JPH1123565A - Corrosion environment scc crack developing prediction method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To predict the development of a stress corrosion crack (SCC) by calculating the crack developing speed based on the crack tip stress expansion coefficient in the specific equation indicating the relation between the crack tip strain speed of the SCC in the stationary progress state and the stress expansion coefficient. SOLUTION: When an SCC crack reaches the critical crack forming time to grow into a developing crack, a critical SCC crack is formed. When the stress expansion coefficient of the critical SCC crack is the SCC lower limit stress expansion coefficient or above, the stationary progress of the SCC crack occurs. The crack progress speed da/dt and the crack progress quantity are calculated in sequence with the equation indicating the relation between the crack tip strain speed dεct /dt and the stress expansion coefficient in an S-curve starting from the SCC lower limit stress expansion coefficient, where (n) is the active dissolution parameter, κ is the filtered water conductivity, ECP is the corrosion potential, and EPR is the effective electrochemical reactivation factor. The progress of the SCC crack can be predicted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for predicting the development of stress corrosion cracking (SCC) cracks in materials exposed to a corrosive environment, and more particularly, to a method for use under high temperature water and neutron irradiation in a nuclear reactor. The present invention relates to a corrosion environment SCC crack growth prediction method suitable for predicting crack growth of a material that generates SCC.

[0002]

2. Description of the Related Art As various types of plants, such as nuclear power plants, become larger and more complex, the environment in which structural materials are used becomes more and more severe. As a result, material damage that may be caused by a corrosive environment becomes apparent. I have.

[0003] Alloy materials such as stainless steel, which are widely used as structural materials, may cause SCC in a high-temperature water environment. When SCC occurs and the SCC crack propagates, the material strength decreases due to the extended SCC crack, and if this falls below the design strength, the structural material may break down. Therefore, it is possible to accurately predict the growth of the SCC crack in a corrosive environment. A method is desired.

[0004] SCC is said to occur under conditions in which three factors, material, stress, and environment, are superimposed.
The evolution mechanism is generally considered to consist of the following four processes as shown in FIG.

(1) A pit is formed at a crack starting point,
This grows by erosion by a corrosive environment.

(2) Microcracks are generated from the grown pits, and the micropits are grown and connected and united.

[0007] (3) The connection crack generated by the connection and coalescence of the microcracks has a small stress intensity factor at the crack tip.
The crack growth rate shows a very slow transition growth.

(4) When the connecting crack reaches a certain crack depth and reaches a critical crack depth, the crack becomes steady growth, and then grows at a certain crack growth speed.

As a conventional method for predicting SCC crack growth in a corrosive environment, for example, “Development and use of Predictive”
Model of Crack Propagation in 304/316
L, A583B / A508 and Inconel 600/18
2 Alloys in 288 ° C. Water “Proc. 3rd Int.
Symp. on Environmental Degradation of Material
s in Nuclear Power Systems-Water Reactors,
In the method described in NACE, p789 (1988), the above processes (1) and (2) are regarded as the crack incubation period and are not considered, and the initial crack depth (a ₀ ) is obtained in the process (3). Is assumed to occur, and the propagation of the SCC crack is calculated from the time of occurrence (t ₀ ).

[0010]

In the conventional method, the occurrence of the SCC crack in the processes (1) and (2) is regarded as a crack incubation period and is not considered, and the time until the occurrence is set to zero.

In the process (3), since the connected crack is a minute crack, the stress intensity at the crack tip is small, so that the crack tip strain rate becomes very slow. SC that strongly depends on depth
The transition propagation behavior of the C crack was included.

As described above, according to the conventional corrosion environment SCC crack growth prediction method, when the generation and growth behavior of the SCC crack is divided into a crack incubation period and a crack growth period, the crack incubation period is ignored and the crack growth period is ignored. Predicts the growth of SCC cracks, including transition progress.

An object of the present invention is to provide an algorithm for quantitatively and more realistically predicting the growth of an SCC crack by integrating both the crack incubation period and the crack growth period in the SCC crack initiation and propagation behavior. Corrosion environment SCC based on
An object of the present invention is to provide a method for predicting crack growth.

[0014]

According to the present invention, there is provided the above-mentioned SCC.
In the process of (4) in the crack initiation and propagation mechanism,
When the SCC crack exceeds the critical crack depth, the crack is steadily grown at a constant crack growth rate. Therefore, the critical crack formation time (t _C ), the critical crack depth (a _C ), the SCC lower critical stress intensity factor ( K _ISCC ) and the corrosion environment S composed of an algorithm (Fig. 1) for calculating the propagation of SCC cracks.
CC crack growth prediction method.

