JP2000275164A - Stress corrosion crack test method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To evaluate the limitation value of factors for causing a stress corrosion crack by executing a low-distortion speed tensile testing in an atmosphere where the inside of an actual machine usage environment is simulated and correlating the stress level to the crack length and depth. SOLUTION: A test piece-gripping part 20 fixes a test piece 1 to a testing device, an actuator 11 applies a low distortion tensile load, and a sensor 14 measures the distortion of the test piece 1. Then, a stress corrosion crack is generated by an evaluation test part 21, and a stress level is correlated to crack length and depth to evaluate the limitation value of factors for causing the stress corrosion crack. In this case, an environment sensor 13 measures the environment (temperature, water quality, or the like) of the evaluation test part 21 formed by a loop 12. Further, the evaluation test part 21 machines its sectional shape so that it has a different sectional area for the direction of an applied load to achieve a stress gradient. In this manner, by using the test piece 1 with the stress gradient, the progress of cracks with different stress levels can be evaluated simultaneously.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a stress corrosion cracking test method for easily evaluating stress corrosion cracking characteristics of a metal material.

[0002]

2. Description of the Related Art As a stress corrosion cracking evaluation test, although not specified in JIS, a U-bending test in G30-94 of ASTM, a C-ring test in G38-73,
G39-90 makes recommendations for bending beam tests, G49-85 makes tensile tests, and NACE makes recommendations for SCC test methods. In addition, as a method for evaluating the sensitization (susceptibility to stress corrosion cracking) of a material instead of directly evaluating crack growth due to stress corrosion cracking, Japanese Patent Application Laid-Open No.
10-26598 and JP-A-7-248305, a method for evaluating sensitization from changes in corrosion potential and corrosion current, and JP-A-8-193942.
A method for evaluating the sensitization of a material by comparing the hardness of the grain boundary with the hardness of the grain within the grain, as disclosed in JP-A-7-113801. JP-A-8-86740 discloses a method for evaluating the degree of sensitization of a material by calculating the presence or absence of dissolution based on the magnitude of the binding energy of surface atoms. As another stress corrosion cracking evaluation method, a method is used in which a cut is made in an actual machine in Japanese Patent Application Laid-Open No. 9-189790, a tensile test is performed using a ligament portion, and the stress corrosion cracking susceptibility is evaluated from the fracture surface. Two
There is a method of predicting and evaluating stress corrosion cracking susceptibility and crack growth from the distribution of pitting corrosion formed on the surface in No. 01270.

[0003]

The main causes of stress corrosion cracking are environmental, residual stress and aging of the material. One of the purposes of the stress corrosion test is to know the limit of this factor that causes stress corrosion cracking. In the conventional method standardized by ASTM and NACE, when determining the occurrence limit of stress corrosion cracking, parameters are required for each of the factors, and the crack growth due to stress corrosion cracking under each condition is evaluated. There were problems such as an increase in the number of test materials and a time required for evaluation. JP-A-10-26598
JP-A-7-248305, JP-A 8-193942, JP-A-7-248305
In the methods of JP-113801 and JP-A-8-86740, it is possible to evaluate the sensitization of the material, but the conventional database must be used for the evaluation of crack growth due to stress corrosion cracking, and the same When a database of conditions does not exist, measures such as using a similar database have been taken. In the case of JP-A-9-189790, there is no problem with respect to the corrosive environment and the deterioration state of the material. However, when used under conditions where the stress level changes, there is a shortage of evaluation test pieces. Evaluation is difficult. JP-A-8-201270 can evaluate the sensitization of the material, but cannot evaluate the crack growth due to stress corrosion cracking.

[0004]

Among the above-mentioned problems, the problem of evaluating not only the degree of sensitization but also the amount of crack propagation due to stress corrosion cracking is a low strain rate tensile test in an atmosphere simulating the actual use environment. And by correlating the stress level with the crack length and depth. Also,
Of the above problems, regarding the crack growth curve by the stress corrosion cracking test, by using a test piece with a stress gradient in the stress corrosion cracking evaluation test part, the crack growth evaluation with different stress levels simultaneously in one test piece Becomes possible.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an example of a stress corrosion cracking test apparatus using a hollow cylindrical test piece and an example of the shape of the test piece.

The basic structure of the stress corrosion cracking test apparatus is a load cell 10 for measuring a load and an actuator 1 for applying a load.
1, a loop 12 for forming a stress corrosion cracking environment, an environment sensor 13 for measuring the environment of a stress corrosion cracking evaluation unit, for example, temperature, corrosion potential, water quality, etc., a sensor 1 for strain measurement
4. In addition, for example, in the case of a stress corrosion cracking test at a high temperature, the heat insulating material 1 for cutting off the heat from the hollow cylindrical test piece 1 with the load cell 10 and the actuator 11
5, a cooling water 16, and a split flange 17 for fixing the hollow cylindrical test piece 1 in the case of a test under high pressure are added to the basic components.

