JP5302870B2 - Fracture stress range estimation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a method for estimating a range of breaking stress capable of obtaining the range of breaking stress of an object material to be evaluated that may be fatigue broken under an oxidizing atmosphere with a high temperature through a rational and relatively versatile examination process. <P>SOLUTION: The method for estimating a range of breaking stress includes the steps of determining a stress expansion coefficient range &utri;Kth at a boundary point where a stable state without crack extension changes to an extension state with rapid crack extension by conducting a crack extension evaluating test, determining the change of the thickness d of an oxide layer in a time t region as d = &alpha;t<SP>&beta;</SP>(&alpha; and &beta;: constant), substituting an approximate expression of d=&alpha;t&beta;in a crack length a in &Delta;Keff=k&times;&Delta;&sigma;&times;(&pi;a)<SP>1/2</SP>, which is a relational expression of the stress expansion coefficient range &Delta;K and a crack length a, substituting &Delta;Keffth of an effective value in the stress expansion coefficient range at the boundary point, in the stress expansion range, and estimating a breaking stress range &Delta;&sigma;f in the object material to be evaluated which is high temperature fatigued by a repeated load received in a high temperature state. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a method for predicting the life of a member to be replaced at a certain cycle in order to receive repeated stress at a high temperature, and more particularly, fracture in a material to be evaluated that undergoes high temperature fatigue under repeated load in a high temperature oxidizing atmosphere. The present invention relates to a fracture stress range estimation method for estimating a stress range Δσf.

  The engine exhaust valve is one of the most representative members that are repeatedly subjected to stress at high temperatures. Even in such an engine exhaust valve, it is necessary to appropriately obtain the replacement time based on the life prediction. Especially for special applications of this type, new materials may be used that have not been known or have not been used for that purpose. It is necessary to estimate the so-called replacement time appropriately.

As a method for analyzing the surface crack depth in a member subjected to repeated stress in this type of high temperature state, Patent Document 1 introduces a method for analyzing the crack depth generated in the water wall pipe of a boiler. In the technique disclosed in Patent Document 1, after measuring the _crack depth a _TC0 of the desired location on the outer surface of the water wall pipe (S1), the stress amplitude σ _a is calculated (S2), and the stress value associated with the start and stop is calculated. The stress amplitude σ _a is calculated from the width using a finite element method or the like, the stress intensity factor range ΔK is calculated from the obtained values (S3), and the diagram i corresponding to the atmospheric conditions in the boiler 1 is selected. (S4) Next, a fatigue surface crack component (da _en / dN) × N corresponding to the stress intensity factor range ΔK is obtained (S5), and the elephant skin-like surface crack of the relevant part at the time of measurement (current) From the depth a _p , the high temperature reoxidation (corrosion) rate da _HC / dt specific to the boiler is calculated backward (S6), and the resulting high temperature reoxidation rate da _HC / dt can be used to predict the future elephant skin shape. The surface crack depth a _TC is sequentially calculated (S8).

JP 2002-156325 A

  However, the technique disclosed in Patent Document 1 requires a stress analysis and requires a diagram i corresponding to the atmospheric conditions in the boiler 1 so that the highly reliable experience required for the analysis is required. A lot of data needs to be accumulated.

  In general, it is known that high-temperature oxidative fatigue failure occurs rapidly when the stress intensity factor range ΔK exceeds a specific threshold value ΔKth. For example, in the case of the Ni-based alloy which is the subject of the present application, the time until the stress intensity factor range ΔK exceeds the threshold value ΔKth is about several tens of thousands of hours, whereas when the threshold value ΔKth is exceeded, several to several tens of hours. Destroy with.

  Therefore, it is preferable to obtain an estimation method for obtaining the fracture stress range of the material to be evaluated that may lead to fatigue fracture under a high-temperature oxidizing atmosphere as described above through a rational and versatile test.

  Grain boundary oxidation occurs on the surface of the metal material in a high temperature oxidation environment. This grain boundary oxide layer progresses with time, works as a precrack, and is very likely to work in the direction of reducing the fatigue life by stress concentration (notch effect). In fact, the inventors have also confirmed that fatigue life is reduced when grain boundary oxidation is progressing.

