GB2403174A

GB2403174A - Flash butt welding of steel rails

Info

Publication number: GB2403174A
Application number: GB0314839A
Authority: GB
Inventors: Vijay Jerath
Original assignee: Corus UK Ltd
Current assignee: Corus UK Ltd
Priority date: 2003-06-26
Filing date: 2003-06-26
Publication date: 2004-12-29
Anticipated expiration: 2023-06-26
Also published as: GB2403174B; EP1641978A1; GB0314839D0; WO2005001204A1

Abstract

A steel rail comprises a plurality of individual rails joined by flash butt welding. The width of the heat affected zone (HAZ) between neighbouring welded rails at the load bearing surface of the rail is less than 30mm, preferably less than 25mm. The flash butt welding process comprises the steps of preheating the rail ends to be joined and subjecting the preheated rail ends to a forging load in excess of 500kN.

Description

24031 74

STEEL RAILS

This invention relates to steel rails.

From the early days of railways, engineers have been striving to achieve optimum track standards with a view to eliminating rail joint maintenance, increasing track life, reducing wear and tear on vehicles, providing facilities for higher operating speeds and heavier axle loads, reducing noise level and improving the overall riding quality.

Over the years considerable progress has been made in reaching these objectives and this is due, at least in part, to the development of flash butt welding techniques.

At the present time, the majority of joints between adjoining rails are made using flash butt welding machines.

In the flash butt welding process, the rail ends to be joined together are held between water cooled copper dies, which act as both clamps and electrodes. One die is fixed and the other, referred to as the platen, is movable towards and away from the fixed die. The first stage in the flash butt welding process is generally termed the "burn-off" or the preflashing stage. During this stage, the rail ends are separated slightly and flashing (i.e. arcing) is initiated to ensure that the rail ends are generally parallel.

The optimum value of the burn-off is the minimum value required to establish a stable temperature distribution of the rail ends.

It has been demonstrated that the amount of burn-off required to produce a stable temperature distribution is dependent upon the thermal diffusivity of the rail material and the flashing pattern, but is essentially À: ee. À: e: ::.:. .:. ::: independent of the material dimensions. "Burn-off" times are typically between 5 and 10 seconds, with 1 to 5mm of platen movement.

The next step in the welding process is the preheating stage. The main aim of the preheating stage is to heat the rail ends to a temperature at which flashing initiates easily, and a suitable temperature gradient is achieved in the rail ends prior to the onset of the final flashing stage.

Preheating is generally achieved by bringing the rail ends into contact at a low pressure, and allowing a high current (typically 60 to 100kA) at 5 to 10V to flow across the rail ends. The rail ends are brought together typically between 7 and 10 times and the rail ends are heated by resistance heating, namely I2R, where I is the magnitude of the preheat current and R the resistivity of the material being welded. In between each preheating cycle of 1 to 10 seconds duration, a short period of flashing between 0.5 and 1.5 seconds is generally allowed to maintain rail end squareness.

Three useful functions are served by the preheating operation of the welding cycle; these being: (a) the temperatures of the rail ends to be joined are raised making it easier to start and sustain subsequent flashing with a lower secondary voltage; (b) a modification of the temperature distribution is produced which persists throughout the flashing operation, and results in a flatter temperature gradient. This, in turn, changes the character of the subsequent upsetting action producing generally a more distributed upset than is the case when no preheat is employed; and (c) the capacity of the welding machine is extended making it possible to weld larger cross sections of material than would otherwise be possible.

ee* e Àe À e e À À eve À c e e À e À e e À * Preheating is followed by a final "flashing" stage. The term "flashing" derives its name from the rapid expulsion of incandescent particles of molten metal from the minute points of contact between the two rail ends. During this stage the rail ends become molten, and the correct conditions for the final upset or forging stroke are achieved. The movable platen of the welding machine during this stage is generally accelerated parabolically, with a resultant increase in the frequency and number of flashing ruptures or arcs across the weld interface. This ensures that the oxygen content at the weld interface is reduced sufficiently to give a semi-protective atmosphere.

The primary purpose of the final flashing stage is to generate enough heat to produce a plastic zone that permits adequate upsetting, i.e. relative movement between the fixed die and the platen. A total flashing distance of between 10 and 15mm, over a period of 5 to 10 seconds is typically employed. It is generally accepted that the lowest practical flashing voltage should be employed in all flashing operations. It has been demonstrated conclusively that the depth of the surface irregularities orncraters" produced in the rail ends is reduced considerably by reducing the flashing voltage.

