Method for detecting residual stress of steel mold and finished part by ultrasonic method

Abstract

The invention discloses a method for ultrasonically detecting residual stress of a heat treatment part. Aiming at the limitation of existing residual stress detection such as a blind hole method and an X-ray method at present, the method solves the problem that only hardness and strength of a heat-treated part are detected in a delivery state and a service period at present, and no good method is provided for paying attention to plasticity; and the surface and the interior of the material are cracked and fractured due to large residual stress caused by stress concentration and fatigue damage. The ultrasonic residual stress measurement method is based on the acoustic elasticity theory, the residual stress is calculated through the change of the acoustic time difference of the critical refraction longitudinal waves, no damage and radiation are caused, precision is high, repeatability is good, the absolute stress value relative to the stress-free state is measured, and the method has the advantages of being high in repeatability, low in cost and capable of measuring the absolute stress. The method is suitable for residual stress detection of heat-treated parts. The method for efficiently and economically ultrasonically detecting the residual stress of the heat treatment part provides a new thought for improving plasticity of a martensitic steel regulation and control heat treatment process and detecting the plasticity and quality of a mold and the part, and is beneficial to solving the existing problems.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment performance detection and process control of steel parts, and specifically relates to a method for ultrasonically detecting heat treatment residual stress of steel parts. Background Art

[0002] Martensitic steel has excellent processing performance and hardenability, and is widely used in industries such as cutting tools, medical treatment, chemical industry, aerospace, and high-end precision molds. As a hardenable steel, residual stress has a great influence on its mechanical properties and service life. The rapid cooling process of quenching will produce phase change stress and thermal stress inside the material, increase residual stress and reduce plastic toughness. Excessive residual stress will cause material fracture or fatigue damage and cracking during subsequent use. It is necessary to release some residual stress through tempering to improve the overall performance. However, the existing blind hole method for residual stress detection will damage the material, and the X-ray method equipment is expensive and has radiation and is complicated to operate. It is greatly affected by the texture organization. Steel molds and other products cannot be destroyed when they are delivered, so the hardness can only be used to detect whether the product quality is qualified. Therefore, in actual production and preparation activities, due to heat treatment process problems, there will be a large number of cases where the material hardness or strength is qualified but the residual stress exceeds the standard, resulting in a significant reduction in the life of subsequent molds and other products, but the cause cannot be found. Ultrasonic stress measurement is based on the acoustic elasticity theory, using the linear proportional relationship between the ultrasonic wave velocity change and the internal stress of the material. By measuring the propagation time or phase change of the ultrasonic wave, the stress change inside the material is obtained. It has now been applied in pressure vessel pipelines, rails, etc., but the ultrasonic stress measurement method is currently still in a blank state for residual stress detection and control methods of mold parts in delivery status and mold parts in service status. The present invention is based on this situation, and applies the ultrasonic stress measurement method to the detection of residual stress of heat-treated workpieces of steel mold parts, and then combines its mechanical properties to better judge the strength and plasticity of the workpiece, so as to adjust the heat treatment process to achieve excellent comprehensive performance, avoid the hardness and strength of the delivered mold parts to meet the standards but the residual stress is too large and the plasticity is poor, and the problems of stress concentration, fatigue damage and fracture in the service process occur, thereby improving the service life. Summary of the invention

[0003] In view of the above problems, the present invention provides a method for ultrasonically detecting residual stress of martensitic steel molds and finished parts.

[0004] The above technical problem of the present invention is mainly solved by the following technical solution: A method for ultrasonically detecting residual stress of steel molds and finished parts, comprising the following steps: Step 1: After heating and keeping the steel warm, the steel is slowly cooled in the furnace to obtain annealed steel, and a tensile specimen of the annealed steel is prepared.

[0005] Step 2: Clamp the tensile specimen of annealed steel on a universal testing machine, fix the ultrasonic stress sensor on the tensile specimen coated with a coupling agent, use an ultrasonic stress detection instrument to record the position of the critical refracted longitudinal wave in the ultrasonic echo signal when the annealed steel is in a zero stress state, load different tensile forces on the tensile specimen, and record the corresponding different stress values ​​respectively. ∆σ The acoustic time difference of the critical refracted longitudinal wave position change ∆t， Draw the acoustic time difference curve corresponding to different stresses to obtain the stress acoustic time difference coefficient K σ value, completing the calibration process before ultrasonic stress testing.

