JP7500166B2 - Damage risk assessment method, system maintenance management method, and risk assessment device - Google Patents

Description

This disclosure relates to a damage risk assessment method, a system maintenance management method, and a risk assessment device.

Heat transfer tubes such as superheaters and reheaters used in boilers are used in high-temperature, high-pressure environments for long periods of time. For this reason, heat transfer tubes are made of heat-resistant materials with excellent high-temperature strength. Heat-resistant materials such as low-alloy steel and stainless steel are used depending on the temperature and pressure at which the heat transfer tube is used. Some heat transfer tubes have areas where the material changes along the way to suit the usage environment, and dissimilar material joints exist at the joints where the material changes. Dissimilar material joints exist in large numbers in the superheaters and reheaters of thermal power boilers.

In dissimilar material joints in heat transfer tubes, damage such as thermal fatigue cracking, creep damage, and fusion damage may occur when used at high temperatures for long periods of time depending on the usage environment. Creep damage occurs when deformation progresses due to the load stress and time during high-temperature use, or when voids appear at the metal grain boundaries, ultimately causing fracture.

Dissimilar material joints are inspected for damage periodically or according to operating conditions, and as part of maintenance management, the heat transfer tubes are repaired as necessary based on the inspection results, and the remaining lifespan is assessed.

Patent Documents 1 and 2 disclose a life assessment method for thermal fatigue cracking. Patent Document 3 discloses a method for evaluating the growth of cracks that occur in the welds of dissimilar material joints. Patent Document 4 discloses a life determination method for creep damage.

JP 2003-90506 A JP 2003-130789 A JP 2015-190950 A JP 2012-173217 A

Takuya Yamashita et al., "Analysis of mechanical factors in interfacial fracture of dissimilar welded joints," Journal of the Iron and Steel Institute of Japan, Vol. 105, No. 1, 2019, pp. 96-104

In order to diagnose the remaining life of the fusion zone and perform preventive maintenance against fusion zone damage, it is essential to clarify the factors that affect this damage in the fusion zone and the damage mechanism. However, as described in Non-Patent Document 1, the influencing factors and mechanism of fusion zone damage that occurs in dissimilar material joints have not been clarified.

The techniques described in Patent Documents 1 to 4 are aimed at thermal fatigue cracking and creep damage, and are unable to predict the risk of fusion damage. In addition, the method in Patent Document 3 evaluates the lifespan after cracks have occurred, and is therefore not effective in preventive maintenance of fusion sections.

Heat exchangers and other equipment used in power plants contain many dissimilar material joints. If the risk of damage to fusion parts cannot be predicted, all dissimilar material joints must be inspected, which is inefficient. Full inspection takes a lot of time, and depending on the inspection period, full inspection may be difficult. In such cases, sampling inspection is an option, but it is difficult to appropriately select areas that should be inspected as a priority.

The present disclosure has been made in consideration of these circumstances, and aims to provide a damage risk assessment method for assessing the risk of fusion damage, a maintenance management method for a system for preventive maintenance of fusion damage, and a risk assessment device.

To solve the above problems, the damage risk assessment method, system maintenance management method, and risk assessment device disclosed herein employ the following measures.

The present disclosure provides a damage risk assessment method for a fusion zone present in a decarburized layer included in a heat-affected zone of a weld of a dissimilar metal joint in which components of different metallic materials are welded together, the damage risk assessment method performing a statistical analysis using explanatory variables including factors that affect the microstructural structure of the heat-affected zone and assessing the risk of damage occurrence based on the results of the statistical analysis.

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

Factors that affect the microstructural composition of the heat-affected zone may include at least the thickness of the components, the number of welding passes for the welded joint between the components, and/or the welding conditions.

The present disclosure provides a risk assessment device that evaluates the risk of damage to a fusion zone present in a decarburized layer included in a heat-affected zone of a weld of a dissimilar metal joint in which components of different metallic materials are welded together, the risk assessment device including an analysis unit that performs statistical analysis using factors that affect the microstructural structure of the heat-affected zone as explanatory variables, and a judgment unit that judges the risk of damage occurring to the weld based on the analysis results of the analysis unit.