Immediately following completion of the final flashing stage, the movable platen of the welding machine is accelerated so that the rail ends are butted together to refusal either under a constant platen speed and/or under impact loading of up to 1000kN. By "refusal" is meant that no further relative movement is possible. The forging load is generally calculated to give an upset pressure at the weld interface of approximately 50N/mm2, to ensure adequate weld consolidation. The welding current is usually supplied during the initial part of the forging operation/stroke to avoid oxidation at the weld interface. In addition, the use of upset current permits general heating of the material between the dies, which reduces the compressive yield strength of the material and permits increased upset distances to be achieved with less force. During À :. . À: e: À À welding of rails, a typical forging distance of between 12 and 20mm is employed.

The magnitude of the upset distance (the distance travailed by the movable platen has to be sufficient to accomplish two main functions: (a) the oxides and molten material has to be extruded all the way from the centre of the thickest section of the weld to its surface; (b) the two rail end frames must be brought into intimate metal-to- metal contact over the entire cross section of the rail ends.

In general, the smoother the flashing interface at the conclusion of the final flashing operation, the easier it is to accomplish both of the above functions. Deep craters, such as those which result from the use of a high flashing voltage, cause entrapment of oxides and molten material and may, therefore, prevent the desired metal-to-metal contact at all points on the interfaces to be achieved.

Following completion of the forging stroke, the molten and soft steel is forced out of the weld joint. This extruded material, generally termed "flash" is removed quickly, typically by means of an automatic weld upset removal tool.

The stripped weld then proceeds to cool down, passing through the austenite to pearlite transformation temperature range (700-500 C) at around lC /s, following natural air cooling.

Natural air cooled welds with critical heat affected zone (HAZ) hardness levels similar to those of CEN 260 and 220 grade parent rails can readily be achieved by this process.

When a heat treatable grade rail e.g. CEN 350 HT is heat treated such that the cooling rate of the rail head through the austenite to pearlite À c ; ce. Àe À transformation temperature range (700-500 C) is of the order of 3-4C /s, then the hardness of the running surface of the rail is observed to be 370 HB. When a flash butt weld is made between two such heat treated rails, the hardness of the critical heat affected zone (HAZ) of the weld will only be 300 HB, unless special equipment is used to provide controlled accelerated cooling of the weld zone, almost immediately after the completion of the weld stripping operation.

Flash butt welds exhibiting softened critical HAZ regions are generally perceived to wear away preferentially during service in track, causing localised dips (weld batter) on the running surface of the rail.

This in turn is believed to lead to the generation of higher dynamic loads, increased propensity for the formation of corrugations, increased wear/tear of wheel sets and bogies, and finally increased noise and passenger discomfort level).

It is known from "Modern Railway Track" by Coenread Esweld - Publishers MRT Productions, 1989, Duisburg, Germany that for railway wheel diameters of between 600 and 1200mm a simplified two- dimensional calculation suffices. The instantaneous contact area between a wheel and the running surface of the rail equates to a rectangular contact patch. In the case of a 1156mm diameter wheel (Class 90 railway wheel used on high speed Network Rail track) this equates to around 15mm in the direction of the rail and 12mm at right angles to the direction of the rail, i.e. a contact patch approximately 15 x 12mm in size.

At the present time, the width of the HAZ region of a flash butt welded rail on the load bearing surface of the rail is typically between 35 and 40mm; this is the dimension of the HAZ region taken in the direction of the rail. The width of the HAZ region is essentially determined by the amount of heat introduced into the rail ends during the preheating stage to achieve a given forging stroke for a given forging load. Presently, a forging stroke of some 12 to 18mm and a forging load of around 450kN . À À À . À c ace . . ace À. e À are perceived to be necessary to produce a satisfactory weld. It is a consequence of such a forging stroke that HAZ region widths of the order of 35 to 40mm are produced at the load bearing surface of flash butt welded rails.

One object of the present invention is to provide a welding process for producing flash butt welded joints between adjoining steel rails in which the disadvantages discussed above are either eliminated or significantly reduced. Another object is to provide flash butt welded rails having significant performance advantages over currently available welded rails.

In one aspect, the invention provides a steel rail comprising a plurality of individual rails joined by flash butt welding, wherein the width of the heat affected zone (HAZ) between neighbouring welded rails at the load bearing surface of the rail is less than 30mm; preferably less than 25mm.

In another aspect, the invention provides a flash butt welded steel rail in which the width of the heat affected zone (HAZ) of each or a majority of welds is less than 30mm; preferably less than 25mm.

