JP2021004787A - Damage risk evaluation method, system maintenance management method and risk evaluation device - Google Patents

Abstract

To provide a damage risk evaluation method and evaluation device for evaluating an occurrence risk of damaging a fusion part, and a system maintenance management method for preventive maintenance of damage of a fusion part, and a risk evaluation device.SOLUTION: The present disclosure provides a method for evaluating a risk of damaging a welded part of a dissimilar joint in which members of different metal materials are welded together, comprising executing statistical analysis using explanatory variables that include factors that affect the tissue composition of a heat-affected part, and evaluating the risk of damage occurrence based on the result of the statistical analysis.SELECTED DRAWING: Figure 4

Description

The present disclosure relates to damage risk assessment methods, system maintenance methods and risk assessment devices.

Heat transfer tubes such as superheaters and reheaters used for boilers and the like are used in a high temperature and high pressure environment for a long period of time. Therefore, a heat-resistant member having excellent high-temperature strength is used for the heat transfer tube. The heat-resistant member is low alloy steel, stainless steel, or the like, and is used properly according to the temperature and pressure at which the heat transfer tube is used. Further, in some heat transfer tubes and the like, a region where the material changes in the middle is provided according to the usage environment, and a dissimilar material joint exists at the joint where the material changes. A large number of dissimilar joints are present in superheaters and reheaters of boilers for thermal power generation.

In the dissimilar joint portion of the heat transfer tube, damage such as thermal fatigue cracking, creep damage, and fusion portion damage may occur due to high temperature and long-term use depending on the usage environment. Creep damage is a phenomenon in which deformation progresses due to load stress and time during high-temperature use, or pores (voids) are generated at the metal grain boundaries and eventually rupture.

Dissimilar joints are inspected for damage on a regular basis or according to operating conditions, and as part of maintenance management, heat transfer tubes are repaired as necessary based on the inspection results, and the remaining life is diagnosed. ..

Patent Documents 1 and 2 disclose a life diagnosis method for thermal fatigue cracking. Patent Document 3 discloses a method for evaluating the growth of cracks generated in a welded portion of a dissimilar joint. Patent Document 4 discloses a method for determining a lifespan for creep damage.

JP-A-2003-90506 Japanese Unexamined Patent Publication No. 2003-130789 JP-A-2015-190950 Japanese Unexamined Patent Publication No. 2012-173217

Takuya Yamashita and 2 others, "Analysis of Mechanical Factors of Interfacial Fracture of Dissimilar Welded Joints", Journal of Steel Association "Iron and Steel", 2019 Vol. 105, No. 1, p. 96-104

In order to diagnose the remaining life at the fusion site and perform preventive maintenance of the fusion site damage, it is indispensable to elucidate the factors affecting the main damage at the fusion site and the damage mechanism. However, as described in Non-Patent Document 1, the influencing factors and mechanisms of fusional damage occurring in dissimilar joints have not been elucidated.

The techniques described in Patent Documents 1 to 4 are intended for thermal fatigue cracking and creep damage, and the risk of fusion portion damage cannot be predicted. Further, since the method of Patent Document 3 evaluates the life after cracks occur, it is not effective for preventive maintenance of the fused portion.

In heat exchangers and the like of power plants, there are many dissimilar joint parts. If the risk of fusion damage cannot be predicted, all dissimilar joints must be inspected, which is inefficient. Since 100% inspection takes a lot of time, 100% inspection may be difficult depending on the inspection period. In that case, sampling inspection will be selected, but it is difficult to appropriately select the part to be inspected with priority.

The present disclosure has been made in view of such circumstances, and is a damage risk evaluation method for evaluating the risk of occurrence of fusion part damage, a system maintenance management method for preventive maintenance of fusion part damage, and risks. It is an object of the present invention to provide an evaluation device.

In order to solve the above problems, the damage risk evaluation method, the system maintenance management method, and the risk evaluation device of the present disclosure employ the following means.

The present disclosure is a method for evaluating the risk of damage to a welded portion of a dissimilar joint in which members of different metal materials are welded together, and statistical analysis is performed using explanatory variables including factors that affect the structural composition of the heat-affected zone. Provided is a damage risk evaluation method for evaluating the risk of damage occurrence based on the result of the statistical analysis.

The member of the different metal material may be a tubular member through which a fluid can flow.

Factors that affect the structure of the heat-affected zone may include at least the wall thickness of the members, the number of welding passes for welding joints between the members, and / or welding conditions.

