Abstract: In response to the failure problem of bearing fracture in automotive water pumps, the morphology, chemical composition, hardness, and gold content of the fractured shaft were inspected and analyzed. It was found that the main reasons for the fracture were the low strength of the pump shaft and the tip notch effect in the cutting groove, which is a typical fatigue fracture. A solution was proposed to change the heat treatment process of water pump bearings and optimize the geometric dimensions of pump shafts, effectively solving the quality problem of water pump bearing fracture.
Keywords: Automobile water pump bearings; Fatigue fracture; Induction hardening
The car water pump is a key component of the engine cooling system, which inputs torque through a pulley at one end of the water pump bearing, driving the impeller at the other end to rotate. In order to meet the development requirements of high-power and high-efficiency engines, higher performance requirements have been put forward for water pump bearings. Fracture, corrosion, peeling, and grease leakage are common failure modes of automotive water pump bearings, among which fracture poses a greater risk. Therefore, it is necessary to analyze the failure modes of fracture and develop corresponding preventive measures.
A certain model of car water pump experienced a bearing shaft breakage problem after approximately 200 hours of vehicle operation, and the user complained seriously. The bearing model of the water pump is WR1938148, and the product structure is shown in Figure 1. The fracture occurs at the back groove of the flange. In order to understand the cause of the bearing fracture of the water pump, the failed parts were inspected and analyzed.
Figure 1
1. Belt pulley 2. Flange plate 3 Water pump bearing 4. Shell 5. Water seal 6. Impeller
1. Fracture analysis
The morphology of the fracture surface is shown in Figure 2, and the fatigue crack propagation steps and stripes are clearly visible. Based on their characteristics, the fracture surface is divided into three regions: the initiation zone, the fatigue crack propagation zone, and the instantaneous fracture zone. Through naked eye and low magnification observation, it was found that there were obvious repeated friction marks in Zone 1, indicating that this is the starting position of fatigue cracks. The lower part of Zone 1 shows radial stripes and obvious tearing marks, indicating that it is a fatigue crack propagation zone. There are obvious tearing marks at the starting position of Zone 2, but they did not appear afterwards, indicating that the crack propagates rapidly at the beginning. As the crack deepens, the propagation speed decreases continuously, and after Z, it remains stagnant for a period of time, leaving a lingering mark on the fracture surface. When the crack extends to the critical value and the remaining section of the shaft cannot overcome the bending torsion composite stress, it causes instantaneous shear fatigue failure mainly dominated by shear stress. Therefore, there are elongated dimples and arc-shaped scratches in Zone 3. Through observation and analysis of the fracture surface, it can be concluded that the cross-section belongs to a typical bending torsion composite fatigue fracture. At the beginning, the initiation and propagation of cracks are mainly caused by bending stress. Once cracks occur, the crack propagation speed slows down from fast to slow, and then enters the stage of crack propagation stagnation. After Z, shear stress mainly causes the fracture failure of the shaft.
Figure 2
1. Crack initiation zone 2. Fatigue crack propagation zone 3. Instantaneous fracture zone
2. Chemical composition inspection
Chemical composition inspection was conducted on the broken axis sampling, and the chemical composition was determined to be qualified according to GB/T18254-2002.
The high carbon in the material can obtain a high content of martensite, ensuring high hardness and wear resistance. At the same time, it is also conducive to the formation of high hardness carbides, further improving hardness and wear resistance. Si and Mn are conventional elements that are beneficial for solid solution strengthening and can also improve the hardenability of steel. On the one hand, Cr can improve the hardenability, and on the other hand, it can also form alloy carbides, making the carbides in the steel small and uniform, thereby greatly improving the wear resistance and contact fatigue strength of the steel.
3. Hardness testing
The heat treatment equipment used is a mesh belt furnace. After testing, the hardness of the broken shaft is 62HRC, which meets the specification requirements.
The Ac1 and Ac3 of GCr15 are 760 ℃ and 900 ℃, respectively, so the quenching temperature is set at 840 ± 10 ℃, and the tempering temperature is determined to be 160 ℃ according to relevant specifications. Regarding the quenching and tempering insulation time, which is related to the power of the heating furnace, the amount of furnace charge, and the effective wall thickness of the parts, it is usually ensured that the parts are uniformly hot penetrated.
