1, the form of failure
The teeth are subjected to different degrees of wear and impact under different working conditions, and different degrees and different forms of failure occur. The tooth is ineffective under normal working conditions for only 3 days (about 36h), which is not desirable from an economic point of view or from an angle of use. It can be seen from the macroscopic photograph of the failed parts that the front working surface of the tooth has obvious rill-like scratches, the tip has a small amount of plastic deformation, no crack, and the front working surface (the surface in contact with the ground) is the thinnest. 4mm, the rear working face is about 8mm.
2, analysis and discussion
(1) Force analysis The tooth working face is in contact with the excavated object, and the force is different in different working stages in a complete excavation process. When the tip of the tooth first contacts the surface of the material, the tip of the tip is subjected to a strong impact due to the faster speed. If the tooth has a low yield strength, it will be plastically deformed at the tip. As the depth of the excavation increases, the force on the teeth will change. When the tooth cuts the material, the tooth moves relative to the material, and a large positive pressing force is generated on the surface, thereby generating a large friction between the tooth working surface and the material. If the material is hard rock, concrete, etc., the friction will be very large. The result of repeated action of this process produces varying degrees of surface wear on the tooth working face, which in turn produces a furrow with a greater depth. The positive pressure of the front working face is obviously larger than that of the rear working face. The front working face is seriously worn. It can be judged that the positive pressure and friction force are the main external mechanical factors of the tooth tooth failure, and play a major role in the process of failure.
(2) Process analysis Two samples were taken from the front and back working faces, respectively, and the hardness test was carried out. It was found that the hardness of the same sample was very different, and the initial judgment was that the material was uneven. The samples were ground, polished, and etched. It was found that there were obvious boundaries on each sample, but the boundaries were different. From a macro perspective, the surrounding is light gray, and the middle part is darker, indicating that the piece is likely to be a cast piece. On the surface, the enclosed part should also be an insert. Hardness tests were performed on both sides of the boundary line on the HRS-150 Digital Rockwell Hardness Tester and the MHV-2000 Digital Micro Hardness Tester, and the difference was obvious (see Table 1). It was confirmed by the above analysis that the tooth was an insert structure. The closed part is the insert and the surrounding part is the base. The two components close to Cr, Mn, Si and other elements of the alloy, its main alloy composition (wt%) of 0.38C, 0.91Cr, 0.83Mn, 0.92Si. The mechanical properties of metallic materials depend on the composition of the materials and the heat treatment process. The composition is similar and the hardness is different, indicating that the tooth is put into use without heat treatment after casting. Subsequent organizational observations also prove this.
(3) Microstructure observation The metallographic observation shows that the matrix is ​​mainly black fine-grained structure, and the inlay structure is composed of white block and black fine piece, and there are many white block structures away from the cross-section area, further microhardness. tests show that the white granular structure of ferrite, black fine lamellar structure is a mixed structure of troostite or troostite and pearlite. The formation of large ferrite in the insert is similar to the formation of part of the phase change zone in the heat affected zone of the weld. Due to the action of molten metal during the casting process, the region is in the austenite and ferrite two-phase region, in which the ferrite is sufficiently grown and its microstructure is maintained at room temperature. Since the tooth wall is relatively thin and the insert is bulky, the central portion of the insert has a low temperature and no large ferrite is formed.
(4) Performance analysis The wear test on the MLD-10 wear tester showed that the wear resistance of the substrate and the insert under the small impact abrasive wear test conditions was better than that of the quenched 45 steel. At the same time, the wear resistance of the substrate and the insert is different, and the substrate is more wear-resistant than the insert. The components on both sides of the base and the insert are close to each other, and it can be seen that the inserts in the teeth mainly function as cold iron. The matrix grains are refined during the casting process to improve their strength and wear resistance. Since the insert is affected by the heat of casting to produce a structure similar to the heat affected zone of the weld, it does not play a role in enhancing the 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 teeth will be significantly improved.
3. Conclusion
(1) The tooth material is low alloy wear-resistant steel, which is suitable for tooth. However, since the necessary heat treatment is not performed, the tooth structure is uneven, the insert does not play its due role, and the overall wear resistance of the tooth is poor, leading to early failure.
(2) After casting, it is recommended to properly normalize the casting to improve the structure and performance and improve the service life. After reasonable heat treatment of the castings, the service life of the teeth is nearly doubled under the same working conditions.
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