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Welding and joining processes Process terminology

The European standard, BS EN ISO 4063:2000 Welding and allied processes - Nomenclature of processes and reference numbers, assigns a unique number to the main welding processes. These are grouped as follows:

Arc welding Resistance welding Gas welding Forge welding Other welding processes Brazing, soldering and braze welding

Each process is identified within the group by a numerical index or reference number. For example, the MIG welding process has a reference number of 131 which is derived as follows:

1 - Arc welding 3 - Gas-shielded metal arc welding 1 - Metal arc inert gas welding

The main arc welding process reference numbers are:

111 MMA with covered electrodes 114 Flux cored wire (self-shielded) 112 Submerged arc 131 MIG (inert gas) 135 MAG (CO 2, active gas) 141 TIG 15 Plasma welding

The reference numbers are used as a convenient way of identifying the welding process in documentation such as welding procedure (EN 288) and welder qualification (EN 287) records.

Process options

Factors which must be taken into account when choosing a suitable welding or joining process are:

material type plate or tubular quality and strength requirements

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degree of mechanization capital cost

Although consideration of these factors will identify the most suitable welding process, the choice within a company may be restricted by the cost of implementing a new process, availability of plant or current workforce skill. Welding and joining processes available to the welding engineer can be separated into the following generic types:

Fusion o arc o gas o power beam o resistance

Thermomechanical o friction o flash o explosive

Mechanical o fasteners

Solid state o adhesive o soldering o brazing

The suitability of the processes for welding and joining materials, joint types and components are shown in Table 1.

In selecting a suitable process, consideration must also be given to the type of application, for example, the portability of equipment, whether it can be used on site, whether it is manual or mechanized, and the overall cost of the welding plant.

Process Index no. Steel Stainless Al Butt joint

Lap joint Plate Tube Portability Manual

Mechanised Automated Site

Arc 1 Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

Gas 3 Yes Possible Possible Yes Yes Yes Yes Yes Yes No Yes

Laser 52 Yes Yes Possible Yes Yes Yes Yes No No Yes No

Resistance 2 Yes Yes Yes Possible Yes Yes Possible Possible Yes Yes No

Friction 42 Yes Yes Yes Yes No Yes No No No Yes No

Brazing 9 No Yes Yes No Yes Yes Possible Yes Yes Possible Yes

Fasteners none Yes Yes Yes No Yes Yes No Possible Yes Yes Yes

Adhesives none Yes Yes Yes No Yes Yes Yes Yes Yes Possible Yes

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Fusion welding processes

When welding using a fusion process, the edges of a component are melted together to form weld metal.

Process Heat source Shield Parent metal thickness mm

Deposition rate Kg/hr

Arc MMA Arc Gas/flux 1-100 1-2

MIG Arc Gas 0.5-100 1-8

TIG Arc Gas 0.1-100 1-4

SAW Arc Flux 5-100 5-20

ES/EG Resistance/arc Gas/flux 5-100 -

Stud Arc - 4-20 -

Gas Oxyfuel Flame Gas 0.6-10 1-2

Power beam Laser Radiation Gas 0.2-100 -

EB Electrogas Vacuum 0.2-100 -

Resistance Spot/Seam Arc - 0.2-10 -

Thermit Thermit Chemical Gas 10-100 -

Table 2 shows heat source, mode of shielding, thickness range and metal deposition rates for a range of fusion processes. Although fusion welding is one of the simplest joining techniques, problems likely to occur include porosity in the weld metal, and cracking in either the weld or heat affected zone (HAZ). Porosity is avoided by ensuring adequate shielding of the weld pool and, for materials such as aluminum, the addition of filler wire.

Consideration of the joint design and the chemistry of the weld metal will prevent weld metal cracking. HAZ cracking which might be caused by hydrogen, is avoided by using low hydrogen consumables (MMA) and controlling the heat input and the rate of cooling of the parent metal.

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The Manual Metal Arc process Manual metal arc welding was first invented in Russia in 1888. It involved a bare metal rod with no flux coating to give a protective gas shield. The development of coated electrodes did not occur until the early 1900s when the Kjellberg process was invented in Sweden and the Quasi-arc method was introduced in the UK. It is worth noting that coated electrodes were slow to be adopted because of their high cost. However, it was inevitable that as the demand for sound welds grew, manual metal arc became synonymous with coated electrodes. When an arc is struck between the metal rod (electrode) and the work piece, both the rod and work piece surface melt to form a weld pool. Simultaneous melting of the flux coating on the rod will form gas and slag which protects the weld pool from the surrounding atmosphere. The slag will solidify and cool and must be chipped off the weld bead once the weld run is complete (or before the next weld pass is deposited).

The process allows only short lengths of weld to be produced before a new electrode needs to be inserted in the holder. Weld penetration is low and the quality of the weld deposit is highly dependent on the skill of the welder.

Types of flux/electrodes

Arc stability, depth of penetration, metal deposition rate and positional capability are greatly influenced by the chemical composition of the flux coating on the electrode. Electrodes can be divided into three main groups:

Cellulosic Rutile Basic

Cellulosic electrodes contain a high proportion of cellulose in the coating and are characterized by a deeply penetrating arc and a rapid burn-off rate giving high welding speeds. Weld deposit can be coarse and with fluid slag, deslagging can be difficult. These electrodes are easy to use in any position and are noted for their use in the 'stovepipe' welding technique.

Features:

deep penetration in all positions suitability for vertical down welding reasonably good mechanical properties

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high level of hydrogen generated - risk of cracking in the heat affected zone (HAZ)

Rutile electrodes contain a high proportion of titanium oxide (rutile) in the coating. Titanium oxide promotes easy arc ignition, smooth arc operation and low spatter. These electrodes are general purpose electrodes with good welding properties. They can be used with AC and DC power sources and in all positions. The electrodes are especially suitable for welding fillet joints in the horizontal/vertical (H/V) position.

Features:

moderate weld metal mechanical properties good bead profile produced through the viscous slag positional welding possible with a fluid slag (containing fluoride) easily removable slag

Basic electrodes contain a high proportion of calcium carbonate (limestone) and calcium fluoride (fluorspar) in the coating. This makes their slag coating more fluid than rutile coatings - this is also fast-freezing which assists welding in the vertical and overhead position. These electrodes are used for welding medium and heavy section fabrications where higher weld quality, good mechanical properties and resistance to cracking (due to high restraint) are required.

Features:

low hydrogen weld metal requires high welding currents/speeds poor bead profile (convex and coarse surface profile) slag removal difficult

Metal powder electrodes contain an addition of metal powder to the flux coating to increase the maximum permissible welding current level. Thus, for a given electrode size, the metal deposition rate and efficiency (percentage of the metal deposited) are increased compared with an electrode containing no iron powder in the coating. The slag is normally easily removed. Iron powder electrodes are mainly used in the flat and H/V positions to take advantage of the higher deposition rates. Efficiencies as high as 130 to 140% can be achieved for rutile and basic electrodes without marked deterioration of the arcing characteristics but the arc tends to be less forceful which reduces bead penetration.

Power source

Electrodes can be operated with AC and DC power supplies. Not all DC electrodes can be operated on AC power sources, however AC electrodes are normally used on DC.

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Welding current

Welding current level is determined by the size of electrode - the normal operating range and current are recommended by manufacturers. Typical operating ranges for a selection of electrode sizes are illustrated in the table. As a rule of thumb when selecting a suitable current level, an electrode will require about 40A per millimeter (diameter). Therefore, the preferred current level for a 4mm diameter electrode would be 160A, but the acceptable operating range is 140 to 180A.

What's new

Transistor (inverter) technology is now enabling very small and comparatively low weight power sources to be produced. These power sources are finding increasing use for site welding where they can be readily transported from job to job. As they are electronically controlled, add-on units are available for TIG and MIG welding which increase the flexibility. Electrodes are now available in hermetically sealed containers. These vacuum packs obviate the need for baking the electrodes immediately prior to use. However, if a container has been opened or damaged, it is essential that the electrodes are redried according to the manufacturer's instructions.

The oxyacetylene process Process features Oxyacetylene welding, commonly referred to as gas welding, is a process which relies on combustion of oxygen and acetylene. When mixed together in correct proportions within a hand-held torch or blowpipe, a relatively hot flame is produced with a temperature of about 3,200 deg.C. The chemical action of the oxyacetylene flame can be adjusted by changing the ratio of the volume of oxygen to acetylene.

Three distinct flame settings are used, neutral, oxidizing and carburising.

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Neutral flame

Oxidising flame

Carburising flame

Welding is generally carried out using the neutral flame setting which has equal quantities of oxygen and acetylene. The oxidising flame is obtained by increasing just the oxygen flow rate while the carburising flame is achieved by increasing acetylene flow in relation to oxygen flow. Because steel melts at a temperature above 1,500 deg.C, the mixture of oxygen and acetylene is used as it is the only gas combination with enough heat to weld steel. However, other gases such as propane, hydrogen and coal gas can be used for joining lower melting point non-ferrous metals, and for brazing and silver soldering.

