Carburizing heat treatment process for roller bit carburizing steel

Abstract

The invention belongs to the field of heat treatment of roller bit carburizing steel, and particularly relates to a carburizing heat treatment process of roller bit carburizing steel. According to the technology, 15MnNi4Mo serves as the material of the roller bit, carburizing (including a strong carburizing period and a diffusion period), two times of high-temperature tempering, quenching and low-temperature tempering are conducted in sequence, parameters such as the temperature, the carbon potential and the heat preservation time of all stages are optimized, the proportion of the strong carburizing time to the diffusion time is adjusted, and the cooling uniformity is guaranteed in a die-free quenching mode. On the premise that the heat preservation temperature and the heating temperature of each process step are both reduced, the energy consumption is obviously reduced; the inner hole deformation, the effective carburized layer deviation value and the Rockwell hardness non-uniform value of the treated roller bit are all reduced, the hardness of the carburized layer and the core part is synchronously improved, the hardness gradient is slowed down, the toughness is enhanced, the double problems of quality and energy consumption of a traditional technology are effectively solved, and the roller bit machining method is suitable for machining roller bits used under complex geological conditions such as deep wells and ultra-deep wells.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment of carburizing steel for roller cone drill bits, specifically a carburizing heat treatment process for carburizing steel for roller cone drill bits, which is applicable to the processing of tricone drill bits used in deep wells, ultra-deep wells and complex geological conditions. Background Technology

[0002] As a key rock-breaking tool in oil and gas resource exploration and development, roller cone drill bits have been widely used since their inception in the early 20th century due to their broad adaptability to various formations and high mechanical drilling speeds. Among them, tri-cone drill bits have become the mainstream model due to their good structural stability and high rock-breaking coverage. In drilling deep wells, ultra-deep wells, and under complex geological conditions, roller cone drill bits must withstand high temperatures, high pressures, corrosive media, and repeated impact loads, placing extreme demands on their comprehensive performance. How to improve the heat treatment process of roller cone drill bits to enhance their wear resistance and service life has always been a problem to be solved. Under traditional heat treatment processes, mining roller cone drill bits are prone to cracking and tooth loss during use. The surface and core hardness are insufficient, and the hardness gradient is too large, resulting in poor wear resistance and greatly affecting service life. See reference [1] Cheng Juqiang. Influence of heat treatment on the microstructure and hardness of carburized layer of mining roller cone drill bits [J]. Rock Drilling Machinery and Pneumatic Tools, 2019, (1): 4; and reference [2] Gao Zhizhe, Chen Xiaoyan, Cheng Fuchao, et al. Influence of intermediate high temperature tempering process on microstructure and properties of 15MnNi4MoA carburized steel [J]. Metal Heat Treatment, 2021, (010): 046. Therefore, it is crucial to formulate a scientific heat treatment process to ensure that the carburized steel of roller cone drill bits obtains ideal performance.

[0003] Existing heat treatment processes for carburized steel roller cone drill bits have several shortcomings. For example, patent CN104195311A proposes a heat treatment process for roller cone drill bits using 15CrNi4Mo material. The carburizing temperature reaches 930℃, and the high-temperature tempering temperature is 640~680℃, including a normalizing process, resulting in high energy consumption. It only involves one high-temperature tempering, limiting the improvement in core hardness and toughness, and failing to address the problem of excessive hardness gradient. Patent CN107099767A proposes a heat treatment process for blasting drill bits, specifically designed for blasting applications. The material contains special components such as vanadium, and a secondary quenching process is used after carburizing, making cooling rate control complex. Patent CN1046196A proposes a rapid heat treatment process for the roller cone teeth of drill bits, with a carburizing temperature reaching 960~980℃. The high-temperature holding time is long, resulting in significantly high energy consumption. It only involves one high-temperature tempering, leading to insufficient core toughness and significant workpiece deformation. Patent CN104070328A discloses a manufacturing process for steel toothed roller cones used in mining drill bits. It employs 15CrNi3Mo material, and after carburizing, only one tempering treatment is performed, resulting in a carburized layer depth of only 0.9~2.2mm. The improvement in surface and core hardness is limited, making it unsuitable for deep well operations. Patent CN108588388A discloses a heat treatment method for roller cones, with a carburizing temperature of 925~930℃ and a high-temperature tempering temperature of 670~690℃. The overall process temperature is too high, resulting in significant energy consumption. Furthermore, it only involves one high-temperature tempering treatment, and the quenching temperature is 850~870℃, leading to poor cooling uniformity and significant deformation of the workpiece's inner hole. Patent CN110564922A discloses an improved quenching process for roller cone steel. Although it includes two high-temperature tempering treatments, the carburizing temperature reaches 950℃, resulting in high energy consumption. The quenching temperature is designed based on the critical zone, primarily focusing on improving surface hardness without considering energy consumption reduction and deformation control. The patent with publication number CN112795730A proposes a heat treatment method to improve the performance of drill bits for down-the-hole drills. It is designed for 4330V material and the process includes isothermal preheating, multiple quenching and tempering. The process is complicated and does not optimize parameters for 15MnNi4Mo material of roller cone drill bits.

