JPH10227754A - High temperature damage evaluation method for temper martensite stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high temperature damage evaluation method for temper martensite stainless steel by which the high temperature damage of the machine parts which are used at a high temperature for a long time, and are made of temper martensite stainless steel, can be evaluated with high accuracy by a simple operation during the use of the same. SOLUTION: A surface of a part to be examined, having the mirror finished surface, is dipped into an electrolyte, the electric potential is applied to the same at a specific potential scanning speed, a peak value of the current value appeared in the almost constant potential on a dissolving current-potential curve obtained corresponding to the dissolution of a Laves phase as a specific deposit, is determined, the formation amount of the Laves phase is determined from an evaluation chart indicating the relationship between the peak current density and the formation amount of the Laves phase, formed by using a specimen on which the ageing treatment has 4 been performed in advance, and an impact value or the rupture strength of a material of the part to be examined, or the operating temperature of the material of the part to be examined, is evalurated on the basis of the relational chart of the Laves phase formation amount, and the impact value or rupture time obtained in advance on the basis of the obtained formation amount of Laves phase, or a relational expression of the Laves phase formation amount-temperature-retension time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-temperature damage evaluation of a tempered martensitic steel capable of evaluating the degree of high-temperature damage of a mechanical part made of a tempered martensitic steel operated at a high temperature for a long time in a service state. About the method.

[0002]

2. Description of the Related Art Conventionally, the following method has been known as a high-temperature damage inspection method during the service of a machine component that is operated at a high temperature for a long time. 1) A non-destructive inspection method for defects such as cracks generated by long-term use using magnetism, ultrasonic waves, radiation, and the like. 2) A method in which a test piece is sampled from the surface of a machine component that has been used for a long time and subjected to a mechanical test such as an impact test or a creep rupture test. 3) A method of observing the metal structure on the surface of the member or the surface of the collected test piece, and observing the morphological change of minute voids and precipitates that occur before a crack is formed.

[0003]

The problems to be solved by the prior art described above by the present invention are as follows. 1) Since the method based on the nondestructive inspection is a method for detecting a crack, the degree of damage before a crack occurs cannot be evaluated. Therefore, in parts where it is considered that there is not much time from crack generation to fracture, for example, in thin-walled heat transfer tubes or large-diameter thin-walled pipes where almost uniform stress is applied over the thickness of the tube, Failure to detect damage before cracking could hinder safe operation of equipment. In addition, no information was obtained on the degree of damage with respect to damage without crack generation, for example, reduction in toughness. 2) The method of performing a mechanical test using a test piece is a method that can solve the above-described problem, but it is necessary to collect a test piece from a member. In many cases, in order to operate the machine after sample collection, it is necessary to replace and repair old parts after collection, and the reliability of recovered parts, the cost required for recovery, and the construction period are limited. It was extremely difficult to collect test pieces. 3) The method of observing the metal structure on the surface of a member or the surface of a test piece is a non-destructive method as compared with the method using a test piece, and the old work of repairing and repairing after inspection is unnecessary. It was necessary to analyze the replica taken from the surface using an apparatus such as an electron microscope, and it was not possible to evaluate the damage on the spot.

The present invention has been made in view of the above-mentioned prior art, and accurately evaluates the high-temperature damage degree of a mechanical part made of tempered martensitic steel which is used for a long time at a high temperature by a simple operation in a service state. It is an object of the present invention to provide a method for evaluating the high-temperature damage of tempered martensitic steel that can be performed.

[0005]

The present inventors have studied various methods for evaluating the high-temperature damage of tempered martensitic steel used at high temperatures, and found that the Laves phase (which is directly related to the high-temperature damage of tempered martensitic steel). Laves phase), and by evaluating the generation state of the Laves phase by an electrochemical method instead of a metallographic method,
It has been found that measurement can be performed easily and accurately in the state of operation, and that the degree of high-temperature damage of the material can be estimated from the state of formation of the Laves phase thus obtained.

