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Failure analysis on speed reducer shaft of sluice gate in nuclear power plant
Engineering Failure Analysis 80 (2017) 453–463
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis on speed reducer shaft of sluice gate in nuclear power plant
MARK
Tong-Tong Bi, Zhen-Guo Yang⁎ Department of Materials Science, Fudan University, Shanghai 200433, PR China
AR TI CLE I NF O
AB S T R A CT
Keywords: Speed reducer shaft Brittle fracture Nuclear power plant Failure analysis
Irradiated fuels of nuclear power plants are usually preserved and cooled in specific water pools containing boric acid solution in nuclear power plants because of residual radioactivity and radiated heat. A sluice gate between two storage pools is used for isolation and connection of irradiated fuels. However, suddenly happened an abnormal fracture of a speed reducer shaft for driving sluice gate in a nuclear power plant with 650 MW capacity in the southern part of China, which could cause potential risk for storage safety of the fuel assemblies. Therefore, comprehensive analysis and investigation on the fractured shaft were carefully carried out in this study. The analysis results revealed that hydrogen embrittlement and temper brittleness induced during carburization and heat treatment were the root causes for the unexpected fracture of the shaft. Countermeasures and recommendations were then proposed and proved effective after implementation.
1. Introduction It is known that irradiated fuels of nuclear power plant are generally preserved and cooled in specific water pools containing boric acid solution because of residual radioactivity and radiated heat. For the isolation between the irradiated fuel pools as well as the control of the amount of boric acid solution under different circumstances, the sluice gate in a nuclear power plant with 650 MW capacity in southern part of China is driven by speed reducer, which is installed at 20-m-high platform from the bottom of water pool, as shown in Fig.1. Fig. 1(a) shows the external appearance of the sluice gate, and Fig. 1(b) illustrates schematic diagram of the structural components in the irradiated fuel pool. The sluice gate, which is manually driven through output end of speed reducer, is open on-off 10 times per year. The input end of the speed reducer is connected with 2.5-m-long shaft with a manual handle and its more detailed parameters are listed in Table 1. In June 2015, when the nuclear power plant was undertaken routine inspection, one of the speed reducer shafts was found fractured during disassembling inspection. It would obviously affect the normal opening on-off of the sluice gate and storage safety of the irradiated fuel matters in neighboring pools was at risk. In a normal condition, the design lifetime of the speed reducer shaft is forty years, unfortunately, the abnormal fracture of the rotating shaft suddenly occurred less than twelve years. Although its failure factors might be ascribed to one or more of many aspects, such as material selection, mechanical property, manufacture process, operational manner, maintaining way, service environment, etc. The primary factor should to be confirmed to as to ascertain safety operation of the nuclear power equipment. Indeed, some failure incidents and researches about relevant shafts have been reported [1–8], showing that improper heattreatment, stress fatigue, materials brittleness and external corrosion were main causes. But the failure of speed reducer shaft on the
⁎
Corresponding author. E-mail address: [email protected] (Z.-G. Yang).
http://dx.doi.org/10.1016/j.engfailanal.2017.07.021 Received 8 December 2016; Received in revised form 1 April 2017; Accepted 13 July 2017 Available online 14 July 2017 1350-6307/ © 2017 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 80 (2017) 453–463
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Fig. 1. Appearance and structure of irradiated fuel pool: (a) overall structure and (b) schematic diagram.
sluice gate of irradiated fuel pools in the nuclear power plant has not been reported. Furthermore, the unexpected failure of the shaft, which occurred under normal condition at low stress and less corrosive medium, was seldom disclosed. To this end, by means of various characterization methods, comprehensive analyses were carefully performed on the fractured speed reducer shaft. Then, some countermeasures and suggestions were proposed so as to prevent the recurrence of the similar incident, based on our previous experiences on the failure analysis [9–20]. Achievement of this study would not only ensure safe operation of the nuclear power plant, but also provide reference values for the safety of similar shafts used in other industrial fields.
Table 1 Operation parameters of speed reducer. Parameters
Model
Weight
Transmission ratio
Manual force
Product manufacturer
Speed reducer
1PKC140N
35 Kg
45
100 N
SEW transmission equipment (Tianjin) Co., Ltd.
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Fig. 2. Sampling and observation of the shaft: (a) position of the shaft (b) overall morphology (c) morphologies of fractures and (d) size of the speed reducer shaft.
2. Materials inspection 2.1. Visual observation and sampling Fig. 2 shows the whole appearance of the fractured shaft. Fig. 2(a) illustrates the position of the fractured shaft at the starting end of the keyway near step connection of the shaft neck. Its length is about 135 mm, as shown in Fig. 2(b), and because of the small size of the neck of shaft, it creates a stress concentration zone just next to the fracture position. Fig. 2(c) shows two parts of the fractured shaft, respectively marking the shorter part with A and the longer part with B for comparative analysis. The size of the speed reducer shaft was marked in the schematic diagram, as illustrated in Fig. 2(d).
