acetylene gases produce an approximately 5600°F (3100°C) flame at the torch tip. This flame melts the edges of the base metals to be joined into a common pool. Sometimes additional filler metal is added to the molten pool from a welding rod. When this common pool cools and the metal freezes solid, the joined metals are fused together and the weld complete.
Oxyacetylene welding was first used industrially in the early years of the twentieth century. Although this process makes excellent welds in steel, it is little used for welding today except for a few specialties because there are other more efficient welding processes available. However, oxyacetylene has many other important uses: cutting, hardening, tempering, bending, forming, preheating, postheating, brazing, and braze welding. Because of the precise control the welder has over heat input and its high-temperature flame, together with its low equipment cost, portability, and versatility, it remains an essential tool. As with all effective tools, using oxyacetylene carries risk. We will cover the theory and use of oxyacetylene equipment so you can use them with confidence and safety. See Figure 1-1.
Figure 1-1 Oxyacetylene setup.
Shielded Metal Arc Welding or Stick Welding
In shielded metal arc welding, an electric circuit is established between the welding power supply, the electrode, the welding arc, the work, the work connection, and back to the power supply. The arc produces heat to melt both the electrode metal and the base metal. Temperatures within the arc exceed 6,000°F (3,300°C). The arc heats both the electrode and the work beneath it. As the electrode moves away from the molten pool, the molten mixture of electrode and base metals solidifies and the weld is complete.
Arc welding machines have been used in this country since the early days of the twentieth century. Arc welding is popular for industrial, automotive, and farm repair because its equipment is relatively inexpensive and can be made portable. More welders have learned this process than any other. Although it will be around for many years, and its annual filler metal poundage continues to grow, it is declining in importance as wire feed welding processes continue to gain popularity and market share. We will cover theory, equipment, electrode rod classification and selection, and safety.
Figure 1-2. Arc welding setup
Wire Feed Welding or MIG Welding
In this process, welding wire within the welding gun is both the electrode and the filler material. Welding begins as the section of electrode wire between the tip and the base metal is heated and deposited into the weld. As the wire is consumed, the feed mechanism supplies more electrode wire at the pre-adjusted rate to maintain a steady arc.
The wire feed processes consume over 70 percent of total filler materials used today, and this percentage continues to grow. While this welding equipment may cost more than arc welding equipment of the same capabilities, it offers higher productivity, and it is easy to learn. Not having to stop a bead, change electrodes, and restart again increases metal deposition rates and reduces weld discontinuities. Also, these processes are readily adapted to robotic/computer-controlled operations. Wire feed processes are relatively easy to learn, especially to those already trained in shielded metal arc welding, once the power source differences and voltage- amperage variables are understood. See Figure 1-3.
Figure 1-3 Wire feed welding setup
Non-Consumable Electrode or TIG Welding
A continuous arc forms between a tungsten electrode on the welding torch and the work. The electrode in this process is not consumed. However, some applications require the use of a filler rod.
Although this process requires more skill than most other processes and does not have high metal deposition rates, improvements in shielding gas mixtures, torch design, and power supply electronics have made it an indispensable tool where high quality welds are essential on aluminum, magnesium, or titanium. It can weld most metals, even dissimilar ones. See Figure 1-4.
Figure 1-4 TIG welding setup
How Are Welding Joints Prepared?
Some elements of welding are common to all types, such as joint preparation, welding terminology, and the like. They will be covered here. For specific techniques, see the chapters dealing with each of the main welding processes.
Joint preparation provides access to the joint interior. Without it the entire internal portion of the joint would not be fused or melted together making the joint weak. Remember that a properly made, full-penetration joint can carry as much load as the base metal itself, but full penetration will only occur with the correct joint preparation.
Usually, joints are prepared by flame cutting, plasma arc cutting, machining, or grinding; however, castings, forgings, shearing, stamping, and filing are also common methods used to prepare material for welding. See figure 1-5.
Figure 1-5 Edge shapes for weld preparation
Figures 1-5 and 1-6 Proper joint preparation is essential to ensure strong welds. Here a portable grinder is used to bevel the edges of two thin sheets of metal
Joint Types
Figure 1-6 Joint types.
Common Welding Types
The V-and U-groove joints are common joints used in welding. The parts of the joints include
•Depth of bevel
•Size of root face
•Root opening
•Groove angle
•Bevel angle
Figure 1-7 Parts of V- and U-groove joint preparations
Joint Preparations for Butt Joints
Figure 1-8 Single-groove and double-groove weld joint
Joint Preparations for Corner Joints
