1. Introduction

The extension in the service time of power units which have exceeded the design service life of 100,000 h is made using the calculation method based on the creep strength data for 200,000 h service and the positive results of comprehensive diagnostic tests, with particular consideration given to those of the critical pressure components of boilers and turbines. Out of them, the components working at above the limit temperature, that is, under creep conditions, are of particular importance. In the assessment of these components, it is important and necessary to assess the condition of their materials [1–7]. Such an assessment is carried out based on nondestructive materials testing, and its result is referred to the available characteristics of materials after service [8–15]. The results of these tests provide good estimation of the condition of material and its exhaustion degree and allow for determination of the time of further safe service until the next inspection [16–28].

However, to reach the level of 200,000 h service for the material of components, not only good estimation of residual life but also its determination based on the destructive testing on a representative test specimen is required in a number of cases.

Yet, it is not always possible in practice. It can be made to assess the condition of the material of the main steam pipeline or transfer pipeline as well as for certain chambers and desuperheaters. However, it must be preceded by a cost-effectiveness calculation for such a procedure.

For the materials after service, the lifetime fracture rule can be presented as

The Procedure for Determining the Time of Safe Service beyond the Design Service Time Based…

where ts is the time of previous service of the material, trs is the time to rupture

Assuming that the further operating conditions will not differ from the previous

trt � tt

trs � ts <sup>¼</sup> ttts

The need to know the time to rupture of the material in the as-received condi-

In the event when the test specimens for destructive testing are taken from the operated material after two different times of service ts1 and ts2 and the creep tests are carried out for both conditions of one material, the time to rupture of the material in the as-received condition trt can be eliminated. Then, the knowledge of

The use of this method is particularly useful in the modernisation of boilers when some critical boiler components operating under creep conditions are replaced and the basic operating parameters are recorded and controlled on-line. The residual life can also be determined by extrapolating the results of creep rupture tests carried out on material after service at a temperature equal to or different from the design service temperature (Tr) and stress values equal to, but usually higher than, those characteristic of the service conditions (σr). Obtained in this way, the results of creep rupture tests with a time of usually approx. 10,000 h are presented in the parametric way by expressing one variable as a product of the function of two other variables, that is, stress and temperature and time, respec-

H ð Þ σ LM ¼ T Cð Þ þ log tre

the material constant at 1/T = 0, and tre is the time to rupture of material after

where H (σ) is the stress function, σ is the stress, T is the temperature in K, C is

This method for determination of residual life is shown graphically in Figure 1. Knowing the operating temperature Tr and the operating stress σ<sup>r</sup> (resulting from the component's geometry and operating pressure pr) of further service, the time to rupture tre can be determined following which the component destruction should be expected. However, the determined time tre is of no practical value. The practical value is the time to end of secondary creep, which is called disposable residual life tre 0.6 and which is part of the residual life. This value is specific to each of the test

The duration of the tests allowing for determination of the residual life tre can be reduced by carrying out the creep tests with measurement of elongation during test in order to determine the creep rate in steady state for the properly selected creep

stress = f(σ) at the constant temperature Tep of further service, the time of further

parameters (σb, Tb). Using the obtained curve of maximum creep rate vs.

safe service can be determined, expressed by the following formula:

ts trs þ tt trt ¼ 1

under service conditions (σe, Te), tt is the time to rupture of the test specimen determined under the accelerated creep test conditions for the material after service (σ<sup>t</sup> > σ<sup>e</sup> and/or Tt > Te), and trt is the time to rupture of the material in the asreceived condition under the accelerated test conditions (σ<sup>e</sup> = σt, T = Tt).

ones, the time equal to the residual life can be determined from

the properties of material in the as-received condition is not required.

tively, using the Larson-Miller parameter in the following form:

tion trt restricts the use of this method.

DOI: http://dx.doi.org/10.5772/intechopen.84986

service.

51

grades of material.

