WO2015059628A1

WO2015059628A1 - Process and mould assembly for aluminothermic welding of rails

Info

Publication number: WO2015059628A1
Application number: PCT/IB2014/065492
Authority: WO
Inventors: G D Macrae; Gd MACRAE
Original assignee: G D Macrae
Priority date: 2013-10-25
Filing date: 2014-10-21
Publication date: 2015-04-30

Abstract

The invention provides a mould assembly [10] for use in fusion welding rail ends of two adjacent rails. The mould assembly [10] comprises two upright side moulds [12] that are fitted on either side of an inter-rail gap [42] defined between the rail ends, wherein each side mould [12] includes at least one elongate, open-ended down gate [14] extending between a top part and a bottom part of the side mould [12] and terminating in a down gate well [16]; and a base mould [18] adapted for receiving the rail ends and for supporting the side moulds [12] on either side of the inter-rail gap [42]; the arrangement being such that the base mould [18] and side moulds [12] cooperate to define an enclosed mould cavity for receiving liquid metal. The mould assembly [10] further comprises at least one pouring basin [20] for receiving liquid metal poured into at least one of the side moulds [12] and arranged in flow communication with the down gate [14], the pouring basin [20] being characterised therein that it includes a level base [22], a bottom diverting slot [24], which is aligned with the down gate [14], and an elongate and raised lip formation [26] positioned between the level base [22] and the bottom diverting slot [24].

Description

PROCESS AND MOULD ASSEMBLY FOR ALUM IN OTHER MIC WELDING OF RAILS

BACKGROUND TO THE INVENTION

This invention is an improvement over PCT patent application number WO201 1/013078 and relates to fusion welding of rails to form substantially continuous railway tracks. In particular, the current invention relates to modifications and improvements over the mould assembly for use in aluminothermic Intercast welding of rail ends as disclosed in WO201 1/013078.

Fusion welding of rail ends, and in particular aluminothermic welding, is one of the established, internationally accepted procedures for in situ welding of rails and utilises the high affinity of aluminum for oxygen for the reduction of iron oxide. In general, the procedure comprises aligning rails to be joined together in an end-to-end manner, leaving a prescribed inter-rail gap between neighbouring rail ends, and fitting preformed refractory moulds around the inter-rail gap to form an enclosed mould cavity intermediate the neighbouring rail ends. An especially designed preheating burner is used to preheat the refractory moulds and the enclosed mould cavity for removal of all water vapour and to preheat the rail ends. Superheated molten steel is then Intercast into the mould cavity from a refractory crucible located above the refractory moulds, and allowed to solidify in the mould cavity to form a rail weld in the inter-rail gap between adjacent rail ends. After solidification, refractory materials and surplus steel are removed from the rail weld, and after further cooling, rail fastenings are replaced and rail heads are grinded back to profile. The quality of the rail weld between the rail ends is of critical importance when it comes to wear resistance of railways and their ability to handle increasing rail traffic and axle loads. In particular, rail welds need to be homogenous, with fine grained, hard weld material concentrated in the rail head, and as little as possible slag-losses of alloying additives. Moreover, intimate turbulent mixing of the casting steel within the mould cavity must be avoided at all costs, otherwise a concentration of alloying additives in the rail head cannot be achieved. In some countries, the weld is ultrasonically and radiographically tested. However, radiographic testing of rail welds is not permitted in all countries and in such cases only ultrasonic testing of the rail welds is performed.

Most commonly-used processes for aluminothermic welding of rails employ top-pouring refractory moulds having a diverting plug, made from silica sand and bound with sodium silicate. Examples of processes are described in patent numbers US 6,227,282; US 2007/02721 14; and US 5,419,484. US 2007/02721 14, for example, provides the steps of surrounding to-be-welded rail ends with a refractory mould, which includes a diverting plug [36] fitted in an upper region of the mould covering a rail head of the to-be-welded rail ends. A liquid metal stream is center-poured from an overhead reaction crucible [5] into the refractory mould [19] over the diverting plug [36], such that it impinges on the diverting plug [36]. The metal stream flows into the mould cavity from the top and subsequently fills the mould cavity to form a rail foot, web and rail head, with two risers extending upwardly from the rail foot.

In US 5,419,484 the mould includes a guiding core [16] which is centrally located in at the upper side of the mould [1 ] and which includes an internal inflow gate [18] through which the liquid metal is poured directly into the mould cavity (refer page 8, column 2, lines 1 - 4).

Center-pouring processes of rail welds, such as those disclosed in US 6,227,282; US 2007/02721 14; and US 5,419,484, suffer from a number of disadvantages.

(i) Turbulence in the mould cavity, gas entrapment and worm holes

Liquid metal enters the mould cavity at a high velocity. In US 2007/02721 14, for example, the metal stream is dispersed over the diverting plug [36] and splashes down the opposing faces of two adjacent rail ends in an almost waterfall-like effect inside the mould cavity. This is done in an effort to achieve complete fusion between the weld metal and the rail ends. Because of the resultant turbulence with which the liquid metal enters the mould cavity, liquid metal entraps some of the air, which is within the mould cavity, as small gas bubbles in the cast material during the casting process. These gas bubbles follow the cooling front and, because of air's lower density compared to the surrounding liquid metal, the gas bubbles attempt to float to the liquid metal surface. However, depending on the cooling rate of the cast metal, these bubbles are trapped inside the cooling liquid metal, resulting in the formation of wormholes and gas pores, which frequently present in the weld foot. This undesirable gas generation, which is directly attributable to the liquid metal turbulence during the casting process, can cause wormholes and gas pores in the weld metal after solidification, which leads to ultrasonic and/or radiographic examination failure of the weld cast.

