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ОглавлениеWELDING PROCESSES AND EQUIPMENT
This chapter reviews the basic equipment used with each of the welding and cutting processes that are presented in detail in subsequent chapters. Process basics provide an understanding of why and how they work. Detailed equipment specifics of each process are covered in the separate chapters.
Oxyacetylene welding is more than 100 years old and is one of the oldest of the welding processes. My old friend, Butch Sosnin, was a welding training consultant and the 1979 president of the American Welding Society. To start all his weldor training, Butch used oxyacetylene because it was slow and students could easily learn welding fundamentals. A student could actually see the molten puddle develop.
Fig. 1.1. Oxyacetylene welding uses oxygen and acetylene gas supplied in cylinders. Regulators reduce the cylinder pressure and hoses deliver the gases to a torch where they are mixed and exit through a small hole in a welding tip. At the hottest part of the flame tip, the gas mixture burns at 5,720 degrees F. Acetylene is the only fuel gas that can achieve this high temperature in a small concentrated area. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
After teaching oxyacetylene welding, he followed with TIG welding, which was similar to oxyacetylene in that the heat and filler additions were independent, and there was time to watch the weld puddle develop.
He proceeded to teach stick and MIG welding after some proficiency in the other two methods was achieved. Both stick and MIG welding are more difficult for a beginner to watch because the puddle process happens so fast!
The sequence of this book follows Butch’s course instruction and presents the welding processes in the order Butch would teach them.
My friend and colleague, Bob Bitzky, former training manager for ESAB Welding and Cutting Products, agrees with Butch’s instructional approach, and starts his new trainees with oxyacetylene followed by TIG welding.
The basic oxyacetylene welding process starts with two cylinders of gas—one oxygen and the other acetylene (Figure 1.1). Regulators reduce the cylinder pressure and hoses bring the gases to a torch where they are mixed and exit through a small hole in a welding torch tip. This mixed gas burns at 5,720 degrees F at the hottest part of the flame tip. Other fuel gases may even generate more total heat, but do not have this concentrated, high temperature at the flame tip.
Fig. 1.2. My old friend Butch Sosnin was a weldor training consultant and the 1979 President of the American Welding Society. Butch insisted on starting his weldor training with the oxyacetylene process because it was relatively slow and students could see the puddle develop. Bob Bitzky, former training manager for ESAB Welding and Cutting Products, supports Butch’s logic, stating, “It still holds true today.”
The oxyacetylene flame is the only one that can truly be used for welding. Other fuel gases can be used for cutting and brazing but are not effective for welding. It is the concentration of heat that allows welding to occur. Don’t be fooled by just temperature comparison with other gases.
Discussing combustion intensity is a way to explain the temperature concentration of various gases. Without going into too many technical details or specifying the units, an oxyacetylene flame produces more than 10,000 while the next best fuel gas produces about 5,000, or half the value. Steel has the highest melting point of materials that are typically welded. It melts at about 2,500 degrees F. The high-heat concentration and 5,720-degree F flame temperature can melt and fuse two pieces of steel.
Although difficult to master, oxyacetylene welding can be used to weld aluminum. However, unlike steel that turns red then white before forming a molten puddle, aluminum does not. Aluminum melts at 1,200 degrees F and gives little indication it is about to melt.
Note the AWS designation for oxyacetylene welding is straightforward—OAW, although few folks use it.
Gas flow rates for oxyfuel welding are relatively high compared to the shielding gas flow rates in TIG and MIG welding. Needle valves in the torch handle adjust the flow of the two gases. Setting the correct mixture is covered in the individual process section. However, carefully read the manufacturer’s instructions because controlling the flow rate of these gases is very important and potentially a significant safety issue. Be sure to follow the manufacturer’s recommendations for adjusting the cylinder regulators. Particularly for the oxygen cylinder, where the pressure adjusting screw must always be backed out before opening the contents valve on the cylinder. Failure to do this properly can cause a surge of high-pressure oxygen to rush into the small chambers of the regulator. Like a Diesel engine, this rapid rise in pressure creates heat and can ignite whatever is in the regulator, including the brass body. In pure oxygen, everything burns, and burns explosively! Follow the manufacturer’s recommendations carefully.
Tungsten inert gas (TIG) welding was first developed in the 1940s to weld aluminum and magnesium. Today it is used to weld many materials, including a variety of steels. TIG creates an arc between a non-consumable tungsten electrode and the workpiece and uses an inert gas, usually argon, to shield the molten puddle. A DC or AC power source supplies the electric power. The tungsten is held in place with a collet inside a TIG torch. The argon shielding gas protects the molten puddle as well as the tungsten, which may be 6,000 degrees F at the tip. The arc itself is much hotter than the oxyacetylene flame. It is 12,000 to 15,000 degrees F on the outer part of the arc to twice that near the tip of the tungsten. A major advantage of the process is the ability to have a stable arc exist from very low currents (3 amps) up to maximum torch capacity, which can be over 500 amps on automatic machine torches.