The method for predicting SCC crack growth in a corrosive environment according to the present invention is characterized in that when the SCC crack reaches a critical crack formation time t _C at which the SCC crack grows into a proliferating crack in a corrosive environment,
Assuming that a crack is formed, when the stress intensity factor (K) of the critical SCC crack is equal to or higher than K _ISCC , steady growth of the SCC crack occurs. At that time, the crack tip strain rate (dε _ct / dt) and the stress intensity factor ( The relationship with K) is determined by an algorithm that sequentially calculates the amount of crack propagation using the above-mentioned relational expressions [1] and [2] given by an S-shaped curve starting from K _ISCC.
It predicts the growth of CC cracks.

According to the present invention, (a) critical crack formation time t _C ,
(B) Critical SSC crack depth a _C , (c) SCC lower critical stress intensity factor K _ISCC , (d) Crack growth rate da / dt and crack tip strain rate dε _ct / dt are described below. .

(A) Critical crack formation time The critical crack formation time t _C can be considered as the time required for an SCC crack to grow into a progressive crack, and the crack opening displacement (one of the fracture toughness tests) The occurrence time can be detected by a COD) sensor, a double cantilever (DCB) sensor, or the like.

FIG. 3 shows the relationship between SCC crack growth and neutron irradiation obtained by installing a DCB sensor having a pre-crack in a nuclear reactor. The neutron dose is the SCC threshold dose (φ
critical SCC cracks are formed in the vicinity when it reaches t) _{thr e,}
The crack has begun steady growth. Therefore, the critical crack formation time t _C in a neutron irradiation environment can be obtained by the following equation [3], where φ is the neutron flux in the reactor.

[0019]

T _C = (φ t) _thre / φ (3) (b) Critical SSC crack depth Critical SCC crack depth a _C is the minimum critical value of the crack depth at which the SCC crack can steadily _propagate. Therefore, when a stress field corresponding to the yield stress σ _y acts, it can be obtained by the following equation [4].

[0020]

(Equation 3)

Here, λ is a constant determined by the shape of the crack, and K _ISCC is an SCC lower limit stress intensity factor. When an SCC crack that progresses in a steady state occurs in an irradiation corrosive environment, the neutron irradiation amount becomes the SCC threshold irradiation amount (φt) _thre
Is reached, the critical SCC given by the above equation [4]
It can be considered that an SCC crack having a crack depth a _C was formed.

(C) SCC lower limit stress intensity factor SCC lower limit stress intensity factor K _ISCC is less than this value.
Crack growth due to CC is not recognized, and when it exceeds this value, it can be considered as a threshold value of the stress intensity factor at which SCC crack growth occurs. FIG. 4 shows the relationship between the crack tip strain rate and the stress intensity factor.

The crack tip strain rate dε _ct / dt is given by an S-shaped curve starting from the SCC lower limit stress intensity factor K _ISCC, and is classified into the following three regions with respect to the value of the stress intensity factor K. It can be considered to show a dependency characteristic.

[0024]

(Equation 4)

Here, C _{1 to} C ₅ are constants determined by the combination of environment and materials.

When predicting SCC crack growth, the critical S
Assuming that the evolving SCC crack generated at the CC crack formation time t _C has the critical SSC crack depth given by the above equation [4], the crack depth a _C and the stress field at the crack tip are obtained. If the obtained stress intensity factor K is equal to or greater than the SCC lower stress intensity factor K _ISCC , it is assumed that the SCC crack will grow, and if it is smaller than K _ISCC , the SCC crack will not grow.

When the SCC crack develops, it is assumed that the relationship between the crack tip strain rate and the stress intensity factor is given by the above-mentioned S-shaped curve.

(D) Relationship between Crack Growth Rate and Crack Tip Strain Rate The crack growth rate da / dt is equal to the crack tip strain rate dε _ct / dt.
And the active dissolution parameter n are given by the following equations [1] and [2].

[0029]

(Equation 5)

The active dissolution parameter n can be determined from the time change of the dissolution current density on the newly formed slip surface at the crack tip caused by the crack propagation. Crack tip strain rate d
ε _ct / dt can be defined as the formation speed of a new slip surface at the crack tip, and is given by the following equation [8].

