximum surface hardness, often neglecting the impact of decarburization on surface toughness, leading to a hard, brittle surface prone to cracking. This invention utilizes this physical phenomenon to intentionally construct a mechanical structure that buffers surface toughness, supports subsurface hardness, and facilitates stress transition in the transition layer.

[0070] It should be further clarified that the limitation on microstructure in this invention refers to the core protected object being the drill pipe joint product itself possessing this specific performance structure. The embodiments of this invention provide a preferred process using a continuous fiber laser with specific parameters; however, those skilled in the art should understand that any technical means capable of forming the same or equivalent microstructure distribution on the thread surface by controlling the surface thermal cycling process falls within the scope of this invention. For example, besides the preferred technical solution provided by this invention, a pulsed laser can be used to achieve more refined control of surface heat input by adjusting the pulse frequency, duty cycle, and peak power, which may also achieve a similar grain refinement effect. Using lasers of different wavelengths (such as CO2 lasers), employing light-absorbing coatings for auxiliary heating, using other protective gases such as nitrogen, or preheating the workpiece to adjust the cooling rate may also achieve a similar grain refinement effect. Regardless of the alternative solution mentioned above, as long as the microstructure characteristics required by this invention are ultimately obtained (i.e., 1μm~5μm acicular martensite on the surface layer, 8μm~12μm lath martensite in the subsurface layer, 20μm~50μm lath martensite in the transition layer, and the hardness peak located in the subsurface), they should all be considered equivalent alternatives to this invention. Therefore, this invention provides a preferred preparation method, but it should not be construed as limiting the only path to obtaining this specific microstructure. Equivalent process substitutions made by those skilled in the art based on the microstructure-property matching principle disclosed in this invention, combined with conventional heat treatment knowledge, should all be included within the scope of protection of this invention.

[0071] Preferably, the material of the joint body is alloy steel, more preferably medium carbon alloy steel, such as 4145H, 4137H or 4140 alloy steel. After heat treatment, the microstructure of the guide side and the bearing side of the joint body is tempered sorbite.

[0072] The reasonable selection of joint body material and control of matrix structure in this invention help ensure the stable performance of the surface hardened layer and also help achieve the aforementioned hardened layer thickness gradient and microstructure gradient.

[0073] Understandably, alloy steel possesses excellent hardenability and comprehensive mechanical properties, enabling it to withstand the complex loads encountered during drilling. Alloying elements such as Cr and Mo in alloy steel enhance its hardenability and corrosion resistance, contributing to a longer overall service life of the joint. If ordinary carbon steel is used, its hardenability is insufficient, making it difficult to form a stable martensitic structure during the rapid cooling process of laser quenching, resulting in large fluctuations in the hardened layer thickness. While high-alloy steel offers even better hardenability, its cost is prohibitively high, and weldability and toughness may decrease, increasing manufacturing difficulty and crack susceptibility. Therefore, selecting medium-carbon alloy steel is the optimal choice for achieving the best balance between performance and cost.

[0074] Understandably, after heat treatment, the microstructure of the guide and load-bearing sides of the joint body is tempered martensite. This is because tempered martensite has a balanced strength and toughness, good thermal conductivity, and can provide good matrix support for the laser-hardened layer on the surface. The tempered martensite microstructure can form a good performance match with the hardened layer on the surface, avoiding interface failure caused by excessive differences in the properties of the matrix and the hardened layer. The uniformity of the matrix microstructure also directly affects the laser hardening effect. If the matrix microstructure is not uniform, it will lead to inconsistent laser absorption rates, resulting in fluctuations in the thickness and hardness of the hardened layer, making it impossible to guarantee the stability of the thickness gradient between high-wear areas and other areas. If the matrix structure is pearlite or ferrite, the strength is insufficient and it cannot withstand high loads. Furthermore, the austenitizing temperature range is unstable during laser heating, resulting in a mixed structure in the hardened layer. If the matrix structure is quenched martensite, the toughness is insufficient and it is prone to brittle fracture. In addition, excessive matrix hardness will reduce the laser energy absorption rate, requiring a significant increase in laser power to reach the austenitizing temperature. This will increase heat input, leading to excessive workpiece deformation and affecting thread accuracy.

[0075] In summary, controlling the material of the joint body to be alloy steel and the matrix structure to be tempered sorbite further enhances the effectiveness of the technical solution of this invention. This matrix state not only ensures the overall strength and toughness of the joint body but also provides a uniform thermophysical performance basis for surface laser quenching, enabling the aforementioned hardened layer thickness gradient, microstructure gradient, and hardness gradient to be stably reproduced, thus ensuring reliability under complex downhole conditions.

[0076] Preferably, the width of the hardened portion on one side of the tooth crest is no greater than 0.5 mm, and can be 0 mm, 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, or any value between them; the surface of the tooth root does not have a hardened layer. More preferably, no hardened layer is formed on either the tooth crest or the tooth root. This invention reasonably controls the thickness of the hardened layer at the tooth crest and tooth root, which helps to reduce the safety hazard of thread root cracking and ensures the structural integrity of the joint under complex downhole conditions.

[0077] It should be noted that, as Figure 3 As shown, the width W of the hardened area on one side of the tooth crest in this application is specified in the context of process implementation as the width of the hardened area that inevitably forms at the tooth crest during the scanning quenching process of the thread guide side and the bearing side, due to material thermal conduction and energy overflow at the edge of the light spot.