[0004] In summary, traditional processes have failed to simultaneously address the quality issues of insufficient surface and core hardness, excessive hardness gradient, easy cracking and tooth breakage, and poor wear resistance in roller cone drill bits, as well as the production problems of excessive heat treatment energy consumption, long total time, and large internal hole deformation. A dedicated heat treatment process that balances quality improvement and energy consumption reduction is needed. Summary of the Invention

[0005] To address the problems of poor quality (prone to cracking and tooth breakage, insufficient surface and core hardness, excessive hardness gradient, and poor wear resistance) and high energy consumption in traditional heat treatment processes for roller cone drill bits, this invention aims to provide a carburizing heat treatment process for carburized steel for roller cone drill bits. This process significantly improves both the hardness of the carburized layer and the core of the drill bit, effectively reduces the overall hardness gradient, and simultaneously enhances toughness. More importantly, while achieving the above-mentioned quality optimization goals, energy consumption is greatly reduced, ultimately solving the dual core problems of poor quality and excessive energy consumption in traditional processes for roller cone drill bits.

[0006] The technical solution of this invention is:

[0007] A carburizing heat treatment process for carburizing steel for roller cone drill bits includes the following steps:

[0008] 1) The roller cone bit is placed in a furnace for carburizing treatment, which includes a strong carburizing period and a diffusion period: the strong carburizing period temperature is 910℃~920℃, the strong carburizing period time is 12h~16h, and the carbon potential is 1.18wt%~1.23wt%; after the strong carburizing period is completed, the roller cone bit is cooled to 880℃~900℃ and enters the diffusion period, the diffusion period time is 3h~7h, and the carbon potential is 0.9wt%~1.1wt%; then it is cooled to 825℃~835℃ and cooled in the furnace to 410℃~430℃ before being taken out of the furnace.

[0009] 2) Perform the first high-temperature tempering treatment on the roller cone drill bit, with the temperature controlled at 620℃~640℃. After holding at this temperature for 2~4 hours under N2 protection, cool it down to 400℃~425℃ and then air cool it to room temperature.

[0010] 3) Perform a second high-temperature tempering treatment on the roller cone drill bit, with the temperature controlled at 620℃~640℃. After holding at this temperature for 2~4 hours under N2 protection, cool it down to 400℃~425℃ and then air-cool it to room temperature.

[0011] 4) After the second high-temperature tempering treatment, the roller cone drill bit is quenched at a temperature of 810~830℃, with a carbon potential of 0.28wt%~0.48wt%. After holding at this temperature for 105min~115min, it is cooled down to 110℃~130℃ by oil cooling.

[0012] 5) After quenching, the roller cone drill bit is subjected to low-temperature tempering at a temperature of 140℃~160℃. After 5h~7h, it is taken out of the furnace and air-cooled to room temperature.

[0013] The carburizing heat treatment process for the carburizing steel of the roller cone drill bit is described above. The carburizing steel of the roller cone drill bit is made of 15MnNi4Mo material.

[0014] In the carburizing heat treatment process of the roller cone drill bit carburizing steel, in step 1), the strong carburizing temperature during the strong carburizing period is 910℃, the strong carburizing time is 720min, and the carbon potential is 1.21wt%; the diffusion temperature during the diffusion period is 880℃, the diffusion time is 420min, and the carbon potential is 0.9wt%; after the diffusion period is completed, the roller cone drill bit is cooled to 830℃ and then cooled to 410℃ in the furnace before being taken out of the furnace.

[0015] In the carburizing heat treatment process of the carburizing steel of the roller cone drill bit, in step 2), the first high-temperature tempering treatment temperature is 630℃, and it is held at this temperature for 2 hours under N2 protection. After cooling down to 410℃, it is air-cooled to room temperature.

[0016] In the carburizing heat treatment process of the aforementioned roller cone drill bit carburizing steel, in step 3), the second high-temperature tempering treatment temperature is 630℃, held at this temperature for 2 hours under N2 protection, then cooled to 410℃ and air-cooled to room temperature.

[0017] In the carburizing heat treatment process of the roller cone drill bit carburizing steel, in step 4), the quenching temperature is 810℃, the carbon potential is 0.38wt%, the holding time is 115min, and then the temperature is cooled down to 120℃ by oil cooling.

[0018] In the carburizing heat treatment process of the roller cone drill bit carburizing steel, in step 4), during the quenching process, the roller cone drill bit carburizing steel workpiece adopts a moldless quenching method to avoid insufficient local cooling caused by mold obstruction, prevent soft spots or uneven structure, and ensure full contact between the workpiece and the quenching medium.

[0019] In the carburizing heat treatment process of the roller cone drill bit carburizing steel, in step 4), the oil-cooled workpiece is cleaned with warm water at a temperature of 50℃~60℃ for 0.5~2h.

[0020] In the carburizing heat treatment process of the aforementioned roller cone drill bit carburizing steel, step 5) involves a low-temperature tempering temperature of 150°C and a holding time of 5 hours, followed by air cooling to room temperature.

