JP3473732B2 - Quantitative evaluation method of creep damage of heat resistant steel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quantitative evaluation method for creep damage of heat-resistant steel, and relates to heat-resistant steel used as a tempered martensite structure, for example, improved 9Cr steel (A
The creep damage of SME standard SA387Cr91) can be quantitatively evaluated by observing dislocations with a transmission electron microscope.

[0002]

2. Description of the Related Art In recent years, steam conditions have been increasing in order to improve the efficiency of large-capacity thermal power generation facilities from the viewpoint of energy saving.
Instead of low alloy steels such as 2.25Cr -1 Mo steel (eg STPA24), improved 9Cr steel alloys with high high temperature strength are used.

This improved 9Cr steel alloy is generally used as tempered martensite in view of workability and creep properties.

Even in the heat-resistant steel used as such tempered martensite, it is necessary to quantitatively know the extent of creep damage due to use in a creep environment.

[0005]

[Problems to be Solved by the Invention] However, the conventional 2.25
Even if the carbide method, which is applied to Cr -1 Mo steel (eg STPA24) to quantify creep damage, is applied as it is, the tempered martensite structure is improved in 9Cr steel alloy because the carbide is stable for a long time. There is a problem that the degree of creep damage cannot be quantitatively determined by the carbide method.

Further, in the improved 9Cr steel alloy of tempered martensite structure, the structure is fine and the strain due to the martensite transformation is present. There is a problem that the degree of damage cannot be obtained quantitatively.

The present invention has been made in view of the above-mentioned problems of the prior art, and is a heat-resistant steel having a tempered martensite structure, such as an improved 9Cr steel, and has a conventional carbide method, a change in crystal orientation or a strain systematically. Even if creep damage cannot be quantified by the method described above, it is possible to quantify creep damage by observing dislocations in cells observed in the lath using a transmission electron microscope. It aims to provide a quantitative evaluation method for creep damage of steel.

[0008]

In the quantitative evaluation method for creep damage of heat-resistant steel according to the present invention, the creep damage is quantified by observing dislocations in cells observed in a lath using a transmission electron microscope. It is what

The dislocations introduced by general plastic deformation are introduced by the dislocation multiplication mechanism existing in the grains such as the Frank-Lead mechanism, and considerable dislocations remain in the grains to work-harden the material.

Dislocations introduced into a material by processing can be roughly classified into two types, one of which is a dislocation (SID: statis) introduced when an ideal single crystal is taken into consideration.
tically induced dislocations. Hereinafter, it is simply referred to as SID. ), It is not necessary to partially distort the crystal, so the same number of dislocations with positive and negative Burgers vectors are introduced.

Therefore, considering the rearrangement reaction at a high temperature, if a rising motion occurs, it disappears even if the distance is short.

The other is dislocation (GND: geometrically nece) which is introduced to maintain the continuity of the polycrystalline body even after deformation.
ssary dislocation. Hereinafter, it is simply referred to as GND. )so,
Dislocations having the same Burgers vector (absolute value and direction) are unevenly distributed in the grains.

Therefore, in order to disappear, it cannot be deleted unless it moves for a long distance by the ascending motion.

On the other hand, in the improved 9Cr steel used as tempered martensite, dislocations are introduced at a high density due to strain associated with the martensitic transformation, and the dislocations observed in tempered martensite are caused by tempering. The dislocation density is considerably lower than that before the heat treatment.

This is because the inside of the grain is
ND does not move out of the grain due to ascending movement,
Considering the generation mechanism, the SID is in a state in which there are the same number of positive and negative dislocations, and it is considered that the SID disappears due to the ascending motion during tempering.

Therefore, by observing the GND, it is possible to understand the contribution of the portion to the creep deformation. That is, the part where the GND disappears is considered to be a region where dislocations can be diffused so that the GND disappears, and it is considered that the contribution of the part to the creep deformation is reduced.

Considering that the amount of creep deformation of each cell is non-uniform even if the material is uniformly deformed, the creep deformation causes some deterioration of the creep strengthening mechanism, and as a result, the partial deformation occurs. It is considered that the creep strength gradually deteriorates.

In the case of this improved 9Cr steel, GND is introduced into all the cells, and since the cell structure is small, the internal dislocations are examined for each cell to find a region with a large dislocation rising motion. A small area can be determined.

Therefore, the determination can be made by obtaining the ratio of the regions where dislocations do not exist in a certain area.

In a structure in which dislocations (GND in this case) have moved to the outside of the cell due to ascending motion, it is considered that no internal stress is generated in principle, and the load stress directly contributes as creep deformation stress. Considering this, the strain rate is expected to increase.

Therefore, it is considered that the proportion of cells having no dislocations is closely related to the creep life.

The quantitative evaluation method for creep damage of heat-resistant steel of the present invention for solving the above-mentioned problems based on the above principle is as follows.
When quantitatively evaluating the creep damage of heat-resistant steel used as a tempered martensite structure, a step of collecting a sample for a transmission electron microscope (TEM) from the measured part, and observing the sample with a TEM The ratio of dislocations disappeared from the area ratio or the number of cells
Of the dislocation vanishing area in the known creep-damaged material of the same composition material obtained in advance .
Ratio, or ratio by number of cells-of creep damage
It is characterized in that the creep damage of the sample is evaluated by the linear relationship and the step of comparison.

