JP2016045037A - Evaluation method of intergranular stress corrosion crack occurrence sensitivity, and intergranular stress corrosion crack occurrence sensitivity evaluation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an evaluation method of intergranular stress corrosion crack occurrence sensitivity capable of evaluating IGSCC occurrence sensitivity of an analyte easily in a short time, and to provide an intergranular stress corrosion crack occurrence sensitivity evaluation device.SOLUTION: Evaluation of εis performed from a distance between dendrite boundary instruction patterns before loading forced displacement of nominal strain ε, and a distance between the dendrite boundary instruction patterns during loading of the forced displacement, and εis determined as many as n region portions on each different spot. Then, εis subjected to statistical processing, and IGSCC occurrence sensitivity of an analyte is evaluated based on a statistical process value.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a method for evaluating the susceptibility to occurrence of intergranular stress corrosion cracks occurring in metals such as austenitic stainless steel, a base material of a nickel base alloy, and a weld metal, and an apparatus for evaluating the susceptibility to occurrence of intergranular stress corrosion cracks.

  As one method for accurately diagnosing the SCC sensitivity of Ni-base alloy welds, Patent Document 1 describes that after polishing the Ni-base alloy weld heat affected zone, it is immersed in an aqueous solution that corrodes the grain boundaries of the alloy. The number of grain boundaries connected to the total number of eroded grain boundaries of the part or the replica taken from the part after electrochemically corroding the Cr-deficient region below the predetermined concentration of the grain boundary of the alloy And a method for evaluating the stress corrosion cracking susceptibility of the weld heat affected zone from the ratio of the two is disclosed.

Japanese Patent Laid-Open No. 11-230928

  For metals such as austenitic stainless steel, nickel-base alloy base metal, and weld metal, when exposed to a corrosive environment for a long time in a state where tensile stress is generated, the stress is applied even if the tensile stress is less than the tensile strength of the material. Corrosion cracking (hereinafter referred to as SCC) may occur.

  For example, when austenitic stainless steel, nickel-base alloy base metal, or weld metal, which has high tensile stress, is exposed to high-temperature pure water for a long period of time, the grain boundary type stress becomes a crystal grain boundary. Corrosion cracking (hereinafter also referred to as IGSCC) occurs. The generation of IGSCC is affected by the corrosive environment, the tensile stress generated on the surface exposed to the corrosive environment, and the IGSCC generation sensitivity of the material. For this reason, it is desirable to employ a material with low IGSCC generation susceptibility for a welded structure made of austenitic stainless steel or nickel base alloy used for a long time in a corrosive environment.

  The conventional method for evaluating the susceptibility to SCC generation of austenitic stainless steel, nickel-base alloy, etc. is fixed to a jig with about 1% strain applied to a test piece with graphite wool laid on the surface, and this is corroded. A CBB (Creviced Bent Beam) test that is immersed in the environment for a long time (about 2000 h) and a method for evaluating SCC sensitivity based on the presence or absence of SCC, or a forced displacement that increases at a low strain rate in a test specimen immersed in the environment There is an SSRT (Slow Strain Rate Technique) method that evaluates the SCC sensitivity from the grain boundary type fracture surface occupancy, load stress, and the like seen on the fracture surface after fracture. JIS G 0576 defines a 42% magnesium chloride stress corrosion cracking test method (Method A) and a 30% calcium chloride stress corrosion cracking test method (Method B) as stress corrosion cracking test methods for stainless steel.

  However, in the evaluation method of IGSCC generation susceptibility based on Patent Document 1 and the conventional SCC generation test described above, the test time for generating cracks may be long. For example, in an environment close to the actual test environment. In the CBB test to be performed, a long test period of about 2000 h is required.

  The present invention provides a method for evaluating the intergranular stress corrosion cracking susceptibility evaluation apparatus and the intergranular stress corrosion cracking susceptibility evaluation apparatus that can easily and easily evaluate the IGSCC generation susceptibility of a specimen.