  Generally, the crack growth rate is determined by the stress intensity factor range ΔK at the crack tip, and the crack growth is not started until ΔK exceeds a certain threshold value (ΔKth). If the grain boundary oxide layer that progresses with time plays a role of pre-cracking, it can be estimated that crack growth starts when the thickness reaches ΔKth with the stress used.

On the other hand, the fatigue life is generally considered as follows.
Fatigue life = Fatigue crack initiation life + Fatigue crack growth life However, in the range where the stress intensity factor range ΔK described above exceeds the threshold value ΔKth, the crack growth rate is very high, and the fracture occurs in several tens of hours at the maximum. To. This fatigue crack growth life is extremely short compared to the time (tens of thousands of hours) in which grain boundary oxidation proceeds, which can be regarded as the fatigue crack initiation life, and can be almost ignored when considering the actual life.
Accordingly, the present inventors have completed the present application by paying attention to the fact that “fatigue life = fatigue crack initiation life” is virtually acceptable.

Further, in a Ni-based alloy as the object of the present application, when a crack growth characteristic evaluation test is performed using a CT test piece, as shown in FIG. 5 later, crack opening displacement COD (crack opening displacement) and test piece The relationship with the load P is not a linear relationship in which the crack opening displacement COD increases simply as the load increases, but is non-linear in which an increase in the crack opening displacement COD is recognized for the first time within a certain load range. It becomes a relationship. In the case of such a material, under a high-temperature atmosphere, the “effective load value ΔPeff” which is smaller than the apparent “load ΔP”, that is, the apparent load “ΔP” to “load” The crack progresses by a load (ΔPeff = ΔP−ΔPb) obtained by subtracting the maximum load Pb ”at which the crack opening displacement hardly increases. Therefore, it is considered that it is appropriate to use the effective value ΔKeff of the stress intensity factor range ΔK, that is, to use ΔKeff = k × Δσ × (πa) ^1/2 for the estimation of the fracture stress range Δσf. Originally, the inventors have completed the present invention.

Specifically, the fracture stress range Δσf is estimated by the following procedure.
1 Boundary point stress intensity factor derivation step Performs a crack growth characteristic evaluation test using a CT specimen of the material to be evaluated, and changes from a stable state where crack growth is not observed to a state where rapid crack growth is observed. The stress intensity factor range ΔKth at the boundary point is obtained.
2 Heat treatment behavior deriving step Heat treatment is performed on the material to be evaluated, and an oxide layer formation rate behavior which is a relation between the oxide layer thickness d formed by the heat treatment and the heat treatment time t is obtained.
3 Change Approximation Formula Deriving Step Based on the oxide layer formation rate behavior obtained in the heat treatment behavior deriving step, the change in the oxide layer thickness d in the time t region is expressed as an approximate expression d = αt ^β (α and β are constants). Ask.
4 Load-crack opening displacement derivation step Crack growth characteristics, which are tests to determine the relationship between crack opening displacement COD (crack opening displacement) and the load P applied to the CT test piece for CT test pieces made of the material to be evaluated Perform an evaluation test. From the obtained load-crack opening displacement relationship, the opening ratio U = ΔPeff / ΔP of the material to be evaluated is obtained, and the effective value of the stress intensity factor range ΔK (assuming ΔKeffth / ΔK = U = ΔPeff / ΔP) ( Effective range) ΔKeffth is obtained.
5 Fracture Stress Range Estimating Step Regarding ΔKeffth = k × Δσ × (πa) ^1/2 (where k is a shape factor), which is a relational expression between the effective value ΔKeffth of the stress intensity factor range ΔK and the crack length a,
Substituting for the crack length a a representative expression of the crack length obtained using the approximate expression d = αt ^β obtained in the oxide layer thickness change approximate expression deriving step,
For the stress intensity factor range, substitute the effective value ΔKeffth of the boundary point stress intensity factor range obtained by the boundary point stress intensity factor derivation step,
A fracture stress range Δσf is estimated in a material to be evaluated that undergoes high temperature fatigue under repeated load in a high temperature state.

