JP5492057B2 - Damage prediction method for heat-resistant steel welds - Google Patents

Description

  The present invention relates to a method for predicting damage to heat-resistant steel, and in particular, predicts damage to welded and heat-affected zones of high-chromium steel welded joints used in high-temperature and high-pressure piping of thermal power plants such as thermal power generation boilers. This is an effective method.

  Since a thermal power plant is operated in a high-temperature and high-pressure environment, a material having excellent high-temperature strength is used for the heat-resistant steel of the equipment members constituting the facility. However, damage due to creep or the like may accumulate due to operation at a high temperature for a long period of time, leading to the life of the material. In designing and operating a thermal power plant, it is important to accurately evaluate the life of heat-resistant steel and improve reliability.

  High heat resistant steel containing about 9-12% chrome (Cr) (mass%, the same shall apply hereinafter) is used as heat resistant steel used in equipment for thermal power plants. Is known to proceed selectively. There are a wide variety of welds in high Cr steels used in thermal power plants. FIG. 13 shows, as an example, a perspective view of a heat-resistant steel (high Cr steel) pipe having a weld in the axial direction of the pipe.

  A welded portion 51 exists in the pipe 50 in the axial direction. FIG. 14 is an enlarged detailed view of a D portion of the welded portion in FIG. The base material 52 is welded with a weld metal 53, and weld heat affected zones (HAZ) 54 a and 54 b exist on both sides of the weld metal 53. The welding heat affected zone 54 is a portion where the base material 52 has undergone a structural change due to a temperature rise during welding.

  The weld heat affected zone 54a, 54b of high Cr heat resistant steel has a lower creep strength than the weld metal 53 and the base metal 52, and damage (generation of creep voids and cracks) concentrates on the heat affected zone in the weld zone. It has been known. FIG. 15 shows an example of the creep void 57 generated in the welding heat affected zone 54a, 54b.

When the pipe 50 is exposed to a state in which stress is applied at a high temperature for a long time, voids called creep voids 57 are generated on the crystal grain boundaries 56. The number of holes increases with the progress of damage due to long-term use, and eventually the adjacent creep voids 57 and 57 are united to form a crack.
FIG. 16 shows the weld 51 where the creep voids 57, 57 are joined together and a crack 58 is generated. The crack 58 generated in the welding heat affected zone 54a, 54b gradually grows and finally penetrates in the plate thickness direction S to cause leakage of internal fluid.

As a method of evaluating the life of the heat-resistant steel, there is a method of inspecting the occurrence of the creep void 57 of the welded portion 51 and estimating the remaining life based on the result.
The following Patent Document 1 (Method for evaluating the life of heat-resistant steel) discloses a method of measuring the number density of creep voids a plurality of times and estimating the remaining life from the rate of change. Patent Document 2 (Creep Life Evaluation Method) below discloses a method of estimating the creep life consumption rate by measuring the maximum value of the crystal void boundary occupancy of the creep void.

In the heat resistance steel life evaluation methods described in these documents, the number density of creep voids of the heat resistant steel to be inspected and the grain boundary occupation ratio of the creep voids are measured and applied to the results obtained in advance. Estimating the remaining life. Therefore, when carrying out these methods, it is necessary to detect and measure the number density of creep voids and the grain boundary occupancy ratio of creep voids every time, and the operation is complicated and a simpler and faster method is required. preferable.
In addition, since there are a wide variety of equipment members of a thermal power plant, there are a wide variety of high Cr steel welds, and the effect of the stress field acting on the shape varies.

  In Patent Document 1 and Patent Document 2 described above, damage is predicted using the number density of creep voids and the grain boundary occupation ratio of creep voids. The relationship between the number density of creep voids and the grain boundary occupancy ratio of creep voids and damage obtained in the stress state) does not hold in a complex stress state such as an actual thermal power plant. It is difficult to accurately estimate the remaining life of heat resistant steel.

  Therefore, in Patent Document 3 (degradation evaluation method for heat-resistant steel and degradation evaluation method for turbine) described below, the generation behavior of creep voids in the welded portion of high Cr steel is affected by the shape of the member, that is, by the stress field. In order to take into account, a degradation evaluation method is disclosed in which the accuracy of damage prediction is improved by using a value obtained by normalizing the number density of creep voids with a stress multiaxiality coefficient.

  However, Patent Document 3 only describes the expression of normalizing the number density of creep voids by the stress multiaxiality coefficient, and does not disclose the specific contents of normalization. Since the remaining life of the heat-resistant steel also changes depending on the standardization method, it is not always possible to estimate the remaining life of the heat-resistant steel with high accuracy. Similarly to Patent Document 1 and Patent Document 2, when carrying out the method, it is necessary to detect the number of creep voids each time and measure the number density of the creep voids.