After extensive research, the inventors have found that the microstructural structure of the heat-affected zone is related to the occurrence of fusion damage in the welded portion of a dissimilar material joint in which components made of different metal materials are welded together. In this disclosure, by incorporating factors that affect the microstructural structure of the heat-affected zone as explanatory variables, highly accurate risk assessment is possible. By inputting the above factors for each specification of the dissimilar material joint (design conditions, use conditions, etc.) and performing statistical analysis, it is possible to narrow down the positions that should be inspected preferentially, even if dissimilar material joints are present in multiple positions. This makes it possible to narrow down the positions to be inspected and reduce the number of dissimilar material joints to be inspected, thereby reducing inspection time and costs while maintaining the reliability of the equipment.

The present disclosure provides a maintenance management method for a system that has multiple welds in dissimilar material joints, which evaluates the risk of damage occurrence using any of the damage risk evaluation methods described above, and limits the inspection positions based on the evaluation results to determine the inspection targets.

By assessing the risk of damage using the damage risk assessment method described above, it is possible to efficiently select dissimilar material joints that require inspection. In addition, the maintenance management method for the system disclosed above is effective in preventing damage to fusion parts.

In one aspect of the disclosure above, at least a portion of the weld of the determined inspection target is subjected to non-destructive testing, and the results of the non-destructive testing are compared with a preset replacement standard value to determine whether replacement is necessary.

Non-destructive testing improves the accuracy of remaining life diagnosis, improving the accuracy of replacement decisions.

In one embodiment of the disclosure above, the results of the damage risk assessment and the non-destructive testing may be accumulated, and the risk of damage may be statistically evaluated based on the database obtained by the accumulation, to determine the next test target.

This makes it possible to assess risk based on past results, improving the accuracy of the assessment.

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

FIG. 1 is a schematic diagram of a superheater of a thermal power plant boiler. FIG. 2 is an enlarged view of the joint in FIG. 1 . FIG. 2 is a flow diagram of a maintenance management method according to the first embodiment. FIG. 2 is a flow diagram of a risk assessment method according to the first embodiment. FIG. 2 is a diagram illustrating an example of an input data set. FIG. 1 is a diagram showing the relationship between design temperature and axial stress due to internal pressure. FIG. 13 is a diagram showing the relationship between usage time and the ratio of outer diameter to wall thickness. FIG. 1 is a block diagram showing a schematic configuration of a risk assessment device. 2 is a functional block diagram showing the functions of the risk assessment device in an expanded form. FIG. FIG. 11 is a flow diagram of a maintenance management method according to a second embodiment. FIG. 11 is a flow diagram of a modification of the maintenance management method of FIG. 10 . FIG. 11 is a flow diagram of a maintenance management method according to a third embodiment. This is a conceptual diagram of a cross-section of a (thick-walled) joint in a dissimilar material joint. This is a conceptual diagram of a cross-section of a (thin-walled) joint in a dissimilar material joint. FIG. 1 is an image diagram of the thermal history during TIG welding of a dissimilar material joint. This is an image of the thermal history and structural changes in the first half pass of a thick-walled weld. This is an image diagram of the thermal history and structural changes in the latter pass portion of a thick-walled weld. This is an image diagram of the thermal history and structural changes of a thin-walled weld. FIG. 13 is a diagram showing the results of an FEM analysis of a dissimilar material joint. FIG. 2 is a schematic cross-sectional view of a thick-walled joint subjected to axial stress. FIG. 21 is a distribution diagram of axial stress in the YY cross section of FIG. 20. FIG. 2 is a partial schematic diagram of a welded portion where microcracks have occurred.

A preferred embodiment of the present disclosure will be described with reference to the drawings. Note that the present disclosure is not limited to these embodiments, and when there are multiple embodiments, the present disclosure also includes configurations in which the respective embodiments are combined.

