Testing and evaluation methods for stress corrosion cracking resistance of metallic material

Abstract

To provide a testing and evaluation method for stress corrosion cracking resistance of metallic materials, which is excellent in evaluation reproducibility and evaluation accuracy.SOLUTION: There is provided a method for testing and evaluating stress corrosion cracking resistance of metallic materials, in which a tensile stress is applied to a test piece so as to produce a predetermined initial strain, and the change in the tensile stress is evaluated in a corrosive gas atmosphere.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for testing and evaluating the stress corrosion cracking properties of metallic materials. [Background technology]

[0002] One of the characteristics of metallic materials is stress corrosion cracking, which occurs due to changes over time when stress is applied or in a corrosive environment due to residual stress. Various methods have been proposed so far as accelerated testing of the stress corrosion cracking resistance. For example, in the field of brass materials, one of a test piece with a female thread and a plug with a male thread is tapered, screwed in with a constant torque, and placed in a corrosive environment such as an ammonia atmosphere under a stress load to evaluate the occurrence of cracks. However, although such an evaluation is excellent as a relative evaluation, it cannot necessarily be said to be an evaluation with reproducibility.

[0003] Patent Document 1 describes a method for evaluating the stress level and the length and depth of cracks that occur in materials using slow strain rate tensile tests. However, since the test specimen has a stress gradient, there are concerns about the stability of the evaluation. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2000-275164 A Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a method for testing and evaluating the stress corrosion cracking resistance of a metallic material, which is excellent in reproducibility and accuracy of the evaluation. [Means for solving the problem]

[0006] The method for testing and evaluating the stress corrosion cracking resistance of the present invention is a method for testing and evaluating the stress corrosion cracking resistance of a metallic material, and is characterized in that a tensile stress is applied to a test piece so as to result in a predetermined initial strain amount, and a change in the tensile stress is evaluated in a corrosive gas atmosphere.

[0007] In the present invention, a slow strain rate tensile tester (SSRT: Slow Strain Rate Technique) may be used, in which a predetermined initial strain is applied to the metal material in advance, and then the stress change due to cracking in a corrosive environment is tested and evaluated. A general tensile testing machine may be used, but in order to apply a stable initial strain, the strain rate should be set to 5.0×10 -4 / s~1.0×10 -5 It is preferable to set it in the range of / s.

[0008] In the present invention, the predetermined initial strain amount may be any one of an elastic region, an inflection point beyond the elastic region, and a region beyond the inflection point. The stress-strain curve in a slow strain rate tensile test of a metallic material shows a linear, proportional displacement in the initial elastic region, followed by an inflection point and then a yield point. For some metals, this yield point does not appear clearly.

[0009] In the present invention, attention has been paid to the fact that when the initial strain amount is set in the elastic region, when it is set near the inflection point, or when it is set in a region beyond the inflection point, the stress subsequently decreases in a corrosive atmosphere, and there is a difference in the progression to fracture.

[0010] Therefore, in the present invention, the method for evaluating the change in tensile stress may be the time to fracture of the test specimen, or the stress transition rate of a first gradient that appears early on and / or the stress transition rate of a second gradient that appears after the first gradient. Effect of the Invention

[0011] In the method for testing and evaluating the stress corrosion cracking resistance of a metal material according to the present invention, a tensile stress is applied to a test piece at a low strain rate, and the subsequent progression of stress change in a corrosive environment is evaluated. Therefore, evaluation can be performed using a progression curve of stress change, fracture time, etc., and the evaluation is highly accurate and has excellent reproducibility. [Brief description of the drawings]

[0012] [Figure 1] An overview of the test equipment is shown. [Diagram 2] Examples of test specimen shapes and an overview of test preparation are shown below. [Diagram 3] An example of setting the initial strain amount is shown below. [Figure 4] The transition of stress is shown. [Diagram 5] Shown is the initial first gradient followed by the second gradient. [Figure 6] The rate of stress transition at the initial gradient is shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The test and evaluation method according to the present invention will be described below with reference to the drawings. Although the present embodiment will be described using brass as the material, the metal material is not limited to this.

[0014] Figure 1 shows an overview of the test method using the SSRT, and Figure 2 shows the size of the test piece and an example of the advance preparation. A test piece is placed inside a test vessel 11, and the upper and lower parts of the test piece are held by upper and lower jigs 12, 13, respectively, and a tensile stress is applied to the test piece at a low strain rate. In this example, the strain rate is set to 2.5×10 -5 Set it to / s. When a predetermined initial strain is reached, tension is stopped at that point. Next, ammonia water with a concentration of 7.4 mol / l was injected into the test vessel 11, creating an ammonia gas phase atmosphere as the corrosive environment. The temperature of the test vessel was set to 298K. The corrosive environment and conditions are selected to suit the metal material being evaluated. In this example, a test piece was cut to the size shown in FIG. 2, and silicone rubber was applied to the area other than the area to be evaluated.