  There are many heat-resistant steel welds in equipment and equipment of thermal power plants, but it is difficult to inspect the degree of damage due to the fact that heat insulation is applied, there is no scaffold, and access is difficult . Further, even if inspection is carried out, it is usually difficult to detect from the surface of the heat-resistant steel because damage often occurs from the inside of the heat-resistant steel in the thickness direction. For these reasons, it is desired to develop a method capable of accurately estimating and predicting damage from the calculation result of stress applied to the heat resistant steel and the metal temperature of the heat resistant steel.

  As a general method for predicting damage from the magnitude of stress, there is a method for estimating damage and life of heat-resistant steel from a single-shaft round bar creep test result (specified in the creep and creep rupture test methods of metal materials of JIS Z 2271). is there.

In FIG. 17, the shape of the creep test piece of a uniaxial round bar is shown.
The round bar creep test piece 59 is made of heat-resistant steel (high Cr steel) to be evaluated, and has a welded portion, that is, a weld metal 61 and a weld heat affected zone 62 at the center of the base material 60. The test piece is subjected to stress σ in the direction indicated by arrows Y1 and Y2 in the drawing at high temperature, the time until breakage is measured, and the relationship between the stress and the breakage time is examined.

FIG. 18 shows an example of creep rupture data obtained by this method. In FIG. 19, the stress which acts on the welding part of a piping axial direction in a thermal power plant is shown.
The stress σC and the stress σL in the circumferential direction of the pipe act on the weld metal 53 and the weld heat affected portions 54a and 54b of the pipe 50, respectively. When the damage of the welded portion of FIG. 19 is estimated using the relationship of FIG. 18, the fracture time tr is estimated from the relationship of FIG. 18 using the circumferential stress σC acting on the pipe, and the welding of FIG. The progress degree of damage of the part 51 is estimated.

Japanese Patent No. 3825378 Japanese Patent No. 3976938 JP 2009-92478 A

  The conventional methods described above have problems that the operation is complicated and the prediction accuracy of the remaining life of the heat resistant steel is not high. The creep rupture data (FIG. 18) used for prediction is obtained from a round bar creep test piece in a state where stress is applied to the uniaxial round bar shown in FIG. However, the weld joint of the heat-resistant steel welded part actually used is, for example, in a biaxial stress state or a triaxial stress state (including stress acting in the radial direction) as shown in FIG. The stress state is different from the test. If the stress state is different, the degree of progress of damage in the welded portion also differs. Accordingly, it is difficult to accurately predict the progress of creep damage and the life of the welded joint as shown in FIG. 19 using the creep rupture data shown in FIG.

  The object of the present invention is to solve the above problems and provide a method capable of accurately predicting damage (occurrence of creep voids and cracks) of a weld in a multiaxial stress state in accordance with a heat-resistant steel weld used in practice. That is.

The subject of this invention is the function (parameter) of following formula (1) which consists of maximum principal stress (sigma) ₁ and stress multiaxiality factor TF in the local stress state of a heat-resistant-steel-welding part.
M = A ・ σ ₁・ TF ^B (1)
(Where A, B: coefficient, σ ₁ : maximum principal stress, TF: stress multiaxiality coefficient), the relationship between the local stress state M and the increase rate of the number density of creep voids, or the local stress state M And the increase rate of the area ratio of the creep voids in advance, and when the damage is actually predicted, the stress analysis of the welded portion to be evaluated is performed to determine the local stress state M in the evaluation target portion. It is solved by predicting the damage of the heat-resistant steel weld using M.
In the three-dimensional problem, there are three main stresses, and they are expressed as σ ₁ , σ ₂ , σ ₃ from the largest. Here, the largest one is described as “maximum principal stress σ ₁ ”.

  Specifically, the number of creep voids in the evaluation target region based on the relationship between the local stress state M obtained in advance and the increase rate of the number density of the creep voids or the relationship between the local stress state M and the increase rate of the area ratio of the creep voids. Obtain the increase in density or creep void area ratio as a function of time, and further determine the time (life) until the number density of creep voids or the area ratio of creep voids reaches a predetermined maximum value. Thus, the degree of damage of the heat-resistant steel weld can be predicted.

That is, the problem of the present invention can be solved by the following means.
The invention according to claim 1 calculates the distribution of stress acting on the heat-resistant steel weld, and predicts damage of the heat-resistant steel weld from the calculated value. It is a damage prediction method of a heat-resistant steel weld using M represented by the following formula (1) as a parameter expressing a stress state.
M = A ・ σ ₁・ TF ^B (1)
Here, A and B are coefficients, σ _{1 is a} maximum principal stress, and TF is a stress multiaxiality coefficient.

  In the invention according to claim 2, the relationship (a) between the parameter M and the increase rate of the number density of creep voids in the heat-resistant steel weld zone, or the parameter M and the increase rate of the creep void area ratio in the heat-resistant steel weld zone. The relationship (b) is obtained in advance, the parameter M in the target part of the heat-resistant steel weld is obtained, and the creep void in the target part is determined from the parameter M in the target part and the relation (a) or relationship (b) obtained in advance. The damage prediction method for a heat-resistant steel weld according to claim 1, wherein the number density of creep voids or the area ratio of creep voids is estimated by determining the rate of increase of the number density or the area rate of creep voids.