[0031]

D _ct / dt = 2ρ _d · b · cos θ · dX / dt (8) where, ρ _d : dislocation density, b: burger spectrum of dislocation, θ: slip direction and stress axis Angle, dX / dt: Indicates the average moving velocity of dislocation.

By experimentally measuring the average moving speed of dislocations on the slip surface, the crack tip strain speed can be determined. FIG. 5 shows the crack tip strain rate dε _ct / dt and the crack growth rate d when the active dissolution parameter n is 0.5.
The relationship with a / dt will be exemplified.

Given the crack tip strain rate and the active dissolution parameter, S is calculated from the above equations [1] and [2].
Using the CC crack growth rate da / dt, the amount of change in the crack depth a is determined sequentially, and the crack depth a is calculated by the following equation.

It is calculated in [9]. It is assumed that the SCC crack does not _propagate when the stress intensity factor K obtained from the crack depth and the stress field at the crack tip becomes smaller than the SCC lower limit stress intensity factor K _ISCC .

[0034]

(Equation 7)

Where a _C is the critical crack depth and t _C is the critical S
The CC crack formation time, da / dt, is the SCC crack growth rate.

[0036]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described based on embodiments.

[Example 1] The result of analysis using a corrosion ring-growth SCC crack growth analysis calculation program developed based on the algorithm of the present invention will be described.

Assuming that there is a residual stress distribution due to welding as shown in FIG. 6 for the welded portion of the stainless steel reactor internal structure, crack growth analysis is performed according to the irradiation SCC crack growth prediction flow shown in FIG. Was.

The SCC crack growth analysis under the irradiation environment
The parameters of environment, material, and dynamics, which are three factors of CC generation, are taken as parameters. In particular, crack growth calculation is performed by incorporating model equations such as irradiation-EPR, irradiation hardening, and irradiation stress relaxation, which are events specific to the irradiation environment. I do.

When a specific site of a plant to be analyzed for crack propagation is selected, as shown in FIG. 7, plant operating conditions such as neutron irradiation rate, plant parameters such as corrosion potential at the target site, and materials such as yield stress are used. Input parameters such as parameters and residual stress distribution.

The neutron dose is the SCC threshold dose (φt) _thre
Is reached, a critical SCC crack is formed, and from the crack depth a and the residual stress field σ at the crack tip, K = F
When the stress intensity factor K obtained by σ√πa is smaller than the lower limit stress intensity factor K _ISCC of the SCC, the SCC crack does not grow, but when the stress intensity factor K is more than K _ISCC , the growth of the SCC crack exceeds the limit.

In the propagation process of the SCC crack, it is assumed that the material of the member deteriorates due to irradiation hardening, irradiation grain boundary segregation, stress relaxation, and the like due to neutron irradiation. As shown in FIG. 4, the crack tip strain rate dε _ct / dt when the SCC crack propagates is determined by the stress intensity factor K at the crack tip, and the stress intensity factor dependency of the crack tip strain rate is determined in the first region. , Mai II area, and III area.

The active dissolution parameter n is given from reactor water conductivity, corrosion potential, and effective electrochemical reactivation rate. The crack growth rate da / dt is obtained from the above-mentioned relational expression [1] comprising the crack tip strain rate and the active dissolution parameter.

The final crack depth a is given by the equation

As shown in [9], the crack depth obtained by integrating the crack growth rate da / dt from the critical crack formation time t _C , ie, t _C = (φt) _thre , is _added to the critical crack depth a _C. I do.

FIG. 8 shows the results of predicting the growth of the SCC crack by the method of this embodiment and the conventional method, respectively.

According to the prediction result of the SCC crack growth according to the method of the present embodiment, the SCC crack does not grow until the number of years has passed considerably, and the years up to that can be regarded as the crack incubation period. In other words, it can be seen that the SCC crack passes through the crack incubation period and then enters the crack growth period, where the crack grows steadily at a constant crack growth rate.

On the other hand, the prediction result by the conventional method is SCC
It can be seen that the depth of the crack has risen early. This is because the conventional method ignores the crack incubation period and sets it to 0. Therefore, by introducing the concept of the critical SCC crack formation time t _C , the SCC crack growth prediction by the method of the present embodiment integrating both the crack incubation period of the generation and propagation behavior of the SCC crack and the crack growth period is SCC Crack growth can be predicted more realistically.

