NAME 345 Welding Technology Lecture 09 SAW, ESW & Resistance Welding

Transcription

1 NAME 345 Welding Technology Lecture 09 Md. Habibur Rahman Lecturer Department of Naval Architecture & Marine Engineering Bangladesh University of Engineering & Technology Dhaka-1000, Bangladesh

2 Submerged Arc Welding (SAW) Submerged arc welding (SAW) is an arc welding process which joins metals by heating the metals to their melting point with an electric arc or arcs set up between bare metal electrode or electrodes and the job. The process is used without pressure and with filter metal from the electrode and sometimes from a supplemental source (welding rod, flux, or metal granules). The arc, the end of electrode and molten pool remains completely hidden and are invisible being submerged under a blanket of granular flux. The continuously fed bare metal electrode melts and acts as filler rod. It is normally automatic process. It is also known as under powder welding or smothered arc welding. 2

3 Submerged Arc Welding (SAW) (Contd.) 3

4 Submerged Arc Welding (SAW) (Contd.) Main Components The wire electrode reel, the wire feel motor equipped with grooved wire feed rolls which are suitable for the demanded wire diameters, a wire straightener as well as a torch head for current transmission (figure). Flux supply is carried out via a hose from the flux container to the feeding hopper which is mounted on the torch head. Depending on the degree of automation it is possible to install a flux excess pickup behind the torch. SAW can be operated using either AC power source or DC power source where the electrode is normally connected to the positive terminal. Welding advance is provided by the welding machine or by workpiece movement. 4

5 Submerged Arc Welding (SAW) (Contd.) Principles of Operation SAW utilizes the heat of an arc between a continuously fed electrode and the work. The heat of the arc melts the surface of the base metal and the end of the electrode. Shielding is obtained from a blanket of granular flux, which is laid directly over the weld area. The flux close to the arc melts and intermixes with the molten weld metal and helps purify and fortify it. The flux forms a glasslike slag that is lighter in weight than the deposited weld metal and floats on the surface as a protective cover. The electrode is fed into the arc automatically and travel can be manual or by machine. The metal transfer mode is less important in SAW. Applications This is an ideal automated welding process. It is ideal for long runs. It is used for producing steel girders or beams used in construction. 5

6 Submerged Arc Welding (SAW) (Contd.) Material Used Two materials are used in submerged arc welding: i. Welding flux: It shields the arc and molten weld metal from atmospheric oxygen and nitrogen. The flux contains deoxidizers and scavengers, which help remove impurities from the weld metal. It introduces alloys into the weld metal. The flux that melts and forms the slag covering must be removed from the weld (easily done after the weld cools). The flux is selected based on the mechanical properties required of the weld deposit. Fluxes may be neutral or active: a. Neutral fluxes: It will not produce any significant changes in weld metal chemistry and used for multi-pass applications. b. Active fluxes: It contains small amounts of manganese and/or silicon used to reduce porosity and weld cracking and used for single pass applications. ii. Consumable electrode: In SAW, it is necessary to select the electrode and flux combination to match the base metal composition and properties. 6

7 Submerged Arc Welding (SAW) (Contd.) Advantages i. Molten flux provides very suitable conditions for high current to flow. Great intensities of heat can be generated and kept concentrated to weld thicker section with deep penetration. ii. Because of high heat concentration, considerably higher welding speeds can be used. iii. Because of high heat concentration and high welding speeds, weld distortion is much less. iv. High metal decomposition rates can be achieved. Single pass welds can be made in thick plates with normal equipment. v. Welding is carried out without sparks, smoke, flash or spatter. vi. Weld metal deposit possesses uniformity, good ductility, corrosion resistance, and good impact strength. vii. Very neat appearance and smooth weld shapes can be got. viii. SAW can be used for welding in exposed areas with relatively high winds. ix. Practically, no edge preparation is necessary for materials under 12 mm in thickness. Limitations i. Since the operator can t see the welding being carried out, he can t judge the progress of welding accurately. ii. The flux needs replacing of the same on iii. iv. the joint which is not always possible. The progress is limited to welding in flat position and on the metal more than 4.8 mm thick. In small thickness burn through is likely to occur. The process requires edge preparation and accurate fit up on the joint. Otherwise the flux may spill through the gap and arc may burn the workpiece edges. v. Flux is subjected to contamination that may cause weld porosity. vi. Weld metal chemistry is difficult to control. A change in welding variables especially when using alloyed fluxes may affect weld metal composition adversely. vii. Cast iron, Al alloys, Mg alloys, Pb and Zn can t be welded by this process. 7

8 Electro-Slag Welding Electro slag welding is a welding process of heavy plates in the vertical position. The coalescence is produced by molten slag which melts the filler metal and the surfaces of the work. It s an arc less process that utilizes resistance heating of the slag pool covering the molten steel. Applications This is generally used to join very thick plates together giving a series of overlapping welds until the two plates are fully joined. It is suitable for automatic welding as it is used on large plates usually. Welding of storage tanks is done by it. It is used to construct big and thick parts of ships. 8

9 Electro-Slag Welding (Contd.) Equipments or Main Parts 1. Electrodes: Generally two types of electrodes that solid and metal-cored are used. Though solid electrodes are more popular than the metal-cored electrodes. 2. Flux: Flux is the most important consumable material of Electro-slag Welding. In its molten state it transforms the electrical energy into heat energy which helps in melting the electrode wire and the base metal to form a weld joint. It is also required to protect the molten weld metal from the atmosphere and to ensure stability. 9

10 Electro-Slag Welding (Contd.) The flux in its molten state is required to conduct electricity but at the same time it should offer sufficient resistance to its flow for generating enough heat to do welding. Arcing results when the resistance is less than the minimal required quantity. The slag must also have minimum viscosity so that it does not be too thick to stop good circulation and nor too thin to result in excessive leakage. 3. Electrode Guide Tube: It is used to guide the electrode wire at desired position where the welding is to be done. 10

