EP0815325A1

EP0815325A1 - Improvements in and relating to steel rails and methods of producing the same

Info

Publication number: EP0815325A1
Application number: EP96904198A
Authority: EP
Inventors: Vijay Jerath; John Anthony Chapman; David James Price
Original assignee: British Steel PLC
Current assignee: Corus UK Ltd
Priority date: 1995-03-20
Filing date: 1996-03-01
Publication date: 1998-01-07
Anticipated expiration: 2016-03-01
Also published as: GB2299044A; EP0815325B1; ZA961892B; GB9505567D0; WO1996029469A1; DE69622698D1; CA2215941A1; ATE221596T1; GB2299044B; AU4838796A

Abstract

A steel rail comprising a plurality of individual rails welded together with any resulting excess material removed from the rail by a stripping process. The weld sites of the rail may be subjected to natural air and/or independent accelerated cooling and grinding or other surface material removal technique and/or peening process.

Description

Improvements in and relating to Steel Rails and Methods of Producing the same

This invention relates to steel rails and to methods of producing the same. More especially, the invention relates to the production of high integrity long welded steel rails.

Railway tracks have traditionally comprised a plurality of rails connected together by bolts and fish plates. In use, such tracks have proved to be noisy, uneven and require considerable maintenance. More recently, individual rails have been welded together in continuous lengths thereby enabling higher train speeds to be achieved at relatively low noise and vibration levels. The welds between adjoining individual rails have, however, previously been subject to occasional bending fatigue failures from the foot of the rail caused inter alia by positive tensile loading stresses experienced by the rail foot during service.

One object of the present invention is to provide a high integrity long welded steel rail which exhibits a bending fatigue life commensurate with that of non-welded rail and an overall improved residual stress pattern across the weld HAZ profile. Another object is to provide a welded steel rail in which the bending fatigue strength of each weld is at least equal to if not greater than the fatigue strength of the parent rails joined by that weld. A further object is to produce a welded steel rail in which the presence of the welds is not apparent from a visual inspection (that is to say an invisibly welded rail). A further object is to provide a method or methods of manufacturing welded steel rail by which the objectives set out above can be achieved.

Typically a long welded rail is a length of the order of 100m to 300m or longer.

According to the present invention in one aspect there is provided a steel rail which comprises a plurality of individual rails welded together with any resulting excess material removed from the rail by a stripping process in which the weld site at the foot of each rail is subjected to grinding.

Each weld site may be coated with a corrosion inhibiting material. Such coating may be applied to the rail foot and/or the rail web and/or the rail head. The coating may, of course, alternatively be applied to the entire weld site.

Welding may be by flash butt welding. Other welding processes may however be employed.

In another aspect, the present invention provides a method of producing a welded steel rail which comprises the steps of welding individual rails together, removing any resulting excess material from the weld sites and subsequently subjecting the weld sites to natural air and/or independent forced cooling and grinding or other surface material technique and/or peening.

As-rolled rail grades such as 700 and 900 (BS11 Normal quality and Grade A/AREA) achieve weld HAZ hardness levels similar to those of the parent rails following natural air cooling of the welds in still air. These rails following welding are, therefore, not normally subjected to accelerated cooling. It has, however, been found advantageous to increase the hardness/strength, in particular, of the welded rail foot by accelerated cooling with a view to improving the bending fatigue strength of the welded rail.

Accelerated cooling may be achieved by applying air, air mist or water under pressure independently to the head and/or the web and/or the foot of the rail at each weld site. Typically, forced cooling is applied through the appropriate phase transformation temperature range, eg austenite to pearlite, austenite to bainite or austenite to martensite. Typically cooling is initiated at a temperature within the high temperature austenite phase of the steel and stopped at a temperature deemed suitable for the completion of a given phase transformation. Accelerated cooling may typically be effected for a period of between 30 and 120 seconds. Forced cooling may be effected by either individual hoods or nozzles positioned above and to the sides of the head of the rail, opposite each web of the rail, and below the foot surface of the rail, each independent hood or nozzle being supplied with air, air mist or water under pressure from a common or a different source. Thus cooling may be effected to, for example, only the rail head,foot or web, or to the entire rail.

Fine grinding or surface material removal by another technique of the welded regions may be applied to the head and/or the underside of the rail foot -and/or to the web of the rail. Invisibly welding of a rail entails, however, grinding or other surface material removal all around the rail profile after welding of the individual rails together.

Robotic material removal techniques may be employed. Surface material removal may be effected either cold or at a temperature below the austenite temperature of the steel from which the rail is produced. Sensors may be provided to ensure that the required material removal depth and surface finish are achieved.

