Full-process closed-loop end hardenability process and quality control method for medium carbon alloy structural steel

Abstract

The invention relates to the technical field of metal material performance detection and heat treatment, and discloses a full-flow closed-loop end hardenability process for medium carbon alloy structural steel and a quality control method. Comprising the first step of sample preparation calibration, the second step of heat treatment process parameter calibration, the third step of heat treatment operation calibration, the fourth step of detection process calibration and the fifth step of result verification and parameter adjustment. According to the method, the calibration range of the heating temperature (835-865 DEG C and within the fluctuation range of + / -6 DEG C), the heating and heat preservation time (48-68 min), the quenching water temperature (17-23 DEG C) and the water column height (63-67 mm) is defined through the step 2, and the parameter execution precision is fixed through thermocouple real-time monitoring and constant temperature device control means, so that the problem that the detection results of the same material are incomparable due to different processes is solved; misjudgment of an enterprise on material quality is avoided, and the risk that qualified raw materials are wasted and unqualified materials flow into a processing link is reduced.

Description

Technical Field

[0001] This invention relates to the field of metal material performance testing and heat treatment technology, specifically to a closed-loop end-hardening process and quality control method for medium carbon alloy structural steel. Background Technology

[0002] End-quench testing is a core method for evaluating the hardenability of structural steel. Its results directly determine the selection of subsequent heat treatment processes and the safety of product use. This is especially true when medium-carbon alloy structural steel is used to manufacture critical components that bear complex loads (such as steering knuckles and shafts), where the end-quench value (e.g., J9) is a key indicator of whether the material meets processing and service requirements. However, in the current end-quench testing process, the lack of uniformity and standardization of heat treatment process parameters has become a core problem leading to deviations in test results and affecting data comparability.

[0003] In different testing scenarios, there is a lack of unified calibration standards for key process steps such as heating temperature, holding time, and quenching medium parameters. On the one hand, the heating furnace temperature setting often fluctuates by 5-15℃, and the holding time can vary by 10-20 minutes (for example, the total heating and holding time in some scenarios is only 35 minutes, while the process that meets the national standard requires more than 50 minutes), resulting in insufficient austenitization or uneven austenitization of the sample core. On the other hand, the quenching water temperature control accuracy is insufficient (the fluctuation range often exceeds ±5℃) and the water column height deviation (more than ±3mm) further exacerbates the dispersion of hardenability test results. This non-standard process directly leads to testing deviations. For the same material, differences in heat treatment processes can result in end-quench values ​​(J9) varying by 3-11 HRC. This not only causes companies to misjudge material quality (e.g., classifying qualified materials as unqualified or vice versa), but also potentially leads to cracking risks during subsequent processing (e.g., cracking rate exceeding the agreed-upon 5% limit after tempering). It also causes stockpiling of raw materials (e.g., high-hardenability materials cannot be processed in a timely manner) and end-customer complaints, severely impacting production stability and supply chain trust. Furthermore, the existing process does not fully consider the impact of residual elements (e.g., Ti, Mo) on hardenability and does not incorporate composition control into the process calibration system, further amplifying the uncertainty of test results. Summary of the Invention

[0004] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a closed-loop end-hardening process and quality control method for medium-carbon alloy structural steel. It has advantages such as unified process parameter benchmarks, elimination of detection deviations, and improved comparability of results, thus solving the problem of large detection deviations.

