Method for evaluating the life of welded parts

Abstract

A method for predicting the life of chromium steel welds from defect information is provided. [Solution] The method for evaluating the life of a welded portion of the present invention includes the steps of preparing a first test piece having a first defect, measuring an ultrasonic echo value A generated at the first defect and obtaining a first correlation with the size of the first defect, preparing a second test piece in which a second defect occurs during an accelerated test, repeatedly performing and interrupting an accelerated test on the second test piece and measuring an ultrasonic echo value B generated at the second defect when the test is interrupted, determining the size of the second defect corresponding to echo value B by referring to the first correlation, measuring the actual life of the second test piece, correcting the life consumption rate, and obtaining a second correlation with the size of the second defect, measuring an ultrasonic echo value C of the welded portion to be evaluated and obtaining the size of a third defect corresponding to echo value C from the first correlation, and obtaining the life consumption rate corresponding to the size of the third defect from the second correlation.

Description

[Technical field]

[0001] The present invention relates to a method for evaluating the life of a welded portion. [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 high-chromium steels used in large-diameter tubes 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 from stress concentration points within the welds. Because it is difficult to evaluate the progress of damage by inspection of the outer surface (penetrant inspection, magnetic particle inspection, replica sampling, etc.), it is expected that phased array ultrasonic inspection will be able to detect signs of damage early and enable remaining life evaluation.

[0004] Phased array ultrasonic inspection is an ultrasonic inspection method that uses multiple vibration elements and electronically controls them to control ultrasonic beams (Patent Document 1). By imaging flaw detection data, defects inside materials can be visually grasped, but since JIS standards have not been established, each company currently creates its own judgment criteria based on UT standards, 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 consideration of the above circumstances, and has an object to provide a method for evaluating the life of a welded portion that makes it possible to predict the life of a welded portion from information on defects present in the welded portion 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 a life of a welded portion according to one embodiment of the present invention is a method for evaluating a life of a welded portion in which members made of chromium steel are welded together, the method 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 transducer, measuring an echo value A generated when the ultrasonic waves are reflected at 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 pieces multiple times, aligning a positional relationship between the second test piece and the ultrasonic transducer with a positional relationship between the first test piece and the ultrasonic transducer when the accelerated test is interrupted, and then irradiating the second test piece with ultrasonic waves using the ultrasonic transducer, 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 a ratio of the test time until the accelerated test is stopped to the test time until 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 a positional relationship between the welded portion to be evaluated, which has a third defect, and the ultrasonic transducer with a positional relationship between the first test piece and the ultrasonic transducer, irradiating the welded portion with ultrasonic waves using the ultrasonic transducer, 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 with reference to the second correlation.

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

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

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

[0012] [Figure 1] 1A to 1C are diagrams for explaining a phased array ultrasonic inspection performed using a first test piece in one embodiment of the present invention. [Diagram 2] FIG. 11 is a diagram for explaining a graph showing the relationship between echo values ​​and defect size obtained by phased array ultrasonic inspection of a first test piece in the embodiment. [Diagram 3] FIG. 1 is a diagram for explaining a graph showing the relationship between the size of defects obtained by an accelerated test and the life consumption rate. [Figure 4] FIG. 2 is a cross-sectional view of a welded portion undergoing phased array ultrasonic inspection in the embodiment; [Diagram 5] 1A is a diagram for explaining a procedure for determining the size of a defect by referring to a first correlation, and FIG. 1B is a diagram for explaining a procedure for determining a life consumption rate by referring to a second correlation. [Figure 6] 1 is a graph showing a first correlation obtained in Example 1. [Figure 7] 1 is a graph showing a second correlation obtained in Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 parts in an enlarged scale for the sake of convenience in order to make the characteristics easier to understand, and the dimensional ratios of each component may not necessarily be the same as in reality. In addition, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto, and may be appropriately modified and implemented 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 in which members made of chromium steel are welded together, the life being determined by the state of creep damage inside the weld. The weld constitutes a part of a structure (piping, etc.) used at high temperatures. The chromium steel may be any alloy steel containing chromium, and is not particularly limited, but examples thereof include high chromium steel (9Cr steel, etc.) with a chromium content of 8% or more.