Peening may, for example, be by a shot or a hammer peening process and may be applied to the underside of the foot of the rail and/or to the head and/or web of the rail or to the entire welded or unwelded (ie parent rail) rail profile.

Peening may be achieved by directing a peening medium under pressure independently to the foot and/or head and/or web of the rail at each weld site. The depth of the residual compressive stress layer may be, for example, between 0.75mm and 1mm and the compressive stresses achieved may be, for example, of the order of 60% to 80% of the yield strength of the rail material in compression. Peening is generally carried out at a temperature below the stress relieving temperature of the steel from which the rail is produced, typically below 250^βC.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which:-

Figure 1 is a transverse section taken through a steel rail and shows substantially typical fatigue initiation sites in the foot of the rail associated with bending fatigue failures.;

Figures 2 and 3 are side views of accelerated cooling apparatus in accordance with the present invention;

Figure 4 graphically illustrates the effect of forced cooling on flash butt weld HAZ hardness of fully pearlitic plain carbon rails;

Figure 5 graphically illustrates the generation of residual stress in a shot peened steel after surface grinding and showing beneficial compressive stress produced following fine grinding; Figure 6 is a side view of peening apparatus in accordance with the invention;

Figure 7 graphically illustrates induced compressive surface stresses and tensile stresses in a shot peened steel material;

Figure 8 graphically illustrates longitudinal residual stress distributions in a roller straightened steel rail, a flash butt welded rail and a ground and shot peened welded rail; and

Figure 9 graphically compares bending fatigue strengths of a parent rail, a normal production weld, a weld ground all around, a weld ground all around and shot peened and a normal production weld after shot peening.

The welded rail illustrated in Figure 1 has a head 1, a web 2 and a foot 3. Typical bending fatigue initiation sites in the illustrated welded rail are indicated by reference numbers 4, 5 and 6. As will be seen, all of these fatigue initiation sites are located in the foot region of the rail. Of the sites illustrated, site 4 located in the base of the foot 3 is found to be the most prevalent, and is generally caused by maximum tensile stresses occasioned by the bending forces generated during service.

As mentioned previously, it is an object of this invention to provide a high integrity long welded steel rail having bending fatigue life and wear performance at least as good, if not better than, those of rails manufactured currently. This objective is currently achieved by flash butt welding (otherwise called electrical resistance welding) individual steel rails together, stripping the resulting flash from the weld sites and then subjecting the weld sites to one, more than one or indeed all of the process steps of natural air or forced cooling, fine grinding and peening. These process steps will now be discussed in more detail.

Flash butt welding is a process in which the rail ends to be joined together are held between water cooled copper grips, which act as both clamps and electrodes. The first stage in the flash butt welding sequence is generally termed the "Burn-Off" or the "Pre-flashing" stage. During this stage, the rail ends are separated slightly and arcing/flashing is initiated between them in order to square up the rail ends. "Burn-Off" times are typically between 5 and 10 seconds, with l-5mm of platten movement. The next stage in the welding sequence is the preheating sta_/e. The main aim of the preheating stage is to heat the rail ends to a sufficient temperature so that flashing initiates easily, and a suitable temperature gradient is achieved in the rail ends, prior to the onset of the final flashing sequence. Preheating is generally achieved by bringing the rail ends into contact, and allowing a high current to flow across the rail ends. The rail ends are brought together and allowed to heat by means of resistance heating.

In between each preheating cycle a short period of flashing is generally allowed to occur in order to maintain rail end squareness. This is followed by the final "flashing" stage. During this stage the rail interfaces become molten, and the correct conditions for the final upset or the forging stroke are achieved. The moving head of the welding machine during this stage is accelerated parabolically, with the 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. A total flashing distance of between 9 and 15 mm, over a period of 5 to 10 seconds, is typically employed for rails. Immediately following the final flashing stage, the movable platten of the welding machine is accelerated so that the rail ends are butted together either under a constant platten speed or under impact loading of up to 600 kN. The load is calculated to give a pressure at the weld interface of approximately 50-60 N/mm², to ensure adequate weld consolidation. The welding current is generally supplied during the initial part of the forging operation to avoid oxidation at the weld interface. During welding of rails, a typical forging distance of between 12 and 20 mm is generally employed. Following the completion of the forging stroke, the molten and soft steel is forced out of the weld joint. This extruded material, generally termed "flash" is then removed quickly by means of an automatic weld upset removal tool.

One recognised advantage of flash butt welding is that good quality welds requiring no external weld metal can be produced in a relatively short period of time, typically within 45 to 90 seconds.

