GB2317356A

GB2317356A - Coating welded steel

Info

Publication number: GB2317356A
Application number: GB9618909A
Authority: GB
Inventors: Leslie Callow; Jane Lomas; Terence Brian Lemmon
Original assignee: Courtaulds Coatings Ltd
Current assignee: International Paint Ltd
Priority date: 1996-09-10
Filing date: 1996-09-10
Publication date: 1998-03-25
Also published as: GB9618909D0

Abstract

A movable carrier 10 bearing an arc welding torch 14, for welding steel plate, and a trailing applicator 21 from which a fusible or thermoset polymer 22 is applied to the weld 20, the residual heat from the weld 20 being sufficient to melt and/or thermoset the polymer 22 to form a coating 32. Means, such as a worm drive 28, adjust the distance between the welding torch 14 and the applicator 21. The polymer 22 may be in the form of a powder, a tape or a solid block. In the case of a block of polymer the applicator may comprise means, such as compressed gas, to urge the block against the weld. The block may be shaped to match the weld joint and/or preheated. Two different polymer coating may be applied, the lower one having better adhesion to the steel and the upper one having better corrosion barrier properties.

Description

Coatinq Welded Steel This invention relates to a process and apparatus for coating welded steel and in particular for coating welded heavy-duty steel plate of the kind used in shipbuilding and other large-scale construction. Such steel plate is typically 3 to 50 mm thick.

During the shipbuilding process steel plates and crosspieces and stiffening members are welded together into panels and blocks of increasing size and eventually into a whole ship, using welds such as butt welds to join two plates and fillet welds to join a stiffening member or crosspiece at right angles to a steel plate. The welding process used for heavy-duty steel plate is usually arc welding. There are several types of arc welding such as metal inert gas (MIG) welding (also known as gas-metal arc welding), metal active gas (MAG) welding, submerged arc welding (SAW), self- shielded metal arc welding (also known as stick-electrode welding), tungsten inert gas (TIG) welding (also known as gas-tungsten arc welding), plasma arc welding and flux-cored arc welding. These various types of welding are described in the article "Welding" by K.F. Graff in Volume 24 of "Kirk-Othmer Encyclopaedia of Chemical Technology", 3rd Edition, published by Wiley-Interscience, especially at pages 502 to 505. As far as possible, the welding is carried out by automatic welding machines in which the welding torch is mounted on a movable carrier operated by remote control.

Steel plate which is to be used in shipbuilding or other heavy construction work is generally pre-coated with a "shop primer" coating which is an anticorrosive coating primarily intended to protect the steel during storage before use or when it is in a part-finished construction.

Advantageously, the shop primer is left on the steel as a primary anti-corrosive coating in the finished construction.

The shop primer is preferably weldable, that is to say it does not emit noxious fumes during welding or generate damaging porosity in the weld, so that it does not need to be removed from areas to be welded before welding. However, if the shop primer is not removed before welding it is certainly burnt away by the welding process, so that the welded steel areas are areas of bare metal which will potentially corrode while the other areas of the steel remain coated by shop primer. The welds need to be coated to prevent corrosion and usually need surface preparation before coating to ensure good adhesion of the coating. This is highly labour-intensive; a major proportion of the manhours, and hence cost, involved in coating a new ship comprises coating the welds. There is a need to reduce the labour involved in coating welds.

JP-B-51-7696 describes a protective coating for welded steelwork in which an adhesive-coated plastics tape containing an anticorrosive agent is applied to the welded area of steelwork and overpainted with chlorinated rubber paint. JP-A-59-156471 describes a process for applying a rust-preventing material to a seam-welded or spot-welded part in which a thermofusing powder rust-preventing material is applied to the welded part having residual heat from the welding, using a nozzle which follows in linkage with the welding roller electrode. JP-A-59-230671 describes corrosion protection of a seam-welded or spot-welded zone by feeding thermoplastic resin to the surface of the weld zone immediately after welding to form a protective film. EP-A309597 describes protecting the area of weld between two steel pipes from corrosion by heating, coating with epoxy resin, heating again and applying heat-shrink film coated with hot-melt adhesive.