[0021] The design concept of this invention is:

[0022] Targeting the material characteristics of 15MnNi4Mo carburizing steel, this invention achieves synergistic optimization of quality and energy consumption by optimizing the combination of heat treatment processes and key parameters. The process employs carburizing (strong carburizing + diffusion) → two high-temperature tempering processes → moldless quenching → low-temperature tempering. This reduces the heating and holding temperatures at each stage, adjusts the ratio of strong carburizing to diffusion time (reducing strong carburizing time and increasing diffusion time), fully refines the microstructure and releases stress through two high-temperature tempering processes, and uses moldless quenching to ensure full contact between the workpiece and the quenching medium, avoiding uneven cooling. Ultimately, while shortening the total heat treatment time, this invention simultaneously improves the surface and core hardness, mitigates the hardness gradient, enhances toughness, and reduces energy consumption and deformation.

[0023] This invention reduces the strong carburizing time and increases the diffusion time. The carbon potential has different effects: it increases the surface carbon content during the strong carburizing period and smooths the carbon concentration gradient during the diffusion period. The temperature and holding time of the carburizing, quenching, high-temperature tempering and low-temperature tempering processes are reasonably adjusted. Under the premise that the holding and heating temperatures of each process step are reduced, the total heat treatment time is reduced from 45 hours to less than 37 hours. This improves the hardness of the core and surface, reduces the overall hardness gradient, improves toughness, and greatly reduces energy consumption.

[0024] The advantages and beneficial effects of this invention are:

[0025] This invention uses 15MnNi4Mo as the material for roller cone drill bits. The process involves sequential carburizing (including a strong carburizing period and a diffusion period), two high-temperature tempering processes, quenching, and low-temperature tempering. By optimizing parameters such as temperature, carbon potential, and holding time at each stage, and adjusting the ratio of strong carburizing to diffusion time, a moldless quenching method is employed to ensure uniform cooling. With reduced holding and heating temperatures at each process step, the total process time is shortened from the traditional 45 hours to less than 37 hours, significantly reducing energy consumption. The processed roller cone drill bit exhibits reduced internal hole deformation, effective carburized layer deviation, and Rockwell hardness unevenness. Surface hardness is increased by 8%, core hardness by 21%, carburized layer depth by 7%, and impact toughness by 15%. The hardness of the carburized layer and core increases synchronously, the hardness gradient is reduced, and toughness is enhanced. This effectively solves the dual problems of quality and energy consumption in traditional processes, making it suitable for machining roller cone drill bits used in complex geological conditions such as deep and ultra-deep wells. Attached Figure Description

[0026] Figure 1 This is a schematic flowchart of the heat treatment process (example) of the present invention.

[0027] Figure 2 This is a flowchart of a traditional heat treatment process (comparative example).

[0028] Figure 3 These are comparative images of the martensitic structure of the carburized layer of the roller cone drill bit carburized steel in Example 1 of the present invention; wherein, (a) is a micrograph of the martensitic structure of the carburized layer of the roller cone drill bit carburized steel in Example 1 of the present invention magnified 50 times, and (b) is a micrograph of the martensitic structure of the carburized layer of the roller cone drill bit carburized steel in the comparative example magnified 50 times.

[0029] Figure 4 These are comparative images of the martensitic structure on the surface of the carburized steel carburized layer of the roller cone drill bit in Example 1 of the present invention; wherein, (a) is a micrograph of the martensitic structure on the surface of the carburized steel carburized layer of the roller cone drill bit in Example 1 of the present invention magnified 1000 times, and (b) is a micrograph of the martensitic structure on the surface of the carburized steel carburized layer of the roller cone drill bit in the comparative example magnified 1000 times.

[0030] Figure 5 These are comparative images of the martensitic structure of the carburized steel core of the roller cone drill bit in Example 1 of the present invention; wherein, (a) is a micrograph of the third-grade martensitic structure of the carburized steel core of the roller cone drill bit in Example 1 of the present invention at 1000x magnification, and (b) is a micrograph of the second-grade martensitic structure of the carburized steel core of the roller cone drill bit in the comparative example at 1000x magnification. Detailed Implementation

[0031] like Figure 1 As shown, this invention proposes a carburizing heat treatment process for carburizing steel used in roller cone drill bits, comprising the following steps:

[0032] 1) The carburized steel for roller cone drill bits is made of 15MnNi4Mo material. The roller cone drill bits are placed in a furnace for carburizing treatment, which includes a strong carburizing period and a diffusion period: the strong carburizing period temperature is 910℃~920℃, the strong carburizing period time is 12h~16h, and the carbon potential is 1.18wt%~1.23wt%; after the strong carburizing period is completed, the roller cone drill bits are cooled to 880℃~900℃ and enter the diffusion period, the diffusion period time is 3h~7h, and the carbon potential is 0.9wt%~1.1wt%; then the temperature is cooled to 825℃~835℃, and the bits are cooled in the furnace to 410℃~430℃ before being taken out of the furnace.

[0033] Preferably, the strong penetration temperature during the strong penetration period is 910℃, the strong penetration time is 720min, and the carbon potential is 1.21wt%; the diffusion temperature during the diffusion period is 880℃, the diffusion time is 420min, and the carbon potential is 0.9wt%. After the diffusion period is completed, the roller cone drill bit is cooled to 830℃ and then cooled to 410℃ in the furnace before being removed from the furnace.

[0034] 2) Perform the first high-temperature tempering treatment on the roller cone drill bit, with the temperature controlled at 620℃~640℃. After holding at this temperature for 2~4 hours under N2 protection, cool it down to 400℃~425℃ and then air-cool it to room temperature.

