Thermal-mechanical coupling test method for evaluating strain-age cracking susceptibility of high temperature alloy

Abstract

The application provides a thermal-mechanical coupling test method for evaluating strain-age crack sensitivity of high-temperature alloy, and belongs to the technical field of welding. The method solves the problem that the existing technology cannot comprehensively quantitatively evaluate the reduction of area. The method comprises the following steps: performing a strain-age crack sensitivity evaluation test through a GTC thermal simulation testing machine, performing a thermal tensile test through different PWHT temperatures and different aging treatment times, measuring the reduction of area of the material under different conditions, obtaining a PWHT temperature-aging time-yield strength surface graph and a heat treatment temperature-aging time-reduction of area surface graph, confirming the fracture mode, quantifying the relationship among the PWHT temperature, the aging time and the reduction of area, and evaluating the strain-age crack sensitivity of the material. The application simulates welding thermal cycles on the Gleeble thermal simulation testing machine under a vacuum state, applies an initial stress to evaluate the reheat crack sensitivity of the material, saves raw materials, is short in test time and accurate in data.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of welding, and particularly relates to a thermal-mechanical coupling test method for evaluating strain age cracking sensitivity of high-temperature alloy. BACKGROUND

[0002] High-temperature alloy is widely used in the fields of aerospace, energy and chemical industry, and its stability and reliability are very important. However, high-temperature alloy is prone to reheat cracking during strain aging, which seriously affects its service life and safety. During post-weld heat treatment, the strain produced when the welding residual stress relaxes exceeds the plastic deformation capacity of the heat-affected zone, which will cause grain boundary reheat cracking. Strain age cracking (SAC) is a form of reheat cracking or post-weld heat treatment cracking, which usually occurs at the grain boundary of the heat-affected zone of the high-restraint weld after post-weld heat treatment, and is a unique crack of precipitation-strengthened nickel-based alloy. Therefore, a method suitable for evaluating the strain age cracking sensitivity of high-temperature alloy is needed.

[0003] The evaluation methods of reheat cracking sensitivity mainly include: 1. iron research sample, which is tested at different heat treatment temperatures, has low precision and large data error; 2. using a plug-in test, using a joint actually welded to make a sample, and obtaining quantitative data by testing under different loads, but the plug-in test has the disadvantage of not being close to the actual production; 3. simulation of coarse grain zone short-term creep breakage test, but the calculation results of the sample cross-section shrinkage rate will be affected by the personnel level and data processing error, thereby affecting the evaluation results of the reheat cracking sensitivity of the material. The commonly used reheat cracking sensitivity test method at present is to use a Gleeble thermal simulation testing machine to perform reheat cracking sensitivity test on a high-temperature tensile sample, change the post-weld heat treatment temperature in a vacuum state, and study the curve relationship between the sample cross-section shrinkage rate and the heat treatment temperature.

[0004] Chinese patent CN107389445A discloses a method for evaluating the reheat cracking sensitivity of a material by stress relaxation test, which can directly test on a thermal simulation testing machine by welding thermal simulation of different process parameters, then simulate stress release in the subsequent heat treatment process, the sample is gradually pulled apart, the relationship between temperature, stress and time is obtained, and the reheat cracking sensitivity of the material is characterized by the curve of fracture temperature-fracture time. However, this method does not comprehensively quantitatively evaluate the cross-section shrinkage rate. The cross-section shrinkage rate is a performance index reflecting the plastic deformation capacity of the heat-affected zone sample under high-temperature external load conditions, and the strain age cracking sensitivity of high-temperature alloy can be quantitatively evaluated by establishing the relationship among PWHT temperature, aging time and cross-section shrinkage rate. SUMMARY

[0005] Therefore, in order to solve the problem that the prior art does not comprehensively quantitatively evaluate the section shrinkage rate, the application provides a thermal-mechanical coupling test method for evaluating strain aging crack sensitivity of high-temperature alloy.

