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Failure analysis of journal bearing used in turboset of a power plant
Materials and Design 52 (2013) 923–931
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Technical Report
Failure analysis of journal bearing used in turboset of a power plant Yongyong He ⇑, Zhongkai Zhao, Tianyu Luo, Xinchun Lu, Jianbin Luo State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 7 April 2013 Accepted 11 June 2013 Available online 26 June 2013
a b s t r a c t This paper presents a failure investigation on the damaged journal bearing, which supports the mainshaft of the Turboset of a power plant. The damage position is located at the hydrodynamic lubrication area of the bearing. A detailed fractographic and metallurgical investigation is conducted on the damaged lining layer of the bearing, which is made of tin-based Babbitt alloys, to analyze the damage mechanism and assess the possible reasons for this failure. The investigation results indicate that the main cause of the failure should be the direct contact and the rubbing between the shaft and the lining layer of the bearing at the damage position. Such contact and rubbing will cause local high temperature in the damage region and also cause the content decrease of the hard inclusion Cu6Sn5 on the Babbitt layer surface, and therefore deteriorate the mechanical performance of the Babbitt layer of the bearing and lead to the serious damage to the Babbitt layer eventually. By this research, some direct and valuable information is obtained for Babbitt lining layer design and operation condition optimization of bearings. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In 1837, Babbitt patented a Sn–Sb–Cu alloy for use in journal bearings, such kinds of the material are therefore named Babbitt alloys and also called white metals. Babbitt alloys can be fundamentally classified into two types: one has lead as its main component, the other tin, called lead-based Babbitt and tin-based Babbitt respectively [1]. Comparatively, tin-based Babbitts are more widely used in preference to lead-based Babbitts due to their more excellent corrosion resistance, easier bonding, and less tendency towards segregation [1]. Babbitt alloys, either tin-based or leadbased ones, have excellent embeddability and conformability characteristics (due to their softness), unsurpassed compatibility with steel shafts, good antifriction properties and unique ability to adapt to misalignment by mild wiping on initial run-in (due to their low melting points) [1–5]. The representative physical and mechanical properties of typical Babbitt alloys are summarized in Ref. [5]. Therefore, Babbitt alloys have been widely used as the lining layer materials of journal bearings used in many industrial applications [5–8]. Over the past decades, many investigations have been done intensively on the tribological and mechanical properties of Babbitt alloys as lining layer materials of journal bearings [1,9–14]. For journal bearings, the desired operation condition should be that a proper thickness lubricant film is formed and maintained between the shaft and the bearing. However, mixed and boundary lubrications are inevitable in the processes of start and stop, or under severe working conditions, such as insufficient lubricant sup⇑ Corresponding author. Tel./fax: +86 10 62787932. E-mail address: [email protected] (Y. He). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.06.027
ply, high operating temperature, and heavy loading [15], which will cause direct contact between the shaft and the lining layer of the bearing and eventually cause various damages to the bearing [15]. Wear and burning of the lining layer are the frequent failures of journal bearings due to abnormal hydrodynamic condition, which always cause shutdown or even catastrophic accident to machinery. Therefore, it is also very meaningful to investigate the damage mechanism of Babbitt alloys as the lining layer of journal bearings. Such investigations can explore the weak points of the properties of Babbitts under improper operation conditions and thus can provide useful information for better design of Babbitts and manufacturing of bearings for better properties against damages. In addition, such investigations can also provide information for adjusting proper operation of bearings so that safe running of bearings can be guaranteed. Unfortunately, the operation condition of journals bearings is always complex, and cannot be simulated really and easily by experiments, and thus the corresponding damages also cannot be simulated and obtained really and accurately by experiments. Therefore, case study by real damaged bearings from real applications is the necessary and useful way to investigate damage mechanism of Babbitt bearings, and such study can provide direct, real and valuable information for the bearing design and operation condition optimization. Recently, we obtained a severe damaged journal bearing from a power plant. It gives us a rare opportunity to carry out such a case study on the damage mechanism of Babbitt layer by a real damaged bearing. The background is illustrated as follows. During the recent scheduled maintenance of the turboset (power: 987 MW; working speed: 3000 r/min) in a power plant, it was reported that there existed some burned black substance deposited on the lining layer surface of the bearing, which supports
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Fig. 1. Photographs of the damaged journal bearing.
the main-shaft of the Turboset, at the position of the hydrodynamic lubrication area of the bearing according to the calculation. Especially, severe burning and fracture damage was observed on the lining layer at the same position of the bearing, as shown in Fig. 1. The lining layer of the bearing is made of Babbitt alloy. This bearing is the cylinder one composed of the upper part and the lower part. The width, the outer radius and the inner radius are 279.6 mm, 319.81 mm and 300.00 mm, respectively. The thickness of the Babbitt layer is about 1.8 mm. The damage position of the bearing is illustrated in Fig. 2 when the bearing is expanded into a plane. The area of the damaged region is about 24.9 mm 19.1 mm. To investigate the damage mechanism of the Babbitt alloy, this paper presents a detailed fractographic and metallurgical investigation on the damaged Babbitt layer of the bearing. The possible reasons for this damage are analyzed and assessed. The investigation could provide a case study of the damage mechanism of Babbitt alloys as lining materials of journal bearings
Fig. 2. Damage position of bearing.
Fig. 4. Specimen preparation illustration of Region 2: (a) cutting order illustration and (b) cutting results.
Fig. 3. Region illustration of specimen preparation.
and also could provide some valuable information for better bearing design, better bearing manufacturing and Turboset running optimization.
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Fig. 5. Fractural section preparation.
2. Specimens preparation and investigation methods For comparison analysis, the specimens were taken from Region 1 and Region 2 of the bearing respectively, as shown in Fig. 3. Region 1 can be regarded as the region where the Babbitt layer of the bearing is healthy and has no damage. Region 2 is the damaged region. From Region 1, 10 specimens of 10 mm 10 mm size were prepared. For Region 2, in order to preserve the damage condition and information of this region as far as possible, a specimen of 40 mm 40 mm size was sectioned from this region first, which covers the whole damage region. Then, this specimen was sectioned into 8 smaller specimens in the order as illustrated in Fig. 4a, i.e. from 1-1 to 6-6 cross section orderly. Fig. 4b shows the sectioning result. The Specimen A in Fig. 4b was used to observe and investigate the fracture and the crack condition in particular. In order to preserve the original condition of the crack, the specimen A was cut by a vee gap first at the other side opposite to the crack side. Then the specimen was broken off along the crack section by using cold quenching of liquid nitrogen, as shown in Fig. 5. X ray diffractometer (XRD) was used to perform the phase analysis of the specimens. After XRD analysis, the metallographic structure and distribution of the surface and the cross section of the specimens were observed and analyzed further by metalloscope. Sweep electron microscopy (SEM) was used to observe and analyze the microstructure of the two regions. SEM was also used to observe and analyze the fracture surfaces to investigate the damage mechanism of the crack region. In addition, energy spectrum analysis was used to analyze the components of various phases in the two regions and further determine if some elements exist which may cause properties degradation of the Babbitt layer in the two regions of the bearing. 3. Results and analysis 3.1. Chemical composition Table 1 gives the chemical compositions of the Babbitt layer of the bearing. From this table, it is known that the Babbitt layer is fabricated from tin-base Babbitt alloy of ZSnSb8Cu4.