[0078] Understandably, existing laser hardening processes often struggle to precisely control the hardening zone, resulting in a hardened layer at the thread root. This hardened layer at the thread root is typically martensite, which is hard but brittle, containing high-density dislocations and substructures, and exhibiting poor toughness. Drill pipe joints require tightening to the recommended tightening torque, at which point the average stress at the critical section is approximately 60% of the material's yield strength. Stress distribution analysis reveals stress concentration at the thread root, with a stress concentration factor of approximately 1.67. After tightening, the local stress level at the thread root can reach the material's yield strength. During oil drilling and production, the tubing string bears not only torque loads but also tensile, bending, and vibration loads. During operation, the stress at the thread root can easily exceed the material's yield strength.

[0079] If a hardened, brittle phase exists at the thread root, under the combined action of internal stress and external load, the hardened, brittle layer will significantly reduce the plastic deformation capacity at the thread root fillet, making it highly susceptible to material yielding and cracking at the thread root, and even leading to joint fracture, posing a significant safety hazard. Therefore, this invention controls the width of the hardened portion on one side of the thread crest to be no more than 0.5 mm, and the surface of the thread root does not have a hardened layer, thereby effectively reducing the risk of cracking and improving the safety of the joint. More preferably, no hardened layer is formed on either the thread crest or the thread root surface, which can eliminate the crack initiation source caused by the hard, brittle phase at the thread root, further contributing to ensuring joint safety.

[0080] Secondly, the present invention provides a method for preparing a drill pipe joint for oil drilling, comprising the following steps: S1. Remove oil and dirt from the surface of the threaded structure; S2. Secure the drill pipe joint; S3. Use a laser beam to scan and quench the guide side and the load-bearing side of the threaded structure; Among them, the laser power of teeth in the high wear area is 15% to 20% higher than that of teeth in other areas, which can be 15%, 16%, 17%, 18%, 19%, 20% and any value between them; or the scanning speed of teeth in the high wear area is 5% to 10% lower than that of teeth in other areas, which can be 5%, 6%, 7%, 8%, 9%, 10% and any value between them.

[0081] S4. Apply rust-preventive oil to the quenched thread structure.

[0082] This invention provides a preferred preparation method that helps to stably obtain the above-mentioned product structure. Reasonable process control of each step further helps to obtain the thickness gradient, microstructure gradient and hardness gradient of the final hardened layer required by this invention.

[0083] Understandably, removing oil from the threaded structure surface in step S1 is necessary because oil absorbs laser energy, leading to uneven heating and even localized overheating, thus affecting the quenching quality. Laser quenching relies on the stable absorption of laser energy by the material surface. If contaminants are present on the surface, inconsistent laser absorption rates will result in fluctuations in the thickness and hardness of the hardened layer, making it impossible to guarantee the stability of the thickness gradient between high-wear areas and other areas. Therefore, a clean surface helps achieve precise gradient quenching.

[0084] Understandably, fixing the drill pipe joint in step S2 to ensure coaxiality is to reduce the risk of deviation in the thread flank quenching position due to excessive coaxiality error. The laser spot size is usually matched with the thread flank width. If the coaxiality error is too large, the laser focus will deviate from the center of the flank, and some energy will irradiate the crest or root of the thread. This will not only fail to form an effective flank hardening layer, but may also increase the risk of cracking due to accidental hardening of the root.

[0085] Understandably, laser quenching in step S3 is the core step. The gradient distribution of the hardened layer thickness is achieved by adjusting the laser power and scanning speed. The hardened layer thickness is positively correlated with the laser energy density and negatively correlated with the speed. When the thickness increases by 20%, the power can be increased by 15%~20% or the linear speed can be appropriately reduced by 5%~10% to compensate for heat conduction loss.

[0086] The laser power in the high-wear area is 15% to 20% higher than that in other areas, which helps to achieve a gradient distribution of the hardened layer thickness. Power directly determines the energy input per unit time and is approximately linearly positively correlated with the hardened layer thickness. If the power difference is less than 15%, the energy input increment is insufficient, and a significant thickness gradient cannot be formed, thus failing to demonstrate the advantages of gradient strengthening. If the power difference is greater than 20%, it may lead to overheating and melting or quenching cracks in the high-wear area, and excessive energy input may result in an excessively wide heat-affected zone, affecting thread accuracy.

[0087] The scanning speed of teeth in high-wear areas is 5% to 10% lower than that of teeth in other areas. This is another way to achieve a gradient distribution of hardened layer thickness. The speed determines the laser interaction time and is sensitive to the effect of heat accumulation. If the speed difference is less than 5%, the increment of interaction time is insufficient, the heat accumulation effect is not obvious, and an effective thickness gradient cannot be formed. If the speed difference is greater than 10%, it may lead to excessive heat input in high-wear areas, grain coarsening, decreased toughness, and reduced production efficiency.

[0088] By setting the power of the toothed area in the high-wear region of the thread structure and the laser power of the other toothed areas in the control system, or by adjusting the scanning speed, the requirement of different laser power in different quenching areas in one stroke can be achieved, which is easy to realize automated production.

[0089] Understandably, applying anti-rust oil to the quenched threaded structure in step S4 is to prevent corrosion in untreated areas and ensure the safety of the joint during storage and transportation. After laser quenching, the surface is in a high-energy state. If not protected in time, it is prone to react with moisture and oxygen in the environment to form surface rust, affecting subsequent performance.

[0090] In summary, the present invention, through the coordinated steps of S1 to S4, particularly the precise control of the power difference (15%~20%) or speed difference (5%~10%) in S3, helps to stably obtain drill pipe joints with gradient hardened layer thickness. This process parameter design ensures sufficient hardened layer thickness increment in high-wear areas while avoiding overheating or insufficient gradient caused by process fluctuations.

