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Failure analysis of TP304H tubes in the superheated steam section of a reformer furnace
Engineering Failure Analysis 79 (2017) 762–772
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of TP304H tubes in the superheated steam section of a reformer furnace
MARK
Shugen Xu⁎, Weige Meng, Chong Wang, Zhiwei Sun, Yuan Zhang College of Chemical Engineering, China University of Petroleum (Huadong), Qingdao 266580, China
AR TI CLE I NF O
AB S T R A CT
Keywords: TP304H tube Stress corrosion cracking Caustic embrittlement Austenitic stainless steel
The cracking failure of TP304H tubes in the superheated steam section of a reformer furnace was analyzed. Through the analysis of macro-appearance, micro-appearance of specimens with cracks, metallurgical structure of specimens from an intact pipe section and cracked pipe section, energy spectrum detection of fracture surface, residual stress measurement, and investigation of the service medium, the cracking mode was described as the stress corrosion cracking (SCC) of austenitic stainless steel. In this case, the materials in the heat affected zone were sensitized by inappropriate welding technology. This together with the higher pH value in the steam due to the failure of a gas-liquid separator led to the final cracking of the reformer furnace tube. So the inappropriate welding technology and the failure of the gas-liquid separator were the main factors in this fracture accident.
1. Introduction As important engineering materials, austenitic stainless steels are widely used in the chemical, textile, transportation, and nuclear fields, [1–3]. Among them, 304H austenitic stainless steel is extensively used in the tubes of boilers due to its good mechanical properties at elevated temperatures [4]. However, its relatively high carbon content makes it more likely to have intergranular corrosion and stress corrosion cracking (SCC) [5]. In modern industrial processes, welding is an important processing link, and the welding joint quality directly affects the quality of the finished equipment [1]. Since the welding temperature of stainless steels is in the range of 550 °C to 850 °C, stainless steel can easily be sensitized [6–7]. The sensitization of austenitic stainless steels is a general and serious problem during welding and high temperature use, resulting in the formation of chromium carbide precipitates near the grain boundaries of the heat affected zone [8–10]. As the diffusion velocity of C is much faster than Cr, a chromium depleted zone is formed near the grain boundaries, and hence the corrosion resistance around the area will be sharply reduced [11–12]. The degree of sensitization is mainly affected by chemical composition, temperature, and time [3]. The residual stress in sensitized austenitic stainless steels can contribute to the initiation of cracking near the grain boundaries, which leads to intergranular stress corrosion cracking (IGSCC) [13–14]. IGSCC occurs in specific combinations of three necessary conditions: a susceptible material, sufficient stress, and a corrosive environment [14]. In addition, because 304H austenitic stainless steel is often used in high temperature environments, with a higher concentration of sodium hydroxide in the sensitive region, the combination of high temperature and sodium hydroxide will lead to caustic embrittlement of SCC. In this study, the cause of the failure of TP304H reformer furnace tubes in the superheated section was studied by the analysis of macro-appearance, micro-appearance, metallurgical structure and energy spectrum, etc.
⁎
Corresponding author. E-mail address: [email protected] (S. Xu).
http://dx.doi.org/10.1016/j.engfailanal.2017.05.033 Received 19 October 2016; Received in revised form 8 March 2017; Accepted 6 May 2017 Available online 08 May 2017 1350-6307/ © 2017 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 79 (2017) 762–772
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Fig. 1. Process flow of hydrogen manufacturing converted furnace.
2. Descriptions of the failure case The process flow of a hydrogen manufacturing converted furnace is shown in Fig. 1. In this figure, F-101 indicates the hydrogen manufacturing converted furnace, D-101 is a medium pressure steam drum, and E-101 is a conversion gas steam producer. S1 and S2 in the pipeline are the monitoring points for the drum steam and medium pressure steam, respectively. Also, the (a) portion marked in red in the converted furnace denotes the failure of the superheated section, and (b), (c), (d), (e) are the heating sections for raw materials, producing section for gas, preheating section for boiler water, and preheating section for air, respectively. It can be seen from the process flow that the flue gas entered from the top of the converted furnace, and exhaust gas was discharged from the left side of horizontal section. Also, the saturated steam from the top of the steam drum entered the superheated section of the converted furnace, after heating to become a superheated steam outflow. The furnace tube material of the superheat section was TP304H with a dimension of φ114 × 8 mm, and the mediums of the tube side and shell side were superheated steam and exhaust gas respectively. The working pressure and temperature parameters of the furnace tube were as follows: the steam pressure of superheated steam was 3.5 MPa, the inlet temperature and outlet temperature of the superheated steam were 260 °C and 440 °C respectively, and the inlet temperature and outlet temperature of the exhaust gas were 870 °C and 650 °C respectively. A large amount of steam leakage was found in the elbow box of the superheat section on April 6, 2016. More than ten cracks were found on both sides of the weld of the furnace tube and elbow at the low temperature section, but the structure was intact at the high temperature section. The cracked pipe was then repaired adopted shield metal arc weld (SMAW). The E308H electrodes were selected as filler metal according to the standard of SH/T 3417–2007 Technical specification of weld engineering for high alloy steel tube of petrochemical tubular heater [15], and the selected electrode diameter was φ 3.2 mm. The SMAW adopted direct current electrode positive (DCEP) with the current of 125A. Besides, the voltage and welding speed were 32 V and 60 cm/min respectively, the repaired furnace tubes did not take any measures to relieve the post weld stress after repair welding. Finally, the equipment leaked again on July 20, 2016. The analytical specimens were selected from the leakage furnace tube. And the specimen location is shown in Fig. 2. Fig. 2(a) shows the overall layout of furnace tubes, and Fig. 2(b) shows the cracked furnace tube.
