first welded from the outside, and then reinforced from the back side with a seal bead. Acetylene Safety Precautions Acetylene safety precautions should be rigidly observed and enforced. Some of the more important precautions to remember are as follows: 1. Store acetylene cylinders in an upright position. They must be securely fastened to prevent shifting or falling. Do not lay on sides, drop, or handle roughly. If horizontal stowage is necessary, or an acetylene cylinder is inadvertently left lying in a horizontal position, it must be placed in an upright position for a minimum of 2 hours before it can be used. (Otherwise, acetone in which the acetylene is dissolved will be drawn out with the gas. ) Avoid damaging the valves or fuse plugs to prevent leakage. 2. Store acetylene cylinders in a well-protected, well-ventilated, dry place, away from heating devices or combustible materials. 3. Use acetylene from cylinders only through pressure-reducing regulators. Do not use acetylene at pressures greater than 15 psi. 4. Open the acetylene valve slowly, 1/4 to 1/2 turn. This will permit an adequate flow of gas. Never open the valve more than 1 1/2 turns of the spindle. 5. Keep sparks, flames, and heat away acetylene cylinders. 6. Turn the acetylene cylinder so that the outlet will point away from the oxygen cylinder. 7. Do not interchange hose, regulators, or from valve other apparatus intended for oxygen with those intended for acetylene. 8. Use only approved hoses and fittings with acetylene equipment. Pure copper, or copper alloys containing 67 to 99 percent copper, must not be used in piping or fittings for handling acetylene (except blowpipe or torch tips). 9. Test for leaks with soapy water–not with an open flame. 10. Make no attempt to transfer acetylene from one cylinder to another, refill an acetylene cylinder, or mix any other gas or gases with acetylene. 11. Keep valves closed on empty cylinders. 12. Should an acetylene cylinder catch fire, use a wet blanket to extinguish the fire. If this fails, spray a stream of water on the cylinder to keep it cool. 13. Crack each cylinder valve for an instant to blow dirt out of the nozzles before attaching the pressure regulator. Do not stand in front of the valve when opening it. 14. Learn to identify standard Navy cylinders by color and decals. GAS TUNGSTEN-ARC WELDING Gas tungsten-arc (GTA) welding is an arc welding process that produces coalescence of metals by heating them with an electric arc between a nonconsumable tungsten electrode and the base metal. The weld pod, arc, electrode, and the heated section of the work pieces are protected from atmospheric contamination by a gaseous shield; otherwise, atmospheric oxygen and nitrogen will combine with the molten weld metal and result in a weak, porous weld. The shielding gas is usually an inert gas, such as helium, argon, or a mixture of gases. The electrode used in GTA welding is generally tungsten or a tungsten alloy because other refractory metals would erode too rapidly at the high arc temperatures involved. GTA welds are stronger, more ductile, and more corrosion-resistant than other types of arc welds. The weld zone has 100-percent protection from the atmosphere; therefore, no flux is required. Since no flux is required, it eliminates flux or slag inclusions in the weld, and there are no sparks, fumes, or spatter. With GTA welding, the welding heat, amount of penetration, and bead shape can be very accurately controlled, and the bead surface is smooth and uniform. Welding Machines Any standard dc or ac welding machine can be used to supply the current for gas tungsten-arc welding. However, it is important that the generator or transformer have good current control in the low range. This is necessary to maintain a stable arc, especially when welding thin gauge materials. Specially designed machines with all of the necessary controls are available for gas tungsten-arc welding. Many of the power supply units are made to produce both ac and dc current. The choice of an ac or dc machine depends on what weld characteristics may be required. Some metals are 15-30