' ..PHILlPS TECHNICAL REVIEW VOLUME 2414

SOME MODERN METHODS OF ARC-WELDING STEEL

by P. C. van der WILLIGEN *). 621.791.8

Electric arc welding is nowadays the most widely used method of obtaining a mechanicallystrong joint between metal parts. The term arc welding covers a very wide variety oftechniques. In this article the author confines himself la the arc welding of steel, includingthe various mild (lmalloyeá) steels as well as low-alloy types. Three Philips deuelopmentsare discussed: Contact electrodes, zircon-basic electrodes and CO2-shielded welding.

The extensive use made of the are welding of steelcan be judged from the annual consumption ofelectrodes. In the Netherlands some 275 millionelectrodes were consumed in 19551). The Nether-lands consumption of steel in that year was 2.5million metric tons, so that per ton of steel noelectrodes were used. The average weight of anelectrode at that time being 40 grams, this meansan electrode consumption of 4.4 kg per ton of steel.The corresponding figure in most other countries

is somewhat lower, a relatively large proportion ofDutch steel consumption being attributable to shipbuilding, where are welding is booming. In the-United States, on the other hand, a big steel con-sumer is the automobile industry, where more useis made of resistance welding; nevertheless, U.S.consumption of welding electrodes in 1955 was noless than 2.9 kg per metric ton of steel.

In view of the enormous scale on which the arewelding of steel is finding application, it is notsurprising that numerous investigations have beenundertaken in this field and a wide variety of weldingmethods developed. We shall discuss here threedevelopments that have taken place in the PhilipsResearch Laboratories at Eindhoven: Contact elec-trodes, zircon-basic electrodes (both types essentiallyoverlap each other, as we shall see) and CO2-

shielded welding. A machine for automatic CO2

welding can be seen in fig. 1; particulars are givenin the relevant section of this article.

To place these developments in the right setting,we shall first give a survey of welding methods clas-sified in accordance with one particular point ofview, without however attempting to be completeand without being concerned too much with chron-ological order. We shall return to our classifica-tion later when comparing the various methods inregard to such points as economy, quality of theweld, and the possibility of automatic operation.

*) Philips Research Laboratories, Eindhoven.1) W. Gerritsen, Smit Mededelingen 14, 125, 1959 (inDutch).

Survey of are-welding methods

In the first years after the invention of are weld-ing (carbon electrodes in 1886, consumable metalelectrodes in 1892) it was not yet realized that toobtain a mechanically sound weld it is essentialto exclude the air from the vicinity of the arc;if the droplets of the weld metal, which are heatedin the are to a temperature several hundred degreesabove the melting point, are exposed to the atmos-phere they absorb some tenths of a percent of ni-trogen and oxygen, resulting in a brittle and porousweld.

All the welding methods to be mentioned hereare therefore designed to shield the weld metal fromthe atmosphere. The methods are shown in fig. 2,classified according to the means of shielding adopted.

An effective way of protecting the molten metal againstthe atmosphere is to weld in vacuum. This is in fact done insome cases, the heat for welding being generated by an elec-tron beam. This method cannot of course be counted amongthe forms of are welding, and it is obviously impracticable forthe great majority of are-welding applications.

A generally useful solution to the problem ofshielding the weld metal was found in about 1910with the development of the coated (or covered)electrode. During the melting process the coatingproduces slag, which envelopes the droplets with amolten shell, and it also evolves gas, which excludesthe air from the vicinity of the are, Initially, upto about 1937, the only known coated electrodesproduced an acid slag and a gas mixture consistingmainly of carbon monoxide and hydrogen 2). Theseelectrodes are eminently satisfactory for weldingmild (unalloyed) steels, but in the case of low-alloysteels, which are nowadays gaining increasing

2) Particulars of the numerous kinds of acid electrodes willbe found, for example, in the Welding Handbook, publishedby the American Welding Society, New York 1960, 4thEdn. This work also contains detailed information on manyare-welding questions that can only be touched upon herein passing.

1962/63, No. 1 FRUITS OF NUCLEAR RESEARCH 13

Even so, the electric power thus produced is stillmore expensive than that delivered by conventionalelectrical power stations.The first British industrial nuclear reactors, built

at Windscale, were solely intended for the produc-tion of plutonium for atomic bombs. The reactorsbuilt subsequently at Calder Hall and Chapelcrosswere primarily intended for plutonium production,the electric power being a by-product. The largereactors now under construction are specificallydesigned for the generation of electricity 21).An objection sometimes made against nuclear

power stations is that the radioactivity of their wasteproducts may be a danger to public health. It isknown with certainty, however, that coal-miningunderground is a frequent cause of disease, in spiteof all the advances made in mining hygiene in thelast decades. The mining of uranium ore can alsobe dangerous to the health of the miners, the escape

21) For a survey of the nuclear power stations under construc-tion or planned in Great Britain, see: U.K. power reactorsbuilding progress, Nucl. Engng. 7, 108-109, 1962 (No. 70).Particulars of the further development of reactors for elec-tricity production are given by W. Vinke and P. J. Krey-ger, Power reactor expcriments, Atoomenergie 4, 33-39,1962 (No. 2).

of radioactive radon being a particular hazard.But by adopting appropriate measures, so far asthey have not already been introduccd, uraniummining can probably be raised to a higher hygieniclevel than coal mining. If this can indeed be done,the nuclear power station will represent a socialadvance on the coal-fired type.

Summary. Inaugural address presented at the Teehnische Hoge-school, Eindhoven. In the past thirty years nuclear researchhas led to the development of numerous techniques, and theauthor discusses some of their applications in various fields.Electrons of very high energy, e.g. from 4 to 35 millionelectron-volts, can be used for medical irradiation, or for gener-ating very hard X-rays, which are used for the samepurpose and also for industrial radiography. The electrons areobtained from accelerators, e.g. linear accelerators or beta-trons. In nuclear reactors, or in cyclotrons, radioactive isotopesare produced which can be used as "tracers" in various inves-tigations (examples: for the study of littoral drift, the absorp-tion of iodine in the thyroid gland). for measurements basedon the absorption of radiation (example: sand concentrationin the outlet pipe of a sand dredger), for industrial radiography(welds in pipelines) or for medical therapy (cobalt irradiator).Mentionis also made of neutron activation analysis. Toillustratethe use of nuclear energy for the generation of electricity, somedata are given on British and American nuclear power stations.The survey is preceded by a consideration of the hazards in-volved in nuclear engineering, and the author returns to thispoint at the end. With appropriate safeguards he considers thedangers to be no greater or even less than are encounteredelsewhere in engineering or daily life.

