CN107843509B - Method for estimating residual endurance life of supercritical unit T/P92 heat-resistant steel based on room-temperature Brinell hardness prediction - Google Patents

Abstract

The invention relates to the field of residual durable life evaluation of iron and steel materials, in particular to a residual durable life evaluation method of supercritical unit T/P92 heat-resistant steel based on room temperature Brinell hardness prediction. The invention establishes a corresponding relationship between the permanent fracture life and the room temperature Brinell hardness HB under different aging states by using the high temperature tensile strength of the material as a bridge, so as to establish the room temperature Brinell hardness and the permanent fracture time of P92 steel under given temperature and stress conditions. Mathematical analysis of the relationship between. The invention can quickly and accurately predict the remaining lasting life of the current material under specific steam parameters through a simple, convenient and non-destructive hardness test, and can directly avoid economic losses caused by downtime or pipe cutting. Features The remaining life of T/P92 heat-resistant steel can be evaluated in a timely manner, which can effectively prevent accidents caused by material aging and failure.

Description

Prediction of residual longevity of T/P92 heat-resistant steel for supercritical units based on Brinell hardness at room temperature life evaluation method

technical field

The invention relates to the field of residual durable life evaluation of iron and steel materials, in particular to a residual durable life evaluation method of supercritical unit T/P92 heat-resistant steel based on room temperature Brinell hardness prediction.

Background technique

Based on safety and economic considerations, the evaluation of high temperature creep life has always been an important topic in the research and development of heat-resistant steel. Metal materials operating under actual conditions are often subjected to very small forces and low temperatures. Using such conditions for experiments, although the service life under service conditions can be directly obtained, it will consume a lot of time, sometimes several years. , even ten years. In order to shorten the time, the experimental stress or temperature must be increased, that is, the accelerated experimental method is used, and then the remaining life under the service conditions is determined by extrapolation. The reliability of the results obtained depends largely on the method of extrapolation, in addition to being related to the experimental technique. At present, the widely used methods are: isotherm extrapolation method, time-temperature parameter method (Larson-Miller parameter method) and least constraint method.

(1) Extrapolation of durable strength by isotherm method

This method refers to the data of short-term tests with higher different stresses at the same experimental temperature, to establish the relationship between stress and fracture time, and to extrapolate the long-term durable strength value at the test temperature. The analysis of a large number of experimental data shows that there is a certain relationship between the fracture time of the specimen and the stress when the material is subjected to a permanent strength test at a constant temperature. The following two empirical relations are widely used:

τ=Aσ- ^B

τ=Ce ^-Dσ

The above formula shows that there is a linear relationship between the log value of fracture time τ and the log value of stress σ or the log value of fracture time τ and stress σ. There is a linear relationship. Of the two, the former (double logarithmic coordinate relationship) is more commonly used. It must be pointed out, however, that the linear extrapolation of persistent strength, whether using a double logarithmic or semi-logarithmic coordinate relationship, is approximate. In the commonly used double logarithmic coordinates, the test point does not really conform to the linear relationship, but is actually a curve with a quadratic inflection, only some areas of the curve are closer to a straight line, so the approximate linear method is used. For different materials and different temperatures, the position and shape of the inflection are different. Long-term tests at high temperatures have shown that in steels with higher structural stability, the transition is not obvious, or only appears after a longer period of time. For some steels with less stable structures, the turning point is very obvious, so the method of straight line extrapolation is rough. However, because the isotherm extrapolation method is simple and easy to implement, under certain conditions, it can still obtain the extrapolated value that is relatively close to the actual value, so it is still widely used. However, the extrapolation time should be limited to no more than 10 times the test time to ensure the relative accuracy of the extrapolated value.