Fig. 6. XRD result of the specimens: (a) Region 1 and (b) Region 2.
3.2. Phase analysis Using XRD, phase analysis was conducted on the specimens from Region 1 and Region 2 respectively. Fig. 6 shows the XRD analysis results. From this figure, it can be seen that, the strongest diffraction peak appears at 2h ¼ 33 , the second one appears at 2h ¼ 46 , and the third one appears at 2h ¼ 31 . From these XRD results and by checking the Powder Diffraction File (PDF), it is known that a-Sn phase exists in the specimens from both Region 1 and Region 2; at 2h ¼ 29 ; 42 ; 52 ; 69 , there exist small peaks, which mean that SnSb phase exists in the specimens; at 2h ¼ 44 ; 61 , there exist smaller peaks, which mean that there exists Cu6Sn5 phase in the specimens. As is well known, the Cu6Sn5 phase is the hard inclusion of tin-based Babbitt alloy [5]. However, according to the structural diagram of the Sn–Sb alloy [16], the SnSb phase should not exist in these specimens theoretically. Therefore, it is inferred that, when casting the Babbitt layer, the cooling speed was not so quick that the reaction occurred between the liquid and a solid solution phase which led to the precipitation of SnSb crystal [17]. This result is also consistent with that of Ref. [18]. Comparing Fig. 6a with b, it can be seen that the content of the Cu6Sn5 phase decreases in the specimen of Region 2. Just as
Table 1 Chemical composition of Babbitt layer of shell. Element
Sn
Sb
Cu
Pb
Zn
Fe
As
Ni
Bi
Cd
Content (%)
–
7.5
3.4
0.014
0.001
0.038
0.011
0.2
<0.001
0.95
Element Content (%)
Ag 0.08
Al <0.0002
In 0.026
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mentioned above, Cu6Sn5 is the hard inclusion of the alloy, which distributes in the Sn matrix and supports the shaft. If direct contact occurs between the lining surface of the bearing and the shaft, wear and even spalling will be produced on the lining surface. Therefore, from the content decease of Cu6Sn5 in the damaged region, it can be inferred that direct contact and rubbing should oc-
curs between the shaft and the bearing in Region 2 which leads to wear, even spalling, of the Cu6Sn5 hard inclusion on the Babbitt layer surface. The spalling of Cu6Sn5 will leave small pits on the surface of the Babbitt layer, which can be observed in following metallographic observation. Wear and spalling damage was also observed in the research of Ref. [19].
Fig. 7. Metallographic diagrams of specimen surface from Region 1.
Fig. 8. Metallographic diagrams of specimen surface from Region 2.
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3.3. Metallographic analysis of specimen surface Fig. 7 shows the metallographic diagrams of the specimen surface from Region 1. From this figure, it can be seen that, the white reticular structures disperse in the black matrix, among which some acicular, granular and star-shaped microstructures distribute densely. Combined with the energy spectrum analysis, it is known that, the white reticular structure of about 30 lm, the star-shaped one of about 30 lm and the granular one of about 1 lm are all the Cu6Sn5 phase which is the main hard inclusion of the alloy. The acicular structure of about 5 lm is the SnSb phase. However, from the standard metallographic structure of the Babbitt alloy [20], it is known that there should be no reticular Cu6Sn5 structure and acicular SnSb structure. In addition, the size of star-shaped Cu6Sn5 is much smaller than that of the standard structure about 160 lm. Therefore, it can be also inferred that, when casting the Babbitt layer, the cooling speed was not so quick that the hard brittle phase Cu6Sn5 precipitated along the grain boundary with reticular distribution and the SnSb phase precipitated from the a solid solution with acicular shape [21]. This inference is also consistent with the phase analysis result in Section 3.2. In addition, compared with the standard of tin-base Babbitt alloy, this Babbitt alloy contains much high Cd content (see Table 1), which will contribute to making Cu6Sn5 grain smaller, promoting the mechanical properties of the alloy and transforming the cuboid SnSb into the acicular one [22]. Fig. 8 shows the metallographic diagrams of the specimen surface from Region 2. From this figure, it can be seen that, the acicular Cu6Sn5 phases of about 15 lm, the granular Cu6Sn5 phases of about 5 lm and the acicular SnSb phases of about 1 lm disperse in the black matrix of a solid solution. Spot segregation phenomenon is obvious. The Cu6Sn5 phases gather regionally by the reticu-
Fig. 11. XRD result of the fracture.
lar manner, and the incipient cracks tend to initiate along the gathered Cu6Sn5 phases. Fig. 8b and c show such kind of the crack more clearly. The crack expands along the brittle second phase Cu6Sn5 and can be identified as the intergranular fracture. It can be inferred that, the brittle second phase Cu6Sn5 deposits along the grain boundary and makes the surface free energy of the grain boundary decrease, and the crack tends to expand along the direction of the lowest energy to cause the intergranular fracture. In addition, comparing Fig. 8 with Fig. 7, it can be seen that, in Fig. 8, the number of the star-shaped Cu6Sn5 grain decreases, the number of the granular Cu6Sn5 grain increases, and the density of the hard inclusion Cu6Sn5 decreases obviously. The size of the Cu6Sn5 grain also decreases. Especially, obvious fragmentation of
Fig. 9. Microscope structure of the surface of Region 2 by SEM.
Fig. 10. Microscope of a big crack in Region 2 by SEM.
