Читать книгу Welding Essentials - William Galvery - Страница 12
ОглавлениеExperience is the name everyonegives to their mistakes.
Oscar Wilde
Introduction
There is a lot of material common to the electrically-based welding processes —SMAW, GMAW, FCAW, and GTAW, so it makes sense to cover them all at once. The joints and positions are also common to OAW. These items are:
•Joint types
•Joint edge preparation
•Parts of a weld
•Selection of joint preparation
•Welding positions
•Other types of welds
•Welding terminology
•Welding cables
•Safety equipment
•Safety practices
Joint Types
What are the five basic joint types?
•Lap joint
•Butt joint
•Corner joint
•T-joint
•Edge joint
See Figure 4–1.
Figure 4–1Joint types
Joint Edge Preparation
Why is edge preparation done?
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.
How are edge shapes for weld joint preparation made?
Usually, they are made 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.
What edge shapes used in preparation for welding?
See Figure 4–2.
Figure 4–2Edge shapes for weld preparation
Edge Preparation Terminology
What are the principal parts of V-and U-groove joint preparations?
•Depth of bevel
•Size of root face
•Root opening
•Groove angle
•Bevel angle
See Figure 4–3.
Figure 4–3Parts of V- and U-groove joint preparations
Edge Shape Combinations
What are the most common combinations of joint preparations for butt joints?
See Figure 4–4.
Figure 4–4Single-groove and double-groove weld joint
What common preparations are used on corner joints?
See Figure 4–5.
Figure 4–5Weld preparation for corner joints
What weld preparations are used on T-joints?
See Figure 4–6.
Figure 4–6Weld preparations for T-joints
What are the weld preparations for edge joints?
See Figure 4–7.
Figure 4–7Weld preparations for edge joints
What are the weld preparations for lap joints?
See Figure 4–8.
Figure 4–8Weld preparations for lap joints
Selection of Joint Preparation
What determines which joint preparation geometry to use?
The factors are:
•Kind of joint loads (tension, compression, shear, or torsion).
•Level of joint loading.
•Static or dynamic loading.
•Thickness and type of the metals joined.
•Welding position.
•One side/both sides access to the weld.
•Skills of welders.
•Trade-offs between joint preparation costs/filler metal costs/welding labor costs; each joint and geometry carries its own combination of total cost.
What is an excellent source of weld joint design and preparation information for steel?
The Structural Welding Code—Steel, ANSI/AWS D 1.1 is a document with over 50 detailed drawings for pre-qualified joint designs using SMAW, GMAW, and FCAW for steel plate. Joints for submerged arc welding (SAW) are also included. There is additional information for joining tubing, weld testing, and inspection. The document is available from the AWS.
Joint Preparation
What are the most common weld preparations?
See Figure 4–9.
Figure 4–9A few typical weld preparations
Parts of the Weld
What are the terms used to describe the parts of a groove weld?
•Effective throat or size of weld
•Face
•Toe
•Face Reinforcement
•Root Reinforcement
See Figure 4–10.
Figure 4–10Parts of a groove weld
What are the terms used to describe the parts of a fillet weld?
•Leg or size of weld
•Face
•Toe
•Convexity
•Root penetration
•Fusion zone
•Leg of a fillet weld
•Root of the weld
•Actual throat and theoretical throat
See Figure 4–11.
CONVEX FILLET WELD
Figure 4–11AParts of a convex fillet weld
CONCAVE FILLET WELD
Figure 4–11BParts of a concave fillet weld
Welding Positions
How are welding positions plate designated?
They are divided by the position of the axis of the weld with respect to the horizontal and whether they are made on plate or pipe. They are used to designate positions for testing of welders and the application of a specific process. See Figure 4–12.
Figure 4–12Groove weld (upper) and fillet weld positions (lower)
What are the welding positions for pipe?
See Figure 4–13. 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 4–13Pipe weld positions
Plug and Slot Welds
What are plug and slot welds and why are they used?
They join two (or more) parts together by welding them at a point other than their edges. Plug and slot welds are used to secure multilevel parking garage and ship decks from shearing forces. 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 4–14.
Figure 4–14Examples of plug and slot welds
Intermittent Welds
What is the difference between a chain intermittent and a staggered intermittent weld?
See Figure 4–15.
Figure 4–15Chain intermittent fillet weld (left) and staggered intermittent fillet weld (right)
Welding Terminology
What terms describe the position of the electrode with respect to the weld?
They are:
•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. See Figure 4–16.
Figure 4–16Orientation of the electrode
What is travel speed?
It is the velocity or speed of the electrode (wire) along the travel axis, usually in inches/minute or cm/minute.
What is a tack weld?
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 work 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 and integral part of the finished weld.
What is a joggle joint?
See Figure 4–17. 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.
Figure 4–17Joggle weld joint preparation
What is the difference between a stringer bead and a weave bead?
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.
What is padding or overlaying and what is it used for?
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 4–18A shaft, bar or pipe and 4–18B is resurfacing a plate.
Figure 4–18AResurfacing on shaft, bar or pipe axial and circumferential welds
Figure 4–18BResurfacing on a plate
What is the purpose of surfacing welds?
Surfacing, also called hard surfacing, is the application of extra hard weld metal (padding) to surfaces subject to severe wear and abrasion. The teeth, buckets, and blades of earth moving equipment are often surfaced, as are the interior chutes of rock crushers. SMAW, GTAW, FCAW, and GMAW processes can all perform surfacing given the proper electrode metal composition.
What does the term boxing mean?
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 4–19.
