# An Experimental Study of Thermal Fatigue on ASTM A 213 Grade T-23 Steel Tube

Authors: G. R. Jinu; P. Sathiya; G. Ravichandran; A. Rathinam
Journal: SRX Engineering
 (2009)

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## Abstract

Super heater tubes are subjected to alternate heating and cooling
in power plants causes crack and eventually fail. This phenomenon
is termed as thermal fatigue. In this paper, a laboratory
simulation for reproducing thermal fatigue phenomenon is developed
to determine the number of cycles of failure occurs in super
heater tubes. Thermal fatigue tests are conducted in
Non-Destructive Tested T23 base and SMAW welded tubes separately.
The tubes are subjected to thermal cycles from800∘C (accelerated temperature) to room temperature
(28∘C). In this work 800∘C is selected in order to achieve the crack much
earlier. The selected temperature is just below the Ac1 temperature. The tubes are subjected to heat by
Oxy-acetylene flame and subsequently quenched with water. The
tests are carried out until open cracks are identified. Surface
cracks are identified in the base and weld tubes after 120 and 80
cycles respectively. The tubes are then sectioned and subjected to
optical microscopy. The causes of failures are thoroughly
investigated using Scanning Electron Microscope (SEM). This study
reveals that localised heating and cooling causes thermal fatigue
which initiates cracks in the tubes.


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## Body

## 1. Introduction

T23 tubes are widely used in pressure part applications in boiler fired by fossil fuels and also in heat recovery steam generators. The creep-rupture properties of grades T22 and T23 have been compared for the temperature range of 500–650°C and the results show that grade T23 has a temperature advantage of approximately 50°C over grade T22. Thermal fatigue is a form of failure that occurs in components subject to alternate heating and cooling. Under thermal fatigue, crack can initiate, propagate, and eventually failure occurs. The reason for cracking is due to the temperature change in the material that induces thermal expansion (or contraction). If surrounding material or external constraints hinder this expansion, thermal stresses arise. These cyclic thermal stresses cause fatigue similar to that of mechanical stresses. This type of failures usually occurs in power industries where turbulent mixing of fluids causes quick thermal transients in boiler heat exchanger tubes as shown in Figure1 [1].Figure 1
Sample piece of economizer inlet header showing crack in header hole and tube connector [1].Headers are an integral part of boilers. High-temperature steam-carrying headers are of significant concern in boilers because they have a finite creep life. The high-temperature headers include the superheater and reheater outlets, which operate at temperatures in excess of 600°C. These headers experience the effects of creep under normal operating conditions. In addition to the material degradation resulting from creep, high-temperature headers can also experience thermal and mechanical fatigue. Creep stresses combined with thermal fatigue stresses can lead to a failure much earlier than creep acting alone [3]. In addition to the effects of temperature variations, the external stresses associated with header expansion and piping loads must be included. In a boiler unit, header expansion may result in fatigue cracks at support attachments, moment restraints, superheater and reheater tubes, and header tubes to stub welds [3]. Failures are reported in critical boiler header components subject to thermal fatigue load applied in pressure vessels [4, 5]. From the above literatures, it is clearly identified that failures may take place in superheater tubes due to thermal fatigue. Hence, it is important to study the effect of thermal fatigue on superheater tubes.Virkkunen [2] conducted experiments on thermal fatigue of austenitic and duplex stainless steels subject to cyclic thermal transients in the temperature range 20°C to 600°C. They conducted experiments by heating the specimens rapidly using high-frequency induction coils and cooled by water spray. The specimens were rotated to achieve uniform heating and cooling. After 500 cycles, microcracks were formed in the sample and soon cracks formed a mosaic-like network which is typical for thermal fatigue as shown in Figures 2 and 3, respectively. The obtained thermal fatigue data was compared with the ASME pressure vessel code (1995) and design curve gives a safe design life for thermal fatigue loading. Similar type of mosaic-like network is obtained in this work also, which indicates the crack developed is due to the thermal fatigue process and not by any other means.Figure 2
Microcracks [2].Figure 3
Mosaic-like network which is a typical thermal fatigue crack [2].Kerezsi et al. [9] developed an experimental setup to conduct thermal fatigue using furnace and quenching rig on a plain carbon steel plate. They studied complex nonlinear stress distributions and environmental effects occurring during thermal shocks and they compared the results with ASME Boiler and pressure vessel code, Section  VIII, Division 2 (1998), fatigue data curve as a guide line. The result shows good agreement with the design curve.Y. H. Choi and S. Y. Choi [10] have evaluated the integrity of socket weld in nuclear piping under the fatigue loading and deflection due to vibration on 304 SS and 316 SS material. The pressure transient ranging from P = 0 to 15.51 MPa and thermal transient ranging from T = 25°C to 288°C were applied. The stress distribution near a socket weld was studied using finite-element method and they compared the results with ASME code, Section III, (2001). Usman and Khan [6] analyzed the failure of T11 heat exchanger tubes used in ammonia plant that was subjected to thermal fatigue from 850°C to room temperature (25°C). They selected 850°C because these superheater tubes are exposed to the process gas temperature of 960°C. They conducted experiments to confirm the site failure results. After 100 thermal cycles, the cross-section of the tube was examined. The microfissures observed in the samples had a close resemblance with cracks observed in the received failed tubes are shown in Figures 4(a) and 4(b).Microcracks obtained in the failed sample.
(a)
Visual examination of microcracks  [6](b)
Microcracks observed using optical microscopy [6]Similar type of failure has occurred in this work, which indicates the crack developed is due to the thermal fatigue. Shiblia and Starr [11] discussed the new high-strength martensitic steel T91 issues and compared some of the perceived benefits with the actual plant experience under creep-thermal fatigue conditions. They suggested that for thin section components, the use of the T91 or other 9Cr martensitic steel tubing is not recommended due to their high level of steam side oxidation and other related oxide characteristics. Brown [12] developed a hybrid method using finite-element method to analyse the creep-fatigue life of boiler components. The developed method provides sufficient conservatism while still maintaining economically designed components for combined cycle gas-fired stations subject to rapid start up and cooling transients. The work also suggests the use of the finite-element method for thermal, stress-analysis aids in the correlation between operational events, and damage mechanisms. King and Riley [7] discussed about their recent experience in the condition assessment of boiler header components used in fossil-fired power plants. The work mentions many hurdles that may happen in boilers including internal borehole and ligament cracking and external tube connector weld cracking during operation as shown in Figures 5 and 6.Figure 5
Closeup view of inside surface header sample piece showing cracking [7].Figure 6
Inside surface of economizer inlet of header showing circumferential ligament field and borehole cracking [7].They concluded that the failure of headers aredue to the material degradation resulting from creep, high-temperature headers which also experiences thermal and mechanical fatigue. This literature shows that thermal fatigue is one of the causes for the failure of headers and superheater tubes.Paterson and Wilson [13] presented a number of practical examples where component life monitoring has been implemented on power plants particularly in high-temperature boiler headers and turbines. Headers are subject to the damage mechanisms ofcreep due to onload temperature and pressure. Fatigue failures are due to start up and shutdown where thermal transient occurs which is similar to this present work. They developed a damage-monitoring system to give substantial benefits to operators in terms of damage reduction and quantitative life assessment. Tokiyoshi et al. [14] has carried out thermal fatigue test on perforated plate. The plate was examined under the high pressure and high thermal stresses during plant operations for prediction of creep fatigue life, finite-element method (FEM) was carried out and compared with experimental results.Kerezsi et al. [15] developed a test method for analyzing the initiation and growth of cracks in pressure vessels and piping equipment. This method simulates the repeated thermal shock conditions produced in operating thermal power station equipment. The work suggests that the primary stress has little or no effect on crack initiation lifetime during repeated thermal shock below the creep range and the environmental interaction is highly influential in the growth of thermal shock cracks. The work also compares the experimental results with prediction methods from the ASME Bolier and pressure vessel code, Section  VIII, Division 2 (1998). Smith et al. [8] had simulated a laboratory procedure using induction coil and quenching rig for reproducing high-temperature cracks found in coal-fired boiler tubes as shown in Figure 7. They proposed that the crack initiation is by an intergranular surface corrosion/thermal stress interaction mechanism and environmentally assisted thermal fatigue crack propagation as shown in Figure 8.Figure 7
Experimental set up employed by Smith et al. [8].Figure 8
Microcracks obtained by Smith et al. [8].Underwood and Banerji [16] have analyzed the failure caused in a 101-C ammonia plant heat exchanger tubes. They suggested that the failures are due to the formation and growth of internal oxide scales or deposits at areas of high heat flux. They presented a case history involving thermal fatigue type of failure. They analyzed the failures using finite-element modeling.From the above literatures, it is clearly found that thermal fatigue is an important phenomenon for the development of cracks in pressure vessel components especially in super-heater tubes. Many works were already done in studying the influence of thermal fatigue on failure of boiler headers using FEM. However, simulating the thermal fatigue behaviour using laboratory setup is available with some constraints (dimensions and other conditions).The objective of this paper is to study the effect of thermal fatigue on base, welded tubes and to determine the numbers of cycles of failure. In this work, a laboratory simulation reproducing thermal fatigue phenomenon is developed and thermal fatigue tests are conducted in both base and weld tubes separately. The tubes are subject to thermal cycles from 800°C to room temperature. The tests are carried out until open cracks are identified. The cause of the failure is thoroughly investigated in this paper.


