Method for evaluating service life of weld zone

Abstract

To provide a method for predicting a service life from defect information in a weld zone of chromium steel.SOLUTION: A method for evaluating a service life of a weld zone includes: a step of preparing a first test piece having a first defect; a step of measuring an ultrasonic echo value A generated at the first defect to obtain a first correlation with the size of the first defect; a step of preparing a second test piece in which a second defect is generated in the course of an accelerated test; a step of repeating execution and interruption of the accelerated test on the second test piece and measuring an ultrasonic echo value B generated at the second defect at the time of interruption; a step of determining a size of the second defect corresponding to the echo value B with reference to the first correlation; a step of measuring the actual service life of the second test piece, correcting a service life consumption rate, and determining the second correlation with the size of the second defect; a step of measuring the ultrasonic echo value C of the weld zone to be evaluated and determining a size of a third defect corresponding to the echo value C from the first correlation; and a step of determining the service life consumption rate corresponding to the size of the third defect from the second correlation.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating the life of a weld. [Background technology]

[0002] Generally, creep damage occurs in metallic materials during high-temperature use. In particular, creep damage progresses in the heat-affected zone (HAZ) of welds in the high-chromium steels used in large-diameter pipes of ultra-supercritical boilers for thermal power generation. Type IV damage, which ranges from the generation, concentration, and connection of voids at metal grain boundaries to microcracks and macrocracks, is considered important.

[0003] It is known that creep damage in high-chromium steel welds occurs in stress concentration areas within the weld. Because it is difficult to assess the progression of damage using external surface inspections (such as penetrant inspection, magnetic particle inspection, and replica sampling), phased array ultrasonic inspection is expected to be able to detect signs of damage early and enable remaining life assessment.

[0004] Phased array ultrasonic testing is an ultrasonic testing method that uses multiple vibration elements and electronically controls them to control the ultrasonic beam (Patent Document 1). By imaging the flaw detection data, defects inside the material can be visually grasped, but since JIS standards have not been established, each company currently creates its own evaluation criteria based on the UT standard, and evaluation is limited to the presence or absence of cracks. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 7013796 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for evaluating the life of a weld that makes it possible to predict the life of a weld from information on defects present in a weld of chromium steel. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention employs the following means.

[0008] (1) A method for evaluating the life of a welded portion according to one aspect of the present invention is a method for evaluating the life of a welded portion obtained by welding together members made of chromium steel, comprising: a first step of preparing a plurality of first test pieces having a predetermined first defect; a second step of irradiating the first test pieces with ultrasonic waves using an ultrasonic vibrator, measuring an echo value A generated when the ultrasonic waves are reflected by the first defect, and analyzing a first correlation between the echo value A and the size of the first defect; a third step of preparing a second test piece having the same composition as the welded portion to be evaluated and in which a second defect occurs during an accelerated test; and a third step of alternately repeating and interrupting the accelerated test on the second test piece multiple times, aligning the positional relationship between the second test piece and the ultrasonic vibrator with the positional relationship between the first test piece and the ultrasonic vibrator when the accelerated test is interrupted, and then irradiating the second test piece with ultrasonic waves using the ultrasonic vibrator, and analyzing a first correlation between the ultrasonic waves and the second defect. a fourth step of measuring an echo value B generated by reflection; a fifth step of determining the size of the second defect corresponding to the echo value B with reference to the first correlation; a sixth step of defining a life consumption rate as the ratio of the test time up to the time the accelerated test is stopped halfway to the test time up to the time when a macrocrack occurs in the second test piece, and analyzing a second correlation between the life consumption rate and the size of the second defect; a seventh step of aligning the positional relationship between the weld to be evaluated, which has a third defect, and the ultrasonic vibrator with the positional relationship between the first test piece and the ultrasonic vibrator, irradiating the weld with ultrasonic waves using the ultrasonic vibrator, measuring the echo value C generated at the third defect, and determining the size of the third defect corresponding to the echo value C with reference to the first correlation; and an eighth step of determining the life consumption rate corresponding to the size of the third defect with reference to the second correlation.

[0009] (2) In the method for evaluating the life of a welded portion described in (1) above, it is preferable that the first correlation referenced in the fifth step and the seventh step is analyzed at a position corresponding to the position closest to the second defect and the third defect.