(2) Time-temperature parameter method

This method is a method of extrapolating persistent strength developed in the 1950s. It points out the relationship between temperature and time, and can find a mathematical expression for a parameter P to represent it. It is a function of the process linking temperature and time, and the parameter P itself is a function of stress, that is, as long as the stress σ of the metal material remains unchanged, for various combinations of temperature T and time τ, the parameter P always remains unchanged. , that is, P=P(σ)=II. The parametric expression can be written in the following general form:

P=P(σ)=f(T, τ)

There are many specific parameter relations. The LM parameter method is the most widely used extrapolation method. It was proposed by Larson-Miller in 1952. The basic idea is that the temperature T(K) has a compensation relationship with the fracture time, that is, for a certain The fracture stress of , temperature and time are equivalent, that is to say, for a certain fracture stress, there is only one P. This relationship can be represented by the LM parameter P _LM . Stress extrapolation was performed using creep rupture data under accelerated experimental conditions to obtain P _LM under service conditions, and the rupture time was then calculated.

lgσ=f(PL-M)=f[10 ^-3 T(C+lgτ)]

The main advantages of this extrapolation method are that it is easy to use, has a wide range of applications and is relatively accurate. The shortcomings are: destructive experiments must be carried out on the components, which is time-consuming and labor-intensive; when the components have obvious voids and microcracks, it is difficult to measure accurate experimental data; it is affected by chemical composition, microstructure and operating conditions. Due to the influence of inhomogeneity, the dispersion of experimental data is large; a large number of experiments are needed to determine the C value for a new material, and the C value is related to stress.

SUMMARY OF THE INVENTION

The purpose of the invention is to provide a method for predicting the remaining durable life of T/P92 heat-resistant steel of a supercritical unit based on Brinell hardness at room temperature, which can quickly and accurately predict the current material in a specific steam through a simple, convenient and non-destructive hardness test. The remaining durable life under the parameters can directly avoid the economic loss caused by downtime or pipe cutting, etc. Because of its convenient and fast characteristics, it can evaluate the remaining life of the material in time and effectively prevent the accident caused by the aging and failure of the material.

For realizing the purpose of the present invention, the technical scheme adopted in the present invention is as follows:

A method for evaluating the remaining durable life of T/P92 heat-resistant steel for a supercritical unit based on Brinell hardness at room temperature, which specifically includes the following steps:

(1) Heat treatment to obtain specimens with different aging damage

For the T/P92 heat-resistant steel that has not been in service, the actual working temperature and time are used to obtain workpieces with different levels of aging damage after equivalent accelerated aging aging heat treatment, and the material is cooled to room temperature. piece;

(2) Hardness determination

A standard hardness tester is used to measure the hardness of the material under different aging damages, and the hardness test specimens are tested in the laboratory according to the national standard GB/T 231.4-2009. There are 3 to 5 specimens in each group, and each aging state is tested. At least 3 times, take the average value;

(3) Determination of high temperature tensile strength

Five groups of samples with characteristic Brinell hardness values were selected to test the high temperature tensile strength of the material under different aging damages. The temperatures included 773K, 823K, 873K, and 923K, and the corresponding mathematical relationship model was established.

σ _R = 1.58HB+547.80-0.61T (1)

In the formula, σ _R is the tensile strength of T/P92 steel when the material is at a certain temperature: 773K≤T≤923K, HB is the room temperature Brinell hardness of the material in any structure state, and T is the application temperature;

(4) Enduring strength test

Using the above 5 groups of samples with different aging and damage states to carry out high temperature endurance experiments and obtain fracture time parameters. enduring experimentation;

(5) Constructing mathematical model of isotherm extrapolation

Isotherm extrapolation formula:

t _r =Aσ ^-B (2)

In the formula, A is a parameter related to the tissue state of the material, also known as the "resistance" of the material, that is, the ability parameter to resist deformation of the material, and B is a parameter related to temperature;

Taking the logarithm of equation (2), we get:

lgt _r =lgA-Blgσ(3)

Due to the obvious difference in the microstructure state of the accelerated aging of the material, A is a parameter that changes with the degree of aging; that is to say, for the use of multiple sets of stresses and their corresponding permanent fracture times under the same aging parameters and the same temperature, the parameters under the current conditions can be obtained. A and B;

(6) Establish the corresponding mathematical relationship between the high temperature tensile strength of the material and the parameter A, the long-term experimental temperature and the parameter B, and obtain the following relationship:

A=2.7*10 ⁶ σ _R ^9.03 +16.71 (4)

B=0.1T-75.68 (5)

(7) Fitting the functional formula of parameters such as hardness-remaining life

Combining formulas (1), (2), (4), and (5), the analytical formula of the mathematical relationship between the prediction of Brinell hardness at room temperature and the remaining life of the current material under any temperature and stress conditions in T/P92 steel can be obtained:

t _r =[2.7*10 ⁶ (1.58HB+547.80-0.61T) ^9.03 +16.71]σ ^75.68-0.1T (6)

In the formula, t _r is the remaining life of the current material, HB is the Brinell hardness of the material at room temperature, T is the temperature used to predict the life, and σ is the stress used to predict the life.