1962/63, No. 1 ARC WELDING OF STEEL 15

importance, theytendin unfavourable circumstancesto cause cracks in the welded joints and other dif-ficulties. This led to the development of basic(low-hydrogen) electrodes, the coating of whichyields a basic slag and a gas which is largely CO andcontains very little hydrogen. An independent in-vestigation in about 1943 at Eindhoven led to thedevelopment of the Contact (iron-powder) elec-trades, which were marketed in 194.7 as a specialversion of the acid-coated electrodes 3). From these,with a view to applying the same principle to thebasic electrodes, the zircon-basic coating wasdeveloped 4).Meanwhile, however, other means were found of

shielding the weld metal from the air: whereas thecoated electrode does this by producing slag ancfgas, it proved possible to protect the weld either byslag or gas alone (fig. 2).

Before going into this point, we shall examine the questionwhether gas shielding alone is fundamentally sound. For if thedeposited metal is shielded without slag, we sacrificc severaladditional functions which slag fulfils or is supposed to fulfil:1) it provides for metallurgical purification of the weld metal;2) sinee some of its constituents readily emit electrons, it hasa stabilizing effect on the are, thereby facilitating welding withalternaring current; 3) it protects the weld, as the are movesaway, against too rapid cooling by the air; 4) it continues toprotect the cooling metal against the action of atmosphericoxygen and nitrogen.The first function mentioned may indeed be important on

occasion, but it can be dispensed with if the deposited metalhas the same (or higher) purity as the parent metal, i.e, thesteel to he welded. The second function can be fulfilled byarc-stabilising substances, added in small quantities to thesteel itself. The other functions, at least as far as steel is con-cerned, have no real significance: coolingby the air is imrnate-rial, in view of the very high heat dissipation through the work-piece itself; and experience shows that the air does no harmto the cooling weld in the case of steel 5).

As long ago as 1925 an electrode was brought out,designed to provide shielding by the slag alone: thiswas the "flux-core electrode", in which substancesthat form slag and also contribute to the stabiliza-tion of the are are contained in an axial cavity ofthe electrode wire. This did not, however, provideadequate protection of the weld metal, and theseelectrodes have therefore virtually dropped out ofuse.

3) P. C. van der Willigen, Welding J. 25, 313S, 1946 andPhilips tech. Rev. 8, 161 and 304.,194.6.

4) P. C. van der Willigen, Welding NewsNo. 89 (March 1958),No. 123 (July 1961) and No. 124 (Sept. 1961).

5) This is not the case in the welding of metals that have agreat affinity for oxygen and nitrogen, e.g, aluminium ortitanium. When such metals are welded by a gas-shieldedmethod, extra gas shielding may he necessary during cool-ing.

Good results, on the other hand, were obtainedwith another method of shielding by slag alone;this was called "submerged-arc welding", and cameout in about 1933 in the United States 6). The arcin this case burns in a cavity inside a coarse-grainedlayer of powder (flux). The are is therefore notvisible, but this is not a serious objection as themethod was in fact primarily developed for auto-matic welding, and has so far remained thè'lnost"widely employed for this purpose. The flux is m~by melting a mixture of slag-forming substancesand, after solidification, grinding it into grains of apartienlar size. The mixture may be .sintered insteadof melted; in both operations, the temperature is sohigh as to remove all substances that give off gas,including possible sources of hydrogen. As we shallsee, the absence of hydrogen is an importantcharacteristic.

In the welding methods with gas shielding only,the absence of hydrogen is also essential. One wouldnot think so, however, judging from the first methodof this kind, atomic-hydrogen welding, dating from1926, which uses an are between two tungstenelectrodes in a hydrogen atmosphere. The hydrogenatoms formed by dissociation in the white-hot arcrecombine outside the arc and thus, together withthe radiation from the arc, melt the workpiece andthe added weld metal. The method is still usedhere and there for metals and alloys other thansteel. Where steel is concerned, however, theappropriate method in this category is argon-arcwelding, developed about 20 years ago in the UnitedStates. The weld metal is shielded here by an inertgas - which in America may be helium as well asargon, but in Europe is always argon - which isdirected around the are from a gas cylinder. In itsoriginal form, in which the are is struck between anon-consumable tungsten electrode and the partsto be welded, the method is still employed on awide scale for special alloy steels and other metals.Since 194·9a method derived from inert-gas weldinghas gained popularity for welding normal steel,the tungsten electrode being replaced by a consum-able bare steel wire, which is fed from a motor-driven reel. In the classification given in fig. 2,this method is denoted as "argon + 4% O2'', theaddition of 3 to 5% oxygen to the inert gas havingproved desirable to avoid porous welds.

The latter method was extended by the processof CO2 welding, which, in 1953, was conceived and

G) See the book mentioned in reference 2); further L. Wolffin "Werkstoff und Schweissung" (Ed. F. Erdmann-Jesnitzer), Akademie-Verlag, Berlin 1951, p. 458.

,--~~~~--- - ~-- -

16 PHILIPS TECHNICAL REVIEW VOLUME 24

developed in the Philips Research Laboratorles 7):the argon, which is rather costly, is replaced hereby carbon dioxide, which is cheap. Although thisis not an inert gas, it provides complete proteetion-for the weld metal when properly used. The methodalso has many other attractive features, and hastherefore gained considerable ground in the courseof a few years 8).The classification in fig. 2 is, of course, far from

complete. In particular, combined shielding by slagand gas is found not only in the above-mentionedcoated electrode, but also in numerous weldingmethods subsequently developed, which were infact based on shielding by gas or by slag alone. Amethod of CO2 welding has been introduced, forexample, in which a little powdered flux is addedjust in front. of the arc by means of a magneticfield. There are also various techniques for specialcases, such as stud welding and spot are welding,which might also be included in this diagram.Passing these over, we shall now turn to the threeabove-mentioned Philips developments, prefacingour discussion with a short account of the basicelectrodes.

Basic electrodes

With acid-coated electrodes, which were the onlycovered electrodes known up to 1937 (and are stillused in enormous numbers), the arc atmosphere isroughly 40% hydrogen, the rest consisting of watervapour, carbon dioxide and carbon monoxide. Thelatter two gases originate from carbonates and car-bohydrates in the coating. H2 and H20 come fromsubstances such as water-glass and kaolin, addedto the coating as binders and moulding compounds.For ordinary mild steel these electrodes are verysuitable, but when they were used for free-cuttingsteel (which contains a fairly high percentage ofsulphur) they gave porous welds. They provedequally unreliable for low-alloy steel, the weldssometimes showing microcracks. Other difficultieswere also encountered, such as under-bead cracking,reduced tensile strength of the deposited metal(noticeable in the occurrence of "fish-eyes" in tensiletest, specimens taken from this metal) and flakingof enamel from the weld in subsequently enamelledparts.7) Netherlands patent application No. 176664, March 1953,

and U.S. patent No. 2 824.948 in the name of P. C. van derWilligen and H. Bienfait. See also P. C. van der Willigenand L. F. Defize, Schweissen und Schneiden 9, 50, 1957.An English translation of the latter article has appearedin Welding News, No. 83, 2-14, 1957.

