Flash, upset, and flanged-joint weld callouts can be confusing because older welding-symbol references do not always match current AWS practice. The most important update is that AWS A2.4:2020 no longer supports the old graphical flash and upset weld symbols. It also uses the edge weld symbol for welds on flanged butt and flanged corner joints, so readers should identify the governing standard and drawing date before interpreting an unfamiliar symbol.
Quick Answer
Under current AWS A2.4:2020, dedicated graphical flash and upset weld symbols are no longer supported; AWS recommends identifying them as FW or UW in the tail when needed. For flanged butt and corner joints, current AWS drawings use the edge weld symbol, while flange geometry is shown on the drawing itself.
Key Takeaways
- AWS A2.4:2020 removed the old dedicated graphical symbols for flash welding and upset welding; use FW or UW in the welding-symbol tail when process identification is needed.
- Older drawings may still show centered flash/upset graphics and legacy flange symbols, so recognizing historical notation remains useful.
- Current AWS practice uses the edge weld symbol for parallel joints and for flanged butt or flanged corner joints.
- Under current AWS rules, flange geometry is part of the drawing rather than the older radius-plus-height flange notation on the welding symbol.
- A welding symbol does not replace the drawing, governing fabrication code, approved WPS, or qualified engineering requirements.
What Flash and Flange Weld Symbols Mean
The phrase flash and flange weld symbols is most useful today when comparing older drawings with current standards. A drawing created from an older symbol reference may show dedicated graphical marks for flash welding, upset welding, edge-flange welds, or corner-flange welds. A current AWS A2.4:2020 drawing uses different conventions for several of these cases.
The AWS A2.4:2020 standard states that dedicated flash and upset welding symbols are no longer supported. When a welding symbol is needed, AWS recommends a reference line and arrow with FW for flash welding or UW for upset welding in the tail.
For flanged sheet-metal joints, current AWS terminology centers on the edge weld symbol. AWS uses that symbol for edge welds on parallel joints and on flanged butt and flanged corner joints. This is different from older references that showed dedicated edge-flange and corner-flange graphics with radius and height information beside them.
Note: “Flanged joint” here refers to sheet or similar material whose edge has been formed or flanged. It does not mean the bolted or welded pipe flanges used in piping systems.
| Item | Legacy Drawing Convention | Current AWS A2.4:2020 Approach |
|---|---|---|
| Flash welding | Dedicated graphical symbol centered on the reference line | Reference line and arrow with FW in the tail when identification is needed |
| Upset welding | Dedicated graphical symbol centered on the reference line | Reference line and arrow with UW in the tail when identification is needed |
| Flanged butt/corner joint | Dedicated flange-style notation may appear | Use the edge weld symbol; flange geometry is part of the drawing |
How Flash and Upset Weld Symbols Are Read
Start by determining whether the drawing follows a current or legacy symbol system. On a current AWS A2.4:2020 drawing, do not expect a dedicated graphical flash or upset weld symbol. Instead, check the tail of the welding symbol for the process designation.
- Locate the arrow and reference line that identify the joint or area.
- Check the tail for FW or UW.
- Read any drawing notes, dimensions, procedure references, or specifications associated with the callout.
- Use the governing WPS and fabrication code for the actual welding variables and acceptance requirements.
AWS A2.4:2020 discontinued the dedicated flash and upset graphics and recommends identifying these processes as FW or UW in the welding-symbol tail when needed.
On a legacy drawing, a centered graphical flash or upset symbol may still appear. In that historical convention, the centered location indicated that the old weld symbol itself did not use the normal above-the-line/below-the-line arrow-side distinction.
Warning: Do not redraw an older centered flash or upset symbol onto a new AWS A2.4:2020 drawing merely because it appears on a legacy print. Confirm the governing standard, drawing revision, contract requirements, and company drafting practice first.
Flange Weld Symbols for Edge Joints
Older welding-symbol references use terms such as edge flange weld and corner flange weld. These terms remain useful when interpreting legacy prints, but current AWS A2.4:2020 practice describes welds on flanged butt and flanged corner joints through the edge weld provisions.