[0091] Preferably, for teeth in high-wear areas, the laser power is 500W~2000W, which can be 500W, 800W, 1000W, 1200W, 1500W, 1800W, 2000W and any value between them, and the scanning speed is 10mm / s~60mm / s, which can be 10mm / s, 15mm / s, 20mm / s, 25mm / s, 30mm / s, 35mm / s, 40mm / s, 45mm / s, 50mm / s, 55mm / s, 60mm / s and above. For teeth in other areas, the laser power is 300W~1500W, which can be 300W, 500W, 800W, 1000W, 1200W, 1500W and any value between them, and the scanning speed is 10mm / s~60mm / s, which can be 10mm / s, 15mm / s, 20mm / s, 25mm / s, 30mm / s, 35mm / s, 40mm / s, 45mm / s, 50mm / s, 55mm / s, 60mm / s and any value between them.

[0092] The specific numerical range of laser power and scanning speed controlled by this invention helps to stably achieve the aforementioned product characteristic of "the hardened layer thickness in high-wear areas being 15%~25% thicker than in other areas". The hardened layer thickness is positively correlated with laser energy density and negatively correlated with scanning speed. By precisely matching the combination of power and speed, an ideal hardened layer thickness can be obtained while avoiding the melting of the substrate.

[0093] Understandably, high-wear areas require a deeper hardened layer to provide sufficient wear resistance reserves, thus necessitating higher power (500W~2000W) or lower speed (10mm / s~60mm / s). If the laser power is less than 500W or the scanning speed is greater than 60mm / s, the energy input is insufficient, the material surface cannot reach the austenitization temperature, or the cooling rate is insufficient, resulting in a shallow hardened layer. Consequently, the wear resistance of high-wear areas cannot be significantly higher than other areas, failing to demonstrate the advantages of gradient strengthening. If the laser power is greater than 2000W or the scanning speed is less than 10mm / s, the energy input is excessive, potentially leading to material surface melting, burning, or quenching cracks. Furthermore, excessive heat input can result in an excessively wide heat-affected zone, affecting thread accuracy and increasing the risk of accidental hardening at the thread root. By controlling the power in high-wear areas between 500W~2000W and the speed between 10mm / s~60mm / s, it is more helpful to ensure sufficient hardened layer thickness and avoid defects caused by extreme process parameters.

[0094] Understandably, other areas experience less stress and require a relatively shallow hardened layer to preserve the toughness of the matrix. Therefore, lower power (300W~1500W) or higher speed (10mm / s~60mm / s) is used. If the laser power is less than 300W, the energy density is below the critical quenching threshold, making it impossible to form a continuous and effective hardened layer, resulting in the matrix being directly exposed to the wear environment. If the laser power is greater than 1500W or the scanning speed is less than 10mm / s, the hardened layer in other areas will be too deep, not only wasting process costs but also reducing the supporting effect of the tough matrix due to the overall excessive hardened layer, thus reducing the joint's ability to resist vibration and impact loads. By controlling the power in other areas to be between 300W and 1500W and the speed to be between 10mm / s and 60mm / s, it is easier to maintain the gradient difference with the high-wear areas while ensuring that the non-high-wear areas have sufficient toughness.

[0095] In summary, this process parameter design enables engineering matching of the hardened layer thickness in high-wear areas with other areas. High-wear areas achieve a thicker hardened layer through high power or low speed, while other areas retain toughness through low power or high speed. This is more conducive to the stable production of drill pipe joints with gradient hardened layer thicknesses.

[0096] Preferably, in step S3, when the light spot type is a flat-top circular light spot, the diameter of the flat-top circular light spot is 85% to 95% of the linear length of the bearing side, and can be 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95% and any value between them. Alternatively, when the light spot type is a rectangular light spot, the width of the rectangular light spot is 85% to 95% of the linear length on the bearing side, and can be 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95% and any value between them.

[0097] This invention, based on the geometric characteristics of the thread profile and the control of laser energy boundaries, selects the linear length of the thread bearing side as the reference for the spot size (the diameter of a flat-topped circular spot or the width of a rectangular spot). In API standard tapered thread structures, the bearing side is typically the narrow face of the thread profile, and its linear length is less than that of the guide side. This area is not only a critical contact surface bearing the main axial load and torque transmission during screwing and tightening, but also a high-incidence area for fretting wear and sticking failure in actual drilling conditions. Setting the spot size based on the linear length of the bearing side ensures that the laser energy covers this narrowest and most critical stress area, while utilizing its geometrically narrow facet characteristics as an energy cutoff boundary to prevent the laser heat-affected zone from spreading to non-target areas.

[0098] Understandably, the present invention sets the laser spot diameter or width to 85%~95% to achieve the best balance between overall strengthening and boundary safety. If the laser spot size is greater than 95% of the linear length of the bearing side, due to the narrowness of the bearing side itself, excess energy is easily spilled to the adjacent thread root fillet. Once the thread root receives excessive laser irradiation, a high-hardness brittle martensite structure will form. When the drill pipe is subjected to high torque and alternating tensile and compressive loads, cracks are easily initiated at the stress concentration point at the thread root and propagate into the matrix, leading to the risk of thread breakage. At the same time, the spilled energy is easily blocked or reflected by the adjacent thread lateral surface, resulting in a decrease in actual absorbed energy and temperature field disorder, affecting the consistency of the hardened layer thickness. If the laser spot size is less than 85% of the linear length of the bearing side, the laser coverage is insufficient to completely cover the narrow surface of the bearing side, resulting in insufficient hardening of the edge area of ​​the thread lateral surface. In actual uncoupling operations, it is easy to form a wear weak zone, accelerating local failure. Moreover, to fill the coverage blind area, it is often necessary to increase the scanning overlap rate, which not only reduces processing efficiency but also causes abnormal grain coarsening due to multiple thermal cycles. By matching the spot size to the linear length of the bearing side and preferably controlling it within the range of 85% to 95%, full coverage reinforcement of the high-stress narrow surface is achieved, and the geometric features of the narrow surface are used to effectively constrain the boundary of the heat-affected zone, avoiding the safety hazards caused by accidental hardening of the tooth root.

