A method of characterizing impact fatigue resistance

Abstract

The present application relates to a kind of methods for characterizing impact fatigue resistance, comprising the following steps: step 1, making impact fatigue test piece: test piece includes upper and lower integrally formed quenching section and non-quenching section, quenching section cross section is required to be smaller than non-quenching section cross section;Step 2, measure and record the original size of test piece;Step 3, carry out impact test: test piece is clamped on test device, and a certain number of impact tests are carried out;Step 4, measure and record the deformation size of test piece after test;Step 5, calculate the deformation of test piece: the size change of test piece before and after test is calculated, the greater the size change, the worse the impact fatigue resistance of test piece, otherwise, the better the impact fatigue resistance of test piece.The present application can effectively characterize the advantages and disadvantages of the impact fatigue resistance of workpiece to a certain extent by combining a large number of workpiece practical applications with a large number of test data, and provide basis for workpiece surface hardening layer depth design.

Description

Technical Field

[0001] This invention relates to the field of fatigue performance technology of metallic materials, and in particular to a method for characterizing impact fatigue performance. Background Technology

[0002] In the machinery industry, a large proportion of workpieces are used in a surface-hardened state. Surface hardening is a surface heat treatment process that hardens the surface of a workpiece to improve its surface hardness, wear resistance, and fatigue strength, while the core of the workpiece still retains high toughness.

[0003] Depending on the usage environment, some workpieces may be subjected to long-term alternating loads of impact pressure-no-force-impact pressure-no-force during service. Under such circumstances, the surface of the workpiece will undergo certain crushing deformation, leading to the failure of the workpiece due to metal fatigue.

[0004] A cutterhead box is installed on the cutterhead of a tunnel boring machine to support the hobbing cutters. The hobbing cutters are housed inside the cutterhead box and can be replaced if damaged. However, the cutterhead box itself is welded to the cutterhead and cannot be replaced. During long-term use, the part of the cutterhead box that supports the hobbing cutter shaft will be deformed and crushed by the impact of the hobbing cutters. Therefore, the design incorporates localized surface hardening of the part of the cutterhead box that supports the hobbing cutters. This aims to improve the cutterhead box's resistance to impact fatigue, resulting in a hardened layer after hardening.

[0005] Currently, the commonly used method is to characterize the fatigue performance of workpiece materials using the SN curve, which is a curve showing the relationship between stress (S) and fatigue life (N) derived from experimental data on the material's fatigue strength. However, this method has poor applicability for surface-quenched workpieces subjected to long-term alternating loads of impact pressure-no-force-impact pressure-no-force. This is because, without clearly understanding the impact of quenching depth on the workpiece's impact fatigue resistance, the SN curve cannot effectively express the impact fatigue resistance of surface-quenched workpieces, nor can it prove whether different depths of the hardened layer after quenching can improve impact fatigue resistance. Summary of the Invention

[0006] To address the problem that existing SN curves cannot effectively express the impact fatigue resistance of surface-hardened workpieces, this invention provides a method for characterizing impact fatigue resistance. By combining analysis with a large number of actual workpiece applications and a large amount of experimental data, this method can effectively characterize the impact fatigue resistance of workpieces with different hardened layer depths to a certain extent.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for characterizing impact fatigue resistance includes the following steps: Step 1: Prepare impact fatigue test specimens: The test specimens consist of an integrally formed quenched section and a non-quenched section. The quenched section is where the test specimen is surface quenched, while the non-quenched section does not require surface quenching. The cross-section of the quenched section is required to be smaller than that of the non-quenched section. Step 2: Measure and record the original dimensions of the test specimen; Step 3: Conduct impact tests: Mount the prepared test specimen on the testing device and conduct a certain number of impact tests on the testing device. The impact force is an alternating load of pressure-no force-pressure-no force, and a certain impact frequency is set. Step 4: Measure and record the deformation dimensions of the test specimen after the test; Step 5: Calculate the deformation of the test piece: Based on the data recorded in Step 2 and Step 4, calculate the dimensional change of the test piece before and after the test. Under the same test conditions, the greater the dimensional change, the worse the impact fatigue resistance of the test piece, and vice versa.