Equipment Oxyacetylene equipment is portable and easy to use. It comprises oxygen and acetylene gases stored under pressure in steel cylinders. The cylinders are fitted with regulators and flexible hoses which lead to the blowpipe. Specially designed safety devices such as flame traps are fitted between the hoses and the cylinder regulators. The flame trap prevents flames generated by a 'flashback' from reaching the cylinders; principal causes of flashbacks are the failure to purge the hoses and overheating of the blowpipe nozzle.

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When welding, the operator must wear protective clothing and tinted coloured goggles. As the flame is less intense than an arc and very little UV is emitted, general-purpose tinted goggles provide sufficient protection.

Operating characteristics The action of the oxyacetylene flame on the surface of the material to be welded can be adjusted to produce a soft, harsh or violent reaction by varying the gas flows. There are of course practical limits as to the type of flame which can be used for welding. A harsh forceful flame will cause the molten weld pool to be blown away, while too soft a flame will not be stable near the point of application. The blowpipe is therefore designed to accommodate different sizes of 'swan neck copper nozzle which allows the correct intensity of flame to be used. The relationship between material thickness, blowpipe nozzle size and welding speed, is shown in the chart. When carrying out fusion welding the addition of filler metal in the form of a rod can be made when required. The principal techniques employed in oxyacetylene welding are leftward, rightward and all-positional rightward. The former is used almost exclusively and is ideally suited for welding butt, fillet and lap joints in sheet thicknesses up to approximately 5mm. The rightward technique finds application on plate thicknesses above 5mm for welding in the flat and horizontal-vertical position. The all-positional rightward method is a modification of the rightward technique and is ideally suited for welding steel plate and in particular pipework where positional welding, (vertical and overhead) has to be carried out. The rightward and all- positional rightward techniques enable the welder to obtain a uniform penetration bead with added control over the molten weldpool and weld metal. Moreover, the welder has a clear view of the weldpool and can work in complete freedom of movement. These techniques are very highly skilled and are less frequently used than the conventional leftward technique.

Solid wire MIG welding Metal inert gas (MIG) welding was first patented in the USA in 1949 for welding aluminium. The arc and weld pool formed using a bare wire electrode was protected by helium gas, readily available at that time. From about 1952 the process became popular in the UK for welding aluminium using argon as the shielding gas, and for carbon steels

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using CO 2 . CO 2 and argon-CO 2 mixtures are known as metal active gas (MAG) processes. MIG is an attractive alternative to MMA, offering high deposition rates and high productivity.

Process characteristics

MIG is similar to MMA in that heat for welding is produced by forming an arc between a metal electrode and the workpiece; the electrode melts to form the weld bead. The main differences are that the metal electrode is a small diameter wire fed from a spool and an externally supplied shielding gas is necessary. As the wire is continuously fed, the process is often referred to as semi-automatic welding.

Metal transfer mode

The manner, or mode, in which the metal transfers from the electrode to the weld pool largely determines the operating features of the process. There are three principal metal transfer modes:

Short circuiting Droplet / spray Pulsed

Short-circuiting and pulsed metal transfer are used for low current operation while spray metal transfer is only used with high welding currents. In short-circuiting or'dip' transfer, the molten metal forming on the tip of the wire is transferred by the wire dipping into the weld pool. This is achieved by setting a low voltage; for a 1.2mm diameter wire, arc voltage varies from about 17V (100A) to 22V (200A). Care in setting the voltage and the inductance in relation to the wire feed speed is essential to minimise spatter. Inductance is used to control the surge in current which occurs when the wire dips into the weld pool.

For droplet or spray transfer, a much higher voltage is necessary to ensure that the wire does not make contact i.e.short-circuit, with the weld pool; for a 1.2mm diameter wire, the arc voltage varies from approximately 27V (250A) to 35V (400A). The molten metal at the tip of the wire transfers to the weld pool in the form of a spray of small droplets (about the diameter of the wire and smaller). However, there is a minimum current level, threshold, below which droplets are not forcibly projected across the arc. If an open arc technique is attempted much below the threshold current level, the low arc forces would be insufficient to prevent large droplets forming at the tip of the wire. These droplets would transfer erratically across the arc under normal gravitational forces. The pulsed mode was developed as a means of stabilising the open arc at low current levels i.e. below the threshold level, to avoid short-

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circuiting and spatter. Metal transfer is achieved by applying pulses of current, each pulse having sufficient force to detach a droplet. Synergic pulsed MIG refers to a special type of controller which enables the power source to be tuned (pulse parameters) for the wire composition and diameter, and the pulse frequency to be set according to the wire feed speed.

Shielding gas

In addition to general shielding of the arc and the weld pool, the shielding gas performs a number of important functions:

forms the arc plasma stabilises the arc roots on the material surface ensures smooth transfer of molten droplets from the wire to the weld pool

Thus, the shielding gas will have a substantial effect on the stability of the arc and metal transfer and the behaviour of the weld pool, in particular, its penetration. General purpose shielding gases for MIG welding are mixtures of argon, oxygen and CO 2 , and special gas mixtures may contain helium. The gases which are normally used for the various materials are:

steels o CO 2 o argon +2 to 5% oxygen o argon +5 to 25% CO 2

non-ferrous o argon o argon / helium

Argon based gases, compared with CO 2 , are generally more tolerant to parameter settings and generate lower spatter levels with the dip transfer mode. However, there is a greater risk of lack of fusion defects because these gases are colder. As CO 2 cannot be used in the open arc (pulsed or spray transfer) modes due to high back-plasma forces, argon based gases containing oxygen or CO 2 are normally employed.

Applications

MIG is widely used in most industry sectors and accounts for more than 50% of all weld metal deposited. Compared to MMA, MIG has the advantage in terms of flexibility, deposition rates and suitability for mechanisation. However, it should be noted that while MIG is ideal for 'squirting' metal, a high degree of manipulative skill is demanded of the welder.

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Submerged-arc Welding

The first patent on the submerged-arc welding (SAW) process was taken out in 1935 and covered an electric arc beneath a bed of granulated flux. Developed by the E O Paton Electric Welding Institute, Russia, during the Second World War, SAW's most famous application was on the T34 tank.

Process features

Similar to MIG welding, SAW involves formation of an arc between a continuously-fed bare wire electrode and the workpiece. The process uses a flux to generate protective gases and slag, and to add alloying elements to the weld pool. A shielding gas is not required. Prior to welding, a thin layer of flux powder is placed on the workpiece surface. The arc moves along the joint line and as it does so, excess flux is recycled via a hopper. Remaining fused slag layers can be easily removed after welding. As the arc is completely covered by the flux layer, heat loss is extremely low. This produces a thermal efficiency as high as 60% (compared with 25% for manual metal arc). There is no visible arc light, welding is spatter-free and there is no need for fume extraction.

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Operating characteristics

SAW is usually operated as a fully-mechanised or automatic process, but it can be semi-automatic. Welding parameters: current, arc voltage and travel speed all affect bead shape, depth of penetration and chemical composition of the deposited weld metal. Because the operator cannot see the weld pool, greater reliance must be placed on parameter settings.

Process variants

According to material thickness, joint type and size of component, varying the following can increase deposition rate and improve bead shape.

Wire

SAW is normally operated with a single wire on either AC or DC current. Common variants are:

twin wire triple wire single wire with hot wire addition metal powder addition

All contribute to improved productivity through a marked increase in weld metal deposition rates and/or travel speeds.

Flux

Fluxes used in SAW are granular fusible minerals containing oxides of manganese, silicon, titanium, aluminium, calcium, zirconium, magnesium and other compounds such as calcium fluoride. The flux is specially formulated to be compatible with a given electrode wire type so that the combination of flux and wire yields desired mechanical properties. All fluxes react with the weld pool to produce the weld metal chemical composition and mechanical properties. It is common practice to refer to fluxes as 'active' if they add manganese and silicon to the weld, the amount of manganese and silicon

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added is influenced by the arc voltage and the welding current level. The the main types of flux for SAW are:

Bonded fluxes - produced by drying the ingredients, then bonding them with a low melting point compound such as a sodium silicate. Most bonded fluxes contain metallic deoxidisers which help to prevent weld porosity. These fluxes are effective over rust and mill scale.

Fused fluxes - produced by mixing the ingredients, then melting them in an electric furnace to form a chemically homogeneous product, cooled and ground to the required particle size. Smooth stable arcs, with welding currents up to 2000A and consistent weld metal properties, are the main attraction of these fluxes.

