Manufacturing Processes lab I Oxyfuel Welding: Oxyacetylene Welding (OAW)

1. Manufacturing Processes lab IOxyfuel Welding:Oxyacetylene Welding (OAW)

2. Oxyfuel welding Oxy-fuel welding of metal is commonly called oxyacetylenewelding (OAW) since acetylene is the predominant choice for a fuel, or often simply gas welding. In gas welding and cutting, the heat needed to melt the metal comes from a fuel gas burning with oxygen in a torch.

3. Oxyfuel Welding (OFW) A torch may have different-sized tips. (green hose) Follow the manufacturer's recommendations for the correct operating pressures for the metal being welded and for the tip size being used. (red hose)

4. Oxyfuel Welding (OFW) • The welding gases used: • Acetylene (most common), • Methylacetylene propadiene (MAPP) = acetylene + propane, • Hydrogen. • (the welding technique is the same for all gases)

5. OxyAcetylen Welding (OAW) • does not require electricity. • is not expensive. • the welding equipment is easy to transport. • is typically slower than the electrical arc-welding processes. • used in the repair of small parts, for maintenance and in body shops. • is used to join iron, steel, cast iron, copper, brass, aluminum, bronze, etc. • dissimilar metals can be joined with OAW. • can also be used for preheating, cutting metal, case hardening, soldering and annealing.

6. Oxygen cylinders Common sizes of oxygen cylinders: 244 cu ft (for industrial plants) 122 cu ft 80 cu ft Cylinders are charged with oxygen at a pressure of 2200 psi and a temperature of 70 F (21 C). Protector cap: screws onto the neck ring of the cylinder to protect the valve from damage when not in use. Safety nut: permits the oxygen to drain slowly if the temperature increases the cylinder pressure beyond its rated safety load.

7. In order to decrease the size of the open spaces in the cylinder, acetylene cylinders are packed with porous materials (balsa wood, charcoal, corn pith, or Portland cement) that is saturated with acetone to allow the safe storage of acetylene. Acetone, a colorless, flammable liquid, is added to the cylinder until about 40 percent of the porous material is saturated. The porous material acts as a large sponge which absorbs the acetone, which then absorbs the acetylene. In this process, the volume of acetone increases as it absorbs the acetylene, while acetylene, being a gas, decreases in volume. The acetylene cylinder has fusible plugs that melt, relieving excess pressure, if the cylinder is subjected to any mechanical pressure or undue heat (such as from a fire). Acetylene cylinders Acetylene is a colorless gas with a very distinctive, nauseating odor that is highly combustible when mixed with oxygen. It is very unstable if compressed to more than 15 psi. Acetylene gas is formed by the mixture calcium carbide and water.

8. Acetylene and oxygen hoses The oxygen hose is always green in color, and the acetylene hose is red. The nut on the acetylene connection has a notch that runs around the center, distinguishing it from the nut on the oxygen connection.

9. Regulators • Two functions of the regulators: • They control the flow of gas from the cylinder to maintain the required working pressure, and • They produce a steady flow of gas under varying cylinder pressures. • Regulators are equipped with two gauges: • A cylinder pressure gauge: • shows the actual pressure, • A working pressure gauge • shows the working or line pressure.

10. Regulators The second scale on the gauge is calibrated to register the contents of the cylinders in cubic feet.

11. Backfire and flashback A backfire is caused by the flame going out suddenly on the torch. A backfire may occur when: the tip is touched against the workpiece, if the flame setting is too low, if the tip is dirty, damage or loose, or if the tip is overheated. When a torch backfires, it could cause a flashback. A flashback is a condition in which the flame burns inside the tip, the torch, or the hose. In case of a backfire, the oxygen and fuel valves must be immediately closed to prevent possible explosion of the cylinders.

12. Check valves A check valve is a valve that allows the flow of the liquid or gas in one direction. A check valve is positioned at the torch inlet, and at the regulator outlet. In welding apparatus, the pressure in the supply hose is higher than the pressure in the torch. If the pressure in the torch becomes higher than that in the supply hose, such as when a flashback occurs, the valve disk (in the check valve) closes, shutting off the supply of the gas to the torch.