11 Electro-Slag Welding (Contd.) Principles of Operation Firstly current flows from welding electrode to base plate. This establishes an arc between electrode and base plate which heats the flux. This heat which is produced during arc formation results in melting the filler metal and deposits into the weld cavity. Now the cooled copper shoe starts its function of solidifying the filler metal into weld cavity. This this is done to avoid flowing of weld metal outside. As the filler metal solidifies into weld cavity, the current flows through it. Then it generates heat due to electric resistance. This heat is further used to continue the melting of the filler metal into weld Cavity. Which means that heat is regenerated, which results in less waste of heat or energy. Roller arrangement continuously provides the filler metal. During welding of the metals both copper shoe and feed mechanism moves upward until the whole cavity is formed. This will create a strong joint in single one pass. The single or multi-pass weld is used according to plate thickness. 11

12 Electro-Slag Welding(Contd.) Advantages i. Cooling rate is very low so there is no problem of cold cracking. ii. There no problem of slag inclusion or porosity. iii. The process is semi-automatic and faster. iv. Heavier section can be welding in single pass. v. High productivity can be achieved. vi. Low cost for joint penetration. Limitations i. Too high heat input to base metal. ii. When the heat input is very high and the weld quality can be rather poor, including low toughness caused by the coarse grains in the fusion zone and the heat affected zone. iii. In electro slag welding, there is some tendency toward hot cracking and notch sensitivity in the heat affected zone. iv. ESW restricted to vertical position welding, because of large molten metal pools and slag. v. It is difficult to close cylindrical welds. vi. Electro slag welding tends to produce large grain sizes. vii. If you are creating joints below 60 mm than Submerged Arc Welding (SAW) is more economical than electro-slag welding process. 12

13 Resistance Welding Resistance welding in one of the oldest of the electric welding processes in use by industry today. The weld is made by a combination of heat, pressure and time. As the name resistance welding implies, it is the resistance of the material to be welded to current flow that causes a localized heating in the part. The pressure exerted by the tongs and electrode tips, through which the current flows, holds the parts to be welded in intimate contact before, during, and after the welding current time cycle. The required amount of time, current flows in the joint is determined by material thickness and type, the amount of current flowing, and the cross-sectional area of the welding tip contact surfaces. 13

14 Resistance Welding (Contd.) In the illustration a complete secondary resistance spot welding circuit is illustrated below. For clarity various parts of the resistance spot welding machine are identified. Heat Generation Heat generation is expressed by Q = I 2 RT Q = Generated heat Resistance welding depends on three factors. They are i. Time of current flow (T) ii. Resistance of the conductor (R) iii. Amperage (I) 14

15 Resistance Spot Welding Principle Electricity is passed between two electrodes with the pieces to be joined in between. As the electricity passes through the material the resistance of the metals cause the metals to melt fusing them together. Operation Two pieces of metal are placed between the electrodes. The electrodes are then closed on the piece causing the circuit to complete. The resistance in the metal between the electrodes causing them to melt and fuse together. 15

16 Resistance Spot Welding (Contd.) Advantages i. Comparatively low cost. ii. Resistance Spot Welding (RSW) method doesn't need highly skilled worker. iii. Distortion or warping of parts is eliminated though it leaves some depressions or indentation. iv. The joint made is highly uniform. v. Automatic or semi-automatic operation both can be done. vi. There is no need for edge preparation. vii. Welding can be done in quick succession. It just needs a few seconds to make the joint. Limitations i. The equipment cost is high so it can has an effect on the initial cost. ii. Skilled welders or technicians are needed for the maintenance and controlling. iii. Some metals need special surface preparation for making the RSW a success. iv. The thick jobs are not easy to weld. Applications Spot welding of thick steel plates has been done and it has replaced the need for riveting. The welding of two or more sheet metals can be joined by mechanical means more economically by using the spot welding methods. We don't need gas tight joints. Spot welding can be used for attaching braces, pads or clips with cases, bases and covers which are mainly product of sheet metal forming. Automobile and aircraft industries relies greatly of spot welding these days. 16

17 Resistance Seam Welding Principle This is exactly the same as spot welding except that a continuous is produced. And here we get continuous weld which is air-tight (If the process is perfect). The seam-welding form of the resistance process is a series of overlapping welds. Operation The first method is similar to spot welding. Here however the spots are overlapped to form a continuous seam. This is called Stitch welding. The second method is to replace the electrodes with rollers that cause a continuous seam to be produces. 17

18 Resistance Seam Welding (Contd.) Advantages i. A continuous overlapping weld produced by the process makes it suitable for joining liquid or gas tight containers and vessels. ii. Energy is efficiently used in resistance seam welding. iii. Filler metals are not required. Hence no associated fumes or gases. This results in clean welds. iv. Roll welding simply joins two work pieces whereas stitch welding produces gas tight and liquid tight joints. Limitations i. It requires complex control system to regulate the travel speed of electrodes as well as the sequence of current to provide satisfactory overlapping welds. The welding speed, spots per inch, and timing schedule are all dependent on each other. ii. It is difficult to weld metals having thickness greater than 3 mm. iii. Relatively higher current is thus required than spot welding. iv. The work pieces to be welded are overlapped sufficiently to prevent metal flowing out from the edges of the pieces during welding under pressure. Applications It is used to fabricate liquid or gas tight sheet metal vessels such as gasoline tanks, automobile mufflers, and heat exchangers. Most of the metals can be welded (Except copper and some high percentage copper alloys) The production of seam welded pipes and tubing (butt seam welding) 18