[0006] To achieve the above object, the application adopts the following technical scheme: a thermal-mechanical coupling test method for evaluating strain aging crack sensitivity of high-temperature alloy, specifically comprising the following steps:

[0007] Step 1: select an aging strengthening high-temperature alloy base material, the sampling position is T / 2 in the middle of the test tube, wherein T is the thickness; the sample size is a Φ6mm*100mm rod-shaped sample, and both ends are provided with M6*12mm threads;

[0008] Step 2: weld a thermocouple to the reduced section of the sample, and coat the thermocouple spot welding position with high-temperature cement to prevent the thermocouple from falling off;

[0009] Step 3: install the sample, vacuumize, and fill in argon protection environment when the vacuum degree reaches 5*10 -3 torr;

[0010] Step 4: heat the sample to a HAZ peak temperature lower than the zero ductility temperature at a speed of 100℃ / s, and after heat preservation for a period of time, cool to a ductility recovery temperature lower than the zero ductility temperature at a speed of 25℃ / s, and stretch the sample to 0.45mm at a constant displacement rate, and cool to room temperature at the same time;

[0011] Step 5: apply a constant stress to the sample at room temperature;

[0012] Step 6: perform post-weld heat treatment, heat the sample to a specified PWHT temperature at a speed of 100℃ / s, and heat preserve for a corresponding time, and then cool to room temperature;

[0013] Step 7: for the sample without aging treatment, keep the two clamps still, measure the diameter at the center of the sample by using a vernier caliper, reheat the sample to the PWHT temperature, and stretch the sample at a displacement rate of 25mm / s until the sample fails;

[0014] for the sample with aging treatment, take out the sample and perform aging treatment at an aging temperature of 675℃ and for an aging time of 300h and 1000h respectively, then heat the sample to 675℃, and stretch the sample at a displacement rate of 25mm / s until the sample fails;

[0015] Step 8: for the failed samples without aging treatment and with aging treatment, measure the fracture diameter in three different directions, and each group of tests is repeated three times, and the measurement results are averaged;

[0016] Step 9: observe the fracture longitudinal section metallographic morphology and SEM morphology by using a metallographic microscope and a scanning electron microscope, and confirm the fracture mode;

[0017] Step 10: Through the test data, a heat treatment temperature-aging time-yield strength surface diagram and a heat treatment temperature-aging time-reduction of area surface diagram are drawn to characterize the strain aging crack sensitivity.

[0018] Further, in step 2, the heating temperature of the sample is not more than 1200 DEG C, and a K-type thermocouple is used; and when the heating temperature is more than 1200 DEG C, an R-type thermocouple is used.

[0019] Further, in step 4, the constant pressure is 0.8-0.9 times of the yield strength, so as to ensure that the sample can have significant creep deformation without immediate fracture during the experiment.

[0020] Further, in step 4, the holding time is 5s.

[0021] Further, in step 6, the post-weld heat treatment temperature is 800 DEG C, 900 DEG C or 1000 DEG C.

[0022] Further, the test method uses a Gleeble thermal simulation testing machine.

[0023] Further, the test method uses the following data indexes: thermocouple temperature, aging time, axial load, travel distance and fracture diameter.

[0024] Further, the test method measures the reduction of area of the material under different conditions through the thermal tensile test under different PWHT temperatures and different aging treatment times, obtains a PWHT temperature-aging time-yield strength surface diagram and a heat treatment temperature-aging time-reduction of area surface diagram, confirms the fracture mode, quantifies the relationship among the PWHT temperature, the aging time and the reduction of area, and evaluates the strain aging crack sensitivity of the material.

[0025] Further, the strain aging crack sensitivity is observed by using a metallographic microscope and a scanning electron microscope to observe the microstructure change of the material during the strain aging process, and is evaluated by analyzing the microstructure and the fracture morphology.

[0026] Further, the strain aging crack sensitivity is determined by determining the threshold value of the reduction of area in the graph, and the region below the threshold value is considered as the strain aging crack sensitive region.

[0027] Compared with the prior art, the heat and force coupling test method for evaluating the strain aging crack sensitivity of a high-temperature alloy has the following beneficial effects:

[0028] 1. The application realizes the quantitative evaluation of the strain aging crack sensitivity of high-temperature alloy by establishing the relationship among PWHT temperature, aging time and section shrinkage rate. The main data indexes include thermocouple temperature, aging time, axial load, stroke distance and fracture diameter. Compared with other methods, the precision is low, the data error is large, and it is not close to the actual production.

[0029] 2. The application has the advantages that the reheat crack sensitivity of the material is evaluated by the welding thermal cycle simulation on the Gleeble thermal simulation testing machine under the vacuum state and the initial stress is applied. The whole test process is very close to the actual production, the raw materials are saved, the test time is short, and the data is accurate. BRIEF DESCRIPTION OF DRAWINGS

[0030] The drawings constituting a part of the application are used to provide further understanding of the application, the schematic embodiments of the application and the description thereof are used to explain the application, and do not constitute improper limitation on the application. In the drawings:

[0031] Figure 1 is a heat treatment temperature-aging time-section shrinkage rate surface diagram;

[0032] Figure 2 is a heat treatment temperature-aging time-yield strength surface diagram;

[0033] Figure 3 is a Gleeble sample processing size diagram. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the application will be clearly and completely described in combination with the drawings of the embodiments of the application. It should be noted that, in the case of no conflict, the embodiments in the application and the features in the embodiments can be combined with each other, and the described embodiments are only part of the embodiments of the application, but not all the embodiments.