The variation in HAZ width over the height of the or each weld preferably lies in the range 10 to 30mm. Preferred ranges are from 17 to 23mm and 15 to 20mm.

In a further aspect, the invention provides a flash butt welding process for steel rails, the process comprising the steps of preheating the rail ends to be joined and applying to the preheated rail ends a forging load in excess of 500kN. The forging load may exceed 650kN. A preferred forging load is between 500 and 800kN.

::: ::e '.. ;: a. À. ::.

cat see, Preheating of the rail ends may be achieved by cyclically bringing the rail ends into contact and causing a current to flow across the rail ends, the number of preheating cycles being sufficient to raise the rail ends to a suitable temperature for final flashing to commence and be sustained and the number of preheating cycles being five or less.

Preferably, only three or four preheating cycles are employed.

Preferably, the flash butt welds are cooled naturally in still air for CEN rail Grades 220 and 260, but controlled enhanced cooling through the austenite to pearlite transformation temperature range (700-500 degree C), for example, using compressed air may or may not be applied for the production of narrow HAZ welds in heat treated CEN 350 HT grade rail (specified rail hardness in the range 350-390 HB). The use of compressed air or other cooling media for post weld cooling of welds in 350 HT or other heat treated rail grades will largely be dictated by the rail hardness requirements specified by a given customer.

Welding times for welds produced in accordance with the invention are typically around 60 seconds. For CEN 56E1 section rail, this compares with a total welding time of around 120 seconds for the production of standard flash butt welds produced presently.

The invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings which display information and data produced by the Applicant during trials conducted to compare the characteristics and performance of narrow flash butt welded rails in accordance with the invention with standard flash butt welded rails of the same compositions but not in accordance with the invention, in which: Figure 1 shows a Schlatter weld monitor chart of welding machine parameters recorded continuously during the production of a standard rail weld not in accordance with the invention; À e : ^: À .- :: ÀP À : . : : À: Figure 2 shows a Schlatter weld monitor chart of welding machine parameters recorded continuously during the production of a narrow rail weld in accordance with the invention; Figure 3 schematically illustrates the sawing procedure employed to obtain longitudinal-vertical sections through the centre of all welded rails produced in the trials; Figure 4 graphically illustrates the relationship of applied load and deflection of welded rails produced when subjecting such rails to 3-point bend tests; Figure 5a is a macrograph of a longitudinal-vertical section through the centre and rail foot tips of a standard Grade 220, 56E1 flash butt weld of a rail not in accordance with the invention; Figure 5b is a macrograph of a longitudinal-vertical section through the centre and rail foot tips of a standard Grade 220, 60E1 flash butt weld of a rail not in accordance with the invention; Figure 6a is a macrograph of a longitudinal-vertical section through the centre and rail foot tips of a Grade 220, 56E1 flash butt weld of a rail in accordance with the invention; Figure 6b is a macrograph of a longitudinal-vertical section through the centre and rail foot tips of a Grade 220, 60E1 flash butt weld of a rail in accordance with the invention; Figure 7a graphically illustrates the hardness profile of a standard Grade 220, 56E1 flash butt weld of a rail not in accordance with the invention; À À t' -. ';@ e:: '.

À À . . Figure 7b graphically illustrates the hardness profile of a standard Grade 220, 60E1 flash butt weld of a rail not in accordance with the invention; Figure 8a graphically illustrates the hardness profile of a Grade 220, 56E1 flash butt weld of a rail in accordance with the invention; Figure 8b graphically illustrates the hardness profile of a Grade 220, 60E1 flash butt weld of a rail in accordance with the invention; Figure 9a is an optical micrograph of the critical HAZ region of a standard Grade 220, 56E1 flash butt weld of a rail not in accordance with the invention; Figure 9b is an optical micrograph of the critical HAZ region of a standard Grade 220, 60E1 flash butt weld of a rail not in accordance with the invention; Figure 10a is an optical micrograph of the critical HAZ region of a Grade 220, 56E1 flash butt weld of a rail in accordance with the invention; Figure 10b is an optical micrograph of the critical HAZ region of a Grade 220, 60E1 flash butt weld of a rail in accordance with the invention.

The trials conducted provide an accurate and detailed comparison between flash butt welded rails in accordance with the invention and standard flash butt welded rails not in accordance with the invention. For ease of explanation, the word "standard" is used herein to indicate conventional welded rails not in accordance with the invention and the word "narrow" is used to indicate narrow HAZ welded rails in accordance with the invention. Two sets of trials were conducted, firstly to compare standard and narrow flash butt welds for Grade 220, 56E1 welded rails À . . À À À e À À À C À C À C ce C À À and secondly standard and narrow flash butt welds for Grade 220, 60E1 welded rails.

