Device and method for testing creep crack growth rate of high-temperature ductile material

Abstract

The invention provides a method for testing the creep crack growth rate of a high-temperature ductile material, which comprises the following steps: processing a target material into a plurality of samples, and respectively prefabricating precracks with different lengths on the plurality of samples; loading the sample into a creep testing machine; calculating creep crack propagation parameters of the sample; and drawing a corresponding curve of the crack propagation rate and the creep crack propagation parameters, and fitting according to a power function to obtain fitting parameters. The invention further provides a device for testing the creep crack growth rate of the high-temperature ductile material. The device and the method have the beneficial effects that a plurality of samples with different prefabricated grain lengths can be tested at the same time, crack propagation rates and loading linear displacement rates corresponding to different crack propagation stages are obtained through two times of unloading, and high-cost crack length real-time measurement equipment and loading linear displacement real-time measurement equipment do not need to be equipped; the test process is greatly simplified, the test time is shortened, and the test cost is reduced.

Description

Technical Field

[0001] The present application relates to the technical field of high temperature mechanical testing, and in particular to a testing device and a testing method for creep crack growth rate of a high temperature ductile material. Background Art

[0002] The materials used in most components of power generation equipment units, such as chromium-containing ferrite steel or austenitic stainless steel, are high-temperature ductile materials. When operating in a high-temperature environment for a long time, viscoplastic creep deformation related to temperature and time will occur, and large-scale creep will occur at the crack tip. It is no longer suitable to use the linear elastic stress intensity factor K and the elastic-plastic fracture toughness J integral to characterize the crack control parameters. In 1976, scholars such as Landers, Begley, and Nikbin proposed the creep crack tip parameter C*, which is used to characterize the crack tip stress and strain rate field in the second stage of large-scale creep. It can better describe the material's ability to resist creep crack extension and can effectively predict the remaining life of high-temperature components in service.

[0003] At present, the creep crack growth rate test is carried out in accordance with "ASTM E1457-15 Measurement of Creep Crack Growth Times in Metals". The test is very time-consuming. The standard recommends running for at least 1000 hours, usually 5000 to 10000 hours. In addition, the crack length and loading line displacement need to be tested in real time during the test, and crack length real-time testing equipment and loading line displacement extensometers are required. However, it is difficult to measure the deformation in two directions in real time for a long time under high temperature conditions. The existing crack length measurement equipment, such as the potentiometric equipment, is complicated to operate and has limited accuracy. Therefore, the application and development of creep crack growth rate tests are restricted, affecting the accuracy and reliability of the remaining life assessment of high-temperature components. Summary of the invention

[0004] In view of the above-mentioned defects, the present invention is beneficial in that it provides a testing device and a testing method for the creep crack growth rate of a high-temperature ductile material to solve the above-mentioned problems.

[0005] The present invention provides a method for testing the creep crack growth rate of a high temperature ductile material, comprising the following steps:

[0006] Processing the target material into N specimens with the same nominal size, each specimen having a specimen number i, i=1, 2, ...N; prefabricating precracks of different lengths on the N specimens;

[0007] Loading the sample into a creep testing machine;

[0008] The sample is heated and kept warm, a load F is applied and kept loaded, and the load is kept until a stable deformation occurs, then the first unloading and cooling is performed, and the first loading line displacement V of the sample is measured. Ii and the first surface crack length a Ii ;

[0009] The sample is heated and kept warm, a load F is applied and kept loaded, and a second unloading and cooling is performed when a significant deformation is observed compared to the first unloading, and the second holding time is recorded as t II , measure the second loading line displacement V of the specimen IIi and the second surface crack length a IIi ;

[0010] Calculate the sample at the second holding time t II The crack extension △a i =a IIi -a Ii The corresponding crack growth rate is

[0011] Calculate the displacement change of the loading line after two unloadings △V i =V IIi -V Ii , the corresponding loading line displacement rate is

[0012] Calculate the creep crack growth parameter C of the specimen i * ,

[0013]

[0014] Where: n is the Norton index in the steady-state creep stage, F is the applied load, B is the specimen thickness, and W is the specimen width;

[0015] Plotting the crack growth rate and creep crack growth parameter C i * The corresponding curve is fitted according to the power function.

[0016]

[0017] Get the fitting coefficient P and fitting index q.

[0018] Preferably, the number N of samples is greater than or equal to 5.

[0019] Preferably, the method further comprises the steps of: connecting the N specimens in series to form a specimen group, and placing the specimen group into a creep testing machine.

[0020] Preferably, thermocouples are bundled at the upper, middle and lower parts of the sample group for temperature measurement and temperature control, and the temperature control accuracy is ±2°C. Preferably, whether to extend the holding time is selected according to the loading line displacement and surface crack length measured after the first unloading and cooling or the second unloading and cooling.