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Fig. 12. Microscope structure of the fracture region: (a) new formed fracture; (b–d): original fracture.
the Cu6Sn5 grain can be observed in the crack regions. In addition, some small pits can be observed on the surface of the damaged region, which also supports the inference in section 3.2. The reasons of these phenomena can be inferred that, the Cu6Sn5 deposits along the grain boundary, therefore the cracks expand along the grain boundary and make the Cu6Sn5 precipitate from the grain boundary and break, which leads to the decrease of the Cu6Sn5 grain in density. In addition, it can be inferred that direct contact and rubbing should occur between the shaft and the bearing at Region 2, which may spall the hard inclusion Cu6Sn5 and lead to the further decrease of the Cu6Sn5 grain finally. This conclusion is consistent with that of Section 3.1. Above analysis results confirm that: direct contact and rubbing should occur between the shaft and the bearing at Region 2; the rubbing causes the spalling of the hard inclusion from the Babbitt
layer surface and also causes local high temperature in the rubbing region, which lead to the degradation of the mechanical performance of the Babbitt layer in the rubbing region and thus promote the crack initiation and expansion; the crack initiation and expansion cause the decrease of the hard inclusion of the Babbitt layer surface and degrade the mechanical performance further, and lead to the severe burning and fracture damage in Region 2 eventually. 3.4. Microscope structure analysis Fig. 9 shows the microscope structure diagrams of the surface of Region 2 by SEM. From Fig. 9b, it can be seen clearly that, the crack tends to expand along the grain boundary, and the crack is of clear bifurcating feature. Fig. 10 shows the microscope structure diagram of a big crack in Region 2. From Fig. 10a, it can be seen that
Fig. 13. Selected points on the fracture for energy spectrum analysis: (a) original fracture and (b) new formed fracture.
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the cracks of 1-1 region and 2-2 region are of obvious quasi-cleavage crack feature. Fig. 10b is the enlarged view of region A in Fig. 10a. From this figure, it can be seen that, the fracture surface is mat and is covered by some film and much granular substance. The granular substance is identified as the Cu6Sn 5 grain by energy spectrum analysis. By further observation, it is found that there exist some micro pits on the fracture surface, which means that the fracture is of ductility to some degree. It is well known that, the intergranular fracture is one kind of the brittle fracture. However, for some material with good ductility, such as aluminum, its intergranular fracture manifests obvious ductility and plastic deforma-
tion. Such intergranular fracture is named as the ductile intergranular fracture [23]. From above analysis, it is concluded that, the crack of the superficial layer is of quasi-cleavage crack feature, which is caused mainly by the mechanical rubbing and wear; while the crack of the deep layer is of ductile intergranular fracture feature, which should be caused by high temperature and high pressure of the lubrication oil squeezed into the cracks. 3.5. Fracture observation and analysis As described in Section 2, the specimen A in Fig. 4b was used for the fracture observation and analysis. As shown in Fig. 5, by breaking off the specimen A along the crack section, the fracture surface and the crack can be observed. It is found that, the new formed fracture surface shows silvery color. And the original fracture surface shows black brown color. Compared with the new formed fracture surface, the original one looks much rougher and is of intergranular fracture feature. And the black brown color indicates that the extreme pressure additive of sulfur in the lubrication oil, which is squeezed into the crack region, has reacted with the copper and the tin of the Babbitt layer to form the black copper sulfide and the brown tin monosulfide. 3.5.1. Phase analysis Fig. 11 shows the XRD results of the fracture surface. From this figure, it can be seen that, the strongest diffraction peak appears at 2h ¼ 32 , the second one appears at 2h ¼ 64 , and the third one appears at 2h ¼ 79 . At 2h ¼ 30 ; 32 ; 52 , there exist smaller diffraction peaks. By checking the Powder Diffraction File (PDF), it is known that these peaks correspond to the SnS compound phase, the content of which is much higher than that of the SbSn phase and the Cu6Sn5 phase in Fig. 11. The SnS phase could be formed Table 2 Energy spectrum chart of point 1. Elements
Content (%)
Mole ratio (%)
C O Mg P S Sn Ca Cu Zn
13.66 13.25 0.50 0.97 6.78 29.42 1.01 30.18 4.23
37.40 27.22 0.68 1.03 6.95 8.15 0.83 15.61 2.13
Elements
Content (%)
Mole ratio (%)
C O S Sn Cu
6.87 6.88 2.70 49.46 34.09
28.04 21.08 4.13 20.43 26.31
Table 3 Energy spectrum chart of point 2.
Table 4 Energy spectrum chart of point 3.
Fig. 14. Energy spectrum diagrams of new formed fracture: (a) point 1; (b) point 2 and (c) point 3.
Elements
Content (%)
Mole ratio (%)
C O S Sn Ca Cu
2.78 6.83 0.79 81.25 4.60 3.76
15.00 27.69 1.60 44.42 7.44 3.84
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by the reaction between the sulfide addictive in the lubrication oil, which is squeezed into the incipient crack, and Sn element of the Babbitt layer. The crystal structure of SnS is rhomboidal. Its melting point reaches as high as 880 °C. However, its plasticity and ductility are all very poor. Therefore, the produced SnS phase in the fracture region of the Babbitt layer will make the crack expand more easily and deteriorate the Babbitt layer’s mechanical properties further.
3.5.2. Microscope structure analysis Fig. 12 shows the microscope structure diagrams of the new formed fracture surface and the original one respectively by SEM. From Fig. 12a, it can be seen that, in the new formed fracture region, there exist a lot of tearing edges and some short and small
quasi-cleavage planes, and the secondary cracks can be observed. The fracture should be quasi-cleavage one. While, from Fig. 12b– d, it can be seen that, the topography of the original fracture region manifests interganular crack feature. In addition, there exist some micro pits and many small granular substances distributing in this region, which could be the precipitated Cu–Sn compound phase along the grain boundary or the carbide and the oxide of Cu, Sn and Sb elements. The detailed chemical compositions can be determined in the next section. Compared with the microscope structures of the ductile fracture of the a solid solution and the plastic fracture of the SnSb compound and Cu6Sn5, it can be seen that the original fracture should be the plastic fracture of Cu6Sn5 mainly [24]. In Fig. 12d, the molten guttate substances indicate that the crack surface experiences an high temperature oxidation process. 3.5.3. Energy spectrum analysis Fig. 13a shows the selected points on the original fracture surface for the energy spectrum analysis. The analysis results are shown in Fig. 14 and Tables 2–4, respectively. It can be seen that, in the original fracture region, the S element content is very high. This result is consistent with the XRD result in Section 3.5.1. In addition, in the point 1 region, the O element content increases greatly, which indicates that the fracture surface is covered by heavy oxide film. This is just the characteristic of high-temperature adhesion. The C element content in the fracture region is the same as that of the crack surface, which indicates that there exist some carbonide inclusions. The elements of Mg and Zn in the fracture region should be introduced by the additive of the lubrication oil, which indicates that the lubrication oil has been squeezed into the cracks deeply. In addition, as it is well known, the P element content has special influence on materials: P element can increase the liquidity of alloy, cause the fluctuation of the Sb element
Table 5 Energy spectrum chart of point 4. Elements
Content (%)
Mole ratio (%)
C O Si S Cl Sn Cu
6.03 5.71 0.59 0.96 0.55 53.68 32.49
26.59 18.89 1.11 1.58 0.82 23.95 27.07
Table 6 Energy spectrum chart of point 5. Elements
Content (%)
Mole ratio (%)
C O Si S Sn Ca Cu
5.26 4.08 0.17 0.70 45.56 1.86 42.37
24.11 14.02 0.33 1.20 21.11 2.55 36.68
Table 7 Energy spectrum chart of point 6.