In the method for estimating the fracture stress range according to the present application, the oxide layer thickness d formed over time in the material to be evaluated is obtained in a change approximation formula deriving step, and a crack that propagates with this oxide layer thickness d is formed. As a result, a relational expression with the stress intensity factor range is obtained. On the other hand, using the stress intensity factor range ΔKth obtained in the boundary point stress intensity factor deriving step, the effective value of the stress intensity factor range ΔKth using the opening ratio U separately obtained in the load-crack opening displacement deriving step. ΔKeffth is obtained, and the fracture stress range Δσf is estimated in the fracture stress range estimation step.
Therefore, according to the present invention, it has become possible to reasonably estimate the range of fracture stress that decreases with age, and thus the fatigue strength and fatigue life. In actual use, it is possible to set an appropriate replacement period using the prediction curve obtained in the present invention by confirming the stress range applied to the actual member and considering an appropriate safety factor.

When the fracture stress range σf is obtained by the procedure as described above, the approximate expression d = αt ^β obtained by the oxide layer thickness change approximate expression derivation step can be used as it is as the crack length a.
That is, the fracture stress range Δσf is obtained based on the following equation: Δσf = ΔKeffth / [k × {π (αt ^β )} ^1/2 ]
Here, k is a shape factor determined based on the shape of the CT specimen.

  In this case, since the approximate expression of the oxide layer thickness is used as it is, the 1 boundary point stress intensity factor deriving step, 2 heat treatment behavior deriving step, 3 change approximate expression deriving step described above are executed, and 4 load− By simply performing the crack opening displacement deriving step, an estimation formula for the fracture stress σf can be obtained substantially.

On the other hand, ΔKeffth = k × Δσf ₀ × from the fracture stress range Δσf _{0 at} the predetermined number of times of the evaluation target material and the effective value ΔKeffth of the stress intensity factor range at the boundary point obtained by the boundary point stress intensity factor derivation step. Based on (πa ₀ ) ^1/2 , an inherent crack length a ₀ that can be assumed to exist at the start of crack initiation considering the fracture stress range Δσf ₀ at a predetermined number of lifetimes is obtained. ,
The fracture stress range Δσf can also be obtained based on the following equation: Δσf = ΔKeffth / [k × {π (αt ^β + a ₀ )} ^1/2 ]
Here, k is a shape factor determined based on the shape of the CT specimen.

In this way, the intrinsic crack length a ₀ is obtained by assuming a virtual initial crack from the effective value ΔKeffth of the stress intensity factor range at the boundary point and the fracture stress range Δσf _{0 at the} predetermined number of times of life. By appropriately assuming the crack length in a short-period region, information on both the inherent crack length a ₀ and the formation of an oxide film in which the film thickness d increases with time can be used. The range can be estimated. The inventors consider that the estimation by this estimation method can perform a reasonable estimation with high accuracy from the short time side such as several tens of hours.

From the above situation, Δσf = ΔKeffth / [k × {π (αt ^β )} ^1/2 ] described above
Δσf = ΔKeffth / [k × {π (αt ^β + a ₀ )} ^1/2 ]
With respect to the fracture stress range Δσf estimated by each, the fracture stress range Δσf between the fracture stress ranges obtained by the estimation methods of both fracture stress ranges is estimated by estimating the fracture stress range in thousands to tens of thousands of hours. It can also be estimated that there is.

  In this configuration, the fracture stress range Δσf can be appropriately estimated in consideration of both fatigue in which the inherent crack length is dominant and fatigue in which the fatigue due to the formation of the oxide film is dominant.

The figure which shows the procedure of the estimation method of the fracture stress range of 1st Embodiment which concerns on this application Diagram showing fatigue crack growth characteristics of Ni-based alloys Diagram showing the shape of CT specimen Diagram showing the relationship between heat treatment time and oxide layer thickness Diagram showing the relationship between crack opening displacement and load The figure which shows the relationship line of the use time calculated | required by 1st Embodiment, and the fracture stress range The figure which shows the procedure of the estimation method of the fracture stress range of 2nd Embodiment which concerns on this application The figure which shows the high temperature fatigue test result of Ni base alloy The figure which shows the relationship line of the use time calculated | required by 2nd Embodiment, and the fracture stress range

Hereinafter, the estimation method of the fracture stress range of the present application will be specifically described based on the result of using the Ni-based alloy for engine exhaust valves, which is the material to be evaluated.
The composition of the Ni-based alloy used this time is as shown in Table 1.

