Damage evaluation method and maintenance evaluation index decision method

Abstract

A first measured value of a specific physical quantity at a target portion is correlated with a damage evaluation index to calculate a damage degree corresponding to the first measured value. The specific physical quantity is measured at least once at a position corresponding to the first measurement position in another time period having a different usage elapsed time from that of the first measurement, and these second and subsequent measured values are correlated with damage degrees calculated based on temporal changes corresponding to the second and subsequent measurements. A new damage evaluation index is approximately calculated based on a relationship between the first, second, and subsequent measured values and the damage degrees corresponding to the first, second, and subsequent measured values.

Claims

1. A damage evaluation method, which evaluates a creep damage ratio of a target portion which changes over time in a structural member, the damage evaluation method comprising: a first process of correlating a first measured value of a number density of creep voids at a first measurement position of the target portion with a damage evaluation curve indicating a relationship between a number density of the creep voids and a creep damage ratio which is generated based on a temporal change, and calculating a creep damage ratio corresponding to the first measured value; a second process of measuring a measured value of the number density of the creep voids at least once, as a second measured value, at a position corresponding to the first measurement position in another time period having a different usage elapsed time from that of the first measured value, and correlating the second and any subsequent measured values with a creep damage ratio calculated based on temporal changes corresponding to the second and any subsequent measured values; a third process of approximately calculating a new damage evaluation curve based on a relationship between the first, second, and any subsequent measured values and the creep damage ratio corresponding to the first, second, and any subsequent measured values; a fourth process of evaluating the creep damage ratio from the first, second, and any subsequent measured values of the number density of the creep voids, based on the new damage evaluation curve calculated in the third process; and a fifth process of determining a lifetime of the structural member based on the fourth process of evaluating the creep damage ratio for performing maintenance on the structural member when appropriate based on the fourth process of evaluating the creep damage ratio.

2. The damage evaluation method according to claim 1, wherein the first process, the second process, and the third process are repeated based on the new damage evaluation curve.

3. The damage evaluation method according to claim 1, wherein the structural member is formed of heat-resistant steel.

4. The damage evaluation method according to claim 3, wherein the target portion is a welding heat-affected portion of the structural member formed of heat-resistant steel.

5. A maintenance evaluation index decision method using the damage evaluation method according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a diagram schematically illustrating a pipe and a welding heat-affected portion (target portion) of a boiler to which a creep damage evaluation method according to an embodiment of the present invention is applied.

(2) FIG. 2 is a flowchart schematically illustrating the order of the creep damage evaluation method according to the embodiment.

(3) FIG. 3 is a diagram illustrating a creep damage evaluation curve according to the embodiment.

(4) FIG. 4 is a diagram illustrating a first process of the creep damage evaluation method according to the embodiment.

(5) FIG. 5 is a diagram illustrating a second process of the creep damage evaluation method according to the embodiment.

(6) FIG. 6 is a diagram illustrating a third process of the creep damage evaluation method according to the embodiment.

(7) FIG. 7 is a diagram illustrating a damage evaluation method in which the first process, the second process, and the third process are repeated.

DESCRIPTION OF EMBODIMENTS

(8) Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(9) In this embodiment, an example in which a damage evaluation method is applied to a welding heat-affected portion (target portion) of a pipe of heat-resistant steel such as a boiler, which is used in a high-temperature and high-pressure environment, to evaluate a creep damage degree will be described.

(10) FIG. 1 illustrates a pipe 1 of a boiler which is an evaluation target of the damage evaluation method according to this embodiment.

(11) Through the inside of the pipe 1, high-temperature vapor flows in a direction indicated by an arrow of FIG. 1. A weld joint portion 10 of the pipe 1 is a joint through which a bent portion 20 and a linear portion 30, which are formed of heat-resistant steel, are welded together.

(12) In the weld joint portion 10, the number density of creep voids is measured at a welding heat-affected portion 11 (target portion) adjacent to the weld joint portion 10 in order to evaluate a creep damage ratio. The number density of creep voids can be obtained by measuring creep voids using, for example, a replica method and obtaining the number of creep voids per predetermined area. The replica method includes performing a predetermined treatment on the surface of the welding heat-affected portion 11 to make a metallographic structure appear; transferring convex and concave shapes of the metallographic structure onto a film; and observing the transferred convex and concave shapes using an optical microscope, a scanning electron microscope, or the like.

(13) In this example of the embodiment, the creep damage ratio is evaluated from the number density of creep voids which is measured at the welding heat-affected portion 11 provided on the weld joint portion 10.

(14) The creep damage ratio is calculated from the following expression.
Creep Damage Ratio=(Usage Elapsed Time)/{(Usage Elapsed Time)+(Remaining Lifetime of Structural Member)}

(15) The usage elapsed time refers to a total time for which a structural member is used in a specific usage environment. The remaining lifetime of a structural member refers to the remaining time until the structural member which is used in a specific environment is broken. The sum of the usage elapsed time and the remaining lifetime of a structural member is the time which is required for the structural member to be broken after the usage in a specific environment, and this sum is called the total lifetime.

