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NCCER Boilermaker

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6.1 AWS Welding Symbols, Joint Geometry, Contours & NDE Designators

Key Takeaways

  • AWS A2.4 standardizes welding symbol architecture comprising the reference line, arrow line, and tail, where the reference line provides the baseline and multiple stacked lines establish chronological operational sequences read from bottom to top.
  • Arrow side significance dictates that symbols placed below the reference line apply to the joint side pointed to by the arrow, whereas symbols placed above the reference line apply to the opposite (other) side.
  • A sharp break (crank) in the arrow line explicitly identifies the specific joint member that requires edge preparation such as beveling or J-grooving, while the unpointed member remains square.
  • Weld dimensions follow strict spatial rules: weld size is placed to the left of the symbol, depth of bevel preparation (S) and effective throat (E) are formatted as S(E), length and center-to-center pitch appear to the right as 'Length - Pitch', and root opening and groove angle are placed inside the symbol.
  • Supplementary symbols designate vital fabrication requirements: a circle at the reference-arrow junction indicates weld-all-around, a flag pointing toward the tail indicates a field weld, and NDE letter codes (RT, UT, MT, PT, VT) on reference lines mandate specific inspection methods.
Last updated: August 2026

AWS Welding Symbols, Joint Geometry, Contours & NDE Designators

Core Concept: Industrial boiler and pressure vessel blueprints rely on the standardized symbolic language established by the American Welding Society (AWS) A2.4: Standard Symbols for Welding, Brazing, and Nondestructive Examination. Boilermakers must interpret welding symbols with absolute precision to determine exact joint preparations, weld types, root dimensions, effective throat thicknesses, sequencing, finishing methods, and quality assurance testing requirements.

1. AWS A2.4 Standard Symbol Architecture

A complete AWS welding symbol consists of several functional elements arranged in standardized spatial relationships around a horizontal baseline.

                                Finish Symbol
                                Contour Symbol
                         Groove Angle; Included Angle
                 Root Opening; Depth of Filling
        Groove Weld Size (S) --+    +-- Length of Weld
        Effective Throat (E) --|    |-- Pitch (Center-to-Center Spacing)
        Depth of Preparation --|    |-- Field Weld Symbol (Flag)
                               |    |
       Specification, Process, |    |   /-- Weld-All-Around Symbol (Circle)
       or other Reference      |    |  /
               \               v    v v
                +-------------+======+-------------+--------> Arrow Line
                |     TAIL    |      |  REFERENCE  |          (Points to Joint)
                +-------------+======+-------------+
                               /    /
              Weld Symbol ----+    /
              (Arrow Side)        /
                                 /
              Multiple Reference Lines Read Bottom-to-Top

Primary Structural Components

  1. Reference Line (Horizontal Baseline): The foundational anchor of every welding symbol. All dimensional data, weld profiles, supplementary icons, and inspection codes are positioned relative to this horizontal line. Standardized orientation is always horizontal on the drawing sheet.
  2. Arrow Line: Extends from one end of the reference line and points directly to the joint or seam where the weld is to be executed. The arrow may touch plate surfaces, joint roots, or centerline axes.
  3. The Tail: An optional V-shaped enclosure attached to the opposite end of the reference line. The tail contains specific technical instructions, such as:
    • Welding Procedure Specification (WPS) numbers (e.g., WPS-BM-104).
    • Welding process abbreviations (e.g., SMAW, GTAW, SAW, FCAW).
    • Filler metal classifications or trade codes (e.g., E7018-H4R, ER70S-6, E309L-16).
    • Rule: If no specific process, code, or specification reference is required, the tail is omitted from the symbol.
                     +---------------------------------------------+
                     |         ARROW SIDE VS. OTHER SIDE RULES     |
                     +---------------------------------------------+
                                            |
               +----------------------------+----------------------------+
               |                                                         |
               v                                                         v
+-------------------------------+                         +-------------------------------+
|    ARROW SIDE (BELOW LINE)    |                         |    OTHER SIDE (ABOVE LINE)    |
+-------------------------------+                         +-------------------------------+
| * Symbols placed BELOW the    |                         | * Symbols placed ABOVE the    |
|   horizontal reference line.  |                         |   horizontal reference line.  |
| * Weld is deposited directly  |                         | * Weld is deposited on the    |
|   on the side of the joint to |                         |   opposite side of the joint  |
|   which the arrow points.     |                         |   away from the arrow point.  |
+-------------------------------+                         +-------------------------------+

Both Sides & No Side Significance

  • Both Sides: When weld symbols appear simultaneously above and below the reference line (such as a double-fillet or double-V groove), welding is performed on both sides of the joint.
  • No Side Significance: Certain symbols are centered directly across the reference line (intersecting the line), indicating that the operation has no side significance (e.g., resistance spot welds, flash welds, or symmetrical stud welding).