The damage risk assessment method, assessment device, and maintenance management method disclosed herein assess and maintain the joints, particularly the fusion parts (fusion part damage), in dissimilar material joints formed by welding together members of different metal materials. In this embodiment, the members of different metal materials are, for example, tubular members. Fluids such as steam can flow inside the tubular members. Such tubular members are, for example, heat transfer tubes of a heat exchanger, and piping that connects various devices.

Note that the components made of different metal materials according to the present disclosure are not limited to tubular components, but may also be applied to structural components.

First, before explaining the damage risk assessment and maintenance management methods, we will explain the dissimilar material joints that are the subject of assessment and maintenance.

Figure 1 shows a schematic diagram of a superheater for a thermal power boiler as an example of this embodiment. In the superheater 1 of Figure 1, multiple heat transfer tubes 3 are connected in parallel via a steam header 2. The heat transfer tubes 3 include a low-temperature tube (tubular member) 5 in a low-temperature section 4 and a high-temperature tube (tubular member) 7 in a high-temperature section 6. The low-temperature section 4 is a region such as the steam inlet side. The high-temperature section 6 is a region such as the downstream side of the steam flow or a region exposed to furnace radiation. In the region exposed to furnace radiation, the temperature of the outer circumferential surface of the heat transfer tube 3 is not the same as the temperature of the steam flowing inside, so there may be some parts where the temperature of the steam flowing through the low-temperature tube 5 is equal to or higher than the temperature of the steam flowing through the high-temperature tube 7.

The low-temperature pipe 5 and the high-temperature pipe 7 are made of different metal materials. In this embodiment, for example, the low-temperature pipe 5 is made of low-alloy steel, such as low-chromium steel, e.g., 1Cr steel. The high-temperature pipe 7 is made of stainless steel, e.g., nickel-chromium-containing steel. The low-temperature pipe 5 and the high-temperature pipe 7 are joined by welding. The joint portion is called the 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, etc.

Figure 2 shows an enlarged view of the joint 8 in Figure 1. The joint 8 includes the base material of the low-temperature pipe 5 and the high-temperature pipe 7, and the weld 9. The weld 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 of a different material from the low-temperature pipe 5 and the high-temperature pipe 7. The weld metal 10 is, for example, a high-nickel-based alloy such as a solid-solution strengthened nickel-based alloy. At the interface between the weld metal 10 and the base material of the low-temperature pipe 5 and the high-temperature pipe 7, the weld metal 10 and the base material are fused, and this is called the fusion zone (not shown). The fusion zone is a region from the dissimilar material interface, which is the boundary surface between the weld metal 10 and the base material of a different material, to the base material side by several μm. The fusion zone is part of the HAZ 11, and the fusion zone in the low-temperature pipe 5 exists in the decarburized layer described later.

The HAZ 11 is an altered part in the base material formed by the thermal cycle of welding. Although not shown in Fig. 2, the HAZ 11 in the low-alloy steel low-temperature pipe 5 includes a coarse-grained region and a fine-grained region (see Figs . 13 and 14 described later). The coarse-grained region of the low-temperature pipe 5 is a structure in which the prior austenite grain size is larger than the grain size of the parent phase. The fine-grained region is a structure in which the prior austenite grain size is smaller than the grain size of the parent phase.

Although not shown in FIG. 2, a decarburized layer (see FIG. 13 and FIG. 14 described later) exists near the interface between different materials in the HAZ 11 of the low-alloy steel low-temperature pipe 5. The decarburized layer is, for example, a ferrite single-phase region with a width of about 20 μm in the axial direction of the HAZ 11 of the low-temperature pipe 5, and is a part of the HAZ 11. Since the weld metal 10 has a higher content of Cr (chromium), which is a carbide-forming element, than the base metal of the low-temperature pipe 5, C (carbon) moves from the base metal side to the weld metal 10, and Cr carbides are formed in the weld metal 10. As a result, a decarburized layer is formed in the part of the base metal to which C has moved. In addition, atoms (Cr, etc.) contained in the weld metal 10 diffuse to the base metal side due to the heat input of welding. The Cr, etc. diffused into the base metal of the low-temperature pipe 5 combines with C. As a result, row-shaped precipitates are generated, and the carbon content in the base metal decreases, so that a decarburized layer is also formed. The decarburized layer is often formed from the center of the wall thickness to the inner surface of the low-temperature pipe 5. The fusion portion basically exists in the decarburized layer.

