JP2018087738A - Method for evaluating remaining life of metal material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for evaluating the remaining life of a metal material, capable of appropriately evaluating the remaining life of the metal material irrespective of the level of deterioration of the metal material.SOLUTION: The method for evaluating the remaining life of an object to be evaluated consisting of a metal material comprises the steps of:(a) obtaining a first parameter on the grain size of the object to be evaluated; (b) obtaining a second parameter on the residual stress of the object to be evaluated; (c) obtaining a strain parameter by multiplying the first parameter by the second parameter; and (d) evaluating the remaining life of the object to be evaluated on the basis of a relation between the strain parameter obtained in the (c) step, a strain parameter previously required and a remaining life or a life consumption rate.SELECTED DRAWING: Figure 7

Description

  The present invention relates to a method for evaluating the remaining life of a metal material.

  Conventionally, a method for evaluating the remaining life of a metal material (steel pipe or the like) used in a high-temperature environment in a thermal power plant or the like has been proposed.

  For example, Patent Document 1 discloses a method for predicting the remaining life of a metal material based on the shape of crystal grains. Specifically, in the method of Patent Document 1, the amount of change in crystal shape is measured based on the major axis of the crystal grain, the width of the crystal grain, the circularity of the crystal grain, and the like, and the remaining life of the metal material is estimated. doing.

Japanese Unexamined Patent Publication No. 63-228062

  However, as a result of various studies by the present inventors, it has been found that there are cases where the above-described method cannot appropriately predict the remaining life of the metal material.

  That is, the state of the metal material changes with the deterioration of the metal material, but the mode of change of the state varies depending on the degree of deterioration of the metal material. For this reason, in the method of Patent Document 1 that predicts the remaining life based on one piece of information indicating the state of the microstructure (for example, the shape of the crystal grains), the remaining life is appropriately predicted depending on the degree of deterioration of the metal material. There are cases where it is not possible.

  The present invention has been made to solve such a problem, and provides a method for evaluating the remaining life of a metal material, which can appropriately evaluate the remaining life of a metal material regardless of the degree of deterioration of the metal material. The purpose is to provide.

  The remaining life of a metal material used in a high temperature environment decreases as the creep strain increases. Therefore, the remaining life of the metal material can be evaluated based on the creep strain of the metal material. Therefore, the present inventor has made various studies on parameters that can appropriately represent changes in creep strain. Specifically, the inventor of the present invention pays attention to the tendency that the average crystal grain size of the metal material becomes smaller and the intragranular strain tends to become larger due to recrystallization and generation of subgrain boundaries in the progress of creep. We proceeded with the examination. The inventor also paid attention to the fact that intragranular strain is caused by lattice strain (uniform strain) and affects the residual stress of the metal material.

  As a result of investigation by the present inventor while paying attention to the above points, it has been found that the residual stress generated in the metal material monotonously increases as the creep strain increases. As described above, it is considered that the uniform lattice strain of the metal material affects the residual stress generated in the metal material. Therefore, for example, uniform lattice strain can be used as a parameter indicating a change in residual stress. In other words, uniform lattice strain can be used as a parameter representing the change in creep strain.

  Further, as a result of investigations by the present inventors, it was found that the two-dimensional X-ray diffraction image of the metal material changes as the average crystal grain size of the metal material changes. Specifically, as the average crystal grain size of the metal material decreases, for example, X in the circumferential direction (hereinafter also referred to as β direction) of the Debye-Scherrer ring obtained as a two-dimensional X-ray diffraction image of the metal material. It was found that the deviation of the line diffraction intensity distribution decreased. Therefore, for example, the deviation of the X-ray diffraction intensity distribution in the circumferential direction of the Debye-Scherrer ring can be used as a parameter representing the change in creep strain.

  As a result of further investigation by the present inventor, it was found that the ratio of the change in the residual stress to the change in the creep strain becomes large, particularly in the initial stage of creep. On the other hand, it was found that the ratio of the change in the average crystal grain size to the change in the creep strain increases at the later stage of creep. That is, as a result of the study by the present inventors, it has been found that the remaining life of the metal material cannot be appropriately evaluated only with one of the parameters related to the residual stress and the crystal grain size. Therefore, as a result of further investigation by the present inventor, it was found that the remaining life of the metal material can be appropriately evaluated by using a parameter that takes into consideration both the change in residual stress and the change in crystal grain size. .

  This invention is made | formed based on said knowledge, and makes it a summary for the remaining life evaluation method of the following metal material.

