Evaluation method for delayed fracture properties

Abstract

To improve evaluation accuracy of delayed fracture properties.SOLUTION: A delayed fracture property evaluation method is provided, comprising conducting a corrosion test on a test piece loaded with a stress, computing fracture mechanics parameters based on sizes of corrosion holes appearing on the test piece during the corrosion test, and evaluating delayed fracture properties based on a result of comparison between the fracture mechanics parameters and thresholds.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating delayed fracture properties. [Background technology]

[0002] One approach to evaluating delayed fracture is proposed as Prior Art 1, which uses the amount of hydrogen that penetrates into metallic materials. In Prior Art 1, electrolysis is performed using a specimen of the material to be tested as the cathode, and hydrogen is charged until the amount of hydrogen is saturated for a charging time determined in a preliminary test. After that, a delayed fracture test is performed and the critical diffusible hydrogen amount is measured, thereby determining the relationship between the amount of hydrogen in the test specimen and the fracture stress.

[0003] Another approach proposed is Prior Art 2, which uses accelerated corrosion testing that simulates an actual use environment. In Prior Art 2, in order to simulate an actual atmospheric corrosion environment, a process including steps (A) and (B) is performed at least once. Step (A) is a process in which a component mainly composed of chlorides is adhered to a metal material. Step (B) is a process in which the metal material that has undergone step (A) is subjected to a drying step (b1), in which the surface of the metal material is dried by changing the relative humidity, and a wetting step (b2), in which the surface is wetted, and this cycle is performed at least once. The relationship between the number of test cycles and delayed fracture properties can be determined by steps (A) and (B). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-122633 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-169918 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-180658 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in both of the above-mentioned prior art 1 and prior art 2, the influence of corrosion pitting on delayed fracture properties is ignored, which reduces the accuracy of evaluation of delayed fracture properties.

[0006] In the above-mentioned prior art 1, an accelerated test is performed in which hydrogen is charged by cathodic electrolysis, so the test is carried out under conditions that differ from the actual environment in which the product is used. For example, the difference between conditions in a hydrogen gas environment and in air manifests itself as a gap in chemical phenomena, such as differences in hydrogen penetration characteristics, the presence or absence of corrosion pitting, the size of pitting, or the pitting pattern, which can be one of the reasons for the decrease in the accuracy of evaluation of delayed fracture strength.

[0007] Furthermore, the above-mentioned prior art 2 merely evaluates delayed fracture properties from the perspective of corrosion, a chemical phenomenon. In other words, corrosion pits formed by corrosion can be considered a type of crack, and it is difficult to evaluate the effect of corrosion pits on delayed fracture properties from the perspective of chemical phenomena alone. Such an evaluation that fails to consider the effect of corrosion pits on delayed fracture properties can be one of the reasons for the decrease in the accuracy of the evaluation of delayed fracture strength.

[0008] In one aspect, the present invention aims to provide a method for evaluating delayed fracture properties that can improve the accuracy of evaluation of delayed fracture properties. [Means for solving the problem]

[0009] A method for evaluating delayed fracture properties according to one embodiment includes the steps of: conducting a corrosion test on a stressed test specimen; calculating a fracture mechanics parameter based on the size of corrosion pits that occur in the test specimen during the corrosion test; and evaluating the delayed fracture properties based on a comparison result between the fracture mechanics parameter and a threshold value. [Effects of the Invention]

[0010] This can improve the accuracy of evaluation of delayed fracture properties. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a flowchart showing the procedure of the method for evaluating delayed fracture properties. [Figure 2] FIG. 2 is a diagram (1) showing an example of a test piece. [Figure 3] FIG. 3 is a diagram (2) showing an example of a test piece. [Figure 4] FIG. 4 is a schematic diagram showing an example of a stress application tool. [Figure 5] FIG. 5 is a diagram showing an example of a corrosion pit. [Figure 6] FIG. 6 is a diagram showing an example of corrosion pit size and stress intensity factor. [Figure 7] FIG. 7 is a schematic diagram (1) showing an example of determining whether the test stress is appropriate. [Figure 8] FIG. 8 is a schematic diagram (2) showing an example of determining whether the test stress is appropriate. [Figure 9] FIG. 9 is a diagram showing an example of the relationship between the threshold value and the amount of hydrogen. [Figure 10] FIG. 10 is a flowchart showing the procedure for evaluating delayed fracture properties. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the method for evaluating delayed fracture properties according to the present application will be described with reference to the accompanying drawings. Each embodiment merely shows one example or aspect, and the range of values, functions, and application scenarios are not limited by such examples. Furthermore, each embodiment can be appropriately combined within the scope of not causing any contradiction in the processing content.

