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

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

The proposed procedure for determination of residual life and disposable residual life, which is the time of safe service beyond the design time, is presented as the algorithm presented in Figure 6. This algorithm consists of six consecutive steps.

be carried out under constant stress on at least three different levels (step 2). Based on the relationships obtained from the results of short-term creep tests, the curves of temporary creep strength at a constant temperature are determined for a few temperature levels (step 3). These creep strength curves allow for determination of the parameter creep strength curve where the parameter is a function of temperature and time to rupture (step 4). The obtained parameter creep resistance curve allows the residual creep strength to be determined for the operating parameters (temperature and stress) of further service (step 5). When the residual creep strength for the parameters of further service is known, the disposable residual life can be determined. It is the safe time of further service for the adopted operating parameters (step 6). The proposed procedure consisting of consecutive steps is presented below on the example of the material after long-term service under creep

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

3.1 Selection of the representative areas for destructive testing and of the test

The selection of the representative areas for destructive testing is made after review of the previous service, results of measurements and diagnostic tests carried out on the object, verification calculations and determination of the temperature, and strain and stress distributions by the finite element method for the most stren-

The test materials selected in this way, with provision of the appropriate access to components for cutting them out and the possibility for repair/replacement, were an elbow and a straight section of the main primary steam pipeline made of 13 HMF

The short-term creep tests were carried out for six levels of test temperature Tb = 600, 620, 640, 650, 660, and 680°C under the constant level of test stress σ<sup>b</sup>

The obtained results of tests on the primary steam pipeline elbow made of 14MoV6–3 steel in the form of logtr = f(Tb) under σ<sup>b</sup> = const, where tr is the time to rupture in creep test, for three levels of stress σb1 = 50 MPa, σb2 = 80 MPa, and

3.3 Temporary creep strength curves at a constant temperature (step 3)

towards a lower temperature allowed for determination of residual life tre for

The extrapolation of the obtained relationships of log tr = f(tb) under σ<sup>b</sup> = const

Based on the constant-temperature residual creep strength curves in the form of log σ<sup>b</sup> = f(logtre), the residual creep strength tre was determined, and on its basis, the Larson-Miller parameter curve was determined in the form of σ<sup>b</sup> = f(L-M). Where: L-M = Tb (C + log tre); Tb, test temperature; C, material constant, tre,

The results of this extrapolation for one level of temperature Tb = const and three levels of stress σb1 = 50, σb2 = 80, and σb3 = 100 MPa allowed the creep strength curves to be plotted in the form of log σ<sup>b</sup> = f (logtre) for Tb1 = 530, Tb2 = 540, Tb3 = 560, and Tb4 = 580°C, which is presented graphically in Figure 8.

conditions far beyond the design time.

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

steel after approx. 200,000 h service.

3.2 Short-term creep tests (step 2)

σb3 = 100 MPa are shown in Figure 7.

temperature equal to the extrapolation one.

residual creep strength (Figure 9).

57

with three different values of σ<sup>b</sup> = 50, 80, and 100 MPa.

3.4 Parameter residual creep strength curve (step 4)

material (step 1)

uous areas [17–19].

The first step is to select the representative areas for destructive testing. At the designated area in the material of test specimen, the short-term creep tests should

#### Figure 6.

Procedure for determination of the time of further safe service beyond the design time according to creep strength characteristics based on short-term creep tests under constant stress level.

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

be carried out under constant stress on at least three different levels (step 2). Based on the relationships obtained from the results of short-term creep tests, the curves of temporary creep strength at a constant temperature are determined for a few temperature levels (step 3). These creep strength curves allow for determination of the parameter creep strength curve where the parameter is a function of temperature and time to rupture (step 4). The obtained parameter creep resistance curve allows the residual creep strength to be determined for the operating parameters (temperature and stress) of further service (step 5). When the residual creep strength for the parameters of further service is known, the disposable residual life can be determined. It is the safe time of further service for the adopted operating parameters (step 6). The proposed procedure consisting of consecutive steps is presented below on the example of the material after long-term service under creep conditions far beyond the design time.

## 3.1 Selection of the representative areas for destructive testing and of the test material (step 1)

The selection of the representative areas for destructive testing is made after review of the previous service, results of measurements and diagnostic tests carried out on the object, verification calculations and determination of the temperature, and strain and stress distributions by the finite element method for the most strenuous areas [17–19].

