JP7366509B2 - Method for estimating remaining creep life of high chromium steel - Google Patents

Description

Patent Law Article 30, Paragraph 2 applies Yaguchi et al., “Estimation of Creep Rupture Property Based on Short-term Data of Long-term Used P91 Steels”, 45th MPA-sEMI NAR 2019, FILDERHALLE in Leinfelden-Echterdingen near Stuttgart

Application of Article 30, Paragraph 2 of the Patent Law Yaguchi et al., “Estimating the remaining creep life of improved 9Cr steel based on short-time creep test data”, 57th High Temperature Strength Symposium, 2019, Tohoku University Qatar Science Campus Hall

The present invention relates to a method for estimating the remaining creep life of high chromium steel. More specifically, the present invention relates to a method for estimating the remaining creep life of high chromium steel based on short time creep test data.

The destructive evaluation method, which is one of the remaining life diagnosis methods for high-temperature equipment, is a method for diagnosing the remaining life of the target material based on tests. ) is considered to have the highest accuracy. However, since the destructive evaluation method has the following conditions and restrictions, its application to actual equipment is currently limited, except for boiler tube extraction materials.

a) Remaining life evaluation is carried out on areas where there is concern about the possibility of damage progression, so in many cases it is not permitted to collect samples for destructive testing.
b) Generally, samples are collected near the surface of the equipment, so there are cases where the test results cannot be said to represent the state of the area being evaluated.
c) Since the creep test conducted as a destructive test generally lasts several thousand hours at the longest, estimation of the remaining life of the actual machine involves a large amount of extrapolation. For some materials, no methodology has been established to properly perform this extrapolation.

In the case of high chromium steel (heat-resistant steel that is blown with about 9 to 12% chromium), which is often used in 600°C class thermal power plants, C) above may apply. It has been reported in many cases that when the long-term creep rupture life of high-chromium steel is evaluated based on short-time test data, the life tends to be longer than the actual life.

Therefore, methods have been proposed such as dividing test data to be evaluated into several regions (Non-patent Document 1) and dividing it by (0.2% proof stress)/2 (Non-Patent Document 2). .

K. Maruyama, J. Nakamura and K. Yoshimi, ASME J. of Pressure Vessel Technology, Vol.138, 031407-1, 2016 K. Kimura, ASME PVP2005-71039, 2005

However, although these methods are effective in the case of materials for which test data is available over a long period of time, they are not suitable for cases where creep characteristics in a long period of time are estimated from data of several points of several thousand hours or less. Gender appears to be unknown. In order to perform destructive evaluation of actual high chromium steel piping, it is necessary to solve the problem of test time constraints.

An object of the present invention is to provide a method for estimating long-term creep characteristics from short-time test data for long-term use high chromium steel.

In order to achieve this objective, the present inventors conducted systematic creep tests on samples taken from a large number of high chromium steel piping (base metal and welded parts) that had been used for a long time in actual equipment. As a result of analyzing the obtained test data, it was found that it can be uniformly organized using the Larson-Miller parameter (hereinafter abbreviated as LMP) without depending on the stress or rupture time region. As a result of sorting it out using LMP, it was discovered that the shape of the main fracture curve of long-term use high chromium steel is constant regardless of the strength of the material, and moves in parallel only in the horizontal axis direction. . Furthermore, the present inventors discovered that if one piece of creep test data is available, it is possible to determine the main rupture curve for that material. We came up with the idea that by acquiring one point of time test data, we could estimate the rupture time in a long-time region.

The method for estimating the remaining creep life of high chromium steel according to the present invention is based on such knowledge and ideas, and is based on creep rupture test data using used high chromium steel as a reference that has been used for more than the design life time. The process involves organizing the relationship between stress and Larson-Miller parameters to obtain the main rupture curve, which will serve as a reference curve, and short-time creep test data for used high-chromium steel materials to be evaluated that have been used for longer than the design life. The main rupture curve for the evaluation material is determined by determining at least one point, and by determining the variable ax, which is the amount of movement of the reference curve in parallel to the horizontal axis direction, on the short-time creep test data of the high chromium steel material to be evaluated. and a step of estimating the rupture time in the long time region from the determined main rupture curve.

According to the method according to claim 1, if at least one point of data is obtained even in a short-time creep test, it becomes possible to determine the main rupture curve for the evaluation material, and it is possible to estimate the rupture time in a long-time region. can.