[0035] Reference is made to Figures 1-3 The present embodiment is a thermal-mechanical coupling test method for evaluating the strain aging crack sensitivity of high-temperature alloy. The strain aging crack sensitivity evaluation test is performed by using a Gleeble 3800-GTC thermal simulation testing machine. The hot tensile test is performed under different PWHT temperatures and different aging times, the section shrinkage rate of the material under different conditions is measured, the PWHT temperature-aging time-yield strength surface diagram and the heat treatment temperature-aging time-section shrinkage rate surface diagram are obtained, the fracture mode is confirmed, the relationship among the PWHT temperature, the aging time and the section shrinkage rate is quantified, and the strain aging crack sensitivity of the material is evaluated.

[0036] Specifically, the following steps are included:

[0037] (1) Select representative age-hardened nickel-based alloy tubular base materials. Sampling location: sampling at position T / 2 in the middle of the test tube; Sample size: Φ6mm×100mm rod-shaped sample with M6×12mm threads at both ends. See details for specific dimensions. Figure 3 .

[0038] (2) Weld the thermocouple to the reduced diameter section of the sample. Apply high-temperature cement to the spot weld of the thermocouple to prevent the thermocouple from falling off. Use a type K thermocouple if the sample heating temperature does not exceed 1200℃, and use a type R thermocouple if it exceeds 1200℃.

[0039] (3) Install the sample, evacuate the vacuum, and when the vacuum reaches 5×10-3 torr, fill it with argon gas to protect the environment (-50kPa).

[0040] (4) Heat the sample at a rate of 100℃ / s to the peak temperature of HAZ (weld heat affected zone) slightly below NDT (zero ductility temperature), hold it at that temperature for a period of time, and then cool it at 25℃ / s to below DRT (ductility recovery temperature). Stretch the sample to 0.45 mm at a constant displacement rate and cool it to room temperature at the same time.

[0041] (5) At room temperature, a constant stress (0.8-0.9 times the yield strength) is applied to the specimen to ensure that the specimen can undergo significant creep deformation without breaking immediately during the experiment.

[0042] (6) Perform post-weld heat treatment. Heat the sample to the specified PWHT (post-weld heat treatment temperature) (800℃, 900℃, 1000℃) at a rate of 100℃ / s and hold for the corresponding time, then cool to room temperature. The test parameters are shown in Table 1.

[0043] (7) Keep the clamps at both ends of the sample stationary, measure the diameter at the center of the sample with a vernier caliper, reheat the sample to the PWHT temperature, and stretch the sample at a displacement rate of 25 mm / s until the sample fails.

[0044] (8) For the aged specimens, the specimens were taken out and aged at an aging temperature of 675℃ for 300h and 1000h respectively. Then the specimens were heated to 675℃ and stretched at a displacement rate of 25mm / s until the specimens failed.

[0045] (9) For both non-aging and aging-treated failed specimens, the fracture diameter was measured in three different directions. Each test was repeated three times, and the average value of the measurement results was taken.

[0046] (10) The fracture mode was confirmed by observing the metallographic morphology and SEM morphology of the longitudinal section of the fracture surface using metallographic microscope and scanning electron microscope.

[0047] (11) Using experimental data, plot the heat treatment temperature-aging time-yield strength surface diagram and the heat treatment temperature-aging time-reduction of area surface diagram to characterize the strain aging crack sensitivity.

[0048] Table 1. Tensile test scheme and test results after post-weld heat treatment and aging treatment.

[0049]

[0050] The working principle of the thermo-mechanical coupling test method for evaluating the susceptibility of strain-aging cracks in high-temperature alloys described in this invention is as follows:

[0051] 1. Based on experimental data, plot the heat treatment temperature-aging time-yield strength surface diagram. Figure 1 ) and heat treatment temperature-aging time-section shrinkage rate surface diagram ( Figure 2 ), to characterize strain aging crack sensitivity.

[0052] (1) The charts show the trend of yield strength variation of materials under different heat treatment temperatures and different aging times.