[0005] (II) Technical Solution To achieve the above-mentioned unified process parameter benchmark, eliminate detection deviations, and improve the comparability of results, this invention provides the following technical solution: a closed-loop end-hardening process and quality control method for medium carbon alloy structural steel, comprising steps 1: sample preparation calibration, step 2: heat treatment process parameter calibration, step 3: heat treatment operation calibration, step 4: detection process calibration, and step 5: result verification and parameter adjustment. Step 1: sample preparation calibration includes steps 1.1 processing method and size control, step 1.2 surface quality treatment, and step 1.3 parallel sample preparation. Step 2: calibration of heat treatment process parameters includes step 2.1 calibration of heating temperature, step 2.2 calibration of heating and holding time, step 2.3 calibration of quenching medium parameters, and step 2.4 calibration of anti-decarburization / carburization measures. Step 3: Heat treatment operation calibration includes step 3.1 sample loading into the furnace, step 3.2 sample unloading from the furnace and quenching transfer, and step 3.3 quenching process monitoring; Step 4: Detection process calibration includes Step 4.1 Detection equipment calibration, Step 4.2 Detection position calibration, and Step 4.3 Repeat detection calibration; Step 5: Result Verification and Parameter Adjustment includes Step 5.1 Comparison of Calibration Results, Step 5.2 Adjustment of Process Parameters, Step 5.3 Auxiliary Control of Component Influence, and Step 5.4 Calibration Records and Cycles.

[0006] Preferably, in step 1.1, processing method and size control, the sample is prepared by sawing, wire cutting or a combination of turning and grinding to ensure that the sample length deviation is controlled within ±2.5mm and the diameter deviation is controlled within ±0.4mm, which meets the basic size requirements of the end quenching test standard. Step 1.2 Surface quality treatment: The test surface of the sample needs to be finely polished, and the surface finish should be controlled at 0.4-1.2μm, free of oxide scale, scratches, pits and decarburized layer; after processing, wipe the surface with anhydrous ethanol to remove oil stains, iron filings and impurities. Step 1.3 Parallel Sample Preparation: Prepare at least 5-8 parallel samples for each batch of calibration to reduce the impact of individual sample processing errors on the overall calibration results and ensure data representativeness.

[0007] Preferably, in step 2.1, heating temperature calibration: the target temperature of the heating furnace is set to 835-865℃, and after heating to the target temperature, it is held for 12-18 minutes to make the temperature inside the furnace uniform; the furnace temperature is monitored in real time using an insertion thermocouple, and the temperature fluctuation is controlled within ±6℃ to avoid local overheating or insufficient temperature. Step 2.2 Heating and Holding Time Calibration: After the sample is placed in the furnace, the temperature is first increased at a rate of 5-8℃ / min (if the furnace supports it), and the heating time is controlled at 22-32min to ensure that the core of the sample reaches the set temperature completely; then the holding stage is entered, and the holding time is set at 26-36min. During the holding period, the furnace temperature is recorded every 5min to prevent temperature drift. Step 2.3 Quenching medium parameter calibration: Industrial clean water is used as the quenching medium, and the water temperature is controlled at 17-23℃ by a constant temperature device; the height of the quenching water column is adjusted to 63-67mm, and the water flow velocity is controlled at 0.8-1.2m / s to ensure that the water column impacts the test surface of the sample vertically and uniformly, without any diversion or turbulence. Step 2.4 Calibration of anti-decarburization / carburization measures: During the heating process, inert gas (nitrogen) is introduced into the furnace or graphite powder is placed as an anti-oxidation medium to reduce decarburization on the sample surface; after calibration, the thickness of the decarburized layer on the sample surface is measured, which should be ≤0.12mm. If it exceeds the range, the amount of protective medium is adjusted.

[0008] Preferably, in step 3.1, sample loading into the furnace, the prepared sample is placed evenly in the effective heating zone of the furnace, with a spacing of not less than 12 mm between samples, and the sample does not directly contact the furnace wall or heating elements to avoid uneven local temperature.

[0009] Preferably, in step 3.2, sample removal from the furnace and transfer after quenching: after the heat preservation is completed, the sample is quickly removed using a special clamp, and the removal time is controlled to be ≤10 seconds; the sample is immediately transferred to the quenching device to ensure that the test surface is completely aligned with the center of the water jet impact, and the quenching time is not less than 18 minutes, during which the sample position is not moved.