8) For the original formof argon-arc weldingthe recommendedinternational designation is TIG (tungsten inert gas),for the variant with consumable bare wire MIG (metalinert gas) and for CO2 welding MAG (metal active gas).

At a time when 110 answer had yet been found tothese undesired effects, efforts were being made tomake electrodes that would yield a basic type ofslag (with CaCOa and CaF2 as the characteristicconstituents of the coating). At first the resultswere negative, and porous welds were obtained.Sound welds were not produced in this way until astart was made, in France, with "baking" the coatedelectrodes at high temperatures (about 450°C).The basic electrode then quickly came into use.In the Netherlands round about 194.2 these elec-trodes were referred to as "St 52" electrodes, sincethey were found particularly useful for a low-alloysteel of that name. As mentioned, this high-qualitystructural steel could not always be satisfactorilywelded using acid-type electrodes. After 1942 itgradually became clear that the above-mentioneddifficulties were attributable to hydrogen 9), andonly then was it understood that it was necessaryto bake the electrodes at high temperature in orderto remove residual hydrogen sources such as water.For this reason, basic electrodes are still frequentlycalled low-hydrogen electrodes to indicate theiressential feature. This, incidentally, is a typicalexample of a successful technique being appliedlong before its underlying principles had properlybeen understood.

Basic electrodes, with their excellent mechanicalproperties, fell short in one important respect:they were not easy to weld with. The slag was noteasily removable, the surface of the bead was notsmooth, the are was not very stable (particularlywhen welding with alternating current and lowopen-circuit voltage), and so on. The latter diffi-culty was due to the fluoride in the coating; becauseof its high electron affinity, the fluorine acceleratedthe deionization of the are and thus hampered itsperiodic re-striking. This snag was overcome byadding to the coating magnesium powder, whichacts as an are stabilizer, or, in the case of anotherbasic electrode, by applying two coatings one ontop of the other, the inner coating containing nofluorine. The other difficulties, however, remained.

The chance of further improving the weldingproperties came unexpectedly in 1943 from aninvention made in another connection.

Contact electrodes

In that year we were working on experimentswhich it was hoped would improve the thermal

0) G. L. Hopkins, Trans. Inst. Welding 7, 76, 1944. A. E.Flanigan, Welding J. 26, 1935, 194.7.See also J. D. Fast,Causes of porosity in welds, Philips tech. Rev. 11, 101-110,1949/50; J. D. Fast, Low-hydrogen welding rods, Philipstech. Rev. 14, 96-101, 1952/53.

1962/63, No. 1 ARC WELDING OF STEEL 17

efficiency of welding electrodes, i.e. reduce the heatgenerated in proportion to the deposited quantityof metal. Possible means to that end seemed to beto reduce the depth of penetration (and hence wasteless heat 011 needless melting of the materialof theworkpiece) by not subjecting all thc metal to thepassage ofthe current. With this in mind we decided,after various experirnents, to distribute uniformlyin the coating a quantity of iron in the form of ironfilings (later iron powder). This resulted in a verythick coating with a high iron content. When westarted welding with these electrodes, we noticedthat after striking an are it was not necessary tohold the electrode at a certain distance from theworkpiece, but that it could be rested upon it, i.e.the electrode could be touch-welded. During weld-ing with the heavily coated electrode a deep cupforms at the end of the coating which automaticallykeeps the are length constant (.fig. 3). Moreover, we

Fig. 3. a) Welding with a normal coated electrode and free arc.b) Welding with a Contact electrode. The coating being thicker,a deeper cup is formed than in (a.). The rim of the cup can thusrest on the surface of the workpiece. The arc here is concen-trated on a thinner core wire, so that the penetration in themiddle is somewhat deeper than in (a).

found that given a suitable percentage of ironpowder with the proper grain size, the coating hassufficient electric conductivity to restart the areautomatically by contact with the workpiece. Sincethese electrodes are in mechanical as well as inelectrical contact with the workpiecc via the coatingwe gave them the name "Contact electrodes" 3).

Further experiments showed that existing typesof electrodes could be adapted to the Contactprinciple, by transferring part of the core iron in afinely powdered form to the coating, in which it isintimately mixed with the other constituents. Theiron-to-slag ratio is then unchanged; the slag hasthe same composition and the weld is given roughlythe same mechanical properties as with the originalelectrode. The advantages are not merely greatlyincreased ease of welding, but also improved shield-ing (due to the deep cup) against the nitrogen in the

atmosphere, and above all greater welding speed.Because of the shielding effect of the cup, the heatradiation by the arc is used more effectively, sothat for a given power more metal is deposited perunit time - the effect aimed at in the originalexperiments, although in a different direction-and less of the molten metal is lost by spa t-ter. As a result (and because the are voltageis somewhat higher) Contact electrodes give anaverage rate of deposition of 0.24 grams of metalper ampere-minute compared with 0.17 grams withconventional coated electrodes (with a free arc).In about 1947 Philips put on the market Contact

versions of acid electrodes. In the following yearssimilar types were produced by manufacturers inother countries, e.g. Russia. In Great Britain andthe United States they first came into use in 1952 10):the welders there were initially opposed to the in-troduction of these electrodes which made weldingeasier and faster. The iron content in the coatingof present-day Contact electrodes generally weighsToughly half of thc iron core. The "depositionefficiency" of the electrodc, i.e. the ratio betweenthe wcight of thc deposited metal and the consumedcore, is consequently about 150%.

Zircon-basic electrodes

Soon after 1943 we attempted to make a Contactversion of the basic type of electrode, whose weldingproperties were in greatest need of improvement.Our efforts at the time were frustrated by the inade-quate viscosity of the slag formed.It will be evident that the viscosity of the slag is

of considerable importance from the point of viewof welding in various positions, e.g. the welding offlat grooves, vertical upward and overhead welding,and standing-fillet welding. In vertical upwardwelding, for example, a fairly thin, fluid slag is per-missible or evcn desirable, as it easily runs off down-wards; for standing-fillet welds, on the other hand,a thin, fluid slag is unsuitable because it collects atthe lower edge of the bead, displacing the metalthere, and thus wcakening the final joint (fig. 4).At the melting point of steel, which is about 1500

°C,it is very difficult to measure the viscosity of theslag exactly with the available instruments becausethe instruments are attacked. Data on this subjectare therefore scarce and unreliable. Nevertheless itcan be said that the viscosity of acid slag at 1500 °Cis in the region of 10 poise, and that of basic slag is

10) In English-speaking countries Contact electrodes are oftenreferred to as iron-powder electrodes. In Germany the term"Hochleistungselektroden" is used.