[0035] Preferably, the first high-temperature tempering treatment is performed at 630°C, held at that temperature for 2 hours under N2 protection, then cooled to 410°C and air-cooled to room temperature.

[0036] 3) Perform a second high-temperature tempering treatment on the roller cone drill bit, with the temperature controlled at 620℃~640℃. After holding at this temperature for 2~4 hours under N2 protection, cool it down to 400℃~425℃ and then air-cool it to room temperature.

[0037] Preferably, the second high-temperature tempering treatment is performed at 630°C, held at that temperature for 2 hours under N2 protection, then cooled to 410°C and air-cooled to room temperature.

[0038] 4) The roller cone drill bit after the second high-temperature tempering treatment is then quenched at a temperature controlled at 810~830℃, with a carbon potential of 0.28wt%~0.48wt%. After holding at this temperature for 105min~115min, it is oil-cooled to 110℃~130℃. Preferably, the quenching temperature is 810℃, the carbon potential is 0.38wt%, and after holding at this temperature for 115min, it is oil-cooled to 120℃.

[0039] During the quenching process, the carburized steel workpiece of the roller cone drill bit adopts a moldless quenching method to avoid insufficient local cooling caused by mold obstruction, prevent soft spots or uneven structure, and ensure that the workpiece is in full contact with the quenching medium to avoid soft spots caused by insufficient local cooling. The use of moldless quenching ensures uniform cooling.

[0040] Step 4) Oil cooling and cleaning processes are required to prevent the core temperature from being too high and causing self-tempering. The cleaning process temperature is 50℃~60℃, the cleaning time is 1 hour, and the core is immediately transferred to low-temperature tempering after cleaning.

[0041] 5) After quenching, the roller cone drill bit is subjected to low-temperature tempering at a temperature controlled between 140℃ and 160℃ for 5 to 7 hours. After tempering, it is removed from the furnace and air-cooled to room temperature. Preferably, the low-temperature tempering temperature is 150℃, the holding time is 5 hours, and after low-temperature tempering, it is air-cooled to room temperature.

[0042] The present invention will now be further described in detail with reference to embodiments and accompanying drawings.

[0043] Example 1

[0044] The material of the roller cone drill bit is 15MnNi4Mo, which includes the following chemical composition in wt% as follows: C 0.151%, Cr 0.19%, Ni 3.35%, Mo 0.35%, Si 0.29%, Mn 0.88%, S 0.006%, P 0.008% and Fe balance.

[0045] In this embodiment, carburizing and quenching are completed in a multi-purpose heat treatment carburizing and quenching furnace, while high-temperature tempering and low-temperature tempering are completed in a high-temperature tempering furnace and a low-temperature tempering furnace, respectively. The specific steps are as follows:

[0046] 1) Carburizing: The roller cone drill bit is placed in a furnace for carburizing treatment, which includes a strong carburizing period and a diffusion period: the strong carburizing period temperature is 910℃, the carbon potential is 1.21wt%, and the strong carburizing time is 720min. After the strong carburizing period is completed, the roller cone drill bit is cooled to 880℃, the carbon potential is 0.9wt%, and it enters the diffusion period with a diffusion time of 420min. After cooling to 830℃, it is cooled to 410℃ with the furnace and then taken out of the furnace.

[0047] 2) First high-temperature tempering: The roller cone drill bit that has been slowly cooled after carburizing is subjected to a first high-temperature tempering treatment. The temperature is controlled at 630℃, and after holding at this temperature for 3 hours under N2 protection, it is cooled down to 410℃ and then air-cooled to room temperature.

[0048] 3) Secondary high-temperature tempering: The roller cone drill bit that has been slowly cooled after the high-temperature tempering is subjected to a secondary high-temperature tempering treatment. The temperature is controlled at 630℃, and after holding at this temperature for 3 hours under N2 protection, it is cooled down to 410℃ and then air-cooled to room temperature.

[0049] 4) Quenching: The roller cone drill bit after the secondary high-temperature tempering treatment is quenched at a temperature of 810℃ with a carbon potential of 0.38wt% and then cooled to 120℃ by oil cooling after 115 minutes.

[0050] Cleaning: Clean the oiled workpiece with warm water at 55℃ for 1 hour.

[0051] 5) Low-temperature tempering: The quenched roller cone bit is subjected to low-temperature tempering at 150℃, and after 5 hours, it is taken out of the furnace and air-cooled to room temperature.

[0052] The roller cone drill bit obtained by the method of this embodiment was tested for performance. The deformation of the inner hole of the roller cone drill bit was reduced from the original average of 0.01 mm to 0.006 mm, the effective carburized layer deviation was reduced from the original 0.1 mm to 0.05 mm, and the Rockwell hardness non-uniformity was reduced from the original 0.5 HRC to 0.3 HRC.

[0053] Surface carbon concentration: 0.87wt%

[0054] Carburized layer depth: 3mm

[0055] Quenching and tempering hardness: HRC64~66

[0056] Grade of martensite and retained austenite in the diffusion layer: Level 2

[0057] Carbide level: Level 3

[0058] Heart Martensite Grade: Grade 3

[0059] Heart hardness: HRC42~44

[0060] Impact toughness: 168J

[0061] like Figure 3 As shown in (a), the martensitic structure of the carburized layer in Example 1 is fine and uniform, with a residual austenite content of only 6 wt%. The carbides are uniformly distributed in granular form and there are no network carbide defects. This indicates that by optimizing the strong carburizing-diffusion time ratio (strong carburizing 720 min + diffusion 420 min) and two high-temperature tempering processes, the precipitation of residual austenite and the network aggregation of carbides are effectively suppressed.

