JPS63228062A - Predicting method for remaining life of metallic material - Google Patents

Abstract

PURPOSE:To observe the used material of an actual machine and to predict the remaining life of the metallic material directly and easily by measuring the shape of crystal grains of the metallic material quantitatively and arranging statistically the quantity of variation in the shape of the crystal grains with the lapse of time. CONSTITUTION:The shape of the crystal grains of the metallic material is measured quantitatively and the remaining life is predicted from the quantity of variation in the shape of the crystal grains. Namely, when the metallic material has a creep damage, the crystal grains expand and deform in the stress direction, so the variation in the shape of the crystal grains with the lapse of time is measured quantitatively to rearranged the quantity of the variation statistically. The replica film 3 of the actual machine member 2 is sampled by a replica method. Then this replica film 3 is observed through a microscope 4 and the crystal grains shape is measured by an image processor 5. Standard deviation based upon distributions of angles in the axial direction of the maximum diameter of, for example, the crystal grains and in the stress direction is found as the shape variation quantity which is a parameter. Then a personal computer 7 computes and evaluates the remaining life from the relation among the standard deviation, the standard deviation of a previously generated data base 6, and the creep damage quantity.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to predicting the remaining life of metal materials, and in particular, predicting the remaining life of metal materials that have suffered creep damage due to use in boilers and other high temperatures and high pressures. Concerning lifespan prediction methods.

[Conventional technology]

It is well known that in equipment used for long periods of time under high temperature and pressure, such as in thermal power plants and chemical plants, the materials used in the actual equipment suffer creep damage and deteriorate during operation. This kind of material deterioration is controlled by metal temperature, applied stress, and usage time, and in consideration of these controlling factors, boilers for thermal power plants usually have a lifespan of 100,000 hours (approximately 15 years of normal operation). The materials used, dimensions, etc. are determined to ensure a long life. However, in such boilers, accidents often occur in which materials are damaged within a short period of time. Possible causes of this include an unexpected rise in metal temperature due to uneven flow of combustion gas, and abnormal material deterioration due to material segregation, such as sigma phase embrittlement. Recently, the design life is 10
The number of power plants that have exceeded 10,000 hours is increasing, and with the shift to base load operation of nuclear power plants, it is predicted that operating conditions such as intermediate load operation and daily startup and shutdown will become more severe. For this reason, it has become necessary to develop a technology that can extend the life of a plant by accurately predicting the remaining life of materials and suggesting when to repair or replace them.

Methods for detecting material deterioration are broadly classified into destructive methods and non-destructive methods. The destructive method is a method in which a part of an actual machine component is sampled, microscopic structure observation, tensile test, creep test, and impact test are conducted, and the remaining life is predicted by combining this with stress analysis. Among these methods, a method for making judgments based on the metallographic structure of a material involves creating standard metallographic structures under various conditions in the laboratory (for example, using Nippon Steel Pipe Engineering Ma6).
(See pages 2, 531-558) The lifespan is estimated by comparing the metal structure of a sample taken from an actual machine member.

In that case, the indicators for Cr-Mo steel are the decomposition and agglomeration of pearlite, and for stainless steel, the precipitation and agglomeration of carbides within one grain boundary, or the state of precipitation of the sigma phase. , stainless steel SUS 321
There is also a technique for predicting the remaining life from the relationship between the amount of sigma phase precipitation and creep damage (Japanese Patent Application Laid-Open No. 58-201066).
No. Publication, and Thermal and Nuclear Power Generation Vol. 1.33, Ki 9 p. 8
99-912). There is also a technology that predicts the remaining life based on the presence of voids due to creep. (for example.

Material Van. 28. (See Na308 pages 372-378)
.

Although the method of quantifying the sigma phase is effective, the material is limited to stainless steel or high Cr steel.

Even in the same stainless steel, there is a problem in that the precipitation state of the sigma phase differs due to minute differences in chemical components.

