Welding Processes - Word 97

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USE OF MATERIALS COURSE WELDING PROCESSES NOTES INDEX Page 1. Metal-Arc Welding with Covered Electrodes ................................. 3 1.1 The Process ...............................3 1.2 Welding Positions .........................4 1.3 Functions of the Electrode Coating ........5 1.4 Types of Electrode Coating ................6 1.5 Electrode Classification System............9 1.6 The Influence of Welding Current .........11 1.7 Arc Length ...............................12 1.8 Low Hydrogen Electrodes ..................14 1.9 Deep Penetration Welding .................15 1.10 Hard Facing ..............................15 1.11 Gravity Welding ........................................................ ......... 16 2. Submerged Arc Welding 18 2.1 The Process ..............................18 2.2 Materials Joined .........................19 2.3 Fluxes ...................................19 2.4 Welding Head Arrangements ................19 2.5 Operating Variables ...................................................... .... 20 2.5.1 Welding Current ..................20 2.5.2 Arc Voltage ......................20 2.5.3 Travel Speed .....................21 2.5.4 Electrode Size ...................21 2.5.5 Electrode Extension ..............22 2.5.6 Type of Electrode ................22 2.5.7 Width and Depth of Flux ..........22 1

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Transcript of Welding Processes - Word 97

Page 1: Welding Processes - Word 97

USE OF MATERIALS COURSEWELDING PROCESSES NOTES

INDEXPage

1. Metal-Arc Welding with Covered Electrodes ................................. 3

1.1 The Process ........................................................................3 1.2 Welding Positions ...............................................................4 1.3 Functions of the Electrode Coating ....................................5 1.4 Types of Electrode Coating ................................................6 1.5 Electrode Classification System..........................................9 1.6 The Influence of Welding Current .....................................11 1.7 Arc Length ........................................................................12 1.8 Low Hydrogen Electrodes .................................................14 1.9 Deep Penetration Welding ................................................15 1.10 Hard Facing ......................................................................15 1.11 Gravity Welding .................................................................16

2. Submerged Arc Welding 18

2.1 The Process .....................................................................182.2 Materials Joined ...............................................................192.3 Fluxes ...............................................................................192.4 Welding Head Arrangements ...........................................192.5 Operating Variables .......................................................... 20

2.5.1 Welding Current ...............................................20 2.5.2 Arc Voltage ......................................................20 2.5.3 Travel Speed ...................................................21 2.5.4 Electrode Size ..................................................21 2.5.5 Electrode Extension .........................................22 2.5.6 Type of Electrode .............................................22 2.5.7 Width and Depth of Flux ..................................22

3. Gas-Shielded Metal-Arc Welding.................................................. 25

3.1 The Process .....................................................................25 3.2 Electrodes .........................................................................26 3.3 Transfer Modes .................................................................27

3.3.1 Spray Transfer .................................................27 3.3.2 Short Circuit or Dip Transfer ............................28 3.3.3 Semi-short Circuiting Arc .................................29 3.3.4 Pulsed Arc Spray .............................................29

3.4 Shielding Gases.................................................................29

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3.5 Operating Variables...........................................................32

3.5.1 Arc Voltage ......................................................32 3.5.2 Arc Length .......................................................32 3.5.3 Current .............................................................32 3.5.4 Travel Speed ...................................................32 3.5.5 Electrode Extension .........................................32

3.5.6 Electrode Size ..................................................33

3.6 Advantages and Limitations of the GMAW Process .........33 3.7 Cored and Self-Shielded Wires ........................................34

4. Tungsten Inert Gas Arc Welding ....................................................37

5. Automatic Welding ..........................................................................39

6. Electroslag Welding.........................................................................40

6.1 The Process .....................................................................40 6.2 Welding with Consumable Guides or Nozzles ..................41

7. Electrogas Welding ..........................................................................43

8. One Side Welding with Backing......................................................44

9. Consumables and Power Sources..................................................47

9.1 Care and storage of consumables ....................................47 9.2 Power Sources .................................................................48

9.3 Arc Blow ............................................................................49

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1. METAL-ARC WELDING WITH COVERED ELECTRODES

1.1 The Process

Known in the USA as Shielded Metal Arc Welding (SMAW) and elsewhere as manual metal arc welding (MMA) this welding process is by far the most widely used, especially for short welds in production, maintenance, repair and construction in the field (see Figure 1). Welds can be made in areas of limited access and the equipment is relatively simple, inexpensive and portable. Welding in any position is possible provided appropriate electrodes are chosen. The process may be applied to the most commonly used metals and alloys such as carbon and alloy steels, stainless steels, copper, nickel and their alloys. It is not suitable for low melting metals such as Tin, Lead or Zinc or the more oxygen reactive metals such as aluminium, titanium and zirconium.

Figure 1 Equipment for Manual Metal Arc Welding

MMA welding is a welding process in which fusion of metals is produced by heat from an electric arc that is maintained between the tip of a flux-coated electrode and the surface of the base metal in the joint being welded.

The core of the electrode consists of a solid metal rod of drawn or cast material which conducts the electric current to the arc and provides filler metal for the joint. The flux coating protects the molten metal from the atmosphere by forming a slag and a gaseous shield and stabilises the arc. The slag helps to smooth and shape the weld bead, and has additional functions, depending on the type of electrode (see Figure 2).

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Most electrodes are 220-450 mm long but may be up to 900 mm with core wire diameters from 1.6 mm to 8.0 mm. The arc is brought about by the difference in electrical potential (voltage) between the electrode and the base metal. In practice, the voltage drop across the arc will be from about 16-40 V, with the current set generally within the range 20-550 amps. Open circuit voltage (OCV), which may be referred to is not that across the arc but that generated by the machine when no welding is being done and it is usually 50-100V. When the arc is struck, the voltage drops to the arc voltage. The power supply can be alternating or direct current and in the latter case the electrode may be connected positively or negatively. Generally AC seems to be favoured in the UK while DC is more commonly employed in the USA.

Figure 2 Manual Metal Arc welding

For flat welding, metal transfer across the arc is attributed to gravity, gas expansion, electric and magnetic forces and surface tension but in other positions gravity will work against the other phenomena. The centre of the arc has a temperature of at least 5000-6000° C, well above the melting point of any metal.

1.2 Welding Positions

The specification of welding positions is important for two reasons. First, the manufacturer needs to define the positions for which his electrodes are suitable. Second, the welder's skills and qualifications are to a large extent determined by the position at which he can produce an acceptable weld.

Thus for example much greater skill is required to weld in an overhead position as compared with a flat or downhand position.

It should be pointed out that the welding position is not limited by the process itself but by the size and type of electrode.

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There are no absolute definitions of welding positions, but, in principle, such definitions are all similar, variations arising only from minor differences in angles. The weld slope may be defined as the angle between the line of the root of the weld and the horizontal. The weld rotation is defined by drawing a line from the root of the weld so that it bisects the weld profile and is at right angles to the weld line. The angle that this line makes with the vertical is the angle of weld rotation. Intermediate positions not specified may be referred to as inclined. (See Figure 3 and Table 1).

Figure 3 Welding Positions

Table 1 Welding Positions

Positions for Plates Slope Rotation LR Symbol

ISO Symbol

AWS Symbol

Flat 0-5° 0-10° D PA 1GHorizontal-vertical 0-5° 30-90° X PC* 2GVertical up 80-90° 0-180° Vu PF 3GVertical down 80-90° 0-180° Vd PG 3GOverhead 0-15° 115-180° O PE* 4G

* Note for fillet welds, horizontal-vertical has the symbol PB and overhead PD.

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1.3 Functions of the Electrode Coating

The functions of the electrode coating are many and varied but the two of most significance are:

(a) to provide a gas to shield the arc and prevent excessive atmospheric contamination of the molten filler metal travelling across the arc;

(b) to improve the smoothness and stability of the arc.

Other important aspects are:

(c) to produce a slag blanket to protect the hot weld metal from the air, to allow slower cooling and to enhance bead shape and surface cleanliness of the weld metal;

(d) to provide fluxes, scavengers and deoxidisers to cleanse the weld and prevent excessive grain growth;

(e) to allow alloying elements to be added to change the composition of the weld metal.

The use of AC will also affect the demands on the coating since the arc is extinguished and must be reignited every half cycle. Therefore the arc atmosphere must contain a suitable ionised gas to make this possible. Coatings containing iron powder may be used to increase the rate of deposition and to improve efficiency in the use of arc energy.

1.4 Types of Electrode Coating

There are three main classes of electrode coating;

The first, 'Cellulosic', contains a large proportion, up to about 35%, of the organic compound Cellulose (C6H10O5)n, together with slag forming items: principally Rutile, a mineral form of Titanium Dioxide TiO2. Cellulose is a naturally occurring constituent of wood which at high temperatures will dissociate into oxides of carbon and water. In ferrous welding the latter will react with iron to produce metallic oxide and the gas hydrogen, which to some extent will be absorbed into the weld. Such coatings will produce a limited amount of slag and they tend to be restricted to smaller diameters and may be used for work in all positions.

