Abstract
BACKGROUND:
A rolling bearing bush alloy of a feed water pump that is part of a waste heat boiler of an oil refinery has failed.
OBJECTIVE:
We try to analyze the reasons that caused the working surface of the bearing bush of the water pump to fall off and then give some suggestions to this failure.
METHODS:
The composition, microstructure, pit, and crack morphology of the bearing bush alloy were analyzed by the X-ray fluorescent analysis, the energy spectrum analysis, the optical microscope and the scanning electron microscope, respectively.
RESULTS:
The content of Pb in the bearing bush alloy was high, and the Cu content was low. The primary crystal Cu6Sn5 was low, and the crystal of SnSb with low density moved upward and segregated. The above phenomenon reduced the fatigue resistance of the babbitt alloy. The bearing bush was subjected to alternating loads in service, and several small cracks were generated on the bearing bush alloy working surface. The cracks continued to expand and connected with each other. Fatigue pitting occurred on the bearing bush working surface, a large number of pits were formed, and several large alloy blocks fell off.
CONCLUSIONS:
The Pb content in the failed bearing bush alloy was too high and did not meet the requirements of the Sn-based babbitt alloys in the national standard. At the same time, the primary crystal Cu6Sn5 formed by Cu and Sn was low due to the low Cu content, and the crystal SnSb with a small density moved upward and segregated. The composition of the babbitt alloy, especially the Cu content, should be strictly controlled to ensure the safe and reliable operation of the bearing.
Failure summary and experimental scheme
During the maintenance of an oil refinery, a feed water pump of a waste heat boiler of the second catalytic workshop was abnormal, and the rolling bearing bush alloy fell off. By consulting relevant bearing failure cases [1,5,6,8–10], the reasons for the failure were analyzed from several aspects of the composition, microstructure, pits, and crack morphology of the bearing bush alloy. The specific scheme included: (1) macro observation of the failure bearing bush working surface; (2) determination of the bearing bush alloy composition through X-ray fluorescent analysis and energy spectrum analysis; (3) observation of the alloy microstructures near the bearing bush working surface and the interface between the bearing bush lining and the bearing cover; and (4) observation of the micro-morphology of the pits and cracks of the bearing bush alloy by a scanning electron microscope.
Sampling was carried out on the failed bearing bush by wire cutting. The sampling position and specimen shape are shown in Fig. 1. Specimens A, B, and C were analyzed by X-ray fluorescent analysis, microstructure observation, scanning electron microscope observation and energy spectrum analysis.
The sampling position and the specimen shape.
The macroscopic morphology of the failed bearing bush alloy working surface.
The circumferential scratch on the bearing bush alloy working surface.
Macro inspection
There were many places where the bearing bush alloy had fallen off. The bearing cover inner surface was the most severely damaged position, and there were a lot of pits on the bearing bush working surface, as shown in Fig. 2. By visual observation, a few circumferential abrasion marks were found, as shown in Fig. 3. Such circumferential scratches could be caused by the bearing bush alloy falling off and scratching during operation. There were several obvious axial scratches at the pits, which were similar to the scraper marks at the oil channels. The scratches at the pits were distributed in an arc-shape on the bearing bush alloy working surface, as shown in Figs 4 and 5. Figure 5 presents a representative area of the surface of friction. It can be seen that the surface of friction consists of a number of wear furrows, which are characteristic of metal wear. The presence of large reoriented areas that can in the process of friction form cracks and crumble is remarkable. Individual cracks were observed to be located across the friction furrows of a worn-out surface. It was determined that a large number of pits on the bearing bush working surface were caused by the normal processing technology of scraping (Scraping is the use of a scraper, reference surface, measuring tool and display agent, in a manual operation, while grinding the points and measuring, while scraping and grinding, so that the workpiece can meet the technological requirements of the size, geometry, surface roughness, sealing ability and so on.) and were not related with failures. Local cracking occurred on the bearing bush alloy working surface, and there were a lot of small cracks and pits, as shown in Figs 6–8. The main crack at the pit was stepped, and several crack branches were derived from the main crack. There were also a large number of small pits in the large pit, as shown in Figs 9 and 10. There was no pit on the side face of the bearing bush alloy, as shown in Fig. 11.
The scratches on the bearing bush alloy working surface.
The axial scratches of the pits on the bearing bush alloy working surface.
The severe cracking on the bearing bush alloy working surface.
The small cracks on the bearing bush alloy working surface.
