Method and Device for Production of Heat Treated Welded Rail for Rail Transport and Rail Produced Therewith

Abstract

The present invention relates to a method and device for the production of heat treated welded rail for rail transport, such as railways and tramways. The invention also relates to a rail produced with the method and/or the device.

Claims

1. A method for the production of heat treated welded rail for rail transport comprising the subsequent steps of: i. providing rail lengths to desired specifications ii. welding two rail lengths together in a welding unit to produce a continuous welded rail, CWR, or producing a longer continuous welded rail by welding one or more additional rail lengths to the continuous welded rail; and iii. post-welding heat treating the entire continuous welded rail in a heat treatment unit by heating the entire continuous welded rail, or by heating all successive cross-sections of the continuous welded rail, to above the Ac.sub.3-temperature to achieve a fully austenitic microstructure in the entire continuous welded rail, followed by holding the continuous welded rail or all successive cross-sections of the continuous welded rail section above Ac.sub.3 for a prescribed time t.sub.a followed by subjecting the continuous welded rail or all successive cross-sections of the continuous welded rail to cooling at a cooling rate using a cooling medium to a prescribed cooling stop temperature T.sub.stop to achieve the desired transformed final microstructure and properties along the entire length of the post-welding heat treated continuous welded rail.

2. The method according to claim 1 wherein the continuous welded rail is heat treated at a constant feed rate of at least 0.5 and/or at most 10 m.Math.min.sup.1.

3. (canceled)

4. The method according to claim 1, wherein the continuous welded rail is produced by welding together lengths of steel having a composition suitable for obtaining a microstructure which is substantially eutectoid or hypereutectoid after heat treatment, wherein the cooling stop temperature is below Ar.sub.1, and wherein the transformed final microstructure treated of the post-welding heat treated continuous welded rail consists substantially of pearlite or pearlite and cementite and substantially free or completely free of martensite and/or bainite phases.

5. The method according to claim 1, wherein the continuous welded rail is made by welding together lengths of steel having a composition suitable for producing a bainitic microstructure after heat treatment, wherein the cooling stop temperature is such that the transformed final microstructure of the post-welding heat treated continuous welded rail is fully bainitic and substantially or entirely free of martensite and substantially or entirely free of pearlite or ferrite phases.

6. The method according to claim 1, wherein the welding process is a flash-butt welding process.

7. A device for performing the method of claim 1, provided with: i. a welding unit for welding two rail lengths together in a welding unit to produce a continuous welded rail, CWR, or to produce a longer continuous welded rail by welding additional rail lengths to the continuous welded rail; ii. a heat treating unit a. for post-welding heat treating the continuous welded rail by subjecting the entire continuous welded rail to substantially the same heat treatment above the Ac.sub.3-temperature to achieve a fully austenitic microstructure in the continuous welded rail in a batch heat treatment unit, or b. for post-welding heat treating the entire continuous welded rail by feeding the entire continuous welded rail through the heat treatment unit for heating all successive cross-sections of the continuous welded rail to above the Ac.sub.3-temperature to achieve a fully austenitic microstructure in all the successive cross-sections, iii. a holding unit for holding the continuous welded rail or all the successive cross-sections above Ac.sub.3 for a time t.sub.a, said holding unit being optionally integrated in the heat treating unit, and iv. a cooling unit for cooling parts of the continuous welded rail (head, base of the foot, web) or the entire continuous welded rail or all the successive cross-sections of the CWR using a cooling medium to a cooling stop temperature T.sub.stop to achieve the desired homogeneous transformed final microstructure and properties in the entire post-welding heat treated continuous welded rail.

8-10. (canceled)

11. The method of claim 1 further comprising removing weld upset or upsets, or parts thereof, prior to post-welding heat treatment.

12. The method of claim 11, wherein removing the weld upset or upsets, or parts thereof, is by stripping, grinding, milling or any combination thereof.

13. The method according to claim 11, wherein the weld upset is removed from the foot of the rail.

14. The method according to claim 12, wherein the weld upset is removed from the foot of the rail.

15. The method of claim 14 wherein the weld upset is further removed from the web and head of the rail.

16. The method of claim 1 further comprising providing the head of the post-welding heat treated continuous welded rail at the locations of a weld with a desired rail head profile.

17. The method of claim 16 wherein providing the desired rail head profile is by grinding or milling.

18. The method according to claim 11, wherein the weld upset is removed from the foot, wherein the weld upset is removed from the foot, web and head of the rail.

19. The method according to claim 12, wherein the weld upset is removed from the foot, wherein the weld upset is removed from the foot, web and head of the rail.

20. The device of claim 7, further comprising a unit for removing part of, or the whole weld upset or weld upset.

21. The device of claim 20, further comprising a head profiling means for providing the head of the continuous welded rail at the locations of the weld or welds with a desired rail head profile.

22. The device of claim 7, further comprising a straightening means to straighten the continuous welded rail and/or optional straightening means to straighten the heat treated continuous welded rail.

23. The device of claim 7, further comprising a straightening means to straighten the continuous welded rail and/or optional straightening means to straighten the heat treated continuous welded rail.

24. A post-welding heat treated rail produced according to the method of claim 1.

25. The rail according to claim 24, wherein no heat affected zones are present at any position along the entire length of the post-welding heat treated rail.