Fig. 15. Energy spectrum diagrams of original fracture: (a) point 1; (b) point 2 and (c) point 3.
Elements
Content (%)
Mole ratio (%)
C O Sn Ca
2.16 4.61 91.07 2.16
13.97 22.34 59.52 4.18
Y. He et al. / Materials and Design 52 (2013) 923–931
content and thus lead to the formation of the SnSb phase; however, when P element content is lower than 0.01%, P element will increase the ductility of alloy; but when P element content is higher than 0.08%, P element will make alloy more brittle and thus cause the performance degradation of alloy [16]. Here, the P element content in the fracture region is much higher than 0.08%. Therefore, the material of the bearing becomes brittle greatly and makes the cracks expand much more easily to cause the fracture damage. Moreover, from the semiquantitative analysis of the energy spectrum, it can be seen that the Sn element content is far from enough for the formation of the Cu6Sn5 phase and the SnS phase. From this result, it can be inferred that CaS, CuS and CuO might be produced, which will cause the performance degradation of the alloy. In the point 2 region, there still exist the elements of C, O and S, the reasons of which have been discussed above. In addition, from the semiquantitative analysis of the energy spectrum, it is found that the element content ratio of Sn to Cu is close to 1.2:1. It means that Ca element and S element have similar varying trend in content, which gives another evidence for that CaS is produced in this region. In the point 3 region, the Cu element content is much low, which is close to that of S element but is much lower than that of Sn element. It can be inferred that the substance in the point 3 region is the a solid solution phase mainly, and the fracture of which is ductile one. By observing the picture of the selected points, it is evaluated that such a solid solution phase covers about 20% area and Cu6Sn5 covers about 80% area of the region. In such condition, the surface always has high friction coefficient [20]. Fig. 13b shows the selected points on the new formed fracture surface for energy spectrum analysis. The analysis results are shown in Fig. 15, Table 5–7 respectively. From these tables, it can be seen that, in the point 4 region, compared with the condition of the original fracture surface, the S element content is very low comparably. The O element content is also much lower, which indicates that there exists no much oxide film produced by high temperature creep but just some oxide film produced when the fresh fracture is opened to the air. In addition, no Mg and no Zn, which are main elements of the lubricant additive, is found in the new formed fracture region because the lubrication oil has not been squeezed into the region yet. In the point 6 region, no Cu element and no S element is detected. But the Sn element content is very high, which should be the a solid solution phase. From the picture of the selected points, it can be evaluated that the Cu6Sn5 phase also covers the most of the area, which is the same as the condition of the original fracture. It means that all the damage region of the bearing has high friction coefficient, which also should be another reason for the crack generation. 4. Conclusions From above investigation and analysis, following conclusions can be drawn for the failure mechanism and the damage reasons of the bearing: (1.) The results of phase analysis, metallographic analysis and microscope structure analysis demonstrate that: Direct contact and rubbing should occur between the shaft and the bearing at the damage area. Such rubbing causes the content decrease of the hard inclusion Cu6Sn5 and also cause local high temperature in the damage region, which lead to the degradation of the mechanical performance of the Babbitt layer at the rubbing area and thus promote the crack initiation and expansion. In addition, the crack initiation and expansion will further cause the content decrease of the hard inclusion of the Babbitt layer surface. Thus, the condition goes into a vicious circle till the severe burning and facture damage is caused to the bearing finally.
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(2.) The results of the fracture observation and analysis show that: the contents of the SnS compound and S element are all very high comparably in the original facture. Especially, the elements of Mg, Zn and P are detected in the original fracture, which can only come from the additive of the lubrication oil. These results further demonstrate that: in the damage region, rubbing causes local high temperature and cracks initiation. The lubrication oil is squeezed in the incipient micro-cracks by high pressure. Then, the sulfide of the lubricant additive reacts with Sn element of the Babbitt layer to form the SnS compound. Such SnS compound has poor plasticity and ductility. Thus, the formed SnS compound makes the cracks expand more easily. In addition, Very high content of P element is detected, which deteriorates the mechanical performance of the Babbitt layer further and also makes the cracks expand more easily.