EP-A-494672 and EP-A-539941 describe a process in which a metal strip is continuously coated with a thermoset resinbased paint by passing it through a pre-heating furnace wherein it is raised to a pre-heat temperature of from 100 to 2104C, then through a coating applicator wherein a solid, substantially solvent-free block of the paint composition is pressed against the strip at a substantially constant pressure of from 10 to 100 kilopascals to cause a liquid layer of the paint composition to be deposited on the strip, then through a roll-type smoothing device to smooth and spread the layer and doctor it to thickness, then through a curing furnace to thermoset the layer, and finally through a quenching bath.

According to one aspect of the present invention a process for welding steel plate by arc welding in which the welding torch is mounted on a movable carrier is characterised in that a fusible polymer is applied to the welded metal from an applicator mounted on the said carrier at a controlled distance behind the welding torch, said distance being such that the fusible polymer is melted by the residual heat of the welded metal and fuses to form a coating on the welded metal.

The welded metal becomes hot at either side of the weld so that the fusible polymer can be melted by the welded metal to form a coating extending up to 5 cm, preferably about 3 cm, either side of the weld line.

An apparatus according to the invention for welding steel plates comprises a welding torch for arc welding mounted on a movable carrier and is characterised in that an applicator is mounted on the carrier so as to follow the welding head along the weld line when the carrier is moved, the applicator being designed to apply a fusible polymer carried on the said carrier to the welded metal along the weld line.

In a preferred process according to the invention, the fusible polymer is in the form of a solid block of fusible polymer. Thus according to another aspect of the invention a process for coating a welded steel construction by applying a fusible polymer to the weld whilst it has residual heat from the welding process is characterised in that a solid block of fusible polymer moving along the weld line behind the welding head is urged against the welded metal so that the surface of the fusible polymer block is continuously melted by the heat of the welded metal, thereby applying a coating of the polymer to the welded metal.

According to another aspect of the invention, a process for welding steel plate by arc welding in which the welding torch is mounted on a movable carrier is characterised in that a thermosetting polymer composition is applied to the welded metal from an applicator mounted on the said carrier at a controlled distance behind the welding torch, said distance being such that the thermosetting polymer forms a coating on the welded metal and is cured by the residual heat of the welded metal.

The movable carrier used for automatic welding is generally a trolley having four wheels (arranged to move alongside the welding path). The trolley can be selfpropelled or manually operated. The movable carrier can alternatively be a robotised arm or an arm mounted on a gantry. The welding carried out by an automatic welding machine using a movable carrier is usually arranged to be downhand welding on a substantially horizontal joint, if possible, although the invention can also be applied to welding of a substantially vertical joint, which is generally carried out by moving the welding apparatus upwards along the joint.

The fusible polymer can be thermoplastic or thermosetting. The softening temperature of the polymer is generally below 300 C. For a thermoplastic polymer the softening temperature is preferably above 60 C to ensure that the polymer solidifies to a hard coating on cooling, and a thermosetting polymer also preferably has a softening temperature above 60 C. Most preferably, the polymer has a softening temperature in the range 100 to 250'C. If the polymer is thermosetting, it preferably has a curing temperature in the range 100 to 250'C. The composition applied to the welded metal can consist of the fusible polymer alone, and it preferably is a clear transparent composition, but it can contain additives such as a curing agent, plasticiser, pigment, filler, corrosion inhibitor, flow aid, adhesion modifying agent and/or wetting agent.

The composition after hardening should preferably be tough rather than brittle, so that it can withstand a light grit blast (a "light sweep") and can also withstand being walked on. The composition after hardening should preferably have a Ball indentation hardness of at least 80, equivalent to about 55 on the Rockwell hardness M scale, so that grit will not stick in it. The polymer composition after hardening should preferably have a glass transition temperature (Tg) of at least 40 C.

Examples of suitable fusible thermoplastic polymers are polyolefins such as polyethylene or polypropylene, ethylene/ vinyl acetate copolymers, polystyrene and styrene copolymers, vinyl polymers such as polyvinyl chloride, acrylic polymers such as a homopolymer or copolymer of one or more alkyl acrylates and/or alkyl methacrylates, and phenoxy resins. Such addition polymers may include minor amounts, for example 1 to 5% by weight, of an adhesionpromoting monomer such as maleic anhydride or an epoxycontaining monomer, for example glycidyl acrylate or methacrylate, or a hydrolysable silane-functional monomer such as gamma-methacryloxy trimethoxy silane, which can be incorporated by copolymerisation or by grafting. Further examples of suitable fusible thermoplastic polymers are polycarbonates, polyamides, polyurethanes and polyesters.