1 is a graph showing the results of a creep test on improved 9Cr steel for long-term use. FIG. 2 is a Larson-Miller parameter schematic diagram showing a typical example of a main fracture curve of an improved 9Cr steel for long-term use that is uniformly organized using Larson-Miller parameters. It is a Larson-Miller parameter schematic diagram showing a typical example of the main fracture curve of a heat-affected zone fracture type (Type IV fracture type) welded joint of modified 9Cr steel for long-term use. FIG. 2 is a schematic diagram illustrating a method for estimating remaining creep life based on short-time creep test data of the present invention. A stress-rupture time (creep rupture) diagram estimated by applying the estimation method of the present invention to a modified 9Cr steel base material that has been used for a long time; (a) shows a material with a relatively short fracture time; (b) shows the case where it is applied to a material with a longer fracture time. A graph comparing the creep rupture time estimated by applying the estimation method of the present invention to a modified 9Cr steel base material that has been used for a long time and the creep rupture time based on the experiment. (a) shows a relatively short fracture. (b) shows the case of a material with a longer fracture time. (a) is a stress-rupture time (creep rupture) diagram estimated by applying the estimation method of the present invention to a heat-affected zone rupture type (Type IV rupture type) welded joint of improved 9Cr steel that has been used for a long time. (b) shows the case where the material is applied to a material with a relatively short failure time, and (b) shows the case where it is applied to a material with a relatively long failure time. A graph comparing the creep rupture time estimated by applying the estimation method of the present invention to a heat-affected zone rupture type (Type IV rupture type) welded joint of improved 9Cr steel that has been used for a long time and the creep rupture time based on experiments. (a) shows a material with a relatively short fracture time, and (b) shows a material with a relatively long fracture time. Stress estimated by applying the estimation method of the present invention to a heat-affected zone fracture type (Type IV fracture type) welded joint and a weld metal fracture type (Type I fracture type) welded joint of improved 9Cr steel that has been used for a long time. - rupture time (creep rupture) diagram. It is a Larson-Miller parameter schematic diagram showing a typical example of the main fracture curve of a weld metal fracture type (Type I fracture type) welded joint of modified 9Cr steel for long-term use. Larson-Miller parameter schematic diagram showing an example of the main fracture curves of a weld metal fracture type (Type I fracture type) welded joint and a heat affected zone fracture type (Type IV fracture type) welded joint of modified 9Cr steel for long-term use. be.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of the present invention will be described in detail below based on embodiments shown in the drawings.

The method for estimating the remaining creep life of high chromium steel according to the present invention is based on the inventors' ``The shape of the main fracture curve of long-term use high chromium steel is constant regardless of the strength and weakness of the material, and ``If only one piece of creep test data is available, it is possible to determine the main rupture curve for that material.'' This is based on the idea that even in the time domain, by acquiring one point of short-time test data, it is possible to estimate the rupture time in the long-time domain.

In other words, the method for estimating the remaining creep life of high chromium steel according to the present invention is based on creep rupture test data using used high chromium steel as a reference that has been used for more than the design life time, and calculates stress and Larson-Miller parameters. A process of obtaining a main rupture curve that is organized in relation to the reference curve, a process of obtaining at least one short-time creep test data of the used high chromium steel material to be evaluated that has been used for more than the design life, and a reference curve. The process of determining the main rupture curve for the evaluation material by determining the variable ax, which is the amount of translation of the horizontal axis on the short-time creep test data of the high chromium steel material to be evaluated, and from the determined main rupture curve. and a step of estimating a rupture time in a long-time region.

Here, the material used to determine the main rupture curve as the reference curve, that is, the long-term use material of high chromium steel, is not limited to a specific material, but it is preferably a weak material, in other words, a material that exhibits creep rupture quickly. It is preferable to use a material that is relatively weak, preferably a waste material that is the most fragile and weak material. Of course, the reference curve may be determined based on a strong material or an intermediate strength material without using the weakest material or the weakest material, but it is more convenient to determine the reference curve based on the weak material. . In addition, the method for estimating the remaining creep life of the present invention is applicable to materials that have been used in actual equipment for more than the design life time (for example, 100,000 hours), and the remaining life of such materials is estimated to be how much longer they can be used. It is meaningful to estimate. This is thought to have been possible because the metal structure has changed and the material has become stable after being used for 100,000 hours in an actual machine.