The present disclosure is a risk evaluation device for evaluating the risk of damage to the welded portion of a dissimilar joint in which members of different metal materials are welded together, and statistics using factors that affect the structural composition of the heat-affected portion as explanatory variables. Provided is a risk evaluation device including an analysis unit for performing analysis and a determination unit for determining the risk of damage to the welded portion based on the analysis result of the analysis unit.

As a result of diligent studies, the present inventors have found that in the welded portion of a dissimilar joint in which members of different metal materials are welded together, the structural composition of the heat-affected zone is related to the occurrence of damage to the fused portion. .. In this disclosure, by incorporating factors that affect the organizational structure of the heat-affected zone as explanatory variables, highly accurate risk evaluation becomes possible. By inputting the above factors and performing statistical analysis for each specification (design conditions, usage conditions, etc.) of dissimilar joints, even if dissimilar joints are present at many positions, the positions to be inspected preferentially are narrowed down. I can put it in. As a result, the position of the inspection target can be narrowed down and the number of dissimilar joints to be inspected can be reduced, and the inspection time and cost can be reduced while maintaining the reliability of the equipment.

The present disclosure is a maintenance management method for a system in which a plurality of welded portions of dissimilar joints are present. The risk of damage is evaluated by the damage risk evaluation method described in any of the above, and the inspection position is limited based on the evaluation result. To provide a maintenance management method for the system that determines the inspection target.

By evaluating the risk of damage occurrence by the damage risk evaluation method described above, it is possible to efficiently select the dissimilar joint portion that requires inspection. Further, the system maintenance management method disclosed above is effective for preventive maintenance of damage to the fusion portion.

In one aspect of the above disclosure, at least a part of the welded portion to be inspected is non-destructively inspected, and the result of the non-destructive inspection is compared with a preset replacement reference value to determine whether or not replacement is necessary. Can be judged.

Non-destructive inspection improves the accuracy of diagnosis of remaining life and improves the accuracy of replacement judgment.

In one aspect of the above disclosure, the results of the damage assessment and the non-destructive inspection are accumulated, the risk of damage is statistically evaluated based on the database obtained by the accumulation, and the next inspection target is determined. You may.

Since risk evaluation based on past results is possible, evaluation accuracy is improved.

According to the present disclosure, the risk of fusion site damage can be evaluated from the statistical analysis results. This enables preventive maintenance.

It is the schematic of the superheater of the boiler for thermal power generation. It is an enlarged view of the joint part of FIG. It is a flow chart of the maintenance management method which concerns on 1st Embodiment. It is a flow chart of the risk evaluation method which concerns on 1st Embodiment. It is a figure which shows an example of an input data set. It is a figure which shows the relationship between the design temperature and the axial stress by an internal pressure. It is a figure which shows the relationship between the use time and the ratio of outer diameter / wall thickness. It is a block diagram of the schematic structure of a risk evaluation device. It is a functional block diagram which expanded and showed the function which a risk evaluation device has. It is a flow chart of the maintenance management method which concerns on 2nd Embodiment. It is a flow chart of the modification of the maintenance management method of FIG. It is a flow chart of the maintenance management method which concerns on 3rd Embodiment. It is a conceptual picture of a cross-sectional photograph of a (thick-walled) joint in a dissimilar joint. It is a conceptual picture of a cross-sectional photograph of a (thin-walled) joint in a dissimilar joint. It is a thermal history image figure at the time of TIG welding in a dissimilar material joint. It is an image diagram of the thermal history and the structure change of the first half pass part of the thick-walled welded part. It is an image diagram of the thermal history and the microstructure change of the latter half pass part of the thick-walled welded part. It is an image diagram of the thermal history and the microstructure change of the thin-walled welded part. It is a figure which shows the FEM analysis result of the dissimilar material joint. It is sectional drawing which shows the case where the axial stress acts on the joint part of a thick-walled system. It is a distribution diagram of the axial stress of the YY cross section of FIG. It is a partial schematic view of the welded part where the microscopic crack occurred.

A preferred embodiment according to the present disclosure will be described with reference to the drawings. It should be noted that the present disclosure is not limited by this embodiment, and when there are a plurality of embodiments, those which are configured by combining each embodiment are also included.

The damage risk evaluation method, evaluation device, and maintenance management method according to the present disclosure evaluate damage to joints, especially fusions (fusion damage) in dissimilar joints formed by welding members of different metal materials. -Target for maintenance. In this embodiment, the members of different metal materials are, for example, tubular members. A fluid such as steam can flow inside the tubular member. Such tubular members are, for example, heat transfer tubes of heat exchangers and pipes connecting various devices.

