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Failure analysis of a turbo-disk
Engineering Failure Analysis 14 (2007) 226–232 www.elsevier.com/locate/engfailanal
Failure analysis of a turbo-disk Zhiwei Yu *, Xiaolei Xu, Yuzhou Gao, Shiyong Liu Electromechanics and Material Engineering College, Dalian Maritime University, Dalian 116026, PR China Received 13 July 2005; accepted 24 October 2005 Available online 7 February 2006
Abstract A failure investigation has been conducted on the turbo-disk used in a locomotive turbochanger, which is made from GH2132 high-temperature alloy. The fracture of three teeth and the crack of 11 teeth took place, which mainly occurred at the third root near the groove. Fractographic studies indicate that the crack originated at the root and propagated along the turning direction of the turbo-disk. In the initial crack propagation zone several different failure mechanisms performed successively, which are the intergranular corrosion crack, the corrosion-fatigue propagation and the dimple fracture. The greater bending stress on the tooth root and the presence of the corrosion medium in the service circumstance should be responsible for the failure of the turbo-disk. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Turbo-disk; Intergranular corrosion; Corrosion fatigue; Dimple fracture; Failure analysis
1. Introduction The fracture and the crack occurred on the teeth of the turbo-disk, which is used in a locomotive turbochanger. The failed turbo-disk is made of GH2132 high-temperature alloy. It was reported by locomotive manufacturer that the turbo-disk had been repaired many times before failure. Accumulated service time is about 30,000 h. The design life of the turbo-disk is 50,000 h. The paper describes the careful fractographic study and metallurgical investigation on the failed turbo-disk. The possible failure reasons were assessed. 2. Experimental methods The chemical composition of the failed turbo-disk material was determined by spectroscopy chemical analysis method. The microcomposition in various zones of the fracture surfaces and the intergranular crack on the sectional specimens taken from the fractures was determined by energy dispersive X-ray spectrometer (EDX). The microstructure of the sectional specimens was observed by scanning electron microscopy (SEM) on a Philips XL-30 scanning electron microscope. The fracture surfaces were analyzed by visual and *
Corresponding author. Tel.: +86 0411 84729613; fax: +86 0411 84728670. E-mail address: [email protected] (Z.W. Yu).
1350-6307/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2005.10.010
Z.W. Yu et al. / Engineering Failure Analysis 14 (2007) 226–232
227
SEM observation to study the failure mechanism. Macrohardness (HRC) of the turbo-disk material was conducted. The tensile properties of the turbo-disk material were measured on an MST-880 machine. 3. Results and discussion 3.1. Macroscopic features The failed turbo-disk is shown in Fig. 1. The fractured teeth are numbered as in Fig. 1. From the remains of the failed turbo-disk the following macro features can be seen: Of the 16 teeth on the failed turbo-disk (which is uncompleted turbo-disk) three teeth had fractured and 11 teeth had cracked. The failure ratio of the teeth is up to 87.5%. The fracture and crack positions are mainly situated at the third root near the groove (shown by arrows). Three fractured teeth (1, 2, 3) neighbor each other. The macrograph of the fracture surfaces of the teeth (1, 2 and 3) is shown in Fig. 2. The fracture surfaces show the typical fatigue fracture appearance with radiating beach marks. From the orientation of the beach marks, it can be concluded that the fatigue fracture origins of the three fractured teeth is situated on the one side of the wheel (by arrows). And the centre of the origin is located at the 1/5 of the tooth length. Crack initiated from the root and propagated along the turning direction of the turbo-disk. Every fracture surface has four different color regions from the tooth root to the inner (Fig. 3a and b), which are black (marked A), light yellow (marked B), brown (marked C) and grey regions (marked D) in turn. The black and the light yellow regions constitute the front fringe of the arc crack propagation region, whose depth on the three fractures are, respectively 3.3, 2.0, 1.1 mm. By calculation, the area of the arc crack propagation region in the three fractures is, respectively 21%, 10%, 4% the whole fracture area, but the grey instantaneous fracture region area is successively 40%, 20% and 10% the whole fracture area. From the crack propagation orientation, the depth of the arc crack propagation and the instantaneous fracture region area, it can be
Fig. 1. Failed turbo-disk.
Fig. 2. Macrograph of the fracture surface of the failed teeth.