In addition, a method based on quantifying creep cavities is also effective, but this method also applies to stainless steel and high Cr steel (for example, H
It is limited to materials with low intragranular ductility such as K40), and materials with high intragranular ductility such as low alloy steel for boilers are difficult to apply because creep cavities are difficult to form.

The non-destructive method is a method of indirectly predicting the remaining life by detecting structural changes such as decomposition of pearlite etc. due to heating and creep, and physical changes accompanying void generation.

In this case, various physical quantities can be considered, but examples of those that have already been put into practical use or are currently being researched include electrical resistance (Japanese Unexamined Patent Publication No. 58-60248), ultrasonic sound velocity (Japanese Unexamined Patent Publication No. 53-120585), (Japanese Unexamined Patent Publication No. 1988-88781), misorientation due to X-rays and coil impedance due to eddy current (Japanese Patent Laid-Open Publication No. 1988-88781).

Non-destructive methods first detect changes in physical quantities such as electrical resistance due to microstructural changes in metal materials, so it is necessary to detect extremely small changes, which requires highly accurate equipment. Become. Additionally, large errors may occur due to handling, measurement environment, etc. Particularly in boilers, unlike turbines, the measurement environment is poor and accurate measurements are difficult.

Furthermore, since the turbine material is a material with a high carbon content such as CrMo-V steel, physical quantities such as electrical resistance are greatly reduced.
o steel), the carbon content is low and the decrease in physical quantities is small, making evaluation difficult. Secondly, in these methods, a master curve is created in a laboratory, and the remaining life is estimated by comparing the master curve with the measurement results of the actual machine. by the way,
The physical quantity to be measured captures minute changes in the material, and the absolute value of the physical quantity changes in response to changes in the initial state of the material or simple heating conditions. For this reason, the amount of damage must be evaluated not as an absolute value of a physical quantity, but as a relative amount to the initial state before damage or to the simple heating state. Therefore, when creating a master curve in a laboratory, it is necessary to measure the lifespan. Materials with the same initial state as the actual machine parts to be used, that is, materials with the same charge are used. However, the reality is that there are no materials left for boilers currently in operation, especially those made more than 10 years ago.

Moreover, there is a possibility that only simple data such as one structure, hardness, and short-term tensile strength remain as data on materials from that time, and there are virtually no physical quantities used for life evaluation. Furthermore, the manufacturing methods used at that time and the current manufacturing methods are different, and it is extremely difficult to reproduce the materials used at that time.

[Problem that the invention seeks to solve]

The above-mentioned conventional technology has the following problems.

The most reliable method is to destructively collect samples from actual equipment and perform creep tests to predict the remaining life, but this is costly and time-consuming, and the scope of investigation is limited. Furthermore, although the method based on single-tissue observation can be evaluated in a short time, quantitative evaluation is difficult because it is performed by comparison with standard photographs.

Furthermore, structural changes are mainly controlled by temperature and time, and the effect of stress is small, so it is difficult to directly link creep damage and structural changes.

Non-destructive methods measure minute changes in the physical quantities of the materials used, which can lead to errors, and physical quantities also change due to environmental temperature, etc., so it is difficult to evaluate by strictly comparing with a laboratory master curve. is difficult. In particular, there is little data on materials from 10 years ago, and it is difficult to recreate materials from that time, so both destructive and non-destructive methods have many problems.

An object of the present invention is to provide a simple method for predicting the remaining life of metal materials, which directly predicts the remaining life by observing the materials used in actual equipment.

[Means for solving problems]

In order to achieve the above object, the present invention is configured to quantitatively measure the shape of crystal grains of a metal material and predict the remaining life based on the amount of change in the shape of the crystal grains.

[Effect]

According to the present invention, when a metal material undergoes creep damage, its crystal grains stretch and deform in the stress direction, so by quantitatively measuring changes in the shape of crystal grains over time, the amount of change can be statistically evaluated. Can be organized.