In the second type of coating, 'Basic', a protective gas is produced by the dissociation of basic carbonates, mainly Calcium Carbonate CaCO3, which at high temperature is converted to Calcium Oxide CaO and Carbon Dioxide CO2. These coatings usually include some Calcium Fluoride or Fluorspar, CaF2, to give fluidity to the slag; and they have the advantage that only small amounts of Hydrogen are generated provided the coating is dry, thus reducing the absorption of Hydrogen by the weld metal. This type of electrode is generally used where low hydrogen contents need to be guaranteed.

The third coating type, 'Rutile', is one consisting mainly of Titanium Dioxide plus various mineral constituents, and a small proportion of Cellulose - up to

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about 15%. Protection of the metal as it is passed across the arc is provided by the dissociation of the Cellulose and the production of large quantities of slag which will coat the metal droplets as they are transferred.

Metal powders (e.g. iron) may be included in the flux coating to raise the efficiency of the process and both basic and rutile coating variations of this type are available. The heavier rutile coatings contain only about 5% cellulose and as both varieties produce large quantities of molten metal and slag they are usually restricted to flat (downhand) and horizontal-vertical fillet welds. Other types of coating are available such as Acid Rutile, Acid and Oxidising but are, however, little used. Although there may be considerable differences in the compositions of the electrode cores depending on the properties desired in the weld, the technique of modifying composition by including alloying elements in the coating can be of great importance.

Electrode Coatings are summarised in Table 2.

Constituents of the coatings may also include: various clays; silica; oxides and carbonates of iron, manganese and calcium; aluminium and magnesium silicates; calcium fluoride, or fluorspar: carbonates and silicates of sodium and potassium; and ferro-manganese as a deoxidiser.

Coating compositions are described in general terms only, the proportions of the different ingredients and even the total number present being the manufacturer's prerogative and his secret. The quality of the product is determined in the judgement of the purchaser and although there are undoubted differences between suppliers, equally, there are variations in the opinions and tastes of individual welders and their employers.

It should be apparent that the coating on the electrode not only has an important influence on the properties of the resulting weld metal but it will also be the principle influence on the welding characteristics themselves, especially affecting such aspects as welding positions.

The functions of the individual constituents in a coating are listed in Table 3.

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Table 2 Electrode Coating Characteristics and Normal ApplicationsClass Composition of Covering Characteristics UsesCellulosic (C) Organic material containing

cellulose with some titanium oxide. Hydrogen releasing.

Thin, easily removable slag. Rather high splatter loss. Considerable envelope of shielding gas. Coarse ripple on weld surface, deeply penetrating arc with rapid burn-off rate.

All classes of mild steel welding in all positions: a.c. or d.c. electrode positive.

Basic (B) Calcium or other basic carbonates and fluorspar bonded with sodium or potassium silicates. Medium coating. Coating compounds contain little hydrogen. CO2 releasing.

Brown slag easy to remove. Medium ripple on weld metal, medium penetration. Fillet profile flat or convex. Deposited metal has high resistance to cold cracking because there is a low hydrogen content in the weld. Electrodes must be stored under warm dry conditions and dried before use.

Suitable for d.c. (electrode positive) or a.c. with OCV of 70V. Used for mild, low alloy high tensile and structural steels, particularly for conditions of high restraint. For flat, vertical and overhead positions, the latter having a flat deposit.

Basic highefficiency (BB)

Similar to basic electrode covering but have additional metallic materials (e.g. iron powder) in the covering which raise the efficiency to 130% and more.

These electrodes are suitable for welding in the flat and horizontal/vertical position with a greatly increased rate of metal deposition. Their high efficiency covering makes them unsuitable for welding in the vertical and overhead positions. They can be used either a.c. or d.c. generally with electrode +ve. Efficiency is indicated by a three-figure digit beginning the additional coding.

Rutile (R) Titanium dioxide (rutile) and other hydrated minerals and/or organic cellulose materials. Coating thickness less than 50% of the core wire diameter.

Easy to use, with smooth weld finish and medium penetration. High level of hydrogen in the weld metal limits their use in thick sections or restrained joints. Suitable for a.c. or d.c. the fast freezing of weld metal and fluid slag makes them suitable for vertical and overhead welding.

Rutile heavy coating (RR)

Similar covering to the previous rutile electrode but containing, in addition metallic substances (e.g. iron powder), which raises the efficiency to 130% or more. Coating thickness at least 50% greater than the core wire diameter.

Similar characteristics to rutile electrodes but generally unsuitable for vertical and overhead welding because of increased slag. Increased rate of metal deposition. Efficiency is indicated by a three-figure digit beginning the additional coding.

Acid (A) Oxides and carbonates of iron and manganese, with deoxidizers such as ferro-manganese.

Generally a thick coating which produces a fluid slag of large volume and solidifies in a ‘puffed-up’ manner, is full of holes and easily detached. Smooth weld finish with small ripples. Good penetration. Weld liable to solidification cracking if plate weldability is not good.

Usually in the flat position only but can be used in other positions; a.c. or d.c.

Acid rutile (AR) Generally a thick coating containing up to 35% rutile. Ilmenite (iron oxide) and titanium oxide is also used.

A fluid slag with other characteristics similar to the acid type of covering.

Similar to the acid type of coating.

Oxidizing (O) Iron oxide with or without manganese oxide and silicates.

Oxidizing slag so that the weld metal has a low carbon and manganese content referred to as ‘dead soft’. Reduction of area and impact values are lower than for other types of electrodes. Core wire melts up inside coating forming a cup so that the electrode can be used for ‘touch-welding’. Low penetration; solid slag often self-deslagging, with weld of neat appearance.

d.c. or a.c. supply with OCV as low as 45V.

Any Other Type (S) This category is for any electrode coverings not included in the foregoing list. Iron powder electrodes do not come into this category but should be indicated by their efficiency with a three digit figure.

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Table 3 Manual Metal Arc Electrode Flux Constituents and their FunctionsConstituent Primary Function Secondary

FunctionIron Oxide Slag Former Arc Stabiliser

Rutile (Titanium Dioxide) Slag Former Arc StabiliserMagnesia (Magnesium

Oxide)Fluxing Agent -

Calcium Fluoride Slag Former Fluxing AgentPotassium Silicate Arc Stabiliser Binder

Other Silicates Slag Formers and Binders

Fluxing Agent

Calcium Carbonate Gas Former Arc StabiliserOther Carbonates Gas Formers -

Cellulose Gas Former -Ferro-Manganese Alloying Deoxidiser

Ferro-Chrome Alloying -Ferro-Silicon Deoxidiser -

1.5 Electrode Classification Systems

Electrode specifications are usually prefixed with E followed by several digits and/or letters and/or chemical symbols.

Covered electrodes for welding carbon and carbon manganese steels

The European system (i.e. EN standards) is principally concerned with the mechanical properties of deposited weld metal, namely yield and tensile strengths and impact toughness. Additional information is included in the classification which relates to the weld metal composition, type of coating, recommended welding positions, welding current requirements, deposition efficiency and hydrogen control. (e.g. E4631NiB54H5)

In the American system, electrodes are classified under the American Welding Society (AWS) specification A 5.1, which is less complicated then the European system. Essentially it is a four digit number with the prefix E designating an electrode. The first two digits define the nominal minimum tensile strength of the deposited metal in thousands of pounds per square inch (kpsi); for mild steel. these will be 60 or 70. The third digit indicates the recommended positions (i.e. the digit 1 signifies suitability for all welding positions, 2 for flat or horizontal fillet welds, and 3 for the flat position only.) The fourth digit determines the electrical power requirements. The last two digits together refer to the type of electrode coating as stipulated in the specification. (e.g. E7018)

Low Alloy Electrodes

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Most of the low alloy steel electrode classifications are based on the low hydrogen or basic type of covering. The composition of the basic covering makes it possible to add a number of alloying elements to produce a range of weld metal analyses and strengths. The increased strength of the weld metal is obtained by the addition of alloying elements, which may be achieved either through the core wire or via the coating.

The European classifications include the chemical symbols such as Mn, Ni, Cr, when they are present as alloying additions, together with an indication of the carbon content, the type of covering, and the hydrogen control if required. Appropriate mechanical properties are specified including tensile strength, proof stress, elongation and if required impact tests at various temperatures. (e.g. ECrMo1LB)

The AWS specification for these electrodes is A5.5 and is very similar to that for carbon steels described above. The first two, or sometimes three, digits of the classification reflect the tensile strength of the weld deposit (e.g. 80, 110 etc.). For example the covering of E9016 will be similar to an E7016 electrode although the tensile strength will be 90 kpsi and the covering will generally contain the alloying elements. (e.g. E9016 - C2)

Electrodes for Alloy and Stainless Steels

The trend for these electrodes is for the weld metal to be at least as high in alloy content as the base material, and in some instances may be considerably higher.

The European standard refers to electrodes for Chromium and Chromium-Nickel steels, and the classification is indicated by the major alloying elements of the weld deposit. The first three character sets indicate the nominal Chromium, Nickel and Molybdenum contents respectively. The letter L or H is then added for low or high carbon content respectively. Other chemical symbols such as Mn, Cu, Nb follow if required. Finally appropriate electrode coating symbols such as B for basic, R for rutile etc. appear. (e.g. 19.12.3.Nb.R)

The AWS Specification for these electrodes is A5.4. As before they are denoted by the prefix E followed by a specification number for the weld metal deposit. These specification numbers are the same as those of the AISI series for stainless and alloy steels. (e.g. E316L)

Table 4 gives details of various alloy and stainless steel compositions relating to the ISO symbol on which the British and European Standards are based. The AWS equivalents are included.