2.2. Chemical compositions As one of the most common used instruments in measuring chemical compositions and contents of metal elements, the photoelectric direct reading spectrometer has high precision for most of metal elements with an atomic number larger than 10 and its measuring precision usually lies at the range of 0.2–2% if standard reference specimen is used for proof test. So chemical compositions of the fractured shaft were investigated by photoelectric direct reading spectrometer, and the results listed in Table 2. The 455
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Table 2 Chemical compositions of the fractured shaft (wt%). Element
C
Si
Mn
P
S
Cr
Ni
Mo
Fractured shaft 20CrMn
0.21 0.17–0.23
0.267 0.17–0.37
1.24 0.90–1.20
0.0110 –
0.029 –
1.19 0.90–1.20
0.106 –
0.0169 –
results show that the material for the speed reducer shaft of the sluice gate is a low carbon alloy steel with a specification of 20CrMn based on the specifications of 20CrMn steel in Chinese National Standards GB/T 3077–1999 [21] and it is one of the most used carburized steel.
2.3. Metallographic structures The sample of the fractured shaft was mounted by epoxy thermosetting resin and polished by metallographic polishing machine until the surface was bright and without scratches. Then discontinuous distributions of gray bar-shaped sulfide inclusions were found obviously under polishing state by the metallographic microscope, as seen in Fig. 3(a). According to the Chinese National Standards GB/T 10561–2005 [22], the class of inclusions were specified as A, more than level 3 individually, by comparing with the morphologies of standard figures with 100 times magnification of metallographic sample. Because the atoms of C would penetrate into the surface layer and form carburization layer on the surface of the shaft when the shaft was carburized [23], the color of carburization layer showed black after the sample was etched by nitric acid alcohol solution. We could observe the annular black carburization layer in Fig. 3(b) and its depth was about 1.2 mm. Similarly, stripped carburization layer could be observed on the
Fig. 3. Observations of the samples of the fractured shaft: (a) morphologies of inclusions, 100 × (b) sample in the transverse direction and (c) sample in the longitudinal direction.
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Fig. 4. Metallographic structures of fractured shaft: (a)surface, 1000× (b)transition part, 500 × (c)near core, 500 × and (d) core, 500 ×.
surface of the sample in the longitudinal direction, as shown in Fig. 3(c). With the help of the metallographic microscope, it was found that the metallographic structures transited gradually from high carbon needle-like martensite structure in surface to low carbon strip-like martensite in core, as seen in Fig. 4. The metallographic structures have representative features of carburized steel, but contain too much coarse grains of needle-like martensite in surface whose length is greater than the fine needle-like martensite. It can be ascribed to the high temperature and long duration in the heattreatment process of the shaft, which leads to increasing brittleness and decreasing toughness and plasticity [24–25], and consequently favors crack initiation and propagation caused by an impact load. 2.4. Micro-hardness Under the conditions that the test load was 980.7 mN (HV0.1) and the holding time was 10 s, the Vickers' micro-hardness of the fractured shaft in different metallographic structures were tested by micro-hardness tester. The maximum permissible errors of the micro-hardness tester are ± 10% and the results are listed in Table 3. It was revealed that the hardness generally increased from the core (401 MPa) to the surface (584 MPa) which were much higher than the standard data (196 MPa) according to the Chinese National Standards GB/T 3077–1999 [21]. 3. Fractographic analysis 3.1. Fracture A 3.1.1. Macroscopic morphologies As shown in the Fig. 5(a), the fracture A with gray surface is relatively flat, which is vertical to the axis. But a few friction streaks Table 3 Vickers micro-hardness of fractured shaft (HV0.1). Position
Core
Near core
Transition 1
Transition 2
Surface 1
Surface 2
Vickers micro Hardness(MPa)
401
423
473
496
522
584
457
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Fig. 5. Macroscopic morphologies of fracture A: (a) overall appearance (b) cracks near keyway (c) overlooked morphology (d) three directions cracks and (e) EDS micro-zone scanning analysis of the foreign substances.
and tawny rusts can be observed, without obvious plastic deformation. Two depressions exist on upper left and lower part of the fractured shaft. On the left of the keyway, a crack which initiates from the surface and grows along the radial direction of the shaft can be observed by 3D stereo microscope (3D-SM), as seen in Fig. 5(b). We observe the surface of the keyway, as shown in Fig. 5(c), two cracks intersect at right angles, marked with crack 1 and crack 2, respectively. With perspective changed, the crack 2 which propagated along the radial direction after the axial propagation formed crack 3. Eventually, the distinctive three directions cracks were formed, including the circumferential propagation, axial propagation and radial propagation, as seen in Fig. 5(d). Foreign substances on the left of crack 2 were identified by EDS micro-zone scanning analysis, as shown in Fig. 5(e). As one element corresponds to one color, and the brighter the color is, the more the content of element exists, the results reveal that foreign substances are mainly composed of C and O, but with just a little Fe, which are not corrosion products, as shown in Fig. 6. 3.1.2. Microscopic morphologies After thoroughly cleaned by ultrasonic cleaner, several micro-zones of fracture A were further studied by SEM and EDS. At the edge of the crack initiation, as shown in Fig. 7(a), some cleavage planes near the keyway with multiple steps exist on the right side of the crack 3. After magnified observation, some rod-shaped and spherical matters are densely distributed on the opposite side of the cleavage planes, as displayed in Fig. 7(b) and four micro-zones (including sites A, B, C and D) were respectively selected to inspect by EDS. Based on the results listed in Table 4, compositions of site A with flat surface were mainly C and Fe, which were from the steel matrix. The compositions of site B were mainly C, O and some traces of Cl, which were from oil contamination possibly. The compositions of granular site C were mainly Fe and O, which were from oxides of matrix materials. The compositions of site D were similar to site C, but with tiny bit of Cl, which were from oil contamination. What's more, S could be detected in different micro-zones on the fracture A, confirming that the sulfide inclusions actually existed which were found in metallographic structures previously (Fig. 3(a)). Along the right side of the cleavage planes, typical rock candy morphologies of brittle fracture could be clearly observed, as shown
Fig. 6. EDS scanning results of foreign substances: (a) element C (b) element O (c) element Fe.