The test materials selected in this way, with provision of the appropriate access to components for cutting them out and the possibility for repair/replacement, were an elbow and a straight section of the main primary steam pipeline made of 13 HMF steel after approx. 200,000 h service.

### 3.2 Short-term creep tests (step 2)

The short-term creep tests were carried out for six levels of test temperature Tb = 600, 620, 640, 650, 660, and 680°C under the constant level of test stress σ<sup>b</sup> with three different values of σ<sup>b</sup> = 50, 80, and 100 MPa.

The obtained results of tests on the primary steam pipeline elbow made of 14MoV6–3 steel in the form of logtr = f(Tb) under σ<sup>b</sup> = const, where tr is the time to rupture in creep test, for three levels of stress σb1 = 50 MPa, σb2 = 80 MPa, and σb3 = 100 MPa are shown in Figure 7.

#### 3.3 Temporary creep strength curves at a constant temperature (step 3)

The extrapolation of the obtained relationships of log tr = f(tb) under σ<sup>b</sup> = const towards a lower temperature allowed for determination of residual life tre for temperature equal to the extrapolation one.

The results of this extrapolation for one level of temperature Tb = const and three levels of stress σb1 = 50, σb2 = 80, and σb3 = 100 MPa allowed the creep strength curves to be plotted in the form of log σ<sup>b</sup> = f (logtre) for Tb1 = 530, Tb2 = 540, Tb3 = 560, and Tb4 = 580°C, which is presented graphically in Figure 8.

### 3.4 Parameter residual creep strength curve (step 4)

Based on the constant-temperature residual creep strength curves in the form of log σ<sup>b</sup> = f(logtre), the residual creep strength tre was determined, and on its basis, the Larson-Miller parameter curve was determined in the form of σ<sup>b</sup> = f(L-M). Where: L-M = Tb (C + log tre); Tb, test temperature; C, material constant, tre, residual creep strength (Figure 9).

The proposed procedure for determination of residual life and disposable residual life, which is the time of safe service beyond the design time, is presented as the algorithm presented in Figure 6. This algorithm consists of six consecutive

The first step is to select the representative areas for destructive testing. At the designated area in the material of test specimen, the short-term creep tests should

Procedure for determination of the time of further safe service beyond the design time according to creep strength

characteristics based on short-term creep tests under constant stress level.

steps.

Creep Characteristics of Engineering Materials

Figure 6.

56

Figure 7.

The results of short-term creep tests on the material of primary steam pipeline elbow made of 13 HMF steel after approx. 200,000 h service under creep conditions in the form of log tre = f(Tb) under σ<sup>b</sup> = const.

The results of the creep tests shown as creep curves ε = f(t) at Tb = const and the ratio of the disposable residual life tbe to the residual life tre depending on the test stress σ<sup>b</sup> allowed to determine the tbe/tre ratio for the stress level equal to the operating stress σ<sup>b</sup> = σep = 50–60 MPa, which is written as tbe = 0.55tre (Figure 3). By knowing the residual life, the disposable residual life can be determined as the time of safe service for the adopted operating parameters. For the material of elbow, the relationships between the disposable residual life tbe and the operating temperature of further service Tep for the adopted constant level of operating stress σep at the selected levels of operating temperature Tep = 540 and 550°C and under stress σ<sup>b</sup> = σep = 60 and 70 MPa are shown in Figure 10a and for the circumferential welded joint at Tep = 530 and 540°C and operating stress σep = 50 MPa in Figure 6, whereas the safe service time of the material of test primary steam pipeline elbow and circumferential welded joint made of 13 HMF steel after approx. 200,000 h service under creep conditions for the operating parameters of further service, which was determined based on the above-mentioned relationships, is summarised

Residual life tre of the material of the test primary steam pipeline elbow made of 13 HMF steel after approx.

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

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

Disposable residual life tbe of the material of primary steam pipeline elbow made of 13 HMF steel after 200,000 h service under creep conditions and stress σep = 60 and 70 MPa equal to the operating one depending

200,000 h service under creep conditions in the form of Larson-Miller parameter curve.

in Table 1.

Figure 10.

59

on operating temperature Tep.

Figure 9.

#### Figure 8.

Residual creep strength curves RZe determined based on the results obtained from extrapolation of creep tests carried out on test pipeline elbow material made of 13 HMF steel after 200,000 h service under creep conditions.