A first aspect of the present invention is to evaluate the high temperature damage of a tempered martensitic steel containing 8 to 12% by weight of Cr by operating according to the following procedures 1) to 7). This is a method for evaluating high-temperature damage of martensitic steel. 1) Polish the surface of the inspection target site of a high-temperature component made of tempered martensitic steel used at a high temperature for a long time to a mirror surface. 2) The other of the mirror-polished surfaces is masked except for the area required for measurement, and the area of the unmasked surface (measurement surface) is measured. 3) The mirror-polished surface is immersed in an electrolytic solution or a gel thereof, and an appropriate reference electrode and a counter electrode are installed, and an electrode wire is connected to the measurement surface to prepare for current measurement. 4) After measuring the natural electrode potential, polarize in a noble direction at a constant potential sweep rate, and record the dissolution current and potential. 5) A peak value of a current value appearing at a substantially constant potential on a dissolution current-potential curve obtained in accordance with dissolution of the Laves phase as a specific precipitate is determined. 6) The generation amount of the Laves phase is obtained from an evaluation diagram showing the relationship between the peak current density (the value obtained by dividing the peak current by the measurement area) and the generation amount of the Laves phase, which is prepared using a test piece that has been subjected to aging treatment in advance. 7) From the obtained amount of generation of the Laves phase, using the relationship diagram between the amount of generation of the Laves phase and the impact value prepared using a test piece aged in advance, the impact value of the material at the site to be investigated was obtained. Thereby, the toughness is evaluated.

A second aspect of the present invention is the method for evaluating the high temperature damage of tempered martensitic steel according to the first aspect of the present invention, wherein the operation of the following procedure 8) is performed instead of the procedure 7). This is a high-temperature damage evaluation method. 8) From the obtained amount of Labes phase generation, the breaking strength of the material at the site to be investigated was determined by using a relationship diagram between the amount of Labes phase generation and the breaking time created using a test piece that had been aged in advance, The creep damage is thereby evaluated.

[0008] A third aspect of the present invention is the method for evaluating the high temperature damage of tempered martensitic steel according to the first aspect, wherein the operation of the following step 9) is performed instead of the step 7). This is a high-temperature damage evaluation method. 9) From the obtained amount of labes phase and the total operation time of the high-temperature parts, the relational expression between the amount of labes phase formed, temperature, and holding time, which was prepared using a test piece that had been subjected to aging treatment and a material used for a long time, was calculated. Evaluate the working temperature of the material at the site to be surveyed.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method of the present invention, first, a surface of a high-temperature component to be inspected is mirror-polished, and then masked with an insulating paint or the like while leaving an area necessary for measurement. The area required for measurement is preferably 0.5 cm ² or more in order to ensure that a sufficiently large number of crystal grains are included in the measurement area in order to obtain a stable measurement value.
A range of about 1 cm ² is preferred. The shape of the measurement surface does not need to be particularly limited, but is preferably rectangular so that the area can be easily measured.

Next, in order to perform an electrochemical measurement, the measurement surface is immersed in an electrolytic solution. As a method thereof, for example, a weir made of clay or the like may be provided around the measurement surface, and the weir may be filled with the electrolyte. Also, if the electrolyte cannot be filled on the measurement surface due to the location, shape, etc. of the site to be surveyed, the electrolyte is gelled and placed in a cylindrical container, etc., and the tip of the gel is placed on the measurement surface. May be brought into contact with each other.

In this way, a counter electrode for measuring the current is provided in the electrolyte or gel containing the measurement surface. Furthermore, the electrode of the target part is formed by spot welding or the like in a portion not in contact with the electrolyte near the measurement surface, and a separately provided reference solution tube is connected to the electrolyte or its gel with a salt bridge to form a solution in the reference solution. Is provided with a reference electrode. If no salt bridge is provided, a reference electrode may be provided at a position sufficiently distant from the electrolyte.

The target part electrode, the reference electrode, and the counter electrode are connected to a potentiostat, and the potential between the target part electrode and the reference electrode is controlled to apply a potential to the target part (polarize) so that the potential between the target part electrode and the counter electrode is changed. And measure the current. The obtained current is divided by the measurement area to obtain a current density.

