Hydrogen embrittlement-preventing carburizing heat treatment process for high-strength self-tapping screw

Abstract

The invention relates to the technical field of metal heat treatment, and discloses an anti-hydrogen embrittlement carburizing heat treatment process for a high-strength self-tapping screw, which comprises the following steps: placing a workpiece in a carburizing furnace, heating to carburizing temperature, and executing multi-cycle pulse carburizing, including a strong carburizing stage and a diffusion stage; a sulfur-containing atmosphere is introduced in the workpiece heating and diffusion stage; after carburization is finished, the workpiece is subjected to high-pressure air cooling, the temperature of the workpiece is reduced to be higher than the martensite phase transformation point, then the workpiece is immediately immersed into an in-situ passivation quenching medium for liquid cooling, and a liquid-phase hydrogen barrier is established in the medium-temperature phase transformation window period; and the quenched workpiece is subjected to low-temperature tempering. According to the invention, through a double cooperation mechanism of high-temperature dynamic sulfur potential control and medium-temperature in-situ passivation quenching, the invasion of hydrogen is inhibited from the source in the whole process, so that the diffusible hydrogen content of the workpiece is extremely low. Therefore, according to the process, the hydrogen embrittlement risk is eliminated, the hydrogen removal baking procedure is not needed, meanwhile, quenching deformation is reduced through segmented controlled cooling, and the product size precision is guaranteed.

Description

Technical Field

[0001] This invention relates to the field of metal heat treatment technology, specifically to a hydrogen embrittlement-resistant carburizing heat treatment process for high-strength self-tapping screws. Background Technology

[0002] High-strength self-tapping screws are critical fasteners widely used in construction, automotive, and machinery manufacturing. To meet their service requirements, these components typically require carburizing, quenching, and low-temperature tempering heat treatment processes to achieve high surface hardness, wear resistance, and good core toughness.

[0003] However, existing conventional carburizing heat treatment processes have serious technical drawbacks when applied to high-strength screws. Conventional gas carburizing (e.g., using propane or methanol as carburizing agents) decomposes at high temperatures, generating a large number of active hydrogen atoms. These hydrogen atoms readily penetrate the steel matrix under high temperature and high carbon potential. Furthermore, during the subsequent rapid quenching process, when the workpiece surface undergoes a martensitic phase transformation, the newly formed crystal lattice surface is highly reactive and will again adsorb hydrogen from the quenching medium (especially aqueous media), leading to a significant increase in the diffusible hydrogen content within the workpiece. These residual diffusible hydrogens are the root cause of hydrogen-induced delayed fracture (i.e., hydrogen embrittlement) in high-strength fasteners, posing a serious safety hazard.

[0004] To address the hydrogen embrittlement problem, the industry's common remedy is to add a lengthy hydrogen removal baking (HDB) process at the final stage of heat treatment. While this process can remove hydrogen from the workpiece to some extent, it also introduces new problems. HDB significantly extends the overall production cycle, reduces equipment turnover, and substantially increases energy consumption, leading to escalating manufacturing costs.

[0005] Meanwhile, conventional quenching processes typically involve immersing the workpiece directly into a liquid medium from the high temperature of carburizing. The huge temperature difference leads to severe thermal shock, generating significant thermal and phase transformation stresses within the workpiece. This can easily cause substantial quenching deformation, affecting the final precision of threads and other precision structures. Furthermore, the prolonged baking process for hydrogen removal is essentially a form of over-tempering, often resulting in a decrease in the surface hardness of the workpiece, thus sacrificing its wear resistance.

[0006] Therefore, developing a heat treatment process that can suppress hydrogen intrusion at the source, eliminate the hydrogen-free baking process, and effectively control quenching deformation is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a hydrogen embrittlement-resistant carburizing heat treatment process for high-strength self-tapping screws. This process solves the problems of high hydrogen embrittlement risk in existing high-strength screw carburizing heat treatment processes, and the low production efficiency, high energy consumption, and large workpiece deformation caused by the hydrogen removal baking process used to eliminate this risk.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a carburizing heat treatment process for high-strength self-tapping screws to prevent hydrogen embrittlement, comprising the following steps:

[0009] S1. The workpiece is placed in a carburizing furnace and heated to the carburizing temperature. During the carburizing process, multi-cycle pulse carburizing is performed. The pulse carburizing includes a strong carburizing stage and a diffusion stage. During the process of heating the workpiece to the carburizing temperature and during the diffusion stage, a sulfur-containing atmosphere is introduced into the carburizing furnace.

[0010] S2. After carburizing, the workpiece is subjected to high-pressure air cooling to reduce the workpiece temperature to above the martensitic phase transformation point. Then, the workpiece is immediately immersed in the in-situ passivation quenching medium for liquid cooling.

[0011] S3. The quenched workpiece is subjected to low-temperature tempering to obtain the high-strength self-tapping screw.

[0012] By adopting the above technical solution, this process inhibits the intrusion and accumulation of hydrogen in the steel matrix from the source by constructing a synergistic hydrogen barrier in two key process stages. Its mechanism of action is as follows:

[0013] First, a dynamic sulfur potential hydrogen intrusion inhibition mechanism was established during the high-temperature carburizing stage. Hydrocarbons (such as propane and methanol) in the gaseous carburizing atmosphere decompose at high temperatures, inevitably generating a large number of active hydrogen atoms, which is the main source of hydrogen intrusion. This scheme inhibits hydrogen intrusion by introducing a sulfur-containing atmosphere through a specific process window, utilizing the effect of sulfur atoms on the steel surface. The specific mechanism is divided into the following processes:

[0014] Surface active site occupancy: When the sulfur-containing atmosphere (such as a gas containing carbon disulfide) is introduced, it decomposes at high temperature, and the released sulfur atoms will preferentially adsorb on the highly active sites on the surface of the steel workpiece.

