Classifications

• • E—FIXED CONSTRUCTIONS
  • E01—CONSTRUCTION OF ROADS, RAILWAYS, OR BRIDGES
  • E01B—PERMANENT WAY; PERMANENT-WAY TOOLS; MACHINES FOR MAKING RAILWAYS OF ALL KINDS
  • E01B11/00—Rail joints
  • E01B11/44—Non-dismountable rail joints; Welded joints
  • E01B11/50—Joints made by electric welding
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K11/00—Resistance welding; Severing by resistance heating
  • B23K11/002—Resistance welding; Severing by resistance heating specially adapted for particular articles or work
  • B23K11/0073—Butt welding of long articles advanced axially
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K11/00—Resistance welding; Severing by resistance heating
  • B23K11/04—Flash butt welding
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K11/00—Resistance welding; Severing by resistance heating
  • B23K11/34—Preliminary treatment
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K2101/00—Articles made by soldering, welding or cutting
  • B23K2101/26—Railway- or like rails

Definitions

• This invention relates to steel rails.
• 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 pre-flashing 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.
• flashing i.e. arcing
• the optimum value of the bum-off is the minimum value required to establish a stable temperature distribution of the rail ends.
• 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 lOOkA) 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 PR, where I is the magnitude of the preheat current and R the resistivity of the material being welded.
• PR resistance heating
• 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 or "craters" produced in the rail ends is reduced considerably by reducing the flashing voltage.
• 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 lOOOkN.
• "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/mm 2 , 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.
• 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.
• a typical forging distance of between 12 and 20mm is employed.
• the magnitude of the upset distance (the distance travelled by the movable platen has to be sufficient to accomplish two main 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.
• 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.
• 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).
• 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.
• a forging stroke of some 12 to 18mm and a forging load of around 450kN 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.
• 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.
• HZ heat affected zone
• 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.
• HZ heat affected zone
• 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.
• 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.
• 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.
• 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).
• 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.
• 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
• 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
• 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.
• 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 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.
• SWEP Schott weld monitor programme
• 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.
• 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.
• 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.
• 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.
• the narrow HAZ welds caused the forging load to increase to ⁇ 650kN; in comparison the forging load for the standard welds was ⁇ 450kN.
• 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.
• HV 30 Permitted Hardness Range
• Tables 3 and 4 show that the narrow welds exhibit enhanced levels of hardness when compared with the standard welds.
• 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 (l ⁇ m surface finish) and etching in 4% Nital
• 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.
• 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.
• the narrow HAZ welded Grade 220, 56 El rail gave a maximum critical HAZ hardness value of ⁇ 29 HV (P+40) as shown in Figure 8a compared with a maximum HAZ hardness value of ⁇ 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.
• FIGS 9a, 9b, 10a and 10B 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 10B. These micrographs were obtained from the fusion line, critical HAZ ( ⁇ 4mm 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.
• 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 - 500°C).
• 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.
• the welding technique for steel rails described herein finds benefit in providing a higher heating rate during welding such that critical fusion occurs at optimum temperature before the heat from the weld joint thermally conducts significantly into the parent material.
• the welding parameters include, for example, a cur nt greater than 60 kA (much greater than prior art welding parameters) and a power of greater than 500 kvA.
• the HAZ width of the weld may be less than 20 mm.
• bainitic steel rails can be welded together to generate an improved hardness profile without negating or otherwise diminishing the properties and resulting advantages of the bainitic steel.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Architecture (AREA)
• Civil Engineering (AREA)
• Structural Engineering (AREA)
• Heat Treatment Of Articles (AREA)

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• 2003
  • 2003-06-26 GB GB0314839A patent/GB2403174B/en not_active Expired - Fee Related
• 2004
  • 2004-06-28 WO PCT/GB2004/002786 patent/WO2005001204A1/en active Application Filing
  • 2004-06-28 EP EP04743134A patent/EP1641978A1/de not_active Withdrawn

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