  Invention of Claim 3 is a damage prediction method of the heat-resistant-steel-welding part of Claim 1 or 2 which calculates the parameter M using the local stress calculated | required from the creep analysis result by a finite element method (FEM). is there.

  According to a fourth aspect of the present invention, the number density of the creep voids or the area ratio of the creep voids is a predetermined maximum from the initial generation of the creep voids as the rate of increase of the number density of the creep voids or the rate of increase of the area ratio of the creep voids. The damage prediction method for a heat-resistant steel weld according to claim 2, wherein the number density of creep voids or the area ratio of creep voids is estimated by using the average increase rate at the end stage as a value.

  In the invention according to claim 5, as the relationship (a), the relationship between the parameter M in the heat-affected zone of the heat-resistant steel weld and the rate of increase in the number density of the creep voids, and the relationship (b) as the heat-resistant steel welding The heat resistant steel welded portion according to claim 2, wherein the relationship between the parameter M in the weld heat affected zone of the welded portion and the rate of increase in the area ratio of the creep void is used, and the target portion of the heat resistant steel welded portion is the weld heat affected zone. This is a damage prediction method.

  The invention according to claim 6 is the relationship (c) between the increase rate of the number density of the creep voids and the average increase rate of the number density of the creep voids per predetermined time in the heat resistant steel welded portion, or the predetermined time in the heat resistant steel welded portion. The relationship (d) between the increase rate of the area ratio of the creep void and the average increase rate of the creep void area ratio is obtained in advance, and the number density of the creep voids in the target site determined from the relationship (a) From the average increase rate and the relationship (c), the increase rate of the number density of the creep voids per predetermined time in the target site is obtained, or the average of the void void area ratio in the target site obtained from the relationship (b) From the increase rate and the relationship (d), the creep void number density is obtained by obtaining the increase rate of the area ratio of the creep voids at predetermined time intervals in the target region. Or claim 4 damage prediction method of the heat-resisting steel weld, wherein the estimating the area ratio of the creep voids.

(Function)
Creep damage (an increase in the number density of creep voids or an increase in the area ratio of creep voids) in a heat-resistant steel weld is greatly affected by the stress state. In particular, it is affected not by unidirectional stress but by tri-directional stress (stress triaxial state). According to the present invention, considering the triaxial stress state in the evaluation target part of the welded portion such as the weld heat affected zone, the relationship between the triaxial stress state obtained in advance and the increase behavior of the number density of the creep voids or triaxial In order to predict the increase in the number density of creep voids or the increase in the area ratio of creep voids using the relationship between the stress state and the increase in the area ratio of creep voids, It becomes possible to predict.

  According to the first aspect of the present invention, by using M represented by the above formula (1) as a parameter expressing the stress state of the heat-resistant steel weld, it is possible to easily and quickly obtain accuracy by calculation such as stress analysis. Good damage (creep voids and cracks) can be predicted.

  According to the invention described in claim 2, in addition to the operation of the invention described in claim 1, the relationship (a) or relationship (b) obtained in advance and the parameter M in the target part of the heat-resistant steel welded portion By determining the increase rate of the number density of creep voids or the increase rate of the area ratio of creep voids at the site, the number density of creep voids or the area ratio of creep voids can be estimated. Without performing complicated operations such as measuring the number density and area ratio, it is possible to make a more accurate prediction according to the actually used heat-resistant steel weld.

For example, it is performed as follows.
In advance, the number density of creep voids or the area ratio of creep voids is detected at each evaluation site of the heat-resistant steel weld to obtain the increase rate of the number density of creep voids or the increase rate of the area ratio of creep voids. The parameter M at each evaluation site of the weld is obtained, and the relationship between the parameter M and the increase rate of the number density of the creep voids (a), or the relationship between the parameter M and the increase rate of the area ratio of the creep voids (b). Ask for.
When the damage is actually predicted, the maximum principal stress σ ₁ and the stress multiaxiality factor TF of the target portion of the heat-resistant steel weld are obtained, and the obtained maximum principal stress σ ₁ and the stress multiaxiality factor TF are obtained. The parameter M is obtained by applying to the above formula (1), and using the relationship (a) or (b) obtained in advance from the obtained parameter M, the increase rate of the number density of the creep voids in the target site or the creep void By determining the increase rate of the area ratio, it is possible to estimate the number density of creep voids or the area ratio of creep voids in the target site.

According to invention of Claim 3, in addition to the effect | action of the invention of the said Claim 1 or 2, the shape of a welding part is obtained by calculating the parameter M from the creep analysis result by a finite element method (FEM). Even if it is complicated, it is applicable, and damage to the heat-resistant steel weld can be easily predicted. When creep analysis is performed by the finite element method (FEM), principal stresses (σ ₁ , σ ₂ , σ ₃ ) and Mises equivalent stress at each point are obtained. When these are substituted into the above equation (1), the stress multiaxiality coefficient TF is obtained.