[0008] Furthermore, in step 1, the test piece is a metal cylinder with a certain height. The cross-section of the quenched section and the non-quenched section is "convex" shaped after being connected and combined. The quenched section is formed by surface quenching, and the surface of the quenched section is covered with a hardened layer.

[0009] Furthermore, in step 3, the test piece is vertically clamped on the test device, and the test device applies alternating impact force to the quenching section to simulate the workpiece's force-no-force cycle. The force is applied in the vertical direction, perpendicular to the upper end face of the quenching section. The number of impact tests set on the test device is N, and the impact frequency is f.

[0010] Furthermore, in step 2, the original dimensions of the test piece that are measured and recorded include the test piece height H1, the cross-sectional area of ​​the quenched section AQ1, and the cross-sectional area of ​​the non-quenched section AT1.

[0011] Furthermore, in step 4, the measured and recorded deformation dimensions of the test piece include the test piece height H2, the cross-sectional area of ​​the quenched section AQ2, and the cross-sectional area of ​​the non-quenched section AT2. After the test piece is subjected to the test impact, its height and cross-section will naturally change, and the degree of deformation of the quenched section and the non-quenched section will be different.

[0012] Furthermore, in step 5, the dimensional changes of the test piece before and after the test are ΔH=H1-H2, ΔAQ=AQ2-AQ1 and ΔAT=AT2-AT1. The larger the values ​​of ΔH, ΔAQ and ΔAT, the greater the deformation of the test piece and the worse its impact fatigue resistance. The smaller the values ​​of ΔH, ΔAQ and ΔAT, the smaller the deformation of the test piece and the better its impact fatigue resistance.

[0013] The beneficial effects of the present invention through the above technical solution are: This invention, based on the characteristics of quenched and unquenched materials, designs the test piece into two segments with different cross-sectional sizes to ensure that both the upper and lower segments of the entire test piece undergo a certain degree of deformation. An alternating load is applied to the test piece using a testing device, and the deformation data before and after the test are recorded. The difference between the deformation data is then calculated, and the magnitude of the difference is used to determine the workpiece's impact fatigue resistance. The entire testing process is simple, requires minimal test data, uses few pieces of equipment, and is highly operable. It can characterize the impact fatigue resistance of workpieces with hardened layers of different depths to a certain extent.

[0014] This invention, through extensive practical application on numerous workpieces, can conveniently and intuitively obtain the deformation of the test pieces. Combined with a large amount of experimental data for analysis, that is, by combining the actual deformation degree of the workpiece with the crushing deformation data, it can intuitively demonstrate the quality of the workpiece's impact fatigue resistance to a certain extent, and provide a basis for the design of the surface hardening layer depth of the workpiece to a certain extent. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of step 1 of a method for characterizing impact fatigue resistance according to the present invention.

[0016] Figure 2 This is a schematic diagram of step 2 of a method for characterizing impact fatigue resistance according to the present invention.

[0017] Figure 3 This is a schematic diagram of step 3 of a method for characterizing impact fatigue performance according to the present invention, where F represents alternating impact force.

[0018] Figure 4 This is a schematic diagram of step 4 of a method for characterizing impact fatigue resistance according to the present invention.

[0019] Figure 5 This is a schematic diagram of experimental data statistics in an embodiment of a method for characterizing impact fatigue resistance according to the present invention.

[0020] Figure 6 This is a schematic diagram of the deformation of a test specimen in an embodiment of a method for characterizing impact fatigue resistance according to the present invention.

[0021] The attached figures are labeled as follows: 1 Test piece, 2 Quenched section, 3 Non-quenched section. Detailed Implementation

[0022] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings: like Figures 1-6 As shown, a method for characterizing impact fatigue resistance includes the following steps: Step 1. Prepare an impact fatigue test piece 1: the test piece 1 is a metal cylinder, and the test piece 1 comprises a quenched section 2 and a non-quenched section 3 which are integrally formed up and down; the quenched section 2 is formed by quenching the upper surface of the test piece 1, and the surface of the quenched section 2 after quenching is covered with a hardened layer of a certain depth.