Other welding techniques may be employed, e.g. squeeze welding.

Accelerated forced cooling is a process for rapidly and controllably reducing the temperature of a product. When used in the context of the present invention, the weld sites, for example, of heat treated, plain carbon pearlitic rail are cooled through the austenite to pearlite transformation temperature range (typically from 700°C to 500°C) at, for example, at up to 7°C/second. Enhanced cooling of the weld sites at the correct rate produces weld HAZ hardness levels which match those of the parent plain carbon heat treated rail. The effect of different forced cooling rates on HAZ hardness can be seen from Figure 4. Thus, the effect of independent forced air cooling of the welded rail head, web and foot weld sites is effected with a view to increasing the hardness and wear resistance of the rail head, to improve the rolling contact fatigue life of the welded rail head and bending fatigue strength of the rail foot, and to improve the overall residual stress pattern across the weld HAZ profile.

Apparatus for effecting forced cooling is illustrated in Figures 2 and 3. In Figure 2 the cooling medium is air and in Figure 3 water. As shown in Figure 2, cooling hoods

7 supported within a frame 9 and each connected by conduits

8 to a source of air under pressure are positioned above and to each side of the rail head, alongside each rail web and below the rail foot. The arrangement is such that one or more (or indeed all) of the hoods may be connected to direct cooling air under pressure onto the adjoining rail surface. The flow of air through the conduits is individually controlled by means of valves positioned, for example, within the conduits.

In the arrangement illustrated in Figure 3, water is supplied under pressure via nozzle 11 to a manifold 12. Cooling water is selectively projected onto the head, web and/or foot of the rail by nozzles 14.

The effect of forced cooling on HAZ hardness of fully pearlitic plain carbon rails is illustrated graphically in Figure 4.

Surface grinding of the welded rail head is carried out as a routine production procedure to match the profile of the parent rail. Alternatively, surface material removal from the rail head may be carried out by another technique. Additional surface grinding (or surface material removed by another technique) to the web and foot of the rail may be effected in the present invention with a view to rendering the weld invisible to the eye. Beneficial grinding (or other method of surface material removal) of the weld sites may be effected to reduce the number of potential fatigue initiation sites, eg surface pitting and by removing all traces of weld flash and decarburized layer from each weld site, particularly that from the rail foot which experiences tensile loading stresses during service.

Surface material removal, for example, by grinding may be carried out by use of a robotic grinder which automatically fine grinds selected parts or each entire weld site of the rail.

Grinding may be effected either cold or at a temperature below the austenite temperature of the rail, typically 700°C. Sensors may be provided to ensure that the required grinding depth and surface finish are achieved.

A graph illustrating the generation of residual stress in a 4340 steel after surface grinding is shown in Figure 5. The beneficial compressive stress introduced by fine grinding is to be noted.

In addition fine grinding or other surface material removal technique particularly of the base of the welded rail foot also enables a full in-line automatic non¬ destructive testing (NDT) and/or additional alternative manual inspection of the welded rail to be carried out. Thus, the rail head, web and foot can readily be ultrasonically tested to a required specification, the rail foot base can be eddy current tested, and the flatness of the rail running surface and the rail head sides can be inspected. Also, manual ultrasonic inspection of each weld site for transverse defects can be effected with considerable ease.

Peening may be effected by, for example, a shot or hammer peening process.

Shot peening is a cold working process in which the surface of a part is bombarded with small spherical media called shot. Each piece of shot striking the material acts as a tiny peening hammer, imparting to the surface a small indentation or dimple. In order for the dimple to be created, the surface fibres of the material must be yielded in tension. Below the surface, the fibres try to restore the surface to its original shape, thereby producing below the dimple, a hemisphere of cold-worked material highly stressed in compression. Overlapping dimples develop an even layer of metal in residual compressive stress. It is known that a compressively stressed zone increases the initiation time required for a crack to form for a given applied tensile stress range. Since nearly all fatigue and stress corrosion failures originate at the surface of a part, compressive stresses induced by shot peening provide considerable increases in part life. The maximum compressive residual stress produced at or under the surface of a part by shot peening is at least as great as half the yield strength of the material being peened. Many materials will also increase in surface hardness due to the cold working effect of shot peening.

Shot peening apparatus for use, for example, with the invention is schematically illustrated in Figure 6. This apparatus comprises a plurality of nozzles 15 positioned above, below and to the sides of the rail. Each nozzle is mounted on a frame 16 and is connected to receive a source of gas under pressure and shot. Each nozzle can be independently controlled whereby the head, web and foot of the rail can be together or selectively peened.