[0015] Suppressing hydrogen dissociation and adsorption: These active sites occupied by sulfur atoms are essential catalytic centers for the dissociation of hydrogen molecules (H2) in the gas phase into hydrogen atoms with high diffusion capacity. Because the active sites are blocked, the rate of the crucial reaction H2→2[H] is significantly reduced.

[0016] Dynamic coordinated control: This scheme does not introduce sulfur throughout the entire process, but only during the heating process and the diffusion stage of carburizing. During the intense carburizing stage, when rapid carbon atom diffusion is required, sulfur introduction is stopped, thus achieving a technical balance between suppressing hydrogen intrusion and ensuring effective carbon atom diffusion.

[0017] Second, during the intermediate-temperature quenching stage, an in-situ passivation hydrogen-blocking mechanism was established during the phase transformation window. When the workpiece cools from the austenitic state and undergoes a martensitic phase transformation, the lattice shear generates a large number of new, clean, and extremely high-energy surfaces. These newly formed surfaces readily react with moisture or decomposition products in conventional quenching media, resulting in secondary hydrogen absorption. This scheme controls this process:

[0018] Reduce reaction driving force: The workpiece temperature is pre-cooled to near the martensitic transformation point (Ms point) by high-pressure gas cooling before immersion in the liquid medium. This significantly reduces the temperature difference between the workpiece and the quenching medium, reducing the intensity of the interfacial reaction and the thermodynamic driving force for secondary hydrogen absorption.

[0019] Formation of a physical isolation layer: The in-situ passivation quenching medium used contains specific surface-active additives. When the martensitic phase transformation generates a new surface, these additives can instantly adsorb or react on the surface, forming a dense passivation film that blocks hydrogen permeation. This passivation film forms instantly at the same time as the phase transformation, achieving a physical blockade of the secondary hydrogen absorption path.

[0020] Therefore, this solution uses the synergistic effect of high-temperature gas-phase hydrogen inhibition and medium-temperature liquid-phase hydrogen inhibition to control the content of diffusible hydrogen introduced into the workpiece during the entire heat treatment process to an extremely low level, thereby fundamentally eliminating the risk of hydrogen embrittlement and making the subsequent hydrogen removal baking process (HDB) no longer a necessary step.

[0021] Preferably, in step S1, the sulfur-containing atmosphere is introduced for pre-passivation when the workpiece is heated to 700-800°C. This temperature range is in the early stage of austenitization, which establishes a basic hydrogen barrier layer on the workpiece surface in advance, and can more effectively suppress the intrusion of hydrogen during subsequent heating and carburizing processes.

[0022] Preferably, during the pre-passivation process, the volume concentration of the sulfur-containing atmosphere is 50-500 ppm, and the duration is 5-10 minutes. This parameter range ensures the formation of an effective hydrogen barrier layer while avoiding the negative impact of excessive sulfur on subsequent carburizing.

[0023] Preferably, in step S1, the carbon potential inside the furnace is controlled at 1.0-1.25%C during the strong infiltration stage and at 0.7-0.9%C during the diffusion stage, and the sulfur-containing atmosphere is introduced. By combining strong infiltration with high carbon potential with diffusion with low carbon potential and supplemental sulfur potential, a balance is achieved between efficient carbon atom infiltration and effective suppression of hydrogen intrusion.

[0024] Preferably, the volume concentration of the sulfur-containing atmosphere introduced during the diffusion stage is 20-200 ppm. A lower concentration of sulfur is sufficient to maintain the hydrogen barrier effect on the surface while minimizing the impact on carbon diffusion.

[0025] Preferably, in step S1, the carburizing temperature is 900-950℃. This temperature range is commonly used in industrial carburizing, balancing carburizing efficiency and grain control.

[0026] Preferably, in step S1, the duration of the strong carburizing phase is 20-30 minutes, and the duration of the diffusion phase is 10-20 minutes. This time ratio constitutes an effective pulsed carburizing cycle.

[0027] Preferably, in step S2, the high-pressure gas cooling uses nitrogen gas at 10-20 bar to cool the workpiece to 350-400°C. This pressure range provides a sufficient cooling rate to prevent the precipitation of non-martensitic structures, while the target cooling temperature is above the Ms point, creating ideal starting conditions for subsequent liquid cooling. Simultaneously, this segmented cooling method significantly reduces the thermal stress on the workpiece and minimizes quenching deformation.

[0028] Preferably, in step S2, the working temperature of the in-situ passivation quenching medium is maintained at 50-70℃. This temperature range ensures the cooling performance of the quenching medium and the activity of the passivation components, while avoiding excessively rapid cooling due to excessively low medium temperature.

[0029] Preferably, in step S3, the low-temperature tempering temperature is 170-200℃, and the holding time is 2-2.5 hours. These tempering process parameters effectively eliminate quenching stress and impart the required combination of strength and toughness to the workpiece. Furthermore, no additional hydrogen removal baking process is performed after the low-temperature tempering, directly demonstrating the advanced nature of this process, significantly shortening the production cycle and reducing energy consumption.