The hollow cylindrical test piece 1 has at least a test piece gripping part 20 fixed to a stress corrosion cracking test apparatus and an evaluation test part 2 which is a part for generating and evaluating stress corrosion cracking.
1 The test piece gripping part 20 is processed according to the shape of the stress corrosion cracking tester. For example, as shown in FIG. 1, when fixed by the split flange 17, the test piece gripping part 20 is attached with a concentric hook larger than the evaluation test part 21. As another example, it is also conceivable to machine a screw on the outer periphery of the test piece gripping section 20. Evaluation test section 2
1 is processed so as to have a stress gradient. For example, as shown in FIG. 1, the cross-sectional shape of the evaluation test section 21 is processed so that the cross-sectional area is different from the load application direction. FIG. 1 also shows an example of a stress gradient in the load application direction at that time. The stress distribution is such that the center of the test piece has a maximum stress value of 22. To consider the effects of a wide range of stress levels,
Gradient 2 from maximum stress value 22 to minimum stress value 24
It is necessary to process No. 3 as small as possible. The same can be said for other test piece shapes.

FIG. 2 shows an example of a stress corrosion cracking test apparatus using an hourglass type or flat type test piece and an example of the shape of the test piece.

The basic structure of the stress corrosion cracking test apparatus includes a load cell 10 for measuring a load, an environment container 30 simulating the stress corrosion cracking environment, and an environment sensor 13 for monitoring the stress corrosion cracking environment. In addition, for example, when performing a stress corrosion cracking test in a water atmosphere containing chlorine under controlled temperature, the heat retention heater 31 for controlling the temperature of the environmental container 30 and the environmental container 30 are protected from a corrosive environment. Environmental protection film 32 is added.

The hourglass type test piece 2 and the flat type test piece 3
As in the case of the hollow test piece 1, the test piece comprises at least a test piece gripping part 20 fixed to the stress corrosion cracking test apparatus and an evaluation test part 21 which is a part for generating and evaluating stress corrosion cracking. In the case of the hourglass type test piece 3, one of the simple test methods is to process a screw on the outer periphery of the test piece gripping portion 20 and fix the screw to a test device. The evaluation test part 21 is a hollow test piece 1
Similarly, processing is performed so as to have a stress gradient. For example, as shown in FIG. 2, the cross-sectional shape of the evaluation test section 21 is processed so that the cross-sectional area is different from the load application direction.

FIG. 3 shows an example of the results of a stress corrosion cracking test of stainless steel in high-temperature and high-pressure water as an example of the stress corrosion cracking test method and a method of summarizing the results. (A) is an example of cutting out a test piece, (b) is a fracture surface, (c) is a crack distribution,
(D) is a crack length distribution. A test using a hollow cylindrical test piece will be described as a representative example. The external shape is an hourglass shape so that the stress corrosion cracking evaluation part is located at the center of the hollow cylindrical test piece 1, and the inner surface is mirror-finished with a constant inner diameter. The tensile test is performed while applying a constant strain rate in the target environment. After the test, the test piece evaluation unit 2 is
As shown in (a), first, only the test piece evaluation part 2 is cut out, and then the cut out test piece evaluation part 2 is cut into four pieces in the vertical direction so that the inner surface can be observed. After cutting out the test piece evaluation part 2, the surface of the evaluation test part 2 is observed using an SEM or an optical microscope. Whether the crack propagation is due to stress corrosion cracking or not is determined by observing the morphology inside the crack using a scanning electron microscope or optical microscope and determining whether the fracture surface is a form peculiar to stress corrosion cracking. I do. For example, when a sensitized austenitic stainless steel undergoes stress corrosion cracking in pure water, the form of the fracture surface due to the stress corrosion cracking becomes a block shape 40. on the other hand,
If a crack occurs due to tension in the same material and the same environment, the fracture surface becomes a ductile tear fracture surface 41. After determining whether all cracks are cracks 40 due to stress corrosion cracking or cracks 41 due to tension, cracks 4 due to stress corrosion cracking are determined.
For 0, the distance from a specific position (in FIG. 3, the reference position is the center position of the test piece evaluation unit), the size and frequency of the crack are examined.

FIG. 4 shows a method for evaluating the stress corrosion crack growth characteristics from the experimental results. The lower limit stress value at which stress corrosion cracking occurs is evaluated from the crack frequency distribution. The position of the crack farthest from the standard point observed by the scanning electron microscope is determined. As shown in FIG. 3C, in most cases, the relationship between the crack distribution and the distance from the standard point is an exponential distribution rule in a region where the distance from the standard point is long. Therefore, it is also possible to evaluate the distance from the standard point where the minimum crack length exists from the approximate expression. Based on the relationship between the distance from the standard point and the stress value shown in FIG. 4 (a), the stress value at the position where the crack is found was evaluated. Is the lower limit stress value.

The crack growth rate due to stress corrosion cracking is determined from the crack length. The onset of crack initiation and propagation can be tentatively determined by the test start time, or the time when the stress value at each crack location is one half of the yield stress,
Alternatively, it is conceivable that the stress value at each crack location exceeds the yield stress, but if you want to accurately evaluate the crack initiation time, attach an electrode to the specimen surface and monitor the voltage value under a constant current. It becomes possible by doing. Assuming that the crack initiation time is the test start time, the crack length is divided by the test time to determine the amount of crack propagation per unit time. The amount of crack propagation in the thickness direction is determined by multiplying the crack length by the aspect ratio. Here, it is desirable to construct a database for the aspect ratio in advance, but if it is unclear, generally 0.5 is used. As for the stress intensity factor K indicating the relationship between the crack and the stress, Equation (1) is established between the depth a of the crack in the thickness direction and the stress value σ at that position.