Applications

SAW is ideally suited for longitudinal and circumferential butt and fillet welds. However, because of high fluidity of the weld pool, molten slag and loose flux layer, welding is generally carried out on butt joints in the flat position and fillet joints in both the flat and horizontal-vertical positions. For circumferential joints, the workpiece is rotated under a fixed welding head with welding taking place in the flat position. Depending on material thickness, either single-pass, two-pass or multipass weld procedures can be carried out. There is virtually no restriction on the material thickness, provided a suitable joint preparation is adopted. Most commonly welded materials are carbon-manganese steels, low alloy steels and stainless steels, although the process is capable of welding some non-ferrous materials with judicious choice of electrode filler wire and flux combinations.

TIG Welding Tungsten inert gas (TIG) welding became an overnight success in the 1940s for joining magnesium and aluminium. Using an inert gas shield instead of a slag to protect the weldpool, the process was a highly attractive replacement for gas and manual metal arc welding. TIG has played a major role in the acceptance of aluminium for high quality welding and structural applications.

Process characteristics

In the TIG process the arc is formed between a pointed tungsten electrode and the workpiece in an inert atmosphere of argon or helium. The small intense arc provided by the pointed electrode is ideal for high quality and precision welding. Because the

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electrode is not consumed during welding, the welder does not have to balance the heat input from the arc as the metal is deposited from the melting electrode. When filler metal is required, it must be added separately to the weldpool.

Power source

TIG must be operated with a drooping, constant current power source - either DC or AC. A constant current power source is essential to avoid excessively high currents being drawn when the electrode is short-circuited on to the workpiece surface. This could happen either deliberately during arc starting or inadvertently during welding. If, as in MIG welding, a flat characteristic power source is used, any contact with the workpiece surface would damage the electrode tip or fuse the electrode to the workpiece surface. In DC, because arc heat is distributed approximately one-third at the cathode (negative) and two-thirds at the anode (positive), the electrode is always negative polarity to prevent overheating and melting. However, the alternative power source connection of DC electrode positive polarity has the advantage in that when the cathode is on the workpiece, the surface is cleaned of oxide contamination. For this reason, AC is used when welding materials with a tenacious surface oxide film, such as aluminium.

Arc starting

The welding arc can be started by scratching the surface, forming a short-circuit. It is only when the short-circuit is broken that the main welding current will flow. However, there is a risk that the electrode may stick to the surface and cause a tungsten inclusion in the weld. This risk can be minimised using the 'lift arc' technique where the short-circuit is formed at a very low current level. The most common way of starting the TIG arc is to use HF (High Frequency). HF consists of high voltage sparks of several thousand volts which last for a few microseconds. The HF sparks will cause the electrode - workpiece gap to break down or ionise. Once an electron/ion cloud is formed, current can flow from the power source.

Note: As HF generates abnormally high electromagnetic emission (EM), welders should be aware that its use can cause interference especially in electronic equipment. As EM emission can be airborne, like radio waves, or transmitted along power cables, care must be taken to avoid interference with control systems and instruments in the vicinity of welding.

HF is also important in stabilising the AC arc; in AC, electrode polarity is reversed at a frequency of about 50 times per second, causing the arc to be extinguished at each polarity change. To ensure that the arc is reignited at each reversal of polarity, HF sparks are generated across the electrode/workpiece gap to coincide with the beginning of each half-cycle.

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Electrodes

Electrodes for DC welding are normally pure tungsten with 1 to 4% thoria to improve arc ignition. Alternative additives are lanthanum oxide and cerium oxide which are claimed to give superior performance (arc starting and lower electrode consumption). It is important to select the correct electrode diameter and tip angle for the level of welding current. As a rule, the lower the current the smaller the electrode diameter and tip angle. In AC welding, as the electrode will be operating at a much higher temperature, tungsten with a zirconia addition is used to reduce electrode erosion. It should be noted that because of the large amount of heat generated at the electrode, it is difficult to maintain a pointed tip and the end of the electrode assumes a spherical or 'ball' profile.

Shielding gas

Shielding gas is selected according to the material being welded. The following guidelines may help:

Argon - the most commonly-used shielding gas which can be used for welding a wide range of materials including steels, stainless steel, aluminium and titanium.

Argon + 2 to 5% H2 - the addition of hydrogen to argon will make the gas slightly reducing, assisting the production of cleaner-looking welds without surface oxidation. As the arc is hotter and more constricted, it permits higher welding speeds. Disadvantages include risk of hydrogen cracking in carbon steels and weld metal porosity in aluminium alloys.

Helium and helium/argon mixtures - adding helium to argon will raise the temperature of the arc. This promotes higher welding speeds and deeper weld penetration. Disadvantages of using helium or a helium/argon mixture is the high cost of gas and difficulty in starting the arc.

Applications

TIG is applied in all industrial sectors but is especially suitable for high quality welding. In manual welding, the relatively small arc is ideal for thin sheet material or controlled penetration (in the root run of pipe welds). Because deposition rate can be quite low (using a separate filler rod) MMA or MIG may be preferable for thicker material and for fill passes in thick-wall pipe welds.

TIG is also widely applied in mechanised systems either autogenously or with filler wire. However, several 'off the shelf' systems are available for orbital welding of pipes, used in the manufacture of chemical plant or boilers. The systems require no manipulative skill, but the operator must be well trained. Because the welder has less control over arc and

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weldpool behaviour, careful attention must be paid to edge preparation (machined rather than hand-prepared), joint fit-up and control of welding parameters.

Plasma Welding Process characteristics

Plasma welding is very similar to TIG as the arc is formed between a pointed tungsten electrode and the workpiece. However, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. Plasma is then forced through a fine-bore copper nozzle which constricts the arc. Three operating modes can be produced by varying bore diameter and plasma gas flow rate:

Microplasma: 0.1 to 15A. The microplasma arc can be operated at very low welding currents. The columnar arc is stable even when arc length is varied up to 20mm.

Medium current: 15 to 200A. At higher currents, from 15 to 200A, the process characteristics of the plasma arc are similar to the TIG arc, but because the plasma is constricted, the arc is stiffer. Although the plasma gas flow rate can be increased to improve weld pool penetration, there is a risk of air and shielding gas entrainment through excessive turbulence in the gas shield.

Keyhole plasma: over 100A. By increasing welding current and plasma gas flow, a very powerful plasma beam is created which can achieve full penetration in a material, as in laser or electron beam welding. During welding, the hole progressively cuts through the metal with the molten weld pool flowing behind to form the weld bead under surface tension forces. This process can be used to weld thicker material (up to 10mm of stainless steel) in a single pass.

Power source

The plasma arc is normally operated with a DC, drooping characteristic power source. Because its unique operating features are derived from the special torch arrangement and separate plasma and shielding gas flows, a plasma control console can be added on to a conventional TIG power source. Purpose-built plasma systems are also available. The plasma arc is not readily stabilised with sine wave AC. Arc reignition is difficult when there is a long electrode to workpiece distance and the plasma is constricted, Moreover, excessive heating of the electrode during the positive half-cycle causes balling of the tip which can disturb arc stability.

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Special-purpose switched DC power sources are available. By imbalancing the waveform to reduce the duration of electrode positive polarity, the electrode is kept sufficiently cool to maintain a pointed tip and achieve arc stability.

Arc starting

Although the arc is initiated using HF, it is first formed between the electrode and plasma nozzle. This 'pilot' arc is held within the body of the torch until required for welding then it is transferred to the workpiece. The pilot arc system ensures reliable arc starting and, as the pilot arc is maintained between welds, it obviates the need for HF which may cause electrical interference.

Electrode

The electrode used for the plasma process is tungsten-2%thoria and the plasma nozzle is copper. The electrode tip diameter is not as critical as for TIG and should be maintained at around 30-60 degrees. The plasma nozzle bore diameter is critical and too small a bore diameter for the current level and plasma gas flow rate will lead to excessive nozzle erosion or even melting. It is prudent to use the largest bore diameter for the operating current level. Note: too large a bore diameter, may give problems with arc stability and maintaining a keyhole.

Plasma and shielding gases

The normal combination of gases is argon for the plasma gas, with argon plus 2 to 5% hydrogen for the shielding gas. Helium can be used for plasma gas but because it is hotter this reduces the current rating of the nozzle. Helium's lower mass can also make the keyhole mode more difficult.

Applications

Microplasma welding

Microplasma was traditionally used for welding thin sheets (down to 0.1 mm thickness), and wire and mesh sections. The needle-like stiff arc minimises arc wander and distortion. Although the equivalent TIG arc is more diffuse, the newer transistorised (TIG) power sources can produce a very stable arc at low current levels.

Medium current welding

When used in the melt mode this is an alternative to conventional TIG. The advantages are deeper penetration (from higher plasma gas flow), and greater tolerance to surface contamination including coatings (the electrode is within the body of the torch). The major disadvantage lies in the bulkiness of the torch, making manual welding more

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difficult. In mechanised welding, greater attention must be paid to maintenance of the torch to ensure consistent performance.