[0030] This invention provides a carburizing heat treatment process for high-strength self-tapping screws to prevent hydrogen embrittlement. It has the following beneficial effects:

[0031] 1. This invention constructs a dual hydrogen barrier in both the gas and liquid phases by employing dynamic sulfur potential control during the high-temperature carburizing stage and utilizing in-situ passivation of the quenching medium during the medium-temperature quenching stage. This synergistic mechanism inhibits the intrusion and accumulation of active hydrogen atoms into the steel matrix from the source throughout the entire process, ensuring that the diffusible hydrogen content in the final product is controlled at an extremely low level. This eliminates the risk of hydrogen-induced delayed fracture in high-strength screws, significantly improving the product's service reliability and safety.

[0032] 2. Because the process of this invention can effectively suppress hydrogen intrusion, the hydrogen content of the workpiece after heat treatment is much lower than that of conventional processes. Therefore, after low-temperature tempering, there is no need to add the time-consuming and energy-intensive hydrogen removal baking (HDB) process required in traditional processes to prevent hydrogen embrittlement. This not only greatly shortens the overall production cycle and improves equipment utilization, but also reduces energy consumption in the production process, thereby achieving effective control of manufacturing costs.

[0033] 3. The segmented controlled cooling method combining high-pressure gas cooling and liquid cooling employed in the quenching step of this invention effectively mitigates the thermal shock of the workpiece during cooling, reduces the temperature gradient across the workpiece cross-section, and mitigates the superposition effect of thermal stress and phase transformation stress. Therefore, this process can reduce the amount of quenching deformation in the workpiece and improve the dimensional stability and accuracy of high-strength screws, especially precision parts such as threads. Simultaneously, because no hydrogen removal baking is required, over-tempering caused by this process is avoided, thus ensuring that the workpiece surface maintains high hardness and wear resistance. Detailed Implementation

[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] Preparation Examples 1-3:

[0036] Preparation Example 1: Preparation of In-situ Passivation Quenching Medium A

[0037] Weigh the following components by weight percentage: 98.49% polyalkylene glycol copolymer, 1.0% triethanolamine, 0.5% sodium nitrite, and 0.01% polydimethylsiloxane.

[0038] The specific preparation method is as follows: The total amount of the polyalkylene glycol copolymer specified in the formula is added to a stainless steel reactor equipped with a stirring and heating jacket. Stirring is started, and the material inside the reactor is heated to 60°C. The specified amount of triethanolamine is slowly added dropwise, and stirring is continued for 30 minutes until it is completely dissolved. Subsequently, powdered sodium nitrite is added in batches, and stirring is continued for 60 minutes until all solid particles are completely dissolved. Finally, the specified amount of polydimethylsiloxane defoamer is added, and stirring continues for 15 minutes. The reactor is sealed, and a vacuum pump is started to reduce the pressure inside the reactor to below 1.0 kPa, while maintaining the temperature of the mixture at 90°C for vacuum dehydration treatment for 2 hours. After dehydration, heating is stopped, and the mixture is cooled to room temperature under vacuum protection to obtain a colorless and transparent in-situ passivation quenching medium A.

[0039] Preparation Example 2: Preparation of In-situ Passivation Quenching Medium B

[0040] Weigh the following components by weight percentage: 95.75% polyalkylene glycol copolymer, 3.0% triethanolamine, 1.2% sodium nitrite, and 0.05% polydimethylsiloxane.

[0041] The specific preparation method is as follows: The total amount of polyalkylene glycol copolymer in the formula is added to a stainless steel reactor equipped with a stirring and heating jacket. Stirring is started, and the material in the reactor is heated to 60°C. The formula amount of triethanolamine is slowly added dropwise, and stirring is continued for 30 minutes until it is completely dissolved. Subsequently, powdered sodium nitrite is added in batches, and stirring is continued for 90 minutes until all solid particles are completely dissolved. Finally, the formula amount of polydimethylsiloxane defoamer is added, and stirring is continued for 15 minutes. The reactor is sealed, and a vacuum pump is started to reduce the pressure inside the reactor to below 1.0 kPa, while maintaining the temperature of the mixture at 90°C for vacuum dehydration treatment for 3 hours. After dehydration, heating is stopped, and the mixture is cooled to room temperature under vacuum protection to obtain a colorless and transparent in-situ passivation quenching medium B.

[0042] Preparation Example 3: Preparation of In-situ Passivation Quenching Medium C

[0043] Weigh the following components by weight percentage: 92.9% polyalkylene glycol copolymer, 5.0% triethanolamine, 2.0% sodium nitrite, and 0.1% polydimethylsiloxane.

[0044] The specific preparation method is as follows: The total amount of polyalkylene glycol copolymer in the formula is added to a stainless steel reactor equipped with a stirring and heating jacket. Stirring is started, and the material in the reactor is heated to 60°C. The formula amount of triethanolamine is slowly added dropwise, and stirring is continued for 30 minutes until it is completely dissolved. Subsequently, powdered sodium nitrite is added in batches, and stirring is continued for 90 minutes until all solid particles are completely dissolved. Finally, the formula amount of polydimethylsiloxane defoamer is added, and stirring is continued for 15 minutes. The reactor is sealed, and a vacuum pump is started to reduce the pressure inside the reactor to below 1.0 kPa, while maintaining the temperature of the mixture at 90°C for vacuum dehydration treatment for 4 hours. After dehydration, heating is stopped, and the mixture is cooled to room temperature under vacuum protection to obtain a colorless and transparent in-situ passivation quenching medium C.

