JP2011196935A - Life expectancy evaluation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a life expectancy evaluation method that enables accurate evaluation of the life expectancy of a member obtained by providing repair welding on a butt welded section.SOLUTION: A life expectancy evaluation method for evaluating the life expectancy of a welded member E obtained by providing a repair welding section 14 at the boundary between a welded section W obtained by butt-welding a first base material 13a and a second base material 13a and the first base material 13a includes evaluating the creep life of the welded section W on the basis of a state of superficial tissue (tissue collection region S) of the boundary (second boundary section B2) between the welded section W and the repair welding section 14.

Description

  The present invention relates to a repair welding method for a welded portion in a high-temperature piping of a boiler in, for example, a thermal power / nuclear power plant or a chemical plant.

  In recent years, for example, in high-temperature piping of boilers in thermal power / nuclear power plants and chemical plants, due to long operation time, equipment deterioration, frequent start / stop, thermal fatigue due to rapid load fluctuations, etc. are fully considered Maintenance management is becoming important.

For example, in large-diameter thick pipes that use high-temperature pressure-resistant metals, nondestructive inspections such as structural inspection and ultrasonic inspection are regularly performed in order to detect deterioration at the welded portion at an early stage. Repairing defective parts.
As a repair technique, a method is employed in which a defective portion is excised from both sides or one side, and overlay welding is applied to the excised portion.

High chrominum steel is used as the large diameter and thick pipe. And long-term use causes creep voids or coarsening of crystal grains in high-temperature pressure-resistant welds. The damage such as creep voids may cause breakage due to strength reduction.
When this creep damage occurs in the welded portion of the thick pipe, it is difficult to remove all of the creep damaged portion in the thickness direction because it is a pipe.

  Therefore, with the repair method described above, the weld heat affected zone (HAZ zone), where creep voids or coarsening of crystal grains occur in particular, is partially removed from the surface side so as to meet the prescribed conditions, and then re-welded (repair) A technique for welding) has been proposed (see Patent Document 1).

  On the other hand, creep voids are connected and united while growing, eventually forming microcracks, and the microcracks are further propagated and connected to cause destruction of the entire member (creep fracture).

  Therefore, in order to operate the plant stably, it is necessary to accurately grasp the remaining life represented by the time until the equipment member creeps. A method for grasping the remaining life has been proposed (see Patent Document 2).

JP 2006-198658 A Japanese Patent No. 4054833

  In the conventional remaining life evaluation method, the vicinity of the boundary with the base material (see the first boundary B1b in FIG. 3) of the repair weld is recognized as a region having low creep strength. The remaining life evaluation was performed.

However, as a result of performing a creep test (JISZ2271) on a member that has undergone repair welding at the butt weld, the inventors have newly developed creep voids or coarsening of crystal grains due to repair welding. It was confirmed that an easy-to-use area was generated.
Specifically, as shown in FIG. 3, creep voids or coarsening of crystal grains is likely to newly occur near the boundary (second boundary B <b> 2) with the repair weld 14 in the butt weld W. It was confirmed.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a remaining life evaluation method capable of accurately evaluating the remaining life of a member subjected to repair welding on a butt weld.

The remaining life evaluation method according to the present invention employs the following means in order to solve the above problems.
The remaining life evaluation method according to the present invention is a method for evaluating the remaining life of a welded member in which a repair weld is applied to the boundary between a welded portion where a first base material and a second base material are butt welded and the first base material. And the creep remaining life of the said welding member is evaluated based on the state of the surface structure of the said weld part, the said repair welding part, and a boundary part, It is characterized by the above-mentioned.

  The creep remaining life of the welded member is evaluated based on the crystal grain size, the number of voids within a predetermined range, the void density, or the void area ratio.

  Further, samples of the surface texture in a plurality of stages having different remaining lives are collected in advance by testing, and the state of the surface texture in the welding member and the state of the structure of the sample are compared, and the creep remaining life of the welding member is determined. It is characterized by evaluating.

  Further, the surface texture is collected by a replica method.

  Moreover, the remaining life evaluation method as described in any one of Claim 1 to 4 is used as a remaining life evaluation method of the welding part of the steam piping connected to the steam boiler which generates water vapor | steam.

  The steam pipe is a steam pipe connected to a superheater or a reheater of the steam boiler.