Keyhole welding

This has several advantages which can be exploited: deep penetration and high welding speeds. Compared with the TIG arc, it can penetrate plate thicknesses up to l0mm, but when welding using a single pass technique, it is more usual to limit the thickness to 6mm. The normal methods is to use the keyhole mode with filler to ensure smooth weld bead profile (with no undercut). For thicknesses up to 15mm, a vee joint preparation is used with a 6mm root face. A two-pass technique is employed and here, the first pass is autogenous with the second pass being made in melt mode with filler wire addition.

As the welding parameters, plasma gas flow rate and filler wire addition (into the keyhole) must be carefully balanced to maintain the keyhole and weld pool stability, this technique is only suitable for mechanised welding. Although it can be used for positional welding, usually with current pulsing, it is normally applied in high speed welding of thicker sheet material (over 3 mm) in the flat position. When pipe welding, the slope-out of current and plasma gas flow must be carefully controlled to close the keyhole without leaving a hole.

Thermal Gouging Thermal gouging is an essential part of welding fabrication. Used for rapid removal of unwanted metal, the material is locally heated and molten metal ejected - usually by blowing it away. Normal oxyfuel gas or arc processes can be used to produce rapid melting and metal removal. However, to produce a groove of specific dimensions, particularly regarding depth and width, the welder must exercise careful control of the gouging operation. If this does not happen, an erratic and badly-serrated groove will result.

Thermal processes, operations and metals which may be gouged or otherwise shaped:

Process operations Thermal process Primary Secondary

Metals

Oxyfuel gas flame Gouging

Grooving Washing Chamfering

Low carbon steels, carbon manganese steels (structural), pressure vessel steels (carbon not over 0.35%), low alloy steels (less than 5%Cr) cast iron (if preheated to 400-450 deg.C)

Manual metal arc Gouging

Grooving Chamfering

Low carbon steels carbon manganese steels (structural), pressure vessel steels, low alloy steels, stainless steels, cast iron, nickel-based alloys

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Air carbon arc Gouging

Grooving Chamfering

Low carbon steels carbon manganese steels (structural), pressure vessel steels, low and high alloy steels, cast iron, nickel-based alloys, copper and copper alloys, copper/nickel alloys, aluminium

Plasma arc Gouging Chamfering Grooving Washing

Aluminium, stainless steels

Note: All processes are capable of cutting/severing operations. Preheat may or may not be required on some metals prior to gouging

Safety

It should be emphasised that because gouging relies on molten metal being forcibly ejected, often over quite large distances, the welder must take appropriate precautions to protect himself, other workers and his equipment. Sensible precautions include protective clothing for the welder, shielding inside a specially-enclosed booth or screens, adequate fume extraction, and removal of all combustible material from the immediate area.

Industrial applications

Thermal gouging was developed primarily for removal of metal from the reverse side of welded joints, removal of tack welds, temporary welds, and weld imperfections. Figure 1 illustrates the value of typical back-gouging applications carried out on arc welded joints., while Fig. 2 shows imperfection removal in preparation for weld repair.

Fig.1 Typical back-gouging applications carried out on arc welded joints

Fig. 2 Imperfection removal in preparation for weld repair

The gouging process has proved to be so successful that it is used for a wide spectrum of applications in engineering industries:

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repair and maintenance of structures - bridges, earth-moving equipment, mining machinery, railway rolling stock, ships, offshore rigs, piping and storage tanks

removal of cracks and imperfections - blow holes and sand traps in both ferrous and non-ferrous forgings and castings

preparation of plate edges for welding

removal of surplus metal - riser pads and fins on castings, excess weld bead profiles, temporary backing strips, rivet washing and shaping operations, demolition of welded and unwelded structures - site work

Thermal gouging is also suitable for efficient removal of temporary welded attachments such as brackets, strongbacks, lifting lugs and redundant tack welds, during various stages of fabrication and construction work.

Gouging processes

Gouging operations can be carried out using the following thermal processes:

oxyfuel gas flame manual metal arc air carbon arc plasma arc

Oxy-fuel Gouging Oxy-fuel or flame gouging offers fabricators a quick and efficient method of removing metal. It can be at least four times quicker than cold chipping operations. The process is particularly attractive because of its low noise, ease of handling, and ability to be used in all positions.

Process description

Flame gouging is a variant of conventional oxyfuel gas welding. Oxygen and a fuel gas are used to produce a high temperature flame for melting the steel. When gouging, the steel is locally heated to a temperature above the 'ignition' temperature (typically 900deg.C) and a jet of oxygen is used to melt the metal - a chemical reaction between pure oxygen and hot metal. This jet is also used to blow away molten metal and slag. It

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should be noted that compared with oxyfuel cutting, slag is not blown through the material, but remains on the top surface of the workpiece.

The gouging nozzle is designed to supply a relatively large volume of oxygen through the gouging jet. This can be as much as 300 litre/min through a 6mm orifice nozzle. In oxyacetylene gouging, equal quantities of oxygen and acetylene are used to set a near-neutral preheating flame. The oxygen jet flow rate determines the depth and width of the gouge. Typical operating parameters (gas pressures and flow rates) for achieving a range of gouge sizes (depth and width) can be seen in the Table.

Typical operating data for manual oxyacetylene flame gouging Gouge dimensions Gas pressure Gas consumption Nozzle orifice

dia.(mm) Width (mm)

Depth (mm)

Acetylene (Bar)

Oxygen (Bar)

Acetylene (Litre/min)

Preheat (Litre/min)

Oxygen (Litre/min)

Travel speed (mm/min)

3 6-8 3-9 0.48 4.2 15 22 62 600 5 8-10 6-12 0.48 5.2 29 31 158 1000 6.5 10-13 10-13 0.55 5.5 36 43 276 1200

When the preheating flame and oxygen jet are correctly set, the gouge has a uniform profile and its surfaces are smooth with a dull blue colour.

Operating techniques

The depth of the gouge is determined principally by the speed and angle of the torch. To cut a deep groove the angle of the torch is stepped up (this increases the impingement angle of the oxygen jet) and gouging speed is reduced. To produce a shallow groove, the torch is less steeply angled, see above, and speed is increased. Wide grooves can be produced by weaving the torch. The contour of the groove is dependent upon the size of the nozzle and the operating parameters. If the cutting oxygen pressure is too low, gouging progresses with a washing action, leaving smooth ripples in the bottom of the groove. If the cutting oxygen pressure is too high, the cut advances ahead of the molten pool - this will disrupt the gouging operation especially when making shallow grooves.

There are four basic flame gouging techniques which are used in the following types of application.

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Progressive gouging

This technique is used to produce uniform grooves. Gouging is conducted in either a continuous or progressive manner. Applications include removal of an unfused root area on the reverse side of a welded joint, part-shaping a steel forging, complete removal of a weld deposit and preparing plate edges for welding.

Spot gouging

Spot gouging produces a deep narrow U-shaped groove over a relatively short length. The process is ideally suited to removal of localised areas such as isolated weld imperfections. Experienced operators are able to observe any imperfections during gouging. These appear as dark or light spots/streaks within the molten pool (reaction zone).

Back-step gouging

Once the material has reached ignition temperature, the oxygen stream is introduced and the torch moved in a backward movement for a distance of 15-20mm. The oxygen is shut off and the torch moved forward a distance of 25-30mm before restarting the gouging operation. This technique is favoured for removal of local imperfections which may be deeply embedded in the base plate.

Deep gouging

It is sometimes necessary to produce a long deep gouge. Such operations are completed using the deep gouging technique, which is basically a combination of progressive and spot gouging.

Manual Metal Arc Gouging The main advantage of manual metal arc (MMA) gouging is that the same power source can be used for welding, gouging, or cutting, simply by changing the type of electrode.

Process description

As in conventional MMA welding, the arc is formed between the tip of the electrode and the workpiece. MMA gouging differs because it requires special purpose electrodes with thick flux coatings to generate a strong arc force and gas stream. Unlike MMA welding

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where a stable weld pool must be maintained, this process forces the molten metal away from the arc zone to leave a clean cut surface.

The gouging process is characterised by the large amount of gas which is generated to eject the molten metal. However, because the arc/gas stream is not as powerful as a gas or a separate air jet, the surface of the gouge is not as smooth as an oxyfuel gouge or air carbon arc gouge.

Electrode

According to the size of gouge specified, there is a wide range of electrode diameters available to choose from. These grooving electrodes are also not just restricted to steels, and the same electrode composition may be used for gouging stainless steel and non-ferrous alloys.

Power source

MMA gouging can be carried out using conventional DC and AC power sources. In DC gouging, electrode polarity is normally negative but electrode manufacturers may well recommend electrode polarity for their brand of electrodes and for gouging specific materials. When using an AC power source, a minimum of 70V open circuit (OCV) is required to stabilise the arc.