[0010] (3) In the method for evaluating the life of a welded portion described in either (1) or (2) above, in the first step, the first test piece may be prepared in which the first defect exists at multiple positions, and in the second step, the first correlation may be analyzed for each position of the first defect. [Effects of the Invention]

[0011] According to the method for evaluating the life of a weld of the present invention, the life of a weld can be predicted from information on defects present in a weld of chromium steel. [Brief explanation of the drawings]

[0012] [Figure 1] 1A to 1C are diagrams illustrating a phased array ultrasonic inspection performed using a first test piece in an embodiment of the present invention. [Figure 2] FIG. 10 is a diagram illustrating a graph showing the relationship between echo values ​​and defect size obtained by phased array ultrasonic inspection of a first test piece in the same embodiment. [Figure 3] FIG. 10 is a diagram illustrating a graph showing the relationship between the size of defects obtained by an accelerated test and the life consumption rate. [Figure 4] FIG. 10 is a cross-sectional view of a welded portion undergoing phased array ultrasonic inspection in the same embodiment. [Figure 5] 1A is a diagram illustrating a procedure for determining the size of a defect by referring to a first correlation, and FIG. 1B is a diagram illustrating a procedure for determining a life consumption rate by referring to a second correlation. [Figure 6] 1 is a graph showing the first correlation obtained in Example 1. [Figure 7] 1 is a graph showing the second correlation obtained in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, a method for evaluating the life of a welded portion according to an embodiment of the present invention will be described in detail with reference to the drawings. Note that the drawings used in the following description may show characteristic portions enlarged for the sake of clarity, and the dimensional ratios of the components may not be the same as those in reality. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate modifications may be made within the scope of the present invention.

[0014] A method for evaluating the life of a weld according to one embodiment of the present invention is a method for evaluating the life of a weld formed by welding together members made of chromium steel, which is determined by the state of creep damage inside the weld. The weld constitutes a part of a structure (such as a pipe) used at high temperatures. The chromium steel may be any alloy steel containing chromium, and is not particularly limited. For example, high-chromium steel (such as 9Cr steel) having a chromium content of 8% or more can be used.

[0015] Here, the term "life" refers to the time until a macrocrack occurs, which can cause a weld to break. A macrocrack is a crack that grows larger as a result of microcracks formed by connecting multiple voids. The "Interpretation of Technical Standards for Thermal Power Generation Equipment" defines criteria for determining the thickness of the weld. Here, a macrocrack refers to a crack that has grown to a length of, for example, approximately 6 mm or more.

[0016] The method for evaluating the life of a welded portion according to this embodiment mainly includes the following steps.

[0017] (first step) A plurality of reference first test pieces are prepared, each made of a material having a sound velocity equivalent to that of the object to be evaluated. The first test pieces have a first defect (such as a void) corresponding to creep damage. The size of the first defect varies for each first test piece. The definition of the size of the first defect is not particularly limited, and may be, for example, the maximum diameter, average diameter (average of the maximum and minimum diameters), or area of ​​the first defect in a predetermined cross section of the first test piece. In this embodiment, an example is shown in which the size of the first defect is defined as the maximum diameter of the first defect. The constituent material of the first test piece is not particularly limited, but is preferably the same as the weld (hereinafter sometimes simply referred to as the weld) to be evaluated.

[0018] (Second process) A first correlation is analyzed between the echo value A measured by irradiating ultrasonic waves by a phased array ultrasonic inspection or the like and the size of a first defect inside the first test piece. Note that if a first test piece having first defects present at multiple positions is prepared in the first step, the first correlation may be analyzed for each position of the first defect in the second step.

[0019] FIG. 1 is a diagram illustrating a case where a phased array ultrasonic inspection is performed and an echo value A is measured. A horizontally bored test piece having a first defect 102 penetrating in one direction is used as the first test piece 101. A probe (probe, ultrasonic vibrator) 103 is disposed on one surface 101a of the first test piece. A plurality of first defects 102 (four in this example) are lined up at predetermined intervals in the depth direction from the one surface 101a, and each first defect 102 penetrates the first test piece 101 in a direction approximately parallel to the one surface 101a.

[0020] 1, ultrasonic waves 103A are irradiated onto the first test piece 101 using an ultrasonic vibrator provided in the probe 103, and an echo value A generated when the ultrasonic waves 103A are reflected by the first defect 102 is measured. The echo value A is assumed to be the maximum value (maximum echo value) of the reflected wave obtained by the measurement.