Examples of suitable fusible thermosetting polymers are a carboxy-functional polyester or acrylic polymer used with an epoxide-functional curing agent such as a polyepoxide or epoxy resin or with a beta-hydroxyalkyl amide or amino resin curing agent, a hydroxy-functional polyester or acrylic polymer used with a blocked polyisocyanate curing agent or an epoxy resin used with an acid-functional curing agent or a dicyandiamide curing agent.

When the fusible polymer is used in the form of a solid block, the solid fusible polymer block can be in the form of a stick trailing from the carrier on which the welding torch is mounted. For downhand welding, the solid polymer block can be gravity-fed so that the weight of the block acts to urge it against the welded metal surface. The solid polymer block can alternatively be urged against the welded metal by a compressed gas, for example compressed air or welding gas, or by spring action. The use of compressed air to urge the polymer block against the welded metal surface is most preferred, since the air pressure can readily be controlled.

The solid fusible polymer block can be shaped to match the welded joint. This may be particularly advantageous when coating a fillet weld in which a steel plate is welded to a stiffening member or another steel plate which is substantially perpendicular to it; the tip of the block can be V-shaped in which the angle of the V is about 90'.

The solid fusible polymer block can be formed of a single fusible polymer composition or can comprise at least two layers of different fusible polymers arranged nearer to and further from the welding head. Alternatively, at least two solid blocks, each of a different fusible polymer, can be mounted on the movable carrier along the weld line behind the welding head. In either case where two different fusible polymers are used the fusible polymer nearer to the welding head, which will be the first polymer applied to the welded metal surface, is preferably chosen to have a better adhesion to the welded metal, and the fusible polymer further from the welding head is chosen to have better corrosion barrier properties. The fusible polymer further from the welding head can for example be less permeable to air and moisture than the polymer nearer to the welding head. The fusible polymer further from the welding head is preferably also readily overcoatable by top coat paints.

Either or both polymers may contain an anticorrosive filler.

If the solid fusible polymer block has three or more layers the layer furthest from the welding head, which becomes the outer layer when applied, can for example be a polymer composition designed to be overcoatable and to adhere well to the next coating to be applied to the steel structure and/or can be a UV-resistant polymer composition.

The fusible polymer for use as a solid block can be any of the thermoplastic polymers described above. It may be particularly preferred to use a blend of thermoplastic polymers of similar chemical composition but of different Tgs. Alternatively, the fusible polymer can for example be a thermosetting polymer composition which comprises a blend of polymers having different Tgs as described in the Examples of EP-A-539941. The difference between the Tgs can for example be up to 100 C and is preferably at least 25 C.

The tip of the solid fusible polymer block may need preheating before the start of a welding run. The apparatus for mounting the polymer block on the carrier can have an electrical-resistance heating element arranged near the tip of the block, or it can include a radio-frequency heater for heating the tip of the block. Alternatively, the tip of the polymer block can be preheated by contact with a hot metal plate which may be, but need not be, mounted on the carrier.

The degree of preheating is preferably sufficient to heat the tip of the block to about or just below its softening temperature (say up to 20"C above or below). Preheating, for example by electrical-resistance or radio-frequency heating, to partially soften the tip of the polymer block can be continued during application of the coating, if desired.

The coating formed after melting of the solid fusible polymer block and hardening on the welded metal surface is generally smooth and clear and adheres well to the welded metal. The rate of application of the fusible polymer to the welded metal is preferably sufficient to form a coating 0.2 to 2.0 mm thick, for example about 1 mm thick, and 1-10 cm wide, for example extending about 3 cm either side of the weld. If desired, the surface of the coating can be roughened before application of the next coating to the steel structure. In practice, a light grit sweep is usually carried out after welding and this may give a slight roughening of the surface. Alternatively, the surface of the applied polymer can be embossed with a pattern which improves adhesion, for example a roller wheel can be mounted on the carrier behind the solid polymer block to emboss a rough pattern on the polymer coating layer.

The fusible polymer can alternatively be used in the form of a tape. For example, a reel of tape of fusible polymer can be mounted on the carrier behind the welding torch. The tape may include a release backing layer which is separated as the tape is removed from the reel and which is taken up on a spool. The applicator for the tape generally includes means for urging the tape from the reel onto the welded metal, which can for example include a guide positioned to ensure that the tape follows the welding head and means for pressing the tape against the welded metal.