The method for measuring crystal grains is to take a sample from the actual machine or non-destructively observe it using the replica method, and measure the amount of change in shape, such as the distribution of the angle between the axial direction of the maximum major axis of the crystal grain and the stress direction. The amount of change in shape can be determined by finding the standard deviation.

〔Example〕

21 is a typical boiler material as an example of the present invention.
Figure 1a ~ when applied to 8cr-IMo steel
This will be explained with reference to FIG.

As shown in FIGS. 1a to 1d, when the crystal grains 1 of the metal material are subjected to creep damage, they elongate in the stress direction.
The structure is configured to quantitatively measure the shape of the crystal grains, detect material deterioration based on the amount of change in the shape of the crystal grains, and predict the remaining life.

When a metal material suffers creep damage, it means that the crystal grains undergo creep deformation.
Creep deformation can be seen as the accumulation of individual deformations of grains. Therefore, creep damage can be detected by quantitatively measuring the deformation of crystal grains over time. Deformation due to creep of grains gradually stretches in the stress direction, so
Creep damage can be detected by sorting by parameters that indicate the degree of elongation. In addition, even if rolling is performed during manufacturing, if the final heat treatment is performed, the grain size will vary depending on the heat treatment conditions and chemical composition, but the shape will be an almost regular polygon in equilibrium, and the material and heat treatment conditions It is not affected by such things. That is, the change in the shape of crystal grains directly corresponds to creep damage, and there is no need to consider the initial state or simple heating state.

The metal structure of 218Cr-I Mo steel is shown in FIGS. 1a to 1d. Figure 1a is unused material, Figure 1b is simply heated material (creep damage rate φC = 0), Figure 1c is creep damaged material (creep damage rate φC = 0.8), Figure 1d is material with creep damage (creep damage rate φC = 0.8).
The figure shows creep-damaged material (creep damage rate φc = 1,
0) Microscopic metal structure.

From these metallographic structures, pearlite is considerably decomposed in the simply heated material and the creep-damaged material, but no difference is observed depending on the creep damage rate φC. However, the shape of the crystal grains in the creep-damaged material is elongated in the stress direction and becomes flat. In addition, creep loss 4jij rate φc = t / t
It is represented by r, where tr is the creep rupture time and t is the creep test elapsed time, and their relationship is shown in FIG. 12 when the test material and temperature are constant.

Focusing on this point, we examined the crystal grain shapes of materials with various types of damage using an image processing device. FIG. 2 shows a method for measuring crystal grain shape parameters, and as shown in FIG. 1, the angle θ between the axial direction of the maximum major axis of the crystal grain and the stress direction is determined. Figures 3a and 3b are the distributions of θ ugliness corresponding to Figures 1b and 1C.
The number of crystal grains measured was 100. In the case of the creep damage rate φc=0 shown in FIG. 3a, since the crystal grains are close to regular polygons, 0 m can be any angle, and the distribution of θI is flat. On the other hand, in the case of the creep damage rate φc = 0.8 shown in Figure 3b, the crystal grains are elongated in the stress direction, so the distribution becomes a normal distribution with a peak in the stress direction (θ country = 0°). ing. In order to express the difference in these distributions, the standard deviation S@ of the theta tumor distribution was calculated, and the relationship with the creep damage rate φC was shown in the fourth section.
It is a diagram. As is clear from this figure, as the creep damage rate φC increases, Sll decreases. In particular, when the creep damage rate φC is 0.5 or more, a remarkable decrease in Sm is observed, and it becomes easy to predict the remaining life using this parameter. By using this diagram, the amount of creep damage to actual machine parts can be evaluated. In addition, Ss was determined when the creep damage rate φC was O using various unused materials and simply heated materials, and the variation was small around 50 degrees in all cases, and no influence of chemical components or heat treatment was observed. Ta. In other words, according to this method, data on unused materials or simply heated materials is unnecessary, and creep damage can be directly evaluated.