First Embodiment
(Maintenance management method)
3 shows a flow diagram of the maintenance management method according to this embodiment. In the maintenance management method according to this embodiment, first, the risk of damage occurrence is evaluated based on the results of a statistical analysis based on the mechanism of fusion part damage. Next, the inspection objects (positions) are narrowed down based on the evaluation results, and the narrowed down inspection objects (located at the positions) are inspected preferentially.

Conventionally, the mechanism of damage at fusion parts was unknown. After extensive research, the inventors have discovered factors that affect the damage mechanism. At the fusion part of a low-temperature pipe 5 made of low-alloy steel in a dissimilar joint, differences in the thermal history experienced during welding change the structural composition of the HAZ. Differences in structural composition lead to differences in stress acting on the interface between the dissimilar materials, ultimately resulting in differences in the risk of damage. By performing statistical analysis based on such a mechanism, the risk of damage occurring at the fusion part can be evaluated. In addition, the results of the risk evaluation can be used to narrow down the inspection targets, improving inspection efficiency.

(Risk assessment method)
FIG. 4 illustrates a flow diagram of the risk assessment method according to this embodiment.

(S1) First, the actual data of the dissimilar material joint of the heat transfer tube is entered into the risk assessment database as explanatory variables.

The explanatory variables include at least factors that affect the microstructural structure of the HAZ (hereafter, influencing factors). "Affecting the microstructural structure" includes at least changing the arrangement of the coarse grain region and the fine grain region. A change in the arrangement of the coarse grain region and the fine grain region affects the increase or decrease in the risk of fusion zone damage. The reason for this will be explained in detail later. The arrangement of the coarse grain region and the fine grain region changes depending on the thermal history, such as the heat input conditions during welding.

As mentioned above, explanatory variables include at least influencing factors, such as wall thickness, number of welding passes, and welding conditions such as heat input and speed. Here, "wall thickness" without a modifier means the nominal wall thickness of the tubular member. In addition, in relation to wall thickness, influencing factors may include, for example, outer diameter/wall thickness and (wall thickness - required minimum wall thickness)/wall thickness. "Required minimum wall thickness" is the wall thickness required for internal pressure based on the heat transfer tube specifications (material, outer diameter, design temperature, etc.).

As mentioned above, the explanatory variables include at least influencing factors, and may further include the conditions of use of the heat transfer tube and actual damage cases. Specifically, they may include whether it is a heated or unheated part, whether there is a history of past repair welding, when the boiler started operating, the acting stress, the design temperature of the heat transfer tube, the material of the heat transfer tube, the length of time it has been used, and whether there is a history of past inspections. "Acting stress" is the axial stress caused by internal pressure.

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

The damage rate of axial stress due to internal pressure is affected by the magnitude of base stress. Whether it is a heated or unheated part affects the magnitude of bending stress. Heated parts are superheaters and reheaters, which are devices that superheat steam, and are located inside the furnace. Unheated parts are header pipes located outside the furnace, known as penthouses or housings, and do not have the function of superheating steam.

According to the results of the inventors' analysis of damage cases in actual boilers, there are many cases where fusion damage is accelerated in areas where repair welding has been performed in the past. Therefore, the presence or absence of a history of repair welding in the past is considered as a factor that accelerates damage.

The objective variable is the presence or absence of damage to the fusion area (bond separation).

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

Statistical analysis can utilize machine learning techniques such as random forests and support vector machines.

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

The inventors have compiled examples of damage to actual boilers in terms of the relationship between design temperature and axial stress due to internal pressure, and the relationship between usage time and the ratio of outer diameter to wall thickness, and the results are shown in Figures 6 and 7. In Figure 6, the horizontal axis is design temperature (°C), and the vertical axis is axial stress due to internal pressure (MPa). In Figure 7, the horizontal axis is usage time (hr), and the vertical axis is the ratio of outer diameter d to wall thickness t.