この表で、「ｂａｌ」は残余分を示す。

In this table, “bal” indicates the remainder.

The purpose of the method for estimating the fracture stress range of the present application is to satisfactorily estimate the fracture stress range in at least several thousand hours to tens of thousands of hours. In the estimation of the fracture stress range Δσf, the fracture stress range in the use time region is estimated under the assumption that “fatigue life = fatigue crack generation life” as described above. Here, in the first embodiment described below, the approximate expression of the oxide layer thickness d is used as it is as the crack length a. On the other hand, in the second embodiment, if the approximate expression of the oxide layer thickness d is used as it is as in the first embodiment, the generated stress is infinite when crack growth starts without an initial crack. In order to avoid this unreasonable nature, the concept of inherent crack inherent in the material is introduced, and the sum of the inherent crack length a ₀ and the oxidized layer thickness d is used as the crack length.
Hereinafter, the first embodiment and the second embodiment will be described in this order.

[First Embodiment]
In FIG. 1, the procedure of the estimation method of the fracture stress range which concerns on this application was shown.
Hereinafter, along with the procedure, the processing contents of each step will be described, and the results of the Ni-based alloy obtained in the step will be described.

1 Boundary point stress intensity factor deriving step (# 1-1)
For CT specimens (compact tension specimens) made of Ni-based alloy, which is the material to be evaluated, a crack growth characteristic evaluation test was performed. From a stable state where no crack propagation was observed, The stress intensity factor range ΔKth at the boundary point where the rapid progress is recognized is obtained.
FIG. 2 shows the stress intensity factor range ΔK and fatigue crack propagation rate da / dN of the Ni-based alloy obtained by executing this crack growth characteristic evaluation test, and shows the shape of the CT specimen. This is shown in FIG. In FIG. 3, the unit of length is mm.

In the crack growth characteristic evaluation test, the stress range applied in the test is Δσ = 0 to 4.3 MPa in accordance with the test method specified in ASTM D5045-93, and the stress intensity factor range ΔK is the crack depth a. Measured and determined according to ΔK = k × Δσ × (πa) ^1/2 , and the fatigue crack propagation rate da / dN was determined by dividing the increase in a by the number of times stress was applied for each plot. Here, k is a shape factor determined according to the shape of the CT test piece, specifically 1.1215.

From the evaluation of crack growth characteristics shown in FIG. 2, the stress intensity factor range ΔKth at the boundary point from the stable state where crack growth is not observed to the growth state where rapid crack growth is recognized is about 8.68 MPa · Estimated as m ^1/2 .

2 Heat treatment behavior derivation step (# 2)
The Ni base alloy evaluation target material is subjected to heat treatment, and an oxide layer formation rate behavior which is a relation between the oxide layer thickness d formed by the heat treatment and the heat treatment time t is obtained.
FIG. 4 shows the relationship between the heat treatment time t (hr) obtained in the heat treatment behavior deriving step and the oxide layer thickness d (μm). In this heat treatment behavior deriving step, the test temperature was set to 900 ° C. with respect to the actual machine temperature of 800 ° C. in order to accelerate the test. As for the influence of time acceleration, the Larson Miller parameter, which is the most commonly used parameter, is used. The Larson mirror parameter P has a relationship of P = absolute temperature × (20 + log (heating time)).
For example, assuming that the actual machine temperature is 800 ° C., the heating time can be shortened as follows by heating to 900 ° C. in an experimental furnace or the like.
800 ° C 900 ° C
2000hr 20.6hr
3000hr 29.9hr
5000hr 47.7hr
10000hr 89.9hr

3. Step for deriving a change approximation formula (# 3)
Based on the oxide layer formation rate behavior obtained in the heat treatment behavior deriving step (# 2), the change in the oxide layer thickness d in the time t region is derived as d = αt ^β (α and β are constants) to obtain the oxide layer An approximate expression of the thickness d (m) is obtained.
For the Ni-based alloy, α = 0.326 × 10 ⁻⁶ and β = 0.6209.

