ARC WELDING WELDING Welding processes are conveniently divided into two classes: fusion welding and solid state welding. During fusion welding a portion of the base materials to be joined is melted and mixed, often with the addition of a filler metal, forming a solid joint after subsequent solidification. As the name implies, during solid state welding the base material is not melted. Joining occurs by a process other than solidification, and very often involves diffusional processes. FUSION WELDING The fusion welding processes are classified in terms of the source of heat used to cause localized melting: electric arc welding, thermal welding and resistance welding. In terms of the absolute number of welds made, fusion welding is the most common welding technique. PRINCIPLE 2 H=I R t ELECTRIC ARC WELDING All of the electric arc welding processes rely on the formation of an arc between an electrode and the base material to provide heat. An electric arc forms when there is an electrical discharge between two metal objects that are not in contact with one another - in the case of arc welding the two objects are the electrode and base metal work piece. The arc ionizes the gas between the electrode and base metal, thus creating a plasma, which causes local melting of base metal. Depending on the technique, the electrode may or may not be consumed, ELECTRODES Electrodes can be classified into Non-Consumable Electrodes and Consumable Electrodes. The composition of electrode depends up on the metal to be welded. For example for welding mild steel electrode of similar composition is used so as to get a homogeneous weld joint. The size (diameter) of electrode depends up on the amount of weld metal to be deposited and the gap between the two plates to be welded. Higher currents will be required when bigger diameter electrodes are used. NON-CONSUMABLE ELECTRODES Non-consumable electrodes are those electrodes, which do not consumed during the welding process. Separate filler metals are necessary to fill the gap between the joints. Non-consumable electrodes are made up of higher melting point materials like carbon (melting point -6700°F), graphite or pure tungsten(melting point - 6150°F). Non-consumable electrodes are used in Carbon Arc Welding and TIG Welding. Using Non-consumable electrodes, good control over the process is possible. As compared to carbon electrodes, tungsten electrodes are more expensive and alloy tungsten electrodes are still more costlier. Alloying tungsten increases emissivity, resistance to contamination, arc stability and contamination. Also electrode consumption is less. CONSUMABLE ELECTRODES Consumable Electrode possess more thermal efficiency than non-consumable electrode. Are consumed during welding operation. May be made of various metals depending upon the purpose and chemical composition of the metals to be welded. Bare electrodes are used in coil form with out coating in MIG Welding. Metal Arc welding make use of coated electrode. Commonly used core wire materials are : mild steel, low alloy steel, nickel steel etc. Consumable Electrodes may be classified into 1. Bare Electrodes : There won’t be any coating of flux in case of bare electrodes. Arc produced by bare electrode is unstable. Joint produced by bare electrodes are not strong enough. Also irregular metal transfer and atmospheric contamination takes place. Bare electrodes are used when strength is not a primary concern. 2. Coated Electrodes : Molten metal is exposed to oxygen and nitrogen in the atmosphere and so undesirable oxides and other substances decreasing the strength of the weld formed. Coated electrodes (Flux Coated) are used to prevent the formation of oxidizes and helps to form slag. Due to Flux coating the molten metal is not exposed to oxygen and nitrogen in the atmosphere resulting in strong bond. Coated electrodes produce very good weld appearances and defect free joints. Commonly used fluxes are asbestos, mica, silica etc. Coated Electrodes are again classified into a) Lightly Coated Electrodes : Thin coating of Flux. eg. Citobest electrode of Advani Oerlikon. b) Medium Coated Electrodes: Medium coating of Flux. eg. Overcord electrode of Advani Oerlikon. c) Heavily Coated Electrodes : Thick coating of Flux. eg. Citofine CARBON-ARC WELDING Carbon-Arc Welding is an arc welding process in which weld is produced by heating the work-piece with an arc setup between the carbon electrode and the work-piece. In this method a rod of carbon is used as negative electrode and work being welded as positive. The arc produced between the 2 electrodes heats the metal to the melting temperature (about 3200° C). The reason to use Carbon electrode is that less heat is generated at the electrode tip than at the work piece and carbon electrode will fuse with the job. In Carbon Arc Welding D.C is used to prevent electrode disintegration and the amount of carbon deposit at the weld metal. SHIELDED (MANUAL) METAL ARC WELDING The shielded or manual metal arc (SMA) process, is widely used for the fabrication of pressure vessels, pipe work and pipeline joints, as well as for the repair and maintenance of industrial machinery. An arc is established between the