GB2177120A - Improvements in and relating to metal coated carbon gouging rods - Google Patents

Abstract

A method of protecting a carbon or graphite gouging rod against oxidation comprises the steps of heating the surface of the rod to a temperature of between 150 DEG C and 400 DEG C and spray coating the heated surface with a layer of molten aluminium by means of an arc spray torch or gun, for example an oxy-gas or electric arc spray torch.

Description

SPECIFICATION Improvements in and relating to metal coated carbon gouging rods This invention is related to metal coated carbon and/or graphite gouging rods and to methods of producing such rods.

Gouging torches are employed in the heavy engineering and shipbuilding industries as a means of creating a metallurgically clean groove capable of accepting weld metal withoutfurther preparation.

They are also extensively used in the foundry industry for dressing and cleaning castings. Essentially in use, electric currenttransmitted to a metal work piece via a carbon or graphite gouging rod forming part ofthe torch, strikes an arc between the live or positive gouging rodandtheearthedornegativeworkpiece, thus melting the metal in the immediate vicinity ofthe arc. The molten metal is removed by a blast of air under pressure delivered through the head of the gouging torch.

Hitherto, carbon and graphite gouging rods have been coated with a thin layer of copper to prevent rapid oxidation ofthe surface carbon and to increase the electrical conductivity of the rods. Whereas a copper coating successfully meets the twin criteria of protection against oxidation and improved conductiv ity, it suffers from a number of disadvantages. Firstly, the price of copper will inevitably rise as a consequence of the present awareness that only a finite amount of copper exists; secondly, the fume given off when using gouging torches employing copper coated rods represents a health hazard to the torch operator; and thirdly, the presence ofcuprous deposits in the weld cavity caused by droplets falling from the gouging rod during use are metallurgically harmful to the subsequent weldment.

Previous attempts to coat carbon and graphite articles with thin protective coatings of aluminium have generally only met with success where an interfacial layer is present to provide a chemical bond firstly with the graphite article and secondly with the aluminium coating.The provision ofthisinterfacial layer is both costly and time consuming.

The present invention sets outto overcome or at least alleviate the disadvantages discussed above.

According to the present invention, there is provided a method of protecting a carbon or graphite gouging rod against oxidation which comprises the steps of heating the surface ofthe carbon rod to a temperature of between 1 500C and 4000C and spray coating the heated surface with a layer of molten aluminium by means of an arcspraytorch orgun,for example an oxy-gas or electric arc spray torch.

In a preferred arrangement, the surface of the carbon or graphite rod is heated to a temperature of between 200"C and 300"C. Priorto heating, the rod surface may be scoured by means of, for example, a wire brush to remove loose deposits.

In another aspect, the present invention provides an aluminium coated carbon orgraphitegouging rod produced in accordance with the methods set out in the preceding two paragraphs.

According to the present invention in a further aspect, there is provided an electricarctorch or gun including an aluminium coated carbon or graphite gouging rod.Thesupplyofcompressedairtothe torch may be controlled automatically by means of a valve located in the flow path of the air supply whose setting is determined by means of a sensor operable to detect the presence of an electric current in the power cable to the torch. Closure of the valve may automatically be delayed for a predetermined period following detection ofthe presence of an electric current in the power cable.

Typically, gouging rods, in accordance with the invention, are produced in lengths ranging from 305mm to 51 Omm and have diameters of between 13mm and 19mm. It is to be understood, however, that rods of lengths and diameters outside the quoted ranges are not excluded from the invention. In use the rods typicallytransmit currents within the range 1000 amps to 2000 amps. Thus, a 13mm diameter gouging rod typically carries 1000 amps, a 16mm diameter gouging rodtypicallycarries 1400 amps and a 19mm diameter gouging rod typically carries 2000 amps.

