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Volume 06 - Welding, Brazing and Soldering Part 12

Tham khảo tài liệu 'volume 06 - welding, brazing and soldering part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | nodularizing element to minimize porosity. Rods containing magnesium can develop a scum during welding un enough flux is used. This scum is hard to remove and can cause contaminated and low-strength welds. The filler metals listed in Table 3 have been used for welding malleable cast iron. However the OFW process can crt wide HAZ of brittle white iron lowering the ductility of the base metal and making it more prone to cracking. Sor foundries repair the rough castings while still having a white iron microstructure before the malleabilizing heat treatrm Fluxes are used to protect the molten metal from oxidation to increase the fluidity of the slag formed and to help disi oxides and other impurities. Fluxes used for OFWwhen utilizing acetylene fuel gas contain sodium borate or boric soda ash and some iron oxide powder. Some fluxes are designed specifically for welding ductile iron by lowering melting point otherwise the fluxes are used for both gray and ductile iron. The weld preparation for OFW must be wider than that for the arc-welding processes to allow for manipulation of welding rod and torch. Groove angles of 90 to 120 are common. A preheating temperature of 540 to 650 1000 to 1200 F is recommended for the oxyfuel welding of cast irons. Tl allows for a quick repair and provides slow cooling. If possible the entire casting should be preheated to minimize I thermal stresses. Gray iron is especially susceptible to cracking due to thermal stresses introduced by loci preheating. If done carefully with a torch localized preheating can be used successfully on small castings. After the proper preheating temperature is reached the repair should be done immediately before the base metal a to the weld area is allowed to cool. The casting should not be allowed to cool below 315 to 425 600 to 800 F . If t casting cools below this temperature during welding or if welding is interrupted the casting should be reheated b welding is resumed. The casting should not be exposed to