[0006] Step 3: Heat the martensitic steel mold or finished part to above the solution temperature and keep it for a period of time so that all alloy elements can be dissolved into the grains; then quickly immerse it in a cooling medium for quenching.

[0007] Step 4: After the quenched steel mold or the surface to be tested of the finished part is polished to a metallic glossy surface, a coupling agent is applied and an ultrasonic stress sensor is placed thereon. The ultrasonic stress detection instrument is used to record the position change of the critical refracted longitudinal wave in the ultrasonic echo signal. ∆t， Multiply by the stress acoustic time difference coefficient K σ It is worthwhile to obtain the residual stress value of quenching heat treatment, and measure and adjust the quenching heat treatment process according to the residual stress value and hardness value.

[0008] Step 5, subjecting the quenched steel mold or finished parts to tempering heat treatment at different temperatures, heating to 200-600°C, then keeping warm and cooling.

[0009] Step 6: After the tempered steel mold or the surface to be tested of the finished part is polished to a metallic glossy surface, a coupling agent is applied and an ultrasonic stress sensor is placed thereon. The ultrasonic stress detection instrument is used to record the position change of the critical refracted longitudinal wave in the ultrasonic echo signal. ∆t， Multiply by the stress acoustic time difference coefficient K σ It is worthwhile to obtain the residual stress value of tempering heat treatment, and to measure and adjust the tempering heat treatment process according to the residual stress value and hardness value.

[0010] Among them, after completing the calibration process before the ultrasonic stress test through steps 1 and 2, the residual stress test can be directly performed on the steel molds or parts in the delivery state and the service state.

[0011] Preferably, in step 1, the steel material has the same grade and composition as the mold or finished part to be tested; the heating and insulation temperature is 800-900°C, the insulation time is 1-4h, and the annealed steel is fully annealed steel.

[0012] Preferably, in steps 2, 4 and 6, the ultrasonic stress sensor can be an integrated one-transmit-one-receive ultrasonic stress sensor with oblique incidence and reception, or it can be a one-transmit-one-receive ultrasonic stress sensor formed by two miniature ultrasonic longitudinal wave probes fixed on a wedge by threads; the ultrasonic stress sensor is fixed on the tensile specimen by strong magnet magnetic fixation and / or rubber band winding fixation.

[0013] Preferably, in steps 2, 4, and 6, the ultrasonic stress detection instrument includes an ultrasonic transceiver probe with one transmit and one receive, a pulse ultrasonic generating module, an ultrasonic signal acquisition module, and a signal acquisition and processing system function. The acoustic time difference resolution of the ultrasonic signal acquisition module is above 1 ns, and the stress test error is below ±10 MPa.

[0014] Preferably, in step 3, the steel mold or finished part is heated to 1000-1150° C. and kept warm for 1-2 hours, and the cooling medium is water or oil.

[0015] Preferably, in step 4, the method for measuring and adjusting the quenching heat treatment process according to the residual stress value and the hardness value includes: taking the quenching residual stress value under the water-cooling medium as the maximum residual stress value of the quenching process to measure the quenching residual stress reduction value under the oil-cooling medium, and adjusting the quenching temperature and the quenching medium and temperature in combination with the hardness value; when the hardness is large and the residual stress is large, changing the cooling medium conditions, reducing the quenching cooling rate on the basis of meeting the hardness requirements to reduce the residual stress; when the hardness is small and the residual stress is small, the quenching cooling rate can be increased.

[0016] Preferably, in step 5, the tempering heat treatment at different temperatures is specifically: low temperature tempering temperature 200-300°C, medium temperature tempering temperature 350-450°C, high temperature tempering temperature 500-600°C.