An edge weld can be specified on a parallel joint, a flanged butt joint, or a flanged corner joint. In current AWS practice, the dimensions that establish the formed flange itself are considered part of the component drawing rather than dimensions encoded by a separate flange-weld graphic.
Edge Flange Symbol Basics
The key distinction is between joint geometry and the weld requirement. The drawing defines the formed or flanged geometry. The welding symbol then tells the fabricator where and how the required edge weld is specified.
Current AWS arrow-side rules apply to edge weld symbols. Information placed below the reference line applies to the arrow side; information placed above applies to the other side. This differs from older flange-symbol explanations that treated the dedicated flange graphic as having no arrow-side or other-side significance.
A useful reading sequence is:
- Identify whether the joint is parallel, flanged butt, or flanged corner.
- Follow the arrow to the joint member or side being referenced.
- Determine whether the edge weld symbol is below or above the reference line.
- Read the weld size, length, pitch, contour, melt-through, and other information only if those items are actually specified.
- Check the drawing dimensions for the flange radius, formed height, gap, root condition, material thickness, and other joint geometry.
Flange Weld Dimensions
Legacy references may show the flange radius and height above the point of tangency beside a dedicated flange symbol. A typical historical notation placed radius and height to the left and separated them with a plus sign. That convention should not be presented as the normal current AWS A2.4:2020 edge-weld dimension system.
Under current AWS edge-weld rules, the edge weld size, when specified, is placed to the left of the edge weld symbol on the same side of the reference line. The dimensions needed to form the flange are drawing dimensions.
| Information | Where to Look on a Current AWS Drawing |
|---|---|
| Edge weld size | To the left of the edge weld symbol when a specific size is required |
| Weld length | To the right of the weld symbol when specified |
| Flange radius/formed height | Component or joint dimensions on the drawing/detail |
| Joint gap or other geometry | Drawing detail, dimensions, or applicable note |
| Procedure/process requirements | Tail, drawing notes, WPS, or referenced specification |
How to Read Flange Weld Dimensions
The first step is to determine which standard and revision the drawing uses. The same-looking joint can be documented differently under a legacy AWS convention, current AWS A2.4:2020, or another system such as ISO 2553.
For a current AWS drawing, read a flanged-joint edge weld in this order:
- Identify the joint geometry on the drawing. Confirm whether the members form a flanged butt or flanged corner joint.
- Follow the arrow. It establishes the referenced joint location and arrow-side member.
- Read the edge weld symbol. Its placement above or below the reference line carries side significance under current AWS conventions.
- Read the weld size. If specified, it appears to the left of the edge weld symbol.
- Read weld length or intermittent-weld information. These details appear to the right when required.
- Check the drawing for flange geometry. Do not expect the current edge weld symbol to define every bend radius, height, gap, or formed-edge dimension.
- Check supplementary symbols and notes. These can add contour, melt-through, field-weld, all-around, process, or procedure information.
Pro Tip: When a drawing looks unfamiliar, check its title block, general notes, referenced standards, and revision date before interpreting an isolated weld symbol. This often resolves whether you are looking at a legacy AWS convention, current AWS A2.4 practice, or an ISO-based system.
The current published international reference is ISO 2553:2019. ISO recognizes different symbolic representation systems, so AWS placement rules should not automatically be imposed on a drawing that explicitly invokes ISO 2553.
Surface Contour for Flash and Flange Welds
Contour requirements should be read according to the current symbol system rather than by applying an old flash-symbol rule to every drawing. AWS A2.4 includes supplementary contour symbols for requirements such as flush, flat, convex, or concave surfaces, with the applicable contour depending on weld type.
If a specific mechanical finishing method is required, a finishing designator can accompany the contour requirement. Common AWS finishing designators include methods such as grinding, machining, chipping, planishing, rolling, and hammering.
For current flash or upset process identification, remember that AWS no longer provides the former dedicated graphical FW/UW weld symbol. Surface treatment, trimming, dimensional limits, or finish requirements may therefore be communicated through notes, details, procedure references, or other applicable drawing information.
This distinction matters because a flash butt weld normally creates expelled or upset material around the joint. According to TWI guidance on flash removal, removal may be required for cosmetic, dimensional, fatigue, or service reasons and can involve shearing, grinding, machining, or trimming.