[0021] Since the measured echo value varies depending on the positional relationship between the first defect 102 and the probe 103, it is preferable to adjust the reference sensitivity using, for example, the first defect 102 located in the center of the thickness of the weld (for example, the third first defect from the top in the figure) as a reference hole, and then adjust the position of the first test piece 101 so that the distance between the probe 103 and the first defect 102 is the same as the positional relationship between the probe and the weld in the second test piece, the object to be evaluated, relative to the fixed reference sensitivity.

[0022] The same measurement is performed on all of the prepared first test pieces 101. The same measurement is also performed on other first test pieces having first defects 102 of different sizes. For each first test piece, a first correlation between the measured echo value A and the size of the first defect 102 is analyzed. Specifically, this is performed as follows.

[0023] FIG. 2 is a diagram illustrating a graph obtained by this measurement. When the probe 103 and the first defect 102 are in the same positional relationship, plotting the measurement results of the size of the first defect (vertical axis) and the echo value A (horizontal axis) at the same depth results in a curve (calibration curve) showing the first correlation as shown in FIG. 2. Therefore, by measuring the echo value, the size of the defect inside the weld can be estimated using the calibration curve without destroying the weld. Note that if the positional relationship between the probe and the first defect or the target first defect changes, a different curve will be obtained. It is preferable to obtain this curve at many positions, assuming that it will be referenced in a later process to estimate the size of the second defect in the second test piece and the size of the third defect in the weld being evaluated.

[0024] 1 illustrates an example in which the echo value A is measured at a position 102A corresponding to the center of the molten metal of the weld (the center of the portion made of weld metal). When the same measurement is performed at positions 102B and 102C corresponding to the heat-affected zone (HAZ), the first test piece 101 is moved horizontally so that the position of the first defect is superimposed on the position corresponding to the heat-affected zone.

[0025] The analysis results are recorded so that they can be referenced in later steps. This recording may be performed using a personal computer or the like.

[0026] (Third step) A second test piece is prepared that has the same composition as the weld to be evaluated (a high-chromium steel composition such as 9Cr steel). A second defect is generated in the second test piece during the accelerated test described below. This second defect grows as the accelerated test progresses, eventually leading to a macrocrack.

[0027] (Fourth step) The second test specimen is subjected to an accelerated test under high temperature and high stress conditions, which alternately begins and ends the test multiple times. For example, each accelerated test is continued until the life consumption rate calculated from the predicted life reaches 10%. That is, the first accelerated test is conducted while the life consumption rate is between 0 and 10% of the predicted life, and after the accelerated test is temporarily stopped, a second accelerated test is conducted while the life consumption rate is between 10 and 20% of the predicted life, and after the third accelerated test is temporarily stopped, a third accelerated test is conducted while the life consumption rate is between 20 and 30%. The accelerated test is repeatedly conducted and stopped in a similar manner. The duration of the accelerated test may be set to any percentage of the life consumption rate. The number of times the accelerated test is repeatedly conducted and stopped can be set arbitrarily.

[0028] When the accelerated test is stopped, ultrasonic waves are irradiated onto the second test piece using an ultrasonic vibrator, and the echo value B generated when the ultrasonic waves are reflected by the second defect is measured. Similar to the echo value A described above, the echo value B here is also the maximum value (maximum echo value) of the reflected wave obtained in the measurement. Note that the ultrasonic waves are irradiated after aligning the positional relationship between the second test piece and the probe (ultrasonic vibrator) with the positional relationship between the first test piece and the probe.

[0029] (Fifth step) The size of the second defect corresponding to the echo value B is determined by referring to the first correlation obtained in the second step (FIG. 2). The first correlation referred to here is preferably the one obtained by analyzing at a position corresponding to the position closest to the second defect.

[0030] (Sixth step) The test time when a macrocrack occurs in the second test piece and the life consumption rate reaches 100% is defined as the actual life of the second test piece, and the life consumption rate of the life predicted in the fourth step is corrected to match the actual life. For example, if the life is predicted to be 5,000 hours, the time required for the life consumption rate to progress by 10% is calculated to be 500 hours. However, if the actual life is 4,000 hours, the calculated life consumption rate of 500 hours is actually 12.5%. In this case, the life consumption rate is corrected from 10% to 12.5%.