[0010] Preferably, in step 3.3, quenching process monitoring: during quenching, the water temperature, water column height, and water flow rate are recorded every 6 minutes. If the parameters exceed the set range (water temperature ±1℃, water column height ±0.5mm), they are immediately adjusted through a constant temperature device or valve to ensure stable quenching conditions.

[0011] Preferably, in step 4.1, the testing equipment is calibrated: before testing, the hardness testing equipment (manual or automatic) is calibrated for accuracy using a standard hardness block (hardness value covering 35-48 HRC). The equipment error must be controlled within ±1.2 HRC. If the error exceeds the standard, the equipment pressure and the position of the testing head must be adjusted and recalibrated. Step 4.2 Detection position calibration: Define the end-quench detection point as the "center + annular area" of the quenched end face of the sample - point 1 at the center, and 3 points evenly distributed in the annular area 4-6mm away from the center, for a total of 4 detection points; mark the detection positions with a marker to avoid detection offset, and clean the surface oxide products before each point is detected; Step 4.3 Repeated test calibration: After the initial test of each sample, a second test is performed at an interval of 8-12 minutes (after the sample temperature has returned to room temperature); all parallel samples must be repeatedly tested. The range of the three test results for the same sample is ≤2.8HRC, and the average value is taken as the final quench value of the sample.

[0012] Preferably, in step 5.1, the calibration result comparison is performed by comparing the average end quench value of all parallel samples with the target range (34-47 HRC). If more than 90% of the sample results are within the target range and the dispersion is ≤3.5 HRC, the calibration is deemed qualified. If it is not qualified, the parameter execution deviation (furnace temperature fluctuation, quenching water temperature exceeding the standard) needs to be investigated.

[0013] Preferably, in step 5.2, process parameter adjustment: if the overall detection value is too high (average exceeding the target upper limit by more than 2 HRC), the heating temperature can be lowered by 5-12℃ or the holding time can be shortened by 3-5 min; if the overall detection value is too low (average below the target lower limit by less than 2 HRC), the heating temperature can be raised by 5-12℃ or the holding time can be extended by 3-5 min. After adjustment, a small batch (3 samples) calibration verification should be performed again.

[0014] Preferably, step 5.3, auxiliary control of composition influence, involves simultaneously controlling the residual element content during calibration—Ti≤0.0065%, Mo≤0.028%—in conjunction with the influence of material composition on hardenability, to reduce the interference of composition fluctuations on end-quench values. If the composition exceeds the standard, the raw materials or smelting process need to be adjusted. Step 5.4 Calibration Records and Cycles: After calibration, record detailed sample information, process parameters, test data, and adjustments to form the "End Quenching Test Heat Treatment Process Calibration Report". The calibration cycle is set to once every 5-7 months. If there is equipment maintenance, process change, or significant deviation in test results, recalibration is required immediately.

[0015] (III) Beneficial Effects Compared with existing technologies, this invention provides a closed-loop end-hardening process and quality control method for medium-carbon alloy structural steel, which has the following beneficial effects: 1. This method presents a closed-loop end-hardening process and quality control method for medium-carbon alloy structural steel. This method standardizes process parameters, eliminates testing deviations, and improves the comparability of results. Step 2 clarifies the calibration ranges for heating temperature (835-865℃, fluctuation within ±6℃), heating + holding time (48-68min), quenching water temperature (17-23℃), and water column height (63-67mm). Real-time thermocouple monitoring and constant temperature control fix the parameter execution accuracy. In practical applications, the end-hardening value deviation under different scenarios can be reduced to within ±2.8HRC, solving the problem of "incomparable test results for the same material due to different processes." This avoids misjudging material quality by enterprises and reduces the risk of waste of qualified raw materials and the inflow of unqualified materials into the processing stage.

