The Babbitt alloy on the sealing tile surface of a nuclear power plant generator was damaged and fell off during operation. The reasons for the falling off of the Babbitt alloy on the sealing tile were analyzed by macroscopic observation, chemical composition analysis, metallographic inspection, hardness test, scanning electron microscopy and energy spectrum analysis. The results show that improper installation of the sealing tile causes friction between the sealing tile and the sealing seat cover, resulting in wear on the sealing tile surface; the chemical composition of the Babbitt alloy material is unqualified, and the β phase is unevenly distributed, resulting in a decrease in its bearing capacity. Under periodic loads, the Babbitt alloy fatigue falls off.
The sealing tile is an important component of the generator sealing oil system, which can ensure that hydrogen does not leak. The Babbitt alloy coating on the surface of the sealing tile of a nuclear power plant generator fell off after running for 1 year, affecting the safe operation of the generator. The generator sealing pad is a carbon steel matrix + liner cast tin-based alloy bimetallic bearing pad. The matrix material is Q345R steel, and the tin-based alloy material is ZSnSb12Cu6Cd1 bearing babbitt alloy. The overall macroscopic morphology of the sealing pad is shown in Figure 1. During the overhaul, it was found that the babbitt alloy on the sealing pad was damaged and fell off, and there was a sign of blackening on the side of the sealing pad that was in contact with the sealing seat cover. The author used a series of physical and chemical test methods to analyze the cause of the babbitt alloy shedding of the sealing pad to avoid such problems from happening again.
1 Physical and chemical test
1.1 Macroscopic observation
The sealing pad lining is divided into three parts: upper, middle and lower. The upper part is the air side and the lower part is the hydrogen side. The damage of the sealing pad lining is mainly concentrated in the middle part, and the damage is irregularly distributed along the circumference of the middle lining. The damage characteristics include block peeling, honeycomb holes and bright pits (see Figure 2).
The sealing tile is composed of two semicircular rings. The installation requires a gap of 0.20-0.25 mm between the sealing tile and the sealing seat cover (see Figure 3). There are two black circular ring marks with a width of about 5 mm on the inner and outer sides of the front of the sealing tile base body, and the center distance between the two circular rings is about 34.5 mm; there are small suspected pit damages on both circular rings, and the distribution of the pits is irregular (see Figure 4). According to the installation requirements of the sealing tile and the sealing seat cover, it is judged that the gap between the sealing tile and the sealing seat cover is insufficient. When in service, the sealing tile rotates so that its front side and the sealing seat cover rub against each other, resulting in the formation of a black circular ring mark on the front of the sealing tile.
1.2 Chemical composition analysis
The chemical composition of the babbitt alloy lining the sealing tile was analyzed by inductively coupled plasma emission spectrometer, and the chemical composition of the sealing tile base was analyzed by direct reading spark spectrometer. The results are shown in Tables 1 and 2 respectively. From Tables 1 and 2, it can be seen that the Cu and As content of the babbitt alloy in the sealing shoe lining are lower than the requirements of GB/T1174-1992 “Casting Bearing Alloy” and JB/T4272-1994 “Technical Conditions for Tin-Based Alloy Bearings for Steam Turbines”; the chemical composition of the sealing shoe matrix meets the requirements of GB/T713-2014 “Steel Plates for Boilers and Pressure Vessels”.
1.3 Metallographic Inspection
According to GB/T13298-2015 “Methods for Metal Microstructure Inspection”, the damaged area and the undamaged area of the sealing shoe were cut, embedded, ground, polished and corroded respectively. The corrosive agent was 4% (volume fraction) nitric acid ethanol solution. The microstructure morphology of the damaged area and the undamaged area of the sealing shoe are shown in Figures 5 and 6 respectively. According to CB1156-1992 “Metallographic Inspection of Tin-Based Bearing Alloys”, the side length of the β phase is measured and the distribution of the β phase is observed, and the quality of the β phase in the microstructure of the babbitt alloy is graded.
As shown in Figure 5, the babbitt alloy in the upper, middle and lower parts of the undamaged area is well combined with the matrix. The babbitt alloy structure is mainly composed of α solid solution (Sb dissolved in Sn solid solution, soft phase) + white needle-shaped ε phase (Cu6Sn5) + white blocky cubic β phase (SnSb, hard phase). There is no difference in the side length level and distribution level of the β phase in the upper, middle and lower parts. The side length level of the β phase in each area is level 1, the β phase is evenly distributed, no segregation phenomenon is observed, and the distribution level is level 1.
As shown in Figure 6, there is no detachment between the Babbitt alloy and the matrix in the upper, middle and lower parts of the damaged area, and the β phase side length level in these areas is level 1; the β phase on the upper surface is agglomerated, there is no β phase on the bonding surface, the β phase segregation is more serious, and the distribution level is level 3; the β phase on the middle surface and bonding surface is square and small and irregularly distributed, the distribution level is level 2, and there are cracks extending along the β phase-free area on the surface and near the bonding surface; the β phase on the lower surface is agglomerated, there is no β phase on the surface, and the β phase distribution level is level 3.
1.4 Hardness test
According to GB/T231.1-2018 “Metallic Material Brinell Hardness Test Part 1: Test Method”, the hardness of the babbitt alloy in the upper, middle and lower areas of the sealing tile lining was tested using a Brinell hardness tester. The diameter of the hardness tester indenter was 2.5 mm, and the test force was 306.5 N. The test results are shown in Table 3. It can be seen from Table 3 that the hardness of the babbitt alloy in the sealing tile lining meets the requirements of GB/T1174-1992.