[0021] Preferably, the Norton index n of the steady-state creep stage is obtained through a creep test of the target material under the same temperature conditions.

[0022] Preferably, the specimen is a step-type compact tensile specimen.

[0023] The present invention also provides a testing device for the creep crack growth rate of high-temperature ductile materials, which is used for connecting specimens in series, comprising: two U-shaped parts, multiple pins and multiple connecting parts, the U-shaped part is provided with a threaded connecting rod with a boss, the threaded connecting rod is threadedly connected to the tensile end of the creep testing machine, the boss is connected to the extensometer, and the specimen is detachably connected to the U-shaped part and the connecting part through the pin.

[0024] The benefit of the present invention is that multiple specimens with different prefabricated groove lengths can be tested at the same time, and the crack growth rate and loading line displacement rate corresponding to different crack growth stages can be obtained by unloading twice. There is no need to equip high-cost real-time crack length measurement equipment and loading line displacement real-time measurement equipment, which greatly simplifies the test process, shortens the test time, and reduces the test cost. It is suitable for creep crack growth rate testing of high-temperature ductile materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is the front view and side view of the specimen in the initial state;

[0026] Figure 2 It is the front view and side view of the specimen group;

[0027] Figure 3 It is the main view of the specimen under the crack extension state;

[0028] Figure 4 It is a corresponding curve of creep crack growth rate and creep crack growth parameter obtained by the testing method of the embodiment of the present invention.

[0029] Component number description:

[0030] 1. Sample

[0031] 11 Pin hole

[0032] 12 Dovetail groove

[0033] 13. Incision

[0034] 2 Threaded connecting rod

[0035] 3 shoulders

[0036] 4 U-shaped part

[0037] 5 Pin

[0038] 6 Connection DETAILED DESCRIPTION

[0039] The specific embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings. These embodiments are only used to illustrate the present invention, but not to limit the present invention.

[0040] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0041] Furthermore, in the description of the present invention, unless otherwise specified, “plurality” means two or more.

[0042] The present invention provides a method for testing the creep crack growth rate of a high temperature ductile material, comprising the following steps:

[0043] S1. Process the target material into N specimens with the same nominal size, each specimen having a specimen number i, i=1, 2, ... N. N is preferably greater than or equal to 5. Specifically, the specimen is a step-type compact tensile specimen (CT specimen), such as Figure 1 As shown, a dovetail groove 12 and a notch 11 are machined on one side of the central axis of the specimen 1 using wire cutting, and two pin holes 11 are opened symmetrically about the central axis. The dovetail groove 12 is used to clamp the open displacement gauge (COD gauge) for pre-crack preparation, the notch 12 is used to guide the crack direction, and the pin hole 11 is used for the pin shaft to pass through and perform subsequent connection and stretching operations. The specimen thickness of specimen 1 is B, the width is W, and the initial spacing of the specimen loading lines is V0.

[0044] S2. At room temperature, clamp a COD gauge on each specimen and install it on a fatigue testing machine to prefabricate fatigue cracks. The length of the pre-crack on the specimen is recorded as a 0i , the relationship between the pre-crack lengths of N CT specimens is: a 01 02 <……a 0N , that is, the larger the sample number, the larger the pre-crack length.

[0045] ​S3. Load the specimens into the creep testing machine. A person skilled in the art can load only one specimen into the creep testing machine at a time, or can connect N specimens in series to form a specimen group, and then assemble the specimens into the creep testing machine. The latter can reduce the test time and improve the test efficiency compared to the former. After assembling the specimens into the creep testing machine, adjust the positions of the specimens to ensure that all specimens are located in the soaking zone of the high-temperature furnace, and bundle thermocouples at the upper, middle, and lower parts of the specimen group for temperature measurement and temperature control, with a temperature control accuracy of ±2. ℃ Finally, install a clamp-on extensometer to monitor the displacement changes of the loading line of the specimen.

[0046] S4. Control the high temperature furnace to heat up to the preset temperature. After keeping the temperature for 0.5 to 1 hour, apply load F to the sample group and keep the load. Observe the deformation of the extensometer. When the load is kept until a stable deformation occurs, unload and cool down for the first time. Record the first load-holding time as t I After the furnace temperature drops to room temperature, remove the sample and measure the first loading line displacement V of the sample. Ii and the first surface crack length a Ii The “stable deformation” here refers to the observation that the curve of the loading line displacement changing with time t tends to a straight line, indicating that the change in the loading line displacement has stabilized, and the target material is in the steady-state creep stage (i.e., the second stage of creep).