In order to solve the above problems, for example, the configuration described in the claims is adopted.
The present invention includes a plurality of means for solving the above-mentioned problems. To give an example, a surface treatment process for subjecting the surface of the subject to mirror polishing, and a nominal treatment for the subject subjected to the surface treatment. strain was loaded forced displacement of epsilon _0, a step of measuring the strain epsilon _i at least two grain boundaries near which occur during the forced displacement load, of at least 2 or more measured in this measuring step strain epsilon _i a calculation step of calculating an epsilon _i processed value from the intergranular stress corrosion of the subject the epsilon _i processed value / epsilon ₀ that the epsilon _i processed value divided by the nominal strain epsilon ₀ obtained in this calculation step as an index And an evaluation process for evaluating cracking susceptibility.

  ADVANTAGE OF THE INVENTION According to this invention, it is possible to evaluate the IGSCC generation | occurrence | production sensitivity of a test object easily and in a short time, for example, it can utilize for selection of the material used by the nuclear reactor power plant structure etc.

It is a figure which shows an example which expandedly observed the to-be-examined object surface before loading the forced displacement of nominal distortion | strain (epsilon) ₀ in 1st Embodiment. It is a figure which shows an example which expandedly observed the to-be-examined object surface in the middle of loading the forced displacement of nominal distortion | strain (epsilon) ₀ in 1st Embodiment. It is a figure which shows an example of the plot of the measurement result of (epsilon) _max-i in 1st Embodiment. It is a figure which shows an example of the grain boundary type stress corrosion cracking generation | occurrence | production sensitivity evaluation apparatus of this invention in 5th Embodiment.

  The grain boundary type stress corrosion cracking susceptibility evaluation method and the grain boundary type stress corrosion cracking susceptibility evaluation apparatus of the present invention evaluate IGSCC generation susceptibility by paying attention to tensile stress which is one of SCC generation factors. is there.

  Specifically, in austenitic stainless steel, nickel-base alloys, and these weld metals, the grain boundary hardness tends to be harder than in the crystal grains, and this occurs when an external force is applied to the specimen. Strain tends to concentrate near the grain boundaries. Therefore, the present inventors pay attention to the characteristic that the strain generated when an external force is applied to these metals is concentrated near the grain boundary, and the IGSCC generation susceptibility using the strain increase rate near the grain boundary as an index. And the idea of evaluating a specimen having a high strain increase rate in the vicinity of the grain boundary as having high IGSCC susceptibility was completed, and the present invention was completed.

  Embodiments of the grain boundary type stress corrosion cracking susceptibility evaluation method and the grain boundary type stress corrosion cracking susceptibility evaluation apparatus of the present invention will be described below with reference to the drawings.

<First Embodiment>
A first embodiment of the method for evaluating the susceptibility to occurrence of intergranular stress corrosion cracking according to the present invention will be described with reference to FIGS.
FIG. 1 shows a specific example of a magnified observation of the surface of a specimen in which columnar grain boundaries and dendrite boundaries are visible by subjecting the specimen surface made of a nickel-base alloy weld metal to mirror polishing and overetching. FIG. 2 is a diagram showing a specific example of an enlarged observation image in which a subject is subjected to a forced displacement with a nominal strain ε ₀ , and FIG. 3 is a graph plotting measurement results of ε _{max-i on} a graph. It is a figure which shows an example.

  First, a specimen made of nickel-base alloy weld metal is prepared.

Next, mirror polishing is performed on the surface of the specimen made of this nickel-base alloy weld metal. Thereafter, an over-etching process is performed on the specimen after mirror polishing (surface treatment process). By performing this mirror polishing and subsequent over-etching treatment, the dendritic boundary can be visually recognized on the surface of the subject, and ε _i can be measured using the dendritic boundary as an index.

Next, before applying a forced displacement of nominal strain ε ₀ to the subject subjected to this surface treatment, the surface in a certain region as shown in FIG. 1 is magnified, and columnar grain boundaries and dendrite boundaries are observed. Confirm. Then, two arbitrary dendrite boundaries located in the vicinity of the columnar grain boundaries are selected, and the distance L _0i between the dendrite boundary designating patterns before the forced displacement load is measured. Measurement of the distance L _0i by this magnified observation is performed in at least two or more regions (total number n).