  According to invention of Claim 4, in addition to the effect | action of the invention of said Claim 2, the increase rate of the number density of the creep void of the relationship (a) or the increase of the area ratio of the creep void of the relationship (b) As the speed, the average increase speed from the initial stage to the end stage of damage is used. The increase in the number density of the creep voids and the increase in the area ratio of the creep voids varies, but an accurate average speed can be calculated by using the average increase speed.

  According to the invention described in claim 5, in addition to the action of the invention described in claim 2, the rate of increase in the number density of creep voids or the increase in the area ratio of creep voids in the weld heat affected zone of the heat resistant steel weld zone. Since the speed can be grasped, it is possible to predict the damage caused by the heat affected zone, which is particularly affected by welding.

In addition, the increase rate of creep voids generated in the heat-resistant steel weld zone is higher in the final stage when the creep voids coalesce with each other than in the initial stage at which they start to occur.
Therefore, according to the invention described in claim 6, in addition to the action of the invention described in claim 4, the increase rate of the number density of creep voids and the average increase rate of the number density of creep voids per predetermined time The relationship (c), or the relationship (d) between the increase rate of the creep void area rate and the average increase rate of the creep void area rate every predetermined time is obtained in advance, and the relationship (a) and the relationship (c), Alternatively, from the relationship (b) and the relationship (d), the increase rate of the number density of creep voids per predetermined time or the increase rate of the area ratio of the creep voids per predetermined time is obtained. The amount of increase in the creep void area ratio can be determined as a function of time. Therefore, since the increase in the number density of the creep voids can be grasped more accurately, the number density of the creep voids or the area ratio of the creep voids at the target site at the time of measurement can be accurately estimated.

  According to the damage prediction method for heat-resistant steel welds of the present invention, damage to the weld heat-affected zone in a complex stress state (occurrence of creep voids and cracks) in a simple and quick manner according to the heat-resistant steel welds actually used. ) Can be accurately predicted. It is also possible to predict a site that requires detailed examination. In addition, excessive damage due to leakage of the internal fluid of the pipe can be prevented in advance.

Specifically, it has the following effects.
According to the first aspect of the present invention, by using M represented by the above formula (1) as a parameter expressing the stress state of the heat-resistant steel weld, it is simple and quick by calculation such as stress analysis. Predicts damage to heat-resistant steel welds with high accuracy.

  According to the invention described in claim 2, in addition to the effect of the invention described in claim 1, the number density of creep voids or the area ratio of creep voids is estimated from the relationship obtained in advance without performing complicated operations. In addition, it is possible to accurately predict the heat-resistant steel welded part that is actually used.

  According to the invention of claim 3, in addition to the effect of the invention of claim 1 or 2, by calculating the parameter M from the creep analysis result by the finite element method, even if the shape of the weld is complicated, Easily predict damage to heat-resistant steel welds.

  According to the invention described in claim 4, in addition to the effect of the invention described in claim 2, an accurate average speed is obtained by using the average increase speed as the increase speed of the relation (a) or the relation (b). And the damage of the heat-resistant steel weld can be predicted with high accuracy.

  According to the invention described in claim 5, in addition to the effect of the invention described in claim 2 above, it is possible to predict the damage caused by the welding heat affected zone particularly affected by welding. The damage can be predicted with good accuracy.

  According to the invention described in claim 6, in addition to the effect of the invention described in claim 4, the increase in the number density of creep voids or the increase in the area ratio of creep voids can be grasped more accurately, so The number density of creep voids or the area ratio of creep voids can be estimated with high accuracy. Therefore, it is possible to predict damage with high accuracy in accordance with the heat-resistant steel welded part that is actually used.