Figures 6 and 7 show that no clear correlation was found between general design conditions such as stress, temperature, outer diameter, wall thickness, and usage time and damage to the fusion zone, and a simple linear regression model was not able to evaluate the risk of damage occurring in the fusion zone. For this reason, in order to evaluate the risk of damage occurring in the fusion zone, it is important to perform statistical analysis using explanatory variables that include at least influential factors.

(Risk assessment device)
Fig. 8 shows a block diagram of a schematic configuration of a risk assessment device for executing the risk assessment method. The risk assessment device is a computer system. The risk assessment 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, an input unit 25 consisting of a keyboard, a mouse, etc., and a display unit 26 consisting of a liquid crystal display device or the like for displaying data. These units are connected via a bus 27, for example. Examples of the storage unit 22 include a magnetic disk, a magneto-optical disk, and a semiconductor memory.

Figure 9 is a functional block diagram showing the functions of the risk assessment device. The risk assessment device includes an analysis unit 28 that performs statistical analysis using at least influencing factors as explanatory variables, and a determination unit 29 that determines the risk of damage occurring at the fusion part based on the analysis results by the analysis unit. The processing performed by each unit shown in Figure 9 is achieved 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 this embodiment differs from the first embodiment in that it further includes a step of determining whether or not a joint made of dissimilar materials in a tubular member such as a heat transfer tube needs to be replaced.

FIG. 10 shows a flow diagram of the maintenance management method according to this embodiment.
First, similarly to the first embodiment, the risk of damage occurrence is evaluated based on the results of the mechanism-based statistical analysis, and inspection objects are narrowed down based on the evaluation results, and the narrowed down inspection objects are inspected preferentially.

The inspection is carried out using non-destructive inspection methods such as ultrasonic testing (UT). UT is capable of detecting flaws by properly determining the flaw detection sensitivity and the flaw detection standard value.

Next, the results of the non-destructive testing are compared with the replacement standard value to determine whether or not the dissimilar material joint needs to be replaced. The replacement standard value is set in advance through preliminary testing, etc. If the results of the non-destructive testing are equal to or greater than the replacement standard value, replacement is carried out, but if the replacement standard value is not met, aging monitoring is carried out.

According to the maintenance management method of this embodiment, the inspection targets are narrowed down by risk assessment, so that heat transfer tubes that require inspection can be efficiently selected from, for example, a large number of heat transfer tubes that exist as tubular components.

(Modification)
FIG. 11 shows a flow chart of a modification of the maintenance management method of FIG.
In this modification, the remaining life of a heat transfer tube group that is determined not to require replacement from among a large number of heat transfer tube groups that exist as tubular members is evaluated. The remaining life can be evaluated by removing a sample from the heat transfer tube group that is at the inspection target (position) and performing a destructive test using the removed tube material. The destructive test can be performed, for example, by a creep test.

For dissimilar material joints that showed no problems in non-destructive testing, taking samples and evaluating the remaining lifespan allows for accurate determination of the timing of the next inspection. In addition, regular inspection schedules can be effectively planned.

Third Embodiment
The maintenance management method according to this embodiment differs from the first embodiment in that the results of the damage occurrence risk assessment and the above-mentioned non-destructive testing are accumulated, and the risk of damage occurrence is statistically evaluated based on the database obtained by the accumulation.

FIG. 12 shows a flow diagram of the maintenance management method according to this embodiment.
First, as in the first embodiment, the risk of damage occurrence is evaluated based on the results of a statistical analysis based on the mechanism, and inspection objects are narrowed down based on the evaluation results, and the narrowed down inspection objects are inspected preferentially. The inspection is performed by a non-destructive inspection method such as UT.

The results of the risk assessment and non-destructive testing are reflected in the risk assessment database, and the database is updated. At this point, it is also a good idea to update the usage time until the next testing timing.

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

Narrow down the subjects for the next test based on the results of the reevaluation model.

According to this embodiment, statistical data can be accumulated and risk assessment can be performed based on past test results, thereby improving the accuracy of the assessment.