As shown in FIG. 1, the crystal grains of the weld metal form columnar crystals in which dendritic structures are assembled. For this reason, there is a strong tendency for columnar grain boundaries to be the path of IGSCC progress. Therefore, when a forced displacement with a nominal strain ε ₀ is applied, the vicinity of the columnar grain boundary (for example, a range from the columnar grain boundary to 10% of the columnar crystal diameter, or a range from the columnar grain boundary to 20 μm). ) to measure the ε _i that occur in.

Specifically, a forced displacement with a nominal strain ε ₀ is applied to the subject by pulling from the left and right. Then, as shown in FIG. 2, in the same area as the area where the distance L _0i between the dendritic boundary indicating patterns before the forced displacement load was previously measured, the area between the dendritic boundary indicating patterns during the forced displacement load with the nominal strain ε ₀ The distance L _1i is measured. This magnified observation is also performed in all (at the total number n) of at least two or more areas in which the distance L _0i is measured before the forced displacement load.

Thereafter, the strain ε _i in the vicinity of the columnar grain boundary is evaluated (measurement step). The strain ε _{i in the} vicinity of the columnar grain boundary is expressed by the equation (1) based on the distance L _0i between the dendritic boundary indicating patterns before the forced displacement load and the distance L _1i between the dendritic boundary indicating patterns during the forced displacement loading. evaluate.

ε _i = (L _1i −L _0i ) / L _0i (1)
Next, from at least two or more strains ε _i measured previously, an upper limit value ε _max (ε for the strain near the grain boundary in the specimen is obtained by extreme value statistics based on the measurement result of the strain maximum value ε _max-i in each region. _i processing value) is calculated (calculation step).

Specifically, the maximum strain value ε _max-i in each region is extracted from the strain ε _i in the vicinity of the grain boundary measured in at least two regions (total number n). After that, the extracted ε _max-i is defined as ε _max−1 , ε _{max− 2} ,..., Ε _{max−n in} order from the smallest value and is set as the coordinate of the horizontal axis of the graph, and y _i in the expression (2) is The measurement result of ε _max-i is plotted on a graph as shown in FIG. 3 as the coordinate of the vertical axis of the graph.

y _i = −ln (−ln (i / (n + 1)) (2)
As shown in FIG. 3, when ε _max-i follows a double exponential distribution, the graph is linear. A linear approximate expression of Expression (4) is created from the plotting result on the graph by the least square method, and α and β are determined.

y _i = αε _max−i + β (3)
In the evaluation of the upper limit value based on the extreme value statistics, y evaluated in Expression (4) is substituted into y _i in Expression (3) based on the area S _{0 of the} measurement region and the total area S of the evaluation target structure. Ε _max−i obtained by this is evaluated as the upper limit value ε _max .

y = -ln (-ln ((T-1) / T) ... Formula (4)
T = (S + S ₀ ) / S ₀
However, the size of the subject to be evaluated may not be determined as one. In this case, the value of y is determined with the value of T as an arbitrary constant (for example, T = 100).

Thereafter, ε _max / ε ₀ is obtained using ε _max evaluated as described above.

Next, the grain boundary type stress corrosion cracking susceptibility of the specimen is evaluated using the previously determined ε _max / ε ₀ as an index (evaluation step). For example, a subject whose ε _max / ε ₀ is higher than ε _max / ε ₀ of other subjects (conventional material amount) is evaluated as a subject having high IGSCC susceptibility, and is equal to or less than the equivalent value. If it is, it will evaluate that it is a test subject with low IGSCC generation | occurrence | production sensitivity.

In the first embodiment of the above-described evaluation method of the intergranular stress corrosion cracking susceptibility of the present invention, the distance between the dendrite boundary indicating patterns before applying the forced displacement with the nominal strain ε ₀ and the dendrite under the forced displacement load. It evaluates epsilon _i and a distance between the boundary indication pattern, obtaining the n-region component of the epsilon _i at different places. Then, ε _i is statistically processed, and the IGSCC occurrence sensitivity of the subject is evaluated based on the statistical processing value.