[0013] <One aspect of the problem-solving approach> The method for evaluating delayed fracture properties according to this embodiment is superior to the above-mentioned prior art 1 and prior art 2 in that it adopts an approach that evaluates the effect of corrosion pitting on delayed fracture properties from the perspective of fracture mechanics.

[0014] As an example, in the method for evaluating delayed fracture properties according to this embodiment, a corrosion test is performed on a test specimen to which stress has been applied. Here, corrosion pits that appear in the test specimen during the corrosion test are considered to be cracks of the same size as the corrosion pits, and fracture mechanics parameters, such as the stress intensity factor, J integral, and crack tip opening displacement, are calculated based on the corrosion pit size. Using these fracture mechanics parameters, the stress that will not cause delayed fracture, i.e., the delayed fracture strength and delayed fracture resistance, are determined. This makes it possible to quantify the effect of the chemical phenomenon of corrosion on delayed fracture properties, as well as the effect of corrosion pits on delayed fracture properties, such as the effect of stress concentration.

[0015] Therefore, according to the method for evaluating delayed fracture properties according to this embodiment, it is possible to improve the accuracy of evaluation of delayed fracture properties.

[0016] <Evaluation method for delayed fracture properties> Next, an example of the method for evaluating delayed fracture properties according to this embodiment will be described below. Fig. 1 is a flowchart showing the procedure of the method for evaluating delayed fracture properties.

[0017] (1) Test piece preparation process 1, in step S1, a test piece is prepared from a metallic material to be subjected to a delayed fracture test. At this time, the test piece may be formed into a shape that can be attached to a stress application tool used in step S2, which will be described later.

[0018] Figures 2 and 3 are diagrams (1) and (2) showing examples of test pieces. As shown in Figure 2, a rectangular metal plate can be prepared as test piece 1A, and as shown in Figure 3, a round metal bar can be prepared as test piece 1B. Note that Figures 2 and 3 only show one example of the shape of the test piece, and it goes without saying that test pieces of the same or similar shape as the shape of an actual product using a metal material can be prepared.

[0019] (2) Stress application process In step S2, stress is applied to the test piece prepared in step S1. Hereinafter, the stress applied to the test piece to be used in the delayed fracture test may be referred to as "test stress." For this type of stress application, a stress application tool as shown in FIG. 4 can be used, as an example only. For example, it is preferable that the stress application tool be one that reproduces the form in which stress is applied to the product corresponding to the test piece in an actual use environment, such as bending, tension, or torsion.

[0020] FIG. 4 is a schematic diagram showing an example of a stress application tool. As an example, FIG. 4 shows a vertical cross section of a stress application tool 2 used to perform a four-point bending test on the test piece 1A shown in FIG. 2. In the example shown in FIG. 4, when a vertically downward load P is applied to the bolt 2B, a vertically downward load 1 / 2P is applied from the support pin p1 via the bolt receiving block 2C, and a vertically downward load 1 / 2P is applied from the support pin p2. Due to the load 1 / 2P from the support pin p1 and the load 1 / 2P from the support pin p2, a vertically upward reaction force 1 / 2P is generated on the support pin p3, and a vertically upward reaction force 1 / 2P is generated on the support pin p4.

[0021] Such a stress application tool 2 can apply a uniform bending moment between the support pins p1 and p2. The support pins p1 to p4 that come into contact with the test piece 1A can be made of a non-metallic material, such as ceramics, from the side that does not come into electrical contact with the bolt 2B or the frame 2F.

[0022] (3) Corrosion testing process In step S3, a corrosion test is performed on the test piece to which stress has been applied in step S2 above in an environment simulating atmospheric corrosion. The corrosion test performed in step S3 may be, for example, a combined cycle test, also known as a CCT (Cyclic Corrosion Tester). The CCT includes chloride deposition, wetting, and drying steps, and the conditions for each step can be set arbitrarily depending on the purpose. For example, as a test cycle, a cycle can be selected in which steps S3.1 to S3.3 below are repeated under corrosion conditions defined in JASO M609, which corresponds to automotive standards. Note that this is merely an example, and techniques described in Patent Documents 2 and 3, for example, may also be used.

[0023] Step S3.1 Chlorine spray: 35±1°C x 2h (5w / v% NaCl aqueous solution) Step S3.2 Drying: 60±1℃×4h (20~30%RH) Step S3.3 Wet: 50±1℃×2h (95%RH or higher)

[0024] (4) Rust removal process In step S4, after the corrosion test in step S3 is completed, the test piece is removed from the stress loading tool and then de-rusted. For example, a commercially available de-rusting solution can be used for the de-rusting.