[0015] The term "life" here refers to the time until a macrocrack occurs, which can cause the weld to break. A macrocrack is a crack that grows larger as a result of microcracks that connect multiple voids, and the "Interpretation of Technical Standards for Thermal Power Generation Equipment" specifies criteria for the thickness of the weld. A macrocrack here refers to a crack that grows 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 made of a material having the same sound velocity as the evaluation target are prepared. The first test pieces have a first defect (void, etc.) corresponding to creep damage. The size of the first defect varies from one first test piece to another. 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 diameter and the minimum diameter), or area of ​​the first defect in a predetermined cross section of the first test piece. In this embodiment, the case where the size of the first defect is defined as the maximum diameter of the first defect is exemplified. The constituent material of the first test piece is not particularly limited, but is preferably the same as the welded part (hereinafter, may be simply referred to as the welded part) of the evaluation target.

[0018] (Second process) A first correlation between an echo value A measured by irradiating ultrasonic waves by a phased array ultrasonic inspection or the like and a size of a first defect inside the first test piece is analyzed. Note that, when a first test piece having first defects 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] 1 is a diagram for explaining a case where a phased array ultrasonic inspection is performed and an echo value A is measured. A horizontal hole test piece having a first defect 102 penetrating in one direction is used as a first test piece 101. A probe (probe, ultrasonic transducer) 103 is disposed on one surface 101a of the first test piece. A plurality of first defects 102 (four in this case) are arranged at predetermined intervals in the depth direction from the one surface 101a, and each of the first defects 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 a first test piece 101 using an ultrasonic transducer provided on a probe 103, and an echo value A generated when the ultrasonic waves 103A are reflected by a 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, for example, by using the first defect 102 located in the center of the thickness of the weld (for example, the first defect third from the top in the figure) as a reference hole, and adjust the position of the first test piece 101 so that the distance between the probe 103 and the first defect 102 with respect to the fixed reference sensitivity is the same as the positional relationship between the probe and the weld in the second test piece, the object to be evaluated.

[0022] The same measurement is performed on all 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, the measurement is performed as follows.

[0023] FIG. 2 is a diagram for explaining a graph obtained by this measurement. When the probe 103 and the first defect 102 are in the same positional relationship, if the measurement results of the size of the first defect (vertical axis) and the echo value A (horizontal axis) at the same depth are plotted, one curve (calibration curve) showing the first correlation as shown in FIG. 2 is obtained. Therefore, by measuring the echo value, the size of the defect inside the welded portion can be estimated using the calibration curve without destroying the welded portion. Note that if the positional relationship between the probe and the first defect or the first defect to be evaluated changes, a different curve will be obtained. It is preferable to obtain this curve at many positions, assuming that it will be referred to 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 welded portion to be evaluated.

[0024] 1 illustrates an example of measuring the echo value A 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 to overlap the position of the first defect with the position corresponding to the heat-affected zone.

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

[0026] (Third step) A second test piece having the same composition as the weld to be evaluated (a high chromium steel such as 9Cr steel) is prepared. 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, and finally becomes a macrocrack.

[0027] (Fourth step) For the second test piece, an accelerated test for accelerating creep damage under high temperature and high stress is performed and stopped alternately multiple times. For example, each accelerated test is performed for a time period until the life consumption rate calculated from the predicted life progresses by 10%. That is, the first accelerated test is performed while the life consumption rate of the predicted life is 0-10%, the accelerated test is temporarily stopped, and then the second accelerated test is performed while the life consumption rate of the predicted life is 10-20%, and the accelerated test is temporarily stopped, and then the third accelerated test is performed while the life consumption rate is 20-30%. In the same manner, the accelerated test is performed and stopped alternately. The execution time of the accelerated test may be set to any percentage of the life consumption rate. The number of times the accelerated test is performed and stopped may be set arbitrarily.

[0028] When the accelerated test is stopped, ultrasonic waves are irradiated to the second test piece using an ultrasonic transducer, and the echo value B generated when the ultrasonic waves are reflected by the second defect is measured. Similar to the above-mentioned echo value A, the echo value B here is also the maximum value (maximum echo value) of the reflected wave obtained by the measurement. Note that the ultrasonic waves are irradiated after the positional relationship between the second test piece and the probe (ultrasonic transducer) is aligned 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 obtained by analyzing 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 is 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 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 is a diagram showing 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) as shown in FIG. 3 is obtained. By using this curve, the life consumption rate of the welded portion can be estimated from the size of the second defect. The analysis result of the second correlation is recorded so that it can be referred to 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 "◯" position in FIG. 4 indicates the position where the first correlation obtained in the second step is created. In this embodiment, it is assumed that the third defect 105 exists near the heat-affected zone 104B on the left side, and an echo value C generated in 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 obtained 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 the definition of the first defect described above. The first correlation referred to here is preferably the one analyzed at the position corresponding to the position closest to the third defect. FIG. 5(a) is a graph showing the curve of the first correlation corresponding to the depth at which the third defect exists among the multiple first correlations obtained. The three curves respectively show the first correlation at the molten metal center (WM=0) 104A, the left heat affected zone (HAZ(L)) 104B, and the right heat affected zone (HAZ(R)) 104C shown in FIG. 4. Here, the center curve obtained at the left heat affected zone (HAZ(L)) 104B3 corresponding 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 obtained.