[0045] Examples 1-3:

[0046] Example 1:

[0047] This embodiment provides a carburizing heat treatment process for preventing hydrogen embrittlement in high-strength self-tapping screws, specifically including the following steps:

[0048] The pretreated SCM435 self-tapping screw workpieces are loaded into the furnace chamber of a multi-functional box-type carburizing furnace, sealed, and then evacuated to below 50Pa. High-purity nitrogen is then backfilled to atmospheric pressure.

[0049] Start the heating program. When the furnace temperature reaches 750°C, maintain this temperature and introduce nitrogen gas containing carbon disulfide into the furnace. Control the volume concentration of carbon disulfide in the furnace atmosphere to 200 ppm and continue for 8 minutes to perform surface pre-passivation.

[0050] After pre-passivation, the temperature is further increased to 920℃, and pulse carburizing begins. During the strong carburizing stage, propane and methanol are introduced to control the carbon potential in the furnace at 1.15%C for 25 minutes. During the diffusion / repair stage, propane is stopped, and only a methanol atmosphere is maintained. Simultaneously, nitrogen containing carbon disulfide is introduced to control the carbon disulfide volume concentration in the furnace atmosphere at 100 ppm for 15 minutes. This cycle is repeated until the target carburized layer depth is reached.

[0051] After carburizing, the workpiece is quickly transferred to the integrated quenching chamber, filled with high-purity nitrogen and pressurized to 15 bar for forced air cooling. When the surface temperature of the workpiece drops to 380°C, the air cooling is stopped and the workpiece is immediately immersed in the in-situ passivation quenching medium B prepared in Preparation Example 2 for liquid cooling. The temperature of the quenching medium is maintained at 60°C.

[0052] After the workpiece cools to room temperature, it is removed and rinsed with hot water, then placed in a tempering furnace and held at 180°C for 2 hours, before being removed and air-cooled. This embodiment does not perform an additional hydrogen removal baking (HDB) process.

[0053] Example 2:

[0054] This embodiment provides a carburizing heat treatment process for preventing hydrogen embrittlement in high-strength self-tapping screws, specifically including the following steps:

[0055] The pretreated SCM435 self-tapping screw workpieces are loaded into the furnace chamber of a multi-functional box-type carburizing furnace, sealed, and then evacuated to below 50Pa. High-purity nitrogen is then backfilled to atmospheric pressure.

[0056] Start the heating program. When the furnace temperature reaches 800°C, maintain this temperature and introduce nitrogen gas containing carbon disulfide into the furnace. Control the volume concentration of carbon disulfide in the furnace atmosphere to 500 ppm and continue for 5 minutes to perform surface pre-passivation.

[0057] After pre-passivation, the temperature is further increased to 950℃, and pulse carburizing begins. During the strong carburizing stage, propane and methanol are introduced to control the carbon potential in the furnace at 1.25%C for 20 minutes. During the diffusion / repair stage, propane is stopped, and only a methanol atmosphere is maintained. Simultaneously, nitrogen containing carbon disulfide is introduced to control the carbon disulfide volume concentration in the furnace atmosphere at 200 ppm for 10 minutes. This cycle is repeated until the target carburized layer depth is reached.

[0058] After carburizing, the workpiece is quickly transferred to the integrated quenching chamber, filled with high-purity nitrogen and pressurized to 20 bar for forced air cooling. When the surface temperature of the workpiece drops to 400°C, the air cooling is stopped and the workpiece is immediately immersed in the in-situ passivation quenching medium C prepared by Preparation Example 3 for liquid cooling. The temperature of the quenching medium is maintained at 70°C.

[0059] After the workpiece cools to room temperature, it is removed and rinsed with hot water, then placed in a tempering furnace and held at 170°C for 2 hours, before being removed and air-cooled. This embodiment does not perform an additional hydrogen removal baking (HDB) process.

[0060] Example 3:

[0061] This embodiment provides a carburizing heat treatment process for preventing hydrogen embrittlement in high-strength self-tapping screws, specifically including the following steps:

[0062] The pretreated SCM435 self-tapping screw workpieces are loaded into the furnace chamber of a multi-functional box-type carburizing furnace, sealed, and then evacuated to below 50Pa. High-purity nitrogen is then backfilled to atmospheric pressure.

[0063] Start the heating program. When the furnace temperature reaches 700°C, maintain this temperature and introduce nitrogen gas containing carbon disulfide into the furnace. Control the volume concentration of carbon disulfide in the furnace atmosphere to 50 ppm and continue for 10 minutes to perform surface pre-passivation.

[0064] After pre-passivation, the temperature is further increased to 900℃, and pulse carburizing begins. During the strong carburizing stage, propane and methanol are introduced to control the carbon potential in the furnace at 1.0%C for 30 minutes. During the diffusion / repair stage, propane is stopped, and only a methanol atmosphere is maintained. Simultaneously, nitrogen containing carbon disulfide is introduced to control the carbon disulfide volume concentration in the furnace atmosphere at 20 ppm for 20 minutes. This cycle is repeated until the target carburized layer depth is reached.

[0065] After carburizing, the workpiece is quickly transferred to the integrated quenching chamber, filled with high-purity nitrogen and pressurized to 10 bar for forced air cooling. When the surface temperature of the workpiece drops to 350°C, the air cooling is stopped and the workpiece is immediately immersed in the in-situ passivation quenching medium A prepared by Preparation Example 1 for liquid cooling. The temperature of the quenching medium is maintained at 50°C.

