Welding technology Arc welding processes Data ultimo aggiornamento

Welding technology Arc welding processes Data ultimo aggiornamento: 6 giugno 2013

Characteristic of the electric arc in welding General information on the use of electric arc welding • The electrical arc is a characteristic thermal source for fusion welding. • The electric arc can be direct or indirect. • The electrode can be fusible or non fusible. • Generally direct current (DC) or alternate current (AC) is employed. • In the first case the polarity can be direct (DCEN) or inverse (DCEP). ELECTRODE + ARC - PIECE DCEP ELECTRODE ARC + PIECE DCEN ELECTRODE -/+ ARC PIECE +/CA Tipical power supply for arc welding IIS Progress Gruppo Istituto Italiano della Saldatura 2

Characteristic of the electric arc in welding Description of the electric arc • The electric arc is the manifestation of the current flow in an ionized gaseous medium, characterized by the following areas: – a bright central area, where there is the flow of electrons and positive ions that, due to its shape, is called arc spindle; – an outer zone, consisting of solid or gaseous particles at high temperature, called halo; – the ends of the anode, in the shape of crater due to the bombardment by the electron current, said anodic spot; Electric arc – the cathode end, pointed, from which the electrons are emitted, called the cathode spot. • The motion of the particles is regularized by forces of magnetic origin (Lorentz forces) that develop from the periphery toward the center of the spindle, which keep the flow of charges concentrated facilitating the ionization. IIS Progress Gruppo Istituto Italiano della Saldatura Electric arc (GTAW) 3

Characteristic of the electric arc in welding Innesco e funzionamento dell’arco elettrico • To strike the electric arc is necessary to pass trough the dielectric: – short circuit; – ignition spark; – ingnition arc. • When the arc is on there is a minimum voltage value which depends on the substances that make up the electrodes, the nature of the atmosphere in which the arc is struck and the arc length. – cathode fall (higher in the case of non-thermionic electrodes); – anodic fall (higher in the case of thermionic electrodes); – fall along the linear zone of the arc, which depends on the electric potential of the gases involved, and is proportional to the arc length. IIS Progress Gruppo Istituto Italiano della Saldatura Voltage drop (graphite electrodes) GTAW 4

Characteristic of the electric arc in welding Electrical characteristic of the arc • The welding current, affects both the flow of particles that the magnetic forces (Lorenz) which focus the passage section: V Arc unstable – for low values of I, the particles are free to move radially (free arc): the arc is unstable and difficult to control; – for higher values of I, the magnetic foces create the condition for a stable arc with direct proportionality between voltage and current (as for a metallic conductor). Arc stable I Minimum current for stable arc Representation on the V vs I graph of the arc curve IIS Progress Gruppo Istituto Italiano della Saldatura 5

Characteristic of the electric arc in welding Main electrical parameters for the arc • • Voltage: influence the amount of energy available for ionization, and therefore the number of charges involved in the arc (variation of the width of the weld); Welding current: influence thermal energy of the arc, concentrating on a smaller section (increase the penetration); Arc length: varying the arc length varies the voltage drop and the minimum current for the stable operation; Welding speed: not influence the operation of the arc, if not for very high values of speed and in CCPI. IIS Progress Gruppo Istituto Italiano della Saldatura Increasing arc lenght Effect of the arc lenght on voltage and current 6

Characteristic of the electric arc in welding General characteristics of the power sources • The voltage supplied must be compatible with those required for the arc welding; • The use of an alternating current is, in general, not recommended; • The voltage-current characteristic must be adapted to the characteristics of the process: – static characteristic at constant voltage (flat) or constant current (falling); – dynamic characteristic adequate. PROCESS FLAT FALLING SMAW GTAW SAW PAW IIS Progress Gruppo Istituto Italiano della Saldatura 7

Characteristic of the electric arc in welding Welding transformers SECONDARY WELDING CIRCUIT PRIMARY POWER MAINS • Transforms the mains voltage (single-phase or three-phase) to values suitable for welding • An electronic circuit may been added at the secondary, to achieve modulation of current MAINS SWITCH CURRENT REGULATOR TRASFORMER VOLTAGE REGULATOR DIODES Schema costruttivo di trasformatore – raddrizzatore Equation for an ideal transformer IIS Progress Gruppo Istituto Italiano della Saldatura 8