(1) A method for evaluating the remaining life of an evaluation object made of a metal material,
(A) obtaining the first parameter relating to the crystal grain size for the evaluation object;
(B) obtaining a second parameter relating to residual stress for the evaluation object;
(C) obtaining a distortion parameter by multiplying the first parameter obtained in the step (a) and the second parameter obtained in the step (b);
(D) a step of evaluating the remaining life of the evaluation object based on the strain parameter obtained in the step of (c) and the relationship between the strain parameter obtained in advance and the remaining life or life consumption rate;
A remaining life evaluation method for a metal material.

(2) The first parameter is a parameter indicating a deviation of the X-ray diffraction intensity distribution in the circumferential direction of the Debye-Scherrer ring obtained as the X-ray diffraction image of the evaluation target.
The remaining life evaluation method of the metal material of said (1).

(3) The second parameter is a parameter indicating a rate of change of uniform lattice strain or lattice constant.
The remaining life evaluation method of the metal material of said (1) or (2).

(4) The step (d) is
(D1) Obtaining the creep strain to be evaluated from the strain parameter obtained in the step (c) and a master curve indicating the relationship between the strain parameter obtained in advance and the creep strain;
(D2) From the creep strain of the evaluation object obtained in the step (d1) and a master curve indicating the relationship between the creep strain obtained in advance and the remaining life or life consumption rate, the remaining life or life of the evaluation object Determining the consumption rate;
The remaining life evaluation method of the metal material in any one of said (1) to (3) containing.

  According to the present invention, the remaining life of the metal material can be appropriately evaluated regardless of the degree of deterioration of the metal material.

FIG. 1 is a diagram for explaining a method of measuring an X-ray diffraction image. FIG. 2 is a diagram showing an X-ray diffraction image. FIG. 3 is a diagram showing an X-ray diffraction image obtained using a two-dimensional detector. FIG. 4 is a diagram showing a relationship between an X-ray irradiation region and a diffraction line. FIG. 5 is a diagram illustrating a relationship between an X-ray irradiation region and a diffraction line. FIG. 6 is a diagram illustrating an example of an X-ray diffraction intensity distribution. FIG. 7 is a diagram illustrating an example of a master curve showing the relationship between the strain parameter and the creep strain. FIG. 8 is a diagram illustrating an example of a master curve showing the relationship between the life consumption rate and the creep strain. FIG. 9 is a diagram illustrating an example of a master curve showing the relationship between the distortion parameter and the lifetime consumption rate.

  The method for evaluating the remaining life of a metal material according to an embodiment of the present invention (hereinafter simply referred to as an evaluation method) is suitably used, for example, when evaluating the remaining life of a metal material used in a high temperature environment. . In the present embodiment, the high temperature environment means, for example, an environment of 500 ° C. or more and less than 1000 ° C., which is a use temperature of a normal thermal power boiler or petroleum refinery equipment.

  Hereinafter, the evaluation method according to the present embodiment will be described. In the following description, the case where the remaining life of the metal material is evaluated by predicting the life consumption rate of the metal material will be described. However, the remaining life may be predicted instead of the life consumption rate.

(Outline of remaining life evaluation method)
The evaluation method according to the present embodiment includes the following steps A to D. Details of steps A to D will be described later.
Step A: Obtain the first parameter related to the crystal grain size for the metal material to be evaluated.
Step B: Obtain the second parameter relating to the residual stress for the evaluation object.
Step C: A distortion parameter is obtained by multiplying the first parameter obtained in Step A by the second parameter obtained in Step B.
Step D: Based on the strain parameter obtained in Step C and the relationship between the strain parameter obtained in advance and the remaining life or the life consumption rate, the remaining life to be evaluated is evaluated.

(Processing contents of each step)
Hereinafter, steps A to D will be described in detail.

(Step A)
In this embodiment, in Step A, the deviation of the X-ray diffraction intensity distribution in the circumferential direction of the Debye-Scherrer ring obtained as the X-ray diffraction image to be evaluated is obtained as the first parameter relating to the crystal grain size. Therefore, in step A, an X-ray diffraction image is first obtained.

