Incoloy 800HT mechanism of the pipe was analyzed. 2013 Elsevier Ltd. All rights reserved.

der hiered pon reer plof 8 g

versized pthe main c

the failure was stress corrosion. Khalaf et al. [11] reported the failure of lube oil feed tube and revealed that the mainwas oversized thermal stress at high temperature. Thermal stress is caused by the thermal expansion and thermal ctivity. Since the yield stress of the structure decreases with the increasing temperature, the thermal stress will causedeformation if it exceeds the yield stress and then leads to the cracking, warping, loosening and other failure form of

1350-6307/$ - see front matter 2013 Elsevier Ltd. All rights reserved.

Corresponding author at: School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China. Tel.: +86 22 27402439.E-mail address: hanyongdian@tju.edu.cn (Y.D. Han).

Engineering Failure Analysis 31 (2013) 375386

Contents lists available at SciVerse ScienceDirect

Engineering Failure Analysishttp://dx.doi.org/10.1016/j.engfailanal.2013.01.013rosion and erosion. Sun et al. [10] investigated the failure of urban pipe and found that the causes were oand corrosion. Attia et al. [12] studied the buried piping made of 316L and cold drain vessel, and found thathe cor-ressureause ofcause

onduc-plasticsion of 800H containing extra Ni, Si, Mn, Ti, etc. The heat and corrosion resistant of the Incoloy 800HT are excellent. Hence,Incoloy 800HT has been widely used to resist high temperature corrosion. The workability is very good and it has outstand-ing resistance to stress corrosion, especially at the temperature above 500 C. These advantages make it possible to reducethe wall thicknesses and consequently improve the behavior of structure submitted to thermal transients [2,3,5]. Becausethis kind of alloy operates at elevated temperature, the thermal stress will arise in the component. Thus, thermal stresscracking is an important issue [68]. Besides, the problem caused by the stress corrosion will also arise. To date, some similarinvestigations on failure analysis have been reported. Gong et al. [9] analyzed the failure of bursting on the inner pipe of ajacketed pipe which was made of SA516Gr55 in a tubular heat exchanger, and found that the failure was caused by tCoordination deformation

1. Introduction

The failure of the components unerally, nickel based alloys are considloys have high strength and corrosicomponents, rockets and nuclear powapplications [14]. With the densitygh temperature was commonly caused by thermal stress and heat corrosion. Gen-romising candidate materials to fulll these challenges. Since the nickel based al-sistance at elevated temperature. Therefore, they are widely used in jet-engineants, as well as in food-handling and chemical-processing equipments, andmarine/cm3, Incoloy 800HT is a kind of alloy of Incoloy 800 series and is a modied ver-Failure analysis of Incoloy 800HT pipe at high temperature

L.Y. Xu a,b, P. Zhu a,b, H.Y. Jing a,b, K. Guo c, S.X. Zhong c, Y.D. Han a,b,a School of Materials Science and Engineering, Tianjin University, Tianjin 300072, Chinab Tianjin Key Laboratory of Advanced Joining Technology, Tianjin 300072, Chinac SINOPEC SABIC Tianjin Petrochemical Co. Ltd., Tianjin 300271, China

a r t i c l e i n f o

Article history:Received 25 October 2012Received in revised form 19 January 2013Accepted 20 January 2013Available online 5 March 2013

Keywords:Numerical simulationHeat stress

a b s t r a c t

In this study, the failure behavior of Incoloy 800HT pipe was investigated. Metallographicanalysis and X-ray Diffraction were conducted to nd the causes of the failure. It wasobserved that the microstructure of the failed area was same as that in the undamagedarea, which indicated that the failure of the pipe was not caused by the change of micro-structure. Then, the nite element method (FEM) was conducted. The results revealed thatonly axial stress during service was larger than the yield stress at the temperature of 890 Cin the inner wall of the pipe. While for the case of 1032 C, both the axial and radial stresseswere larger than the yield stress. Based on the stress and deformation analysis, the failure

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Fig. 1. (a) Assembly drawing of the pipe; and (b) the pipes in service.

Fig. 2. Photos of the failed area: (a) overall view; and (b) local view.

376 L.Y. Xu et al. / Engineering Failure Analysis 31 (2013) 375386

component, which limits the reliability and lifetime of the components [13]. Thus, it is valuable to understand the failuremechanism of heat stress in engineering practice.