[0025] (5) Corrosion pit size measurement process In step S5, the size of corrosion pits is measured from the test piece that was derusted in step S4. Here, the width and depth of the corrosion pit are used as examples of indices for defining the corrosion pit size, but the indices are not limited to width and depth. For example, an indicator such as √Area, which represents the volumetric shape of the corrosion pit, or the area of ​​the corrosion pit may also be measured. To measure the corrosion pit size, an observation tool with a resolution sufficient to observe corrosion pits appearing on the surface of the test piece, such as an optical microscope or an electron microscope, can be used.

[0026] FIG. 5 is a diagram showing an example of a corrosion pit. FIG. 5 is a schematic top view of the test piece 1A observed in step S5 after steps S2 to S4, i.e., a view of the surface of the test piece 1A as viewed vertically from above. As shown in FIG. 5, six corrosion pits, C1 to C6, are observed on the surface of the test piece 1A. The corrosion pit size is measured for each of these six corrosion pits, C1 to C6. For example, the width of the corrosion pit is measured as the longest diameter of the corrosion pit. Furthermore, the depth of the corrosion pit is measured as the vertical distance from the surface of the test piece 1A (the Z direction in the figure). Measurement of the corrosion pit size is not limited to an operation using an observation tool, but may also be achieved by sensing using an image sensor or by image processing, as described below.

[0027] (6) Stress intensity factor calculation process In step S6, a stress intensity factor is calculated based on the corrosion pit size measured in step S5. Here, to calculate the stress intensity factor, a calculation formula suitable for the type of stress applied to the test piece in step S2, such as bending, tension, or torsion, can be used. As just one example, when the type of stress applied is bending, formula (1) described in Reference 1 below can be used. In formula (1) below, "σ" refers to stress, and "F" refers to a shape factor. K=σF(πa)^(1 / 2) (1)

[0028] Reference 1: Analysis of stress intensity factors for surface cracks subjected to arbitrarily distributed forces (3rd report, Analysis of influence coefficients for semi-elliptical cracks in round bars and its application)

[0029] The depth a measured in step S5 is directly included in the above formula (1). Furthermore, the shape factor F in the above formula (1) is defined as a function of the depth a and the width b, and therefore indirectly includes the depth a and the width b measured in step S5.

[0030] In this way, by regarding corrosion pits as cracks (initial fractures) the same size as the corrosion pits and applying the above formula (1), the stress intensity factor can be quantified using the corrosion pit size. FIG. 6 is a diagram showing an example of corrosion pit size and stress intensity factor. FIG. 6 shows the corrosion pit depths a1 to a6 and widths b1 to b6 as examples of corrosion pit size for each of the six corrosion pits C1 to C6 shown in FIG. 5. Furthermore, FIG. 6 shows the stress intensity factors K1 to K6 calculated from the corrosion pit depths a1 to a6 and widths b1 to b6 for each of the corrosion pits C1 to C6.

[0031] (7) Delayed fracture strength determination process In step S7, the delayed fracture strength is determined using the stress intensity factor calculated in step S6. As just one example, whether the stress intensity factor calculated in step S6 is equal to or less than a threshold value determines whether the stress applied to the test piece in step S2 is appropriate for the delayed fracture strength. For example, if the stress intensity factor is equal to or less than the threshold value, the test stress applied to the test piece in step S2 is determined to be the delayed fracture strength. On the other hand, if the stress intensity factor exceeds the threshold value, the test stress applied to the test piece in step S2 is determined to be inappropriate for the delayed fracture strength.

[0032] In one respect, the threshold value mentioned above refers to the crack propagation resistance value determined by the amount of hydrogen accumulated in the test piece, and can be determined, for example, by a method according to the actual use environment of the product to be designed. As an example, the threshold value mentioned above can be calculated according to formula (2) described in the following reference 2. The "K" in formula (2) below th " indicates the lower threshold stress intensity factor for delayed fracture crack propagation in the metallic material used to prepare the test specimen, and may correspond to an example of the threshold value mentioned above. Note that "HRC" in the following formula (2) indicates the Rockwell hardness value of the metallic material. K th =0.00155×HRC^4 - 0.29738×HRC^3 + 21.3419×HRC^2 - 680.224×HRC + 8143.62 (2)

[0033] Reference 2: Threshold stress intensity factor for delayed fracture crack propagation in spring steel and design method to prevent delayed fracture

[0034] FIG. 7 is a schematic diagram (1) showing an example of determining whether a test stress is appropriate. In FIG. 7, the calculation results of stress intensity factors K1 to K6 calculated from the corrosion pit size for each of corrosion pits C1 to C6 that occurred in test piece 1A loaded with a test stress of 700 MPa are shown in table format. Furthermore, for the sake of convenience of explanation, in FIG. 7, the calculation results of stress intensity factors K1 to K6 for corrosion pits C1 to C6 are charted as a bar graph to facilitate comparison of magnitude relationships. Note that in FIG. 7, the threshold value K1 to K6 to be compared with the stress intensity factors is shown. th An example is given where the value is set to 9.0.

