Metal treatment – Process of modifying or maintaining internal physical... – Producing or treating layered – bonded – welded – or...
Utility Patent
1992-05-14
2001-01-02
Kastler, Scott (Department: 1308)
Metal treatment
Process of modifying or maintaining internal physical...
Producing or treating layered, bonded, welded, or...
C148S512000, C148S569000
Utility Patent
active
06168676
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods and apparatus for the reconstitution of metal that includes microfractures, and more particularly the metal of a rail of a railway track or a wheel of a rail vehicle.
BACKGROUND AND SUMMARY OF THE INVENTION
The deterioration of railroad rail and wheel surfaces (the rail-wheel problem) is generally caused by two mechanisms of material removal. These occur at the wheel and rail contact points. The two processes are (1) microscopic material loss due to friction generated abrasion; and (2) macroscopic material loss due to flaking, spalling and shelling.
The first mechanism, friction generated abrasion, results in a gradual loss of surface material predominantly on the wheel flanges and inside surfaces of the high rail; particularly on curves. This type of frictional wear can result in very rapid rail and wheel deterioration if lubrication is not present. This aspect has now become particularly troublesome with the ever increasing appearance of heavily loaded unit trains.
This being the case, automatic flange-lubricators are increasingly being installed on railroad lines, particularly before sharp curves. Lubrication of the wheel flange and inside rail surface has resulted in an extension of the useful lifetime of these items by as much as a factor of three. An additional but equally important side benefit that has been realized by rail lubrication is a near 30% reduction in fuel costs for the prime mover. Consequently, rail lubrication is rapidly becoming a universally accepted procedure.
However, the second type of material removal, generally categorized by descriptive terms such as dandruff, flaking, spalling and shelling, can be a much more rapid and debilitating process (once it has started), since unlike the case of microscopic abrasive wear, relatively large particles of material are dislodged.
The term dandruff is characterized by the generation of small particles, typically less than 1 or 2 square mm in area and perhaps a maximum of 0.1 mm thick. In flaking, the particle size ranges up to about 1 or 2 square cm in area and up to 1 mm in thickness. Spalling and shelling usually refer to the generation and loss of surface particulates that are larger still.
As might be expected, this second type of material loss can be very detrimental to rail service, since once it has started it progresses very rapidly. This follows as a consequence of the large nonuniformity created in the rail-wheel contact surface by the loss of a flake. The condition results in impact loading of the members, accompanied by a further acceleration in the phenomenon. The generation of corrugations is often a subsequent symptomatic feature of this flaking deterioration process. These phenomena generally affect both wheel and rail members with equal severity.
In certain instances, microcracks can propagate large distances in the horizontal plane and then suddenly execute a 90° turn into the bulk material. Such a phenomenon can quickly lead to very deep vertical cracks developing into the bulk rail member. This process commonly known as a squat, can if not detected and repaired, subsequently lead to a very serious complete transverse break in the rail track.
As of the present time there has been no effective in situ remedy for these flaking, shelling or squat processes, other than to grind off the top of the rail head and wheel surfaces to a considerable depth. Grinding is an expensive process and, because of the loss of rail material, requires replacement of the entire rail about every seven years. In many cases companion microcracks propagate deep within the bulk material and the items must then be replaced.
Experiment has shown that the reason for the development of this flaking or shelling phenomenon, which has become much more severe now that lubrication has been adopted, is due to the propagation of fatigue-generated microcracks.
Because of a significant overstressing with concomitant yielding and plastic flow of the members, the maximum stress usually does not occur at the surface, but rather at a plane a few mm below the cap.
In particular, since the individual wheel loading is now typically 32,000 pounds and further since the vertical contact area is generally only about the size of a dime (about 1 square cm in area), the stress to which the steel rail and wheel material is subjected is in the range of 150,000 to 200,000 psi. As a consequence the surface material on both rail and wheel is cyclically loaded far beyond its elastic limit, upon passage of each train wheel. The process gradually causes cyclic stress fatigue of the surface and near-surface material, which in turn leads to the formation of a large number of microcracks.
If these cracks remain dry the coefficient of friction within them will stay at about 0.5. Since the coefficient of friction at the rail-wheel interface is typically only about 0.3 for dry rail and still less at 0.18 for lubricated rail, these microcracks are locked in somewhat and do not propagate excessively due to tip stress concentration.
However, when the track inside face is lubricated, grease invariably migrates onto the surface of the rail head also. This grease is in turn gradually forced into these microcracks; thereby also lowering their coefficient of friction to 0.18. Thus if the rail-wheel interface ever becomes nonlubricated or dry, a situation soon develops whereby the coefficient of friction in the microcrack is considerably lower than that at the rail surface.
This condition permits lateral differential movement of the sides of the microcrack with the generation of a concomitant extreme stress concentration at the crack tip. The situation leads to rapid crack propagation in a direction parallel to the rail top surface. Ultimately these cracks propagate large horizontal distances just beneath the surface; and thereby produce large area thin flakes, which eventually become dislodged. The process leaves behind a relatively deep depression in the rail-wheel interface.
The occurrence of such defects accelerates the propagation of other microcracks due to the impact loading effect from the nonuniform surface. Once the flaking process has started, quick rail and wheel grinding is mandatory; otherwise the phenomenon will rapidly deteriorate these members to a nonserviceable condition.
As noted above, grinding of the rail surface below the microcracks is the only known solution to the problem. But this is expensive both due to the cost of servicing and using grinding equipment and the need for eventual replacement of the entire rail.
Melting and solidification of the rail in situ, for example by induction heating, has also been considered as a solution, but this method has been rejected as a practical solution for a number of reasons, the most important of which is the impossibility of selectively heating only the cracked part of the rail. Simply put, induction heating melts the entire rail, leaving it a formless blob.
Solutions to the rail wheel interface problem must also meet constraints imposed by limitations on track access time on busy rail lines. Track access time is a parameter of major importance in the rail industry. Consequently, the application speed or processing rate of any in situ rail refurbishing activity is an important factor in the development of the specifics of that process.
Because of this dominating processing-rate constraint, and the large depth-of-penetration required for effectiveness to be discussed further below, relatively slow and very shallow laser surface cladding process are not believed useful for solving the problem.
As a first consideration, the minimum acceptable processing speed (from a track access time consideration) is about one meter per second. Secondly, because of extreme loading at the rail-wheel interface contact point, stress concentrations far beyond the elastic limit of the rail steel extend deep below the rail surface. Thus, to prevent a pealing-off or de-lamination of the processed region under heavy cyclic loading in subsequent service, it is believed esse
Kastler Scott
Lambert Anthony R.
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