In other words, the shapes of the main fracture curves of the strong material and the weak material are almost the same and are in a parallel relationship. If they are parallel, determine the main rupture curve as one reference and obtain the creep experimental data for one point of the material to be evaluated. By moving parallel (moving _horizontally ), the main rupture curve for that material can be determined. is found. Thereby, the main fracture curve of the material to be evaluated can be determined. Specifically, by using the main rupture curve (reference curve) obtained from the waste material, scraping off a small amount of the material currently used in the actual machine, and obtaining data at one point through a short-time creep test, the stress and In the Larson-Miller parameter schematic diagram organized in relation to the Larson-Miller parameters, the "principal rupture curve" of the reference material is horizontally plotted up to one point of data obtained in a short-time creep test of the material to be evaluated. By moving, it is possible to obtain the main fracture curve of the material having the strength of the destination, that is, the material to be evaluated. Then, using the determined main rupture curve of the material to be evaluated, it is possible to estimate the long-time creep rupture life of the material to be evaluated. In other words, even in a creep test in a short time range, by using the parallel relationship, it is possible to estimate the rupture time in a long time range. Here, the creep remaining life estimation method of the present invention assumes that high chromium steel materials have been used in actual equipment for a long time, for example, over the design life (100,000 hours), and does not target new materials. In other words, it is to find out how long a product that has been used for, for example, 100,000 hours or more will last.

In this embodiment, a method for estimating long-term creep characteristics from short-time test data was studied using the following procedure, targeting an improved 9Cr steel for long-term use as an example of high chromium steel.

The present inventors first analyzed the results of a creep test using scrap high chromium steel that had been used for a long time beyond its design life. Specifically, a systematic creep test was conducted on samples taken from a large number of improved 9Cr steel pipes (base metal and welded parts) that had been used for a long time in a 600°C class thermal power plant. Here, the piping is a high-temperature reheat steam pipe, and the maximum operating time is about 150,000 hours. The test piece was taken from the center of the pipe wall thickness so that the axis was in the circumferential direction.

Figure 1 shows a representative example of test results regarding the base material. Here, the rupture time on the horizontal axis is the value obtained in the creep test, and the load history is not taken into account. The alphabets in the diagram are abbreviations to identify each pipe, and the average characteristics based on the evaluation formula (K. Kimura and M. Yaguchi, ASME PVP2016-63355, 2016) for improved 9Cr steel (SCMV28 steel) are added for reference. did. Due to differences in creep life characteristics (differences between heats) during the manufacturing of each material, as well as differences in usage conditions and operating hours at the power plant, in Figure 1, for the same test temperature and stress, A difference in the rupture time can be seen. However, this difference does not vary randomly, and the rupture time clearly varies depending on the material.

When this test data was analyzed, it was found that it could be uniformly organized using the Larson-Miller parameter (hereinafter abbreviated as LMP) without depending on the stress or rupture time region. Figure 2 shows a representative example of the results of organizing the creep test data for each material. Here, for the constant C in LMP, the same C value (CB) is used for all materials. Materials L and Q differ in fracture time by about one order of magnitude under the same test conditions (temperature, stress), but in FIG. 2, the difference is almost independent of the position in the horizontal axis direction. That is, the difference between materials L and Q is approximately the same regardless of the size of LMP. This can be interpreted as the main fracture curve being translated in the horizontal axis direction depending on the material. In addition, in the case of new high chromium steel, it is difficult to appropriately estimate the long life unless the material constants in the life evaluation formula are used depending on the stress and fracture time range (Non-patent Document 1, (see 2). This is because the (apparent) activation energy changes depending on the stress and rupture time range, and in LMP, it means that the C value changes depending on the range (30 or more in the short time range, 20 or more in the long time range). before and after). Therefore, since the main fracture curve as shown in FIG. 2 is not obtained, it is considered that the method for estimating the creep remaining life according to the present invention is not applicable. On the other hand, in the case of high chromium steel used for a long time, the LMP C value was around 20 regardless of the area. In other words, even in the short time range creep test, the C value was comparable to that of the new high chromium steel in the long time range. This is because the structure deteriorates/recovers due to long-term use in actual equipment (holding at high temperatures for long periods of time), and the long-term use material has a "structure under the long-term creep test conditions of new material" before starting the test. It is assumed that this is because the situation was similar to that of Therefore, in the case of long-term use materials, it is considered that the combination of short-time test data and the shape of the main fracture curve in the long-term region was effective.