4 Fracture Stress Range Estimation Step 4-1 Basic Concept Regarding ΔKeff = k × Δσ × (πa) ^1/2 that is a relational expression between the effective value ΔKeff of the stress intensity factor range and the crack length a (Δσ is stress), Substituting the approximate expression d = αt ^β obtained in the oxide layer thickness change approximate expression deriving step for the crack length a, and obtaining the effective value ΔKeff in the stress intensity factor range by the boundary point stress intensity coefficient deriving step. The effective value ΔKeffth of the stress intensity factor range at the boundary point is substituted, and the fracture stress range Δσf in the evaluation target material that undergoes repeated load in a high temperature state and undergoes high temperature fatigue is estimated.

The specific expression expansion is as follows.
From normal fracture mechanics, the relationship between the stress range Δσ, the crack length a, and the effective value ΔKeff of the stress intensity factor range is expressed as ΔKeff = k × Δσ × (πa) ^1/2 .
If the stress range where fracture occurs is Δσf, then ΔKeffth = k × Δσf × (πa) ^1/2 .

Substituting the approximate expression d = αt ^β obtained in the oxide layer thickness change approximate expression deriving step into the crack length a, ΔKeffth = 1.215 × Δσf × (0.326 × 10 ⁻⁶ πt ^0.6209 ) ^{1 / 2} .
This formula is a basic form of the life prediction formula according to the present case.

4-2 Load-crack opening displacement derivation step (# 4)
In the present application, with respect to a CT test piece composed of a material to be evaluated, the relationship between the crack opening displacement COD (crack opening displacement) and the load P is obtained using the crack growth characteristic evaluation test result, and the obtained load is obtained. From the relationship between the range ΔP and the crack opening displacement COD, the opening ratio U = ΔPeff / ΔP is obtained.
In the case of the Ni-based alloy, the opening ratio U = ΔPeff / ΔP is (3.338417-1.9111169) /3.38417=0.441117 from the relationship between the load (MN) and the crack opening displacement (mm) in FIG. became.

In the crack growth characteristic evaluation test, in a high temperature state, a compressive stress field is usually formed in the vicinity of the crack tip portion of the CT specimen due to the effect of high temperature heating, and the apparent appearance shown in FIG. The crack propagates with an effective load ΔPeff smaller than the load ΔP. Therefore, this effect is removed.
As shown above, in such a nonlinear material, ΔKeff = k × Δσ × (πa) ^1/2 is adopted as a relational expression between the effective value ΔKeff of the stress intensity factor range and the crack length a. It is preferable to do this. Therefore, the stress intensity factor range ΔKth at the boundary point previously obtained in the boundary point stress intensity factor deriving step (# 1-1) is replaced with the effective value ΔKeffth, and this effective value is used (# 1-2).

From the above, when the effective value ΔKeffth of the stress intensity factor range at the boundary point described above is rewritten using the fracture stress range Δσf and the crack length αt ^β , the following is obtained.
ΔKeffth = k × Δσf × (παt ^β ) ^1/2 (Formula 1)

As a result, the fracture stress range Δσf is
Δσf = ΔKeffth / [ k × (παt ^β ) ^1/2 ] (Expression 2) (# 6).
Regarding the Ni-based alloy to be evaluated, Δσf = 3.83 ^/ [1.1215 ^{× (} 0.326 ^× 10 ⁻⁶ πt ^0.6209 ) ^1/2 ].

FIG. 6 shows the relationship between the fracture stress range Δσf thus determined and time t.
The measured value of the fracture stress range indicated by “■” described as “actual product use” is that the Ni-based alloy is removed from the engine exhaust valve for about 5000 hours in a high-temperature oxidizing atmosphere where the temperature changes from 800 ° C. to room temperature. It is the result of measuring the fracture stress after using as a surface material. The measurement results of the fracture stress range curve and the actual product were in good agreement.

[Second Embodiment]
As previously indicated, in the second embodiment, the concept of “inherent crack length” inherent to the material is introduced, and this “inherent crack length” and “thickness of oxide layer” are considered. Use “crack length”.