## 2. Experimental Work

### 2.1. Specimen Preparation

Two attested ASTM A 213 grade T-23 tubes of 300 mm length, 45 mm outer diameter, and 10 mm thickness are taken for this experimental study. One of the tube is taken as base tube as shown in Figure9.Figure 9
T-23 base tube before subject to thermal fatigue.Another tube is sectioned into two equal halves of 150 mm length and then edges are prepared for butt-welding. The joint designs are selected as a single “V” joint. The edge preparation angle is 30° and the root face is 1 mm as shown in Figure10. The joint fit up is tacked with MMAW as shown in Figure 11.Figure 10
Specimen after edge preparation.Figure 11
Specimen after tack welding.The tack-welded tube is preheated to 220°C by GTAW preheating method. The root is GTA welded with TGS 2 CW filler wire with 85-ampere current and voltage of 17 V is used in this study. The first and the second fill passes are welded with stick electrodes of 2.5 mm and 3.2 mm in diameter with 90–115 amperes, while the remaining three passes are welded by SMAW process with thermanit 23 electrodes of 125–135 amperes current. The necessary preheating is done at 220°C for 57 seconds. After the completion of the welding, radiographic examination is done to ensure the quality of welds and the results are found to be acceptable range as per the ASME pressure vessel code requirements. The welded tube is shown in Figure12.Figure 12
Weld specimen before subjected to thermal fatigue.The chemical composition and mechanical properties of the T23 tube, T23 filler wire and electrode are given in Tables1 and 2, respectively.Table 1
Chemical composition of ASTM A 213 GRADE T-23 material and T23 filler wire and electrode.
MaterialsElements (wt%)CMnPSSiCrMoVWNbBNAlNiASTM A 213 GRADE T-230.04–0.100.10–0.600.030.010.501.92–2.600.05–0.300.20–0.301.45–1.750.02–0.08.0005–.00600.030.03—GTAW filler wire0.080.54——0.272.140.080.211.580.031.002.011—0.04Table 2
Mechanical properties of ASTM A 213 GRADE T-23 material and T23 filler wire and electrode.
MaterialsUltimate tensilestrength, UTS MPa0.2%Yield strength, YS MPa% of ElongationASTM A 213 GRADE T-2351040020GTAW filler wire62052020.2SMAW Electrode55342125The microstructures of the base tube before subject to thermal fatigue is shown in Figure13. The microstructures of the welded tube before subject to thermal fatigue is shown in Figures 14(a) and 14(b).Figure 13
Microstructure of base before subject to thermal fatigue.Microstructure of base (a) and weld (b) regions in the weld tube before subject to thermal fatigue.
(a)(b)From Figure13, T23 base tube has a bainitic-martensitic structure. It is a result of precise heat treatment operation after heating the metal to above Ac3 temperature. Ac1 is located between 800°C and 820°C, and Ac3 temperature is between 960°C and 980°C. The cooling rate for T23 is 200 K/s. After the bainitic-martensite structure is formed it is normalized at 1060°C for the dissociation of most of the precipitates. Tempering at 760°C allows the growth of precipitates like chromium carbide, vanadium and niobium carbonitrides, and so forth. This process helps to improve creep resistance at high-temperatures. The grains are little coarse and irregular in shape. The high strength of this alloy comes from the bainitic structure. It is a mixture of bainite and martensite with little delta ferrite to give the required ductility. The carbides that are formed in the grain boundaries that will give the necessary strength and the bainite give the hardness. The grain boundaries are clearly visible but the grain growth is irregular producing an irregular structure.The microstructures of weld tube before subject to thermal fatigue are shown in Figures14(a) and 14(b).The difference in the grain size can be seen in the above microstructure. The dark-colored patch indicates the bainite-martensite structure. The white areas indicate the delta ferrite. When compared to the base the weld contains more delta ferrite structure. From Figures14(a) and 14(b) it is observed the grains are very fine in the base material Figure 14(a) and the weld Figure 14(b) shows coarse grains.


### 2.2. Experimental Setup

An apparatus has been purposed by built to conduct thermal fatigue tests. The test set-up employed in this study consists of the following main physical components:(1)
the oxy-acetylene heating set up,(2)
the specimen mounting frame,(3)
the specimen holding system,(4)
quenching rig,(5)
laser sensor temperature measuring system.The specimen is locally heated up by Oxy-acetylene flame from the outer surface of the tube. The peak temperature can be suddenly attained by this process. The experimental setup is shown in Figure15.Figure 15
Experimental setup.


#### 2.2.1. Oxy-acetylene Heating Setup

The Oxy-acetylene heating set up is used as a heating source. The oxygen and acetylene pressure is regulated to 0.5 and 1 bar, respectively. As shown in Figure16, the oxy-acetylene flame is moved in clockwise direction from point “A” and ends at the same point. The torch is continuously shown in the heating surface until 800°C is attained. Time taken to attain 800°C is 5 minutes.Figure 16
Specimen mounting frame with heating.