Acknowledgement The work is partially supported by the Natural Science Foundation of China (Grant No. 51275263). References [1] Zeren A, Feyzullahoglu E, Zeren M. A study on tribological behaviour of tinbased bearing material in dry sliding. Mater Des 2007;28:318–23. [2] Hamrock BJ. Fundamentals of fluid film lubrication. McGraw-Hill; 1994. [3] Rigney DA. Comments on the sliding wear of metals. Tribol Int 1997;30:361–7. [4] Jacobson B. The Stribeck memorial lecture. Tribol Int 2003;36:781–9. [5] Booser E, Khonsari M. Applied tribology bearing design and lubrication. NY: Wiley; 2001. [6] Neale MJ. Bearings: a tribology handbook. Scotland: Butterworth/Heinemann; 1993. [7] Wu H, Bi Q, Zhu S, Yang J, Liu W. Friction and wear properties of Babbitt alloy 16-16-2 under sea water environment. Tribol Int 2011;44:1161–7. [8] Zeren A. Embeddability behaviour of tin-based bearing material in dry sliding. Mater Des 2007;28:2344–50. [9] Feyzullahoglu E, Zeren A, Zeren M. Tribological behaviour of tin-based materials and brass in oil lubricated conditions. Mater Des 2008;29:714–20. [10] Kim SS, Yu HN, Hwang IU, Lee DG, Kim SN, Suzuki K, et al. The sliding friction of hybrid composite journal bearing under various test conditions. Tribol Lett 2009;35:211–9. [11] Kamal M, El-Bediwi AB, El-Shobaki MR. Influence of alloying elements on structure and some physical properties of quenched Sn–Sb alloy. Radiat Eff Defects Solids 2006;161:549–57. [12] Diouf P, Jones A. Investigation of bond strength in centrifugal lining of babbitt on cast iron. Metall Mater Trans A 2010;41:603–9. [13] Bora MÖ, Coban O, Sinmazcelik T, Günay V, Zeren M. Instrumented indentation and scratch testing evaluation of tribological properties of tin-based bearing materials. Mater Des 2010;31:2707–15. [14] Moazami Goudarzi M, Jenabali Jahromi SA, Nazarboland A. Investigation of characteristics of tin-based white metals as a bearing material. Mater Des 2009;30:2283–8. [15] Wang Q. Seizure failure of journal-bearing conformal contacts. Wear 1997;210:8–16. [16] Shun XM. Study on metallurgical structure and casting flaw of Sn matrix Babbitt alloy. Int Combust Eng Parts 1992;3:1–8. [17] MSIT Materials Science International Team. Selected copper systems: phase diagrams, crystallographic and thermodynamic data. Springer; 2006. [18] Diouf P, Jones A. Investigation of bond strength in centrifugal lining of Babbitt on cast iron. Metal Mater Trans A 2010;41A:603–9. [19] Ishihara S, Tamura K, Goshima T. Effect of amount of antimony on sliding wear resistance of white metal. Tribol Int 2010;43:935–8. [20] Gao Q. Metallographic atlas collection of nonferrous metal. Beijing: Metallurgical Industry Press; 2005. [21] Tu HL, Yu Y. Physico-chemical examination of metal materials. Beijing (China): Chemical Industry Press; 2007. [22] Li JH, Lin DC. Metallographic altas of metal materials. Beijing (China): Chinese Machine Press; 2007. [23] Shanghai Jiao Tong University Group. Fracture analysis of metal. Beijing (China): National Defence Industry Press; 1979. [24] Sadykov FA, Barykin NP, Valeev ISh, Danilenko VN. Influence of the structural state on mechanical behavior of tin Babbitt. J Mater Eng Perform 2003;12(1):29–36.
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Technical Report
Failure analysis of journal bearing used in turboset of a power plant Yongyong He ⇑, Zhongkai Zhao, Tianyu Luo, Xinchun Lu, Jianbin Luo State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 7 April 2013 Accepted 11 June 2013 Available online 26 June 2013
a b s t r a c t This paper presents a failure investigation on the damaged journal bearing, which supports the mainshaft of the Turboset of a power plant. The damage position is located at the hydrodynamic lubrication area of the bearing. A detailed fractographic and metallurgical investigation is conducted on the damaged lining layer of the bearing, which is made of tin-based Babbitt alloys, to analyze the damage mechanism and assess the possible reasons for this failure. The investigation results indicate that the main cause of the failure should be the direct contact and the rubbing between the shaft and the lining layer of the bearing at the damage position. Such contact and rubbing will cause local high temperature in the damage region and also cause the content decrease of the hard inclusion Cu6Sn5 on the Babbitt layer surface, and therefore deteriorate the mechanical performance of the Babbitt layer of the bearing and lead to the serious damage to the Babbitt layer eventually. By this research, some direct and valuable information is obtained for Babbitt lining layer design and operation condition optimization of bearings. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In 1837, Babbitt patented a Sn–Sb–Cu alloy for use in journal bearings, such kinds of the material are therefore named Babbitt alloys and also called white metals. Babbitt alloys can be fundamentally classified into two types: one has lead as its main component, the other tin, called lead-based Babbitt and tin-based Babbitt respectively [1]. Comparatively, tin-based Babbitts are more widely used in preference to lead-based Babbitts due to their more excellent corrosion resistance, easier bonding, and less tendency towards segregation [1]. Babbitt alloys, either tin-based or leadbased ones, have excellent embeddability and conformability characteristics (due to their softness), unsurpassed compatibility with steel shafts, good antifriction properties and unique ability to adapt to misalignment by mild wiping on initial run-in (due to their low melting points) [1–5]. The representative physical and mechanical properties of typical Babbitt alloys are summarized in Ref. [5]. Therefore, Babbitt alloys have been widely used as the lining layer materials of journal bearings used in many industrial applications [5–8]. Over the past decades, many investigations have been done intensively on the tribological and mechanical properties of Babbitt alloys as lining layer materials of journal bearings [1,9–14]. For journal bearings, the desired operation condition should be that a proper thickness lubricant film is formed and maintained between the shaft and the bearing. However, mixed and boundary lubrications are inevitable in the processes of start and stop, or under severe working conditions, such as insufficient lubricant sup⇑ Corresponding author. Tel./fax: +86 10 62787932. E-mail address: [email protected] (Y. He). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.06.027
ply, high operating temperature, and heavy loading [15], which will cause direct contact between the shaft and the lining layer of the bearing and eventually cause various damages to the bearing [15]. Wear and burning of the lining layer are the frequent failures of journal bearings due to abnormal hydrodynamic condition, which always cause shutdown or even catastrophic accident to machinery. Therefore, it is also very meaningful to investigate the damage mechanism of Babbitt alloys as the lining layer of journal bearings. Such investigations can explore the weak points of the properties of Babbitts under improper operation conditions and thus can provide useful information for better design of Babbitts and manufacturing of bearings for better properties against damages. In addition, such investigations can also provide information for adjusting proper operation of bearings so that safe running of bearings can be guaranteed. Unfortunately, the operation condition of journals bearings is always complex, and cannot be simulated really and easily by experiments, and thus the corresponding damages also cannot be simulated and obtained really and accurately by experiments. Therefore, case study by real damaged bearings from real applications is the necessary and useful way to investigate damage mechanism of Babbitt bearings, and such study can provide direct, real and valuable information for the bearing design and operation condition optimization. Recently, we obtained a severe damaged journal bearing from a power plant. It gives us a rare opportunity to carry out such a case study on the damage mechanism of Babbitt layer by a real damaged bearing. The background is illustrated as follows. During the recent scheduled maintenance of the turboset (power: 987 MW; working speed: 3000 r/min) in a power plant, it was reported that there existed some burned black substance deposited on the lining layer surface of the bearing, which supports
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Fig. 1. Photographs of the damaged journal bearing.
the main-shaft of the Turboset, at the position of the hydrodynamic lubrication area of the bearing according to the calculation. Especially, severe burning and fracture damage was observed on the lining layer at the same position of the bearing, as shown in Fig. 1. The lining layer of the bearing is made of Babbitt alloy. This bearing is the cylinder one composed of the upper part and the lower part. The width, the outer radius and the inner radius are 279.6 mm, 319.81 mm and 300.00 mm, respectively. The thickness of the Babbitt layer is about 1.8 mm. The damage position of the bearing is illustrated in Fig. 2 when the bearing is expanded into a plane. The area of the damaged region is about 24.9 mm 19.1 mm. To investigate the damage mechanism of the Babbitt alloy, this paper presents a detailed fractographic and metallurgical investigation on the damaged Babbitt layer of the bearing. The possible reasons for this damage are analyzed and assessed. The investigation could provide a case study of the damage mechanism of Babbitt alloys as lining materials of journal bearings
Fig. 2. Damage position of bearing.