  For example, as shown in FIG. 1, the X-ray diffraction image is obtained by making X-rays incident on the surface of the evaluation object. In the present embodiment, a plurality of Debye-Scherrer rings as shown in FIG. 2 are obtained as X-ray diffraction images. Debye Scherrer ring is obtained for each of several face indices. In FIG. 1, the direction perpendicular to the surface to be evaluated is indicated by an arrow b (hereinafter, also referred to as a vertical direction b), and the X-ray incident direction when viewed from the vertical direction b is indicated by an arrow a (hereinafter, referred to as “a”). Also referred to as the incident direction a). A direction perpendicular to the incident direction a and the vertical direction b is indicated by an arrow c. The Debye-Scherrer ring is obtained by a known method, and a detailed description thereof is omitted.

  With reference to FIG. 1, in this embodiment, an X-ray diffraction image is obtained using a two-dimensional detector. FIG. 3 is a diagram showing an example of an X-ray diffraction image obtained by a two-dimensional detector. In FIG. 3, as an example, a part of four diffraction lines (Debye Scherrer rings) is shown. 3 corresponds to a range of the same angle in the circumferential direction (hereinafter also referred to as β direction) of each diffraction line (in the example of FIG. 3, a range of 30 °).

  With reference to FIG. 4A, in the present embodiment, when an X-ray irradiation area on the evaluation target surface is viewed from a direction perpendicular to the evaluation target surface (see arrow b in FIG. 1), Is longer than the length in the direction c (hereinafter also referred to as the orthogonal direction c) perpendicular to the incident direction a. By irradiating X-rays in this way, a diffraction line having a uniform width can be detected by a two-dimensional detector as shown in FIG. 4B. Thereby, X-ray diffraction intensity can be calculated | required more correctly. The length in the incident direction a of the X-ray irradiation region on the surface to be evaluated is, for example, 0.3 to 10.0 mm, and the length in the orthogonal direction c is, for example, 0.1 to 0.8 mm. .

  In addition, as shown to Fig.5 (a), the length in the orthogonal direction c of the irradiation area | region of X-rays may be larger than the length in the incident direction a. However, in this case, as shown in FIG. 5 (b), the diffraction lines detected by the two-dimensional detector are wider at both ends than at the center, so that the measurement accuracy may be reduced. There is.

In step A, the standard deviation of the X-ray diffraction intensity distribution in the circumferential direction of the Debye-Scherrer ring obtained as described above is obtained. Specifically, first, an X-ray diffraction intensity distribution in the circumferential direction of any Debye-Scherrer ring among a plurality of Debye-Scherrer rings is obtained. In this embodiment, as shown in FIG. 6, an X-ray diffraction intensity distribution in a predetermined range in the β direction (for example, a range of 30 °: a range surrounded by a broken line in FIG. 3) is obtained. Further, based on the diffraction intensity I obtained from the obtained X-ray diffraction intensity distribution and the average value μ thereof, a standard deviation σ is obtained from the following formula.

  In the present embodiment, the relative standard deviation (RSD) is further obtained by dividing the standard deviation σ obtained as described above by the average value μ. In the above description, the standard deviation of the population is obtained, but the standard deviation of the sample may be obtained. For example, the standard deviation may be obtained after removing the background from the X-ray diffraction intensity distribution. For example, as shown in FIG. 6, the background can be removed using an appropriate approximate curve according to the shape of the background, measurement conditions, and the like. Since various known methods can be used as the background removal method, detailed description is omitted.

  As described above, in this embodiment, in step A, the relative standard deviation of the X-ray diffraction intensity distribution in the circumferential direction of the Debye-Scherrer ring is obtained as the first parameter.

(Step B)
In this embodiment, in Step B, uniform lattice distortion is obtained as the second parameter relating to residual stress. In the present embodiment, for example, the diffraction grating surface spacing is obtained based on the X-ray diffraction image obtained in step A, and the difference between the obtained diffraction grating surface spacing and the grating surface spacing in an unstrained state is calculated as described above. By dividing by the lattice plane spacing in the unstrained state, the uniform lattice strain can be obtained. In the present embodiment, for example, the literature value of the Ni plane spacing can be used as the lattice plane spacing in the unstrained state. For example, in the case of obtaining the uniform lattice distortion by measuring the {331} plane spacing based on the X-ray diffraction image to be evaluated, 0.0808 nm is used as the literature value for the Ni plane spacing. In addition, the calculation method of uniform lattice distortion is not limited to the above-mentioned example, Various well-known methods can be used.

(Step C)
In Step C, a distortion parameter is obtained by multiplying the relative standard deviation obtained in Step A by the uniform lattice distortion obtained in Step B. For example, when the relative standard deviation obtained in step A is “60” and the uniform lattice distortion obtained in step B is “−0.1”, the distortion parameter obtained in step C is “−6”. is there.