18 PHILlPS TECHNICAL REVIEW VOLUME 24·

roughly half this value. (The viscosity of both isthus much higher than that of the molten steelitself, which is 0.025 poise.) Every welder knowsthat basic slag is much less viscous than acid slag:he would say that the latter has the consistency oftreacle and the former that of water. Ifnow, in order

b

Fig. 4. For depositing a standing-fillet weld an electrode isrequired that forms a viscous slag. If the slag is too fluid, itpushes the molten metal at the lower side of the bead to oneside; this can be seen in (a), while (b) shows the desired shape.

to make a Contact version, iron powder is addedto the coating of a basic electrode, the already thinslag becomes even more fluid. Attempts to thickenthe slag by adding, for example, a high percentageof rutile (TiOz) to the coating, yielded the desiredresult but reduced the basicity of thc slag 11),thc consequence being poorer mechanical propertiesand a greater tendency for the weld to show porosity.After many years of research, a good basic Con-

tact electrode was at last produced in 19574) byadding to the coating about 20% zirconium oxide(Zr02). It proved impracticable to measure theviscosity of the slag from this coating with anyreliability, not only because of the difficulty men-tioned but also because of the added complicationthat the Zr02 particles in the "zircon-basic" slagstend to settle out during the measurement (thespecific gravity of Zr02 is 5.6 g/cm3, that of thebasic slag about 3 g/cm3). This very fact, however,indicates that the slag must have a higher viscosity,for it is evident that the Zr02 crystals, whosemelting point is 2700 °C, have not reacted with theother constituents and are present as such in thefluid slag. A suspension of solid particles in a liquidhas a viscosity 17s which is greater than the viscosity170 of the liquid itself. This is expressed by the Ein-stein viscosity equation:

17s = 170 (1 +~cp) ,

where cp is the total volume fraction of the dispersedparticles.

11) The basicity is given by the molecular ratio of basic andacid oxides; in our case these are mainly CaO and Si02respectively, with CaO: Si02 "'" l.S.

The viscosity-enhancing effect of the Zr02 proveduseful both for making a Contact version of thebasic electrode and also for the normal versions.With the new "zircon-basic" electrode, type Ph 86,standing-fillet welds can easily be made. The Con-tact version of this electrode, type C 6, which has adeposition efficiency of about 160% and is nowmarketed alongside the free-arc version, is part.icu-larly suitable for the rapid filling of Vee joints.Owing to the greater viscosity of the slag, zircon-basic electrodes cannot however be used for verticalupward welding.

7853

For welding in various positions an important point besidesthe viscosity is the melting range of the slag (i.e. the differencein temperature between the softening point and the meltingpoint, which lies between 1200 and 14-00°C for most slags) 12).This melting range can be measured by heating pieces of slagon a platinum sheet in a [urnace and observing the changeof shape through an optical pyrometer. The value found forvarious acid slags is lOO-200°C, and only about 4,0°C for normalbasic slags. The zircon-basic slag is found to have a muchlonger melting range, more like that of acid slags.

Returning Ior a moment to the Zr02 particles present insuspension, a side-effect of their presence is that the slag islight grey in colour where it has not been exposed to the air;this is, as it were, an identification mark of the zircon-basicelectrode. See ji.g. 5, which shows photographs of the undersideof two kinds of slag.

The presence of Zr02 crystals in the slag has also been de-monstrated by X-ray diffraction diagrams. At the norrna!basicity of 1.5, Zr02 evidently behaves neutrally; at a higherbasicity it appears that the compound CaZr03 is formed,

C6

Ph 36 7838

Fig. 5. Slag formed by a zircon-basic and a basic electrode(C 6 and Ph 36, respectively). The photo shows the undersideof the slag, which was in contact with the bead. The slag ofC 6 appears light grey in the middle where it was not exposedto the air; this is due to the dispersed Zr02 particles (whichare also concentrated near the surface, due to sedirnenta tionof the particles). If the bead is deposited not on a flat plate,as here, but in a groove, the air has less access to the slag andthe light-grey colour is perceptible up to the edges.

12) G. J. Pogodin-Alexejew, Theorie der Schweissprozesse,VerI. Technik, Berlin 1953, p. 188.

1962/63, No. 1 ARC WELDING OF STEEL 19

Notable features of the C 6 and Ph 86 electrodesare the ease with which the slag is removable evenfrom narrow grooves, and the smoothness of the weld.The surface of the weld metal is less ridged, and thetransition between weld and parent metal issmoother than with the basic electrode (fig. 6). It isdue to this that fatigue tests of welded joints in the

Fig. 6. Appearance of an unprepared butt weld in 13 mm platewith 2 mm gap; one made with a zircon-basic Contact electrodeC 6 and the other with a normal basic electrode Ph 36. Thesurface of the latter weld is uneven and the weld metal doesnot spread ou t very well.

as-welded condition (i.e. not smooth-ground orstress-relieved by heat treatment) havc shown valuesof fatigue strength about 30% higher than similarwelds made with normal basic electrodes.As mentioned at the beginning, the introduction

of basic electrodes was stimulated by the increasinguse of low-alloy steel, which makes lighter construe-tions possible but at the same time imposes highdemands on the mechanical properties of the welds.Another tendency in modern engineering is the useof thicker and thicker plates, e.g. for reactor pres-sure vessels, for very large ships and in the chemicalindustry. For welding such plates, too, basic elec-trodes have proved equally successful. We havecarried out extensive tests to determine the notchtoughness of welded joints in very thick plate(76 mm) made with type C 6 and Ph 86 electrodes.At room temperature these electrodes were foundto give practically the same high notch-toughnessvalue as the normal basic electrode Ph 36 S. At lowtemperatures, on the other hand. the zircon-basic

electrodes give appreciably better values; see thegraphs in fig. 7a and b, which also give sketches ofthe type of impact bars used and some data onthe welds. The plates measured 400 X 400 X 76 mm.The total number of passes in each welded joint was24. After each pass we reversed the workpiece andwaited until the temperature had dropped below100 oe, after which it was cooled with water anddried. This method gave a better approximation tothe situation during the welding of very thick platesin practice.