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Fig. 7. SEM morphologies of fracture A: (a) cleavage morphology and (b) magnification of cleavage morphology. Table 4 The EDS results of micro-zones on the fracture A (wt%). Element
Fe
C
Mn
Si
Cr
O
Cl
S
V
Na
Mg
Al
Site Site Site Site
83.32 42.35 33.85 64.60
12.59 10.61 6.31 –
2.92 1.29 18.95 2.50
0.61 0.31 0.79 0.63
0.55 0.46 11.48 1.23
– 44.61 11.66 26.90
– 0.25 – 0.31
– 0.12 0.07 0.24
– – 0.40 –
– – – 1.81
– – – 0.70
– – – 0.58
A B C D
in Fig. 8(a). The appearance of the fracture is bright and has strong sense of stereoscopic feelings, which are typical characteristics of brittle fracture. As exhibited in Fig. 8(b) and (c), the cracks obviously propagate along the grain boundaries, forming a kind of intergranular cracking morphologies. There were some micro holes distributing in grain boundaries and their shapes were relatively regular circles. Those defects were typical features of hydrogen embrittlement, so we could confirm that the micro holes were produced by hydrogen aggregation in the grain boundaries during the carburizing process of the shaft [26–29]. 3.2. Fracture B 3.2.1. Macroscopic morphologies Fig. 9 shows the macroscopic appearance of fracture B, whose surface was relatively flat observed by 3D-SM, and there were several convex areas which were corresponding to the concave areas on the surface of fracture A. The concave-convex areas of two fractures were matched well, moreover, no obvious corrosion products were found, as shown in Fig. 9(a). Fibrous substances which were kinds of external foreign substances adhered to the surface of the keyway, as shown in Fig. 9(b). Several lacerated depressions could be observed on the lower side of the facture B, which were caused by collisions in the moment of ultimate fracture, as seen in Fig. 9(c). 3.2.2. Microscopic morphologies With the help of SEM, it was found that the rock candy morphologies of brittle fracture existed on the surface of fracture B, as shown in Fig. 10(a). Three micro-zones marked with No.1, 2 and 3 were individually chosen from surface to core for further observation. As seen in Figs. 10(b), (c) and (d), typical intergranular cracking morphologies generally existed in different micro-zones, which were similar to the morphologies observed on the fracture A. A certain number of grain separations occurred in different
Fig. 8. Hydrogen embrittlement morphologies: (a) 500 × (b) 2000× and (c) 2000×.
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Fig. 9. Macroscopic morphologies of fracture B: (a) appearance (b) morphology near the keyway and (c) morphology of the lower side.
Fig. 10. Morphologies of hydrogen embrittlement: (a) selected micro-zones (b) No.1, 5000 × (c) No.2, 5000 × and (d) No.3, 5000×.