Figure (i) hereunder is an X-ray of a rail weld foot which was poured using a center- pouring process, and clearl illustrates the formation of gas pores in the rail foot.

after center-pou

Figure (ii) hereunder illustrates the formation of wormholes in the weld cast using a center-pouring process.

Turbulence and droplets oxidization

A further disadvantage associated with the center-pouring processes of the prior art and the resultant liquid metal turbulence inside the mould cavity is that, due to the high temperature of the metal stream, dispersed liquid metal droplets oxidize on their surfaces. These oxidized liquid metal droplets must then reform to a complete metal liquid before arriving at the bottom of the mould cavity. However, often the liquid metal stream surrounding these droplets cool down so quickly once it hits the much-cooler mould surface that the oxidized metal droplets are not reformed to metal liquid in time, resulting in unwanted inclusions becoming entrapped in the rail weld. Such oxidized metal droplets are highly undesirable and lead to metal fatigue and failure.

Silica plug destruction

A further disadvantage associated with, for example US 2007/02721 14, is the silica plug [36]. Liquid metal hits the silica plug at a casting temperature of about 2000°C. The liquid metal is highly erosive and the silica plug is literally "washed away" during the casting process, resulting in the entrapment of sand inclusions in the weld cast. These sand inclusions are impurities which decrease strength and durability in the weld cast. (iv) Stress raiser caused by riser fracturing

In US 2007/02721 14, the metal risers [38] sit on top of the weld foot and is hammered-off the weld cast after casting, while the cast material is at a black heat. A disadvantage associated with this design is that fracturing of the riser from the weld foot creates a cup inside the weld foot - the riser breaks off inside the weld foot. This causes a stress raiser in the weld foot, which inevitably leads to fracturing of the rail weld far below the expected tensile strength of the material. Moreover, the risers must be hammered-off at a critical temperature, after partial solidification, because the weld material becomes extremely hard and difficult to remove after complete solidification. Expert on-site knowledge is required to know exactly when sufficient partial solidification is achieved for this step.

It is because of these disadvantages that WO 201 1 /013078 introduced an in situ fusion welding process consisting of a single process step. In particular, WO 201 1/013078 introduced a process and mould assembly for side-pouring liquid metal into an inter-rail gap between the rail ends, the process and mould assembly being characterised in reducing turbulence during the liquid metal casting so as to concentrate alloying additives in a weld head, thereby to produce a rail weld that includes a rail head of hard weld material, and a rail foot that is less prone to breakage. The applicant has now identified three critical improvements to be made to the process and mould assembly of WO 201 1 /013078.

SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a mould assembly for use in fusion welding rail ends of two adjacent rails, the mould assembly comprising - two upright side moulds that are fitted on either side of an inter-rail gap defined between the rail ends, each side mould including at least one elongate, open-ended down gate extending between a top part and a bottom part of the side mould and terminating in a down gate well;

a base mould adapted for receiving the rail ends and for supporting the side moulds on either side of the inter-rail gap; the arrangement being such that the base mould and side moulds cooperate to define an enclosed mould cavity for receiving liquid metal; and

at least one pouring basin for receiving liquid metal poured into at least one of the side moulds and arranged in flow communication with the down gate, the pouring basin being characterised therein that it includes a level base; a bottom diverting slot which is aligned with the down gate; and an elongate, raised lip formation positioned between the level base and the bottom diverting slot.

The elongate lip formation may be raised to extend above the level base for preventing immediate flow of first cast liquid metal from the pouring basin into the down gate, the arrangement being such that a head of liquid metal is first created within the pouring basin upon immediate first casting of liquid metal into the pouring basin, after which the liquid metal only starts running down the down gate after the head of liquid metal rises above and over the raised lip formation.

The pouring basin may be a refractory or ceramic bowl which is removably located inside the top part of a side mould.

The mould assembly further may be characterised therein that the down gate well has a half- spherical well floor. Each side mould may include downwardly depending base mould engaging means extending from the base of the side mould and dimensioned to engage complimentarily dimensioned side mould engaging means, which extend upwardly from the base mould, when the side mould sits on top of the base mould. The mould assembly may be characterised therein that when the side moulds sit on top of the base mould, the base of the side moulds are level with the top of the rail foot.

The base mould may include a substantially central rail foot well in which liquid metal enters the base mould underneath a rail end to form the rail foot; the at least one spherical down gate well laterally disposed relative to the rail foot well; side mould engaging means extending upwardly from the base mould and dimensioned to engage complimentarily dimensioned base mould engaging means on the side moulds for supporting the side moulds; and two parallel rail end supporting walls on which the rail ends of to-be-welded rails sit during casting, the rail end supporting walls bordering the rail foot well on either side thereof such that they determine the depth of the rail foot well, the base mould being characterised therein that the rail foot well has a critical depth of between 2mm and 10mm, and preferably a depth of between 5mm and 7mm; and the rail end supporting walls are tapered down to the rail foot well, preferably through a curved surface of predetermined radius.

The base mould further may include a fracture rib between the rail foot well and each of the down gate wells. The side moulds further may be characterised therein that they define a head rising room in an upper part of the side moulds for receiving liquid metal rising from the inter-rail gap and the down gates.