The flash butt welding machine employed for the production of all of the welded rails for the trials was a Schlatter GAAS 80, DC welder. For every flash butt weld produced on the machine both a paper and an electronic SWEP (Schlatter weld monitor programme) chart was generated. Examples of such charts are shown in Figures 1 and 2. The welding machine parameters recorded continuously during the production of each weld included primary current, number and duration of preheats, forging load, material displacement and welding time. It should be noted that the scale of the chart of Figure 1 is one half of that of the chart of Figure 2.

From Figure 1, it can be seen that the data recorded in the SWEP chart of a standard weld involved ten preheats each having a duration of between 4 seconds and 4.6 seconds at a current of between 56.3kA and 68.3KA, a forging load of around 417kN followed by natural air cooling.

The forging stroke was 16.6mm and the welding time was around 120 seconds.

The data of Figure 1 is to be compared with that recorded by the SWEP chart of Figure 2 for a narrow flash butt weld in accordance with the invention. This trial was aimed at reducing the total HAZ width through the application of a higher forging load and reduced number of preheats.

During this trial the forging load was set to 800kN in order to forge the weld to "refusal" (that is until no further relative movement between the adjoining rails was possible). The actual forging load was around 678kN suggesting that the material at the end of the forging stroke was too cold to be forged any further. Production of this weld involved only three preheats each having a duration of between 2.3 and 2.9 seconds at a current of between 61.6kA and 63.9kA, a forging load of around 678kN, A: À .:: :::: ::: '. .:-À :.:: followed by natural air cooling. A forging stroke of 9. 81mm was achieved and the overall welding time was reduced to around 60 seconds.

A comparison of the data specified in the SWEP charts of Figures 1 and 2 show, for welded rails in accordance with the invention, a reduced number of preheats (3 compared with 10) and a reduction in overall welding time leading to lower power consumption and increased productivity; increased forging load and a reduced forging stroke.

The reduced number of preheats, had a significant effect in lowering the total HAZ width of the weld because of the lower heat input.

The HAZ width achieved by the trials for narrow welded rails was from 16 to 23mm compared with a HAZ width of 33 to 40mm for standard welded rails. Comparisons between the HAZ widths produced for both standard and narrow welds are set out below in Tables 1 and 2.

TABLE 1

HAZ width data for Grade 220, 56E1 welded rails Rail 25mm Centre 25mm Base Foot Foot running below We I d QA surface ru nnIng web above rai I sa m pie Sa m p I e ( m m) ( mrma) e ( m m) (ramim ) ot f(omom) ( m m) ( m m) ( memo) n Standard 33-40 wei d 37 33 40 3 8 3 7 3 9 3 5 (va ri ation Narrow 16-22 HAZ 22 16 22 16 22 23 22 (variation weld 6mm) s' . ;e Àe e: À:e e

TABLE 2

HAZ width data for Grade 220, 60E1 welded rails Total Ra i I 25 m m Reentry 25m mBase FootFoot HAZ Weld QA running below of rail above of rail sample Sample width for runn ng base of A c o d e s u rf a c e s u r f a c e w e b r a i i f o o t f o o t m m (Bm m) f (sumcm o) Standard 35- 41 weld 37 35 41 41 40 36 43 (variation 6mm) Narrow 22-27 HAZ 27 22 26 25 27 27 29 (variation weld 6mm) The narrow HAZ welds caused the forging load to increase to v650kN; in comparison the forging load for the standard welds was 450kN.

Several of the welds produced during the course of the trials were sampled for full HAZ shape, hardness and me/allographic assessment.

The sawing procedure employed to obtain the required longitudinalvertical sections through the centre of each of several approx 200mm welded rail lengths is shown in Figure 3. In the section illustrated, the flash butt weld is indicated by a series of parallel lines. Similar sections were also taken from both sides of the rail foot, 10mm inboard of the foot tips. This procedure accords with the requirements of the draft European Specification for flash butt welding of rails prEN 14587-1, October 2002.

The QA identities and welding conditions are given in Tables 3 and 4 below.