Preferably the surface of carbon or graphite rods to be coated in accordance with the invention are formed with impressions such as grooves, ridges or indentsto assist bonding of the protective coating of aluminium to the rod surface. Such grooves or ridges may extend longitudinally along the length of each rod in a direction generally parallel to the rod axis, laterally generally at right angles thereto or inclined with respect to the rod's longitudinal axis. In the latter case, the grooves or ridges may define sprial channels extending along and about the rod surface. Rods formed with such ridges or grooves may be extruded through a suitably grooved die.

In an alternative embodiment, thedepressions may take the form of individual indents or recesses formed, for example, by passing the carbon or graphite rods between suitably spiked rollers.

In preparation forthe protective coating of alumi nium,the surface ofthe extruded carbon orgraphite rod is initially scoured using,forexample, a wire brush and is then pre-heatedto a temperature lying in the range of 1 50"C to 400 C. A preferred pre-heating temperature range is 200 Cto 300 C. Molten aluminium at a temperature of approximately 700 C is directed ontothesurface of the heated carbon or graphite rod intheform of a spray issuing from an oxy-gas or electric arc spray torch. In addition to cleansing and driving off any moisture which may be present on the surface of the carbon or graphite rod, the pre-heating stage provides sufficient time forthe liquid aluminium to penetrate the surface pores of the graphite rod and/or the depressions formed in the rod surface priorto solidification. Without such preheating the protective aluminium coating will not attach itself effectively to the rod surface with the result thatthe coating becomes detached either in use ofthe rod or before such use.

Typically, the aluminium coating applied to a carbon or graphite rod of 13mm diameter is of a thickness of 150 microns, that applied to a rod of 16mm is 200 microns andthatappliedto a rod of 19mm is 250 microns. Experiments have shown that such coatings are securely attached to the carbon surface ofthe rod and do not become detached during use.

One embodiment of an electric arc torch including a gouging rod in accordance with the invention will now be described with reference to the accompanying diagrammatic drawings, in which:~ Figure 1 is a side elevational view of an electric arc torch utilising a gouging rod in accordance with the invention; and Figure 2 schematically illustrates an automatic switching system for use with the torch illustrated in Figure 1.

The torch illustrated in Figure 1 includes a springbiased clamp 1 for retaining a gouging rod 2 between a pair ofelectrically conductive jaws 3. The torch has an insulated handle 4and the clamp jaws 3 can be opened bymeansofa pivotable operating arm 5. The jaws 3 are connected to a mains power supply by a cable 6 located within a conduit 7 through which air under pressure is also conveyed coaxiallywith the cable 6.

The compressed air is discharged from the jaws 3 as a series of discrete jetsthrough a series of apertures 8.

In use, the air jets are directed onto the workpieceto remove liquid metal from the groove produced by the torch.

The use of compressed air is an integral part of the gouging process and its supply is conventionally governed by a manually operated switch sitiuated on the torch. Because the noise level associated with the air emergentfrom the torch is very high (greater than 100 dB(A)),thetotal time during which the air is switched on markedly affects the noise dose of the operator and, obviously, the amount of air consumed.

As mentioned previously, the main purpose of the air in the gouging process is the removal of molten metal created during arcing. If the air is running at other times, not only is it creating a noise problem, but also the air is being wasted. With a manually operated switching system, occasions when the air is running to no good purpose include those during which the air supply is switched on priorto arcing, those during which the air supply is left on following arcing and those during which air leakage occurs, due to switch malfunction.

All the above lead to increases in noise dose and compressed air usage.

The automatic switching system illustrated in Figure 2 sets out to meetthe above problems. In addition, it provides a switchless, hence more reliable and cheaper, arc torch.

The illustrated system includes a current sensor 9 connected to the power conduit 7 at a location between a rectifier 10 and the torch, and a solenoid valve 11 connected between the air supply 12 and the torch.

The solenoid valve 11 is naturally held in the closed position; hence, no air flows to the torch unless the solenoid valve is activated. However, as soon as an arc is struck, the presence of a current in the power cable 6 is detected bythe sensor9 to provide a voltage output from the sensorto energize a relay operable to activate the solenoid valve 11 to its open position.