The above characteristics were also observed in creep test data of heat-affected zone (HAZ) fracture type (hereinafter referred to as Type IV fracture type) welded joints of modified 9Cr steel that had been used for a long time. Figure 3 shows a representative example of the results of organizing creep test data for Type IV fracture welded joints. The value of the LMP constant C was approximately the same regardless of the stress or rupture time range, and the main rupture curve was parallel to the horizontal axis direction depending on the strength of the material. Here, the shape and constant C of the main fracture curve need to be distinguished between the base metal and the welded joint. In addition, for welded joints, the shapes of the main fracture curves were slightly different between the weld metal fracture type (hereinafter referred to as Type I type) and Type IV type.

From the above results, if we consider that "the shape of the main fracture curve of improved 9Cr steel for long-term use is constant regardless of the strength and weakness of the material, and moves in parallel only in the horizontal axis direction," it is necessary to have one piece of test data. For example, it is possible to determine the principal fracture curve for the material. This means that if one piece of test data is acquired even in the short time range, the rupture time in the long time range can be estimated.

A schematic diagram of this estimation method is shown in FIG. The specific steps are as follows. First, Equation 1 below is applied to the data of the reference material in the base material of FIG. 1, and each constant is determined by regression calculation.

Here, T is the test temperature (° C.), _tr is the rupture time (h), σ is the stress (MPa), and C _B , a ₀ , a ₁ , and a ₂ are material constants. The main rupture curve calculated using this constant is hereinafter referred to as a "reference curve." The main fracture curve of each material is considered to be expressed by the following equation 2, which is obtained by translating the "reference curve" in the horizontal axis direction.

Here, _aX is a variable representing the strength of each material, and is the amount of movement of the main fracture curve in the horizontal axis direction in FIG. In Equation 2 above, there is only one unknown quantity, _aX , so if there is one piece of test data, the value of _aX can be determined from Equation 3 below.

したがって、当該材料の任意の温度・応力での破断時間は、決定した材料定数と下記の式４により推定できる。

Therefore, the rupture time of the material at any temperature and stress can be estimated using the determined material constant and the following equation 4.

Furthermore, the rupture time of each material at any temperature and stress can be estimated using the same procedure for Type IV rupture type welded joints. That is, first, the material constants C _WJ , a ₃ , a ₄ , and a ₅ in the following equation 5 are determined based on the reference material data, and then the value of the variable a _X is determined based on the test data of each material.

Regarding the above estimation method, material constants and variables _aX were determined as follows.
1) In the case of base material Material Q in Fig. 1 was used as the “reference material”. This is because data has been acquired up to the low stress side (50 MPa) at 650℃, data has been acquired at 650℃ and 600℃ for the same stress, and weak materials are the focus of remaining life evaluation. Because of the reason. For the data for determining the value of the variable _aX , the rupture time under conditions of 650° C. and 100 MPa was used for each material. These conditions were selected from a practical standpoint, and the maximum rupture time is about 600 hours.

2) For welded joints
For Type IV fracture type welded joints, Q material (welded joints) was used as the "reference material." The reason for choosing this material is the same as for the base material. For the data for determining the value of the variable _aX , the rupture time under the conditions of 650° C. and 70 MPa was used for each material.

Note that although the above-described embodiment is an example of a preferred embodiment of the present invention, the present invention is not limited thereto, and various modifications can be made without departing from the gist of the present invention. For example, in the above embodiment, the method for estimating the creep remaining life of the base material of modified 9Cr steel piping and Type IV fracture type welded joints that have been used for a long time in thermal power plants has been mainly described. For example, but not limited to, estimating the creep life of other high chromium steels that have been used for more than a design life period where it is meaningful to estimate the creep life of welded joints of Type IV failure type. Needless to say, it can also be applied.

(Example 1)
[Base material]
FIG. 5 shows a typical example of the results obtained by applying the estimation method according to the above-described embodiment to an improved 9Cr steel base material that has been used for a long time. The life estimation curve and test data are good in all cases, from materials with a relatively short rupture time (Figure 5(a)) to materials with a relatively long rupture time (Figure 5(b)). It was confirmed that the results matched with high accuracy. As mentioned above, the data used to determine the constants (variables) of the life estimation curve is only one point at 650℃ and 100MPa, but it is possible to properly reproduce not only data at 650℃ but also data for tens of thousands of hours at 600℃. We were able to.

