Welding/Overview of Techniques
There are several different ways in which to weld. The most common in industrial workplaces is 'SMAW', or "Shielded Metal Arc Welding". The electrode or 'stick' is placed into an electrode holder (gun or clamp style) which is then scraped or tapped onto the grounded weld piece to produce an 'electric arc', which produces enough heat to melt the 'rod' and form it into the weld piece. This process utilizes electricity to push out a "glob" of molten metal onto a plate(s) of metal, the 'workpiece'. Pipeline and structural welders use this method the most because of its deep 'penetration' (how much it digs into the base metal), and pressure handling capabilities.
The next most common method of welding is Gas Metal Arc Welding(GMAW) also called MIG or MAG welding (MIG- Metal Inert Gas and MAG- Metal Active Gas), GMAW utilizes the same principal as SMAW, but the electrode is not a 'rod', but a wire on a spool, which is fed through a 'gun'. The welder may prefer this method because of its ability to travel along the weld surface faster, and has a similar outcome in quality of the weld. However, spools of wire can be expensive, and the use of a 'shielding gas' is needed to keep the weld free of contaminants (oxidization, particles, bubbles, etc.).
A similar method of GMAW is available, Flux Cored Arc Welding(FCAW) can either be self shielded, or gas shielded just like GMAW welding. The welder must chip slag off the face of the weld, as it stays on the cooled metal surface once the weld is completed. The main difference can be seen in the selection of the electrode wire for this process. GMAW uses a solid wire and will be listed on the spool as for example ER70S. S standing for solid wire. Flux cored is tubular and will be listed as ER71T. T standing for tubular wire.
Gas selection can also vary. 100% CO2, 75% Argon/25% CO2(75/25), and Argon/Helium/CO2 (Trimix). CO2 is commonly used for FCAW as it causes the arc to be hotter, increasing penetration. 75/25 is the most common shielding gas for GMAW as it is relatively inexpensive, and has excellent welding characteristics. Trimix is only used for GMAW-SS or MIG welding stainless steel.
The reader should note that the processes talked about in this chapter are the most commonly used methods that a welder may use. There are however, other methods that will be discussed in a later chapter.
Oxy-Fuel Cutting and Welding
Oxy Fuel cutting and welding is a skill all in itself. The terms, oxy- (standing for oxygen,) and fuel (the assortment of gases that produce combustion) speak for themselves. Oxy-Acetylene welding is the most common fuel to use, but others can include natural gas, propane, MAPP gas, gasoline or any number of 'hot' gases.
Generally, the principal for oxy-fuel welding or cutting is the same. Oxygen is supplied in ratio to the fuel gas, and is then lit by a sparking lighter. Valves on the torch handle control the flame (more oxygen to make an oxidizing flame, or more gas to make a carbonizing flame, or in between to make a neutral flame). Welding with oxy-fuel usually desires a 'neutral flame', with no 'feather'. A 'feather' is cooler then the 'cone', the flame at the very tip of the torch. 'Feathers' can be used to determine how carbonizing a flame is. A slightly carbonizing flame is desirable when welding with brass rod, or 'brazing', thus, the 'feather' should be present, but only slightly above the 'cone', about a half of an inch. For direct welding of steel and mild steels, no 'feather' should be present, but use of a hot 'cone' is efficient. For steel, the welder may desire to apply the use of a 'filler rod', or a long, skinny rod of metal similar to the base metal. This helps to build up a weld face and strengthen the joint. This is done by heating the base metal into a molten puddle at the base of the joint, and then 'dipping' or adding the filler rod directly to the puddle.
Cutting involves the same principal, but requires (in most cases) a special torch handle and head. In a regular torch head, there is only one 'orifice', or hole, for which the gas and oxygen can pass through. In a cutting torch, however, there is the one 'orifice' in the center, and a circle of orifices around it. The purpose for this is simple. The torch is lit regularly, and adjusted to a hot neutral flame. The flame is wider, more forceful and louder (although not much). The torch handle has a lever on it that allows the welder to have an extra "burst" of oxygen when he or she desires. This burst blows the hot metal out of the way, allowing a gap to be created, thus the cut is formed. This burst of oxygen is easier to be applied if there is more room to push the oxygen through, thus the extra orifices are essential.
TIG (GTAW) Welding
TIG welding is a form of arc welding, with the same principal to that of MIG/MAG or SMAW. An electric current is supplied, and is fed through a handle. TIG stands for 'tungsten inert gas' and GTAW for 'Gas Tungsten Arc Welding', both meaning the same thing. Tungsten is used as an 'electrode', which is semi-permanent in the torch. It carries the electric current and is used to establish and keep an arc. Tungsten is used because of its conductivity, versatility and high melting temperature. The electrode will become contaminated, however, if accidentally dipped into the weld pool. TIG is preferred by many because of its ability to weld "almost any metal". As long as a filler material (rod) is available, that particular type of metal can be welded with TIG. TIG welding is achieved by operating the torch in a similar manner to oxy-fuel welding, but the operator controls the heat by means of electricity. There are 'remotes' that the welder can use for this task, or it can be set directly on some machines, so it will not fluctuate. Fluctuation is desired in some cases, where the base metal must be cooled sufficiently at regular intervals so as to not burn a hole. It should be noted that TIG is usually DC- (DC with electrode negative) which places the majority of the heat on the base material, or work piece. This allows complete fusion of the base metal and filler rod, without burning out the tungsten electrode.
For other, more advanced welding techniques, see Welding.