According to a second aspect of the invention there is provided a mould assembly for use in fusion welding rail ends of two adjacent rails, the mould assembly comprising - two upright side moulds that are fitted on either side of an inter-rail gap defined between the rail ends, each side mould including at least one elongate, open-ended down gate extending between a top part and a bottom part of the side mould and terminating in a down gate well;

at least one pouring basin for receiving liquid metal poured into at least one of the side moulds and arranged in flow communication with the down gate; and

a base mould adapted for receiving the rail ends and for supporting the side moulds on either side of the inter-rail gap such that the base mould and side moulds cooperate to define an enclosed mould cavity for receiving liquid metal;

the base mould including a substantially central rail foot well in which liquid metal enters the base mould underneath a rail end to form the rail foot; at least one down gate well laterally disposed relative to the rail foot well; side mould engaging means extending upwardly from the base mould and dimensioned to engage complimentarily dimensioned base mould engaging means on the side moulds for supporting the side moulds; and two parallel rail end supporting walls on which the rail ends of to-be-welded rails sit during casting, the rail end supporting walls bordering the rail foot well on either side thereof such that they determine the depth of the rail foot well, the base mould being characterised therein that the rail foot well has critical depth of between 2mm and 10mm, and preferably a depth of between 5mm and 7mm; and the rail end supporting walls are tapered down to the rail foot well, preferably through a curved surface of predetermined radius. According to a third aspect of the invention there is provided a mould assembly for use in fusion welding rail ends of two adjacent rails, the mould assembly comprising - two upright side moulds that are fitted on either side of an inter-rail gap defined between the rail ends, each side mould including at least one elongate, open-ended down gate extending between a top part and a bottom part of the side mould and terminating in a down gate well;

at least one pouring basin for receiving liquid metal poured into at least one of the side moulds and arranged in flow communication with the down gate; and a base mould adapted for receiving the rail ends and for supporting the side moulds on either side of the inter-rail gap such that the base mould and side moulds cooperate to define an enclosed mould cavity for receiving cast liquid metal,

the base mould including a substantially central rail foot well in which liquid metal enters the base mould underneath a rail end to form the rail foot; at least one down gate well laterally disposed relative to the rail foot well; side mould engaging means extending upwardly from the base mould and dimensioned to engage complimentarily dimensioned base mould engaging means on the side moulds for supporting the side moulds; and two parallel rail end supporting walls on which the rail ends of to-be-welded rails sit during casting, the base mould being characterised therein that the down gate well has a half- spherical well floor.

According to a fourth aspect of the invention there is provided a process for fusion welding rail ends of two adjacent rails, the process comprising the steps of - aligning the to-be-welded rails so as to define an inter-rail gap between the rail ends; providing a mould assembly comprising - two upright side moulds that are fitted on either side of the inter-rail gap, each side mould including at least one elongate, open-ended down gate extending between a top part and a bottom part of the side mould and terminating in a down gate well;

a pouring basin for receiving liquid metal in at least one of the side moulds and arranged in flow communication with the down gate; and

pouring liquid metal into the pouring basin such that the liquid metal is transported from the pouring basin, down the down gate of the side mould and into the base mould to form a rail foot, from where the liquid metal rises into the inter-rail gap to form the web and rail head of the rail weld.

The pouring basin may be a refractory bowl which is characterised therein that it includes a level base, a bottom diverting slot which is aligned with the down gate, and an elongate, raised lip formation positioned between the level base and the bottom diverting slot. The pouring basin may be positioned in the side mould such that the diverting slot is arranged directly above the down gate of the side mould. The process may comprise the step of pouring liquid metal from an overhead crucible, which may be positioned above the side mould and off-centre from the rail head, onto the level base of the pouring basin, thus forming a head of liquid metal in the pouring basin. As soon as the liquid metal head rises above the raised lip formation, the liquid metal may flow over the lip formation, through the bottom diverting slot and down the down gate into the spherical down gate well, which is located directly below the down gate of the side mould. Upon entering the spherical down gate well, velocity of the liquid metal is reduced, thus allowing the liquid metal to enter the inter-rail gap with reduced velocity and little or no turbulence.

The base mould includes a substantially central rail foot well in which liquid metal enters the base mould underneath a rail end to form the rail foot; at least one down gate well laterally disposed relative to the rail foot well; side mould engaging means extending upwardly from the base mould and dimensioned to engage complimentarily dimensioned base mould engaging means on the side moulds for supporting the side moulds; and two parallel rail end supporting walls on which the rail ends of to-be-welded rails sit during casting, the rail end supporting walls bordering the rail foot well on either side thereof such that they determine the depth of the rail foot well, the base mould being characterised therein that the rail foot well has a critical depth of between 2mm and 10mm, and preferably a depth of between 5mm and 7mm; and the rail end supporting walls are tapered down to the rail foot well, preferably through a curved surface of predetermined radius.

In a preferred embodiment of the invention, the base mould includes two spherical down gate wells located in the lateral opposite ends of the base mould and positioned directly below the down gates of the side moulds. Each down gate well is dimensioned to receive liquid metal from the down gate in the side mould, so as to reduce the velocity of the liquid metal and in turn to direct the stream of liquid metal towards the rail foot well in such a way that it reduces turbulence of the liquid metal. The inter-rail gap is filled from the bottom upwards, through liquid metal that enters the rail foot well and rises not only to fill the inter- rail gap, but also the down gate of the opposite side mould.

The base mould further includes a fracture rib between the rail foot well and each of the down gate wells, which allows, after solidification and at black heat, for the down gates and down gate wells to be removed with an appropriate tool so as not to damage the weld.

The process includes the steps of - preheating the enclosed mould cavity and rail ends with a preheating burner;

inserting the pouring basin in the top part of one of the two side moulds such that the bottom diverting slot is aligned with the down gate in the side mould;

pouring liquid metal onto the level base of the pouring basin such that a head of liquid metal is first created in the pouring basin upon immediate first casting of the liquid metal into the pouring basin, until the head of liquid metal rises above and flows over the raised lip formation and enters the down gate of a side mould through the bottom diverting slot in the pouring basin, from where the liquid metal flows into the spherical down-gate well and then into the rail foot well of the base mould to form a rail foot, before the liquid metal rises upwardly from the rail foot well into the inter-rail gap to form the web and head of the rail weld. After solidification and when the liquid metal in the inter-rail gap has reached a black heat, the mould assembly and excess metal is removed.