(16) Heat-resistant steel to be used is appropriately selected according to the usage temperature and applied stress of a pipe or a structural member. Examples of a representative material to be used include 2 Cr steel; and 9 Cr steel and 12 Cr steel which are known as high-strength ferritic steel. In a portion requiring higher corrosion resistance and creep strength, other ferritic steels, austenitic steels, Fe-based alloys, Ni-based alloys, and the like may be used.

(17) Hereinafter, the order of the creep damage evaluation method according to this embodiment will be described.

(18) The creep damage evaluation method according to this embodiment is performed according to the order of a flowchart illustrated in FIG. 2. That is, an optimum creep damage evaluation curve is obtained by decreasing a difference between a relationship of the damage ratio corresponding to the number density of creep voids and the damage evaluation index. Using this optimum creep damage evaluation curve, the creep damage ratio is determined.

(19) The order for obtaining the creep damage evaluation curve includes, for example, a first process S10, a second process S20, a third process S30, and a determination process S40. In the determination process S40, the validity of the creep damage evaluation curve is determined.

(20) (First Process S10)

(21) The first process S10 is a process of calculating the creep damage ratio from the number density of creep voids. A method of calculating the creep damage ratio will be described below.

(22) FIG. 3 illustrates a creep damage evaluation curve 40 representing a relationship between the number density of creep voids at a welding heat-affected portion of heat-resistant steel, which is used in a high-temperature environment, and the creep damage ratio. This creep damage evaluation curve 40 is obtained in advance, for example, through a laboratory experiment or using cumulative data or the like in a database.

(23) A first measurement of the number density of creep voids is performed at the welding heat-affected portion 11 of the pipe 1 which is actually used. It is preferable that the number of the measurement positions at this time be one or more.

(24) The first measured value of the number density of creep voids is correlated with the creep damage evaluation curve 40 of FIG. 3 to calculate a creep damage ratio.

(25) Specifically, when the number of the first measurement positions of creep voids is three, measured values D1-1, D2-1, and D3-1 at the respective measurement positions are plotted as illustrated in FIG. 4, and creep damage ratios R1-1, R2-1, and R3-1 corresponding to the measured values D1-1, D2-1, and D3-1 of the creep damage evaluation curve 40 are calculated. Next, using the creep damage ratios and the usage elapsed time, the total lifetime of the welding heat-affected portion 11 at each measurement position is calculated.

(26) (Second Process S20)

(27) The second process S20 is a process of correlating second and subsequent measured values of the number density of creep voids with creep damage ratios.

(28) In another time period having a different usage elapsed time from that of the first measurement, second and subsequent measurements of the number density of creep voids are preformed at least once at the welding heat-affected portion 11 of the pipe 1. The measurement positions at this time are positions corresponding to the first measurement positions.

(29) The positions corresponding to the first measurement positions refer to, for example, the same positions as the first measurement positions or the positions of which the damage degrees are the same as those of the first measurement positions.

(30) In this embodiment, as illustrated in FIG. 5, the creep damage ratio at each measurement point is calculated using the usage elapsed time and the number density of creep voids in the second measurement and the total lifetime calculated in the first process.

(31) Next, as illustrated in FIG. 5, the measured values (D1-2, D2-2, D3-2) of the number density of creep voids in the second measurement are correlated with the creep damage ratios (R1-2, R2-2, R3-2) calculated from the above-described second and subsequent measured values of the number density of creep voids.

(32) FIG. 5 illustrates a case where the second measurement is performed at three positions in the same manner as that of the first measurement, and the three measured values of the number density of creep voids are correlated with the creep damage ratios once. In the second process, the number density of creep voids may be measured at least once, and three and subsequent measurements may be performed.

(33) (Third Process S30)

(34) The third process S30 is a process of calculating a new creep damage evaluation curve.

(35) As illustrated in FIG. 6, a new creep damage evaluation curve 50 is approximately calculated based on a relationship between the first, second, and subsequent measured values of the number density of creep voids and the damage creep ratios corresponding to the above measured values.

(36) In order to approximately calculate the new creep damage evaluation curve 50 based on the measurement data, for example, regression analysis may be used. Specifically, the approximation may be performed by log approximation, polynomial approximation, a least-square method, or the like.

(37) In the creep damage evaluation curve 50 of FIG. 6, the first and second measured values of the number density of creep voids are plotted. A slight difference between the damage evaluation curve 50 and the measured values of the number density of creep voids can be confirmed.

(38) (Determination Process S40)

(39) The determination process S40 is a process of determining whether or not the new creep damage evaluation curve 50 obtained in the third process S30 is valid.