Although most MMA welding power sources can be used for gouging, the current rating and OCV must be capable of accommodating current surges and longer arc lengths.

Typical operating data for MMA gouging Gouging dimensions

Electrode diameter (mm)

Current (A) Depth

(mm) Width (mm)

Gouging speed (mm/min)

3.2 210 2 6 1200

4.0 300 3 8 1000

4.8 350 4 10 800

Operational characteristics

The arc is struck with an electrode which is held at a normal angle to the workpiece (15 degrees backwards from the vertical plane in line with proposed direction of gouging). Once the arc is established, the electrode is immediately inclined in one smooth and continuous movement to an angle of around 15-20 degrees to the plate surface. With the

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arc pointing in the direction of travel, the electrode is pushed forward slightly to melt the metal. It should then be pulled back to allow the gas jet to displace the molten metal and slag. This forward and backward motion is repeated as the electrode is guided along the line to complete the gouge.

To produce a consistent depth and width of gouge, a uniform rate of travel must be maintained, together with the angle of electrode: 10-20 degrees. If the electrode angle becomes too steep, in excess of about 20 degrees, the amount of slag and molten metal will increase. This is a result of the arc penetrating too deeply. Digging the electrode into the metal causes problems in controlling the gouging operation and will produce a rough surface profile. For gouging in positions other than vertical, the electrode is always pushed forward. With vertical surfaces, the electrode is directed and pushed vertically downwards.

Application

MMA gouging is used for localised gouging operations, removal of defects for example, and where it is more convenient to switch from a welding electrode to a gouging electrode rather than use specialised equipment. Compared with alternative gouging processes, metal removal rates are low and the quality of the gouged surface is inferior. When correctly applied, MMA gouging can produce relatively clean gouged surfaces. For general applications, welding can be carried out without the need to dress by grinding. However when gouging stainless steel, a thin layer of higher carbon content material will be produced - this should be removed by grinding.

Plasma Arc Gouging The use of the plasma arc as a gouging tool dates back to the 1960s when the process was developed for welding. Compared with the alternative oxyfuel and MMA gouging techniques, plasma arc has a needle-like jet which can produce a very precise groove, suitable for application on almost all ferrous and non- ferrous materials.

Process description

Plasma arc gouging is a variant of the plasma arc process. The arc is formed between a refractory (usually tungsten) electrode and the workpiece. Intense plasma is achieved by constricting the arc using a fine bore copper nozzle. By locating the electrode behind the nozzle, the plasma-forming gas can be separated from the general gas supply used to cool the torch/assist the plasma gas to blow away molten metal (dross) from the groove.

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The temperature and force of the constricted plasma arc is determined by the current level and plasma gas flow rate. Thus, the plasma can be varied to produce a hot gas stream or a high power, deeply penetrating jet. This ability to control quite precisely the size and shape of a groove is very useful for removing unwanted defects from a workpiece surface.

Whilst gouging, normal precautions should be taken to protect the operator and other workers in the immediate area from the effects of intense arc light and hot metal spray. Unlike the oxyfuel and MMA processes, the plasma arc's high velocity jet will propel fume and hot metal dross some considerable distance from the operator. When using a deeply penetrating arc, noise protection is an essential requirement.

Equipment

The power source for sustaining this gouging arc must have a high open circuit voltage, usually well in excess of 100V. The torch is connected to the negative polarity of the power source and the workpiece must be connected to the positive. The plasma torch is the same as the one used for cutting; it will be either gas or water cooled and have the facility for single and dual gas operation.

Electrodes are normally tungsten for argon and argon-based gases. However, when using air as the plasma gas, special purpose, for example hafnium tipped copper, electrodes must be used to withstand the more aggressive, oxidising arc.

Plasma and cooling gases

Plasma gas can be argon, helium, argon - H 2 , nitrogen or air. Argon - 35%H 2 is normally recommended as a general- purpose plasma gas for cutting most materials. Alternative plasma gases are argon and helium. Argon, a colder gas, will reduce metal removal rates. Helium, which generates a hot but less intense arc than argon - H 2 , can produce a wider and shallower groove. Nitrogen and air are also used as plasma gases, especially for gouging C-Mn steels. Although gas costs will be substantially reduced, the groove surface profile will be inferior to that which can be achieved with argon - H 2 gas. Air is not recommended for gouging aluminium as this requires an inert or reducing gas. Argon, nitrogen or air are all used as cooling gases. Use of argon will normally produce the best quality of gouge, but nitrogen or air will reduce operating costs.

Operating techniques

Gouging is effected by moving the torch forward at a steady controlled rate. It is carried out in a progressive manner to remove metal over a distance of 200 to 250mm. The jet can then be repositioned, either to deepen or widen the groove, or to continue gouging for a further 200 to 250mm. Principal process parameters are current level, gas flow rate, and speed of gouging. These settings determine groove size and metal removal rate. In a typical gouging operation on C-Mn steel, metal is removed at about 100 kg/hr at a speed of 0.5 m/min, and groove size will be around 12mm wide and 5mm deep.

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The torch stand-off and its angle to the surface of the workpiece have a major influence on speed of travel, groove profile and quality of surface. The torch is normally held at a distance of 20mm from the workpiece and inclined backwards to the direction of gouging at an angle of 40 to 45 degrees. Gouging will remove up to approximately 6mm depth of metal in a single pass.

The torch stand-off should not be reduced to less than 12mm, to avoid spatter build-up on the nozzle from the molten particles ejected from the groove. At standoff distances greater than 25mm, arc/gas forces are reduced and this lessens the depth of penetration of the jet. By reducing the torch angle to the workpiece surface, the plasma jet can be encouraged to 'skate' along the surface of the workpiece; this produces a shallower and wider groove. By increasing the angle of the torch the plasma jet is directed into the workpiece surface, resulting in a deeper and narrower groove.

Air Carbon Arc Gouging The main difference between this gouging technique and the others is that a separate air jet is used to eject molten metal to form the groove.

Process description

Air carbon arc gouging works as follows. An electric arc is generated between the tip of a carbon electrode and the workpiece. The metal becomes molten and a high velocity air jet streams down the electrode to blow it away, thus leaving a clean groove. The process is simple to apply (using the same equipment as MMA welding), has a high metal removal rate, and gouge profile can be closely controlled. Disadvantages are that the air jet causes the molten metal to be ejected over quite a large distance and, because of high currents (up to 2000A) and high air pressures (80 to 100 psi), it can be very noisy.

Application

As air carbon arc gouging does not rely on oxidation it can be applied to a wide range of metals. DC (electrode positive) is normally preferred for steel and stainless steel but AC is more effective for cast iron, copper and nickel alloys. Typical applications include back gouging, removal of surface and internal defects, removal of excess weld metal and preparation of bevel edges for welding.

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Electrode

The electrode is a non-consumable graphite (carbon) rod which has a copper coating to reduce electrode erosion. Electrode diameter is selected according to required depth and width of gouge. Cutting can be precisely controlled and molten metal/dross is kept to a minimum.

Power source

A DC power supply with electrode positive polarity is most suitable. AC power sources which are also constant current can be used but with special AC type electrodes. The power source must have a constant current output characteristic. If it does not, inadvertant touching of the electrode to the workpiece will cause a high current surge sufficient to 'explode' the electrode tip. This will disrupt the operation and cause carbon pick-up. As arc voltage can be quite high (up to 50V), open circuit voltage of the power source should be over 60V.

Air supply

The gouging torch is normally operated with either a compressed air line or seperate bottled gas supply. Air supply pressure will be up to 100psi from the air line but restricted to about 35psi from a bottled supply. Providing there is sufficient air flow to remove molten metal, there are no advantages in using higher pressure and flow rates.

Carbon pickup

Although carbon is picked up by the molten metal, the air stream will remove carbon-rich metal from the groove to leave only minimal contamination of the sidewalls. Poor gouging technique or insufficient air flow will result in carbon pick-up with the risk of metallurgical problems, e.g high hardness and even cracking.

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Typical operating data for air carbon arc gouging:

Gouging dimensions Electrode

diameter (mm)

Current ANote: DC electrode Depth

(mm) Width (mm)

Carbon electrode consumed (mm/min)

Gouging speed (mm/min)

6.4 275 6-7 9-10 120 609

8.0 350 7-8 10-11 114 711

9.5 425 9-10 12-13 100 660 Manual

13.0 550 12-13 18-19 76 508

8.0 300-400 2-9 3-8 100 1650-840

9.5 500 3-12 3-10 142 1650-635

13.0 850 3-15 3-13 82 1830-610 Automatic

16.0 1250 3-19 3-16 63 1830-710

Operation

Gouging is commenced by striking the electrode tip on to the workpiece surface to initiate the arc. Unlike manual metal arc (MMA) welding the electrode tip is not withdrawn to establish arc length. Molten metal directly under the electrode tip (arc) is immediately blown away by the air stream. For effective metal removal, it is important that the air stream is directed at the arc from behind the electrode and sweeps under the tip of the electrode. The width of groove is determined by the diameter of electrode, but depth is dictated by the angle of electrode to the workpiece and rate of travel. Relatively high travel speeds are possible when a low electrode angle is used. This produces a shallow groove: a steep angle results in a deep groove and requires slower travel speed. Note, a steeply angled electrode may give rise to carbon contamination.

