Method for evaluating life of welded zone

Abstract

To provide a method for predicting life from information on a softened structure in molten metal in a welded zone of chrome steel.SOLUTION: A method for evaluating a life of a welded zone includes the steps of: providing a first welding part having a first softened structure; alternately repeating execution and halfway stopping of an acceleration test on the first welded zone multiple times, applying an ultrasonic wave to the first welded zone and measuring an echo value that is generated before starting the acceleration test and at each halfway stopping, and calculating an amount of increase A in the echo value; analyzing a correlation between the amount of increase A in the echo value and a life consumption rate at each halfway stopping using a ratio of a test time to a halfway stopping time of the acceleration test to a test time to an occurrence of a microscopic fracture in the first welded zone as the life consumption rate; applying an ultrasonic wave to a second welded zone having a second softened structure and measuring an echo value that occurs, and calculating an amount of increase B from the echo value measured in an early stage of the life of the second welded zone; and calculating a consumption rate of a life time corresponding to the amount of increase B from the correlation.SELECTED DRAWING: Figure 3

Description

[Technical Field]

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

[0002] Generally, metallic materials undergo creep damage during high-temperature use. In particular, Type IV damage in the heat-affected zone (HAZ) of welds is considered important in the high-chromium steels used in large-diameter pipes of ultra-supercritical boilers for thermal power generation. However, in recent years, coarsened ferrite structures have been confirmed in high-chromium steel weld metals that have been used for nearly 100,000 hours in actual plants. This ferrite structure is significantly softer than the other weld metals and the base material, and is therefore called a softened structure.

[0003] The softened structure has the characteristic that the average grain size increases and the hardness decreases as creep damage progresses. It is known that welds with softened structure have a shorter creep rupture time than welds without softened structure, and since this cannot be evaluated using conventional HAZ damage evaluation, a new evaluation method is required.

[0004] Phased array ultrasonic testing is an ultrasonic testing method that uses multiple ultrasonic 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. 7252918 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 enables evaluation of the life of a weld caused by softened structures 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 weld according to one aspect of the present invention is a method for evaluating the life of a weld formed by welding together members made of chromium steel, the method comprising: a first step of preparing a reference first welded portion having a first softened structure in a weld metal portion; a second step of alternately repeating an accelerated test and interrupting it a plurality of times on the first welded portion, irradiating the first welded portion with ultrasonic waves to generate echo values, measuring the echo values ​​before the start of the accelerated test on the first welded portion and at each interruption, and calculating an increase A in the echo value at each interruption relative to the echo value before the start; The method includes a third step of analyzing the correlation between the increase A in the echo value and the life consumption rate at each of the interruptions, where the ratio of the test time up to the time when the accelerated test is stopped to the test time up to the time when a macrocrack occurs is defined as a life consumption rate; a fourth step of measuring the echo value generated by irradiating an ultrasonic wave to a second welded portion to be evaluated, which has a second softened structure in a welded metal portion, and calculating an increase B from the echo value measured at an early stage of the life of the second welded portion; and a fifth step of determining the life consumption rate corresponding to the increase B in the echo value by referring to the correlation.

[0009] (2) In the method for evaluating the life of a welded portion described above in (1), it is preferable that the echo value measured in the early stage of the life of the second welded portion is the echo value measured during a period in which the life consumption rate is less than 30%. [Effects of the Invention]

[0010] According to the method for evaluating the life of a weld of the present invention, it is possible to provide a method for evaluating the life of a weld that enables evaluation of the life of a weld caused by softened structures in a weld of chromium steel. [Brief explanation of the drawings]

[0011] [Figure 1] 1A and 1B are diagrams illustrating a welded portion used in an embodiment of the present invention and a phased array ultrasonic inspection performed on the welded portion. [Figure 2] 10 is an example of an image of a cross section of a weld in one embodiment of the present invention. [Figure 3] FIG. 10 is a diagram illustrating a graph showing the relationship between the increase in echo value and the life consumption rate obtained from an accelerated test of the welded portion of the embodiment. [Figure 4] 10 is a graph showing the correlation between the increase in echo value and the consumption rate of the life time. DETAILED DESCRIPTION OF THE INVENTION

[0012] 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 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. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate changes can be made within the scope of the present invention.

[0013] 1 is a diagram illustrating a welded portion 101 (a first welded portion 101A for reference and a second welded portion 101B to be evaluated) used in one embodiment of the present invention, and a phased array ultrasonic inspection performed on the welded portion 101. An ultrasonic transducer (probe) 106 is arranged on one surface 101a of the welded portion 101.