Fig. 4. Specimen preparation illustration of Region 2: (a) cutting order illustration and (b) cutting results.
Fig. 3. Region illustration of specimen preparation.
and also could provide some valuable information for better bearing design, better bearing manufacturing and Turboset running optimization.
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Fig. 5. Fractural section preparation.
2. Specimens preparation and investigation methods For comparison analysis, the specimens were taken from Region 1 and Region 2 of the bearing respectively, as shown in Fig. 3. Region 1 can be regarded as the region where the Babbitt layer of the bearing is healthy and has no damage. Region 2 is the damaged region. From Region 1, 10 specimens of 10 mm 10 mm size were prepared. For Region 2, in order to preserve the damage condition and information of this region as far as possible, a specimen of 40 mm 40 mm size was sectioned from this region first, which covers the whole damage region. Then, this specimen was sectioned into 8 smaller specimens in the order as illustrated in Fig. 4a, i.e. from 1-1 to 6-6 cross section orderly. Fig. 4b shows the sectioning result. The Specimen A in Fig. 4b was used to observe and investigate the fracture and the crack condition in particular. In order to preserve the original condition of the crack, the specimen A was cut by a vee gap first at the other side opposite to the crack side. Then the specimen was broken off along the crack section by using cold quenching of liquid nitrogen, as shown in Fig. 5. X ray diffractometer (XRD) was used to perform the phase analysis of the specimens. After XRD analysis, the metallographic structure and distribution of the surface and the cross section of the specimens were observed and analyzed further by metalloscope. Sweep electron microscopy (SEM) was used to observe and analyze the microstructure of the two regions. SEM was also used to observe and analyze the fracture surfaces to investigate the damage mechanism of the crack region. In addition, energy spectrum analysis was used to analyze the components of various phases in the two regions and further determine if some elements exist which may cause properties degradation of the Babbitt layer in the two regions of the bearing. 3. Results and analysis 3.1. Chemical composition Table 1 gives the chemical compositions of the Babbitt layer of the bearing. From this table, it is known that the Babbitt layer is fabricated from tin-base Babbitt alloy of ZSnSb8Cu4.
Fig. 6. XRD result of the specimens: (a) Region 1 and (b) Region 2.
3.2. Phase analysis Using XRD, phase analysis was conducted on the specimens from Region 1 and Region 2 respectively. Fig. 6 shows the XRD analysis results. From this figure, it can be seen that, the strongest diffraction peak appears at 2h ¼ 33 , the second one appears at 2h ¼ 46 , and the third one appears at 2h ¼ 31 . From these XRD results and by checking the Powder Diffraction File (PDF), it is known that a-Sn phase exists in the specimens from both Region 1 and Region 2; at 2h ¼ 29 ; 42 ; 52 ; 69 , there exist small peaks, which mean that SnSb phase exists in the specimens; at 2h ¼ 44 ; 61 , there exist smaller peaks, which mean that there exists Cu6Sn5 phase in the specimens. As is well known, the Cu6Sn5 phase is the hard inclusion of tin-based Babbitt alloy [5]. However, according to the structural diagram of the Sn–Sb alloy [16], the SnSb phase should not exist in these specimens theoretically. Therefore, it is inferred that, when casting the Babbitt layer, the cooling speed was not so quick that the reaction occurred between the liquid and a solid solution phase which led to the precipitation of SnSb crystal [17]. This result is also consistent with that of Ref. [18]. Comparing Fig. 6a with b, it can be seen that the content of the Cu6Sn5 phase decreases in the specimen of Region 2. Just as
Table 1 Chemical composition of Babbitt layer of shell. Element
Sn
Sb
Cu
Pb
Zn
Fe
As
Ni
Bi
Cd
Content (%)
–
7.5
3.4
0.014
0.001
0.038
0.011
0.2
<0.001
0.95
Element Content (%)
Ag 0.08
Al <0.0002
In 0.026
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mentioned above, Cu6Sn5 is the hard inclusion of the alloy, which distributes in the Sn matrix and supports the shaft. If direct contact occurs between the lining surface of the bearing and the shaft, wear and even spalling will be produced on the lining surface. Therefore, from the content decease of Cu6Sn5 in the damaged region, it can be inferred that direct contact and rubbing should oc-
curs between the shaft and the bearing in Region 2 which leads to wear, even spalling, of the Cu6Sn5 hard inclusion on the Babbitt layer surface. The spalling of Cu6Sn5 will leave small pits on the surface of the Babbitt layer, which can be observed in following metallographic observation. Wear and spalling damage was also observed in the research of Ref. [19].
Fig. 7. Metallographic diagrams of specimen surface from Region 1.
Fig. 8. Metallographic diagrams of specimen surface from Region 2.
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3.3. Metallographic analysis of specimen surface Fig. 7 shows the metallographic diagrams of the specimen surface from Region 1. From this figure, it can be seen that, the white reticular structures disperse in the black matrix, among which some acicular, granular and star-shaped microstructures distribute densely. Combined with the energy spectrum analysis, it is known that, the white reticular structure of about 30 lm, the star-shaped one of about 30 lm and the granular one of about 1 lm are all the Cu6Sn5 phase which is the main hard inclusion of the alloy. The acicular structure of about 5 lm is the SnSb phase. However, from the standard metallographic structure of the Babbitt alloy [20], it is known that there should be no reticular Cu6Sn5 structure and acicular SnSb structure. In addition, the size of star-shaped Cu6Sn5 is much smaller than that of the standard structure about 160 lm. Therefore, it can be also inferred that, when casting the Babbitt layer, the cooling speed was not so quick that the hard brittle phase Cu6Sn5 precipitated along the grain boundary with reticular distribution and the SnSb phase precipitated from the a solid solution with acicular shape [21]. This inference is also consistent with the phase analysis result in Section 3.2. In addition, compared with the standard of tin-base Babbitt alloy, this Babbitt alloy contains much high Cd content (see Table 1), which will contribute to making Cu6Sn5 grain smaller, promoting the mechanical properties of the alloy and transforming the cuboid SnSb into the acicular one [22]. Fig. 8 shows the metallographic diagrams of the specimen surface from Region 2. From this figure, it can be seen that, the acicular Cu6Sn5 phases of about 15 lm, the granular Cu6Sn5 phases of about 5 lm and the acicular SnSb phases of about 1 lm disperse in the black matrix of a solid solution. Spot segregation phenomenon is obvious. The Cu6Sn5 phases gather regionally by the reticu-
Fig. 11. XRD result of the fracture.