Oscillating the electrode in a circular or restricted weave motion during gouging can greatly increase gouging width. This is useful for removal of a weld or plate imperfection that is wider than the electrode itself. It is important, however, that weave width should not exceed four times the diameter of the electrode.The groove surface should be relatively free of oxidised metal and can be considered ready for welding without further preparation. Dressing by grinding the side-walls of the gouge should be carried out if a carbon rich layer has been formed. Also, dressing by grinding or another approved method will be necessary if working on crack-sensitive material such as high strength, low alloy steel.

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Equipment for Oxyacetylene Welding Essential equipment components

Torch

The basic oxyacetylene torch comprises:

torch body (or handle) two separate gas tubes (through the handle connected to the hoses) separate control valves mixer chamber flame tube welding tip

NB The cutting torch requires two oxygen supplies to the nozzle, one mixed with fuel gas for preheating and a separate oxygen flow for cutting.

Hoses

Hoses are colour-coded red for acetylene and blue (UK) or green (US) for oxygen. Oxygen fittings on the hose have a right-hand thread while acetylene is left-handed.

Gas regulators

The primary function of a gas regulator is to control gas pressure. It reduces the high pressure of the bottle-stored gas to the working pressure of the torch, and this will be maintained during welding.

The regulator has two separate gauges: a high pressure gauge for gas in the cylinder and a low pressure gauge for pressure of gas fed to the torch. The amount of gas remaining in the cylinder can be judged from the high pressure gauge. The regulator, which has a pressure adjusting screw, is used to control gas flow rate to the torch by setting the outlet gas pressure. Note Acetylene is supplied in cylinders under a pressure of about 15 bars psi but welding is carried out with torch gas pressures typically up to 2 bars.

Flame traps

Flame traps (also called flashback arresters) must be fitted into both oxygen and acetylene gas lines to prevent a flashback flame from reaching the regulators. Non-return spring-loaded valves can be fitted in the hoses to detect/stop reverse gas flow. Thus, the valves can be used to prevent conditions leading to flashback, but should always be used in conjunction with flashback arresters.

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A flashback is where the flame burns in the torch body, accompanied by a whistling sound. It will occur when flame speed exceeds gas flow rate and the flame can pass back through the mixing chamber into the hoses. Most likely causes are: incorrect gas pressures giving too low a gas velocity, hose leaks, loose connections, or welder techniques which disturb gas flow.

Identification of gas cylinders

An oxygen cylinder is colour-coded black and the acetylene cylinder is maroon. Oxygen and acetylene are stored in cylinders at high pressure. Oxygen pressure can be as high as 230 bars. Acetylene, which is dissolved in acetone contained in a porous material, is stored at a much lower pressure, approximately 15 bars.

The appropriate regulator must be fitted to the cylinders to accommodate cylinder pressures. To avoid confusion, oxygen cylinders and regulators have right-hand threads and acetylene cylinders and regulators have left-hand ones.

Typical gas pressures and flow rates for C-Mn steel:

Acetylene Oxygen Steel thickness (mm)

Nozzle size Pressure

(bar) Consumption (l/min)

Pressure (bar)

Consumption (l/min)

0.90 1 0.14 0.50 0.14 0.50

1.20 2 0.14 0.90 0.14 0.90

2.00 3 0.14 1.40 0.14 1.40

2.60 5 0.14 2.40 0.14 2.40

3.20 7 0.14 3.30 0.14 3.30

4.00 10 0.21 4.70 0.21 4.70

5.00 13 0.28 6.00 0.28 6.00

6.50 18 0.28 8.50 0.28 8.50

8.20 25 0.42 12.00 0.42 12.00

10.00 35 0.63 17.00 0.63 17.00

13.00 45 0.35 22.00 0.35 22.00

25.00 90 0.63 42.00 0.63 42.00

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Selection of correct nozzles

Welding torches are generally rated according to thickness of material to be welded. They range from light duty (for sheet steel up to 2mm in thickness) to heavy duty (for steel plate greater than 25mm in thickness). Each torch can be fitted with a range of nozzles with a bore diameter selected according to material thickness. Gas pressures are set to give correct flow rate for nozzle bore diameter. Proportions of oxygen and acetylene in the mixture can be adjusted to give a neutral, oxidising or carburising flame. (See the description of oxyacetylene processes) Welding is normally carried out using a neutral flame with equal quantities of oxygen and acetylene.

Equipment safety checks

Before commencing welding it is wise to inspect the condition and operation of all equipment. As well as normal equipment and workplace safety checks, there are specific procedures for oxyacetylene. Operators should verify that:

flashback arresters are present in each gas line hoses are the correct colour, with no sign of wear, as short as possible and not

taped together regulators are the correct type for the gas a bottle key is in each bottle (unless the bottle has an adjusting screw)

It is recommended that oxyacetylene equipment is checked at least annually - regulators should be taken out of service after five years. Flashback arresters should be checked regularly according to manufacturer's instructions and, with specific designs, it may be necessary to replace if flashback has occurred.

For more detailed information the following legislation and codes of practice should be consulted:

UK Health and Safety at Work Act 1974 Pressure Systems and Transportable Gas Containers Regulations British Compressed Gases Association, Codes of Practice BOC Handbook

Equipment for MMA Welding Although the manual metal arc (MMA) process has relatively basic equipment requirements, it is important that the welder has a knowledge of operating features and performance to comply with welding procedures for the job and, of course, for safety reasons.

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Essential equipment

The main components of the equipment required for welding are:

power source electrode holder and cables welder protection fume extraction

Tools required include: a wire brush to clean the joint area adjacent to the weld (and the weld itself after slag removal); a chipping hammer to remove slag from the weld deposit; and, when removing slag, a pair of clear lens goggles or a face shield to protect the eyes (lenses should be shatter-proof and noninflammable).

Power source

The primary function of a welding power source is to provide sufficient power to melt the joint. However with MMA the power source must also provide current for melting the end of the electrode to produce weld metal, and it must have a sufficiently high voltage to stabilise the arc.

MMA electrodes are designed to be operated with alternating current (AC) and direct current (DC) power sources. Although AC electrodes can be used on DC, not all DC electrodes can be used with AC power sources.

As MMA requires a high current (50-30OA) but a relatively low voltage (10-50V), high voltage mains supply (240 or 440V) must be reduced by a transformer. To produce DC, the output from the transformer must be further rectified. To reduce the hazard of electrical shock, the power source must function with a maximum no-load voltage, that is, when the external (output) circuit is open (power leads connected and live) but no arc is present. The no-load voltage rating of the power source is as defined in BS 638 and must be in accordance with the type of welding environment or hazard of electrical shock. The power source may have an internal or external hazard reducing device to reduce the no-load voltage; the main welding current is delivered as soon as the electrode touches the workpiece. For welding in confined spaces, you should use a low voltage safety device to limit the voltage available at the holder to approximately 25V.

There are four basic types of power source:

AC transformer DC rectifier AC/DC transformer-rectifier DC generator

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AC electrodes are frequently operated with the simple, single phase transformer with current adjusted by means of tappings or sliding core control. DC rectifiers and AC/DC transformer-rectifiers are controlled electronically, for example by thyristors. A new generation of power sources called inverters is available. These use transistors to convert mains AC (50Hz) to a high frequency AC (over 500 Hz) before transforming down to a voltage suitable for welding and then rectifying to DC. Because high frequency transformers can be relatively small, principal advantages of inverter power sources are undoubtedly their size and weight when the source must be portable.

Electrode holder and cables

The electrode holder clamps the end of the electrode with copper contact shoes built into its head. The shoes are actuated by either a twist grip or spring-loaded mechanism. The clamping mechanism allows for quick release of the stub end. For efficiency the electrode has to be firmly clamped into the holder, otherwise poor electrical contact may cause arc instability through voltage fluctuations. Welding cable connecting the holder to the power source is mechanically crimped or soldered.

It is essential that good electrical connections are maintained between electrode, holder and cable. With poor connections, resistance heating and, in severe cases, minor arcing with the torch body will cause the holder to overheat. Two cables are connected to the output of the power source, the welding lead goes to the electrode holder and the current return lead is clamped to the workpiece. The latter is often wrongly referred to as the earthlead. A separate earth lead is normally required to provide protection from faults in the power source. The earth cable should therefore be capable of carrying the maximum output current of the power source.