  This makes it possible to evaluate the susceptibility to IGSCC generation by loading a forced displacement, and unlike the conventional method for evaluating susceptibility to IGSCC generation by the SCC generation test, it is not necessary to generate cracks in the subject, and in a short time. Can be tested and evaluated. Therefore, the susceptibility of the subject to IGSCC occurrence can be evaluated easily and in a short time, and can be suitably applied to, for example, selection of a material to be used in a nuclear power plant structure or the like.

  Although the specimen is a nickel-base alloy weld metal, the specimen is not limited to this, and may be either austenitic stainless steel, a nickel-base alloy base material, these weld metals, or both of these metals. it can.

<Second Embodiment>
Next, a second embodiment of the evaluation method of the grain boundary type stress corrosion cracking susceptibility of the present invention will be described.

  In the IGSCC occurrence susceptibility evaluation method of this embodiment, first, a specimen is prepared, and mirror polishing and overetching are performed (surface treatment process).

Next, observe the surface of the specimen after mirror polishing + over-etching by magnifying observation, select two arbitrary dendrite boundaries located near the columnar grain boundary, and between the dendrite boundary designating patterns before forced displacement loading The distance L _0i is measured. Measurement of the distance L _0i by this magnified observation is performed at least in two or more places in the observed region.

Next, a forced displacement with a nominal strain ε ₀ is applied to the subject, and the distance L _1i between the dendritic boundary indicating patterns in the forced displacement load at the location where the distance L _0i has been measured previously is measured. The strain ε _i near the boundary is evaluated (measurement process).

Then, at least two or more strain epsilon _i previously measured, calculates the average value epsilon _ave of the measured ε _i (ε _i value) and determine the ε _ave / ε ₀ (calculation step).

Next, using the previously obtained ε _ave / ε ₀ as an index, the grain boundary type stress corrosion cracking susceptibility of the specimen is evaluated (evaluation step).

  Also in the second embodiment of the method for evaluating the susceptibility to occurrence of intergranular stress corrosion cracking according to the present invention, substantially the same effect as in the first embodiment of the method for evaluating the susceptibility to occurrence of intergranular stress corrosion cracking can be obtained. .

<Third Embodiment>
Next, a third embodiment of the evaluation method of the grain boundary type stress corrosion cracking susceptibility of the present invention will be described.

  In the IGSCC occurrence susceptibility evaluation method of this embodiment, first, a specimen is prepared, and mirror polishing and overetching are performed (surface treatment process).

Next, two arbitrary dendrite boundaries located in the vicinity of the columnar grain boundary are selected, and the distance L _0i between the dendrite boundary designating patterns before the forced displacement load is measured. Measurement of the distance L _0i by this magnified observation is performed at least in two or more places in the observed region.

Next, a forced displacement with a nominal strain ε ₀ is applied to the subject, and the distance L _1i between the dendritic boundary indicating patterns in the forced displacement load at the location where the distance L _0i has been measured previously is measured. The strain ε _i near the boundary is evaluated (measurement process).

Next, an average value ε _ave of the measured ε _i is calculated from at least two strains ε _i measured in advance. Further, the standard deviation σ is obtained assuming that the strain ε _i follows a normal distribution, and the value a times the standard deviation σ is added to the average value ε _ave as the strain near the grain boundary in the specimen (ε _ave + a × σ) / ε ₀ (ε _i processing value) is obtained (calculation step).

Next, the grain boundary type stress corrosion cracking susceptibility of the specimen is evaluated using the previously obtained (ε _ave + a × σ) / ε ₀ as an index (evaluation step).

  Also in the third embodiment of the evaluation method for the susceptibility to occurrence of intergranular stress corrosion cracking according to the present invention, substantially the same effect as in the first embodiment of the evaluation method for the susceptibility to occurrence of intergranular stress corrosion cracking can be obtained. .

<Fourth Embodiment>
Next, a fourth embodiment of the method for evaluating the susceptibility to occurrence of intergranular stress corrosion cracking according to the present invention will be described.

  In the IGSCC generation susceptibility evaluation method of this embodiment, first, a specimen is prepared and subjected to a mirror polishing process (surface treatment process).