[0099] Furthermore, this invention selects the linear length of the thread bearing side as the reference for the spot size (diameter of a flat-topped circular spot or width of a rectangular spot). This is not only to control the energy boundary but also to help ensure the stable reproduction of microstructures with specific gradients. During laser quenching, the nucleation and growth of microstructures are highly dependent on the temperature field distribution in the heating zone and the subsequent cooling rate gradient. When the spot size is highly matched with the linear length of the bearing side (85%~95%), the laser energy can uniformly cover the entire bearing surface in a single-channel scan, forming a stable and continuous one-dimensional heat conduction path along the thickness direction. In this state, the surface layer, directly exposed to laser irradiation and forced convection cooling by protective gas, achieves an extremely high instantaneous cooling rate, causing the supercooled austenite to rapidly shear into fine acicular martensite while retaining a suitable amount of residual austenite; the subsurface layer, benefiting from the delayed cooling effect of surface heat conduction, is in the optimal phase transformation cooling window and fully develops into lath martensite with both strength and toughness; the transition layer, due to the buffering effect of the matrix heat capacity, cools the slowest, allowing the grains to grow moderately and form a coarse lath structure.

[0100] Understandably, if the spot size is less than 85% of the linear length of the bearing side, a single scan cannot completely cover the bearing surface, necessitating multi-pass overlapping scans. The overlapping area undergoes secondary or even multiple thermal cycles. The already formed fine-grained martensite will soften under tempering or abnormally coarsen under subsequent overlapping heat input, disrupting the original cooling rate gradient and leading to disordered grain size distribution, making it impossible to obtain a clearly defined three-layer gradient structure. Simultaneously, the residual stress superposition in the overlapping area significantly increases the tendency for surface microcracks. If the spot size is greater than 95% of the linear length of the bearing side, the overflow energy will irradiate the adjacent thread flanks or root, not only causing a decrease in energy reflection and absorption rate but also introducing strong transverse thermal interference at the thread flank edges. This edge thermal disturbance disrupts the original unidirectional heat conduction mode along the thickness direction, leading to localized overheating or uneven cooling of the surface layer, easily inducing grain coarsening or the formation of non-equilibrium mixed-grain structures. This causes the surface layer grain size to exceed the upper limit, and the subsurface layer lath bundles to coarsen, ultimately resulting in gradient structure failure and hardness peak shift.

[0101] In summary, matching the spot size based on the linear length of the bearing side essentially provides an ideal thermal cycling environment for the laser quenching process. Geometric matching eliminates thermal accumulation interference and edge thermal disturbance, ensuring that the cooling rate gradient from the surface to the substrate is strictly controlled. This provides a thermodynamic basis for stably obtaining specific gradient microstructures of fine-grained acicular martensite, lath martensite, and coarse lath martensite. This size matching relationship, in conjunction with the aforementioned power / velocity gradient control, jointly guarantees the service performance of the hardened layer in the drill pipe joint.

[0102] Preferably, in step S3, the illumination angle of the laser beam is 30°~70°, which can be 30°, 40°, 50°, 60°, 70° and any value between them, and the laser beam spot type is a rectangular spot or a flat-topped circular spot.

[0103] The present invention controls the selection of laser beam irradiation angle and spot type in order to utilize laser energy more efficiently and make the thickness of the hardened area more uniform, which is more conducive to the stable realization of the aforementioned gradient hardened layer thickness.

[0104] Understandably, to utilize laser energy more efficiently and achieve a more uniform thickness in the hardened area, it is optimal to irradiate the surface being treated with the laser perpendicularly. However, in practice, the flank angle of commonly used drill pipe joint threads is 30°~45°, with a certain taper, and the bearing-side teeth often obstruct each other. Therefore, the angle between the laser beam and the axis is limited to 30°~70°. If the angle is less than 30°, the incident angle is too small, and the laser beam may be blocked by adjacent teeth, failing to effectively irradiate the root of the bearing or guide side, resulting in insufficient hardened layer thickness at the root of the tooth flank and inability to form a complete protective area. If the angle is greater than 70°, the incident angle is too large, nearly parallel to the tooth flank surface, reducing energy absorption efficiency, with most energy being reflected, and potentially causing the laser spot to be elongated on the tooth flank surface, resulting in uneven energy density distribution, affecting quenching uniformity, and causing fluctuations in the hardened layer thickness. By controlling the irradiation angle within 30°~70°, it is helpful to ensure that the laser energy effectively acts on the tooth flank surface, obtaining a hardened layer of uniform thickness.

[0105] Understandably, rectangular or flat-topped circular laser spots are chosen because these two types of spots have uniform energy distribution, which is beneficial for obtaining a hardened layer of consistent thickness. Rectangular spots have uniform energy distribution, resulting in better hardness uniformity. Their microstructure is primarily lath martensite, leading to more balanced toughness and better impact resistance than circular spots. Flat-topped circular spots have relatively uniform energy from center to edge, with a clear cutoff at the edge, resulting in a small and controllable heat-affected zone. Energy is concentrated in the target area, providing higher precision, thus yielding a relatively uniform hardened layer. If a traditional Gaussian circular laser spot is used, the energy is highest at the center and gradually decreases outwards along a Gaussian curve. This results in a large heat diffusion range, significant heat-affected zone at the edges, and a tendency for transition zones or thermal damage to appear at the edges. After quenching, a crescent-shaped hardened layer forms in the thickness direction of the target area, with a deep hardened layer in the middle but a very shallow one at the edges. This leads to uneven hardened layer thickness, unstable wear resistance, and an inability to guarantee uniform and stable wear resistance on the threaded mating surface. Therefore, rectangular or flat-topped circular laser spots are preferred to ensure the uniformity of hardened layer thickness and hardness in the tooth flank width direction.