According to this quantitative evaluation method for creep damage of heat-resistant steel, a sample is taken from the part to be measured and observed with a transmission electron microscope (TEM), and the dislocation disappearance ratio in the observed structure is calculated as the area ratio, or The ratio of the number of cells is used to determine the area of dislocation disappearance in the material of known creep damage of the same composition material that was previously determined .
Ratio, or ratio by number of cells-of creep damage
By making a comparison with the linear relationship, it becomes possible to evaluate the creep damage from any of the samples having a tempered martensite structure.

Further, in the quantitative evaluation method for creep damage of heat-resistant steel according to claim 2 of the present invention, in addition to the structure according to claim 1, dislocations in a material of the same composition which is obtained in advance and has known creep damage. The ratio of disappearance of creep damage to the relationship between creep damage and the resistance to creep damage that should be evaluated quantitatively.
Even if the usage conditions and test conditions of hot steel differ,
Ratio of disappearance of dislocations-collinear straight line of creep damage
It is characterized in that it is obtained from the relationship expressed above .

According to this quantitative evaluation method of creep damage of heat-resistant steel, it has been confirmed that when a relationship for calibration is to be obtained in advance, a certain relationship can be obtained regardless of the test conditions, and the applicable range is expanded. become able to.

[0026]

[0027]

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be specifically described below with reference to the drawings. The quantitative evaluation method for creep damage of heat-resistant steel of the present invention is as shown in FIG.
The creep life is known by using a calibration graph that is obtained in advance and is composed of the following three components. 1. The dislocation disappearance ratio in the TEM observation structure is determined for a material that has been subjected to known creep damage (preparation of a graph for calibration).

A plurality of samples with known creep strains are prepared by the creep test, and a micro test piece for a transmission electron microscope (TEM) is prepared from the samples. At this time, be careful not to add new distortion.

This micro test piece is observed by TEM.
When observing with a TEM, when observing a thin portion of the micro test piece, dislocations are missing on the surface of the thin film, and it does not correspond to observing an actual structure. Therefore, it is necessary to observe a thick portion to some extent.

Here, the improved 9Cr steel whose composition is shown in Table 1 was used as the heat-resistant steel used as the tempered martensite structure.

[0032]

[Table 1]

Using this improved 9Cr steel, as shown in FIG. 2, one having a creep strain of 12 kgf / mm @ 2 at 600.degree. C. and one having a creep strain of 6 kgf / mm @ 2 at 650.degree. Five kinds of samples having different creep life consumption rates (use time relative to creep rupture time) were observed by a transmission electron microscope (TEM) by a stress acceleration test or the like.

In the observation with the transmission electron microscope, the thick portion of the micro test piece, for example, a portion of 0.2 μm or more
It was observed and photographed at a magnification of 20,000 times.

In this case, the sample is rotated slightly and photographed under the diffraction condition so that dislocations can be observed as well as possible. If the diffraction conditions are matched to the dislocations in one cell, the dislocations in the other cells cannot be completely seen.
With reference to the obtained photograph, the sample is slightly rotated and the region where the dislocation exists is recorded on the photograph.

Note that cell growth may be observed at the end of creep deformation. In this case, the area ratio may be used for the calculation. Before creep deformation, dislocations are uniformly distributed in the cell, but after creep deformation, it is also observed that they are rearranged in a net shape, and there are cases where they are not regarded as cells. Be recorded.

The dislocation disappearance ratio P is calculated for each sample. The dislocation disappearance ratio P is calculated as the ratio of the number of dislocation disappeared cells to the number of all observed cells, or the ratio of the dislocation (GND) disappeared area to all the observed areas.

Here, as a criterion for judging cells in which dislocation (GND) disappears, the case where several dislocations are observed at 1 μm 2 is defined as dislocation (GND) disappearance. In addition,
The fractured test piece was observed while avoiding the necked portion.

FIG. 2 shows the relationship between the dislocation disappearance ratio P thus obtained and the creep damage (creep life consumption rate).

As can be seen from the figure, there is a constant relationship between the dislocation disappearance ratio P and the creep life consumption rate, that is, linearity.

The creep test conditions are 600 ° C. and 12
The same straight line can be used to indicate the difference between the one with kgf / mm2 (●) and the one with 6 kgf / mm2 at 650 ° C (○). Without P is 0.8 to 0.9 (P is 0.9 at the necking part)
.About.1.0), which was calculated as a region in which 80 to 90% of GND disappeared, and the values were the same. 3 to 5 show transmission electron micrographs of various test pieces.

In FIG. 3, the creep test conditions are 600 ° C. and 12
The creep life consumption rate is 20% at kgf / mm2,
High-density dislocations are observed in the cells where the dislocations disappear and the cells in the lath.

