JPH02263160A - Method of evaluating remaining life of ferritic heat resisting steel - Google Patents

Abstract

PURPOSE:To estimate the life of a high-temp. apparatus by evaluating the life thereof simply by drawing replicas and extracted replicas from the surface of a high-temp. member. CONSTITUTION:The creep rupture test of a sample material is carried out in order to divide the changes in the structure arising from the accumulation of creep damages by ranks according to the degrees of the progression of the damages and the test is interrupted at each of the various life ratios to the rupture, then the replicas and the extracted replicas are drawn and the optical microsopic structure investigation and deposit distribution condition investigation are carried out. The optical microsopic structure investigation and deposit distribution condition investigation with the material interrupted of the above-mentioned creep rupture test are then carried out and the conditions of the respective changes in the structures are classified to the damage segments corresponding to the degrees of the damages in contrast with standard structures. The reference chart for evaluating the damages is obtd. by connecting the boundaries where the changes in the structure corresponding to the progression of the damages are admitted and the overall damage sections obtd. from the correlations of the various conditions of the changes in the structure.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a nondestructive testing method applied to the maintenance and management of ferritic heat-resistant steel, which is used as high-temperature equipment for a long period of time.

[Conventional technology]

Conventionally, methods for evaluating the lifespan of heat-resistant steels used in high-temperature equipment include methods for removing long-term used materials and subjecting them to destructive tests such as creep rupture tests, methods based on theoretical analysis, and methods for evaluating the lifespan of heat-resistant steels by There are non-destructive testing methods such as ultrasonic flaw detection and magnetic particle flaw detection to detect defects such as mark cracks.

[Problems to be Solved by the Invention] The destructive testing method described above has been widely used as the most accurate life evaluation method because it actually performs destructive tests on materials used for long periods of time as equipment components. In particular, large-scale pipe removal and restoration work was required, especially in thick-walled areas, and creep rupture tests required approximately six months. Also,
In the theoretical analysis method, the operating temperature of the relevant part is required, but it has been difficult to accurately determine the operating temperature, especially for heat transfer steel pipes of boilers for thermal power generation.

The present invention, in view of the above-mentioned technology, provides a simple non-destructive testing method that can be applied to the maintenance and management of ferritic heat-resistant steel, which is used as high-temperature equipment for long periods of time.

[Means for Solving the Problem] The present inventors have discovered that the decrease in creep rupture strength caused by holding ferritic heat-resistant steel in a high-temperature heating cart for a long period of time is related to structural changes. Based on this knowledge, the present invention uses the replica and extraction replica method, which is a non-destructive microstructure inspection method for the surface of high-temperature components, to observe changes in the structure and precipitate distribution with an optical microscope due to long-term use,
By investigating the optical microscopic structure and the distribution of precipitates of materials that have been subjected to various degrees of creep damage in laboratory tests, we compared them with the standard optical microscopic structure and standard precipitate distribution that were created. The structure was divided into 4 to 5 categories based on the above-mentioned laboratory tests. (By applying the classification according to the degree of change in the optical microscopic structure and the distribution of precipitates of the high-temperature member and the stress value applied to the part to the cow diagram) This allows the degree of creep damage to be evaluated.

That is, the present invention provides a method for evaluating the remaining life of ferritic heat-resistant steel used as high-temperature equipment, in which optical microscopic structures and precipitates observed from replicas and extracted replicas sampled from the heat-resistant steel are used. A process of determining the state of deterioration of the structure by comparing the state of deterioration of each structure with a standard structure in which the state of deterioration of each structure is ranked in multiple stages due to the accumulation of creep damage, and Determine the extent of creep damage by applying the tissue deterioration state determined in the above process, the investigation, and the applied stress at the position to the life evaluation standard diagram showing the relationship between the distribution situation, applied stress, and the extent of creep damage. This is a method for evaluating the remaining life of ferritic heat-resistant steel.