[0035] As shown in Fig. 7, in the example of a test stress of 700 MPa, the stress intensity factors of corrosion pits C1, C4, C5, and C6 are above the threshold K th On the other hand, the stress intensity factor K2 (= 9.5 [MPa / √m]) of the corrosion pit C2 and the stress intensity factor K3 (= 10 [MPa / √m]) of the corrosion pit C3 are below the threshold K th Therefore, the test stress of 700 MPa is judged to be inappropriate for delayed fracture strength.

[0036] In this case, the test stress applied to the test piece in step S2 can be reduced, and the delayed fracture test can be performed again. As an example, let us assume that the test stress applied to the test piece 1A prepared in step S1 in step S2 is reduced by 100 MPa from 700 MPa to 600 MPa, and the processes of steps S1 to S7 are performed again. The stress intensity factors K of the corrosion pits C11 to C16 of the test piece 1A thus retested are as follows: 11 ~K 16 The calculation results are shown in Figure 8.

[0037] Fig. 8 is a schematic diagram (2) showing an example of determining whether the test stress is appropriate. Fig. 8 shows the stress intensity factor K calculated from the pit size for each of the pits C11 to C16 that occurred in the test piece 1A when a test stress of 600 MPa was applied. 11 ~K 16The calculation results are shown in a table format. Furthermore, in FIG. 8, for the sake of convenience of explanation and to facilitate comparison of magnitude relationships, the stress intensity factors K 11 ~K 16 The calculation results are charted as a bar graph. As in Figure 7, Figure 8 also shows the threshold K th An example is given where the value is set to 9.0.

[0038] As shown in Figure 8, in the case of a test stress of 600 MPa, the stress intensity factor K 11 ~K 16 is the threshold K th Therefore, the test stress of 600 MPa is determined as the delayed fracture strength.

[0039] In the examples shown in FIGS. 7 and 8, the threshold value K is set based on the Rockwell hardness. th can be set based on the amount of hydrogen that penetrates the test specimen. th 9 is a graph showing an example of the relationship between the threshold value K th Graph G1 is shown, with the horizontal axis representing the hydrogen content [ppm] and the horizontal axis representing the hydrogen content [MPa / √m]. As shown in FIG. 9, the threshold value K is set to a larger value as the hydrogen content decreases, while the threshold value K is set to a smaller value as the hydrogen content increases. th It can be set as:

[0040] In this way, the threshold K th In the case where the threshold value is adaptively selected according to the amount of hydrogen, the process of step S7 may include the following steps S7.1 to S7.3. That is, in step S7.1, the amount of hydrogen in the test piece is analyzed by thermal desorption analysis (TDA). Then, in step S7.2, a threshold value is calculated based on the amount of hydrogen obtained in the analysis of step S7.1 and the Rockwell hardness of the metallic material. thThen, in step S7.3, the stress intensity factor K of the corrosion pit calculated in step S6 above is set. 1~n is the threshold K set in step S7.2 above. th Depending on whether or not the test stress applied in step S2 is below this, it is determined whether or not the test stress is suitable as delayed fracture strength.

[0041] In addition, the threshold value K in step S7.2 above th The setting of can be realized by adding an explanatory variable of the hydrogen content to at least one of the first to fourth terms of the above formula (2) according to the relationship between the threshold and the hydrogen content shown in Figure 9. In addition, the threshold K calculated based on the Rockwell hardness in the above formula (2) can be th This can also be achieved by correcting the above in accordance with the relationship between the threshold and the amount of hydrogen shown in FIG.

[0042] <One aspect of the effect> As described above, the method for evaluating delayed fracture properties according to this embodiment regards corrosion pits that occur in test specimens during corrosion tests as cracks of the same size as the corrosion pits, and determines the delayed fracture strength using fracture mechanics parameters calculated based on the corrosion pit size. This makes it possible to quantify not only the effect of the chemical phenomenon of corrosion on delayed fracture properties, but also the effect of corrosion pits on delayed fracture properties, such as the effect of stress concentration. Therefore, the method for evaluating delayed fracture properties according to this embodiment can improve the accuracy of evaluation of delayed fracture properties.