It will be appreciated that the foregoing descriptions relate only to their barest essentials. It cannot be emphasised too strongly that when dealing with Standard Specifications it is important that they are studied in their official form and their requirements fully understood.

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Table 4 Stainless Steel Compositions

ISO Symbol

Composition of deposited metalSimilar AWS (2)

standardC Max% Cr % Ni % Mo % Other

elements13 0.12 11-14 E 41017 0.10 15-18 E 43030 0.10 17-30

19.9 0.08 18-21 8-11 E 30819.9 L 0.04 18-21 8-11 E 308L

19.9 Nb 0.08 18-21 8-11 Nb (1) E 34719.9 L Nb 0.04 18-21 8-11 Nb (1)

19.12.2 0.08 17-20 11-14 2-2.5 E 31619.12.2 L 0.04 17-20 11-14 2-2.5 E 316L19.12.2

Nb0.08 17-20 11-14 2-2.5 Nb (1) E 318

19.13.4 0.08 17-21 11-15 3.5-5.5

19.13.4 L 0.04 22-26 11-15 3.5-5.5

E 317

23.12 0.15 22-26 11-1523.12 L 0.04 22-26 11-15 E 309

23.12 Nb 0.12 22-25 11-15 Nb (1)

23.12.2 0.12 22-25 11-15 2-318.8 Mn 0.20 17-20 7-10 Mn 5-8

25.20 0.20 24-28 18-22 E 31025.20 L 0.04 24-28 18-22

25.20 Nb 0.12 24-28 18-22 Nb (1)

25.20.2 0.12 25-28 18-22 2-325.20 C 0.25/0.45 24-28 18-2220.9 Nb 0.13 18-21 8-10 0.35-

0.65Nb (1) E 349

29.9 0.15 28-32 8-12 E 31218.36 0.25 14-19 33-38

(1) NB content = min 8xC content and max. 1.2%. part of Nb can be replaced by Ta.

(2) American Welding Society.

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1.6 The Influence of Welding Current

Tables are available as guides to the approximate currents to be used with various types and sizes of electrodes, although the actual values employed will depend to a great extent on the work to be done. Generally the higher the current in the range given for a particular electrode size, the deeper the penetration and the faster the rate of deposition. Too high a current can lead to spatter and undercutting but too low a current will result in insufficient penetration and too small a deposit of weld metal. As a rule the arc voltage will be observed to increase slightly with increase in electrode diameter.

Operating requirements are invariably clearly stated by the electrode manufacturer and these are best adhered to; however, as a guide, Table 5 shows some suggested values.

Table 5 Typical Electrode CurrentsElectrode Diameter (mm)

1.6 2.5 3.25 4 5 6

Normal RecoveryType Electrode

30-50A 50-110A 110-150A

140-200A

200-260A

220-340A

High RecoveryElectrode

- - 130-160A

190-220A

260-320A

330-380A

The diameter of the electrode to be used will depend on the welding position, thickness and the type of joint. In the overhead, vertical and horizontal-vertical positions, owing to the effects of gravity, molten metal tends to run out of the joint and control by way of smaller molten metal pool as provided by lower currents and smaller diameter electrodes is necessary.

The welding current may be DC positive or negative, or AC. Some electrodes may be used with DC or AC, but others will be limited in this respect. The energy cost is lower when welding with AC but as this represents only a very minor part of the total welding costs it is unlikely to be a significant factor when choosing the current type. Generally all electrodes can be used with DC which provides a steadier arc and smoother metal transfer than AC. It also produces a good wetting action and a uniform weld bead shape. It is considered better for vertical and overhead work, and where a short arc is advantageous. Thin sheet is easier to weld with DC. However, there is the disadvantage of ‘arc blow’ where magnetic effects influence the direction of the arc making it difficult to control, especially when welding near the edges of ferro magnetic metals using high currents. (See Section 12). This problem does not arise with AC.

When the electrode is connected to DC negative (American terminology: DC straight polarity, DCSP) about two thirds of the heat is at the work-piece

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which will give deeper penetration. On the other hand if the electrode is connected to DC positive (US: DC reverse polarity, DCRP) two thirds of the heat will be at the electrode, thus increasing the electrode melting rate but reducing penetration.

1.7 Arc Length

The arc length is the distance from the molten tip of the electrode core wire to the surface of the molten weld pool. Proper arc length is important in obtaining a sound welded joint. Metal transfer from the tip of the electrode to the weld pool is not a smooth, uniform action, and instantaneous arc voltage varies as droplets of molten metal are transferred across the arc, even with constant arc length.

However, any variation in voltage will be minimal when welding is done with the proper amperage and arc length. The latter requires constant and consistent electrode feed.

The correct length varies according to the electrode classification, diameter, and covering composition; it also varies with amperage and welding position. Arc length increases with increasing electrode diameter and amperage and as a general rule, it should not exceed the diameter of the core wire of the electrode. The arc usually is shorter than this for electrodes with thick coverings, such as iron powder or ‘drag’ electrodes.

Too short an arc will be erratic and may short circuit during metal transfer. Too long an arc will lack direction and intensity, which will tend to spatter the molten metal as it moves from the electrode to the weld. The spatter may be heavy and deposition efficiency low. Also, the gas and flux generated by the covering are not as effective in shielding the arc and the weld metal from air. The poor shielding can cause porosity and contamination of the weld metal by oxygen or nitrogen, or both and the quality of the weld will be poor.

Control of arc length is largely a matter of welder skill, involving the welder's knowledge, experience, visual perception and manual dexterity. Although the arc length does change to some extent with changing conditions certain fundamental principles can be given as a guide to the proper arc length for a given set of conditions.

For downhand welding, particularly with heavy electrode coverings, the tip of the electrode can be dragged lightly along the joint. The arc length, in this case, is automatically determined by the depth of the cup at the tip of the electrode and the melting rate of the electrode. For vertical or overhead welding, the arc length is always gauged by the welder. The proper arc length, in such cases, is the one that permits the welder to control the size and motion of the molten weld pool. The same is true for the root passes in groove and fillet welds.

The various classifications of electrodes have widely different operating characteristics, including arc length. It is important, therefore, for the welder to be familiar with the operating characteristics of the types of electrodes he

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uses in order to recognise the proper arc length and to know the effect of different arc lengths. The effect of a long and a short arc on bead appearance with a mild steel electrode is illustrated (see Figure 4).

Figure 4 Effects of varying current, arc length (arc voltage) and travel speed illustrated by surfaces and cross-sectional views of shielded metal-arc welds: left to right - current, arc length and travel speed normal; current too low; current too high; arc length too short; arc length too long; travel speed too slow; travel speed too high

Table 6 Influence of Arc Length on Weld Metal AnalysisArc Type C N2 Mn Si S P

Short 0.085 0.009 0.72 0.53 0.018 0.016

Normal 0.080 0.015 0.71 0.54 0.017 0.027

Long in still air 0.075 0.048 0.64 0.39 0.018 0.025

Long in windy conditions 0.055 0.069 0.63 0.15 0.018 0.018

(Deposition with a normal basic-coated electrode)

1.8 Low Hydrogen Electrodes

At high temperatures hydrogen, unlike oxygen and nitrogen, does not form any compounds with iron and has a high solubility in the austenite phase. Hydrogen has a lower solubility in steel after transformation at lower temperatures, on cooling. This hydrogen will cause embrittlement in steel. In a weld and the surrounding regions the presence of hydrogen will also increase the tendency to cracking. It is important in all critical structures to keep hydrogen to a minimum. The problem can be minimised by employing basic-coated electrodes which have been baked in manufacture and subsequently kept dry. Here the coating consists of calcium, and other carbonates, and fluorspar bonded with sodium or potassium silicate. In the heat of the arc, the carbonates dissociate releasing carbon dioxide which acts as the shielding gas. To further reduce moisture content they are frequently baked immediately before use. Low hydrogen electrodes are

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normally tested to demonstrate a weld metal hydrogen content of less than 15 cm3/100g of deposited metal.

Table 7 indicates the effect of different coatings and baking temperatures on hydrogen content of the weld metal.

Table 7 Typical Hydrogen ContentsCoating type Hydrogen cm3 per 100 g

deposited metalCellulosic > 70

Rutile > 20Basic - Dried 100-150°C 10 - 15Basic - Dried 350-450°C 3 - 10

A Basic slag is relatively thick and viscous which makes the electrodes comparatively difficult to use. They can however be employed for welding in all positions and the weld metal has excellent mechanical properties. Such electrodes are often used for welding structures exposed to high stresses and are usually specified when there are requirements for impact values at low temperatures. Basic, low hydrogen coatings are used for electrodes to deposit high strength steel weld metal.

1.9 Deep Penetration Welding

With common welding practices it may be expected that penetration will be of the order of 1 mm per 100 amps of current. A deep penetration electrode is defined in BS 499 as ‘A covered electrode in which the covering aids the production of a penetrating arc to give a deeper than normal fusion in the root of the joint.’ Such electrodes can be used to produce faultless butt welds in square butt joints which have been set up correctly.