The process includes the further step of sealing the side moulds and base mould where they engage each other and the rails by firmly placing luting putty within luting grooves defined between the side moulds, base mould and the rail ends.

The side moulds are further characterised therein that they define a head rising room in an upper part of the side moulds for receiving liquid metal rising from the inter-rail gap and the down gates.

SPECIFIC EMBODIMENT OF THE INVENTION

Without wishing to be bound by it, the invention will now further be described and illustrated with reference to the following non-limiting examples only and the attached FIGURES, wherein -

FIGURE 1 is an exploded perspective view of a mould assembly according to the invention;

FIGURE 2 is an assembled perspective view of the mould assembly of FIGURE 1 ;

FIGURE 3 is the base mould only of the mould assembly of FIGURE 1 ;

FIGURE 4 is a cross-sectional view of the base mould of FIGURE 3;

FIGURE 5 is a cross-sectional view of the base mould assembly of FIGURE 2.

The invention relates to a new in situ fusion welding process for welding adjacent rail ends (not shown) to form a substantially continuous railway track, and to a mould assembly [10] for use in such a process. The process consist of a single process step and is aimed at introducing liquid metal into an inter-rail gap [42] between two neighbouring to-be-welded rail ends in a controlled manner and with the least amount of turbulence and velocity, so as to concentrate alloying additives in a weld head and to minimize casting defects.

A mould assembly according to the invention is designated by reference numeral [10]. The mould assembly [10] comprises two upright side moulds [12] that are fitted on either side of an inter-rail gap [42] defined between the neighbouring rail ends. Each side mould [12] includes at least one elongate, open-ended down gate [14] extending between a top part and a bottom part of the side mould [12] and terminating in a down gate well [16]. The down gate [14] is tapered down between the top part and bottom part of the side mould [12] such that the radius of the down gate [14] approximate the top part of the side mould [12] is approximately 1 .5 times that of the radius of the down gate [14] approximate a bottom part of the side mould [12] where it meets the down gate well [16].

The mould assembly also comprises a base mould [18] which is adapted for receiving the rail ends (not shown) and for supporting the side moulds [12] on either side of the inter-rail gap [42], the arrangement being such that the base mould [18] and side moulds [12] cooperate to define an enclosed mould cavity for receiving cast liquid metal.

The mould assembly [10] further comprises a pouring basin [20] for receiving liquid metal poured into at least one of the side moulds [12], the pouring basin [20] being arranged in flow communication with the down gate [14]. The pouring basin [20] is a refractory or ceramic bowl which is removably located inside the top part of a side mould [12]. The pouring basin [20] is characterised therein that it includes a level base [22], a bottom diverting slot [24] which is aligned with the down gate [14], and an elongate and raised lip formation [26] which is positioned between the level base [22] and the bottom diverting slot [24]. The elongate lip formation [26] is raised to extend above the level base [22] for preventing immediate flow of first cast liquid metal from the pouring basin [20] into the down gate [14]. In particular, the arrangement is such that a head of liquid metal is first created in the pouring basin [20] upon immediate first casting of liquid metal into the pouring basin [20], after which the liquid metal only starts running down the down gate [14] after the head of liquid metal rises above and over the raised lip formation [26]. The applicant believes that this modification to the pouring basin [20] reduces flow speed of the liquid metal into the down gate [14], which in turn reduces turbulence in the down gate well [16]. The mould assembly [10] is further characterised therein that the down gate well [16] of the base mould [18] has a half-spherical well floor [28]. In prior art arrangements, such as in WO 201 1/013078, the down gate well [16] typically has a planar well floor. Liquid metal enters the down gate [14] with a high velocity turbulent force and hits the planar floor of the down gate well at an approximately 90° angle. The turbulent force splashes the liquid metal upward, forming small liquid metal droplets, resulting in circumferential surface oxidation of the liquid metal droplets. This causes solid, oxidised inclusions in a final cast product. By modifying the down gate well [16] to have a spherical well floor [28], turbulence and splashing of the liquid metal as it hits the spherical well floor [28] of the down gate well [16] is reduced, with a resultant reduction in oxidised inclusions in the final cast product.

Each side mould [12] includes downwardly depending base mould engaging means [30] extending from the base of the side mould [12] and dimensioned to engage complimentarily dimensioned side mould engaging means [32], which extend upwardly from the base mould [18], when the side mould [12] sits on top of the base mould [18]. When the side moulds [12] sit on top of the base mould [18], the base of each side mould [12] is level with the top of a rail foot. The base mould [18] includes a substantially central rail foot well [34] in which liquid metal enters the base mould [18] underneath a rail end to form the rail foot; the at least one spherical down gate well [16] laterally disposed relative to the rail foot well [34]; side mould engaging means [32] extending upwardly from the base mould [18] and dimensioned to engage complimentarily dimensioned base mould engaging means [30] on the side moulds [12] for supporting the side moulds [12]; and two parallel rail end supporting walls [36] on which the rail ends of to-be-welded rails sit during casting, the rail end supporting walls [36] bordering the rail foot well [34] on either side thereof such that they determine the depth of the rail foot well [34]. The base mould [18] is characterised therein that the rail foot well [34] has critical depth [X] of between 2mm and 10mm, and preferably a depth of between 5mm and 7mm. In other words, the rail end supporting walls [36] have a critical height of between 2mm and 10mm, and preferably between 5mm and 7mm above the rail foot well [34]. Moreover, the rail end supporting walls [36] taper into the rail foot well [34] at an angle, and preferably at a predetermined radius [35] (refer FIGURE 3).