micro-zones and then made the grain boundaries between grains be wider. In view of relatively regular circle shapes of all micro holes, we could further confirm that they were produced by hydrogen aggregation. All of them were typical features of hydrogen embrittlement caused by hydrogen aggregation during carburizing process and it fully proved that the hydrogen embrittlement was closely related to the brittle fracture of the shaft. 4. Failure analysis Based on the above-mentioned analysis and testing results, it was found that the rock candy morphologies were clearly observed in different micro-zones of the fractured shaft, and they were typical features of brittle fracture. In the following, we carefully carried out comprehensive analysis and discussion so as to find out the root cause for the premature fracture of the shaft. 4.1. Analysis of crack initiation From the view of metallographic structures of the fractured shaft (Fig. 4), the coarse grains of needle-like martensite in the surface structure with high brittleness and low toughness were easy to cause crack initiation. Depending on the macroscopic morphologies 460
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(Figs. 5 and 9), we found that the fracture of shaft was correspondingly flat and vertical to the axis, with features of brittle fracture presented. The cracks initiated from the surface of the keyway where stresses were concentrated, then propagated along the circumferential, axial and radial directions individually, forming the three directions cracks under a possible impact load. Based on microscopic morphologies (Figs. 7, 8 and 10), some cleavage planes were observed on the surface of the fracture, indicating that the brittleness of shaft was fairly high. And the cracks propagated along the grain boundaries and formed the intergranular cracking and rock candy morphologies of the brittle fracture of the shaft. 4.2. Analysis of materials brittleness As above results showed, the coarse grains of needle-like martensite existing in the surface layer of the metallographic structures greatly reduced the toughness of the shaft and favored its brittleness. The hardness of the shaft was more than 400 MPa, and the surface hardness even reached 584 MPa (Table 3), which were much higher than the standard data. Over-high surface hardness led to increasing brittleness and decreasing toughness of the shaft and favored crack initiation in the stress concentration zones under an impact load. Beyond that, intergranular cracking and the rock candy morphologies (Figs. 7, 8 and 10) ascribed to the temper brittleness of the shaft confirmed the brittle fracture of the shaft much further. What's more, a certain amount of micro holes, grain separations and wide grain boundaries widely existed in different micro-zones of the fractured shaft. Because carburized steels were easy to cause hydrogen aggregation on the grain boundaries during carburizing process and formed relatively regular circle-shaped holes [30]. Those defects can fully reveal that hydrogen embrittlement caused by hydrogen aggregation was a main cause for the premature failure of the shaft. 4.3. Analysis of torsional strength The maximum shear strength (τmax) on the surface of the shaft was calculated on account of Hibbeler [31], whose formulas were expressed as follows:
T = F1 × L
(1)
16T πd3
(2)
τmax =
Where T = manual torque (N.mm), F1 = manual force (N), L = radius of motion path (mm), τmax = maximum shear strength (MPa), d = radius of shaft (mm). On the basis of Table 1, manual force (F1) was about 100 N, and according to the schematic diagram of mechanism parts, as shown in Fig. 11, the radius of motion path (L) was 120 mm, and the radius of shaft (d) was 19.02 mm, so the maximum shear strength (τmax) was 9 MPa. Normally, the shear strength of a shaft could be estimated by tensile strength in accordance with the following empirical formula:
τb = (0.8~1.0) × σb
(3)
Where τb = shear strength (MPa), σb = tensile strength (MPa). In the light of Chinese National Standards GB/T 3077-1999 [21], the tensile strength of the materials of the shaft (σb) was 930 MPa, thus the shear strength was between 740–930 MPa. Obviously, the maximum shear strength loaded by manual force was far less than the shear strength of the shaft, which demonstrated that the shaft fractured at low stress. Furthermore, the shaft fractured at low stress in a brittle manner and it had a good relationship with the stress concentration
Fig. 11. Schematic diagram of mechanism parts.
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based on its fracture position. As the handbook of stress concentration factors and other researches [32–34] point out, the stress concentration factors are sharply increased with smaller of ratio of round radius to diameter of the shaft. In addition to those, the surface partial oxidization of the rotating parts of the sluice gate or blockage of foreign substances would cause the increase of opening-up load. Those would result in the manual force to be suddenly applied and produce an impact load on the shaft. So it was easy to induce crack initiation along the grain boundaries in the stress concentration zones of the shaft (Fig. 3(a)), leading to brittle fracture of the shaft occurred at low stress. 5. Conclusions and recommendations 5.1. Conclusions 1. By chemical compositions analysis and metallographic structures examination, it was confirmed that the material of speed reducer shaft of the sluice gate was 20CrMn low carbon alloy steel and contained a certain amount of inclusions and hydrogen embrittlement which were apt to induce cracking. 2. A certain amount of micro holes, grain separations and wide grain boundaries existing in the different micro-zones of the fractured shaft could fully prove that hydrogen embrittlement resulted from hydrogen aggregation during carburizing process was a root cause for the premature failure of the shaft. 3. The coarse grains of needle-like martensite, over-high surface hardness and rock candy morphologies of the shaft, were all ascribed to the temper brittleness resulting from improper heat-treatment process and those were other root causes for the brittle fracture of the shaft at low stress. 4. The surface partial oxidation of rotating parts of the sluice gate would cause a bigger resistant force for the opening-up of the sluice gate and impact load was artificially induced. Impact load exerted by manual operation was easy to induce crack initiation along brittle grain boundaries of the shaft and that was another cause for the brittle fracture of the shaft. 5.2. Recommendations 1. Adopting suitable alloy steel with good toughness as the shaft material, such as ultra-low carbon stainless steel 316 L or low carbon alloy steels like 20CrMnMo, 20CrMnTi can prevent the crack initiation from an impact load. 2. In order to prevent the recurrence of the similar incident, proper measurements should be utilized to reduce brittleness of the shaft, such as improving material quality of the shaft, controlling inclusion content, applying reasonable heat-treatment process as well as effective hydrogen removing process to eliminate temper brittleness of the shaft. 3. The manual load applied at the handle of the shaft must be slowly exerted so as to prevent introducing of artificial impact load during opening on-off of the sluice gate. 4. The good maintenance and timely inspection of the sluice gate should be strengthened to enhance the safety and reliability for the operation of the sluice gate. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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Failure analysis on speed reducer shaft of sluice gate in nuclear power plant
MARK
Tong-Tong Bi, Zhen-Guo Yang⁎ Department of Materials Science, Fudan University, Shanghai 200433, PR China
AR TI CLE I NF O
AB S T R A CT
Keywords: Speed reducer shaft Brittle fracture Nuclear power plant Failure analysis
Irradiated fuels of nuclear power plants are usually preserved and cooled in specific water pools containing boric acid solution in nuclear power plants because of residual radioactivity and radiated heat. A sluice gate between two storage pools is used for isolation and connection of irradiated fuels. However, suddenly happened an abnormal fracture of a speed reducer shaft for driving sluice gate in a nuclear power plant with 650 MW capacity in the southern part of China, which could cause potential risk for storage safety of the fuel assemblies. Therefore, comprehensive analysis and investigation on the fractured shaft were carefully carried out in this study. The analysis results revealed that hydrogen embrittlement and temper brittleness induced during carburization and heat treatment were the root causes for the unexpected fracture of the shaft. Countermeasures and recommendations were then proposed and proved effective after implementation.