In many modern methods of welding the weldermust take precautions to avoid inhaling toomuch of the fumes. This also applies to the basic

~,----------------------------,kgm/cm2

0-40 -20 0 20 iOoe

Q

15kgm/cm2

c::::A=::J

10

5.0°----COl

10 °

5C6~O

o/" __-oo---Ph86 DO____ x

x

0L__L-_~4~0--L_-~~0~~~0--~~20~-L~4~ooe7966

Fig. 7. a) Average notch toughness of a welded joint made withthree types of electrode, as a function of temperature. Thenotch toughness was measured on Charpy test specimens7 X 10 mm" of the shape shown top left (U notch), taken froma welded joint in 3 inch thick plate, the prep aration being asshown bottom right.b) As (a) using Charpy test specimens 8Xl0 mm- with a Veenotch as shown top left.

20 PHILIPS TECHNICAL REVIEW VOLUME 24

electrodes, the vapours from which contain a smallamount of fluoride. Table I shows the total amountof fluorine per 100 grams of deposited metal meas-ured in the welding vapours from basic and zircon-basic electrodes. The quantities found for zircon-basic electrodes are considerably smaller, which wasto be expected since the coating of zircon-basicelectrodes contains much less fluoride.

Table I.Measured quantity of fluorine in welding vapour fromvarious electrodes.

Electrode,type and thickness

mg F in welding vapourper 100 g deposited metal

C 6 (4 mm)

Ph 86 (5 mm)

Ph 36 S (5 mm)

55·56

75·77

97·139

CO2-shielded welding

The development now to be described constitutesan entirely different approach to the problems ofare welding.

In 1952 we started experimental welding in mix-tures of CO and CO2, based upon our knowledge ofthe composition of the are atmosphere in weldingwith basic electrodes. Wc soon stopped adding CO,which is relatively expensive and is moreoverpoisonous. Welding 'with an are atmosphereconsisting entirely of CO2 had already been triedin 1926, but the result at that timc was a brittleand porous weld. We shall describe bclow how thisproblem was finally solved, but first it will be usefulto outline the whole method as at present employed.

Main features of CO2 welding

As mentioned at the beginning of this article,CO2 wclding is based on the use of consumablc barewire which is unwound from a reel and fed by amotor-drive to the arc. This method makes it pos-sible to weld long joints uninterruptedly, and canreadily be made automatic or semi-automatic.

The principal difference between welding withseparate electrodes and welding with consumablebare wire consists in the way in which the currentis supplied. The current is fed to the wire close tothe tip by means of a feed tube as sketched infig. 8.This makes it possible to work with much highercurrent densities, and hence at greater speed, thanwhen using electrodes where the current is normallyapplied to the rear end, i.e. at a distance of 35 or45 cm from the are, An electrode of 2 mm core-wire diameter, for example, is welded with a currentof at the most 70 A; a higher current would make

the electrode too hot, perhaps even red-hot, givingrise to decomposition of the coating and other dif-ficultiee. In CO2 welding with consumable wire ofthe same thickness, the welding currents can be ashigh as 700 A.

A machine for fully automatic CO2 welding wasshown in ftg. 1. The motor (M in this ftgure) feedsthe wire from the reel to the welding are throughtwo hardened-steel rollers. The wire is clamped witha variable pressure between the rollers, and thespeed of the motor can be continuously varied by amanual control (K). The nozzle (E), in the axis ofwhich is located the feed tube and out of which

20mm

7967

Fig. 8. Sketch of the bottom section of the welding head.E is the water-cooled nozzle from which the CO2 is supplied.The consumable bare wire A is fed through tube .8. Thecurrent is supplied to the wire via the same tube, themouth of which is only about 25 mm from the wire tip.To ensure good contact it is essential not to straighten the wirebeforehand.

The wire projecting from the tube .8 is pro-heated by thecurrent it carries. This effect, which is extremely im-portant with the high current used and the small diameter ofthe wire, can largely be controlled by varying the length ofthe wire protruding (here about 25 mm). This effect wasrecognized in 1943 by T. Hehenkamp and described in U.S.patent No. 2475835.

the CO2 gas flows through an annular opening, IS

double-walled and is fitted with closed-circuit watercooling.

Complete equipment for semi-automatic CO2 weld-ing can be seen in fig. 9.

1962/63, No. 1 ARC WELDING OF STEEL

Fig. 9. Completc equipment for semi-automatic CO2 welding. An electric motor feeds theconsumable bare wire from the reel through the long flexible pipe to the welding gun,which the operator passes along the joint to be welded. The current (DC up to 300 A in thismachine) is supplied near to the wire tip; the carbon dioxide (from one of the two gascylinders) flows through a nozzle in the gun and envelopes the wire electrode and the are.

21

the deposition rate is be-tween 0.30 and 0.40 gramsper ampere-minute. Afurther advantage of CO2

welding is the particularlyfavourable shape of thepenetration: whereas ar-gon-shielded welding pro-duces a V-shaped pene-tration (fig. lOa), CO2-

shielded welding producesa U-shaped penetration(fig. lOb) which is more-over, using the same cur-rent, several millimetresdeeper than with argon.The upshot is that CO2

welding, generally speak-ing, permits less bevellingof the parts to be welded,and thus narrower groovesthat can be ftlled in fewerpasses.All this makes CO2 weld-

ing a very fast process.The gain in welding speedcompared with methodsemploying separate elec-trodes is of course en-hanced by the fact that itis no longer necessary tostop every minute or soto insert a new electrode.Compared with the con-tinuous method of sub-mcrged-arc welding (de-scrihed at the end of thisarticle) a favourable fea-ture of CO2welding is thatno time is wasted U1

removing slag from thebead.

Apart from the weldingspeed it is important toconsider the mechanicalproperties of the welds ob-tained. We shall presently

The heat generated by the current is very effi-ciently used in all gas-shielded methods of welding,since no slag needs to be melted. Consequently thedeposition rate (at a given current) is considerablyhigher in these methods than, for example, whenusing the electrodes discussed above: in CO2 welding

examine in some detailthe manner in which the metal is shielded in CO2

welding, but it may be mentioned at this stage thatthe deposited metal has roughly the same mechan-ical properties as obtained when using basic elec-trodes. As to the properties of the weld, if good useis made of the deep penetration which is a feature of

22 PI-HLIPS TECHNICAL REVIEW VOLUME 24·

a

b

Fig. ID. Etched cross-section of weld beads, deposited with2 mm thick bare wire on a flat plate 13 mm thick, at a currentof 480 A and a rate of travel of 4·0 cm per minute: (ft) weldedin argon + 5% oxygen, (b) welded in CO2, In (a) the penetra-tion is V-shaped, 7 mm deep; in (b) it is U-shaped, 10 mm deep.