As an electrolytic solution for immersing the measurement surface, for example, 1M
Can be used. This electrolytic solution can be gelled by a method such as mixing about 20 g of a superabsorbent acrylic resin with 1 liter of the aqueous solution.

The current is measured as follows. First, a state where no potential is applied from the outside is maintained for 5 minutes. Here, a saturated calomel electrode (SCE: Satylated Ca
lmmerElectrode), the spontaneous potential is about -50
It becomes 0 mV. Then, it is polarized in the noble direction at a constant potential sweep speed (potential is applied while gradually changing the size), and at this time, the current flowing between the target part electrode and the counter electrode is monitored, and this value is measured. The potential-current density curve is obtained by using the value obtained by dividing the area by the current density as the current density. Specifically, for example, a potential is applied (polarized) at a sweep rate of about 30 mV / min up to about +400 mV.

The Laves phase (here, Fe ₂ Mo and Fe ₂ W) appearing in steel containing 8 to 12% Cr shows a high current density peak at a potential of about +200 to 300 mV when this electrolytic solution is used. The Laves phase dissolves at the potential. In these steels, a Laves phase is not generated as it is manufactured, but a Laves phase is generated as it is heated for a long time,
Will increase. Therefore, in the method of the present invention, a test piece (aged test piece) which has been subjected to a heating test for various holding times at various temperatures in a laboratory in advance in the above-mentioned steel is used, and the potential is determined by the method described above. −2 after obtaining the current density curve
The current density of the peak appearing at a potential of 00 to 300 mV (I
_P) was measured, their values and Charpy impact value, to previously obtain a relation between the creep rupture time or heating temperature and time, just Charpy impact value measuring the I _P of stbm by using it, The creep rupture time and the use temperature can be obtained, and the high temperature damage can be evaluated.

[0016] That is, for each of the test specimens subjected to the heating test described above, by observing the metallic structure, the area ratio of the Laves phase was measured using an image processing apparatus, the I _P and Laves phase area ratio create a relational diagram, determine the impact value by performing a Charpy impact test at any temperature were taken Charpy impact test piece, to create a relational diagram between I _P and Charpy impact value, the creep rupture test specimen After processing, a creep rupture test was performed under arbitrary conditions to determine the rupture time, and I _P
Create a relationship diagram between the time and the creep rupture time. And
And a I _P measurements and these relational diagram of the measurement target sites can be determined Laves phase area ratio, Charpy impact value, the creep rupture time and the like.

The operation and effect of the present invention will be described below. In tempered martensitic steel, the Laves phase does not form as it is in the heat treatment, but gradually forms and grows by prolonged exposure to high temperatures. In addition, the formation position is often at the crystal grain boundary, and once formed, it is likely to become coarse and brittle as compared to the base, so that the toughness is reduced. Furthermore, since the Laves phase contains Mo and W, which are solid solution strengthening elements, it is known that the formation thereof reduces the solid solution concentration of the matrix of these solid solution strengthening elements and lowers the creep strength. I have. That is, the generation and growth of the Laves phase are the main factors of high-temperature damage of heat-resistant steel.

For this reason, as described in the section of the prior art, an attempt has been made to observe the metal structure and evaluate the generation amount of the Laves phase. Since the present invention non-destructively detects and evaluates the state of the generation of the Laves phase, it does not involve high-temperature damage and crack generation before crack initiation which could not be detected by the conventional nondestructive inspection method. High temperature damage can now be detected. Further, unlike the mechanical test as shown in the prior art, there is no need to collect a test piece from a mechanical part, so that it is no longer necessary to repair and repair the machine after the evaluation.

