JP2010203812A - Method for evaluating life time of high strength ferritic steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a life evaluation method capable of accurately evaluating the life time even of a high strength ferritic steel regardless thermal history thereof. <P>SOLUTION: The life time of a high strength ferritic steel is evaluated by measuring hardness of a first evaluation part suffered from creep damage by temperature and a stress applied with a lapse of time and a second evaluation part free from creep damage without a stress applied thereto, in the high strength ferritic steel; calculating the difference between the hardness of the first evaluation part and the hardness of the second evaluation part; and estimating the creep life time consumption rate of the first evaluation part based on the difference between the hardness of the first evaluation part and the hardness of the second evaluation part. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is a method for evaluating the life of high-strength ferritic steel, and in particular, a ferritic high-strength heat-resistant steel having a chromium (Cr) content of 9% or more used in high-temperature pressure-resistant parts such as thermal power generation and nuclear power generation facilities, That is, the present invention relates to a method for evaluating the life of high-strength ferritic steel.

  High temperature members such as boiler tubes used in thermal power generation and nuclear power generation facilities and their welded parts are used for a long time while receiving external force at high temperature (creep temperature range) and high pressure, so that creep damage progresses. As a result, voids are generated, and cracks are generated due to the connection of the generated voids.

  Therefore, utilizing the fact that the amount of creep voids generated and the hardness of the high temperature member change as the creep damage progresses, the life consumption rate (use time / breaking time) and void generation confirmed in advance as shown in FIG. The life of a high-temperature member is evaluated by comparing the relationship between the amount and the relationship between the life consumption rate and hardness as shown in FIG. 6 with the measurement result of the creep void by actual structure observation, and the replacement time is determined. It was. However, the initial hardness, which is the standard for changes, generally depends on the various thermal histories received in the part manufacturing process, as well as variations in chemical composition within the range of the standard and differences between lots such as heat treatment during steelmaking. Each has a different value. For this reason, accurate life evaluation cannot be performed without considering the initial hardness.

  Japanese Patent Laid-Open No. 2001-208654 (Patent Document 1) discloses a measurement result obtained by measuring a plurality of times at time intervals in order to accurately predict the life without measuring the initial hardness of the high temperature member. It is disclosed that the amount of decrease in hardness due to creep damage is calculated from the difference between the two, and the lifetime is predicted by obtaining the creep damage accumulation rate for a time interval.

JP 2001-208654 A

  However, the decrease in hardness due to the use of members used at high temperatures is caused not only by creep damage but also by thermal history. That is, the hardness of high-strength ferritic steel not only decreases due to stress being applied at high temperatures and creep damage, but also decreases due to prolonged exposure to high temperatures even in no-load conditions. To do. As described above, the change in hardness associated with the use of high-strength ferritic steel also includes factors due to factors other than creep damage, so the above-described method using the fact that hardness changes with the progress of creep damage remains unchanged. When the life evaluation is performed using this, there is a problem that a difference occurs between the life of the evaluation result and the actual life, and the accuracy of the life evaluation is lowered.

  The present invention has been made to solve the above problems, and provides a life evaluation method for high-strength ferritic steel that can accurately evaluate the life regardless of thermal history even for high-strength ferritic steel. With the goal.

In order to solve the above problems, the present invention employs the following means.
The present invention relates to a first evaluation portion that is subject to creep damage due to stress being applied with the passage of temperature and time among the high-strength ferritic steels to be evaluated, and a second that is not subjected to creep damage due to stress being not applied. And measuring the difference between the hardness of the first evaluation part and the hardness of the second evaluation part, and based on the difference, the creep life of the first evaluation part Provided is a life evaluation method for high strength ferritic steel that evaluates the life of the high strength ferritic steel by estimating a consumption rate.

  The high strength ferritic steel to be evaluated undergoes creep damage and decreases its hardness when stress is applied with the passage of temperature and time. Moreover, even if stress is not loaded, the hardness falls also by being exposed to high temperature for a long time. Therefore, for example, in high-strength ferritic steel used for boiler tubes of power generation facilities, etc., the first evaluation site that undergoes creep damage due to stress being applied over time and with time, A second evaluation part that is not damaged is selected, and the hardness of the first evaluation part and the second evaluation part are measured. From this measurement result, by calculating the difference between the hardness of the first evaluation site and the hardness of the second evaluation site, only the amount of decrease in hardness due to creep damage caused by stress is extracted. be able to. Then, by estimating the creep life consumption rate of the first evaluation part based on the difference between the hardness of the first evaluation part and the hardness of the second evaluation part, the life of the high strength ferritic steel is accurately evaluated. be able to.