The deep penetration electrodes are sometimes given the classification P, as in E435P, and have a cellulose type of covering. If using a DC arc the electrode should be connected to the negative pole so that the maximum heat goes to the work-piece. For such electrodes an arc voltage of 60-70V is usual compared with 20-30V for the normal type of electrode. In general the weld metal will contain a large proportion of melted parent metal and will therefore have a composition closely related to that of the parent metal.

1.10 Hard Facing

‘MMA welding is a often used for applying surface layers’ to metals to improve the resistance to abrasion, impact, corrosion and heat. The advantage of the method is that the surface can be deposited on a cheaper base metal to give wear resistance or other qualities, exactly where required, with great financial savings. Also worn parts can be built up with substantial reductions in time and replacement costs. Very hard surfaces are normally required for good abrasion resistance but high hardness values

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are usually accompanied by poor resistance to impact. Conversely good impact resistance is not allied to extreme hardness and it is therefore necessary to determine which quality is of greater importance.

Similarly consideration must be given to the requirements of corrosion and heat resistance and to the composition of the base metal, the need for pre-heating and the possibility of post-welding heat treatment. The Stellite series of alloys which may be nickel or cobalt based are well known for hardfacing applications which may include caterpillar tracks, excavator buckets, railway points, rock crushers etc. Cutting tools for lathes and milling machines etc. can be made by depositing a layer of high speed tool steel onto a shank of lower carbon steel.

It will be realised that the parent metal will dilute the deposited metal and to minimise this effect three hard facing layers should be applied where possible. The total thickness of the hard facing layers should normally not exceed about 6 mm and where a thicker deposit is required it should first be built up with low hydrogen weld metal.

1.11 Gravity Welding

This is a simple method for economically welding long fillets in the flat position using gravity to feed the electrode and to traverse the weld pool along the joint. An operator can look after two or more machines at any one time, for example, one on each side of a plate, giving symmetrical welds and reducing stress and distortion. The electrode holder is mounted on a ball-bearing carriage and slides smoothly down a guide bar, the angle of which can be adjusted to give faster or slower traverse and thus vary the length of deposit of the electrode and the leg length of the weld (see Figure 5).

Figure 5 Gravity Welding

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Electrodes of 700 mm and more in length are available in diameters of 3.5, 4.0, 4.5, 5.0 and 5.5 mm using currents of 220-315 A with rutile, rutile-basic and acid coatings suitable for various grades of steel.

Gravity welding is generally used for fillets with leg lengths of 5-8 mm, the lengths being varied by altering the length of deposit per electrode. An AC power source is used for each unit with an OCV of 60V and arc voltage about 40V with currents up to 300A. Sources are available for supplying up to 6 units (3 pairs) manageable by one welding operator and so arranged that when the current setting for one unit is chosen, the remaining units are supplied at this value. In general the system is particularly suitable for welding, for example, long parallel stiffeners on large unit panels, enabling one operator to make three or four times the deposit length compared with manual welding. Its main application is in ship building for fillet welding in the horizontal-vertical position.

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2. SUBMERGED ARC WELDING

2.1 The Process

Abbreviated as SAW, this is a welding process where an arc is struck between a continuous bare wire and the parent plate. The arc, electrode end and the molten pool are submerged in an agglomerated or fused powdered flux which turns into a slag in its lower layers when subjected to the heat of the arc, thus protecting the weld from contamination. The wire electrode is fed continuously by a feed unit of motor-driven rollers which usually are voltage-controlled to ensure an arc of constant length. The flux is fed from a hopper fixed to the welding head, and a tube from the hopper spreads the flux in a continuous elongated mound in front of the arc along the line of the intended weld and of sufficient depth to submerge the arc completely so that there is no spatter, the weld is shielded from the atmosphere, and there are no ultra-violet or infra-red radiation effects (see Figure 6). Unmelted flux is reclaimed for use. The use of powdered flux restricts the process to the flat and horizontal-vertical welding positions.

Submerged arc welding is noted for its ability to employ high weld currents owing to the properties and functions of the flux. Such currents give deep penetration and high dilution where twice as much parent metal as wire electrode is melted. Generally a DC positive current is employed up to about 1000 amps. At higher currents, AC is often preferred to avoid the problem of arc blow. Difficulties sometimes arise in ensuring conformity of the weld with a predetermined line owing to the obscuring effect of the flux. Where possible, a guide wheel to run in the joint preparation is positioned in front of the welding head and flux hoppers.

Submerged arc welding is widely used in the fabrication of ships, pressure vessels, line pipe, railway wagons and anywhere long welds are required. It can be used to weld thicknesses from 1.5 mm upwards.

Figure 6 Schematic diagram of Submerged Arc Welding

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2.2 Materials Joined

Submerged arc welding may be used for joining many ferrous and non-ferrous metals and alloys and to apply cladding to base metals to improve wear and corrosion resistance. Electrodes are available producing weld metal suitable for use with plain carbon steel, special alloy steel, stainless steel, non-ferrous alloys, mainly Nickel based, and special alloys for surfacing applications. Combinations of carbon steel electrodes and fluxes are specified to give the desired properties to the resulting weld metal. Alloy steels can be welded with alloy steel electrodes using neutral fluxes or with carbon steel electrodes using fluxes containing the alloying elements.

2.3 Fluxes

The fluxes may be defined as granular mineral compounds mixed to various formulations. The so called fused fluxes are produced when the constituents are dry mixed and melted in an electric furnace and thereafter granulated by pouring the molten mixture into water. Subsequently, these particles are crushed and screened to yield a uniform glass-like product. Such fluxes have the advantages of homogeneity and they are less hygroscopic than other types. They allow fines (fine powders) to be removed without changes in composition and they can easily be recycled through the system. There are however limitations in composition as some components such as basic carbonates would be unable to withstand the melting process.

Alternatively, the powdered flux constituents may be bonded by mixing the dry constituents with Potassium or Sodium Silicate. This wet mixture is then pelletised, dried, crushed and screened to size. This method has the advantage that deoxidisers and alloying elements can easily be added to the flux to adjust the weld metal composition. It will allow a thicker flux layer when welding and it can be identified by colour coding. Its disadvantages are that it is generally more hygroscopic, that gas may be evolved from the slag as it is melted, and there may be changes in weld metal chemical composition from the segregation of fine particles produced by the mechanical handling of the granulated flux.

2.4 Welding Head Arrangements

There are several variations of machine and automatic submerged arc welding that will permit higher deposition rates with good control of the weld bead size and penetration. Various multiple electrode systems that use one or more power sources with different types of circuit connections are available. For example, two electrodes can be positioned in tandem so that their arcs will produce a single molten weld pool. In this configuration, the arcs may be operated from the same power source by connecting them either in series or in parallel, or they may be operated from separate power sources. In the latter case, one system uses a DC source for the lead arc and an AC source for the trail arc. Another system uses two AC power sources with an adjustable phase-shift control to adjust the interaction between the two AC arcs.

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2.5 Operating Variables

Knowledge and control of the operating variables in submerged arc welding are essential if high production rates and welds of good quality are to be obtained consistently (see Figure 7).

These variables, in the approximate order of their importance, are:

1. Welding current 2. Type of flux and particle distribution 3. Welding voltage 4. Welding speed 5. Electrode size 6. Electrode extension 7. Type of electrode 8. Width and depth of the layer of flux

2.5.1 Welding Current

Welding current is the most influential variable because it controls the rate at which the electrode is melted, the depth of penetration, and the amount of base metal melted. If the current is too high at a given travel speed, the depth of fusion or penetration will be too great. The resulting weld may have a tendency to melt through the metal being joined. High current also leads to waste of electrodes in the form of excess weld metal. This over welding increases weld shrinkage and usually causes greater distortion. If the current is too low, inadequate penetration or incomplete fusion may result.

Some rules to remember concerning welding current are:

1. Increasing current increases penetration and melting rate.2. Excessively high current produces a digging arc; undercut; or a high,

narrow bead.3. Excessively low current produces an unstable arc.

2.5.2 Arc Voltage

Arc voltage adjustment varies the length of the arc between the electrode and the molten weld metal. If the arc voltage increases, the arc length increases; if the arc voltage decreases, the arc length decreases.

The arc voltage has little effect on the electrode deposition rate which is determined mainly by the welding current. The voltage principally determines the shape of the weld bead cross section and its external appearance.

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Increasing the arc voltage with constant current and travel speed will:

1 . Produce a flatter and wider bead2. Increase flux consumption3. Tend to reduce porosity caused by rust or scale on steel.4. Help to bridge excessive root opening when fit-up is poor.5. Increase pickup of alloying elements from the flux when they are

present.

Excessively high arc voltage will:

1. Produce a wide bead shape that is subject to solidification cracking.

2. Make slag removal difficult in groove welds.3. Produce a concave shaped fillet weld that may be subject to

cracking.4. Increase undercut along the edge(s) of fillet welds.5. Over alloy the weld metal, via the flux.

Lowering the arc voltage produces a ‘stiffer’ arc which improves penetration in a deep weld groove and resists arc blow. An excessively low voltage produces a high, narrow bead and causes difficult slag removal along the bead edges.