[0053] (2) The graph shows the trend of the cross-sectional shrinkage rate of the material under different heat treatment temperatures and different aging times.

[0054] (3) If the mechanical properties in the chart decrease during the aging process, such as a decrease in yield strength and a decrease in the reduction of area, it indicates that the material has a high sensitivity to strain-aging cracks.

[0055] (4) The strain aging crack sensitivity can be evaluated by observing the microstructure changes of the material during the strain aging process using metallographic microscopes, scanning electron microscopes, etc., and by analyzing the microstructure and fracture morphology.

[0056] (5) The aforementioned microstructure and fracture morphology, including changes in grain size and fracture morphology, may indicate that the material underwent recovery, recrystallization, or changes in its microstructure during aging, thereby increasing its susceptibility to strain-aging cracks. (After plastic deformation, metals undergo changes in structure, microstructure, and properties when heated at different temperatures. Recovery occurs at lower temperatures, while recrystallization of the matrix and grain growth occur at higher temperatures. Through recovery and recrystallization, the metal or alloy transforms from a thermodynamically unstable cold-deformation state to a more thermodynamically stable new microstructure.)

[0057] (6) The strain aging crack sensitivity mentioned above can be determined in the chart by the threshold of the reduction of area. The area below the threshold is considered to be the strain aging crack sensitive area.

[0058] (7) The strain aging crack sensitivity mentioned above can be reduced by comparing the crack sensitivity trends under different heat treatment temperatures and different aging time combinations to find the optimal heat treatment process parameters.

[0059] 2. The coupling principle of strain-aging crack susceptibility involves the interaction and influence of multiple factors, including strain, aging time, temperature, material composition, and microstructure. The main coupling principles are:

[0060] (1) Coupling of strain and aging time: The formation of strain-aged cracks is closely related to strain and aging time. Under strain, defects such as dislocations and grain boundaries inside the material will rearrange and aggregate, forming crack initiation sites. At the same time, the extension of aging time will promote the further development and evolution of these defects, thereby increasing the possibility of crack formation.

[0061] (2) Coupling of Strain and Heat Treatment Temperature: Heat treatment temperature is an important factor affecting the susceptibility of materials to strain-aging cracks. Post-weld heat treatment is primarily aimed at eliminating residual welding stress and improving the microstructure and properties of the welded joint. During post-weld heat treatment, the holding temperature is a key factor affecting properties such as yield strength and reduction of area. A suitable holding temperature can reduce the yield strength of the welded joint metal to a certain level, thereby achieving the effect of eliminating residual stress.

[0062] (3) Coupling of strain and microstructure: The microstructure of a material has a significant impact on its strain-aging crack susceptibility. Microstructure parameters such as grain size, phase composition, and dislocation density affect the mechanical properties and crack propagation behavior of the material. Under strain, these microstructures change, thereby affecting the strain-aging crack susceptibility of the material.

[0063] During the aging process, the strain generated by the external load will interact with the changes in microstructure such as grains and precipitates over time, thereby affecting the material's properties (yield strength, reduction of area). This coupling relationship between microstructure evolution and mechanical property changes can be characterized by curves, surface plots, etc.

[0064] The effects of heat treatment temperature, aging time, and stress also influence aging crack susceptibility. Coupled curves are plotted to observe trends in material properties under different temperatures and aging times. Reduction of area, aging time, and heat treatment temperature interact; analyzing these relationships using coupled curves allows for a more accurate assessment of the material's aging crack susceptibility.

[0065] In summary, characterizing strain-aging crack susceptibility from graphs requires comprehensive consideration of multiple factors, including changes in mechanical properties (yield strength, reduction of area, etc.), microstructure changes, aging time, and heat treatment temperature. Furthermore, the coupling principle of strain-aging crack susceptibility involves multiple interactions and influences, necessitating a comprehensive consideration of the effects of strain, aging time, temperature, and microstructure. By deeply studying and understanding these factors and their interaction mechanisms, we can more effectively assess and control the strain-aging crack susceptibility of materials.

[0066] The embodiments of the present invention disclosed above are merely illustrative of the invention. These embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention.