Figure 4–19Boxing weld
Welding on Thick Plates
What is a root pass weld and where is it used?
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.
What is a back 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 4-20.
Figure 4–20-21Back weld and backing weld
What is a 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 4–20-21.
Why are weld backing plates used and what materials are used for them?
A backing plate is used to contain 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 weldment. 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 4–22.
Figure 4–22Weld backing plate
What is a runoff plate or tab and why are they used?
It is a plate of the same material of the work being joined which 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 4–23.
Figure 4–23Runoff plate or tab
How can a large weld be made when the electrode deposition is much smaller than the weld width?
By using multiple passes of parallel weld beads. See Figure 4–24.
Figure 4–24Multiple passes to join thick material
Slag Removal
Why is slag removal between weld bead applications important?
Slag must be fully removed 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.
Welding Cable Sizing
How do you determine what size welding cable is adequate for a task?
For copper cables, look up the cable size required in welding lead sizing chart based on the power supply-to-work distance and the current setting in Table 4–1. Tables are also available for aluminum conductors.
Table 4–1 Copper welding lead sizing
Work-Lead Connections
What are the common designs of work-lead connections?
•Spring-loaded
•Screw-clamp
•Magnetic attachment
•Tack welded connectors
See Figure 4–25.
Figure 4–25Work-lead connection methods include: spring-loaded, screw-type, magnet type, and tack-welded connections
Why is it important to have a good work-lead connection?
A poor work-lead connection will generate heat between the connection and the work. It is best to make a solid connection on freshly cleaned base metal. Use a grinder or wire wheel to get through rust, paint, and mill scale. If the welding electrode holder overheats, this is an indication of a poor work-lead connection.
Welding Hand Tools
What hand tools is the welder likely to need?
•Chipping hammer to remove welding slag
•Wire brush for cleaning welds
•Hammer and cold chisel to break tack welds
•Pliers for moving hot metal safely
•Wire cutters to trim electrode wire (GMAW and FCAW only)
See Figure 4–26.
Figure 4–26Various hand tools
Safety Equipment
•Welding helmet with the proper lens shade for the process and amperage.
•Leather capes and sleeves or jacket called skins or leathers, to protect the welder’s clothing from sparks, especially while welding overhead.
•Welder’s cap to protect from sparks getting behind the welding helmet and into the welder’s hair. See Figure 4–27.
Figure 4–27Welding safety equipment
•Breathing apparatus to provide the welder with fresh air in confined spaces with inadequate ventilation. Safety glasses under the welding helmet.
What is and what are the advantages of the electronic faceplates?
•An electronic faceplate or lens is one of the most recent and important safety devices developed in the welding industry. These devices are designed to be clear, or nearly clear, then darken the instant arc is established; when purchasing be aware of the time the lens takes to darken 1/25,000 of a second or faster is recommended.
•The welder does not need to raise and lower his helmet when performing a series of welds: he can always see where he is with the helmet down.
•The beginner does not have to master holding his electrode steady when he drops his helmet. This permits beginners to perform better welds earlier in their training.
•Electronic faceplates offer continuous eye protection from infrared radiation coming off red-hot metal even when they are not in the darkened mode. It is just easier on the eyes and the welder is less likely to incur eye injury from inadvertent arc strikes.
Safety
What is the authoritative source of welding safety information?
Consult the AWS (American Welding Society) booklet Safety in Welding Cutting and Allied Processes, Z49.1.
What safety issues must we remember with SMAW, GMAW, FCAW, and GTAW?
•Protection of face and eyes from sparks and radiation with a helmet and lens of appropriate shade number (darkness).
•Protection of all of the welders skin from arc and weld material radiation by covering it with cotton, wool, specially treated canvas jackets or leather garments; ultra violet radiation is carcinogenic.
•Personnel in the welding area must be protected from the welding arc and sparks by protective screens. Never view the welding being performed through the protective screens alone; the only way to safely view welding is through the proper shade lens and welding helmet or goggles.
•Beware of hazards from gases and insure adequate ventilation; inert shielding gases may cause suffocation in confined areas.
•Provide adequate ventilation from welding process smoke and the metal vapors, particularly heavy metals like zinc and cadmium that are toxic; keep your head out of the welding plume.
•Leathers or specially treated canvas jackets must be worn when welding vertically or overhead to protect the welder from the falling hot metal, sparks and slag.
•A welder’s hat will prevent both radiation burns to the head and hot sparks, falling slag and hot metal burns.
•High-top boots can prevent hot sparks and slag from burning your feet.
•Never weld with pant cuffs sparks falling into cuffs will burn pants.
•Make sure your welding gloves are dry and have no holes.
•Keep hands and body insulated from both the work and the metal electrode holder.
•Do not change the polarity switch position while the machine is under welding current load.
•Welding machines must be turned off when not attended.
•Must not stand on a wet surface when welding to prevent electric shock.
•Welding cables and electrode holders must be inspected for broken insulation regularly to prevent electric shock.
•Welding power supplies on AC lines must be properly grounded and emergency shut-off switch location known and accessible.
•Welding area must be dry and free of flammable materials.
•Protect your ears from welding and grinding noise with ear plugs or ear protectors.
•Any compressed gas cylinders must be properly secured and out of the spark stream.
•Must avoid wrapping welding cable around their arms or bodies in case a vehicle snags the cables.
•Never cut or weld on containers without taking precautions, see Chapter 1, Safety.
•Shielded metal arc welding the welder must plan for disposal of electrode stubs: they are hot enough to cause burns and to start fires and must not be dropped from heights because of the hazard to others.