Cables are covered in a smooth and hard-wearing protective rubberised flexible sheath. This oil and water resistant coating provides electrical insulation at voltages to earth not exceeding 100V DC and AC (rms value). Cable diameter is generally selected on the basis of welding current level, As these electrode types are When welding, the welder air movement should be from duty cycle and distance of the work from the power source. The higher the current and duty cycle, the larger the diameter of the cable to ensure that it does not overheat (see BS 638 Pt 4). If welding is carried out some distance from the power source, it may be necessary to increase cable diameter to reduce voltage drop.

Care of electrodes

The quality of weld relies upon consistent performance of the electrode. The flux coating should not be chipped, cracked or, more importantly, allowed to become damp.

Storage

Electrodes should always be kept in a dry and well-ventilated store. It is good practice to stack packets of electrodes on wooden pallets or racks well clear of the floor. Also, all unused electrodes which are to be returned should be stored so they are not exposed to

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damp conditions to regain moisture. Good storage conditions are 10 degrees C above external air temperature. As the storage conditions are to prevent moisture from condensing on the electrodes, the electrode stores should be dry rather that warm. Under these conditions and in original packaging, electrode storage time is practically unlimited. It should be noted that electrodes are now available in hermetically sealed packs which obviate the need for drying. However, if necessary, any unused electrodes must be redried according to manufacturer's instructions.

Drying of electrodes

Drying is usually carried out following the manufacturer's recommendations and requirements will be determined by the type of electrode.

Cellulosic coatings

As these electrode coatings are designed to operate with a definite amount of moisture in the coating, they are less sensitive to moisture pick-up and do not generally require a drying operation. However, in cases where ambient relative humidity has been very high, drying may be necessary.

Rutile coatings

These can tolerate a limited amount of moisture and coatings may deteriorate if they are overdried. Particular brands may need to be dried before use.

Basic and basic/rutile coatings

Because of the greater need for hydrogen control, moisture pick-up is rapid on exposure to air. These electrodes should be thoroughly dried in a controlled temperature drying oven. Typical drying time is one hour at a temperature of approximately 150 to 300 degrees C but instructions should be adhered to.

After controlled drying, basic and basic/rutile electrodes must be held at a temperature between 100 and 150 degrees C to help protect them from re-absorbing moisture into the coating. These conditions can be obtained by transferring the electrodes from the main drying oven to a holding oven or a heated quiver at the workplace.

Protective clothing

When welding, the welder must be protected from heat and light radiation emitted from the arc, spatter ejected from the weld pool, and from welding fume.

Hand and head shield

For most operations a hand-held or head shield constructed of lightweight insulating and non-reflecting material is used. The shield is fitted with a protective filter glass,

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sufficiently dark in colour and capable of absorbmg the harmful infrared and ultraviolet rays. The filter glasses conform to the strict requirements of BS 679 and are graded according to a shade number which specifies the amount of visible light allowed to pass through - the lower the number, the lighter the filter. The correct shade number must be used according to the welding current level, for example:

Shade 9 - up to 40A Shade 10 - 40 to 80A Shade 11 - 80 to 175A Shade 12 - 175 to 300A Shade 13 - 300 to 500A

Clothing

For protection against sparks, hot spatter, slag and burns, a leather apron and leather gloves should be worn. Various types of leather gloves are available, such as short or elbow length, full fingered or part mitten.

Fume extraction

When welding within a welding shop, ventilation must dispose harmlessly of the welding fume. Particular attention should be paid to ventilation when welding in a confined space such as inside a boiler, tank or compartment of a ship.

Fume removal should be by some form of mechanical ventilation which will produce a current of fresh air in the immediate area. Direction of the air movement should be from the welder's face towards the work. This is best achieved by localised exhaust ventilation using a suitably designed hood near to the welding area.

Equipment for MIG Welding The MIG process is a versatile welding technique which is suitable for both thin sheet and thick section components. It is capable of high productivity but the quality of welds can be called into question. To achieve satisfactory welds, welders must have a good knowledge of equipment requirements and should also recognise fully the importance of setting up and maintaining component parts correctly.

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Essential equipment

In MIG the arc is formed between the end of a small diameter wire electrode fed from a spool, and the workpiece. Main equipment components are:

power source wire feed system conduit gun

The arc and weldpool are protected from the atmosphere by a gas shield. This enables bare wire to be used without a flux coating (required by MMA). However, the absence of flux to 'mop up' surface oxide places greater demand on the welder to ensure that the joint area is cleaned immediately before welding. This can be done using either a wire brush for relatively clean parts, or a hand grinder to remove rust and scale. The other essential piece of equipment is a wire cutter to trim the end of the electrode wire.

Power source

MIG is operated exclusively with a DC power source. The source is termed a flat, or constant current, characteristic power source, which refers to the voltage/welding current relationship. In MIG, welding current is determined by wire feed speed, and arc length is determined by power source voltage level (open circuit voltage). Wire burn-off rate is automatically adjusted for any slight variation in the gun to workpiece distance, wire feed speed, or current pick-up in the contact tip. For example, if the arc momentarily shortens, arc voltage will decrease and welding current will be momentarily increased to burn back the wire and maintain pre-set arc length. The reverse will occur to counteract a momentary lengthening of the arc.

There is a wide range of power sources available, mode of metal transfer can be:

dip spray pulsed

A low welding current is used for thin-section material, or welding in the vertical position. The molten metal is transferred to the workpiece by the wire dipping into the weldpool. As welding parameters will vary from around 100A \ 17V to 200A \ 22V (for a 1.2mm diameter wire), power sources normally have a current rating of up to 350A. Circuit inductance is used to control the surge in current when the wire dips into the weldpool (this is the main cause of spatter). Modern electronic power sources automatically set the inductance to give a smooth arc and metal transfer.

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In spray metal transfer, metal transfers as a spray of fine droplets without the wire touching the weldpool. The welding current level needed to maintain the non short-circuiting arc must be above a minimum threshold level; the arc voltage is higher to ensure that the wire tip does not touch the weldpool. Typical welding parameters for a 1.2mm diameter wire are within 250A \ 28V to 400A \ 35V. For high deposition rates the power source must have a much higher current capacity: up to 500A.

The pulsed mode provides a means of achieving a spray type metal transfer at current levels below threshold level. High current pulses between 25 and 100Hz are used to detach droplets as an alternative to dip transfer. As control of the arc and metal transfer requires careful setting of pulse and background parameters, a more sophisticated power source is required. Synergic pulsed MIG power sources, which are advanced transistor-controlled power sources, are preprogrammed so that the correct pulse parameters are delivered automatically as the welder varies wire feed speed.

Welding current and arc voltage ranges for selected wire diameters operating with dip and spray metal transfer:

Dip transfer Spray transfer Wire diameter (mm) Current (A) Voltage (V) Current (A) Voltage (V)

0.6 30 - 80 15 - 18

0.8 45 - 180 16 - 21 150 - 250 25 - 33

1.0 70 - 180 17 - 22 230 - 300 26 - 35

1.2 100 - 200 17 - 22 250 - 400 27 - 35

1.6 120 - 200 18 - 22 250 - 500 30 - 40

Wire feed system

The performance of the wire feed system can be crucial to the stability and reproducibility of MIG welding. As the system must be capable of feeding the wire smoothly, attention should be paid to the feed rolls and liners. There are three types of feeding systems:

pinch rolls push-pull spool on gun

The conventional wire feeding system normally has a set of rolls where one is grooved and the other has a flat surface. Roll pressure must not be too high otherwise the wire will deform and cause poor current pick up in the contact tip. With copper coated wires, too high a roll pressure or use of knurled rolls increases the risk of flaking of the coating

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(resulting in copper build up in the contact tip). For feeding soft wires such as aluminium dual-drive systems should be used to avoid deforming the soft wire.

Small diameter aluminium wires, 1mm and smaller, are more reliably fed using a push-pull system. Here, a second set of rolls is located in the welding gun - this greatly assists in drawing the wire through the conduit. The disadvantage of this system is increased size of gun. Small wires can also be fed using a small spool mounted directly on the gun. The disadvantages with this are increased size, awkwardness of the gun, and higher wire cost.

Conduit

The conduit can measure up to 5m in length, and to facilitate feeding, should be kept as short and straight as possible. (For longer lengths of conduit, an intermediate push-pull system can be inserted). It has an internal liner made either of spirally-wound steel for hard wires (steel, stainless steel, titanium, nickel) or PTFE for soft wires (aluminium, copper).

Gun

In addition to directing the wire to the joint, the welding gun fulfils two important functions - it transfers the welding current to the wire and provides the gas for shielding the arc and weldpool.