Fig. 1.3. TIG welding uses an inert gas, usually argon, to shield the molten puddle, which is created by an arc between a non-consumable tungsten electrode and the workpiece. Electric power is supplied by a DC or AC source. A major advantage of the process is the ability to have a stable arc at settings as low as low 3 amps.
For gas tungsten arc welding, the official AWS designation for TIG welding is GTAW. The common term TIG for tungsten inert gas is used throughout the book. Note: If taking welding courses and the instructor insists on the use of GTAW, use it!
Oscar Kjellberg, the founder of ESAB, invented and was issued a patent for stick electrode welding in 1904. Its use grew rapidly through the 1920s, 1930s, and 1940s and became the leading welding method until displaced by MIG welding.
Stick electrode welding is simple to use, requiring only a power source and an inexpensive electrode holder. It is still preferred when welding outdoors because it can make acceptable welds in significantly more wind than gas shielding processes. The AWS Bridge Welding Code, for example, specifies maximum wind speeds of 5 mph for MIG and TIG welding, while stick welding is allowed up to 20 mph. However, when TIG welding, a gas lens should be used for up to a 4-mph wind. In addition, TIG is more sensitive to the need for quality shielding than MIG.
Stick welding utilizes a simple power source, such as an AC welding transformer. DC power is also widely used and for welding outdoors; portable engine-driven DC generators are often employed. All these power sources are called constant current, referring to setting the desired welding current level, which stays relatively fixed regardless of the arc voltage. Thus, welding starts at a high voltage needed to initiate an arc, and reduces to 20 to 30 welding volts. In addition, when the stick electrode is shorted to the work at the start, the maximum current is only slightly more than the preset valve. While welding, as the arc length varies with the manipulation of the melting electrode, the current remains relatively constant. Therefore, if the arc length is varied, the voltage changes, but the welding current, which controls weld penetration, remains close to the preset level. In addition to the power source, the stick electrode holder is the only other equipment needed to make a stick weld.
Fig. 1.4. Stick electrode welding is simple—it only requires a power source and an inexpensive electrode holder. The heart of the process is the stick electrode that has a metal rod in the center, which is coated with a mixture of flux ingredients, small amounts of metal alloy, and a liquid binder. The coated rod is baked to harden the binder.
The heart of the process is the stick electrode itself. For steel welding, the center core is typically a non-alloyed steel rod. The rod is cut into short lengths, such as 14 inches. Flux ingredients are mixed with small amounts of metal alloy and a binder, often liquid sodium silicate. The dough-like mixture is extruded around the core wire. The coated rod is baked to harden the binder. A short section at the end has the coating removed, so the electrical power can be transferred to the core rod. When an arc is struck between the core wire and the workpiece, the flux melts and some gaseous products, such as carbon dioxide, are formed, and this helps protect the weld puddle from oxidation and nitrogen contamination. The flux ingredients melt and form a slag that floats to the top of the weld puddle and protects it from atmospheric contamination as it cools. A variety of electrode types are available. Some can weld high-strength steels and match their strength and toughness.
Fig. 1.5. Oscar Kjellberg, founder of ESAB, invented (and received a patent for) stick electrode welding in 1904. Stick welding became more prevalent through the 1920s, 1930s, and 1940s and became the leading welding method until displaced by MIG welding. ESAB grew to be a worldwide leader in the welding field.
Note the official AWS designation for stick welding is SMAW for shielded metal arc welding. The common term “stick welding” is used throughout this book.
In 1950, Gibson, Muller, and Nelson, working at the Airco development laboratories, patented the MIG (metal inert gas) welding process. Over the ensuing decades it evolved, and today it is used to deposit more than 60 percent of the filler metal in the United States.
Fig. 1.6. The MIG welding process utilizes a constant-voltage power supply. The output is similar to a car battery in which the voltage stays relatively constant as current rises. A small-diameter wire, typically .035 or .045 inch, is fed from a spool through a flexible cable and MIG gun. The wire exits the gun and current flows to an arc, which forms between the wire and workpiece.