FIG. 7 shows the procedure of the method for estimating the fracture stress range according to the present application, corresponding to FIG.
In the procedure shown in this figure, an inherent crack length deriving step (# 15) is added to the procedure shown in FIG. In the second embodiment, the crack length a obtained by adding the inherent crack length a ₀ derived in the inherent crack length deriving step (# 15) and the oxide layer thickness d is defined as the stress expansion at the boundary point. It is introduced into the relational expression between the effective value of the coefficient range and the crack length.

  Boundary point stress intensity factor derivation step (# 11-1, # 1-1 shown in FIG. 1), heat treatment behavior derivation step (# 12, # 2 shown in FIG. 1), change approximate expression derivation step (# 13, FIG. 1) 3), a load-crack opening displacement deriving step (# 14, # 4 shown in FIG. 1), and a step of converting the boundary point stress intensity factor range ΔKth into its effective value ΔKeffth (# 11-2, FIG. # 1-2) shown in FIG. 1 has substantially the same contents although the step numbers are different.

1 Boundary point stress intensity factor deriving step (# 11-1)
For CT specimens (compact tension specimens) made of Ni-based alloy, which is the material to be evaluated, a crack growth characteristic evaluation test was performed. From a stable state where no crack propagation was observed, The stress intensity factor range ΔKth at the boundary point where the rapid progress is recognized is obtained.

As previously indicated, the stress intensity factor range at the boundary point from the stable state where crack growth is not observed to the state where rapid crack growth is observed, based on the evaluation of crack growth characteristics shown in FIG. ΔKth was estimated to be about 8.68 MPa · m ^1/2 .

2 Heat treatment behavior derivation step (# 12)
The Ni base alloy evaluation target material is subjected to heat treatment, and an oxide layer formation rate behavior which is a relationship between the thickness of the oxide layer formed by the heat treatment and the heat treatment time is obtained.

3. Step for deriving a change approximation formula (# 13)
Based on the oxide layer formation rate behavior obtained in the heat treatment behavior deriving step (# 12), the change in the oxide layer thickness d in the time t region is derived as d = αt ^β (α and β are constants) to obtain the oxide layer An approximate expression of the thickness d (m) is obtained.
Regarding the Ni-based alloy shown in FIG. 4, α = 0.326 × 10 ⁻⁶ and β = 0.6209.

4 Load-crack opening displacement derivation step (# 14)
In the present application, the relationship between the crack opening displacement COD and the load P is obtained for the CT test piece composed of the material to be evaluated using the crack growth characteristic evaluation test result. From the relationship between the obtained load range ΔP and the crack opening displacement COD, the opening ratio U = ΔPeff / ΔP is obtained.
In the case of the Ni-based alloy, the opening ratio U = ΔPeff / ΔP is (3.338417-1.9111169) /3.38417=0.441117 from the relationship between the load (MN) and the crack opening displacement (mm) in FIG. became. Using this aperture ratio U = ΔPeff / ΔP, the boundary point stress intensity factor range ΔKth obtained in the boundary point stress intensity factor derivation step is converted into its effective value ΔKeffth, and this effective value is used (# 11-2). ).

5 Intrinsic crack length derivation step (# 15)
This intrinsic crack length deriving step (# 15) includes an effective value of the fracture stress range Δσf _{0 at} the predetermined number of times of the evaluation target material and the boundary point stress intensity factor range ΔKeffth obtained by the boundary point stress intensity factor deriving step. From ΔKeffth = k × Δσf ₀ × (πa ₀ ) ^1/2 , it can be assumed that the crack is present at the start of crack propagation, taking into account the fracture stress range Δσf ₀ at the predetermined number of times of life. This is a step for obtaining the inherent crack length a ₀ .

This will be described in more detail below.
In this step, a fatigue limit (fracture stress range at a predetermined number of lifetimes of the evaluation target material) Δσf ₀ in the evaluation target environment of the evaluation target material is obtained, and an inherent crack length a ₀ is derived from the fatigue limit.
The inherent crack length a ₀ is the stress at t = 0 based on the basic equation ΔKeff = k × Δσ × [π (a ₁ + a ₀ )] ^1/2 when considering the inherent crack length a ₀ The magnification factor range ΔKeff = k × Δσ × (π (a ₀ )) ^1/2 is used. In the above basic formula considering the inherent crack length, a ₁ is a crack element (an oxide film element in this application) that grows with time, and a ₀ is a crack element that is assumed to exist when t = 0. (Generally referred to as the inherent crack length).
Such an inherent crack length a ₀ is generally obtained by assuming a fatigue limit (fracture stress Δσf ₀ ) at a high temperature of 10 ⁸ times life and obtaining the 10 ⁸ times life of the material to be evaluated. That is, it can be obtained from the above 10 ⁸ times lifetime based on the above ΔKeff = k × Δσ × [π (a ₀ )] ^1/2 .

