Composite heat treatment process for niobium-vanadium microalloyed bolt

Abstract

The invention discloses a niobium-vanadium microalloyed bolt composite heat treatment process. The process comprises the following steps: step 1, raw material pretreatment and component verification; step 2, gradient preheating treatment; 3, multi-section austenitizing heating is carried out; step 4, carrying out segmented isothermal precooling treatment; 5, double-medium segmented quenching treatment is carried out; step 6, intermediate tempering and stress release; seventhly, subzero treatment strengthening is conducted; 8, gradient final tempering treatment is carried out; ninthly, finished product cleaning and performance detection are conducted; according to the method, through multi-section austenitizing heating and gradient tempering, sufficient dissolution and dispersive precipitation of niobium-vanadium microalloy elements are achieved. The temperature system is designed according to the dissolution characteristics of vanadium and niobium elements in the double-stage austenitizing, and it is ensured that the two elements are completely fused into an austenite matrix; the gradient final tempering provides multi-level nucleation and growth conditions for a precipitated phase, the precipitation strengthening effect is remarkably improved, and the problem that the element utilization rate of an existing process is low is solved.

Description

Technical Field

[0001] This invention relates to the field of bolt manufacturing technology, specifically a composite heat treatment process for niobium-vanadium microalloyed bolts. Background Technology

[0002] As a core connecting component in mechanical structures, bolts' mechanical properties directly determine the operational safety and durability of equipment. In the field of high-end equipment manufacturing, niobium-vanadium microalloyed steel, through Nb(C,N) precipitation strengthening and V(C,N) grain refinement, possesses both high strength and good toughness, making it a preferred material for large-size, high-strength bolts. However, the heat treatment process for this type of steel requires extremely high precision in controlling temperature, time, and cooling rate. Existing technologies struggle to fully utilize the effectiveness of microalloying elements, resulting in shortcomings in bolt performance.

[0003] Chinese patent CN105907938A discloses a "heat treatment process for high-strength bolts," which involves normalizing: placing the product in a normalizing furnace at a temperature of 880-900℃ for 40-50 minutes, then removing it and cooling it in fine sand to 500℃; quenching: placing the naturally cooled product in a quenching furnace at a temperature of 840-860℃ for 30-40 minutes, followed by a first cooling and a second cooling process. This process has significant drawbacks: First, the single normalizing temperature cannot achieve sufficient dissolution and uniform distribution of niobium and vanadium elements, resulting in insufficient precipitate quantity during subsequent tempering and quenching, thus limiting the improvement of bolt tensile strength; Second, direct high-temperature tempering after quenching without an intermediate heat preservation stage leads to incomplete release of internal stress, making the bolts prone to early fatigue fracture under alternating loads; Third, the use of a single type of machine oil as the cooling medium makes it difficult to match the phase transformation requirements of microalloyed steel, resulting in the formation of pearlite structure in the core and reducing the overall consistency of mechanical properties.

[0004] Chinese patent CN102851479A discloses a "heat treatment process for high-strength bolts," which involves heating the workpiece at 820℃ for 50 minutes, followed by a water-cooling and oil-quenching process. This involves removing the heated workpiece from the furnace and first immersing it in water for 10-15 seconds to lower the temperature to below 400℃, then further cooling it in oil to below 50℃. While this process improves the low-temperature toughness of bolts, it has significant drawbacks: First, the isothermal quenching time is as long as 50 minutes, resulting in low production efficiency and difficulty in meeting mass production needs. Second, it lacks a specific heating regime designed for niobium-vanadium microalloying elements, leading to large and unevenly distributed precipitated phases, resulting in poor matching between the bolt's yield strength and tensile strength. Third, the process lacks surface decarburization control measures, resulting in low surface hardness at the bolt thread, making it prone to thread stripping failure.

[0005] Therefore, there is an urgent need to develop a composite heat treatment process for niobium-vanadium microalloyed bolts to solve the problems mentioned above. Summary of the Invention

[0006] The purpose of this invention is to provide a composite heat treatment process for niobium-vanadium microalloyed bolts to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a niobium-vanadium microalloyed bolt composite heat treatment process, comprising the following steps:

[0008] Step 1: Raw material pretreatment and composition verification: The bolt blanks are surface cleaned, chemically analyzed, and internally inspected to remove unqualified blanks;

[0009] Step 2, Gradient preheating treatment: The billet is preheated to 750℃ using a three-stage heating process and then held at that temperature;

[0010] Step 3, Multi-stage austenitizing heating: The temperature is raised to 860℃ and 920℃ in two stages, and then held at these temperatures to achieve complete dissolution of niobium and vanadium.

[0011] Step 4: Segmented isothermal pre-cooling treatment: The billet temperature is homogenized by salt bath and air cooling;

[0012] Step 5, Dual-medium segmented quenching treatment: Quenching is performed sequentially using water-based quenching fluid and machine oil, with the cooling rate controlled;

[0013] Step 6, Intermediate Tempering and Stress Relief: Hold at 300℃ to eliminate residual quenching stress;

[0014] Step 7, Cryogenic Treatment Strengthening: Holding at -80℃ promotes the transformation of residual austenite;

[0015] Step 8: Gradient final tempering treatment: Performance is adjusted by tempering at three stages: 400℃, 500℃, and 550℃.

[0016] Step 9, Finished Product Cleaning and Performance Testing: Perform surface cleaning, dimensional inspection, mechanical property testing, and internal structure testing on the bolts.

