welding and welding symbols

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In general, welding may be described as a process ofuniting two pieces of metal or alloy by raising thetemperature of the surfaces to be joined so that theybecome plastic or molten. This may be done with orwithout the application of pressure and with or withoutthe use of added metal. This definition excludes themore recently developed method of cold-welding, inwhich pressure alone is used. Cold-welding, however,has a limited application, and is used principally foraluminium and its alloys, and not for steel.

There are numerous methods of welding, but theycan be grouped broadly into two categories. Forgewelding is the term covering a group of weldingprocesses in which the parts to be joined are heated toa plastic condition in a forge or other furnace, and arewelded together by applying pressure or impact, e.g.by rolling, pressing, or hammering. Fusion welding isthe process where the surfaces to be joined are meltedwith or without the addition of filler metal. The termis generally reserved for those processes in whichwelding is achieved by fusion alone, without pressure.

Forge welding will be dealt with first. Pressurewelding is the welding of metal by means of mechanicalpressure whilst the surfaces to be joined are maintainedin a plastic state. The heating for this process is usuallyprovided by the process of resistance welding, wherethe pieces of metal to be joined are pressed togetherand a heavy current is passed through them.

Projection welding is a resistance-welding processin which fusion is produced by the heat obtained fromthe resistance to flow of electric current through thework parts, which are held together under pressure bythe electrodes providing the current. The resulting weldsare localized at predetermined points by the design ofthe parts to be welded. The localization is usuallyaccomplished by projections or intersections.

Spot welding is a resistance-welding process of joiningtwo or more overlapping parts by local fusion of a smallarea or ‘spot’. Two copper-alloy electrodes contacteither side of the overlapped sheets, under known loadsproduced by springs or air pressure. Stitch welding isspot welding in which successive welds overlap. Seamwelding is a resistance-welding process in which theelectrodes are discs. Current is switched on and offregularly as the rims of the discs roll over the work, withthe result that a series of spot welds is at such points.If a gas-tight weld is required, the disc speed and timecycle are adjusted to obtain a series of overlappingwelds.

Flash-butt welding is a resistance-welding processwhich may be applied to rod, bar, tube, strip, or sheetto produce a butt joint. After the current is turned on,the two parts are brought together at a predeterminedrate so that discontinuous arcing occurs between thetwo parts to be joined. This arcing produces a violentexpulsion of small particles of metal (flashing), and apositive pressure in the weld area will exclude air andminimize oxidation. When sufficient heat has beendeveloped by flashing, the parts are brought togetherunder heavy pressure so that all fused and oxidizedmaterial is extruded from the weld.

Fusion-welding processes can now be dealt with.The heat for fusion welding is provided by either gasor electricity. Gas welding is a process in which heatfor welding is obtained from a gas or gases burning ata sufficiently high temperature produced by anadmixture of oxygen. Examples of the gases used areacetylene (oxy-acetylene welding), hydrogen (oxy-hydrogen welding), and propane (oxy-propane welding).In air-acetylene welding, the oxygen is derived fromthe atmosphere by induction.

Electrical fusion welding is usually done by theprocess of ‘arc welding’. Metal-arc welding is weldingwith a metal electrode, the melting of which providesthe filler metal. Carbon arc welding is a process of arcwelding with a carbon electrode (or electrodes), inwhich filler metal and sometimes flux may be used.Submerged-arc welding is a method in which a barecopper-plated steel electrode is used. The arc is entirelysubmerged under a separate loose flux powder whichis continually fed into and over the groove which ismachined where the edges to be welded are placedtogether. Some of the flux powder reacts with the moltenmetal: part fuses and forms a refining slag whichsolidifies on top of the weld deposit; the remainder ofthe powder covers the weld and slag, shielding themfrom atmospheric contamination and retarding the rateof cooling.

Argon-arc welding is a process where an arc is struckbetween an electrode (usually tungsten) and the workin an inert atmosphere provided by directing argoninto the weld area through a sheath surrounding theelectrode. Heliarc welding uses helium to provide theinert atmosphere, but this process is not used in theUnited Kingdom, because of the non-availability ofhelium. Several proprietary names are used for weldingprocesses of this type, e.g. Sigma (shielded inert-gasmetal-arc) welding uses a consumable electrode in an

Chapter 26

Welding and welding symbols


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Welding and welding symbols 211

argon atmosphere. Atomic-hydrogen arc welding is aprocess where an alternating-current arc is maintainedbetween tungsten electrodes, and each electrode issurrounded by an annular stream of hydrogen. In passingthrough the arc, the molecular hydrogen is dissociatedinto its atomic state. The recombination of the hydrogenatoms results in a very great liberation of heat whichis used for fusing together the metals to be joined.Stud welding is a process in which an arc is struckbetween the bottom of a stud and the base metal. Whena pool of molten metal has formed, the arc isextinguished and the stud is driven into the pool toform a weld.

