Method for calculating inner layer thickness of oxide layer of martensitic heat-resistant steel under high pressure steam

Abstract

The application discloses a calculation method of inner layer thickness of an oxidation layer of a martensitic heat-resistant steel part under high-pressure steam. The method is based on a metal oxidation kinetics model, a large amount of actual power plant operation and laboratory simulation experiment data, mathematical correction of a formula, linear fitting and curve fitting, and a calculation method of inner layer thickness of an oxidation layer of a 9% Cr martensitic heat-resistant steel under high-pressure steam. The application can conveniently and quickly calculate the inner layer thickness of the oxidation layer of the 9% Cr martensitic heat-resistant steel under high-pressure steam according to steam pressure and operation time, the result is accurate, the remaining life of the high-temperature part can be evaluated without pipe cutting and measurement in actual power plant operation, the safe operation of the unit is ensured, the cost is reduced, and the application has important industrial application value.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of martensitic heat-resistant steel, and particularly relates to a calculation method for inner layer thickness of an oxidation layer of martensitic heat-resistant steel under high-pressure steam. BACKGROUND

[0002] The martensitic heat-resistant steel includes T / P91, T / P92, E911, T / P93 (9Cr-3W-3Co), T / P122 and other 9-12% Cr heat-resistant steel. The martensitic heat-resistant steel has excellent high-temperature creep strength, good thermal conductivity and low linear expansion coefficient, and is widely used for manufacturing main steam pipes, headers, superheaters, reheaters and other important high-temperature components of supercritical (ultra-supercritical) units. With the increase of the working steam pressure of the unit, the high-temperature high-pressure steam oxidation resistance of the martensitic heat-resistant steel becomes one of the key factors affecting the service life of the high-temperature components. During the long-term operation of the high-temperature components, the growth of the thickness of the oxidation layer will lead to the decrease of the effective wall thickness of the pipe wall, and the stress of the pipe wall will also increase accordingly; meanwhile, the oxidation layer will cause the deterioration of the thermal conductivity of the pipe wall, so that the average operating temperature of the pipe wall is increased, and the pipe wall is in an over-temperature service state for a long time, and when the deterioration reaches a certain degree, the pipe explosion accident will occur. Therefore, it is necessary to evaluate the service life of the components and make early warning, and it is necessary to predict the oxidation layer thickness of the components such as the superheater and the reheater under high-temperature high-pressure steam in order to reduce the accident.

[0003] The calculation of the thickness of the oxidation layer needs the help of the oxidation kinetics model of the heat-resistant steel under high-temperature high-pressure steam. At present, the steam oxidation kinetics models of the martensitic heat-resistant steel are mostly obtained based on the experimental results of the oxidation weight gain method, and cannot directly calculate the thickness of the oxidation layer. Although a few literatures report the high-pressure steam oxidation kinetics models of the martensitic heat-resistant steel based on the growth of the thickness of the oxidation layer, the oxidation layer includes the inner layer and the outer layer, and these literatures do not distinguish the thickness of the outer layer and the thickness of the inner layer. The applicant finds that only the growth of the thickness of the inner layer of the oxidation layer will lead to the thinning of the pipe wall and affect the service life of the pipe, and therefore it is more practical to predict the thickness of the inner layer of the oxidation layer.

[0004] Another main deficiency in the oxidation kinetics of the heat-resistant steel is that the current research is limited to the influence of the time variable on the thickness of the oxidation layer under the fixed steam pressure, and the influence of the change of the steam pressure is not considered. In fact, the steam pressures in the components such as the superheater and the reheater in the unit are quite different, and the oxidation rate will be quite different when the steam pressures are different. Therefore, if the influence of the change of the steam pressure is not considered, the accuracy of the prediction result will be seriously affected. SUMMARY

[0005] The present application provides a kind of 9%Cr martensitic heat-resistant steel pipe oxide layer inner layer thickness calculation method to solve the problems existing in prior art, according to the running time and pressure, the oxide layer inner layer thickness of 9%Cr martensitic heat-resistant steel under high pressure steam is calculated quickly and conveniently, the result is accurate, in actual power plant operation, it can not be cut pipe to measure, and the residual life of high temperature component can be evaluated, the safe operation of unit is guaranteed, the cost is reduced, and it has important industrial application value.