3. Experimental analysis 3.1. Macro appearance Two specimens were obtained by cutting the leaking furnace tube, which were used for macro tests, are shown in Fig. 3. Fig. 3(a) and (b) show the specimen of straight pipe with cracks. The crack was located in the heat affected zone of the girth weld, and its length was about 1/4 of the furnace tube perimeter. The specimen of the elbow with cracks is shown in Fig. 3(c). Three cracks were found on its surface, one crack paralleled to the girth weld, the other two cracks paralleled to repair welding seam. These cracks were all in the heat affected zone and no crack was found on weld seam. Fig. 3(d) shows the fracture morphology around the girth weld. The fracture was even with an obvious morphology of brittle fracture, and the whole fracture was old and dark. It can be seen that the cracks have completely split. The welding condition of the specimen is shown in Fig. 3(e) and (f). There were some welding defects such as heterogeneous weld, incomplete penetration, and the inner wall of straight pipe and elbow was not aligned. 763
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Fig. 2. Furnace pipe of hydrogen manufacturing converted furnace.
Fig. 3(g) and (h) show the penetration test of the specimen. The cracks were found on the inner and outer walls, and the cracks of the inner wall were much longer than those of the outer wall. In addition, the cracks were propagated from inside toward the outside along the wall thickness. According to the macro appearances discussed above, we initially judged that the cracks originated from the inner wall. 3.2. Chemical composition According to design handbook, the reformer furnace tube was made of TP304H austenitic stainless steel, and its chemical composition was analyzed by direct reading spectrometer. The detected results and its standard requirements of furnace tubes are shown in Table 1. From the test results it can be seen that the composition met the requirement of ASTM A-312 M of steel except for the relatively low content of carbon. 3.3. Metallurgical structure In order to test the metallurgical structure of the elbow, samples were collected from the base metal with no crack a distance from the weld seam, the crack tip of the inner wall, the crack tip of the outer wall, and the crack along the wall thickness, as shown in Fig. 4. According to Fig. 4(a) and (b), the metallurgical structures in the vicinity of base metal and heat affected zone were equiaxed austenite. The metallurgical structure of the crack tip of inner and outer wall is shown in Fig. 4(c) and (d), respectively. Fig. 4(e) displays the appearance of crack propagation along the thickness under the polished state. After etching by aqua regia, the appearance of the crack initiation and crack tip region are shown in Fig. 4(f) and (g), respectively. It can be seen from Fig. 4(f) and (g) that the cracks propagated from the inner wall toward the outer wall and there are many branched cracks, which show the obvious intergranular cracking and the characteristics of SCC. In addition, two samples were taken from the heat affected zone and base metal with no crack, which were used to test the sensitization of the heat affected zone according to ASTM 262 practice A. The two samples were electrochemically etched in 10%(wt.) oxalic acid with a current density of 1A/cm2 for 90 s, and the metallurgical structures are shown in Fig. 4(h) and (i). It can be seen from the figures that many grains are completely surround by ditches in the heat affected zone, but the ditch structure was not found in the base metal. Fig. 4(j) shows the oxalic acid etch results on the base metal and heat affected zone, this figure displays the transitional change of ditches from the base metal to the heat affected zone. According to the metallurgical structures discussed above, it can be seen that the materials in the heat affected zone of furnace tube have been sensitized. 3.4. Micro appearance The fracture was observed by scanning electron microscope (SEM) as shown in Fig. 5. Fig.5(a) shows the appearance of the uncleaned fracture surface, and some obvious corrosion products can be seen. In addition, the appearance of the cleaned fracture surface is shown in Fig. 5(b). The fracture surface presents a typical intergranular fracture. Also, some secondary intergranular cracks were found on the fracture, and they show obvious characteristic of IGSCC. 764
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Fig. 3. Macro appearance of the cracked furnace tube and elbow.
Table 1 Chemical composition of reformer furnace tube (wt%). Element
C
Si
Mn
P
S
Ni
Cr
Measured ASTM A-312 M
0.031 0.04–0.10
0.417 ≤ 1.00
1.128 ≤ 2.00
0.033 ≤0.045
0.012 ≤ 0.030
8.494 8.00–11.00
18.24 18.00–20.00
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Fig. 4. Metallurgical structure and crack growth morphology.
3.5. Energy spectrum analysis Energy spectrum detection of the uncleaned fracture surface was carried out. The scanning positions are shown in Fig. 6 and 7 presents the scanning spectrum. The percentage of each component is shown in Table 2. From Table 2, it can be seen that the corrosion products contained more Na element and O element. The detection results indicated that the alkaline substance NaOH that is sensitive to SCC of austenitic stainless steel was in this medium. 766
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Fig. 4. (continued)
3.6. Residual stress measurement Because the furnace tubes of austenitic stainless steel do not need to relieve the post weld stress in the environment without stress corrosion sensitive medium according to the standard of SH/T 3417–2007 Technical specification of weld engineering for high alloy steel tube of petrochemical tubular heater [15], the furnace tubes do not adopt any measures to relieve the welding residual stress in this case, and the residual stresses in the vicinity of the welds can be determined by X-ray diffraction (XRD) using an X-350A residual stress analyzer with Chromium Kβ radiation and (311) diffracting plane. The tube voltage of the X-ray was 24 kV with a target current of 7 mA. The scanning range and scanning step of 2θ were 140°-162° and 0.1° respectively. The selected Ψ angles were 0°, 24.2°, 35.3°, and 45°, and the X-ray stress constant was -366 MPa/°. Fig. 8 presents the apparatus for measuring residual stress. The axial residual stress was measured as 93.4 MPa on the outer wall.
