In various heavy-loaded, high-speed, and low-speed sliding bearings, it is very common for the babbitt alloy material to burn and fail due to the instability of the lubricating oil film. Through surface oil film analysis, surface crack morphology observation, chemical composition inspection, cross-sectional metallographic structure inspection, element spectrum analysis, etc. of the babbitt alloy on the surface of a failed bearing, it was found that the carbon deposition on the surface of the babbitt alloy caused the babbitt alloy on the surface of the bearing to be unevenly stressed. Under the action of friction, the surface of the babbitt alloy can undergo plastic flow, forming macroscopic friction marks, and cracking occurs in the babbitt alloy repair welding area with low hardness and welding defects, and expands along the layer depth direction; at the same time, the high content of Na+ in the oil film on the surface of the babbitt alloy forms an acidic solution with water, causing the babbitt alloy to undergo electrochemical corrosion and form corrosion pits.
Tin-based babbitt alloy is composed of α phase with tin-based solid solution as the matrix, hard phase β and dispersed Cu and Sn. It has good wear resistance, corrosion resistance and embeddability. Therefore, in large steam turbines, compressors, ships and other industrial fields, babbitt alloy is often used as sliding bearings. However, in various heavy-loaded, high-speed and low-speed sliding bearings, it is very common for babbitt alloy materials to burn and fail due to the instability of the lubricating oil film.
When used for turbine bearings, there is a layer of oil film between the shaft and the bearing to avoid direct contact and friction between the two. However, in actual work, when the shaft rotates at high speed, the bearing surface is subjected to a certain periodic alternating load, especially during startup and shutdown, the lubricating oil film is often destroyed, and semi-dry friction or even dry friction will occur at this time. At this time, friction occurs between the sliding bearing and the journal, generating friction heat. When the friction heat is too large, the Babbitt alloy on the surface will burn and melt, causing the Babbitt alloy material to burn and fail.
This work found the root cause of the cracking and even shedding of the Babbitt alloy layer on the surface of a failed bearing during the operation of the power plant by checking the chemical composition of the Babbitt alloy on the surface of a failed bearing, observing the macroscopic and microscopic morphology, checking the cross-sectional metallographic structure and analyzing the energy spectrum. Improvement measures are proposed to perform more detailed operation and maintenance of the bearing with Babbitt alloy to avoid the failure of the Babbitt alloy.
1 Overview of failed bearings
During operation, the thrust bearing is subjected to long-term axial force, generally below 32 tons, and the transient impact force can reach 200 tons. In order to ensure the strength of the working surface, a layer of Babbitt alloy is cast on the surface of the thrust bearing by vibration, and the non-Babbitt alloy end face is aligned with 0. 1 mm, and then machine the surface of the babbitt alloy to ensure the thickness of the babbitt alloy surface and the parallelism with the non-babbitt alloy end face within 0.03 mm. There is an oil film between the babbitt alloy on the bearing surface and the journal, and the babbitt alloy soft matrix (α phase) is concave, and the hard point (β phase) is convex, forming a small gap on the contact surface of the two as an oil storage space, reducing the friction between the two and heat transfer, ensuring that the working temperature of the bearing is below 100 ℃.
The failed bearing analyzed in this work showed a slow growth trend in the bearing temperature before the overhaul of the unit. On-site dissection of the thrust bearing for observation, it can be found that carbon deposits have obviously appeared on the surface of the thrust bearing, the parallelism of the bearing surface is poor, and the force has changed. After removing the carbon on the surface, the babbitt alloy on the surface of some bearings cracked or even fell off.
2 Analysis of the causes of bearing failure
2.1 Macroscopic inspection of cracks in babbitt alloy on the bearing surface
Through macroscopic inspection of the inspected bearing block, it was found that the babbitt alloy had fallen off at one place on the surface of the bearing block, and the surface of the babbitt alloy at another place was obviously worn. Visible cracks can be found in the wear area, and there are multiple electrolytic pits on its surface.
2.2 Chemical composition analysis of babbitt alloy on the bearing surface
The chip sample was taken from the surface of the alloy layer near the crack of the babbitt alloy using a milling cutter, and the composition of the babbitt alloy was analyzed using an inductively coupled plasma atomic emission spectrometer ICP-OES and a spectrophotometer in accordance with GB/T 10574.13-2017, etc. The results are shown in Table 1. It can be seen that the Pb content of the babbitt alloy is higher than the required value of the specification, and the Cu content is lower than the required value of the specification. This is because there will be a certain component segregation in the solidification process of the cast babbitt alloy. This analysis was conducted on the surface of the babbitt alloy layer. Due to the component segregation, the results of the two elements Cu and Pb in the analysis results are different from the standard values.
2.3 Scanning electron microscope observation of cracks on the bearing surface
The morphology of the babbitt alloy at various positions on the bearing surface was observed using a scanning electron microscope, as shown in Figure 1. It can be seen that the alloy layer has cracked at the wear position, and multiple microcracks are found around it; the babbitt alloy on the alloy layer surface has partially peeled off, and obvious electrolytic pits can also be observed. After the local position is magnified, it can also be observed that part of the surface has peeled off. After energy spectrum analysis, it is found that the hard phase β phase (SnSb) on the surface should be peeled off, as shown in Figure 2.