lar manner, and the incipient cracks tend to initiate along the gathered Cu6Sn5 phases. Fig. 8b and c show such kind of the crack more clearly. The crack expands along the brittle second phase Cu6Sn5 and can be identified as the intergranular fracture. It can be inferred that, the brittle second phase Cu6Sn5 deposits along the grain boundary and makes the surface free energy of the grain boundary decrease, and the crack tends to expand along the direction of the lowest energy to cause the intergranular fracture. In addition, comparing Fig. 8 with Fig. 7, it can be seen that, in Fig. 8, the number of the star-shaped Cu6Sn5 grain decreases, the number of the granular Cu6Sn5 grain increases, and the density of the hard inclusion Cu6Sn5 decreases obviously. The size of the Cu6Sn5 grain also decreases. Especially, obvious fragmentation of
Fig. 9. Microscope structure of the surface of Region 2 by SEM.
Fig. 10. Microscope of a big crack in Region 2 by SEM.
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Fig. 12. Microscope structure of the fracture region: (a) new formed fracture; (b–d): original fracture.
the Cu6Sn5 grain can be observed in the crack regions. In addition, some small pits can be observed on the surface of the damaged region, which also supports the inference in section 3.2. The reasons of these phenomena can be inferred that, the Cu6Sn5 deposits along the grain boundary, therefore the cracks expand along the grain boundary and make the Cu6Sn5 precipitate from the grain boundary and break, which leads to the decrease of the Cu6Sn5 grain in density. In addition, it can be inferred that direct contact and rubbing should occur between the shaft and the bearing at Region 2, which may spall the hard inclusion Cu6Sn5 and lead to the further decrease of the Cu6Sn5 grain finally. This conclusion is consistent with that of Section 3.1. Above analysis results confirm that: direct contact and rubbing should occur between the shaft and the bearing at Region 2; the rubbing causes the spalling of the hard inclusion from the Babbitt
layer surface and also causes local high temperature in the rubbing region, which lead to the degradation of the mechanical performance of the Babbitt layer in the rubbing region and thus promote the crack initiation and expansion; the crack initiation and expansion cause the decrease of the hard inclusion of the Babbitt layer surface and degrade the mechanical performance further, and lead to the severe burning and fracture damage in Region 2 eventually. 3.4. Microscope structure analysis Fig. 9 shows the microscope structure diagrams of the surface of Region 2 by SEM. From Fig. 9b, it can be seen clearly that, the crack tends to expand along the grain boundary, and the crack is of clear bifurcating feature. Fig. 10 shows the microscope structure diagram of a big crack in Region 2. From Fig. 10a, it can be seen that
Fig. 13. Selected points on the fracture for energy spectrum analysis: (a) original fracture and (b) new formed fracture.
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the cracks of 1-1 region and 2-2 region are of obvious quasi-cleavage crack feature. Fig. 10b is the enlarged view of region A in Fig. 10a. From this figure, it can be seen that, the fracture surface is mat and is covered by some film and much granular substance. The granular substance is identified as the Cu6Sn 5 grain by energy spectrum analysis. By further observation, it is found that there exist some micro pits on the fracture surface, which means that the fracture is of ductility to some degree. It is well known that, the intergranular fracture is one kind of the brittle fracture. However, for some material with good ductility, such as aluminum, its intergranular fracture manifests obvious ductility and plastic deforma-
tion. Such intergranular fracture is named as the ductile intergranular fracture [23]. From above analysis, it is concluded that, the crack of the superficial layer is of quasi-cleavage crack feature, which is caused mainly by the mechanical rubbing and wear; while the crack of the deep layer is of ductile intergranular fracture feature, which should be caused by high temperature and high pressure of the lubrication oil squeezed into the cracks. 3.5. Fracture observation and analysis As described in Section 2, the specimen A in Fig. 4b was used for the fracture observation and analysis. As shown in Fig. 5, by breaking off the specimen A along the crack section, the fracture surface and the crack can be observed. It is found that, the new formed fracture surface shows silvery color. And the original fracture surface shows black brown color. Compared with the new formed fracture surface, the original one looks much rougher and is of intergranular fracture feature. And the black brown color indicates that the extreme pressure additive of sulfur in the lubrication oil, which is squeezed into the crack region, has reacted with the copper and the tin of the Babbitt layer to form the black copper sulfide and the brown tin monosulfide. 3.5.1. Phase analysis Fig. 11 shows the XRD results of the fracture surface. From this figure, it can be seen that, the strongest diffraction peak appears at 2h ¼ 32 , the second one appears at 2h ¼ 64 , and the third one appears at 2h ¼ 79 . At 2h ¼ 30 ; 32 ; 52 , there exist smaller diffraction peaks. By checking the Powder Diffraction File (PDF), it is known that these peaks correspond to the SnS compound phase, the content of which is much higher than that of the SbSn phase and the Cu6Sn5 phase in Fig. 11. The SnS phase could be formed Table 2 Energy spectrum chart of point 1. Elements
Content (%)
Mole ratio (%)
C O Mg P S Sn Ca Cu Zn
13.66 13.25 0.50 0.97 6.78 29.42 1.01 30.18 4.23
37.40 27.22 0.68 1.03 6.95 8.15 0.83 15.61 2.13
Elements
Content (%)
Mole ratio (%)
C O S Sn Cu
6.87 6.88 2.70 49.46 34.09
28.04 21.08 4.13 20.43 26.31
Table 3 Energy spectrum chart of point 2.
Table 4 Energy spectrum chart of point 3.
Fig. 14. Energy spectrum diagrams of new formed fracture: (a) point 1; (b) point 2 and (c) point 3.
Elements
Content (%)
Mole ratio (%)
C O S Sn Ca Cu
2.78 6.83 0.79 81.25 4.60 3.76
15.00 27.69 1.60 44.42 7.44 3.84
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by the reaction between the sulfide addictive in the lubrication oil, which is squeezed into the incipient crack, and Sn element of the Babbitt layer. The crystal structure of SnS is rhomboidal. Its melting point reaches as high as 880 °C. However, its plasticity and ductility are all very poor. Therefore, the produced SnS phase in the fracture region of the Babbitt layer will make the crack expand more easily and deteriorate the Babbitt layer’s mechanical properties further.