(Step D)
In Step D, the remaining life to be evaluated is evaluated using the relationship between the strain parameter and the remaining life or the life consumption rate obtained in advance. In this embodiment, step D includes the following step D1 and step D2. In the present embodiment, two types of master curves obtained in advance are used as the relationship between the distortion parameter and the lifetime consumption rate. Hereinafter, step D1 and step D2 will be described.

(Step D1)
In step D1, the creep distortion to be evaluated is obtained based on the distortion parameter obtained in step C using a master curve indicating the relationship between the distortion parameter and creep distortion as shown in FIG.

The master curve used in step D1 is obtained in advance as follows, for example. First, a plurality of model materials having different creep strains are prepared. And about each model material, while measuring a creep distortion, a Debye-Scherrer ring is obtained as an X-ray diffraction image. Next, the X-ray diffraction intensity distribution in the circumferential direction of the Debye-Scherrer ring is determined for each model material by a method similar to the method described in step A and step B, as shown in Table 1 below. Determine relative standard deviation (RSD) and uniform lattice distortion. Table 1 also shows the interplanar spacing obtained for calculating the uniform lattice distortion. The Debye-Scherrer ring in which the relative standard deviation is obtained when creating the master curve and the Debye-Scherrer ring in which the relative standard deviation is obtained in the above step A are Debye-Scherrer rings corresponding to the same surface index. .

  Next, as shown in Table 1, the distortion parameter of each model material is obtained by multiplying the obtained relative standard deviation (RSD) and uniform lattice distortion in the same manner as the method described in Step C above. A master curve (approximate curve) as shown in FIG. 7 can be obtained on the basis of the distortion parameter of each model material thus obtained and the creep strain of each model material. Note that the approximate curve serving as the master curve can be obtained using an appropriate function corresponding to the strain parameters and creep strain of a plurality of model materials. The master curve shown in FIG. 7 is an example of a master curve created using a sigmoid function. In the present embodiment, for example, the same crystalline metal material as the evaluation target is used as the model material.

(Step D2)
In step D2, the life consumption rate of the evaluation object is obtained based on the creep strain obtained in step D1, using a master curve showing the relationship between the life consumption rate and creep strain as shown in FIG. Thereby, the remaining life of an evaluation object can be evaluated. The life consumption rate Lc can be obtained by using the following equation as a ratio of the test elapsed time T to the time Tr until the test piece reaches the creep rupture in the creep test. In FIG. 8, the lifetime consumption rate calculated | required by the following formula is shown by the percentage.
Lc = T / Tr

Note that the master curve used in step D2 may be created based on data from past experiments, etc., and a new experiment is performed using a plurality of model materials to collect data, and based on that data You may create it. For example, as shown in Table 1 above, the life consumption rate of each model material for which the creep strain is obtained may be obtained by experiments, and a master curve may be created based on the creep strain and the life consumption rate of each model material. The master curve can be obtained by using an appropriate function based on experimental data, for example. In step D2, a master curve indicating the relationship between the remaining life and the creep strain may be used. The remaining life Lr can be obtained, for example, in the creep test using the following formula using the time Tr until the test piece reaches the creep rupture and the test elapsed time T.
Lr = 1−T / Tr

(Function and effect)
As described above, in this embodiment, the change in intragranular strain can be observed from a microscopic viewpoint (uniform lattice strain), and the crystal grain size change (Debye- The relative standard deviation of the diffraction intensity distribution in the circumferential direction of the Scherrer ring can be evaluated. Specifically, in this embodiment, a strain parameter representing a change in intragranular strain is calculated based on the uniform lattice strain and the relative standard deviation. Thereby, regardless of the degree of deterioration of the metal material, it is possible to appropriately grasp the change in intragranular strain. Further, the remaining life of the metal material is evaluated using the strain parameter. Thereby, the remaining life of the metal material can be appropriately evaluated regardless of the degree of deterioration of the metal material. Specifically, the remaining life of a metal material is quantitatively determined using a common master curve in both the initial stage of creep where the change in interplanar spacing is significant and the later stage of creep where the change in crystal grain size is significant. It becomes possible to evaluate.

  For example, when evaluating the remaining life of a plant, information on the microstructure of the metal material when the life consumption rate is about 60% or less is useful. Therefore, it is necessary to evaluate the remaining lifetime using parameters that can be observed when the lifetime consumption rate of the metal material is about 60% or less. Considering this point, the evaluation method according to the present embodiment using the above-described parameter indicating the change in intragranular strain is considered to be an excellent method for evaluating the remaining life of the plant.