Characteristic of the electric arc in welding Electronic power sources (inverter) • The switch of high frequency power is placed before the transformer (primary). • The transformer is supplied with the high frequency current, and is consequently smaller. • The current can be modulated, even with adaptive features IIS Progress Gruppo Istituto Italiano della Saldatura 9

Characteristic of the electric arc in welding Typical values of current and voltage SMAW Voltage [V] GTAW SAW GMAW Short arc GMAW Spray arc Current/diameter [A/mm] IIS Progress Gruppo Istituto Italiano della Saldatura 10

Characteristic of the electric arc in welding Shielded Metal arc wedling • • Arc welding with coated electrodes requires an electrode made of a core wire of steel and a coating. The arc burns between the end of the coated electrode and the workpiece. The steel core melts into the weld pool and provides the necessary filler material. The coating produces shielding gases and slag during the welding which protect the arc and the weld pool, thus making it possible to produce welds unaffected by the harmful atmospheric air. Arc welding with a coated electrode is a typical manual arc welding process. . IIS Progress Gruppo Istituto Italiano della Saldatura Covered electrodes 11

Characteristic of the electric arc in welding Types of coating • • C - Cellulose electrodes (organic) A - Acidic electrodes R - Rutile electrodes B - Basic electrodes Pinch with remote control unit (Courtesy of ESAB) IIS Progress Gruppo Istituto Italiano della Saldatura 12

Characteristic of the electric arc in welding Main features • • • The slag, which is produced when the coating melts, lies on top of the weld protecting and shaping the weld during the solidification. The slag must be removed after welding of each run. The electrode has a certain length, usually 350 -450 mm. If it were longer the vertical resistance for the welding current would be too great. When the electrode has melted, the welder has to change to a new electrode. These many interruptions of the welding may cause an increased risk of errors and a bad productivity. Arc welding with coated electrodes (SMAW) is primarily used for welding in unalloyed, low-alloyed and high-alloyed steels in thickness ranges from about 2 mm, for instance welding of steel constructions, vessels under pressure, construction of ships and all other similar construction in one unit production or small serial production. IIS Progress Gruppo Istituto Italiano della Saldatura Pinch, electrodes and welding cable 13

Characteristic of the electric arc in welding Coated electrodes comparison IIS Progress Gruppo Istituto Italiano della Saldatura 14

Characteristic of the electric arc in welding Main properties (most common covering types) IIS Progress Gruppo Istituto Italiano della Saldatura 15

Characteristic of the electric arc in welding Welding circuit and power sources Power unit features • DC/AC output, constant current • Short circuit current is generally low • Open circuit voltage can reach 80 V • Actually, widely used units transformer + rectifier type or inverter type IIS Progress Gruppo Istituto Italiano della Saldatura 16

Characteristic of the electric arc in welding Behavior of the welding arc • • Most coated electrodes can be used both with the electrode connected to + (DCEP/DCRP) and – (DCEN/DCSP) pole. However, acidic and rutile electrodes are normally connected to negative polarity on the electrode and positive polarity on the workpiece. Basic electrodes are welded with positive polarity on the electrode. The manufacturers of electrodes normally indicate the recommended polarity on the electrode package. The iron core is the weld metal which is to fill the weld preparation. During melting the core material will melt quicker than the coating, providing the crater that gathers the production of gas in a forward directed flow which helps leading the droplets into the weld pool. IIS Progress Gruppo Istituto Italiano della Saldatura SMAW metal transfer 17

Characteristic of the electric arc in welding Behavior of the welding arc • • The gas flow is so large that in combination with the electromagnetic force - the pinch effect - it is able to lift the droplets so overhead welding can be done. The size and the number of the droplets depend on the type and thickness of the coating. – A thick coating provides small droplets - spray transfer, while a thin coating provides big droplets that can shortcircuit during the droplet transfer. • • The slag is either transferred as a thin film around the droplets or as separate droplets. Due to the high temperatures in the arc, a part of the metal is vaporised and will concentrate in the weld pool while some of it is released as welding fumes. The droplets, which are formed in the crater come close with the melted coating that coats the droplets with a thin film of slag. This thin film is able to transfer alloying elements to the droplet. It is also possible to improve the alloy of the weld metal by means of alloying elements in the coating. IIS Progress Gruppo Istituto Italiano della Saldatura 18