Is a graph showing the relationship between the average rate of increase and stress parameters of the number density of creep voids _{(M = A · σ 1 ·} TF B) used in the damage prediction method of the heat-resisting steel welds of an embodiment of the present invention. It is the side view which showed the position which evaluated the local stress in the welding heat affected zone of piping used for the damage prediction method of the heat-resistant-steel-welding part of one Embodiment of this invention. It is the figure which showed distribution of the local stress in the welding heat affected zone of FIG. It is the figure which showed distribution of the stress multiaxiality coefficient TF in the welding heat affected zone of FIG. It is a flowchart of Example 1 of the damage prediction method of the heat-resistant-steel-welding part of this invention. It is the figure which showed the relationship between the increase rate Nv of the number density of a creep void for every predetermined time used in Example 1 of the damage prediction method of the heat-resistant-steel-welding part of this invention, and the average increase rate Nvm of the number density of a creep void. . It is the figure of piping used in order to confirm the precision of the damage prediction method (Example 1) of the heat-resistant-steel-welding part of this invention. FIG. 8 is a detailed view of part B in FIG. 7. It is the figure which showed the FEM stress analysis model used in order to confirm the precision of the damage prediction method (Example 1) of the heat-resistant-steel-welding part of this invention. It is the figure which showed the comparison with the change of the number density of the creep void estimated by the damage prediction method (Example 1) of the heat-resistant-steel-welding part of this invention, and an experimental value. It is the figure which showed the comparison with the distribution in the thickness direction of the number density of the creep void estimated by the damage prediction method (Example 1) of the heat-resistant-steel-welding part of this invention, and the experimental value. Is a diagram showing the relationship between damage prediction method of the heat-resisting steel welds average increase rate and the stress parameter area ratio of creep voids used in (Example 2) (M = A · σ 1 · TF B) of the present invention . It is a perspective view of the general heat-resistant steel (high Cr steel) piping which has a welding part in the axial direction of piping. It is detail drawing of the D section of FIG. It is the figure which showed the creep void which generate | occur | produced in the welding heat affected zone of the piping of FIG. It is the figure which showed the crack which generate | occur | produced in the welding heat affected zone of the piping of FIG. It is the figure which showed the creep test piece of the single axis | shaft round bar used with the damage prediction method of the conventional heat-resistant steel weld. It is the figure which showed the example of the creep rupture data calculated | required using the round bar single axis | shaft creep test piece of FIG. It is the figure which showed the stress which acts on the welding part of the axial direction of piping of FIG.

The following explains the principle of predicting the damage of heat-resistant steel welds.
FIG. 1 shows an increase amount (average increase rate) per unit time of the number density of creep voids (number of creep voids per unit area) used for damage prediction by the damage prediction method of the present embodiment and stress parameters ( An example of the relationship of M = A · σ ₁ · TF ^B ) is shown.

The method of obtaining FIG. 1 is as follows.
That is, the creep test of the pipe 6 of the test body as shown in FIG. In the welding heat affected zone 8a to 8d, the maximum principal stress σ ₁ and the stress multiaxiality factor TF differ depending on the position in the thickness direction. The growth state of creep voids is observed with a microscope at six points in the thickness direction, and the average increase rate Nvm of the number density of creep voids is measured. On the other hand, the maximum principal stress σ ₁ and the stress multiaxiality coefficient TF at these six points are obtained by triaxial stress analysis. These are plotted on a log-log graph, and coefficients A and B are determined by linear regression.

The average increase speed Nvm shown on the vertical axis in FIG. 1 is obtained as follows.
That is, several (for example, five) creep tests of the pipe 6 of the test specimen as shown in FIG. 7 are prepared. The experiment is started under the same experimental conditions. When the life is 1000 h, the experiment is canceled every 1 hour until 1000 h and the rest every 200 h, and the state of creep void generation inside the pipe 6 is observed. Note that the observation sites are all welding heat affected zones 8a to 8d. The relationship between the number density N of creep voids and time is determined from the five test pieces. The gradient of this relationship is the increase rate Nv (number / mm ² / h) of the individual density of creep voids, and the average value of these is the average increase rate Nvm.
It is also possible to measure the number density of creep voids by a replica method (a method in which a metal structure of an evaluation site is copied to a replica film and observed with a scanning electron microscope (SEM) or the like and evaluated thereby). In this case, only the surface can be measured.

  Moreover, although the evaluation site | part of the damage of the piping 6 is not restricted to the welding heat affected zone 8a-8d, it is good also as a weld metal and base material of welded part 7a, 7b, but the welding heat affected zone 8a-8d is other welded parts. In comparison, the creep strength is low, and damage is concentrated on the heat affected zone in the welded portion. Therefore, it is preferable to use the weld heat affected zone 8a to 8d, which is particularly affected by welding, as the evaluation site.

   The average increase rate Nvm is an average value from a region where damage is small (a region where creep voids start to be generated) to a large region (a region where creep voids coalesce to form a crack). The increase in the number density of creep voids and the increase in the area ratio of creep voids varies, but by calculating the velocity as the average from the initial stage (new material, etc.) to the end of life (evaluating the velocity with the longest possible sampling time) ), An accurate average speed can be calculated.

The horizontal axis in FIG. 1 is a parameter represented by the following formula (1), and represents a triaxial stress state of local stress in the welding heat affected zone 4.
M = A ・ σ ₁・ TF ^B (1)
Here, A and B are coefficients, σ ₁ is the principal stress, TF is a stress multiaxiality coefficient TF (Triaxiality Factor) representing the triaxiality of the stress, and is defined by the following equation (2) .
TF = (σ ₁ + σ ₂ + σ ₃ ) / σ _m (2)
Here, σ ₁ , σ ₂ , and σ ₃ (shown from the larger one) are principal stresses in the circumferential direction, axial direction, and radial direction, respectively, in the case of piping. σ _m is Mises' equivalent stress.

