Welding Joint Design, Welding Symbols
WELD JOINT DESIGN
The term weld joint design refers to the way pieces of metal are put together or aligned with each other. The five basic joint designs are butt joints, lap joints, tee joints, outside corner joints, and edge joints.
Butt joint—In a butt joint the edges of the metal meet so that the thickness of the joint is approximately equal to the thickness of the metal. The metal surfaces are usually parallel with each other, although there can be some difference in thickness or misalignment of the plates. Butt joints can be welded from one side or both sides with some form of groove weld.
Lap joint—In a lap joint the edges of the metal overlap so that the thickness of the joint is approximately equal to the combined thickness of both pieces of metal. The distance the surfaces overlap each other may vary from a fraction of an inch to several inches or even feet. Lap welds are usually joined by making a fillet weld along the edge of one plate, joining it to the surface of the other. There are several alternate ways of welding lap joints where the weld is made through one or both pieces of metal joining the lap in the center of the overlap. Some examples of this would be plug welds, seam welds, and stir welds. Welds can be made on one side or both sides of the joint.
Tee joint—In a tee joint the edge of a piece of metal is placed on the surface of another piece of metal. Usually the parts are placed at a 90° angle with each other. Tee joints can be welded with a fillet weld applied to the surfaces, or a weld can be made in a precut groove in the edge of the joining plate. In a few cases, a fillet weld can be made on the top of a groove weld on a tee joint. Welds can be made on one side or both sides of the joint.
Outside corner joint—In an outside corner joint, the edges of the metal are brought together at an angle, usually around 90° to each other. The edges can meet at the corner evenly or they can overlap. The outside corner joint can be welded on both sides, with the outside being made as a groove weld and the inside as a fillet weld.
Edge joint—In an edge joint the metal surfaces are placed together so the edges are even. One or both plates may be formed by bending them at an angle. Edge joints are usually welded only on one side.
Welding drawings and specifications usually tell you exactly which joint design will be used for all of the welds to be made. Often a welding engineer or designer has determined the best type of joint to be used. However, on small projects or on some repair welding jobs, you will be making the decision as to the joint design to be used. The way the pieces of metal fit together may determine the joint design that must be used. For example, the part shown in can be made using only tee joints. Joints on every weldment are not as easily determined. If you are the one choosing the weld joint design, you must consider a number of factors. Some of the factors involve the type and thickness of metal being welded, the welding position, welding process, finished weld properties, and any code requirements. The selection of the best joint design for a specific weldment requires that you carefully consider all of the various factors. Each factor, if considered alone, could result in a part that might not be able to be fabricated or meet the strength requirements. For example, a narrower joint angle requires less filler metal, and that results in lower welding cost. But if the angle is too small for the welding process being used, the weld cannot be made strong enough. A large weld may be stronger, but it may result in the part being distorted so badly that it becomes useless. The purpose of a welded joint is to join parts together so that the completed weldment can withstand the stresses. The forces acting on a weld cause stresses. Forces cause stresses in five ways: tensile, compression, bending, torsion, and shear. If the stresses are excessive, the part can fail. The ability of a welded joint to withstand these forces depends both upon the joint design and the weld integrity. Some joints can withstand some types of forces better than others. Some of the factors that affect the selection of a specific weld joint design include welding process, edge preparation, joint dimensions, metal thickness, metal type, welding position, codes or standards, and cost.
Welding process
The welding process to be used has a major effect on the selection of the joint design. Each welding process has characteristics that affect its performance. Some processes are easily used in any position; others may be restricted to one or more positions. The rate of travel, penetration, deposition rate, and heat input also affect the welds used on some joint designs. For example, a square butt joint can be made in very thick plates using either electroslag or electrogas welding, but not many other processes can be used on such a joint design.
Edge preparation
The area of the metal’s surface that is melted during the welding process is called the faying surface. The faying surface can be shaped before welding to increase the weld’s strength; this is called edge preparation. The edge preparation may be the same on both members of the joint, or each side can be shaped differently. Reasons for preparing the faying surfaces for welding include the following:
Codes and standards—Some codes and standards require specific edge preparations.
Metals—Some metals must be grooved to successfully weld them, such as thick magnesium, which must be U-grooved; or cast iron cracks, which must be drill-stopped and grooved.
Deeper weld penetration—With the metal removed by grooving or beveling the metal’s edge, it is easier for the molten weld metal to completely fuse through the joint. In some cases, it is possible to make a through-thickness weld from one side.
Smooth appearance—The weld’s surface can be ground smooth with the base metal so that the weld “disappears.” This can be done for appearance or so that the weld does not interfere with the sliding or moving of parts along the surface.
Increased strength—A weld should be as strong as or stronger than the base metal being joined. By having 100% joint fusion and an appropriate amount of weld reinforcement, the weld can meet its strength requirement.
Joint dimensions
In some cases the exact size, shape, and angle can be specified for a groove. If exact dimensions are not given, you may make the groove any size that you feel necessary; but remember, the wider the groove, the more welding it will require to complete.