[0017] Preferably, in step 6, the method for measuring and adjusting the tempering heat treatment process according to the residual stress value and the hardness value includes: taking the quenching residual stress value under the water-cooled medium as the maximum residual stress value of the quenching process to measure the quenching residual stress reduction value under the tempering heat treatment at different temperatures, and adjusting the tempering temperature and the holding time in combination with the hardness value; on the basis of meeting the hardness, the tempering temperature can be appropriately increased to reduce the residual stress during low-temperature tempering and high-temperature tempering. Due to the precipitation of carbides, the residual stress and hardness of medium-temperature tempering may be higher than those of low-temperature tempering, but the residual stress can still be reduced by appropriately increasing the temperature within the medium-temperature tempering range.

[0018] The present invention provides a method for ultrasonically detecting residual stress of heat-treated parts. It has the following beneficial effects: (1) For steel molds or parts in the delivery state and service state, the product strength can only be tested by hardness, but the residual stress cannot be verified. Excessive residual stress will greatly reduce plasticity, leading to accelerated fatigue damage, surface cracks and other defects or even fractures, which will affect the service life. The residual stress of heat-treated parts can be monitored and the corresponding heat treatment can be performed to reduce the residual stress and increase the service life.

[0019] (2) Compared with the existing residual stress detection blind hole method that will damage the material, the X-ray method is expensive and has radiation and is complicated to operate. It is greatly affected by the texture and has large errors and cannot measure macroscopic stress. The ultrasonic method is convenient and fast, and can measure residual stress within seconds. The residual stress value obtained by the ultrasonic method is an absolute stress value relative to the stress-free state, not a relative value. After completing the calibration process before the ultrasonic stress test, the residual stress of the steel mold or parts in the delivery state and the service state can be directly tested. This method is a non-destructive test. This method fills the gap in the residual stress detection and control method of the delivery state mold parts and the service state mold parts using ultrasonic stress measurement method.

[0020] (3) Compared with the existing detection methods of quenching and tempering processes of steel molds or finished parts, which only focus on strength and adjust the quenching and tempering processes only by referring to strength data, and cannot better characterize and refer to their plasticity, this method uses ultrasonic method to monitor the residual stress after the existing quenching process and tempering process, which can better understand its residual stress and plasticity, and can adjust the quenching temperature, quenching medium and tempering temperature and tempering time according to the size of the values. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A diagram showing the steps of the method of the present invention.

[0022] Figure 2 is a schematic diagram of the ultrasonic stress device principle (a) principle block diagram; (b) critical refracted longitudinal wave. Figure 3 This is the critical refracted longitudinal wave pattern in ultrasonic echo during ultrasonic stress detection. DETAILED DESCRIPTION

[0023] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Example 1

[0024] The embodiment of the present invention provides a method for ultrasonically detecting and regulating residual stress of heat-treated parts, which is as follows: The Cr13 steel plate was heated to 860°C and kept at that temperature for 2 hours. After being slowly cooled in the furnace, annealed steel was obtained, and tensile specimens of the annealed steel were prepared.

[0025] The tensile specimen of annealed Cr13 steel was clamped on a universal testing machine, and the ultrasonic stress sensor was fixed on the tensile specimen coated with a coupling agent. The ultrasonic stress detection instrument was used to record the position of the critical refracted longitudinal wave in the ultrasonic echo signal when the annealed steel was loaded with zero stress. Different stresses were applied to the tensile specimen, and the corresponding different stress values ​​were recorded respectively. ∆σ The acoustic time difference of the critical refracted longitudinal wave position change ∆t， Draw the acoustic time difference curve corresponding to different stresses to obtain the stress acoustic time difference coefficient K σ value, and the calibration is completed.

[0026] The steel mold was heated to 1000℃ and kept for 1h, and quenched with water and oil respectively.

[0027] After the quenched steel mold is polished to a metallic glossy surface, the ultrasonic stress sensor is placed after the coupling agent is applied. The ultrasonic stress detection instrument is used to record the position change of the critical refracted longitudinal wave in the ultrasonic echo signal. ∆ t， Multiply by the stress acoustic time difference coefficient K σ It is worth to obtain the residual stress value of quenching heat treatment. The residual stress value of the steel mold after water medium quenching is 370.3MPa, and the residual stress value of the steel mold after oil medium quenching is 297.5MPa.

[0028] The oil-quenched steel mold is kept at 200°C for 2 hours for low-temperature tempering, and the cooling method is air cooling.