Benefits obtained by shot peening are the result of the effect of the compressive stress and the cold working induced. In the present invention, peening is effected to welded rail and/or unwelded rail, i.e. parent rail, thereby creating compressive stresses which act to minimise the onset of crack initiation during service.

In one example, shot peening to the specification MI - S-13165C was applied to the rails in the present invention. This peening process employed a shot size of 1.375mm (MI 550) at an intensity of 0.012-0.014C. A range of shot sizes and intensity levels can, nevertheless, be employed to obtain beneficial compressive stresses.

Typical compressive stresses achieved by the process are, for example, of the order of 60 to 80% of the yield strength of the rail material in compression. The depth of the residual compressive layer produced by peening is, for example, typically between 0.75mm and 1.0mm. Peening is generally effected at a temperature below the stress relieving temperature of the steel from which the rail is produced, typically below 250°C.

The effect of induced compressive and tensile stresses in a shot peened material with no external load is illustrated in Figure 7 of the drawings and Figure 8 graphically illustrates longitudinal residual stress distribution in a conventional rail, a weld and a shot peened weld.

A comparison of the results of 4-point bend fatigue testing of conventional rails and welded rails in accordance with the present invention can be seen from Figure 9. This Figure shows that the high integrity welds produced in accordance with this invention and labelled "3" "4" and "5" possess bending fatigue strength levels significantly in excess of that of the parent or unwelded rail labelled "1".

Bending fatigue data for as-rolled welded rails which have been either ground or ground and shot peened have been found to be at least as good as, if not better than, those of the parent rails. These rails have shown similar overall trends to those observed in the case of mill heat treated rails (MHT) as exemplified in Figure 9.

The techniques mentioned above concerning the production of high integrity invisible welds may also be applied readily to all grades of as-rolled and heat treated pearlitic rails and to any other additional rail grades developed in the future.

In addition, the techniques mentioned above may be applied readily to the bainitic and martensitic steels disclosed in our co-pending patent applications 950097.1 and 9313060.

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

Claims

1. A steel rail which comprises a plurality of individual rails welded together with any resulting excess material removed from the rail by a stripping process in which the weld site at the foot of each rail is subjected to grinding.

2. A rail as claimed in claim 1 wherein the weld sites are cooled.

3. A rail as claimed in claim 2 wherein the weld sites are subjected to natural air cooling.

4. A rail as claimed in claim 2 wherein the weld sites are subjected to independent accelerated cooling.

5. A rail as claimed in any one of claims 1 to 4 wherein the entire weld sites are subjected to grinding.

6. A rail as claimed in any one of the preceding claims wherein the weld sites are subjected to peening.

7. A rail as claimed in any one of the preceding claims wherein each weld site is coated with a corrosion inhibiting material.

8. A rail as claimed in claim 7 wherein the coating is applied to the rail foot and/or the rail web and/or the rail head.

9. A method of producing a welded steel rail which comprises the steps of welding individual rails together, removing any resulting excess material from the weld sites and subsequently subjecting the weld sites to natural air and/or independent forced cooling and grinding or other surface material technique and/or peening.

10. A method as claimed in claim 9 wherein accelerated cooling is achieved by applying air, air mist or water under pressure independently to the head and/or the web and/or the foot of the rail at each weld site.

11. A method as claimed in claim 10 wherein accelerated cooling is effected for a period of between 30 and 120 seconds.

12. A method as claimed in claim 10 or claim 11 wherein forced cooling is effected by directly cooling medium through nozzles positioned above and to the sides of the head of the rail, opposite each web of the rail, and below the foot surface of the rail.

13. A method as claimed in any one of claims 9 to 12 wherein peening medium is applied to the underside of the foot of the rail and/or to the head and/or web of the rail.

14. A method as claimed in claim 13 wherein peening is achieved by directing a peening medium under pressure independently to the foot and/or head and/or web of the rail at each weld site.

15. A method as claimed in claim 13 or claim 14 wherein the depth of the residual compressive stress layer achieved is between 0.75mm and 1mm and the compressive stresses achieved are between 60% to 80% of the yield strength of the rail material in compression.

16. A rail as claimed in any one of claims 13 to 15 wherein peening is carried out at a temperature below the stress relieving temperature of the steel from which the rail is produced.

17. A steel rail substantially as herein described and as described with reference to Figures 2 and 3 or Figure 5 of the accompanying drawings.

18. A method of producing steel rail substantially as herein described and as described with reference to Figures 2 and 3 or Figure 5 of the accompanying drawings.