#### 2.2.2. Specimen Mounting Frame

As shown in Figure16, two ends of the tube are welded with a thick ridge plate and then the two plates are welded with a bottom plateso that the linear expansion of the tube during heating is arrested. A steel rod is welded on the top surface of the plate for holding the specimen mounting frame to immerse it in the quenching rig.


#### 2.2.3. Specimen Holding System

The specimen holding system consists of leverage to handle the hot specimen for immersing it in the quenching rig as shown in Figure17.Figure 17
Specimen holding system and quenching rig.


#### 2.2.4. Quenching Rig

The quenching rig consists of a tank filled with water. After the completion of the heating cycle, the specimen is immersed in the quenching rig with the help of holding system as shown in Figure17. The temperature of the specimen is suddenly decreased to room temperature after 5 minutes.


#### 2.2.5. Temperature Measuring Laser Sensor

The temperature is measured using laser sensor at the end of every cycle to check whether 800°C is attained or not.


### 2.3. Experimental Procedure

The specimen is prepared as discussed above and heat is applied from the outer surface of tube, which are the actual conditions of a heat exchanger tube exposed to the thermal fatigue on pressure vessels. The flame is shown in the heating zone manually, starting from one point of the heating zone and ends in the same point so that heat will be locally distributed from the outer surface tube to the inner surface as shown in Figure16. After heating, the temperature of the specimen is measured using laser thermometer to check whether the attained temperature reaches 800°C. The time taken to attain 800°C is 5 minutes. The flame is then turned off. Then the hot specimen is lifted with the help of the specimen-holding system and immersed into the quenching rig. The specimen is taken out from the quenching rig when the temperature of the specimen reaches the room temperature. All of the above processes constitute one thermal fatigue cycle. For one thermal cycle, 10 (5 + 5) minutes are taken which include both the heating and the cooling cycle. The experiments are conducted until open cracks are identified. The specimen is then removed from the specimen-mounting frame, NDT test is carried out, and the specimen is then sectioned for metallographic study.


## 2.1. Specimen Preparation

Two attested ASTM A 213 grade T-23 tubes of 300 mm length, 45 mm outer diameter, and 10 mm thickness are taken for this experimental study. One of the tube is taken as base tube as shown in Figure9.Figure 9
T-23 base tube before subject to thermal fatigue.Another tube is sectioned into two equal halves of 150 mm length and then edges are prepared for butt-welding. The joint designs are selected as a single “V” joint. The edge preparation angle is 30° and the root face is 1 mm as shown in Figure10. The joint fit up is tacked with MMAW as shown in Figure 11.Figure 10
Specimen after edge preparation.Figure 11
Specimen after tack welding.The tack-welded tube is preheated to 220°C by GTAW preheating method. The root is GTA welded with TGS 2 CW filler wire with 85-ampere current and voltage of 17 V is used in this study. The first and the second fill passes are welded with stick electrodes of 2.5 mm and 3.2 mm in diameter with 90–115 amperes, while the remaining three passes are welded by SMAW process with thermanit 23 electrodes of 125–135 amperes current. The necessary preheating is done at 220°C for 57 seconds. After the completion of the welding, radiographic examination is done to ensure the quality of welds and the results are found to be acceptable range as per the ASME pressure vessel code requirements. The welded tube is shown in Figure12.Figure 12
Weld specimen before subjected to thermal fatigue.The chemical composition and mechanical properties of the T23 tube, T23 filler wire and electrode are given in Tables1 and 2, respectively.Table 1
Chemical composition of ASTM A 213 GRADE T-23 material and T23 filler wire and electrode.
MaterialsElements (wt%)CMnPSSiCrMoVWNbBNAlNiASTM A 213 GRADE T-230.04–0.100.10–0.600.030.010.501.92–2.600.05–0.300.20–0.301.45–1.750.02–0.08.0005–.00600.030.03—GTAW filler wire0.080.54——0.272.140.080.211.580.031.002.011—0.04Table 2
Mechanical properties of ASTM A 213 GRADE T-23 material and T23 filler wire and electrode.
MaterialsUltimate tensilestrength, UTS MPa0.2%Yield strength, YS MPa% of ElongationASTM A 213 GRADE T-2351040020GTAW filler wire62052020.2SMAW Electrode55342125The microstructures of the base tube before subject to thermal fatigue is shown in Figure13. The microstructures of the welded tube before subject to thermal fatigue is shown in Figures 14(a) and 14(b).Figure 13
Microstructure of base before subject to thermal fatigue.Microstructure of base (a) and weld (b) regions in the weld tube before subject to thermal fatigue.
(a)(b)From Figure13, T23 base tube has a bainitic-martensitic structure. It is a result of precise heat treatment operation after heating the metal to above Ac3 temperature. Ac1 is located between 800°C and 820°C, and Ac3 temperature is between 960°C and 980°C. The cooling rate for T23 is 200 K/s. After the bainitic-martensite structure is formed it is normalized at 1060°C for the dissociation of most of the precipitates. Tempering at 760°C allows the growth of precipitates like chromium carbide, vanadium and niobium carbonitrides, and so forth. This process helps to improve creep resistance at high-temperatures. The grains are little coarse and irregular in shape. The high strength of this alloy comes from the bainitic structure. It is a mixture of bainite and martensite with little delta ferrite to give the required ductility. The carbides that are formed in the grain boundaries that will give the necessary strength and the bainite give the hardness. The grain boundaries are clearly visible but the grain growth is irregular producing an irregular structure.The microstructures of weld tube before subject to thermal fatigue are shown in Figures14(a) and 14(b).The difference in the grain size can be seen in the above microstructure. The dark-colored patch indicates the bainite-martensite structure. The white areas indicate the delta ferrite. When compared to the base the weld contains more delta ferrite structure. From Figures14(a) and 14(b) it is observed the grains are very fine in the base material Figure 14(a) and the weld Figure 14(b) shows coarse grains.


## 2.2. Experimental Setup

An apparatus has been purposed by built to conduct thermal fatigue tests. The test set-up employed in this study consists of the following main physical components:(1)
the oxy-acetylene heating set up,(2)
the specimen mounting frame,(3)
the specimen holding system,(4)
quenching rig,(5)
laser sensor temperature measuring system.The specimen is locally heated up by Oxy-acetylene flame from the outer surface of the tube. The peak temperature can be suddenly attained by this process. The experimental setup is shown in Figure15.Figure 15
Experimental setup.


### 2.2.1. Oxy-acetylene Heating Setup

The Oxy-acetylene heating set up is used as a heating source. The oxygen and acetylene pressure is regulated to 0.5 and 1 bar, respectively. As shown in Figure16, the oxy-acetylene flame is moved in clockwise direction from point “A” and ends at the same point. The torch is continuously shown in the heating surface until 800°C is attained. Time taken to attain 800°C is 5 minutes.Figure 16
Specimen mounting frame with heating.