electrode and the base metal at the joint line. The arc melts a portion of the base metal and the electrode to form a weld pool. The molten metal is protected from the surrounding atmosphere by decomposition of the electrode coating which forms a gaseous cloud. There are two types of Carbon Arc Welding. They are 1) Single Carbon Electrode Welding 2) Twin Carbon Electrode Welding DIFFERENCE BETWEEN SINGLE CARBON ELECTRODE WELDING AND TWIN CARBON ELECTRODE WELDING Advantages : Heat input to the work-piece can be easily controlled. Work-piece distortion is negligible. Process can be mechanized. Suitable for thinner pieces. Disadvantages : Separate filler metal is needed slowing the process. Chances of carbon deposit. Applications : Welding of Sheet Steel, Copper Alloys, brass , bronze and aluminum. On many applications, Carbon Arc Welding has been replaced by TIG Welding. SHIELDED (MANUAL) METAL ARC WELDING Flux Shielded Metal Arc Welding is an arc welding process in which weld is produced by heating the work-piece with an arc setup between the flux coated electrode and the work-piece. Steel when exposed to air forms oxides and nitrides. These impurities weaken the weld. To prevent this molten metal is shielded by enveloping it completely with an inert gas or flux. In this method a metal rod is used as negative electrode and work being welded as positive. Arc melts the electrode and the job. The flux coating melts, produces a gaseous shield to prevent the atmospheric contamination of the molten weld metal. The arc produced between these two electrodes heats the metal to the melting temperature (about 2400-2600° C). Both A.C and D.C can be used. SHIELDED (MANUAL) WELDING METAL ARC Steps : 1. Before welding work-pieces are suitably prepared (mating surfaces are cleaned to remove scales). 2. Work-pieces are positioned. 3. Arc is struck. 4. Once arc is obtained, electrode is progressed at a constant speed along the area to be welded. Advantages : Flux Shielded Metal Arc Welding is the simplest of all the arc welding process. Equipment is portable. Big range of metals and alloys can be welded. Good weld quality can be obtained. Cost is fairly low. Disadvantages : Mechanization is difficult due to the limited length of electrode. Process is slow. Metal transfer is not clear. Applications : Used for fabrication work and maintenance work. All commonly employed metals and alloys can be welded. The process finds applications in a) Ship building b) Pipes and penstock joining. c) Building and bridges construction. d) Automotive and Aircraft industry. METAL INERT GAS ARC WELDING (MIG) MIG make use of the high heat produced by the electric arc between the consumable electrode and material to be welded. Gas Metal Arc Welding is a shielded metal arc process. The electrode is continuously fed through a gun. The current ranges from 100 to 400 A depending upon the diameter of the wire. The speed of melting of the wire may be up to 5m/min. Usually constant voltage D.C machine is used for MIG Welding. Welding Gun is either water cooled or air cooled. Welding wire is often bare. CO2, argon or argon helium mixtures are often used as shielding gases. Shielding is done to prevent contamination of weld. Advantages : Does not require much skill. Continuous welding at high speeds can be carried out. Deeper penetration is possible. Process can be mechanized. Thick and thin sections can be welded easily. Large metal deposition rates can be obtained. No flux is used. Faster compared to TIG and Metal Arc Welding. Disadvantages : Welding Equipment is much complex. Difficult to weld small corners. Slightly complex than TIG. Applications : Used for welding of carbon, silicon and low alloy steels, stainless steels, aluminum, magnesium, copper, nickel and their alloys, titanium etc. Used for manufacture of refrigerator parts. Used in industries like aircraft, automobile, pressure vessel and ship building. GAS-TUNGSTEN-ARC WELDING(GTAW) OR TUNGSTEN INERT GAS WELDING (TIG) : Principle Gas Tungsten Arc Welding is a shielded metal arc process. TIG Make use of the high heat produced by the electric arc between the nonconsumable tungsten electrode and material to be welded. Tungsten Electrode is used only to generate an arc. Filler metal may be or may not be used. Shielding is obtained by an inert gas such as helium or argon or mixture of two. Shielding is done to prevent contamination of weld. Usually A.C machine is used for TIG Welding (for nonferrous alloys) except for ferrous alloys d.c is used. End of the welding gun is water cooled. Leftward Welding technique is usually used. Base metals welded are carbon and alloy steels, stainless steels, heat resisting alloys, refractory metals, aluminum alloys, copper alloys, magnesium alloys, nickel alloys etc. TUNGSTEN INERT GAS WELDING (TIG) : TUNGSTEN INERT GAS WELDING (TIG) : Advantages : More different types of metals can be welded such as carbon steel, nickel steel, aluminum, brass, bronze, titanium. Unlike metals can be welded to each other like mild steel, stainless steel, brass to copper. Heat affected zone is very low. No flux is used. Clear visibility of arc. Smooth welds can be obtained. Disadvantages : Under similar