2. Background

In the present study, the system was composed of Incoloy 800HT and ceramic. The ceramic was used to insulate heat inthe cavity of the Incoloy 800HT (shown in Fig. 1a). There was a V type weld along the outside wall of the pipe. The pipe wasused to transport pintsch gas (mainly containing hydrocarbon) at the temperature of 844 C. And the operating pressure wasabout 0.0798 MPa during the operating process. The photo of operating process was shown in Fig. 1b. During the operatingperiod, the maximum temperature may reach 890 C (some pipes may reach 1032 C) for a short time.

After about 1000 h, an obvious necking was observed using the endoscope in the inner wall of many pipes (shown inFig. 2a and b) and thus led to the stoppage of operation.

According to the drawing shown in Fig. 1a and the deformed pipe shown in Fig. 2, it was surprised to nd that the de-formed place occurred just under the welded joints in the inner wall. However, the damage caused by hot corrosion wasnot apparent. Consequently, the objective of this paper is to nd the causes of failure of the pipe during operation.

3. Experiments

The chemical composition of Incoloy 800HT was listed in Table 1. The samples were extracted from the failed area andundamaged area in the inner wall of Incoloy 800HT to conduct metallographic analysis test and XRD. Before test, the sampleswere washed by the ultrasonic agitation in acetone.

Table 1Chemical composition of Incoloy 800HT (wt%).

Alloy C Al Si S Ti Cr Mn Fe Ni Co CuIncoloy 800HT 0.077 0.51 0.34

Fig. 4. X-ray Diffraction of the failed area.

Table 2Result of tensile test for Incoloy 800HT.

25 C 844 C 890 C

Yield stress (MPa) 355 107 96Tensile stress (MPa) 624 141 130

Table 3Result of expansion coefcient test for ceramic and Incoloy 800HT.

Name 844 oC (ppm/K) 890 oC (ppm/K)Incoloy 800HT 15.0209 15.940Ceramic 4.2113 4.4210

Table 4Parameters of welding.

Voltage (V) 10Current (A) 100Welding speed (mm/min) 15Interpass temperature (C) 100Preheating temperature (C) 150Cooling condition air coolingheat transfer coefcient w/(m2 K) 20Height of the weldout (mm) 6

Fig. 5. Material properties of Incoloy 800HT.

378 L.Y. Xu et al. / Engineering Failure Analysis 31 (2013) 375386

L.Y. Xu et al. / Engineering Failure Analysis 31 (2013) 375386 3793.1. Macro-analysis

From Fig. 2, it can be clearly observed that there was large plastic deformation in the inner wall which was necked. How-ever, the features of necked and undamaged surfaces were almost the same. And the failure caused by hot corrosion was notobvious.

3.2. Metallographic analysis

In order to further investigate the failure causes, a metallographic experiment was also conducted on the failed andundamaged area. The photos were shown in Fig. 3a and b.

Fig. 6. Temperature and stress distribution after welding: (a) temperature; (b) plots of the temperature prole of points 1 and 2 over time; (c) radial stress;(d) circumferential stress; and (e) axial stress.

380 L.Y. Xu et al. / Engineering Failure Analysis 31 (2013) 375386As known to all, the normal microstructure of the Incoloy 800HT was austenite. From Fig. 3a, the metallographic struc-tures were homogenous and there was not any impurities or particles precipitated from the grain spreading along the grainboundary. By calculating the size of the grain, the result was about 24 lm2. While from Fig. 3b, the grain size was about22 lm2, which meant that it was smaller than that in undamaged area and the metallographic structures were homogenoustoo. Besides, the cavities, which may grow and gather together under stress and then lead to the crack, were not seen in thegraphic. By analyzing, the metallographic structures of the two areas were also austenite. In other words, the metallographicstructures were not changed, which indicated that the failure was not caused by the change of the metallographic structure.Besides, the creep cavities were not found, which meant that the failure was not caused by the creep deformation.

3.3. XRD analysis of the corrosion products

A preliminary analysis of an unexposed 800HT was studied by Yin [2]. It was observed that the Incoloy 800HT consistedentirely of a single-phase Cr0.19Fe0.7Ni0.11 without grain boundary precipitates. In order to investigate whether the phase ofthe corrosion products changed, a sample was extracted from the failed area to conduct the XRD analysis. The result wasshown in Fig. 4.

