How to Read Basic Welding Symbols
BEGINNER’S GUIDE TO READING WELDING SYMBOLS
Welding symbols provide for a convenient way of depicting the way a particular joint is welded. However, in order to have accurate welds, the symbols must be both properly drawn and interpreted. The current U.S. standard for welding symbols was created by the American Welding Society and approved by the American National Standards Institute.
While the basic welding-symbol system is standardized, it is necessary to allow for some flexibility as particular circumstances may differ between shops and field operations. Therefore, this article will focus on how to read welding symbols for typical applications with the ASME Code.
THE REFERENCE LINE
An essential and required part of all welding symbols is the reference line as it is the anchor that all of the other welding symbols are connected to. This is a horizontal line with any other necessary information needed to create the weld drawn on or around the reference line. The reference line must be drawn near or on the joint the line describes. A multiple reference-line symbol may be used if the sequence of an operation must be specified.
The arrow is the second required part of a welding symbol. The arrow is placed at one end of the reference line, connecting the reference line to the joint that will be welded. The arrow may be drawn at either end of the reference line. Any information relating to the arrow end of the reference line is placed below the line. The opposite side of the joint is referred to as the “other side” and information relating to the other side of the joint is placed above the reference line.
These positional associations exist whether or not the arrow is attached to the right-end or left-end of the reference line, and does not change as the angle between the reference line and arrow varies. More than one reference line may be connected to the same arrow, with the reference line nearest to the arrow specifying the initial operation and followed by the additional operations if any. Additional operations are specified by the succession of reference lines reading up or down from the arrow. The below and above rule applies regardless of the arrow's direction.
There are often two sides to the joint where the arrow points, indicating two potential spots to place a weld. As an example, if two steel plates are joined into a T shape, a weld may be placed on either side of the T at the stem. To minimize the number of required symbols, it is acceptable to use multiple arrows in a single welding diagram, provided each joint the arrow points to is meant to be welded in exactly the same manner. Since a symbol indicates welding of only the joint the arrow points to, and a change of direction or geometry constitutes the end of a joint, more than one arrow can be extremely helpful, particularly when used around a closed corner.
A third important element of the welding symbol is the “tail.” The tail is shown as a greater-than or less-than sign, connected to the reference line at the opposite end of the arrow. Any information that has no specific provision anywhere else in the symbol is written to the right or left of the tail, as needed and appropriate. References to approved welding procedure specifications, or WPS, are one example of information that may be placed in the tail of the symbol. Since a WPS can contain all of the information needed for a specific joint, a welding symbol containing an arrow, reference line and tail with relevant WPS description is typically sufficient to fully detail the welding of the joint.
Even if all the information is included in a welding symbol WPS, additional information for things like grooves may also be included. A groove-weld symbol is indicated by a double line, half-circle, a “V” or hash marks. The groove symbol is sometimes placed below the reference line to indicate a single weld on only the arrow side of the joint, above the reference line to specify only a single weld on the other side of the joint or symbols may be added both above and below the reference line to specify a double weld.
COMPLETE JOINT PENETRATION
Since there are many applications were welds are required that provide complete joint penetration, or CJP, there are a number of ways to indicate this. One example is to use an arrow and a reference line and add “CJP” in the symbol’s tail. Such a symbol may be suitable when it is uncertain as to what particular equipment will be available at the worksite.
Another way to identify CJP is to include either a single-groove or double-groove symbol, with the same symbol on both sides of the reference line, with no indicated dimensions to indicate the depth of the weld size. It should be noted that partial joint penetration, or PJP, can be indicated by adding the depth-of-bevel measurement and the weld’s required size to the left of the groove symbol, or to both sides of a double weld.
Fillet welds are used extensively in constructing boilers and pressure applications. The fillet symbol is a right triangle located on the reference line, where the triangle’s perpendicular line is always placed to the left. The dimension indicating the size of a fillet weld is set to the left of the fillet symbol and on the same side of the reference line.
Compared to groove-type welds, fillet welds don’t necessarily run for the entire length of the joint. The length of a fillet weld is specified by placing the dimension of the weld to the right of the fillet symbol. Dimension lines are necessary if the length of the weld must be exact. The omission of measurements indicates the fillet weld runs the full length of the joint. Intermittent fillet welds can be indicated on both sides of a joint by making a fillet-weld symbol on both sides of the reference line.