4. Material metallographic inspection
The specimen was prepared using a broken axis for metallographic examination. Figure 3a shows the microstructure of the center, martensite is evaluated as Grade II, and Figure 3b shows the microstructure of the hardened layer carbides, as well as Grade II carbides after quenching and tempering. Its structure can be divided into black zone and white zone. The black zone is cryptocrystalline martensite dominated by lath martensite, and the white zone is cryptoneedle martensite dominated by twin martensite.
Figure 3 500X
Wetting method: 4% nitric acid alcohol solution erosion
From the above results, it can be seen that the microstructure of the shaft is composed of fine crystalline martensite, cryptocrystalline martensite, a small amount of fine needle shaped martensite, a small amount of residual carbides, and an appropriate amount of residual austenite, resulting in higher hardness and lower toughness in the center. Meanwhile, in practical applications, materials always inevitably have original defects (such as microcracks, slag inclusions, segregation, etc.), which disrupt the continuity and uniformity of the material structure, leading to a decrease in the mechanical strength of the material. Under the radial load impact of the belt pulley, the above defects gradually develop into cracks, leading to fatigue failure and ultimately Z fracture.
5. Solution
Through inspection and analysis, it is determined that the fractured shaft belongs to a typical bending torsion composite fatigue fracture, and its chemical composition, hardness, metallographic structure, and heat treatment process all meet the requirements of relevant technical specifications. However, the insufficient toughness of the quenched shaft leads to low bending and torsional fatigue strength. Secondly, according to the analysis in Figure 1, the load of the pulley is transmitted to the water pump bearing shaft through the flange, and there is a tip notch effect in the back of the flange, which is a stress concentration area and is prone to crack sources. Therefore, the quality problem of water pump bearing fracture can be solved by changing the heat treatment process of water pump bearings and optimizing the geometric dimensions of pump shafts.
5.1 Optimization of heat treatment process
To solve the problem of insufficient toughness of the quenched shaft, surface high-frequency induction quenching process was adopted.
Surface induction quenching, rapid heating and short time holding will reduce the carbon content of austenite, so it is easier to obtain lath martensite with substructure and high density dislocation rather than needle martensite, so the toughness is improved compared with that before. Moreover, induction hardening can generate compressive stress on the surface, and parts that are hard on the outside and tough on the inside can effectively improve their fatigue strength, thereby avoiding the recurrence of shaft breakage. Microscopic analysis reveals that the microstructure obtained from quenching consists of cryptocrystalline martensite, fine and uniformly distributed carbides, and a small amount of residual austenite. The retained small amount of residual austenite can relax stress and act as a buffer. Due to its softness and toughness, residual austenite can partially absorb the rapid expansion energy of martensitization, alleviate phase transformation stress, and prevent the occurrence of cracks. After quenching, low-temperature tempering should be carried out in a timely manner. Generally, tempering must be carried out within three hours, otherwise internal stress can easily cause cracking.
Experimental tests have shown that the surface hardness of the shaft is 58-63HRC, the depth is (0.8-4.0) (550HV), and the core hardness is not greater than 179-271HBS, which is a relatively ideal state.
5.2 Optimization of product structure
To solve the tip notch effect of the back cutting groove of the flange and improve the mechanical strength of the shaft, the cutting groove of the shaft was moved to the front end of the flange. The optimized product structure is shown in Figure 4. The diameter of the load-bearing part of the optimized rear axle has increased, and the grooves that were previously prone to stress concentration no longer bear the load of the pulley. The structure is more scientific and reasonable, effectively improving the load-bearing capacity of the axle.
Figure 4
1. Belt pulley 2. Flange plate 3 Water pump bearing 4. Shell 5. Water seal 6. Impeller
6. Conclusion
The early fracture failure of WR1938148 water pump bearings is a typical fatigue fracture. Through inspection and analysis of the failed parts, Z finally improved the fatigue strength and bearing capacity of the shaft through high-frequency induction quenching heat treatment process and optimized pump shaft geometric dimensions, thereby avoiding the recurrence of pump bearing fracture. Induction quenching has advantages such as uniform heating, small deformation, stable quality, high degree of automation, energy conservation and environmental protection, which cannot be compared to ordinary quenching treatment.
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2024-05-17