Fig. 5. Fracture appearance.
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Fig. 6. Scanning area.
4. Reasons of cracking 4.1. Material and structural factors The metallographic structures showed that the grain size of the heat affected zone and the grain boundaries around the crack were
Fig. 7. Scanning spectrum of the fracture.
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Table 2 Energy spectrum analysis results of the fracture (wt%). Element
C
P
O
Na
Si
Ti
Ca
Cr
Mn
Fe
Ni
Position-1 Position-2
4.11 4.11
0.30 0.35
31.36 36.84
2.30 3.45
0.88 1.44
0.23 0.20
0.28 0.30
10.62 9.33
1.02 0.87
45.69 38.12
3.21 4.99
coarse, and moreover, some black carbides precipitated at some of the grain boundaries. According to the metallographic structures discussed above, it can be determined that the materials in heat affected zone were sensitized, and intergranular corrosion and IGSCC occurred in the heat affect zone of the sensitive medium. In this case, the cracks only existed in the heat affected zone and no crack was found in areas a distance from the weld seam. So the material parameters of SCC can be attributed to the sensitization of heat affected zone by the residual heat of welding. In addition, the welding points had some welding defects such as heterogeneous welds, incomplete penetration, and the inner wall of straight pipe and elbow was not aligned. These defects could easily produce the residual stress and the accumulation of the medium, which provided the condition for the stress corrosion of the austenitic stainless steel. 4.2. Medium factors Tables 3 and 4 represent the composition of the drum steam and medium pressure steam from March1, 2016 to April 7, 2016, respectively. From these tables it can be seen that the maximum pH value of steam medium in the steam drum reached 11.35, which indicated that the medium contained more OH−. In medium pressure steam, the content of sodium ions was abnormal on March 5 and March 28, their values were 720 μg/L and 290 μg/L, respectively. The alkaline elements in the medium agreed with the results of the energy spectrum analysis. In addition, there were a few chloride ions and phosphate ions in the steam medium. Tables 5 and 6 display the steam composition in June, compared with the data before repair welding (Tables 3 and 4), it can be seen that the steam compositions were alkaline and they were similar before and after repair welding. The higher content of impurity elements in steam may be related to the bad separation efficiency of the gas-liquid separator. In the boiler, generally boiler compound is injected to adjust pH value and soften water quality [16]. The gas-liquid separator is used to separate the liquid drops in the steam, in order to avoid the boiler compound entering the steam. But if the separation efficiency of gas-liquid separator is bad, liquid drops will accompany the steam entering the superheated section. The liquid drops will rapidly evaporate under the heating of the exhaust gas, and will lead to the incrassation of salt in the inner wall of the furnace tube. Therefore, the salinity in the lower temperature section of the steam inlet is higher than the outlet side. The range of 60 °C to 300 °C is most sensitive to SCC. The temperature of the steam inlet was 265 °C, so the internal wall temperature was in the sensitive range. Due to the salinity and temperature, SCC occurred at the low temperature section, and no cracks were found in the high temperature zone. SCC of austenitic stainless steel can be caused by alkaline substances and chloride ions in steam. Considering that chloride ions
Fig. 8. The apparatus for measuring residual stress.
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Table 3 Test results of the steam drum (before repair welding). Date
Time
pH value
Conductivity μS/cm
Phosphate radical mg/L
Chloride ions mg/L
March 1, 2016 March 2, 2016 March 3, 2016 March 4, 2016 March 5, 2016 March 6, 2016 March 7, 2016 March 8, 2016 March 9, 2016 March 10, 2016 March 11, 2016 March 12, 2016 March 13, 2016 March 14, 2016 March 15, 2016 March 16, 2016 March 17, 2016 March 18, 2016 March 19, 2016 March 20, 2016 March 21, 2016 March 22, 2016 March 23, 2016 March 24, 2016 March 25, 2016 March 26, 2016 March 27, 2016 March 28, 2016 March 29, 2016 March 30, 2016 March 11, 2016 April 1, 2016 April 2, 2016 April 3, 2016 April 4, 2016 April 5, 2016 April 6, 2016 April 7, 2016
8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00