CO2 welding, the deposited metal will be mixedwith a high percentage of molten parent metal.This is one of the main points governing the mechan-ical properties of the weld. Also of importance isthe number of layers: severallayers entail repeatedannealing of the previous beads, and this as a ruleimproves the structure and increases the toughnessof the weld. The deep pcnctration in CO2 weldingdoes, however, call for extra precautions in somecases. If the width of the penetration is less thanthe depth, which may happen if the arc is vcryshort and the current high, internal shrinkage cracks(hot cracking) may occur. This phenomenon is alsoknown in other welding methods. The remedy in allcases is to increase the are length and/or to widcnthe groove to be welded.

Shielding the metal in CO2

When the molten metal III the are is envelopedby CO2, it is completely shielded against the nitrogenin the atmosphere. Although the metal dropletstransferred in the are are also thereby shieldedagainst oxygen in the air, they are neverthelesshighly susceptible to oxidation: the CO2 itself disso-ciates at a relatively low temperature into COand oxygen, and at the temperature of the arcthere is likely to be almost complete dissociation,i.e. a high partial oxygen pressure (see fig. 13).

The oxidizing action of the dissociated CO2 wasevidently the reason for the porosity of the weldswhich caused the failure of the experiments in 1926.The experiments begun in 1952 were based on the

idea that the oxidizing action could be compensatedby adding deoxidizing agents to the steel wire, i.e.alloying elements such as titanium, silicon, mangan-ese, etc., the oxides of which possess a high heatof formation and which therefore largely bind the"available" oxygen by, e.g., the following reactions:

Si + 2 CO2 _:E:; Si02 + 2 CO ,

Mn + CO2 _:E:;" MnO + CO •

In view of the high partial oxygen pressure expected(even higher than 50% in the case of complete dis-sociation in the arc) we thought at first that fairlyhigh percentages of additions would be necessary.The experiments soon showed, however, that theCO2 had little oxidizing action, less even than theair (partial oxygen pressure 20%) - a surprisingfact to which we shall return below. Under favour-able conditions the addition of 0.3% Si + 0.3% Mnwas found to be sufficient to prevent porosity inthe weld metal i). The results of a representativeexperiment, illustrating the relatively low oxidation,are summarized in Table 11. In this experiment aparticular kind of weld was made successively inthree different gas atmospheres: CO2, air, and

Table Il. Burn-off of silicon from consumable steel wire, thick-ness 1.6 mm, welded in three different gas atmospheres. Inall three cases the wire contained 0.9% Si, together with 0.10/0C, 1.6% Mn, 0.02% Pand 0.02% S. The metal was depositedin a wide Vee join t in 16 mm thick steel plate, using directcurrent of 34·0 A (wire positive) at 32-33 V are voltage and atravel speed of 40 cm/min. The joint was filled in eight passes,with interpass cooling to 100 DC. Gas feed at nozzle 25 litrespel' minute.

Shielding gas Si burn-off inconsumed wire

%

Si in depositedmetal%

CO2

AirArgon + 20% O2

0.460.300.27

4.86770

argon + 20% 02' r <(he latter atmosphere oxygenwas present with roughly the same partialpressure as in air, so that this experiment couldshow whether the nitrogen in the air had any-thing to do with the oxidation. The welding wirehad roughly the same composition as that nowused in the CO2 welding process; it contained 0.9%Si and 1.6% Mn, together with various other ele-ments. The tabulated results of the analysis of thedeposited metal show that in CO2-shielded 'weldingrelatively little Si disappears (the "burn-off" ofSi is by far the smallest). Further experimentsdemonstrated that, as regards the burn-off of Si andMn, the CO2 behaves like argon plus about 9% 02'This favourable behaviour makes it possible to

use relatively low percentages of Si and Mn, which

1962/63, No. 1 ARC WELDING OF STEEL 23

can readily be added as alloying elements to thesteel wire. Of course, one cannot expect the addedSi and Mn to give a 100% shielding of the iron (forreasons of reaction kinetics), even though the Si, forexample, is by no means completely consumed(see Table II): a small quantity of Fe is oxidized,and all reaction products together form silicates,which are found in the form of a thin film of slagon the weld bead. If a second bead is to be appliedto the first, this film is in many cases not brushedoff before welding.In regard to the composition of the steel of the

weld wire, it is also necessary to take account ofthe carbon which is always present. A very lowcarbon content in the deposited metal cannot berealized - even if it were desirable - since, whenwelding with a wire containing very little carbon(e.g. 0.03%), some carbonizing of thc depositedmetal always occurs (up to about 0.05%). The ex-planation is that the transferred droplets areenveloped in a blanket of almost pure CO, fromwhich it is possible for iron to take up carbon.

In view of its toxicity, we cannot be indifferentto the further fate of this CO, produced by dissocia-tion of CO2 and also by the above-mentioned reac-tions with Si and Mn etc. One might be inclined tosuppose that the CO would be oxidized by theoxygen in the surrounding air. Measurements ofthe quantity of CO present, with and without theadmission of air to the surroundings of the CO2

are, have shown ï) that this is only partly the case:because of the marked dilution of CO with CO2,

some of the CO does not undergo combustion andremains in the welding vapours. At a supply of 20litres of CO2 per minute, the fumes from the weldingsite are found to contain 0.5 litre of CO per minute.Air conditioning is therefore just as essential withCO2 welding as with most other welding processes,at least in small workshops.

Droplet transfer in CO2 welding

When developing a welding process it is desirableto have a clear idea of the way in which the moltenmetal is transferred in the arc to the workpiece.Right from the beginning of our work on CO2

welding we therefore studied the droplet transferwith the aid of a high-speed 16 mm cine camera 13).

13) Scc the articles mentioned in reference '). The sarne set-upwas used for studying the droplet transfer of coated elec-trodes: P. C. van del' Willigen and L. F. Defize, Philipstech. Rev. 15, 122, 1953/54. See also: L. F. Dcfize and P. C.van der Willigen, Droplet transfer during arc welding invarious shielding gases, Brit. Welding J. 7, 297-305,1960. (The Sir William J. Larke medal of the Institute ofWelding for 1961 was awarded to Messrs. Van del' Willigenand Defize for the latter article. - Ed.)

As a result we were able to give a reasonably satis-factory explanation for the relatively slight oxidizingaction discussed in the foregoing.The camera used can take up to 3000 exposures

per second; projecting the film at normal speed(24 frames per second), the welding process is thusseen in slow motion, being slowed down more than100 times. Fig. 11 shows the set-up employed. Aweld bead is dcposited on a flat steel plate. The areremains stationary and the plate is moved, so thatthe cine camera and the illumination can be keptin a fixed position. When filming we used a long arc,

F

7855

Fig. 11. Set-up for filming droplet transfer in CO, welding.C are with tip of consumablc wire automatically fed from thereel D. The workpiece P moves in the direction of the observer.F 16 mm cine camera capable of 3000 exposures per second.H water-cooled high-pressure mercury lamp, which, with theaid of a lens, provides a sufficiently brilliant background forthe arc during filming.

the melting tip of the wire being about 7 mm abovethe surface of the plate, with the obj ect of observingas much as possible of what was happening in thearc. In actual welding practice a short are is alwayspreferable, in order among other things to reducespatter loss (this applies to all arc-welding methods).The tip of the wire is then located about 2 mm oreven less above the surface of the workpiece. Thisis possible owing to the fact that, at the enormouscurrent density used in CO2 welding (about 20000A/cm2), the "are pressure" blows, as it were, acavity into the parent metal immediately after ithas been melted, thus creating its own are length.The arc then burns largely in the cavity, therebyreducing spatter loss and also energy losses due toradiation.