[Function] According to the method of the present invention, life evaluation is performed by simply collecting replicas and extracted replicas from the surface of the high-temperature member, so there is no need for large-scale specimen collection and restoration work unlike in destructive tests. Furthermore, since the lifespan is evaluated simply by observing structural changes, there is no need for a long test unlike a destructive test. Furthermore, since life evaluation is performed based on structural changes that directly correspond to damage to high-temperature parts that have been used for a long time, the accuracy of life evaluation is based on variations in temperature data and unused material strength data used as in the theoretical analysis method. No problem arises.

〔Example〕

Below, as an example of the present invention, an example of evaluating the remaining life of 2ZCr1Mo steel used as a high-temperature equipment member will be shown.

First, in order to rank the structural changes associated with the accumulation of creep damage according to the degree of damage progression, a creep rupture test was conducted on the specimen material under the following conditions.
The test was interrupted at various life ratios (0,]tr) up to r), replicas and extracted replicas were collected, and an optical microscopic structure investigation and a precipitate distribution state investigation were conducted.

600"CX 10kgf/mm2 600°CX 9kgf/mm"60f)"CX 8kgf/mm2 600°CX 7kgf/mm2 600"CX 6kgf/n+il" 650°CX 5kgf/mm2 650°C×4kgf/llll112650 °CX
3 kgf/+*m2 Based on these tissue investigation results, each tissue was classified into 4 to 5 stages according to the progression of damage.

Figures 1 and 2 show optical microscope structures (magnification: 30
0) and the precipitate distribution state (1500x magnification) of the standard structure of the structure change with respect to the progress of creep damage.

In the optical microscopic structure shown in Fig. 1, there are five stages depending on the presence or absence of precipitation of carbide 2 within one ferrite grain, the presence or absence of contraction and dispersion of carbide within pearlite 3, and the presence or absence of precipitation-free zone 5 around grain boundary precipitate 4. Divided into.

In addition, the distribution of precipitates in Figure 2 shows the presence or absence of grain boundary precipitates 4, the presence or absence of grain boundary precipitate-free zones 5, and the presence or absence of carbides in pearlite 6.
It was divided into four stages focusing on changes in shape.

Next, we conducted an optical microscope structure investigation and a precipitate distribution investigation on the material that had been interrupted in the creep rupture test mentioned above, and compared each structure change state to the degree of damage by comparing it with the standard structure shown in Figures 1 and 2. The damage was classified into corresponding damage categories.

The results are shown in Figure 3. Tissue changes that correspond to the progression of damage are observed, and the overall damage classification obtained from the correlation of each tissue change state connects the changing boundaries (dotted lines in Figure 3). It was made into a damage evaluation baseline diagram.

FIG. 3 has the following technical significance. The above test conditions (600°CX10kgf/mm2.600-CX
9kgf/mm2. 600°CX 8kgf/+++
n”, 600°CX7kgf/++v+2.60
0°CX 6kgf/mm2. 650°CX5kgf
/mm”, 650°CX 4kgf/m+n265
A creep rupture test is carried out at 0° C. Next, a creep rupture test was conducted on a test piece made from the same sample material under the above test conditions, and 0. I
The test was interrupted every Lr and the replica method was applied from the surface of the test piece.
The surface structure is transferred using the extraction replica method, and the structure is investigated using an optical microscope and the distribution of precipitates is investigated. Through these procedures, the life consumption rate under each stress condition is 10% (-
〇, ILr), 20! (-〇、2 trL 30Z
(=0.3tr).

, 9o% (= 0.9 tr) , 1 (IOZ (=
Since the results of the optical microscopic structure investigation and precipitate distribution investigation results of the test piece (tr) can be obtained, the optical microscopic structure of each test piece can be compared with the standard microstructures shown in Figures 1 and 2 above. Obtain damage classification and damage classification of precipitate distribution. Therefore, if we plot the damage classification of each test piece from 1 to 1 on a graph with the logarithm of the creep rupture life consumption rate on the horizontal axis and the stress on the vertical axis, Figure 3 is obtained. Those with a microscopic structure of IM and a precipitate distribution of I are classified as comprehensive damage classification A, and are classified as IIs and I, respectively.
p is indicated by B, and I[1M LIP is indicated by C.