(Damage Mechanism)
The mechanism of fusion site damage is explained below.
The occurrence of fusion zone damage is influenced by the microstructural composition of the HAZ, the acting stress, and the presence of decarburized layers and precipitates.

[1] Structure and acting stress of HAZ Figures 13 and 14 show conceptual diagrams based on cross-sectional photographs of joints in dissimilar metal joints. Figure 13 shows a cross-section of a joint 31 of a thick-walled dissimilar metal joint, and Figure 14 shows a cross-section of a joint 32 of a thin-walled dissimilar metal joint. In Figures 13 and 14, A is the weld metal, B is the coarse-grained region, C is the fine-grained region, D is the base metal (low-temperature side), E is the decarburized layer, and F is the dissimilar material interface. In dissimilar metal joints, welding passes are performed in a process stacked from the inner surface side of the pipe to the outer surface side of the pipe. In this embodiment, the thick-walled system means that the wall thickness is 5 mm or more, and the thin-walled system means that the wall thickness is thinner than that of the thick-walled system.

Comparing Figures 13 and 14, there is a difference in the structure of the HAZ 11. In the thick-walled joint 31 in Figure 13, a fine-grained region C is located from the inner pipe surface near the dissimilar material interface F to the center of the pipe thickness. A coarse-grained region B is located on the outer pipe surface side of the fine-grained region C. On the other hand, in the thin-walled joint 32 in Figure 14, the fine-grained region C and the coarse-grained region B are aligned in a row, and the coarse-grained region B is located on the dissimilar material interface F side.

Figure 15 shows an image of the thermal history during TIG welding at the fusion zone of a dissimilar joint (base material: low chromium steel). 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 begins when heated, and Ac3 is the temperature at which the transformation from ferrite + cementite to austenite is completed when heated. Ac1 and Ac3 for low chromium steel are, for example, 742°C and 889°C.

The grain size of the HAZ generally depends on the prior austenite grain size. Therefore, when the thermal history is as shown in Figure 15, the grain size of the HAZ is thought to be determined by the maximum temperature reached when it finally becomes austenite single phase (i.e., after it finally exceeds Ac3).

Figures 16 to 18 show images of the thermal history and structural changes of the fusion zone. Figure 16 shows the first half weld pass part (inner surface of the pipe) where the first weld pass process of the weld joint of the thick-walled weld is carried out, Figure 17 shows the second half weld pass part (outer surface of the pipe) where the second weld pass process of the thick-walled weld is carried out, and Figure 18 shows an image of the thin-walled weld. In Figures 16 to 18, the horizontal axis is time and the vertical axis is temperature. Figure 17 does not take into account the temperature rise due to the welding process of the previous layer. For comparison, Figure 18 shows the thermal history of the first half weld pass part of the thick-walled weld (same as Figure 16) superimposed on the thermal history of the first half weld pass part of the thin-walled weld.

The inner surface of the pipe near the interface of dissimilar materials in thick-walled welds is where the weld passes overlap. As shown in Figure 16, on the inner surface of the pipe in thick-walled welds, as the weld passes overlap, the position of the next weld pass advances toward the outer surface of the pipe, and the heat input of the weld pass that is welded after the weld pass ultimately lowers the temperature after exceeding the Ac3 point. This is thought to result in the grain size becoming smaller in the HAZ on the inner surface of the pipe, and the formation of a fine-grained region. In Figure 16, the structure of the first half weld pass portion of the thick-walled weld is bainite, martensite, or a mixed structure.

On the other hand, as shown in Figure 17, the welding pass performed after the welding pass is located on the outer surface of the pipe, and the temperature on the outer surface of the pipe remains high and ultimately exceeds the Ac3 point. This is thought to cause the prior austenite grains to coarsen, forming a coarse-grained region. In Figure 17, the structure of the latter welding pass portion of the thick-walled weld is bainite, martensite, or a mixture of these.