2.5.3 Travel Speed

With any combination of welding current and voltage, the effects of changing the travel speed conform to a general pattern. If the travel speed is increased:

1. Power or heat input per unit length of weld is decreased;2. Less filler metal is applied per unit length of weld, and consequently

less excess weld metal;3. Penetration decreases.

Thus, the weld bead becomes smaller.

2.5.4 Electrode Size

Electrode size affects the weld bead shape and the depth of penetration at a given current. Small electrodes are used with semi-automatic equipment to provide flexibility of movement. They are also used for multiple electrode, parallel power equipment.

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Electrode size also influences the deposition rate. At any given amperage setting, a small diameter electrode will have a higher current density and a higher deposition rate of molten metal than a larger diameter electrode. However, a larger diameter electrode can carry more current than a smaller electrode, so the larger electrode can ultimately produce a higher deposition rate at higher amperage. If a desired electrode feed rate is higher (or lower) than the feed motor can maintain, changing to a larger (or smaller) size electrode will permit the desired deposition rate.

For a given electrode size, a high current density results in a ‘stiff' arc that penetrates into the base metal. Conversely, a lower current density in the same size electrode results in a ‘soft’ arc that is less penetrating.

2.5.5 Electrode Extension

The electrode extension is the distance the continuous electrode protrudes beyond the contact tip. At high current densities, resistance heating of the electrode between the contact tip and the arc can be utilised to increase the electrode melting rate. The longer the extension, the greater the amount of heating and the higher the melting rate. This resistance heating is commonly referred to as I2R heating which when increased will enhance deposition rates by as much as 25-50%. Such adjustments will limit the power available at the weld itself resulting in reduced penetration and bead width. To counteract these effects increases in electrode extension should be accompanied by appropriate increases in voltage.

2.5.6 Type of Electrode

An electrode with a low electrical conductivity, such as stainless steel, can with a normal electrode extension experience greater resistance heating. Thus for the same size electrode and current, the melting rate of a stainless steel electrode will be higher than that of a carbon steel electrode.

2.5.7 Width and Depth of Flux

The width and depth of the layer of granular flux influence the appearance and soundness of the finished weld as well as the welding action. If the granular layer is too deep, the arc is too confined and a rough weld with a rope-like appearance is likely to result. The gases generated during welding cannot readily escape, and the surface of the molten weld metal is irregularly distorted. If the granular layer is too shallow, the arc will not be entirely submerged in flux. Flashing and spattering will occur. The weld will have a poor appearance, and it may be porous.

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Figure 7 Effect on submerged Arc Operating Variables

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3. GAS-SHIELDED METAL-ARC WELDING

3.1 The Process

Known in the USA as Gas Metal Arc Welding (GMAW) this process can be further distinguished by the character of the shielding gas: inert or chemically active. This results in the terms Metal-Arc Inert Gas (MIG) and Metal-Arc Active Gas (MAG) welding. This process is now in common use having displaced some of the more traditional manual welding techniques. In this process, the arc, surrounded by a protective gas, is struck between a consumable wire electrode and the work.

The process is suitable for welding aluminium, magnesium alloys, plain and low-alloy steels, stainless and heat-resistant steels, copper and bronze, the variations being filler wire and type of shielding gas.

The continuous consumable electrode wire is mechanically fed from a spool to a manually or mechanically controlled gun through a flexible guide tube by motor-driven rollers of adjustable speed. The rate of burn-off of the electrode wire must be balanced by the rate of wire feed which determines the current used.

In addition, a shielding gas or gas mixture is fed to the gun together with welding current supply, cooling water flow and return (if the gun is water cooled) and a control cable from the gun switch to control contactors (see Figures 8 and 9).

Figure 8 Components of gas shielded metal arc welding process

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Figure 9 Gas metal arc welding terminology

Pure argon cannot be used as a shielding gas for mild, low-alloy and stainless steel because of arc instability but now sophisticated gas mixtures of argon, helium, carbon dioxide and oxygen have greatly increased the range of the process. Carbon dioxide alone is widely employed as a shield when welding carbon and low alloy steels. The method has many applications and its use is likely to increase in the future.

3.2 Electrodes

The composition of the electrode and base metal should be as nearly alike as practicable. In some cases this requirement can be met but in others, to obtain satisfactory welding and weld metal characteristics, an appreciable composition change is needed. Deoxidisers and other scavengers are nearly always added to minimise porosity or to ensure that the presence of oxygen, hydrogen or nitrogen is neutralised. These gases may be part of the shielding gas or reach the weld pool from the surrounding atmosphere. In steel electrodes, deoxidisers may be Mn, Si or Al, in Nickel alloys Ti or Si and in Copper alloys Ti, Si or P. Their use is especially important with shielding gases containing oxygen.

Manganese and silicon are used as deoxidisers in many cases in steel but triple deoxidised wire using aluminium, titanium and zirconium gives high-quality welds and is especially suitable for use with CO2 gas shield.

Generally wire diameters are quite small compared with other types of welding, ranging from 1.0 mm to 1.6 mm, although up to 3.0 mm or down to 0.5 mm may be used occasionally. The high currents employed in MIG and MAG welding combined with the small diameter wire result in very high melting rates varying from about 40 mm/sec up to 340 mm/sec. The wires must therefore be supplied in long continuous strands, suitably hardened (stiffened), for non-stop smooth feeding through the equipment. Ferrous wires are usually coated with copper to provide some corrosion resistance and to improve electrical contact in the welding equipment.

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3.3 Transfer Modes

3.3.1 Spray Transfer

In manual metal arc welding, metal is transferred in globules or droplets from the electrode to work. If the current is increased to the continuously fed, gas-shielded wire, the rate at which such droplets are projected across the arc increases and they become smaller in volume and the transfer is then in the form of a fine spray (see Figure l0a).

The type of gas being used as a shield greatly affects the values of current at which spray transfer occurs but they are usually more than 200 amps. Much greater current densities are required with CO2 than with argon mixtures to obtain the same droplet rate. The arc is continuous during operation, arc energy output is high, the rate of deposition of metal is high, penetration is deep and there is considerable dilution from the parent metal. If current becomes excessively high, turbulence can be induced in the gas shield, leading to oxidation, and oxide film entrapment in the weld metal when welding aluminium. For spray transfer there is a high voltage drop across the arc (30-45 V) and a high current density in the wire electrode making the process suitable for thicker sections, mostly in the flat position.

The high currents used produce strong magnetic fields and a very directional arc. With argon shielding the forces on the droplets are well balanced during transfer so that they move smoothly from wire to work with little spatter. With CO2 shielding the forces on the droplet are less balanced so that the arc is less smooth and spatter tendency is greater.

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(a) Spray transfer: arc volts 27-45 V. Shielding gases: argon, argon- 1 or 2% oxygen, argon- 20% CO2, argon- 2% oxygen- 5% CO2. High current and deposition rate, used for flat welding of thicker sections

(b) Short-circuit or dip transfer: arc volts 15-22 V. Shielding gases as for spray transfer. Lower heat output and lower deposition rate than spray transfer. Minimises distortion, low dilution. Used for thinner sections and positional welding of thicker sections

Figure 10 Types of arc transfer

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3.3.2 Short Circuit or Dip Transfer

With lower arc volts (15-22V) and currents usually less than 200 amps, transfer takes place in globular form but with intermittent short-circuiting of the arc (see Figure 10b). The wire feed rate must just exceed the burn-off rate so that the intermittent short-circuiting will occur. When the wire touches the pool and short-circuits the arc there is a momentary rise of current, which must be sufficient to make the wire tip molten, a neck is then formed in it due to magnetic pinch effect and it melts off in the form of a droplet being sucked into the molten pool aided by surface tension. The arc is then re-established, gradually reducing in length as the wire feed rate gains on the burn-off until short-circuiting again occurs. The power source must supply sufficient current on short-circuit to ensure melt-off or otherwise the wire will stick in a solidified weld. It must also be able to provide sufficient voltage immediately after short-circuit to re-establish the arc.

The short-circuit frequency depends upon:

the arc voltage and current type of shielding gas diameter of wire power source characteristics

but will be about 50 to 200 times per second. The heat output of this type of arc is much less than that of the spray transfer type and makes the process suitable for the welding of thinner sections and for all positional welding, in addition to multi-run thicker sections, and it gives much greater welding speed than metal-arc-welding with covered electrodes on light gauge steel, for example. Dip transfer has the lowest weld metal dilution value of all the arc processes. However, welds may be more prone to lack of fusion defects, particularly when CO2 is used as the gas shield.

In order to keep stable welding conditions with a low voltage arc (17-20 V) which is being rapidly short-circuited, the power source must have the right characteristics. If the short-circuit current is low the electrode will freeze to the plate when welding with low currents and voltages. If the short-circuit current is too high a hole may be formed in the plate or excessive spatter may occur due to scattering of the arc pool when the arc is re-established.

The power supply must fulfil the following conditions:

1. During short-circuit the current must increase enough to melt the wire tip but not so much that it causes spatter when the arc is re-established.

2. The inductance of the circuit must store enough energy during the short-circuit to help to start the arc again and assist in maintaining it during the decay of voltage and current

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3.3.3 Semi-short Circuiting Arc

In between the spray transfer and dip transfer ranges is an intermediate range in which the frequency of droplet transfer is approaching that of spray yet at the same time short-circuiting is taking place, but is of very short duration. This semi-short circuiting arc has certain applications, as for example the automatic welding of medium-thickness steel plate with CO2 as the shielding gas.