As hot liquid metal enters the rail foot well [34] during casting, the liquid metal is required not only to flow into the rail foot well [34] underneath a rail end, but also to fuse with the rail end so as to form a strong and solid inter-rail weld. However, as the hot liquid metal, which is typically cast at around 2000°C, enters the rail foot well [34], it is brought into contact with a significantly cooler rail end, which can seldom be heated to above 600°C to 800°C. This temperature gradient between the cast liquid metal entering the rail foot well [34], and the rail end inevitably reduces the temperature of the liquid metal as it flows into the rail foot well [34] underneath the rail end. The applicant has established that if the rail foot well [34] is less than 2mm deep, the cooling effect of the rail end on the hot liquid metal in the rail foot well [34] is so significant that the liquid metal and rail end do not fuse properly, causing cluster porosity in the final cast product, which causes stresses and induces fatigue failures in use. The side mould engaging means [32] is positioned on lateral opposite ends of the base mould [18], which ends extend beyond the rail foot such that the side moulds [12] sit on top of the lateral opposite ends of the base mould [18]. The base mould [18] further includes a fracture rib [38] between the rail foot well [34] and each of the down gate wells [16]. The fracture rib [38] is characterised therein that it tapers into the rail foot well [34] at an angle [37], and preferably at a predetermined radius (refer FIGURE 5). The side moulds [12] further define a head rising room [40] in an upper part of the side moulds [12] for receiving liquid metal rising from the inter-rail gap [42] and the down gates [14].

The process for fusion welding rail ends of two adjacent rails according to the invention comprises the steps of aligning neighbouring to-be-welded rails so as to define an inter-rail gap [42] between the rail ends; providing a mould assembly [10] as hereinbefore described; and pouring liquid metal into the pouring basin [20] such that the liquid metal is transported from the pouring basin [20], down the down gate [14] of the side mould [12] and into the base mould [18] to form a rail foot, from where the liquid metal rises into the inter-rail gap [42] to form the web and rail head of the rail weld.

The process comprises the further steps of pouring liquid metal from an overhead crucible (not shown), which is positioned above the side mould [12] and off-centre from the rail head, onto the level base [22] of the pouring basin [20], thus forming a head of liquid metal in the pouring basin [20]. As soon as the liquid metal head rises above the raised lip formation [26], the liquid metal flows over the lip formation [26], through the bottom diverting slot [24] and down the down gate [14] into the spherical down gate well [16], which is located directly below the down gate [14] of the side mould [12]. Upon entering the spherical down gate well [16], velocity of the liquid metal is reduced, thus allowing the liquid metal to enter the inter- rail gap [42] with reduced velocity and little or no turbulence.

The mould assembly [10] is characterised therein that when the side moulds [12] sit on top of the base mould [18], the base of each side mould [12] is level with the top of the rail foot. This differs from prior art mould assemblies where the bases of the side moulds [12] are usually level with the bottom of the rail foot. The applicant has found that with the new arrangement, improved close fitting is achieved when the two side moulds [12] are clamped together against the rails.

The process includes the additional steps of preheating the enclosed mould cavity and rail ends with a preheating burner; inserting the pouring basin [20] in the top part of one of the two side moulds [12] such that the bottom diverting slot [24] is aligned with the down gate [14] in the side mould [12]; pouring liquid metal into the pouring basin [20] such that the liquid metal is transported from the pouring basin [20], down the down gate [14] of the side mould [12], into the spherical down gate well [16] and then into the rail foot well [34] of the base mould [18] to form a rail foot, from where the liquid metal rises upwardly from the rail foot well [34] into the inter-rail gap [42] to form the web and head of the rail weld. After solidification and when the liquid metal in the inter-rail gap [42] has reached a black heat, the mould assembly [10] and excess metal is removed.

The process includes the further step of sealing the side moulds [12] and base mould [18] where they engage each other and the rails by firmly placing luting putty within luting grooves defined between the side moulds [12], base mould [18] and the rail ends.

The side moulds [12] define a head rising room in an upper part of the side moulds [12] for receiving molten material rising from the inter-rail gap [42] and the down gates [14]. In use, the overhead crucible, containing an aluminothermic welding mixture, is positioned above the top of at least one side mould [12]. Once all the mould parts have been positioned and clamped around the inter-rail gap [42] between the two rails to be welded, and the pouring basin [20] has been inserted into position, the aluminothermic welding mixture is ignited. After expiration of the appropriate reaction time, liquid metal is released from the crucible and directed into the refractory pouring basin [20]. All the liquid metal falling into the pouring basin [20] is discharged through the bottom diverting slot [24] into the down gate [14] of the side mould [12]. The metal flow is received in the spherical bottom down gate well [16] of the base mould [18] and is diverted into the inter-rail gap [42], filling the inter-rail gap [42] from the rail foot well [34] upwards and rising to the rail head and further upwards to fill the head rising room.

Alumina slag, which also forms during the reaction of the aluminothermic welding mixture in the crucible, is discharged from the crucible directly after the discharge of the molten metal. As a result of the lower specific gravity of the alumina slag, liquid alumina slag floats on top of the molten metal after the inter-rail gap [42] and down gates [14] have been filled.

The side moulds [12] are so constructed to contain all the alumina slag in the head rising room and on top of the molten metal. This alumina slag acts as an insulator and assists in keeping the top head riser section liquidous for a longer period, thus facilitating progressive solidification to take place from thinner sections in the rail foot through the rail web area and lastly in the thicker rail head. The invention addresses the disadvantages associated with the prior art as detailed in paragraphs (i) - (iv) ("Background to the Invention") above.

