Excavator Tooth System is the major part of excavator and also the quick-wear one. It is a combined one which consists of adaptor and bucket teeth. Both of this 2 parts connect with pin.
Owing to the tooth could be the abrasion failure part of tooth system so changing it would be feasible. An outdoor excavator suffered initial failure due to the severe wore during the process of using. For this batch of tooth, the failure mode and failure reason had been analyzed and improved measures had been offered.
Under various working condition, the tips inflict abrasion and impact at different degree, different kinds of failure in diverse form. At the normal operating conditions, the life time of this type of tooth have only 3 days(about 36 hours). It does not cater the requirements both in the aspects of economy or usage. It can be seen from the macro-photograph that there has a relatively obvious furrow-shaped scratch at the surface of the tooth fore part. At the top, there are a few plastic deformation without crack. The fore working surface(the side touch the ground) is the thinnest, about 4 centimeter while the rear one is about 8 centimeter.
Ⅱ. Analysis And Discussion
- Force Analysis
In a complete course of excavating, the force condition vary from different stages while the working surface of tooth contact with objects by digging. Owing to the fast speed while tooth touch the surface of object firstly, the top of the tooth suffer a strong compact. If the yield strength is weak, the top will be prone to become plastic deformation. The force condition are along with the change of excavation depth. When using the tooth to cut the materials, the relative movement between them will produce huge extrusion forces. Thus, a powerful friction may be made by the tooth working surface and materials. If the materials are the objects relatively hard like rock, concrete and so on, the friction will be powerful. The result of reciprocal action of this process, the various degree of abrasion could be generate at the working surface of the tooth so as to lead a furrow at quite deepness. The positive pressure of the fore working surface is bigger than the rear one apparently. The fore one damaged severely. Accordingly, the judgment could be figure out that the positive and friction are the main outer mechanical factor, which lead the core function at the procedure of the failure.
- Craft Analysis
Acquired 2 abraded sample from the working surface of the fore and the rear separately and then had a hardness test. It can be found that although test on the same one the test result of hardness may lead to discrepancy because of heterogeneous materials. It is just the initial judgment. After grinding, polishing, corroding, there are overt line on the each sample while at different place. From the macro phase, the surrounding is gray while the central part is in dead color which could be illustrated that this is a cast-in bonding goods. From the working surface, the part being wrapped may also the insert. The hardness test executed on the HRS-150 Digital Rockwell apparatuso and MHV-2000 Digital micro-hardness tester. It mainly tested the hardness of the 2 sides between the boundary line. The result apparently differ from each other. To crown it all, the tooth is the insert could be verified. There into, the enclosed part is the insert while the surrounding part is matrix. The ingredients of both are proximate, alloying the Cr、Mn、Si and other elements, the core alloy ingredients are 0.38C、0.91Cr、0.83Mn、0.92Si. The mechanical property of the metallic material depends on the components and the technical of heat-treatment. The proximity of the components leads to the imparity of the hardness, which indicated that the tooth came into used after casting but without heat-treatment. The point was also verified by observation of the organization lately.
- Organization Analysis
According to the observation, it could be indicated that the matrix is mainly the black silvered-shape tissue. The insert parts consists of white lump(cover the most areas) and black shred, far away from the cross-sectional areas. The further experiment of micro hardness testified the white lump tissue is the ferrite while the black silvered one is the troostite or the mixed tissue of troostite and pearlite. In the insert, the formation of the chunk ferrite is similar with the formation of the central part of welding HAZ(Heat affected zone). This area is at the two-phased region due to the molten metal thermogenesis during the casting process. At here, the ferrite fully grown up and keep the temperature of form of tissue at a room temperature. The temperature of insert center is partially low and cannot form the chunk ferrite because of the huge volume of insert and the thin wall of the tooth.
- Performance Analysis
Through the abrasion test on the MLD-10 tribological machine indicated that the wear resistance of matrix and insert are better than quenched 45 steel under the experiment of trivial impacts abrasive grains. Meanwhile, the resistance of matrix and insert differ from each other, matrix is tough than insert. The ingredients of 2 sides of them are similar. So it is obvious that the main effect of the tooth insert is to chill. In the course of casting, grain refinement is to advance its hardness and resistance. It may not exert the improved intensity and resistance due to the insert , which influenced by casting thermal, generated the organization similarly like welding heat influence area. After casting, the appropriate heat treatment to advance the organization of matrix and insert would apparently enhance the resistance and the life span of the bucket teeth.
- The teeth material is low – alloy wear – resistant steel, which is pretty suitable for teeth. The results of missing the essential heat treatment could be as follows: the in-homogeneous of teeth organization, the insert not exert the due effect, the whole bad resistance of teeth. All of those leads the premature failure of teeth.
- After casting formation, appropriate normalizing to the casting to improve the organization and performance, and also prolong the life span. Under the same working condition, the teeth, which had the reasonable heat treatment, lasts twice as long as other teeth.
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