3.3.4 Pulsed Arc Spray

This system allows all-position welding at higher energy levels than short circuit transfer. The power source provides two current levels, a steady ‘background’ level too low to produce spray transfer and a ‘pulsed peak’ current which is superimposed on the background at regular intervals. The pulsed peak is well above the transition current and usually one drop of metal is transferred during each pulse. The combination of two current levels produces a steady arc with axial spray transfer at currents below those required for conventional spray arc welding.

3.4 Shielding Gases

As oxygen and CO2 are not inert gases the term Metal-Arc Inert Gas (MIG) is not applicable when either of these gases is mixed with Argon, or CO 2 is used on its own. The term Metal-Arc Active Gas (MAG) should be used in these cases, if greater distinction is required than that provided by the general term ‘Gas-Shielded Metal-Arc Welding’.

Argon is used as a shielding gas because it is chemically inert and forms no compounds. It is especially useful in welding non-ferrous metals and alloys but in welding steel it exhibits an uneven negative pole at the work piece, (the electrode being positive) to give an irregular weld profile. Argon plus 1% or 2% oxygen gives a higher arc temperature and the oxygen acts as a wetting agent to the molten pool making it more fluid and stabilising the arc.

Helium is sometimes added to mixed gases. Its presence increases the arc voltage and consequent heat input. Mixing it with Argon, Oxygen or CO2 controls the pool temperature, increases wetting and stabilises the arc.

Carbon Dioxide CO2 has the advantage of being the cheapest shielding gas and it can be used for welding both alloy and plain carbon steels up to 0.4%C. There is some dissociation of CO2 in the arc producing carbon monoxide and oxygen which requires the filler wire to be adequately deoxidised to prevent porosity. Some wires rely solely on Mn and Si for this deoxidation. Others include the more efficient elements Al, Ti and Zr in varying proportions. Generally the arc is less smooth with CO2 than with Ar-CO2 and Ar-O2. The arc conditions are more critical and there is more spatter.

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Argon plus 5% CO2 or Argon + 20% CO2 for steel improves the wetting action, reduces surface tension and makes the pool more fluid. Both mixtures are excellent with spray or dip transfer, they give a smoother less critical arc than pure CO2 and reduce spatter; but naturally they are more expensive than pure CO2.

Recommended gases and gas mixture for various metals and alloys are shown in Table 8 below:Metal Type Gas Shield RemarksCarbon and low-alloy steels

CO2

Ar-15/20%CO2

Ar-5%CO2

Ar-5%O2

Ar-5%CO2 - 2%O2

For dip transfer, and spray transfer spatter problems. Use deoxidized wire.

For dip or spray transferMinimum spatter

For dip and spray transferSpray transfer. High impact properties.

For pulsed arc and thin sections.

Stainless Steels Ar-1/2%O2

75%He 23.5%Ar 1.5%CO2

75%He - 24%Ar 1%O2

Spray transferHigh quality dip transfer. For thin sections and positional work.

Good profile.

Aluminium and its alloys

ArgonHelium

75% He - 25%Ar

Stable with little spatterHotter arc, less pre-heat, more spatterStable arc, high heat input. Good penetration. Recommended for thicknesses above 16 mm.

Magnesium and its alloys

Argon75% He 25%Ar

Stable arcHotter arc. Less porosity.

Copper and its alloys

Argon

Helium75% He 25%Ar

For sections up to 9.5 mm thickness

For medium and heavy sections. High heat input.

Nickels and its alloys

Argon

70% Ar 30%He25% Ar 75%He

Sections up to 9.5 mm thickness Pulsed arcHigh heat input less cracking in thicker sections of 9% Ni

CupronickelTitanium,Zirconium and alloys

Argon70% Ar 30%HeHigh purity argon

Stable arcStable arc with less cracking riskVery reactive metals. High purity shielding gases are essential.

Note: 02 increases the wetting action.

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3.5 Operating Variables

3.5.1 Arc Voltage

It is easier to set and maintain welding conditions with a constant voltage power source (see Power Sources chapter) which will permit little variation of voltage or arc length during welding. These are predetermined to a large extent by the shielding gas and the metal to be welded. Such voltage adjustments are usually incorporated into the welding machine in which the amperage is controlled by the wire feed speed. Within the limits of these conditions it may be expected that increase in voltage will tend to flatten the weld bead and increase the width of the fusion zone. Decrease in voltage will result in a narrower weld bead with a higher reinforcement and deeper penetration. Excessively high voltage may cause porosity, spatter and undercutting whereas excessively low voltage may cause porosity and overlap at the weld edges.

3.5.2 Arc Length

An increase in arc length, that is the distance from the electrode tip to the work, will cause an increase of arc voltage and vice-versa. In practice any such changes would be instantly corrected by the constant voltage supply system.

3.5.3 Current

If all other variables are held constant, welding current varies with the wire speed or melting rate. At lower amperage ranges the relationship is nearly linear but in the upper ranges this ceases to be so largely due to resistance heating of the electrode ‘stick out’ beyond the contact tube. Generally, increase in welding current alone will -

(a) Increase the depth and width of the weld penetration.(b) Increase deposition rate.(c) Increase the size of the weld bead.

3.5.4 Travel Speed

A decrease in speed will increase the deposit of the filler metal per unit length producing a large shallow weld pool. The welding arc impinges on this pool rather than the base metal as it advances. This limits penetration but gives a wide weld bead. Increase in speed will reduce the thermal energy transmitted to the base metal and melting is therefore slowed and restricted to the surface. Thus both penetration and bead width are decreased.

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3.5.5 Electrode Extension

Electrode extension is the distance between the last point of electrical contact and the tip of the electrode (see Figure 9). As this distance increases so does the electrical resistance of the electrode extension and the consequent increase in resistance heating causes the electrode temperature to rise. Thus less welding current is required to melt the electrode at a given feed rate.

3 5.6 Electrode Size

Each electrode diameter of a given composition has a usable current range. The welding current range is limited by undesirable effects, such as the absence of wetting at very low values, and also spatter, porosity, and poor bead appearance with excessively high values.

The electrode melting rate is a function of current density. If two wires of different diameters are operated at the same current, the smaller will have the higher melting rate and deposit larger quantities of molten metal.

Penetration is also a function of current density. For example, a 1 mm diameter electrode will produce deeper penetration than a 1.5 mm diameter electrode when it is used at identical current. However, the weld bead profile will be wider with the larger electrode. The reverse is also true when a small weld bead profile is specified. Since smaller diameter wires are more costly on a weight basis, for each application there is a wire size that will give minimum cost welds. Cored wires give a greater deposition rate as a result of increased current density.

3.6 Advantages and Limitations of the GMAW Process

Advantages:

1. The continuous electrode wire feed allows greater continuity of welding than with Manual Metal-Arc Welding with covered electrodes. This alone leads to high weld metal deposition rates. The latter is further increased by the higher arc efficiency of the process, and because there is little or no slag to be removed.

2. Welding is possible in all positions which is not the case with Submerged Arc welding.

3. Deeper penetration is possible than with Manual Metal Arc welding.

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Limitations:

1. Welding equipment is more costly, complex and less portable than that used for Manual Metal Arc welding.

2. Access to the welding location can be restricted by the shape of the welding gun and its attached feed tubes and cables.

3. Outside applications are limited as the shielding gas can be disrupted even by low speed winds.

4. Weld metal cooling rates are higher owing to the absence of slag, affecting the metallurgical and mechanical properties of the weld.

This GMAW process has not displaced Submerged Arc and electroslag methods for welding thick steel sections but complements them. It offers the most competitive method for repetition welding and thicknesses up to 75 mm can be joined in steel using fully automatic heads.

3.7 Cored and Self-Shielded Wires

Solid wires are limited in use by composition. Unlike MMA consumables where alloy variation can be made in changes to the flux coating, the composition of solid wires is fixed. In the past this problem restricted the use of GMAW welding.

With GMAW equipment flux-cored wires are becoming increasingly popular for welding ferrous metals as they can combine the productivity of continuous welding with the metallurgical benefits derived from using a flux.

Flux-cored arc welding offers two major process variations that differ in the method used to shield the arc and weld pool from atmospheric contamination (oxygen and nitrogen). One type, self-shielded, protects the molten metal to some extent through the decomposition and vaporisation of the flux core by the heat of the arc. The other type, gas shielded, makes use of a protective gas flow in addition to the flux core action to shield the arc and the weld pool. With both methods, the electrode core material provides a relatively thin slag covering to protect the solidifying weld metal.

In the gas shielded method, the shielding gas (usually, but not exclusively, carbon dioxide) protects the molten metal from the oxygen and nitrogen of the air by forming an envelope around the arc and over the weld pool (see Figure 11). Little need exists for denitrification of the weld metal because nitrogen from the air is mostly excluded. Although most of the air is excluded, some oxygen is present in the protective atmosphere. It may be present as an additive to argon or from dissociation of CO2 to form carbon monoxide and oxygen. The compositions of the electrodes are designed to tolerate small amounts of oxygen in the shielding gas. Thus, flux-cored electrodes are normally designed specifically either to be self-shielding or for use with gas shielding.