[0006] In order to achieve the above object, the technical scheme provided by the present application is:

[0007] A kind of martensitic heat-resistant steel under high pressure steam oxide layer inner layer thickness calculation method is provided, the martensitic heat-resistant steel is 9%Cr heat-resistant steel, and the calculation formula of oxide layer inner layer thickness under high pressure steam is:

[0008] Y=a+bt+cp+dt 2 +hpt+ip 2

[0009] Wherein, Y is the thickness of the inner layer of the oxide layer, unit μm;P is steam pressure, unit MPa;T is time, unit h;A, b, c, d, h, i is fitting coefficient.

[0010] According to the above scheme, the steam pressure under high pressure steam is 5.0-25.0MPa.

[0011] According to the above scheme, the running time under high pressure steam is 1,000-150,000h.

[0012] According to the above scheme, the high pressure steam temperature under high pressure steam is 550-650℃.

[0013] According to the above scheme, the value of fitting coefficient a is 22.21.

[0014] According to the above scheme, the value of fitting coefficient b is 0.0009334.

[0015] According to the above scheme, the value of fitting coefficient c is-0.8198.

[0016] According to the above scheme, the value of fitting coefficient d is-7.655×10 -10 .

[0017] According to the above scheme, the value of fitting coefficient h is 1.79×10 -5 .

[0018] According to the above scheme, the value of fitting coefficient i is 0.1152.

[0019] The application provides application of the above calculation method in evaluating service life of martensite heat-resistant steel pipes running under high-pressure steam in a power plant.

[0020] The inner layer thickness of the oxidation layer of the 9% Cr martensite heat-resistant steel of the high-temperature component can be calculated by the above formula, the oxidation corrosion thinning degree of the inner wall of the component can be reflected, the residual life of the component can be evaluated, and safe operation of the unit is ensured.

[0021] The application has the following beneficial effects:

[0022] 1. The application firstly studies the inner layer thickness of the oxidation layer of the martensite heat-resistant steel under the supercritical or ultra-supercritical high-pressure steam, the inner layer thickness of the oxidation layer of the 9% Cr martensite heat-resistant steel under the high-temperature high-pressure steam can be conveniently and quickly calculated according to the running time and the steam pressure, the application has higher practical value, the result is more accurate, the pipe does not need to be cut for measurement in the actual power plant operation, cost is saved, and the inner layer thickness of the oxidation layer of the pipe is calculated without affecting the operation; the inner layer thickness of the oxidation layer can reflect the oxidation corrosion thinning degree of the inner wall of the martensite heat-resistant steel pipe, the residual life of the component can be evaluated, safe operation of the unit is ensured, and the application has important industrial application value.

[0023] 2. The calculation method of the application takes into account the steam pressure and the running time, both of which have great influence on the thickness of the oxidation layer, the error can be controlled within 8%, the calculation result is accurate, and the application has important reference value. DETAILED DESCRIPTION

[0024] Figure 1 The figure is a relationship diagram of the inner layer thickness Y of the oxidation layer, the selected pressure p and the time t in the embodiment of the application.

[0025] Figure 2 The figure is a cubic repeated fitting prediction diagram of the inner layer thickness Y of the oxidation layer, the selected pressure p and the time t actual data in the embodiment of the application. DETAILED DESCRIPTION

[0026] The technical scheme of the application is further explained and described below through specific embodiments.

[0027] The relationship formula of the inner layer thickness Y of the oxidation layer and the steam pressure p

[0028] In order to explore the influence of the pressure p and the time t on the formation of the oxidation layer of the high-temperature heating surface of the power plant boiler, the experimental data of the inner layer thickness of the oxidation layer of the 9% Cr martensite heat-resistant steel under different time t and different pressure p are screened and processed, and the results are as follows:

[0029] Table 1 Inner layer thickness of the oxidation layer of the 9% Cr martensite heat-resistant steel under different time and different steam pressure