[0062] like Figure 4 As shown in (a), the surface martensite of Example 1 is fine needle-like, with a dense and non-porous structure and obvious grain refinement. This indicates that through the synergistic effect of optimizing the quenching temperature at 810℃ and two high-temperature tempering processes, the surface carbides are fully dissolved and re-precipitated, forming a uniform and fine martensite structure with a corresponding surface hardness of 65HRC.

[0063] like Figure 5 As shown in (a), the core martensite level of Example 1 is grade 3, and the lath martensite structure is uniform and continuous. By reducing the quenching temperature (810℃) and extending the diffusion time, the uniformity of the core carbon concentration is improved (core carbon concentration 0.35wt%), the core hardness reaches 46HRC, the impact toughness is 168J, and the strength and toughness are well matched.

[0064] Example 2

[0065] The material of the roller cone drill bit is 15MnNi4Mo, which includes the following chemical composition in wt% as follows: C 0.151%, Cr 0.19%, Ni 3.35%, Mo 0.35%, Si 0.29%, Mn 0.88%, S 0.006%, P 0.008% and Fe balance.

[0066] In this embodiment, carburizing and quenching are completed in a multi-purpose heat treatment carburizing and quenching furnace, while high-temperature tempering and low-temperature tempering are completed in a high-temperature tempering furnace and a low-temperature tempering furnace, respectively. The specific steps are as follows:

[0067] 1) Carburizing: The roller cone drill bit is placed in a furnace for carburizing treatment, which includes a strong carburizing period and a diffusion period: the strong carburizing period temperature is 915℃, the carbon potential is 1.18wt%, and the strong carburizing time is 840min. After the strong carburizing period is completed, the roller cone drill bit is cooled to 890℃, the carbon potential is 1.05wt%, and it enters the diffusion period, the diffusion time is 300min, and it is cooled to 825℃ and then removed from the furnace at 420℃.

[0068] 2) First high-temperature tempering: The roller cone drill bit that has been slowly cooled after carburizing is subjected to a first high-temperature tempering treatment. The temperature is controlled at 620℃, and after holding at this temperature for 3 hours under N2 protection, it is cooled down to 400℃ and then air-cooled to room temperature.

[0069] 3) Secondary high-temperature tempering: The roller cone drill bit that has been slowly cooled after the high-temperature tempering is subjected to a secondary high-temperature tempering treatment. The temperature is controlled at 620℃, and after holding at this temperature for 3 hours under N2 protection, it is cooled to 400℃ and then air-cooled to room temperature.

[0070] 4) Quenching: The roller cone drill bit after the secondary high-temperature tempering treatment is quenched at a temperature of 820℃ with a carbon potential of 0.28wt% and then cooled to 110℃ by oil cooling after 105 minutes.

[0071] 5) Cleaning: Clean the oiled workpiece with warm water at 50℃.

[0072] 6) Low-temperature tempering: The quenched roller cone bit is subjected to low-temperature tempering at 140℃, and after 7 hours, it is taken out of the furnace and air-cooled to room temperature.

[0073] The roller cone drill bit obtained by the method of this embodiment was tested for performance. The deformation of the inner hole of the roller cone drill bit was reduced from the original average of 0.01 mm to 0.008 mm, the effective carburized layer deviation was reduced from the original 0.1 mm to 0.07 mm, and the Rockwell hardness unevenness was reduced from the original 0.5 HRC to 0.4 HRC.

[0074] Surface carbon concentration: 0.95wt%

[0075] Carburized layer depth: 2.84mm

[0076] Quenching and tempering hardness: HRC60~64

[0077] Grade of martensite and retained austenite in the diffusion layer: Level 3

[0078] Carbide level: Level 3

[0079] Heart hardness: HRC38~42

[0080] Impact toughness: 155J

[0081] Example 3

[0082] The material of the roller cone drill bit is 15MnNi4Mo, which includes the following chemical composition in wt% as follows: C 0.151%, Cr 0.19%, Ni 3.35%, Mo 0.35%, Si 0.29%, Mn 0.88%, S 0.006%, P 0.008% and Fe balance.

[0083] In this embodiment, carburizing and quenching are completed in a multi-purpose heat treatment carburizing and quenching furnace, while high-temperature tempering and low-temperature tempering are completed in a high-temperature tempering furnace and a low-temperature tempering furnace, respectively. The specific steps are as follows:

[0084] 1) Carburizing: The roller cone drill bit is placed in a furnace for carburizing treatment, which includes a strong carburizing period and a diffusion period: the strong carburizing period temperature is 920℃, the carbon potential is 1.23wt%, and the strong carburizing time is 960min. After the strong carburizing period is completed, the roller cone drill bit is cooled to 900℃, the carbon potential is 1.1wt%, and it enters the diffusion period, the diffusion time is 180min, and it is cooled to 835℃ and then cooled to 430℃ in the furnace before being taken out.