[0017] Preferably, step one specifically includes:

[0018] Surface cleaning: The bolt blanks are treated with a combination of mechanical grinding and ultrasonic cleaning. Mechanical grinding uses an 80-grit corundum wheel to remove surface oxide scale, oil stains and burrs. After grinding, the surface roughness is controlled below Ra1.6. Ultrasonic cleaning uses a mixture of industrial cleaning agent and water with a volume ratio of 1:20. The cleaning temperature is 50-60℃ and the cleaning time is 15-20 minutes. After cleaning, compressed air is used to dry the surface to avoid residual impurities affecting the heat treatment quality.

[0019] Composition Analysis: The chemical composition of the billet is analyzed using a direct-reading spectrometer. The content of core elements must meet the following requirements: C 0.38-0.45%, Si 0.17-0.37%, Mn 0.70-1.00%, Nb 0.02-0.06%, V 0.10-0.20%, Cr 0.90-1.20%, Mo 0.15-0.25%, with the remainder being Fe and unavoidable impurities. Three testing points are randomly selected from each batch of billets, and each testing point is tested twice. The average value is taken as the final result. Billets that do not meet the composition requirements are directly rejected.

[0020] Internal flaw detection: An ultrasonic flaw detector is used to detect internal defects in the billet. The detection range covers the entire cross section of the billet, and the sensitivity is adjusted according to the Φ2mm transverse hole standard. Billets with internal cracks, looseness, or inclusion defects are immediately isolated and prohibited from entering subsequent processes.

[0021] Preferably, step two specifically includes:

[0022] Furnace loading control: Place qualified billets into the box-type resistance furnace, with a spacing of not less than 15mm between billets and a distance of not less than 50mm from the furnace wall to ensure uniform heating; the loading temperature should not exceed 200℃ to avoid excessive temperature difference and initial stress.

[0023] Gradient heating: A three-stage heating regime is adopted. The first stage heats the temperature from room temperature to 400℃ at a heating rate of 5℃ / min; the second stage heats the temperature from 400℃ to 650℃ at a heating rate of 3℃ / min; and the third stage heats the temperature from 650℃ to 750℃ at a heating rate of 2℃ / min.

[0024] Temperature control: After reaching 750℃, heat preservation is carried out. The heat preservation time is calculated based on the maximum diameter of the billet, with 30 minutes of heat preservation for every 10mm. During the heat preservation period, the temperature fluctuation inside the furnace is controlled within ±5℃. Ten minutes before the end of heat preservation, the surface temperature of the billet in different areas of the furnace is measured to ensure that the temperature difference does not exceed 10℃.

[0025] Preferably, step three specifically includes:

[0026] Heating to the first austenitizing temperature: from the preheating temperature of 750℃ to 860℃, the heating rate is 4℃ / min. This stage mainly achieves the initial dissolution of vanadium.

[0027] First heat preservation stage: heat preservation at 860℃, heat preservation time is calculated based on the maximum diameter of the billet, heat preservation for 20 minutes per 10mm, to promote the full dissolution of V(C,N) phase;

[0028] Heating to the second austenitizing temperature: from 860℃ to 920℃, at a heating rate of 3℃ / min, during which niobium is dissolved and uniformly distributed;

[0029] The second heat preservation stage: heat preservation at 920℃, the heat preservation time is calculated based on the maximum diameter of the billet, heat preservation for 25 minutes for every 10mm, to ensure that the Nb(C,N) phase is completely dissolved and the austenite grain size is controlled at level 5-8; during the heat preservation period, the atmosphere in the furnace is monitored every 30 minutes to maintain the carbon potential in the furnace at 0.40-0.45% and prevent surface decarburization.

[0030] Preferably, step four specifically includes:

[0031] First pre-cooling stage: The austenitized billet is quickly transferred to a salt bath furnace. The salt bath medium is a mixture of 50% KNO3 and 50% NaNO3, the temperature is 600℃, and the pre-cooling time is 10-15 minutes, so that the temperature difference between the surface and the core of the billet is reduced to within 30℃.

[0032] The second pre-cooling stage: The billet is taken out of the salt bath furnace and pre-cooled in the air to 450-480℃ for 5-8 minutes. The surface temperature is monitored in real time by an infrared thermometer. Once the set temperature is reached, the quenching process begins immediately.

[0033] Preferably, step five specifically includes:

[0034] First medium quenching: Immerse the pre-cooled billet in water-based quenching liquid (PAG content 15-20%), quenching liquid temperature 25-30℃, stirring rate 300r / min, quenching time calculated according to the maximum diameter of the billet, quenching for 4 minutes for every 10mm, so that the surface temperature of the billet drops rapidly to below 200℃.

[0035] Second medium quenching: Take the billet out of the water-based quenching liquid and immediately transfer it to the machine oil quenching medium. The machine oil temperature is 40-50℃ and the quenching time is 15-20 minutes, so that the temperature of the core of the billet slowly drops to below 100℃, reducing the internal stress of quenching.

[0036] Post-quenching inspection: After quenching, use a hardness tester to test the surface hardness of the billet. The hardness value should reach HRC58-62. At the same time, observe whether there are cracks on the surface. Billets with cracks or unqualified hardness are marked and processed separately.

[0037] Preferably, step six specifically includes:

[0038] Furnace loading requirements: The quenched billets should be evenly placed in the tempering furnace, with a spacing of not less than 20mm between billets to avoid uneven heating caused by stacking;

[0039] Tempering heating and holding: Heat to 300℃ at a rate of 5℃ / min, and hold for 40 minutes for every 10mm of the billet diameter. This stage is mainly to eliminate the residual stress generated during quenching and to avoid cracking in subsequent processing.