### 2.2.2. Specimen Mounting Frame

As shown in Figure16, two ends of the tube are welded with a thick ridge plate and then the two plates are welded with a bottom plateso that the linear expansion of the tube during heating is arrested. A steel rod is welded on the top surface of the plate for holding the specimen mounting frame to immerse it in the quenching rig.


### 2.2.3. Specimen Holding System

The specimen holding system consists of leverage to handle the hot specimen for immersing it in the quenching rig as shown in Figure17.Figure 17
Specimen holding system and quenching rig.


### 2.2.4. Quenching Rig

The quenching rig consists of a tank filled with water. After the completion of the heating cycle, the specimen is immersed in the quenching rig with the help of holding system as shown in Figure17. The temperature of the specimen is suddenly decreased to room temperature after 5 minutes.


### 2.2.5. Temperature Measuring Laser Sensor

The temperature is measured using laser sensor at the end of every cycle to check whether 800°C is attained or not.


## 2.2.1. Oxy-acetylene Heating Setup

The Oxy-acetylene heating set up is used as a heating source. The oxygen and acetylene pressure is regulated to 0.5 and 1 bar, respectively. As shown in Figure16, the oxy-acetylene flame is moved in clockwise direction from point “A” and ends at the same point. The torch is continuously shown in the heating surface until 800°C is attained. Time taken to attain 800°C is 5 minutes.Figure 16
Specimen mounting frame with heating.


## 2.2.2. Specimen Mounting Frame

As shown in Figure16, two ends of the tube are welded with a thick ridge plate and then the two plates are welded with a bottom plateso that the linear expansion of the tube during heating is arrested. A steel rod is welded on the top surface of the plate for holding the specimen mounting frame to immerse it in the quenching rig.


## 2.2.3. Specimen Holding System

The specimen holding system consists of leverage to handle the hot specimen for immersing it in the quenching rig as shown in Figure17.Figure 17
Specimen holding system and quenching rig.


## 2.2.4. Quenching Rig

The quenching rig consists of a tank filled with water. After the completion of the heating cycle, the specimen is immersed in the quenching rig with the help of holding system as shown in Figure17. The temperature of the specimen is suddenly decreased to room temperature after 5 minutes.


## 2.2.5. Temperature Measuring Laser Sensor

The temperature is measured using laser sensor at the end of every cycle to check whether 800°C is attained or not.


## 2.3. Experimental Procedure

The specimen is prepared as discussed above and heat is applied from the outer surface of tube, which are the actual conditions of a heat exchanger tube exposed to the thermal fatigue on pressure vessels. The flame is shown in the heating zone manually, starting from one point of the heating zone and ends in the same point so that heat will be locally distributed from the outer surface tube to the inner surface as shown in Figure16. After heating, the temperature of the specimen is measured using laser thermometer to check whether the attained temperature reaches 800°C. The time taken to attain 800°C is 5 minutes. The flame is then turned off. Then the hot specimen is lifted with the help of the specimen-holding system and immersed into the quenching rig. The specimen is taken out from the quenching rig when the temperature of the specimen reaches the room temperature. All of the above processes constitute one thermal fatigue cycle. For one thermal cycle, 10 (5 + 5) minutes are taken which include both the heating and the cooling cycle. The experiments are conducted until open cracks are identified. The specimen is then removed from the specimen-mounting frame, NDT test is carried out, and the specimen is then sectioned for metallographic study.


## 3. Results and Discussion

The experiments are carried out until open cracks were identified. Surface cracks are identified in the base and weld tubes after 120 and 80 cycles, respectively.


### 3.1. Visual Inspection in Failed Tubes

The failed base and weld tubes are visually inspected. The developed crack in the tubes are measured and presented in what follows.


#### 3.1.1. Visual Inspection in Failed Base Tube

The macrograph of the failed base tube is shown in Figure18. From Figure 18, it is observed that many surface cracks are obtained in the base tube subjected to thermal fatigue after 120 cycles. A 20 mm long longitudinal crack is visually identified in the failed base tube at the heated zone. The tube is bulged in the heating zone due to localised heating. The failed tube is then subjected to liquid penetrant test to identify the small surface cracks.Figure 18
Visual inspection of failed base specimen after subjected to thermal fatigue.


#### 3.1.2. Visual Inspection in Failed Weld Tube

The macrograph of the failed weld tube is shown in Figures19(a) and 19(b). From Figures 19(a) and 19(b), it is observed that many surface cracks are present in the base tube subjected to thermal fatigue after 80 cycles.Visual inspection of failed weld specimen after subjected to thermal fatigue.
(a)(b)Transverse cracks of 80 mm long are visually identified in the failed weld tube at the heated zone.


### 3.2. Liquid Penetrant Test

To identify the small surface cracks on the failed tubes, the base and weld tubes are subjected to liquid penetrant test.


#### 3.2.1. Liquid Penetrant Test on Failed Base Tube

The macrograph of  the failed base specimen subjected to liquid penetrant test  is shown in Figure20 and it clearly shows that many  longitudinal cracks are found  in the heated zone and from the longitudinal crack, small transverse cracks are also identified, which were not seen in the visual examination.Figure 20
Liquid penetrant examination of failed base specimen.


#### 3.2.2. Liquid Penetrant Test on Failed Weld Tube

Figure21 shows the macrograph of the failed weld specimen subjected to liquid penetrant test and it clearly shows that many transverse cracks are developed in the heated zone which propagates through out the circumference of the weld and from the transverse crack small longitudinal cracks are also seen which were not seen in the visual examination.Figure 21
Liquid penetrant examination of failed weld specimen.The failed tubes are then sectioned and subjected to metallographic study.


### 3.3. Specimen Preparation for Metallographic Study

The sectioned view of the failed base and weld tube is shown in Figures22(a) and 22(b). The failed tubes are sectioned in two halves in longitudinal direction and again one half of the tube is divided into two sections. The projected portion from the two sides of the heating zone in the tube is removed further so that only the heat-affected zone is taken for metallographic study. Specimens for metallographic examinations are prepared by polishing successively in 220, 320, 400, 600, 800 emery grits, followed by a cloth disc polishing. The specimens are etched with 2% Nital solution. The specimens are then subjected to microscopic and SEM study and the results are discussed.Sectioned view of the failed base and weld tubes.
(a)(b)


### 3.4. MacroStudy

Before examining in the microanalysis the polished samples are examined to macroscopically study. The macrophotographs are taken in the outer surface of the failed tubes to get the closer view possible of the cracks obtained in the surface while the microstructure are taken in the cross-section of the failed tube to study the depth of penetration of the crack.


#### 3.4.1. Macrostudy on the Failed Base Tube

Figure23(a) shows the magnified view of the surface crack obtained in the failed base tube subjected to thermal fatigue and it is clearly identified that many longitudinal cracks are found in the heated zone, and from the longitudinal crack small transverse cracks are also developed. Figure 23(b) shows the magnified view of the single longitudinal crack obtained in the failed base tube.Surface crack obtained in the failed base tube.
(a)(b)


#### 3.4.2. Macrostudy on the Failed Weld Tube

Figure24(a) shows the magnified view of the surface crack obtained in the failed weld tube subjected to thermal fatigue and it is clearly identified that many transverse cracks are found in the weld joint, and from the transverse crack small longitudinal cracks are also developed. Figure 24(b) shows the magnified view of the surface crack obtained in the failed weld tube.Surface crack obtained in the failed weld tube.
(a)(b)


### 3.5. Optical Microscopy

The samples are prepared according to the ASME standards. The polished and etched samples are examined under optical microscopy and SEM. The microstructures are taken on different locations of the tubes in the defect and defect-free surfaces.