The slow-motion films show that the electrodematerial is transferred in a CO2 atmosphere in theform of very coarse droplets, the diameter of which isbetween 3 and 4 mm, independent of the diameterof the wire used; see Table 11J. The droplets growasymmetrically on the tip of the wire towards theside of the bead already deposited. This obliquemelting of the wire appears to be bound up with the

24 PI-IILIPS TECHNICAL REVIEW VOLUME 24

Table Ill. Average size of transferred metal droplets in CO2welding with wire of various thicknesses.

--

Wire Current Current Dropletdiameter density diameter

mm A A/cm2 mm

2.5 4·90 10000 3.7

2.0 315 10000 4.0

1.2 110 10000 3.6

fact that at the fairly rapid relative displacementof are and workpiece (e.g. LlO cm/min) the are is incontact with the hottest part of the workpiece, i.e.the metal last melted, and that the tip of the wirereceives radiation mainly from that side. At a givenmoment the growing droplet separates, the arejumps over from the droplet to the wire, and thedroplet falls spinning (this can clearly be seen in thefilm) along a curved path into the weld pool. Sincethe arc creates its own length, aswehave seen above,the coarse droplets do not give rise to short-circuiting. Fig. 12 gives some frames from a colourfilm taken with the camera pointed obliquely down-wards, showing the cavity blown by the are, andthe oblique melting of the wire.It was known from a previous investigation 14)

(and confirmed by our experiments) that the droplettransfer in argon is quite different, particularly whena small percentage of oxygen has been added tothe argon: small droplets are formed at the tip ofthe melting wire and split off in the direction of thewire. Increasing the current reduces the size of thcdroplets, until at last they are 110 longer separatelydistinguishable 15).

Fig. 12. Frames from a 16 mm colour film of CO2 welding,using a short are (as in practice). The film speed was 2000frames per second, the camera being directed obliquely down-wards, giving a view of the cavity blown by the are in theworkpiece. On the left can be seen the growing end of tbe weldbead; right, above the weld pool, the molten tip of the wireand the droplet passing in the are. (This is particularly clearit, the photos on the right, which also show under the weldpool the reflection of the electrode wire in the gleaming sur-face of the workpiece.)

14) H. T. Herbst and T. McElrath, Welding J. 30, 1084., 1951.15) Our investigation related to helium as well as argon:

as far as droplet transfer is concerned, He resembles Aat high currents, and CO2 at low currents.

The slow-motion pictures reveal another remark-able difference between argon and CO2: with argonthe are plasma envelopes the tip of the wire,including the hanging droplet, but with CO2 onlypart of the droplet is covered, no more than halfits surface area. Without going too much into thedetails, some of which have still not been explained,we should mention here that this difference in plasmacoverage is probably attributable to a differencein the contraction of the plasma 16). Both with argonand CO2 there is the familiar contraction due tomagnetic attraction of the current paths ("pincheffect"), but the contraction is increased by localcooling of the gas 17), especially in the neighbourhoodof the relatively cold wire tip, and this effect willbe most marked in gases with the highest thermalconductivity - a property which is particularlynoticeable in gases which dissociate at the high aretemperatures, such as H2 and N2 and above allCO2 (see fig. 13).

0 -:V /I ( /I I 7

JL I 7

~If iJl /~87 If!I I I

I I /I I 1/

./ J l/

90

80

70

60

50

40

30

20

ID

2 3 7 9 IOxl03 OK5 6 87856

Fig. 13. Dissociation of various poly-atomic gases, as a func-tion of temper a ture , at a pressure of 1 atm. (After: H. C.Ludwig, Welding J. 38, 296S, 1959.)

The diffcrences found between argon and CO2

in regard to droplet size and plasma contraction areundoubtedly related to one another. If the cross-section of the contact surface between plasma anddroplet is smaller than the cross-section of the neckof the droplet, as in CO2, the result is an electro-

16) Sec last article of 13).17) ln the "plasma burner", which is now in use for producing

exceptionally high temperatures (e.g. 10000 °C), a water-cooled copper wall is employed for constricting the areplasma. See, for example, Welding and Metal Fabrication27, 287, 1959.

1962/63, No. 1 ARC WELDING OF STEEL 25

dynamic force directed upwards, which is increasedby the pressure ofthe gas in the arc. This enables thedroplet to grow to quite a size until the pinch effectin the neck, together with the force of gravity andsurface tension, cause the droplet to separate andthe are to jump back to the wire (see above). Ifthe contact surface is larger than the cross-sectionof the droplet neck, however, as in argon(fig.14), theresultant force is directed downwards. The drop-lets then break off and fall into the weld poolwithout having a chance to grow large.

We have dwelt on these effects at some lengthbecause, in our opinion, they make the relativelyslight oxidation of the steel in the CO2 atmosphereto some extent understandable. For owing to thelarge volume of each droplet transferred, and alsoto the fact that the droplet is only partially incontact with the extremely hot plasma, the surfaceexposed to oxidation is relatively small. I t is prob-able, however, that other factors have a bearingon the oxidation, as for example the very short timein which the droplet forms and passes in the are,and the violent perturbation of the molten material.Our hypothesis is not yet capable of providing aquantitative explanation of the processes involved,especially the measured burn-off of Si and Mn.

Fig. 14. Droplet formation on the wire tip, a) CO2 welding,b) welding in argon + 5% oxygen. In (a) the arc plasma(shaded area) covers only a relatively small part of the dropletsurface. Electrodynamic and other forces produce a resultantupward force, enabling the droplet to grow to an appreciablesize; in (b) the resultant force is in the downward direction,and the droplets remain small.

Comparison of CO2 welding with other methods

In the engineering industry there are manymachining operations that resemble one another tosome extent but which exist in their own right sideby side in fields that often overlap, as for exampleboring, drilling, broaching, punching, milling, ream-ing, etc. One cannot say that anyone of thesemethods is "the best", and the same applies to themany and various methods of welding. Any com-parison between them should rather serve as anattempt to delineate their useful scope.

