Ultrasonic fatigue test piece preparation method and ultrasonic fatigue test method

Abstract

To provide a preparation method of a test piece utilizable for an ultrasonic fatigue test even for a steel type low in hardenability, such as medium carbon steel.SOLUTION: A preparation method of an ultrasonic fatigue test piece includes: performing an induction hardening treatment in which induction heating is performed on a test-piece collection target part of a test-piece base material of a steel material to be evaluated to heat the test-piece collection target part from a surface to a center portion, followed by cooling; performing a tempering treatment on the test-piece base material after the induction hardening treatment; and collecting a test piece for an ultrasonic fatigue test from the test-piece collection target part of the test-piece base material after the tempering treatment.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for preparing an ultrasonic fatigue test piece to be used in an ultrasonic fatigue test, and to an ultrasonic fatigue test method using the test piece. [Background technology]

[0002] In an effort to achieve carbon neutrality, high-frequency heating, which produces less carbon dioxide, is attracting attention as a heat treatment method for steel. Medium-carbon steel is an example of a steel type suitable for high-frequency heating. Therefore, the use of medium-carbon steel types, including medium-carbon steel and alloy steels with similar components but with additional alloying elements, is expected to expand in the future, and various steel types are being developed.

[0003] On the other hand, evaluating the size of nonmetallic inclusions is necessary to determine the reliability of medium-carbon steel for bearing applications. This is because fatigue failure can occur due to stress concentration sources in high-strength steels used for bearing applications. Inclusions in steel are primarily unavoidably generated during the steel manufacturing process and remain unremoved. To assess the degree of cleanliness of steel, a technology for accurately evaluating the inclusion content in steel is needed. Among these evaluation techniques, ultrasonic fatigue testing is used to detect inclusions as fracture initiation points and observe them. This method allows for large test specimens, enabling evaluation in large volumes and enabling faster and more accurate detection of large inclusions. Therefore, it is necessary to be able to appropriately evaluate inclusions in the above-mentioned medium-carbon steels using ultrasonic fatigue testing. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-149121 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the low hardenability of the aforementioned medium-carbon steels poses a bottleneck when preparing evaluation specimens for ultrasonic fatigue testing. When large ultrasonic fatigue test specimens are prepared using a conventional furnace-heated quenching method, the hardness of the specimens may be insufficient or may be uneven.

[0006] Regarding the hardness of test specimens, a method for evaluating inclusions using ultrasonic fatigue testing has been proposed that allows evaluation if the hardness of the test specimen is approximately 27 HRC or more in Rockwell hardness (approximately 280 HV or more in Vickers hardness) (Patent Document 1). However, this method requires extending the fatigue test time and implementing additional processes to bring the test specimen to fracture, making the evaluation method complicated.

[0007] Furthermore, if the hardness of the test specimen is uneven, conducting ultrasonic testing under repeated loading conditions that match the strength of the soft parts may prevent fatigue fractures from initiating from inclusions in the hard parts.On the other hand, even if ultrasonic fatigue testing is conducted under repeated loading conditions that match the strength of the hard parts, the soft parts may heat up due to the influence of internal friction caused by ultrasonic fatigue, further reducing their hardness.This prevents the test specimen from resonating, which is essential for ultrasonic fatigue testing, and may prevent the inclusions from being properly evaluated.

[0008] Therefore, an object of the present invention is to provide a method for producing ultrasonic fatigue test pieces that can produce test pieces suitable for ultrasonic fatigue testing even for steel types with low hardenability, such as medium carbon steel. [Means for solving the problem]

[0009] The present invention was completed after extensive investigations to solve the above problems, and details thereof are as follows.

[0010] (1) High-frequency heating is performed on the test specimen extraction target portion of the test specimen base material of the steel material to be evaluated, heating the test specimen extraction target portion from the surface to the center, and then performing high-frequency hardening treatment in which cooling is performed; The test piece base material after the induction hardening treatment is subjected to a tempering treatment, a method for producing an ultrasonic fatigue test piece, comprising: extracting a test piece for an ultrasonic fatigue test from the test piece extraction target portion of the test piece base material after the tempering treatment;

[0011] (2) The method for producing an ultrasonic fatigue test piece according to (1) above, characterized in that the high-frequency hardening treatment is used to make the average hardness of the test piece extraction target portion of the test piece base material equal to or greater than a predetermined hardness.