(a) Pouring basin [201 reduces velocity and temperature The pouring basin [20] absorbs the brunt of the velocity and temperature of the liquid cast metal and controls the flow of liquid metal into the down gate [14]. The controlled flow results in significantly reduced splashing and turbulence in the liquid metal during the casting process. Any turbulence that may occur presents in the pouring basin [20] and then the down gates [14], and not inside the mould cavity, resulting in no splashing whatsoever of liquid metal inside the mould cavity.

Figure (iii) hereunder is a comparison of the velocity with which liquid metal enters the mould cavity using center-pouring (prior art) and side-pouring (invention) processes respectively.

Side-pouring

Centre-pouring

Figure (iii): Velocity profiles: center-pouring vs side-pouring of inter-rail weld casts

Down gate well absorbs turbulence for smooth flow into the mould cavity

Velocity and turbulence (if any) of the liquid cast metal is further absorbed within the down gate well [16] prior to entering the mould cavity. This allows the liquid metal within the down gate well [16] to turn on itself before entering the mould cavity, thus further increasing the smooth inflow of liquid metal into the mould cavity with no turbulence, no oxidised metal droplets, and no gas entrapment in the liquid metal inside the mould cavity. By the time the liquid metal enters the mould cavity it has a fraction of the velocity (and no turbulence) compared to the liquid metal velocity of a center-pouring process.

Air displaced upwardly in the mould cavity with no gas entrapment in the cast material

The mould cavity is filled from the side, with liquid metal being fed into the mould cavity from the down gate [14], after which the mould cavity is smoothly filled from the bottom upwards. Liquid metal smoothly rises upward in the mould cavity, gently pushing out the air, which is inside the mould cavity, upwardly and out of the mould cavity. With no turbulence of liquid metal inside the mould cavity, there is no gas entrapment, gas pores or wormhole formation inside the weld cast.

Figure (iv) hereunder is an X-ray of a rail weld cast which was poured using the side- pouring process according to the invention, from which it is clear that there are no gas pores or wormholes inside the weld cast, which has a homogenous, fine-grained structure. Compared to Figures (i) and (ii) above, the difference in the weld cast quality is unmistakable.

Pouring basin [201 remains intact

The pouring basin [20] does not get destroyed during the casting process and remains completely intact, thus eliminating the inclusion of impurities (e.g. sand particles) in the weld cast.

No stress raisers caused by riser fracturing

In the present invention the risers (cast metal inside the down gates [14] after the casting process) are located next to the weld foot. The risers are cut off with an angle grinder after solidification, leaving the weld foot intact with a smooth upper surface. This eliminates the problem encountered with stress raisers (refer par. iv above).

Ultrasonic testing of the weld foot possible

Moreover, by cutting off the risers, as provided for in the claimed mould assembly, it is now for the first time ever possible to conduct ultrasonic testing of the weld foot. Ultrasonic testing requires two clean cut, flat faces on either side of the foot, which was up to now not possible, because of the non-flat ends of a weld foot cast. Previously, ultrasonic testing of the rail weld head was possible, but the rail weld foot could only be tested radio graphically. Radio graphic testing requires a radioactive source on-site, specialised expertise, safely protocols and high costs. The present mould assembly and process now for the first time permits ultrasonic testing of the rail foot, which involves a small hand-held apparatus which can be run over the weld on-site to detect internal defects in the weld foot.

Entrapping slag inside the mould cavity to reduce cooling-down tempo

The mould assembly and process of the invention has optimised use of the slag resulting from the casting process. In prior art technologies, mould assemblies include slag pans which are located on either side of the moulds for receiving the slag. The slag pans and slag content are removed after the casting process. Volume shrinkage is a common result of cooling processes in metal casting technologies, in which a solid metal cast can have a volume of between 2% and 3% less than in the liquid state. The same applies to casting of inter-rail welds.

A further advantage of the mould assembly and side-pouring process of the invention, is that the slag is encased in a head rising room in an upper part of the mould assembly, thus providing thermal insulation to the cast liquid metal. This in turn aids in directional solidification of the cast metal - the weld cast solidifies from the bottom (foot) upwards, eliminating shrinkage in the final cast, while the risers serve as feeders. Entrapment of the slag within the mould cavity permits more controlled solidification behaviour in the present invention as compared to, for example US 2007/02721 14 and US 5,419,484 (discussed above) in that the cast metal is kept warmer for longer and cools down slower. A weld cast done according to a process of US 2007/02721 14 or US 5,419,484, solidifies in about 7 minutes after casting, whereas a weld cast according to the invention solidifies in about 9 - 10 minutes. This results in reduced thermal stresses inside the weld cast.

Side-pouring never done before in rail weld casting

Various center-pouring processes of rail welds have been around for the past 30 to 40 years. No-one has ever before thought of side-pouring the liquid metal into down gates first, as opposed to directly into the mould cavity. Rail welds that are cast by both the technologies disclosed in US 2007/02721 14 and US 5,419,484 result in a failure rate of up to 25%. By contrast, by using the process and mould assembly of the present invention, the applicant has managed to reduce the failure rate to less than 2% over a comparative period of time. Internationally this is the first major development in rail welding technology for the last 30 - 40 years.

It will be appreciated that alternative embodiments of the invention may be possible without departing from the spirit or scope of the invention as defined in the claims.

Claims

A mould assembly [10] for use in fusion welding rail ends of two adjacent rails, the mould assembly [10] comprising - two upright side moulds [12] that are fitted on either side of an inter-rail gap [42] defined between the rail ends, each side mould [12] including at least one elongate, open-ended down gate [14] extending between a top part and a bottom part of the side mould [12] and terminating in a down gate well [16];

a base mould [18] adapted for receiving the rail ends and for supporting the side moulds [12] on either side of the inter-rail gap [42]; the arrangement being such that the base mould [18] and side moulds [12] cooperate to define an enclosed mould cavity for receiving liquid metal; and

at least one pouring basin [20] for receiving liquid metal poured into at least one of the side moulds [12] and arranged in flow communication with the down gate [14], the pouring basin [20] being characterised therein that it includes a level base [22], a bottom diverting slot [24], which is aligned with the down gate [14], and an elongate, raised lip formation [26] positioned between the level base [22] and the bottom diverting slot [24].