Further, in the present invention, the amount of precipitation of the Laves phase related to high-temperature damage is measured by an electrochemical method, unlike the conventional method using a metal structure.
Therefore, according to the method of the present invention, the electrochemical measurement is performed at the installation site of the mechanical component, and the result can be obtained immediately, so that the degree of high-temperature damage of the mechanical component can be evaluated on the site.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. It is used as a high-temperature member for boilers for thermal power generation.
Using a test piece of 1C-9Cr-1Mo-0.05Nb-0.2V steel, 1-3000 at 600 ° C. and 625 ° C.
h was subjected to a heating test. 10 mm from the heated test piece
A 10 mm × 5 mm test piece was sampled, and a 10 mm square surface was mirror-polished using abrasive paper and diamond paste, and a platinum wire was spot-welded to the back surface. The area other than the 8 mm × 8 mm area of the mirror-polished surface was covered with an insulating paint, and the back side and the platinum wire were further covered with the insulating paint.
After coating, the area of the uncoated polished surface was measured.

FIG. 1 is a schematic diagram of a measuring apparatus according to an embodiment of the present invention. The configuration of the measuring device is Japanese Industrial Standard JIS
According to G0579. First, a platinum electrode wire spot-welded to the test piece 1 is covered with a glass tube, and the potential between the platinum electrode and the platinum electrode 2 is measured with a potentiostat 3 while being isolated from the solution. Further, a potential is applied between the test piece 1 and the counter electrode 2, and the potential between the reference electrode 5 made of a saturated ginger electrode and the test piece 1 installed through the salt bridge 4 filled with agar is measured. The value was recorded on the personal computer 6.
For the measurement, the test piece was immersed in an electrolyte solution that had been sufficiently deoxygenated by passing nitrogen, and was swept from the natural electrode potential to +400 mV on the noble side at 0.5 mV / sec, and the current was measured. As a result of exploring a solution for detecting the dissolution of the Laves phase, 1 M
As the potassium hydroxide solution, the temperature of the electrolytic solution was adjusted to 30 ± 1 ° C. in a thermostat (not shown).

FIG. 2 shows the as-received material and the aged material at 600 ° C. × 10000 h (heat-treated test piece) measured in this way.
FIG. 3 is a diagram showing a polarization curve of the present invention. Here, the horizontal axis is the corrosion potential,
The vertical axis is the current density obtained by dividing the current by the measured area of the test piece. In the aging material, a peak was observed at a potential of about +210 mV / SCE (SCE indicates that a saturated calomel electrode was used as a reference electrode). Therefore, in order to make it easier to observe the metal structure, the polished surface of the aging material is chemically etched in advance with an aqueous solution of picric acid hydrochloride, and the above-mentioned electrochemical measurement is performed. The peak potential is about +210 mV /
As a result of observing the structure of the test piece which was held for a certain period of time in the SCE and the measurement was interrupted, the Labes phase was no longer observed, and it was apparent that the Labes phase was dissolved (disappeared) by holding at this potential. Became.

Therefore, the above electrochemical measurement was performed on various aging materials, and the current density at +210 mV / SCE was calculated as I
Measured as _p . Further, these test pieces were subjected to chemical etching, and the area ratio was determined using a precipitate in which molybdenum and iron were mainly detected as a Laves phase using a scanning Auger electron spectrometer. It said showing the relationship between Laves phase area ratio as determined by electrochemical peak current density I _p and the conventional method by measuring in FIG. A good correlation was observed between the Laves phase area ratio determined by a method focusing on the metal structure, which is a conventional method, and the peak current density electrochemically measured. By this method, an analyzer such as an electron microscope was used. It was found that the production amount of the Laves phase could be measured on the spot without any problem.

[0024] FIG. 4 is a diagram showing the relationship between 2mmV notch Charpy impact value and the peak current density I _p at 0 ℃ measured for acceptance Mom material and aging material. The impact value decreases as the peak current density I _p increases (ie, as the Laves phase increases), and the non-destructive measurement of the generation amount of the Laves phase according to the method of the present invention allows a long time. It was found that the decrease in toughness due to heating can be evaluated.

FIG. 5 is a schematic diagram showing an example of the relationship between the creep rupture time at 650 ° C. and 100 MPa and the peak current density I _p measured for the as-received material and the aged material. The magnitude of the peak current density I _p (i.e., the amount of Laves phase) correlates well with the reduction in creep rupture strength, it is clear that can evaluate the decrease in creep rupture strength due to prolonged heating by the method of the present invention .