TABLE 3

This table compares HAZ hardness and cooling data for standard Grade 220, 56E1 welded rails The letter"P" stands for"Average hardness of the parent material".

e;. Àe e:: a. :' Preheat _. . Mean Cooling Max mum M'n mum Weld QA Weld No. T(Om) e HAZ Hardness Mater al 5(o80cot/ooc ) Standard Natural 10 4.5 271 (P+19) 225 (P-27) 252 1-1.2 Weld air cooled Narrow Natural 3 2.3-3 299 (P+40) 231 (P-28) 261 1-1.2 HAZ air Weld cooled The narrow HAZ weld led to an increase in the forging load to 650kN; in comparison the forging load of the standard weld was 450kN.

TABLE 4

This table compares HAZ hardness and cooling data for standard and narrow Grade 220,60E! welded rails Maximum Minimum Mean t Weld QA Weld No. of HAZ HAZ Paren Code Condition reheats Hardness Hardness P HVt30 HV 30 HMvr3dOneass Standard Natural air 10 291 (P+21) 249 (P-21) 270 Narrow Natural air 307 (p+36) 261 (p-10) 271 The Permitted Hardness Range (HV 30) for welded rails in the draft European Specification is: P-30 to P+60, where P = Average hardness of the parent rail material.

Tables 3 and 4 show that the narrow welds exhibit enhanced levels of hardness when compared with the standard welds.

Having achieved optimum welding machine parameters, a number of welded rails were subjected to a 3-point bend test. Examples of load A: :: :: :: À . . À À À À deflection data for some of the welds are given below. The data presented in Table 5 below show that all of the test welds easily satisfied the stipulated Network Rail minimum bend test load requirement of 1230 (kN).

Table 5

3-Point bend test load/deflection data for Grade 220 56E1 welded rails Forging Cooling Weld Id Preheats load condition Load (kN) Deflection (mm) Standard 10 450kN Natural air 1363 68 Standard 10 450kN Natural air 1360 66 Narrow 670kN Natural air 1370 55 Narrow 670kN Natural air 1385 61 Natural air Narrow 3 670kN cooled 1350 55 Narrow 670kN Natural air 1453 71 The amount of deflection achieved during these 3-point bend tests for standard and narrow welds of the present invention are graphically illustrated in Figure 4. Graph line A applies to tests conducted on a standard welded rail and graph line B applies to tests conducted on a narrow welded rail in accordance with the invention.

Bending fatigue tests conducted on welded rails in accordance with the invention produced during the trials, over a minimum of 5 million cycles at stress cycle ranges of around 240 and 250 MPa were successfully completed with no failures. These tests showed that the fatigue characteristics of the tested welded rails were at least as good, if not better than those of standard welded rails. Further tests at higher stress À . e À cycles are currently under way with a view to establishing the bending fatigue strength/threshold of narrow HAZ welded rails.

Figures 5a and 5b respectively show macrographs obtained from longitudinal-vertical sections of standard Grade 220, 56E1 and Grade 220, 60E1 flash butt welded rails following polishing (1pm surface finish) and etching in 4% Nital, and Figures 6a and 6b respectively show macrographs obtained from identical sections of narrow HAZ Grade 220, 56E1 and Grade 220, 60E1 welded rails in accordance with the invention.

These Figures clearly show that the HAZ regions of the narrow welded rails are not only significantly reduced in width but also that their edges are parallel to a greater extent over their respective heights.

Typically, a reduction of almost 40% in the HAZ width at the rail running surface is achieved for welded rails in accordance with the invention when compared with a standard welded rail.

The longitudinal-vertical sections obtained through the centre of the welds produced during the trials were subjected to Vickers hardness (HV30) surveys, which were carried out at a depth of 4mm below the running surface of the rail, at 2mm indent spacing. The resultant Vickers hardness profiles are illustrated in Figures 7a and 7b for standard welds in Grade 220, 56 E1 and 60 E1 rails, and 8a and 8b for narrow HAZ welds in same Grade and section rails.

The welds whose hardness profiles are illustrated in these Figures were produced using both the conventional/standard welding practice. The narrow HAZ practice involved 3 preheats (Figure 2) and a forging load of 650kN. This compared with the employment of 10 preheats (Figure 3) and a forging load of 450kN for the production of standard welds. The utilization of considerably lower heat inputs in the case of the narrow HAZ welded rails resulted in significantly higher maximum critical HAZ hardness values. For example, the narrow HAZ welded Grade 220, 56 E1 I ee À Àe rail gave a maximum critical HAZ hardness value of 299 HV (P+40) as shown in Figure 8a compared with a maximum HAZ hardness value of,v 270 HV (Figure 7a) for the standard weld. This higher HAZ hardness of the narrow HAZ welded rail was within that stipulated in the draft European specification of P+60 ("P" being the average hardness of the parent material) and can be attributed to a significantly higher cooling rate through the austenite to pearlite transformation temperature range (700-500 degree C). The higher cooling rate of the narrow HAZ welded rail being a consequence of the lower heat input/narrower HAZ width.