[0106] In summary, by limiting the laser beam irradiation angle to 30°~70° and selecting a rectangular or flat-topped circular spot, quenching blind zones caused by geometric obstruction and fluctuations in hardened layer thickness due to uneven energy distribution can be avoided. This process parameter design results in a more uniform thickness in the hardened area, providing a stable process guarantee for the aforementioned product characteristic of "the hardened layer thickness in high-wear areas being 15%~25% thicker than in other areas," thus ensuring the reliability of the drill pipe joint under complex downhole conditions.

[0107] Preferably, in step S3, argon gas is used for protection, and the argon gas flow rate is 10L / min to 20L / min, which can be 10L / min, 12L / min, 14L / min, 16L / min, 18L / min, 20L / min and any value between them.

[0108] This invention uses argon gas for protection to prevent the risk of surface oxidation caused by high temperatures, ensuring the quality and corrosion resistance of the threaded surface after quenching. During laser quenching, the threaded surface temperature rises instantaneously above the austenitizing temperature. If exposed to air, it will react with oxygen to form oxide scale, affecting surface quality and corrosion resistance. Argon, as an inert gas, can effectively isolate the thread from air and prevent oxidation.

[0109] Argon flow rate directly affects the stability of the protective atmosphere and its cooling effect on the molten pool. If the argon flow rate is less than 10 L / min, the protective gas flow velocity is insufficient, failing to completely isolate the air. Especially at high laser scanning speeds, surrounding air can easily be drawn into the protected area, potentially leading to surface oxidation. The resulting oxide scale not only affects surface finish but also reduces corrosion resistance during subsequent use and may even affect laser energy absorption, causing fluctuations in the hardened layer thickness. If the argon flow rate is greater than 20 L / min, the excessive flow velocity may result in gas waste, and excessively high flow rates can generate turbulence, interfering with laser beam stability, leading to uneven energy distribution, or excessively rapid cooling, increasing residual stress and affecting microstructure transformation. Controlling the argon flow rate between 10 L / min and 20 L / min helps maintain a stable thermal cycle while ensuring adequate protection, preventing the risk of surface oxidation due to high temperatures, and simultaneously improving corrosion resistance. This protection method eliminates the need for cooling media such as water or oil required for traditional quenching, thus offering high efficiency and energy savings, meeting the requirements of green manufacturing.

[0110] In summary, by limiting the argon flow rate to 10L / min~20L / min, surface oxidation defects caused by insufficient protection and process instability caused by excessive gas flow are effectively avoided. This process parameter design makes the atmosphere protection during the quenching process more reliable, providing environmental protection for the stable realization of the aforementioned microstructure gradient and hardness gradient of the hardened layer, and ensuring the reliability of the drill pipe joint under complex downhole conditions.

[0111] The embodiments of the present invention are described in detail below. These embodiments are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0112] Example 1 This embodiment provides a method for preparing an external threaded joint for oil drilling pipes, taking the APINC50 joint as an example, such as... Figure 1 , Figure 3 , Figure 4 , Figure 5 , Figure 6 As shown, it includes the following steps: S1. Pretreatment: Remove oil stains from the surface of external thread 1. Use acetone for ultrasonic cleaning for 10 minutes to remove surface oil stains.

[0113] S2. Clamping and positioning: Fix the external threaded connector on the machine tool chuck, ensuring that the thread axis 3 is coaxial with the laser scanning axis, with a coaxiality error ≤0.02mm.

[0114] S3. Laser quenching: The external thread laser 11 is used to scan and quench the thread guide tooth side 9 and the thread bearing tooth side 6 of the external thread 1.

[0115] It should be noted that in step S3, the scanning trajectory of the quenching laser beam 12 emitted by the external thread laser 11 is along the external thread quenching thread region 2. This region covers the entire effective meshing surface of the thread bearing tooth side 6 and the thread guiding tooth side 9. At the same time, through the coordinated control of the spot size and the irradiation angle, it is ensured that the laser energy is accurately applied to the tooth side surface, avoiding excessive irradiation to the thread crest 7 and the thread root arc 8. Combined with the geometric guidance of the thread taper line 10, when the quenching laser beam 12 scans from the large end to the small end along the generatrix direction of the external thread quenching thread region 2, it can naturally adapt to the change in tooth side inclination angle, ensuring the stability of the gradient distribution of the hardened layer thickness along the axial direction.

[0116] The specific parameters are as follows: Connector material: Quenched and tempered 4145H alloy steel; Thread profile: APIV-038R thread profile; Linear length of the bearing tooth side: 2.8 mm; Laser power: The laser power in the high-wear area (the first 3 teeth at the large end, accounting for about 15% of the total axial length) is 1180W, and the laser power in other areas is 1000W, with a power difference of 18%. Scanning speed: 40mm / s; Spot type: rectangular spot, spot size is 2.5mm × 2.5mm; Irradiation angle: The laser irradiation angle on the guide side is 41°, and the laser irradiation angle on the load-bearing side is 38°; Protective gas: Argon protection, flow rate 15L / min; Scanning path: Starting from the large end of the external thread 1, the laser moves towards the small end, first quenching the thread guide tooth side 9, then returning to the large end, and then quenching the thread bearing tooth side 6. During the process, the linear velocity is kept constant. Through the control system, the laser power in the first three thread teeth area and the laser power in the remaining thread teeth area are set to achieve the requirement of different laser power in different quenching areas in one stroke.