The Broken Arrow (Member Preparation Priority)

When a single-bevel groove, single-J groove, or flare-bevel groove weld requires only one specific member of the joint to be beveled or prepared, the arrow line features a sharp, intentional break (a crank or dogleg). The broken arrow always points directly toward the specific plate, nozzle neck, or forging that must receive the edge preparation, leaving the mating member with a square edge.

   BROKEN ARROW (Bevel Left Plate Only)          STRAIGHT ARROW (No Preference)

          |             |                               |             |
          | Beveled     | Square                        | Plate A     | Plate B
          | Member      | Member                        |             |
          |\            |                               |             |
          | \           |                               |             |
          |  \          |                               |             |
          +---+         +---+                           +---+         +---+
              ^                                             ^
               \   <-- Arrow breaks                          \   <-- Straight arrow
                \      toward beveled                         \
                 \     component                               \
           +------+                                      +------+
           |                                             |

Multiple Reference Lines (Operational Sequence)

Complex pressure vessel joints often require sequential operations: initial root welding, back-gouging, secondary groove completion, post-weld examination, and surface cladding. AWS A2.4 utilizes multiple stacked reference lines attached to a single arrow to establish strict chronological order:

Execution Order: First Operation (Bottom Line)⟶Second Operation (Middle Line)⟶Final Operation (Top Line)\text{Execution Order: } \text{First Operation (Bottom Line)} \longrightarrow \text{Second Operation (Middle Line)} \longrightarrow \text{Final Operation (Top Line)}

  1. Bottom Line (Closest to Arrow): First operational step (e.g., root pass GTAW single-V groove with ceramic backing).
  2. Middle Line: Second operational step (e.g., back gouge arrow side to sound metal and deposit back weld SMAW).
  3. Top Line: Final operational step (e.g., 100% Radiographic Examination RT 100%).

2. Basic Weld Symbols & Joint Geometry

Boilermakers encounter standard AWS weld symbols that represent physical joint cross-sections.

Weld TypeAWS Symbol GeometryJoint Description & Trade Application
Fillet WeldRight triangle with vertical leg always on the leftLap joints, T-joints, corner joints, nozzle reinforcing pads, tray support ring attachments.
Square GrooveTwo parallel vertical lines ($
V-GrooveV-shape ($V$)Single or double-V groove butt joints in boiler tubes and shell plates ($1/4\text{ to }3/4\text{ inch}$ thick).
Bevel GrooveVertical line on left, angled line on right ($/$)
U-GrooveCurved U-profile with flat bottom ($U$)Heavy-wall pressure vessel longitudinal and circumferential seams ($> 3/4\text{ inch}$) to minimize filler metal volume.
J-GrooveVertical line on left, curved J on right ($\supset$)
Flare-V GrooveTwo curved arcs facing each other ($()$)Formed flanged heads or two adjacent rounded tubes in waterwall panels.
Flare-Bevel GrooveVertical line on left, curved arc on right ($)$)
Plug or Slot WeldRectangle ($\sqcap$ or $\sqcup$)Holes or slots in overlapping plates filled with weld metal to transfer shear load.
Surfacing / OverlayTwo adjacent half-circles / arcs below line ($uu$)Corrosion-resistant alloy cladding (e.g., Inconel 625 overlay on carbon steel tube sheets).
Backing / Back WeldSemicircle on side opposite groove symbolBacking weld: Deposited before groove weld is placed. Back weld: Deposited on root side after back-gouging groove weld.
                               BASIC WELD SYMBOLS SUMMARY

   Fillet         Square Groove     V-Groove        Bevel Groove       U-Groove
     |\                | |             \ /               | /              \_/
     | \               | |              v                |/              (   )
   -------           -------         -------          -------          ---------

    J-Groove       Flare-V          Flare-Bevel     Surfacing          Back/Backing
     |                 ( )              | )                               ---
     |_)              (   )             |)              ( ( (            (   )
   -------           -------          -------         ---------        ---------

3. Weld Dimensioning Conventions

Dimensional callouts on welding symbols adhere to rigid spatial rules defined by AWS A2.4. Misinterpreting the location of a number can lead to structural failure or non-compliance with ASME Section VIII / Section I codes.