[0030] Time t / h Steam pressure p / Mpa Oxidation layer inner layer thickness Y / μm 1169.916 5.0 35.422 1466.175 15.0 48.916 1767.934 20.0 69.157 1228.343 25.0 107.108 10018.521 5.0 42.169 10031.581 15.0 58.193 10048.766 20.0 79.277 10084.509 25.0 123.133 50267.199 5.0 75.060 50004.621 15.0 102.892 50034.178 20.0 139.157 50081.607 25.0 197.349 100222.328 5.0 117.229 100264.946 15.0 169.518 100308.250 20.0 222.651 100064.232 25.0 273.253 150177.458 5.0 159.398 150242.071 15.0 238.675 150270.941 20.0 274.096 122611.575 25.0 287.590

[0031] The above data is plotted to obtain the graph of the inner thickness Y of the oxide layer versus the selected pressure p and time t as shown in Fig. 1. Figure 1

[0032] It can be found that there is a binary quadratic function relationship between the thickness Y and the pressure p and time t, and the following formula is obtained by three-dimensional nonlinear surface fitting:

[0033] Y = a + bt + cp + dt + hpt + ip 2 2 (1)

[0034] Step 1: find the coefficients i, h, c of the term containing p

[0035] When a particular time t is selected, the term containing t is a constant value, and formula (1) is converted into a parabolic equation about p. The coefficients i, h, c of the term containing p can be obtained by substituting the data and then fitting.

[0036] i = 0.1152 ± 0.03315

[0037] h = 1.79 × 10 -5 ± 4.74 × 10 -6

[0038] c = -0.8198 ± 1.0842

[0039] Step 2: find the coefficients d, b of the term containing t

[0040] Similarly, when a particular pressure p is selected, the term containing p is a constant value, and formula (1) is converted into a parabolic equation about t. The coefficients d, b of the term containing t can be obtained by substituting the data and then fitting.

[0041] d = -7.655 × 10 -10 ± 7.7735 × 10 -10

[0042] b = 0.0009334 ± 0.0001543

[0043] Step 3: After the determination of the five coefficients, the coefficient a needs to be determined finally. Substitute the above obtained coefficients into formula (1), and then substitute all the data into three-dimensional nonlinear surface fitting to obtain the coefficient a = 22.21 ± 9.65. Therefore, formula (1) is rewritten as:

[0044] Y = 22.21 + 0.0009334t + (-0.8198p) + (-7.655 × 10 -10 t 2 ) + 1.79 × 10 -5 tp + 0.1152p 2 (2)​​

[0045] Step 4: Attach Figure 1 Repeated fitting was performed, and the error rates of the coefficients in equation (2) were verified. A surface plot of the expression for equation (2) was created, and all data were substituted into the plot. If the data points basically fell on the surface, it indicated that the prediction results of equation (2) basically matched the actual results. Since the values ​​of each coefficient had a certain fluctuation range, a third fitting was performed to improve accuracy and reduce error. Finally, the appendix was obtained. Figure 2 The actual data is shown in the triplet fitting prediction plot. It can be found that the data points under different working conditions basically fall on the three prediction surfaces (boundaries), with an average error rate of 5%. This indicates that the relationship between the thickness Y and the pressure p and time t under actual working conditions basically conforms to the function variation law described by equation (2). Therefore, the coefficients are determined as follows:

[0046] a = 22.21

[0047] b = 0.0009334

[0048] c = -0.8198

[0049] d = -7.655 × 10 -10

[0050] h = 1.79 × 10 -5

[0051] i = 0.1152

[0052] The final fitting formula for the inner oxide layer thickness Y versus vapor pressure p is as follows:

[0053] Y=22.21 + 0.0009334t + (-0.8198p)+(-7.655×10 -10 t 2 )+1.79×10 -5 tp+0.1152p 2 (3)

[0054] In the above formula, the unit of time t is h, the unit of steam pressure p is MPa, and the unit of oxide layer inner layer thickness Y is μm.

[0055] Example 1

[0056] Comparison of the calculation method involved in this invention with the results of the T91 oxidation experiment.

[0057] Nishimura et al. measured that the inner layer thickness of the oxide layer of T91 steel is about 71 μm after oxidation for about 1,451 h under a steam pressure of 25 MPa. The inner layer thickness of the oxide layer calculated by substituting the experimental conditions into formula (3) proposed in the embodiment of the present application is about 75.7171 μm, and the error percentage is 6.6%.