The problem related to the assessment of creep resistance in case of materials operated for a long time under creep conditions is the time required to carry out the creep tests to assess the residual life and the remaining safe time of service. This paper shows how to use long-term creep tests at the temperature and stress parameters corresponding to the actual operating ones, which allow for determination of the actual residual life, to make such an assessment. The method for using shortterm creep tests for such an assessment where test duration is reduced by increasing one of the parameters in relation to the actual ones is also presented.

## 2. Creep tests

#### 2.1 Long-term creep tests

In the assessment of residual life and disposable residual life, the main source of information is still creep tests in spite of the fact that their disadvantage is a long time until the test results are obtained. Creep tests are usually carried out under uniaxial tension and at strain rates from 10 to 9 to 10–12 s�<sup>1</sup> (corresponding to the actual service conditions) on test specimens taken directly from the material of component after long-term service at a temperature exceeding the limit one tg (te > tg).

Long-term creep tests can be divided into:


Based on the results of creep rupture tests without measurement of elongation during test, the residual life can be determined using one of the two procedures:


The lifetime fracture rule relies on the assumption that changes in temperature T or stress σ during service can be divided into degrees characterised by specific temperature Ti and stress σi.

Thus, the lifetime fracture rule concept by Robinson assumes that

$$\sum\_{\mathbf{i}} \left[ \frac{\mathbf{t}(\sigma, \mathbf{T})}{\mathbf{t}\_{\mathbf{r}}(\sigma, \mathbf{T})} \right] = \mathbf{1}$$

where T is the time of stress σ at a temperature T, tr is the time to rupture under stress σ and at temperature T, and i is the number of degrees of stress and temperature T from the beginning of service until rupture of the material.

For each of these degrees, the creep rate in steady state (secondary creep) or the time to rupture tri based on the experimental data measured under preset stress and temperature is determined. The total creep strain is assumed to be the sum of strains corresponding to the consecutive degrees, that is, ε<sup>c</sup> ¼ ∑<sup>i</sup> εi.

The Procedure for Determining the Time of Safe Service beyond the Design Service Time Based… DOI: http://dx.doi.org/10.5772/intechopen.84986

For the materials after service, the lifetime fracture rule can be presented as

$$\frac{\mathbf{t}\_{\mathbf{t}\_{\mathbf{t}}}}{\mathbf{t}\_{\mathbf{t}\_{\mathbf{t}}}} + \frac{\mathbf{t}\_{\mathbf{t}}}{\mathbf{t}\_{\mathbf{t}\_{\mathbf{t}}}} = \mathbf{1}$$

where ts is the time of previous service of the material, trs is the time to rupture under service conditions (σe, Te), tt is the time to rupture of the test specimen determined under the accelerated creep test conditions for the material after service (σ<sup>t</sup> > σ<sup>e</sup> and/or Tt > Te), and trt is the time to rupture of the material in the asreceived condition under the accelerated test conditions (σ<sup>e</sup> = σt, T = Tt).

Assuming that the further operating conditions will not differ from the previous ones, the time equal to the residual life can be determined from

$$\mathbf{t}\_{\mathbf{r}\_s} - \mathbf{t}\_s = \frac{\mathbf{t}\_t \mathbf{t}\_s}{\mathbf{t}\_{\mathbf{r}\_s} - \mathbf{t}\_t}$$

The need to know the time to rupture of the material in the as-received condition trt restricts the use of this method.

In the event when the test specimens for destructive testing are taken from the operated material after two different times of service ts1 and ts2 and the creep tests are carried out for both conditions of one material, the time to rupture of the material in the as-received condition trt can be eliminated. Then, the knowledge of the properties of material in the as-received condition is not required.

The use of this method is particularly useful in the modernisation of boilers when some critical boiler components operating under creep conditions are replaced and the basic operating parameters are recorded and controlled on-line.