The mould assembly [10] according to claim 1 wherein the elongate lip formation [26] is raised to extend above the level base [22] for preventing immediate flow of first cast liquid metal from the pouring basin [20] into the down gate [14], the arrangement being such that a head of liquid metal is first created within the pouring basin [20] upon immediate first casting of liquid metal into the pouring basin [20], after which the liquid metal only starts running down the down gate [14] after the head of liquid metal rises above and over the raised lip formation [26]. The mould assembly [10] according to anyone of claims 1 or 2 wherein the pouring basin [20] is a refractory or ceramic bowl which is removably located inside the top part of a side mould [12].

The mould assembly [10] according to claim 1 wherein each side mould [12] includes downwardly depending base mould engaging means [30] extending from the base of the side mould [12] and dimensioned to engage complimentarily dimensioned side mould engaging means [32], which extend upwardly from the base mould [18], when the side mould [12] sits on top of the base mould [18], the mould assembly [10] being characterised therein that when the side moulds [12] sit on top of the base mould [18], the base of each side mould [12] is level with the top of the rail foot.

The mould assembly [10] according to claim 1 wherein the base mould [18] includes a substantially central rail foot well [34] in which liquid metal enters the base mould [18] underneath a rail end to form the rail foot; side mould engaging means [32] extending upwardly from the base mould [18] and dimensioned to engage complimentarily dimensioned base mould engaging means [30] on the side moulds [12] for supporting the side moulds [12]; at least one down gate well [16] which is laterally disposed relative to the rail foot well [34]; and two parallel rail end supporting walls [36] on which the rail ends of to-be-welded rails sit during casting, the rail end supporting walls [36] bordering the rail foot well [34] on either side thereof such that they determine the depth of the rail foot well [34], the base mould [18] being characterised therein that the down gate well [16] has a half-spherical well floor [28], and the rail foot well [34] has a critical depth of between 2mm and 10mm.

The mould assembly [10] according to claim 5 wherein the rail foot well [34] has a critical depth of between 5mm and 7mm. The mould assembly [10] according to claim 5 wherein the base mould [18] includes a fracture rib [38] between the rail foot well [34] and each of the down gate wells [16], the fracture rib [38] being characterised therein that it tapers into the rail foot well [34] at an angle, and preferably at a radius [37].

The mould assembly [10] according to claim 5 wherein the rail end supporting walls [36] taper into the rail foot well [34] at an angle, and preferably at a radius [35].

The mould assembly [10] according to claim 1 wherein the down gate [14] is tapered down between the top part and bottom part of the side mould [12] such that the radius of the down gate [14] approximate the top part of the side mould [12] is approximately 1 .5 times that of the radius of the down gate [14] approximate a bottom part of the side mould [12] where it meets the down gate well [16].

The mould assembly [10] according to claim 1 wherein the side moulds [12] define a head rising room [40] in an upper part of the side moulds [12] for receiving liquid metal rising from the inter-rail gap [42] and the down gates [14].

at least one pouring basin [20] for receiving liquid metal poured into at least one of the side moulds [12] and arranged in flow communication with the down gate [14]; and a base mould [18] adapted for receiving the rail ends and for supporting the side moulds [12] on either side of the inter-rail gap [42] such that the base mould [18] and side moulds [12] cooperate to define an enclosed mould cavity for receiving liquid metal;

the base mould [18] including a substantially central rail foot well [34] in which liquid metal enters the base mould [18] underneath a rail end to form the rail foot; at least one down gate well [16] laterally disposed relative to the rail foot well [34]; side mould engaging means [32] extending upwardly from the base mould [18] and dimensioned to engage complimentarily dimensioned base mould engaging means [30] on the side moulds [12] for supporting the side moulds [12]; and two parallel rail end supporting walls [36] on which the rail ends of to-be-welded rails sit during casting, the rail end supporting walls [36] bordering the rail foot well [34] on either side thereof such that they determine the depth of the rail foot well [34], the base mould [18] being characterised therein that the rail foot well [34] has critical depth of between 2mm and 10mm.

12. The mould assembly [10] according to claim 1 1 wherein the rail foot well [34] has critical depth of between 5mm and 7mm.

The mould assembly [10] according to claim 1 1 wherein the rail end supporting walls [36] taper into the rail foot well [34] at an angle, and preferably at a radius [35].

A mould assembly [10] for use in fusion welding rail ends of two adjacent rails, the mould assembly [10] comprising - two upright side moulds [12] that are fitted on either side of an inter-rail gap [42] defined between the rail ends, each side mould [12] including at least one elongate, open-ended down gate [14] extending between a top part and a bottom part of the side mould [12] and terminating in a down gate well [16]; at least one pouring basin [20] for receiving liquid metal poured into at least one of the side moulds [12] and arranged in flow communication with the down gate [14]; and

a base mould [18] adapted for receiving the rail ends and for supporting the side moulds [12] on either side of the inter-rail gap [42] such that the base mould [18] and side moulds [12] cooperate to define an enclosed mould cavity for receiving cast liquid metal,

the base mould [18] including a substantially central rail foot well [34] in which liquid metal enters the base mould [18] underneath a rail end to form the rail foot; at least one down gate well [16] laterally disposed relative to the rail foot well [34]; side mould engaging means [32] extending upwardly from the base mould [18] and dimensioned to engage complimentarily dimensioned base mould engaging means [30] on the side moulds [12] for supporting the side moulds [12]; and two parallel rail end supporting walls [36] on which the rail ends of to-be-welded rails sit during casting, the base mould [18] being characterised therein that the down gate well [16] has a half-spherical well floor [28].