In contrast, in thin-walled welds, as shown in Figure 18, the thin wall thickness means that the heat input from the final layer of the weld pass affects the inner surface of the pipe. As a result, the temperature after exceeding the Ac3 point becomes higher than that of the inner surface of a thick-walled weld. Therefore, in thin-walled welds, the prior austenite grain size is less likely to become small, and a difference in crystal grain size in the thickness direction between the inner and outer surfaces of the pipe is less likely to occur. In Figure 18, the structure of thin-walled welds is bainite, martensite, or a mixed structure.

In addition, as shown in Figure 18, the heat transfer area of thin-walled welds is smaller than that of thick-walled welds, so the cooling rate is slower and the overall temperature is less likely to decrease, which is thought to be a factor in preventing a difference in crystal grain size in the pipe thickness direction on the inner and outer sides of the pipe. In thick-walled welds, the heat transfer area is larger than in thin-walled welds, and the cooling rate on the inner side of the pipe is faster, resulting in a structure with a larger crystal grain size.

As described above, the structure of the HAZ (the arrangement of coarse and fine grain regions) changes depending on the thermal history. The thermal 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, etc.

The material and wall thickness of tubular components such as heat transfer tubes vary depending on the conditions of use. Even if the tubes are made of the same material, the wall thickness of the tube will be thicker if the conditions of use are high temperature and high pressure. When joining thick-walled tubular components by welding, the number of welding passes is greater. On the other hand, the number of welding passes for thin-walled tubular components is fewer than for thick-walled tubular components. A difference in the number of welding passes results in a difference in the thermal history that the welded part is subjected to.

Each structure in the joint (weld metal A, coarse grain region B, fine grain region C, base material D) has a different creep deformation resistance. Creep deformation resistance is the ease of deformation in high temperature regions. If creep deformation resistance is large, the axial stress generated in the fusion part of tubular components such as heat transfer tubes during operation of a boiler, etc., will also be large. If creep deformation resistance is small, the above-mentioned axial stress will be small.

It is generally believed that the creep deformation resistance of the coarse-grained region B of the HAZ is higher than that of the fine-grained region C. However, it was unclear whether the HAZ of dissimilar joints showed a similar tendency. The inventors took tiny test pieces from each structure of a dissimilar joint using 1Cr steel as the low alloy steel and investigated the creep properties, confirming that the creep deformation resistance of the fine-grained region C is higher than that of the coarse-grained region B. According to these results, in thick-walled tubular members, when axial stress acts during operation, high stress is generated in the fine-grained region C, i.e., on the inner surface of the pipe or near the center of the pipe thickness.

Figure 19 shows the results of a finite element method (FEM) analysis of the fusion zone near the interface of dissimilar materials in thick-walled and thin-walled welds, where all conditions except for the microstructural composition were the same. In the figure, the horizontal axis is the axial stress in the fusion zone near the interface of dissimilar materials, and the vertical axis is the wall thickness (distance from the inner surface of the pipe).

According to Fig. 19, the axial stress acting on the inner surface of the tube is larger in the thick-walled welded part. Furthermore, the FEM analysis also revealed that the higher the base acting stress (axial stress due to internal pressure), the higher the axial stress acting on the fusion part. This confirmed that the magnitude of the acting stress is one of the factors that affect fusion part damage. Furthermore, according to the results of the inventors' analysis of damage cases in actual boilers, the inner surface of the tube coincides with the location where fusion part damage (bond peeling ) occurs.

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

Axial stress of the pipes caused by internal pressure acts on the joint of dissimilar material joints. In reality, thermal stress also acts in addition to stress due to internal pressure, but here we will only consider stress due to internal pressure.

In the structure shown in Figure 20, when axial stress is applied at high temperatures, creep deformation occurs, causing the tube to stretch in the axial direction. As mentioned above, the coarse-grained region has a lower creep deformation resistance than the fine-grained region. Therefore, the coarse-grained region deforms more than the fine-grained region. The fine-grained region is wider and undergoes less deformation.