[0047] S5. Control the high temperature furnace to heat up to the same predetermined temperature as step S4. After keeping the temperature for 0.5 to 1 hour, apply the same load F as step S4 to the sample group and keep the load. Observe the deformation of the extensometer. When the load is kept until the deformation is obvious compared with the first unloading, perform the second unloading and cooling. Record the second load-holding time as t II After the furnace temperature drops to room temperature, remove the sample and measure the second loading line displacement V of the sample. IIi and the second surface crack length a IIi The “apparent deformation” here refers to the change in displacement of the loading line that can be measured by the extensometer compared to the first unloading.

[0048] Those skilled in the art can choose whether to extend the holding time according to the loading line displacement and surface crack length measured after the first unloading and cooling or the second unloading and cooling. If the measured loading line displacement and surface crack length are too small, the holding time can be appropriately extended as needed.

[0049] S6. Calculate the sample's second holding time t II The crack extension △a i =a IIi -a Ii , the corresponding crack growth rate

[0050] S7. Calculate the displacement change of the loading line after two unloadings △Vi =V IIi -V Ii , the corresponding loading line displacement rate is

[0051] S8. Calculate the creep crack growth parameter C of the specimen i * ,

[0052]

[0053] Where: n is the Norton index in the steady-state creep stage, F is the applied load, B is the specimen thickness, and W is the specimen width. Among them, n is obtained through the creep test of the target material under the same temperature conditions. The specific acquisition steps are as follows: The creep test refers to the standard "GBT 2039-2012 Uniaxial Tensile Creep Test Method for Metallic Materials", and the target material is processed into a group of round rod-type creep specimens with the same nominal size, and 4 to 5 stress values ​​σ are set under the same temperature conditions as step S4. i , install the sample on the creep testing machine, clamp the sample with a high temperature extensometer to measure the strain ε i , and conduct constant load creep tests respectively. Record the strain ε i The corresponding curve with time t, the minimum creep rate in the steady-state creep stage is obtained as For the above stress σ i and minimum creep rate conduct The power function is fitted to obtain the fitting index n, which is the Norton index in the steady-state creep stage.

[0054] S9. Plotting the crack growth rate and creep crack growth parameter C i * The corresponding curve is fitted according to the power function.

[0055]

[0056] Get the fitting coefficient P and fitting index q.

[0057] like Figure 2 As shown, the present invention also provides a test device for creep crack growth rate of high temperature ductile materials, which is used to realize the series connection of N specimens 1 in the above step S3, and includes two U-shaped parts 4, multiple pins 5 and multiple connecting parts 6. The U-shaped part 4 is provided with a threaded connecting rod 2 with a boss 3, the threaded connecting rod 2 is threadedly connected to the tensile end of the creep tester, and the boss 3 is connected to the extensometer. Specifically, the connecting part 6 is in the form of a sheet, and two through holes are formed thereon, and each of the two clamping arms of the U-shaped part 4 has a through hole, and the pin hole 11 of the specimen 1 is detachably connected to the through holes on the U-shaped part 4 and the connecting part 6 through the pin 5.

[0058] The installation method of the sample group includes: the upper end of the first sample is installed between the two clamping arms of the U-shaped part located at the upper end through the first pin shaft, the lower end of the first sample is installed at the upper end of the first connecting part through the second pin shaft, and the lower end of the first connecting part is connected to the upper end of the second sample through the third pin shaft, and the installation is carried out in sequence until the lower end of the last sample is installed between the two clamping arms of the U-shaped part located at the lower end through the last pin shaft, so as to realize the series connection of N samples 1.

[0059] The following is a specific embodiment of the present invention:

[0060] In this embodiment, the target material is a Ni-based alloy, the test temperature is 650° C., and the Norton index n=6.8 of the material at 650° C. is obtained through a creep test.

[0061] S1, such as Figure 1 As shown, the target material is processed into 5 specimens 1 with the same nominal size, the specimen numbers are 1-5#, the specimen thickness B of each specimen 1 is 10 mm, the specimen width W is 20 mm, and the initial spacing V0 of the specimen loading lines is 5 mm.

[0062] S2. Clamp the COD gauge in the dovetail groove 12 of the specimen 1 and install it in the fatigue testing machine to prepare pre-cracks of different lengths. The pre-crack length a 01 02 03 04 05 .

[0063] S3, connect the five samples in series Figure 2 The sample group shown is then assembled into the creep tester. Thermocouples are bundled at the upper, middle and lower positions of the sample group for temperature measurement and temperature control, with a temperature control accuracy of ±2°C. A clamp-on extensometer is installed on the shoulder 3 to monitor the displacement change of the sample loading line.