The number of crystal grains measured at one time on the θ-type side was 100, but it is conceptually possible that the shape of the crystal grains changes over time. As is clear from Figures 1a and 1b, it is not clear even when looking at individual crystal grains. Therefore, it is necessary to increase the number of measurements made by a machine that eliminates individual differences and to use statistical methods. Results of various studies regarding the number of crystal grains measured.

It was revealed that when the number of crystal grains is 80 or more, the variation in Sin becomes small and the influence of the number of measured crystal grains disappears, so the number was set at 100 to give a margin.

Since the materials shown in the examples are materials that have been subjected to various types of damage in creep tests, the stress direction is clear, but in actual machine members, the stress direction may not be clear. Therefore, even if the creep damage is large, the θ distribution may be as shown in FIG. 5a. This is because an arbitrary angle is assumed to be the stress direction. Although the standard deviation of this distribution is very large, considering the continuity of angles (90° and -90° are in the same direction), Figures 5a and 5b are the same distribution. Therefore, by converting the angle by 1 degree (that is, converting Figure 5a to Figure 5b), find the distribution of the 0 layer with the minimum standard deviation, and calculate the standard deviation as the Sugliness of the material. did. By using this method, there is the advantage that the maximum stress direction of the actual machine member can be determined. In addition, in the example, the crystal grain shape was measured by directly observing the sample, but as shown in Figure 6, even with the replica method, which transfers the irregularities of the actual product onto a thin film, the crystal grains are clearly visible and identical. Can be evaluated.

FIG. 7 shows the procedure when applying this method to an actual machine member. Since the non-destructive method is the principle for the actual machine member 2, the thin film is collected here by the replica method, but if it is possible to collect a sample, the accuracy will be better if the sample is collected. In the replica method, the evaluation area is polished to a mirror finish using a grinder, etc., and then etched using an etching solution suitable for the material.

In this case, it is better to make the etching a little longer in order to clearly show the crystal grains. The surface texture can be transferred by applying a replica film dissolved in a solvent to the etched area and peeling it off after drying. This replica film 3 is examined using a microscope 4.
By measuring the crystal grain shape using the image processing device 5, the standard deviation SIl, which is a damage parameter, is determined.
Calculate.

This Sm and S+ of database 6 created in advance
The remaining life is calculated and evaluated using the computer 7 from the relationship between m and the amount of creep damage.

Although the idea is the same, it was confirmed that the following parameters are also effective for quantifying changes in the shape of crystal grains.

(a) Relationship between major axis and width of crystal grains As shown in Figure 8, measure the major axis Qx projected onto the axis in the stress direction and the width Qy projected onto the axis perpendicular to the stress direction, and calculate the ratio. Qx
The relationship between the average of
The average value of individual crystal grains is shown. As is clear from this figure, when the creep damage rate φC is O and Qx/Q'/ is 1, the vertical and horizontal lengths of the grains are the same, and the creep damage rate φC is 0.5 or less. Then, the average value of Qx/Qy is approximately 1, and when the creep damage rate φC increases to 0.5 or more, ΩX becomes larger than Qy, so Qx/
The value of my increases, so the remaining life can be predicted using Qx/Qy as a parameter. Also, the first
As shown in Figure 0, simply the maximum major axis QL of the major axis of the crystal grain
Even if the ratio Q[,/ΩS of the maximum width Qs on the orthogonal axis is used as a parameter, a diagram similar to that shown in FIG. 9 can be obtained.

In Fig. 9, the average value of Qx/Qy is used for organizing, but Qx/Qy<1 or Q
The ratio of the number of crystal grains such that x / Q y > 1,
In other words, the ratio of the number of vertically elongated or laterally elongated crystal grains to all crystal grains also has a good correlation with the creep damage rate, and can be used as a parameter for predicting remaining life.