Optical micrographs of the critical HAZ regions of standard and narrow welds produced during the course of the trials are shown in Figures 9a, 9b, 10a and JOB. These micrographs were obtained from the fusion line, critical HAZ (Grimm away from the fusion line) and parent material regions, from a depth of 4mm below the running surface of the rail, in accordance with the stipulated specification. The micrographs shown in Figures 9a and 9b are of standard Grade 220, 56E1 and 220, 60E1 welded rails, and those depicted in Figures 10a and 10b relate to narrow HAZ welded rails of same rail grades and sections in accordance with the invention.

The microstructures of the narrow HAZ welds show a marked refinement of the prior austenite grain size, which is regarded to be a beneficial attribute as far as improved fracture toughness, rolling contact fatigue behaviour and ductility levels are concerned.

Advantages which accrue from the present invention include: - a reduction in the welding time by around 50% - leading to both a lower power consumption per weld and the possibility of increased productivity levels, if required; - a marked increase in the post weld cooling rate through the austenite to pearlite transformation temperature range (700 ce. ;:: :..

500 C). This is regarded to be of paramount importance as far as the Corus Rail welding plants are concerned, since this stage of the weld production process is one of the major bottlenecks in terms of being able to achieve increased productivity levels; an overall improvement in the bend strength levels in comparison with standard production welds; a markedly narrower HAZ (width being 30 to 50% lower compared with standard production weld, and as such the railway wheel should "see" less of the weld. In addition, in the case of the narrow HAZ weld, the railway wheel when located on top of a given weld will be supported by a larger amount of harder material present on either side, and just outside the extent of the visible HAZ, in comparison with an equivalent situation for the standard weld), leading to an overall improved ride quality;- a harder HAZ (critical HAZ hardness 40 points higher compared with parent rails, which would be expected to lead to an overall improved rolling contact fatigue (RCF) and wear resistance of the weld); and enhanced weld ductility.

Flash butt welding in accordance with the invention can be carried out either in a suitably equipped depot or in situ using a mobile flash butt welding machine.

It will be appreciated that the foregoing is merely exemplary of narrow HAZ welded rails in accordance with the invention and methods of producingsuch rails and that various modifications can readily be made thereto without departing from the true scope of the invention.

Claims

1. A steel rail comprising a plurality of individual rails joined by flash butt welding, wherein the width of the heat affected zone (HAZ) between neighbouring welded rails at the load bearing surface of the rail is less than 30mm.

2. A rail as claimed in claim 1 wherein the width of the heat affected zone is less than 25mm.

3. A flash butt welded steel rail in which the width of the heat affected zone (HAZ) of each or a majority of welds is less than 30mm.

4. A rail as claimed in claim 3 wherein the width of the heat affected zone is less than 25mm.

5. A rail as claimed in any one of the preceding claims wherein the variation in HAZ width over the height of the or each weld lies in the range 10 to 30mm.

6. A rail as claimed in claim 5 wherein the range is from 17 to 23mm.

7. A rail as claimed in claim 6 wherein the range is from 15 to 20mm.

8. A flash butt welding process for steel rails, the process comprising the steps of preheating the rail ends to be joined and applying to the preheated rail ends a forging load in excess of 500kN.

9. A process as claimed in claim 8 wherein the forging load exceeds 650kN.

10. A process as claimed in claim 8 wherein the forging load is between 500 and 800kN.

11. A process as claimed in any one of claims 8 to 10 wherein preheating of the rail ends is achieved by cyclically bringing the rail ends into contact and causing a current to flow across the rail ends, the number of preheating cycles being sufficient to raise the rail ends to a suitable temperature for final flashing to commence and be sustained.

12. A process as claimed in claim 11 wherein the number of preheating cycles is five or less.

13. A process as claimed in claim 11 wherein the number of preheating cycles is four or less.

14. A process as claimed in claim 11 wherein there are three preheating cycles.

15. A process as claimed in any one of claims 8 to 14 wherein the flash butt welds are cooled naturally in still air.

16. Flash butt welded rails and a process for producing such rails substantially as herein described and as described with reference to the accompanying diagrammatic drawings.