[0040] Cooling control: After the heat preservation is completed, the furnace is cooled to below 150°C, and then the furnace is removed and air-cooled to room temperature. The cooling rate is controlled within 2°C / min to prevent the generation of new thermal stress.

[0041] Preferably, step seven specifically comprises:

[0042] Pre-cooling preparation: Place the billet after intermediate tempering into a deep cooling box and keep it at 0℃ for 30 minutes to make the billet temperature uniform;

[0043] Cryogenic cooling: The temperature is reduced to -80℃ at a rate of 1℃ / min. The temperature inside the chamber is monitored in real time during the cooling process to ensure that the temperature fluctuation does not exceed ±3℃.

[0044] Cryogenic insulation: Hold at -80℃ for 2-3 hours to promote the transformation of residual austenite to martensite and refine the size of precipitated phases;

[0045] Temperature recovery: After cryogenic treatment, the temperature is increased to room temperature at a rate of 2℃ / min to avoid thermal shock caused by rapid temperature rise.

[0046] Preferably, step eight specifically includes:

[0047] First stage of tempering: The cryogenically treated billet is placed in a tempering furnace and heated to 400℃ at a heating rate of 4℃ / min. The holding time is calculated based on the maximum diameter of the billet, with 35 minutes of holding time for every 10mm. This stage mainly adjusts the stability of the martensite structure.

[0048] The second stage of tempering: the temperature is increased to 500℃ at a rate of 3℃ / min, and the holding time is calculated based on the maximum diameter of the billet, with 45 minutes of holding time for every 10mm, to promote the diffuse precipitation of niobium, vanadium, carbonitridium compounds.

[0049] The third stage of tempering: the temperature is increased to 550℃ at a rate of 2℃ / min, and the holding time is calculated based on the maximum diameter of the billet, with 30 minutes of holding time for every 10mm, to achieve a balance between strength and toughness; during the holding period, the carbon potential in the furnace is maintained at 0.35-0.40% to prevent surface decarburization;

[0050] Tempering and cooling: After the final tempering is completed, the furnace is cooled to below 200°C, and then the furnace is removed and air-cooled to room temperature. The cooling rate is controlled within 3°C / min.

[0051] Preferably, step nine specifically comprises:

[0052] Surface cleaning: The tempered bolts are cleaned using a shot blasting machine with cast steel shot of 0.8-1.2mm in diameter, shot blasting pressure of 0.3-0.4MPa, and cleaning time of 5-8 minutes to remove surface oxide scale and residue; after cleaning, the surface is blown with compressed air to ensure no shot residue remains.

[0053] Dimensional inspection: The nominal diameter, length and thread size of the bolts are inspected using vernier calipers and thread gauges. The dimensional deviations must meet the requirements of GB / T196-2003 standard.

[0054] Mechanical property testing: Three bolts are randomly selected from each batch for mechanical property testing, including tensile strength, yield strength, elongation after fracture, and impact toughness; tensile strength is tested using a universal testing machine, and yield strength is determined based on 0.2% residual deformation; impact toughness is tested using a Charpy V-notch impact tester at room temperature; all performance indicators must meet design requirements.

[0055] Internal structure inspection: The cross-sectional structure of the bolts is observed using a metallographic microscope. The martensite content is not less than 95%, the precipitated phase size is controlled within 50-100nm, and the grain size is not less than grade 7. If bolts with unqualified structure are found, the batch needs to be sampled twice for inspection. If they are still unqualified, the entire batch is scrapped.

[0056] Compared with the prior art, the beneficial effects of the present invention are:

[0057] This invention achieves the full dissolution and dispersed precipitation of niobium-vanadium microalloying elements through multi-stage austenitizing heating and gradient tempering. The two-stage austenitizing process is designed with temperature regimes tailored to the dissolution characteristics of vanadium and niobium, ensuring complete integration of both elements into the austenitic matrix. The gradient final tempering provides multi-level nucleation and growth conditions for the precipitated phases, significantly improving the precipitation strengthening effect and solving the problem of low element utilization in existing processes. The combination of dual-medium segmented quenching and cryogenic treatment effectively balances the strength and toughness of the bolts. Rapid cooling of the water-based quenching fluid ensures the martensitic transformation of the surface and subsurface layers, while slow cooling of the machine oil reduces core stress. Cryogenic treatment further promotes the transformation of residual austenite to martensite and refines the microstructure, avoiding the performance inconsistencies caused by a single quenching medium. The introduction of intermediate tempering and gradient preheating processes constructs a comprehensive stress control system. Gradient preheating reduces the thermal stress caused by the initial temperature difference of the billet, intermediate tempering specifically eliminates the residual stress from quenching, and gradient tempering after deep cooling further optimizes the stress distribution, significantly reducing the risk of bolt cracking during service and solving the problem of incomplete stress release in existing processes. Attached Figure Description

[0058] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0059] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0060] Please see Figure 1 This invention provides a composite heat treatment process for niobium-vanadium microalloyed bolts, comprising the following steps:

[0061] Step 1: Raw Material Pretreatment and Composition Verification: The bolt blanks undergo surface cleaning, chemical composition testing, and internal flaw detection; substandard blanks are removed. Specifically, this includes:

[0062] Surface cleaning: The bolt blanks are treated with a combination of mechanical grinding and ultrasonic cleaning. Mechanical grinding uses an 80-grit corundum wheel to remove surface oxide scale, oil stains and burrs. After grinding, the surface roughness is controlled below Ra1.6. Ultrasonic cleaning uses a mixture of industrial cleaning agent and water with a volume ratio of 1:20. The cleaning temperature is 50-60℃ and the cleaning time is 15-20 minutes. After cleaning, compressed air is used to dry the surface to avoid residual impurities affecting the heat treatment quality.