[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 a 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] Since the first correlation is common to welds of the same composition, the first and second steps can be omitted in the life evaluation of the second and subsequent welds of the same composition. Also, since the second correlation is common to welds of the same composition (high chromium steel (9Cr steel, etc.)), the third, fourth, fifth, and sixth steps can be omitted in the life evaluation of the second and 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 can cause fracture, it is considered unnecessary to perform a life assessment on them. Most growing defects exist in the heat-affected zone, and most non-growing defects exist in the molten metal zone. Therefore, life assessment may be performed preferentially on 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 object of evaluation in the eighth step is only grown defects.

[0038] The discrimination between grown defects and non-grown defects is carried out, for example, by comparing the echo value of the welded portion 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 part according to the present embodiment enables the creep life of a welded part of a chromium steel, which depends on the growth of defects, to be estimated at any timing, and the life consumption rate to be evaluated. The method for evaluating the life of a welded part according to the present embodiment is based on the inventor's findings that there is an inherent correlation between the echo value obtained by irradiating an ultrasonic wave on a defect, the size of the defect corresponding to the echo value, and the life consumption rate due to the growth of the defect.

[0040] According to the method for evaluating the life of a welded portion of the present embodiment, the correlation between the size of defects and the life consumption rate can be analyzed (understood) using a test piece that serves as a reference for the evaluation target, and the life consumption rate of the welded portion can be easily and accurately evaluated from the size of creep damage (defects) observed in the welded portion to be evaluated. Therefore, the progress of creep damage can be known at an early stage, and the best measures can be taken, such as by considering repair or replacement of the welded portion. EXAMPLES

[0041] The effects of the present invention will be more clearly understood by the following examples. Note that the present invention is not limited to the following examples, and can be modified as appropriate without departing from the spirit and scope of the present invention.

[0042] Example 1 According to the above embodiment, a life evaluation was performed on a welded part of chrome steel (fire-technical material: fire STPA28). Specifically, among defects present inside the welded part, a defect located at a depth of 30 mm from the surface where the probe was placed near the heat-affected zone was selected, and the life of the welded part determined by this defect was evaluated by the following procedure.

[0043] Using the first test piece as a reference for the weld, a phased array ultrasonic inspection 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 at a depth of 30 mm from the surface where the probe was placed, in the heat-affected zone on both sides (left and right) of the center of the molten metal. The values ​​on the horizontal axis of the graph are not displayed.

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

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

[0046] It was found that the life consumption rate corresponding to the size of this third defect (αmm) was 72% from the curve of the second correlation in Fig. 7. According to the method for evaluating the life of a weld of the present invention, the echo value of any defect can be measured by a similar procedure, thereby evaluating the life consumption rate of the weld due to that defect. [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 portion in which members made of chromium steel are welded together, comprising the steps of: 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 transducer, 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 a size of the first defect; a third step of preparing a second test piece having the same composition as the weld to be evaluated, the second test piece having a second defect during the accelerated test; a fourth step of alternately repeating the execution and interruption of the accelerated test on the second test piece a plurality of times, aligning a positional relationship between the second test piece and the ultrasonic transducer with a positional relationship between the first test piece and the ultrasonic transducer when the accelerated test is interrupted, irradiating the second test piece with ultrasonic waves using the ultrasonic transducer, and measuring an echo value B generated when the ultrasonic waves are reflected by the second defect; a fifth step of determining a size of the second defect corresponding to the echo value B with reference to the first correlation; a sixth step of determining a life consumption rate as a ratio of a test time until a test time until a macrocrack occurs in the second test piece to a test time until the accelerated test is interrupted, and analyzing a second correlation between the life consumption rate and the size of the second defect; a seventh step of aligning a positional relationship between the welded portion to be evaluated, which has a third defect, and the ultrasonic transducer with a positional relationship between the first test piece and the ultrasonic transducer, irradiating the welded portion with ultrasonic waves using the ultrasonic transducer, measuring an echo value C generated at the third defect, and determining a 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 span of a weld as described in claim 1, characterized in that the first correlation referenced in the fifth step and the seventh step is analyzed at a position corresponding to a position closest to the second defect and the third defect.

3. A method for evaluating the life span of a welded portion as described in any one of claims 1 and 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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