[0012] (3) The method for producing an ultrasonic fatigue test piece according to (1) above, characterized in that the high-frequency hardening treatment is a moving hardening method in which the high-frequency heating and cooling are repeated while the test piece base material or the high-frequency heating coil is moved relative to one another.

[0013] (4) The method for producing an ultrasonic fatigue test piece according to (3) above, characterized in that the tempering treatment is carried out immediately after the cooling in the induction hardening treatment by the moving hardening method.

[0014] (5) The method for producing an ultrasonic fatigue test piece according to (2) above, wherein the predetermined hardness is 450 HV or more in Vickers hardness.

[0015] (6) The method for producing an ultrasonic fatigue test piece according to (1) above, wherein the steel material is a carbon steel or an alloy steel having a carbon content of 0.35 mass % or more and 0.8 mass % or less.

[0016] (7) The method for producing an ultrasonic fatigue test piece according to (6) above, wherein the steel material contains more than 0 mass% and 0.8 mass% or less of chromium.

[0017] (8) The method for producing an ultrasonic fatigue test piece according to (1) above, characterized in that the high-frequency hardening treatment is carried out after annealing the base material of the test piece.

[0018] (9) A test piece is prepared by the method for preparing an ultrasonic fatigue test piece according to any one of (1) to (8) above, an ultrasonic fatigue testing method, characterized in that an ultrasonic fatigue test is performed on the test piece. [Effects of the Invention]

[0019] According to the present invention, a method for producing an ultrasonic fatigue test piece can be provided that can produce a test piece suitable for ultrasonic fatigue testing even for steel types with low hardenability, such as medium carbon steel. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a flowchart showing a method for producing an ultrasonic fatigue test piece according to the present embodiment. [Figure 2] FIG. 1 shows a diagram for explaining the part of the test piece base material where high-frequency heating is performed (the part from which the test piece is taken). [Figure 3] FIG. 1 is a schematic diagram showing a test piece for an ultrasonic fatigue test according to an example. DETAILED DESCRIPTION OF THE INVENTION

[0021] This embodiment will be described with reference to the drawings. In the description of the method for producing an ultrasonic fatigue test piece of this embodiment, a case will be described in which a medium carbon steel with low hardenability is used as the steel material to be evaluated. Figure 1 is a flowchart showing the method for producing an ultrasonic fatigue test piece of this embodiment.

[0022] In the following description of the present embodiment, a numerical range expressed using "to" means a numerical range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0023] First, a test piece base material of a size and shape that allows for heating to the center by high frequency is cut out from a medium carbon steel material to be evaluated by ultrasonic fatigue testing (step 101). The test piece base material is the test piece from which the test piece to be used in the ultrasonic fatigue test is ultimately cut. The test piece base material may be of any appropriate shape and size. In this embodiment, a cylindrical round bar test piece is prepared as the test piece base material.

[0024] Next, if necessary, a process for adjusting the microstructure of the round bar test specimen is performed (step 102). When high-frequency heating is performed, rapid heating and rapid cooling are performed, which may cause quench cracking. The microstructure adjustment process is performed to prevent this quench cracking. For example, annealing or spheroidizing annealing can be used as the structure adjustment. Note that this step may be omitted if this process is not necessary, such as when using a steel material that is less likely to cause quench cracking. In that case, for example, as-rolled steel material or steel material that has been normalized can be used.

[0025] Next, induction hardening is performed as a heat treatment for the test specimen base material. The induction hardening process includes induction heating (step 103) and cooling (step 104) of the test specimen base material. First, induction heating is a method of directly heating the test specimen base material by generating eddy currents in the test specimen base material using electromagnetic induction. In the induction heating process of this embodiment, a deep heating process is performed, in which the test specimen base material is heated entirely, partially to the center, using high frequency waves. Figure 2 illustrates the portion of the test specimen base material that undergoes induction heating. As shown in Figure 2, the test specimen extraction target portion (the hatched portion enclosed by a dashed line), which is a portion of the test specimen base material, is heated from the surface to the center, i.e., the entire test specimen extraction target portion. The test specimen extraction target portion is the portion of the test specimen base material from which a test specimen for ultrasonic fatigue testing is extracted. The test specimen extraction target portion can be set to an appropriate range depending on the size of the test specimen to be extracted. The test specimen extraction target portion is preferably set in the longitudinal center portion of the base material, but may be set in other portions. The direction of extraction of the base material from which the test specimens are taken may be parallel to the rolling direction of the steel or perpendicular to the rolling direction of the steel. In this way, rather than heat-treating the surface of the test specimen base material as is common in induction hardening, the entire test specimen is partially heated and hardened to the center (induction heat treatment). Partial hardening can be performed, for example, by using the traveling hardening method, which involves relatively moving the test specimen base material (steel) or the induction heating coil or cooling device to heat and cool only the desired portion. The traveling hardening method involves repeatedly moving the test specimen base material or the induction heating coil, etc., and repeatedly heating and cooling the entire desired area, thereby induction heat treating the entire desired area. The induction hardening process (induction heating treatment and subsequent cooling treatment) adjusts the overall hardness (average hardness) of the test specimen base material from which the test specimen is taken to a predetermined hardness suitable for ultrasonic fatigue testing.