[0015] First, metal material A was tested and evaluated. Metal material A is an example of a brass material, and its components in mass% are as follows: The components are Cu: 59.3%, Pb: 3.2%, Fe: 0.2%, Sn: 0.29%, Bi: 0.011%, Mn: 0.001%, and the remainder is Zn. Figure 3 shows the stress-strain curves obtained by performing a slow strain rate tensile test in air using n=2 test pieces. In the initial elastic region, it shows a linear change, then passes through inflection point b, where it reaches the yield point and breaks. Therefore, the initial strain amount in this test was selected as follows: First, as condition b, the strain was set to 0.040 and the stress to 240 MPa in accordance with the inflection point. Condition a was above the inflection point, with a strain of 0.075 and a stress of 296 MPa. In addition, condition c was set to a stress of 184 MPa and a strain of 0.030 so as to be symmetrical with condition a and centered on condition b.

[0016] When the initial strain reached a, b, or c, respectively, the tension was stopped, and 82.0 ml of 7.4 mol / l ammonia water was injected as shown in Figure 1 until the liquid level reached 10.0 mm. The stress transition curve at that time is shown in Figure 4. Comparing the transition curves under conditions a, b, and c, it is clear that the larger the initial strain, the shorter the time to fracture. This means that the inflection point of the metal material to be evaluated is measured in advance, and the stress corrosion cracking resistance can be evaluated using the time to fracture of a test piece in a corrosive environment at the initial strain amount corresponding to this inflection point. At this time, a more detailed evaluation can be performed by selecting the initial strain amount in the elastic region or in the region beyond the inflection point. The smaller the initial strain, the more gentle the gradient of the stress decrease. Furthermore, by focusing on the change in the stress transition curve, it was found that it can be decomposed into a linear regression equation consisting of an initial first gradient 1 and a subsequent second gradient 2, as shown in Figure 5.

[0017] For this reason, we focused on the first gradient 1 and compared the differences in metal materials. The metal material A is as described above. Metal material B has the following composition. The components are Cu: 58.7%, Pb: 2.3%, Fe: 0.13%, Sn: 0.32%, Bi: 0.001%, Mn: 0.002%, and the remainder is Zn. Metal material C has the following composition. The components are Cu: 59.3%, Pb: 0.003%, Fe: 0.1%, Sn: 0.23%, Bi: 2.13%, and the remainder is Zn. The first gradient of the stress transition rate R that occurs at the beginning of the stress drop dec The calculation results (regression equation) are shown in Figure 6. It can be seen that the stress transition speed differs depending on the metal material. Metallic materials A and B are brass materials containing Pb, and metallic material C is a brass material containing Bi. In the graph of Figure 6, the stress transition rate R de c, and to the right of it are shown the average velocities at each initial strain a, b, and c. As described above, from the graphs of FIG. 4 to FIG. 6, it is clear that the stress corrosion cracking resistance can be evaluated with higher accuracy by using the inclination of the first gradient or the second gradient, although the time to fracture of the test piece may be used.

[0018] In addition, when comparing the microstructure cross sections of the specimen where the test was stopped at the first gradient stage shown in Figure 5 with the specimen where the test was stopped at the second gradient stage, a difference was found in the direction of crack propagation. In the first gradient condition, the cracks propagated in a roughly horizontal direction perpendicular to the tensile direction, whereas in the second gradient condition, horizontal cracks and vertically diagonal cracks were mixed.

[0019] From the above, the following can be said according to the testing and evaluation method of the present invention. The stress corrosion cracking resistance can be evaluated based on the time it takes for the test piece to break. The stress corrosion cracking resistance can be evaluated from the value of the stress transition rate (regression equation) obtained from the transition in which the stress in the test piece decreases.

Claims

1. A method for testing and evaluating the stress corrosion cracking resistance of a metallic material, comprising: A tensile stress is applied to the test piece so that a specified initial strain is achieved. A method for testing and evaluating stress corrosion cracking resistance, comprising evaluating the change in tensile stress in a corrosive gas atmosphere.

2. 2. The method for testing and evaluating stress corrosion cracking resistance according to claim 1, wherein the predetermined initial strain amount is any one of an elastic region, an inflection point beyond the elastic region, and a region beyond the inflection point.

3. 3. The method for testing and evaluating stress corrosion cracking resistance according to claim 2, wherein the method for evaluating the change in tensile stress is the time to fracture of the test piece.

4. 3. The method for testing and evaluating stress corrosion cracking resistance according to claim 2, characterized in that the method for evaluating the change in tensile stress is the stress transition rate of a first gradient that appears early and / or the stress transition rate of a second gradient that appears after the first gradient.

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