#### 3.5.1. Study on the Failed Base Tube

The metallographic samples prepared from the failed base tube are studied thoroughly by optical microscope in both the defect and defect-free regions.(1) Study on the Defect Free Surfaces on the Failed Base Tube
The microstructures of defect-free surfaces of the failed base tube are shown in Figure25.
Figure25 shows the optical microstructure of failed base tube taken in the defect free surface. The microstructure of T23 failed base tube shows that the grains are little coarse and irregular in shape. The failed thermal fatigued base tube shows similar microstructure as observed in the base tube before being subjected to thermal fatigue. This reveals that during thermal fatigue there is no phase transformation taking place in the base tube subjected to thermal fatigue.Figure 25
Microstructure of the failed base in defect free region.(2) Study on the Defect Surfaces on the Failed Base Tube
Through the optical microscope, the defect surfaces in the T23 base tube are identified and presented in Figures26(a)–26(d). From the Figures 26(a)–26(d) it is clearly identified that large number of cracks are present in the failed base tube. The cracks have developed from the heating surface (outer surface) of the tube and propagate towards the inner surface. Cracks are observed in the failed base tube have a close resemblance with cracks observed in the failed tubes [6]. In Figure 26(a), mild decarburization to the depth of one grains is observed.  The obtained cracks in the failed base tubes show blunt tip cracks as shown in Figure 26. This crack shows typical characteristics of thermal fatigue failure.Cracks identified in the failed base tube.
(a)(b)(c)(d)


#### 3.5.2. Study on the Failed Weld Tube

The metallographic samples prepared from thefailed weld tube are studied thoroughly by optical microscope in both the defect and defect-free regions.(1) Study on the Defect Free Surfaces on the Weld Tube
The microstructures of defect-free surfaces of the failed weld tube are shown in Figures27(a) and 27(c).      The microstructures of the weld are taken on the outer surface, HAZ, and inner surface of the tube as shown in Figures 27(a) and 27(b). When compared to the grains observed in inner surface of the tube, the outer surface and HAZ shows very finer grains. This difference is due to the application of the heat from the outer surface of the tube to the inner surface. Formation of martensite is more as observed in the failedwelded tubes when compared to the base tubes.Microstructure of weld region in defect free areas of the failed weld tube.
(a)
Weld metal after thermal fatigue in outer surface.(b)
Weld metal after thermal fatigue in inner surface.(c)
Weld metal after thermal fatigue in HAZ.(2) Study on the Defect Surfaces on the Weld Tube
Through the microstructures, the defect surfaces in the weld tube is identified and presented in Figures28(a)–28(e). From Figures 28(a)–28(e), it is clearly identified that large number of cracks are present in the failed weld tube. The cracks are developed from the heating surface (outer surface) of the tube and propagate towards the inner surface. Cracks observed in the weld tube which had a close resemblance with cracks observed in the failed tubes as reported in [6]. The obtained cracks in the failed base tubes show that blend tip cracks and parallel cracks are also observed as shown in Figure 28. This crack shows typical characteristics of thermal fatigue failure.Cracks identified in the failed weld tube.
(a)(b)(c)(d)(e)


### 3.6. SEM Study

The cracks observed with the optical microscope have been observed with an SEM study to analyse the failures in detail. SEM analyses are carried out in defect surfaces of the base and weld tube.


#### 3.6.1. Defect Surface on Failed Base Tube

The cracks observed in the SEM photography of the failed base tube are shown in Figures29(a) and 29(b). From Figures 29(a) and 29(b), the cracks are developed from the outer surface of the tube and propagate towards the inner surface. Similarly it travels through or across the circumference of tube heated region. When it is compared to the cracks obtained in the failed weld tube, the width and depth of the cracks are less as shown in Figures 29(a) and 29(b). The cracks have been opened and examined as shown in Figures 30(a) and 30(b). They revealed smooth-oxidized crack surfaces. From Figure 31, Mosaic-like crack formation is observed when it is magnified. From Figures 29(a) and 29(b) it is clearly identified that the initial position of the crack width is high, when it propagates towards the inner surface, the crack growth rate increases due to the temperature difference and rate of cooling. Due to these reasons, cracks are developed.Crack propagation in the failed base tube.
(a)(b)Smooth-oxidized crack surfaces.
(a)(b)Figure 31
Mosaic-like crack formation.


#### 3.6.2. Defect Surface on Failed Weld Tube

Figure32 shows the microstructure taken in an unaffected region of the failed weld tube.  The defects present in the failed weld tube are shown in Figures 33(a) and 33(b). From Figure 33, the cracks are developed from the outer surface of the tube and propagate towards the inner surface. Similarly, it travels in the circumference of the tube-heated region. When compared to the base crack, the size of the crack obtained in the weld is large. Mosaic-like crack formation is observed when it is magnified as shown in Figures 34(a) and 34(b) which is a typical thermal fatigue crack. From Figures 33(a) and 33(b), the initial crack width is large, when it moves towards the inner surface, the crack growth rate increases due to the temperature difference and rate of cooling. Due to these reasons, cracks are developed.Figure 32
Unaffected weld tube structure.Figure 33
Crack propagation in the failed-welded tube.Smooth oxidized crack surfaces and Mosaic-like crack formation.
(a)
Smooth-oxidized crack surfaces(b)
Mosaic-like crack formationOn evaluating the microstudy and SEM results, it is clearly identified that the cracks are developed from the heating surface (outer surface) of the tube and propagate towards the inner surface, which eventually cause failure of the tube. The cracks occurring in the tube is due to long-time exposure in high service temperature. Mosaic-like crack formation is observed inside the crack at higher magnification. The cracks obtained in the tube specimen is the typical thermal fatigue crack which resembles the failures reported in tubes subjected to thermal fatigue [6].


### 3.7. Hardness

Vickers’s hardness measurements are taken on the base and welded tube, before subjected to thermal fatigue and after failure occurs. The hardness location taken in the base and welded specimen is shown in Figures35 and 36. The load applied is 10 kg and the hardness values taken in the base and welded specimen are tabulated in Tables 3 and 4.Table 3
Hardness taken in the base tube (HV, 10 kg).
PositionBaseHeating ZoneNear the CrackBefore subjected thermal fatigue258,260——After failure occurs252,254327,327339,334Table 4
Hardness taken in the weld tube.
PositionBaseWeldHAZHeating ZoneNear the CrackBefore subjected thermal fatigue264, 268274, 282296, 284——After failure occurs333,333360,348339,351360,348370,360Figure 35
Hardness locations taken in the failed base tube.Figure 36
Hardness locations taken in the failed weld tube.From Tables3 and 4, the hardness value is much higher nearer in the cracked zone. Due to the higher hardness, the initiated cracks due to the thermal fatigue can propagate towards the circumferential areas and through the thickness of the tube. The hardness value of the weld near the crack zone is nearly closer, due to repeated heating and cooling. The grains get finer near the outer surface of the tube where heat is applied which are discussed in micro structural study, hence hardness value increases when compared to the base material.