(40) For example, when the new creep damage evaluation curve 50 and the relationship between the measured values of the number density of creep voids and the creep damage ratios are out of a predetermined range, it is preferable that the first process, the second process, and the third process be further performed using the new creep damage evaluation curve 50. These processes are repeated until the obtained creep damage evaluation curve and the relationship between the first and second measured values of the number density of creep voids and the creep damage ratios are within the desired predetermined range. Then, as illustrated in FIG. 7, a creep damage evaluation curve 60 where a difference from the measured values is small is obtained (in FIG. 7, the creep damage evaluation curve 60 obtained after performing the first process, the second process, and the third process N times is illustrated).

(41) In this example, the creep damage evaluation curve 60 matches well with the relationship between the first and second measured values of the number density of creep voids and the creep damage ratios.

(42) When the new creep damage evaluation curve and the relationship between the first and second measured values of the number density of creep voids and the creep damage ratios are within the predetermined range, it is evaluated that an optimum damage evaluation curve is obtained.

(43) Regarding the determination of whether or not the creep damage evaluation curve is valid, for example, when differences between the creep damage ratios, which are calculated from the measured values of the number density of creep voids, and damage ratios of the creep damage evaluation curve are less than a predetermined ratio (for example, 5%), it is determined that an optimum damage evaluation curve is obtained. It is preferable that whether or not the creep damage evaluation curve is valid be comprehensively determined based on, for example, a past database or a safety factor of a welding heat-affected portion (target portion) of a structural member.

(44) In the creep damage evaluation method according to this embodiment, a high-accuracy new creep damage evaluation curve is obtained through the first process S10, the second process S20, and the third process S30. Since a non-dimensionalized creep damage degree is used in this creep damage evaluation index, the first measured value and the second and subsequent measured values measured at different positions can be compared with each other based on the same index indicated by the damage degree.

(45) In the determination process S40, whether or not the creep damage evaluation curve is valid can be determined. When it is determined that a creep damage evaluation curve is valid, this creep damage evaluation curve is set as the optimum creep evaluation curve.

(46) When it is determined that a creep damage evaluation curve is not valid, the first process, the second process, and the third process are repeated based on the newly obtained creep damage evaluation curve until it is determined that the newly obtained creep evaluation curve is valid. As a result, the optimum creep damage evaluation curve is obtained, and the accuracy can be further increased.

(47) Based on the creep damage evaluation curve obtained as above, the creep damage ratio can be evaluated from the measured values of the number density of creep voids with higher accuracy. As a result, the remaining lifetime can be accurately determined even in a portion, such as a welding heat-affected portion of heat-resistant steel, where creep damage is likely to occur. As a result, maintenance can be performed at an appropriate time, and the cost required for the maintenance of equipment can be reduced.

(48) In this embodiment, since the creep damage evaluation curve is used, the relationship between the number density of creep voids and the creep damage ratio is visually easily understood. The creep damage ratio can be easily and effectively determined from the creep damage evaluation curve and the measured values of the number density of creep voids.

(49) Hereinabove, the creep damage evaluation method, which is the embodiment of the present invention, of a welding heat-affected portion of a pipe of a boiler or the like has been described, but the present invention is not limited thereto. The present invention can be appropriately modified within a range not departing from the technical scope of the present invention.

(50) In the above-described embodiment, the method of evaluating a creep damage ratio has been described. However, the present invention may be applied to cases where the degree of damage, which occurs due to a temporal change, of a target portion of a structural member is evaluated, for example, where the degree of fatigue, abrasion, or corrosion of the structural member is evaluated.

(51) In the above-described embodiment, the number density of creep voids is used as the specific physical quantity. However, other specific physical quantities can be measured as long as they are specific physical quantities which change over time according to the damage degree of a target portion, for example, the hardness and elongation of a target portion, the number of creep voids and change rate at a specific portion of crystal grains, or the defect property of the inside of a sheet which can be detected through a ultrasonic inspection or a radiological inspection.

(52) In the above-described embodiment, the case where the damage evaluation curve is used has been described, but the present invention is not limited to the evaluation curve. A numerical table that shows the correlation between the specific physical quantity and the damage degree or media that is electromagnetically stored the numerical data may be used.

(53) In the above-described embodiment, the case where the pipe 1 is formed of heat-resistant steel has been described. However, the structural member is formed of any material as long as it has a target portion which is damaged according to a temporal change.

(54) In the above-described embodiment, the position at which the number density of creep voids is measured is a welding heat-affected portion of a pipe. However, a position which is damaged according to a temporal change, for example, a blade groove portion of a turbine or a base material portion of a heat-transfer pipe and a pipe may be measured.

(55) In the above-described embodiment, in the first process, the total lifetime of a welding heat-affected portion (target portion) is calculated, but the remaining lifetime thereof may be calculated.

DESCRIPTION OF REFERENCE NUMERALS

(56) 10 WELD JOINT PORTION 11 WELDING HEAT-AFFECTED PORTION (TARGET PORTION) 40, 50, 60 CREEP DAMAGE EVALUATION CURVE (DAMAGE EVALUATION INDEX)