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Fig. 3. SEM observations on fracture surface of the tooth No. 1: (a) general view of the fractured tooth, (b) general view showing different fracture mechanism, (c) intergranular fracture, (d) corrosion-fatigue striations, (e) interface between the intergranular and fatigue regions, (f) dimple fracture, (g) strain fatigue fracture and (h) dimple and intergranular fracture.
concluded that the fracture sequence of the three teeth is 1, 2, then 3 in turn. The three teeth bore greater bending load when fracturing. It is noted that the crack positions of the other cracked teeth are in agreement with the crack origin position of the fractured teeth (Fig. 1). Additionally, some shorter cracks were found at the first and the second root (Fig. 4) and on another side of the turbo-disk backward the crack origin, a few of teeth appear cracks at the third root.
Z.W. Yu et al. / Engineering Failure Analysis 14 (2007) 226–232
229
Fig. 4. Morphology of the cracked tooth.
The striking marks can be observed on the tooth face of the cracked tooth near the fractured tooth (by arrows in Fig. 1). However, the crack of this tooth has no relation to the striking of the fractured teeth, which would be further demonstrated by the observation on the sectional specimens of the cracked tooth. 3.2. Microscopic features SEM observations on the three fracture surfaces indicate that the four different color regions on the macrofractures correspond to the different fracture modes. The black region near the root exhibits the intergranular corrosion fracture morphology (Fig. 3c) and the light yellow region linked to the black region shows the brittle fatigue striations (Fig. 3d), which attributed to the corrosion-fatigue characteristic [2,3]. An obvious interface between the intergranular fracture region and the fatigue fracture can be observed (Fig. 3e). The brown and grey regions show mainly dimple morphology (Fig. 3f), and wider fatigue striations in the some region which attribute to the strain fatigue fracture can be found (Fig. 3g). In the instantaneous fracture region near the root for tooth No. 1 and No. 2, the intergranular fracture can be observed and the propagation depth of the intergranular crack is about 0.25 mm (Fig. 3h). From the crack propagation marks, it can be concluded that the intergranular crack had occurred before the crack propagated to the region. Frequent change in the fracture mechanism is related to the abrupt change in the stress station and the circumstance medium.
Fig. 5. Microstructure of the turbo-disk material.
230
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3.3. Microstructure examination 3.3.1. Microstructure of the base material The microstructure of the failed turbo-disk material was examined by SEM. The results show that the microstructure is mainly composed of homogeneous austenite (Fig. 5), which is quite common for this grade of alloy. 3.3.2. Observation on the sectional specimens The fractures (teeth 1, 2, and 3) were cut transversely, and then the sectional specimens were prepared. Observations show that the intergranular fracture position and depth of the three fractured teeth completely correspond to the microstructure of the sectional specimen, as shown in Fig. 6a (tooth 1). In the intergranular fracture region, an obvious edges and corners morphology can be observed. The intergranular crack propagation depth corresponds generally to the size of one grain along the direction normal to the fracture (Fig. 6b). The intergranular corrosion crack depths in the sectional specimens correspond to the depth of the intergranular crack region on the fractures. It must be mentioned the intergranular precipitates were not found in the microstructure of base material. It is suggested that occurrence of the intergranular fracture do not result from the intergranular precipitates. The external stress and corrosion medium in service circumstance should be responsible for the intergranular fracture. The observations on the sectional specimens of the cracked teeth
Fig. 6. Sectional morphology of the fractured teeth No. 1: (a) low-powered sectional morphology and (b) high-powered sectional morphology of intergranular region.
Fig. 7. Sectional morphology of the cracked teeth: (a) low-powered sectional morphology and (b) high-powered sectional morphology of intergranular region.