## 3.1. Visual Inspection in Failed Tubes

The failed base and weld tubes are visually inspected. The developed crack in the tubes are measured and presented in what follows.


### 3.1.1. Visual Inspection in Failed Base Tube

The macrograph of the failed base tube is shown in Figure18. From Figure 18, it is observed that many surface cracks are obtained in the base tube subjected to thermal fatigue after 120 cycles. A 20 mm long longitudinal crack is visually identified in the failed base tube at the heated zone. The tube is bulged in the heating zone due to localised heating. The failed tube is then subjected to liquid penetrant test to identify the small surface cracks.Figure 18
Visual inspection of failed base specimen after subjected to thermal fatigue.


### 3.1.2. Visual Inspection in Failed Weld Tube

The macrograph of the failed weld tube is shown in Figures19(a) and 19(b). From Figures 19(a) and 19(b), it is observed that many surface cracks are present in the base tube subjected to thermal fatigue after 80 cycles.Visual inspection of failed weld specimen after subjected to thermal fatigue.
(a)(b)Transverse cracks of 80 mm long are visually identified in the failed weld tube at the heated zone.


## 3.1.1. Visual Inspection in Failed Base Tube

The macrograph of the failed base tube is shown in Figure18. From Figure 18, it is observed that many surface cracks are obtained in the base tube subjected to thermal fatigue after 120 cycles. A 20 mm long longitudinal crack is visually identified in the failed base tube at the heated zone. The tube is bulged in the heating zone due to localised heating. The failed tube is then subjected to liquid penetrant test to identify the small surface cracks.Figure 18
Visual inspection of failed base specimen after subjected to thermal fatigue.


## 3.1.2. Visual Inspection in Failed Weld Tube

The macrograph of the failed weld tube is shown in Figures19(a) and 19(b). From Figures 19(a) and 19(b), it is observed that many surface cracks are present in the base tube subjected to thermal fatigue after 80 cycles.Visual inspection of failed weld specimen after subjected to thermal fatigue.
(a)(b)Transverse cracks of 80 mm long are visually identified in the failed weld tube at the heated zone.


## 3.2. Liquid Penetrant Test

To identify the small surface cracks on the failed tubes, the base and weld tubes are subjected to liquid penetrant test.


### 3.2.1. Liquid Penetrant Test on Failed Base Tube

The macrograph of  the failed base specimen subjected to liquid penetrant test  is shown in Figure20 and it clearly shows that many  longitudinal cracks are found  in the heated zone and from the longitudinal crack, small transverse cracks are also identified, which were not seen in the visual examination.Figure 20
Liquid penetrant examination of failed base specimen.


### 3.2.2. Liquid Penetrant Test on Failed Weld Tube

Figure21 shows the macrograph of the failed weld specimen subjected to liquid penetrant test and it clearly shows that many transverse cracks are developed in the heated zone which propagates through out the circumference of the weld and from the transverse crack small longitudinal cracks are also seen which were not seen in the visual examination.Figure 21
Liquid penetrant examination of failed weld specimen.The failed tubes are then sectioned and subjected to metallographic study.


## 3.2.1. Liquid Penetrant Test on Failed Base Tube

The macrograph of  the failed base specimen subjected to liquid penetrant test  is shown in Figure20 and it clearly shows that many  longitudinal cracks are found  in the heated zone and from the longitudinal crack, small transverse cracks are also identified, which were not seen in the visual examination.Figure 20
Liquid penetrant examination of failed base specimen.


## 3.2.2. Liquid Penetrant Test on Failed Weld Tube

Figure21 shows the macrograph of the failed weld specimen subjected to liquid penetrant test and it clearly shows that many transverse cracks are developed in the heated zone which propagates through out the circumference of the weld and from the transverse crack small longitudinal cracks are also seen which were not seen in the visual examination.Figure 21
Liquid penetrant examination of failed weld specimen.The failed tubes are then sectioned and subjected to metallographic study.


## 3.3. Specimen Preparation for Metallographic Study

The sectioned view of the failed base and weld tube is shown in Figures22(a) and 22(b). The failed tubes are sectioned in two halves in longitudinal direction and again one half of the tube is divided into two sections. The projected portion from the two sides of the heating zone in the tube is removed further so that only the heat-affected zone is taken for metallographic study. Specimens for metallographic examinations are prepared by polishing successively in 220, 320, 400, 600, 800 emery grits, followed by a cloth disc polishing. The specimens are etched with 2% Nital solution. The specimens are then subjected to microscopic and SEM study and the results are discussed.Sectioned view of the failed base and weld tubes.
(a)(b)


## 3.4. MacroStudy

Before examining in the microanalysis the polished samples are examined to macroscopically study. The macrophotographs are taken in the outer surface of the failed tubes to get the closer view possible of the cracks obtained in the surface while the microstructure are taken in the cross-section of the failed tube to study the depth of penetration of the crack.


### 3.4.1. Macrostudy on the Failed Base Tube

Figure23(a) shows the magnified view of the surface crack obtained in the failed base tube subjected to thermal fatigue and it is clearly identified that many longitudinal cracks are found in the heated zone, and from the longitudinal crack small transverse cracks are also developed. Figure 23(b) shows the magnified view of the single longitudinal crack obtained in the failed base tube.Surface crack obtained in the failed base tube.
(a)(b)


### 3.4.2. Macrostudy on the Failed Weld Tube

Figure24(a) shows the magnified view of the surface crack obtained in the failed weld tube subjected to thermal fatigue and it is clearly identified that many transverse cracks are found in the weld joint, and from the transverse crack small longitudinal cracks are also developed. Figure 24(b) shows the magnified view of the surface crack obtained in the failed weld tube.Surface crack obtained in the failed weld tube.
(a)(b)


## 3.4.1. Macrostudy on the Failed Base Tube

Figure23(a) shows the magnified view of the surface crack obtained in the failed base tube subjected to thermal fatigue and it is clearly identified that many longitudinal cracks are found in the heated zone, and from the longitudinal crack small transverse cracks are also developed. Figure 23(b) shows the magnified view of the single longitudinal crack obtained in the failed base tube.Surface crack obtained in the failed base tube.
(a)(b)


## 3.4.2. Macrostudy on the Failed Weld Tube

Figure24(a) shows the magnified view of the surface crack obtained in the failed weld tube subjected to thermal fatigue and it is clearly identified that many transverse cracks are found in the weld joint, and from the transverse crack small longitudinal cracks are also developed. Figure 24(b) shows the magnified view of the surface crack obtained in the failed weld tube.Surface crack obtained in the failed weld tube.
(a)(b)


## 3.5. Optical Microscopy

The samples are prepared according to the ASME standards. The polished and etched samples are examined under optical microscopy and SEM. The microstructures are taken on different locations of the tubes in the defect and defect-free surfaces.