The MIG process utilizes a constant-voltage power supply. Similar to a car battery in output characteristics, the voltage stays relatively constant as current rises. A small-diameter solid wire, typically .030 to .045 inch, feeds from a spool through a flexible cable and MIG gun. The MIG gun has a copper nozzle that directs shielding gas to protect the weld from oxidation and nitrogen contamination. The wire exits the front of the gun through a copper contact tip where it picks up electrical power. Current flows from the copper contact tip to the small diameter wire. As current passes through the wire on the way to the workpiece, resistant heating increases the temperature to perhaps to 500 degrees F. Then an arc forms between the end of the wire and the workpiece, and as a result, the arc melts both the wire and the workpiece. Unlike TIG welding that requires careful hand manipulation to maintain the arc length, MIG arc length is maintained automatically. However, the weldor must still control the distance from the MIG nozzle to the work to achieve proper welding performance. This is covered in detail, with examples, in Chapter 6.
MIG welding can be used for steel, stainless steel, aluminum, and some other materials. The official AWS designation for MIG welding is GMAW (gas metal arc welding), but the common term MIG is used throughout this book. For those outside of the United States, the term MIG is only used when 100 percent argon or argon-helium shielding gas mixtures as used when welding aluminum. For any shielding gas that includes oxygen or an oxygen compound, such as carbon dioxide, MAG (metal active gas) is used. My purest friends cringe when I use MIG at AWS Section talks. I tell them it is far better than calling the process wire welding!
The title of this section was purposely changed from “Oxyacetylene Welding” to “Oxyfuel Cutting.” The AWS term for the process is OFC (oxyfuel cutting). OAW is commonly used for welding because acetylene is the only gas that can be effectively used for welding, while a number of other fuel gases can be used for cutting. In fact, natural gas can be used for automatic cutting machines.
Fig. 1.7. Similar to the oxyacetylene welding torch, a cutting torch uses a special tip. Mixed oxygen and fuel gas exit multiple holes around the outer edge of the tip producing high-temperature flames. A large center hole in the cutting tip emits a high flow rate of pure oxygen, and this oxygen does the cutting. The flame preheats the steel and the oxidation of iron generates the heat to maintain the cut.
The process for cutting steel is shown in Figure 1.7. Similar to a welding torch, a cutting torch utilizes a special tip that has multiple small holes around the outside of a larger center hole. These small holes flow mixed oxygen and fuel gas and produce the high-temperature preheat flames. A larger center hole in the cutting tip has a high flow rate of pure oxygen only, and that oxygen does the cutting.
The hot outer flames preheat the steel, and the oxygen converts the hot iron to iron oxide. This chemical oxidation generates the heat to keep the cut going! Therefore, any fuel gas can be used to start the process, but depending on which fuel gas, it may take longer to start the cutting action. However, with a steady hand, the cut can be made.
Acetylene is often the preferred fuel gas. If you don’t have a steady hand, the cutting may stop with other fuel gases. When using the hotter, more intense acetylene flames even with slight hesitation it continues cutting, and that makes it easier to operate.
I found that out by experience when I was troubleshooting a welding job and needed to take a sample of a 1-inch-thick weld on the airplane. All of the weldors and cutters were at lunch so I offered to make the sample cut. I picked up the torch and proceeded. My experience had been cutting with oxyacetylene. This torch was using propane as a fuel gas. After a number of starts and stops, the cut was made, but it wasn’t pretty!
Bob Gage, working for the Linde Development Labs (my former employer), invented plasma cutting in 1957. Welding was discussed in the patent, but the process has gained much more popularity for cutting. Initially, the process used nitrogen cutting gas; today, manual systems mostly use compressed air as the plasma gas. The process creates an arc between a non-consumable electrode in the plasma torch and the workpiece. However, unlike TIG welding, the arc is forced to go through a very small hole that concentrates the heat and raises the temperature of the exiting plasma gas. The exiting gas in the center of the arc column reaches more than 30,000 degrees F, and that melts and blows away any conductive material. The innovative design and the rapidly swirling plasma gas in the nozzle throat allows the much-lower-melting-point copper nozzle to remain unharmed from a 30,000 degrees F arc coming through its small-diameter orifice.
Fig. 1.8. A plasma cutting torch is similar in some ways to a TIG torch. An arc is formed between a non-consumable electrode and the workpiece. However, the arc is forced to go through a very small hole, which concentrates the heat and raises the arc temperature to more than 30,000 degrees F. That is more than five times hotter than an oxyacetylene flame, and therefore it can cut through most materials and thicknesses.
When air or nitrogen are used as the plasma gas, some nitrogen compounds form in a thin area near the surface of the cut material. If the cut edges are going to be welded, they should be ground to remove this thin layer.
The AWS designation for this process is PAC, for plasma arc cutting. However, few folks use that acronym. In this book I use the shortened form, plasma cutting.
Fig. 1.9. Bob Gage, working for the Linde Development Labs (my old employer), invented plasma cutting in 1957. Welding was mentioned in the patent, but its use for cutting has gained much more popularity. Initially, plasma cutting used nitrogen as the plasma gas. Today, however, manual systems mostly use compressed air as the plasma and cutting gas.