[0058] Example 2

[0059] Comparison of the calculation method of the present application with the results of T92 oxidation experiments.

[0060] Muraki et al. measured that the inner layer thickness of the oxide layer of T92 steel is about 119 μm after oxidation for about 31,010 h under a steam pressure of 25 MPa. The inner layer thickness of the oxide layer calculated by substituting the experimental conditions into formula (3) proposed in the embodiment of the present application is about 115.8006 μm, and the error percentage is 2.6%.

[0061] Example 3

[0062] Application of the calculation method of the present application in actual power plant environment.

[0063] The steam pressure of a supercritical unit used in a power plant is 19.2 MPa, and the pipeline adopts T91 material. The inner layer thickness of the oxide layer in the pipeline is measured to be about 65 μm after running for about 8,911 h. The inner layer thickness of the oxide layer calculated by substituting the running parameters into formula (3) proposed in the embodiment of the present application is about 60.2564 μm, and the error percentage is 7.2%.

[0064] Example 4

[0065] Application of the calculation method of the present application in actual power plant environment.

[0066] The pipeline of a superheater of a boiler in a power plant adopts T91 material, and the steam pressure is 10 MPa. The inner layer thickness of the oxide layer in the pipeline is measured to be about 32 μm after running for about 4,000 h. The inner layer thickness of the oxide layer calculated by substituting the running parameters into formula (3) proposed in the embodiment of the present application is about 29.9694 μm, and the error percentage is 6.3%.

[0067] The above examples all show that the inner layer thickness of the oxide layer of 9% Cr martensitic steel calculated by the method of the present application is in good agreement with the actual measurement results, and the error is within 8%.

[0068] The technical solution of the present application is not limited to the above examples, and any technical solution obtained by equivalent replacement falls within the scope of the present application.

Claims

1. A method of calculating the inner layer thickness of an oxide layer of a martensitic heat-resistant steel under high-pressure steam, characterized by: The martensitic heat-resistant steel is a 9% Cr heat-resistant steel, and a calculation formula of an inner layer thickness of an oxidation layer under high-pressure steam is: Y = a + b t + c p + d t 2 + h pt +i p 2 wherein, Y is the inner layer thickness of the oxide layer, in μm; p is the steam pressure, in MPa, and the steam pressure ranges from 5.0 to 25.0 MPa; t is the time, in h; a, b, c, d, h, i are fitting coefficients.

2. The method of claim 1, wherein the thickness of the inner layer of the oxide layer of the martensitic heat-resistant steel under high-pressure steam is calculated. The time range is 1,000-150,000 h.

3. The method of claim 1, wherein the thickness of the inner layer of the oxide layer of the martensitic heat-resistant steel under high-pressure steam is calculated. The temperature range of the high-pressure steam is 550-650 DEG C.

4. The method of claim 1, wherein the thickness of the inner layer of the oxide layer of the martensitic heat-resistant steel under high-pressure steam is calculated. The value of the fitting coefficient a is 22.

21.

5. The method of claim 1, wherein the thickness of the inner layer of the oxide layer of the martensitic heat-resistant steel under high-pressure steam is calculated. The value of the fitting coefficient b is 0.0009334.

6. The method of claim 1, wherein the thickness of the inner layer of the oxide layer of the martensitic heat-resistant steel under high-pressure steam is calculated. The value of the fitting coefficient c is -0.8198.

7. The method of claim 1, wherein the thickness of the inner layer of the oxide layer of the martensitic heat-resistant steel under high-pressure steam is calculated. The value of the fitting coefficient d is -7.655 x 10 -10 .

8. The method of claim 1, wherein the thickness of the inner layer of the oxide layer of the martensitic heat-resistant steel under high-pressure steam is calculated. The value of the fitting coefficient h is 1.79 x 10 -5 .

9. The method of claim 1, wherein the thickness of the inner layer of the oxide layer of the martensitic heat-resistant steel under high-pressure steam is calculated. The value of the fitting coefficient i is 0.1152.

10. Application of the calculation method in claim 1 to evaluating the service life of a martensitic heat-resistant steel component operating under high-pressure steam in a power plant.

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