### 3.5.1. Study on the Failed Base Tube

The metallographic samples prepared from the failed base tube are studied thoroughly by optical microscope in both the defect and defect-free regions.(1) Study on the Defect Free Surfaces on the Failed Base Tube
The microstructures of defect-free surfaces of the failed base tube are shown in Figure25.
Figure25 shows the optical microstructure of failed base tube taken in the defect free surface. The microstructure of T23 failed base tube shows that the grains are little coarse and irregular in shape. The failed thermal fatigued base tube shows similar microstructure as observed in the base tube before being subjected to thermal fatigue. This reveals that during thermal fatigue there is no phase transformation taking place in the base tube subjected to thermal fatigue.Figure 25
Microstructure of the failed base in defect free region.(2) Study on the Defect Surfaces on the Failed Base Tube
Through the optical microscope, the defect surfaces in the T23 base tube are identified and presented in Figures26(a)–26(d). From the Figures 26(a)–26(d) it is clearly identified that large number of cracks are present in the failed base tube. The cracks have developed from the heating surface (outer surface) of the tube and propagate towards the inner surface. Cracks are observed in the failed base tube have a close resemblance with cracks observed in the failed tubes [6]. In Figure 26(a), mild decarburization to the depth of one grains is observed.  The obtained cracks in the failed base tubes show blunt tip cracks as shown in Figure 26. This crack shows typical characteristics of thermal fatigue failure.Cracks identified in the failed base tube.
(a)(b)(c)(d)


### 3.5.2. Study on the Failed Weld Tube

The metallographic samples prepared from thefailed weld tube are studied thoroughly by optical microscope in both the defect and defect-free regions.(1) Study on the Defect Free Surfaces on the Weld Tube
The microstructures of defect-free surfaces of the failed weld tube are shown in Figures27(a) and 27(c).      The microstructures of the weld are taken on the outer surface, HAZ, and inner surface of the tube as shown in Figures 27(a) and 27(b). When compared to the grains observed in inner surface of the tube, the outer surface and HAZ shows very finer grains. This difference is due to the application of the heat from the outer surface of the tube to the inner surface. Formation of martensite is more as observed in the failedwelded tubes when compared to the base tubes.Microstructure of weld region in defect free areas of the failed weld tube.
(a)
Weld metal after thermal fatigue in outer surface.(b)
Weld metal after thermal fatigue in inner surface.(c)
Weld metal after thermal fatigue in HAZ.(2) Study on the Defect Surfaces on the Weld Tube
Through the microstructures, the defect surfaces in the weld tube is identified and presented in Figures28(a)–28(e). From Figures 28(a)–28(e), it is clearly identified that large number of cracks are present in the failed weld tube. The cracks are developed from the heating surface (outer surface) of the tube and propagate towards the inner surface. Cracks observed in the weld tube which had a close resemblance with cracks observed in the failed tubes as reported in [6]. The obtained cracks in the failed base tubes show that blend tip cracks and parallel cracks are also observed as shown in Figure 28. This crack shows typical characteristics of thermal fatigue failure.Cracks identified in the failed weld tube.
(a)(b)(c)(d)(e)


## 3.5.1. Study on the Failed Base Tube

The metallographic samples prepared from the failed base tube are studied thoroughly by optical microscope in both the defect and defect-free regions.(1) Study on the Defect Free Surfaces on the Failed Base Tube
The microstructures of defect-free surfaces of the failed base tube are shown in Figure25.
Figure25 shows the optical microstructure of failed base tube taken in the defect free surface. The microstructure of T23 failed base tube shows that the grains are little coarse and irregular in shape. The failed thermal fatigued base tube shows similar microstructure as observed in the base tube before being subjected to thermal fatigue. This reveals that during thermal fatigue there is no phase transformation taking place in the base tube subjected to thermal fatigue.Figure 25
Microstructure of the failed base in defect free region.(2) Study on the Defect Surfaces on the Failed Base Tube
Through the optical microscope, the defect surfaces in the T23 base tube are identified and presented in Figures26(a)–26(d). From the Figures 26(a)–26(d) it is clearly identified that large number of cracks are present in the failed base tube. The cracks have developed from the heating surface (outer surface) of the tube and propagate towards the inner surface. Cracks are observed in the failed base tube have a close resemblance with cracks observed in the failed tubes [6]. In Figure 26(a), mild decarburization to the depth of one grains is observed.  The obtained cracks in the failed base tubes show blunt tip cracks as shown in Figure 26. This crack shows typical characteristics of thermal fatigue failure.Cracks identified in the failed base tube.
(a)(b)(c)(d)


## 3.5.2. Study on the Failed Weld Tube

The metallographic samples prepared from thefailed weld tube are studied thoroughly by optical microscope in both the defect and defect-free regions.(1) Study on the Defect Free Surfaces on the Weld Tube
The microstructures of defect-free surfaces of the failed weld tube are shown in Figures27(a) and 27(c).      The microstructures of the weld are taken on the outer surface, HAZ, and inner surface of the tube as shown in Figures 27(a) and 27(b). When compared to the grains observed in inner surface of the tube, the outer surface and HAZ shows very finer grains. This difference is due to the application of the heat from the outer surface of the tube to the inner surface. Formation of martensite is more as observed in the failedwelded tubes when compared to the base tubes.Microstructure of weld region in defect free areas of the failed weld tube.
(a)
Weld metal after thermal fatigue in outer surface.(b)
Weld metal after thermal fatigue in inner surface.(c)
Weld metal after thermal fatigue in HAZ.(2) Study on the Defect Surfaces on the Weld Tube
Through the microstructures, the defect surfaces in the weld tube is identified and presented in Figures28(a)–28(e). From Figures 28(a)–28(e), it is clearly identified that large number of cracks are present in the failed weld tube. The cracks are developed from the heating surface (outer surface) of the tube and propagate towards the inner surface. Cracks observed in the weld tube which had a close resemblance with cracks observed in the failed tubes as reported in [6]. The obtained cracks in the failed base tubes show that blend tip cracks and parallel cracks are also observed as shown in Figure 28. This crack shows typical characteristics of thermal fatigue failure.Cracks identified in the failed weld tube.
(a)(b)(c)(d)(e)


## 3.6. SEM Study

The cracks observed with the optical microscope have been observed with an SEM study to analyse the failures in detail. SEM analyses are carried out in defect surfaces of the base and weld tube.


### 3.6.1. Defect Surface on Failed Base Tube

The cracks observed in the SEM photography of the failed base tube are shown in Figures29(a) and 29(b). From Figures 29(a) and 29(b), the cracks are developed from the outer surface of the tube and propagate towards the inner surface. Similarly it travels through or across the circumference of tube heated region. When it is compared to the cracks obtained in the failed weld tube, the width and depth of the cracks are less as shown in Figures 29(a) and 29(b). The cracks have been opened and examined as shown in Figures 30(a) and 30(b). They revealed smooth-oxidized crack surfaces. From Figure 31, Mosaic-like crack formation is observed when it is magnified. From Figures 29(a) and 29(b) it is clearly identified that the initial position of the crack width is high, when it propagates towards the inner surface, the crack growth rate increases due to the temperature difference and rate of cooling. Due to these reasons, cracks are developed.Crack propagation in the failed base tube.
(a)(b)Smooth-oxidized crack surfaces.
(a)(b)Figure 31
Mosaic-like crack formation.