9.58 8.86 9.06 8.65 9.25 9.40 10.17 9.75 9.89 9.85 9.96 9.53 9.05 9.92 9.57 10.11 10.38 10.05 9.55 10.49 10.58 10.55 9.95 9.72 10.17 10.07 10.18 10.38 10.27 10.72 10.27 10.58 11.02 10.91 10.63 10.73 10.88 11.35
68.3 47.3 93.8 40.9 38.9 47.6 77.6 77.8 63.9 79.2 75.8 69.4 41.7 73.3 59.4 93.9 122.7 103.3 71.7 124.1 148.1 116.0 95.5 69.7 83.9 88.8 78.7 118.3 110.6 159.7 118.4 160.2 166.3 162.4 143.6 104.0 103.3 139.4
5.53 5.24 6.31 4.13 3.18 4.37 6.31 29.36 6.39 6.18 6.45 4.35 2.86 5.65 3.61 7.46 10.14 9.58 7.97 9.03 11.00 9.72 8.03 6.21 7.48 4.22 4.91 7.42 10.26 14.64 9.84 11.88 10.82 9.30 9.64 6.80 5.22 7.74
3.18 0.71 9.19 1.77 0.71 0.71 2.12 17.68 1.77 1.41 1.06 1.41 0.71 1.77 1.06 1.41 1.77 1.41 1.77 1.41 2.48 2.12 1.41 1.06 2.83 1.41 4.24 2.12 1.77 1.77 2.48 1.77 2.12 2.12 2.86 1.79 3.22 3.58
Table 4 Test results of medium pressure steam (before repair welding). Date
Time
Conductivity μS/cm
Silicon dioxide μg/L
Sodium content μg/L
Chloride ions mg/L
March 5, 2016 March 7, 2016 March 12, 2016 March 14, 2016 March 19, 2016 March 21, 2016 March 26, 2016 March 28, 2016 April 2, 2016 April 4, 2016
8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00
38.9 29.1 26.5 24.5 74.4 34.7 27.6 24.3 2.90 3.05
89.2 12.9 11.4 6.3 48.4 96.0 15.8 66.7 43.8 4.6
720 53.0 34.0 15.6 94.3 31.2 51.7 290 36.3 70.1
1.06 1.77 1.77 1.41 2.48 1.77 1.77 1.06 0.35 1.79
were not found on energy spectrum analysis, and the content of chlorine ions was lower in the steam, the primary medium factor of SCC was regarded as alkaline substances, and chloride ions were seen as a secondary factor. 4.3. Stress factors The existence of tensile stress is one of the prerequisites for SCC of austenitic steel [16]. Residual stresses during the welding process, thermal stress during the running process, and mechanical stress during internal pressure, provide convenient conditions for SCC. 770
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Table 5 Test results of steam drum (after repair welding). Date
Time
Conductivity μS/cm
Silicon dioxide μg/L
Sodium content μg/L
Chloride ions mg/L
June 1, 2016 June 2, 2016 June 3, 2016 June 4, 2016 June 5, 2016 June 6, 2016 June 7, 2016 June 8, 2016 June 9, 2016 June 10, 2016 June 11, 2016 June 12, 2016 June 13, 2016 June 14, 2016 June 15, 2016 June 16, 2016 June17, 2016 June 18, 2016 June 19, 2016 June 20, 2016 June 21, 2016 June 22, 2016 June 23, 2016 June 24, 2016 June 25, 2016 June 26, 2016 June 27, 2016 June 28, 2016 June 29, 2016 June 30, 2016
8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00
9.44 9.99 10.04 10.12 10.25 9.73 9.5 8.96 9.33 10.08 9.6 9.69 9.43 9.9 9.24 9.24 9.13 9.77 9.84 9.6 9.84 9.37 8.97 10.05 9.25 9.64 9.23 9.25 8.66 8.98
72.9 113.1 108 108.7 132.9 105.7 73.3 43.2 80 122.8 76 72.8 77.7 77.5 67.5 65.1 71.2 81.3 81.1 78.4 77.4 83.7 81.8 114.1 74.5 115.1 73.8 65 68.9 84.1
8.06 12.45 11.82 12.12 15.13 11.59 7.07 3.28 6.94 11.8 7.41 6.51 7.21 6.42 6.43 5.52 5.57 7.77 7.45 6.68 7.33 7.47 7.39 9.61 6.43 8.23 5.02 3.96 5.49 6.36
1.06 1.06 1.06 1.77 1.77 1.77 1.41 1.77 3.18 2.83 1.77 1.77 1.77 1.77 1.77 2.12 1.77 2.12 3.54 3.89 4.42 3.18 3.18 2.48 1.77 2.12 1.77 1.77 1.78 1.78
Table 6 Test results of medium pressure steam (After repair welding). Date
June June June June June June June June
4, 2016 6, 2016 11, 2016 13, 2016 18, 2016 20, 2016 25, 2016 27, 2016
Time
Conductivity μS/cm
Silicon dioxide μg/L
Sodium content μg/L
Chloride ions mg/L
8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00
25.2 23.6 17.6 17.19 28.3 27 20.3 25.1
4.8 6 7.7 9 26.8 36.3 29.3 2.9
193 158 139 101 770 760 50.8 200
1.77 2.12 1.77 1.41 4.95 3.54 1.77 3.18
5. Conclusions (1) All cracks were in the heat affected zone and no cracks were found in other parts. Therefore, the material factors of SCC can be attributed to the deterioration of corrosion resistance of materials in the heat affected zone during the welding process. (2) The alkaline substances and chloride ions were the medium factor of SCC, and the main cause was attributed to alkaline substances. Also, the higher pH value in the steam may be related to carried liquid due to the failure of the gas-liquid separator. (3) Tensile stress is only a basic guarantee of SCC. Studies have shown that stress corrosion cracking can be caused by lower tensile stress. But, a small amount of stress is unavoidable such as welding residual stress, mechanical stress caused by internal pressure, and thermal stress, and hence stress was not the immediate cause of SCC in this case.