According to the present invention, the degree of damage is confirmed in the region where the creep strength is weakest (near the boundary between the welded portion and the repair welded portion) among the members subjected to repair welding on the butt welded portion. It is possible to accurately grasp the remaining life of the product.
Moreover, the remaining life of a steam boiler can be evaluated accurately by applying this remaining life evaluation method to the repair welding part of the steam piping connected to a steam boiler.

It is a piping system diagram of a power plant. It is sectional drawing of the bending steam piping which carried out butt welding. It is sectional drawing of the butt welding part of a bending steam piping. It is a flowchart which shows the procedure which evaluates the remaining life of a welding member. It is a figure (photograph) which shows the structure | tissue in the structure | tissue collection area | region obtained by the creep test of the sample member. FIG. 4 is a relationship diagram of stress-lifetime at a boundary between a base material and a weld.

Hereinafter, a life evaluation method according to an embodiment of the present invention will be described with reference to the drawings.
(Power plant and steam boiler)
In the piping system diagram of the power plant shown in FIG. 1, the power plant 1 supplies a steam boiler 2 that generates superheated steam by burning fuel, and superheated steam generated in the steam boiler 2 through a pipe that will be described later. The steam turbine 3 is configured to include a generator 4 driven by the steam turbine 3.

  The steam boiler 2 is a kind of heat exchanger that generates steam by heating water by burning fuel such as coal, for example, from a superheater 10 that generates primary superheated steam and a high-pressure turbine T1 described later. And a reheater 12 that reheats the introduced steam to generate secondary superheated steam.

  The steam turbine 3 includes a high-pressure turbine T1 as a primary turbine driven by the primary superheated steam generated by the superheater 10 and a secondary turbine driven by the secondary superheated steam reheated by the reheater 12. A pressure turbine T2 and a low pressure turbine T3 driven by exhaust from the intermediate pressure turbine T2 are provided.

  The outlet side of the superheater 10 in the steam boiler 2 is connected to one end side of the high-pressure turbine T1 through the first steam pipe 6, and the other end side of the high-pressure turbine T 1 is connected to the steam boiler through the second steam pipe 7. The two reheaters 12 are connected in communication with the inlet side. Thereby, the primary superheated steam supplied from the superheater 10 is introduced into one end side of the high-pressure turbine T1, and the superheated steam exhausted from the other end side of the high-pressure turbine T1 is introduced into the reheater 12 to be resuperheated. It has come to be.

  Further, the outlet side of the reheater 12 is connected to one end side of the intermediate pressure turbine T <b> 2 by a third steam pipe 8, and the other end side of the intermediate pressure turbine T <b> 2 has a low pressure by a fourth steam pipe 9. The turbine T3 is connected in communication.

  The rotating shafts of the respective turbines T1, T2, and T3 are connected to the rotating shaft of the generator 4 coaxially with the turbines, and the rotating shafts are rotationally driven by the turbines T1, T2, and T3 to generate electric power.

  Steel pipes made of high chromium steel are used for the steam pipes 6 and 8 connected to the superheater 10 and the reheater 12 of the steam boiler 2 in the power plant 1.

The high chromium steel is a ferritic heat resistant steel containing about 9 to 12 wt% chromium and is also called a high chromium heat resistant steel. For example, it contains about 1 to 2 wt% Mo (molybdenum), and a small amount of V (niobium oxide), Nb (niobium) or the like is added to improve the creep strength.
In particular, the service temperature is about 600 to 650 ° C. or less, and it is used in power generation boilers, turbines, nuclear reactors and the like.

Next, the operation of the power plant 1 will be described.
The water introduced into the steam boiler 2 is superheated by the superheater 10 and becomes primary superheated steam. The primary superheated steam is sent through the first steam pipe 6 and introduced into one end side of the high-pressure turbine T1 to operate the high-pressure turbine T1. The superheated steam exhausted from the other end side of the high-pressure turbine T1 is sent to the second steam pipe 7 and introduced into the reheater 12 to become secondary superheated steam of about 600 to 650 ° C. or less.

The secondary superheated steam exhausted from the outlet side of the reheater 12 and introduced into one end side of the intermediate pressure turbine T2 operates the intermediate pressure turbine T2, and then the low pressure turbine T3 via the fourth steam pipe 9. The low-pressure turbine T3 is operated.
In this way, the high pressure turbine T1, the medium pressure turbine T2, and the low pressure turbine T3 that constitute the steam turbine 3 are operated to drive the generator 4 to generate electric power.