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In the self-shielded method, (see Figure 12) although some shielding is obtained from vaporised flux ingredients there is greater need for the addition of deoxidizing and denitrifying constituents to the filler metal and flux. This explains why self-shielded electrodes can operate in the strong air currents frequently encountered when welding outdoors.

Figure 11 Gas Shielded Flux Cored Arc Welding

Figure 12 Self Shielded Flux Cored Arc Welding

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The self-shielded method is used with long electrode extensions (20-95 mm) which tend to produce shallow weld beads whereas the gas-shielded method with electrode extensions 19-38 mm is suited to the production of narrow deeply penetrating welds. The process may be used to weld plain carbon and low alloy steels and stainless steels. Cored wires may contain proportions of metal powder to improve deposition rates. Such wires may be Argon/20% CO2 gas shielded with the electrode DC negative to give a smooth arc with little spatter. Cored wires of all types can usually be applied to welding in all positions.

A major advantage of cored over solid wires is the ability to change the weld metal composition by alloy additions to the flux. This has therefore provided a variety of consumable compositions on a scale similar to SMAW (MMA).

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4. TUNGSTEN INERT GAS ARC WELDING

Known in the USA as Gas Tungsten Arc Welding (GTAW), TIG welding is a process where melting is produced by heating with an arc struck between a non-consumable tungsten electrode and the work-piece. Inert shielding of the electrode and weld zone is necessary to prevent oxidation of the tungsten electrode (see Figure 13). Filler metal may or may not be needed. Tungsten is used because its melting point is 3370°C, well above any other common metal.

Figure 13 Gas Tungsten Arc Welding

The TIG process is very good for joining thin base metals and as the electrode is not consumed, fusion alone, without the addition of a filler metal, may be employed if desired. It is suitable for almost all metals but is not generally used for those with low melting points such as Lead and Tin. The method is especially useful in welding the reactive metals with very stable oxides such as Aluminium, Magnesium, Titanium and Zirconium. A very high quality weld is normally produced and it is often used for joining very expensive metals and for critical service uses.

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Process limitations are:

(1) It is slower than most other arc welding processes.

(2) Tungsten may contaminate the weld to give inclusions.

(3) Inert gases are expensive, usually being Argon or Helium, or a mixture of the two.

For these reasons TIG welding is generally not competitive with other methods for welding heavier gauges of metal.

TIG welding can be markedly affected by variations in current, voltage and power source characteristics. The most important aspects are:

(1) Generally the best welding results are obtained with DC electrode negative.

(2) Fusion is hindered by refractory oxides such as those of Aluminium or Magnesium but these can be removed by using AC or DC electrode positive.

(3) With a DC positively connected electrode, heat is concentrated at the anode or positive terminal and therefore a positive electrode needs to be of greater diameter than one connected negatively so that the extra heat is dissipated.

(4) The current carrying capacity of a positive electrode is about one tenth that of a negative one and it is therefore limited to welding sheet metal.

Common applications for the TIG process include welding longitudinal seams in thin walled pressure pipes and tubes on continuous forming mills usually in alloy and stainless steel without filler metals. Also, using filler metals, in producing heavier gauge pipe and tubing for the chemical, petroleum and power generating industries and in the aircraft industry for airframes, jet engines and rocket motor cases.

It is convenient here to compare once again the American terminology. DC negative is known as Direct Current Straight Polarity (DCSP) and DC positive as Direct Current Reverse Polarity (DCRP).

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5. AUTOMATIC WELDING

There has been a great increase in the number of automatic processes designed to speed up welding production. Automatic welding gives high rates of metal deposition because high currents from 400 to more than 2000 amps can be used, compared with the normal limit of about 600 amps with manual arc welding. Automatic arc control gives uniformly good weld quality and finish and the high heat input reduces distortion and the number of runs for a given plate thickness is reduced. Twin welding heads still further reduce welding time and when used, for example, one on each side of a plate being fillet welded, distortion is reduced. The welding head may be:

1. Fixed with the work arranged to move beneath it.

2. Mounted on a boom and column which can either be of the positioning type in which the work moves or the boom can traverse at welding speed over the fixed work.

3. Gantry mounted so that it can traverse over the stationary work.

4. Self propelled on a motor-driven carriage.

5. Mounted in the 'hand' of a robot.

The processes, namely TIG, MIG and CO2 (gas shielded metal arc) with their modifications, are extensively used fully automatically. Heads are now available which, by changing simple components, enable one item of equipment to be used for MIG (inert gas), CO2 and tubular wire, and submerged arc processes.

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6. ELECTROSLAG WELDING

6.1 The Process

As the thickness of the metal to be welded increases, multi-pass techniques become less economical. Even the use of automatic welding with high current and large weld beads in the flat position, can give a weld pool so large that it runs ahead of the electrode out of control resulting in inadequate fusion. The difficulties with large weld beads can be overcome by turning the plates into the vertical position and arranging the gap between them so that the welding process becomes akin to continuous casting.

Developed in Russia, the Electroslag process is used for butt welding steel sections usually above 60 mm in thickness although plates down to 10 mm thick have been welded. The sections to be joined are fixed in the vertical position and the part of the joint line where welding is to commence, is enclosed with water-cooled copper plates or dams which serve to confine the molten weld metal and slag between the edges of the plates (see Figure 14). The dams are pressed tightly against each side of the joint to prevent leakage. There may be one or more electrode wires depending upon the thickness of the section and they are fed continuously from spools. The self-adjusting arc is struck on to a starting plate beneath a coating of powder flux which is melted in about half a minute. The arc becomes extinguished and the current is then transferred, not as an arc but through the liquid slag, which gives the same order of voltage drop as would occur across the arc. Further melting results from resistance heating of the liquid slag.

Figure 14 Principle of electroslag or vertical submerged melt welding

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During welding some slag is lost in forming a skin between the molten metal and the copper dams, and a flow of flux powder, carefully metered to avoid disturbing the welding conditions, is fed in to match the consumption. The vertical traverse may be obtained by mounting the welding head on a carriage which is motor-driven and travels up a rack on a vertical column in alignment with the joint to be welded. The rate of travel is controlled so that the electrode nozzle and copper dams are kept in the correct position relative to the molten pool. Since the electrode is at right angles to the pool, variations in fit-up are not troublesome. For thick sections the electrode is oscillated across the molten pool, or more than one electrode is used. The gap between the plates is generally between 20 and 40 mm. Welding speeds are usually l metre per hour or faster. The welds produced are generally free from slag inclusions, porosity and cracks, although too high a welding speed can cause centre line cracking. The process is rapid, preparation costs are reduced, and there is no de-slagging. Cored wires containing deoxidizers and alloying elements can be used when required.

Preparation of the faces to be welded is not critical and a flame cut surface is quite acceptable. The slag temperature is about 1900°C internally.

An AC or DC positive power source in the range 300-750 A is suitable, such as is used for automatic processes. Open circuit voltage is of the order of 70 -80 V, with arc voltages of 30-50 V, higher with AC than DC.

6.2. Welding With Consumable Guides Or Nozzles

Consumable guide welding is a simplified version of the electroslag process for welding thick plate in the vertical or near vertical position, for joints of limited length: usually up to 2 m. The gap between plates is 25-30 mm, but when welding thicknesses less than 20 mm the restriction on the minimum gap being so as to ensure that the guide tube does not touch the plate edges and there is sufficient space for insulating wedges if these are needed to position the guide tube. Water-cooled copper shoes act as dams to confine the molten metal, and give it the required weld profile. As with electroslag welding the current passes through molten slag and generates enough heat to melt the electrode end, the guide tube and edges of the parts being joined ensuring a good fusion weld (see Figure 15).

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Figure 15 Consumable guide layout showing water-cooled dams

If a plain uncoated guide tube is used, flux is added to cover the electrode and guide end before welding commences. Otherwise, the process is started and operated in a similar manner to normal electroslag welding. Although there is no arc present after the starting phase of the process, the slag surface of the molten pool should be viewed through dark glasses (as in gas cutting) because of its brightness.

The equipment for welding is considerably simpler than that for normal electroslag welding, chiefly because the welding head and wire feed mechanism do not need to be moved up the joint as the weld is made. It is possible to weld where there is access from one side only, or indeed where there is a permanent backing bar on both sides of the joint. It is cheaper and more adaptable than other similar processes, faster than metal-arc welding of thick plate, joint preparation is cheaper, uniform heat distribution through the joint reduces distortion problems, and there are no spatter losses.

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7. ELECTROGAS WELDING

In its mechanical aspects and its application to welding practice, electrogas welding resembles conventional electroslag welding, from which it was developed. Electrically, electrogas welding differs from electroslag welding in two ways:

1. The heat is produced by an electric arc and not by the electrical resistance of a slag, and

2. Only direct current can be used, whereas either alternating or direct current can be used for electroslag welding.

Figure 16 Electrogas Welding

The equipment used for electrogas welding (see Figure 16) closely resembles that for conventional electroslag welding. Therefore, a change from one process to the other requires only a change from shielding gas to flux, or from flux to shielding gas (80% Argon + 20% carbon dioxide). Thus selection between processes is based on cost and application requirements, not on capital expenditure. The system is capable of greater welding speeds than the electroslag method and it can be stopped and restarted more easily. Flux cored electrodes are sometimes employed.