Acknowledgment The authors wish to express their gratitude for the financial support by National Natural Science Foundation of China (51404284), Fundamental Research Funds for the Central Universities (15CX05011A), and Applied Fundamental Research Funds of Qingdao City (15-9-1-95-jch). Thanks to Dr. Edward C. Mignot, Shandong University, for linguistic advice. 771
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of TP304H tubes in the superheated steam section of a reformer furnace
MARK
Shugen Xu⁎, Weige Meng, Chong Wang, Zhiwei Sun, Yuan Zhang College of Chemical Engineering, China University of Petroleum (Huadong), Qingdao 266580, China
AR TI CLE I NF O
AB S T R A CT
Keywords: TP304H tube Stress corrosion cracking Caustic embrittlement Austenitic stainless steel
The cracking failure of TP304H tubes in the superheated steam section of a reformer furnace was analyzed. Through the analysis of macro-appearance, micro-appearance of specimens with cracks, metallurgical structure of specimens from an intact pipe section and cracked pipe section, energy spectrum detection of fracture surface, residual stress measurement, and investigation of the service medium, the cracking mode was described as the stress corrosion cracking (SCC) of austenitic stainless steel. In this case, the materials in the heat affected zone were sensitized by inappropriate welding technology. This together with the higher pH value in the steam due to the failure of a gas-liquid separator led to the final cracking of the reformer furnace tube. So the inappropriate welding technology and the failure of the gas-liquid separator were the main factors in this fracture accident.
1. Introduction As important engineering materials, austenitic stainless steels are widely used in the chemical, textile, transportation, and nuclear fields, [1–3]. Among them, 304H austenitic stainless steel is extensively used in the tubes of boilers due to its good mechanical properties at elevated temperatures [4]. However, its relatively high carbon content makes it more likely to have intergranular corrosion and stress corrosion cracking (SCC) [5]. In modern industrial processes, welding is an important processing link, and the welding joint quality directly affects the quality of the finished equipment [1]. Since the welding temperature of stainless steels is in the range of 550 °C to 850 °C, stainless steel can easily be sensitized [6–7]. The sensitization of austenitic stainless steels is a general and serious problem during welding and high temperature use, resulting in the formation of chromium carbide precipitates near the grain boundaries of the heat affected zone [8–10]. As the diffusion velocity of C is much faster than Cr, a chromium depleted zone is formed near the grain boundaries, and hence the corrosion resistance around the area will be sharply reduced [11–12]. The degree of sensitization is mainly affected by chemical composition, temperature, and time [3]. The residual stress in sensitized austenitic stainless steels can contribute to the initiation of cracking near the grain boundaries, which leads to intergranular stress corrosion cracking (IGSCC) [13–14]. IGSCC occurs in specific combinations of three necessary conditions: a susceptible material, sufficient stress, and a corrosive environment [14]. In addition, because 304H austenitic stainless steel is often used in high temperature environments, with a higher concentration of sodium hydroxide in the sensitive region, the combination of high temperature and sodium hydroxide will lead to caustic embrittlement of SCC. In this study, the cause of the failure of TP304H reformer furnace tubes in the superheated section was studied by the analysis of macro-appearance, micro-appearance, metallurgical structure and energy spectrum, etc.
⁎
Corresponding author. E-mail address: [email protected] (S. Xu).
http://dx.doi.org/10.1016/j.engfailanal.2017.05.033 Received 19 October 2016; Received in revised form 8 March 2017; Accepted 6 May 2017 Available online 08 May 2017 1350-6307/ © 2017 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 79 (2017) 762–772
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Fig. 1. Process flow of hydrogen manufacturing converted furnace.
2. Descriptions of the failure case The process flow of a hydrogen manufacturing converted furnace is shown in Fig. 1. In this figure, F-101 indicates the hydrogen manufacturing converted furnace, D-101 is a medium pressure steam drum, and E-101 is a conversion gas steam producer. S1 and S2 in the pipeline are the monitoring points for the drum steam and medium pressure steam, respectively. Also, the (a) portion marked in red in the converted furnace denotes the failure of the superheated section, and (b), (c), (d), (e) are the heating sections for raw materials, producing section for gas, preheating section for boiler water, and preheating section for air, respectively. It can be seen from the process flow that the flue gas entered from the top of the converted furnace, and exhaust gas was discharged from the left side of horizontal section. Also, the saturated steam from the top of the steam drum entered the superheated section of the converted furnace, after heating to become a superheated steam outflow. The furnace tube material of the superheat section was TP304H with a dimension of φ114 × 8 mm, and the mediums of the tube side and shell side were superheated steam and exhaust gas respectively. The working pressure and temperature parameters of the furnace tube were as follows: the steam pressure of superheated steam was 3.5 MPa, the inlet temperature and outlet temperature of the superheated steam were 260 °C and 440 °C respectively, and the inlet temperature and outlet temperature of the exhaust gas were 870 °C and 650 °C respectively. A large amount of steam leakage was found in the elbow box of the superheat section on April 6, 2016. More than ten cracks were found on both sides of the weld of the furnace tube and elbow at the low temperature section, but the structure was intact at the high temperature section. The cracked pipe was then repaired adopted shield metal arc weld (SMAW). The E308H electrodes were selected as filler metal according to the standard of SH/T 3417–2007 Technical specification of weld engineering for high alloy steel tube of petrochemical tubular heater [15], and the selected electrode diameter was φ 3.2 mm. The SMAW adopted direct current electrode positive (DCEP) with the current of 125A. Besides, the voltage and welding speed were 32 V and 60 cm/min respectively, the repaired furnace tubes did not take any measures to relieve the post weld stress after repair welding. Finally, the equipment leaked again on July 20, 2016. The analytical specimens were selected from the leakage furnace tube. And the specimen location is shown in Fig. 2. Fig. 2(a) shows the overall layout of furnace tubes, and Fig. 2(b) shows the cracked furnace tube.