Common Uses for Flash Welds in Fabrication
Flash butt welding is an electrical resistance welding process in which components are normally positioned end-to-end. Electrical resistance generates heat at contacting points while the parts move together. After sufficient heating and burn-off, an upset or forge force consolidates the joint.
The process can be rapid and highly automated, and it normally does not require filler metal. It can also handle a wide range of section sizes and shapes when the machine, joint design, clamping, material, and procedure are suitable.
Heavy-Duty Joint Production
One of the best-documented heavy-duty applications is rail welding. Flash butt welding can join rail sections across the complete joint cross-section and is widely associated with fixed-plant and mobile rail-welding systems.
Joint quality depends on more than simply reaching welding temperature. Important variables include flashing voltage, flashing speed, burn-off, upset force, upset distance, alignment, clamp spacing, material condition, and heat balance between the parts.
For components with similar material and cross section, a symmetrical setup can help provide balanced heating. Parts with different electrical or thermal properties require procedure adjustments rather than an assumption that one standard setting will work for every combination.
Seamless Part Assembly
Flash butt welding can create a continuous joined component without adding filler metal, but describing every finished part as “seamless” can be misleading because the joint still has a weld zone and normally develops an upset or flash profile.
Documented applications include:
- Road-vehicle wheel rims formed into rings and flash welded.
- Railway rails and crossings.
- Aerospace components such as engine rings and undercarriage parts.
- Drill-pipe fittings used in oil-sector applications.
- Other rings, sections, and components whose geometry allows adequate clamping and forging.
TWI notes that flash butt welding can be applied to carbon steels, stainless steels, nickel alloys, aluminum alloys, copper alloys, and titanium, but the correct procedure depends strongly on material properties. For example, conductive materials can demand higher current or different upset conditions, while titanium may need inert shielding.
High-Speed Fabrication Runs
Flash butt welding is well suited to repetitive production because machines are commonly automatic or semi-automatic and process variables can be controlled consistently. It can therefore support high throughput when a component is designed for the process.
That does not mean every flash-welded product needs little finishing. The expelled flash or upset may remain acceptable in some applications, while other designs require hot trimming, grinding, machining, or another removal method.
Component designers also need to allow for the material length consumed during flashing and upset. Where the final overall length is critical, that burn-off allowance must be built into the starting dimensions.
When Flange Weld Symbols Need Drawing Notes
Drawing notes or detailed views are needed whenever the basic welding symbol does not communicate all information required to fabricate and inspect the joint.
For a current AWS edge weld on a flanged joint, the formed flange dimensions belong to the drawing. Notes or details may therefore define items such as bend radius, flange height, material thickness, joint gap, tolerances, local preparation, finish requirements, process restrictions, or a referenced welding procedure.
Notes are especially useful when a joint has more than two members or when the geometry is difficult to communicate with the basic symbol alone. AWS A2.4 allows drawing notes to provide weld information that does not need to be repeated in every individual welding symbol.
Note: A weld callout defines drawing intent; it does not by itself establish amperage, voltage, travel speed, upset force, inspection acceptance criteria, or every other production variable. Those requirements may come from the WPS, code, specification, contract documents, or engineering notes.
Common Mistakes in Flash Weld Symbols
The most serious mistakes come from mixing symbol editions or treating an old drafting convention as a current requirement.
- Using the legacy centered flash graphic on a new AWS A2.4:2020 drawing. Current AWS recommends FW in the tail when flash welding must be identified.
- Using UW or FW as though they define a complete procedure. They identify the process; procedure details still belong in the WPS, notes, or referenced specification.
- Assuming all drawings follow AWS. ISO 2553 and company-specific drafting systems may use different conventions.
- Ignoring the drawing revision. Legacy symbols can be valid evidence of the designer’s intent on an older controlled drawing even though the current AWS edition uses a different representation.
- Inferring missing dimensions. If size, length, joint geometry, or another requirement is not shown, do not invent it from the symbol’s appearance.
- Confusing process flash with a flange. Flash butt welding and a flanged sheet-metal joint describe different things even though both words may appear in older welding-symbol discussions.