The members of different metal materials according to the present disclosure are not limited to tubular members, and may be applied to structural members.

First, prior to the explanation of the damage risk evaluation and the maintenance management method, the dissimilar joints to be evaluated and maintained will be explained.

FIG. 1 shows a schematic view of a superheater of a boiler for thermal power generation as an example of the present embodiment. In the superheater 1 of FIG. 1, a plurality of heat transfer tubes 3 are connected in parallel via a steam header 2. The heat transfer tube 3 includes a low temperature tube (tubular member) 5 in the low temperature section 4 and a high temperature tube (tubular member) 7 in the high temperature section 6. The low temperature section 4 is a region such as the steam inlet side. The high temperature portion 6 is a region on the wake side of the steam flow or a region that receives radiation from the fireplace. Since the temperature of the outer peripheral surface of the heat transfer tube 3 is not the same as the temperature of the steam flowing inside the heat transfer tube 3 in the region where the heat transfer tube 3 is exposed to radiation, the temperature of the steam flowing inside the heat transfer tube 3 is the temperature of the steam flowing through the low temperature tube 5. There may be a portion where the temperature is equal to or higher than the steam temperature flowing through the high temperature pipe 7.

The low temperature tube 5 and the high temperature tube 7 are made of different metal materials. In the present embodiment, for example, the low temperature pipe 5 is made of low alloy steel such as low chromium steel such as 1Cr steel. The high temperature tube 7 is made of stainless steel such as nickel / chromium-containing steel. The low temperature pipe 5 and the high temperature pipe 7 are joined by welding. The joint portion is called a joint portion 8. The welding temperature is, for example, about 2000 ° C. The low temperature pipe 5 is not limited to low alloy steel, and may be carbon steel or the like.

FIG. 2 shows an enlarged view of the joint portion 8 of FIG. The joint portion 8 includes a base material of the low temperature pipe 5 and the high temperature pipe 7, and a welded portion 9. The welded portion 9 is a general term for a portion including the weld metal 10 and the heat-affected zone (HAZ) 11. The weld metal 10 is a metal having a material different from that of the low temperature pipe 5 and the high temperature pipe 7. The weld metal 10 is a high nickel-based alloy such as a solid solution reinforced nickel-based alloy. At the interface between the weld metal 10 and the base metal of the low temperature pipe 5 and the high temperature pipe 7, the weld metal 10 and the base metal are fused, and this is called a fusion portion (not shown). The fusion portion is a region from the interface between different materials, which is the boundary surface between the weld metal 10 and the base material of a different material, to the base material side of several μm. The fusion part is a part of HAZ11, and the fusion part in the low temperature tube 5 exists in the decarburization layer described later.

HAZ11 is an altered portion in the base metal formed by undergoing a thermal cycle by welding. Although not shown in FIG. 2, the HAZ 11 in the low alloy steel low temperature tube 5 includes a coarse grain region and a fine grain region (see FIGS. 11 and 12 described later). The coarse grain region of the low temperature tube 5 is a structure in which the particle size of the former austenite is larger than the particle size of the parent phase. The fine grain region is a structure in which the particle size of the former austenite is smaller than the particle size of the matrix.

Further, although not shown in FIG. 2, a decarburized layer (see FIGS. 11 and 12 described later) is present in the vicinity of the interface between different materials in HAZ11 in the low alloy steel low temperature pipe 5. The decarburized layer is, for example, a ferrite single-phase region having a width of about 20 μm in the pipe axis direction of the HAZ11 of the low temperature pipe 5, and is a part of the HAZ11. Since the content of Cr (chromium), which is a carbide-forming element, is higher in the weld metal 10 than in the base metal of the low temperature tube 5, C (carbon) moves from the base metal side to the weld metal 10 and is contained in the weld metal 10. Cr carbide is formed in. As a result, in the base metal, a decarburized layer is formed in the portion where C has moved. Further, due to the heat input of welding, atoms (Cr and the like) contained in the weld metal 10 are diffused to the base metal side. Cr and the like diffused in the base material of the low temperature tube 5 are combined with C. As a result, a row of precipitates is generated, and the amount of carbon in the base metal is reduced, so that a decarburized layer is formed. The decarburized layer is often formed from the central portion of the pipe thickness of the low temperature pipe 5 to the inner surface of the pipe. The fusion part basically exists in the decarburized layer.