applications MIG is faster than TIG. Tungsten if transferred can contaminate the same. Costly. Applications : Welding sheet metals and thinner sections. Used in precision welding in atomic energy, aircraft and instrument industries. ATOMIC HYDROGEN ARC WELDING Principle : In this process arc is struck between the terminals of two tungsten electrode and the work piece does not form any terminal. Heat is generated by striking an arc between the electrodes and work piece under the shield of hydrogen. Electric arc splits molecular hydrogen into atomic hydrogen which is not stable and has a strong tendency to combine. When it combines in molecular form generating heat. This combination raises the heat up to 4200°C. Atomic hydrogen features of both arc and flame welding process. Combined energy of arc and a chemical reaction is utilized for welding. ATOMIC HYDROGEN ARC WELDING Steps : 1. Hydrogen gas supply and current are switched on. 2. Arc is struck by bringing the two tungsten electrodes in touch with each other and separating them by a predetermined distance (say 1.5mm). Arc is held over the job till a molten pool forms. Advantages : Process is fast. No flux or separate shielding gas is used. Hydrogen its self acts as a shielding gas. Welding of thin materials is also possible. Uniform welds can be obtained. Disadvantages : Speed is less compared to MAW or MIG. Cost is more. Applications : Process can be used for welding of most of metals and alloys like plain carbon steel, alloy steel, aluminum, copper, nickel and their alloys. SUBMERGED ARC WELDING (SAW) ( HIDDEN ARC WELDING) Submerged Arc Welding is an arc welding process in which coalescence (joint) is produced by heating the work-piece with an arc setup between a bare metal electrode and the work-piece. In Submerged Arc Welding, the arc is submerged under a layer of Flux and so the arc is invisible. Flux is fed through a Flux Hopper. The upper portion of flux is in contact with the atmosphere. The Flux may be made of silica, metal oxides or other compounds. Bare electrode (Steel stainless steel or copper etc) is fed through the gun. Normally d.c is employed for Submerged Welding, but a.c is also used. Instead of flux covered electrode, granular flux and a bare electrode is used. SAW is an automatic process for the production of high quality butt welds. SUBMERGED ARC WELDING (SAW) Steps : 1. Trigger is pulled and the flux starts depositing on the joint to be welded. 2. Arc is struck by touching the work-piece with the electrode of by using a high frequency unit. 3. In all cases arc is struck under the cover of flux. Flux is a non-conductor of electricity, but once it melts due to the action of heat it becomes highly conductive and hence current flow will be maintained between work-piece and electrode. 4. Electrode at a predetermined speed is fed to the joint to be welded. Advantages : Often automated, so faster. Deep penetration and high quality weld is possible. Less distortion. Operator can work without safety equipment. Wire electrodes are inexpensive. No sparks. Practically no edge preparation is necessary. Smoot welds can be obtained. Disadvantages : Since the operator cannot see the welding being carried out, he cannot judge accurately the progress of welding. Can’t be used for plates less thickness. Slag has to be removed continuously. Can’t be used for welding cast iron due to high heat input. Cast iron, Al alloys, Mg Alloys, Pb and Zn cannot be welded by this process. Applications : Fabrication of pipes, penstocks, pressure vessels, boilers, structural shapes etc. Used in automotive, aviation, ship-building and nuclear power industry. For welding of metals like mild steel, medium and high tensile low alloy steels. FLUX-CORED ARC WELDING (FCAW) Principle : Flux-Cored Arc Welding is an arc welding process in which coalescence (joint) is produced by heating the work-piece with an arc setup between a continuous tubular consumable electrode and the work-piece. Equipment consists of a constant-voltage d.c source, a wire feeder and a light weight welding gun. Flux is contained with in the electrode. Additional shielding may be obtained y an externally supplied gas(CO2) or gas mixture. The flux provides the necessary shielding for the pool. The heat of the arc melts the surface of base metal and the end of the electrode. Welding gun is similar to MIG welding gun. Sometimes additional shielding is provided with a gas. FCAW is a modification of MIG/CO2 in which a solid wire is replaced by a flux-cored electrode. FLUX-CORED ARC WELDING (FCAW) Advantages : Provides high quality weld at lower costs. Welds variety of steel over a wide thickness range. Visible arc-easy to weld. Reduced distortion compared to SMAW. Disadvantages : Used only to weld ferrous metals, primarily steels. FCAW produces a slag covering which has to be removed. Equipment is costly. But increased productivity compensates for this. Applications : FCAW is widely used for medium thickness steel fabrication work. Used for welding in bridges, ship building etc. Used for welding of a. Low to medium carbon steels b. Low alloy high strength steels c. Cast iron d. Stainless steels (certain) e. Quenched and tempered steels.