The described evaluation method, the experimental method for predicting the remaining life by using room temperature hardness is only suitable for predicting the remaining permanent life of T/P92 steel used between 773K and 923K.

In the evaluation method, the empirical formula for predicting the remaining life by using room temperature hardness has a self-accurate function, and the hardness and durability data of any T/P92 heat-resistant steel in the applicable temperature range are further accurate to the empirical formula.

The design idea of the present invention is:

The invention establishes a corresponding relationship between the permanent fracture life and the room temperature Brinell hardness HB under different aging states by using the high temperature tensile strength of the material as a bridge, so as to establish the room temperature Brinell hardness and the permanent fracture time of P92 steel under given temperature and stress conditions. Mathematical analysis of the relationship between.

The advantages and beneficial effects of the present invention are:

The invention can quickly and accurately predict the remaining lasting life of the current material under specific steam parameters through a simple, convenient and non-destructive hardness test, and can directly avoid economic losses caused by downtime or pipe cutting. Features The remaining life of T/P92 heat-resistant steel can be evaluated in a timely manner, which can effectively prevent accidents caused by material aging and failure.

Description of drawings

Figure 1 shows the change of Brinell hardness at room temperature of P92 heat-resistant steel under different aging times. In the figure, the abscissa agedtime is the aging time (h); the ordinate is the Brinell hardness HB.

Figure 2 shows the high temperature tensile strength diagram of P92 heat-resistant steel under different aging times at the characteristic hardness value. In the figure, the abscissa aged time is the aging time (h); the ordinate Tensile Strength is the tensile strength (MPa).

Figure 3 is a graph showing the corresponding relationship between high temperature tensile strength and room temperature hardness. In the figure, the abscissa is the Brinell hardness HB; the ordinate σ _R is the tensile strength (MPa).

Figure 4 is a double logarithmic plot of lgσ-lgt _r . In the figure, the abscissa lgσ represents the logarithm of the stress used to predict the life; the ordinate parameter lgt _r represents the logarithm of the remaining life of the current material.

Figure 5 is a graph showing the corresponding relationship between high temperature tensile strength and parameter A. In the figure, the abscissa σ _R is the tensile strength (MPa) and the ordinate A represents the parameters related to the microstructure state of the material.

Figure 6 is a graph showing the corresponding relationship between the persistent experimental temperature and parameter B. In the figure, the abscissa T is the temperature (K); the ordinate parameter B represents the parameters related to temperature.

Detailed ways

In order to understand the technical content of the present invention more clearly, this material is taken as an example for further description. It should be understood that the following embodiments are for illustrative purposes only, and are not intended to limit the scope of the present invention.

Example

Utilizing P92 heat-resistant steel not in service in a power plant to realize the method for evaluating the remaining lasting life of T/P92 heat-resistant steel of a supercritical unit based on room temperature Brinell hardness prediction according to the present invention, the specific implementation method is as follows:

(1) Heat treatment to obtain specimens with different aging damage

For the T/P92 heat-resistant steel that has not been in service, the actual working temperature and time are used to obtain workpieces with different levels of aging damage after equivalent accelerated aging aging heat treatment, and the material is cooled to room temperature. piece;

In this embodiment, the equivalent accelerated aging aging heat treatment refers to: aging at a temperature of 30-40°C below the AC1 point for 5-1000h; after the aging is completed, perform secondary aging at 650°C to ensure rapid precipitation Laves phase, the final realization is equivalent to the aging structure under different periods of actual working conditions.

