Will the bucket teeth of an excavator fail?

【1】Failure mode Bucket teeth are subjected to different degrees of wear and impact under different working conditions, resulting in different degrees and forms of failure. This bucket tooth failed after only 3 days (about 36 hours) of use under normal working conditions. This does not meet the requirements from both economic and practical perspectives. From the macroscopic photos of this batch of failed parts, it can be seen that there are obvious plow-shaped scratches on the front working surface of the bucket tooth, a small amount of plastic deformation at the tip, and no cracks. The front working surface (the surface in contact with the ground) is the thinnest, about 4mm, and the rear working surface is about 8mm. 【2】Analysis and discussion (1) Stress analysis The working surface of the bucket tooth is in contact with the excavated material. The stress conditions at different working stages in a complete excavation process are different. When the tooth tip first contacts the material surface, the bucket tooth tip is subjected to a stronger impact due to the high speed. If the yield strength of the bucket tooth is low, plastic deformation will occur at the tip. As the excavation depth increases, the stress conditions of the bucket tooth will change. When the bucket teeth cut the material, the bucket teeth and the material move relative to each other, generating a large positive extrusion pressure on the surface, thereby generating a large friction force between the bucket tooth working surface and the material. If the material is a hard rock block, concrete, etc., the friction force will be very large. As a result of the repeated action of this process, the bucket tooth working surface will produce different degrees of surface wear, and then produce deep furrows. The positive pressure of the front working surface is significantly greater than that of the rear working surface, and the front working surface is severely worn. It can be judged that the positive pressure and friction are the main external mechanical factors of bucket tooth failure and play a major role in the process of failure.

(2) Process analysis

Two samples were taken from the front and rear working surfaces respectively, and they were ground flat for hardness testing. It was found that the hardness of the same sample varied greatly, and the preliminary judgment was that the material was uneven. The samples were ground, polished, and corroded, and it was found that there was a clear boundary on each sample, but the boundary position was different. From a macroscopic point of view, the surrounding area was light gray and the middle part was darker, indicating that the part was probably an insert casting. From the surface, the surrounded part should also be an insert. Hardness tests on both sides of the boundary using an HRS-150 digital Rockwell hardness tester and an MHV-2000 digital microhardness tester revealed a significant difference (see Table 1). This analysis confirms that the bucket tooth is an insert structure. The enclosed portion is the insert, and the surrounding portion is the matrix. The two components have similar compositions, alloyed with elements such as Cr, Mn, and Si. The main alloying components (mass fraction, %) are 0.38C, 0.91Cr, 0.83Mn, and 0.92Si. The mechanical properties of metal materials depend on their composition and heat treatment process. The difference in hardness despite similar composition indicates that the bucket tooth was used without heat treatment after casting. Subsequent microstructure observations also confirm this.

(3) Microstructure analysis Metallographic observations show that the matrix is ​​mainly composed of black flakes, and the insert structure consists of white blocks and black flakes. The white block structure is more in the area away from the cross section. Further microhardness tests show that the white block structure is ferrite and the black flake structure is troostite or a mixed structure of troostite and pearlite. The formation of large ferrite in the insert is similar to the formation of some phase transformation zones in the heat affected zone of welding. Under the action of liquid metal heat during the casting process, this area is in the austenite and ferrite two-phase region. In this area, the ferrite grows fully and its microstructure is maintained at room temperature. Since the bucket tooth wall is relatively thin and the insert volume is large, the temperature in the center of the insert is low and no large ferrite is formed. (4) Performance analysis Wear tests on the MLD-10 wear tester show that the wear resistance of the matrix and insert under small impact abrasive wear test conditions is better than that of quenched 45 steel. At the same time, there is a difference in the wear resistance of the matrix and the insert, and the matrix is ​​more wear-resistant than the insert. The composition of the matrix and the insert on both sides is similar, which shows that the insert in the bucket tooth mainly plays the role of cold iron. The matrix grain is refined during the casting process to improve its strength and wear resistance. Because the insert is affected by the casting heat and produces a structure similar to the heat affected zone of welding, it does not play a role in enhancing wear resistance. If appropriate heat treatment is performed after casting to improve the structure of the matrix and the insert, the wear resistance and service life of the bucket tooth will be significantly improved.

【3】Conclusion (1) The bucket tooth material is low-alloy wear-resistant steel, which is relatively suitable for bucket teeth. However, due to the lack of necessary heat treatment, the bucket tooth structure is uneven, the insert does not play its due role, and the overall wear resistance of the bucket tooth is poor, resulting in early failure.

(2) It is recommended to properly normalize the casting after casting to improve the structure and performance and increase the service life. After reasonable heat treatment of the casting, the service life of the bucket tooth is increased by nearly 2 times under the same working conditions.