13. Flash arrestors A flash arrestor is a safety device that prevents an explosion or a backfire in the torch or torch head from reaching the regulator and the acetylene cylinder. Flash arrestors: torch-mounted and regulator-mounted Torch-mounted flash arrestor: is a check valve that prevents a reverse gas flow from reaching the cylinder. Regulator-mounted flash arrestor: is a combination check valve and flame barrier. The barrier metal is a porous flame-retardant material that allows gas to flow through, but blocks out a flame. Note: when acetylene and oxygen are mixed and ignited, the flame can reach the temperature of 5700F to 6300F (3150C to 3482C).

14. Care of welding tips • Frequent torch use causes carbon to form in the passage of the tip. • To clean the torch: • File the end of the tip flat with a metal file. • Insert a properly-sized tip cleaner into the tip and pull it straight out. Repeat until the tip is clean.

15. How to weld

16. Cutting • Oxy-fuel cutting of metal is a similar process, using a different type of gas torch, called a blowtorch. (But, colloquially, many people also call a welding torch a blowtorch.) Welding torch Cutting torch

17. Cutting Here the metal is heated until it glows, and then a long lever on the torch is pressed to blow an excess of oxygen into the gas mixture, to blow and melt the metal with the resulting extra heat, much of which comes from the metal burning rather than from the gas burning. Sometimes a metal-cutting blowtorch is colloquially called a gas-axe or hot wrench.

18. Cutting • Never use a cutting torch where sparks will be a hazard. • Sweep floors clean and wet them before cutting. • Provide a bucket of water or sand to catch dripping slag. • Use fire-resistant guards, partitions or screens if necessary. • Take extra precautions while working in greasy, dirty, or gaseous atmosphere to prevent explosions. • Keep combustible materials at least 35 ft. away from cutting operations. • Never cut near ventilators. • Never use oxygen to dust off clothing or work pieces. • Never use oxygen as a substitute for compressed air.

19. Safety • All oxygen piping and fittings must withstand a maxpressure of 150 psi. • Locate the nearest fire extinguisher before performing any welding or cutting operation. • Keep oxyacetylene equipment clean and free of oil. • Keep heat, flame, and sparks away from combustibles. • Prevent leaks in cylinders. • Open cylinder valvesslowly. • Purge Oxygen and acetylene hoses before lighting torch. • Never move cylinders without protective caps in place.

20. Safety • Wear personal protective equipment. Wear welding gloves, helmet, leather apron, welding chaps, leather shoes, welding goggles, and other personal protective equipment to help prevent weld burns and injury. Make sure the welding goggles or face shield have at least a No. 4 filter lens. • Do not wear clothing made of synthetic fibers while welding. • Fasten cylinders securely. Do not handle cylinders roughly. Chain cylinders in an upright position to a wall or cart. When regulators are not on cylinders, keep safety caps in place. Caps will prevent damage to cylinder valves.

21. Safety • Never use oil on welding equipment. Oil and grease may ignite spontaneously, when in contact with oxygen. • Open cylinder valves correctly. Open the valve on the acetylene cylinder no more than three-fourths of a turn (maximum one turn) so it can be closed quickly in case of emergency. Open the valve on the oxygen tank fully. While welding or cutting, leave the valve wrench in position. • Acetylene cylinder should never be laid down as the corrosive nature of the acetone can erode the seals in the tanks.

22. Safety • Keep the tip pointed away from your body. Do not saturate your clothing with oxygen or acetylene. Before and while lighting the flame, keep the tip pointed away from your body. • Light the flame with an approved lighter. Using matches to light the torch brings fingers too close to the tip. • Set the operating pressure carefully. Never use acetylene at a pressure over 15 psi. • Never stand in front of a regulator while you are opening a tank valve.

23. Safety • Treat the flame with respect. Keep the flame and heat away from the cylinder, hoses, and people. Never lay down a lighted torch. Be sure the flame is out before laying down the torch. Never walk around with a lighted torch. • Control flashbacks and backfires. Make certain that reverse flow-check valves and flash arrestors are installed on the oxygen and acetylene lines. • Do not leave the work area until the cylinder valves are closed. Be sure the cylinder valves are closed and pressure is relieved from the hoses before you leave the work area.

24. Safety • Do not weld or cut on containers that have held flammable materials. • Store oxygen cylinders away from acetylene cylinders. A non-combustible wall at least 5 feet high should be used to separate cylinders. • Handle hot metal with pliers or tongs. Do not leave hot metal on the welding table because your classmate may touch it and be burned. • Check connections for leaking gases. To prevent fires or explosions, use soapy water to check connections for leaks.