[0016] 2. This method describes a closed-loop end-hardening process and quality control method for medium-carbon alloy structural steel. This method standardizes the entire process, reduces human error, and ensures testing stability. Through detailed calibration in steps 1 (sample surface finish 0.4-1.2μm, 5-8 parallel samples), 3 (sample removal time ≤10 seconds, quenching position alignment), and 4 (test point positioning "center + annular area," repeat test range ≤2.8HRC), the method minimizes the impact of human operational differences on the results. For example, strict control of sample surface finish avoids hardness testing deviations caused by grinding defects (reducing errors within ±1.5HRC). Repeated testing and parallel sample design further reduce data randomness, ensuring a stable end-hardening test pass rate of over 90%, effectively preventing subsequent processing cracking caused by unstable testing (cracking rate after tempering can be controlled within 5% of the agreed requirements).

[0017] 3. The closed-loop end-hardening process and quality control method for medium-carbon alloy structural steel. This method links composition and process calibration to improve the adaptability of results and match actual usage requirements. In step 5, this method explicitly includes residual elements (Ti≤0.0065%, Mo≤0.028%) in the calibration system. Through the synergistic optimization of composition control and process parameter adjustment, the interference of trace elements on hardenability is reduced (for every 0.001% decrease in Ti content, the end-hardening value can be stably reduced by 0.5-1 HRC). This design makes the end-hardening test results more consistent with the actual smelting level of the material, avoiding the situation where "the process is qualified but the material is unqualified in actual use" due to composition fluctuations. It ensures that the test results are accurately matched with the processing requirements of the end parts (steering knuckle tempering process) and reduces abnormal feedback during customer field trials.

[0018] 4. This method for closed-loop end-quenching process and quality control of medium carbon alloy structural steel establishes a closed-loop calibration mechanism to ensure long-term accuracy and support production continuity. Step 5 sets a calibration cycle of 5-7 months and requires immediate recalibration after equipment maintenance or process changes. Simultaneously, the entire process is traceable through the archiving of calibration reports (retention period ≥ 3 years). This closed-loop management effectively prevents process parameter drift caused by furnace aging and quenching device sealing failure, ensuring long-term stability of end-quenching test results. After application, the annual end-quenching test pass rate can be maintained above 94%, avoiding production interruptions due to test failures (stockpiled raw materials blocked, batch re-judgment), and ensuring the continuity and stability of the supply chain. Detailed Implementation

[0019] 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.