It could be seen that the phase of the necked area in the inner wall and the base metal was austenite. Consequently, itcould be concluded that the damage was not caused by hot corrosion, which may lead to the change of the metallographicstructure.

4. Numerical simulation analysis

Since the failure of the pipe was not caused by the hot corrosion or the phase change, it is necessary to simulate the pipefrom welding process to operating process to see whether it was caused by thermal stress during service. Before the test,some tests were conducted to nd out the 0.2% yield strength (YS), the ultimate tensile strength (UTS) and the expansion

Fig. 6. (continued)

L.Y. Xu et al. / Engineering Failure Analysis 31 (2013) 375386 381coefcients of the Incoloy 800HT at temperature of 25 C, 844 C and 890 C, respectively. The expansion coefcients of theceramic were also tested.

4.1. High temperature tensile test

The test specimen was prepared in accordance with the ASTM test method E8M. The mechanical properties of the spec-imens were investigated using an Instron Microtester, at a cross-head speed of 0.5 mm/min at the temperature of 25 C,

Fig. 7. Distribution of temperature and stress after operating at 890 C for 1 h: (a) scaleplate; (b) radial stress; (c) circumferential stress; (d) axial stress; (e)axial stress in 3D model; (f) value of radial stress along inner wall; and (g) value of axial stress along inner wall.

382 L.Y. Xu et al. / Engineering Failure Analysis 31 (2013) 375386844 C and 890 C. Through the test, the stressstrain curves were obtained. Consequently, the 0.2% yield strength (YS) andthe ultimate tensile strength (UTS) were reaped from the curve. The result was shown in Table 2.

4.2. Expansion coefcient test

The expansion coefcients (CTEs) were tested by the Automatic thermo-mechanical analyzer TMA 2940, and were ob-tained by measuring the displacement of the specimen as a function of temperature at 844 C and 890 C, respectively.

Fig. 7. (continued)

The lengths of the specimens were 10 mm with the diameter of 4 mm. The rate of heating was 5 C/min. The height of eachspecimen was about 30 mm. For each kind of material, at least three groups of data should be obtained. And their valueswere averaged. The CTE results were shown in Table 3.

4.3. Finite element modeling

Thermal stress simulations by the nite element method (FEM) were widely used to analyze the damages caused bymechanical factors [14]. The pipe was manufactured by welding before service. Hence, it was necessary to simulate the weld-ing process. Since the pipe was axisymmetric, it was reasonable to simplify the model as 2D axial symmetry model. Hence, anite element (FE) model of the problem was built using ABAQUS to study the stress during welding and operating process.The method of sequentially coupled thermal-stress analysis was used to simulate both welding and operating process. Theheat transfer element type was used to simulate the heat transferring process. Then the element type was converted to stresselement type for calculation of the stress eld. During the welding and operating process, both ends of the pipe were fullconstrained and the ceramic was contacted to the Incoloy 800HT. The parameters used were shown in Table 4. After welding,it was cooled down in the air for 10 h and the temperature was below 200 C

During operation after welding, operating pressure was about 0.0798 MPa. And the temperature in the inner wall was

L.Y. Xu et al. / Engineering Failure Analysis 31 (2013) 375386 383844 C for about 1000 h. However, the pipe will be worked at scorched state for about 1 h at the temperature of 890 C,and some pipes may reach 1032 C. Then the pipe was returned to operate at 844 C for about 100 h. During the processesmentioned above, the heat would be exchanged between the outer wall of the pipe and the air. The material properties ofIncoloy 800HT were shown in Fig. 5.

4.4. Results and discussion

The processes of welding and operating were simulated. The distribution of temperature and stress in the inner wall wasobtained. Points 1 and 2 near the V groove (showed in Fig. 6a) were created to show the temperature prole over time andthe result was shown in Fig. 6b.

From the results (showed in Fig. 6ce) in inner wall during the welding process, it can be obviously observed that theresidual stresses in the radial direction, axial direction and circumferential direction in the inner wall were too low to causethe failure. In other words, the failure was not caused during the welding process. After welding, the inner wall of the pipeworked at 844 C for 1000 h and then the temperature increased to 890 C for 1 h and the results were shown in Fig. 7bdand the value of radial stress and axial stress in the inner wall were shown in Fig. 7f and g. After that, the pipe was returnedto work at 844 C for about 100 h. However, the axial stress near the free end (see endpoint B in Fig. 8) in the inner wall was115 MPa, which was higher than its yield stress at 890 C (96 MPa). The radial stress was about 85 MPa.