In FIG. 4, the creep test conditions are 600 ° C. and 12
The creep life consumption rate is 100% at kgf / mm2, and dislocation disappeared laths are almost observed.

FIG. 5 shows creep test conditions of 650 ° C. and 6 kg.
The creep life consumption rate is 100% at f / mm2,
A cell in which dislocations are partially eliminated is observed.

2. For materials with unknown creep damage For materials with unknown creep damage 1 above,
Then, the ratio P at which dislocation (GND) disappears is obtained.

3. The creep life consumption rate can be known by comparing the graph for the configuration obtained in 1 with the rate P in which dislocations disappeared in the sample obtained from the actual machine whose creep damage is unknown and obtained in 2 above.

As described above, the creep test (including the interruption test) is performed under various conditions in advance, the relationship between the dislocation disappearance ratio P and the life consumption rate is obtained, and the graph for calibration is prepared. Therefore, if the dislocation disappearance ratio P is obtained for the same material whose creep life damage is unknown, the creep life consumption rate can be known by comparison with the calibration graph.

According to this quantitative evaluation method of creep damage, since the ratio P of dislocation disappearance is almost zero in the state before use, the graphs for calibration under the various use conditions are the same. If so, use environment, organization before use,
Even if the mechanical properties are unknown, it is possible to obtain the lifespan consumption rate under a fixed usage environment.On the other hand, even if the calibration graphs are not the same, if the usage conditions can be estimated by other methods, The life consumption rate of

In the above embodiment, the modified Cr steel alloy is used as an example of the material used as the tempered martensitic structure, but the material is not limited to this and other materials used as the tempered martensitic structure. Can be similarly applied to.

Although the ratio of disappearance of dislocations is calculated by the area, some threshold value may be set and the number of cells having dislocations may be used instead of the area.

[0051]

As described above in detail with reference to the embodiments, according to the method for quantitatively evaluating creep damage of heat-resistant steel according to claim 1 of the present invention, a sample is taken from the portion to be measured and the transmission electron Observed with a microscope (TEM), the ratio of dislocation disappearance in the observed tissue is the ratio of the area or the ratio of the number of cells.
In determined, the ratio of the areas in which dislocation is lost in the material which received the known creep damage of the same composition material obtained in advance,
Or straight between the ratio and the creep damage due to the ratio of the number of cells
By comparing the line relationship, be a sample of martensitic tempered from both, it is possible to quantitatively evaluate the creep damage.

Further, according to the quantitative evaluation method for creep damage of heat-resistant steel according to claim 2 of the present invention, when a relationship for calibration is obtained in advance, a certain relationship can be obtained regardless of the test conditions. The range can be expanded.

[0053]

[Brief description of drawings]

FIG. 1 is a flow chart according to an embodiment of a quantitative evaluation method for creep damage of heat resistant steel of the present invention.

FIG. 2 is a graph of the relationship (for calibration) between the dislocation disappearance ratio and the creep life consumption rate according to an embodiment of the quantitative evaluation method for creep damage of heat-resistant steel of the present invention.

FIG. 3 shows a creep test condition according to one embodiment of the quantitative evaluation method for creep damage of heat-resistant steel of the present invention, which is 600 ° C.
It is a transmission electron micrograph with a creep life consumption rate of 20% at 12 kgf / mm 2.

FIG. 4 shows a creep test condition of 600 ° C. according to an embodiment of the quantitative evaluation method for creep damage of heat-resistant steel of the present invention,
It is a transmission electron micrograph with a creep life consumption rate of 100% at 12 kgf / mm 2.

FIG. 5 shows a creep test condition of 650 ° C. according to one embodiment of the quantitative evaluation method for creep damage of heat-resistant steel of the present invention,
It is a transmission electron micrograph with a creep life consumption rate of 100% at 6 kgf / mm 2.

─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-10-227784 (JP, A) Materials and Processes, 1993, Volume 6, No. 6, 1646-1649 Materials and Processes, 1996, 9th Volume, No. 6, 1305 (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 33/00-33/46

Claims (2)

(57) [Claims] 1. A step of collecting a sample for a transmission electron microscope (TEM) from a portion to be measured when quantitatively evaluating creep damage of heat-resistant steel used as a tempered martensite structure, and the TEM. The step of obtaining the dislocation disappearance ratio in the observed structure from the ratio of the areas or the ratio of the number of cells, and the area where the dislocation disappeared in the material of the same composition with known creep damage that was obtained in advance. Ratio or the ratio of the number of cells to a ratio-creep damage linear relationship, and a step of comparing the creep damage of the sample is evaluated. 2. A ratio of disappearance of dislocations in a material which has been previously obtained and has a known creep damage of the same composition material.
The relationship between creep damage is quantitatively evaluated by the creep damage.
When the use conditions and test conditions of the heat-resistant steel to be evaluated differ
Even the obtained dislocation disappearance ratio-creep damage
The method for quantitatively evaluating creep damage of heat-resistant steel according to claim 1, characterized in that it is obtained from the relationship represented by the same straight line .