Such pressing means can for example be an air jet blowing the tape onto the welded metal or a suction head mounted ahead of the reel of tape and sucking the tape onto the welded metal. Alternatively, a mechanical device can press the tape onto the welded metal, for example a roller wheel can follow the tape guide.

The tape can be formed of a single layer of any of the fusible polymers listed above, or it can be a laminated tape, in which case the layer of tape contacting the welded metal is formed of fusible polymer. The other layer or layers of a laminated tape need not be of fusible polymer and can be chosen to give desired properties, for example it may be impermeable to air, moisture and ions to act as a barrier to corrosion or may be selected to be easily overcoatable with the next coating to be applied to the steel structure. The tape can contain a non-polymer layer if desired, for example it can contain a metal layer such as aluminium foil to act as a barrier against corrosion. The tape can for example be a 3-layer laminate comprising a layer of fusible polymer which acts as a hot-melt adhesive, a metal foil and an outer layer designed to be overcoatable.

As well as being based on a polymer which is easily overcoatable (acrylic ester polymers, for example, are readily overcoated by most paints used in marine constructions) the outer layer can have a textured surface to improve adhesion of subsequently applied coatings.

The fusible polymer can alternatively be in the form of a powder of particle size 15 to 250 microns; it can for example be a thermoplastic polymer powder or a composition in which the powder particles comprise a thermosetting polymer and a curing agent therefor which is activated by heat as used in powder coatings. The use of polymer powder is however not preferred for safety reasons; the proximity of a cloud of ignitable polymer dust to a welding torch is a safety hazard and potential explosion hazard. The applicator for a fusible polymer powder which is mounted on the carrier can be a spray gun of the type used for applying powder coatings or it can be a hopper, preferably a vibrating hopper, having holes in its lower surface through which the powder is sprinkled onto the welded metal. If fusible polymer powder is used, the carrier should preferably have a shield mounted between the welding torch and the powder applicator and a suction device mounted alongside the powder applicator so that the suction prevents flow of air from the polymer powder to the welding arc.

If the coating to be applied to the welded metal is thermosetting , it is not essential that it is in the form of a solid fusible polymer, although the use of a fusible solid polymer block is still the most preferred process.

The thermosetting polymer can alternatively be in the form of a paste or a highly viscous liquid thermosetting polymer composition. The thermosetting polymer composition applied to the welded metal forms a coating on the welded metal and is cured by the residual heat of the welded metal.

If the thermosetting polymer composition is not in solid form, it is usually preferred that the thermosettable polymer and the curing agent therefor are kept separate until being mixed just before application to the welded metal, to prevent premature curing. They can for example be stored in separate packages mounted on the carrier. In one preferred arrangement, the thermosettable polymer and the curing agent therefor are separately formed into viscous compositions which are mixed in equal volumes to produce the thermosetting polymer composition, and these viscous compositions are stored in parallel dispensers mounted on the carrier. The parallel dispensers are operated by a common plunger which acts to extrude the compositions from the dispersers; the plunger can for example be actuated by air pressure It is not essential that the viscous compositions are to be mixed in equal volumes; if they are not, the ratio of the cross-sectional areas of the parallel dispensers should be the volume ratio required for mixing the viscous compositions. As the two compositions are extruded, they are mixed by a static mixer. As an alternative to the parallel dispensers, two pressurised dispensers controlled to give the required ratio of volumes of thermosettable polymer composition and curing agent composition can feed a similar mixing apparatus through pipes.

The thermosetting polymer composition is preferably free from organic solvents, which are a safety hazard in the vicinity of a welding torch. The viscous liquid composition is preferably of the 100% solids or solventless type but can alternatively be a viscous paste containing a non-flammable solvent such as water. Examples of viscous thermosetting polymer compositions are epoxy resin compositions, for example using an amino-functional curing agent such as an amino-functional polyamide, polyurethane compositions, for example comprising a polyisocyanate prepolymer and a hydroxyl- or amino- functional polymer.

The distance on the carrier between the welding head and the applicator is chosen so that the temperature of the welded metal when contacted by the fusible polymer is sufficient to melt the polymer without degrading the polymer. The applicator can for example be mounted a fixed distance behind the welding head appropriate to the type of welding carried out by that welding head, or the position of the applicator can be adjustable and can be adjusted before the start of each welding run. In one preferred apparatus the applicator is movable on the carrier towards or away from the welding torch so that the fusible polymer can be arranged to contact welded metal of a desired temperature so that the polymer is fused and then hardened and/or cured without becoming too mobile or being heat-damaged. The applicator can for example be movable by a worm drive which can be manually operated or can be automatically controlled depending on the temperature of the welded metal at one or more measured positions behind the welding head. A device for measuring the temperature of the welded metal is preferably mounted on the carrier between the welding torch and the applicator. The temperature-measuring device can for example comprise a thermocouple mounted on a sliding shoe arranged to contact the welded metal.