However, for the pipe whose temperature increased to 1032 C (results were shown in Fig. 9ac and the value of radialstress and axial stress in the inner wall were shown in Figs. 9e to 7f), the axial stress near the free end in the inner wall in-creased to 88.4 MPa, which was higher than the yield stress at 1032 C (75 MPa, which was calculated according to theextrapolation method conducted by using the yield stress of 844 C and 890 C [15]). Besides, the radial stress was about80 MPa, which indicated that two directions of stress exceeded the yield stress.

Each point position in the inner wall could be referred in Fig. 7a. In order to observe the axial stress apparently, the three-dimensional models were created to show the axial stress in Figs. 7e and 9d.

From the point of mechanics, only the axial stress near the endpoint B was higher than the yield stress for the pipe oper-ated at 890 C. Although the radial stress was very large near the endpoint A, it was constrained and was difcult to deform.However, for the case of 1032 C, the axial stress near the endpoint B was larger than its yield stress. Besides, the radial stressalso exceeded the yield stress. Under the effect of stresses, the Mises stress was about 107 MPa, which led to the failure of the

Fig. 8. Scheme of specimen deformation.

384 L.Y. Xu et al. / Engineering Failure Analysis 31 (2013) 375386pipe according to the strength theory. And this was why most pipes working at 890 C were not failed, while some pipeswere failed.

Fig. 9. Distribution of stress after operating at 1032 C for 1 h: (a) radial stress; (b) circumferential stress; (c) axial stress; (d) axial stress in 3D model; (e)value of radial stress along inner wall and (f) value of axial stress along inner wall.

L.Y. Xu et al. / Engineering Failure Analysis 31 (2013) 375386 385From the point of deformation, the temperature of endpoint A of the pipe along the thickness direction would be almostthe same during the operating process. The area near endpoint A would expand along the radial direction. Since the thicknessnear endpoint A was very large, the area near the endpoint A in the inner wall would deform to the left when the endpoint Aexpanded, so that the whole system could remain stable. The farther the distance from endpoint A in the inner wall, thesmaller the effect of this coordination deformation would be. While the endpoint B of the inner wall was free in the axialdirection, it would expand freely along the axial direction. Meanwhile, since the endpoint B was free in the axial direction,it was more difcult for heat to transfer in the inner wall. Therefore, the temperature would be higher than other areas in thepipe. Consequently, the heat stress in the endpoint B would be higher than other places. Besides, the endpoint B could

Fig. 9. (continued)

Fig. 10. Node change between deformed state and undeformed state (green line represents the deformed state). (For interpretation of the references tocolor in this gure legend, the reader is referred to the web version of this article.)

deform freely and the area near the endpoint A would deform to the left. The area marked by green circle would be the crit-ical point of coordinative deformation and free deformation (shown in Fig. 8). Hence, the area would be in dangerous. Thegridding of the pipe between deformed and undamaged state veried the explanation above (shown in Fig. 10).

5. Conclusions

The analyses of the failure of the pipe were investigated. The obtained results can be summarized as follows:

(1) The corrosion and the welding process have insignicant effect on the failure of the pipe.(2) The microstructure of the Incoloy 800HT kept constant during the welding and operating process. And the creep cav-

ities were not seen in the failed area, which meant that the failure was not caused by the creep.(3) The heat stresses of radial and axial direction of the pipes working at 1032 C were too large and the Mises stress

exceeded the yield stress, which caused the failure of the pipe.

Acknowledgements

386 L.Y. Xu et al. / Engineering Failure Analysis 31 (2013) 375386The authors acknowledge the research funding by National Natural Science Foundation of China (Grant No. 51275341),Key Project in the Science & Technology Pillar Program of Tianjin (Grant No. 11ZCKFGX03000), Program for New CenturyExcellent Talents in University (NCET-11-0375) and Specialized Research Fund for the Doctoral Program of Higher Educationof China (20110032130002).

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Failure analysis of Incoloy 800HT pipe at high temperature1 Introduction2 Background3 Experiments3.1 Macro-analysis3.2 Metallographic analysis3.3 XRD analysis of the corrosion products

4 Numerical simulation analysis4.1 High temperature tensile test4.2 Expansion coefficient test4.3 Finite element modeling4.4 Results and discussion

5 ConclusionsAcknowledgementsReferences