Metal thickness
As the metal becomes thicker, you must change the joint design to ensure a sound weld. On thin sections it is often possible to make full penetration welds using a square butt joint. Square butt joints take less preparation time and less welding time. But with thicker plates or pipe, the edge must be prepared with a groove on one or both sides. The edge may be shaped with either a bevel, V-groove, J-groove, or U-groove. When welding on thick plate or pipe, it is often impossible for the welder to get 100% penetration without some type of groove being used. The groove may be cut into just one of the plates or pipes or both. On some plates it can be cut both inside and outside of the joint. The groove may be ground, flame cut, gouged, sawed, or machined on the edge of the plate before or after the assembly. Bevels and V-grooves are best if they are cut before the parts are assembled. J-grooves and U-grooves can be cut either before or after assembly. The lap joint is seldom prepared with a groove because little or no strength can be gained by grooving this joint. For most welding processes, plates that are thicker than 3/8 in. (10 mm) may be grooved on both the inside and outside of the joint. Plate in the flat position is usually grooved on only one side unless it can be repositioned or it is required to be welded on both sides. Tee joints in a thick plate are easier to weld and will have less distortion if they are grooved on both sides. Sometimes plates are either grooved and welded or just welded on one side and then back-gouged and welded. Back gouging is a process of cutting a groove in the back side of a joint that has been welded. Back gouging can ensure 100% joint fusion at the root and remove discontinuities of the root pass.
Metal type
Because some metals have specific problems with thermal expansion, crack sensitivity, or distortion, the joint design selected must help control these problems. For example, magnesium is very susceptible to postweld stresses, and the U-groove works best for thick sections.
Welding position
The most ideal welding position for most joints is the flat position because it allows for larger molten weld pools to be controlled. Usually the larger that a weld pool can be, the faster the joint can be completed. When welds are made in any position other than the flat position, they are referred to as being done out-of-position. Some types of grooves work better in out-of-position welding than others; for example, the bevel joint is often the best choice for horizontal butt welding.
Plate Welding Positions—The American Welding Society has divided plate welding into four basic positions for grooves (G) and fillet (F) welds as follows:
Flat 1G or 1F—When welding is performed from the upper side of the joint, and the face of the weld is approximately horizontal.
Horizontal 2G or 2F—Th e axis of the weld is approximately horizontal, but the type of weld dictates the complete defi nition. For a fi llet weld, welding is performed on the upper side of an approximately vertical surface. For a groove weld, the face of the weld lies in an approximately vertical plane.
Vertical 3G or 3F—Th e axis of the weld is approximately vertical.
Overhead 4G or 4F—When welding is performed from the underside of the joint.
Pipe Welding Positions—The American Welding Society has divided pipe welding into five basic positions:
Horizontal rolled 1G—When the pipe is rolled either continuously or intermittently so that the weld is performed within 0° to 15° of the top of the pipe.
Horizontal fi xed 5G—When the pipe is parallel to the horizon, and the weld is made vertically around the pipe.
Vertical 2G—The pipe is vertical to the horizon, and the weld is made horizontally around the pipe.
Inclined 6G—Th e pipe is fi xed in a 45°-inclined angle, and the weld is made around the pipe.
Inclined with a restriction ring 6GR—Th e pipe is fi xed in a 45° inclined angle, and there is a restricting ring placed around the pipe below the weld groove.
Code or standards requirements
The type, depth, angle, and location of the groove is usually determined by a code or standard that has been qualified for the specific job. Organizations such as the American Welding Society (AWS), American Society of Mechanical Engineers (ASME), and the American Bureau of Ships are a few of the agencies that issue such codes and specifications. The most common codes or standards are the AWS D1.1 and the ASME Boiler and Pressure Vessel (BPV) Section IX. The joint design for a specific set of specifications is often known as prequalified. These joints have been tested and found to be reliable for the weldments for specific applications. The joint design can be modified, but the cost to have the new design accepted under the standard being used is often prohibitive.
Cost
Almost any weld can be made in any material in any position if cost is not a factor. A number of items affect the cost of producing a weld. Joint design can be a major way to control welding cost. Changes in the design can reduce cost while still meeting the weldment’s strength requirements. Making the groove angle smaller can help, reducing the welding filler metal required to complete the weld as well as reducing the time required to fill the larger groove opening. Good joint design must be a consideration for any project to be competitive and cost effective.
FILLET WELDS
NOTE: A fillet weld is approximately triangular in shape. It is used to join lap joints, tee joints, or corner joints where the joint is at an approximate right angle.