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show that the crack propagated along the turning direction of the turbo-disk after the third root cracked (Fig. 7a). In the initiative crack propagation region, the sectional morphology exhibits the intergranular fracture feature showing an obvious edges and corner (Fig. 7b). It is further indicated that crack of the teeth did not result from the striking by the fractured teeth. The teeth cracked independently in service. Yet, only because the crack propagation rate was is relatively slow, the teeth did not fracture fully. Additionally, some shorter cracks appear at the second root on the tooth face toward the turning direction of the turbo-disk and the intergranular fracture morphology can be observed inside the crack. 3.4. Chemical composition Table 1 gives the chemical composition of the failed turbo-disk material, compared with the specified. It can be seen that the chemical composition of the failed turbo-disk material is within the specified range. The composition of the products in various regions on the fracture surfaces (the intergranular fracture zone and fatigue fracture region) and within the intergranular cracks of the sectional specimen was determined by EDAX (Table 2). It can be seen that elements O, S, Cl, K, Ca can be found in the intergranular crack of the sectional specimen. The covering on the intergranular fracture region (region A) are mainly composed of elements O, Zn, Na, P, S, Ca. However, the amount of these elements in the fatigue fracture region (the region B) is very small. The impurity in the intergranular cracks and the covering on the intergranular fracture region should be the products of gas and the circumstance medium. The composition difference in the covering of the intergranular and the fatigue fracture regions is related to the change of the service circumstance. 3.5. Mechanical properties Standard cylindrical tensile specimens were machined from the position shown in Fig. 1. The tensile properties were evaluated by breaking the specimen in tension. The tensile properties (average of the two measurements) and macrohardness (average of the three measurements) are shown in Table 3, which correspond to the technical demands. Table 1 Chemical composition of the turbo-disk material (wt%) Element
C
Al
Mo
Ti
V
Cr
Ni
Si
Mn
P
S
Analysis As-specified
0.03 60.08
0.29 60.40
1.39 1.00–1.50
1.98 1.75–2.30
0.35 0.10–0.50
15.51 13.5–16.0
25.36 24.0–27.0
0.62 61.00
1.54 62.00
0.015 0.030
0.018 0.020
Table 2 Chemical composition of the products at the crack and the covering on the fracture (wt%) Position
C
O
Boundary
1.95
11.26 /
Intergranular fracture regions
11.62 22.72 0.89 1.29 0.26 0.47 0.93 0.83 0.21 / 11.32 23.41 1.42 1.07 0.15 0.41 1.00 0.82 0.17 / 13.65 21.82 2.66 1.42 0.54 0.80 2.04 1.59 0.34 /
Fatigue fracture regions
7.18 5.22 4.21
6.85 7.59 4.88
Zn
Na
Al
Si
P
/
0.20 0.44 /
Mo
S
Cl
K
Ca
Ti
V
Cr
/
0.72 3.67 0.51 0.34 1.32 0.26 14.93 1.13 8.74 0.43 0.46 0.86 0.07 3.07 0.33 0.48 0.83 0.08 3.40 0.92 1.03 0.91 0.12 5.17
Mn
Ni
1.02 3.99 1.38 4.50 1.07 /
Mg / 0.14 0.13 0.44
0.21 0.58 0.17 0.49 0.45 1.27 0.06 0.16 0.16 0.18 1.61 0.32 12.64 1.20 20.34 / / 0.53 0.30 0.62 0.34 1.17 0.12 0.06 0.09 0.14 1.71 0.34 13.48 1.39 22.68 / 0.09 0.22 0.28 0.63 0.23 1.75 / / 0.09 0.16 1.83 0.34 13.78 1.56 23.24 0.13
Table 3 Tensile properties of the turbo-disk material
Tensile properties (measured) Tensile properties (requirement)
Tensile strength rb (MPa)
Yield strength r0.2 (MPa)
Elongation d5 (%)
Reduction in area w (%)
HRC
1105 P950
820 P630
19.8 P20
40.5 P40
29 28–34
232
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4. Failure causes analysis From the observation and examination in Section 3, it is inferred that the composition, microstructure of the material are within the technical specification. The mechanical properties mainly correspond to the specified in spite of having a long time service. As described in Section 3, intergranular corrosion, fatigue corrosion occurred on the teeth of the turbodisk. However, absence of the intergranular precipitates in base material indicates that the intergranular fracture has no relation to the microstructure change of the base material in service. The previous data indicate that intergranular corrosion damage was closely related to the service environment condition, such as the relative humidity of the service region [1]. Through the analysis of the corrosion products on the fracture and intergranular crack of the sectional specimens, it can be concluded that S, Zn, etc. resulting from the products of gas and Cl, Ca, etc. resulting from the circumstance medium in service region especially in the coastal area may promotes the intergranular corrosion. According to the working principle of the turbo-disk, teeth of the turbo-disk mainly bear the radial centrifugal force, which lead to the bending stress on the tooth root. The intergranular crack would occur on the weaken grain boundary by intergranular corrosion and corrosion-fatigue propagation perform under the action of the bending to result in premature failure of the turbo-disk. 5. Conclusions 1. The material of the failed turbo-disk is GH2132 high temperature alloy, which corresponds to the specified material. Mechanical properties and microstructure of the material are within the technical specification. The internal defects of the turbo-disk, including raw material and manufacturing or process defects can be excluded. 2. Fracture and crack ratio of the teeth for the turbo-disk are high, which are 87.5%. The fracture and crack positions of the teeth are mainly concentrated at the third root near the groove. 3. The crack originated at the root and propagated along the turning direction of the turbo-disk. In the initiative crack propagation zone, different fracture modes are dominated. The failure modes are successively the intergranular corrosion crack ! the corrosion-fatigue propagation ! the dimple fracture. 4. The greater bending stress at the tooth root of the turbo-disk and the presence of the corrosion medium in the service circumstance are responsible for the fracture and the crack of the teeth of the turbo-disk.