Further, the amount X of precipitation of the Laves phase at each temperature (T) and time (t) is represented by the following equation, and the coefficient K
(T) has temperature dependency. Therefore, when K (t) is determined from the amount of Laves phase precipitation obtained by the method of the present invention and the operation time of the mechanical part, the K
From the temperature dependence of (T), the working temperature of the machine component can be determined. Xeq is a coefficient that does not depend on temperature in the test temperature range.

X = X _eq [1-exp (-kt ⁿ )]

[0027]

As described above in detail, according to the method of the present invention, the toughness is reduced and the creep rupture is caused by using the tempered martensitic steel which is frequently used for high temperature parts such as thermal power plants at a high temperature for a long time. The degree of high-temperature damage, such as a decrease in strength, can be accurately determined in the service state, and the actual temperature of machine parts that cannot directly measure the temperature during use, such as heat transfer tubes and adhered hardware that touch the flame inside the boiler, can be measured. Since the operating temperature can be determined,
It can contribute to safe operation of equipment operated at high temperatures for a long time.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a measuring device according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of polarization curves of an as-received material and an aged material at 600 ° C. for 10,000 hours.

FIG. 3 is a schematic diagram showing a relationship between a peak current density _Ip according to the method of the present invention and a Laves phase area ratio obtained by a conventional method.

FIG. 4 is a schematic diagram showing the relationship between a 2 mm V notch Charpy impact value at 0 ° C. and a peak current density I _p measured for an as-received material and an aging material.

FIG. 5: 650 measured for as-received and aged materials
4 shows the relationship between the creep rupture strength at 100 ° C. and 100 MPa and the peak current density _Ip .

Claims (3)

[Claims] 1. A method for evaluating the high-temperature damage of a tempered martensitic steel containing 8 to 12% by weight of Cr, wherein the operation is performed according to the following steps 1) to 7). High temperature damage evaluation method. 1) Polish the surface of the inspection target site of a high-temperature component made of tempered martensitic steel used at a high temperature for a long time to a mirror surface. 2) The other of the mirror-polished surfaces is masked except for the area required for measurement, and the area of the unmasked surface (measurement surface) is measured. 3) The mirror-polished surface is immersed in an electrolytic solution or a gel thereof, and an appropriate reference electrode and a counter electrode are installed, and an electrode wire is connected to the measurement surface to prepare for current measurement. 4) After measuring the natural electrode potential, polarize in a noble direction at a constant potential sweep rate, and record the dissolution current and potential. 5) A peak value of a current value appearing at a substantially constant potential on a dissolution current-potential curve obtained in accordance with the dissolution of the Laves phase (Laves phase) as a specific precipitate is determined. 6) The generation amount of the Laves phase is obtained from an evaluation diagram showing the relationship between the peak current density (the value obtained by dividing the peak current by the measurement area) and the generation amount of the Laves phase, which is prepared using a test piece that has been subjected to aging treatment in advance. 7) From the obtained amount of generation of the Laves phase, using the relationship diagram between the amount of generation of the Laves phase and the impact value prepared using a test piece aged in advance, the impact value of the material at the site to be investigated was obtained. Thereby, the toughness is evaluated. 2. The method for evaluating the high-temperature damage of a tempered martensitic steel according to claim 1, wherein the operation of the following step 8) is performed instead of the step 7). Method. 8) From the obtained amount of Labes phase generation, the breaking strength of the material at the site to be investigated was determined by using a relationship diagram between the amount of Labes phase generation and the breaking time created using a test piece that had been aged in advance, The creep damage is thereby evaluated. 3. The method for evaluating high-temperature damage of a tempered martensitic steel according to claim 1, wherein the operation of the following step 9) is performed instead of the step 7). Method. 9) From the obtained amount of labes phase and the total operation time of the high-temperature parts, the relational expression between the amount of labes phase formed, temperature, and holding time, which was prepared using a test piece that had been subjected to aging treatment and a material used for a long time, was calculated. Evaluate the working temperature of the material at the site to be surveyed.