The apparatus according to the invention may include a device for cleaning the weld before the fusible polymer or thermosetting polymer is applied, that is to say the device is mounted on the carrier behind the welding torch but in front of the applicator, and in front of the temperature measuring device if used. In types of arc welding using a consumable welding flux, such as shielded metal arc welding and flux-cored arc welding, a layer of slag is formed on top of the welded metal. The welding flux used is preferably one having low water-solubility and particularly a low level of water-soluble ionic materials, in which case it may remain on the welded metal surface. Alternatively, the slag can be removed, in which case the cleaning device for doing this can for example be a rotating wire brush. In other types of welding such as metal active gas welding a layer of dusty brown iron oxide may be formed on top of the welded metal. This can be removed by a mechanical cleaning device or by suction, although we have found that removal is not generally necessary since the fusible polymer forms a strongly adherent coating when the iron oxide is not removed. In submerged arc welding, the welding takes place under a blanket of granules of flux. A welding torch for submerged arc welding is usually followed by a suction device for removing any welding flux which has remained as loose granules. The apparatus of the invention may also include a cleaning device for removing the slag formed by fused flux granules, for example a rotating brush or a "snowplough" device shaped to cut through the fused flux and throw it to both sides out of the path of the fusible polymer applicator.

As stated above, the fusible polymer composition applied to the weld is preferably a clear polymer composition, in which case the weld can be visually inspected after it has been coated with the polymer composition. Alternatively, or additionally, the weld can be automatically visually inspected before the fusible or thermosetting polymer is applied. An imaging system can be mounted on the carrier and linked to an optical recognition system to detect weld defects. The imaging system can for example be a video camera. The camera may be used solely for recording the surface condition of the weld prior to coating or it may be used with suitable computer-based software to detect typical weld defects. Alternatively, a person may evaluate the recordings or computer-processed data for defects, either in real time or subsequently. When a weld defect is detected, the coating applicator may be automatically retracted to avoid coating the defective weld or the position of the weld defect may be automatically marked for later correction of the defect.

The invention is illustrated by the accompanying drawings, of which: Figure 1 is a diagrammatic plan view of a welding and coating apparatus according to the invention and Figure 2 is a side elevation of the apparatus of Figure 1.

The apparatus of Figures 1 and 2 comprises a trolley (10) having wheels (12, 13). A welding torch (14) is carried on a rigid arm (16) extending forwards from a mast (18) anchored in the trolley. The power supply, gas supply and supply of welding consumables extend from a remote source via the mast (18) and the arm (16) to the welding torch (14). The welding torch (14) extends sideways and downwards from the arm (16) so that the inner wheels (13) of the trolley (10) move alongside the weld (20) formed by the torch (14).

An applicator (21) for material for coating the weld (20) comprises a stick of fusible thermoplastic polymer (22) carried within a holder (24). The holder (24) is carried on an arm (26) mounted rigidly on a worm drive (28) which screws into or away from a housing (30) mounted on the trolley (10), whereby the applicator (21) can be moved towards and away from the welding torch (14). The stick of polymer (22) is carried loosely within the holder (24) and is free to fall under gravity as polymer at the tip (23) of the stick is melted by the heat of the welded metal to form a coating (32) on the weld (20) and adjoining parent metal, so that fusible polymer (22) is continuously presented to the welded metal.

Claims

1. A process for welding steel plate by arc welding in which the welding torch is mounted on a movable carrier, characterised in that a fusible polymer is applied to the welded metal from an applicator mounted on the said carrier at a controlled distance behind the welding torch, said distance being such that the fusible polymer is melted by the residual heat of the welded metal and fuses to form a coating on the welded metal.

2. A process according to claim 1, characterised in that the fusible polymer is applied to the welded metal in the form of a solid block of fusible polymer.

3. A process according to claim 1, characterised in that the fusible polymer is applied to the welded metal in powder form.