1. Introduction It is known that irradiated fuels of nuclear power plant are generally preserved and cooled in specific water pools containing boric acid solution because of residual radioactivity and radiated heat. For the isolation between the irradiated fuel pools as well as the control of the amount of boric acid solution under different circumstances, the sluice gate in a nuclear power plant with 650 MW capacity in southern part of China is driven by speed reducer, which is installed at 20-m-high platform from the bottom of water pool, as shown in Fig.1. Fig. 1(a) shows the external appearance of the sluice gate, and Fig. 1(b) illustrates schematic diagram of the structural components in the irradiated fuel pool. The sluice gate, which is manually driven through output end of speed reducer, is open on-off 10 times per year. The input end of the speed reducer is connected with 2.5-m-long shaft with a manual handle and its more detailed parameters are listed in Table 1. In June 2015, when the nuclear power plant was undertaken routine inspection, one of the speed reducer shafts was found fractured during disassembling inspection. It would obviously affect the normal opening on-off of the sluice gate and storage safety of the irradiated fuel matters in neighboring pools was at risk. In a normal condition, the design lifetime of the speed reducer shaft is forty years, unfortunately, the abnormal fracture of the rotating shaft suddenly occurred less than twelve years. Although its failure factors might be ascribed to one or more of many aspects, such as material selection, mechanical property, manufacture process, operational manner, maintaining way, service environment, etc. The primary factor should to be confirmed to as to ascertain safety operation of the nuclear power equipment. Indeed, some failure incidents and researches about relevant shafts have been reported [1–8], showing that improper heattreatment, stress fatigue, materials brittleness and external corrosion were main causes. But the failure of speed reducer shaft on the
⁎
Corresponding author. E-mail address: [email protected] (Z.-G. Yang).
http://dx.doi.org/10.1016/j.engfailanal.2017.07.021 Received 8 December 2016; Received in revised form 1 April 2017; Accepted 13 July 2017 Available online 14 July 2017 1350-6307/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Appearance and structure of irradiated fuel pool: (a) overall structure and (b) schematic diagram.
sluice gate of irradiated fuel pools in the nuclear power plant has not been reported. Furthermore, the unexpected failure of the shaft, which occurred under normal condition at low stress and less corrosive medium, was seldom disclosed. To this end, by means of various characterization methods, comprehensive analyses were carefully performed on the fractured speed reducer shaft. Then, some countermeasures and suggestions were proposed so as to prevent the recurrence of the similar incident, based on our previous experiences on the failure analysis [9–20]. Achievement of this study would not only ensure safe operation of the nuclear power plant, but also provide reference values for the safety of similar shafts used in other industrial fields.
Table 1 Operation parameters of speed reducer. Parameters
Model
Weight
Transmission ratio
Manual force
Product manufacturer
Speed reducer
1PKC140N
35 Kg
45
100 N
SEW transmission equipment (Tianjin) Co., Ltd.
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Fig. 2. Sampling and observation of the shaft: (a) position of the shaft (b) overall morphology (c) morphologies of fractures and (d) size of the speed reducer shaft.
2. Materials inspection 2.1. Visual observation and sampling Fig. 2 shows the whole appearance of the fractured shaft. Fig. 2(a) illustrates the position of the fractured shaft at the starting end of the keyway near step connection of the shaft neck. Its length is about 135 mm, as shown in Fig. 2(b), and because of the small size of the neck of shaft, it creates a stress concentration zone just next to the fracture position. Fig. 2(c) shows two parts of the fractured shaft, respectively marking the shorter part with A and the longer part with B for comparative analysis. The size of the speed reducer shaft was marked in the schematic diagram, as illustrated in Fig. 2(d).