The application of weldingsymbols to workingdrawings

The following notes are meant as a guide to the methodof applying the more commonly used welding symbolsrelating to the simpler types of welded joints onengineering drawings. Where complex joints involvemultiple welds it is often easier to detail such cons-tructions on separate drawing sheets.

Each type of weld is characterized by a symbolgiven in Table 26.1 Note that the symbol is representativeof the shape of the weld, or the edge preparation, butdoes not indicate any particular welding process anddoes not specify either the number of runs to be deposit-ed or whether or not a root gap or backing material isto be used. These details would be provided on a weldingprocedure schedule for the particular job.

It may be necessary to specify the shape of the weldsurface on the drawing as flat, convex or concave anda supplementary symbol, shown in Table 26.2, is thenadded to the elementary symbol. An example of eachtype of weld surface application is given in Table 26.3.

A joint may also be made with one type of weld ona particular surface and another type of weld on theback and in this case elementary symbols representingeach type of weld used are added together. The lastexample in Table 26.3 shows a single-V butt weldwith a backing run where both surfaces are required tohave a flat finish.

A welding symbol is applied to a drawing by usinga reference line and an arrow line as shown in Fig.26.1. The reference line should be drawn parallel tothe bottom edge of the drawing sheet and the arrowline forms an angle with the reference line. The side ofthe joint nearer the arrow head is known as the ‘arrowside’ and the remote side as the ‘other side’.

The welding symbol should be positioned on thereference line as indicated in Table 26.4.

Sketch (a) shows the symbol for a single-V buttweld below the reference line because the externalsurface of the weld is on the arrow side of the joint.

Sketch (b) shows the same symbol above thereference line because the external surface of the weldis on the other side of the joint.

From of weld Illustration BS symbol

Butt weld between flangedplates (the flanges beingmelted down completely)

Square butt weld

Single-V butt weld

Single-bevel butt weld

Single-V butt weld withbroad root face

Single-bevel butt weld withbroad root face

Single-U butt weld

Single-J butt weld

Backing or sealing run

Fillet weld

Plug weld (circular orelongated hole, completelyfilled)

Spot weld (resistance orarc welding) or projectionweld

Seam weld

(b) Arc

(a) Resistance

Table 26.1 Elementary weld symbols




212 Manual of Engineering Drawing

Table 26.2 Supplementary symbols

Shape of weld surface BS symbol

flat (usually finished flush)

convex

concave

Table 26.3 Some examples of the application of supplementarysymbols

Form of weld Illustration BS symbol

Flat (flush) single-V buttweld

Convex double-V buttweld

Concave fillet weld

Flat (flush) single-V buttweld with flat (flush)backing run

Sketch (c) shows the symbol applied to a double-Vbutt weld.

Sketch (d) shows fillet welds on a cruciform jointwhere the top weld is on the arrow side and the bottomweld is on the other side

The positioning of the symbol is the same fordrawings in first or third angle projection.

Additional symbols can be added to the referenceline as shown in Fig. 26.2. Welding can be done in the

Graphic SymbolicIllustration representation representation

(a)

(b)

(c)

(d)

Table 26.4 Significance of the arrow and the position of the weldsymbol

Joint

1 2 3

1 is the arrow line2 is the reference line3 is the symbol

Fig. 26.1

factory or on site when plant is erected. A site weld isindicated by a flag. A continuous weld all round ajoint is shown by a circle at the intersection of thearrow and the reference line. Note that if a continuousweld is to be undertaken at site then both symbolsshould be added to the drawing.

The introductory notes relating to welding processesare of a general nature. There are many specializedmethods listed in BS 499. Each process is given anindividual identification number and group headingsare as follows; (a) Arc welding, (b) Resistance welding,(c) Gas welding, (d) Solid phase welding; Pressurewelding, (e) Other welding processes, (f) Brazing,soldering and braze welding.

A welding procedure sheet will usually give detailsof the actual process to be used on a particular joint.On the drawing, a reference line with an arrow pointing

Fig. 26.2 Indication of (a) site welds and (b) continuous welds

(a)

(b)



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