Next, the plastic strain (ε _0i before the forced displacement load) estimated from the pattern before loading of the forced displacement of the nominal strain ε _{0 on} the surface of the subject after mirror polishing is performed by electron backscatter diffraction (EBSD). ). Thereafter, the plastic strain (ε _1i during the controlled displacement load) estimated from the pattern during the forced displacement load with the nominal strain ε ₀ is measured by electron backscatter diffraction. Thereafter, the strain ε _i in the vicinity of the columnar grain boundary is evaluated by the equation (5) based on these ε _0i before the forced displacement load and ε _1i during the forced displacement load.

ε _i = ε _1i −ε _0i (5)
Next, from at least two or more strains ε _i measured previously, the upper limit value ε _max (ε _i (ε _{i) of the} strain near the grain boundary in the specimen is determined by extreme value statistics based on the measurement result of the strain maximum value ε _max-i in each region. Processing value) is calculated (calculation step).

Next, the grain boundary type stress corrosion cracking susceptibility of the specimen is evaluated using the previously determined ε _max / ε ₀ as an index (evaluation step).

  Also in the fourth embodiment of the evaluation method for the susceptibility to occurrence of intergranular stress corrosion cracking according to the present invention, substantially the same effect as in the first embodiment of the evaluation method for the susceptibility to occurrence of intergranular stress corrosion cracking can be obtained. .

  That is, the susceptibility of the subject to IGSCC can be evaluated easily and in a short time, and can be suitably applied to, for example, selection of a material used in a nuclear reactor plant structure or the like.

In the present embodiment, the case where the ε _i processing value is obtained by the extreme value statistics has been described. However, the method for obtaining the ε _i processing value is a method for obtaining ε _ave as in the second embodiment or the third embodiment. Other methods such as a method for obtaining ε _ave + a × σ can be used.

<Fifth Embodiment>
A fifth embodiment of the grain boundary type stress corrosion cracking susceptibility evaluation apparatus of the present invention will be described with reference to FIG. FIG. 4 is a diagram for explaining a specific example of the IGSCC occurrence susceptibility evaluation apparatus of the present invention.

  As shown in FIG. 4, the intergranular stress corrosion cracking susceptibility evaluation apparatus includes a displacement load unit 3, an enlargement observation unit 4, a calculation unit 5, and a display unit 6.

The displacement load unit 3 loads a forced displacement of a nominal strain ε ₀ of pressing from the lower side to the upper side of the subject 10 in which the crystal grain boundary can be visually recognized by surface treatment such as mirror polishing or over etching.

The magnification observation unit 4 acquires a magnification observation image necessary for observing the strain ε _i in the vicinity of the grain boundary generated in the subject 10 before and during the loading of the forced displacement by the displacement load unit 3. Examples of the magnified observation unit include a microscope and a scanning electron microscope for electron backscatter diffraction.

The computing unit 5 evaluates at least two or more strains ε _i in the vicinity of the crystal grain boundaries from the magnified observation image of the surface of the subject 10 with or without the forced displacement load acquired by the magnification observation unit 4. Moreover, this evaluation was epsilon _i calculates the epsilon _i processed value from computing the epsilon _i processed value / epsilon ₀ divided by epsilon ₀ strain nominal this epsilon _i processed value. Furthermore, IGSCC generation sensitivity is evaluated using this ε _i treatment value / ε ₀ as an index. Examples of the calculation method of the ε _i processing value include a method of obtaining an ε _i processing value by extreme value statistics, a method of obtaining ε _ave, and a method of obtaining ε _ave + a × σ.

The display unit 6 displays the ε _i processing value / ε ₀ obtained by the calculation unit 5. On this display unit 6, the ε _i processing value / ε ₀ of another subject can be displayed side by side with the ε _i processing value / ε ₀ obtained by the calculation unit 5 to facilitate comparison.

  In the fifth embodiment of the grain boundary type stress corrosion cracking susceptibility evaluation apparatus of the present invention, substantially the same effect as that of the first embodiment of the grain boundary type stress corrosion cracking susceptibility evaluation method described above can be obtained.