3. Experimental analysis 3.1. Macro appearance Two specimens were obtained by cutting the leaking furnace tube, which were used for macro tests, are shown in Fig. 3. Fig. 3(a) and (b) show the specimen of straight pipe with cracks. The crack was located in the heat affected zone of the girth weld, and its length was about 1/4 of the furnace tube perimeter. The specimen of the elbow with cracks is shown in Fig. 3(c). Three cracks were found on its surface, one crack paralleled to the girth weld, the other two cracks paralleled to repair welding seam. These cracks were all in the heat affected zone and no crack was found on weld seam. Fig. 3(d) shows the fracture morphology around the girth weld. The fracture was even with an obvious morphology of brittle fracture, and the whole fracture was old and dark. It can be seen that the cracks have completely split. The welding condition of the specimen is shown in Fig. 3(e) and (f). There were some welding defects such as heterogeneous weld, incomplete penetration, and the inner wall of straight pipe and elbow was not aligned. 763
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Fig. 2. Furnace pipe of hydrogen manufacturing converted furnace.
Fig. 3(g) and (h) show the penetration test of the specimen. The cracks were found on the inner and outer walls, and the cracks of the inner wall were much longer than those of the outer wall. In addition, the cracks were propagated from inside toward the outside along the wall thickness. According to the macro appearances discussed above, we initially judged that the cracks originated from the inner wall. 3.2. Chemical composition According to design handbook, the reformer furnace tube was made of TP304H austenitic stainless steel, and its chemical composition was analyzed by direct reading spectrometer. The detected results and its standard requirements of furnace tubes are shown in Table 1. From the test results it can be seen that the composition met the requirement of ASTM A-312 M of steel except for the relatively low content of carbon. 3.3. Metallurgical structure In order to test the metallurgical structure of the elbow, samples were collected from the base metal with no crack a distance from the weld seam, the crack tip of the inner wall, the crack tip of the outer wall, and the crack along the wall thickness, as shown in Fig. 4. According to Fig. 4(a) and (b), the metallurgical structures in the vicinity of base metal and heat affected zone were equiaxed austenite. The metallurgical structure of the crack tip of inner and outer wall is shown in Fig. 4(c) and (d), respectively. Fig. 4(e) displays the appearance of crack propagation along the thickness under the polished state. After etching by aqua regia, the appearance of the crack initiation and crack tip region are shown in Fig. 4(f) and (g), respectively. It can be seen from Fig. 4(f) and (g) that the cracks propagated from the inner wall toward the outer wall and there are many branched cracks, which show the obvious intergranular cracking and the characteristics of SCC. In addition, two samples were taken from the heat affected zone and base metal with no crack, which were used to test the sensitization of the heat affected zone according to ASTM 262 practice A. The two samples were electrochemically etched in 10%(wt.) oxalic acid with a current density of 1A/cm2 for 90 s, and the metallurgical structures are shown in Fig. 4(h) and (i). It can be seen from the figures that many grains are completely surround by ditches in the heat affected zone, but the ditch structure was not found in the base metal. Fig. 4(j) shows the oxalic acid etch results on the base metal and heat affected zone, this figure displays the transitional change of ditches from the base metal to the heat affected zone. According to the metallurgical structures discussed above, it can be seen that the materials in the heat affected zone of furnace tube have been sensitized. 3.4. Micro appearance The fracture was observed by scanning electron microscope (SEM) as shown in Fig. 5. Fig.5(a) shows the appearance of the uncleaned fracture surface, and some obvious corrosion products can be seen. In addition, the appearance of the cleaned fracture surface is shown in Fig. 5(b). The fracture surface presents a typical intergranular fracture. Also, some secondary intergranular cracks were found on the fracture, and they show obvious characteristic of IGSCC. 764
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Fig. 3. Macro appearance of the cracked furnace tube and elbow.
Table 1 Chemical composition of reformer furnace tube (wt%). Element
C
Si
Mn
P
S
Ni
Cr
Measured ASTM A-312 M
0.031 0.04–0.10
0.417 ≤ 1.00
1.128 ≤ 2.00
0.033 ≤0.045
0.012 ≤ 0.030
8.494 8.00–11.00
18.24 18.00–20.00
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Fig. 4. Metallurgical structure and crack growth morphology.
3.5. Energy spectrum analysis Energy spectrum detection of the uncleaned fracture surface was carried out. The scanning positions are shown in Fig. 6 and 7 presents the scanning spectrum. The percentage of each component is shown in Table 2. From Table 2, it can be seen that the corrosion products contained more Na element and O element. The detection results indicated that the alkaline substance NaOH that is sensitive to SCC of austenitic stainless steel was in this medium. 766
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Fig. 4. (continued)
3.6. Residual stress measurement Because the furnace tubes of austenitic stainless steel do not need to relieve the post weld stress in the environment without stress corrosion sensitive medium according to the standard of SH/T 3417–2007 Technical specification of weld engineering for high alloy steel tube of petrochemical tubular heater [15], the furnace tubes do not adopt any measures to relieve the welding residual stress in this case, and the residual stresses in the vicinity of the welds can be determined by X-ray diffraction (XRD) using an X-350A residual stress analyzer with Chromium Kβ radiation and (311) diffracting plane. The tube voltage of the X-ray was 24 kV with a target current of 7 mA. The scanning range and scanning step of 2θ were 140°-162° and 0.1° respectively. The selected Ψ angles were 0°, 24.2°, 35.3°, and 45°, and the X-ray stress constant was -366 MPa/°. Fig. 8 presents the apparatus for measuring residual stress. The axial residual stress was measured as 93.4 MPa on the outer wall.