Hence, air is allowed to the torch. When the arc is switched off, the lack of a current flowing is detected by the sensor 9 and the relay de-energised to return the solenoid to its normal closed position, thus stopping the airflowtothetorch. Closure ofthe solenoid valve may be delayed: indeed, in many cases it is advisable to have a 2-5 second delayto allowairto cool the electrode tip immediately after arcing.

Because the air is not running when arcing has ceased, there are obvious reductions in noise dose and air wastage.

In conclusion, the use ofthedescribed automatic air switching device enables a reduction in the noise dose to be obtained; a reduction in compressed air usage to be achieved; and provides a switchlesstorch with its inherent superior reliability and life.

As mentioned previously, aluminium coated gouging rods enjoy a number of advantages over conventional copper coated rods. Firstly, it has been estabiished that the cuprous oxide fume given off when using copper gouging rods is hazardous to the health oftheoperator. Fumes given off when using aluminium coated gouging rods consist essentially of oxides of iron and aluminium which are not toxic or harmful to the user of the torch. In addition, itis well known that the presence of copper is detrimental to a weldment. In use of copper coated gouging rods, it is inevitable that molten copper will be deposited within the weld groove and subsequently become part of the weldment applied to that groove. On the other hand, the presence of aluminium is not harmful to a weldment and indeed its presence can be beneficial.A further advantage of aluminium coated gouging rods is one of cost. Ecologists are now aware that only a finite amount of copper is available. This inevitably means that the price of copperwill rise as its scarcity becomes more apparent. No such problems exist in respect of aluminium where the availability of bauxite is unlimited.

Furthermore, the use of spray metallisation equip mentfor applying the protective coating issignificant- ly less costlythan the deposition equipment required for coating the carbon articles with copper. Thus, even allowing for the need to apply a slightly thicker coating of aluminium in orderto match the electrical conductivity properties of copper, the use of aluminium shows a significant cost saving overthe use of copper.

It is to be understoodthattheforegoing is merely exemplary of gouging rods and electric arc torches in accordance with the invention and that various modifications can be made thereto without departing from the scope ofthe invention.

Claims (13)

1. A method of protecting a carbon or graphite gouging rod against oxidation which comprises the steps of heating the surface ofthe rod to atempera- ture of between 150 C and 4000C and spray coatingthe heated surface with a layer of molten aluminium.

2. A method as claimed in claim 1 wherein the layer of molten aluminium is applied by means of an arcspraytorchorgun.

3. A method as claimed in claim 2 wherein the torch comprises an oxy-gas or electric arc spray torch.

4. A method as claimed in any one of claims 1 to 3 wherein the surface of the carbon or graphite rod is heated to a temperature of between 200OCand 300on.

5. A method as claimed in any one of claims 1 to 4 wherein, prior to heating, the rod surface is scoured to remove loose deposits.

6. An aluminium coated carbon or graphite gouging rod produced in accordancewith a method as claimed in any one of claims 1 to 5.

7. A gouging rod as claimed in claim 6 formed with impressions to assist bonding ofthe protective coating of aluminium to the rod surface.

8. A gouging rod as claimed in claim 7 wherein the impressions comprise grooves or ridges which extend longitudinally along the length ofthe rod in a direction generally parallel to the rod axis, laterally generally at right angles thereto or inclined with respect to the rod's longitudinal axis.

9. A gouging rod as claimed in claim 7 wherein the impressions comprise individual indents formed by passing the rod, priorto coating, between spiked rollers.

10. An electric arctorch including an aluminium coated carbon or graphite gouging rod.

11. Atorch as claimed in claim l0whereinthe supply of compressed airto the torch is controlled automatically by means of a valve located in the flow path of the air supply whose setting is determined by means of a sensor operable to detectthe presence of an electric current in the power cable to the torch.

12. Atorch as claimed in claimed 11 wherein closure ofthe valve is automatically delayed for a predetermined period following detection ofthe presence of an electric current in the power cable.

13. An electric arc torch substantially as herein described and as herein described with reference to the accompanying drawings.