#### 3.5 Safe time of service beyond the design time (step 5)

In contrast to the actual lifetime equal to the time to rupture of the material subject to creep, which is the residual life tre, the disposable residual life tbe is of practical importance. The disposable life is the time until which a structural component can be safely operated under the assumed temperature and stress conditions.

The disposable residual life is part of the residual life. To determine its share in the residual life, defined as the time to the end of the secondary creep tIIe in relation to the time to rupture tre, the rupture creep tests with measurement of elongation during test at a constant temperature Tb = 500°C similar to the operating one were carried out.

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 9.

Residual life tre of the material of the test primary steam pipeline elbow made of 13 HMF steel after approx. 200,000 h service under creep conditions in the form of Larson-Miller parameter curve.

The results of the creep tests shown as creep curves ε = f(t) at Tb = const and the ratio of the disposable residual life tbe to the residual life tre depending on the test stress σ<sup>b</sup> allowed to determine the tbe/tre ratio for the stress level equal to the operating stress σ<sup>b</sup> = σep = 50–60 MPa, which is written as tbe = 0.55tre (Figure 3).

By knowing the residual life, the disposable residual life can be determined as the time of safe service for the adopted operating parameters. For the material of elbow, the relationships between the disposable residual life tbe and the operating temperature of further service Tep for the adopted constant level of operating stress σep at the selected levels of operating temperature Tep = 540 and 550°C and under stress σ<sup>b</sup> = σep = 60 and 70 MPa are shown in Figure 10a and for the circumferential welded joint at Tep = 530 and 540°C and operating stress σep = 50 MPa in Figure 6, whereas the safe service time of the material of test primary steam pipeline elbow and circumferential welded joint made of 13 HMF steel after approx. 200,000 h service under creep conditions for the operating parameters of further service, which was determined based on the above-mentioned relationships, is summarised in Table 1.

#### Figure 10.

Disposable residual life tbe of the material of primary steam pipeline elbow made of 13 HMF steel after 200,000 h service under creep conditions and stress σep = 60 and 70 MPa equal to the operating one depending on operating temperature Tep.

3.5 Safe time of service beyond the design time (step 5)

tions.

58

Figure 8.

conditions.

Figure 7.

Creep Characteristics of Engineering Materials

carried out.

In contrast to the actual lifetime equal to the time to rupture of the material subject to creep, which is the residual life tre, the disposable residual life tbe is of practical importance. The disposable life is the time until which a structural component can be safely operated under the assumed temperature and stress condi-

Residual creep strength curves RZe determined based on the results obtained from extrapolation of creep tests carried out on test pipeline elbow material made of 13 HMF steel after 200,000 h service under creep

The results of short-term creep tests on the material of primary steam pipeline elbow made of 13 HMF steel after

approx. 200,000 h service under creep conditions in the form of log tre = f(Tb) under σ<sup>b</sup> = const.

The disposable residual life is part of the residual life. To determine its share in the residual life, defined as the time to the end of the secondary creep tIIe in relation to the time to rupture tre, the rupture creep tests with measurement of elongation during test at a constant temperature Tb = 500°C similar to the operating one were


### Table 1.

Safe service time of the material of the test primary steam pipeline elbow and circumferential welded joint made of 13 HMF steel after approx. 200,000 h service under creep conditions for the operating parameters of further service.

Their designation and values for the test material of elbow and

under creep conditions for the operating stress σ<sup>e</sup> and operating temperature Te.

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

tre determined and knowledge of the previous time of service.

service (column 8).

Figure 11.

characteristics

parameters of further service.

61

circumferential welded joint are summarised in Table 2, columns 2–7. The table also presents the operating stress σ<sup>r</sup> values calculated for the test materials after

Residual life tre of the test primary steam pipeline components made of 13 HMF steel after 200,000 h service

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

The life tr is defined as the total time of the previous service te and residual life tre for the parameters of the previous service (σe, Te), that is, tr = te + tre. Since the exhaustion degree based on the above-mentioned definition can be written as te/tr and tr = te + tre, then, in practice, te/tr = te/(te + tre). Using this relationship, the exhaustion degree (Table 2, column 10) was determined based on the residual life

4. Method for determination of the minimum component wall thickness necessary to transfer the actual service loads of the material after long-term service under creep conditions with known creep strength