2.2. Chemical compositions As one of the most common used instruments in measuring chemical compositions and contents of metal elements, the photoelectric direct reading spectrometer has high precision for most of metal elements with an atomic number larger than 10 and its measuring precision usually lies at the range of 0.2–2% if standard reference specimen is used for proof test. So chemical compositions of the fractured shaft were investigated by photoelectric direct reading spectrometer, and the results listed in Table 2. The 455
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Table 2 Chemical compositions of the fractured shaft (wt%). Element
C
Si
Mn
P
S
Cr
Ni
Mo
Fractured shaft 20CrMn
0.21 0.17–0.23
0.267 0.17–0.37
1.24 0.90–1.20
0.0110 –
0.029 –
1.19 0.90–1.20
0.106 –
0.0169 –
results show that the material for the speed reducer shaft of the sluice gate is a low carbon alloy steel with a specification of 20CrMn based on the specifications of 20CrMn steel in Chinese National Standards GB/T 3077–1999 [21] and it is one of the most used carburized steel.
2.3. Metallographic structures The sample of the fractured shaft was mounted by epoxy thermosetting resin and polished by metallographic polishing machine until the surface was bright and without scratches. Then discontinuous distributions of gray bar-shaped sulfide inclusions were found obviously under polishing state by the metallographic microscope, as seen in Fig. 3(a). According to the Chinese National Standards GB/T 10561–2005 [22], the class of inclusions were specified as A, more than level 3 individually, by comparing with the morphologies of standard figures with 100 times magnification of metallographic sample. Because the atoms of C would penetrate into the surface layer and form carburization layer on the surface of the shaft when the shaft was carburized [23], the color of carburization layer showed black after the sample was etched by nitric acid alcohol solution. We could observe the annular black carburization layer in Fig. 3(b) and its depth was about 1.2 mm. Similarly, stripped carburization layer could be observed on the
Fig. 3. Observations of the samples of the fractured shaft: (a) morphologies of inclusions, 100 × (b) sample in the transverse direction and (c) sample in the longitudinal direction.
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Fig. 4. Metallographic structures of fractured shaft: (a)surface, 1000× (b)transition part, 500 × (c)near core, 500 × and (d) core, 500 ×.
surface of the sample in the longitudinal direction, as shown in Fig. 3(c). With the help of the metallographic microscope, it was found that the metallographic structures transited gradually from high carbon needle-like martensite structure in surface to low carbon strip-like martensite in core, as seen in Fig. 4. The metallographic structures have representative features of carburized steel, but contain too much coarse grains of needle-like martensite in surface whose length is greater than the fine needle-like martensite. It can be ascribed to the high temperature and long duration in the heattreatment process of the shaft, which leads to increasing brittleness and decreasing toughness and plasticity [24–25], and consequently favors crack initiation and propagation caused by an impact load. 2.4. Micro-hardness Under the conditions that the test load was 980.7 mN (HV0.1) and the holding time was 10 s, the Vickers' micro-hardness of the fractured shaft in different metallographic structures were tested by micro-hardness tester. The maximum permissible errors of the micro-hardness tester are ± 10% and the results are listed in Table 3. It was revealed that the hardness generally increased from the core (401 MPa) to the surface (584 MPa) which were much higher than the standard data (196 MPa) according to the Chinese National Standards GB/T 3077–1999 [21]. 3. Fractographic analysis 3.1. Fracture A 3.1.1. Macroscopic morphologies As shown in the Fig. 5(a), the fracture A with gray surface is relatively flat, which is vertical to the axis. But a few friction streaks Table 3 Vickers micro-hardness of fractured shaft (HV0.1). Position
Core
Near core
Transition 1
Transition 2
Surface 1
Surface 2
Vickers micro Hardness(MPa)
401
423
473
496
522
584
457
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Fig. 5. Macroscopic morphologies of fracture A: (a) overall appearance (b) cracks near keyway (c) overlooked morphology (d) three directions cracks and (e) EDS micro-zone scanning analysis of the foreign substances.
and tawny rusts can be observed, without obvious plastic deformation. Two depressions exist on upper left and lower part of the fractured shaft. On the left of the keyway, a crack which initiates from the surface and grows along the radial direction of the shaft can be observed by 3D stereo microscope (3D-SM), as seen in Fig. 5(b). We observe the surface of the keyway, as shown in Fig. 5(c), two cracks intersect at right angles, marked with crack 1 and crack 2, respectively. With perspective changed, the crack 2 which propagated along the radial direction after the axial propagation formed crack 3. Eventually, the distinctive three directions cracks were formed, including the circumferential propagation, axial propagation and radial propagation, as seen in Fig. 5(d). Foreign substances on the left of crack 2 were identified by EDS micro-zone scanning analysis, as shown in Fig. 5(e). As one element corresponds to one color, and the brighter the color is, the more the content of element exists, the results reveal that foreign substances are mainly composed of C and O, but with just a little Fe, which are not corrosion products, as shown in Fig. 6. 3.1.2. Microscopic morphologies After thoroughly cleaned by ultrasonic cleaner, several micro-zones of fracture A were further studied by SEM and EDS. At the edge of the crack initiation, as shown in Fig. 7(a), some cleavage planes near the keyway with multiple steps exist on the right side of the crack 3. After magnified observation, some rod-shaped and spherical matters are densely distributed on the opposite side of the cleavage planes, as displayed in Fig. 7(b) and four micro-zones (including sites A, B, C and D) were respectively selected to inspect by EDS. Based on the results listed in Table 4, compositions of site A with flat surface were mainly C and Fe, which were from the steel matrix. The compositions of site B were mainly C, O and some traces of Cl, which were from oil contamination possibly. The compositions of granular site C were mainly Fe and O, which were from oxides of matrix materials. The compositions of site D were similar to site C, but with tiny bit of Cl, which were from oil contamination. What's more, S could be detected in different micro-zones on the fracture A, confirming that the sulfide inclusions actually existed which were found in metallographic structures previously (Fig. 3(a)). Along the right side of the cleavage planes, typical rock candy morphologies of brittle fracture could be clearly observed, as shown
Fig. 6. EDS scanning results of foreign substances: (a) element C (b) element O (c) element Fe.