3.5.2. Microscope structure analysis Fig. 12 shows the microscope structure diagrams of the new formed fracture surface and the original one respectively by SEM. From Fig. 12a, it can be seen that, in the new formed fracture region, there exist a lot of tearing edges and some short and small
quasi-cleavage planes, and the secondary cracks can be observed. The fracture should be quasi-cleavage one. While, from Fig. 12b– d, it can be seen that, the topography of the original fracture region manifests interganular crack feature. In addition, there exist some micro pits and many small granular substances distributing in this region, which could be the precipitated Cu–Sn compound phase along the grain boundary or the carbide and the oxide of Cu, Sn and Sb elements. The detailed chemical compositions can be determined in the next section. Compared with the microscope structures of the ductile fracture of the a solid solution and the plastic fracture of the SnSb compound and Cu6Sn5, it can be seen that the original fracture should be the plastic fracture of Cu6Sn5 mainly [24]. In Fig. 12d, the molten guttate substances indicate that the crack surface experiences an high temperature oxidation process. 3.5.3. Energy spectrum analysis Fig. 13a shows the selected points on the original fracture surface for the energy spectrum analysis. The analysis results are shown in Fig. 14 and Tables 2–4, respectively. It can be seen that, in the original fracture region, the S element content is very high. This result is consistent with the XRD result in Section 3.5.1. In addition, in the point 1 region, the O element content increases greatly, which indicates that the fracture surface is covered by heavy oxide film. This is just the characteristic of high-temperature adhesion. The C element content in the fracture region is the same as that of the crack surface, which indicates that there exist some carbonide inclusions. The elements of Mg and Zn in the fracture region should be introduced by the additive of the lubrication oil, which indicates that the lubrication oil has been squeezed into the cracks deeply. In addition, as it is well known, the P element content has special influence on materials: P element can increase the liquidity of alloy, cause the fluctuation of the Sb element
Table 5 Energy spectrum chart of point 4. Elements
Content (%)
Mole ratio (%)
C O Si S Cl Sn Cu
6.03 5.71 0.59 0.96 0.55 53.68 32.49
26.59 18.89 1.11 1.58 0.82 23.95 27.07
Table 6 Energy spectrum chart of point 5. Elements
Content (%)
Mole ratio (%)
C O Si S Sn Ca Cu
5.26 4.08 0.17 0.70 45.56 1.86 42.37
24.11 14.02 0.33 1.20 21.11 2.55 36.68
Table 7 Energy spectrum chart of point 6.
Fig. 15. Energy spectrum diagrams of original fracture: (a) point 1; (b) point 2 and (c) point 3.
Elements
Content (%)
Mole ratio (%)
C O Sn Ca
2.16 4.61 91.07 2.16
13.97 22.34 59.52 4.18
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content and thus lead to the formation of the SnSb phase; however, when P element content is lower than 0.01%, P element will increase the ductility of alloy; but when P element content is higher than 0.08%, P element will make alloy more brittle and thus cause the performance degradation of alloy [16]. Here, the P element content in the fracture region is much higher than 0.08%. Therefore, the material of the bearing becomes brittle greatly and makes the cracks expand much more easily to cause the fracture damage. Moreover, from the semiquantitative analysis of the energy spectrum, it can be seen that the Sn element content is far from enough for the formation of the Cu6Sn5 phase and the SnS phase. From this result, it can be inferred that CaS, CuS and CuO might be produced, which will cause the performance degradation of the alloy. In the point 2 region, there still exist the elements of C, O and S, the reasons of which have been discussed above. In addition, from the semiquantitative analysis of the energy spectrum, it is found that the element content ratio of Sn to Cu is close to 1.2:1. It means that Ca element and S element have similar varying trend in content, which gives another evidence for that CaS is produced in this region. In the point 3 region, the Cu element content is much low, which is close to that of S element but is much lower than that of Sn element. It can be inferred that the substance in the point 3 region is the a solid solution phase mainly, and the fracture of which is ductile one. By observing the picture of the selected points, it is evaluated that such a solid solution phase covers about 20% area and Cu6Sn5 covers about 80% area of the region. In such condition, the surface always has high friction coefficient [20]. Fig. 13b shows the selected points on the new formed fracture surface for energy spectrum analysis. The analysis results are shown in Fig. 15, Table 5–7 respectively. From these tables, it can be seen that, in the point 4 region, compared with the condition of the original fracture surface, the S element content is very low comparably. The O element content is also much lower, which indicates that there exists no much oxide film produced by high temperature creep but just some oxide film produced when the fresh fracture is opened to the air. In addition, no Mg and no Zn, which are main elements of the lubricant additive, is found in the new formed fracture region because the lubrication oil has not been squeezed into the region yet. In the point 6 region, no Cu element and no S element is detected. But the Sn element content is very high, which should be the a solid solution phase. From the picture of the selected points, it can be evaluated that the Cu6Sn5 phase also covers the most of the area, which is the same as the condition of the original fracture. It means that all the damage region of the bearing has high friction coefficient, which also should be another reason for the crack generation. 4. Conclusions From above investigation and analysis, following conclusions can be drawn for the failure mechanism and the damage reasons of the bearing: (1.) The results of phase analysis, metallographic analysis and microscope structure analysis demonstrate that: Direct contact and rubbing should occur between the shaft and the bearing at the damage area. Such rubbing causes the content decrease of the hard inclusion Cu6Sn5 and also cause local high temperature in the damage region, which lead to the degradation of the mechanical performance of the Babbitt layer at the rubbing area and thus promote the crack initiation and expansion. In addition, the crack initiation and expansion will further cause the content decrease of the hard inclusion of the Babbitt layer surface. Thus, the condition goes into a vicious circle till the severe burning and facture damage is caused to the bearing finally.
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(2.) The results of the fracture observation and analysis show that: the contents of the SnS compound and S element are all very high comparably in the original facture. Especially, the elements of Mg, Zn and P are detected in the original fracture, which can only come from the additive of the lubrication oil. These results further demonstrate that: in the damage region, rubbing causes local high temperature and cracks initiation. The lubrication oil is squeezed in the incipient micro-cracks by high pressure. Then, the sulfide of the lubricant additive reacts with Sn element of the Babbitt layer to form the SnS compound. Such SnS compound has poor plasticity and ductility. Thus, the formed SnS compound makes the cracks expand more easily. In addition, Very high content of P element is detected, which deteriorates the mechanical performance of the Babbitt layer further and also makes the cracks expand more easily.
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