  In the life evaluation method for the high strength ferritic steel described above, a high strength ferritic steel with a predetermined load and a high strength ferritic steel without a load are heated at a predetermined temperature for a predetermined time, and the respective hardnesses are set at predetermined time intervals. Measured multiple times in, and generate a master curve in advance showing the relationship between the creep life consumption rate and the difference in hardness of high-strength ferritic steel with a predetermined load and high-strength ferritic steel without load, By estimating the creep life consumption rate of the first evaluation site based on the master curve and the difference between the hardness of the first evaluation site and the hardness of the second evaluation site, high strength ferritic steel It is preferable to evaluate the lifetime.

  Using the same high-strength ferritic steel as the high-strength ferritic steel to be evaluated, a high-strength ferritic steel with a predetermined load and a high-strength ferritic steel without a load are heated at a predetermined temperature for a predetermined time, Hardness is measured multiple times at specified time intervals, and the master shows the relationship between the creep life consumption rate and the difference in hardness between high-strength ferritic steel with a given load and high-strength ferritic steel without any load. By generating the curve in advance, the difference between the hardness of the first evaluation site and the hardness of the second evaluation site can be applied to the master curve, and the creep life consumption rate can be estimated easily.

  Here, the predetermined temperature, the predetermined time, and the predetermined load are preferably determined according to the use environment of the product to which the high-strength ferritic steel to be evaluated is applied, and a master curve corresponding to the use environment is preferably generated. For example, when the high-strength ferritic steel is a boiler tube of a power generation facility, it is determined according to the operating conditions of the power generation facility. In general, a boiler tube for power generation equipment formed of high-strength ferritic steel is assumed to be subjected to a stress load of about 50 MPa when used in the power generation equipment, and is considered to be operated at a temperature of about 600 °. In addition, the predetermined time is set to a temperature acceleration condition in which an expected creep rupture time is about 1,000 to 30,000 hours, a time substantially equivalent to that of an actual power generation facility having an expected rupture time of about 100,000 hours or more, etc. can do.

  In the above-described method for evaluating the life of high-strength ferritic steel, the second evaluation site is formed of the same high-strength ferritic steel as the first evaluation site, and is heated at the same temperature for the same time. It is preferable that

  High-strength ferritic steel has different stresses depending on its usage and usage conditions. For example, when high-strength ferritic steel is applied to a boiler tube, the boiler tube may be bent. Therefore, in such a case, it is assumed that the most stressed part and the bending part where the creep damage is assumed to progress most are set as the first evaluation part, and the stress hardly acts and the creep damage does not proceed. The site | part which is not made | formed by the bending process made can be made into a 2nd evaluation part.

  On the other hand, it is assumed that there is no place where stress is not applied to the boiler tube to be evaluated, etc., so when manufacturing products using high-strength ferritic steel such as boiler tube, a test site where stress is not applied is formed in advance. In addition, this test site can be used as the second evaluation site. In addition, if the test site cannot be formed during the manufacture of products using high-strength ferritic steel such as boiler tubes, the test object is to be evaluated against the test piece formed of the same high-strength ferritic steel as the first evaluation site. The high-strength ferritic steel is heated at the same temperature for the same time, and this can be used as the second evaluation site.

  According to the present invention, even if it is high strength ferritic steel, life evaluation can be performed accurately regardless of its thermal history.

It is explanatory drawing which shows the relationship between the hardness of the stress load part of the high strength ferritic steel for producing | generating the master curve of embodiment of this invention, the hardness of a stress no load part, and a creep rupture lifetime consumption rate. It is explanatory drawing which shows the relationship between the hardness variation | change_quantity of a stress load part and a stress no load part, and a creep rupture life consumption rate in the high strength ferritic steel for producing | generating the master curve of embodiment of this invention. It is a flowchart which shows the lifetime evaluation method of the high strength ferritic steel concerning embodiment of this invention. It is explanatory drawing which shows the example of the boiler tube which consists of high strength ferritic steel. It is explanatory drawing which shows the relationship between a creep void number density and a creep rupture lifetime consumption rate. It is explanatory drawing which shows the relationship between the Vickers hardness of a metal material, and a creep rupture lifetime consumption rate.