[0085] 2) High-temperature tempering: The roller cone drill bit that has been slowly cooled after carburizing is subjected to a high-temperature tempering treatment. The temperature is controlled at 640℃, and after holding at this temperature for 3 hours under N2 protection, it is cooled to 425℃ and then air-cooled to room temperature.

[0086] 3) Secondary high-temperature tempering: The roller cone drill bit that has been slowly cooled after the high-temperature tempering is subjected to a secondary high-temperature tempering treatment. The temperature is controlled at 640℃, and after holding at this temperature for 3 hours under N2 protection, it is cooled down to 425℃ and then air-cooled to room temperature.

[0087] 4) Quenching: The roller cone drill bit after the secondary high-temperature tempering treatment is quenched at a temperature of 830℃ with a carbon potential of 0.48wt% and then cooled to 130℃ by oil cooling after 110 minutes.

[0088] 5) Cleaning: Clean the oiled workpiece with warm water at 60℃.

[0089] 5) Low-temperature tempering: The quenched roller cone bit is subjected to low-temperature tempering at 160℃, and after 6 hours, it is taken out of the furnace and air-cooled to room temperature.

[0090] The roller cone drill bit obtained by the method of this embodiment was tested for performance. The deformation of the inner hole of the roller cone drill bit was reduced to 0.009 mm, the effective carburized layer deviation was reduced to 0.092 mm, and the Rockwell hardness unevenness was reduced to 0.45 HRC.

[0091] Surface carbon concentration: 1.03wt%

[0092] Carburized layer depth: 2.82mm

[0093] Quenching and tempering hardness: HRC60~64

[0094] Grade of martensite and retained austenite in the diffusion layer: Level 4

[0095] Carbide level: 4

[0096] Heart hardness: HRC38~42

[0097] Impact toughness: 148J

[0098] Comparative Example

[0099] The material of the roller cone drill bit is 15MnNi4Mo, which includes the following chemical composition in wt% as follows: C 0.151%, Cr 0.19%, Ni 3.35%, Mo 0.35%, Si 0.29%, Mn 0.88%, S 0.006%, P 0.008% and Fe balance.

[0100] like Figure 2As shown, carburizing and quenching in the comparative example were completed in a multi-purpose heat treatment furnace for carburizing and quenching, while high-temperature tempering and low-temperature tempering were completed in high-temperature tempering furnaces and low-temperature tempering furnaces, respectively. The specific steps are as follows:

[0101] 1) Carburizing stage: The roller cone drill bit is placed in the furnace for carburizing treatment, which includes a strong carburizing period and a diffusion period. The strong carburizing temperature during the strong carburizing period is 924℃, the carbon potential is 1.24wt%, and the strong carburizing time is 890min. After the strong carburizing period, the roller cone drill bit is cooled to 918℃, and the carbon potential is 1.08wt% to enter the diffusion period, with a diffusion time of 230min. Then, it is cooled to 855℃ and held for 30min. After that, it is cooled with the furnace to 445℃ and taken out of the furnace. The high-temperature section has a long holding time and a relatively high carbon potential.

[0102] 2) High-temperature tempering stage: The roller cone drill bit that has been slowly cooled after carburizing is subjected to high-temperature tempering treatment. The temperature is controlled at 675℃ and held at that temperature for 12 hours under N2 protection. Then, it is cooled down to 446℃ and air-cooled to room temperature. Insufficient tempering leads to residual stress in the structure.

[0103] 3) Quenching stage: The roller cone drill bit after high-temperature tempering is quenched at a temperature of 845℃ with a carbon potential of 0.42wt%. After holding for 115 minutes, it is cooled down to 140℃ by oil cooling. High quenching temperature and long holding time can easily lead to coarse grains.

[0104] 4) Cleaning stage: Clean the oiled workpiece with warm water at 70℃ for 1.2 hours;

[0105] 5) Low-temperature tempering stage: The cleaned roller cone drill bit is subjected to low-temperature tempering at 223℃. After holding at this temperature for 12 hours, it is removed from the furnace and air-cooled to room temperature.

[0106] The roller cone drill bit obtained by the comparative method was tested for performance. The average inner hole deformation of the roller cone drill bit was 0.011 mm, the effective carburized layer deviation was 0.12 mm, and the Rockwell hardness non-uniformity was 0.53 HRC.

[0107] Surface carbon concentration: 1.05wt%

[0108] Carburized layer depth: 2.81mm

[0109] Quenching and tempering hardness: HRC60~64

[0110] Grade of martensite and retained austenite in the diffusion layer: Level 4

[0111] Carbide level: 4

[0112] Heart hardness: HRC36~38

[0113] Impact toughness: 146J

[0114] like Figure 3 As shown in (b), the comparative example shows a coarse martensitic structure in the carburized layer, with a retained austenite content as high as 14.5 wt%, and locally visible network carbides. Due to the high carburizing temperature and short diffusion time of the traditional process, the carbon concentration gradient is large, resulting in insufficient microstructure transformation, which affects the hardness and wear resistance of the carburized layer.

[0115] like Figure 4 As shown in (b), the surface martensite in the comparative example is coarse and needle-like, with a loose microstructure and uneven distribution of retained austenite along the grain boundaries. The traditional process uses a high quenching temperature (855℃) and does not involve a second high-temperature tempering, which leads to the growth of martensite grains and the inability of retained austenite to decompose effectively. The surface hardness is only 60HRC, resulting in insufficient wear resistance.