In the above case, as shown in Figure 21, after creep deformation caused by axial stress at high temperatures, the outer and inner surfaces of the tube attempt to balance their elongation, resulting in a lower deformation resistance on the outer surface and a smaller load on the outer surface, while the inner surface has a higher deformation resistance and a larger load on the inner surface. This results in a higher axial stress on the inner surface. By comparison, the axial stress distribution during elastic deformation is constant on both the outer and inner surfaces of the tube, as shown by the dashed line.

In addition, tubular members such as thick-walled heat transfer tubes have a smaller likelihood of wall thickness t up to the required minimum wall thickness tsr (wall thickness t - required minimum wall thickness tsr) than thin-walled tubular members. Therefore, the axial stress due to internal pressure is relatively high.

[2] Decarburized layer and precipitates FIG. 22 shows a partial schematic diagram of a weld where a crack has occurred in the fusion zone.
The weld 9 includes a weld metal 10 and a HAZ 11. A decarburized layer 33 is present on the dissimilar material interface F side in the HAZ 11.

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 weld 9 due to the heat input during welding. The fusion portion is basically present within the decarburized layer 33. The decarburized layer 33 has a smaller creep deformation resistance than the surrounding structure. Therefore, creep strain is more likely to accumulate.

During operation of a boiler or the like, tubular components such as heat transfer tubes are exposed to high temperatures. This causes atoms (such as Cr) contained in the weld metal 10 to diffuse to the base metal side of the low-temperature portion 4 of the weld 9. As a result, rows of precipitates 36 are formed in the decarburized layer 33 region.

The accumulation of creep strain makes it easier for atomic vacancies to accumulate at the interface between the base material and the precipitates 36 that have formed in the decarburized layer 33. Therefore, (creep) voids are preferentially generated in the decarburized layer. As the generation of voids 35 progresses, the voids connect with each other and eventually become microcracks 34. As the microcracks 34 grow, they lead to fracture.

Reference Signs List 1 Superheater 2 Steam header 3 Heat transfer tube 4 Low temperature section 5 Low temperature tube 6 High temperature section 7 High temperature tube 8 Joint 9 Welded section 10 Weld 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 Determination unit 31 Thick-walled joint 32 Thin-walled joint 33 Decarburized layer 34 Microcrack 35 Void 36 Precipitate

Claims (9)

A method for evaluating a risk of damage to a fusion zone present in a decarburized layer included in a heat-affected zone of a weld of a dissimilar metal joint in which members of different metal materials are welded together, comprising:
A damage risk assessment method comprising: performing a statistical analysis using explanatory variables including factors that affect the microstructural structure of the heat-affected zone; and assessing the risk of damage occurrence based on the results of the statistical analysis. The damage risk assessment method according to claim 1, wherein the component made of a different metal material is a tubular component through which a fluid can flow. The damage risk assessment method according to claim 1 or claim 2, wherein the factors include the thickness of the component. A damage risk assessment method according to any one of claims 1 to 3, wherein the factors include the number of welding passes of the welded joint. A damage risk assessment method according to any one of claims 1 to 4, wherein the factors include welding conditions. A maintenance management method for a system having a plurality of welds of dissimilar material joints, comprising:
Evaluating the risk of damage occurrence by the damage risk assessment method according to any one of claims 1 to 5,
A system maintenance management method that limits inspection locations and determines inspection targets based on evaluation results. non-destructively inspecting at least a portion of the welded portion of the inspection target determined;
7. The system maintenance management method according to claim 6, further comprising the step of comparing the results of said non-destructive inspection with a preset replacement reference value to determine whether or not replacement is necessary. Accumulating the results of the risk assessment of damage occurrence and the non-destructive testing;
8. The system maintenance management method according to claim 7, further comprising the step of statistically evaluating the risk of damage occurrence based on the database obtained by accumulation, and determining which items to inspect next. A risk assessment device for assessing a risk of damage to a fusion zone present in a decarburized layer included in a heat-affected zone of a weld of a dissimilar metal joint in which members of different metal materials are welded together,
an analysis unit that performs statistical analysis using factors that affect the microstructural composition of the heat-affected zone as explanatory variables;
a determination unit that determines a risk of damage occurring at the welded portion based on an analysis result of the analysis unit;
A risk assessment device comprising:

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