Acknowledgment This project was funded by National Natural Science Foundation of China. References [1] Xie ML, Zhong PD, Xi NS, Zhang Y, Tao CH. Analysis of failure of fir-tree serrations of stage II turbine disks. Eng Fail Anal 2000;7:249–60. [2] ASM metal handbook. Failure analysis and preventation. Metal Park (OH): American Society for Metals; 1986. [3] Metal handbook. Fractography and atlas of fractographs, vol. 9. 8th ed. Metals Park (OH): American Society for Metals; 1974. Yu Zhiwei, Professor, is working in Electronicmechanics and Materials Engineering College, Dalian Maritime University. He is mainly engaging in failure analysis and surface modification of the materials. Xu Xiaolei, Professor, is working in Electronicmechanics and Materials Engineering College, Dalian Maritime University. She is mainly engaging in failure analysis and surface modification of the materials. Yu Zhou Gao, Assistant professor, is working in Electronicmechanics and Materials Engineering College, Dalian Maritime University. He is mainly engaging in failure analysis and surface modification of the materials. Shiyong Liu, Professor, is working in Electronicmechanics and Materials Engineering College, Dalian Maritime University. He is mainly engaging in failure analysis and surface modification of the materials.
Failure analysis of a turbo-disk Zhiwei Yu *, Xiaolei Xu, Yuzhou Gao, Shiyong Liu Electromechanics and Material Engineering College, Dalian Maritime University, Dalian 116026, PR China Received 13 July 2005; accepted 24 October 2005 Available online 7 February 2006
Abstract A failure investigation has been conducted on the turbo-disk used in a locomotive turbochanger, which is made from GH2132 high-temperature alloy. The fracture of three teeth and the crack of 11 teeth took place, which mainly occurred at the third root near the groove. Fractographic studies indicate that the crack originated at the root and propagated along the turning direction of the turbo-disk. In the initial crack propagation zone several different failure mechanisms performed successively, which are the intergranular corrosion crack, the corrosion-fatigue propagation and the dimple fracture. The greater bending stress on the tooth root and the presence of the corrosion medium in the service circumstance should be responsible for the failure of the turbo-disk. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Turbo-disk; Intergranular corrosion; Corrosion fatigue; Dimple fracture; Failure analysis
1. Introduction The fracture and the crack occurred on the teeth of the turbo-disk, which is used in a locomotive turbochanger. The failed turbo-disk is made of GH2132 high-temperature alloy. It was reported by locomotive manufacturer that the turbo-disk had been repaired many times before failure. Accumulated service time is about 30,000 h. The design life of the turbo-disk is 50,000 h. The paper describes the careful fractographic study and metallurgical investigation on the failed turbo-disk. The possible failure reasons were assessed. 2. Experimental methods The chemical composition of the failed turbo-disk material was determined by spectroscopy chemical analysis method. The microcomposition in various zones of the fracture surfaces and the intergranular crack on the sectional specimens taken from the fractures was determined by energy dispersive X-ray spectrometer (EDX). The microstructure of the sectional specimens was observed by scanning electron microscopy (SEM) on a Philips XL-30 scanning electron microscope. The fracture surfaces were analyzed by visual and *
Corresponding author. Tel.: +86 0411 84729613; fax: +86 0411 84728670. E-mail address: [email protected] (Z.W. Yu).