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Fig. 7. SEM morphologies of fracture A: (a) cleavage morphology and (b) magnification of cleavage morphology. Table 4 The EDS results of micro-zones on the fracture A (wt%). Element
Fe
C
Mn
Si
Cr
O
Cl
S
V
Na
Mg
Al
Site Site Site Site
83.32 42.35 33.85 64.60
12.59 10.61 6.31 –
2.92 1.29 18.95 2.50
0.61 0.31 0.79 0.63
0.55 0.46 11.48 1.23
– 44.61 11.66 26.90
– 0.25 – 0.31
– 0.12 0.07 0.24
– – 0.40 –
– – – 1.81
– – – 0.70
– – – 0.58
A B C D
in Fig. 8(a). The appearance of the fracture is bright and has strong sense of stereoscopic feelings, which are typical characteristics of brittle fracture. As exhibited in Fig. 8(b) and (c), the cracks obviously propagate along the grain boundaries, forming a kind of intergranular cracking morphologies. There were some micro holes distributing in grain boundaries and their shapes were relatively regular circles. Those defects were typical features of hydrogen embrittlement, so we could confirm that the micro holes were produced by hydrogen aggregation in the grain boundaries during the carburizing process of the shaft [26–29]. 3.2. Fracture B 3.2.1. Macroscopic morphologies Fig. 9 shows the macroscopic appearance of fracture B, whose surface was relatively flat observed by 3D-SM, and there were several convex areas which were corresponding to the concave areas on the surface of fracture A. The concave-convex areas of two fractures were matched well, moreover, no obvious corrosion products were found, as shown in Fig. 9(a). Fibrous substances which were kinds of external foreign substances adhered to the surface of the keyway, as shown in Fig. 9(b). Several lacerated depressions could be observed on the lower side of the facture B, which were caused by collisions in the moment of ultimate fracture, as seen in Fig. 9(c). 3.2.2. Microscopic morphologies With the help of SEM, it was found that the rock candy morphologies of brittle fracture existed on the surface of fracture B, as shown in Fig. 10(a). Three micro-zones marked with No.1, 2 and 3 were individually chosen from surface to core for further observation. As seen in Figs. 10(b), (c) and (d), typical intergranular cracking morphologies generally existed in different micro-zones, which were similar to the morphologies observed on the fracture A. A certain number of grain separations occurred in different
Fig. 8. Hydrogen embrittlement morphologies: (a) 500 × (b) 2000× and (c) 2000×.
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Fig. 9. Macroscopic morphologies of fracture B: (a) appearance (b) morphology near the keyway and (c) morphology of the lower side.
Fig. 10. Morphologies of hydrogen embrittlement: (a) selected micro-zones (b) No.1, 5000 × (c) No.2, 5000 × and (d) No.3, 5000×.