[0043] Furthermore, the method for evaluating delayed fracture properties according to this embodiment can be implemented by changing the parameters other than the corrosion pit size that determine the delayed fracture strength, such as the test cycle conditions of the corrosion test and the threshold K th Therefore, the method for evaluating delayed fracture properties according to this embodiment makes it possible to design delayed fracture strength that is suited to the actual use environment.

[0044] <Application example> The above embodiment is merely an example, and various applications are possible.

[0045] For example, the method for evaluating delayed fracture properties according to the above embodiment may be realized as information processing in which each of steps S5 to S7 shown in FIG. 1 is executed by a computer.

[0046] Fig. 10 is a flowchart showing the procedure for the evaluation process of delayed fracture properties. As shown in Fig. 10, loop process 1 is executed in which the process of step S101 is repeated a number of times corresponding to the K test piece images specified in the evaluation request as evaluation targets for the delayed fracture test.

[0047] That is, a processor of a given computer measures the size of each corrosion pit contained in a test piece image read from storage or the like (step S101). Note that, although an example in which the processing of step S101 is repeatedly executed is shown in Fig. 10, the processing of step S101 may be executed in parallel for each of K test piece images. By such loop processing 1, measurement results of the corrosion pit size for each of the corrosion pits contained in the test piece image are obtained for each of the K test piece images.

[0048] Then, the processor calculates an estimate of the maximum corrosion pit size by performing extreme value statistics based on the measurement results of the corrosion pit size for each of the K test piece images in loop process 1 (step S102). Then, the processor calculates the stress intensity factor K by substituting the maximum corrosion pit size obtained by the extreme value statistics into the above formula (1). max is calculated (step S103).

[0049] Then, the processor calculates the stress intensity factor K max is the threshold K th At this time, it is determined whether the stress intensity factor K max is the threshold K thIf it is equal to or less than the test stress (Yes in step S104), the processor determines the test stress as the delayed fracture strength (step S105). In this case, the processor outputs the delayed fracture strength to an arbitrary output destination, such as an external device, software, or service (step S106), and ends the process.

[0050] On the other hand, the stress intensity factor K max is the threshold K th If it is not equal to or less than the above (No in step S104), it is determined that the test stress is inappropriate for the delayed fracture strength. In this case, the processor outputs an alert to an arbitrary output destination to the effect that the test stress is inappropriate for the delayed fracture strength (step S107), and ends the process.

[0051] As described above, the method for evaluating delayed fracture properties according to the above embodiment can be realized as information processing executed by a computer.

[0052] <Modification> The above embodiment is merely an example, and various modifications are possible.

[0053] For example, the delayed fracture strength can be determined by any design criteria. 1~n is the threshold K th Only when no corrosion pits exceeding K exist can the test stress be determined as the delayed fracture strength. i is the threshold K th The probability that the number of specimens with corrosion pits exceeding the threshold P th1 The test stress can be determined as the delayed fracture strength if it is less than or equal to the stress intensity factor K i is the threshold K th The probability that the number of corrosion pits exceeding K is P th2 The test stress can also be determined as the delayed fracture strength if it is:

[0054] Furthermore, the effects of each embodiment described in this specification are merely examples and are not intended to be limiting, and other effects may also be obtained.

Claims

1. A method of performing a corrosion test on a stressed test specimen; calculating a fracture mechanics parameter based on the size of corrosion pits formed in the test piece during the corrosion test; evaluating delayed fracture properties based on the comparison result between the fracture mechanics parameters and a threshold value; Including, The evaluation method for delayed fracture properties is characterized in that the evaluation step includes a step of determining whether the stress applied to the test piece is suitable as delayed fracture strength based on whether the fracture mechanics parameter is equal to or less than the threshold value.

2. 2. The method for evaluating delayed fracture properties according to claim 1, wherein the fracture mechanics parameter is a stress intensity factor when the corrosion pit is treated as a crack having the same size as the corrosion pit.

3. 3. The method for evaluating delayed fracture properties according to claim 1, wherein the threshold value is set based on the amount of hydrogen measured from the test piece after the corrosion test.

4. A step of conducting a corrosion test on the stressed test specimen; calculating a fracture mechanics parameter based on the size of corrosion pits formed in the test piece during the corrosion test; evaluating delayed fracture properties based on the comparison result between the fracture mechanics parameters and a threshold value; Including, The method for evaluating delayed fracture properties is characterized in that the corrosion test is carried out in an environment simulating atmospheric corrosion.

Images