[0063] Composition Analysis: The chemical composition of the billet is analyzed using a direct-reading spectrometer. The content of core elements must meet the following requirements: C 0.38-0.45%, Si 0.17-0.37%, Mn 0.70-1.00%, Nb 0.02-0.06%, V 0.10-0.20%, Cr 0.90-1.20%, Mo 0.15-0.25%, with the remainder being Fe and unavoidable impurities. Three testing points are randomly selected from each batch of billets, and each testing point is tested twice. The average value is taken as the final result. Billets that do not meet the composition requirements are directly rejected.

[0064] Internal flaw detection: An ultrasonic flaw detector is used to detect internal defects in the billet. The detection range covers the entire cross section of the billet. The sensitivity is adjusted according to the Φ2mm transverse hole standard. Billets with internal cracks, looseness or inclusion defects are immediately isolated and prohibited from entering subsequent processes.

[0065] Step 2, Gradient Preheating Treatment: The billet is preheated to 750℃ using a three-stage heating process and held at that temperature; specifically:

[0066] Furnace loading control: Place qualified billets into the box-type resistance furnace, with a spacing of not less than 15mm between billets and a distance of not less than 50mm from the furnace wall to ensure uniform heating; the loading temperature should not exceed 200℃ to avoid excessive temperature difference and initial stress.

[0067] Gradient heating: A three-stage heating regime is adopted. The first stage heats the temperature from room temperature to 400℃ at a heating rate of 5℃ / min; the second stage heats the temperature from 400℃ to 650℃ at a heating rate of 3℃ / min; and the third stage heats the temperature from 650℃ to 750℃ at a heating rate of 2℃ / min.

[0068] Temperature control: After reaching 750℃, heat preservation is carried out. The heat preservation time is calculated based on the maximum diameter of the billet, with 30 minutes of heat preservation for every 10mm. During the heat preservation period, the temperature fluctuation inside the furnace is controlled within ±5℃. Ten minutes before the end of heat preservation, the surface temperature of the billet in different areas of the furnace is measured to ensure that the temperature difference does not exceed 10℃.

[0069] Step 3, Multi-stage austenitizing heating: The temperature is raised to 860℃ and then 920℃ in two stages, followed by holding at these temperatures to ensure complete dissolution of niobium and vanadium. Specifically:

[0070] Heating to the first austenitizing temperature: from the preheating temperature of 750℃ to 860℃, the heating rate is 4℃ / min. This stage mainly achieves the initial dissolution of vanadium.

[0071] First heat preservation stage: heat preservation at 860℃, heat preservation time is calculated based on the maximum diameter of the billet, heat preservation for 20 minutes per 10mm, to promote the full dissolution of V(C,N) phase;

[0072] Heating to the second austenitizing temperature: from 860℃ to 920℃, at a heating rate of 3℃ / min, during which niobium is dissolved and uniformly distributed;

[0073] The second heat preservation stage: heat preservation at 920℃, the heat preservation time is calculated based on the maximum diameter of the billet, heat preservation for 25 minutes for every 10mm, to ensure that the Nb(C,N) phase is completely dissolved and the austenite grain size is controlled at level 5-8; during the heat preservation period, the atmosphere in the furnace is monitored every 30 minutes to maintain the carbon potential in the furnace at 0.40-0.45% to prevent surface decarburization.

[0074] Step 4: Segmented isothermal pre-cooling treatment: This involves using a salt bath and air cooling to homogenize the billet temperature; specifically:

[0075] First pre-cooling stage: The austenitized billet is quickly transferred to a salt bath furnace. The salt bath medium is a mixture of 50% KNO3 and 50% NaNO3, the temperature is 600℃, and the pre-cooling time is 10-15 minutes, so that the temperature difference between the surface and the core of the billet is reduced to within 30℃.

[0076] Second pre-cooling stage: The billet is taken out of the salt bath furnace and pre-cooled in the air to 450-480℃ for 5-8 minutes. The surface temperature is monitored in real time by an infrared thermometer. Once the set temperature is reached, the quenching process begins immediately.

[0077] Step 5, Dual-medium segmented quenching treatment: Quenching is performed sequentially using water-based quenching fluid and machine oil, controlling the cooling rate; specifically:

[0078] First medium quenching: Immerse the pre-cooled billet in water-based quenching liquid (PAG content 15-20%), quenching liquid temperature 25-30℃, stirring rate 300r / min, quenching time calculated according to the maximum diameter of the billet, quenching for 4 minutes for every 10mm, so that the surface temperature of the billet drops rapidly to below 200℃.

[0079] Second medium quenching: Take the billet out of the water-based quenching liquid and immediately transfer it to the machine oil quenching medium. The machine oil temperature is 40-50℃ and the quenching time is 15-20 minutes, so that the temperature of the core of the billet slowly drops to below 100℃, reducing the internal stress of quenching.

[0080] Post-quenching inspection: After quenching, use a hardness tester to test the surface hardness of the billet. The hardness value should reach HRC58-62. At the same time, observe whether there are cracks on the surface. Billets with cracks or unqualified hardness are marked and processed separately.

[0081] Step Six, Intermediate Tempering and Stress Relief: Hold at 300℃ to eliminate residual quenching stress; specifically:

[0082] Furnace loading requirements: The quenched billets should be evenly placed in the tempering furnace, with a spacing of not less than 20mm between billets to avoid uneven heating caused by stacking;

[0083] Tempering heating and holding: Heat to 300℃ at a rate of 5℃ / min, and hold for 40 minutes for every 10mm of the billet diameter. This stage is mainly to eliminate the residual stress generated during quenching and to avoid cracking in subsequent processing.