1350-6307/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2005.10.010
Z.W. Yu et al. / Engineering Failure Analysis 14 (2007) 226–232
227
SEM observation to study the failure mechanism. Macrohardness (HRC) of the turbo-disk material was conducted. The tensile properties of the turbo-disk material were measured on an MST-880 machine. 3. Results and discussion 3.1. Macroscopic features The failed turbo-disk is shown in Fig. 1. The fractured teeth are numbered as in Fig. 1. From the remains of the failed turbo-disk the following macro features can be seen: Of the 16 teeth on the failed turbo-disk (which is uncompleted turbo-disk) three teeth had fractured and 11 teeth had cracked. The failure ratio of the teeth is up to 87.5%. The fracture and crack positions are mainly situated at the third root near the groove (shown by arrows). Three fractured teeth (1, 2, 3) neighbor each other. The macrograph of the fracture surfaces of the teeth (1, 2 and 3) is shown in Fig. 2. The fracture surfaces show the typical fatigue fracture appearance with radiating beach marks. From the orientation of the beach marks, it can be concluded that the fatigue fracture origins of the three fractured teeth is situated on the one side of the wheel (by arrows). And the centre of the origin is located at the 1/5 of the tooth length. Crack initiated from the root and propagated along the turning direction of the turbo-disk. Every fracture surface has four different color regions from the tooth root to the inner (Fig. 3a and b), which are black (marked A), light yellow (marked B), brown (marked C) and grey regions (marked D) in turn. The black and the light yellow regions constitute the front fringe of the arc crack propagation region, whose depth on the three fractures are, respectively 3.3, 2.0, 1.1 mm. By calculation, the area of the arc crack propagation region in the three fractures is, respectively 21%, 10%, 4% the whole fracture area, but the grey instantaneous fracture region area is successively 40%, 20% and 10% the whole fracture area. From the crack propagation orientation, the depth of the arc crack propagation and the instantaneous fracture region area, it can be
Fig. 1. Failed turbo-disk.
Fig. 2. Macrograph of the fracture surface of the failed teeth.
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Fig. 3. SEM observations on fracture surface of the tooth No. 1: (a) general view of the fractured tooth, (b) general view showing different fracture mechanism, (c) intergranular fracture, (d) corrosion-fatigue striations, (e) interface between the intergranular and fatigue regions, (f) dimple fracture, (g) strain fatigue fracture and (h) dimple and intergranular fracture.
concluded that the fracture sequence of the three teeth is 1, 2, then 3 in turn. The three teeth bore greater bending load when fracturing. It is noted that the crack positions of the other cracked teeth are in agreement with the crack origin position of the fractured teeth (Fig. 1). Additionally, some shorter cracks were found at the first and the second root (Fig. 4) and on another side of the turbo-disk backward the crack origin, a few of teeth appear cracks at the third root.
Z.W. Yu et al. / Engineering Failure Analysis 14 (2007) 226–232
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Fig. 4. Morphology of the cracked tooth.
The striking marks can be observed on the tooth face of the cracked tooth near the fractured tooth (by arrows in Fig. 1). However, the crack of this tooth has no relation to the striking of the fractured teeth, which would be further demonstrated by the observation on the sectional specimens of the cracked tooth. 3.2. Microscopic features SEM observations on the three fracture surfaces indicate that the four different color regions on the macrofractures correspond to the different fracture modes. The black region near the root exhibits the intergranular corrosion fracture morphology (Fig. 3c) and the light yellow region linked to the black region shows the brittle fatigue striations (Fig. 3d), which attributed to the corrosion-fatigue characteristic [2,3]. An obvious interface between the intergranular fracture region and the fatigue fracture can be observed (Fig. 3e). The brown and grey regions show mainly dimple morphology (Fig. 3f), and wider fatigue striations in the some region which attribute to the strain fatigue fracture can be found (Fig. 3g). In the instantaneous fracture region near the root for tooth No. 1 and No. 2, the intergranular fracture can be observed and the propagation depth of the intergranular crack is about 0.25 mm (Fig. 3h). From the crack propagation marks, it can be concluded that the intergranular crack had occurred before the crack propagated to the region. Frequent change in the fracture mechanism is related to the abrupt change in the stress station and the circumstance medium.
Fig. 5. Microstructure of the turbo-disk material.
