Читать книгу The Art of Welding - William Galvery - Страница 9
ОглавлениеT he term “welding” comprises a number of different processes for fusing metals. For the home hobbyist or do-it-yourselfer, oxyacetylene welding, shielded metal arc welding (also called stick welding), wire-feed welding processes (also called MIG), and nonconsumable electrode welding (also called TIG) are the most popular. This chapter also covers the basics of joint preparation and the most common types of welds. For procedures that relate to specific welding processes, see the relevant chapter.
WHAT ARE THE MOST POPULAR WELDING PROCESSES FOR THE DO-IT-YOURSELFER?
HOW ARE WELDING JOINTS PREPARED?
WHAT ARE SOME OF THE BASIC TYPES OF WELDS?
WHAT ARE THE SPECIAL PROCEDURES FOR WELDING THICK PLATES?
WHAT ARE SOME COMMON WELDING POSITIONS?
What are the most popular welding processes for the do-it-yourselfer?
Oxyacetylene Welding
When combined in the correct proportions in the welding torch handle, oxygen and 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
Figure 1-9 Weld preparation for corner joints
Joint Preparations for T-joints
Figure 1-10 Weld preparations for T-joints
Joint Preparations for Edge Joints
Figure 1-11 Weld preparations for edge joints
Joint Preparations for Lap Joints
Figure 1-12 Weld preparations for lap joints
Common Weld Preparations
Figure 1-13 A few typical weld preparations
What are some basic types of welds?
The Groove Weld
As the name implies and the illustration below shows, this is a weld made in a groove between work pieces. See “Common Weld Preparations,” on the opposite page, for some typical weld dimensions.
See Figure 1-14.
Figure 1-14 Parts of a groove weld
Fillet Weld
Fillet welds are triangular in shape and used to join materials that are at right angles to one another in a lap, T-, or corner joint. The face of the weld can be convex or concave. See Figure 1-15 A&B.
Figure 1-15A Parts of a convex fillet weld
Figure 1-15B Parts of a concave fillet weld
Plug and Slot Welds
These welds join two (or more) parts together by welding them at a point other than their edges. They are particularly useful in sheet metal and auto-body work where welds can be completely concealed by grinding and painting. A hole or slot is made in the work-piece facing the welder and weld is made inside the hole. Filler metal completely fills the hole or slot and penetrates into the lower work-piece(s) securing them together. Plug welds are round and slot welds are elongated and rounded at the ends. See Figure 1-16.
Figure 1-16 Examples of plug and slot welds
Intermittent Welds
Note the positions of the welds shown below. See Figure 1-17.
Figure 1-17 Chain intermittent fillet weld (left) and staggered intermittent fillet weld (right)
Welding Terminology
These terms describe the position of the electrode in respect to the weld.
Axis of the weld—an imaginary line drawn parallel to the weld bead through the center of the weld.
Travel angle—is the smallest angle formed between the electrode and the axis of the weld.
Work angle—for a T-joint or corner joint, the smallest angle formed by a plane, defined by the electrode (wire) and the axis of the weld, and the work piece.
Push angle during forehand welding—this is the travel angle during push welding when the electrode (wire) is pointing toward the direction of weld progression.
Drag angle during backhand welding—this is the travel angle during drag welding when the when the electrode (wire) is pointing away from the direction of weld progression.
Travel speed—the velocity or speed of the electrode (wire) along the travel axis, usually in inches/minute or cm/minute.
See Figure 1-18.
Figure 1-18 Orientation of the electrode
TackWeld
Welders place small, initial welds along joints to hold the work pieces in place so the parts remain in alignment when they are welded. Tack welds hold work firmly in position, but can be broken with a cold chisel in the event further adjustment is needed. Beginning welders tend to make them too small. One inch is the standard length of a tack weld. A tack should be as strong as the weld itself as it becomes an integral part of the finished weld.
Figure 1-19 Make sure the work is aligned properly; keep it aligned by welding small sections at intervals to tack the metal in place
Figure 1-21 Joggle weld joint preparation
Joggle Joints
Joggle joints are used where a strong joint and flat surface is needed to join two pieces of sheet metal or light plate. There are hand tools available to put the joggle into sheet metal. They are useful whenever a finished surface concealing the weld is needed and where a butt joint would not work with thin sheet metal. See Figure 1-21.