[0023] When preparing the test piece 1, the cross-sectional area of the test piece 1 is required to be different, that is, the cross-section of the quenched section 2 is required to be smaller than that of the non-quenched section 3, so that the cross-section after the quenched section 2 and the non-quenched section 3 are connected and combined is in a "convex" shape, as Figure 1 shown.

[0024] It should be noted that under the same impact load condition, the quenched section 2 has high strength and is not prone to deformation, while the non-quenched section 3 is generally in a quenched and tempered state, has lower strength than the quenched section 2 and is prone to deformation. In order to ensure that both the quenched section 2 and the non-quenched section 3 produce a certain degree of deformation during the impact fatigue test, the test piece 1 is designed into a shape in which the cross-sectional area of the quenched section 2 is smaller than that of the non-quenched section 3.

[0025] Step 2. Measure and record the original dimensions of the test piece 1: including measuring the height H1 of the test piece 1, the cross-sectional area AQ1 of the quenched section 2 and the cross-sectional area AT1 of the non-quenched section 3, as Figure 2 shown.

[0026] Step 3. Carry out an impact test: clamp the prepared test piece 1 on a test device, and clamp the test piece 1 vertically on the test device.

[0027] Carry out a certain number of impact tests on the test device, the number of impact tests is recorded as N; the impact force is an alternating load of pressure - no force - pressure - no force, that is, the test device applies an alternating impact force to the quenched section 2, the force application direction is vertical and perpendicular to the upper end surface of the quenched section 2, and a certain impact frequency is set, which is recorded as f, as Figure 3 shown.

[0028] Step 4. Measure and record the deformed dimensions of the test piece 1 after test: after the impact test, the test piece 1 will produce a certain degree of crushing deformation, the deformed dimensions to be measured and recorded include the height H2 of the test piece 1, the cross-sectional area AQ2 of the quenched section 2 and the cross-sectional area AT2 of the non-quenched section 3, as Figure 4 shown.

[0029] Step 5. Calculate the deformation amount of the test piece 1: according to the data recorded in step 2 and step 4, calculate the dimensional change amount of the test piece 1 before and after the test: the overall height difference of the test piece 1 ΔH=H1-H2, the cross-sectional area difference of the quenched section 2 ΔAQ=AQ2-AQ1, and the cross-sectional area difference of the non-quenched section 3 ΔAT=AT2-AT1.

[0030] Based on the differences obtained above, the impact fatigue resistance of the metal can be represented: under the same test conditions, the greater the dimensional change, the worse the impact fatigue resistance of test piece 1, and vice versa. That is, the larger the values ​​of ΔH, ΔAQ, and ΔAT, the greater the deformation of test piece 1 and the worse its impact fatigue resistance; the smaller the values ​​of ΔH, ΔAQ, and ΔAT, the smaller the deformation of test piece 1 and the better its impact fatigue resistance.

[0031] To verify the effect of surface hardening depth, i.e. hardened layer depth, on the impact fatigue resistance of a workpiece, this invention was used to prepare three test pieces 1 with hardened layer depths of 0mm, 5mm, 10mm, and 20mm for each of the quenched section 2, and these pieces were numbered as follows: the three test pieces 1 with a hardened layer depth of 0mm were numbered 1-1, 1-2, and 1-3; the three test pieces 1 with a hardened layer depth of 5mm were numbered 2-1, 2-2, and 2-3; the three test pieces 1 with a hardened layer depth of 10mm were numbered 3-1, 3-2, and 3-3; and the three test pieces 1 with a hardened layer depth of 20mm were numbered 4-1, 4-2, and 4-3.

[0032] The hardened layer depth is also the height of the quenching section 2. Different quenching depth requirements are achieved by controlling the quenching frequency and time. Cutting open test piece 1 and performing corrosion and hardness measurement confirms the hardened layer depth. A hardened layer depth of 0mm indicates that the material is in a tempered state and has not undergone surface quenching. Using test piece 1 with a hardened layer depth of 0mm as the baseline test piece 1, and comparing it with other test pieces 1, verifies that adding surface quenching to the workpiece can improve its impact fatigue resistance.