The residual life can also be determined by extrapolating the results of creep rupture tests carried out on material after service at a temperature equal to or different from the design service temperature (Tr) and stress values equal to, but usually higher than, those characteristic of the service conditions (σr). Obtained in this way, the results of creep rupture tests with a time of usually approx. 10,000 h are presented in the parametric way by expressing one variable as a product of the function of two other variables, that is, stress and temperature and time, respectively, using the Larson-Miller parameter in the following form:

$$\mathbf{H} \left( \boldsymbol{\mathfrak{o}} \right) \mathbf{L} \mathbf{M} = \mathbf{T} \left( \mathbf{C} + \log \mathbf{t}\_{\mathrm{re}} \right)$$

where H (σ) is the stress function, σ is the stress, T is the temperature in K, C is the material constant at 1/T = 0, and tre is the time to rupture of material after service.

This method for determination of residual life is shown graphically in Figure 1. Knowing the operating temperature Tr and the operating stress σ<sup>r</sup> (resulting from the component's geometry and operating pressure pr) of further service, the time to rupture tre can be determined following which the component destruction should be expected. However, the determined time tre is of no practical value. The practical value is the time to end of secondary creep, which is called disposable residual life tre 0.6 and which is part of the residual life. This value is specific to each of the test grades of material.

The duration of the tests allowing for determination of the residual life tre can be reduced by carrying out the creep tests with measurement of elongation during test in order to determine the creep rate in steady state for the properly selected creep parameters (σb, Tb). Using the obtained curve of maximum creep rate vs. stress = f(σ) at the constant temperature Tep of further service, the time of further safe service can be determined, expressed by the following formula:

Yet, it is not always possible in practice. It can be made to assess the condition of the material of the main steam pipeline or transfer pipeline as well as for certain chambers and desuperheaters. However, it must be preceded by a cost-effectiveness

The problem related to the assessment of creep resistance in case of materials operated for a long time under creep conditions is the time required to carry out the creep tests to assess the residual life and the remaining safe time of service. This paper shows how to use long-term creep tests at the temperature and stress parameters corresponding to the actual operating ones, which allow for determination of the actual residual life, to make such an assessment. The method for using shortterm creep tests for such an assessment where test duration is reduced by increasing

In the assessment of residual life and disposable residual life, the main source of information is still creep tests in spite of the fact that their disadvantage is a long time until the test results are obtained. Creep tests are usually carried out under uniaxial tension and at strain rates from 10 to 9 to 10–12 s�<sup>1</sup> (corresponding to the actual service conditions) on test specimens taken directly from the material of component after long-term service at a temperature exceeding the limit one tg

• Creep rupture tests without measurement of elongation during test

Based on the results of creep rupture tests without measurement of elongation during test, the residual life can be determined using one of the two procedures:

The lifetime fracture rule relies on the assumption that changes in temperature T

or stress σ during service can be divided into degrees characterised by specific

tð Þ σ; T trð Þ σ; T 

where T is the time of stress σ at a temperature T, tr is the time to rupture under stress σ and at temperature T, and i is the number of degrees of stress and temper-

For each of these degrees, the creep rate in steady state (secondary creep) or the time to rupture tri based on the experimental data measured under preset stress and temperature is determined. The total creep strain is assumed to be the sum of

¼ 1

Thus, the lifetime fracture rule concept by Robinson assumes that

∑ i

ature T from the beginning of service until rupture of the material.

strains corresponding to the consecutive degrees, that is, ε<sup>c</sup> ¼ ∑<sup>i</sup> εi.

• Creep test with measurement of elongation during test

one of the parameters in relation to the actual ones is also presented.

calculation for such a procedure.

Creep Characteristics of Engineering Materials

2. Creep tests

(te > tg).

50

2.1 Long-term creep tests

• Lifetime fracture rule

temperature Ti and stress σi.

Long-term creep tests can be divided into:

• Extrapolation of creep rupture test results

Figure 1.