Claims

1. A thermo-mechanical coupling test method for evaluating the susceptibility of strain-aged cracks in high-temperature alloys, characterized in that: Specifically, the following steps are included: Step 1: Select age-hardening high-temperature alloy base material; sampling location: sampling at the middle position T / 2 of the test tube, where T is the thickness; sample size: Φ6mm×100mm rod-shaped sample with M6×12mm threads at both ends; Step 2: Weld the thermocouple to the reduced diameter section of the sample, and apply high-temperature cement to the spot weld area of ​​the thermocouple to prevent it from falling off. Step 3: Install the sample, evacuate to a vacuum level of 5×10⁻⁶. -3 During torr, argon gas is introduced to protect the environment; Step 4: Heat the sample at a rate of 100℃ / s to the peak temperature of the weld heat-affected zone below the zero ductility temperature, hold it at that temperature for a period of time, and then cool it at a rate of 25℃ / s to below the ductility recovery temperature. Stretch the sample to 0.45 mm at a constant displacement rate and cool it to room temperature at the same time. Step 5: Apply a constant stress to the sample at room temperature; Step 6: Perform post-weld heat treatment. Heat the sample to the specified post-weld heat treatment temperature at a rate of 100℃ / s, hold it at that temperature for the corresponding time, and then cool it to room temperature. Step 7: Keep the clamps at both ends of the unaged sample stationary, measure the diameter at the center of the sample with a vernier caliper, reheat the sample to the PWHT temperature, and stretch the sample at a displacement rate of 25 mm / s until the sample fails. For the aged specimens, the specimens were taken out and aged at an aging temperature of 675℃ for 300h and 1000h respectively. Then the specimens were heated to 675℃ and stretched at a displacement rate of 25mm / s until the specimens failed. Step 8: For both non-aging and aging-treated failed specimens, the fracture diameter was measured in three different directions. Each test was repeated three times, and the average value of the measurement results was taken. Step 9: Observe the metallographic morphology and SEM morphology of the longitudinal section of the fracture surface using a metallographic microscope and a scanning electron microscope to confirm the fracture mode; Step 10: Using experimental data, plot the heat treatment temperature-aging time-yield strength surface diagram and the heat treatment temperature-aging time-reduction of area surface diagram to characterize the susceptibility to strain aging cracks.

2. The thermo-mechanical coupling test method for evaluating the susceptibility of strain-aging cracks in high-temperature alloys according to claim 1, characterized in that: In step 2, if the sample heating temperature does not exceed 1200℃, a type K thermocouple is used; if it exceeds 1200℃, a type R thermocouple is used.

3. The thermo-mechanical coupling test method for evaluating the susceptibility of strain-aging cracks in high-temperature alloys according to claim 1, characterized in that: The constant pressure in step 4 is 0.8-0.9 times the yield strength to ensure that the specimen can undergo significant creep deformation without immediately breaking during the experiment.

4. The thermo-mechanical coupling test method for evaluating the susceptibility of strain-aging cracks in high-temperature alloys according to claim 1, characterized in that: This test method uses the Gleeble thermal simulation test machine.

5. The thermo-mechanical coupling test method for evaluating the susceptibility of strain-aging cracks in high-temperature alloys according to claim 1, characterized in that: The data indicators used in this test method include: thermocouple temperature, aging time, axial load, stroke distance, and fracture diameter.

6. The thermo-mechanical coupling test method for evaluating the susceptibility of strain-aging cracks in high-temperature alloys according to claim 1, characterized in that: This experimental method measures the reduction of area of ​​the material under different conditions through hot tensile tests at different PWHT temperatures and aging times. It yields PWHT temperature-aging time-yield strength surface plots and heat treatment temperature-aging time-reduction of area surface plots, confirms the fracture mode, quantifies the relationship between PWHT temperature, aging time, and reduction of area, and evaluates the strain-aging crack susceptibility of the material.

7. The thermo-mechanical coupling test method for evaluating the susceptibility of strain-aging cracks in high-temperature alloys according to claim 1, characterized in that: Strain aging crack sensitivity is evaluated by observing the microstructure changes of materials during strain aging using metallographic microscopy and scanning electron microscopy, and by analyzing the microstructure and fracture morphology.

8. The thermo-mechanical coupling test method for evaluating the susceptibility of strain-aging cracks in high-temperature alloys according to claim 1, characterized in that: Strain aging crack sensitivity is defined in the chart by determining a threshold for the reduction of area; areas below this threshold are considered strain aging crack sensitive areas.

9. The thermo-mechanical coupling test method for evaluating the susceptibility of strain-aging cracks in high-temperature alloys according to claim 1, characterized in that: The heat preservation time in step 4 is 5 seconds.

10. The thermo-mechanical coupling test method for evaluating the susceptibility of strain-aging cracks in high-temperature alloys according to claim 1, characterized in that: In step 6, the post-weld heat treatment temperature is 800℃, 900℃, or 1000℃.

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