There are two types of welding guns: 'air' cooled and water cooled. The 'air' cooled guns rely on the shielding gas passing through the body to cool the nozzle and have a limited current-carrying capacity. These are suited to light duty work. Although 'air' cooled guns are available with current ratings up to 500A, water cooled guns are preferred for high current levels, especially at high duty cycles.

Welding current is transferred to the wire through the contact tip whose bore is slightly greater than the wire diameter. The contact tip bore diameter for a 1.2mm diameter wire is between 1.4 andt 1.5mm. As too large a bore diameter affects current pick up, tips must be inspected regularly and changed as soon as excessive wear is noted. Copper alloy (chromium and zirconium additions) contact tips, harder than pure copper, have a longer life, especially when using spray and pulsed modes.

Gas flow rate is set according to nozzle diameter and gun to workpiece distance, but is typically between 10 and 30 l/min. The nozzle must be cleaned regularly to prevent excessive spatter build-up which creates porosity. Anti-spatter spray can be particularly effective in automatic and robotic welding to limit the amount of spatter adhering to the nozzle.

Protective equipment

A darker glass than that used for MMA welding at the same current level should be used in hand or head shields.

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Recommended shade number of filter for MIG/MAG welding:

Welding current A Shade number MIG Heavy metal MIG Light metal MAG

10 under 100 under 100 under 80

11 1001 - 175 100 - 175 80 - 125

12 175 - 300 175 - 250 125 - 175

13 300 - 500 250 - 350 175 - 300

14 over 500 350 - 500 300 - 500

15 over 500 over 450

Equipment for Submerged-arc Welding The submerged-arc welding(SAW) process is similar to MIG where the arc is formed between a continuously-fed wire electrode and the workpiece, and the weld is formed by the arc melting the workpiece and the wire. However, in SAW a shielding gas is not required as the layer of flux generates the gases and slag to protect the weld pool and hot weld metal from contamination. Flux plays an additional role in adding alloying elements to the weld pool.

Essential equipment

Essential equipment components for SAW are:

power source wire gun flux handling protective equipment

As SAW is a high current welding process, the equipment is designed to produce high deposition rates.

Power source

SAW can be operated using either a DC or an AC power source. DC is supplied by a transformer-rectifier and AC is supplied by a transformer. Current for a single wire

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ranges from as low as 200A (1.6mm diameter wire) to as high as 1000A (6.0mm diameter wire). In practice, most welding is carried out on thick plate where a single wire (4.0mm diameter) is normally used over a more limited range of 600 to 900A, with a twin wire system operating between 800 and 1200A.

In DC operation, the electrode is normally connected to the positive terminal. Electrode negative (DCEN) polarity can be used to increase deposition rate but depth of penetration is reduced by between 20 and 25%. For this reason, DCEN is used for surfacing applications where parent metal dilution is important. The DC power source has a 'constant voltage' output characteristic which produces a self-regulating arc. For a given diameter of wire, welding current is controlled by wire feed speed and arc length is determined by voltage setting.

AC power sources usually have a constant-current output characteristic and are therefore not self-regulating. The arc with this type of power source is controlled by sensing the arc voltage and using the signal to control wire feed speed. In practice, for a given welding current level, arc length is determined by wire burnoff rate, i.e. the balance between the welding current setting and wire feed speed which is under feedback control.

Square wave AC square wave power sources have a constant voltage output current characteristic. Advantages are easier arc ignition and constant wire feed speed control.

Welding gun

SAW can be carried out using both manual and mechanised techniques. Mechanised welding, which can exploit the potential for extremely high deposition rates, accounts for the majority of applications.

Manual welding

For manual welding, the welding gun is similar to a MIG gun, with the flux which is fed concentrically around the electrode, replacing the shielding gas. Flux is fed by air pressure through the handle of the gun or from a small hopper mounted on the gun. The equipment is relatively portable and, as the operator guides the gun along the joint, little manipulative skill is required. However, because the operator has limited control over the welding operation (apart from adjusting travel speed to maintain the bead profile) it is best used for short runs and simple filling operations.

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Mechanised welding - single wire

As SAW is often used for welding large components, the gun, wire feeder and flux delivery feed can be mounted on a rail, tractor or boom manipulator. Single wire welding is mostly practised using DCEP even though AC will produce a higher deposition rate for the same welding current. AC is used to overcome problems with arc blow, caused by residual magnetism in the workpiece, jigging or welding machine.

Wire stickout, or electrode extension - the distance the wire protrudes from the end of the contact tip - is an important control parameter in SAW. As the current flowing between the contact tip and the arc will preheat the wire, wire burnoff rate will increase with increase in wire stickout. For example, the deposition rate for a 4mm diameter wire at a welding current of 700A can be increased from approximately 9 kg/hr at the normal 32mm stickout, to 14 kg/hr at a stickout length of 178mm. In practice, because of the reduction in penetration and greater risk of arc wander, a long stickout is normally only used in cladding and surfacing applications where there is greater emphasis on deposition rate and control of penetration, rather than accurate positioning of the wire.

For most applications, electrode stickout is set so that the contact tube is slightly proud of the flux layer. The depth of flux is normally just sufficient to cover the arc whose light can be seen through the flux.

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Recommended and maximum stickout lengths:

Wire stickout Wire diameter mm Current range ANormal mm Maximum mm

0.8 100 to 200 12 -

1.2 150 to 300 20 -

1.6 200 to 500 20 -

2.0 250 to 600 25 63

3.2 350 to 800 30 76

4.0 400 to 900 32 128

4.75 450 to 1000 35 165

Mechanised welding - twin wire

Tandem arc connections

SAW can be operated with more than one wire. Although up to five wires are used for high deposition rates, e.g. in pipe mills, the most common multi-wire systems have two wires in a tandem arrangement. The leading wire is run on DCEP to produce deep penetration. The trailing wire is operated on AC which spreads the weld pool, which is ideal for filling the joint. AC also minimises: interaction between the arcs, and the risk of lack of fusion defects and porosity through the deflection of the arcs (arc blow). The wires are normally spaced 20mm apart so that the second wire feeds into the rear of the weld pool.

Gun angle

In manual welding, the gun is operated with a trailing angle, i.e. with the gun at an angle of 45 degrees (backwards) from the vertical. In single wire mechanised welding operations, the gun is perpendicular to the workpiece. However, in twin wire operations the leading gun is normal to the workpiece, with the trailing gun angled slightly forwards between an angle of 60 and 80 degrees. This reduces disturbance of the weld pool and produces a smooth weld bead profile.

Flux handling

Flux should be stored in unopened packages under dry conditions. Open packages should be stored in a humidity-controlled store. While flux from a newly-opened package is ready for immediate use, flux which has been opened and held in a store should first be dried according to manufacturer's instructions. In small welding systems, flux is usually held in a small hopper above the welding gun. It is fed automatically (by gravity or

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mechanised feed) ahead of the arc. In larger installations the flux is stored in large hoppers and is fed with compressed air. Unused flux is collected using a vacuum hose and returned to the hopper.

Note: Care must be taken in recycling unused flux, particularly regarding the removal of slag and metal dust particles. The presence of slag will change the composition of the flux which, together with the wire, determines the composition of the weld metal. The presence of fine particles can cause blockages in the feeding system.

Protective equipment

Unlike other arc welding processes, SAW is a clean process which produces minimum fume and spatter when welding steels. (Some noxious emissions can be produced when welding special materials.) For normal applications, general workshop extraction should be adequate.

Protective equipment such as a head shield and a leather apron are not necessary. Normal protective equipment (goggles, heavy gloves and protective shoes) are required for ancillary operations such as slag removal by chipping or grinding. Special precautions should be taken when handling flux - a dust respirator and gloves are needed when loading the storage hoppers.

Equipment for TIG Welding Job Knowledge for Welders No. 6 describes the TIG welding process. Using an inert gas shield instead of a slag to protect the weldpool, this technology is a highly attractive alternative to gas and manual metal arc welding and has played a major role in the acceptance of high quality welding in critical applications.

Essential equipment

In TIG, the arc is formed between the end of a small diameter tungsten electrode and the workpiece. The main equipment components are:

power source torch backing system protective equipment

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Power source

The power source for TIG welding can be either DC or AC but in both the output is termed a drooping, or constant current, characteristic; the arc voltage / welding current relationship delivers a constant current for a given power source setting. If the arc voltage is slightly increased or decreased, there will be very little change in welding current. In manual welding, it can accommodate the welder's natural variations in arc length and, in the event of the electrode touching the work, an excessively high current will not be drawn which could fuse the electrode to the workpiece.

The arc is usually started by HF (High Frequency) sparks which ionise the gap between the electrode and the workpiece. HF generates airborne and line transmitted interference, so care must be taken to avoid interference with control systems and instruments near welding equipment. When welding is carried out in sensitive areas, a non-HF technique, touch starting or 'lift arc', can be used. The electrode can be short circuited to the workpiece, but the current