[0026] The predetermined hardness (average hardness) is preferably 450 HV or more in Vickers hardness. If the entire test specimen extraction target portion of the test specimen base material has a Vickers hardness of 450 HV or more, fracture originating from inclusions can be more reliably generated in the ultrasonic fatigue test described below for test specimens extracted from the test specimen, allowing for more accurate and rapid evaluation of inclusions. For the same reasons, a hardness of 540 HV or more is more preferable. The upper limit of the hardness is not particularly limited, but it may be 800 HV or less. In this embodiment, the average hardness of the entire test specimen extraction target portion is the average of hardness values ​​measured at multiple locations in the test specimen extraction target portion. Specifically, at an arbitrary position in the test specimen extraction target portion, Vickers hardness is measured at 20 points in the radial direction (along an arbitrary radius) at 0.5 mm intervals from the outer periphery to the center in a cross section perpendicular to the axial direction (longitudinal direction) of the test specimen base material, and the arithmetic mean of the 20 measured values ​​is the value.

[0027] In addition to satisfying the above-mentioned predetermined average hardness, it is also preferable that the minimum hardness of the test piece base material be 400 HV or more as an index of hardness unevenness. In other words, a minimum hardness of within minus 50 HV of the average hardness can be considered to be a state with little unevenness. The minimum hardness is the smallest hardness of the above-mentioned 20 hardness points.

[0028] The conditions for the high-frequency heat treatment are not particularly limited, but can be appropriately set depending on the chemical composition of the steel so that the specimen extraction area is heated to the center of the specimen base material and the required hardness is achieved after quenching. For example, the test specimen base material of the target steel can be pre-heated to determine the appropriate frequency and heating time required to harden the entire specimen extraction area to 450 HV or higher. The test specimen base material can then be heated under these conditions. Furthermore, when setting the conditions, the test specimen base material after heat treatment can be cut to check the internal hardness of the test specimen base material or checked at multiple locations. Heat treatment conditions can be set so that the specimen can be hardened to the required hardness all the way to the center while suppressing hardness variations. Note that high-frequency hardening by heating the entire test specimen base material is not recommended. This is because, with medium-carbon steel, hardness at the center decreases, resulting in hardness variations, which makes it difficult to properly perform ultrasonic fatigue testing. Therefore, it is preferable to perform induction hardening treatment by the moving hardening method on the test specimen extraction target portion, which is a part of the test specimen base material.

[0029] The equipment for performing the high-frequency heating treatment is not particularly limited as long as it is equipped with a high-frequency heating coil shaped to heat the test piece base material, a device for moving the high-frequency heating coil or a device for moving the test piece (in the case of a moving quenching method), a cooling device for spraying water or oil for quenching (and a device for moving it if necessary), a device for cooling by immersion in water or oil instead of spray cooling, and a high-frequency power source.

[0030] The test piece base material that has been subjected to the high-frequency heating treatment is subjected to a cooling treatment for hardening (step 104). The cooling treatment method is not particularly limited, but for example, a coolant may be sprayed onto the test piece base material, or the test piece base material may be immersed in a coolant to cool it. The coolant may be water or a water-soluble coolant.

[0031] Although not shown in the flowchart of FIG. 1, the test piece base material may be tempered after the induction hardening process (steps 103 and 104). The tempering process may be performed immediately after cooling after the induction hardening process. There are no particular restrictions on the tempering method, but tempering by immersion in an oil bath or tempering by induction hardening may be performed.