Stringer and Weave Beads
In a stringer bead the path of the electrode is straight, with no appreciable side to side movement, and parallel to the axis of the weld, while a weave bead has a side-to-side motion which makes the weld bead wider (and the heat-affected zone larger) than that made with a stringer bead.
Padding
Padding is when weld filler metal is applied to a surface to build up a plate or shaft, to make a plate thicker, or to increase the diameter of a shaft. It is used either to restore a dimension to a worn part or to apply an extra hard wear surface. See Figure 1-20A shaft, bar, or pipe and 1-20B is resurfacing a plate.
Figure 1-20A Resurfacing on shaft, bar, or pipe axial and circumferential welds.
Figure 1-20B Resurfacing on a plate
Boxing
Boxing is when a fillet weld is continued around a corner. Normally a fillet weld is made from one abrupt end of the joint to the other abrupt end of the joint. See Figure 1-21.
Figure 1-21 Boxing weld
What are the special procedures for welding thick plates?
Root Pass Weld
A root pass uses weld filler metal to close the root space between the weld faces. It is especially helpful in welding pipe and thick plates where only one side of the weld is accessible and no backing material is used.
Back and Backing Weld
A back weld is applied after a groove weld is completed. The back weld is made to insure full penetration through the material being joined. Before we apply the back weld we must grind or gouge into the bottom of the groove weld until we reach sound weld metal then we may apply the back weld to the bottom of the groove weld. See Figure 1-22.
Figure 1-22 and 23 Back weld and backing weld
A backing weld is applied to the bottom or root of a groove weld before the groove weld is applied. Because the root or bottom of the weld is made first it becomes a backing for the groove. The difference between a back and backing weld is the sequence of welding. Before the groove weld is completed the backing weld must be ground or gouged to sound weld. See Figure above.
Backing Plates
A backing plate contains the large weld pool when joining two thick sections that are accessible from only one side. It takes the place of a root pass. The backing plate also shields the weld pool from atmospheric contamination coming in from the back of the weld. Backing plates are usually tack welded to the two sections of the weld, but there are proprietary ceramic tapes and metal-glass tapes that perform the same function and do not need to be tacked into place. Copper and other materials are also used as backing plates. See Figure 1-24.
Figure 1-24 Weld backing plate
Runoff Plates
This is a made of the same material as the work being joined. The plate is tack welded to the joint at the start and/or end of the groove joint. The runoff plate contains a groove like the pieces being joined. It prevents the discontinuities caused by beginning and ending the welding process. See Figure 1-25.
Figure 1-25 Runoff plate or tab
Multiple Pass Technique
Use multiple passes of parallel weld beads when you are faced with making a large weld but the electrode deposition is much smaller than the weld width. See Figure 1-26.
Figure 1-26 Multiple passes to join thick material
What are some common welding positions?
They are determined by the position of the axis of the weld with respect to the horizontal and whether they are made on plate or pipe. See Figure 1-27.
Figure 1-27 Groove weld (upper) and fillet weld positions (lower)
Welding Positions for Pipe
See Figure 1-28. Note the difference between welding positions A , and C: In position A, (1G) the pipe may be rotated about its longitudinal axis to provide access to any part of the weld joint allowing the welder the opportunity to weld the entire pipe in the flat (1G) position; in position C, (5G) the pipe is fixed and cannot rotate forcing the welder to weld upward or downward vertically, flat on the top and overhead on the bottom; position B is pipe in a vertical position and welded on the horizontal plane; pipe in D is on a 45° angle and all positions (flat, horizontal, vertical, and overhead) are welded when pipe is in this position; the final position is pipe at a 45° with a restrictor in place (the restrictor allows the welder to weld only from one side of the restrictor) making this the most difficult of all welding positions.
Figure 1-28 Pipe weld positions
Slag Removal
Remove slag between weld passes or the remaining slag will form inclusions within the weld metal and weaken it. Slag is usually removed with a slag hammer and wire brush angle grinders or pneumatic peening tools may also be used. Sometimes a wire wheel is used. Pipe welding, grinders and power wheels are used between each welding pass to assure a slag-free surface on which to begin the next pass.