The small pits on the bearing bush alloy working surface.
The step pit of the bearing bush alloy.
The main cracks and the secondary cracks of the bearing bush alloy.
The side face of the bearing bush alloy.
X-ray fluorescent analysis
In order to determine the chemical composition of the bearing bush alloy, X-ray fluorescent analysis was conducted on the surface of specimen A. The result is shown in Table 1. According to Table 1, the bearing bush alloy was the Sn-based babbitt alloy. But, the failed bearing bush alloy did not contain Cu, and the Sb content was much higher than the standard value [3]. Comparing the components of the babbitt alloy, it was determined that the primary acicular and star-shaped crystal Cu6Sn5 formed by Cu and Sn was low due to insufficient Cu in the bearing bush lining, and the crystal SnSb with a low density moved upward and caused a density segregation in the crystallization process [7 ].
The X-ray fluorescent analysis results of the bearing bush alloy working surface (wt%)
The X-ray fluorescent analysis results of the bearing bush alloy working surface (wt%)
The points a, b, and c on the side face of specimen C were sequential selected from the working surface of the bearing bush alloy to the interface between the bearing bush alloy and the bearing cover in the radial direction, as shown in Fig. 12. The energy spectrum analysis was conducted on the above three points, and the results are shown in Table 2. According to the results of the energy spectrum analysis and the national standard GB/T 1174-1992, the Pb content was much higher than the standard value of ZSnSb12Pb10Cu4 which has the highest Pb content in different babbitt alloys, and the Cu content was lower than the required range of 2.5% to 5% in the babbitt alloy [3]. Therefore, the composition of the failed bearing bush alloy was unsatisfactory.
The energy spectrum analysis positions.
The energy spectrum analysis results of the bearing bush alloy (wt%)
The primary acicular and star-shaped crystal Cu6Sn5 was low due to insufficient Cu in the failed bearing bush alloy. The crystal SnSb with a low density moved upward and caused the density segregation in the crystallization process. The mechanical properties and the fatigue resistance of the babbitt alloy were reduced due to the above factors [7 ].
After specimen B was etched with a 4% nitric acid alcohol solution, the microstructures of the bearing bush alloy near the bearing bush alloy working surface and the interface between the bearing bush alloy and the bearing cover were observed under a metallographic microscope. The microstructure of the bearing bush alloy near the interface is shown in Fig. 13. It was mainly a Sn-based 𝛼 solid solution (soft matrix), and the white 𝛽-phase SnSb was low. The microstructure of the bearing bush alloy near the working surface is shown in Fig. 14. The hard and brittle 𝛽-phase SnSb (white square or polygonal compound) was dense and interconnected, and such a microstructure seriously reduced the mechanical properties and the fatigue resistance of the bearing bush alloy. The root cause of the reduction of the fatigue resistance was the lower Cu content in the failed bearing bush alloy according to the above tests of chemical composition.
The microstructure of the bearing bush alloy near the interface between the bearing bush alloy and the bearing cover.
The microstructure of the bearing bush alloy near the working surface.
The morphology of the pit on specimen C was observed by a scanning electron microscope after ultrasonic cleaning. It was found that the morphology of each pit was basically similar. Two pits were selected for further observation. A pit on the edge of the specimen was observed, as shown in Fig. 15. The slope of the pit was stepped, and there were a lot of small pits on the slope. The primary acicular and star-shaped crystals Cu6Sn5 and square SnSb crystals are shown in Fig. 15(c). The morphology of another pit was observed, as shown in Fig. 16. Fatigue striations were found on the slope of the pit, and there were also a large number of small pits on the slope.
The morphology of a pit on specimen C.
The morphology of another pit.
In engineering practice, cracking and spalling are common modes of fatigue failure for babbitt steel bimetal sliding bearings, which are the same with this study [2]. As the wear progressed, the temperature gradually increased, and the strength and hardness of the babbitt alloy decreased greatly with the increase of temperature [11]. Under the action of periodic impact load, once the impact load exceeded the fatigue strength of the babbitt alloy, micro-cracks usually appeared on the bearing working surface, and simultaneously, the lubricating oil entered the gaps of micro-cracks and accelerated the propagation of cracks under the impact load. In addition, the temperature rose, thermal stress occurred inside and on the surface of the babbitt alloy, which was also one of the factors that caused micro-cracks on the bearing working surface. Under the action of the pulsating oil film pressure, the cracks propagated along the normal direction of the bearing working surface. When the cracks extended close to the joint interface, they continued to extend in a direction parallel to the joint interface. After the cracks met each other, sheet-peeling occurred.
