Electric heating – Metal heating – For deposition welding
Reexamination Certificate
2000-03-22
2002-07-02
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
For deposition welding
C219S076150, C175S374000
Reexamination Certificate
active
06414258
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a base carrier for a tracklaying vehicle, which has a crawler belt provided with bushings each mounted between a pair of opposed links; and a sprocket comprising a plurality of teeth engaging with the bushings respectively. The invention also relates to hard facing methods for sprocket teeth and bushings which constitute a base carrier.
BACKGROUND OF THE INVENTION
Tracklaying vehicles such as bulldozers are usually operated on an unleveled ground and therefore their base carriers are subjected to extremely severe wear conditions through working because of the presence of sand and rocks. The wear rates of the crawler bushings and sprocket teeth are comparatively fast since they are subjected to considerably great contact pressure and used in such a situation that they are repeatedly brought into sliding contact with each other with sand and rocks entrapped therebetween. In the case of a bulldozer for use in mining oil sand for example, soil and sand are more likely to stick to its base carriers because of the high viscosity of oil sand. This aggravates the entrapment of sand and soil so that the base carriers for such a bulldozer wear at much faster rates than those of ordinary bulldozers. Therefore, there are strong demands for the development of base carrier parts improved in wear resistance.
In an attempt to impart wear resistance to parts which are locally required wear-resistance, a wear-resistant weld overlay having a composite structure composed of a matrix metal (untreated soft material) and hard particles or the like is applied to the surface of a base material. As one example of such a technique, a surface hardening method for tool joints is disclosed in Japanese Patent Publication (KOKOKU) Gazette No. 56-35986 (1981) according to which hard particles are dropped from a point proximate to the arc into a molten weld pool so as to precipitate on the bottom of the matrix and so as not to protrude from the surface of the a=matrix. In this surface hardening method, the effects of the sedimentation of the particles are: (1) since the particles do not protrude from the bead surfaces, a casing, that is, the mating member the joint, does not get damaged; and (2) the hardness of the metal (parent phase metal) which holds the precipitated cemented carbide particles increases, leading to improved wear resistance in the region where the cemented carbide particles have deposited. Another example is disclosed in U.S. Pat. No. 4,097,711. This patent proposes a surface treatment method for steel rollers comprising the steps of carburizing the surface of a material up to a depth equal to at least a half of the required depth of surface treatment; supplying particles for surface treatment to a molten weld pool and evenly dispersing them to obtain an effective hardness through the thickness of the whole molten part; applying thermal treatments to this molten part in order to obtain the required properties in the surface structure as well as in the base material. In this surface treatment method, it is admitted that the effect of the uniform dispersion of the particles is (1) an improvement in wear resistance and the effects of the carburization are (1) an improvement in the fatigue strength of the steel roller body and (2) an improvement in the wear resistance of the steel roller body.
However, these surface hardening methods reveal the following drawbacks when they are applied, as they are, to the base carrier parts of a tracklaying vehicle such as sprockets and bushing.
(1) During traveling, the sprocket and bushings of the base carrier intermittently come into contact with each other so that loads are repeatedly imposed on these parts. In the above conventional methods, cracks running transversely to the overlay beads are often created during formation of the weld overlay and stresses are repeatedly concentrated on the cracks, aggravating them. This causes exfoliation or partial chipping of the weld overlay and development of the cracks into the parts, resulting in breakage of the parts in themselves. Such cracks are created in the weld overlay not only during formation of the weld overlay, but also during travel of the tracklaying vehicle due to repetitive loads in cases where the weld overlay is inhomogeneous or the shape of the overlay surface is uneven. In the latter case, there would also occur exfoliation and chipping of the weld overlay, and, still more, breakage of the parts per se. However, the above conventional methods do not teach any techniques for preventing cracking which occurs during formation of the weld overlay; cracking which occurs during travel due to the inhomogeneous structure of the weld overlay or unevenness in the surface of the weld overlay; and exfoliation, chipping and extension of cracks.
(2) Cracking due to repetitive loads during travel occurs not only in the weld overlay but also on the surface of the bushing body. Such cracking develops similarly to the above-mentioned cracking, sometimes leading to breakage of the bushing. U.S. Pat. No. 4,097,711 teaches a carburizing process capable of improving fatigue strength, according to which a weld overlay is formed after carburization and then quenching/tempering is carried out. In other words, a weld overlay is applied to high-carbonized steel. This process, however, presents the problem that the area near the boundary (the toe of the weld overlay) between the weld overlay and the body of the par is hardened and liable to cracking during weld overlay formation.
(3) Since the hard particles dispersed in the weld overlay have much higher wear resistance than the matrix (parent phase metal) which holds the hard particles, wear progresses with the hard particles coming to project from the weld overlay and the worn surface with the protrudent hard particles scrapes off a mating member to be engaged with the part, with the result that wear is still more expedited. The above conventional method does not discuss the problem attributable to wear patterns and its solution as well as the relationship between wear patterns and the preferable structure of the weld overlay.
The present invention is directed to overcoming the foregoing problems and a first object of the invention is therefore to provide a base carrier for a tracklaying vehicle, the base carrier being designed such that prompt conformability with a mating member is ensured by utilizing wear which occurs at the initial stage of traveling; exfoliation nor chipping is not caused in the weld overlay in engagement of the part with its mating member; cracking which has occurred during overlaying is prevented from extending, leading to breakage of the part; no new cracking occurs nor develops when the part is in operation; and the mating member is not adversely affected by the part during travel.
A second object of the invention is to provide a hard facing method for sprocket teeth which method meets the first object and has the capability of further improving wear resistance by providing a weld overlay with a property of prompt conformability relative to a mating member by utilizing wear at the initial stage of traveling; preventing occurrence of cracking as much as possible during overlaying in order to prevent exfoliation and chipping of the weld overlay during operation; stabilizing the quality of the weld overlay, that is, the amount and distribution of hard particles and the structure and hardness of a parent phase metal; and rendering the wear surface smooth during service so that damage to the mating member and falling and chipping of the part, which are attributable to scraping of the mating member due to the projection of the hard particles, are all prevented.
A third object of the invention is to provide a hard facing method for bushings, which method meets the first object and has the capability of further improving wear resistance by providing a weld overlay with a property of prompt conformability relative to a mating member by utilizing wear at the initial stage of traveling
Armstrong Westerman & Hattori, LLP
Dunn Tom
Edmondson L.
Komatsu Ltd.
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