micro-zones and then made the grain boundaries between grains be wider. In view of relatively regular circle shapes of all micro holes, we could further confirm that they were produced by hydrogen aggregation. All of them were typical features of hydrogen embrittlement caused by hydrogen aggregation during carburizing process and it fully proved that the hydrogen embrittlement was closely related to the brittle fracture of the shaft. 4. Failure analysis Based on the above-mentioned analysis and testing results, it was found that the rock candy morphologies were clearly observed in different micro-zones of the fractured shaft, and they were typical features of brittle fracture. In the following, we carefully carried out comprehensive analysis and discussion so as to find out the root cause for the premature fracture of the shaft. 4.1. Analysis of crack initiation From the view of metallographic structures of the fractured shaft (Fig. 4), the coarse grains of needle-like martensite in the surface structure with high brittleness and low toughness were easy to cause crack initiation. Depending on the macroscopic morphologies 460
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(Figs. 5 and 9), we found that the fracture of shaft was correspondingly flat and vertical to the axis, with features of brittle fracture presented. The cracks initiated from the surface of the keyway where stresses were concentrated, then propagated along the circumferential, axial and radial directions individually, forming the three directions cracks under a possible impact load. Based on microscopic morphologies (Figs. 7, 8 and 10), some cleavage planes were observed on the surface of the fracture, indicating that the brittleness of shaft was fairly high. And the cracks propagated along the grain boundaries and formed the intergranular cracking and rock candy morphologies of the brittle fracture of the shaft. 4.2. Analysis of materials brittleness As above results showed, the coarse grains of needle-like martensite existing in the surface layer of the metallographic structures greatly reduced the toughness of the shaft and favored its brittleness. The hardness of the shaft was more than 400 MPa, and the surface hardness even reached 584 MPa (Table 3), which were much higher than the standard data. Over-high surface hardness led to increasing brittleness and decreasing toughness of the shaft and favored crack initiation in the stress concentration zones under an impact load. Beyond that, intergranular cracking and the rock candy morphologies (Figs. 7, 8 and 10) ascribed to the temper brittleness of the shaft confirmed the brittle fracture of the shaft much further. What's more, a certain amount of micro holes, grain separations and wide grain boundaries widely existed in different micro-zones of the fractured shaft. Because carburized steels were easy to cause hydrogen aggregation on the grain boundaries during carburizing process and formed relatively regular circle-shaped holes [30]. Those defects can fully reveal that hydrogen embrittlement caused by hydrogen aggregation was a main cause for the premature failure of the shaft. 4.3. Analysis of torsional strength The maximum shear strength (τmax) on the surface of the shaft was calculated on account of Hibbeler [31], whose formulas were expressed as follows:
T = F1 × L
(1)
16T πd3
(2)
τmax =
Where T = manual torque (N.mm), F1 = manual force (N), L = radius of motion path (mm), τmax = maximum shear strength (MPa), d = radius of shaft (mm). On the basis of Table 1, manual force (F1) was about 100 N, and according to the schematic diagram of mechanism parts, as shown in Fig. 11, the radius of motion path (L) was 120 mm, and the radius of shaft (d) was 19.02 mm, so the maximum shear strength (τmax) was 9 MPa. Normally, the shear strength of a shaft could be estimated by tensile strength in accordance with the following empirical formula:
τb = (0.8~1.0) × σb
(3)
Where τb = shear strength (MPa), σb = tensile strength (MPa). In the light of Chinese National Standards GB/T 3077-1999 [21], the tensile strength of the materials of the shaft (σb) was 930 MPa, thus the shear strength was between 740–930 MPa. Obviously, the maximum shear strength loaded by manual force was far less than the shear strength of the shaft, which demonstrated that the shaft fractured at low stress. Furthermore, the shaft fractured at low stress in a brittle manner and it had a good relationship with the stress concentration
Fig. 11. Schematic diagram of mechanism parts.
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based on its fracture position. As the handbook of stress concentration factors and other researches [32–34] point out, the stress concentration factors are sharply increased with smaller of ratio of round radius to diameter of the shaft. In addition to those, the surface partial oxidization of the rotating parts of the sluice gate or blockage of foreign substances would cause the increase of opening-up load. Those would result in the manual force to be suddenly applied and produce an impact load on the shaft. So it was easy to induce crack initiation along the grain boundaries in the stress concentration zones of the shaft (Fig. 3(a)), leading to brittle fracture of the shaft occurred at low stress. 5. Conclusions and recommendations 5.1. Conclusions 1. By chemical compositions analysis and metallographic structures examination, it was confirmed that the material of speed reducer shaft of the sluice gate was 20CrMn low carbon alloy steel and contained a certain amount of inclusions and hydrogen embrittlement which were apt to induce cracking. 2. A certain amount of micro holes, grain separations and wide grain boundaries existing in the different micro-zones of the fractured shaft could fully prove that hydrogen embrittlement resulted from hydrogen aggregation during carburizing process was a root cause for the premature failure of the shaft. 3. The coarse grains of needle-like martensite, over-high surface hardness and rock candy morphologies of the shaft, were all ascribed to the temper brittleness resulting from improper heat-treatment process and those were other root causes for the brittle fracture of the shaft at low stress. 4. The surface partial oxidation of rotating parts of the sluice gate would cause a bigger resistant force for the opening-up of the sluice gate and impact load was artificially induced. Impact load exerted by manual operation was easy to induce crack initiation along brittle grain boundaries of the shaft and that was another cause for the brittle fracture of the shaft. 5.2. Recommendations 1. Adopting suitable alloy steel with good toughness as the shaft material, such as ultra-low carbon stainless steel 316 L or low carbon alloy steels like 20CrMnMo, 20CrMnTi can prevent the crack initiation from an impact load. 2. In order to prevent the recurrence of the similar incident, proper measurements should be utilized to reduce brittleness of the shaft, such as improving material quality of the shaft, controlling inclusion content, applying reasonable heat-treatment process as well as effective hydrogen removing process to eliminate temper brittleness of the shaft. 3. The manual load applied at the handle of the shaft must be slowly exerted so as to prevent introducing of artificial impact load during opening on-off of the sluice gate. 4. The good maintenance and timely inspection of the sluice gate should be strengthened to enhance the safety and reliability for the operation of the sluice gate. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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