1.5 Scanning electron microscopy (SEM) and energy spectrum analysis
The morphology of the damaged area of the babbitt alloy surface of the sealing tile was observed using a scanning electron microscope, and the results are shown in Figure 7. As shown in Figure 7, the honeycomb defect area on the Babbitt alloy surface is composed of a large number of reticular cracks, and some cracks are embedded with impurities; the bottom of the bright pit is relatively flat, and the characteristic morphology of crack extension can be seen. These cracks form a reticular structure, which further causes the Babbitt alloy to peel off; the quasi-cleavage characteristics and fatigue streak characteristic morphology can be seen at the bottom of the large piece of detachment.
The energy spectrum analysis results show that the impurities in the cracks in the honeycomb defect area are mainly composed of elements such as Fe, C, and O. It is speculated that the wear products formed by the contact between the sealing shoe gland and the sealing shoe enter the Babbitt alloy matrix, and cracks are initiated and extended at these locations. The sealing shoe will be subjected to periodic loads during service, and the initial cracks formed by the embedded impurities in the Babbitt alloy will undergo fatigue expansion under the action of periodic loads.
The sealing shoe matrix was observed using SEM, and the results are shown in Figure 8. As shown in Figure 8, the blackened and pitted areas are composed of small pits embedded with foreign matter; there are machining traces in the non-blackened area, while there are no machining traces in the blackened area of the substrate, indicating that the front surface of the sealing tile and the sealing seat cover are worn during service, resulting in a decrease in the surface roughness of the substrate.
The foreign matter at the bottom of the pit in the blackened area, the blackened area substrate and the non-blackened area substrate were analyzed using an energy spectrum analyzer. The foreign matter in the pit is mainly composed of C and O elements, and it is judged to be the residue of sealing oil; the oxygen content on the surface of the blackened area is higher than that of the non-blackened area, indicating that the cause of the blackening is that the surface is oxidized after the friction between the sealing tile substrate and the sealing seat cover causes heat.
2 Comprehensive analysis
When installing the sealing tile, a gap of 0.20-0.25 mm should be left between the sealing tile and the sealing seat cover so that the sealing tile and the seat cover will not wear during operation. However, there are traces of wear and blackening on the damaged sealing tile substrate, indicating that the sealing tile and the seat cover rotated relative to each other during actual operation, causing wear of the sealing tile and the sealing seat cover. The wear products enter the flow channels of the upper, middle and lower parts of the sealing tile along with the sealing oil. When the sealing tile rotates, these wear products are embedded in the matrix of the Babbitt alloy of the middle sealing tile, causing defects on the surface of the Babbitt alloy. There are signs of blackening on the entire front circle of the sealing tile substrate. The reason is that the heat generated by wear causes the sealing oil temperature to rise, resulting in a decrease in matrix viscosity, a thinning of the oil film, and a decrease in the performance of the boundary oil film, which further affects the service performance of the sealing tile.
For tin-based babbitt alloy, in the Sn-Sb binary alloy system, only when the mass fraction of Sb element is greater than 7.5%, the blocky β phase SnSb will appear in the structure; while Cu element is almost insoluble in Sn element, but a small amount of Cu element can be dissolved in α solid solution. Only when Cu element is excessive in α solid solution can fine ε phase (Cu6Sn5) be generated. ε phase is the crystallization core of β phase SnSb cubic crystal. During the crystallization process, ε phase first crystallizes out of the solution in a network form, thus preventing the subsequent precipitated β phase from floating due to its light weight and causing segregation, making the formed crystals more uniform and fine. The Cu content in the babbitt alloy of the damaged sealing tile lining is lower than the standard requirement, which makes the ε phase in the material less and causes the segregation of β phase. The As content in the babbitt alloy of the damaged sealing tile lining is lower than the standard requirement. The As element is usually dissolved in the matrix, which plays a role in refining the grains and improving the strength of the alloy. Its low content will reduce the mechanical properties of the babbitt alloy.
The normal babbitt alloy matrix should be distributed with uniform, dispersed, and fine β-phase cubic crystals. A small amount of rod-shaped, needle-shaped or point-shaped crystals are allowed. The crystals should have no obvious directionality and be perpendicular to the matrix. Segregation and accumulation are not allowed. If the hard particles of the β phase are large or unevenly distributed, a single crystal in the alloy will be subjected to excessive pressure and break during service. The more uneven the distribution of the β phase is, the greater the force on a single crystal will be, and deformation and slip will occur in the crystal, which will destroy the grain boundary and reduce the fatigue strength of the alloy. Only when the β phase crystals are fine and evenly distributed can the alloy have a better load-bearing capacity. The uneven distribution of β phase in the damaged area of Babbitt alloy causes the bearing capacity of Babbitt alloy to deteriorate. Wear products exist on the surface of the middle area, which forms initial defects and initiates cracks after embedding. However, the β phase in the damaged area cannot effectively prevent crack propagation, so that the cracks propagate along the surface and inside of Babbitt alloy, eventually causing the Babbitt alloy to fall off.
3 Conclusion
When installing the generator sealing tile, the installation gap between the sealing tile and the sealing seat cover is insufficient, causing the sealing tile and the sealing seat cover to wear and heat up. The presence of wear products deteriorates the quality of the sealing oil and causes cracking on the surface of Babbitt alloy. The chemical composition of the inner lining Babbitt alloy material is unqualified, and the β phase is unevenly distributed, resulting in poor bearing capacity. Under the action of periodic load, the sealing tile falls off due to fatigue.
