2. IT 208Chapter 141 ELECTRICAL WELDING Resistance Welding – The two parts are pressed together and an alternating current (A/C) is passed through the contact zone. Spot welding – used extensively on sheet metals (holds handles on pots, car body together) Ribbon welding rollers. - parts to be welded are drawn between electrodes rollers while electricity is applied.

3. IT 208Chapter 142 Arc Welding A sustained arc generates the heat for melting the work piece and filler material. –Consumable electrodes –Non-consumable electrodes

4. IT 208Chapter 143 Consumable electrodes Flux Core Arc Welding (FCAW) developed in the early 1950s as an adaptation to SMAW to overcome limitation imposed by the use of a stick electrodes. Uses a spool of filler wire fed through the hand-piece. A core of flux is inside the wire Two versions –Self-shielded flux-cored arc welding – includes not only fluxes but also ingredients that generate shielding gases for protecting the arc. –Gas-shielded flux-cored arc welding – developed primarily for welding steels, obtains a shielding from externally supplied gases, similar to GMAW

5. IT 208Chapter 144 Consumable electrodes Submerged Arc Welding (SAW) – uses a continuous, consumable bare wire electrode, and arc shielding is provided by a cover of granular flux. Low-carbon, low alloy, and stainless steels can be readily welded by SAW. Electrogas Welding (EGW) – uses a continuous consumable electrode (either flux-cored wire or bare wire with externally supplied shielding gases) and molding shoes to contain the molten metal. Shielded Metal Arc Welding (SMAW) (stick) – arc is struck between the rod (shielded metal covered by flux) and the work pieces to be joined, the impurities rise to the top of the weld in the form of slag (18- 19a, handout pg. 40)

6. IT 208Chapter 145 A sustained arc, shielded by molten slag, is maintained in consumable-electrode welding by the (a) shielded metal-arc, (b) submerged arc, and (c) electrogas methods.

7. Oxyfuel Gas Welding Welding process that uses fuel gas combined with oxygen to produce flame This flame heat melts the metals at the joint Acetylene fuel is used in gas welding process Primary combustion process C 2 H 2 + O 2 2CO + H 2 + heat This reaction dissociates into carbon monoxide and hydrogen. Secondary combustion process 2CO + H 2 + 1.5 O 2 2CO 2 + H 2 O + heat

8. Types of flames Neutral flame Oxidising flame Carburising flame Filler Metals : Additional material to weld the weld zone Available as rod or wire They can be used bare or coated with flux The purpose of the flux is to retard the

9. Torch used in Oxyacetylene Welding Fig : (a) General view of and (b) cross-section of a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a park lighter or a pilot light; then the oxygen valve is opened and the flame adjusted. (c) Basic equipment used in oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left-handed, whereas those for oxygen are right- handed. Oxygen regulators are usually painted green, acetylene regulators red.

10. Pure Acetylene and Carburizing Flame profiles

11. Neutral and Oxidizing Flame Profiles

12. Flame definition The neutral flame (Fig. 4-1) is produced when the ratio of oxygen to acetylene, in the mixture leaving the torch, is almost exactly one-to- one. It’s termed ”neutral” because it will usually have no chemical effect on the metal being welded. It will not oxidize the weld metal; it will not cause an increase in the carbon content of the weld metal. The excess acetylene flame (Fig. 4-2), as its name implies, is created when the proportion of acetylene in the mixture is higher than that required to produce the neutral flame. Used on steel, it will cause an increase in the carbon content of the weld metal. The oxidizing flame (Fig. 4-3) results from burning a mixture which contains more oxygen than required for a neutral flame. It will oxidize or ”burn” some of the metal being welded.