[0066] After the workpiece cools to room temperature, it is removed and rinsed with hot water, then placed in a tempering furnace and held at 200°C for 2.5 hours, before being removed and air-cooled. This embodiment does not perform an additional hydrogen removal baking (HDB) process.

[0067] Comparative Examples 1-5:

[0068] Comparative Example 1:

[0069] Compared to Example 1, the difference lies in the following: the use of industry-standard conventional carburizing heat treatment process. Specifically, the carburizing step does not use a sulfur-containing atmosphere for pre-passivation and dynamic repair, but instead uses a conventional gas carburizing process at 920°C; the quenching step does not use segmented controlled cooling and in-situ passivation quenching media, but instead directly immerses the workpiece in conventional rapid bright quenching oil; after tempering, an additional hydrogen removal baking (HDB) process is added, which involves holding the workpiece at 190°C for 8 hours.

[0070] Comparative Example 2:

[0071] The only difference from Comparative Example 1 is that the final hydrogen removal baking (HDB) process at 190°C for 8 hours is not performed; all other steps are the same.

[0072] Comparative Example 3:

[0073] Compared with Example 1, the only difference is the quenching step. Instead of segmented controlled cooling and the in-situ passivation quenching medium of this invention, the workpiece is directly immersed in conventional rapid bright quenching oil from 920°C for quenching; all other steps remain the same.

[0074] Comparative Example 4:

[0075] Compared to Example 1, the only difference is the carburizing step. Instead of using a sulfur-containing atmosphere for pre-passivation and dynamic repair, a conventional gas carburizing process is employed, without any control over the sulfur-containing atmosphere; all other steps remain the same.

[0076] Comparative Example 5:

[0077] Compared to Example 1, the difference lies in that high-pressure nitrogen is not used for segmented controlled cooling in the quenching step. Instead, after carburizing, the workpiece is directly immersed at 920°C into the in-situ passivation quenching medium B prepared in Preparation Example 2 for cooling, while the rest are the same.

[0078] Test Examples 1-5:

[0079] Test Example 1: Feasibility Test of the Invention

[0080] To verify that the process of the present invention can still achieve effective carburizing of workpieces under the condition of introducing a sulfur-containing atmosphere, the depth and hardness distribution of the carburized layer were tested on the samples prepared in Examples 1-3.

[0081] Experimental steps:

[0082] Sampling and Preparation: Samples were randomly selected from the screws treated in Examples 1, 2, and 3. The threaded portion was cut along a direction perpendicular to the screw axis, and a section was cold-mounted using epoxy resin. After the resin had completely cured, the cross-section of the inlay was successively polished with 240#, 400#, 800#, and 1200# wet sandpaper until the cross-section was smooth and free of scratches. Subsequently, fine polishing was performed on a polishing machine using diamond polishing paste with a grit size of 1.0 μm.

[0083] Hardness gradient measurement: Place the polished sample on the stage of the Vickers microhardness tester. Set the test load to 0.98 N (HV0.1). Starting from the outer contour of the workpiece surface, move the tester perpendicular to the surface towards the center, measuring the Vickers hardness value at each point every 0.05 mm until the hardness value at the center tends to stabilize.

[0084] Data recording and calculation: The hardness value at a distance of 0.05 mm from the surface was recorded as the surface hardness, and the average value of the core hardness stable zone was recorded as the core hardness. Based on the measured hardness gradient curve, the vertical depth at which the hardness value drops to 550 HV was determined as the effective hardened layer depth of the sample.

[0085] Experimental results:

[0086] Table 1. Test results of surface hardness, core hardness, and effective hardened layer depth of the sample in the examples.

[0087] Sample number Surface hardness (HV0.1) Core hardness (HV0.1) Effective hardened layer depth (mm) Example 1 821 412 0.94 Example 2 845 426 1.11 Example 3 794 407 0.86

[0088] Results analysis:

[0089] In the field of steel heat treatment, sulfur is generally regarded as an element that inhibits surface reactions. Therefore, introducing a sulfur-containing atmosphere into the carburizing process raises technical concerns about hindering the effective transfer of carbon atoms into the steel matrix.

[0090] The purpose of this test is to verify that the proposed process can overcome this technical challenge. The test data in Table 1 show that after processing in Examples 1-3, all samples achieved a clear hardness gradient distribution. Their surface hardness reached above 790 HV0.1, while the core remained at a lower level above 400 HV0.1, forming a typical carburized layer characteristic of being hard on the outside and tough on the inside. Simultaneously, each example formed an effective hardened layer depth of over 0.8 mm, meeting the metallurgical requirements of high-strength screws for the carburized layer.

[0091] The results confirm the feasibility of the process of this invention. The data show that a kinetic window was successfully established through dynamic synergistic control, employing high carbon potential during the intense carburizing stage and supplementing with low-concentration sulfur potential during the diffusion stage. Within this window, the inhibitory effect of sulfur on hydrogen molecule dissociation dominates, while the inhibitory effect on hydrocarbon decomposition and carbon atom transfer is effectively controlled within a range that does not affect the final carburizing effect. Therefore, this test example confirms that the proposed process can achieve its core function of preventing hydrogen embrittlement while ensuring that the workpiece obtains qualified carburized layer microstructure and properties.

[0092] Test Example 2: Comparison of Basic Mechanical Properties

[0093] To evaluate the impact of the process of the present invention on the final mechanical properties of the workpiece and to compare it with the prior art, the surface hardness, core hardness and breaking torque of the samples treated in Examples 1-3 and Comparative Examples 1-5 were tested.