[0032] Next, a test piece for ultrasonic fatigue testing is cut out and collected from the test piece collection target portion of the test piece base material that has been subjected to high-frequency heating and cooling (step 105). In the method of this embodiment, the test piece can be cut out to an appropriate size depending on the ultrasonic fatigue testing machine to be used. Furthermore, the test piece may be designed to have an appropriate resonant frequency depending on the frequency of the ultrasonic fatigue testing machine. The shape of the test piece is not particularly limited as long as it can be used to perform ultrasonic fatigue testing. For example, a so-called "dumbbell" shape, in which a parallel portion is provided in the center of a rod-shaped test piece, may be used. When estimating the maximum inclusion diameter contained in the volume of an arbitrary steel material using extreme value statistical analysis, multiple test pieces may be collected from the test piece base material and ultrasonic fatigue testing may be performed on each test piece.

[0033] In the test specimen preparation method of this embodiment, induction hardening can heat the test specimen base material up to the center, so the test specimen extracted from the test specimen extraction target area is hardened throughout. For example, by using a moving quenching method, it is possible to prepare a test specimen in which the entire test specimen is appropriately hardened to the desired hardness. In addition, in the case of induction heating, compressive residual stress is imparted to the surface of the test specimen, which has the advantage of making it easier to suppress fracture from the surface of the test specimen that does not originate from the inclusion to be evaluated. This concludes the ultrasonic fatigue test specimen preparation method of this embodiment.

[0034] The type of steel material used in the method for producing an ultrasonic fatigue test piece according to the present embodiment is not particularly limited, but as mentioned above, it is more effective to apply it to steel materials with low hardenability. A steel material with low hardenability is a steel material with a relatively low content of alloying elements, and a typical example is medium-carbon steel with a carbon content of approximately 0.5 mass% or less. The method according to the present embodiment can be applied to carbon steels with a carbon content of 0.35 mass% or more and 0.8 mass% or less, including such medium-carbon steels. When carbon steel is used, it can be classified as, for example, carbon steel for machine structures (SC steel). Furthermore, steel materials with low hardenability may also be carbon-based alloy steels (specialty steels) with the same carbon content as the above-mentioned carbon steels but with the addition of other alloying elements.

[0035] As mentioned above, when test specimens of low-hardenability steels, such as medium-carbon steels, are prepared by conventional furnace heating, the hardness after quenching may be insufficient or the hardness of the test specimen may be uneven. In such cases, for example, it may be necessary to extend the ultrasonic fatigue test time or to perform special treatment to induce fracture, or fracture in the ultrasonic fatigue test itself may be difficult. In contrast, the test specimen preparation method of this embodiment can harden the entire specimen, from the surface to the center, even for low-hardenability medium-carbon steels, and more reliably cause fracture of the test specimen at the inclusion origin. Therefore, accurate and rapid inclusion evaluation is possible even for low-hardenability steels.

[0036] Preferably, the target steel material is a steel type that is less susceptible to quench cracking. For example, it is preferable to add chromium (Cr) to the above-mentioned medium carbon steel or alloy steel, and use a steel material containing more than 0 mass% and 0.8 mass% or less of Cr. If this steel is annealed or spheroidized and then induction hardened, it becomes less susceptible to quench cracking.

[0037] Next, the ultrasonic fatigue testing method of this embodiment will be described. In the ultrasonic fatigue testing of this embodiment, the ultrasonic fatigue testing is performed on a test piece, and inclusions that appear on the fracture surface are evaluated.

[0038] Specifically, a test piece is first prepared for the target steel material using the ultrasonic fatigue test piece preparation method described above. Then, an ultrasonic fatigue test is performed using the prepared test piece. The ultrasonic fatigue test is performed using an ultrasonic fatigue testing machine that applies vibrations of approximately 20,000 Hz to the test piece and applies repeated axial loads of tension and compression in the axial direction of the test piece. The conditions for the ultrasonic fatigue test are not limited to those described above, and may be set appropriately depending on the shape of the test piece, the physical properties of the steel material, etc.

[0039] Then, after the test piece is subjected to repeated fatigue using ultrasonic vibration and breaks, the fracture surface is observed to evaluate the inclusion that initiated the fracture. The fracture surface can be observed using a scanning electron microscope (SEM), and the size of the inclusion can be measured from the SEM image. If a fisheye pattern appears on the observed fracture surface, the inclusion in the center is the inclusion that initiated the fracture.