[0117] S4. Post-treatment: Apply rust-preventive oil to the quenched thread structure.

[0118] Example 2 This embodiment provides a method for preparing an internal threaded joint for oil drilling pipes, such as... Figure 2 , Figure 3 , Figure 7 , Figure 8 As shown, it includes the following steps: S1. Pretreatment: Remove oil stains from the surface of the internal thread 4. Use acetone for ultrasonic cleaning for 10 minutes to remove surface oil stains.

[0119] S2. Clamping and positioning: Fix the internal threaded connector on a special fixture to ensure that the thread axis is coaxial with the laser scanning axis, with a coaxiality error of ≤0.02mm.

[0120] S3, Laser quenching: The guide side and bearing side of the internal thread 4 are scanned and quenched using an internal thread laser 13.

[0121] It should be noted that in step S3, the scanning trajectory of the internal thread laser 13 is along the internal thread quenching region 5, which covers all effective meshing surfaces of the bearing and guiding sides of the internal thread 4. Through spot size matching and irradiation angle optimization, the laser energy can accurately cover the tooth flank surface, while avoiding accidental hardening of the thread crest 7 and thread root arc 8. Combined with the geometric constraints of the thread taper line 10, when the quenching laser beam 12 scans from the large end to the small end along the generatrix direction of the internal thread quenching region 5, it can adapt to changes in the spatial position of the tooth flank, ensuring the stability of the gradient distribution of the hardened layer thickness along the axial direction.

[0122] The specific parameters are as follows: Connector material: Quenched and tempered 4137H alloy steel; Thread profile: APIV-038R thread profile; Linear length of the bearing tooth side: 2.8 mm.

[0123] Laser power: The laser power in the high-wear area (the first 3 teeth at the large end, accounting for about 15% of the total axial length) is 960W, and the laser power in other areas is 800W, with a power difference of 20%. Scanning speed: 35mm / s; Spot type: flat-topped circular spot, spot diameter is 2.5mm; Irradiation angle: The laser irradiation angle on the guide side is 44°, and the laser irradiation angle on the load-bearing side is 41°; Protective gas: Argon protection, flow rate 18 L / min; Scanning path: Starting from the large end of the internal thread, the laser moves towards the small end, first quenching the guide side, then returning to the large end, and then quenching the bearing side.

[0124] S4. Post-treatment: Apply rust-preventive oil to the quenched thread structure.

[0125] Example 3 The difference between this embodiment and embodiment 1 is that the laser power in the laser quenching process parameters in step S3 remains unchanged at 1000W throughout; the scanning speed in the high wear area (the first 3 teeth at the large end, accounting for about 15% of the total length of the axis) is 36mm / s, and the scanning speed in other areas is 40mm / s, with a speed difference of 10%.

[0126] Example 4 The difference between this embodiment and Embodiment 1 is that the high wear area is defined as the first four teeth at the large end, accounting for approximately 20% of the total length of the axis.

[0127] Example 5 The difference between this embodiment and Embodiment 1 is that the high wear area is defined as the first 6 teeth at the large end, accounting for approximately 30% of the total length of the axis.

[0128] Example 6 The difference between this embodiment and Embodiment 1 is that the laser power in the high-wear area (the first 3 teeth at the large end, accounting for about 15% of the total axial length) is 500W, while the laser power in other areas is 417W, with a power difference of about 20%. Example 7 The difference between this embodiment and Embodiment 1 is that the laser power in the high-wear area (the first 3 teeth at the large end, accounting for about 15% of the total axial length) is 2000W, while the laser power in other areas is 1667W, with a power difference of about 20%. Comparative Example 1 The difference between this comparative example and Example 1 is that the laser power in step S3 laser quenching process parameters remains constant at 1000W throughout the process, and the scanning speed remains constant at 40mm / s throughout the process.

[0129] Comparative Example 2 This comparative example simulates the traditional laser hardening process for threaded surfaces in the prior art. The difference between this example and Example 1 is that: In step S1, after the pretreatment is completed, a phosphating light-absorbing coating is applied to the thread surface to improve the laser absorption rate. In step S3, the laser quenching process parameters are as follows: Laser power: Remains constant at 1200W throughout, without distinguishing between high-wear areas and other areas; Scanning speed: kept constant at 40 mm / s throughout; Spot type: Gaussian circular spot, spot diameter is 3mm; Irradiation angle: The laser irradiation angle on the guide side is 41°, and the laser irradiation angle on the load-bearing side is 38°; Protective gas: Argon protection, flow rate 15L / min.

[0130] Test case The drill pipe joints prepared in the examples and comparative examples were subjected to performance tests. The test items included the thickness of the hardened layer, microstructure, hardness distribution, and wear resistance.

[0131] 1. Hardened layer thickness test Optical microscopy (metallographic method) was used. First, a cross-sectional sample was taken along the thread axis, mounted, polished, and then etched with a 4% nitric acid-alcohol solution. The boundary between the hardened layer and the matrix was observed at 200× magnification. The hardened layer thickness was measured at three different locations on each thread, and the average value was taken. The hardened layer thickness was defined as the vertical distance from the thread surface to the boundary between the martensitic structure and the tempered sorbite structure of the matrix.