[0020] This solution provides a technical solution, specifically, a closed-loop end-hardening process and quality control method for medium carbon alloy structural steel, including the following method, step 1: sample preparation and calibration; By standardizing the sample processing methods, dimensional accuracy, and surface quality, interference from processing defects on the test results can be avoided. Step 1.1 Processing method and size control: The sample is prepared by sawing, wire cutting or "turning + grinding" combination processing method to ensure that the sample length deviation is controlled within ±2.5mm and the diameter deviation is controlled within ±0.4mm, which meets the basic size requirements of the end quenching test standard; Step 1.2 Surface quality treatment: The test surface of the sample needs to be finely polished, and the surface finish should be controlled at 0.4-1.2μm, free of oxide scale, scratches, pits and decarburized layer; after processing, wipe the surface with anhydrous ethanol to remove oil stains, iron filings and impurities. Step 1.3 Parallel Sample Preparation: Prepare at least 5-8 parallel samples for each batch of calibration to reduce the impact of individual sample processing errors on the overall calibration results and ensure data representativeness; Step 2: Calibration of heat treatment process parameters; Clearly define the calibration range and control requirements for key parameters of heating, heat preservation, and quenching to ensure process consistency; Step 2.1 Heating temperature calibration: Set the target temperature of the heating furnace to 835-865℃. After heating to the target temperature, hold for 12-18 minutes to ensure uniform temperature inside the furnace. Use an insertion thermocouple to monitor the furnace temperature in real time, and control the temperature fluctuation within ±6℃ to avoid local overheating or insufficient temperature. Step 2.2 Heating and Holding Time Calibration: After the sample is placed in the furnace, the temperature is first increased at a rate of 5-8℃ / min (if the furnace supports it), and the heating time is controlled at 22-32min to ensure that the core of the sample reaches the set temperature completely; then the holding stage is entered, and the holding time is set at 26-36min. During the holding period, the furnace temperature is recorded every 5min to prevent temperature drift. Step 2.3 Quenching medium parameter calibration: Industrial clean water is used as the quenching medium, and the water temperature is controlled at 17-23℃ by a constant temperature device; the height of the quenching water column is adjusted to 63-67mm, and the water flow velocity is controlled at 0.8-1.2m / s to ensure that the water column impacts the test surface of the sample vertically and uniformly, without any diversion or turbulence. Step 2.4 Calibration of anti-decarburization / carburization measures: During the heating process, inert gas (nitrogen) is introduced into the furnace or graphite powder is placed as an anti-oxidation medium to reduce decarburization on the sample surface; after calibration, the thickness of the decarburized layer on the sample surface is measured, which should be ≤0.12mm. If it exceeds the range, the amount of protective medium is adjusted. Step 3: Heat treatment operation calibration; Standardize operating procedures to reduce the impact of human error on process execution; Step 3.1 Sample loading operation: Place the prepared sample evenly in the effective heating zone of the furnace, with a spacing of not less than 12mm between samples, and ensure that the sample does not come into direct contact with the furnace wall or heating elements to avoid uneven local temperature. Step 3.2 Sample removal and quenching transfer: After the heat preservation is completed, the sample is quickly removed using a special clamp, and the removal time is controlled within ≤10 seconds; the sample is immediately transferred to the quenching device, ensuring that the test surface is completely aligned with the center of the water jet impact, and the quenching time is not less than 18 minutes, during which the sample position is not moved. Step 3.3 Quenching process monitoring: During quenching, record the water temperature, water column height and water flow rate every 6 minutes. If the parameters exceed the set range (water temperature ±1℃, water column height ±0.5mm), adjust them immediately through the constant temperature device or valve to ensure stable quenching conditions. Step 4: Calibration of the testing process; Calibrate the testing equipment and operational details to ensure the accuracy and repeatability of the test results; Step 4.1 Calibration of testing equipment: Before testing, the hardness testing equipment (manual or automatic) shall be calibrated for accuracy using a standard hardness block (hardness value covering 35-48HRC). The equipment error shall be controlled within ±1.2HRC. If the error exceeds the standard, the equipment pressure and the position of the testing head shall be adjusted and recalibrated. Step 4.2 Detection position calibration: Define the end-quench detection point as the "center + annular area" of the quenched end face of the sample - point 1 at the center, and 3 points evenly distributed in the annular area 4-6mm away from the center, for a total of 4 detection points; mark the detection positions with a marker to avoid detection offset, and clean the surface oxide products before each point is detected; Step 4.3 Repeated testing and calibration: After the initial test of each sample, a second test is performed at an interval of 8-12 minutes (after the sample temperature has returned to room temperature); all parallel samples must be tested repeatedly. The range of the three test results for the same sample is ≤2.8HRC, and the average value is taken as the final quench value of the sample. Step 5: Result verification and parameter adjustment; Optimize process parameters based on calibration results to form a closed-loop management system. Step 5.1 Comparison of calibration results: Compare the average end quenching value of all parallel samples with the target range (34-47 HRC). If more than 90% of the sample results are within the target range and the dispersion is ≤3.5 HRC, the calibration is considered qualified. If it is not qualified, the parameter execution deviation (furnace temperature fluctuation, quenching water temperature exceeding the standard) needs to be investigated. Step 5.2 Adjustment of process parameters: If the overall detection value is too high (average exceeding the target upper limit by more than 2 HRC), the heating temperature can be lowered by 5-12℃ or the holding time can be shortened by 3-5 min; if the overall detection value is too low (average below the target lower limit by less than 2 HRC), the heating temperature can be increased by 5-12℃ or the holding time can be extended by 3-5 min. After adjustment, small batch (3 samples) calibration and verification should be performed again. Step 5.3 Auxiliary control of composition influence: In combination with the influence of material composition on hardenability, the residual element content is controlled simultaneously during the calibration process—Ti≤0.0065%, Mo≤0.028%—to reduce the interference of composition fluctuations on the end quench value. If the composition exceeds the standard, the raw materials or smelting process need to be adjusted. Step 5.4 Calibration Records and Cycle: After calibration, record the sample information, process parameters, test data and adjustments in detail to form the "End Quenching Test Heat Treatment Process Calibration Report"; the calibration cycle is set to once every 5-7 months. If there is equipment maintenance, process change or significant deviation in test results, recalibration is required immediately. Furthermore, this method standardizes process parameters, eliminates detection bias, and improves the comparability of results. Step 2 clarifies the calibration ranges for heating temperature (835-865℃, fluctuation within ±6℃), heating + holding time (48-68min), quenching water temperature (17-23℃), and water column height (63-67mm). Real-time thermocouple monitoring and constant temperature control fix the parameter execution accuracy. In practical applications, the deviation of end-quench values ​​under different scenarios can be reduced to within ±2.8HRC, solving the problem of "incomparable test results for the same material due to different processes," avoiding misjudgments of material quality by enterprises, and reducing the risk of waste of qualified raw materials and the inflow of unqualified materials into the processing stage. Furthermore, this method standardizes the entire process, reduces human error, and ensures testing stability. Through detailed calibration in steps 1 (sample surface finish 0.4-1.2 μm, 5-8 parallel samples), 3 (sample removal time ≤ 10 seconds, quenching position alignment), and 4 (test point positioning "center + annular area," repeatability range ≤ 2.8 HRC), the method minimizes the impact of human operational differences on the results. For example, strict control of sample surface finish avoids hardness testing deviations caused by grinding defects (reducing errors within ±1.5 HRC). Repeatability testing and parallel sample design further reduce data randomness, ensuring a stable end-quench test pass rate of over 90%, effectively preventing subsequent processing cracking caused by unstable testing (cracking rate after tempering can be controlled within 5% of the agreed requirements). Furthermore, this method correlates composition with process calibration to improve the adaptability of the results and match actual usage requirements. In step 5, this method explicitly includes residual elements (Ti≤0.0065%, Mo≤0.028%) in the calibration system. Through the synergistic optimization of composition control and process parameter adjustment, the interference of trace elements on hardenability is reduced (for every 0.001% decrease in Ti content, the end-quench value can be stably reduced by 0.5-1 HRC). This design makes the end-quench test results more consistent with the actual smelting level of the material, avoiding the situation where "the process is qualified but the material is unqualified in actual use" due to composition fluctuations. It ensures that the test results are accurately matched with the processing requirements of the end parts (steering knuckle tempering process) and reduces abnormal feedback during customer field trials. Furthermore, this method establishes a closed-loop calibration mechanism to ensure long-term accuracy and support production continuity. This method sets a calibration cycle of 5-7 months in step 5 and requires immediate recalibration after equipment maintenance or process changes. At the same time, the entire process is traceable through the archiving of the "Calibration Report" (retention period ≥ 3 years). This closed-loop management can effectively prevent process parameter drift caused by furnace aging and quenching device sealing failure, ensuring long-term stability of end-quench test results. After application, the annual end-quench test pass rate can be maintained above 94%, avoiding production interruptions due to test failure (blocking of raw material inventory, batch re-judgment), and ensuring the continuity and stability of the supply chain.