[0040] Furthermore, ultrasonic fatigue tests are performed on multiple test pieces cut out from the test piece base material to determine the inclusion diameter for each piece, and extreme value statistical analysis is performed on the multiple inclusion diameters, making it possible to estimate the maximum inclusion diameter contained in the volume of any steel material.

[0041] In the ultrasonic fatigue testing method described above, the test specimens prepared by the ultrasonic fatigue test specimen preparation method described above are used, so the test specimens are sufficiently hardened to the center, and the ultrasonic fatigue testing method can more reliably fracture the test specimen starting from the inclusions. Therefore, inclusions can be evaluated more quickly and accurately even in steels with low hardenability. [Example]

[0042] Hereinafter, the embodiments will be described in more detail with reference to examples, which should not be construed as limiting the scope of the present disclosure.

[0043] Test pieces of the examples and comparative examples were actually prepared, and ultrasonic fatigue tests were carried out on each test piece to evaluate inclusions, and the accuracy, speed, etc. of the inclusion evaluation were confirmed.

[0044] Example 1 A cylindrical round bar test specimen was prepared as the test specimen base material using normalized S53C medium-carbon steel (the microstructure adjustment process in step 102 was omitted). The dimensions of the round bar test specimen were a diameter (φ) of 20 mm and a length (L) of 150 mm. The test specimen was extracted from the area extending from 25 mm to 125 mm from one end of the round bar test specimen (a 100 mm long section). This test specimen extraction area of ​​the round bar test specimen was subjected to induction hardening. Specifically, induction heating using a moving quenching method and cooling for quenching were performed to achieve a deep hardening state throughout the test specimen extraction area. The entire test specimen extraction area was heated using an induction heating coil designed to take into account the diameter of the test specimen extraction area. Induction heating was performed at 1000°C for 20 seconds. Cooling was performed using oil-jet cooling. The induction hardening process consisted of one induction heating and one subsequent cooling. The tempering was carried out in oil at 180°C for 1.5 hours.

[0045] After induction hardening, tempered round bar test pieces were cut at an arbitrary position on the test piece extraction area along a plane perpendicular to the axial direction (longitudinal direction) of the test piece. The Vickers hardness of the cut cross section was measured at 20 points at 0.5 mm intervals from the outer periphery to the center (along an arbitrary radius). The average hardness and the minimum hardness, which serves as an index of hardness unevenness, were confirmed. The minimum value of the 20 measured values ​​was used as the minimum hardness. The same confirmation was carried out for the other examples and comparative examples below.

[0046] The ultrasonic fatigue test specimen of Example 1 was obtained by cutting out the test specimen shown in Fig. 3 from the test specimen sampling target portion of the round bar test specimen after high-frequency heating, cooling, and tempering. The test specimen had a cylindrical dumbbell shape with a parallel part having a length of 20.0 mm and a diameter of 7.0 mm.

[0047] Example 2 An ultrasonic fatigue test piece of Example 2 was prepared in the same manner as in Example 1, except that the high-frequency heating conditions were changed to 950° C. for 40 seconds.

[0048] (Comparative Example 1) Round bar test pieces (φ=20 mm, L=150 mm) made of the same medium carbon steel (S53C) as in Example 1 were subjected to a furnace-heated through-quenching and tempering treatment. For through-quenching, the round bar test pieces were first heated in a furnace at 830°C for 20 minutes, and then quenched in oil held at 60°C. Subsequently, they were tempered in oil at 180°C for 1.5 hours.

[0049] Test pieces of the same shape and size as those in Example 1 were cut out from the round bar test pieces after quenching and tempering treatment to obtain ultrasonic fatigue test pieces for Comparative Example 1.

[0050] (Comparative Example 2) An ultrasonic fatigue test piece for Comparative Example 2 was obtained in the same manner as in Comparative Example 1, except that the heating conditions for through-quenching the round bar test piece were a temperature of 870°C and a holding time of 10 minutes.

[0051] (Ultrasonic fatigue testing) Ultrasonic fatigue tests were conducted on the ultrasonic fatigue test pieces of the examples and comparative examples prepared as described above. The ultrasonic fatigue tests were conducted at a frequency of 20,000 Hz, a stress ratio R of minus 1, and a stress amplitude of 800 MPa until the test pieces broke. The ultrasonic fatigue test was considered to have ended when the ultrasonic fatigue tester stopped during the test without breaking the test pieces.