[0132] 2. Microstructural Characterization On the cross-section of the hardened layer, the microstructure transformation interface is identified by scanning electron microscopy (SEM) or electron backscatter diffraction (EBSD): the surface layer is a region dominated by acicular martensite and retained austenite, and the average size of no less than 50 grains in this region is determined by grain morphology and orientation difference; the subsurface layer is a region adjacent to the surface layer, dominated by lath martensite and with significantly increased grain size, and is measured using the same method; the transition layer is a region of gradual microstructure transformation between the subsurface layer and the tempered sorbite matrix, where the grains coarsen along the thermal diffusion direction, and its average grain size is measured.

[0133] The actual thickness range of each layer varies slightly due to process parameters, and should be determined based on the microstructure characteristics.

[0134] 3. Hardness test Hardness testing employed a micro Vickers hardness tester to measure the hardness gradient. Starting from the surface, three effective measurement points were selected in representative areas with clearly defined microstructures for each of the surface layer, subsurface layer, and transition layer, along the thickness direction of the hardened layer. The average value was then converted to HRC hardness. The load was set to 300g with a holding time of 10s. It should be noted that the original hardness test data in this application were measured using micro Vickers hardness (HV0.3). The main reason is that the total thickness of the hardened layer in this invention is only 0.1mm~0.5mm. If macroscopic Rockwell hardness (HRC) testing were used, the indentation length generated by a 150kgf load would penetrate the entire hardened layer and be affected by the matrix, failing to accurately characterize the true hardness of each sublayer in the gradient layer. Therefore, this application uses a small load Vickers hardness test of 0.3kgf (marked as HV0.3) for layer testing, which can distinguish the hardness differences between the surface layer, subsurface layer, and transition layer. The obtained HV values ​​were converted to HRC hardness according to general standards. (Appendix) Figure 12 The vertical axis uses HV0.3 units to visually display the gradient change trend of the original test data. Its Vickers hardness value range, after standard conversion, is substantially consistent with the HRC hardness gradient technology required by this invention.

[0135] 4. Wear resistance test Wear resistance testing employed a ring-block wear test to evaluate the differences in wear resistance caused by different processes. Standard test blocks were cut from samples treated with the same laser quenching process, quenched, and then used for wear resistance testing. Test parameters were set as follows: load 200 N, rotation speed 100 r / min, wear time 30 min, dry friction conditions. The test index was the volumetric wear loss of the test block, measured using a three-dimensional profilometer to measure the wear mark width and calculate the volumetric wear of the test block.

[0136] 5. Impact resistance performance analysis Impact resistance testing employed a drop hammer impact test to assess the toughness of the hardened layer. V-notch specimens were prepared using the load-bearing side of the threaded structure, and impact tests were conducted after hardening. The test parameter was the impact absorbed energy. Three measurements were taken at three points for each specimen, and the average value was recorded. Simultaneously, the fracture morphology was observed to evaluate the fracture mode.

[0137] 6. Anti-hooking performance test The anti-galling performance test employed a thread roll-on / roll-off cycle test to simulate the actual service conditions of the drill pipe joint. Drill pipe joints prepared in each embodiment and comparative example were paired male and female for anti-galling performance testing. Test parameters were set as follows: roll-on torque 60% of the recommended torque, rotation speed 10 r / min, lubrication condition wet friction (applying standard thread grease), and the test index was the number of roll-on / roll-off cycles before galling failure. Galling failure was defined as the appearance of obvious scoring marks on the thread surface. Three joints were tested for each sample, and the average value was taken. Specific test results are shown in Table 1.

[0138] Table 1

[0139] As shown in Table 1, the drill pipe joint prepared in this embodiment of the invention successfully achieved a gradient distribution of the hardened layer thickness along the axial direction. The hardened layer thickness in the high wear area is 15% to 25% thicker than that in other areas, and a three-layer gradient microstructure of surface layer, subsurface layer, and transition layer is formed. The surface layer consists of fine-grained acicular martensite and retained austenite, the subsurface layer consists of lath martensite, and the transition layer consists of coarse lath martensite. The hardness peak is located in the subsurface layer, and the hardness of the surface layer and the transition layer is lower than that of the subsurface layer. This gradient matching of microstructure and hardness significantly improves the overall performance of the joint. The wear volume loss and impact absorption energy are significantly higher than those of Comparative Example 1 (no thickness gradient) and Comparative Example 2 (single martensite structure with the highest surface hardness). The wear resistance and impact toughness are significantly improved.

[0140] The anti-gluing performance test results show that the drill pipe joints prepared in the examples, under 60% of the recommended torque and dry friction conditions, achieved more than 100 cycles of coupling and uncoupling (102 to 138 cycles), meeting the technical requirements of this invention. In contrast, Comparative Example 1, due to the lack of a hardened layer thickness gradient design, had insufficient wear resistance in the large end area, resulting in only 65 cycles of coupling and uncoupling; Comparative Example 2, due to the use of traditional laser quenching process to form a single martensitic structure with a hard and brittle surface, was prone to microcracks and propagation under alternating loads, resulting in the lowest number of coupling and uncoupling cycles at only 42.

[0141] Example 2, due to its use of a flat-topped circular light spot to form a dense microstructure, exhibits the best overall performance. Examples 6 and 7 demonstrate that even within the limits of process parameters, the present invention maintains superior overall performance compared to existing technologies. In summary, the present invention, through the synergistic effect of gradient hardened layer thickness design and gradient microstructure control, effectively solves the technical problems of mismatch between hardened layer thickness distribution and actual wear conditions, as well as the difficulty in achieving a balance between hardness and toughness in the hardened layer, thus achieving a balance between wear resistance and toughness.

[0142] The technical effects of this application will be explained below with reference to specific images.