[0094] Experimental steps:

[0095] Hardness test:

[0096] Surface hardness: The Rockwell hardness (HRC) of the screw head was measured using a Rockwell hardness tester on a flat surface. Three points were measured for each sample, and the average value was taken.

[0097] Core hardness: After cutting a cross-sectional sample of the screw, and after setting, grinding and polishing, the hardness (HRC) at its geometric center is measured using a Rockwell hardness tester.

[0098] Torque test:

[0099] Using a digital torque tester, the screw to be tested is fixed in a steel test block conforming to ISO 898-1 standard.

[0100] Apply torque at a constant rate until the screw breaks.

[0101] The maximum torque value displayed by the instrument when the screw breaks is recorded; this is the breaking torque.

[0102] Experimental results:

[0103] Table 2. Basic mechanical property test results of the examples and comparative samples

[0104] Sample number Surface hardness (HRC) Heart hardness (HRC) Destructive torque (N·m) Example 1 62.4 42.8 18.3 Example 2 63.1 43.5 18.7 Example 3 61.8 41.9 17.8 Comparative Example 1 60.3 42.4 17.4 Comparative Example 2 62.8 43.1 18.5 Comparative Example 3 62.2 42.6 17.2 Comparative Example 4 61.9 42.1 16.9 Comparative Example 5 62.5 42.9 17.1

[0105] Results analysis:

[0106] The samples from Examples 1-3 all exhibited high surface hardness (≥61.8 HRC), standard core hardness (41.9-43.5 HRC), and high breaking torque values. This indicates that the process of the present invention can endow the workpiece with excellent comprehensive mechanical properties.

[0107] Comparative Example 1 is an industry-standard process that incorporates a prolonged hydrogen removal baking (HDB) to eliminate the risk of hydrogen embrittlement. Data shows that its surface hardness (60.3 HRC) is significantly lower than that of the embodiments of this invention. This is attributed to the fact that the prolonged HDB treatment is essentially an over-tempering process, leading to a partial loss of surface hardness and thus affecting its breaking torque. The process of this invention avoids the HDB step by blocking hydrogen absorption at the source, thus maintaining a higher surface hardness.

[0108] The instantaneous mechanical properties of Comparative Example 2 (standard process without HDB), particularly the breaking torque of 18.5 N·m, are similar to those of Example 1, indicating that the presence of hydrogen does not significantly affect its instantaneous strength in the absence of external load delay. However, this data does not reflect its risk of hydrogen embrittlement under service conditions.

[0109] The destructive torque of Comparative Example 3 (atmosphere of the present invention + conventional oil quenching) and Comparative Example 4 (conventional atmosphere + quenching of the present invention) was lower than that of Example 1. This confirms that in the present invention, the dynamic sulfur potential hydrogen inhibition during the high-temperature carburizing stage and the in-situ passivation of the phase transformation window during the medium-temperature quenching stage have a synergistic effect on obtaining optimal performance, and the absence of either step will lead to a decrease in performance.

[0110] The destructive torque of Comparative Example 5 (without segmented controlled cooling) was also lower than that of Example 1. This indicates that the thermal shock and internal stress caused by direct liquid quenching from high temperature have a negative impact on the final load-bearing capacity of the workpiece, thus demonstrating the effectiveness of the gas-liquid dual-phase quenching process used in this invention in controlling quenching stress.

[0111] Test Example 3: Comparative Test of Hydrogen Embrittlement Sensitivity

[0112] To evaluate the effectiveness of the process of the present invention in suppressing hydrogen-induced delayed fracture (hydrogen embrittlement), static torque loading tests were performed on the samples treated in Examples 1-3 and Comparative Examples 1-5.

[0113] Experimental steps:

[0114] Sample grouping: 10 samples were randomly selected from the screws treated in Examples 1-3 and Comparative Examples 1-5.

[0115] Torque loading: Using a static torque loading test device, screws from each sample are screwed into a steel test block conforming to ISO 898-7. A constant torque is applied, which is 90% of the guaranteed load torque for the corresponding screw specification.

[0116] Timing and Observation: Place the loaded test device in a room temperature (20-25℃) environment and start the timing. Regularly inspect and record the time required for each sample from the start of loading to the occurrence of fracture.

[0117] Test termination: If the sample does not break after 200 hours of continuous loading, the test for the sample is terminated, and the breakage time is recorded as "200 hours".

[0118] Experimental results:

[0119] Table 3. Static loading fracture time test results of the examples and comparative samples

[0120] Sample number Mean fracture time (hours) Example 1 >200 Example 2 >200 Example 3 >200 Comparative Example 1 >200 Comparative Example 2 3.7 Comparative Example 3 21.5 Comparative Example 4 14.8 Comparative Example 5 >200

[0121] Results analysis:

[0122] The samples in Examples 1-3 did not fracture under continuous high torque loading for 200 hours. This indicates that the process combining dynamic sulfur potential hydrogen inhibition and in-situ passivation of the phase transition window employed in this invention effectively blocks hydrogen intrusion from both the carburizing and quenching stages, resulting in extremely low diffusible hydrogen content in the final product, thus exhibiting excellent resistance to hydrogen-induced delayed fracture.