In FIG. 2, the side view which shows the position which evaluated the local stress in the welding heat affected zone 4 of the piping 1 used for the damage prediction method of one Embodiment of this invention is shown.
In the heat-affected zone 4a of the heat-resistant steel pipe 1 shown in FIG. 2, the creep analysis (FIG. 9) by the finite element method was performed, and the result of examining the stress distribution along the stress evaluation line 5 from the outer surface of the pipe is shown in FIG. Shown in

  The circumferential, axial, and radial stresses shown in FIG. 3 are local stresses on the stress evaluation line 5. The horizontal axis in FIG. 3 is a value (x / t) obtained by dividing the distance x from the outer surface of the pipe 1 by the thickness t and making it dimensionless. The pipe 1 is subjected to stress σL in the axial direction and σC in the circumferential direction due to internal pressure, but the deformation rates of the creep voids of the base material 2, the weld metal 3, and the weld heat affected zones 4a and 4b are respectively Since they are different, complicated local stresses are generated in the welding heat affected zones 4a and 4b. Speaking of the welding heat affected zone 4a, the stress at the central portion A of the wall thickness is the largest, and all the axial, circumferential and radial directions (not shown) are in a triaxial tensile stress state. Strictly speaking, it is triaxial, but since the stress in the radial direction is smaller than the others, it may be considered almost biaxial.

FIG. 4 shows the distribution of the stress multiaxiality coefficient TF in the welding heat affected zone 4a of the heat resistant steel pipe 1 of FIG. 2, and the stress multiaxiality coefficient TF on the stress evaluation line 5 and the distance (x / T). The stress multiaxiality coefficient TF is obtained by applying the principal stresses σ ₁ , σ ₂ , and σ ₃ in FIG. ₃ to the above equation (2).
As described above, since the stress at the point A at the thickness center is large and is in a triaxial state, the TF representing the triaxiality of the stress is the largest at the point A at the center of the thickness.

From the observation results of the generation of creep voids and the growth behavior in the weld heat affected zone of heat-resistant steel, the present inventors determined the local stress at each point (each evaluation site) of the heat affected zone shown in FIG. 3 and FIG. The relationship between the increase in the number density of creep voids or the increase in the area ratio of creep voids in each evaluation site is as shown in FIG. 1, and by using this relationship, damage to the weld heat affected zone can be accurately performed. It was found that can be predicted.
And the relationship of FIG. 1 is measured and prepared in advance, and is applied to the prediction method of the heat-resistant steel weld of Example 1 shown below.

(Specific example of prediction method for heat-resistant steel welds)
A method for predicting the damage of the heat-resistant steel weld using the aforementioned prediction principle will be specifically described. FIG. 5 shows a flowchart of an embodiment of the damage prediction method.

When the operation of the thermal power generation boiler is started, first, the magnitude Δt of the time step (measurement time interval) for calculation is determined. The calculation time step Δt is determined as follows, for example.
When calculation up to 100,000 hours (prediction of damage progress) is to be performed, it is considered that the time step of calculation is sufficient if it is 100 divisions. Therefore, in this case, Δt = 100,000 hours / 100 = 1000 h.

Then, the number N of creep voids and the measurement time t are set to initial values (zero). Next, the creep analysis by the finite element method is performed on the evaluation target part, and the stress distribution at the measurement time t is obtained. That is, the main stresses σ ₁ , σ ₂ , and σ ₃ in the evaluation target part are obtained.

  The creep void does not occur immediately after the start of the operation of the thermal power generation boiler, and begins to occur after a predetermined time has elapsed. Therefore, it is confirmed whether or not the first measurement time t is equal to or longer than the preset creep void generation start time. If the first measurement time t is less than the preset creep void generation start time, the first measurement time t is advanced by Δt (the new measurement time is t + Δt), and the creep analysis at the new measurement time t is performed. To do.

On the other hand, if the creep void generation start time is equal to or greater than the preset creep void generation start time, the stress parameter M represented by the above formula (1) is calculated in each evaluation target portion of the welding heat affected zone. Necessary principal stress σ ₁ and stress multiaxiality coefficient TF are calculated by creep analysis by the finite element method (FIG. 9), and the stress parameter M is calculated from these values.

  Furthermore, the average increase rate Nvm of the number density of creep voids is determined from the stress parameter M using the relationship of FIG. Nvm in FIG. 1 is an average value from the initial stage where creep voids start to occur to the final stage where creep voids coalesce and become cracks (when the number density N of creep voids reaches a predetermined maximum value).

  FIG. 6 shows a relationship between an increase rate Nv of the number density of creep voids per predetermined time and an average increase rate Nvm of the number density of creep voids. The creep void number density increase rate Nv) / (creep void number density average increase rate Nvm). FIG. 6 shows a correction method from the average increase rate when the increase rate of the number density of creep voids is not constant and it is desired to consider acceleration.