Characteristic of the electric arc in welding Storing of coated electrodes • All types of electrodes should be stored in a dry environment, but some types of coating have a larger tendency to absorbing water than others. • Particularly the basic electrodes are highly hygroscopic. • Rutile electrodes only absorb humidity very slowly, but it is also impossible to remove by redrying them without damaging the coating. • Therefore, when welding with rutile electrodes it should be considered that they hold hydrogen in the weld metal. IIS Progress Gruppo Istituto Italiano della Saldatura 19

Characteristic of the electric arc in welding Main welding parameters A. Current, Voltage, arc lenght are correct C. High current E. Arc too long G. Speed to high IIS Progress Gruppo Istituto Italiano della Saldatura B. Low current D. Arc too short F. Speed too low 20

Characteristic of the electric arc in welding Submerged Arc welding • The joining of metals is produced by heating with an arc or arcs between a bare metal electrode or electrodes and the work. • The arc is shielded by a blanket of granular fusible material on the work. • Filler metal is obtained from the electrode with a possible a supplementary welding wire. • The flux close to the arc melts and intermixes with the molten weld metal, helping to purify and alloy it. • The flux forms a glass-like slag that is lighter in weight than the deposited weld metal and floats on the surface as a protective cover; therefore the arc is not visible. IIS Progress Gruppo Istituto Italiano della Saldatura SAW (scheme) 21

Characteristic of the electric arc in welding Advantages, limitations and major use • Advantages – – high quality of the weld metal. extremely high deposition rate and speed. smooth, uniform finished weld with no spatter. little or no smoke, no arc flash, thus minimal need for protective clothing. – normally, no involvement of manipulative skills. • Limitations – welding positions – in semiautomatic submerged arc welding, the inability to see the arc and puddle can be a disadvantage in reaching the root of a groove weld and properly filling or sizing. IIS Progress Gruppo Istituto Italiano della Saldatura Flux and fused slag 22

Characteristic of the electric arc in welding Advantages, limitations and major use • The submerged arc process is widely used in heavy steel plate fabrication work – welding of structural shapes, – longitudinal seam of larger diameter pipe, – manufacture of machine components for all types of heavy industry, – manufacture of vessels and tanks for pressure and storage use). • The high metallurgical properties make it a common process also for pressure vessels manufacturing (with every king of steel and weldable nickel alloy) • It is also used for surfacing and buildup work, maintenance, and repair. IIS Progress Gruppo Istituto Italiano della Saldatura On field SAW application 23

Characteristic of the electric arc in welding Field of applications and use • Submerged arc welding is used to weld low- and medium-carbon steels, low-alloy high-strength steels, quenched and tempered steels, many stainless steels and some nickel alloys. • Experimentally, it has been used to weld certain copper alloys, nickel alloys, and even uranium. IIS Progress Gruppo Istituto Italiano della Saldatura 24

Characteristic of the electric arc in welding Equipment IIS Progress Gruppo Istituto Italiano della Saldatura 25

Characteristic of the electric arc in welding Welding equipment • The power source for submerged arc welding must be rated for a 100 percent duty cycle, with size from 300 to 1500 A • DC power is used for partly mechanised and automatic applications, but AC power may be used with high current levels for avoiding arc bowing. • For partly mechanised application, a welding gun and cable assembly are used to carry the electrode and current and to provide the flux at the arc. • For automatic welding, the torch is attached to the wire feed motor and includes current pickup tips for transmitting the welding current to the electrode wire. Partly mechanised SAW – The flux hopper is normally attached to torch, and may have magnetically operated valves which can be opened or closed by the control system. IIS Progress Gruppo Istituto Italiano della Saldatura 26

Characteristic of the electric arc in welding Welding consumables SUBMERGED ARC FLUXES FUSED FLUXES MIXED FLUXES ACIDIC AGGOLMERATED FLUXES NEUTRAL BASIC The weld metal properties depend strongly on the flux wire combination (a process essential variable) IIS Progress Gruppo Istituto Italiano della Saldatura 27

Characteristic of the electric arc in welding Narrow gap welding NG SAW - Tandem welding unit IIS Progress Gruppo Istituto Italiano della Saldatura 28

Characteristic of the electric arc in welding Narrow gap welding IIS Progress Gruppo Istituto Italiano della Saldatura 29

Characteristic of the electric arc in welding Multiple wire welding – Two different techniques are available: – Tandem – Twin arc Welded pipe longitudinal welding Panel – line (coutesy Fincantieri spa) IIS Progress Gruppo Istituto Italiano della Saldatura 30