2.4 Inspection of metallographic structure and hardness uniformity of Babbitt alloy layer
The metallographic structure of the cross section of the Babbitt alloy layer was observed by sampling at the crack position on the bearing surface, and the cross-sectional structure was observed after corrosion with 4% nitric acid (HNO3) + alcohol corrosive solution. It can be seen that the Babbitt alloy layer cracked at the position where the surface β phase was less, and looseness and other defects could be observed at the crack position. Further observation of the crack position revealed that there was an uneven structure at the crack position of the Babbitt alloy. There was less β phase at the crack position of the Babbitt alloy layer, and the segregated ε phase was dispersed in a dotted manner, with a lower hardness, and the structure morphology was significantly different from that of the adjacent position. The Babbitt alloy structure in the uncracked area [α phase + β phase (SnSb) + ε phase (Cu6Sn5), in which the gray matrix is the Sn solid solution α phase, the white square crystals are β phase, and the dispersed needle phase is ε phase] has a completely different morphology. The babbitt alloy layer of the bearing bush was repaired due to casting defects. The difference in structure should be caused by the inconsistent cooling rate of the babbitt alloy repaired in the cracked area and the original cast babbitt alloy. At the same time, obvious pitting pits can be observed on the surface of the babbitt alloy. The interface between the babbitt alloy layer and the bush body is well bonded, and there is no peeling phenomenon (delamination) between the alloy layer and the bush body, as shown in Figure 3.
The thickness and hardness of the babbitt alloy were tested by sampling at different positions on the tile surface. It can be seen that the thickness of the babbitt alloy layer at different positions is in the range of 1.39 to 1.47 mm, and the thickness is relatively uniform; the Rockwell hardness uniformity of the babbitt alloy at different positions was checked by using a Rockwell hardness tester. There are certain differences in the hardness values at different positions, and the Rockwell hardness is basically between 28.2 and 32.5 HRC. Since the thickness of the alloy layer is only about 1.40 mm, which is lower than the minimum thickness of the object to be tested in the test method standard, the hardness result is for reference.
2.5 Energy spectrum analysis of babbitt alloy surface
The energy spectrum analysis of the electrolytic pits on the surface of the babbitt alloy is performed, and the results are shown in Figure 4. It is found that the content of Na and O elements at this location is relatively high. Combined with the results of the carbon composition analysis on the bearing surface (see Table 2), it can be seen that the content of metal elements such as Na, Zn, Cu, and Mg in the carbon on the surface of the babbitt alloy is significantly higher than that of the new oil, indicating that the impurity elements gradually deposit on the surface of the bearing during operation, resulting in local enrichment of the metal impurity elements.
The depth of the electro-corrosion pit was detected, and the results are shown in Figure 5. The depths of the electro-corrosion pits are 57.8 and 44.8 μm, respectively.
2 Analysis and discussion
Due to the excessively fast cooling rate, the crystallization process of the babbitt alloy repaired on the bearing surface is significantly shortened, and the hard phase β has no time to precipitate, forming an oversaturated tin-based solid solution, and the dispersed Cu and Sn structures are completely different from the casting, and the hardness is also lower than the babbitt alloy at the casting. When the bearing is in operation, the higher C and O content in the oil film causes carbon deposits to gradually form on the bearing surface, the parallelism of the bearing thrust plate deteriorates, the contact with the journal is uneven, and the oil film is destroyed. On the one hand, it causes serious friction between the Babbitt alloy and the journal, and the accumulation of friction heat causes the surface temperature to rise. When the temperature approaches and reaches the melting point of the matrix, the matrix begins to soften or even melt, and plastic flow can occur on the surface of the matrix under the action of friction, forming friction marks that can be seen macroscopically, causing the Babbitt alloy to crack or even peel off in the repair welding area with softer hardness and welding defects. On the other hand, the uneven contact between the bearing and the journal causes the oil film on the bearing surface to be blocked, and the impurity elements in the operation process are deposited on the bearing surface. The high content of Na elements and the water formed by the liquefaction of water vapor during the operation of the unit form an acidic solution with the oil. The low pH value causes the hydrolysis of tin oxides, and the Babbitt alloy matrix undergoes electrochemical corrosion, forming electrolytic pits.
In a Cl-containing solution, the dissolved oxygen content and OH- concentration will directly affect the cathode process of tin corrosion. During the operation of the bearing, the Na content in the oil is far higher than the standard requirements, and the water formed by the liquefaction of water vapor during the operation of the unit forms an acidic solution with the oil. The low pH value causes the hydrolysis of tin oxides, causing the Babbitt alloy matrix to undergo electrochemical corrosion. The main anode reactions in the corrosion process are shown in formulas (1) and (2), and the cathode reaction is shown in formula (3): See formulas (1), (2), and (3) in the figure.
3 Conclusions
(1) The fundamental reason for the failure of the bearing is that the impurity elements in the oil film are too high, resulting in serious carbon accumulation on the bearing surface, uneven contact with the journal, and cracking and even peeling first occurred at the softer Babbitt alloy of the repair weld.
(2) The Na content in the oil on the surface of the bearing is too high, causing electrochemical corrosion of the Babbitt alloy during operation. The content of corrosive elements in the oil should be strictly controlled in the future to avoid electrochemical corrosion between the Babbitt alloy and the bearing.