For work from 18-75 mm thickness, the electrogas and conventional electroslag systems are closely competitive. However, for sections thicker than 75 mm electroslag welding is usually more practical. It is widely used in ship building and the site fabrication of storage tanks.

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8. ONE SIDE WELDING WITH BACKING

When butt joints are to be made the problem arises of obtaining an absolutely sound weld throughout the thickness. If the gap between the butt faces is too narrow it will be insufficiently penetrated whereas if it is too wide it will be impossible to bridge during the welding operation. The problem can be solved to some extent in V-butt joints by using a root or stringer run which effectively seals the gap and provides a base for subsequent weld metal. Such a procedure however requires a very accurate preparation and fit up of the joint before welding and this is not always possible, nor can the standard of such a root run be guaranteed completely. This problem can be solved by welding from both sides which after the first pass necessitates the back of the weld being cleaned and chipped before a second weld is applied to that side. This second weld is often arranged to be the final or sealing weld.

The cost of back chipping and making a sealing run has become very high especially in recent years so that it is desirable to weld plates and large cylinders with runs from one side only. To achieve this a temporary backing can be used, with which an acceptable under-bead profile is also obtained even when fit-up and alignment are not good. The essential purposes of the backing are to provide a base on which the first layer of weld is deposited and to prevent the escape of molten metal through the root. Consideration of the welded structure during the design stage can do much to relieve this necessity, since it is often possible to arrange for joints and reinforcing members to coincide and for the latter to act as built in backing bars. Otherwise a backing strip should be made of metallurgically compatible material and if it does not interfere with the operation of the structure it may be left in place. Alternatively it must be removed. Figure 17a shows some simple joints, some with backing, others such as fillet and lap joints which by their design provide their own backing.

A backing weld onto a single groove at the back of the joint may sometimes be adequate when applied by a different welding process, e.g. TIG, which may remove the need for back chipping although some protection against oxidation may be necessary by gas purging. The deposited metal must naturally be the same as the weld proper.

Copper backing bars may also be employed but they should be of a sufficient mass to avoid fusion or be water cooled. They may be grooved or profiled to give the weld a desirable contour. Granular fluxes or an appropriate refractory powder can act as backing given a suitable support.

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Figure 17(a) Types of butt welds

Ceramic tile backing may also be used for slag forming welded processes and can be applied to vertical and horizontal vertical butt joints (see Figure 17b). The recess in the tile allows the slag to form below the under-bead and it can be stripped off after removal of the adherent aluminium foil.

Figure 17(b) Ceramic tile backing strip

Fibreglass backing strip consisting of four to six layers of closely woven flexible material gives good support to the root run and is usually employed with a copper or aluminium backing bar. Large structures, such as ships, employing submerged arc welding sometimes use a backing of sintered silica sand about 600 mm long by 50 mm wide and 10 mm thick reinforced with steel wires. It has fibreglass tape fitted to its upper surface to support the root and adhesive at the outer edges for attachment. Additionally an aluminium support may be used if necessary (see Figure 17c).

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Figure 17(c) Fibreglass tape covered backing strip

Backing systems can be applied to butt welding in all positions and are widely used not only for flat welds where large pools of molten metal are formed, but also for example, in circumferential pipe welds and long vertical welds in ship building. They can be employed with any of the metal-arc processes, with TIG welding and even with electrogas and electroslag (consumable guide) welding.

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9 CONSUMABLES AND POWER SUPPLIES

9.1 Storage And Care Of Consumables

In storage, the main enemies of electrodes and fluxes are mechanical damage and moisture. Careless handling of covered electrodes, such as those used for manual metal-arc welding, can lead to removal of areas of the flux cover and such affected materials should not be used for welding. Similarly, exposure to excessive amounts of moisture can lead to rusting of the core wire with a lifting of the flux coating. This also requires the electrodes to be discarded.

The flux covering on modern electrodes tends to be porous and will absorb moisture to some extent depending on the atmospheric humidity. Electrode coverings of the cellulose type can absorb an appreciable quantity of moisture with little effect on their properties. They should not be over dried or charring of the coating may result. Mineral coated electrodes do not naturally absorb so much moisture and can be dried out if damp. The electrodes should be well spaced out in an oven and subjected to a temperature of about 110°C for 10-60 minutes depending on their size. Cellulose type electrodes will require only about 15 minutes to dry.

Low hydrogen electrodes are specially designed to contain relatively little in the way of hydrogen containing compounds including moisture. They need to be kept in a dry, heated, well ventilated store at about 12°C above the external air temperature. Where necessary they should be oven-dried before use, at temperatures ranging from 150-450°C, depending on the permissible hydrogen content of the weld and the manufacturer's recommendations. Some modern electrodes are vacuum packed and generally need no further drying if used within a specified time of opening.

Care must also be given to fluxes supplied for submerged arc welding which, although they may be dry when packaged, may be exposed to high humidity in store. In such cases they should be dried in accordance with the manufacturer's recommendations before use, or porosity or cracking may result.

Ferrous wire coils supplied as continuous feeding electrodes are usually copper coated. This provides some corrosion resistance, ensures good electrical contacts and helps in smooth feeding.

Rust and mechanical damage should be avoided in such products as they will both interrupt smooth feeding of the electrode. Rust will be detrimental to weld quality generally, and to equipment condition in the case of GMAW.

Contamination by carbon containing materials such as oil, grease, paint and drawing lubricants is especially harmful with ferrous metals. Here carbon pick-up in the weld metal can cause a marked and usually undesirable change in properties. Such contaminants may also result in hydrogen being absorbed in the weld pool.

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In general it is a wise welder who studies and follows the manufacturer's recommendations for consumables.

9.2 Power Sources

In arc welding it is principally the current which determines the amount of heat generated and this controls the melting of the electrode and parent metal and also such factors as penetration and bead shape and size. Voltage and arc length are however more or less interchangeable factors with increasing voltage leading to increasing arc length and vice-versa.

There are two methods of automatic arc control -

(1) Constant voltage known as the self-adjusting arc.

(2) Drooping characteristic or controlled arc (constant current)

Constant Voltage DC Supply

Power can be supplied from a welding generator with level characteristic or from a three-phase or one-phase transformer and rectifier arranged to give output voltages of approximately 14-50 V and ranges of current according to the output of the unit.

The voltage-current characteristic curve, which should be flat or level in a true constant voltage supply, is usually designed to have a slight droop (see Figure 18a).

Figure 18 (a) & (b) Volt-ampere curves of constant voltage and drooping characteristic sources

This unit will maintain an almost constant arc voltage irrespective of the current flowing. The wire feed motor has an adjustable speed control with which the wire feed must be pre-set for a given welding operation. Once pre-set the motor feeds the wire to the arc at constant speed. For the arc to function correctly the rate of wire feed must be exactly balanced by the burn-off rate to keep the arc length constant. Suppose the normal arc

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length is that with voltage VM indicated at M in diagram (a), and the current for this length is IM amperes. If the arc shortens (manually or due to slight variation in motor speed) to S (the voltage is now VS) the current now increases to IS, increasing the burn-off rate, and the arc is lengthened to M. Similarly if the arc lengthens to L, current decreases to IL and burn-off rate decreases. and the arc shortens to M.

Drooping Characteristic DC Supply (Constant Current)

With this system the DC supply is obtained from a welding generator with a drooping characteristic or more usually from a transformer-rectifier unit. The characteristic curve of this type of supply (see Figure 18b) shows that the voltage falls considerably as the current increases, hence the name. If normal arc length M has voltage VM and if the arc length increases to L, the voltage increases substantially to VL. If the arc is shortened the constant current which is often given to this type of supply. In continuously fed systems the variations in voltage due to changing arc length are fed through control gear to the wire feed motor, the speed of which is thus varied so as to keep a constant arc length, the motor speeding up as the arc lengthens and slowing down as the arc shortens. With this arrangement, therefore, the welding current must be selected for given welding conditions and the control circuits are more complicated than those for the constant voltage method.

The constant voltage type of generation is commonly provided for continuously fed systems such as MIG/MAG, submerged arc, flux cored, electroslag and electrogas. This self-regulating ability will ensure a constant arc length for the processes. The constant amp method is employed in MMA and TIG processes and may be used for some submerged arc applications but these will require control mechanisms to monitor motor and consequently electrode feed speed.

9.3 Arc Blow

High currents such as those used in submerged arc welding may cause the phenomenon known as arc blow. Direct current flowing in a circuit produces a magnetic field around the conductors and such a field can cause deflection of the arc. Arc blow becomes progressively more uncontrollable with a noisy, wavering arc and heavy spatter especially when approaching the edges of the work or welding in enclosed corners. Arc blow does not occur with AC owing to the constant reversal of current cancelling out the effect.

Sometimes arc blow is very difficult to eliminate and possible remedies include changing the angle of the electrode when deflection begins, changing the position of the welding return connection, welding in a different direction, wrapping the welding cable in a few turns around the work if possible, or if particularly troublesome using smaller gauge electrodes and a greater number of runs. A reduction of voltage may also relieve the problem.

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