(2) Hardness determination

A standard hardness tester is used to measure the hardness of the material under different aging damages, and the hardness test specimens are tested in the laboratory according to the national standard GB/T 231.4-2009. There are 3 to 5 specimens in each group, and each aging state is tested. At least 3 times, take the average value;

(3) Determination of high temperature tensile strength

Select 5 groups of samples with characteristic Brinell hardness values to test the tensile strength of materials at high temperature (including 773K, 823K, 873K, 923K) under different aging damage, and establish the corresponding mathematical relationship model, the relationship is as follows:

σ _R = 1.58HB+547.80-0.61T (1)

In the formula, σ _R is the tensile strength of T/P92 steel when the material is at a certain temperature T (773K≤T≤923K), HB is the room temperature Brinell hardness of the material in any structure state, and T is the application temperature.

(4) Enduring strength test

Use the above 5 groups of samples with different aging and damage states to carry out high temperature (including 550 ° C, 600 ° C, 650 ° C) endurance experiments and obtain fracture time parameters. Each group needs at least 2 different stress conditions at 3 different experimental temperatures. The results of the persistent experiments are shown in Table 1.

Table 1 Persistence performance of P92 at different aging levels

(5) Constructing mathematical model of isotherm extrapolation

Commonly used isotherm extrapolation formula:

t _r =Aσ ^-B (2)

In the formula, A is a parameter related to the organizational state of the material, which can also be called the "resistance" of the material, that is, the ability parameter to resist deformation of the material, and B is a parameter related to temperature.

Taking the logarithm of equation (2), we get:

lgt _r =lgA-Blgσ(3)

Due to the obvious difference in the tissue state of accelerated aging of materials, A is a parameter that changes with the degree of aging. That is to say, for the same aging parameters and the same temperature, the parameters A and B under the current conditions can be obtained by using multiple sets of stresses and their corresponding permanent fracture times. The results are shown in Table 2.

Table 2 Material parameter A and temperature parameter B in P92

(6) Establish the corresponding mathematical relationship between the high temperature tensile strength of the material and the parameter A, the long-term experimental temperature and the parameter B, and obtain the following relationship:

A=2.7*10 ⁶ σ _R ^9.03 +16.71 (4)

B=0.1T-75.68 (5)

(7) Fitting the functional formula of parameters such as hardness-remaining life

Combining formulas (1), (2), (4) and (5), the analytical formula of the mathematical relationship between the prediction of Brinell hardness at room temperature and the remaining life of the current material under any temperature and stress conditions in T/P92 steel can be obtained:

t _r =[2.7*10 ⁶ (1.58HB+547.80-0.61T) ^9.03 +16.71]σ ^75.68-0.1T (6)

In the formula, t _r is the remaining life of the current material, HB is the Brinell hardness of the material at room temperature, T is the temperature used to predict the life, and σ is the stress used to predict the life.

In the experiment, a sample of P92 heat-resistant steel with a hardness of HB148 was selected for the endurance test at 600°C/140MPa, and its permanent fracture time was 1397.42h; using formula (6) to calculate and predict the remaining endurance life of the sample under the corresponding endurance conditions was 1357.59h, The error is 2.9%, which well proves the accuracy of the prediction method of the present invention.

As shown in Figure 1, it can be seen from the Brinell hardness change diagram of P92 heat-resistant steel at room temperature under different aging times that the Brinell hardness HB decreases continuously with the aging time, and the initial decrease is relatively fast, and then gradually tends to This result can correspond well with the aging evolution process of the material structure, which means that it is suitable to use the hardness to characterize the current structure characteristics of the material. In addition, using different equivalent accelerated aging temperature tests, it was verified that the heat treatment time of the material was shorter and the effect was better at 800 °C.

As shown in Figure 2, it can be seen from the high temperature tensile strength diagram of P92 heat-resistant steel under different aging times at the characteristic hardness value, as the aging time prolongs, the high temperature tensile strength of the material also decreases to a certain extent and decreases. The trend is basically consistent with the hardness change.

As shown in Figure 3, it can be seen from the corresponding relationship between high temperature tensile strength and room temperature hardness that the hardness of different aging degrees and high temperature tensile strength have a good corresponding relationship, which can establish the hardness and high temperature under different aging conditions. The mathematical relationship of tensile strength, that is, the high temperature tensile strength of a material can be characterized by hardness.

As shown in Figure 4, it can be seen from the lgσ-lgt _r double logarithmic coordinate diagram that under the same aging parameters and the same temperature, the parameters A and B, where A is the intercept of the relationship and B is the absolute value of the slope of the relationship.

