**3.3 Assessment of residual life and disposable residual life of the material of welded joints operated for a long time under creep conditions**

The most critical element of the materials characteristics is the creep resistance level of the welded joints being operated. This creep resistance is responsible for

**43**

**4. Summary**

*Creep Characteristics of Engineering Materials DOI: http://dx.doi.org/10.5772/intechopen.86078*

**14MoV6-3** Parent

**14MoV6-3** Parent

**Table 3.**

material

Welded joint

material

Welded joint

**Steel grade Test area Material used in testing Adopted operating** 

**Type of component Service time**

> Primary steam pipeline 200,000 h

> Primary steam pipeline 200,000 h

**parameters of further service**

> **temperature Tr, °C**

ϕ327 × 40 60 540 **300,000 165,000**

ϕ327 × 40 50 540 **140,000 77,000**

**Stress σr, MPa**

**Determined residual life, h**

**200,000 110,000**

**35,000 19,000**

**Estimated disposable residual life, h**

their ability to transfer the actual service loads of the component, which occur under operating conditions. A measure of creep resistance is the level of residual creep resistance, which is able to meet these expectations. However, in the engineering practice, the safe service of the welded joints operated under the preset temperature and pressure conditions is important. The time of safe service is the disposable residual life, which is part of the residual life being determined.

*Residual life and disposable residual life based on the results of short-term creep tests on the materials of the main primary steam pipelines made of 14MoV6-3 steel after long-term service far beyond the design service time.*

**Dimensions of Dout × gn, mm**

The examples of residual life, which is the time to rupture under the temperature and stress conditions equal to operating ones, determined in short-term creep tests and compared to the residual life of the parent material after service are shown in **Figure 9**. The determined residual life and disposal residual life for the presented examples are summarised in **Table 3**. The comparison of the residual life and disposable residual life determined for the material of two primary steam pipelines made of 14MoV6-3 steel after 200,000 h service shows significant differences. These differences probably result mainly from differences in the as-received condition of the operated material and welded joint as well as different service history. This confirms the need of an individual assessment, mainly based on the assessment of

Based on many power units in use, the results of creep tests on the butt-welded joints after long-term service beyond the design time carried out on the parent material and butt-welded joints of the components operating under creep conditions invalidate the generally accepted principle that the residual creep strength of the material of welded joint is no less than 80% the residual creep strength of the basic material. The principle of the relationship of residual creep strength between them transferred from that applicable to the basic material and welded joint in the as-received condition is not supported by the investigations of real cases. Also, in many investigated cases, the residual life of the welded joint is significantly lower than that obtained for the parent material. Therefore, for the components and materials operated far beyond the design time, it is necessary to determine the residual life not only for the basic material of the representative test specimen of the

the actual condition of the component and its material.

component but also for the welded joint (**Table 3**).

#### **Figure 9.**

*Residual life of parent material and butt-welded joints of components working far beyond the design service time based on results of short-term creep tests in the form of log tre = f(tb) at σb = const for two different main primary steam pipelines made of 14MoV6-3 steel after 200,000 h service. σB- test stress.*


#### **Table 3.**

*Creep Characteristics of Engineering Materials*

In accordance with the results of long-term own research, for the materials that most often exist in the components operated under creep conditions far beyond the design service time, such as 13CrMo4-5, 14MoV6-3, 10CrMo9-10 low-alloy steels and X20CrMoV121 high-chromium steel, the highest trend towards the loss of plastic properties is shown by the components made of 14MoV6-3 steel and then those made of X20CrMoV121 steel. The knowledge of these material features is necessary for the selection of water-pressure test parameters and the method for overhaul works and repairs. It should be noted that for components in long-term use, the leak water tests can only be carried out. The use of strength water test may result in destruction of the component being operated. Such a test can only be carried out for the new equip-

It can be concluded that with an increase in the exhaustion degree, the mechanical properties of the material being operated are reduced. However, no unambiguous relationships between the level of properties, both strength and plastic, and the residual life and exhaustion degree caused by creep were observed so far. According

**3.3 Assessment of residual life and disposable residual life of the material of** 

The most critical element of the materials characteristics is the creep resistance level of the welded joints being operated. This creep resistance is responsible for

*Residual life of parent material and butt-welded joints of components working far beyond the design service time based on results of short-term creep tests in the form of log tre = f(tb) at σb = const for two different main* 

*primary steam pipelines made of 14MoV6-3 steel after 200,000 h service. σB- test stress.*

**welded joints operated for a long time under creep conditions**

ment, and the governing standards apply to new equipment only.

to the present knowledge, such a correlation does not exist.

**42**

**Figure 9.**

*Residual life and disposable residual life based on the results of short-term creep tests on the materials of the main primary steam pipelines made of 14MoV6-3 steel after long-term service far beyond the design service time.*

their ability to transfer the actual service loads of the component, which occur under operating conditions. A measure of creep resistance is the level of residual creep resistance, which is able to meet these expectations. However, in the engineering practice, the safe service of the welded joints operated under the preset temperature and pressure conditions is important. The time of safe service is the disposable residual life, which is part of the residual life being determined.

The examples of residual life, which is the time to rupture under the temperature and stress conditions equal to operating ones, determined in short-term creep tests and compared to the residual life of the parent material after service are shown in **Figure 9**.

The determined residual life and disposal residual life for the presented examples are summarised in **Table 3**. The comparison of the residual life and disposable residual life determined for the material of two primary steam pipelines made of 14MoV6-3 steel after 200,000 h service shows significant differences. These differences probably result mainly from differences in the as-received condition of the operated material and welded joint as well as different service history. This confirms the need of an individual assessment, mainly based on the assessment of the actual condition of the component and its material.

## **4. Summary**

Based on many power units in use, the results of creep tests on the butt-welded joints after long-term service beyond the design time carried out on the parent material and butt-welded joints of the components operating under creep conditions invalidate the generally accepted principle that the residual creep strength of the material of welded joint is no less than 80% the residual creep strength of the basic material. The principle of the relationship of residual creep strength between them transferred from that applicable to the basic material and welded joint in the as-received condition is not supported by the investigations of real cases. Also, in many investigated cases, the residual life of the welded joint is significantly lower than that obtained for the parent material. Therefore, for the components and materials operated far beyond the design time, it is necessary to determine the residual life not only for the basic material of the representative test specimen of the component but also for the welded joint (**Table 3**).

*Creep Characteristics of Engineering Materials*