(Steam piping)
Next, the steam pipes 6 and 8 used in the power plant 1 will be described with reference to FIG.
FIG. 2 shows a cross-sectional view of the bent steam pipe E butt welded. The bent steam pipe E (elbow) is constituted by joining a plurality of steel pipes made of high chromium steel, and a welded portion W that is butt welded is formed at the joined portion (plural places). .

(Repair welding)
Next, the repair welding will be described with reference to FIGS. 3 (a) and 3 (b).
FIGS. 3A and 3B are diagrams schematically showing a cross section of a butt weld portion of a bent steam pipe.
3A and 3B, the welded portion W has an upper portion indicating the outer peripheral surface of the steam pipe E and a lower portion indicating the inner peripheral surface (side surface of the pipe).

The welded portion W is a butt welded end portion of a steel pipe made of high chromium steel, and is a portion where the end portions of the first base material 13a and the second base material 13b are joined by conventional butt welding.
A first boundary B1a is formed between the welded portion W and the first base material 13a. Similarly, a first boundary B1b is formed between the welded portion W and the second base material 13b.

  The first boundary portion B1a and the first boundary portion B1b include welding heat affected portions 16a and 16b. These welding heat affected zone 16a, 16b are formed in the 1st base material 13a and the 2nd base material 13b so that the outside of the 1st boundary part B1a and the 1st boundary part B1b may be met.

In repair welding, first, the welded portion W is inspected from the outer periphery of the bent steam pipe E by nondestructive inspection such as ultrasonic flaw detection.
As a result, as shown in FIG. 3A, for example, damage C such as creep voids or coarsening of crystal grains is searched in the vicinity of the first boundary B1b.

Therefore, a part of the weld W, that is, a portion where damage C such as creep voids or coarsening of crystal grains exists is removed (see FIG. 3B described later).
Specifically, in order to completely remove the found damage C, the region deeper than the damage C is removed in the thickness direction (the direction of the thickness T). At this time, only the vicinity of the first boundary portion B1b between the welded portion W and the second base material 13b is removed.

Next, as shown in FIG. 3B, a new weld metal is repaired in the removed region to form a repair weld 14.
The new weld metal, that is, the repair weld 14 is the same material as the weld W. Moreover, as a formation method (welding method) of the repair welding part 14, conventional welding methods, such as arc welding and MIG welding, are used.

(Remaining life evaluation method)
Next, the procedure of the life evaluation method for the steam pipe E will be described with reference to FIG.
FIG. 4 is a flowchart showing a procedure for evaluating the remaining life of the welded member.

First, in Step 1 (ST1), for example, in the laboratory, a sample member similar to the welded portion W (including the repair welded portion 14) of the steam pipe E (a member in which the repaired weld 14 is applied to the butt welded portion W) is prepared. A creep test is performed on the sample member.
Then, the structure of the surface (structure collection region S) of the boundary (second boundary portion B2) between the welded portion W and the repair welded portion 14 of the sample member is varied for example during the progress of the creep test. Collect multiple times.
Note that the tissue collection region S is a region illustrated in FIG.

Then, a ratio at the time of sampling the tissue sampling region S with respect to the lifetime obtained by the creep test of the sample member (that is, the ratio to the lifetime: remaining lifetime) is obtained.
For example, with respect to the life time, the tissue in the tissue collection region S at the time of 0% life (at the start of the test), 50% life, 80% life, and 100% life (at break) is collected.
Specifically, when the lifetime is 100,000 hours, the tissues in the tissue collection region S are sampled when 0 hour, 50,000 hours, 80,000 hours, and 100,000 hours have elapsed since the start of the creep test. .

FIGS. 5A to 5D are diagrams (photographs) showing a structure in the tissue collection region S obtained by a creep test of a sample member (four-stage sample photographs having different remaining lives).
5 (a) is at 0% life (at the start of the test), FIG. 5 (b) is at 50% life, FIG. 5 (c) is at 80% life, and FIG. 5 (d) is at 100% life (fracture). It is an organization chart (photograph) of organization collection field S in (time).
In addition, the scale attached | subjected to Fig.5 (a)-(c) has shown 100 micrometers. The scale attached to FIG. 5 (d) indicates 10 μm.