15. A process for fusion welding rail ends of two adjacent rails, the process comprising the steps of - aligning the to-be-welded rails so as to define an inter-rail gap [42] between rail ends;

providing a mould assembly [10] comprising - two upright side moulds [12] that are fitted on either side of the inter- rail gap [42], each side mould [12] including at least one elongate, open- ended down gate [14] extending between a top part and a bottom part of the side mould [12] and terminating in a down gate well [16];

a base mould [18] adapted for receiving the rail ends and for supporting the side moulds [12] on either side of the inter-rail gap [42]; the arrangement being such that the base mould [18] and side moulds [12] cooperate to define an enclosed mould cavity for receiving liquid metal; and a pouring basin [20] for receiving liquid metal in at least one of the side moulds [12], the pouring basin [20] being a refractory bowl which is characterised therein that it includes a level base [22], a bottom diverting slot [24] which is aligned with the down gate [14], and an elongate, raised lip formation [26] positioned between the level base [22] and the bottom diverting slot [24], the pouring basin [20] being positioned in the side mould [12] such that the diverting slot [24] is arranged directly above the down gate of the side mould [12]; and

pouring liquid metal into the pouring basin [20] such that the liquid metal is transported from the pouring basin [20], down the down gate of the side mould [12] and into the base mould [18] to form a rail foot, from where the liquid metal rises into the inter-rail gap [42] to form the web and rail head of the rail weld.

6. The process according to claim 15 wherein the liquid metal is cast into the pouring basin [20] from an overhead crucible positioned above the side mould [12] and off- centre from the rail head, the liquid metal being cast onto the level base [22] of the pouring basin [20] in order to form a head of liquid metal in the pouring basin [20], the arrangement being such that as the liquid metal head rises above the raised lip formation [26], the liquid metal flows over the lip formation [26], through the bottom diverting slot [24] and down the down gate [14] into the down gate well [16], which is located directly below the down gate [14] of the side mould [12].

The process according to claim 15 wherein the base mould [18] includes a substantially central rail foot well [34] in which liquid metal enters the base mould [18] underneath a rail end to form the rail foot; at least one down gate well [16] laterally disposed relative to the rail foot well [34]; side mould engaging means [32] extending upwardly from the base mould [18] and dimensioned to engage complimentarily dimensioned base mould engaging means [30] on the side moulds [12] for supporting the side moulds [12]; and two parallel rail end supporting walls [36] on which the rail ends of to-be-welded rails sit during casting, the rail end supporting walls [36] bordering the rail foot well [34] on either side thereof such that they determine the depth of the rail foot well [34], the base mould [18] being characterised therein that the rail foot well [34] has a critical depth of between 2mm and 10mm, and preferably a depth of between 5mm and 7mm.

18. The process according to anyone of claims 15 or 17 wherein the base mould [18] includes two spherical down gate wells [16] located in the lateral opposite ends of the base mould [18] and positioned directly below the down gates [14] of the side moulds [12], each down gate well [16] being dimensioned to receive liquid metal that is poured through the down gate [14] in the side mould [12], so as to reduce the velocity of the liquid metal and in turn to direct the stream of liquid metal towards the rail foot well [34] in such a way that it reduces turbulence of the liquid metal, the process being characterised therein that the inter-rail gap [42] is filled from the bottom upwards, through liquid metal that enters the rail foot well [34] and rises not only to fill the inter-rail gap, but also the down gate of the opposite side mould [12].

19. The process according to claim 15 wherein the base mould [18] includes a fracture rib [38] between the rail foot well [34] and each of the down gate wells [16], which allows, after solidification of the liquid metal cast and at black heat, for the down gates [14] and down gate wells [16] to be removed with an appropriate tool so as not to damage the inter-rail weld cast, the fracture rib [38] being characterised therein that it tapers into the rail foot well [34] at an angle, and preferably at a radius [37].

20. The process according to anyone of claims 15 or 17 including the steps of - sealing the side moulds [12] and base mould [18] where they engage each other and the rails by firmly placing luting putty within luting grooves defined between the side moulds [12], base mould [18] and the rail ends;

preheating the enclosed mould cavity and rail ends with a preheating burner; inserting the pouring basin [20] in the top part of one of the two side moulds [12] such that the bottom diverting slot [24] of the pouring basin [20] is aligned with the down gate [14] in the side mould [12];

pouring liquid metal onto the level base [22] of the pouring basin [20] such that a head of liquid metal is first created in the pouring basin [20] upon immediate first casting of the liquid metal into the pouring basin [20], until the head of liquid metal rises above and flows over the raised lip formation [26] and enters the down gate of the side mould [12] through the bottom diverting slot [24] in the pouring basin [20], from where the liquid metal flows into the spherical down-gate well [16] and then into the rail foot well [34] of the base mould [18] to form a rail foot, before the liquid metal rises upwardly from the rail foot well [34] into the inter-rail gap [42] to form the web and head of the rail weld;

allowing the liquid cast metal in the inter-rail gap [42] to reach black heat; and removing the mould assembly [10] and excess metal from the weld cast.

21 . A mould assembly [10] for use in fusion welding rail ends of two adjacent rails substantially as herein illustrated and exemplified with reference to the accompanying Figures 1 to 5.

22. A process for fusion welding rail ends of two adjacent rails substantially as herein illustrated and exemplified with reference to the accompanying Figures 1 to 5.