230
Z.W. Yu et al. / Engineering Failure Analysis 14 (2007) 226–232
3.3. Microstructure examination 3.3.1. Microstructure of the base material The microstructure of the failed turbo-disk material was examined by SEM. The results show that the microstructure is mainly composed of homogeneous austenite (Fig. 5), which is quite common for this grade of alloy. 3.3.2. Observation on the sectional specimens The fractures (teeth 1, 2, and 3) were cut transversely, and then the sectional specimens were prepared. Observations show that the intergranular fracture position and depth of the three fractured teeth completely correspond to the microstructure of the sectional specimen, as shown in Fig. 6a (tooth 1). In the intergranular fracture region, an obvious edges and corners morphology can be observed. The intergranular crack propagation depth corresponds generally to the size of one grain along the direction normal to the fracture (Fig. 6b). The intergranular corrosion crack depths in the sectional specimens correspond to the depth of the intergranular crack region on the fractures. It must be mentioned the intergranular precipitates were not found in the microstructure of base material. It is suggested that occurrence of the intergranular fracture do not result from the intergranular precipitates. The external stress and corrosion medium in service circumstance should be responsible for the intergranular fracture. The observations on the sectional specimens of the cracked teeth
Fig. 6. Sectional morphology of the fractured teeth No. 1: (a) low-powered sectional morphology and (b) high-powered sectional morphology of intergranular region.
Fig. 7. Sectional morphology of the cracked teeth: (a) low-powered sectional morphology and (b) high-powered sectional morphology of intergranular region.
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show that the crack propagated along the turning direction of the turbo-disk after the third root cracked (Fig. 7a). In the initiative crack propagation region, the sectional morphology exhibits the intergranular fracture feature showing an obvious edges and corner (Fig. 7b). It is further indicated that crack of the teeth did not result from the striking by the fractured teeth. The teeth cracked independently in service. Yet, only because the crack propagation rate was is relatively slow, the teeth did not fracture fully. Additionally, some shorter cracks appear at the second root on the tooth face toward the turning direction of the turbo-disk and the intergranular fracture morphology can be observed inside the crack. 3.4. Chemical composition Table 1 gives the chemical composition of the failed turbo-disk material, compared with the specified. It can be seen that the chemical composition of the failed turbo-disk material is within the specified range. The composition of the products in various regions on the fracture surfaces (the intergranular fracture zone and fatigue fracture region) and within the intergranular cracks of the sectional specimen was determined by EDAX (Table 2). It can be seen that elements O, S, Cl, K, Ca can be found in the intergranular crack of the sectional specimen. The covering on the intergranular fracture region (region A) are mainly composed of elements O, Zn, Na, P, S, Ca. However, the amount of these elements in the fatigue fracture region (the region B) is very small. The impurity in the intergranular cracks and the covering on the intergranular fracture region should be the products of gas and the circumstance medium. The composition difference in the covering of the intergranular and the fatigue fracture regions is related to the change of the service circumstance. 3.5. Mechanical properties Standard cylindrical tensile specimens were machined from the position shown in Fig. 1. The tensile properties were evaluated by breaking the specimen in tension. The tensile properties (average of the two measurements) and macrohardness (average of the three measurements) are shown in Table 3, which correspond to the technical demands. Table 1 Chemical composition of the turbo-disk material (wt%) Element
C
Al
Mo
Ti
V
Cr
Ni
Si
Mn
P
S
Analysis As-specified
0.03 60.08
0.29 60.40
1.39 1.00–1.50
1.98 1.75–2.30
0.35 0.10–0.50
15.51 13.5–16.0
25.36 24.0–27.0
0.62 61.00
1.54 62.00
0.015 0.030
0.018 0.020
Table 2 Chemical composition of the products at the crack and the covering on the fracture (wt%) Position
C
O
Boundary
1.95
11.26 /
Intergranular fracture regions
11.62 22.72 0.89 1.29 0.26 0.47 0.93 0.83 0.21 / 11.32 23.41 1.42 1.07 0.15 0.41 1.00 0.82 0.17 / 13.65 21.82 2.66 1.42 0.54 0.80 2.04 1.59 0.34 /
Fatigue fracture regions