[0084] Cooling control: After the heat preservation is completed, the furnace is cooled to below 150℃, and then the furnace is removed and air-cooled to room temperature. The cooling rate is controlled within 2℃ / min to prevent the generation of new thermal stress.

[0085] Step 7, Cryogenic Treatment Strengthening: Holding at -80℃ promotes the transformation of residual austenite; specifically:

[0086] Pre-cooling preparation: Place the billet after intermediate tempering into a deep cooling box and keep it at 0℃ for 30 minutes to make the billet temperature uniform;

[0087] Cryogenic cooling: The temperature is reduced to -80℃ at a rate of 1℃ / min. The temperature inside the chamber is monitored in real time during the cooling process to ensure that the temperature fluctuation does not exceed ±3℃.

[0088] Cryogenic insulation: Hold at -80℃ for 2-3 hours to promote the transformation of residual austenite to martensite and refine the size of precipitated phases;

[0089] Temperature recovery: After cryogenic treatment, the temperature is increased to room temperature at a rate of 2℃ / min to avoid thermal shock caused by rapid temperature rise;

[0090] Step 8, Final Gradient Tempering: Performance is adjusted through three-stage tempering at 400℃, 500℃, and 550℃; specifically:

[0091] First stage of tempering: The cryogenically treated billet is placed in a tempering furnace and heated to 400℃ at a heating rate of 4℃ / min. The holding time is calculated based on the maximum diameter of the billet, with 35 minutes of holding time for every 10mm. This stage mainly adjusts the stability of the martensite structure.

[0092] The second stage of tempering: the temperature is increased to 500℃ at a rate of 3℃ / min, and the holding time is calculated based on the maximum diameter of the billet, with 45 minutes of holding time for every 10mm, to promote the diffuse precipitation of niobium, vanadium, carbonitridium compounds.

[0093] The third stage of tempering: the temperature is increased to 550℃ at a rate of 2℃ / min, and the holding time is calculated based on the maximum diameter of the billet, with 30 minutes of holding time for every 10mm, to achieve a balance between strength and toughness; during the holding period, the carbon potential in the furnace is maintained at 0.35-0.40% to prevent surface decarburization;

[0094] Tempering and cooling: After the final tempering is completed, the furnace is cooled to below 200°C, and then the furnace is removed and air-cooled to room temperature. The cooling rate is controlled within 3°C / min.

[0095] Step Nine, Finished Product Cleaning and Performance Testing: The bolts undergo surface cleaning, dimensional inspection, mechanical property testing, and internal structure analysis; specifically:

[0096] Surface cleaning: The tempered bolts are cleaned using a shot blasting machine with cast steel shot of 0.8-1.2mm in diameter, shot blasting pressure of 0.3-0.4MPa, and cleaning time of 5-8 minutes to remove surface oxide scale and residue; after cleaning, the surface is blown with compressed air to ensure no shot residue remains.

[0097] Dimensional inspection: The nominal diameter, length and thread size of the bolts are inspected using vernier calipers and thread gauges. The dimensional deviations must meet the requirements of GB / T196-2003 standard.

[0098] Mechanical property testing: Three bolts are randomly selected from each batch for mechanical property testing, including tensile strength, yield strength, elongation after fracture, and impact toughness; tensile strength is tested using a universal testing machine, and yield strength is determined based on 0.2% residual deformation; impact toughness is tested using a Charpy V-notch impact tester at room temperature; all performance indicators must meet design requirements.

[0099] Internal structure inspection: The cross-sectional structure of the bolts is observed using a metallographic microscope. The martensite content is not less than 95%, the precipitated phase size is controlled within 50-100nm, and the grain size is not less than grade 7. If bolts with unqualified structure are found, the batch needs to be sampled twice for inspection. If they are still unqualified, the entire batch is scrapped.

[0100] Example:

[0101] This invention provides a composite heat treatment process for niobium-vanadium microalloyed bolts. Taking 42CrMoVNb bolt blanks as an example, the specific steps are as follows:

[0102] Step 1: Raw material pretreatment and composition verification

[0103] Surface cleaning: The bolt blank is mechanically ground with an 80-mesh corundum grinding wheel, and the grinding path is along the axial direction of the blank to remove the surface oxide scale and burrs; then the blank is placed in an ultrasonic cleaning tank, and a cleaning solution prepared by mixing industrial cleaning agent and water at a volume ratio of 1:20 is added. The cleaning temperature is set to 55℃ and the cleaning time is 18 minutes; after cleaning, the blank is taken out and blown along the threads and end face with 0.6MPa compressed air until there is no liquid residue on the surface.

[0104] Composition analysis: The chemical composition of the billet was analyzed using a direct-reading spectrometer. One testing point was selected at each end and the middle of the billet. Data was collected at each testing point using the "three-point sampling method" and the average value was taken. The test results showed: C 0.42%, Si 0.25%, Mn 0.85%, Nb 0.04%, V 0.15%, Cr 1.05%, and Mo 0.20%, which meets the composition requirements.

[0105] Internal flaw detection: An ultrasonic flaw detector with a probe diameter of 20mm was used. After calibrating the sensitivity according to the standard of Φ2mm transverse hole, the entire cross-section of the billet was inspected. During the flaw detection process, the probe was moved at a uniform speed of 30mm / s. If an internal porosity defect was found in one billet, it was immediately rejected and marked.