[0014] The life evaluation method according to this embodiment is a method for evaluating the life of a welded portion 101 formed by welding together members made of chromium steel, the life being determined by the state of a softened structure (ferritic structure) 105 formed in the welded portion 103, of which the welded portion 102 is made up of a welded portion 103 and a heat-affected zone 104. The welded portion 101 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 having a chromium content of 8% or more.

[0015] Here, the term "life" refers to the time until a macrocrack occurs, which can cause the weld 102 to fracture. A macrocrack occurs when multiple particles that make up the softened structure 105 grow larger and soften, causing the crack to form. The "Interpretation of Technical Standards for Thermal Power Generation Equipment" defines criteria based on the thickness of the weld. Here, the term 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 the welded portion 102 according to this embodiment mainly includes the following steps.

[0017] (first step) A plurality of first welded regions 101A are prepared as references for the second welded region 101B to be evaluated. Each of the plurality of first welded regions 101A has a first softened structure 105A in the welded metal portion 102A. The first softened structure 105A is a locally softened structure caused by creep damage, and softening progresses as creep damage progresses, causing cracks to occur. Figure 2 is an example of a cross-sectional image of a welded region having a softened structure.

[0018] The size of the first softened tissue 105A varies depending on the first welded region 101A and the position of the first softened tissue 105A. The definition of the size of the first softened tissue 105A is not particularly limited, and may be, for example, the maximum diameter or average diameter (average of the maximum diameter and the minimum diameter) of the constituent particles of the first softened tissue 105A in a predetermined cross section of the first welded region 101A, or the area of ​​the first softened tissue 105A. The constituent material of the first welded region 101A is not particularly limited, but is preferably the same as that of the second welded region 101B.

[0019] The definition of the hardness of the first softened tissue 105A is not particularly limited, and may be, for example, Vickers hardness. For example, a region having a Vickers hardness of 190 HV or less may be defined as the first softened tissue 105A.

[0020] (Second process) An accelerated test for accelerating creep damage under high temperature and high stress conditions is repeatedly performed and interrupted multiple times on the first welded portion 101A. For example, each accelerated test is performed for a period of time until the life consumption rate calculated from the predicted life reaches 10%. That is, a first accelerated test is performed while the life consumption rate of the predicted life is between 0 and 10%, and after the accelerated test is temporarily interrupted, a second accelerated test is performed while the life consumption rate of the predicted life is between 10 and 20%, and after the first accelerated test is temporarily interrupted, a third accelerated test is performed while the life consumption rate is between 20 and 30%. Similarly, the accelerated test is alternately performed and interrupted. 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 performed and interrupted may be set as desired.

[0021] As shown in Figure 1, ultrasonic transducer 106 is used to irradiate first welding area 101A with ultrasonic waves 106S, and the echo values ​​generated when ultrasonic waves 106S are reflected by first softened tissue 105A are measured before the start of an accelerated test of first welding area 101A (described later) and at each stop. The echo value here refers to the maximum value of the reflected wave obtained in the measurement (maximum echo value). From the measured echo values, an increase A in the echo value at each stop of the accelerated test relative to the echo value before the start of the accelerated test is calculated.

[0022] Here, "before the start of the accelerated test" refers to the early stage of the lifespan when softened tissue is present but has barely grown, specifically the period when the lifespan consumption rate is less than 30%. In other words, the echo value measured before the start of the accelerated test is the echo value measured during the period when the lifespan consumption rate is less than 30%.

[0023] (Third step) The test time when a macrocrack occurs in the welded portion 102A and the life consumption rate reaches 100% is defined as the actual life of the first welded portion 101A, and the life consumption rate of the life predicted in the second 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%.

[0024] The correlation between the echo value increase A and the corrected life consumption rate at each interruption is analyzed (graphed). Figure 3 is an example of a graph showing the analysis results. The horizontal axis of the graph represents the life consumption rate (%), and the vertical axis of the graph represents the echo value increase. As shown in this graph, if the three echo value increase amounts E1, E2, and E3 are known, the corresponding life consumption rates R1, R2, and R3 can be calculated.

[0025] (Fourth step) 1, ultrasonic waves 106S are irradiated to a second welded portion 101B to be evaluated, which has second softened tissue 105B, using an ultrasonic transducer 106 such as a phased array ultrasonic tester. The echo value generated when the ultrasonic waves 106S are reflected by the second softened tissue 105B is measured, and an increase B from the echo value measured at an early stage of the life of the second welded portion 101B is calculated.

[0026] The echo value measured in the early stage of the life of the second welded portion 101B is defined as an echo value measured during a period when the life consumption rate is less than 30%.