[0123] The sample in Comparative Example 2 (conventional process without hydrogen removal) fractured within an average of 3.7 hours, a typical hydrogen embrittlement failure mode, demonstrating that conventional carburizing and quenching processes inevitably introduce a large amount of hydrogen, posing a serious safety hazard. Although Comparative Example 1 also achieved hydrogen embrittlement resistance through prolonged hydrogen removal baking (HDB), as shown in Test Example 2, this came at the cost of sacrificing surface hardness and significantly increasing production cycle and energy consumption.

[0124] The samples in Comparative Examples 3 and 4 both failed within a short period of time, confirming that the high-temperature carburizing stage and the medium-temperature quenching stage are both critical windows for hydrogen intrusion. Hydrogen embrittlement cannot be effectively suppressed by controlling hydrogen intrusion at only a single stage. This highlights the completeness and necessity of the synergistic hydrogen intrusion scheme throughout the entire process of this invention.

[0125] It is noteworthy that the sample in Comparative Example 5 (without segmented controlled cooling) also did not exhibit hydrogen embrittlement fracture. This indicates that the core hydrogen-blocking mechanism of this invention (dynamic sulfur potential + in-situ passivation quenching fluid) is effective. However, as shown in Test Example 2 and subsequent test examples, segmented controlled cooling is crucial for optimizing the final mechanical properties and controlling quenching deformation.

[0126] Test Example 4: Quantitative Analysis of Diffusible Hydrogen Content

[0127] To quantitatively analyze the content of diffusible hydrogen remaining in workpieces after different processes, thermal desorption analysis (TDA) was used to test the samples treated in Examples 1-3 and Comparative Examples 1-5. Diffusible hydrogen is the main factor leading to hydrogen embrittlement.

[0128] Experimental steps:

[0129] Sample preparation: Before testing, cut a 10-15 mm long sample from the middle of each group of screw samples. Clean the sample surface with acetone using ultrasonic cleaning to remove oil, then dry it in cold air. Immediately weigh the weighed samples using an electronic balance with an accuracy of 0.01 mg.

[0130] Thermal desorption analysis: The weighed sample is quickly placed in the quartz tube of the thermal desorption analyzer. After the system is evacuated, high-purity argon (Ar) is introduced as the carrier gas. The program is set to heat from room temperature (approximately 25°C) to 300°C at a constant rate of 100°C / h. Within this temperature range, the desorbed hydrogen is mainly diffusible hydrogen.

[0131] Hydrogen content detection: The hydrogen signal in the carrier gas is monitored in real time using a quadrupole mass spectrometer connected to the analyzer.

[0132] Data calculation: The total amount of hydrogen desorption is calculated by integrating the mass spectrometer signal. Then, based on the weight of the sample, the diffusible hydrogen content per unit mass of the workpiece is calculated, and the result is expressed in ppm (parts per million, by weight).

[0133] Experimental results:

[0134] Table 4. Test results of diffusible hydrogen content in the examples and comparative samples

[0135] Sample number Diffused hydrogen content (ppm) Example 1 0.12 Example 2 0.09 Example 3 0.15 Comparative Example 1 0.18 Comparative Example 2 1.85 Comparative Example 3 0.84 Comparative Example 4 0.67 Comparative Example 5 0.21

[0136] Results analysis:

[0137] The samples from Examples 1-3 all showed extremely low diffusible hydrogen content (≤0.15ppm), a value close to the original hydrogen content level of high-purity steel. This result directly confirms the effectiveness of the process of the present invention in blocking hydrogen intrusion. The mechanism is as follows: the dynamic sulfur potential control during the high-temperature carburizing stage forms and maintains an effective hydrogen barrier on the steel surface; while in the subsequent medium-temperature quenching stage, the in-situ passivation quenching medium forms a physical barrier again during the martensitic phase transformation window, blocking secondary hydrogen enrichment. The synergistic effect of the two stages ultimately achieves full-process suppression of hydrogen.

[0138] Comparative Example 2 (conventional process without hydrogen removal) had a hydrogen content as high as 1.85 ppm, which corresponds perfectly to the rapid hydrogen embrittlement phenomenon observed in Test Example 2.2, indicating that the conventional process introduces a large amount of harmful hydrogen. Comparative Example 1 reduced the hydrogen content to 0.18 ppm through prolonged hydrogen removal baking, achieving a safety level comparable to that of this invention, but this came at the cost of sacrificing production efficiency and increasing energy consumption.

[0139] The hydrogen contents of Comparative Examples 3 and 4 were 0.84 ppm and 0.67 ppm, respectively, which were much higher than those of the embodiments of the present invention, but lower than those of Comparative Example 2. This clearly demonstrates that the dynamic sulfur potential hydrogen inhibition step and the in-situ passivation quenching step in the present invention both have independent hydrogen inhibition effects, but neither single step can control the hydrogen content to a sufficiently low safe level. The optimal effect can only be achieved through the combination of the two.

[0140] The hydrogen content of Comparative Example 5 (0.21 ppm) was also at a low level, proving the effectiveness of the core hydrogen barrier mechanism of the present invention. However, its value was slightly higher than that of Example 1, which is related to the fact that the workpiece was directly immersed in the liquid at high temperature, resulting in a more intense interfacial reaction.

[0141] Test Example 5: Comparison of Quenching Deformation

[0142] To evaluate the effect of the segmented controlled cooling step in the process of the present invention on the dimensional stability of the workpiece, a comparative test was conducted on the dimensional changes of the samples after heat treatment in Example 1 and Comparative Example 5.

[0143] Experimental steps:

[0144] Initial Measurements: Samples were randomly selected from the same batch of screws to be processed and divided into two groups, corresponding to Example 1 and Comparative Example 5, respectively. The total length and thread outer diameter of each sample were measured using a high-precision image measuring instrument, and the initial dimensions were recorded.