As shown in Figure 5, it can be seen from the corresponding relationship between the high-temperature tensile strength and parameter A that the high-temperature tensile strength of the material has a good corresponding relationship with the parameter A. Because the high-temperature tensile strength of the material can characterize the different aging of the material The microstructure state under the characteristic, which means that the parameter A is the microstructure state parameter of the material in the isotherm extrapolation formula and can be characterized by the high temperature tensile strength.

As shown in Figure 6, it can be seen from the graph of the corresponding relationship between the persistent experimental temperature and the parameter B that the parameter B is only related to the temperature and not to the aging state of the material, and this parameter tends to a certain value in different aging states.

Claims (2)

1. A method for predicting the residual endurance life of supercritical unit T/P92 heat-resistant steel based on room-temperature Brinell hardness is characterized by comprising the following steps:

(1) test piece with different aging damages obtained by heat treatment

For T/P92 heat-resistant steel which is not in service, test pieces with different aging damage levels are obtained after equivalent accelerated aging heat treatment, the test pieces are cooled to room temperature, and a part of the test pieces are prepared into a plurality of hardness test pieces;

(2) measurement of hardness

Measuring the hardness of the test pieces under different aging damages by using a standard hardness tester, carrying out hardness test on the hardness test pieces in a laboratory according to the national standard GB/T231.4-2009, wherein each group of test pieces is 3-5, the test is carried out for at least 3 times in each aging state, and averaging;

(3) high temperature tensile Strength determination

Selecting 5 groups of test pieces with characteristic Brinell hardness values, testing the high-temperature tensile strength under different aging damages at the temperature of 773K, 823K, 873K and 923K, and establishing a corresponding mathematical relationship model, wherein the relationship is as follows:

σ_R＝1.58HB+547.80-0.61T (1)

in the formula, σ_RIs that the temperature of the test piece is: T/P92 steel with T being more than or equal to 773K and less than or equal to 923KTensile strength, HB is the room temperature Brinell hardness of the test piece in any tissue state, and T is the application temperature;

(4) endurance test

Performing high-temperature endurance experiments on the 5 groups of test pieces with different aging damage states of characteristic Brinell hardness values to obtain fracture time parameters, wherein the temperature comprises 550 ℃, 600 ℃ and 650 ℃, and each group is subjected to endurance experiments under at least 2 different stress conditions at 3 different experiment temperatures;

(5) construction of mathematical model of isotherm extrapolation

Isotherm extrapolation formula:

t_r＝Aσ^-B (2)

wherein A is a parameter related to the tissue state of the test piece, also called the resistance of the test piece, namely the parameter of the capability of resisting the deformation of the test piece, and B is a parameter related to the temperature;

taking logarithm of formula (2) to obtain:

lgt_r＝lgA-Blgσ (3)

the difference of the accelerated aging tissue states of the test piece is obvious, so that A is a parameter which changes along with the aging degree; that is to say, parameters A and B under the current condition are obtained by utilizing a plurality of groups of stresses and corresponding permanent fracture time under the same aging parameter and the same temperature;

(6) establishing a mathematical relationship between the high-temperature tensile strength of the test piece and the parameter A, and between the lasting experiment temperature and the parameter B, and obtaining the following relational expression:

A＝2.7*10⁶σ_R ^9.03+16.71 (4)

B＝0.1T-75.68 (5)

(7) fitting a function of hardness-residual life isoparametric

Combining the formulas (1), (2), (4) and (5) to obtain the mathematical relationship analytical formula of the room temperature Brinell hardness and the current test piece residual life under any temperature and stress conditions in the T/P92 steel:

t_r＝[2.7*10⁶(1.58HB+547.80-0.61T)^9.03+16.71]σ^75.68-0.1T (6)

in the formula, t_rIs at presentThe residual life of the test piece, HB is the Brinell hardness of the test piece at room temperature, T is the temperature used for predicting the life, and sigma is the stress used for predicting the life;

the evaluation method is only suitable for predicting the residual endurance life of the T/P92 steel used in the range between 773K and 923K.

2. The evaluation method according to claim 1, wherein the mathematical relationship analysis formula has a self-precision function, and the hardness and durability data of any T/P92 heat-resistant steel in the applicable temperature range further refine the mathematical relationship analysis formula.

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