[0029] After the tempered steel mold or the surface to be tested of the finished part is polished to a metallic glossy surface, the ultrasonic stress sensor is placed after the coupling agent is applied, and the ultrasonic stress detection instrument is used to record the position change of the critical refracted longitudinal wave in the ultrasonic echo signal. ∆t， Multiply by the stress acoustic time difference coefficient K σ The residual stress value after tempering heat treatment is obtained, and the residual stress after tempering is 183.6MPa.

[0030] The tempering process temperature was adjusted to 300°C, and the oil-quenched steel mold was kept at 300°C for 2 hours for low-temperature tempering. The cooling method was air cooling, and the measured residual stress after tempering was 170.2MPa.

[0031] The tempering process temperature was adjusted to 500°C, and the oil-quenched steel mold was kept at 500°C for 2 hours for high-temperature tempering. The cooling method was air cooling, and the measured residual stress after tempering was 90.5MPa.

[0032] The tempering process temperature was adjusted to 600°C, and the oil-quenched steel mold was kept at 600°C for 2 hours for high-temperature tempering. The cooling method was air cooling, and the measured residual stress after tempering was 42.3 MPa. Example 2

[0033] The embodiment of the present invention provides a method for ultrasonically detecting residual stress of heat-treated parts, which is as follows: The Cr13 steel plate was heated to 860°C and kept at that temperature for 2 hours. After being slowly cooled in the furnace, annealed steel was obtained, and tensile specimens of the annealed steel were prepared.

[0034] The tensile specimen of annealed Cr13 steel was clamped on a universal testing machine, and the ultrasonic stress sensor was fixed on the tensile specimen coated with a coupling agent. The ultrasonic stress detection instrument was used to record the position of the critical refracted longitudinal wave in the ultrasonic echo signal when the annealed steel was loaded with zero stress. Different stresses were applied to the tensile specimen, and the corresponding different stress values ​​were recorded respectively. ∆σ The acoustic time difference of the critical refracted longitudinal wave position change ∆t， Draw the acoustic time difference curve corresponding to different stresses to obtain the stress acoustic time difference coefficient K σ value, and the calibration is completed.

[0035] The fully annealed Cr13 steel was tested using ultrasonic stress equipment, and the measured residual stress after tempering was -5~5MPa.

[0036] The ultrasonic stress equipment was used to test the Cr13 steel after quenching and high temperature tempering, and the residual stress was measured to be 70~90MPa.

[0037] The Cr13 steel mold after quenching and high temperature tempering is used in production and is subjected to hot and cold shocks of aluminum alloy and release agent. After a certain period of circulation, the residual stress detected by ultrasonic stress equipment is 152~187MPa; the residual stress of Cr13 steel mold with surface defects detected by ultrasonic stress equipment is 273~315MPa; The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for a person skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to replace some of the technical features therein with equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions claimed to be protected by the present invention.

Claims

1. A method for ultrasonically detecting residual stress of steel molds and finished parts, characterized in that: The steps include: Step 1, heating and keeping the steel, and then slowly cooling it in the furnace to obtain annealed steel, and preparing a tensile specimen of the annealed steel; Step 2: Clamp the tensile specimen of annealed steel on a universal testing machine, fix the ultrasonic stress sensor on the tensile specimen coated with a coupling agent, use an ultrasonic stress detection instrument to record the position of the critical refracted longitudinal wave in the ultrasonic echo signal when the annealed steel is in a zero stress state, load different tensile forces on the tensile specimen, and record the corresponding different stress values ​​respectively. ∆σ The acoustic time difference of the critical refracted longitudinal wave position change ∆t， Draw the acoustic time difference curve corresponding to different stresses to obtain the stress acoustic time difference coefficient K σ value, completing the calibration process before ultrasonic stress testing; Step 3, heating the steel mold or finished part to above the solution temperature and maintaining it for a period of time so that all alloy elements can be dissolved into the grains; then quickly immersing it in a cooling medium for quenching; Step 4: After the quenched steel mold or the surface to be tested of the finished part is polished to a metallic glossy surface, a coupling agent is applied and an ultrasonic stress sensor is placed thereon. The ultrasonic stress detection instrument is used to record the position change of the critical refracted longitudinal wave in the ultrasonic echo signal. ∆t， Multiply by the stress acoustic time difference coefficient K σ It is worthwhile to obtain the residual stress value of quenching heat treatment, and measure and adjust the quenching heat treatment process according to the residual stress value and hardness value; Step 5, subjecting the quenched steel mold or finished parts to tempering heat treatment at different temperatures, heating to 200-600°C, then keeping warm and cooling; Step 6: After the tempered steel mold or the surface to be tested of the finished part is polished to a metallic glossy surface, a coupling agent is applied and an ultrasonic stress sensor is placed thereon. The ultrasonic stress detection instrument is used to record the position change of the critical refracted longitudinal wave in the ultrasonic echo signal. ∆t， Multiply by the stress acoustic time difference coefficient K σ It is worthwhile to obtain the residual stress value of tempering heat treatment, and measure and adjust the tempering heat treatment process according to the residual stress value and hardness value; Among them, after completing the calibration process before the ultrasonic stress test through steps 1 and 2, the residual stress test can be directly performed on the steel molds or parts in the delivery state and the service state.