  Embodiments of a method for evaluating the life of high-strength ferritic steel according to the present invention will be described below with reference to the drawings.

  The life evaluation method for high-strength ferritic steel according to the present invention is used in, for example, thermal power generation and nuclear power generation facilities and is applied to boiler tubes made of high-strength ferritic steel. Therefore, hereinafter, a boiler tube made of high-strength ferritic steel in a power generation facility will be described as a life evaluation target.

  Prior to the life evaluation, a creep rupture test and an interruption test were conducted using unused test pieces made of high-strength ferritic steel used for the boiler tube to be evaluated. A master curve is generated in advance. Specifically, two test pieces made of the same high-strength ferritic steel as the boiler tube to be evaluated are prepared, and the hardness before heating is measured for the two test pieces. In addition, it is necessary to extract this test piece so that the stress applied to the boiler tube during the operation of the actual power generation equipment is the same as the direction of the stress applied to the test piece during the test.

  Subsequently, a load is not applied to one test piece A, and a predetermined load P is applied to the other test piece B, and the test piece A is heated at a predetermined temperature for a predetermined time. Measure the hardness multiple times. At this time, it is necessary to heat the two test pieces A and B at the same temperature. As the predetermined load P, for example, it is conceivable to perform a test in which a load of about 65 MPa is applied in consideration of an action stress in an actual power generation facility. The predetermined temperature is a temperature equivalent to that of actual power generation equipment, and can be arbitrarily determined. However, for high-strength ferritic steel, about 600 ° is assumed. The predetermined time is set to a temperature acceleration condition in which an expected creep rupture time is about 1,000 to 30,000 hours, a time substantially equivalent to an actual power generation facility having an expected rupture time of about 100,000 hours or more, and the like. be able to.

  Based on the measurement results of the hardness of the unloaded specimen A and the hardness of the specimen B to which the load P is applied, as shown in FIG. 1, the hardness of the specimen A and the creep rupture life consumption rate (t / tr ) And the relationship between the hardness of the specimen B and the creep rupture life consumption rate (t / tr). Here, t represents the test time, and tr represents the creep rupture time of the metal material. The unloaded specimen A is not loaded and does not break due to creep damage, but its hardness changes when exposed to a high temperature for a predetermined time. On the other hand, the test piece B to which the load P is applied is exposed to a high temperature for a predetermined time, and the hardness changes due to the progress of creep damage due to the load P being applied. That is, as shown in FIG. 1, the hardness of the test piece B to which the load P is applied is reduced by being heated and stressed. For this reason, as shown in FIG. 2, by taking the difference between the measurement result of the hardness of the test piece B and the measurement result of the hardness of the test piece A, the amount of change in hardness due to the stress load (reduction amount). And the creep rupture life consumption rate. Therefore, the relationship between the amount of change in hardness due to this stress load and the creep rupture life consumption rate is taken as the master curve.

  Next, for example, a method for evaluating the life of a boiler tube made of high-strength ferritic steel in thermal power generation or nuclear power generation equipment will be described with reference to FIG. FIG. 3 is a flowchart of the life evaluation method for high-strength ferritic steel according to the present embodiment.

  In evaluating the life of a boiler tube that has been used for a predetermined time at a high temperature by operating the power generation facility for a predetermined time, first, an evaluation part of the boiler tube is determined. Specifically, in step S11, the stress load state is investigated, and a portion of the boiler tube where the stress is most applied, that is, a portion where a high load is applied and the degree of damage is estimated to be high is designated as the stress load portion ( Select as the first evaluation site. In addition, a portion of the boiler tube where stress is not applied or is least affected, that is, a portion that is assumed to be unloaded or low loaded and that the degree of damage is low is selected as a stress unloaded portion (second evaluation portion). To do.