[First Embodiment]
(Maintenance management method)
FIG. 3 shows a flow chart of the maintenance management method according to the present embodiment. In the maintenance management method according to the present embodiment, first, the risk of damage occurrence is evaluated based on the result of statistical analysis based on the mechanism of fusion part damage. Next, the inspection target (position) is narrowed down based on the evaluation result, and the narrowed down (position) inspection target is preferentially inspected.

Conventionally, the mechanism of fusion damage has been unknown. As a result of diligent studies, the present inventors have found factors that influence the damage mechanism. In the fusion portion of the low-alloy steel low-temperature pipe 5 of the dissimilar joint, the difference in heat history received during welding changes the structural composition of HAZ. Differences in tissue composition lead to stress differences acting on different material interfaces, ultimately creating a difference in damage risk. By performing statistical analysis based on such a mechanism, the risk of damage at the fusion site can be evaluated. In addition, the inspection efficiency can be improved because the inspection target can be narrowed down by using the result of the risk evaluation.

(Risk evaluation method)
FIG. 4 illustrates a flow chart of the risk evaluation method according to the present embodiment.

(S1) First, the actual machine data of the dissimilar joint of the heat transfer tube is input to the risk evaluation database as an explanatory variable.

The explanatory variables include at least factors that influence the tissue composition of HAZ (hereinafter, influence factors). "Affecting the tissue composition" includes at least changing the arrangement of the coarse grain area and the fine grain area. Changes in the arrangement of coarse and fine granules affect the increase or decrease in the risk of fusion damage. The reason for this will be explained in detail later. The arrangement of the coarse grain area and the fine grain area changes depending on the heat history such as the heat input state at the time of welding.

As described above, the explanatory variables include at least an influencing factor, and the influencing factors are welding conditions such as wall thickness, number of welding passes, heat input and speed, and the like. Here, "thickness" without a modifier means the nominal wall thickness of the tubular member. Further, in relation to the wall thickness, for example, outer diameter / wall thickness and (wall thickness-minimum required wall thickness) / wall thickness may be included in the influencing factors. The "minimum required wall thickness" is the wall thickness required for the internal pressure from the specifications (material, outer diameter, design temperature, etc.) of the heat transfer tube.

As mentioned above, the explanatory variables may include at least the influencing factors, as well as the conditions of use of the heat transfer tube and actual damage cases. Specifically, whether it is a heated part or a non-heated part, whether or not there is a past repair welding history, the operation start time of the boiler, the acting stress, the design temperature of the heat transfer tube, the material of the heat transfer tube, the usage time, and the past inspection history. It may include the presence or absence of. The "working stress" is the axial stress due to the internal pressure.

FIG. 5 shows an example of an input data set. In the figure, t is the wall thickness, d is the outer diameter, and tsr is the minimum required wall thickness (thickness shell requirement). The wall thickness, the ratio of outer diameter d / wall thickness t, and the ratio of (wall thickness t-minimum required wall thickness tsr) / wall thickness t affect the difference in the structure of HAZ.

As for the axial stress due to the internal pressure, the magnitude of the base stress affects the damage rate. Whether it is a heated part / a non-heated part (whether it is a heated part or a non-heated part) affects the magnitude of the bending stress. The heating unit is a superheater or reheater that is a device for superheating steam, and is located in a fireplace. The non-heating part is a pipe base part outside the fireplace called a penthouse, a housing, etc., and does not have a function of overheating steam.

According to the results of the present inventors organizing the damage cases of the actual boiler machine, there are many cases where the damage to the fusion part is accelerated at the place where the repair welding is performed in the past. Therefore, the presence or absence of past repair welding history is considered as an accelerating factor for damage.

The objective variable is the presence or absence of fusion damage (bond peeling).

(S2) Next, a statistical analysis based on the mechanism of fusion damage is performed using explanatory variables including at least influencing factors.

For statistical analysis, machine learning such as random forest and support vector machine can be used.

(S3) Finally, the risk of damage at the fusion site is evaluated based on the results of statistical analysis.

Figures 6 and 7 show the results of the inventors arranging the damage cases of the actual boiler in terms of the relationship between the design temperature and the axial stress due to the internal pressure, and the relationship between the usage time and the outer diameter / wall thickness ratio. .. In FIG. 6, the horizontal axis represents the design temperature (° C.) and the vertical axis represents the axial stress (MPa) due to the internal pressure. In FIG. 7, the horizontal axis is the usage time (hr), and the vertical axis is the ratio of outer diameter d / wall thickness t.