[0033] The parameters were set on the test apparatus, and the impact test was conducted according to the following parameters: impact force F = 2750 N, impact stroke = 20 mm, impact frequency f = 30 Hz, and impact time = 3600 s. Under the same test conditions, the test was carried out according to the characterization method of the present invention, and the corresponding parameter changes were measured and recorded. The average value of each parameter was calculated to obtain the following results: Figure 5 The experimental data statistics table is shown, and the deformation images of test piece 1 are obtained, as shown. Figure 6 As shown.

[0034] according to Figure 5 The statistical table shown, combined with Figure 6 It can be seen that: 1. After the impact fatigue test, the cross-sectional areas of both the quenched section 2 and the non-quenched section 3 of test piece 1 increased, while the total height of test piece 1 showed a decreasing trend, and the deformation was mainly concentrated at the upper and lower ends of test piece 1. 2. With the increase of the hardened layer depth of the quenched section 2, the deformation of the cross-sectional area at the upper and lower ends of test piece 1 and the deformation of the total height of test piece 1 both decreased.

[0035] It can be intuitively judged that the impact fatigue resistance of test piece 1 is improved with the increase of the hardened layer depth. It can characterize the impact fatigue resistance of the workpiece to a certain extent, and at the same time provide a basis for the design of the hardened layer depth of the workpiece surface to a certain extent.

[0036] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Therefore, all equivalent changes or modifications made to the structure, features and principles described in the claims of the present invention should be included within the scope of the present invention.

Claims

1. A method for characterizing impact fatigue resistance, characterized in that, Includes the following steps: Step 1: Prepare impact fatigue test specimen (1): The test specimen (1) includes an integrally formed quenched section (2) and a non-quenched section (3). The cross-section of the quenched section (2) is required to be smaller than the cross-section of the non-quenched section (3). Step 2: Measure and record the original dimensions of the test specimen (1); Step 3: Conduct impact test: clamp the prepared test piece (1) on the test device and conduct a certain number of impact tests on the test device. The impact force is an alternating load of pressure-no force-pressure-no force, and a certain impact frequency is set. The test piece (1) is vertically clamped on the test device. The test device applies alternating impact force to the quenching section (2). The force direction is vertical and perpendicular to the upper end face of the quenching section (2). The number of impact tests set on the test device is N, and the impact frequency is f. Step 4: Measure and record the deformation dimensions of the test piece (1) after the test; Step 5: Calculate the deformation of test piece (1): Based on the data recorded in Step 2 and Step 4, calculate the dimensional change of test piece (1) before and after the test. Under the same test conditions, the larger the dimensional change, the worse the impact fatigue resistance of test piece (1) is, and vice versa.

2. The method for characterizing impact fatigue resistance according to claim 1, characterized in that, In step 1, the test piece (1) is a metal cylinder, and the cross section of the quenched section (2) and the non-quenched section (3) is "convex" shaped after being connected and combined. The quenched section (2) is formed by surface quenching, and the surface of the quenched section (2) is covered with a hardened layer.

3. The method for characterizing impact fatigue resistance according to claim 1, characterized in that, In step 2, the original dimensions of the test piece (1) are measured and recorded, including the height H1 of the test piece (1), the cross-sectional area AQ1 of the quenched section (2), and the cross-sectional area AT1 of the non-quenched section (3).

4. The method for characterizing impact fatigue resistance according to claim 3, characterized in that, In step 4, the measured and recorded deformation dimensions of the test piece (1) include the height H2 of the test piece (1), the cross-sectional area AQ2 of the quenched section (2), and the cross-sectional area AT2 of the non-quenched section (3).

5. The method for characterizing impact fatigue resistance according to claim 4, characterized in that, In step 5, the dimensional changes of the test piece (1) before and after the test are ΔH=H1-H2, ΔAQ=AQ2-AQ1 and ΔAT=AT2-AT1. The larger the values ​​of ΔH, ΔAQ and ΔAT, the greater the deformation of the test piece (1) and the worse its impact fatigue resistance. The smaller the values ​​of ΔH, ΔAQ and ΔAT, the smaller the deformation of the test piece (1) and the better its impact fatigue resistance.

Images