[0064] S4. Control the high temperature furnace to heat up to 650℃. After keeping the temperature for half an hour, apply a load of F=5760N to the sample group. Observe the deformation of the extensometer. When the deformation is stable, unload and cool down. Record the first holding time as t I After the furnace temperature drops to room temperature, all samples are removed and the first loading line displacement V of each sample is measured in turn using an optical measuring device. Ii and the first surface crack length a Ii , the measurement position is as follows Figure 3 shown.

[0065] ​​​​S5. Control the high temperature furnace to heat up to 650℃. After keeping the temperature for half an hour, apply a load of F=5760N to the sample group. Observe the deformation of the extensometer. When the deformation is obvious compared with the first unloading, perform the second unloading and cooling. Record the second holding time as t II After the furnace temperature drops to room temperature, remove all samples and measure the second loading line displacement V of each sample. IIi and the second surface crack length a IIi .

[0066] S6. Calculate the second holding time t II The crack extension △a i =a IIi -a Ii The crack growth rates of the five specimens are

[0067] S7. Calculate the displacement change of the loading line after two unloadings △V i =V IIi -V Ii The loading line displacement rates of the five specimens are

[0068] S8. Calculate the creep crack growth parameter C of the five specimens i * , the calculation formula is:

[0069]

[0070] Where n is the Norton index in the steady-state creep stage, F is the applied load, B is the specimen thickness, and W is the specimen width.

[0071] S9, such as Figure 4 As shown, the crack growth rate is plotted and creep crack growth parameter C i * The corresponding curve is fitted according to the power function:

[0072]

[0073] The fitting results were obtained: fitting coefficient P = 0.776, fitting index q = 0.714.

[0074] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and substitutions can be made without departing from the technical principles of the present invention. These improvements and substitutions should also be regarded as the scope of protection of the present invention.

Claims

1. A method for testing the creep crack growth rate of high temperature ductile materials, characterized in that: The following steps are involved: The target material is processed into N specimens with the same nominal size, and the specimen number of each specimen is i, i = 1, 2...N; Prefabricating precracks of different lengths on the N specimens respectively; Loading the sample into a creep testing machine; The sample is heated and kept warm, a load F is applied and kept loaded, and the load is kept until a stable deformation occurs, then the first unloading and cooling is performed, and the first loading line displacement V of the sample is measured. Ii and the first surface crack length a Ii ; The sample is heated and kept warm, a load F is applied and kept loaded, and a second unloading and cooling is performed when a significant deformation is observed compared to the first unloading, and the second holding time is recorded as t II , measure the second loading line displacement V of the specimen IIi and the second surface crack length a IIi ; Calculate the sample at the second holding time t II The crack extension △a i =a IIi -a Ii The corresponding crack growth rate is Calculate the displacement change of the loading line after two unloadings △V i =V IIi -V Ii , the corresponding loading line displacement rate is Calculate the creep crack growth parameter C of the specimen i * , Where: n is the Norton index in the steady-state creep stage, F is the applied load, B is the specimen thickness, and W is the specimen width; Plotting the crack growth rate and creep crack growth parameter C i * The corresponding curve is fitted according to the power function. Get the fitting coefficient P and fitting index q.

2. The testing method according to claim 1, characterized in that: The number N of samples is greater than or equal to 5.

3. The testing method according to claim 1, characterized in that: The test method also includes the steps of: connecting the N samples in series to form a sample group, and placing the sample group into a creep testing machine.

4. The testing method according to claim 3, characterized in that: Thermocouples are respectively bundled at the upper, middle and lower parts of the sample group for temperature measurement and temperature control, and the temperature control accuracy is ±2°C.

5. The testing method according to claim 1, characterized in that: Whether to extend the load holding time is selected according to the loading line displacement and the surface crack length measured after the first unloading and cooling or the second unloading and cooling.

6. The testing method according to claim 1, characterized in that: The Norton index n of the steady-state creep stage is obtained through a creep test of the target material under the same temperature conditions.

7. The testing method according to claim 1, characterized in that: The specimen is a step-type compact tensile specimen.

8. A test device for creep crack growth rate of high temperature ductile materials, characterized in that: Used for connecting test specimens (1) in series, the device comprises: two U-shaped parts (4), a plurality of pins (5) and a plurality of connecting parts (6), wherein the U-shaped part (4) is provided with a threaded connecting rod (2) with a boss (3), the threaded connecting rod (2) is threadedly connected to the tensile end of a creep testing machine, the boss (3) is connected to an extensometer, and the test specimen (1) is detachably connected to the U-shaped part (4) and the connecting part (6) via the pin (5).

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