[0116] like Figure 5 As shown in (b), the core martensite level of the comparative example is level 2, the lath martensite is coarse and unevenly distributed, the carbon concentration of the core in the traditional process is only 0.17wt%, the quenching temperature is too high, resulting in coarse grains in the core, the core hardness is 38HRC, the impact toughness is 146J, and it is prone to fracture failure under deep well impact load.

[0117] In the examples and comparative examples, the deformation of the inner hole of the roller cone drill bit was measured using an inner diameter dial indicator; the effective carburized layer was measured using the test bar peeling method; the Rockwell hardness was measured using a Rockwell hardness tester; the relevant standards used were: GB4324.30-1984 "Heavy-duty gears, carburized quality inspection"; GB 5124.1-2008 "Heavy-duty gears, carburized metallographic inspection"; GB T229-2020 "Charpy pendulum impact test method for metallic materials".

[0118] The products obtained in Examples 1-3 and the comparative examples were tested for hardened layer depth, surface microstructure, core microstructure, surface hardness and core hardness in accordance with JB / T 25744-2010 "Metallographic Inspection of Carburized, Quenched and Tempered Steel Parts". The results are shown in Tables 1 and 2.

[0119] Table 1

[0120] Carburized layer depth / mm Surface martensite / grade Cardiac tissue / grade Surface hardness / HRC Heart hardness / HRC Residual austenite / wt% Example 1 3mm 3 3 65HRC 46HRC 6 Example 2 2.84mm 3 2 63HRC 44HRC 8 Example 3 2.82mm 4 2 62HRC 42HRC 10 Comparative Example 2.8mm 4 2 60HRC 38HRC 14.5

[0121] As shown in Table 1, the carburized layer depth of Examples 1-3 reaches 2.82-3 mm, which is increased compared to the comparative example, and can meet the requirements of high-strength wear. The lower the surface martensite level, the finer the structure. Examples 1-2 are all at level 3, indicating that the surface hardness and wear resistance are better. The core structure level of Example 1 is level 3, indicating that the lath martensite in the core is more uniform and continuous, and the strength and toughness are better matched. The surface hardness of Example 1 reaches 65 HRC, which is 8% higher than the 60 HRC of the comparative example. The core hardness of Example 1 reaches 46 HRC, which is 21% higher than the 38 HRC of the comparative example, achieving dual optimization of surface wear resistance and core impact resistance. The residual austenite content of Example 1 is only 6 wt%, which is much lower than the 14.5 wt% of the comparative example. The residual austenite content of Examples 1-3 is 6-10 wt%. The reduction of residual austenite can avoid dimensional deformation and hardness reduction caused by microstructure transformation during use, and improve the dimensional stability of the roller cone drill bit. In Examples 1-3, as the carburizing temperature increased from 910℃ to 920℃ and the strong carburizing time was extended from 720 min to 960 min, the surface martensite grade increased from grade 3 to grade 4, the retained austenite content increased from 6 wt% to 10 wt%, and the surface and core hardness decreased slightly. This trend indicates that Example 1 represents the optimal process design, and its parameter combination (910℃ strong carburizing, 420 min diffusion) can maximize the balance between microstructure fineness, hardness, and retained austenite content.

[0122] Table 2

[0123] Surface carbon concentration / wt% Core carbon concentration / wt% Impact toughness / J Example 1 0.87 0.35 168 Example 2 0.95 0.26 155 Example 3 1.03 0.22 148 Comparative Example 1.05 0.17 146

[0124] As shown in Table 2, the surface carbon concentration in Examples 1-3 was 0.87-1.03 wt%, lower than the 1.05 wt% of the comparative example, avoiding the precipitation of network carbides caused by excessively high carbon concentration and reducing the risk of brittle surface cracking. The core carbon concentration in Examples 1-3 reached 0.22-0.35 wt%, significantly higher than the 0.17 wt% of the comparative example. The improved carbon concentration uniformity allowed for a more complete martensitic transformation in the core, resulting in simultaneous enhancement of core hardness and toughness. The impact toughness of Example 1 reached 168 J, a 15% increase compared to the 146 J of the comparative example. This improvement in impact toughness directly solved the defect of easy cracking and tooth loss in roller cone drill bits in traditional processes, enabling it to withstand repeated impact loads in deep well drilling. In Examples 1-3, as the surface carbon concentration increased from 0.87 wt% to 1.03 wt%, the core carbon concentration decreased from 0.35 wt% to 0.22 wt%, and the impact toughness decreased from 168 J to 146 J. This principle indicates that by optimizing the carbon potential and time ratio of strong infiltration and diffusion (strong infiltration carbon potential 1.21wt%, diffusion carbon potential 0.9wt%), the present invention achieves a smooth transition of carbon concentration from the surface to the core, avoiding the problem of excessively high carbon concentration in the surface and insufficient carbon concentration in the core in traditional processes, and ultimately achieving a synergistic effect of reducing hardness gradient and improving impact toughness.

[0125] As can be seen from Tables 1 and 2, the products prepared in Examples 1-3 using the heat treatment process of the present invention all have better performance than the products in the comparative examples.