According to FIGS. 6 and 7, no clear correlation was found between general design conditions such as stress, temperature, outer diameter, wall thickness and usage time and fusion damage, and the simple linear regression model shows the fusion. The risk of damage in the area could not be evaluated. Therefore, in order to evaluate the risk of fusion damage, it is important to perform statistical analysis using explanatory variables that include at least influencing factors.

(Risk evaluation device)
FIG. 8 shows a block diagram of a schematic configuration of a risk evaluation device for executing the above risk evaluation method. The risk evaluation device is a computer system (computer system). The risk evaluation device includes a CPU 21, a storage unit 22 for storing programs executed by the CPU 21, a main memory 23 that functions as a work area when each program is executed, a communication unit 24 for connecting to a network, a keyboard, a mouse, and the like. It includes an input unit 25 including an input unit 25 and a display unit 26 including a liquid crystal display device for displaying data. Each of these parts is connected via, for example, a bus 27. Examples of the storage unit 22 include magnetic disks, magneto-optical disks, semiconductor memories, and the like.

FIG. 9 is a functional block diagram showing the functions of the risk evaluation device in an expanded manner. The risk evaluation device includes an analysis unit 28 that performs statistical analysis using at least an influential factor as an explanatory variable, and a determination unit 29 that determines the risk of damage occurrence in the fusion unit based on the analysis result by the analysis unit. The processing realized by each unit shown in FIG. 9 is realized by the CPU 21 reading the evaluation program stored in the storage unit 22 into the main memory 23 and executing it.

[Second Embodiment]
The maintenance management method according to the present embodiment is different from the first embodiment in that it further includes a step of determining whether or not the joint portion of a tubular member such as a heat transfer tube needs to be replaced by a different material joint.

FIG. 10 shows a flow chart of the maintenance management method according to the present embodiment.
First, as in the first embodiment, the risk of damage occurrence is evaluated based on the statistical analysis result based on the mechanism, the inspection target is narrowed down based on the evaluation result, and the narrowed inspection target is preferentially inspected.

The inspection is carried out by a non-destructive inspection method such as an ultrasonic flaw detection inspection method (UT: Ultrasonic Testing). The UT can detect scratches by appropriately determining the flaw detection sensitivity and the scratch detection reference value.

Next, the result of the non-destructive inspection is compared with the replacement reference value to determine whether or not the joint portion needs to be replaced by a dissimilar joint. The replacement reference value is set in advance by a preliminary test or the like. If the result of non-destructive inspection is equal to or higher than the replacement standard value, replacement will be carried out, and if it is less than the replacement standard value, it will be monitored over time.

According to the maintenance management method according to the present embodiment, since the inspection target is narrowed down by the risk evaluation, it is possible to efficiently select a heat transfer tube that needs to be inspected from, for example, a large number of heat transfer tube groups existing as tubular members.

(Modification example)
FIG. 11 shows a flow chart of a modified example of the maintenance management method of FIG.
In this modification, the remaining life of a heat transfer tube group determined to be unnecessary to be replaced is evaluated from among a large number of heat transfer tube groups existing as tubular members, for example. The remaining life can be evaluated by extruding a sample from the heat transfer tube group at the inspection target (position) and performing a destructive test using this extubated material. The rupture test can be carried out by, for example, an inspection by a creep test.

For dissimilar joints that had no problem in non-destructive inspection, the next inspection can be set accurately by taking a sample and evaluating the remaining life. In addition, the regular inspection schedule can be effectively planned.

[Third Embodiment]
The first embodiment of the maintenance management method according to the present embodiment is to accumulate the results of the risk evaluation of damage occurrence and the above-mentioned non-destructive inspection, and statistically evaluate the risk of damage occurrence based on the database obtained by the accumulation. Different from the form.

FIG. 12 shows a flow chart of the maintenance management method according to the present embodiment.
First, as in the first embodiment, the risk of damage occurrence is evaluated based on the result of statistical analysis based on the mechanism, the inspection target is narrowed down based on the evaluation result, and the narrowed inspection target is preferentially inspected. The inspection is carried out by a non-destructive inspection method such as UT.

The results of risk evaluation and non-destructive inspection are reflected in the risk evaluation database, and the database is updated. Here, it is advisable to update the usage time until the next inspection timing.

Next, a statistical analysis is performed based on the updated risk assessment database to reassess the damage risk. For statistical analysis, machine learning such as deep learning, random forest, and support vector machine may be used. Machine learning uses the updated risk assessment database to build a reassessment model.

Based on the result of the re-evaluation model, narrow down the next inspection target.

According to this embodiment, statistical data can be accumulated and risk evaluation can be performed based on past test results, so that the evaluation accuracy is further improved.