26. The rail according to claim 24, wherein: the difference between the minimum hardness in HV30 of the post-welding heat treated CWR and the average hardness in HV30 of the post-welding heat treated CWR is lower than 10% of the average hardness value in HV30, or wherein the difference between the maximum hardness in HV30 of the post-welding heat treated CWR and the average hardness in HV30 of the post-welding heat treated CWR is lower than 15% of the average hardness value in HV30, or wherein the difference between the minimum hardness of the post-welding heat treated CWR and the average hardness in HV30 of the post-welding heat treated CWR is lower than 10% of the average hardness value in HV30 and the difference between the maximum hardness in HV30 of the post-welding heat treated CWR and the average hardness in HV30 of the post-welding heat treated CWR is lower than 15% of the average hardness value in HV30; and wherein the hardnesses in HV30 are determined in accordance with ISO 6507-1:2005.

27. A post-welding heat treated rail produced using the device of claim 7.

28. The rail of claim 27, wherein no heat affected zones are present at any position along the entire length of the post-welding heat treated rail.

29. The rail according to claim 27 wherein: the difference between the minimum hardness in HV30 of the post-welding heat treated CWR and the average hardness in HV30 of the post-welding heat treated CWR is lower than 10% of the average hardness value in HV30, or wherein the difference between the maximum hardness in HV30 of the post-welding heat treated CWR and the average hardness in HV30 of the post-welding heat treated CWR is lower than 15% of the average hardness value in HV30, or wherein the difference between the minimum hardness of the post-welding heat treated CWR and the average hardness in HV30 of the post-welding heat treated CWR is lower than 10% of the average hardness value in HV30 and the difference between the maximum hardness in HV30 of the post-welding heat treated CWR and the average hardness in HV30 of the post-welding heat treated CWR is lower than 15% of the average hardness value in HV30; and wherein the hardnesses in HV30 are determined in accordance with ISO 6507-1:2005.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1a-d shows the microstructure of the heat treated welded rail of the example above at the fusion line (a) and 4 (b), 8 (c) and 20 (d) mm from the fusion line.

[0056] FIG. 2 is the hardness profile of the microstructure of FIG. 1. It is clearly visible that the hardness profile is very flat, with only a lower value at the fusion line and a couple of higher values next to the fusion line.

[0057] FIG. 3 is the hardness profile of a post-welding heat treated-rail (solid line) compared to a pre-welding heat treated rail (dashed line) as a function of the distance to the weld (0 being the fusion line). Comparison of these profiles shows a very big difference in the hardness profile near the weld leading to the conclusion that the post-welding heat treated rail has a much more even hardness distribution along the length of the rail.

[0058] FIG. 4 is a schematic and non-limiting drawing of a device according to the invention wherein R represents an amount of rail lengths which are welded together in a welding unit (WU) to form a CWR and subsequently heat treated in a heating unit (HU) and cooled in a cooling unit (CU). The WU is drawn to be in-line with the HU in this figure, but most likely the WU and the HU are decoupled processes for practical reasons. A schematic temperature profile is given as well. The location of the welds is indicated schematically with a W.

[0059] FIG. 5 is a plot of residual stress values along the length of the welded rail from the weld to 250 mm from the weld, showing the longitudinal residual stress on the surface of the foot at the centre-line of the rail and the vertical residual stress on the surface of the web of the rail at the rail mid-web position. It is evident from this Figure that the residual stress in the rail produced according to the invention is much lower and more even than the one in the process where the weld is not heat treated, which is the state-of-the-art process as described in [0007]. In the Figure the solid lines represent the post-welding heat treated rail and the dashed lines represents the pre-welding heat treated rail. The triangles show the vertical residual stress (in MPa) of the web, and the diamonds show the longitudinal residual stress (in MPa) of the foot of the rail, all as a function of the distance (in mm) from the weld.

[0060] FIG. 6 shows the difference between the method according to the prior art (a) and the method according to the invention (b). In FIG. 6a the heat treated (HT) rails are welded together, forming a heat affected zone at the location of each weld. The method according to the prior art then heat treats the weld (indicated with the dashed ellipsoid) in order to restore the properties and microstructure back to the level of the original heat treated rails. However, this local heat treatment is likely to result in local variations in internal stresses and creates new zones next to the area of local heat treatment where the local heat treatment influences the microstructure of the pre-weld heat treated (HT) rail. In the method according to the invention the non-heat treated (NHT) rails are welded together (see also FIG. 7), also forming a heat affected zone at the location of each weld. But after this the entire rail is heat treated, either in one go (batch mode), or each cross-section after cross-section (continuous mode). Since the composition of the weld is substantially identical to the composition of the rail, this post-weld heat treatment leads to a homogeneous microstructure along the rail, with the possible exception of the fusion line. When studying the microstructure only the fusion line is discernable (see also FIG. 1). This is indicated in FIG. 6b with the dashed line. Beside the fusion line there is no discernable difference between the post-weld heat treated HAZ and the heat treated rail in the method according to the invention.

[0061] FIG. 7 shows the method according to the invention after the welding (a) and after the post-weld heat treatment (b) with the invisible welds. FIGS. 8 and 9 show compositions of steels that can be processed with the method according to the invention.

[0062] FIG. 10 shows a macro-photograph of a Heat Affected Zone (left) after welding two rails together and after annealing according to the invention (right). The upper part of the images shows the effect in the thick parts of the rail (head) and the lower parts show the effect in the thin parts of the rail (foot). The right hand image shows the microstructure after annealing according to the invention, where only the fusion line is still visible, but the rest of the HAZ has gone completely.