The most important element necessary to determine the minimum required component wall thickness goe, which will be able to transfer the required service load (σep, Tep), is to have the characteristics of residual creep strength for the material of test component after service in the form of log σ = f(tre) for the

operating temperature Tep. This is the first step in the adopted procedure. Step 2 is to determine the residual creep strength RZe/Tep/tep for the adopted temperature of further service Te and the assumed time of further service te based on log σ = f(tre) at Tep = const. The value of RZe/Tep/tep obtained from the characteristics makes it possible to determine the acceptable stress k for the parameters of further service adopted in this way, which is step 3. To calculate the required minimum wall thickness goe for the adopted parameters of further service, the nominal outside diameter DOut of the component, the operating pressure pep, and the construction weakening coefficient z should be defined, which is step 4. Step 5 is the determination of the required minimum wall thickness goe of the component for the adopted

And the last, sixth, step is to compare the obtained value of the calculated required minimum wall thickness goe to the measured minimum actual thickness

gact min. If for the adopted time of further safe service tep:

## 3.6 Estimation of exhaustion degree caused by creep (step 6)

The exhaustion degree for materials working under creep conditions was defined in Chapter 3 of the study as the time of service until the time to rupture of this material under the temperature and stress operating conditions (te/tr).

For the reviewed case, the residual life tre (Table 2, column 9) was determined based on the developed relationship of log σ = f(tre) for the operating temperature of service Te. It was determined for the level of operating stress σ<sup>e</sup> of the test components as shown in Figure 11.

To determine the operating stress level using the formula

$$
\sigma\_v = \frac{p\_v \cdot \left[D\_z - \mathbf{g}\_{vz\,\mathrm{min}} \cdot \left(2 - z\right)\right]}{2\mathbf{g}\_{vz\,\mathrm{min}} \cdot z}
$$

where σ<sup>e</sup> is the operating stress, gact min is the actual minimum wall thickness of component, pe is the operating pressure of component, DOut is the nominal outside diameter, and z is the weakening coefficient, it is necessary to know the specific quantities in the formula.


#### Table 2.

Exhaustion extent te/tr of the material of test primary steam pipeline components made of 13 HMF steel after 200,000 h service under creep conditions for the operating parameters of service.

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 11.

3.6 Estimation of exhaustion degree caused by creep (step 6)

To determine the operating stress level using the formula

components as shown in Figure 11.

Creep Characteristics of Engineering Materials

Table 1.

further service.

quantities in the formula.

Table 2.

60

The exhaustion degree for materials working under creep conditions was defined in Chapter 3 of the study as the time of service until the time to rupture of

Safe service time of the material of the test primary steam pipeline elbow and circumferential welded joint made of 13 HMF steel after approx. 200,000 h service under creep conditions for the operating parameters of

Component Operating temperature Tep, °C Operating stress σep, MPa

Elbow 540 127,000 104,000 56,000

Circumferential welded joint Operating temperature Tep, °C Operating stress σep, MPa

55 60 70 Safe time of further service tbe, h

50 80 Safe time of further service tbe, h

550 62,000 51,000 28,000

530 143,000 48,400 540 77,000 26,000

For the reviewed case, the residual life tre (Table 2, column 9) was determined based on the developed relationship of log σ = f(tre) for the operating temperature of service Te. It was determined for the level of operating stress σ<sup>e</sup> of the test

where σ<sup>e</sup> is the operating stress, gact min is the actual minimum wall thickness of component, pe is the operating pressure of component, DOut is the nominal outside diameter, and z is the weakening coefficient, it is necessary to know the specific

Exhaustion extent te/tr of the material of test primary steam pipeline components made of 13 HMF steel after

200,000 h service under creep conditions for the operating parameters of service.

this material under the temperature and stress operating conditions (te/tr).

Residual life tre of the test primary steam pipeline components made of 13 HMF steel after 200,000 h service under creep conditions for the operating stress σ<sup>e</sup> and operating temperature Te.

Their designation and values for the test material of elbow and circumferential welded joint are summarised in Table 2, columns 2–7. The table also presents the operating stress σ<sup>r</sup> values calculated for the test materials after service (column 8).

The life tr is defined as the total time of the previous service te and residual life tre for the parameters of the previous service (σe, Te), that is, tr = te + tre. Since the exhaustion degree based on the above-mentioned definition can be written as te/tr and tr = te + tre, then, in practice, te/tr = te/(te + tre). Using this relationship, the exhaustion degree (Table 2, column 10) was determined based on the residual life tre determined and knowledge of the previous time of service.