Characteristic of the electric arc in welding Multiple wire welding IIS Progress Gruppo Istituto Italiano della Saldatura 31

Characteristic of the electric arc in welding Multiple wire welding IIS Progress Gruppo Istituto Italiano della Saldatura 32

Characteristic of the electric arc in welding Multiple wire welding IIS Progress Gruppo Istituto Italiano della Saldatura 33

Characteristic of the electric arc in welding Applications: tube butt welding / overlaying IIS Progress Gruppo Istituto Italiano della Saldatura 34

Characteristic of the electric arc in welding Applications: pressure vessels Adaptative control IIS Progress Gruppo Istituto Italiano della Saldatura 35

Characteristic of the electric arc in welding Cladding (stainless steel, nickel alloys) IIS Progress Gruppo Istituto Italiano della Saldatura 36

Characteristic of the electric arc in welding Introduction • Gas metal arc welding (GMAW or MIG welding) is an electric arc welding process which joins metals by heating them with an arc established between a continuous filler metal (consumable) electrode and the work. • Shielding of the arc and molten weld pool is obtained entirely from an externally supplied gas or gas mixture IIS Progress Gruppo Istituto Italiano della Saldatura 37

Characteristic of the electric arc in welding Introduction • The process is sometimes referred to as MIG or CO 2 welding • Recent developments in the process include operation at low current densities and pulsed direct current, application to a broader range of materials, and the use of reactive gases, particularly CO 2, or gas mixtures • This latter development has led to the formal acceptance of the term gas metal arc welding (GMAW) for the process because both inert and reactive gases are used. The term MIG welding is still more commonly used IIS Progress Gruppo Istituto Italiano della Saldatura 38

Characteristic of the electric arc in welding Introduction • MIG welding is operated in semiautomatic, machine, and automatic modes • It is utilized particularly in high production welding operations • All commercially important metals such as carbon steel, stainless steel, aluminum, and copper can be welded with this process in all positions by choosing the appropriate shielding gas, electrode, and welding conditions Partly mechanised GMAW IIS Progress Gruppo Istituto Italiano della Saldatura 39

Characteristic of the electric arc in welding Advantages • High quality welds can be produced much faster than with SMAW or TIG welding • No chance for the entrapment of slag in the weld metal • There is very little loss of alloying elements as the metal transfers across the arc. Only minor weld spatter is produced, and it is easily removed • The process is versatile and can be used with a wide variety of metals and alloys, including aluminium, copper, magnesium, nickel, and many of their alloys, as well as iron and most of its alloys IIS Progress Gruppo Istituto Italiano della Saldatura 40

Characteristic of the electric arc in welding Equipment • Gas metal arc welding equipment consists of a welding gun, a power supply, a shielding gas supply, and a wire-drive system which pulls the wire electrode from a spool and pushes it through a welding gun • A source of cooling water may be required for the welding gun. • In passing through the gun, the wire becomes energized by contact with a copper contact tube, which transfers current from a power source to the arc • The MIG process is used for semiautomatic, machine, and automatic welding. Semiautomatic MIG welding is often referred to as manual welding IIS Progress Gruppo Istituto Italiano della Saldatura 41

Characteristic of the electric arc in welding Equipment: constant current power supply • • With this type, the welding current is established by the appropriate setting on the power supply Arc length (voltage) is controlled by the automatic adjustment of the electrode feed rate The use of this type of power supply in conjunction with a constant wire electrode feed results in a selfcorrecting arc length system With a pulsed direct current power supply, the power source pulses the dc output from a low background value to a high peak value Welding Equipment output V I Courtesy Lincoln Electric Company IIS Progress Gruppo Istituto Italiano della Saldatura 42

Characteristic of the electric arc in welding Wire feeding IIS Progress Gruppo Istituto Italiano della Saldatura 43

Characteristic of the electric arc in welding Arc power and polarity • The vast majority of MIG welding applications require the use of direct current reverse polarity (DCRP) • This type of electrical connection yields a stable arc, smooth metal transfer, relatively low spatter loss, and good weld bead characteristics for the entire range of welding currents used • Direct current straight polarity (DCSP) is seldom used, since the arc can become unstable and erratic • When dcsp is employed, penetration is lower with straight polarity than with reverse polarity direct current • AC has found no commercial acceptance with the MIG welding process for two reasons – the arc is extinguished during each half cycle as the current reduces to zero, and it may not reignite if the cathode cools sufficiently – rectification of the reverse polarity cycle promotes the erratic arc operation IIS Progress Gruppo Istituto Italiano della Saldatura 44