[0106] Step 2: Gradient preheating treatment

[0107] Furnace loading control: Place qualified billets into the box-type resistance furnace. The billets are arranged in a single layer and evenly, with a spacing of 20mm between adjacent billets and 60mm from the furnace wall. The initial temperature inside the furnace is 180℃ when loading.

[0108] Gradient heating: Start the heating program. The first stage heats from 180℃ to 400℃ at a heating rate of 5℃ / min, taking 44 minutes. The second stage heats from 400℃ to 650℃ at a heating rate of 3℃ / min, taking 83 minutes. The third stage heats from 650℃ to 750℃ at a heating rate of 2℃ / min, taking 50 minutes.

[0109] Temperature control: Temperature holding begins after reaching 750℃, with a maximum billet diameter of 30mm and a holding time of 90 minutes. During the holding period, the furnace temperature is recorded every 20 minutes, with a temperature fluctuation range of 748-752℃. Ten minutes before the end of the holding period, the surface temperature of the billet at different locations is measured using an infrared thermometer, with a temperature difference of 8℃, which meets the requirements.

[0110] Step 3: Multi-stage austenitizing heating

[0111] Heating to the first austenitizing temperature: from 750℃ to 860℃, heating rate 4℃ / min, time 27.5 minutes.

[0112] First heat preservation stage: heat preservation at 860℃ for 60 minutes. This stage mainly promotes the dissolution of V(C,N) phase.

[0113] Heating to the second austenitizing temperature: from 860℃ to 920℃, heating rate 3℃ / min, taking 20 minutes.

[0114] The second holding stage: The furnace was held at 920℃ for 75 minutes. During the holding period, the carbon potential inside the furnace was maintained at 0.42% using a carbon potential controller. After the holding period, samples were taken for observation, and the austenite grain size was grade 6, which meets the requirements.

[0115] Step 4: Segmented isothermal precooling treatment

[0116] First pre-cooling stage: The austenitized billet is quickly transferred to a salt bath furnace. The salt bath medium is a mixture of 50% KNO3 and 50% NaNO3, the temperature is 600℃, the transfer time is no more than 30 seconds, and the pre-cooling time is 12 minutes.

[0117] The second pre-cooling stage: The billet is taken out of the salt bath furnace using a special fixture and pre-cooled naturally in the air at an ambient temperature of 25°C. After 6 minutes of pre-cooling, the surface temperature is measured by an infrared thermometer and is 465°C, which meets the pre-cooling requirements.

[0118] Step 5: Dual-medium segmented quenching treatment

[0119] First medium quenching: The pre-cooled billet is immediately immersed in a PAG water-based quenching liquid tank with a quenching liquid concentration of 18%, a temperature of 28℃, a stirrer speed of 300r / min, and a quenching time of 10 minutes.

[0120] Second medium quenching: Take the billet out of the water-based quenching fluid and quickly transfer it to the oil quenching tank. The oil temperature is 45℃ and the quenching time is 18 minutes.

[0121] Post-quenching inspection: After quenching, the surface hardness was tested with a Rockwell hardness tester at three points. The hardness values ​​were HRC60, HRC59, and HRC61, with an average of HRC60. Visual inspection showed no cracks on the surface, which met the requirements.

[0122] Step Six: Intermediate Tempering and Stress Relief

[0123] Furnace loading requirements: Place the quenched billets into the tempering furnace with a spacing of 25mm between billets to avoid stacking.

[0124] Tempering and holding: The temperature was raised from room temperature to 300°C at a rate of 5°C / min, taking 60 minutes, and then held for 120 minutes.

[0125] Cooling control: After the heat preservation is completed, the heating device is turned off and the furnace is cooled to 140°C, which takes 280 minutes. Then it is taken out of the furnace and air-cooled to room temperature at a cooling rate of 1.8°C / min.

[0126] Step 7: Cryogenic Treatment for Enhancement

[0127] Pre-cooling preparation: Place the intermediate tempered billet into a deep cooling box, set the temperature to 0℃, and hold for 30 minutes.

[0128] Cryogenic cooling: Start the cooling program and reduce the temperature to -80℃ at a rate of 1℃ / min, which takes 80 minutes. During the cooling process, the temperature inside the chamber fluctuates between -82℃ and -78℃.

[0129] Cryogenic insulation: Insulate at -80℃ for 2.5 hours.

[0130] Temperature recovery: The temperature is raised from -80°C to room temperature at a rate of 2°C / min, taking 40 minutes. After being taken out of the oven, it is left to stand for 30 minutes to allow the temperature to stabilize completely.

[0131] Step 8: Gradient Final Tempering Process

[0132] First stage of tempering: The cryogenically treated billet is placed in a tempering furnace and heated to 400°C at a rate of 4°C / min for 75 minutes, followed by a holding time of 105 minutes.

[0133] The second stage of tempering: the temperature was increased from 400°C to 500°C at a rate of 3°C / min, taking 33 minutes, and the holding time was 135 minutes.

[0134] The third stage of tempering: the temperature is increased from 500℃ to 550℃ at a rate of 2℃ / min, taking 25 minutes, followed by a holding time of 90 minutes. During the holding period, the carbon potential inside the furnace is controlled at 0.38%.

[0135] Tempering and cooling: After the heat preservation is completed, the furnace is cooled to 180°C, which takes 283 minutes. Then it is taken out of the furnace and air-cooled to room temperature at a cooling rate of 2.7°C / min.

[0136] Step Nine: Finished Product Cleaning and Performance Testing

[0137] Surface cleaning: Place the tempered bolts into a shot blasting machine, use 1.0mm cast steel shot, shot blasting pressure 0.35MPa, cleaning time 6 minutes; after cleaning, blow the threaded parts with 0.5MPa compressed air to ensure no shot residue.