Determination of residual life using extrapolated Larson-Miller parameter curve for creep strength of the material after service at above limit temperature.

$$\mathbf{t}\_{\mathbf{e}\_{\mathrm{p}}} = \frac{\mathbf{e}\_{\mathbf{c}\_{\mathrm{disp}}}}{\mathbf{e}^{\mathrm{"up"}}},$$

where εc acpt is the acceptable total strain (adopted based on the operating experience and creep test results)—creep rate for parameters of further service (εe, Te) provided that the relation ε<sup>e</sup> ≤ εc acpt is met.

The method for determination of creep rate for the parameters of further service is shown graphically in Figure 2.

The creep rupture tests with measurement of elongation during test are also used to determine:


2.2 Short-term creep tests

depending on test stress.

Figure 3.

53

estimation of residual life.

from the material of a power system component and:

The disadvantage of the method for determination of residual life using longterm creep tests is the time of waiting for test results, which takes at least 2–3 years. To reduce the duration of these tests and the assessment of residual life, the socalled short-term creep tests with a duration from a few dozen to max 3000– 10,000 h are used in the engineering practice. This opens up the possibility for obtaining test results within a maximum of several months and providing good

The results of creep rupture tests with recording of elongation. (a) Creep curves at constant temperature Tb = const. (b) Strain corresponding to the end of secondary creep, elongation at rupture, and ν coefficient

The Procedure for Determining the Time of Safe Service beyond the Design Service Time Based…

DOI: http://dx.doi.org/10.5772/intechopen.84986

The acceleration of the creep process and the reduction in the duration of tests is obtained in creep tests carried out under uniaxial tension on test specimens taken

• Under constant test stress equal to the operating stress and at different test temperature levels much higher than the operating temperature

different test stress levels much higher than the operating stress

• At constant test temperature equal to the operating temperature and under

Tb, test temperature; Tr, operating temperature; σb, test stress; σr, operating stress; tr, time to rupture; Rz av, average temporary creep strength (result of longterm creep tests); tr(r)s, time to rupture for the operating parameters obtained by

The tests are carried out at a constant test temperature Tb, which is similar to the operating one, and at stress σ<sup>b</sup> values higher than those corresponding to the operating one. As a result of each of the tests, the creep curves ε = f (t) at σ<sup>b</sup> = const are obtained (Figure 3a). The test results are plotted on the diagram in the coordinate system υ = f(σ)ε<sup>c</sup> = f(σ)εII =f(σ) at Tb ≈ Te = const (Figure 3b). The characteristics obtained in this way allow for determination of the disposable residual life, which is the time of further safe service for the adopted operating parameters.

Figure 2.

Relationship between creep rate ε� <sup>s</sup> in steady state of the material after service under creep conditions and test stress for determination of the time of further safe service tep.

The Procedure for Determining the Time of Safe Service beyond the Design Service Time Based… DOI: http://dx.doi.org/10.5772/intechopen.84986

#### Figure 3.

tep <sup>¼</sup> <sup>ε</sup>cdop ε:sep

The method for determination of creep rate for the parameters of further service

The creep rupture tests with measurement of elongation during test are also

• The time to end of secondary creep tII and the corresponding elongation εII

• The share υ of the time of secondary creep tII in the overall time to rupture tr

The tests are carried out at a constant test temperature Tb, which is similar to the operating one, and at stress σ<sup>b</sup> values higher than those corresponding to the operating one. As a result of each of the tests, the creep curves ε = f (t) at σ<sup>b</sup> = const are obtained (Figure 3a). The test results are plotted on the diagram in the coordinate system υ = f(σ)ε<sup>c</sup> = f(σ)εII =f(σ) at Tb ≈ Te = const (Figure 3b). The characteristics obtained in this way allow for determination of the disposable residual life, which is the time of further safe service for the adopted operating parameters.

<sup>s</sup> in steady state of the material after service under creep conditions and test

• The time to rupture tr and the corresponding elongation ε<sup>c</sup>

where εc acpt is the acceptable total strain (adopted based on the operating experience and creep test results)—creep rate for parameters of further service

Determination of residual life using extrapolated Larson-Miller parameter curve for creep strength of the

(εe, Te) provided that the relation ε<sup>e</sup> ≤ εc acpt is met.

is shown graphically in Figure 2.

material after service at above limit temperature.