(Modification)
In the above-described embodiment, the case where the relative standard deviation is used as the index indicating the deviation of the X-ray diffraction intensity distribution (the first parameter related to the crystal grain size) has been described, but other indices such as the standard deviation may be used. In the above-described embodiment, the case where uniform lattice strain is used as the second parameter regarding the residual stress has been described. However, the rate of change of the lattice constant may be used as the second parameter.

  In the above-described embodiment, the case where two master curves obtained in advance as the relationship between the strain parameter and the life consumption rate (or remaining life) are used in Step D has been described. However, the strain parameter and the life used in Step D are described. The relationship with the consumption rate (or remaining life) is not limited to the above example. For example, instead of the master curve described with reference to FIGS. 7 and 8, a master curve showing the relationship between the distortion parameter and the life consumption rate as shown in FIG. 9 may be used. In this case, in step D, the life consumption rate to be evaluated can be obtained using the distortion parameter obtained in step C and the above master curve indicating the relationship between the distortion parameter and the life consumption rate. As described above, in this embodiment, the remaining life of the evaluation target can be more easily evaluated with one master curve.

  Note that the master curve shown in FIG. 9 can be created in advance, for example, by calculating the distortion parameters of a plurality of model materials by the above-described method, and by obtaining the lifetime consumption rates of the plurality of model materials by experiments. Similarly to the other master curves, the master curve shown in FIG. 9 is obtained using an appropriate function based on, for example, experimental data. Although illustration is omitted, in step D, a master curve indicating the relationship between the strain parameter and the remaining life may be used.

In the examples, the lifetime consumption rates of four metal materials (Ni-based alloys) in different states were determined by the above-described evaluation method and experiment. When calculating the lifetime consumption rate of the metal material by the above-described evaluation method, in step D, the master curve described in FIG. 9, that is, the master curve indicating the relationship between the strain parameter and the lifetime consumption rate was used. Table 2 below shows the life consumption rate (predicted value) obtained by the above-described evaluation method and the life consumption rate (actual value) obtained by experiment. In Table 2 below, the uniform lattice strain, relative standard deviation (RSD), and strain parameters obtained in the process of calculating the lifetime consumption rate are also shown for reference.

  As shown in Table 2, the lifetime consumption rate predicted by the evaluation method according to the present embodiment was not significantly different from the actually measured value. From this result, it can be seen that according to the evaluation method according to the present embodiment, the remaining life of the metal material can be appropriately evaluated. Although detailed description is omitted, it has been confirmed that the lifetime consumption rate calculated by the method described in the above steps D1 and D2 is not significantly different from the actually measured value.

  According to the present invention, the remaining life of the metal material can be appropriately evaluated regardless of the degree of deterioration of the metal material. The present invention can be suitably used for evaluating the remaining life of various metal materials such as carbon steel, stainless steel, Ni-base alloy, and Fe-Ni base alloy.

Claims (4)

A method for evaluating the remaining life of an evaluation object made of a metal material,
(A) obtaining the first parameter relating to the crystal grain size for the evaluation object;
(B) obtaining a second parameter relating to residual stress for the evaluation object;
(C) obtaining a distortion parameter by multiplying the first parameter and the second parameter;
(D) a step of evaluating the remaining life of the evaluation object based on the strain parameter obtained in the step of (c) and the relationship between the strain parameter obtained in advance and the remaining life or life consumption rate;
A remaining life evaluation method for a metal material. The first parameter is a parameter indicating a deviation of an X-ray diffraction intensity distribution in a circumferential direction of a Debye-Scherrer ring obtained as an X-ray diffraction image of the evaluation target.
The method for evaluating the remaining life of the metal material according to claim 1. The second parameter is a parameter indicating a rate of change of uniform lattice strain or lattice constant.
The remaining life evaluation method of the metal material of Claim 1 or 2. The step (d)
(D1) Obtaining the creep strain to be evaluated from the strain parameter obtained in the step (c) and a master curve indicating the relationship between the strain parameter obtained in advance and the creep strain;
(D2) From the creep strain of the evaluation object obtained in the step (d1) and a master curve indicating the relationship between the creep strain obtained in advance and the remaining life or life consumption rate, the remaining life or life of the evaluation object Determining the consumption rate;
The remaining life evaluation method of the metal material in any one of Claim 1 to 3 containing these.

Images