[0021] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A closed-loop end-hardening process and quality control method for medium-carbon alloy structural steel, comprising steps 1: sample preparation and calibration, 2: heat treatment process parameter calibration, 3: heat treatment operation calibration, 4: testing process calibration, and 5: result verification and parameter adjustment, characterized in that: Step 1: Sample preparation and calibration includes Step 1.1 Processing method and size control, Step 1.2 Surface quality treatment and Step 1.3 Parallel sample preparation; Step 2: calibration of heat treatment process parameters includes step 2.1 calibration of heating temperature, step 2.2 calibration of heating and holding time, step 2.3 calibration of quenching medium parameters, and step 2.4 calibration of anti-decarburization / carburization measures. Step 3: Heat treatment operation calibration includes step 3.1 sample loading into the furnace, step 3.2 sample unloading from the furnace and quenching transfer, and step 3.3 quenching process monitoring; Step 4: Detection process calibration includes Step 4.1 Detection equipment calibration, Step 4.2 Detection position calibration, and Step 4.3 Repeat detection calibration; Step 5: Result Verification and Parameter Adjustment includes Step 5.1 Comparison of Calibration Results, Step 5.2 Adjustment of Process Parameters, Step 5.3 Auxiliary Control of Component Influence, and Step 5.4 Calibration Records and Cycles.

