**3.2 Assessment of mechanical properties of the material of welded joints operated for a long time under creep conditions**

Another element of the materials characteristics is the mechanical properties of welded joints after service and their level. Most often, the level of tensile strength and yield point at room temperature and of tensile strength at elevated temperature of such welded joints after service, for a time much longer than the design time, corresponds to the required minimum values for the parent material in the asreceived condition, or their actual values are not much lower. The example of such characteristics is shown in **Figure 4**. The level of these properties is very important for the determination of the rate of start-ups and shutdowns, which is connected with the level of instantaneous internal stresses resulting from the temperature gradient, particularly of thick-walled components. It is also important to know the formability of the material of welded joints, which can be reflected by the

**41**

**Figure 8.**

*zone and (c) weld.*

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

elongation in tensile test, its impact energy and brittle fracture appearance transition temperature both in the weld and the heat-affected zones. The example of elongation test results depending on the test temperature of welded joints compared to the parent material after long-term service far beyond the design service time is shown in **Figure 5**, whereas **Figure 8** presents the comparison of mechanical properties of the parent material and butt-welded joint with regard to the requirements

*The relationship between the impact energy measured on V-notch test specimens and the test temperature of butt-welded joint components of steam pipeline made of 14MoV6-3 steel after 200,000 h service and the brittle fracture appearance transition temperature determined for (a) parent material, (b) heat-affected* 

The test results for impact energy measured on V-notch test specimens depending on the test temperature of parent material and weld of homogeneous butt-welded joint of the pipeline components after long-term service under creep conditions are compared in **Figure 7**, whereas **Figure 8** shows the relationship between the impact energy measured on V-notch test specimens and the test temperature of the parent material, heat-affected zone and weld of the butt-welded joint of the main steam pipeline made of 14MoV6-3 steel after 20,000 h service as well as brittle fracture appearance transition temperatures determined for the materials of these areas. This example illustrates how diversified the level of brittle fracture appearance transition temperature of the material of individual welded

joint components after long-term service under creep conditions can be.

for the parent material in the as-received condition.

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

*Creep Characteristics of Engineering Materials*

**3.2 Assessment of mechanical properties of the material of welded joints operated** 

*The comparison of test results for impact energy measured on V-notch test specimens depending on test temperature of parent material and weld of homogeneous butt-welded joint of pipeline components after long-*

*term service under creep conditions: (a) made of 14MoV6-3 steel, (b) made of 10CrMo9-10 steel.*

*The comparison of test results for mechanical properties of the parent material and homogeneous butt-welded joint of pipeline components made of 14MoV6-3 after long-term service under creep conditions for 200,000 h.*

Another element of the materials characteristics is the mechanical properties of welded joints after service and their level. Most often, the level of tensile strength and yield point at room temperature and of tensile strength at elevated temperature of such welded joints after service, for a time much longer than the design time, corresponds to the required minimum values for the parent material in the asreceived condition, or their actual values are not much lower. The example of such characteristics is shown in **Figure 4**. The level of these properties is very important for the determination of the rate of start-ups and shutdowns, which is connected with the level of instantaneous internal stresses resulting from the temperature gradient, particularly of thick-walled components. It is also important to know the formability of the material of welded joints, which can be reflected by the

**for a long time under creep conditions**

**40**

**Figure 6.**

**Figure 7.**

#### **Figure 8.**

*The relationship between the impact energy measured on V-notch test specimens and the test temperature of butt-welded joint components of steam pipeline made of 14MoV6-3 steel after 200,000 h service and the brittle fracture appearance transition temperature determined for (a) parent material, (b) heat-affected zone and (c) weld.*

elongation in tensile test, its impact energy and brittle fracture appearance transition temperature both in the weld and the heat-affected zones. The example of elongation test results depending on the test temperature of welded joints compared to the parent material after long-term service far beyond the design service time is shown in **Figure 5**, whereas **Figure 8** presents the comparison of mechanical properties of the parent material and butt-welded joint with regard to the requirements for the parent material in the as-received condition.

The test results for impact energy measured on V-notch test specimens depending on the test temperature of parent material and weld of homogeneous butt-welded joint of the pipeline components after long-term service under creep conditions are compared in **Figure 7**, whereas **Figure 8** shows the relationship between the impact energy measured on V-notch test specimens and the test temperature of the parent material, heat-affected zone and weld of the butt-welded joint of the main steam pipeline made of 14MoV6-3 steel after 20,000 h service as well as brittle fracture appearance transition temperatures determined for the materials of these areas. This example illustrates how diversified the level of brittle fracture appearance transition temperature of the material of individual welded joint components after long-term service under creep conditions can be.

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

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