[0031] A second correlation between the corrected life consumption rate and the size of the second defect is analyzed. FIG. 3 shows an example of a graph obtained by this analysis. When the measurement results of the size of the second defect (vertical axis) and the life consumption rate (horizontal axis) are plotted, a curve (master curve) such as that shown in FIG. 3 is obtained. By using this curve, the life consumption rate of the weld can be estimated from the size of the second defect. The analysis results of the second correlation are recorded so that they can be referenced in a later process. This recording may be performed using a personal computer or the like.

[0032] (Seventh step) A welded portion to be evaluated, which has a third defect (creep damage), is irradiated with ultrasonic waves using a phased array ultrasonic inspection or the like. FIG. 4 is a cross-sectional view of a welded portion 104 irradiated with ultrasonic waves 103A. The "◯" positions in FIG. 4 indicate the positions where the first correlation obtained in the second step is created. In this embodiment, it is assumed that the third defect 105 is located near the heat-affected zone 104B on the left side, and an echo value C generated at the third defect 105 irradiated with ultrasonic waves 103A is measured. Note that the irradiation of ultrasonic waves 103A is performed after the positional relationship between the welded portion 104 and the probe (ultrasonic transducer) 103 is aligned with the positional relationship between the first test piece 101 and the probe 103.

[0033] The size of the third defect 105 corresponding to the measured echo value C is determined by referring to the first correlation obtained in the second step. The definition of the size of the third defect 105 is the same as that of the first defect described above. The first correlation referenced here is preferably the one analyzed at the position closest to the third defect. Figure 5(a) is a graph showing curves of the first correlations corresponding to the depth at which the third defect exists, among the multiple first correlations obtained. The three curves represent the first correlations at the center of the molten metal (WM=0) 104A, the left heat-affected zone (HAZ(L)) 104B, and the right heat-affected zone (HAZ(R)) 104C shown in Figure 4, respectively. Here, the middle curve obtained at the left heat-affected zone (HAZ(L)) 104B3, which corresponds to the position closest to the third defect 105, is selected. From this curve, the size of the third defect 105 corresponding to the measured echo value C can be determined.

[0034] (Eighth process) The second correlation obtained in the sixth step is used to determine the life consumption rate corresponding to the size of the third defect 105 determined in the seventh step. Figure 5(b) is a graph showing the curve of the second correlation. From this curve, the life consumption rate corresponding to the determined size of the third defect 105 can be determined.

[0035] Because the first correlation is common to welds of the same configuration, steps 1 and 2 can be omitted when evaluating the life of the second or subsequent welds of the same configuration. Also, because the second correlation is common to welds of the same composition (high chromium steel (9Cr steel, etc.)), steps 3, 4, 5, and 6 can be omitted when evaluating the life of the second or subsequent welds of the same composition.

[0036] Defects that occur inside welds can include not only growing defects (creep damage) that grow over time, but also non-growing defects (initial defects, etc.). Since non-growing defects are not defects that cause fracture, it is considered unnecessary to assess their lifespan. Most growing defects exist in the heat-affected zone, and most non-growing defects exist in the molten metal zone. Therefore, life assessment can be given priority to tertiary defects that exist near the heat-affected zone.

[0037] It is also possible to distinguish between grown defects and non-grown defects in advance and then perform a life evaluation only on grown defects. Specifically, the seventh step further includes a step of distinguishing between grown defects and non-grown defects, and the target of evaluation in the eighth step is only grown defects.

[0038] The growing defect and the non-growing defect are distinguished, for example, by comparing the echo value of the weld to be evaluated with the echo value of a phased array ultrasonic inspection previously obtained.

[0039] As described above, the method for evaluating the life of a welded portion of this embodiment makes it possible to estimate the creep life of a chromium steel welded portion, which depends on defect growth, at any timing, and to evaluate the life consumption rate. The life evaluation method of this embodiment was obtained based on the inventor's finding that there is an inherent correlation between the echo value obtained by irradiating an ultrasonic wave on a defect, the defect size corresponding to the echo value, and the life consumption rate due to defect growth.