Fig. 5. Fracture appearance.
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Fig. 6. Scanning area.
4. Reasons of cracking 4.1. Material and structural factors The metallographic structures showed that the grain size of the heat affected zone and the grain boundaries around the crack were
Fig. 7. Scanning spectrum of the fracture.
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Table 2 Energy spectrum analysis results of the fracture (wt%). Element
C
P
O
Na
Si
Ti
Ca
Cr
Mn
Fe
Ni
Position-1 Position-2
4.11 4.11
0.30 0.35
31.36 36.84
2.30 3.45
0.88 1.44
0.23 0.20
0.28 0.30
10.62 9.33
1.02 0.87
45.69 38.12
3.21 4.99
coarse, and moreover, some black carbides precipitated at some of the grain boundaries. According to the metallographic structures discussed above, it can be determined that the materials in heat affected zone were sensitized, and intergranular corrosion and IGSCC occurred in the heat affect zone of the sensitive medium. In this case, the cracks only existed in the heat affected zone and no crack was found in areas a distance from the weld seam. So the material parameters of SCC can be attributed to the sensitization of heat affected zone by the residual heat of welding. In addition, the welding points had some welding defects such as heterogeneous welds, incomplete penetration, and the inner wall of straight pipe and elbow was not aligned. These defects could easily produce the residual stress and the accumulation of the medium, which provided the condition for the stress corrosion of the austenitic stainless steel. 4.2. Medium factors Tables 3 and 4 represent the composition of the drum steam and medium pressure steam from March1, 2016 to April 7, 2016, respectively. From these tables it can be seen that the maximum pH value of steam medium in the steam drum reached 11.35, which indicated that the medium contained more OH−. In medium pressure steam, the content of sodium ions was abnormal on March 5 and March 28, their values were 720 μg/L and 290 μg/L, respectively. The alkaline elements in the medium agreed with the results of the energy spectrum analysis. In addition, there were a few chloride ions and phosphate ions in the steam medium. Tables 5 and 6 display the steam composition in June, compared with the data before repair welding (Tables 3 and 4), it can be seen that the steam compositions were alkaline and they were similar before and after repair welding. The higher content of impurity elements in steam may be related to the bad separation efficiency of the gas-liquid separator. In the boiler, generally boiler compound is injected to adjust pH value and soften water quality [16]. The gas-liquid separator is used to separate the liquid drops in the steam, in order to avoid the boiler compound entering the steam. But if the separation efficiency of gas-liquid separator is bad, liquid drops will accompany the steam entering the superheated section. The liquid drops will rapidly evaporate under the heating of the exhaust gas, and will lead to the incrassation of salt in the inner wall of the furnace tube. Therefore, the salinity in the lower temperature section of the steam inlet is higher than the outlet side. The range of 60 °C to 300 °C is most sensitive to SCC. The temperature of the steam inlet was 265 °C, so the internal wall temperature was in the sensitive range. Due to the salinity and temperature, SCC occurred at the low temperature section, and no cracks were found in the high temperature zone. SCC of austenitic stainless steel can be caused by alkaline substances and chloride ions in steam. Considering that chloride ions
Fig. 8. The apparatus for measuring residual stress.
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Table 3 Test results of the steam drum (before repair welding). Date
Time
pH value
Conductivity μS/cm
Phosphate radical mg/L
Chloride ions mg/L
March 1, 2016 March 2, 2016 March 3, 2016 March 4, 2016 March 5, 2016 March 6, 2016 March 7, 2016 March 8, 2016 March 9, 2016 March 10, 2016 March 11, 2016 March 12, 2016 March 13, 2016 March 14, 2016 March 15, 2016 March 16, 2016 March 17, 2016 March 18, 2016 March 19, 2016 March 20, 2016 March 21, 2016 March 22, 2016 March 23, 2016 March 24, 2016 March 25, 2016 March 26, 2016 March 27, 2016 March 28, 2016 March 29, 2016 March 30, 2016 March 11, 2016 April 1, 2016 April 2, 2016 April 3, 2016 April 4, 2016 April 5, 2016 April 6, 2016 April 7, 2016
8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00
9.58 8.86 9.06 8.65 9.25 9.40 10.17 9.75 9.89 9.85 9.96 9.53 9.05 9.92 9.57 10.11 10.38 10.05 9.55 10.49 10.58 10.55 9.95 9.72 10.17 10.07 10.18 10.38 10.27 10.72 10.27 10.58 11.02 10.91 10.63 10.73 10.88 11.35