2. The closed-loop end-hardening process and quality control method for medium carbon alloy structural steel according to claim 1, characterized in that: Step 1.1 Processing method and size control: The sample is prepared by sawing, wire cutting or "turning + grinding" combination processing method to ensure that the sample length deviation is controlled within ±2.5mm and the diameter deviation is controlled within ±0.4mm, which meets the basic size requirements of the end quenching test standard; Step 1.2 Surface quality treatment: The test surface of the sample needs to be finely polished, and the surface finish should be controlled at 0.4-1.2μm, free of oxide scale, scratches, pits and decarburized layer; after processing, wipe the surface with anhydrous ethanol to remove oil stains, iron filings and impurities. Step 1.3 Parallel Sample Preparation: Prepare at least 5-8 parallel samples for each batch of calibration to reduce the impact of individual sample processing errors on the overall calibration results and ensure data representativeness.

3. The closed-loop end-hardening process and quality control method for medium carbon alloy structural steel according to claim 1, characterized in that: Step 2.1 Heating temperature calibration: Set the target temperature of the heating furnace to 835-865℃. After heating to the target temperature, keep it at the temperature for 12-18 minutes to make the temperature inside the furnace uniform. Use an insertion thermocouple to monitor the furnace temperature in real time, and control the temperature fluctuation within ±6℃ to avoid local overheating or insufficient temperature. Step 2.2 Heating and Holding Time Calibration: After the sample is placed in the furnace, the temperature is first increased at a rate of 5-8℃ / min (if the furnace supports it), and the heating time is controlled at 22-32min to ensure that the core of the sample reaches the set temperature completely; then the holding stage is entered, and the holding time is set at 26-36min. During the holding period, the furnace temperature is recorded every 5min to prevent temperature drift. Step 2.3 Quenching medium parameter calibration: Industrial clean water is used as the quenching medium, and the water temperature is controlled at 17-23℃ by a constant temperature device; the height of the quenching water column is adjusted to 63-67mm, and the water flow velocity is controlled at 0.8-1.2m / s to ensure that the water column impacts the test surface of the sample vertically and uniformly, without any diversion or turbulence. Step 2.4 Calibration of anti-decarburization / carburization measures: During the heating process, inert gas (nitrogen) is introduced into the furnace or graphite powder is placed as an anti-oxidation medium to reduce decarburization on the sample surface; after calibration, the thickness of the decarburized layer on the sample surface is measured, which should be ≤0.12mm. If it exceeds the range, the amount of protective medium is adjusted.

4. The closed-loop end-hardening process and quality control method for medium carbon alloy structural steel according to claim 1, characterized in that: Step 3.1 Sample loading operation: Place the prepared sample evenly in the effective heating zone of the furnace, with a spacing of not less than 12mm between samples, and ensure that the sample does not directly contact the furnace wall or heating elements to avoid uneven local temperature.