Characteristic of the electric arc in welding Metal transfer • Filler metal can be transferred from the electrode to the work in two principal ways: – when the electrode contacts the molten weld pool, thereby establishing a short circuit, which is known as short circuiting transfer (short circuiting arc welding) – when discrete drops are moved across the arc gap under the influence of gravity or electromagnetic forces drop transfer can be either globular or spray type GMAW Aluminum (5082 H 114 alloy) IIS Progress Gruppo Istituto Italiano della Saldatura 45

Characteristic of the electric arc in welding Short circuiting transfer • The electrode contacts the molten weld pool at a steady rate in a range of 20 to over 200 times each second • As the wire touches the weld metal, the current increases. It would continue to increase if an arc did not form • The rate of current increase must be high enough to maintain a molten electrode tip until filler metal is transferred. It should not occur so fast that it causes spatter by disintegration of the transferring drop of filler metal • The open circuit voltage of the power source must be low enough so that an arc cannot continue under the existing welding conditions IIS Progress Gruppo Istituto Italiano della Saldatura Short circuiting transfer 46

Characteristic of the electric arc in welding Globular transfer • With a positive electrode (DCRP), globular transfer takes place when the current density is relatively low, regardless of the type of shielding gas • However, carbon dioxide (CO 2) shielding yields this type of transfer at all usable welding currents. Globular transfer is characterized by a drop size of greater diameter than that of the electrode • Globular, axially directed transfer can be achieved in a substantially inert gas shield without spatter. The arc length must be long enough to assure detachment of the drop before it contacts the molten metal • Globular transfer However, the resulting weld is likely to be unacceptable because of lack of fusion, insufficient penetration, and excessive reinforcement IIS Progress Gruppo Istituto Italiano della Saldatura 47

Characteristic of the electric arc in welding Spray arc transfer Spray - arc • In a gas shield of at least 80 percent argon or helium, filler metal transfer changes from globular to spray type as welding current increases for a given size electrode • For all metals, the change takes place at a current value called the globularto-spray transition current • Spray type transfer has a typical fine • arc column and pointed wire tip associated with it • Molten filler metal transfers across the arc as fine droplets, with diameter equal to or less than the electrode diameter. The metal spray is axially directed The reduction in droplet size is also accompanied by an increase in the rate of droplet detachment: – Metal transfer rate may range from less than 100 to several hundred droplets per second as the electrode feed rate increases from approximately 40 to 340 mm/s IIS Progress Gruppo Istituto Italiano della Saldatura 48

Characteristic of the electric arc in welding Transition between globular and spray arc IIS Progress Gruppo Istituto Italiano della Saldatura 49

Characteristic of the electric arc in welding Pulsed arc • To obtain weld puddle control, it is necessary to be able to change the current level while welding • This is done by the machine, which can be programmed to change from a high current (HC) to a low current (LC) on a repetitive basis, known as pulsed welding • In pulsed current welding there are two current levels, the high current and low current, sometimes called background current. By programming a control circuit, the output of the machine continuously switches from the high to the low current • The level of both high and low current is adjustable. In addition, the length of time for the high and low current pulses is adjustable. This gives the welder the necessary control over the arc and weld puddle IIS Progress Gruppo Istituto Italiano della Saldatura Pulsed arc 50

Characteristic of the electric arc in welding Flux cored arc welding (FCAW) • Flux-cored, tubular electrode welding has evolved from the MIG welding process to improve arc action, metal transfer, weld metal properties, and weld appearance • It is an arc welding process in which the heat for welding is provided by an arc between a continuously fed tubular electrode wire and the workpiece • Shielding is obtained by a flux contained within the tubular electrode wire or by the flux and an externally supplied shielding gas IIS Progress Gruppo Istituto Italiano della Saldatura FCAW welding 51

Characteristic of the electric arc in welding Flux cored arc welding (FCAW) • Flux-cored arc welding is widely used for welding ferrous metals and is particularly good for applications in which high deposition rates are needed • At high welding currents, the arc is smooth and more manageable when compared in using large diameter gas metal arc welding electrodes with CO 2 • The arc and weld pool are clearly visible to the welder. A slag coating is left on the surface of the weld bead, which must be removed Self shielded flux cored arc welding • Some spatter is created and some smoke produced IIS Progress Gruppo Istituto Italiano della Saldatura 52