The babbitt alloy consists of the hard phase and the soft phase, and the 𝛽 phase (SnSb) and 𝜂 phase (Cu6Sn5) which are both the hard phases and are dispersed in the soft phase a solid solution. The 𝛽 phase is cuboidal, and the size of 𝛽 phase grains in the babbitt layer is different due to the difference in the content of trace elements or the preparation processes. The 𝜂 phase is needle-like or star-shaped, forming a hard phase skeleton of the babbitt alloy, which prevents segregation of the 𝛽 phase during crystallization. After the babbitt alloy is welded on the steel back, an intermetallic compound layer with Fe and Sn as the main elements is formed at the joint interface to form a transition layer from babbitt lining to steel back [12]. This transition layer can enhance the bonding strength of babbitt-steel bimetal material. When babbitt alloy acts as the lining and 𝜂 phase is concentrated on the babbitt side of the transition layer, the crack propagation is inhibited and the bonding strength is improved [13]. If the 𝛽 phase grains are fine and uniform in distribution, in addition to expanding in the 𝛽 phase or along the grain boundary of the 𝛽 phase, the cracks will propagate by tearing the a solid solution and breaking the 𝜂 phase, which will have a greater resistance [12]. If the hard and brittle 𝛽 phase grains are coarse and the segregation is serious, the stress concentration is likely to occur and the crack is likely to be generated and expanded. Figure 14 shows that the 𝛽 phase grains were bad and unevenly distributed in a solid solution. Therefore, when the cracks propagated, the expansion resistance was inevitably small, and so the material had low fatigue strength. Therefore, the finer the crystal grains are and the more uniform the grain distribution is, the higher the fatigue strength is. By observing Fig. 13 which shows the metallographic structure of the babbitt-steel bimetal bearing material, it could be seen that around the joint interface there was not a segregation layer of acicular 𝜂 phase, which increased the resistance of crack propagation in the vicinity of the interface and improved the bonding force of babbitt-steel bimetal material. The elemental composition near the interface was analyzed by energy spectrum analysis, as shown in Table 2, and it was found that there was an intermetallic transition layer mainly composed of Fe, Pb and Sn elements in the vicinity of the interface. There was not a clear accumulation of copper element on the babbitt side of the transition layer, indicating the inexistence of 𝜂 phase.
The fracture surfaces of the destroyed babbitt were studied with the help of a scanning electron microscope. Figures 15 and 16 present the typical photograph of the fracture surface. Figures 15(a), 16(a) show the microscope structures of two big cracks. From Figs 15(a), 16(a), it can be seen that the crack is obvious quasi-cleavage crack feature. Figure 15(b) is the enlarged view of the big crack in Fig. 15(a). From this figure, it can be seen that, the fracture surface is mat and is covered by some film and much granular substance, which could be the precipitated Cu–Sn compound phase or the carbide and the oxide of Cu, Sn and Sb elements [4]. By further observation, it is found that there exist some micro pits on the fracture surface, as shown in Fig. 16(c), which means that the fracture is of ductility to some degree. 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 fracture feature, which should be caused by high temperature and high pressure of the lubrication oil squeezed into the cracks.
Conclusions and recommendations
The Pb content in the failed bearing bush alloy was too high, and did not meet the requirements of the Sn-based babbitt alloys in the national standard. At the same time, the primary crystal Cu6Sn5 formed by Cu and Sn was low due to the low Cu content, and the crystal SnSb with a small density moved upward and segregated. Therefore, the mechanical properties and the fatigue resistance of the bearing bush alloy were reduced. The bearing bush was subjected to alternating loads in service, and several small cracks were generated on the bearing bush alloy working surface. Lubricating oil penetrated into the small cracks. Under the loads of the bearing, the lubricating oil could not escape and formed an oil wedge, and then the small cracks continue to expanded and connected with each other. Fatigue pitting occurred on the bearing bush working surface, a large number of pits were formed, and several large alloy blocks fell off. The composition of the babbitt alloy, especially the Cu content, should be strictly controlled to ensure the safe and reliable operation of the bearing.
Footnotes
Acknowledgements
The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 51705265). The authors also thank Dr. Edward C. Mignot, Shandong University, for linguistic advice.
Conflict of interest
None to report.