Chptr 11 Oxygen Fuel Gas Welding Procedures
(4) Magnesium castings to be welded should be preheated with a torch or in a furnace before welding is started. The entire casting should be brought up to a preheat temperature of abut 650°F (343°C). This temperature can be approximated with blue carpenter's chalk which will turn white at about 600°F (316°C). After welding, the casting should be stress relieved in a furnace for 1 hour at 500°F (260°C). If no furnace is available, a gas flame should be used to heat the entire casting until the stress relieving temperature is reached. The casting should then be allowed to cool slowly, away from all drafts. WARNING Precleaning and postcleaning acids used in magnesium welding and brazing are highly toxic and corrosive. Goggles, rubber gloves, and rubber aprons must be worn when handling the acids and solutions. Do not inhale fumes and mists. When spilled on the body or clothing, wash immediately with large quantities of cold water. Seek medical attention. Never pour water into acid when preparing solutions; instead, pour acid into water. Always mix acid and water slowly. Cleaning operations should be performed only in well ventilated areas. f. Cleaning After Gas Welding. All traces of flux must be removed from parts immediately after the completion of gas welds. First, scrub with a stiff bristle brush and hot water, and then immerse for 1 to 2 minutes in a chrome pickling solution consisting of 1-1/2 lb (0.7 kg) sodium dichromate and 1-1/2 pt (0.7 1) nitric acid with enough water to make a gallon. The temperature of the solution should be 70 to 90°F (21 to 32°C). After chrome pickling, the parts should be washed in cold running water. They should be boiled for 2 hours in a solution of 8 oz (226.8 g) of sodium dichromate in 1 gal. (3.8 1) of water. Parts should then be rinsed and dried. 11-27. MAGNESIUM BRAZING a. General. (1) Furnace, torch, and flux dip brazing can be used. Furnace and torch brazing are generally limited to M1A alloys. Flux dip brazing can be used on AX10, AX31B, K1A, M1A, and ZE10A alloys. (2) Brazed joints are designed to permit the flux to be displaced from the joint by the brazing filler metal as it flows into the joint. The best joints for brazing are butt and lap. Suitable clearances between parts are essential if proper capillary filling action is to take place. The suggested clearance is from 0.004 to 0.010 in. (0.102 to 0.254 mm). In furnace brazing, beryllium is added to the brazing alloy to avoid ignition of the magnesium. b. Equipment and Materials. file:///C|/Documents%20and%20Settings/Dad/Desktop/welding-theory/ch11.htm (62 of 68)1/8/2006 4:11:26 PM