[0138] Dimensional inspection: The nominal diameter was measured with a 0-50mm vernier caliper and the actual measurement was 30.02mm; the thread accuracy was measured with a thread gauge and met the 6g grade requirement; the length was measured with a 150-200mm vernier caliper and the actual measurement was 150.05mm, all of which meet the standard.

[0139] Mechanical property testing: Three bolts were randomly selected for testing. The tensile strength, yield strength, elongation after fracture, and impact toughness all met the design requirements. After the test, the fracture surface was observed and showed ductile fracture characteristics.

[0140] Internal structure testing: A cross-sectional sample of the bolt was taken, and after grinding, polishing, and etching, it was observed under a metallographic microscope. The martensite content was 97%, the precipitated phase size was about 70nm, and the grain size was grade 7, which met the microstructure requirements.

[0141] The niobium-vanadium microalloyed bolts processed using the process of this invention have significantly improved mechanical performance balance and microstructure stability compared to bolts using existing processes, and the production cycle is shortened, fully meeting the requirements of high-end equipment for high-strength bolts.

[0142] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite heat treatment process for niobium-vanadium microalloyed bolts, characterized in that: Includes the following steps: Step 1: Raw material pretreatment and composition verification: The bolt blanks are surface cleaned, chemically analyzed, and internally inspected to remove unqualified blanks; Step 2, Gradient preheating treatment: The billet is preheated to 750℃ using a three-stage heating process and then held at that temperature; Step 3, Multi-stage austenitizing heating: The temperature is raised to 860℃ and 920℃ in two stages, and then held at these temperatures to achieve complete dissolution of niobium and vanadium. Step 4: Segmented isothermal pre-cooling treatment: The billet temperature is homogenized by salt bath and air cooling; Step 5, Dual-medium segmented quenching treatment: Quenching is performed sequentially using water-based quenching fluid and machine oil, with the cooling rate controlled; Step 6, Intermediate Tempering and Stress Relief: Hold at 300℃ to eliminate residual quenching stress; Step 7, Cryogenic Treatment Strengthening: Holding at -80℃ promotes the transformation of residual austenite; Step 8: Gradient final tempering treatment: Performance is adjusted by tempering at three stages: 400℃, 500℃, and 550℃. Step 9, Finished Product Cleaning and Performance Testing: Perform surface cleaning, dimensional inspection, mechanical property testing, and internal structure testing on the bolts.

2. The composite heat treatment process for niobium-vanadium microalloyed bolts according to claim 1, characterized in that: Step one specifically includes: Surface cleaning: The bolt blanks are treated with a combination of mechanical grinding and ultrasonic cleaning. Mechanical grinding uses an 80-grit corundum wheel to remove surface oxide scale, oil stains and burrs. After grinding, the surface roughness is controlled below Ra1.

6. Ultrasonic cleaning uses a mixture of industrial cleaning agent and water with a volume ratio of 1:

20. The cleaning temperature is 50-60℃ and the cleaning time is 15-20 minutes. After cleaning, the bolts are dried with compressed air. Composition Analysis: The chemical composition of the billet is analyzed using a direct-reading spectrometer. The content of core elements must meet the following requirements: C 0.38-0.45%, Si 0.17-0.37%, Mn 0.70-1.00%, Nb 0.02-0.06%, V 0.10-0.20%, Cr 0.90-1.20%, Mo 0.15-0.25%, with the remainder being Fe and unavoidable impurities. Three testing points are randomly selected from each batch of billets, and each testing point is tested twice. The average value is taken as the final result. Billets that do not meet the composition requirements are directly rejected. Internal flaw detection: An ultrasonic flaw detector is used to detect internal defects in the billet. The detection range covers the entire cross section of the billet, and the sensitivity is adjusted according to the Φ2mm transverse hole standard. Billets with internal cracks, looseness, or inclusion defects are immediately isolated and prohibited from entering subsequent processes.

3. The composite heat treatment process for niobium-vanadium microalloyed bolts according to claim 1, characterized in that: Step two specifically involves: Furnace loading control: Place qualified billets into the box-type resistance furnace, with a spacing of not less than 15mm between billets and a distance of not less than 50mm from the furnace wall; the loading temperature shall not exceed 200℃; Gradient heating: A three-stage heating regime is adopted. The first stage heats the temperature from room temperature to 400℃ at a heating rate of 5℃ / min; the second stage heats the temperature from 400℃ to 650℃ at a heating rate of 3℃ / min; and the third stage heats the temperature from 650℃ to 750℃ at a heating rate of 2℃ / min. Temperature control: After reaching 750℃, heat preservation is carried out. The heat preservation time is calculated based on the maximum diameter of the billet, with 30 minutes of heat preservation for every 10mm. During the heat preservation period, the temperature fluctuation inside the furnace is controlled within ±5℃. Ten minutes before the end of heat preservation, the surface temperature of the billet in different areas of the furnace is measured to ensure that the temperature difference does not exceed 10℃.

4. The composite heat treatment process for niobium-vanadium microalloyed bolts according to claim 1, characterized in that: Step three specifically involves: Heat to the first austenitizing temperature: from the preheating temperature of 750℃ to 860℃, at a heating rate of 4℃ / min; First heat preservation stage: heat preservation at 860℃, heat preservation time is calculated based on the maximum diameter of the billet, 20 minutes for every 10mm of heat preservation; Heat to the second austenitizing temperature: from 860℃ to 920℃, at a heating rate of 3℃ / min; The second holding stage: hold at 920℃ for 25 minutes per 10mm of billet, with the austenite grain size controlled at level 5-8. During the holding period, monitor the furnace atmosphere every 30 minutes to maintain the carbon potential in the furnace at 0.40-0.45%.