4. A process according to any of claims 1 to 3, characterised in that the rate of application of the fusible polymer to the welded metal is sufficient to form a coating 0.2 to 2 mm thick

5. A process according to claim 1, characterised in that the fusible polymer is applied to the welded metal in the form of a tape of fusible polymer.

6 A process according to claim 5, characterised in that the tape is a laminated tape in which the layer contacting the welded metal is formed of a fusible polymer.

7. Apparatus for welding steel plates comprising a welding torch for arc welding mounted on a movable carrier, characterised in that an applicator is mounted on the carrier so as to follow the welding head along the weld line when the carrier is moved, the applicator being designed to apply a fusible polymer carried on the said carrier to the welded metal along the weld line.

8. Apparatus according to claim 7, characterised in that the fusible polymer is in the form of a solid block and the applicator comprises means for urging a block of fusible polymer against the welded metal.

9. Apparatus according to claimS, characterised in that the means for urging the block of fusible polymer against the welded metal comprises a compressed gas applying pressure to the block of fusible polymer.

10. Apparatus according to claim 8 or claim 9, characterised in that heating means are arranged to heat the tip of the block of fusible polymer before it contacts the welded metal.

11. Apparatus according to claim 7, characterised in that the fusible polymer is in the form of a reel of tape of fusible polymer and the applicator includes means for urging tape from the reel onto the welded metal.

12. Apparatus according to any of claims 7 to 11, characterised in that the applicator is movable on the carrier towards or away from the welding torch.

13. Apparatus according to claim 12, characterised in that the means for moving the applicator comprises a worm drive for moving the applicator relative to the welding torch.

14. Apparatus according to claim 12 or claim 13, characterised in that a device for measuring the temperature of the welded metal is mounted on the carrier between the welding torch and the applicator.

15. Apparatus according to claim 14, characterised in that the device for measuring the temperature of the weld comprises a thermocouple mounted on a sliding shoe arranged to contact the welded metal.

16. Apparatus according to any of claims 7 to 15, characterised in that an imaging system linked to an optical recognition system to detect weld defects is mounted on the carrier.

17. A process for coating a welded steel construction by applying a fusible polymer to the weld while it has residual heat from the welding process, characterised in that a solid block of fusible polymer moving along the weld line behind the welding head is urged against the welded metal so that the surface of the fusible polymer block is continuously melted by the heat of the welded metal, thereby applying a coating of the polymer to the welded metal.

18. A process according to claim 17, characterised in that the tip of the block of fusible polymer is preheated to partially soften the fusible polymer before it contacts the welded metal.

19. A process according to claim 17 or claim 18, characterised in that the weld is a fillet weld and the block of fusible polymer is shaped to match the welded joint.

20. A process according to claim 17 or claim 18, characterised in that the solid block of fusible polymer comprises at least 2 layers of different fusible polymers arranged nearer to and further from the welding head.

21. A process according to any of claims 17 to 19, characterised in that at least two solid blocks of fusible polymer moving along the weld line behind the welding head are applied successively to the welded metal.

22. A process according to claim 20 or claim 21, characterised in that the fusible polymer nearer to the welding head has better adhesion to the welded metal than the fusible polymer further from the welding head and the fusible polymer further from the welding head has better corrosion barrier properties than the fusible polymer nearer to the welding head.

23. A process for welding steel plate by arc welding employing a welding torch mounted on a movable carrier, characterised in that a thermosetting polymer composition is applied to the welded metal from an applicator mounted on the said carrier at a controlled distance behind the welding torch, said distance being such that the thermosetting polymer composition forms a coating on the welded metal and is cured by the residual heat of the welded metal.

24. R process according to claim 23, characterised in that the thermosetting polymer composition comprises a non-solid thermosettable polymer and a curing agent therefor which are stored in separate packages mounted on the carrier and are mixed just before application to the welded metal.

25. A process according to claim 24, characterised in that the thermosettable polymer and the curing agent are stored as viscous compositions in parallel dispensers which are mounted on the carrier and from which they are extruded by a common plunger.

26. A process for welding steel plate or coating a welded steel construction, carried out substantially as hereinbefore described.

27. Apparatus for welding steel plates substantially as hereinbefore described or as illustrated in Figs 1 and 2 of the drawings accompanying the application.

28. Welded steel plates coated along the weld(s) with a layer of fusible polymer when produced by a process as claimed in any of claims 1 to 6 or 17 to 26 or by means of an apparatus as claimed in any of claims 7 to 16 or 27.