Creep Characteristics of Engineering Materials

used to determine:

Figure 1.

<sup>υ</sup> <sup>¼</sup> tII tr 

Figure 2.

52

Relationship between creep rate ε�

stress for determination of the time of further safe service tep.

The results of creep rupture tests with recording of elongation. (a) Creep curves at constant temperature Tb = const. (b) Strain corresponding to the end of secondary creep, elongation at rupture, and ν coefficient depending on test stress.

#### 2.2 Short-term creep tests

The disadvantage of the method for determination of residual life using longterm creep tests is the time of waiting for test results, which takes at least 2–3 years. To reduce the duration of these tests and the assessment of residual life, the socalled short-term creep tests with a duration from a few dozen to max 3000– 10,000 h are used in the engineering practice. This opens up the possibility for obtaining test results within a maximum of several months and providing good estimation of residual life.

The acceleration of the creep process and the reduction in the duration of tests is obtained in creep tests carried out under uniaxial tension on test specimens taken from the material of a power system component and:


Tb, test temperature; Tr, operating temperature; σb, test stress; σr, operating stress; tr, time to rupture; Rz av, average temporary creep strength (result of longterm creep tests); tr(r)s, time to rupture for the operating parameters obtained by

short-term creep tests as a result of extrapolation; tr(r), time to rupture for the operating parameters obtained by long-term creep tests of up to 100,000 h.

The manner of presenting results of short-term creep tests conducted under constant stress equal to the operating one (σb = σr = const) for different levels of test temperature Tb and at a constant temperature equal to the operating one (Tb = tr = const) and under different levels of test stress σb as well as the reliability of their estimation is shown graphically in Figures 4 and 5.

Based on at least 40 years' experience of the Creep Test Laboratory of the Institute for Ferrous Metallurgy in comparing results of short-term creep tests and extrapolating them with results of long-term tests of even up to 100,000 h, they were found to be useful for and applicable to the estimation of life and residual life. The determination of residual life for the material after service does not require knowledge of its history and previous operating conditions, and the only requirement is to define the operating parameters of its further service (σrp, Trp). Strong convergence of the results obtained in short- and long-term creep tests allows the method of short-term creep tests conducted under constant stress equal to the operating one to be used in practice. A disadvantage of this method is the restriction with regard to the applied test temperature levels arising from precipitation processes that occur in the material. The temperature levels must therefore be selected individually for each grade of material being tested.

Therefore, in the diagnostic practice, only short-term creep tests under constant stress and at different levels of temperature significantly higher than the operating one should be used to determine the residual life of the materials of components operating under creep conditions (pipelines, steam superheater coils). Such tests are recommended for use after exceeding the design service time, which is most often equal to 100,000 h. When the design time is exceeded by 50% (1.5 of to), these tests are obligatory.

The reliability of the obtained results of short-term creep tests depends on the requirements for compliance with constant level of preset test temperature Tb over the measurement length of the test specimen throughout the duration of the test. The determination of residual life with an error of no more than 20% in relation to the time determined based on long-term creep tests ensures that short-term creep tests are carried out at the test temperature Tb (much higher than the operating

one) with an accuracy equal to or higher than 0.5°C over the measurement length

The reliability of the estimation of time to rupture tr for the operating parameters based on the results of shortterm creep tests at a constant temperature equal to the operating one Tb = Tr = const and under different levels of stress σb higher than the operating one. Tb, test temperature; Tr, operating temperature; σb, test stress; σr, operating stress; tr, time to rupture; Rz av, average temporary creep strength (result of long-term creep tests); tz(r)s, time to rupture for the operating parameters obtained in short-term creep tests as a result of extrapolation; tr(r), time to rupture for the operating parameters obtained in long-term creep tests of up to 100,000 h.