The relationship of FIG. 6 is obtained from the relationship between the number density N of creep voids and the increase rate Nv of the individual density of creep voids when the average increase rate Nvm of FIG. 1 is obtained. Therefore, when the average increase speed Nvm in FIG. 1 is obtained, the relationship in FIG. 6 is also obtained in advance.
As can be seen from FIG. 6, the speed at the end stage where the creep voids coalesce with each other, that is, the rate of increase in the number density of the creep voids per predetermined time, is increased compared to the initial stage where creep voids start to occur. .

  Therefore, using the relationship shown in FIG. 6 prepared in advance, the creep void number density increase rate corresponding to the creep void number density N at the measurement time t from the creep void number density average increase rate Nvm. Nv is obtained. For example, in the case of FIG. 5, since the initial value of N is 0, Nv when N = 0 is obtained. Then, the increase rate Nv is multiplied by the time step Δt to obtain an increase amount ΔN of the number density of creep voids in this step. By adding this ΔN to N, the number density N of creep voids at the measurement time t can be calculated.

The above method is summarized as follows.
In a certain calculation step (time step), the average increase rate Nvm of the number density of creep voids is obtained from the stress parameter M. Using the average increase rate Nvm of the number density of creep voids and the number density N of creep voids in the previous step, the increase rate Nv of the number density of creep voids at this time is obtained from FIG. Creep void number density increase rate Nv is multiplied by Δt to obtain ΔN (creep void number increase in this time step), and creep void number density N in the previous step is added to ΔN to creep in this time step The void number density is N.
Therefore, it becomes possible to predict the damage of the heat-resistant steel weld at the measurement time t with higher accuracy.

The above procedure is repeated until the number density N of the creep voids reaches a limit value Nc (for example, about 800 / mm ² ) at which the predetermined maximum value is reached. Note that the limit value Nc is a value at the end stage when the creep voids coalesce to form a crack, and varies depending on the type of heat-resistant steel and the part to be evaluated. By this calculation, the number density N of creep voids at time t (0 to the time until reaching the limit value) can be calculated. That is, the relationship as shown in FIG. 10 is required.

  The prediction of the damage of the heat-resistant steel weld can be made from the number density N of creep voids at the measurement time t determined from the above calculation. The time until the creep void number density N reaches the limit value Nc can be predicted to be the life of the heat-resistant steel.

(Accuracy confirmation of the above method)
In FIG. 7, the figure of the heat-resistant steel piping used in order to confirm the precision of a present Example is shown, and the detailed view of the B section of FIG. 7 is shown in FIG.

  In order to confirm the effectiveness of the damage prediction method shown in FIG. 1 and FIG. 5, the prediction result by the method of this example and the experimental result were compared using the heat-resistant steel pipe 6 of the test body shown in FIG. 7. The pipe 6 in FIG. 7 is a heat-resistant steel pipe 6 that has a weld 7 and a weld heat affected zone 8 in the axial direction and receives an internal pressure P. FIG. 8 shows details of part B of FIG. The number density of creep voids determined by the method of this example and the experiment was compared along the line indicated by the dotted line 9 in the weld heat affected zone 8a.

FIG. 9 shows a finite element method (FEM) analysis model 10 for obtaining a detailed stress distribution of the pipe 6 of FIG.
Using this analysis model 10, the stress parameter M was obtained according to the flowchart of FIG. 5, and the time change of the number density of the creep voids was predicted from the relationship of FIG. In the experiment, the test was stopped halfway, the pipe 6 was cut, and the creep void generation state (cross section) inside the pipe 6 was observed.

  FIG. 10 shows the change over time in the number density of creep voids at point C in FIG. The solid line shows the result obtained by the method of this example, and the plotted points show the result obtained by experiments. Also from FIG. 10, it was confirmed that both the predicted value and the experimental value agree well.

FIG. 11 shows the distribution of the number density of creep voids along the dotted line 9 in FIG.
The horizontal axis of FIG. 11 is a value (x / t) obtained by dividing the distance x from the outer surface of the pipe 6 by the thickness t and making it dimensionless.
Also in FIG. 11, it was confirmed that the predicted value indicated by the solid line and the experimental value indicated by the plot point are in good agreement. From the above results, it was confirmed that the increase in the number density of creep voids can be predicted with high accuracy by the method of this example.

Therefore, according to the method of the present embodiment, it is possible to accurately predict the damage of the weld heat affected zone in a complicated stress state in accordance with the actually used heat resistant steel weld zone. And it becomes possible to estimate the damage of a heat-resisting-steel-welding part from the relationship and calculated value which were calculated | required previously in this way.
And the site | part which needs a more detailed test | inspection can also be estimated, and the necessity of the test | inspection of a heat-resistant steel welding part and replacement | exchange of heat-resistant steel itself can be grasped | ascertained. For example, the inspection includes a replica method (which can be applied only to the surface) and ultrasonic flaw detection (which can also inspect the inside of the wall).