Characteristic of the electric arc in welding Flux cored arc welding • • • Flux-cored electrode wires are designed to operate on either DCEP or DCEN. The wires designed for use with an external gas shielding system are generally designed for use with DCEP Some self-shielding flux-cored ties are used with DCEP while others are developed for use with DCEN – Electrode positive current gives better penetration into the weld joint – Electrode negative current gives lighter penetration and is used for welding thinner metal or metals where there is poor fit-up. The weld created by DCEN is wider and shallower than the weld produced by DCEP Different types of flux cored electrodes are available: – Basic (mainly used for high alloyed steels) – Rutile (mainly used for low alloyed steels, in conjunction with CO 2 or Ar- CO 2 mixtures) – Metal Cored (with high productivity) – Seld schielded (generally used for on-site welding) IIS Progress Gruppo Istituto Italiano della Saldatura 53

Characteristic of the electric arc in welding Gas and mixtures for welding • Inert and active gases may both be used for GMAW • Active gases such as carbon dioxide, argon-oxygen mixture, and argoncarbon dioxide mixtures are used for almost all applications (CO 2 is the most common) • The choice of the proper shielding gas for a specific application is based on: – – – Pipeline FCAW welding the type of metal to be welded arc characteristics and metal transfer availability cost of the gas mechanical property requirements penetration and weld bead shape IIS Progress Gruppo Istituto Italiano della Saldatura 54

Characteristic of the electric arc in welding Fields of application of different shielding gases IIS Progress Gruppo Istituto Italiano della Saldatura 55

Characteristic of the electric arc in welding Gas tungsten arc welding • The gas tungsten arc welding is an arc welding process working with an arc between a tungsten (nonconsumable) electrode and the workpiece. • The area of the arc is enveloped in a protective gas shield which protects the fused zone and electrode from contamination. • Filler metal may or may not be used. Manual GTAW • The possible filler rod is added by hand; in the mechanised process, the filler wire is added by external feed rollers. • Different designations are used for the process: TIG, GTAW, ISO 4063 141 IIS Progress Gruppo Istituto Italiano della Saldatura 56

Characteristic of the electric arc in welding Application field of TIG process • Thickness range for TIG process • The range of TIG changes from 0, 15 [mm] to 10 [mm]. The TIG is particularly used for the first pass where isn’t possible the back gouging and back weld. • However, when we want achieve a particular soundness of the welding seam is possible to weld big thicknesses with multipass technique. • TIG process may be manual, fully mechanised, automatic, robotized • Typical automatic applications are tubes to tube-sheets welding and orbital circumferential butt welds of tubes. IIS Progress Gruppo Istituto Italiano della Saldatura Orbital GTAW 57

Characteristic of the electric arc in welding TIG Welding equipment • The main components of TIG equipment are: – power supply and electric circuit; – shielding gas system with: • gas cylinder • flowmeter and gas saver – gas circuit to the torch – cable carrier with: • electric circuit • water cooling system (when necessary) • gas circuit • welding torch IIS Progress Gruppo Istituto Italiano della Saldatura 58

Characteristic of the electric arc in welding Pulsed current General advantages are 1. Good penetration with less heat input 2. Less distortion; 3. Good control of the pool when welding out of position; 4. Ease of welding thin materials; 5. Ease of welding materials of dissimilar thickness levels; 6. Depth/width ratio till 2 to 1 in the welding of stainless steels reduction of hot cracks; 7. Seam concavity reduction because high currents and the short time of the pulsed mode gives the possibility to the weld pool of rapidly cooling; 8. Small heat affected zone; 9. Gas inclusions reduction because the pulsed arc shakes the weld pool and permits the gas discharge. IIS Progress Gruppo Istituto Italiano della Saldatura 59

Characteristic of the electric arc in welding Suitability of current supply type (EN ISO 6848) IIS Progress Gruppo Istituto Italiano della Saldatura 60

Characteristic of the electric arc in welding Shielding gases: argon • Argon has low thermal conductivity which means it is not a good conductor of heat. This results in a more compact, higher density arc. The core part of the arc is more hot in comparison with helium and for this reason the weld metal drops that pass in the arc zone are more fluid. • Arc density refers to the concentration of energy in the arc. With argon this energy is confined to a narrow or more “pinpointed” area and then also the penetration is deep ad narrow. IIS Progress Gruppo Istituto Italiano della Saldatura 61