Figure 6 shows a typical example comparing experimental results and estimated results regarding the creep rupture time of each material. It was confirmed that for all materials, the rupture time was estimated with an accuracy within the range of Factor of 2 (the ratio of predicted results/experimental results was 1/2 to 2). Based on these results, it is expected that by combining this estimation method with destructive tests for remaining life evaluation, the time required to obtain test data can be reduced by several tens to one hundred times.

[Type IV fracture type welded joint]
FIG. 7 shows a typical example of the results obtained by applying the estimation method according to the embodiment described above to a Type IV fracture welded joint of improved 9Cr steel that has been used for a long time. The life estimation curve and test data are good in all cases, from materials with a relatively short rupture time (Fig. 7 (a)) to materials with a long rupture time (Fig. 7 (b)). It was confirmed that the results matched with high accuracy. The data used to determine the constants (variables) of the life estimation curve is only one point, 650℃ and 70MPa, but it is possible to appropriately reproduce data not only at 650℃ but also for tens of thousands of hours at 600℃. Ta.

Figure 8 shows a representative example comparing experimental results and estimated results regarding the failure time of each Type IV failure type welded joint. It was confirmed that for all materials, the rupture time was estimated to an accuracy within a factor of 2. Based on these results, it is expected that the time required to obtain test data for welded joints can be significantly reduced by combining this estimation method with destructive tests for remaining life evaluation.

[Type I fracture type welded joint]
The shape of the fracture curve of Type I welded joint was slightly different from that of Type IV. A comparative example of the creep rupture curves of Type I and Type IV is shown in Figure 9. The slope of the creep rupture curve of Type I is slightly more horizontal than that of Type IV. Therefore, when applying the shape of a Type IV creep rupture curve to a Type I material, the difference between the rupture curve and the actual life may increase as the conditions move away from the data used to determine the constant.

[Estimation of remaining life of Type I fracture type welded joint]
When sorting and analyzing only Type I welded joints, the shape of the main fracture curve of each material is generally constant regardless of the strength of the material, and it tends to move in parallel only in the horizontal axis direction. was observed (Fig. 10). By using this curve, it was possible to appropriately estimate the long-time rupture time from the short-time data even for Type I welded joints. However, in the case of Type I welded joints, depending on the material, there were cases where the fracture position changed from the weld metal to the HAZ over a long period of time. In this case, as shown in FIG. 11, the estimated value of the rupture time in the long-time region (actual machine usage region) differs depending on whether Type I or Type IV is used as the shape of the main rupture curve. For this reason, it is necessary to consider estimating which fracture mode the material to be evaluated will exhibit in the area of actual machine use.

As described above, the effectiveness was confirmed in the case where a material that had been used for 100,000 to 150,000 hours at around 600°C broke in a creep test at 600°C and 650°C for tens of thousands of hours. In the case of modified 9Cr steel base metal and welded joints used for a long time, the LMP C value was around 20 regardless of the area. That is, even in the short time range creep test, the C value was comparable to that of the new high chromium steel in the long time range. This is because the structure deteriorates/recovers due to long-term use in actual equipment (holding at high temperatures for long periods of time), and the long-term use material has a "structure under the long-term creep test conditions of new material" before starting the test. It is assumed that this is because the situation was similar to that of Therefore, in the case of long-term use materials, it is considered that the combination of short-time test data and the shape of the main fracture curve in the long-term region was effective.

Also, from the above experimental results,
(1) The difference in creep life between modified 9Cr steel base metal and HAZ fracture welded joints that have been used for a long time tends to shift the life curve in the relationship diagram between stress and Larson-Miller parameters in parallel in the horizontal axis direction. Do you get it.
(2) Based on this feature, long-term creep life can be estimated from short-term data. When the estimated results were compared with the experimental results, the creep life could be estimated with high accuracy within the range of the data.

Claims (1)

The process of calculating the main rupture curve, which becomes the reference curve, by organizing the relationship between stress and Larson-Miller parameters, based on creep rupture test data using standard used high chromium steel materials that have been used for more than the design life time. and,
a step of obtaining at least one short-time creep test data of the used high chromium steel material to be evaluated that has been used for more than the design life time;
Determining a main rupture curve for the evaluation material by determining a variable ax that is the amount of movement of the reference curve in parallel in the horizontal axis direction on the short-time creep test data of the high chromium steel material to be evaluated;
A method for estimating the remaining creep life of high chromium steel, comprising the step of estimating a rupture time in a long time region from the determined main rupture curve.

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