2. The method for ultrasonically detecting residual stress of steel molds and finished parts according to claim 1, characterized in that: In step 1, the steel material has the same grade and composition as the mold or finished part to be tested; the heating and insulation temperature is 800-900° C., the insulation time is 1-4 hours, and the annealed steel is fully annealed steel.

3. The method for ultrasonically detecting residual stress of steel molds and finished parts according to claim 1, characterized in that: In steps 2, 4, and 6, the ultrasonic stress sensor can be an integrated one-transmit-one-receive ultrasonic stress sensor with oblique incidence and reception, or it can be two miniature ultrasonic longitudinal wave probes fixed on a wedge block by threads to form a one-transmit-one-receive ultrasonic stress sensor; the ultrasonic stress sensor is fixed on the tensile specimen by strong magnet magnetic fixation and / or rubber band winding fixation.

4. The method for ultrasonically detecting residual stress of steel molds and finished parts according to claim 1, characterized in that: In steps 2, 4, and 6, the ultrasonic stress detection instrument includes an ultrasonic transceiver probe with one transmitter and one receiver, a pulse ultrasonic generating module, an ultrasonic signal acquisition module, and a signal acquisition and processing system. The ultrasonic signal acquisition module has a sound time difference resolution of more than 1 ns and a stress test error of less than ±10 MPa.

5. The method for ultrasonically detecting residual stress of steel molds and finished parts according to claim 1, characterized in that: In step 3, the steel mold or finished part is heated to 1000-1150° C. and kept at this temperature for 1-2 hours, and the cooling medium is water, oil or a water-steam mixture.

6. The method for ultrasonically detecting residual stress of steel molds and finished parts according to claim 1, characterized in that: In step 4, the method for measuring and adjusting the quenching heat treatment process according to the residual stress value and the hardness value includes: taking the quenching residual stress value under the water-cooling medium as the maximum residual stress value of the quenching process to measure the quenching residual stress reduction value under the oil-cooling medium, and adjusting the quenching temperature and the quenching medium and temperature in combination with the hardness value; when the hardness is large and the residual stress is large, the cooling medium conditions can be changed to reduce the quenching cooling rate; when the hardness is small and the residual stress is small, the quenching cooling rate can be increased.

7. The method for ultrasonically detecting residual stress of steel molds and finished parts according to claim 1, characterized in that: In step 5, the tempering heat treatment at different temperatures is specifically: low temperature tempering temperature 200-300°C, medium temperature tempering temperature 350-450°C, high temperature tempering temperature 500-600°C.

8. The method for ultrasonically detecting residual stress of steel molds and finished parts according to claim 1, characterized in that: In step 6, the method for measuring and adjusting the tempering heat treatment process according to the residual stress value and the hardness value includes: taking the quenching residual stress value under the water-cooled medium as the maximum residual stress value of the quenching process to measure the quenching residual stress reduction value under the tempering heat treatment at different temperatures, and adjusting the tempering temperature and the holding time in combination with the hardness value; the tempering temperature can be appropriately increased during low-temperature tempering and high-temperature tempering to reduce the residual stress.

9. The method for ultrasonically detecting residual stress of steel molds and finished parts according to claim 1, characterized in that: The steel is martensitic steel.

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