[0143] Figure 9 The electron microscope image of the surface layer of the thread structure after laser quenching of the external thread provided in the embodiment of the present invention shows that the surface layer structure is mainly composed of fine acicular martensite and a small amount of retained austenite, with the average grain size controlled in the range of 1μm to 5μm. This fine grain structure can effectively hinder crack propagation, and an appropriate amount of retained austenite helps to relieve contact stress and provide toughness buffer for the thread surface.

[0144] Figure 10The electron microscope image of the subsurface layer of the thread structure after laser quenching of the external thread provided in the embodiment of the present invention shows that the subsurface layer structure is first lath martensite with an average grain size of 8μm~12μm. This layer is located in the hardness peak region and has a high hardness and good toughness combination, which can provide strong hardness support for the surface layer and undertake the main wear resistance function.

[0145] Figure 11 Electron microscope images of the surface layer, subsurface layer, and transition layer of the thread structure after laser quenching of the external thread provided in this embodiment of the invention visually present the gradient microstructure distribution of the hardened layer from the outside to the inside of the surface layer, subsurface layer, and transition layer. The grain size gradually transitions from fine grains on the surface to coarse lath martensite (20μm~50μm) in the transition layer. This smooth microstructure gradient effectively alleviates the stress abrupt change between the hardened layer and the matrix, and avoids the hardened layer from peeling or cracking due to excessive hardness difference.

[0146] Figure 12 The image shows a hardness comparison between the bearing side and the guide side of the internal thread after laser quenching, as provided in an embodiment of the present invention. The test results show that both the bearing side and the guide side exhibit a hardness gradient distribution characteristic with the highest subsurface hardness, the middle surface layer, and the lowest transition layer. In this embodiment, the subsurface layer hardness can reach between 600HV0.3 and 700HV0.3 (which translates to a subsurface layer hardness between 55.2HRC and 60.1HRC), and the hardness distribution trend on both sides is consistent. This verifies that the present invention can stably achieve the expected hardness gradient matching through process control, ensuring that the thread mating surface has both high wear resistance and excellent impact toughness.

[0147] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A drill pipe connector for oil drilling, comprising a connector body and a threaded structure disposed at the end of the connector body, the threaded structure comprising a crest, a root, and a guide side and a bearing side connecting the crest and the root, characterized in that, The surfaces of the guide side and the bearing side are provided with a hardened layer formed by laser quenching. The thickness of the hardened layer is gradient-distributed along the axial direction of the thread structure, and the thickness of the hardened layer located in the high wear region of the thread structure is 15% to 25% thicker than the thickness of the hardened layer located in other regions of the thread structure excluding the high wear region. The high-wear area is defined as the region that extends from the large end of the threaded structure towards the small end, accounting for 15% to 30% of the total length of the threaded structure's axis.

2. The drill pipe joint for oil drilling according to claim 1, characterized in that, The high-wear area includes 3 to 6 teeth arranged sequentially from the large end of the thread structure towards the small end, starting from the axial end near the large end of the thread structure. The thickness of the hardened layer in the high wear area is 0.12mm~0.5mm, and the thickness of the hardened layer in other areas of the thread structure other than the high wear area is 0.1mm~0.42mm.

3. The drill pipe joint for oil drilling according to claim 1, characterized in that, The hardened layer comprises, from the outside to the inside, a surface layer, a subsurface layer, and a transition layer. The surface layer consists of acicular martensite and retained austenite, with an average grain size of 1 μm to 5 μm. The subsurface layer consists of first lath martensite, with an average grain size of 8 μm to 12 μm. The transition layer consists of second lath martensite, with an average grain size of 20 μm to 50 μm.

4. The drill pipe joint for oil drilling according to claim 3, characterized in that, The hardened layer has a hardness gradient from the outside to the inside; the hardness of the surface layer and the transition layer is lower than that of the subsurface layer.

5. The drill pipe joint for oil drilling according to claim 4, characterized in that, The surface layer has a hardness of 50 HRC to 60 HRC; the subsurface layer has a hardness of 55 HRC to 65 HRC; and the transition layer has a hardness of 45 HRC to 55 HRC.

6. The drill pipe joint for oil drilling according to claim 1, characterized in that, The material of the joint body is alloy steel. After heat treatment, the microstructure of the guide side and the bearing side of the joint body is tempered sorbite.

7. The drill pipe joint for oil drilling according to claim 1, characterized in that, The width of the hardened area on one side of the tooth crest is no more than 0.5 mm, and the surface of the tooth root does not have a hardened layer.

8. A method for preparing a drill pipe joint for oil drilling as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1. Remove oil and dirt from the surface of the threaded structure; S2. Secure the drill pipe joint; S3. Use a laser beam to scan and quench the guide side and the load-bearing side of the threaded structure; Specifically, the laser power of teeth in high-wear areas is 15% to 20% higher than that of teeth in other areas, or the scanning speed of teeth in high-wear areas is 5% to 10% lower than that of teeth in other areas. S4. Apply rust-preventive oil to the quenched thread structure.

9. The preparation method according to claim 8, characterized in that, In step S3, for teeth in high wear areas, the laser power is 500W~2000W and the scanning speed is 10mm / s~60mm / s; for teeth in other areas, the laser power is 300W~1500W and the scanning speed is 10mm / s~60mm / s. Furthermore, the laser beam spot type is either a rectangular spot or a flat-topped circular spot; When the light spot type is a flat-topped circular light spot, the diameter of the flat-topped circular light spot is 85% to 95% of the linear length of the bearing side; or, when the light spot type is a rectangular light spot, the width of the rectangular light spot is 85% to 95% of the linear length of the bearing side.

10. The preparation method according to claim 8, characterized in that, In step S3, the illumination angle of the laser beam is 30°~70°; And / or, in step S3, argon gas is used for protection, with an argon gas flow rate of 10 L / min to 20 L / min.

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