[0145] Heat treatment: The two groups of samples were treated according to the processes of Example 1 (using gas-liquid segmented controlled cooling) and Comparative Example 5 (direct immersion in liquid quenching from high temperature).

[0146] Final measurement: After the samples have cooled to room temperature and been cleaned, the total length and thread outer diameter of each sample are measured again at the same location as the initial measurement using the same high-precision imaging measuring instrument.

[0147] Data calculation: Calculate the dimensional difference of each sample before and after heat treatment, which is the dimensional change of the sample.

[0148] Experimental results:

[0149] Table 5. Comparison of dimensional changes after heat treatment between Sample 1 and Comparative Example 5

[0150] Sample number Total length change (μm) Change in thread outer diameter (μm) Example 1 12.4 8.7 Comparative Example 5 35.8 26.2

[0151] Results analysis:

[0152] The data in Table 5 show that, compared with Comparative Example 5, the sample processed using the process of Example 1 showed a significant reduction in dimensional changes in both total length and thread outer diameter.

[0153] This result verifies the effectiveness of the segmented controlled cooling process in controlling quenching deformation in this invention. In Comparative Example 5, the workpiece was directly immersed from the high carburizing temperature (920°C) into a liquid medium. The significant temperature difference between the workpiece surface and the cooling medium resulted in severe thermal shock. This intense cooling generated substantial thermal stress within the workpiece, which, combined with the subsequent martensitic transformation stress, was the primary cause of the significant deformation of the workpiece.

[0154] The process in Example 1 was optimized through a high-pressure gas cooling stage. While the cooling rate of high-pressure gas is fast in the high-temperature region, it is still more gradual than liquid cooling, reducing the temperature gradient across the workpiece cross-section and thus lowering thermal stress. When the workpiece is cooled to near the martensitic transformation point (Ms point) before being immersed in the liquid medium, the temperature difference between the workpiece and the medium is significantly reduced, avoiding the severe thermal shock at the onset of the phase transformation. Therefore, the segmented controlled cooling process effectively manages the thermal stress and phase transformation stress of the workpiece throughout the cooling process, achieving effective control over the dimensional stability of the final product, which is crucial for ensuring the precision of threads and other precision structures.

Claims

1. A carburizing heat treatment process for preventing hydrogen embrittlement in high-strength self-tapping screws, characterized in that, Includes the following steps: S1. Place the workpiece in a carburizing furnace, heat it to the carburizing temperature, and perform multi-cycle pulse carburizing during the carburizing process. The pulse carburizing includes a strong carburizing stage and a diffusion stage. During the process of heating the workpiece to the carburizing temperature and during the diffusion stage, a sulfur-containing atmosphere is introduced into the carburizing furnace. S2. After carburizing, the workpiece is subjected to high-pressure air cooling to reduce the workpiece temperature to above the martensitic phase transformation point. Then, the workpiece is immediately immersed in the in-situ passivation quenching medium for liquid cooling. S3. The quenched workpiece is subjected to low-temperature tempering to obtain the high-strength self-tapping screw.

2. The anti-hydrogen embrittlement carburizing heat treatment process for high-strength self-tapping screws according to claim 1, characterized in that, In step S1, the sulfur-containing atmosphere is introduced for pre-passivation when the workpiece is heated to 700-800°C.

3. The anti-hydrogen embrittlement carburizing heat treatment process for high-strength self-tapping screws according to claim 2, characterized in that, During the pre-passivation process, the volume concentration of the sulfur-containing atmosphere is 50-500 ppm, and the duration is 5-10 minutes.

4. The anti-hydrogen embrittlement carburizing heat treatment process for high-strength self-tapping screws according to claim 1, characterized in that, In step S1, the carbon potential inside the furnace is controlled at 1.0-1.25%C during the strong percolation stage; During the diffusion stage, the carbon potential inside the furnace is controlled at 0.7-0.9%C, and the sulfur-containing atmosphere is introduced.

5. The anti-hydrogen embrittlement carburizing heat treatment process for high-strength self-tapping screws according to claim 4, characterized in that, The volume concentration of the sulfur-containing atmosphere introduced during the diffusion stage is 20-200 ppm.

6. The anti-hydrogen embrittlement carburizing heat treatment process for high-strength self-tapping screws according to claim 1, characterized in that, In step S1, the carburizing temperature is 900-950℃.

7. The anti-hydrogen embrittlement carburizing heat treatment process for high-strength self-tapping screws according to claim 1, characterized in that, In step S1, the duration of the strong penetration stage is 20-30 minutes, and the duration of the diffusion stage is 10-20 minutes.

8. The anti-hydrogen embrittlement carburizing heat treatment process for high-strength self-tapping screws according to claim 1, characterized in that, In step S2, the high-pressure air cooling uses 10-20 bar of nitrogen to cool the workpiece to 350-400°C.

9. The anti-hydrogen embrittlement carburizing heat treatment process for high-strength self-tapping screws according to claim 1, characterized in that, In step S2, the working temperature of the in-situ passivation quenching medium is maintained at 50-70℃.

10. The anti-hydrogen embrittlement carburizing heat treatment process for high-strength self-tapping screws according to claim 1, characterized in that, In step S3, the temperature of the low-temperature tempering is 170-200℃, and the holding time is 2-2.5 hours; and no additional hydrogen removal baking process is performed after the low-temperature tempering.