Machinery's Handbook, 31st Edition
WELDING
1575
WELDING A number of variables set welding and related processes apart from mechanical fasten- ing methods used to join material. However, practically all welding and related processes currently in use involve heat input and/or upset in the areas where materials are being joined. Modern welding methods may employ manual, machine, semi-automatic, auto- matic, or robotic controls to deliver welding energy at the point of fusion. Welding of metals generally requires that materials be heated to a molten state so they can be fused together. The method of applying heat or upset to produce the desired results may involve just a simple flame. The autogenous method—where a torch burns a mixture of (usually) acetylene and oxygen gases to heat the components—is still used for certain work. However, most welding operations today use an electric arc and/or pressure, or they involve the application of complex electric waveforms. In arc welding, a low-voltage, high-current arc is struck between the end of an electrode and the work, generating in- tense heat that immediately melts the surfaces. Lasers and electrons are also used as the heating medium for some welding operations. In the welding process, a filler wire or rod is held in the heated zone to add material that replaces the metal consumed by the process. (This usually produces a slightly raised area, which subsequently can be dressed down to a level surface if needed.) Most, if not all, metals are weldable, and the majority can make capable connections. Some welds make joints that are stronger than the base materials being joined, while other welds weaken the workpiece’s overall mechanical properties. Though typically the welded joint has proper- ties similar to the base materials. Metals that can be welded include: carbon and low-alloy steels, stainless steels, tool and die steels, high-alloy steels, cast iron, nickel and cobalt alloys, copper alloys, aluminum alloys, magnesium alloys, titanium, zirconium, hafnium, tantalum, columbium, and oth- ers. Dissimilar welds—joining two different metal types (for example, carbon steel to stainless steel)—can be made but may prove difficult or produce undesirable results. Electrodes, Fluxes, and Processes Electrodes for welding may be made of a tungsten or other alloy that does not melt at welding temperatures (nonconsumable) or of an alloy similar to that of the work so that it melts and acts as the filler wire (consumable). In welding with a nonconsumable electrode, filler metal is added to the pool as welding proceeds. Filler metals that will produce welds having strength properties similar to those of the work are used where high-strength welds are specified. Briefly, the effects of the main alloying elements in welding filler wires and electrodes are: carbon adds strength but may cause brittle weld metal if cooling is rapid, so low-carbon wire is preferred; silicon adds strength and reduces oxidation, changes fluidity, and gives a flatter weld bead; manganese strengthens and assists deoxidation, plus it reduces effects of sulfur, lowering the risk of hot cracking; sulfur may help form iron sulfide, which increases the risk of hot cracking; and phosphorus, may contribute to hot cracking. Fluxes in (usually) granular form are added to the weld zone, as coatings on the filler wire or as a core in the tube that forms the (consumable) electrode. The flux melts and flows in the weld zone, shielding the arc from the oxygen in the atmosphere, and often contains materials that clean impurities from the molten metal and prevent grain growth during recrystallization. Processes.— There are approximately 100 welding and allied welding processes but the four manual arc welding processes: gas metal arc welding (GMAW) (which is also commonly known as MIG for metal inert gas), flux-cored arc (FCAW), shielded metal arc (SMAW), gas tungsten arc welding (GTAW), account for over 90 percent of the arc welding used in production, fabrication, structural, and repair applications. FCAW and
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