[Example 2] In the case where there is a residual stress distribution due to welding as shown in FIG. SCC crack growth analysis was performed. FIG.
FIG. 0 shows the results of predicting the growth of the SCC crack by the method of the present embodiment and the conventional method, respectively.

The prediction of the SCC crack growth by the method of the present embodiment results in that the SCC crack does not grow steadily,
Results consistent with real-world events were obtained. This is because the stress intensity factor at the critical SCC crack tip is smaller than the SCC lower stress intensity factor K because the residual stress field due to welding is sufficiently small.
_It is smaller than _{ISCC and} no SCC crack growth occurred.

On the other hand, the prediction by the conventional method results in that the SCC crack grows with age. This is because in the conventional method, the crack growth amount is sequentially calculated assuming a constant initial crack depth from the beginning (time = 0). Therefore, it can be seen that the SCC crack growth prediction of the present invention, in which the concept of the critical crack depth a _C and the SCC lower stress intensity factor K _ISCC is introduced, can perform a more reliable prediction than the conventional method.

[0051]

As described above, the method for predicting the SCC crack growth in the corrosive environment according to the present invention provides a critical crack formation time t _C ,
Since it is based on an algorithm that introduces the concept of critical crack depth a _C and SCC lower limit stress intensity factor K _ISCC , SC
It is possible to quantitatively predict the growth of the SCC crack by integrating both the crack incubation period in the generation and growth behavior of the C crack and the crack growth period.

Further, regarding the prediction of the SCC crack growth,
Compared with the conventional method, more realistic prediction is possible and highly reliable prediction is possible.

[Brief description of the drawings]

FIG. 1 is a flowchart illustrating an algorithm of a method for predicting the growth of a corrosion environment SCC crack according to the present invention.

FIG. 2 is an explanatory view schematically showing the generation and propagation mechanism of an SCC crack.

FIG. 3 is a characteristic diagram showing a relationship between crack propagation and neutron irradiation dose.

FIG. 4 is a characteristic diagram showing a relationship between a crack tip strain rate and a stress intensity factor.

FIG. 5 is a characteristic diagram showing a relationship between a crack growth rate and a crack tip strain rate.

FIG. 6 is a view showing a residual stress distribution of a welded part in Example 1.

FIG. 7 is a flowchart illustrating an analysis procedure for predicting irradiation environment SCC crack growth.

FIG. 8 is a graph showing prediction results of SCC crack growth in Example 1.

FIG. 9 is a view showing a residual stress distribution of a welded part in Example 2.

FIG. 10 is a graph showing prediction results of SCC crack growth in Example 2.

Claims (2)

[Claims] In a corrosive environment, stress corrosion cracking (S
CC) A critical SCC crack is formed at a critical crack formation time (t _C ) until the crack grows into a growing crack, and the stress intensity factor of the critical SCC crack is SCC lower stress intensity factor K
Assuming that steady growth of the SCC crack occurs in the case of _ISCC or more, the relationship between the crack tip strain rate (dε _ct / dt) and the stress intensity factor (K) at that time is determined by the stress intensity factor (K) at the tip of the SCC crack. Area I (K _ISCC ≦ K
<K _I-II ), a region II (K _I-II ≦ K <K _II-III ) and a region III (K _II-III ≦ K). According to [1] and [2], The crack growth rate (da / dt) is calculated, and the crack growth amount is calculated by the algorithm.
Corrosion environment SC characterized by predicting the growth of C crack
C crack growth prediction method. 2. In a corrosive environment in a nuclear reactor, a critical SCC crack is formed when a neutron dose reaches an SCC threshold dose [(φt) _thre ], and a stress intensity factor of the critical SCC crack is lower than SCC. Assuming that steady growth of the SCC crack occurs when the SCC crack is equal to or greater than the critical stress intensity factor (K _ISCC ), the relationship between the crack tip strain rate (dε _ct / dt) and the stress intensity factor (K) at that time is determined by Based on the stress intensity factor (K) at the tip, region I (K _ISCC ≦ K <K
_I-II ), region II (K _I-II ≦ K <K _II-III ) and region II
The crack growth rate (da / dt) is obtained from the above formulas [1] and [2] given by an S-shaped curve composed of three regions of region I (K _II-III ≤ K), and the crack growth amount Algorithm for calculating the SCC in a neutral irradiation environment
Corrosion environment SCC characterized by predicting crack growth
Crack growth prediction method.