FIG. 12 shows the relationship between the average increase rate of the creep void area ratio and the stress parameter M.
The area ratio of the creep void may be calculated by measuring the area of the micrograph by using the replica method or by using a cross section actually cut in the same manner as the above-described method of obtaining the average increase rate Nvm. The area ratio of the creep void is a value obtained by dividing the area occupied by the creep void shown in FIG. 15 by a certain observation area. By determining the area occupied by creep voids, the area ratio of creep voids can be determined in the same manner as the method for determining the number density of creep voids.

  In Example 1, the increase in damage was predicted from the increase in the number density N of creep voids. However, even when the area ratio of creep voids is used for damage evaluation of heat-resistant steel welds, the relationship shown in FIG. In the same manner as in Example 1, it is possible to predict an increase in the area ratio of creep voids, that is, an increase in damage to the heat-resistant steel weld.

The “creep void number density”, “creep void number density increase rate”, and “creep void number density average increase rate” in the flow of FIG. 5 are respectively “creep void area ratio” and “creep void area ratio”. By replacing with “rate of increase in rate” and “average rate of increase in area ratio of creep voids”, damage to the heat-resistant steel weld can be predicted in the same manner as in Example 1.
In this embodiment, the same effect as that of the first embodiment can be obtained.

  According to the present invention, it is an effective method for predicting damage not only to a thermal power generation plant but also to a high chromium weld, particularly a weld heat affected zone used for high temperature / high pressure piping.

DESCRIPTION OF SYMBOLS 1 Piping 2 Base material 3 Weld metal 4 Welding heat affected zone 5 Stress evaluation line 6 Piping (test body)
7 Welded portion 8 Welded heat affected zone 9 Dotted line 10 Finite element analysis model 50 Piping 51 Welded portion 52 Base material 53 Weld metal 54 Welded heat affected zone 56 Grain boundary 57 Creep void 58 Crack 59 Round bar creep test piece 60 Mother Material 61 Weld metal 62 Weld heat affected zone

Claims (6)

In the damage prediction method for heat-resistant steel welds, which calculates the distribution of stress acting on the heat-resistant steel welds and predicts the damage of the heat-resistant steel welds from the calculated values.
A damage prediction method for heat-resistant steel welds, wherein M represented by the following formula (1) is used as a parameter expressing the stress state of the heat-resistant steel welds.
M = A ・ σ ₁・ TF ^B (1)
Here, A and B are coefficients, σ _{1 is a} maximum principal stress, and TF is a stress multiaxiality coefficient. The relationship between the parameter M and the increase rate of the number density of creep voids (a) in the heat resistant steel weld zone or the relationship (b) between the parameter M and the increase rate of the creep void area ratio in the heat resistant steel weld zone is obtained in advance. Every
The parameter M at the target site of the heat-resistant steel weld is obtained, and the increase rate of the number density of the creep voids at the target site or the creep void of the parameter M from the target site and the previously determined relationship (a) or relationship (b) The damage prediction method for heat-resistant steel welds according to claim 1, wherein the number density of creep voids or the area ratio of creep voids is estimated by obtaining an increasing rate of the area ratio.   3. The damage prediction method for a heat-resistant steel weld according to claim 1, wherein the parameter M is calculated using a local stress obtained from a creep analysis result by a finite element method (FEM).   As the rate of increase in the number density of the creep voids or the rate of increase in the area ratio of the creep voids, the average rate of increase in the creep void number density or creep void area ratio from the initial generation to the predetermined maximum value at the initial stage. The damage prediction method for heat-resistant steel welds according to claim 2, wherein the number density of creep voids or the area ratio of creep voids is estimated by using. As the relationship (a), the relationship between the parameter M in the heat affected zone of the heat resistant steel weld zone and the increase rate of the number density of the creep voids, and as the relationship (b), the parameter at the weld heat affected zone of the heat resistant steel weld zone. Using the relationship between M and the creep void area rate increase rate,
The damage prediction method for a heat-resistant steel welded portion according to claim 2, wherein a target part of the heat-resistant steel welded portion is a welding heat affected zone. Relationship between the rate of increase in the number density of creep voids per predetermined time in the heat resistant steel weld zone and the average rate of increase in the number density of creep voids (c), or the area ratio of creep voids per predetermined time in the heat resistant steel weld zone The relationship (d) between the increase rate and the average increase rate of the creep void area ratio is obtained in advance,
From the average increase rate of the number density of creep voids in the target site determined from the relationship (a) and the relationship (c), to determine the increase rate of the number density of creep voids in the target site every predetermined time, or From the average increase rate of the area ratio of creep voids in the target site determined from the relationship (b) and the relationship (d), by determining the increase rate of the area ratio of creep voids in the target site for each predetermined time,
The damage prediction method for heat-resistant steel welds according to claim 4, wherein the number density of creep voids or the area ratio of creep voids is estimated.

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