Figure 13 shows Qx for 5US321 HTB.
The relationship between /Qy and damage rate φC is obtained. 2'
/+Cr Although the curve is slightly different from that of IMo steel, the damage ratio φC is 0.5 or more and QL/Q-d gradually increases, making it possible to evaluate the remaining life.

(b) Circularity The circularity of a crystal grain is a parameter that can estimate creep damage. The 0 circularity can be expressed in various ways, an example of which is the contour ratio. The 0 contour ratio is (the circumference of an equivalent circle) /(peripheral length of crystal grain). This contour ratio and creep damage rate φ
FIG. 11 shows the relationship with C. As is clear from this figure, creep damage can also be predicted from the contour ratio. Other parameters that represent circularity include, for example, 1perimeter)2/(area).The reason why circularity is used as a parameter is that when the damage is small, the crystal grains are close to a circle, and the damage This is because as the curve increases, it stretches in the stress direction and the circularity increases.

Note that the parameters shown in the other embodiments have small changes in parameters due to the creep damage rate φC and have low accuracy, but may become effective parameters when the material changes.

〔Effect of the invention〕

According to the present invention, by quantitatively measuring the shape of the crystal grains of a metal material and comparing the amount of change, the remaining life of the metal material can be easily and quantitatively predicted using a non-destructive method, thereby significantly reducing cost and time. It does not require high-precision equipment, nor does it require initial data on the materials used or simple heating material data.
It has great industrial utility value because it can be applied to many polycrystalline metal materials.

[Brief explanation of drawings]

Figure 1a is a photograph showing the metallographic structure of an unused 218Cr IMo steel, and Figure 1b is a photograph showing the metallographic structure of a simply heated material (creep damage rate φc = 0). Figure 1c shows the creep damaged material (creep damage rate φC=O,
Figure 1d is a photograph showing the metallographic structure of the creep damaged material (creep damage rate φe = 1.0), Figure 2 is the measurement of the parameter θ- of grain shape change of the present invention. Figures explaining the method, Figures 3a and 3b are Figures 1b and 3b.
Figure 4 is a graph showing the relationship between the standard deviation S of the distribution of θ■ and the creep damage rate, and Figures 5a and 5b are stress directions. A graph showing an example of the distribution of θ ugliness when it is unknown and when it is known. Figure 6 is a photograph showing the metal structure under a microscope using the replica method. Figure 7 is a prediction of the remaining life based on the crystal grain shape of an actual machine part. A circuit diagram showing the procedure. Figure 8 is a diagram explaining the method for measuring the parameters Qx and Qy of grain shape changes, Figure 9 is a graph showing the relationship between Qx/Qy and creep damage rate, and Figure 10 is the parameter QL + I29 of grain shape changes. Figure 1 explaining the measurement method of
Figure 1 is a graph showing the relationship between parameter contour ratio of grain shape change and creep damage rate, Figure 12 is a graph showing the relationship between creep rupture and test elapsed time, and Figure 13 is 5US3.
It is a graph showing the relationship between Qx/Qy and creep damage rate of 21HTB steel. 1...Crystal grain.

Claims (5)

[Claims] (1) A method for predicting the remaining life of a metal material, which comprises quantitatively measuring the shape of crystal grains of the metal material and predicting the remaining life based on the amount of change in the shape of the crystal grains. (2) The remaining life of the metal material according to claim 1, wherein the amount of shape change is expressed by the standard deviation with respect to the distribution of the angle between the long axis direction of the maximum major axis of the crystal grain and the stress direction. Prediction method. (3) The amount of shape change is expressed by the ratio of the maximum major axis of the crystal grains to the maximum width diameter on an axis perpendicular to the major axis thereof. Lifespan prediction method. (4) The method for predicting the remaining life of a metal material according to claim 1, wherein the amount of shape deformation is expressed by the major axis of the crystal grain in the stress direction and the width in the direction perpendicular to the stress direction. . (5) The method for predicting the remaining life of a metal material according to claim 1, wherein the amount of shape change is expressed by the circularity of crystal grains.