FIG. 8 shows the results of a high temperature fatigue test of a new Ni-based alloy and an aged product according to the present application. The 10 ⁸ times fatigue limit of this material is 153 Mpa for a new product and about 80 Mpa for an aged product (a product used for 3000 hours). When determining the specific crack length a ₀ evaluated material, originally for specific crack length a ₀ in the material there is only small Without crack is intended to be assumed, and 153MPa case of obtaining the specific crack length a ₀ It is appropriate to do.

As shown above, the aperture ratio U = ΔPeff / ΔP of this material is 0.44117, and the threshold value of the stress intensity factor range ΔK is ΔKth = 8. 68 . Therefore, the effective value of the stress intensity factor range at the boundary point where the crack starts to propagate is ΔKeffth = 0.44117 × 8.68 = 3.83 .

Using the effective value ΔKeffth of the stress intensity factor range at the obtained boundary point and the previously obtained fracture stress range Δσf _{0 at} 10 ⁸ times life, the intrinsic crack length a ₀ was estimated as follows.

ΔKeffth = k × Δσf ₀ × (πa ₀ ) ^1/2 ,
3. 83 = 1.1215 × 153 × (πa ₀ ) ^1/2
a ₀ = 0.1589 × 10 ⁻³ (m).

6 Fracture Stress Range Estimating Step In the second embodiment, as previously indicated, the sum of the oxide layer thickness d and the intrinsic crack length a ₀ is used as the crack length a.
The effective value ΔKeff of the stress intensity factor range and the crack length a = d + a ₀ are connected by the following basic formula.
ΔKeff = k × Δσ × [π (d + a ₀ )] ^1/2
Considering the above threshold value ΔKeffth of the effective stress intensity factor range and the approximate expression of the oxide layer thickness d, the fracture stress range Δσf can be expressed by the following equation (# 16).
ΔKeffth = k × Δσf × {π (αt ^β + a ₀ )} ^1/2 (Formula 3)

As a result, the fracture stress range Δσf is
Δσf = ΔKeffth / [k × {π (αt ^β + a ₀ )} ^1/2 ] (# 17).

For Ni-based alloys that were evaluated,
Δσf = 3. 83 / [ 1.1215 × ^{π (0.326 × 10 ⁻⁶ × t ^0.6209 + 0.1589 × 10 ⁻³ )} ^1/2 ]
It became.

FIG. 9 shows the relationship between the fracture stress range Δσf thus determined and the time t.
The measured value of the fracture stress range indicated by “■” described as “actual product use” is that the Ni-based alloy is removed from the engine exhaust valve for about 5000 hours in a high-temperature oxidizing atmosphere where the temperature changes from 800 ° C. to room temperature. It is the result of measuring the fracture stress after using as a surface material. The measurement results of the fracture stress range curve and the actual product were in good agreement.

  Therefore, it has become possible to accurately predict the fracture stress range that decreases with age. When actually used, the stress range applied to the actual member is clarified, and an appropriate replacement cycle can be set using the prediction curve obtained by the present invention by considering an appropriate safety factor. .

[Another embodiment]
As described above, the fracture stress range Δσf is different between the first embodiment and the second embodiment. Therefore, both embodiments can be weighted, and the actual fracture stress range can be estimated as a result of the weighting.
That is, the fracture stress range in the first embodiment Δσf = ΔKeffth / [k × {π (αt ^β )} ^1/2 ]
And the fracture stress range in the second embodiment Δσf = ΔKeffth / [k × {π (αt ^β + a ₀ )} ^1/2 ]
With respect to the fracture stress range Δσf estimated by each, the fracture stress range Δσf between the fracture stress ranges obtained by the estimation methods of both fracture stress ranges is estimated by estimating the fracture stress range in thousands to tens of thousands of hours. It can also be estimated that there is.
In this way, the fracture stress range Δσf can be appropriately estimated by taking into account both the fatigue in which the inherent crack length a ₀ is dominant and the fatigue in which the fatigue due to the formation of the oxide film is dominant.