Figure 4 shows the optical microscopic structure (magnification: 300x) of a replica taken from the surface of 2 Va Cr-I Mo steel that was used for a long time as a heat exchanger tube in a thermal power generation boiler, and the precipitate distribution state of the extracted replica (magnification: 1500x). A schematic diagram of the tissue is shown. In the microscopic structure, carbide 2 is precipitated within each ferrite grain, and the carbide within pearlite 3 is shrinking, as shown in the mark m! shown in Figure 1. In contrast to the No. 11 weave, the damage category of the microscopic tissue was classified as ■9. In addition, although grain boundary precipitates 4 were precipitated in the precipitate distribution, no shrinkage of carbides 6 within pearlite occurred, and no grain boundary precipitate-free zone was observed, as shown in Figure 2. When compared with the standard structure, the damage classification of precipitate distribution was classified as [P].

So, damage classification of each tissue created based on Figure 3,
Using the relationship between stress and creep rupture life consumption rate, the lifespan of the surveyed locations was evaluated.

Note that internal pressure stress was acting on the surveyed position, and the equivalent stress calculated from the average diameter formula was 5.0 kgf/mm2. Figure 5 shows the life evaluation results, and the damage classification of the microscopic structure at the survey location is ■. and damage division lip of precipitate distribution and applied stress5. From Okgf/+ua2, the creep rupture life consumption rate at the investigated position was estimated to be 8 to 30%.

After this investigation, the heat exchanger tube including the investigation location was extruded and a creep rupture test was conducted at a test temperature of 650°C. The results are shown in FIG. The figure also shows the results of the creep rupture test for the unused material, and compared to this, the creep rupture strength of the extubated material was lower.

Therefore, based on the creep rupture of unused material and used material at a stress of 6.0 kgf/n+m2, the creep rupture life consumption rate determined by the following formula was evaluated by the method of the present invention.

X100 (%) Figure 7 shows a comparison of creep rupture life consumption rates. 7th
From the figure, the creep rupture life consumption rate evaluated by the creep rupture test and the creep rupture life consumption rate according to the present invention are almost the same. We were able to non-destructively evaluate the remaining life of the

〔Effect of the invention〕

As detailed above, according to the method of the present invention, the remaining life of ferritic heat-resistant steel that has been used for a long time as a high-temperature equipment component can be easily determined by observing the structure of the outer surface without destroying the component. Since the method can be provided, it becomes possible to estimate the lifespan of high-temperature equipment, and maintenance management of aging equipment can be performed efficiently.

[Brief explanation of drawings]

Figure 1 shows 2% Crl prepared as an example of the present invention.
Fig. 2 is a schematic diagram of a standard optical microscope structure related to the progress of creep damage in Mo steel. Figure 3 shows the optical microscopic structure of 2% Cr-I Mo steel prepared as an example of the present invention, the relationship between damage classification of precipitate distribution, applied stress, and creep rupture life consumption rate. Evaluation standard IT diagram; Figure 4 is a schematic diagram showing the optical microscopic structure and precipitate distribution of 2% Cr-I Mofi, which has been used for a long time as a heat transfer steel for thermal power generation boilers; Figure 5 is the above-mentioned Figures 1 and 2 show the optical microscopic structure and precipitate classification of the material used for a long time, and the life evaluation results based on the damage classification results. Figure 6 shows the long-time use material and the unused material. FIG. 7 is a comparison diagram of the life evaluation results by the creep rupture test and the life evaluation result by the method of the present invention. (Ki 11J/+fDx)α2 CD

Claims (1)

[Claims] In a method for evaluating the remaining life of ferritic heat-resistant steel used as high-temperature equipment, the optical microstructure and the distribution of precipitates observed from replicas and extracted replicas taken from the heat-resistant steel are evaluated for creep damage. A process of determining the state of deterioration of each structure due to accumulation by comparing it with a standard structure ranked in multiple stages, and a process of determining the state of deterioration of each structure with an optical microscope, the distribution of precipitates, applied stress, and creep damage. The degree of creep damage is determined by applying the deterioration state of the structure determined in the above process and the applied stress at the investigation position to a life evaluation standard diagram showing the relationship between the degree and the degree of creep damage. Lifespan evaluation method.