68.3 47.3 93.8 40.9 38.9 47.6 77.6 77.8 63.9 79.2 75.8 69.4 41.7 73.3 59.4 93.9 122.7 103.3 71.7 124.1 148.1 116.0 95.5 69.7 83.9 88.8 78.7 118.3 110.6 159.7 118.4 160.2 166.3 162.4 143.6 104.0 103.3 139.4
5.53 5.24 6.31 4.13 3.18 4.37 6.31 29.36 6.39 6.18 6.45 4.35 2.86 5.65 3.61 7.46 10.14 9.58 7.97 9.03 11.00 9.72 8.03 6.21 7.48 4.22 4.91 7.42 10.26 14.64 9.84 11.88 10.82 9.30 9.64 6.80 5.22 7.74
3.18 0.71 9.19 1.77 0.71 0.71 2.12 17.68 1.77 1.41 1.06 1.41 0.71 1.77 1.06 1.41 1.77 1.41 1.77 1.41 2.48 2.12 1.41 1.06 2.83 1.41 4.24 2.12 1.77 1.77 2.48 1.77 2.12 2.12 2.86 1.79 3.22 3.58
Table 4 Test results of medium pressure steam (before repair welding). Date
Time
Conductivity μS/cm
Silicon dioxide μg/L
Sodium content μg/L
Chloride ions mg/L
March 5, 2016 March 7, 2016 March 12, 2016 March 14, 2016 March 19, 2016 March 21, 2016 March 26, 2016 March 28, 2016 April 2, 2016 April 4, 2016
8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00
38.9 29.1 26.5 24.5 74.4 34.7 27.6 24.3 2.90 3.05
89.2 12.9 11.4 6.3 48.4 96.0 15.8 66.7 43.8 4.6
720 53.0 34.0 15.6 94.3 31.2 51.7 290 36.3 70.1
1.06 1.77 1.77 1.41 2.48 1.77 1.77 1.06 0.35 1.79
were not found on energy spectrum analysis, and the content of chlorine ions was lower in the steam, the primary medium factor of SCC was regarded as alkaline substances, and chloride ions were seen as a secondary factor. 4.3. Stress factors The existence of tensile stress is one of the prerequisites for SCC of austenitic steel [16]. Residual stresses during the welding process, thermal stress during the running process, and mechanical stress during internal pressure, provide convenient conditions for SCC. 770
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Table 5 Test results of steam drum (after repair welding). Date
Time
Conductivity μS/cm
Silicon dioxide μg/L
Sodium content μg/L
Chloride ions mg/L
June 1, 2016 June 2, 2016 June 3, 2016 June 4, 2016 June 5, 2016 June 6, 2016 June 7, 2016 June 8, 2016 June 9, 2016 June 10, 2016 June 11, 2016 June 12, 2016 June 13, 2016 June 14, 2016 June 15, 2016 June 16, 2016 June17, 2016 June 18, 2016 June 19, 2016 June 20, 2016 June 21, 2016 June 22, 2016 June 23, 2016 June 24, 2016 June 25, 2016 June 26, 2016 June 27, 2016 June 28, 2016 June 29, 2016 June 30, 2016
8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00
9.44 9.99 10.04 10.12 10.25 9.73 9.5 8.96 9.33 10.08 9.6 9.69 9.43 9.9 9.24 9.24 9.13 9.77 9.84 9.6 9.84 9.37 8.97 10.05 9.25 9.64 9.23 9.25 8.66 8.98
72.9 113.1 108 108.7 132.9 105.7 73.3 43.2 80 122.8 76 72.8 77.7 77.5 67.5 65.1 71.2 81.3 81.1 78.4 77.4 83.7 81.8 114.1 74.5 115.1 73.8 65 68.9 84.1
8.06 12.45 11.82 12.12 15.13 11.59 7.07 3.28 6.94 11.8 7.41 6.51 7.21 6.42 6.43 5.52 5.57 7.77 7.45 6.68 7.33 7.47 7.39 9.61 6.43 8.23 5.02 3.96 5.49 6.36
1.06 1.06 1.06 1.77 1.77 1.77 1.41 1.77 3.18 2.83 1.77 1.77 1.77 1.77 1.77 2.12 1.77 2.12 3.54 3.89 4.42 3.18 3.18 2.48 1.77 2.12 1.77 1.77 1.78 1.78
Table 6 Test results of medium pressure steam (After repair welding). Date
June June June June June June June June
4, 2016 6, 2016 11, 2016 13, 2016 18, 2016 20, 2016 25, 2016 27, 2016
Time
Conductivity μS/cm
Silicon dioxide μg/L
Sodium content μg/L
Chloride ions mg/L
8:00 8:00 8:00 8:00 8:00 8:00 8:00 8:00
25.2 23.6 17.6 17.19 28.3 27 20.3 25.1
4.8 6 7.7 9 26.8 36.3 29.3 2.9
193 158 139 101 770 760 50.8 200
1.77 2.12 1.77 1.41 4.95 3.54 1.77 3.18
5. Conclusions (1) All cracks were in the heat affected zone and no cracks were found in other parts. Therefore, the material factors of SCC can be attributed to the deterioration of corrosion resistance of materials in the heat affected zone during the welding process. (2) The alkaline substances and chloride ions were the medium factor of SCC, and the main cause was attributed to alkaline substances. Also, the higher pH value in the steam may be related to carried liquid due to the failure of the gas-liquid separator. (3) Tensile stress is only a basic guarantee of SCC. Studies have shown that stress corrosion cracking can be caused by lower tensile stress. But, a small amount of stress is unavoidable such as welding residual stress, mechanical stress caused by internal pressure, and thermal stress, and hence stress was not the immediate cause of SCC in this case.
Acknowledgment The authors wish to express their gratitude for the financial support by National Natural Science Foundation of China (51404284), Fundamental Research Funds for the Central Universities (15CX05011A), and Applied Fundamental Research Funds of Qingdao City (15-9-1-95-jch). Thanks to Dr. Edward C. Mignot, Shandong University, for linguistic advice. 771
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