Next, in step 2 (ST2), the surface (structure collection region) of the boundary (second boundary B2) between the welded portion W and the repair welded portion 14 in the bent steam pipes 6 and 8 (steam pipe E) of the power plant 1. The tissue of S) is collected by, for example, the replica method (see FIG. 3B).
Note that the replica method is a method in which a film is attached to a region where a surface structure of a material has been revealed by polishing and corrosion, and irregularities are transferred to observe the material structure.

  Finally, in step 3 (ST3), the tissue collected from the tissue collection region S of the steam pipes 6 and 8 (steam piping E) and the tissue collected from the tissue collection region S of the sample member (remaining lifespan is different 4). The organization of the stage).

  For example, using various measuring devices, the crystal grain size, the number of creep voids, the creep void area ratio, the creep void density, and the like are measured and compared with those values in the sample member. Thereby, the remaining life of the welded part W of the steam pipes 6 and 8 (steam pipe E) can be estimated and evaluated.

For example, the case where the remaining life of the welded portion W of the steam pipes 6 and 8 (steam pipe E) is estimated and evaluated by comparing the crystal grain size (degree of coarsening of the crystal grains) will be described.
The larger the crystal grain size, that is, the degree of coarsening of the crystal grain, indicates that the recrystallization of the material has progressed and the material deterioration has progressed. As a method for measuring the size of crystal grains, a line analysis method using an optical microscope is suitable.

The crystal grain size (degree of coarsening of crystal grains) is the structure shown in FIG. 5 (a) at the time of 0% life, the structure shown in FIG. 5 (b) at the life of 50%, and the structure shown in FIG. 5 (c). Then, it can be confirmed in advance (measurement side) that the life is 80%, and the structure shown in FIG.
For example, the crystal grain size (degree of coarsening of crystal grains) of the structure collection region S of the steam pipes 6 and 8 (steam pipe E) is approximately the same as the crystal grain size at the time of 50% life of the sample member. It can be estimated and evaluated that the lifetime of the steam pipes 6 and 8 (steam pipe E) is 50% or more, in other words, the remaining life is within 50,000 hours.

  Furthermore, for example, when it is estimated and evaluated that 80% or more of the lifetime has passed (remaining lifetime is within 20,000 hours), stop using the steam pipes 6 and 8 (steam pipe E), It is possible to determine whether or not to perform full-scale repair such as replacement of the steam pipes 6 and 8 (steam pipe E).

  Thus, by comparing the structure of the prepared sample member during the creep test (four-stage sample photos with different remaining lives) with the structure of the actual machine (steam piping 6, 8 (steam piping E)), the actual machine It is possible to easily estimate / evaluate the remaining lifetime.

(Creep test for repair welds)
Here, in the life evaluation method described above, the structure of the surface (structure collection region S) of the boundary (second boundary part B2) between the welded portion W and the repair welded portion 14 is collected, and the remaining life is determined based on the structure state. Explain the basis for estimating and evaluating.
Specifically, it will be described with reference to FIGS. 6A to 6C and FIG. This creep test (JISZ2271) was performed on the welded portion W shown in FIG.

The welded portion W shown in FIG. 3 (b) is the one subjected to conventional repair welding, that is, only the region near the damage C (the region including the first boundary B1b between the welded portion W and the second base material 13b). It has been removed. In other words, only the region including the first boundary B1b is removed while leaving the first boundary B1a.
And the repair welding part 14 is formed by repairing a new weld metal in the removed area | region.
In addition, as above-mentioned, the boundary with the repair welding part 14 among the welding parts W is called 2nd boundary part B2.

FIGS. 6A to 6C are stress-lifetime relationship diagrams at the boundary between the base material and the weld.
6A to 6C, the vertical axis represents stress (MPa), and the horizontal axis represents lifetime (h) (log-log graph).
A curve M1 indicates a creep strength curve of the base material itself (high chromium steel). Curve B1 shows the creep strength curve of the first boundary B1 (first boundary B1a, B1b). A curve B2 indicates a creep strength curve of the second boundary portion B2.

FIG. 6A shows the creep strength at 675 ° C.
As shown in FIG. 6A, when a tensile stress of 4 Mpa is applied at 675 ° C., assuming that the lifetime of the base material itself is 1, the lifetime of the first boundary B1 is 2/3, It can be confirmed that the lifetime of the two boundary portions B2 is about 1/4 (that is, a short lifetime).