(Damage mechanism)
The mechanism of fusion damage will be described below.
The occurrence of fusion damage is affected by the microstructural composition of HAZ, acting stress, the presence of decarburized layers and precipitates.

[1] Structural structure and acting stress of HAZ Figures 13 and 14 show conceptual pictures based on cross-sectional photographs of joints in dissimilar joints. FIG. 13 is a cross section of the joint portion 31 of the thick-walled dissimilar material joint, and FIG. 14 is a cross section of the joint portion 32 of the thin-walled dissimilar material joint. In FIGS. 13 and 14, A is a weld metal, B is a coarse grain region, C is a fine grain region, D is a base material (low temperature portion side), E is a decarburized layer, and F is a dissimilar material interface. Welding joints in dissimilar material joints are carried out by repeating the steps from the inner surface side of the pipe to the outer surface side of the pipe. The thick-walled type in the present embodiment means that the wall-thickness is 5 mm or more, and the thin-walled type means that the wall-thickness is thinner than the thick-walled type.

Comparing FIGS. 13 and 14, there is a difference in the organizational structure of HAZ11. In the thick-walled joint portion 31 of FIG. 13, the fine grain region C is located from the inner surface side of the pipe near the interface F of different materials to the central portion of the pipe thickness. There is a coarse grain area B on the outer surface side of the tube of the fine grain area C. On the other hand, in the thin-walled joint portion 32 of FIG. 14, the fine grain region C and the coarse grain region B are arranged in a row, and the coarse grain region B is located on the dissimilar material interface F side.

FIG. 15 shows a thermal history image at the time of TIG welding at the fusion portion (base material: low chrome steel) of the dissimilar material joint. In the figure, the horizontal axis is time, the vertical axis is temperature, Ac1 is the temperature at which the transformation from ferrite + cementite to austenite starts during heating, and Ac3 is the temperature at which the transformation from ferrite + cementite to austenite is completed during heating. .. Ac1 and Ac3 of low chromium steel are, for example, 742 ° C and 889 ° C.

The grain size of HAZ generally depends on the old austenite grain size. Therefore, in the case of the thermal history as shown in FIG. 15, it is considered that the crystal grain size of HAZ is determined by the maximum temperature reached when the austenite becomes a single phase at the end (that is, after the ac3 is finally exceeded).

Figures 16 to 18 show an image of the thermal history and tissue changes of the fused portion. FIG. 16 shows the first half welding pass portion (on the inner surface side of the pipe) where the process of the welding pass at the beginning of the welding joint of the thick-walled welding portion is performed, and FIG. 17 shows the welding path at the rear of the thick-walled welding portion. The latter half welding path portion (on the outer surface side of the pipe) where the process is performed, FIG. 18 is an image diagram of a thin-walled welded portion. In FIGS. 16-18, the horizontal axis is time and the vertical axis is temperature. In FIG. 17, the temperature rise due to the welding process of the front layer is not taken into consideration. In FIG. 18, for comparison, the heat history of the first half welding pass portion in the thin-walled welded portion is superimposed on the heat history of the first half welding pass portion in the thick-walled welded portion (same as FIG. 16). ..

The inner surface side of the pipe near the interface between different materials in the thick-walled welded portion is the portion where the welding paths overlap. As shown in FIG. 16, on the inner surface side of the pipe of the thick-walled welded portion, the welding paths overlap and the position of the next welding path to be constructed advances to the outer surface side of the pipe, and the welding path to be welded after the welding pass. Due to the influence of the heat input, the temperature after finally exceeding the Ac3 point becomes low. Therefore, it is considered that the crystal grain size of HAZ on the inner surface side of the tube becomes smaller and a fine grain region is formed. In FIG. 16, the structure of the first half welding path portion of the thick-walled welded portion is bainite, martensite, or a mixed structure thereof.

On the other hand, as shown in FIG. 17, the position of the welding path to be constructed next to the welding path on the pipe outer surface side is the pipe outer surface side, and the temperature on the pipe outer surface side finally exceeds the Ac3 point remains high. Therefore, it is considered that the old austenite grains are coarsened and a coarse grain region is formed. In FIG. 17, the structure of the latter half welding path portion of the thick-walled welded portion is bainite, martensite, or a mixed structure thereof.