If we first compare CO2 welding with the methodsusing basic and zircon-basic electrodes, we note thatwhile they are all suitable for mild steel and low-alloy steel, CO2 welding entails higher investmentcosts and therefore will not be considered wherethe volume of welding work is small. Moreover,zircon-basic electrodes give a smoother weld and willaccordingly be preferred where a particularly trimfinish is required. Another limitation of CO2 welding,at least in the present state of the art, is the needto use direct current, which again calls for moreexpensive welding equipment. Alternating currentis less suitable because CO2 has a high ionizationpotential, which means that a very high open-circuit voltage would be required from the weldingtr ansformer to ensure reliable re-striking. (Thisdrawback can be eliminated by using special wirecontaining as additive or coating a substancehaving a high thermionic emission, e.g. caesium.)

In the case of mass production, however, or wherevery long joints have to be welded, the investmentcosts are outweighed by the greater speed of CO2

welding. This process can be used to best advantagefor automatic or semi-automatic work, to which itis ideally adapted. In fact, where fully automaticwork is concerned, there is only one other methodthat can compare with CO2 welding, and that issubmerged-arc welding, which has hitherto beenthe most widely adopted automatic welding process.

Submerged-arc welding was described very brieflyat the beginning of this articlc. Fig. 15 gives aclearer idea ofits operation. Bare wire is continuouslyfcd from a reel and melted in an are which isblanketed by the uninterrupted deposition of a

7B57

7858

Fig. 15. Submerged-are welding. A consumable bare wire.B current supply. G tube through which the slag-forming gran-ular flux P is continuously deposited over the arc. The arcitself is therefore invisible and burns in a cavity inside thelayer of flux. S weld bead with layer of slag. L bead from whichslag has been removed. H adjustable pointer for directing thearc to the groove to be welded.

26 PIHLIPS TECHNICAL REVIEW VOLUME 24

granular slag-forming flux. The fact that the are isthus submerged largely governs the' characteristicsof the process: 1) very high currents can he usedsince the intense radiation is not troublesome;2) relatively good use is made of the are heat, whichis retained inside the fusible material; 3) the methodlends itself well only to fully automatic welding, forif the operator wanted to control the welding headmanually he would have to work by feel, beingunable to see the arc. This is the price to be paid forthe advantage of not having to shield the eyes. Asregards point (1) currents of 1000 A and higherwere initially used, permitting very high weldingspeeds and often making it possible to producethick welded joints in a single pass. It was laterestablished that the structure of such a single-passweld was too coarse, to the detriment ofthe mechan-ical properties. Extremely high currents are there-fore no longer the rule, but the welding speeds arestill very high.In comparing CO2 welding and submerged-arc

welding, then, it must he stated that neithermethod differs much as far as welding speed is con-cerned. Preference for one method or the othcr willtherefore depend rather on secondary factors, someof which we will mention here.In the CO2-shielded method the nozzle through

which the gas streams gradually becomes constrictedby spatter during welding, and therefore has tobe regularly cleaned. Submerged-arc welding, beingfree from spatter, does not have this drawback. Onthe other hand, time is wasted with this method inremoving the surplus flux and the slag formed aroundthe weld, particularly in the case of narrow joints,where the slag has to be chipped away. This has thefurther disagreeable consequence of making itdifficult to keep the working area clean. One of thevirtues of gas-shielded welding is precisely the factthat the gas, after having served its purpose, dis-appears of its own accord or can easily he contin-uously exhausted.We have already pointed out that the melting

of the slag costs heat. This reduces the rate of de-position in grams per amper,? per minute. Table IV,which includes some deposition rates mentionedearlier in this article, shows that the highest valuesare obtained in CO2 welding.The profile of the weld bead in CO2 welding is

often quite convex. This is especially noticeable insquare butt welds; in submerged-arc welding thesurface of the weld is smoother.

Submerged-arc welding sometimes gives troublein a humid atmosphere, because the flux absorbsmoisture. CO2 welding, on the other hand, may be

Table IV. Deposition rate of metal in grams per ampereper ~nute (approximate values) in various welding methods.

Coated electrodes with free are

Contact electrodes

0.17

0.24

0.24-0.30

0.30-0.40

Submerged-arc welding

CO2-shielded welding

hampered by side winds, making it necessary tofit up a screen or curtain, or to increase the gassupply.

Finally, as the flux in submerged-arc welding isheld in place by gravity, vertical upwards and over-head welding are of course impossible. These posi-tions present no problems in CO2 welding.

Although the CO2 method of welding hasmade headway all over the world in a remarkablyshort time, and is still gaining ground, it will beclear from the survey presented here that we arenot predicting that it will eventually supersedesubmerged-arc welding. It is rather to be assumedthat engineering will have room for both methods,just as there will continue to be scope for coatedwelding electrodes and for the numerous variantsof processes in which the weld metal is shielded byslag, by gas or by combinations of both.

Summary. The many existing methods of are-welding steelcan be classified according to the way in which the moltenmetal is shielded from the atmosphere. This can he done byslag (as in submerged-arc welding), by gas (as in argon-arcor CO2welding), or by slag and gas combined (e.g. using coatedelectrodes). Three developments are discussed in the lattertwo categories, due to Philips Research Laboratories in Eind-hoven: Contact electrodes, zircon-basic electrodes and CO2welding.

Contact (iron-powder) versions of acid-coated electrodeshave been widely used since 1947 for their case and speed ofwelding and other useful properties. A Contact version of basic(low-hydrogen) electrodes, which are especially suitable forlow-alloy steels, was difficult to make because of the low vis-cosity of the slag. The addition of Zr02 to the coating broughtan improvement, and the "zircon-basic" electrodes developedon this basis (type Ph 86, and the Contact type, C 6) produceexcellent results in down-hand welding, both as regards theappearance of the weld and its mechanical properties.

In CO2 welding, consumable bare wire, contaiuing about0.9% Si and 1.6% Mn as deoxidizing additives, is fed by anelectric motor from a reel to the are, which is shieldedby carbondioxide gas issuing from a nozzle around the tip of the wire.Although a very high partial oxygen pressure is to be expected,due to dissociation of the CO2 in the are, the above percentage,or even lower, of Si and other additives is surprisingly enoughcapable of almost entirely preventing oxidation ofthe depositedmetal. This is attributcd to the large size of the dropletstransferred in the are, as observed with a high-speed cinecamera, and which is probably bound up with the marked con-traction of the are plasma (in argon, for example, the contrac-tion is muchless marked and the droplets are therefore smaller).

Like submerged-arc welding, CO2 welding lends itself verywell to automatic operation. In a comparison of the variousmethods of are welding a rough indication is given of theirappropriate fields of application.