Characteristic of the electric arc in welding Shielding gases: helium • Helium is present in the atmosphere in quantity of 5 parts for one million. • In the USA and in Canada there are natural sources where Helium is produced with an high grade of purity. • Helium used for welding has a gas purity grade greater than 99, 995 [%]. • Unlike argon, helium has high thermal conductivity. Due to this higher thermal conductivity, the arc column expands, reducing current density in the arc. • The arc column will become wider and more flared out than the arc column with argon shielding gas. • The consequence is a wider penetration in comparison with Argon shielding gas and less risks of undercuts on the boundary of the welding seam. • Greater are also the penetration and the dilution. IIS Progress Gruppo Istituto Italiano della Saldatura Wider and deeper penetration with Helium 62

Characteristic of the electric arc in welding Ar – H mixtures • Just as helium is mixed with argon to take advantage of the best features of both gases, hydrogen is mixed with argon to further constrict the arc and produce a cleaner weld with a greater depth to width ratio (penetration). • This mix is used primarily for welding austenitic stainless steels and some nickel alloys. The addition of hydrogen to argon also increases travel speed. • It should be noted that an argon hydrogen mix will introduce the risk of hydrogen cracking, undercuts and metal porosity particularly in multipass welds. • This type of mixture, in the most used quantities, (95% of Ar and 5% of H 2) is not inert, but slightly reducing. IIS Progress Gruppo Istituto Italiano della Saldatura 63

Characteristic of the electric arc in welding Back shielding • Just as the surface of the weld bead must be protected from atmospheric contamination, the backside must be protected as well. In the case of pipe welding or other full penetration butt joints, this can be accomplished with a backing dam, as seen in the following picture. • Often a backing dam can be something as simple as heavy paper used to close off the ends of the pipe through which a gas hose is passed to fill the pipe with shielding gas. • In some cases where the weld occurs too fast for the torch supplied shielding gas to protect the pool and tungsten, a trailing shield may be used. Trailing shields are available as separate devices that attach to the torch or torch nozzle. Back or trail shielding is required on reactive metals like titanium, duplex steels, stainless steels, etc. This type shielding keeps the welds bright and shiny without discoloration and oxidation, thus reducing rework due to contamination. IIS Progress Gruppo Istituto Italiano della Saldatura Back shielding 64

Characteristic of the electric arc in welding Secondary shielding • In titanium (and other reactive metals) welding may be used a supplementary gas shielding system (trailer gas) with a cable, on the torch that permits, with a porous mass the shielding of the seam already solidified. Argon to torch Argon Porous material Welding Torch direction IIS Progress Gruppo Istituto Italiano della Saldatura 65

Characteristic of the electric arc in welding Special applications • TIG process is used in automatic applications: for example in the tube to tube-sheet welding and in the butt circumferential welds of pipes. TIG torch Tube-sheet Tube Different welding edge preparation of tube to tube-sheet welding IIS Progress Gruppo Istituto Italiano della Saldatura 66

Characteristic of the electric arc in welding Special applications (tube to tube-sheet welding) Coffee processing plant, AISI 304 L steel Cu-Ni tubesheet for the salt industry IIS Progress Gruppo Istituto Italiano della Saldatura 67

Characteristic of the electric arc in welding Orbital welding TIG torch Circumferential weld Tube AISI 347 GTAW orbital welding IIS Progress Gruppo Istituto Italiano della Saldatura 68

Characteristic of the electric arc in welding Hot wire TIG • • • The main difference, in comparison with standard TIG, is the wire filler metal preheating with a supplementary power source. The electric arc works with lower heat inputs The welded materials are the same of the standard TIG: carbon and low alloyed steels, stainless steels (duplex and superduplex), nickel and titanium alloys. The process is particularly used for welding of tubes with medium-high thickness and for special parts (fittings) cladding Up from 15 [mm] of thickness is the narrow gap (NG) preparation is used, with an increase of productivity up to 300%. IIS Progress Gruppo Istituto Italiano della Saldatura 69

Characteristic of the electric arc in welding Key – hole (HI-Pro) TIG • Is special cold wire TIG technique process that permits: - more simple edges preparations (also square bevels) - reduction or elimination of filler metal - higher productivity. • In the example a comparison between standard TIG and key hole TIG welding of 12 [mm] thickness in AISI 304 L IIS Progress Gruppo Istituto Italiano della Saldatura 70