5. The composite heat treatment process for niobium-vanadium microalloyed bolts according to claim 1, characterized in that: Step four specifically involves: First pre-cooling stage: The austenitized billet is quickly transferred to a salt bath furnace. The salt bath medium is a mixture of 50% KNO3 and 50% NaNO3, the temperature is 600℃, and the pre-cooling time is 10-15 minutes, so that the temperature difference between the surface and the core of the billet is reduced to within 30℃. The second pre-cooling stage: The billet is taken out of the salt bath furnace and pre-cooled in the air to 450-480℃ for 5-8 minutes. The surface temperature is monitored in real time by an infrared thermometer. Once the set temperature is reached, the quenching process begins immediately.

6. The composite heat treatment process for niobium-vanadium microalloyed bolts according to claim 1, characterized in that: Step five specifically involves: First medium quenching: Immerse the pre-cooled billet in water-based quenching liquid at a temperature of 25-30℃ and a stirring rate of 300r / min. The quenching time is calculated based on the maximum diameter of the billet, with 4 minutes of quenching per 10mm, so that the surface temperature of the billet drops rapidly to below 200℃. Second medium quenching: Take the billet out of the water-based quenching liquid and immediately transfer it to the machine oil quenching medium. The machine oil temperature is 40-50℃ and the quenching time is 15-20 minutes, so that the temperature of the core of the billet slowly drops to below 100℃. Post-quenching inspection: After quenching, use a hardness tester to test the surface hardness of the billet. The hardness value should reach HRC58-62. At the same time, observe whether there are cracks on the surface. Billets with cracks or unqualified hardness are marked and processed separately.

7. The composite heat treatment process for niobium-vanadium microalloyed bolts according to claim 1, characterized in that: Step six specifically involves: Furnace loading requirements: The quenched billets should be evenly placed in the tempering furnace, with a spacing of not less than 20mm between the billets; Tempering heating and holding: Heat to 300℃ at a rate of 5℃ / min, and hold for 40 minutes per 10mm of billet diameter. Cooling control: After the heat preservation is completed, the furnace is cooled to below 150℃, and then the furnace is removed and air-cooled to room temperature. The cooling rate is controlled within 2℃ / min.

8. The composite heat treatment process for niobium-vanadium microalloyed bolts according to claim 1, characterized in that: Step seven specifically involves: Pre-cooling preparation: Place the billet after intermediate tempering into a deep cooling box and hold it at 0℃ for 30 minutes to make the billet temperature uniform; Cryogenic cooling: The temperature is reduced to -80℃ at a rate of 1℃ / min. The temperature inside the chamber is monitored in real time during the cooling process to ensure that the temperature fluctuation does not exceed ±3℃. Cryogenic insulation: Hold at -80℃ for 2-3 hours to promote the transformation of residual austenite to martensite and refine the size of precipitated phases; Temperature recovery: After the cryogenic process is completed, the temperature is increased to room temperature at a rate of 2℃ / min.

9. The composite heat treatment process for niobium-vanadium microalloyed bolts according to claim 1, characterized in that: Step eight specifically involves: First stage of tempering: Place the cryogenically treated billet into the tempering furnace, heat it to 400℃, heating rate 4℃ / min, and hold for 35 minutes for every 10mm of the billet's maximum diameter. The second stage of tempering: the temperature is increased to 500℃ at a rate of 3℃ / min, and the holding time is calculated based on the maximum diameter of the billet, with 45 minutes of holding time for every 10mm, to promote the diffuse precipitation of niobium, vanadium, carbonitridium compounds. Third stage tempering: Increase to 550℃ at a rate of 2℃ / min, and hold for 30 minutes for every 10mm of billet diameter, calculated based on the maximum diameter of the billet; maintain the carbon potential in the furnace at 0.35-0.40% during the holding period. Tempering and cooling: After the final tempering is completed, the furnace is cooled to below 200°C, and then the furnace is removed and air-cooled to room temperature. The cooling rate is controlled within 3°C / min.

10. The composite heat treatment process for niobium-vanadium microalloyed bolts according to claim 1, characterized in that: Step nine specifically involves: Surface cleaning: The tempered bolts are cleaned using a shot blasting machine with cast steel shot of 0.8-1.2mm in diameter, shot blasting pressure of 0.3-0.4MPa, and cleaning time of 5-8 minutes to remove surface oxide scale and residue; after cleaning, the surface is blown with compressed air to ensure no shot residue remains. Dimensional inspection: The nominal diameter, length and thread size of the bolts are inspected using vernier calipers and thread gauges. The dimensional deviations must meet the requirements of GB / T196-2003 standard. Mechanical property testing: Three bolts are randomly selected from each batch for mechanical property testing, including tensile strength, yield strength, elongation after fracture, and impact toughness; tensile strength is tested using a universal testing machine, and yield strength is determined based on 0.2% residual deformation; impact toughness is tested using a Charpy V-notch impact tester at room temperature; all performance indicators must meet design requirements. Internal structure inspection: The cross-sectional structure of the bolts is observed using a metallographic microscope. The martensite content is not less than 95%, the precipitated phase size is controlled within 50-100nm, and the grain size is not less than grade 7. If bolts with unqualified structure are found, the batch needs to be sampled twice for inspection. If they are still unqualified, the entire batch is scrapped.

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