[0052] (Inclusion evaluation by SEM) The fracture surfaces of the test pieces of the examples and comparative examples were observed using an SEM to identify the inclusions that initiated the fracture. If a fish-eye pattern was observed on the fracture surface, the inclusion at the center of the fish-eye pattern was identified as the inclusion that initiated the fracture.

[0053] The size (diameter) of the inclusions was also measured from the SEM images by measuring the projected area of ​​the inclusions on the fracture surface from the SEM images and calculating the square root of that projected area (√area).

[0054] Table 1 shows the preparation conditions for the test specimens in the examples and comparative examples, the results of the average hardness and minimum hardness (an index of hardness unevenness) of the cross section of the round bar test specimens, and the results of the ultrasonic fatigue test and inclusion evaluation. The results of the ultrasonic fatigue test indicate the number of repeated stress applications at which fracture occurred. If the ultrasonic fatigue tester stopped without fracture, it is indicated as "no fracture." Furthermore, for the inclusion evaluation, if inclusions were confirmed by examining the fracture surface of the test specimen with an SEM, it is indicated as "pass," and if not, it is indicated as "fail."

[0055] [Table 1]

[0056] For Examples 1 and 2, the average hardness of the test specimen extraction portion of the test specimen base material was 450 HV or more, confirming that sufficient hardness was obtained. It was also confirmed that the difference in hardness between Examples 1 and 2 was within 50 HV, indicating that the specimens were hardened to the core and had no hardness unevenness. On the other hand, for Comparative Example 1, the difference in hardness was large, exceeding 50 HV, resulting in hardness unevenness. Furthermore, for Comparative Example 2, the average hardness was less than 450 HV, and the difference in hardness also greatly exceeded 50 HV, resulting in even greater hardness unevenness.

[0057] In addition, in both Examples 1 and 2, the test pieces were broken without the ultrasonic fatigue tester stopping. In Examples 1 and 2, inclusions were confirmed on the fracture surface. On the other hand, in Comparative Examples 1 and 2, no fracture occurred, and inclusion evaluation was not possible.

[0058] From the above, it was confirmed that in all Examples, it was possible to quickly evaluate inclusions even when evaluating steel materials with poor hardenability. Furthermore, since inclusions were confirmed in all Examples, it was also confirmed that inclusion evaluation can be performed more accurately and reliably.

Claims

1. High-frequency heating is performed on a test specimen extraction target portion of a test specimen base material of the steel material to be evaluated, heating the test specimen extraction target portion from the surface to the center, and then performing high-frequency hardening treatment in which cooling is performed; The test piece base material after the induction hardening treatment is subjected to a tempering treatment, a method for producing an ultrasonic fatigue test piece, characterized in that a test piece for an ultrasonic fatigue test is taken from the test piece taking target portion of the test piece base material after the tempering treatment.

2. 2. The method for producing an ultrasonic fatigue test piece according to claim 1, wherein the high-frequency hardening treatment is performed to make the average hardness of the test piece extraction target portion of the test piece base material equal to or greater than a predetermined hardness.

3. 2. The method for producing an ultrasonic fatigue test piece according to claim 1, wherein the high-frequency hardening treatment is a moving hardening method in which the high-frequency heating and cooling are repeated while the test piece base material or the high-frequency heating coil is moved relative to one another.

4. 4. The method for producing an ultrasonic fatigue test piece according to claim 3, wherein the tempering treatment is carried out immediately after the cooling in the induction hardening treatment by the moving hardening method.

5. 3. The method for producing an ultrasonic fatigue test piece according to claim 2, wherein the predetermined hardness is 450 HV or more in Vickers hardness.

6. 2. The method for producing an ultrasonic fatigue test piece according to claim 1, wherein the steel material is a carbon steel or an alloy steel having a carbon content of 0.35 mass % or more and 0.8 mass % or less.

7. 7. The method for producing an ultrasonic fatigue test piece according to claim 6, wherein the steel material contains more than 0 mass% and 0.8 mass% or less of chromium.

8. 2. The method for producing an ultrasonic fatigue test piece according to claim 1, wherein the induction hardening is performed after the base material of the test piece is annealed.

9. A test piece is prepared by the method for preparing an ultrasonic fatigue test piece according to any one of claims 1 to 8, an ultrasonic fatigue testing method, characterized in that an ultrasonic fatigue test is performed on the test piece.

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