[0126] The results show that the carburizing heat treatment process for the carburized steel of the roller cone drill bit of the present invention, by adjusting the ratio of strong carburizing time to diffusion time and temperature, and simultaneously adjusting the quenching and tempering stage temperatures and holding times, reduces the total process time of the comparative example from 45 hours to less than 37 hours. Furthermore, it lowers the holding temperatures of each process stage, significantly reducing energy consumption caused by excessively long periods at high temperatures. This results in a reduction of the residual austenite content in the carburized layer, increased surface and core hardness, reduced carbon concentration gradient, improved overall toughness, reduced internal hole deformation of the roller cone drill bit, and improved wear resistance. Further, by studying the effect of carburizing time on microstructure properties, a carburizing heat treatment regime was formulated to improve the performance of the roller cone drill bit from the surface to the core. Compared with the comparative example, Example 1, while significantly reducing energy consumption, improves the service performance of the carburized steel of the roller cone drill bit, achieving a significant increase in surface hardness of 8%, core hardness of 21%, carburized layer depth of 7%, and impact toughness of 15%.

[0127] The specific embodiments listed in this specification are merely illustrative of the technical solutions of the present invention and should not be construed as limiting the scope of patent protection. Within the scope defined by the claims of this invention, any equivalent substitutions or obvious modifications that a person skilled in the art could conceive of based on this specification should be considered to be included within the scope of protection of this invention.

Claims

1. A carburizing heat treatment process for carburizing steel used in roller cone drill bits, characterized in that, Includes the following steps: 1) The roller cone bit is placed in a furnace for carburizing treatment, which includes a strong carburizing period and a diffusion period: the strong carburizing period temperature is 910℃~920℃, the strong carburizing period time is 12h~16h, and the carbon potential is 1.18wt%~1.23wt%; after the strong carburizing period is completed, the roller cone bit is cooled to 880℃~900℃ and enters the diffusion period, the diffusion period time is 3h~7h, and the carbon potential is 0.9wt%~1.1wt%; then it is cooled to 825℃~835℃ and cooled in the furnace to 410℃~430℃ before being taken out of the furnace. 2) Perform the first high-temperature tempering treatment on the roller cone drill bit, with the temperature controlled at 620℃~640℃. After holding at this temperature for 2~4 hours under N2 protection, cool it down to 400℃~425℃ and then air cool it to room temperature. 3) Perform a second high-temperature tempering treatment on the roller cone drill bit, with the temperature controlled at 620℃~640℃. After holding at this temperature for 2~4 hours under N2 protection, cool it down to 400℃~425℃ and then air-cool it to room temperature. 4) After the second high-temperature tempering treatment, the roller cone drill bit is quenched at a temperature of 810~830℃, with a carbon potential of 0.28wt%~0.48wt%. After holding at this temperature for 105min~115min, it is cooled down to 110℃~130℃ by oil cooling. 5) After quenching, the roller cone drill bit is subjected to low-temperature tempering at a temperature of 140℃~160℃. After 5h~7h, it is taken out of the furnace and air-cooled to room temperature.

2. The carburizing heat treatment process for carburizing steel for roller cone drill bits according to claim 1, characterized in that, The carburized steel for roller cone drill bits is made of 15MnNi4Mo material.

3. The carburizing heat treatment process for carburizing steel for roller cone drill bits according to claim 1, characterized in that, In step 1), the strong penetration temperature during the strong penetration period is 910℃, the strong penetration time is 720min, and the carbon potential is 1.21wt%; the diffusion temperature during the diffusion period is 880℃, the diffusion time is 420min, and the carbon potential is 0.9wt%; after the diffusion period is completed, the roller cone drill bit is cooled to 830℃ and then cooled to 410℃ in the furnace before being taken out of the furnace.

4. The carburizing heat treatment process for carburizing steel for roller cone drill bits according to claim 1, characterized in that, In step 2), the first high-temperature tempering treatment is performed at 630°C, and the temperature is maintained for 2 hours under N2 protection. After cooling to 410°C, the temperature is then air-cooled to room temperature.

5. The carburizing heat treatment process for carburizing steel for roller cone drill bits according to claim 1, characterized in that, In step 3), the second high-temperature tempering treatment is performed at 630°C, held at this temperature for 2 hours under N2 protection, then cooled to 410°C and air-cooled to room temperature.

6. The carburizing heat treatment process for carburizing steel for roller cone drill bits according to claim 1, characterized in that, In step 4), the quenching temperature is 810℃, the carbon potential is 0.38wt%, and the holding time is 115min before oil cooling to 120℃.

7. The carburizing heat treatment process for carburizing steel for roller cone drill bits according to claim 1, characterized in that, In step 4), during the quenching process, the carburized steel workpiece of the roller cone drill bit adopts a moldless quenching method to avoid insufficient local cooling caused by mold obstruction, prevent soft spots or uneven structure, and ensure full contact between the workpiece and the quenching medium.

8. The carburizing heat treatment process for carburizing steel for roller cone drill bits according to claim 1, characterized in that, In step 4), the oil-cooled workpiece is cleaned with warm water at a temperature of 50℃~60℃ for 0.5~2 hours.

9. The carburizing heat treatment process for carburizing steel for roller cone drill bits according to claim 1, characterized in that, In step 5), the temperature for low-temperature tempering is 150℃, the holding time is 5h, and after low-temperature tempering, it is air-cooled to room temperature.

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