  That is, the susceptibility of the subject to IGSCC generation can be evaluated easily and in a short time, and can be suitably used, for example, for selection of a material to be used in a reactor internal structure of a nuclear power plant.

<Others>
In addition, this invention is not restricted to said embodiment, A various deformation | transformation and application are possible. The above-described embodiment has been described in detail for easy understanding of the present invention, and is not necessarily limited to the one having all the configurations described.

For example, the direction of the load of the forced displacement with the nominal strain ε ₀ is the load caused by the tensile stress from the left and right on the subject as in the first embodiment, and the pressing from the lower side to the upper side as in the fifth embodiment. The forced displacement with the nominal strain ε ₀ may be applied to the subject.

1 ... Columnar grain boundaries,
2 ... Dendrite boundary,
3 ... displacement load section,
4 ... Magnification observation part,
5 ... arithmetic unit,
6 ... display part,
10 ... Subject.

Claims (7)

A surface treatment process for subjecting the subject surface to mirror polishing;
A measuring step of applying a forced displacement of nominal strain ε _{0 to} the subject subjected to the surface treatment, and measuring at least two strains ε _i in the vicinity of the crystal grain boundaries generated during the forced displacement load;
A calculation step of calculating an ε _i treatment value from the at least two strains ε _i measured in the measurement step;
Having an evaluation step of evaluating the intergranular stress corrosion cracking susceptibility of the subject to index epsilon _i processed value / epsilon ₀ obtained by dividing the epsilon _i processed value by the nominal strain epsilon ₀ obtained in this calculation step Evaluation method of grain boundary type stress corrosion cracking susceptibility. In the evaluation method of the intergranular stress corrosion cracking susceptibility according to claim 1,
In the measuring step, the strain ε _i is measured in at least two regions,
In the calculation step, as the ε _i treatment value, the upper limit value ε _{max of the} strain near the grain boundary in the specimen is obtained by extreme value statistics based on the measurement result of the strain maximum value ε _max-i in each region measured in the measurement step. And
In the evaluation step, the grain boundary type stress corrosion cracking susceptibility of the specimen is evaluated using the ε _max / ε ₀ as an index. In the evaluation method of the intergranular stress corrosion cracking susceptibility according to claim 1,
In the calculating step, as the epsilon _i processed value, calculates the average value epsilon _ave of the measured the epsilon _i,
In the evaluation step, the susceptibility of the grain boundary type stress corrosion cracking of the specimen is evaluated using the ε _ave / ε ₀ as an index. In the evaluation method of the intergranular stress corrosion cracking susceptibility according to claim 1,
In the calculation step calculates the average value epsilon _ave of the measured epsilon _i, and the standard deviation sigma as epsilon _i follows a normal distribution, the ε _{ave + a} × σ as the epsilon _i processed value,
In the evaluation step, the grain boundary type stress corrosion cracking susceptibility of the specimen is evaluated by using (ε _ave + a × σ) / ε ₀ as an index. In the evaluation method of the intergranular stress corrosion cracking susceptibility according to claim 1,
In the surface treatment step, in addition to the mirror polishing, over-etching is performed on the specimen after mirror polishing. An evaluation method of the intergranular stress corrosion cracking susceptibility. In the evaluation method of the intergranular stress corrosion cracking susceptibility according to claim 1,
The test object is one of austenitic stainless steel, a nickel-base alloy base material, a weld metal thereof, and both of these metals. A displacement load section for applying a forced displacement with a nominal strain ε ₀ to the subject whose crystal grain boundaries are visible;
A measuring unit for measuring at least two strains ε _i in the vicinity of the crystal grain boundary generated on the surface of the subject during the load of forced displacement by the displacement load unit;
This calculates the epsilon _i processed value the measured by the measurement unit from at least two strain epsilon _i, a calculator for calculating the epsilon _i processed value / epsilon ₀ divided by the epsilon _i processed value the nominal strain epsilon ₀ to ,
A grain boundary type stress corrosion cracking susceptibility evaluation apparatus, comprising: a display unit that displays the ε _i processing value / ε ₀ obtained by the calculation unit.

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