**3.1 Structure assessment of the material of welded joints operated for a long time under creep conditions**

The selected example showing the state of structure of homogeneous buttwelded joints after long-term service compared to the structure of the parent material of a component after service far beyond the design service life is shown in **Figure 3** and summarised in **Table 1**.

#### **Figure 3.**

*Structure of the material of homogeneous butt-welded joint components of the main primary steam pipeline made of 14MoV6-3 steel after approximately 200,000 h service under creep conditions parent material marked PM1, PM2; weld material marked WELD heat-affected zone marked HAZ1, HAZ2.*

**Figure 3** reveals the images of microstructure and the results of hardness measurements of components of the butt-welded joint in the main primary steam pipeline, whereas the description, state of microstructure of the parent material and its exhaustion degree as well as the class of internal damages of the heat-affected zones and weld for the selected example of welded joint are summarised in **Table 1**. The image of microstructure of the welded joint components after service, exceeding


#### **Table 1.**

*State of microstructure, degree of exhaustion and hardness of the material of homogeneous butt joint components of test critical pressure components of power units after long-term service under creep conditions beyond the design service time, based on examples.*


**39**

**Figure 5.**

**Figure 4.**

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

suitability for further service.

contained in PN-75/H-84024.

*The comparison of test results for yield point at room (Re) and elevated (Re*

*under creep conditions in relation to the as-received condition of the material.*

twice the value of the design service time, most often corresponds to the exhaustion degree which allows the material to be used in further service. It mainly results from significant difference between the design thickness and the actual thickness of the assessed components (go < gact), which causes that the actual operating stress is lower than the design one (σact < σ), the as-received condition of the test materials and the correct previous service. Also, the frequent small differences in hardness level in the individual areas of the joints are a positive phenomenon affecting the

**Table 2** presents an analysis of the chemical composition of the tested sections. The mechanical properties of the material obtained from pipeline sections were shown in graphical form in **Figures 4–8**. In order to assess the properties of the tested steels and their welded joints, reference was made to the requirements

*material and butt-welded joint of the primary steam pipeline made of 14MoV6-3 steel after 240,000 h service* 

*The comparison of test results for A5t elongation at room and elevated temperature of the shell material and* 

*butt-welded joint of the primary steam pipeline made of 14MoV6-3 steel after 240,000 h service.*

*t*

*) temperature of the parent* 

#### **Table 2.** *Analysis of chemical composition.*

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

*Creep Characteristics of Engineering Materials*

**Component Grade of material Service time**

Primary steam pipeline 14MoV6-3 (13HMF) 200,000 h

**Figure 3** reveals the images of microstructure and the results of hardness measurements of components of the butt-welded joint in the main primary steam pipeline, whereas the description, state of microstructure of the parent material and its exhaustion degree as well as the class of internal damages of the heat-affected zones and weld for the selected example of welded joint are summarised in **Table 1**. The image of microstructure of the welded joint components after service, exceeding

**Material condition—exhaustion degree**

Ferritic-bainitic structure. Partially coagulated bainitic areas. Few precipitates of various sizes at the ferrite grain boundaries. Precipitates inside ferrite grains, mostly very fine, distributed evenly No discontinuities and micro-cracks are observed in the structure Bainitic areas: class I, precipitates: class a Damaging processes: class O **Class 1/2, exhaustion degree: approximately 0.3**

Bainite with a small amount of ferrite. Significant number of fine precipitates in the bainitic areas and ferrite No discontinuities and micro-cracks are found in the structure

Bainitic areas of various sizes with a small number of ferrite. Bainitic areas with precipitates of various sizes No discontinuities and micro-cracks found in the structure

Mixture of bainite and ferrite. Fine precipitates: in the bainite areas—distributed unevenly, in ferrite distributed evenly. No discontinuities and micro-cracks found in the structure

**C Mn Si P S Cr Mo V Ni**

0.18 0.70 0.35 0.040 0.040 0.60 0.65 0.35 0.30

0.16 0.61 0.33 0.020 0.020 0.48 0.51 0.31 0.08

0.14 0.51 0.29 0.018 0.019 0.52 0.51 0.28 0.09

**Hardness HV10**

156

153

198

211

181

**Material used in testing Description of microstructure**

**Test area**

Parent material **marked PM1**

Parent material **marked PM2**

Heat-affected zone **marked HAZ1**

> Weld **marked WELD**

Heat-affected zone **marked HAZ2**

*beyond the design service time, based on examples.*

*State of microstructure, degree of exhaustion and hardness of the material of homogeneous butt joint components of test critical pressure components of power units after long-term service under creep conditions* 

PN-75/H-84024 0.10 0.40 0.15 Max Max 0.30 0.50 0.22 Max

**Standard Chemical composition, %**

**38**

**Table 2.**

**Table 1.**

Cr-Mo-V (13HMF)

Examined element MR1

Examined element MR 2

*Analysis of chemical composition.*

twice the value of the design service time, most often corresponds to the exhaustion degree which allows the material to be used in further service. It mainly results from significant difference between the design thickness and the actual thickness of the assessed components (go < gact), which causes that the actual operating stress is lower than the design one (σact < σ), the as-received condition of the test materials and the correct previous service. Also, the frequent small differences in hardness level in the individual areas of the joints are a positive phenomenon affecting the suitability for further service.

**Table 2** presents an analysis of the chemical composition of the tested sections. The mechanical properties of the material obtained from pipeline sections were shown in graphical form in **Figures 4–8**. In order to assess the properties of the tested steels and their welded joints, reference was made to the requirements contained in PN-75/H-84024.

#### **Figure 4.**

*The comparison of test results for yield point at room (Re) and elevated (Re t ) temperature of the parent material and butt-welded joint of the primary steam pipeline made of 14MoV6-3 steel after 240,000 h service under creep conditions in relation to the as-received condition of the material.*

### **Figure 5.**

*The comparison of test results for A5t elongation at room and elevated temperature of the shell material and butt-welded joint of the primary steam pipeline made of 14MoV6-3 steel after 240,000 h service.*

#### **Figure 6.**

*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.*

#### **Figure 7.**

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