5. The closed-loop end-hardening process and quality control method for medium carbon alloy structural steel according to claim 1, characterized in that: Step 3.2 Sample removal and quenching transfer: After the heat preservation is completed, the sample is quickly removed using a special clamp, and the removal time is controlled within ≤10 seconds; the sample is immediately transferred to the quenching device to ensure that the test surface is completely aligned with the center of the water jet impact, and the quenching time is not less than 18 minutes, during which the sample position is not moved.

6. The closed-loop end-hardening process and quality control method for medium carbon alloy structural steel according to claim 1, characterized in that: Step 3.3 Quenching process monitoring: During quenching, record the water temperature, water column height and water flow rate every 6 minutes. If the parameters exceed the set range (water temperature ±1℃, water column height ±0.5mm), adjust them immediately through the constant temperature device or valve to ensure stable quenching conditions.

7. The closed-loop end-hardening process and quality control method for medium carbon alloy structural steel according to claim 1, characterized in that: Step 4.1 Testing equipment calibration: Before testing, the hardness testing equipment (manual or automatic) is calibrated for accuracy using a standard hardness block (hardness value covering 35-48 HRC). The equipment error must be controlled within ±1.2 HRC. If the error exceeds the standard, the equipment pressure and the position of the testing head must be adjusted and recalibrated. Step 4.2 Detection position calibration: Define the end-quench detection point as the "center + annular area" of the quenched end face of the sample - point 1 at the center, and 3 points evenly distributed in the annular area 4-6mm away from the center, for a total of 4 detection points; mark the detection positions with a marker to avoid detection offset, and clean the surface oxidation products before each point is detected; Step 4.3 Repeated test calibration: After the initial test of each sample, a second test is performed at an interval of 8-12 minutes (after the sample temperature has returned to room temperature); all parallel samples must be repeatedly tested. The range of the three test results for the same sample is ≤2.8HRC, and the average value is taken as the final quench value of the sample.

8. The closed-loop end-hardening process and quality control method for medium carbon alloy structural steel according to claim 1, characterized in that: Step 5.1 Comparison of calibration results: Compare the average end quenching value of all parallel samples with the target range (34-47HRC). If more than 90% of the sample results are within the target range and the dispersion is ≤3.5HRC, the calibration is deemed qualified. If it is not qualified, the parameter execution deviation (furnace temperature fluctuation, quenching water temperature exceeding the standard) needs to be investigated.

9. The closed-loop end-hardening process and quality control method for medium carbon alloy structural steel according to claim 1, characterized in that: Step 5.2 Process parameter adjustment: If the overall detection value is too high (average exceeds the target upper limit by more than 2 HRC), the heating temperature can be lowered by 5-12℃ or the holding time can be shortened by 3-5 min; if the overall detection value is too low (average is below the target lower limit by less than 2 HRC), the heating temperature can be raised by 5-12℃ or the holding time can be extended by 3-5 min. After adjustment, small batch (3 samples) calibration verification should be performed again.

10. The closed-loop end-hardening process and quality control method for medium carbon alloy structural steel according to claim 1, characterized in that: Step 5.3 Auxiliary control of composition influence: In combination with the influence of material composition on hardenability, the residual element content is controlled simultaneously during the calibration process—Ti≤0.0065%, Mo≤0.028%—to reduce the interference of composition fluctuations on the end quench value. If the composition exceeds the standard, the raw materials or smelting process need to be adjusted. Step 5.4 Calibration Records and Cycles: After calibration, record detailed sample information, process parameters, test data, and adjustments to form the "End Quenching Test Heat Treatment Process Calibration Report". The calibration cycle is set to once every 5-7 months. If there is equipment maintenance, process change, or significant deviation in test results, recalibration is required immediately.