[0040] According to the method for evaluating the life of a weld of this embodiment, by analyzing (understanding) the correlation between the size of defects and the life consumption rate using a test piece that serves as a reference for the evaluation, the life consumption rate of the weld can be easily and accurately evaluated from the size of creep damage (defects) observed in the weld to be evaluated. Therefore, the progress of creep damage can be known early, and the best countermeasures can be taken, such as repairing or replacing the weld. [Example]

[0041] The effects of the present invention will be more clearly understood from the following examples. Note that the present invention is not limited to the following examples and can be practiced with appropriate modifications within the scope of the present invention.

[0042] Example 1 According to the above-mentioned embodiment, a lifespan evaluation of a welded portion of chromium steel (fire-technical steel: fire STPA28) was carried out. Specifically, among defects present inside the welded portion, a defect located near the heat-affected zone and 30 mm deep from the surface where the probe was placed was selected, and the lifespan of the welded portion determined by this defect was evaluated using the following procedure.

[0043] Using the first test piece, which served as a reference for the weld, phased array ultrasonic testing was performed and a first correlation analysis was performed. Figure 6 is a graph showing the curve of the first correlation obtained for the first defect located 30 mm deep from the surface where the probe was placed, in the heat-affected zone on both sides (left and right) of the center of the weld. The values ​​on the horizontal axis of the graph are not displayed.

[0044] An accelerated test was conducted using a second test piece with the same composition as the weld, and a second correlation analysis was performed. Figure 7 is a graph showing the obtained second correlation curve. The values ​​on the vertical axis of the graph are not displayed.

[0045] The measured echo value (maximum echo value) for the selected third defect in the weld was B%. From the first correlation curves in Figure 6, the first correlation curve (middle curve) corresponding to the selected third defect, it was found that the size (maximum diameter) of the third defect corresponding to the measured echo value was α mm.

[0046] From the second correlation curve in Figure 7, it was found that the life consumption rate corresponding to the size of this third defect (αmm) was 72%. According to the method for evaluating the life of a weld of the present invention, the echo value of any defect can be measured using a similar procedure, and the life consumption rate of the weld due to that defect can be evaluated. [Explanation of symbols]

[0047] 101 First test piece 101a: One side of the first test piece 102 First Defect 103 Probe (ultrasonic transducer) 103A...Ultrasonic 104... Welded section 104A... Molten metal center 104B...Heat affected zone (left side) 104C...Heat affected zone (right side) 105...Third defect

Claims

1. A method for evaluating the life of a welded joint formed by welding together members made of chromium steel, comprising: a first step of preparing a plurality of first test pieces having a predetermined first defect; a second step of irradiating the first test piece with ultrasonic waves using an ultrasonic vibrator, measuring an echo value A generated when the ultrasonic waves are reflected by the first defect, and analyzing a first correlation between the echo value A and the size of the first defect; a third step of preparing a second test piece having the same composition as the weld to be evaluated, in which a second defect occurs during the accelerated test; a fourth step of alternately repeating the accelerated test and interrupting it multiple times on the second test piece, aligning the positional relationship between the second test piece and the ultrasonic vibrator with the positional relationship between the first test piece and the ultrasonic vibrator when the accelerated test is interrupted, irradiating the second test piece with ultrasonic waves using the ultrasonic vibrator, and measuring an echo value B generated when the ultrasonic waves are reflected by the second defect; a fifth step of determining the size of the second defect corresponding to the echo value B by referring to the first correlation; a sixth step of defining a life consumption rate as a ratio of a test time up to the time when the accelerated test is stopped to a test time up to the time when a macrocrack occurs in the second test piece, and analyzing a second correlation between the life consumption rate and the size of the second defect; a seventh step of aligning the positional relationship between the welded portion to be evaluated, which has a third defect, and the ultrasonic vibrator with the positional relationship between the first test piece and the ultrasonic vibrator, irradiating the welded portion with ultrasonic waves using the ultrasonic vibrator, measuring an echo value C generated at the third defect, and determining the size of the third defect corresponding to the echo value C with reference to the first correlation; and an eighth step of determining a life consumption rate corresponding to the size of the third defect by referring to the second correlation.

2. The method for evaluating the life of a weld as described in claim 1, characterized in that the first correlation referenced in the fifth step and the seventh step is obtained by analyzing at a position corresponding to a position closest to the second defect and the third defect.

3. 3. A method for evaluating the life of a weld as described in claim 1 or 2, characterized in that in the first step, the first test piece is prepared in which the first defect exists at multiple positions, and in the second step, the first correlation is analyzed for each position of the first defect.

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