7.18 5.22 4.21
6.85 7.59 4.88
Zn
Na
Al
Si
P
/
0.20 0.44 /
Mo
S
Cl
K
Ca
Ti
V
Cr
/
0.72 3.67 0.51 0.34 1.32 0.26 14.93 1.13 8.74 0.43 0.46 0.86 0.07 3.07 0.33 0.48 0.83 0.08 3.40 0.92 1.03 0.91 0.12 5.17
Mn
Ni
1.02 3.99 1.38 4.50 1.07 /
Mg / 0.14 0.13 0.44
0.21 0.58 0.17 0.49 0.45 1.27 0.06 0.16 0.16 0.18 1.61 0.32 12.64 1.20 20.34 / / 0.53 0.30 0.62 0.34 1.17 0.12 0.06 0.09 0.14 1.71 0.34 13.48 1.39 22.68 / 0.09 0.22 0.28 0.63 0.23 1.75 / / 0.09 0.16 1.83 0.34 13.78 1.56 23.24 0.13
Table 3 Tensile properties of the turbo-disk material
Tensile properties (measured) Tensile properties (requirement)
Tensile strength rb (MPa)
Yield strength r0.2 (MPa)
Elongation d5 (%)
Reduction in area w (%)
HRC
1105 P950
820 P630
19.8 P20
40.5 P40
29 28–34
232
Z.W. Yu et al. / Engineering Failure Analysis 14 (2007) 226–232
4. Failure causes analysis From the observation and examination in Section 3, it is inferred that the composition, microstructure of the material are within the technical specification. The mechanical properties mainly correspond to the specified in spite of having a long time service. As described in Section 3, intergranular corrosion, fatigue corrosion occurred on the teeth of the turbodisk. However, absence of the intergranular precipitates in base material indicates that the intergranular fracture has no relation to the microstructure change of the base material in service. The previous data indicate that intergranular corrosion damage was closely related to the service environment condition, such as the relative humidity of the service region [1]. Through the analysis of the corrosion products on the fracture and intergranular crack of the sectional specimens, it can be concluded that S, Zn, etc. resulting from the products of gas and Cl, Ca, etc. resulting from the circumstance medium in service region especially in the coastal area may promotes the intergranular corrosion. According to the working principle of the turbo-disk, teeth of the turbo-disk mainly bear the radial centrifugal force, which lead to the bending stress on the tooth root. The intergranular crack would occur on the weaken grain boundary by intergranular corrosion and corrosion-fatigue propagation perform under the action of the bending to result in premature failure of the turbo-disk. 5. Conclusions 1. The material of the failed turbo-disk is GH2132 high temperature alloy, which corresponds to the specified material. Mechanical properties and microstructure of the material are within the technical specification. The internal defects of the turbo-disk, including raw material and manufacturing or process defects can be excluded. 2. Fracture and crack ratio of the teeth for the turbo-disk are high, which are 87.5%. The fracture and crack positions of the teeth are mainly concentrated at the third root near the groove. 3. The crack originated at the root and propagated along the turning direction of the turbo-disk. In the initiative crack propagation zone, different fracture modes are dominated. The failure modes are successively the intergranular corrosion crack ! the corrosion-fatigue propagation ! the dimple fracture. 4. The greater bending stress at the tooth root of the turbo-disk and the presence of the corrosion medium in the service circumstance are responsible for the fracture and the crack of the teeth of the turbo-disk.
Acknowledgment This project was funded by National Natural Science Foundation of China. References [1] Xie ML, Zhong PD, Xi NS, Zhang Y, Tao CH. Analysis of failure of fir-tree serrations of stage II turbine disks. Eng Fail Anal 2000;7:249–60. [2] ASM metal handbook. Failure analysis and preventation. Metal Park (OH): American Society for Metals; 1986. [3] Metal handbook. Fractography and atlas of fractographs, vol. 9. 8th ed. Metals Park (OH): American Society for Metals; 1974. Yu Zhiwei, Professor, is working in Electronicmechanics and Materials Engineering College, Dalian Maritime University. He is mainly engaging in failure analysis and surface modification of the materials. Xu Xiaolei, Professor, is working in Electronicmechanics and Materials Engineering College, Dalian Maritime University. She is mainly engaging in failure analysis and surface modification of the materials. Yu Zhou Gao, Assistant professor, is working in Electronicmechanics and Materials Engineering College, Dalian Maritime University. He is mainly engaging in failure analysis and surface modification of the materials. Shiyong Liu, Professor, is working in Electronicmechanics and Materials Engineering College, Dalian Maritime University. He is mainly engaging in failure analysis and surface modification of the materials.