  Here, as shown in FIG. 4A, for example, when the boiler tube 10 is bent, the stress-loaded portion and the stress-unloaded portion are the most stress-loaded portions. Therefore, the bent part can be used as the stress load part 11, and the non-bent part can be used as the stress unload part 12. Further, as shown in FIG. 4B, the load portion 11 is selected from the portion where the stress of the boiler tube is most loaded, and at the time of manufacturing the boiler tube 10, no stress is generated using high-strength ferritic steel of the same lot. The load part 12 can also be formed in advance. Further, as shown in FIG. 4 (C), the portion of the boiler tube where the stress is most applied is the stress load portion 11 and is made of the same high strength ferritic steel as the boiler tube, and at the same temperature as the boiler tube for the same time. A heated test piece may be prepared in advance, and this test piece may be used as the stress-unloaded portion 12.

  Subsequently, in step S12, the hardness of the stress loading part and the stress unloading part determined in step S11 is measured, respectively, and in step S13, the difference between the hardness of the stress loading part and the hardness of the stress unloading part is taken. The difference is the amount of change in hardness due to the stress load on the boiler tube. In the next step S14, the fracture life consumption rate of the boiler tube is obtained by plotting the amount of change in hardness calculated in step S13 on the master curve obtained earlier, thereby evaluating the life of the boiler tube.

  As described above, the decrease in the hardness of the high-strength ferritic steel is caused not only by creep damage but also by the thermal history. By measuring the hardness and taking the difference, it is possible to easily determine the amount of change in hardness caused solely by creep damage in the stress-loaded portion, and from this amount of change, the creep rupture life consumption rate By grasping, it is possible to accurately evaluate the life of high-strength ferritic steel.

  In the above-described embodiment, the hardness measurement of the stress load part and the stress no load part is performed once, but the present invention is not limited to this, and the life is determined based on the measurement result without being limited to this. It can also be evaluated. In this case, the accuracy of the evaluation result is improved. On the other hand, in order to measure the hardness, a predetermined preparation such as surface polishing is required for the measurement site, but prior to that, for example, when the measurement site is in a narrow section and difficult to access, a protective cover is When it is necessary to remove it, there may be cases where the measurement location is high and it is necessary to build a scaffold for the worker. For this reason, when the life evaluation is performed accurately with one hardness measurement as in this embodiment, there is an effect that the working time and the working cost can be reduced.

  If the boiler tube does not have an unloaded part and stress is applied at any part, such as when the hardness of the unloaded part cannot be measured, it is heated in advance at the same temperature for the same time in the unloaded state. It is also possible to measure the hardness of the test piece and use this hardness as the hardness of the unloaded portion of the boiler tube.

  Moreover, before the boiler tube is used, the amount of change in hardness is zero at any location. Therefore, for example, by measuring the hardness of the stress-loaded portion in advance before use and taking the difference from the hardness of the stress-loaded portion measured after use, the boiler tube has no stress-free portion. Even if it is a case where the hardness of a load part cannot be measured, rough life evaluation can be performed by using the curve which shows the hardness change of a stress load state.

10 Boiler tube 11 Stress loading part 12 Stress unloading part

Claims (3)

Among the high-strength ferritic steels to be evaluated, a first evaluation site that undergoes creep damage due to stress being applied with the passage of temperature and time, and a second evaluation site that undergoes no creep damage without being stressed Measure the hardness of each
Calculate the difference between the hardness of the first evaluation site and the hardness of the second evaluation site,
A life evaluation method for high-strength ferritic steel, wherein the life of the high-strength ferritic steel is evaluated by estimating a creep life consumption rate of the first evaluation site based on the difference. A high-strength ferritic steel with a predetermined load and a high-strength ferritic steel without a load are heated at a predetermined temperature for a predetermined time, and each hardness is measured a plurality of times at predetermined time intervals. A master curve indicating the relationship between the hardness of high-strength ferritic steel with a predetermined load and the difference in hardness of high-strength ferritic steel without load is generated in advance,
By estimating the creep life consumption rate of the first evaluation site based on the master curve and the difference between the hardness of the first evaluation site and the hardness of the second evaluation site, high strength ferritic steel The life evaluation method for high-strength ferritic steel according to claim 1, wherein the life is evaluated.   The second evaluation part is a test piece formed of the same high strength ferritic steel as the first evaluation part and heated at the same temperature for the same time. The life evaluation method of high strength ferritic steel described in 1.

Images