On the other hand, in the thin-walled welded portion, as shown in FIG. 18, since the wall thickness is thin, the heat input of the final layer of the welding path affects the inner surface of the pipe. As a result, the temperature after exceeding the Ac3 point becomes higher than that on the inner surface of the thick-walled system. Therefore, in the thin-walled welded portion, the old austenite particle size is unlikely to be small, and the difference in crystal grain size in the pipe thickness direction between the inner surface side and the outer surface side of the pipe is less likely to occur. In FIG. 18, the structure of the thin-walled welded portion is bainite, martensite, or a mixed structure thereof.

Further, as shown in FIG. 18, in the thin-walled welded portion, the heat transfer area is smaller than that of the thick-walled welded portion, so that the cooling rate is slow and the overall temperature is difficult to decrease. It is considered that this affects the fact that the difference in crystal grain size in the thickness direction is less likely to occur. Since the heat transfer area of the thick-walled welded portion is larger than that of the thin-walled welded portion and the cooling rate on the inner surface side of the pipe is high, the structure has a large crystal grain size.

As described above, the structural composition of HAZ (arrangement of coarse grain area and fine grain area) changes depending on the heat history. The heat history of the joint depends on the wall thickness of the tubular member such as the heat transfer tube, the number of welding passes, the welding conditions, and the like.

For tubular members such as heat transfer tubes, the material and wall thickness of the tubes differ depending on the conditions of use. Further, even if the pipe is made of the same material, the wall thickness of the pipe becomes thick when the usage conditions are high temperature and high pressure. When joining thick-walled tubular members by welding, the number of welding passes increases. On the other hand, the number of welding passes in the thin-walled tubular member is smaller than that in the thick-walled tubular member. If the number of welding passes is different, the heat history received by the welded portion will be different.

Each structure (welded metal A, coarse grain area B, fine grain area C, base material D) contained in the joint has a different creep deformation resistance. Creep deformation resistance is the ease of deformation in a high temperature region. When the creep deformation resistance is large, the axial stress generated in the fusion portion of the tubular member such as the heat transfer tube during the operation of the boiler or the like is also large. When the creep deformation resistance is small, the axial stress is small.

It is generally said that the creep deformation resistance of the coarse grain area B and the fine grain area C of HAZ is higher in the coarse grain area B. However, it was unclear whether HAZ of dissimilar material joints showed the same tendency. According to the results of investigating the creep characteristics by collecting minute test pieces from each structure of the dissimilar joint using 1Cr steel as the low alloy steel, the present inventors have found that the creep deformation resistance of the fine grain region C is coarse grain. It has been confirmed that it is higher than that of region B. According to this confirmation result, in a thick tubular member, when an axial stress acts during operation, a high stress is generated in the fine grain region C, that is, on the inner surface side of the pipe or near the center of the pipe thickness.

FIG. 19 shows the results of a finite element method (FEM) analysis of the fusion portion near the interface between different materials of the thick-walled welded portion and the thin-walled welded portion in which all the conditions other than the structure structure are satisfied. In the figure, the horizontal axis is the axial stress of the fusion portion near the interface between different materials, and the vertical axis is the wall thickness (distance from the inner surface of the pipe).

According to FIG. 19, the thick-walled welded portion has a larger axial stress acting on the inner surface side of the pipe. Further, in the FEM analysis, it was also found that the higher the base acting stress (axial stress due to the internal pressure), the higher the axial stress acting on the fusion portion. From this, it was confirmed that the magnitude of the acting stress is also one of the factors that affect the damage to the fusion part. According to the results of the present inventors organizing the damage cases of the actual boiler, the inner surface side of the pipe coincides with the position where the fusion portion damage (bond dissociation) occurs.

FIG. 20 shows a schematic cross-sectional view when an axial stress is applied to a joint portion of a thick-walled tubular member. FIG. 21 shows the distribution of axial stress in the YY cross section of FIG. In FIG. 21, the horizontal axis is the axial stress of the pipe, and the vertical axis is the wall thickness direction position. A is a weld metal, B is a coarse grain region, C is a fine grain region, and D is a base metal (low temperature portion side).

Axial stress of the pipe generated by internal pressure acts on the joint of dissimilar material joints. Actually, in addition to the stress due to internal pressure, thermal stress also acts, but here we consider only the stress due to internal pressure.

In the structure of FIG. 20, when axial stress acts at a high temperature, creep deformation causes deformation that extends in the axial direction of the pipe. As described above, the coarse grain region has a lower creep deformation resistance than the fine grain region. Therefore, the coarse grain area is deformed more than the fine grain area. The fine grain area is wide and the deformation is small.

