Process and device for hardening a rail

Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal

Reexamination Certificate

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Details

C148S645000, C148S581000, C148S585000, C266S114000

Reexamination Certificate

active

06432230

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. §119 of Austrian Patent Application No. 939/2000, filed on May 29, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for hardening a rail or part thereof (e.g., the rail head) by forced cooling and a device for carrying out a corresponding process.
2. Discussion of Background Information
For the growing railway traffic with its increasing axle loads rails should, on the one hand, have high wear resistance in the area that comes into contact with the train wheels and, on the other hand, have high resistance to fracture in view of the high bending load acting on the rail.
Thermally tempering the (steel) material or changing the structure from an austenitic structure into a microstructure that is stable at room temperature during at least temporary force-cooling is known to harden the rail and/or the rail head (EP-358362 A1, AT 402941 B, EP186373 B, WO 94/02652).
Cooling can be effected by subjecting at least a part of the rail surface to coolant, a so-called splash cooling or spray cooling, or by at least partially submerging the rail into a coolant bath, wherein the advantageous use of the rolling heat is known in the art.
Depending on the cooling method used, pass-through processing devices (AT 323224 B, EP 186373 B1), cooling bed transport devices (DE 4237991 A1) and submerging devices (DE 4003363 C1, AT 402941 B) are known for forced cooling of the rail or parts thereof.
When alloys with an appropriate chemical composition are used, rails with increased hardness and wear resistance in the area of the surface of the rail head and with sufficient resistance to fracture can be produced by using hardening processes in the appropriate devices.
A possible lack of homogeneity of the structural distribution over the cross-section as a function of the length of the rail must be considered a great disadvantage of the known hardening processes and cooling devices for rails. In other words, if the portion of the surface area of the relevant tempering structure and/or the position of the structural constituents in the rail cross-section is uneven over the length of the rail this has an increasingly detrimental effect on the quality of the rail. Even if process parameters are maintained precisely and cooling devices are controlled precisely, unexpected differences in the quality of the rail can occur, resulting in some individual rails that do not meet the quality requirements of an extremely sophisticated quality control.
SUMMARY OF THE INVENTION
The present invention is directed to overcoming the above problems by providing a process of the above mentioned type which results in a constant structural distribution over length across the rail cross-section and a high rail quality.
The present invention furthermore is directed to providing a device for hardening a rail or parts thereof through which the local cooling intensity of the surface regions of the cross-section can be kept constant over the length of the rail.
One aspect of the present invention is a process for hardening a rail or part thereof by transforming it from an austenitic structure into a different microstructure that is stable at room temperature, the process comprising aligning, horizontally positioning, and fixing in axial alignment to secure against bending, the rail in its austenitic state; and while keeping the rail fixed in axial alignment and secured against bending, force-cooling the rail or part thereof to allow the austenitic structure to be transformed into said different microstructure.
In one embodiment of the above process the rail is force-cooled from a first temperature above its Ac
3
-point to a second temperature which is below said Ac
3
-point. In another embodiment of the process only the rail head is force-cooled.
It may also be advantageous if alignment, positioning and fixing in axial alignment are conducted immediately after the last finish rolling pass.
In the above process the rail may, for example, be fixed in a standing position, with the rail head pointing (straight) upward, or it may be fixed in a hanging position, with the rail head pointing (straight) downward.
Force-cooling of the rail or part thereof may be accomplished by spray cooling, for example by spray cooling while using equally high cooling intensities in the surface regions of the cross-section symmetrically to the height axis of the rail viewed in longitudinal direction.
In another embodiment of the process of the invention the rail or part thereof may be force-cooled by immersion thereof into a cooling liquid.
In a further embodiment the rail or part thereof may force-cooled intermittently with respect to at least one of time and location with regard to a surface region of the cross-section.
After the force-cooling the rail may be released and kept at an elevated temperature, and/or it may be left to cool in ambient air.
In many cases the length of the rail will be at least about 50 m, e.g., at least about 90 m, and frequently the rail or part thereof will be force-cooled over approximately its entire length.
In another aspect the rail is fixed in axial alignment by means of fixing elements arranged in longitudinal direction of the rail at a distance to each other of not more than about 1 m, e.g., not more than about 0.5 m. These fixing elements may be designed to keep the rail foot in a fixed position.
The Ac
3
-point referred to above is the temperature of iron or an alloy thereof at which upon heating a purely austenitic (&ggr;) microstructure is present. For more details as regards the definition of the Ac
3
-point (and microstructures that are stable at room temperature) reference may, for example, be made to F. Rapatz, “Die Edelstähle”, Springer-Verlag Berlin, Germany, 1962, pp. 2-25 (see particularly pages 3 and 12), the disclosure of which is expressly incorporated by reference herein in its entirety.
The advantages achieved through the process of the invention include that alignment occurs in the austenitic structural state and heat is then removed from the rail surface at an increased rate, while it is fixed (clamped) in axial alignment. During the intensive cooling of the rail cross-section or parts thereof the rail remains fixed in axially straight position which helps to keep the specific cooling intensity constant in the axial direction. Extensive research shows that, if even a slight bending of the rail occurs during intensive cooling of the rail or at least parts thereof, the local cooling rate curve in the surface region can change. This has a major effect on the formation of the structure during the change from the austenitic state of the alloy. In a thermal tempering of the rail an axially aligned (flush) horizontal mounting according to the invention secures a constant profile of material properties over the cross-section and over the length of the rail.
A particularly economical embodiment of the process of the present invention is obtainable when aligning (straightening), positioning and fixing in axial alignment of the rail are performed immediately after the last finish rolling pass with utilization of the rolling heat.
For a specific site technology, but also for a desired microstructural distribution over the cross-section of the rail, it may also be advantageous to fix (clamp) the rail in a standing position, with the rail head pointing straight upward. Here, it is advantageous to remove the heat from the rail by spray cooling using equally high cooling intensities in the surface regions of the cross-section symmetrically to the vertical axis of the rail viewed in the longitudinal direction.
In order to improve manufacturing reliability of rails with the desired property profile and to achieve special wear resistance of the surface in contact with the train wheels, it may be advantageous if the rails are mounted hanging with the rail head pointing vertically downward. Another

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