Reading Complex Flange Weld Joints
Complex flanged joints should be read from the complete drawing rather than from one isolated symbol. Under current AWS A2.4, the edge weld symbol can apply to parallel joints, flanged butt joints, and flanged corner joints, including joints with more than two members.
When more than two members are present, first identify the members included in the joint from the drawing view or section. Then interpret the welding symbol’s arrow-side/other-side placement and any size, length, or supplementary information.
| Feature | Current AWS Reading Rule |
|---|---|
| Arrow | Points to the joint, location, or member relevant to the callout |
| Edge weld symbol position | Below the reference line = arrow side; above = other side |
| Edge weld size | Placed to the left when a specific size is required |
| Flange geometry | Defined by drawing dimensions/details rather than by the edge weld symbol itself |
| Multiple members | Confirm the included members from the joint detail and apply the edge-weld callout accordingly |
| Complete penetration with visible root reinforcement | May be specified with the applicable edge weld and melt-through convention |
For an edge weld on a flanged butt or flanged corner joint that requires complete joint penetration plus visible root reinforcement, AWS provides for the melt-through symbol on the opposite side of the reference line. Do not add this requirement simply because the joint is flanged; it must be part of the design callout.
Frequently Asked Questions
How do you interpret weld symbols?
First identify the standard referenced by the drawing. Then follow the arrow to the joint, identify the weld symbol, determine whether information applies to the arrow side or other side, read dimensions and supplementary symbols, and check the tail, drawing notes, WPS, and referenced specifications. The complete welding symbol must be interpreted together with the drawing rather than in isolation.
What is the symbol for a welded flange?
Under current AWS A2.4:2020, an edge weld on a flanged butt or flanged corner joint is specified with the edge weld symbol. The flange geometry itself is part of the drawing. Older references may show dedicated edge-flange or corner-flange graphics, so the drawing’s governing standard and revision must be checked.
What are the three welding symbols?
There is no AWS rule limiting welding to three symbols. AWS A2.4 includes major weld-symbol families for groove, fillet, plug or slot, spot or projection, seam, stud, surfacing, back or backing, and edge welds, along with supplementary symbols. “Fillet, groove, and edge” are three common examples, not the complete system.
What do 1F, 2F, 3F, and 4F mean in welding?
They are fillet-weld position designations: 1F is flat, 2F is horizontal, 3F is vertical, and 4F is overhead. The letter F identifies a fillet weld; comparable groove-weld position designations use G, such as 1G through 4G.
Are flash and upset weld symbols still used in AWS A2.4?
The old dedicated graphical symbols are not supported in AWS A2.4:2020. The standard recommends using a reference line and arrow with FW for flash welding or UW for upset welding in the tail when the process needs to be identified. Older controlled drawings may still contain the legacy graphics.
Are AWS and ISO welding symbols identical?
No. AWS A2.4 and ISO 2553 both provide systems for communicating weld requirements, but their conventions and representation systems are not identical. Always use the standard specified on the drawing instead of translating a symbol by appearance alone.
Conclusion
Flash and flanged-joint callouts are easiest to interpret when the drawing’s standard and revision are identified first. The key current AWS A2.4:2020 change is that the former graphical flash and upset symbols are no longer supported; FW and UW can be placed in the welding-symbol tail instead. Current AWS practice also uses the edge weld symbol for flanged butt and flanged corner joints, with flange geometry shown on the drawing. Recognizing older conventions remains valuable, but they should be labeled as legacy rather than taught as current rules.
Sources
- American Welding Society — AWS A2.4:2020 — current U.S. standard for welding, brazing, and nondestructive-examination symbols.
- AWS A2.4:2020 official preview — documents the 2020 revision that removed dedicated flash and upset welding symbols and recommends FW/UW tail notation.
- ISO 2553:2019 — international standard for symbolic representation of welded joints on drawings.
- TWI — What Is Flash Butt Welding? — process fundamentals, heating, flashing, and upset/forge operation.
- TWI — Materials and Applications for Flash Butt Welding — material suitability and documented applications including rails, wheel rims, aerospace parts, and drill pipe.
- Miller — Welding Positions — 1F, 2F, 3F, and 4F position definitions.