In the above case, as shown in FIG. 21, after creep deformation due to the action of axial stress at high temperature, the deformation resistance is low on the outer surface side of the pipe in an attempt to balance the elongation on the outer surface side of the pipe and the inner surface side of the pipe. The load generated on the outer surface side of the pipe becomes smaller, and the deformation resistance is higher on the inner surface side of the pipe, and the load generated on the inner surface side of the pipe becomes larger accordingly. As a result, the axial stress on the inner surface side of the pipe increases. On the other hand, for comparison, the axial stress distribution during elastic deformation is constant on the outer surface side of the pipe and the inner surface side of the pipe as shown by the broken line.

Further, a tubular member such as a thick-walled heat transfer tube has a smaller likelihood of a wall thickness t up to a required minimum wall thickness tsr (wall thickness t-required minimum wall thickness tsr) than a thin-walled tubular member. Therefore, the axial stress due to the internal pressure becomes relatively high.

[2] Decarburized layer and precipitates FIG. 22 shows a partial schematic view of a welded portion in which cracks are generated in the fused portion.
The welded portion 9 contains the weld metal 10 and the HAZ 11. The decarburized layer 33 is present on the F side of the different material interface in HAZ11.

The decarburized layer 33 is formed by the diffusion of atoms from the weld metal 10 to the base material of the low temperature portion 4 of the welded portion 9 due to heat input during welding. The fusion portion basically exists in the decarburized layer 33. The decarburized layer 33 has a smaller creep deformation resistance than the surrounding structure. Therefore, creep strain tends to accumulate.

During the operation of a boiler or the like, tubular members such as heat transfer tubes are exposed to high temperatures. As a result, atoms (Cr and the like) contained in the weld metal 10 are diffused to the base metal side of the low temperature portion 4 of the welded portion 9. As a result, a row of precipitates 36 is generated in the decarburized layer 33 region.

Accumulation of creep strain facilitates the accumulation of atomic vacancies at the interface between the precipitate 36 deposited in the decarburized layer 33 and the base metal. Therefore, (creep) voids are preferentially generated in the decarburized layer. As the formation of voids 35 progresses, the voids are connected to each other and finally become a microscopic crack 34. When the microcrack 34 grows, it breaks.

1 Superheater 2 Steam header 3 Heat transfer tube 4 Low temperature part 5 Low temperature tube 6 High temperature part 7 High temperature tube 8 Joint part 9 Welded part 10 Welded metal 11 Heat affected zone (HAZ)
21 CPU
22 Storage unit 23 Main memory 24 Communication unit 25 Input unit 26 Display unit 27 Bus 28 Analysis unit 29 Judgment unit 31 Thick-walled joint 32 Thin-walled joint 33 Decarburized layer 34 Microscopic crack 35 Void 36 Precipitate

Claims (9)

This is a method for evaluating the risk of damage to the welded part of a dissimilar joint in which members of different metal materials are welded together.
A damage risk evaluation method in which statistical analysis is performed using explanatory variables including factors that affect the organizational structure of the heat-affected zone, and the risk of damage occurrence is evaluated based on the results of the statistical analysis. The damage risk evaluation method according to claim 1, wherein the member of the different metal material is a tubular member through which a fluid can flow. The damage risk evaluation method according to claim 1 or 2, wherein the factor includes the wall thickness of the member. The damage risk evaluation method according to any one of claims 1 to 3, wherein the factor includes the number of welding passes of the welded joint. The damage risk evaluation method according to any one of claims 1 to 4, wherein the factor includes welding conditions. It is a maintenance management method for a system in which there are multiple welds of dissimilar joints.
The risk of damage occurrence is evaluated by the damage risk evaluation method according to any one of claims 1 to 5.
A system maintenance management method that limits the inspection position based on the evaluation results and determines the inspection target. Non-destructive inspection was performed on at least a part of the welded portion to be inspected.
The system maintenance management method according to claim 6, wherein the result of the non-destructive inspection is compared with a preset replacement reference value to determine whether or not replacement is necessary. Accumulate the results of the risk assessment of damage occurrence and the non-destructive inspection,
The system maintenance management method according to claim 6 or 7, wherein the risk of damage occurrence is statistically evaluated based on the database obtained by the accumulation, and the next inspection target is determined. It is a risk evaluation device that evaluates the risk of damage to the welded part of a dissimilar joint in which members of different metal materials are welded together.
An analysis unit that performs statistical analysis using factors that affect the organizational structure of the heat-affected zone as explanatory variables,
A determination unit that determines the risk of damage to the welded portion based on the analysis results of the analysis unit, and a determination unit.
A risk assessment device equipped with.

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