  We were able to obtain a method for estimating the range of fracture stress that can be obtained through a reasonable and relatively versatile testing process for the fracture stress range of the evaluation target material that can lead to fatigue failure in a high-temperature oxidizing atmosphere. .

Claims (4)

A method for estimating a fracture stress range for estimating a fracture stress range Δσf in a material to be evaluated that undergoes high temperature fatigue under repeated loading in a high temperature oxidizing atmosphere,
A crack growth characteristic evaluation test is performed on the CT specimen composed of the material to be evaluated, and a stable state in which crack propagation is not observed is changed to a progress state in which rapid crack growth is recognized. A boundary point stress intensity factor deriving step for obtaining a stress intensity factor range ΔKth of the boundary point;
A heat treatment behavior deriving step of performing a heat treatment on the material to be evaluated, and obtaining an oxide layer formation rate behavior that is a relationship between a thickness of the oxide layer formed by the heat treatment and a heat treatment time;
Based on the oxide layer formation rate behavior obtained in the heat treatment behavior deriving step, the change in the oxide layer thickness d in the time t region is derived as d = αt ^β (α and β are constants). An approximate expression derivation step;
A load-crack opening displacement derivation step for conducting a crack growth characteristic evaluation test, which is a test for obtaining a relationship between a crack opening displacement COD and a load P applied to the CT test piece, with respect to a CT test piece composed of the material to be evaluated. And calculating the opening ratio U = ΔPeff / ΔP of the material to be evaluated from the relationship of load-crack opening displacement obtained in the load-crack opening displacement derivation step,
From the stress intensity factor range ΔKth obtained in the boundary point stress intensity factor deriving step and the obtained opening ratio U of the evaluation target material, an effective value ΔKeffth of the stress intensity factor range at the boundary point is expressed as ΔKeffth = ΔKth. X as U,
Regarding ΔKeff = k × Δσ × (πa) ^1/2 , which is a relational expression between the effective value ΔKeff of the stress intensity factor range, where k is a shape factor, and the crack length a,
Substituting the crack length a using the approximate expression d = αt ^β obtained by the oxide layer thickness change approximate expression derivation step into the crack length a,
Valid values ΔKeff of the stress intensity factor range, by substituting the effective value ΔKeffth the stress intensity factor range of the boundary points obtained,
A method for estimating a fracture stress range for estimating a fracture stress range Δσf in a material to be evaluated that undergoes high temperature fatigue under repeated load in a high temperature state. The fracture stress range estimation method according to claim 1, wherein the fracture stress range Δσf is obtained based on the following formula: Δσf = ΔKeffth / [k × {π (αt ^β )} ^1/2 ]
Here, k is a shape factor determined based on the shape of the CT specimen. ΔKeffth = k × Δσf ₀ × (πa) from the fracture stress range Δσf _{0 at} the predetermined number of times of the evaluation target material and the effective value ΔKeffth of the stress intensity factor range at the boundary point obtained by the step of deriving the boundary point stress intensity factor ₀ ) Based on ^1/2 , an inherent crack length a ₀ that can be assumed to exist at the start of crack initiation considering the fracture stress range Δσf ₀ at a predetermined number of lifetimes,
The fracture stress range estimation method according to claim 1, wherein the fracture stress range Δσf is obtained based on the following formula: Δσf = ΔKeffth / [k × {π (αt ^β + a ₀ )} ^1/2 ]
Here, k is a shape factor determined based on the shape of the CT specimen. The estimation method of the fracture stress range according to claim 2 and the estimation method of the fracture stress range according to claim 3 are executed to estimate the fracture stress range in thousands to tens of thousands of hours.
A method for estimating a fracture stress range in which the fracture stress range Δσf is estimated to be between the fracture stress ranges obtained by the fracture stress range estimation method according to claim 2 or 3.

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