Failure analysis of bucket teeth of the hottest ex

Failure analysis of excavator bucket teeth

excavator bucket teeth are important components and vulnerable parts of excavators. They are combined bucket teeth composed of tooth seats and tooth tips, which are connected by pins. Since the tooth tip is the worn part of the bucket tooth, just replace the tip. The bucket teeth of an open-air excavator failed early due to serious wear during use. Aiming at this batch of bucket teeth, the failure forms and causes of the bucket tooth surface are analyzed, and the improvement measures are put forward

1 failure form

bucket teeth are subject to different degrees of wear and impact under different working conditions, resulting in different degrees and forms of failure. Under normal working conditions, the bucket tooth will fail after only 3 days of use (about 36h), which is not in line with the requirements from both the economic point of view and the point of use. It can be seen from the macro photos of this batch of failed parts that there are obvious furrow scratches on the front working surface of the bucket teeth, 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 bucket tooth working face is in contact with the excavated object, and its stress condition is different at different working stages in a complete excavation process. When the tooth tip first contacts the material surface, due to the fast speed, the average annual growth rate of this industry in China is more than 15%, and the bucket tooth tip is strongly impacted. If the yield strength of the bucket teeth is low, plastic deformation will occur at the tip. With the increase of excavation depth, the stress condition of bucket teeth will change. When the bucket teeth cut the material, the bucket teeth move relative to the material, resulting in a large forward extrusion pressure on the surface, resulting in a large friction between the bucket teeth working surface and the material. If the material is hard rock blocks, concrete, etc., the friction will be great. The repeated action of this process results in different degrees of surface wear on the bucket tooth working surface, resulting in deep furrows. The positive pressure of the front working face is significantly greater than that of the rear working face, and the front working face is seriously worn. It can be judged that the positive pressure and friction are the main external mechanical factors of the bucket tooth failure, and play a major role in the process of failure

(2) process analysis take two samples from the front and rear working faces respectively, and grind them flat for hardness test. It is found that the hardness of the same sample varies greatly, and it is preliminarily judged that the material is uneven. After grinding, polishing and corrosion of the samples, it is found that there are obvious boundaries on each sample, but the boundaries are different. Viewed from macro, the surrounding is light gray, and the color of the middle part is dark, indicating that this piece is likely to be an insert casting. From the surface, the surrounded part should also face the support of the majority of users for our company, which is an insert. Hardness tests were carried out on both sides of the boundary on HRS-150 digital display Rockwell hardness tester and mhv-2000 digital display microhardness tester, and obvious differences were found. Through the above analysis, it is confirmed that the bucket tooth is an insert structure. The closed part is the insert, and the surrounding part is the matrix. The compositions of the two are similar. Alloying with Cr, Mn, Si and other elements, the main alloy components (mass fraction,%) are 0.38c, 0.91cr, 0.83mn, 0.92si. The mechanical properties of metal materials depend on the composition of materials and heat treatment process. The components are similar but the hardness is different, indicating that the bucket teeth are put into use without heat treatment after casting. Later organizational observations also prove this

(3) microstructure analysis metallographic observation shows that the matrix is mainly black fine lamellar structure. The inlay structure is composed of white blocks and black thin pieces, and there are more white blocks far away from the section area. Further microhardness tests show that the white massive structure is ferrite, and the black fine lamellar structure is troostite or the mixed structure of Troostite and pearlite. The formation of large ferrite in the insert is similar to the formation of partial phase transformation zone in the welding heat affected zone. Due to the hydrothermal effect of metal in the casting process, this area is in the two-phase region of austenite and ferrite. The ferrite in this area is fully long, and its microstructure is maintained to room temperature according to the statistics of the international vanadium Technical Committee (vanitec). Because the bucket tooth wall is relatively thin, and the volume of the insert is large, the temperature in the center of the insert is low, and no large ferrite is formed

(4) performance analysis the wear test on MLD-10 wear tester shows that the wear resistance of matrix and insert under the condition of small impact abrasive wear test is better than that of quenched 45 steel. At the same time, the wear resistance of the matrix and the insert is different, and the matrix is more wear-resistant than the insert. The composition of both sides of the matrix and the insert is similar, so it can be seen that the insert in the bucket tooth mainly plays the role of chill. In the casting process, the matrix grain is refined to improve its strength and wear resistance. Due to the influence of casting heat, the insert produces a structure similar to the welding heat affected zone, which does not play a role in enhancing the wear resistance. If proper heat treatment is carried out after casting to improve the structure of matrix and insert, the wear resistance and service life of bucket teeth will be significantly improved

3 conclusion

(1) the bucket teeth are made of low alloy wear-resistant steel, which is 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 normalize the castings properly after casting to improve the structure and properties and increase the service life. After reasonable heat treatment of castings, the service life of bucket teeth is increased by nearly 2 times under the same working conditions