[0027] The size and hardness of second softened tissue 105B can be defined in the same way as the size and hardness of first softened tissue 105A.

[0028] (Fifth step) The correlation between the echo value increase amount A obtained in the third step and the life consumption rate is referenced to determine the life consumption rate corresponding to the measured echo value increase amount B. The life consumption rate can be used to evaluate the life of the second welded portion 101B.

[0029] As described above, the method for evaluating the life of a welded part of this embodiment makes it possible to estimate the creep life of a chromium steel welded part, which depends on the growth of softened tissue, at any timing and evaluate the life consumption rate. The method for evaluating the life of a welded part of this embodiment focuses on the fact that the echo value obtained by ultrasonic irradiation increases as the softened tissue grows, and is based on the inventor's finding that there is an inherent correlation between the echo value obtained from the softened tissue at the time of evaluation relative to the echo value obtained from the softened tissue in the early stage of the life of the welded part and the life consumption rate.

[0030] According to the method for evaluating the life of a weld of this embodiment, by analyzing (understanding) the correlation between the increase in the echo value of ultrasonic waves accompanying the growth of softened tissue and the life consumption rate using a reference weld, the life consumption rate of the weld can be easily and accurately evaluated from the echo value measured on a weld of equivalent configuration. Therefore, the progress of creep damage can be known at an early stage, and the best countermeasures can be taken, such as repairing or replacing the weld. [Example]

[0031] 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.

[0032] Example 1 In accordance with the above embodiment, the second welded portion of chromium steel (fire engineering material: fire STPA28) was subjected to an evaluation of the lifespan related to softened structure by the following procedure.

[0033] When an image showing the echo values ​​measured at the weld of the second welded portion at the time of evaluation, which was the initial stage of the life of the second welded portion, was examined, it was found that softened tissue that was not present in the initial stage had developed in some areas at the time of evaluation.

[0034] Multiple reference first welded regions were prepared, each with a similar structure to the second welded region and an early-stage softened structure (first step). An accelerated test was performed using the prepared first welded regions. Ultrasonic waves were irradiated onto the molten metal of the first welded region, and the generated echo values ​​were measured before the accelerated test began and at each stop. The increase A in the echo value at each stop relative to the echo value before the test was calculated (second step). The life consumption rate was calculated at each stop of the accelerated test, and the correlation between the increase A in the echo value and the life consumption rate was analyzed (third step). Figure 4 is a graph showing the correlation obtained from this analysis. The values ​​on the vertical axis of the graph are not displayed.

[0035] At the time of evaluation, the echo value generated by irradiating the second welded portion to be evaluated with ultrasonic waves was A1%. The increase from the echo value A0% measured at the initial stage of the life of the second welded portion was ΔA% (= A1 - A0 (%)) (fourth step). By referring to the correlation graph shown in Figure 4, the life consumption rate B when the increase in echo value is ΔA% can be determined to be 87% (fifth step). According to the method for evaluating the life of a welded portion of the present invention, the increase in echo value for any softened structure can be measured using a similar procedure, and the life consumption rate of the welded portion due to that softened structure can be evaluated. [Explanation of symbols]

[0036] 101... Welded part 101A...First welding area 101B: Second welding area 101a... One side of the welded area 102 Welded section 102A First weld 102B Second weld 103 Welding section 104...Heat-affected zone 105...soft tissue 105A...First softening tissue 105B...Second softened tissue 106 Ultrasonic vibrator 106S···Ultrasonic

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 reference first welded portion having a first softened structure in a weld metal portion; a second step of alternately repeating execution and interruption of an accelerated test on the first welding portion a plurality of times, measuring an echo value generated by irradiating the first welding portion with ultrasonic waves before the start of the accelerated test on the first welding portion and at each interruption, and calculating an increase A in the echo value at each interruption relative to the echo value before the start; a third step of defining a life consumption rate as a ratio of the test time up to the time when the accelerated test is stopped midway to the test time up to the time when a macrocrack occurs in the first welded portion, and analyzing the correlation between the increase A in the echo value and the life consumption rate at each of the time when the test is stopped midway; a fourth step of measuring an echo value generated by irradiating an ultrasonic wave to a second welded portion to be evaluated, the second welded portion having a second softened structure in a welded metal portion, and calculating an increase B from the echo value measured at an early stage of the life of the second welded portion; and a fifth step of determining a life consumption rate corresponding to the increase B in the echo value by referring to the correlation.

2. 2. A method for evaluating the life of a weld as described in claim 1, wherein the echo value measured in the early stage of the life of the second welded portion is the echo value measured during a period in which the life consumption rate is less than 30%.

Images