[0014]

K = Fσ√ (πa) (1) where F is a coefficient due to the shape. Finally Figure 4
The relationship between the crack growth rate and the stress intensity factor shown in FIG.

Further, the degree of sensitization of the material subjected to the test can be evaluated by comparing the crack growth rate under the same test conditions as that of the unsensitized material.

[0016]

The main causes of stress corrosion cracking are environmental, residual stress and aging of the material. One of the purposes of the stress corrosion test is to know the limit of this factor that causes stress corrosion cracking. In the conventional method standardized by ASTM and NACE, when determining the occurrence limit of stress corrosion cracking, parameters are required for each of the factors, and the crack growth due to stress corrosion cracking under each condition is evaluated. There were problems such as an increase in the number of test materials and a time required for evaluation. However, the limit value of each factor that causes stress corrosion cracking is evaluated by conducting a low strain rate tensile test in an atmosphere simulating the environment in which the actual machine is used, and correlating the stress level with the crack length and depth. It becomes possible to do. In addition, regarding the crack growth curve by the stress corrosion cracking test, the structure used for the stress corrosion cracking test has a stress gradient so that it is possible to evaluate crack growth at different stress levels simultaneously on a single test piece. become.

[Brief description of the drawings]

FIG. 1 is a view showing a stress corrosion cracking test apparatus and a test piece (hollow cylindrical type).

FIG. 2 shows a stress corrosion cracking test apparatus and test pieces (hourglass type,
FIG.

FIG. 3 is a view showing the results of a stress corrosion cracking test.

FIG. 4 is a diagram illustrating a method for evaluating a stress corrosion cracking test result.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... hollow cylindrical test piece, 2 ... hourglass type test piece, 3 ... flat type test piece, 10 ... load cell, 11 ... actuator, 12
... Loop, 13 ... Environment sensor, 14 ... Strain measurement sensor, 15 ... Insulation material, 16 ... Split flange, 20 ...
Test piece gripping part, 21 ... Evaluation test part, 22 ... Test piece maximum stress value, 23 ... Test piece stress gradient, 24 ... Test piece minimum stress value, 25 ... Stress distribution curve, 30 ... Environment container, 31 ... Heat keeping heater , 32: Environmental container protective film, 40: Stress corrosion cracking fracture surface, 41: Ductile tear fracture surface, 42: Crack frequency curve, 43
... crack length distribution curve, 50 ... crack maximum distance position, 51 ...
Lower limit stress value, 52: lower limit stress intensity factor, 53: sensitized material stress corrosion cracking crack growth curve, 54: non-sensitized material stress corrosion cracking crack growth curve, 55: sensitization degree.

Claims (5)

[Claims] In a stress corrosion cracking test method for evaluating the susceptibility of a metal material to stress corrosion cracking, a test section for evaluating stress corrosion cracking is provided on a part of a tensile test piece in an atmosphere simulating the use environment of an actual machine. The test section is provided with a stress gradient, the tensile test piece is subjected to a tensile test at a constant strain rate, and stress corrosion cracking is determined from the distribution of the form, size, and depth of the test section to evaluate stress corrosion cracking. A stress corrosion cracking test method characterized by evaluating the critical stress intensity factor, crack growth rate, and material sensitization that occur. 2. The stress corrosion cracking test method according to claim 1, wherein the shape of the test piece is a hollow cylindrical shape, and the inside of the hollow cylinder is simulated in an actual machine, and at the same time, the inner diameter of the evaluation test portion of the test piece is made constant. The test piece, which has a stress gradient by changing the outer diameter in the direction in which the load is applied, is left in a real machine simulated environment, or the shape of the test piece is an hourglass type, and the stress gradient is provided in the load application direction to simulate the real machine Either leave it in the environment, or leave it in the simulated environment of a real machine by making the shape of the test piece flat and changing the cross-sectional area of the test piece in the load application direction to give a stress gradient. A stress corrosion cracking test method characterized by performing a low strain rate test using a piece. 3. The stress corrosion cracking test method according to claim 1, wherein a critical stress at which stress corrosion cracking occurs in a target environment is evaluated from a crack initiation point of a stress corrosion cracking test piece evaluation part. Stress corrosion cracking test method. 4. The stress corrosion cracking test method according to claim 1, wherein the stress corrosion crack growth rate is evaluated based on the crack length and the shape of the crack in the stress corrosion crack test piece evaluation part. Test method. 5. The stress corrosion cracking test method according to claim 1, wherein when the stress corrosion cracking susceptibility evaluation is performed in the stress corrosion cracking test piece evaluation section after the test, the index for evaluating the susceptibility is the crack occurrence frequency, A stress corrosion cracking test method characterized by a crack length.