Dimensions of fillet welds are shown on the same side of the reference line as the weld symbol and are shown to the left of the symbol. When both sides of a joint have the same size fillet welds, one or both may be dimensioned. When both sides of a joint have different size fillet welds, both are dimensioned. When the dimensions of one or both welds differ from the dimensions given in the general notes, both welds are dimensioned. The size of a fillet weld with unequal legs is shown in parentheses to the left of the weld symbol. The length of a fillet weld, when indicated on the welding symbol, is shown to the right of the weld symbol. In intermittent fillet welds, the length and pitch increments are placed to the right of the weld symbol. Intermittent welds are often used in sheet metal to both reduce the heat input to the joint and to stop cracking from continuing through the joint. Each individual weld serves as a crack-stopper. If a crack starts in a single intermittent weld, it has to restart in the next weld before it can continue on down the joint. For these reasons you must make the intermittent welds as designed, even though making a continuous weld might be faster. The first number represents the length of the weld, and the second number represents the pitch or the distance between the center of two welds.
NOTE: Unequal legged fillet welds are used when one piece of metal is much thinner than the other. The short leg of the fillet allows less heat input to the thinner metal and reduces the chance of burnthrough.
PLUG WELDS
NOTE: A plug weld is made by welding through a round hole in the top plate to fuse the bottom plate. The hole in the top plate may or may not be filled completely by the weld. Plug welds are used to make lap joints.
Holes in the arrow side member of a joint for plug welding are indicated by placing the weld symbol below the reference line. Holes in the other side member of a joint for plug welding are indicated by placing the weld symbol above the reference line. The diameter or size is located to the left of the symbol (A). The angle of the sides of the hole, if not square, is given above the symbol (B). The depth of buildup, if not completely flush with the surface, is given in the symbol (C). The center-to-center dimensioning or pitch is located on the right of the symbol (D).
SPOT WELDS
NOTE: A spot weld is approximately round and is created between the two overlapping surfaces being joined.
Dimensions of spot welds are indicated on the same side of the reference line as the weld symbol. Such welds are dimensioned either by size or strength. The size is designated as the diameter of the weld expressed in fractions or in decimal hundredths of an inch. The size is shown with or without inch marks to the left of the weld symbol. The centerto-center spacing (pitch) is shown to the right of the symbol. The strength of spot welds is shown as the minimum shear strength in pounds (Newton’s) per spot and is shown to the left of the symbol. When a definite number of spot welds are desired in a certain joint, the quantity is placed above or below the weld symbol in parentheses.
SEAM WELDS
NOTE: Seam welds are continuous along the overlapping surfaces. They can be made by producing a series of overlapping spot welds or be one continuous resistance weld.
Dimensions of seam welds are shown on the same side of the reference line as the weld symbol. Dimensions relate to either size or strength. The size of seam welds is designated as the width of the weld expressed in fractions or decimal hundredths of an inch. The size is shown with or without the inch marks to the left of the weld symbol. When the length of a seam weld is indicated on the symbol, it is shown to the right of the symbol. When seam welding extends for the full distance between abrupt changes in the direction of welding, a length dimension is not required on the welding symbol. The strength of seam welds is designated as the minimum acceptable shear strength in pounds per linear inch. The strength value is placed to the left of the weld symbol.
GROOVE WELDS
NOTE: A groove weld is made in the space cut into the joint between two pieces being joined.
Joint strengths can be improved by making some type of groove preparation before the joint is welded. There are seven types of grooves. The groove can be made in one or both plates or on one or both sides. By cutting the groove in the plate, the weld can penetrate deeper into the joint, helping to increase the joint strength without restricting flexibility. The grooves can be cut in base metal in a number of different ways. The groove can be oxyacetylene cut, air carbon arc cut, plasma arc cut, machined, sawed, and so forth. The various features of groove welds are as follows:
Single-groove and symmetrical double-groove welds that extend completely through the members being joined. No size is included on the weld symbol.
Groove welds that extend only partway through the parts being joined. The size as measured from the top of the surface to the bottom (not including reinforcement) is included to the left of the welding symbol.
The size of groove welds with a specified effective throat is indicated by showing the depth of groove preparation with the effective throat appearing in parentheses and placed to the left of the weld symbol. The size of square groove welds is indicated by showing the root penetration. The depth of chamfering and the root penetration is read in that order from left to right along the reference line.
The root opening of groove welds is the user’s standard unless otherwise indicated. The root opening of groove welds, when not the user’s standard, is shown inside the weld symbol.
The root face’s main purpose is to minimize the burnthrough that can occur with a feather edge. The size of the root face is important to ensure good root fusion.
The size of flare groove welds is considered to extend only to the tangent points of the members.
FLANGE WELDS
NOTE: Flange welds are used on thin metal as a way of stiffening the edge so there is less distortion. It can also make it easier to make a weld on these thin sections without excessive burnthrough. A flange weld is made along the edge of the metal that has been bent upward, forming a ridge or flange.
The following welding symbols are used for light-gauge metal joints where the edges to be joined are bent to form a flange or flare.
Edge flange welds are shown by the edge flange weld symbol.
Corner flange welds are indicated by the corner flange weld symbol.
Dimensions of flange welds are shown on the same side of the reference line as the weld symbol and are placed to the left of the symbol. The radius and height above the point of tangency are indicated by showing both the radius and the height separated by a plus sign.
The size of the flange weld is shown by a dimension placed outward from the flanged dimensions.