### 3.6.2. Defect Surface on Failed Weld Tube

Figure32 shows the microstructure taken in an unaffected region of the failed weld tube.  The defects present in the failed weld tube are shown in Figures 33(a) and 33(b). From Figure 33, the cracks are developed from the outer surface of the tube and propagate towards the inner surface. Similarly, it travels in the circumference of the tube-heated region. When compared to the base crack, the size of the crack obtained in the weld is large. Mosaic-like crack formation is observed when it is magnified as shown in Figures 34(a) and 34(b) which is a typical thermal fatigue crack. From Figures 33(a) and 33(b), the initial crack width is large, when it moves towards the inner surface, the crack growth rate increases due to the temperature difference and rate of cooling. Due to these reasons, cracks are developed.Figure 32
Unaffected weld tube structure.Figure 33
Crack propagation in the failed-welded tube.Smooth oxidized crack surfaces and Mosaic-like crack formation.
(a)
Smooth-oxidized crack surfaces(b)
Mosaic-like crack formationOn evaluating the microstudy and SEM results, it is clearly identified that the cracks are developed from the heating surface (outer surface) of the tube and propagate towards the inner surface, which eventually cause failure of the tube. The cracks occurring in the tube is due to long-time exposure in high service temperature. Mosaic-like crack formation is observed inside the crack at higher magnification. The cracks obtained in the tube specimen is the typical thermal fatigue crack which resembles the failures reported in tubes subjected to thermal fatigue [6].


## 3.6.1. Defect Surface on Failed Base Tube

The cracks observed in the SEM photography of the failed base tube are shown in Figures29(a) and 29(b). From Figures 29(a) and 29(b), the cracks are developed from the outer surface of the tube and propagate towards the inner surface. Similarly it travels through or across the circumference of tube heated region. When it is compared to the cracks obtained in the failed weld tube, the width and depth of the cracks are less as shown in Figures 29(a) and 29(b). The cracks have been opened and examined as shown in Figures 30(a) and 30(b). They revealed smooth-oxidized crack surfaces. From Figure 31, Mosaic-like crack formation is observed when it is magnified. From Figures 29(a) and 29(b) it is clearly identified that the initial position of the crack width is high, when it propagates towards the inner surface, the crack growth rate increases due to the temperature difference and rate of cooling. Due to these reasons, cracks are developed.Crack propagation in the failed base tube.
(a)(b)Smooth-oxidized crack surfaces.
(a)(b)Figure 31
Mosaic-like crack formation.


## 3.6.2. Defect Surface on Failed Weld Tube

Figure32 shows the microstructure taken in an unaffected region of the failed weld tube.  The defects present in the failed weld tube are shown in Figures 33(a) and 33(b). From Figure 33, the cracks are developed from the outer surface of the tube and propagate towards the inner surface. Similarly, it travels in the circumference of the tube-heated region. When compared to the base crack, the size of the crack obtained in the weld is large. Mosaic-like crack formation is observed when it is magnified as shown in Figures 34(a) and 34(b) which is a typical thermal fatigue crack. From Figures 33(a) and 33(b), the initial crack width is large, when it moves towards the inner surface, the crack growth rate increases due to the temperature difference and rate of cooling. Due to these reasons, cracks are developed.Figure 32
Unaffected weld tube structure.Figure 33
Crack propagation in the failed-welded tube.Smooth oxidized crack surfaces and Mosaic-like crack formation.
(a)
Smooth-oxidized crack surfaces(b)
Mosaic-like crack formationOn evaluating the microstudy and SEM results, it is clearly identified that the cracks are developed from the heating surface (outer surface) of the tube and propagate towards the inner surface, which eventually cause failure of the tube. The cracks occurring in the tube is due to long-time exposure in high service temperature. Mosaic-like crack formation is observed inside the crack at higher magnification. The cracks obtained in the tube specimen is the typical thermal fatigue crack which resembles the failures reported in tubes subjected to thermal fatigue [6].


## 3.7. Hardness

Vickers’s hardness measurements are taken on the base and welded tube, before subjected to thermal fatigue and after failure occurs. The hardness location taken in the base and welded specimen is shown in Figures35 and 36. The load applied is 10 kg and the hardness values taken in the base and welded specimen are tabulated in Tables 3 and 4.Table 3
Hardness taken in the base tube (HV, 10 kg).
PositionBaseHeating ZoneNear the CrackBefore subjected thermal fatigue258,260——After failure occurs252,254327,327339,334Table 4
Hardness taken in the weld tube.
PositionBaseWeldHAZHeating ZoneNear the CrackBefore subjected thermal fatigue264, 268274, 282296, 284——After failure occurs333,333360,348339,351360,348370,360Figure 35
Hardness locations taken in the failed base tube.Figure 36
Hardness locations taken in the failed weld tube.From Tables3 and 4, the hardness value is much higher nearer in the cracked zone. Due to the higher hardness, the initiated cracks due to the thermal fatigue can propagate towards the circumferential areas and through the thickness of the tube. The hardness value of the weld near the crack zone is nearly closer, due to repeated heating and cooling. The grains get finer near the outer surface of the tube where heat is applied which are discussed in micro structural study, hence hardness value increases when compared to the base material.


## 4. Conclusion

From this research work, the following conclusions are drawnThe effects of thermal fatigue on ASTM A 213 GRADE T-23 base, weld tubes are studied effectively and the number of cycles to failure occurs are obtained experimentally.(1)
While thermal cycling surface cracks are identified in the base and weld tubes after 120 and 80 cycles, respectively. This shows that the base tube will withstand 40 more cycles than the welded tubes subjected to thermal fatigue.(2)
From the visual examination a 20 mm long longitudinal crack are visually identified in the failed base tube in the heated zone. The tube is bulged in the heating zone which is clearly seen. A transverse crack of 80 mm long is visually identified in the failed weld tube at the heated zone. This shows that during thermal fatigue in the base tube, longitudinal crack is developed and in the welded tube transverse crack is obtained.(3)
Liquid penetrant test shows that apart from the longitudinal crack small transverse crack is also observed in the failed base tube. Similarly, in the welded tube apart from the transverse crack, small longitudinal crack is also observed in the failed-welded tube.(4)
From the macro- and the microexaminations, cracks are observed in the failed base and the welded specimens. The cracks have developed from the outer surface of the tube and propagate towards the inner surface of the tube.(5)
The SEM examination revealed smooth oxidized crack surfaces and Mosaic-like crack formation is observed inside the crack at higher magnification.(6)
The hardness test shows that at the point nearer to the crack, the hardness value is much higher. Due to the higher hardness cracks developed by the thermal fatigue process propagate toward the circumferential areas and through the thickness of the tube. The hardness values of the weld near the crack zone is nearly closer, because due to repeated heating and cooling, the grains get finer and the hardness values have increased when compared to the base material.(7)
The development of these cracks is due to the temperature variation from high-temperature to room temperature, which is similar in the power plant during the start-up and shut-down operations.



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*Source: 309529-2009-08-31.xml*