The Procedure for Determining the Time of Safe Service beyond the Design Service Time Based…

DOI: http://dx.doi.org/10.5772/intechopen.84986

The main step in the proposed method for determination of the time of further safe service is to determine the residual life. Its determination is based on creep test results. These tests have been so far the only known method for determination of service life for actual operating parameters of the materials in use. In creep tests, the decisive factor for their duration is the time to rupture. It cannot be reduced in the development of material characteristics. However, such a possibility exists for the assessment of specific material both in the as-received condition and after service. Nevertheless, the research methods used for this purpose have to be veri-

3. Method for determination of the time of safe service for material after long-term service under creep conditions beyond the design

of the test specimen, regardless of what the test temperature level is.

service time

55

Figure 5.

fied with results of long-term creep tests [16].

#### Figure 4.

Reliability of the estimation of time to rupture tr for the operating parameters based on the results of short-term creep tests under constant test stress equal to the operating one σ<sup>b</sup> = σ<sup>r</sup> = const and at different test temperatures Tb higher than the operating one.

The Procedure for Determining the Time of Safe Service beyond the Design Service Time Based… DOI: http://dx.doi.org/10.5772/intechopen.84986

#### Figure 5.

short-term creep tests as a result of extrapolation; tr(r), time to rupture for the operating parameters obtained by long-term creep tests of up to 100,000 h. The manner of presenting results of short-term creep tests conducted under constant stress equal to the operating one (σb = σr = const) for different levels of test temperature Tb and at a constant temperature equal to the operating one (Tb = tr = const) and under different levels of test stress σb as well as the reliability

Based on at least 40 years' experience of the Creep Test Laboratory of the Institute for Ferrous Metallurgy in comparing results of short-term creep tests and extrapolating them with results of long-term tests of even up to 100,000 h, they were found to be useful for and applicable to the estimation of life and residual life. The determination of residual life for the material after service does not require knowledge of its history and previous operating conditions, and the only requirement is to define the operating parameters of its further service (σrp, Trp). Strong convergence of the results obtained in short- and long-term creep tests allows the method of short-term creep tests conducted under constant stress equal to the operating one to be used in practice. A disadvantage of this method is the restriction with regard to the applied test temperature levels arising from precipitation processes that occur in the material. The temperature levels must therefore be selected

Therefore, in the diagnostic practice, only short-term creep tests under constant stress and at different levels of temperature significantly higher than the operating one should be used to determine the residual life of the materials of components operating under creep conditions (pipelines, steam superheater coils). Such tests are recommended for use after exceeding the design service time, which is most often equal to 100,000 h. When the design time is exceeded by 50% (1.5 of to), these tests

The reliability of the obtained results of short-term creep tests depends on the requirements for compliance with constant level of preset test temperature Tb over the measurement length of the test specimen throughout the duration of the test. The determination of residual life with an error of no more than 20% in relation to the time determined based on long-term creep tests ensures that short-term creep tests are carried out at the test temperature Tb (much higher than the operating

Reliability of the estimation of time to rupture tr for the operating parameters based on the results of short-term creep tests under constant test stress equal to the operating one σ<sup>b</sup> = σ<sup>r</sup> = const and at different test temperatures

of their estimation is shown graphically in Figures 4 and 5.

Creep Characteristics of Engineering Materials

individually for each grade of material being tested.

are obligatory.

Figure 4.

54

Tb higher than the operating one.

The reliability of the estimation of time to rupture tr for the operating parameters based on the results of shortterm creep tests at a constant temperature equal to the operating one Tb = Tr = const and under different levels of stress σb higher than the operating one. Tb, test temperature; Tr, operating temperature; σb, test stress; σr, operating stress; tr, time to rupture; Rz av, average temporary creep strength (result of long-term creep tests); tz(r)s, time to rupture for the operating parameters obtained in short-term creep tests as a result of extrapolation; tr(r), time to rupture for the operating parameters obtained in long-term creep tests of up to 100,000 h.

one) with an accuracy equal to or higher than 0.5°C over the measurement length of the test specimen, regardless of what the test temperature level is.
