Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-05-19
2002-06-04
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S512000, C148S612000, C148S639000, C148S565000, C148S566000, C148S567000, C148S902000
Reexamination Certificate
active
06398881
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns the production of highly wear-resistant ledeburitic surface layers of cast-iron machine components. The present invention is useful in all cast-iron components subject to wear as a result of lubricated friction. The invention is particularly advantageous for use in the production of engine components, such as camshafts, cam followers, rocker arms, cylinder liners, or the like.
2. Discussion of Background Information
Ledeburitic surface layers have very good wear resistance to sliding friction under hydrodynamic or mixed friction conditions.
It is known to produce such layers for camshafts by TIG remelting (e.g., Heck: Influence of Process Control in Remelt Chilling on the Surface Layer Properties of Camshafts Made of Ledeburitic Cast Iron, Dissertation, Munich, 1983). For this, a TIG burner is guided relatively slowly at approximately 125 to 225 mm/min at a right angle to the feed direction with a low oscillation frequency of approximately 0.7 to 2.2 Hz in pendulum fashion along the camshaft circumference. The power density used is roughly 3000 W/cm
2
. Thus, heating speeds of approximately 200-750 K/s are achieved. In order to avoid cracks, preheating to temperatures of approximately 400° C. is used.
The cams produced in this fashion have a coarse solidification structure which consists of relatively coarse ledeburitic cementite and pearlite in the metal matrix. Moreover, tempered zones are generated which are characterized by unfavorable damage to the remelted structure because of repeated temperature loading as a result of the slow pendulum action of the TIG burner.
A disadvantageous effect with cams produced in this manner is the fact that wear resistance is too low. The cause of the low wear resistance lies in the coarse grain structure and the additional coarsening of the structure within the tempered zones.
The major shortcoming of the method is that the solidification speed is too slow. The cause for this consists in the power density is too low, which makes it necessary to work with relatively low feed rates.
To counter this shortcoming, it is known to also use modern high-energy surface layer remelting methods such as laser beam remelting (e.g.: M. S. Mordike: “Principles and Application of Laser Surface Refinement of Metals”, Dissertation, Clausthal-Zellerfeld, 1991; Patent DE 42 37 484) or electron beam remelting (e.g., Patent DE 43 09 870) for ledeburitic remelting of camshafts. For this, an appropriately shaped energy beam (e.g., rectangular; two rectangular radiation fields separated in the feed direction; scanning spot grids; grids with different power densities) with a feed rate which is constant or a function of the local radius curvature is guided over the camshaft such that one melting pool extending over the entire width of the cam is created, or a plurality of melting pools extending only slightly in the feed direction. Here, power densities of 10
3
to 10
5
W/cm
2
are used. The feed rates are 500 to 2500 mm/min. To avoid cracks in the melt zones, it seemed indispensable to use intensive preheating to temperatures of approximately 360 to 550° C. This occurs as a rule in expensive through-type furnaces.
The remelted cam regions have a remelted zone 0.3 mm to an average of approximately 0.8 mm deep. The remelted zone includes ledeburitic cementite and pearlite in the metallic matrix. When the austenitizing temperature is exceeded in the zone directly below the remelted zone, a new pearlitic zone of slightly higher hardness than that of the starting state is formed because of the slow cooling. The drop in hardness begins, consequently, immediately at the edge of the remelted zone and is relatively steep.
The shortcoming of cams produced in this manner is that they do not achieve the actual wear resistance possible for such a finely dispersed structural formation of the ledeburitic cementite. The reason for this is that the pearlite in the metallic matrix has lower wear resistance than the cementite and, consequently, represents the weak point of structure.
The shortcoming of the method is that pearlite develops both within the remelted zone and in the underlying new austenitizing zone. The cause for this is that, due to the high preheating temperatures of 360° C. to 550° C., the cooling speed in the temperature range of approximately 600° C. to 450° C. is already so low despite the high solidification speed that the residual austenite breaks down completely to form relatively coarse pearlite.
However, an optimum surface layer structure for wear resistance requires a layer structure consisting of a thin surface layer which is capable of accommodating the adhesive stresses occurring with tribologic loading, plastic deformations, and cyclic elastic-plastic microstrains, and an underlying support layer which accommodates the strains as a result of Herzian stresses. Consequently, an additonal shortcoming of this method is that this support layer can also only be formed by a remelted layer. The greater remelting depth necessary for this results in economic disadvantages due to the low feed rate required.
A cam with a surface layer structure better suited for wear resistance became known with patent EP 0 161 624. The cam surface layer includes a cementite layer with a large proportion of cementite and, under it, a martensitic layer, whereby the remelted layer has a depth of 0.3 to 1.5 mm and the underlying hardening zone has a thickness of 0.3 to 2.0 mm.
In this method, the cams, without preheating are brought to melting by a TIG arc and then solidify by self-quenching. In a subsequent patent (EP 0 194 506), to accelerate the cooling, water or a water air mixture is passed through the central oil bore in the lengthwise axis of the camshaft.
It is possible, without consequences for crack formation, to do without preheating, since the work is performed with a very low power of 1360-2600 W at very low rotational speeds of 0.7 to 1.0 rpm. This corresponds approximately to feed rates of 80 to 130 mm/min. At these slow feed rates, the heat introduced runs in front of the remelting spot and also penetrates very deeply into the cam during the remelting. Thus, the quenching speed is reduced so much that the crack formation stress is no longer reached during cooling. However, because of the low feed rate, the solidification speed is also reduced, which results in a coarser formation of the ledeburitic cementite compared to laser or electron beam remelted cams.
Despite the low cooling speed, cams treated in this manner have improved wear resistance compared with the TIG remelted cams with preheating. The only reason for this can be that the pearlite formed in the metallic matrix is clearly more finely laminated because of the higher cooling speed during its creation. The potential of possible improvement of properties due to a finely dispersed cementite formation can, however, not be realized.
Consequently, the shortcoming of cams produced in this manner is that they have no wear-optimal surface layers. The cause of this is the relatively coarse formation of the solidification structure as a result of the low solidification speed and the formation of tempered zones.
The low power density and slow feed rate result in a solidification speed too low for the formation of a finely dispersed structure. Another disadvantage is that the structure is macroscopically non-homogeneous and periodically has even coarser grain structures. The reason is the repeated local temperature exposure of already greatly cooled regions to far above the austenitizing temperature as a result of the vary low oscillation motion of the TIG burner.
SUMMARY OF THE INVENTION
The present invention provides a camshaft better protected against wear caused by sliding friction as well as a method for production thereof.
The invention further reports formation of a grain structure and a surface layer structure for camshafts and similarly loaded cast-iron components which are better suited to the use conditions of sliding friction loads
Brenner Berndt
Duschek Carsten
Naunapper Dietmar
Wetzig Andreas
Fraunhofer-Gesellschaft zur Förderung der Angewandten Forschung
Greenblum & Bernstein P.L.C.
Yee Deborah
LandOfFree
Wear-resistant camshaft and method of producing the same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Wear-resistant camshaft and method of producing the same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Wear-resistant camshaft and method of producing the same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2960476