FIG. 6B is a diagram showing the creep strength at 650 ° C.
As shown in FIG. 6B, when a tensile stress of 6 Mpa is applied at 650 ° C., assuming that the lifetime of the base material itself is 1, the lifetime of the first boundary B1 is 1/2, It can be confirmed that the lifetime of the two boundary portions B2 is about 1/6 (that is, a short lifetime).

FIG.6 (c) is a figure which shows the creep strength in 625 degreeC.
As shown in FIG. 6C, when a tensile stress of 8 Mpa is applied at 625 ° C., assuming that the lifetime of the base material itself is 1, the lifetime of the first boundary B1 is 1/2, It can be confirmed that the lifetime of the two boundary portions B2 is about 1/4 (that is, a short lifetime).

  As apparent from FIGS. 6A to 6C, the second boundary portion B2, that is, the vicinity of the boundary between the welded portion W and the repair welded portion 14, is recognized as the region having the lowest creep strength. That is, it is recognized that the vicinity of the boundary between the first welded portion (welded portion W) and the welded portion (repair welded portion 14) added later is the region having the lowest creep strength.

  Therefore, it can be seen that when the remaining life is estimated / evaluated by the state (change, transition) of the metal structure, it is desirable to collect the structure in the region having the lowest creep strength (structure collection region S).

As described above, by comparing the structure of the sample member prepared in advance during the creep test (four-stage sample photograph) and the structure of the actual machine, the actual machine (steam pipes 6 and 8 (steam pipe E)) The remaining life can be easily estimated and evaluated.
In particular, by comparing the structures of the regions with the lowest creep strength (where creep voids or coarsening of crystal grains are likely to occur), it is possible to accurately determine the remaining life to the creep fracture point in the repair weld. .

  And by applying this remaining life evaluation method to the repair welded portion of the steam pipe connected to the steam boiler that generates steam, the remaining life of the steam boiler can be accurately grasped.

  The repair welding method shown in the above-described embodiment is an example, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.

For example, as a method for measuring the metal structure, an ultrasonic method, an SPD method, an EBSP method, or the like may be used in addition to the replica method.
Regardless of which measurement method is used, the remaining life of the welded portion is estimated and evaluated based on the surface texture of the welded portion W, the repair welded portion 14, and the boundary portion (second boundary portion B2). Anything is acceptable.

Further, the present invention is not limited to the case of comparing the crystal grain size, and may be the case of comparing the creep void area ratio, the number of creep voids, and the creep void density within a predetermined range.
Moreover, although the case where the four-stage metal structure photograph in which the remaining life in the structure | tissue extraction area | region S of a sample member differs was demonstrated, you may use a multi-stage metal structure photograph etc. further.

  Further, the treatment after estimating and evaluating the remaining life of the steam pipes 6 and 8 (steam pipe E) (determination criteria for performing full-scale repair or the like) can be arbitrarily set.

  DESCRIPTION OF SYMBOLS 1 ... Power plant, 2 ... Steam boiler, 6,8, E ... Steam piping (welding member), 10 ... Superheater, 12 ... Reheater, 13a ... First base material, 13b ... Second base material, 14 ... Repair weld, B1 (B1a, B1b) ... first boundary, B2 ... second boundary, C ... damage, W ... weld, S ... tissue collection area (surface)

Claims (6)

A method for evaluating the remaining life of a welded member in which a repair weld is applied to the boundary between the first base metal and a weld portion where the first base material and the second base material are butt-welded,
The remaining life evaluation method characterized by evaluating the creep remaining life of the said welding member based on the state of the surface structure of the said weld part, the said repair welding part, and a boundary part.   The remaining life evaluation method according to claim 1, wherein the remaining life of creep of the welded member is evaluated based on crystal grain size, number of voids within a predetermined range, void density, or void area ratio.   Samples of the surface texture in a plurality of stages having different remaining lives are collected in advance by testing, and the creep remaining life of the welding member is evaluated by comparing the state of the surface structure of the welding member with the state of the structure of the sample. The remaining life evaluation method according to claim 1.   The remaining life evaluation method according to any one of claims 1 to 3, wherein the surface texture is collected by a replica method.   The remaining life evaluation method according to any one of claims 1 to 4, wherein the remaining life evaluation method according to any one of claims 1 to 4 is used as a remaining life evaluation method for a welded portion of a steam pipe connected to a steam boiler that generates steam. .   The remaining life evaluation method according to claim 5, wherein the steam pipe is a steam pipe connected to a superheater or a reheater of the steam boiler.