+-----------------------------------------------------------------------------------------+
|                         AWS A2.4 DIMENSION PLACEMENT RULES                              |
+-----------------------------------------------------------------------------------------+
| 1. LEFT OF SYMBOL   -> Weld Size, Depth of Bevel Preparation (S), Effective Throat (E) |
| 2. INSIDE SYMBOL    -> Root Opening (R), Included Groove Angle (degree)                |
| 3. RIGHT OF SYMBOL  -> Length of Weld, Pitch (Center-to-Center Spacing of Segments)    |
+-----------------------------------------------------------------------------------------+

Fillet Weld Dimensioning

  • Equal Leg Fillet: A single dimension placed to the left of the fillet symbol indicates equal leg size (e.g., 3/8 specifies a $3/8\text{-inch}$ vertical leg and a $3/8\text{-inch}$ horizontal leg).
  • Unequal Leg Fillet: Two dimensions in parentheses or separated by an 'x' placed to the left (e.g., (1/4 x 1/2) specifies one leg length of $1/4\text{ inch}$ and the other leg length of $1/2\text{ inch}$). The drawing detail clarifies which member receives the longer leg.

Groove Weld Dimensioning: Depth of Preparation (S) vs. Effective Throat (E)

Groove weld sizes distinguish between the depth of cut machined into the plate and the actual structural throat thickness achieved by the weld metal:

Format: S(E)⟹Depth of Bevel Preparation S (Effective Weld Throat Size E)\text{Format: } S(E) \quad \Longrightarrow \quad \text{Depth of Bevel Preparation } S \text{ (Effective Weld Throat Size } E\text{)}

  • $S$ (Without Parentheses): The depth of groove preparation (the depth of the machined bevel or J-groove).
  • $(E)$ (Enclosed in Parentheses): The effective throat—the total depth of joint penetration from the outer weld surface to the deepest point of root fusion.
  • Example: 1/2 (5/8) to the left of a single-V groove symbol indicates that the plate is beveled to a depth of $1/2\text{ inch}$, but the welding procedure must achieve an effective throat penetration of $5/8\text{ inch}$ (penetrating $1/8\text{ inch}$ past the root land).
               GROOVE DIMENSIONS: S(E)           INTERMITTENT WELDS: L - P

                  Included Angle: 60 deg
                       Root Opening: 1/8"
                                                    Length: 3"   Pitch: 8"
                    1/2 (5/8)  \ 60 deg /                 |\    3 - 8
                              --\-1/8--/-                 | \  
                     ^    ^         v                   --+--\---------
                     |    |                               Arrow Side
         Depth of Prep    Effective Throat Size

Length and Pitch (Intermittent Fillet Welds)

Intermittent welds are segmented welds separated by unwelded spaces, used on structural stiffeners, boiler casing buckstays, and tank baffle plates.

  • Format: Length - Pitch placed immediately to the right of the weld symbol (e.g., 3 - 8).
    • Length ($L$): The linear length of each individual weld segment (e.g., $3\text{ inches}$).
    • Pitch ($P$): The center-to-center distance between adjacent weld segments (e.g., $8\text{ inches}$). It is NEVER the clear space between welds.
    • Clear Space Between Segments=Pitch−Length=8"−3"=5"\text{Clear Space Between Segments} = \text{Pitch} - \text{Length} = 8" - 3" = 5"
  • Chain Intermittent: Welds on both sides of the joint are directly opposite each other. The fillet symbols on the arrow side and other side are aligned directly across the reference line.
  • Staggered Intermittent: Welds on one side alternate with welds on the opposite side. The fillet symbols on the arrow side and other side are offset / staggered along the reference line.
   CHAIN INTERMITTENT FILLET WELD              STAGGERED INTERMITTENT FILLET WELD

         |\    3 - 8 (Other Side)                    |\    3 - 8 (Other Side)
         | \                                         | \  
   ------+--\-----------------                 ------+--\-----------------
         |  /                                              |  /
         | /   3 - 8 (Arrow Side)                          | /   3 - 8 (Arrow Side)

   [=== 3" ===]       [=== 3" ===]             [=== 3" ===]       [=== 3" ===]
   -------------------------------             -------------------------------
   [=== 3" ===]       [=== 3" ===]                    [=== 3" ===]       [=== 3" ===]
   <--- 8" --->                                <--- 8" --->

4. Supplementary Symbols, Contours & Finish Designators

Supplementary symbols modify the baseline weld symbol to convey critical job-site installation, geometric shape, and surface finishing instructions.

+-----------------------------------------------------------------------------------------+
|                              AWS SUPPLEMENTARY SYMBOLS                                  |
+-----------------------------------------------------------------------------------------+
| * WELD-ALL-AROUND  : A circle at the intersection of the reference line and arrow line. |
|                      Specifies continuous welding around the entire perimeter 360 deg.  |
| * FIELD WELD       : A flag at the junction, pointing TOWARD THE TAIL (away from arrow).|
|                      Specifies weld is made at the erection site, not in the shop.      |
| * MELT-THROUGH     : A filled, solid half-circle on the side OPPOSITE the groove symbol.|
|                      Indicates 100% complete joint penetration with root bead reinforcement.|
| * BACKING STRIP    : A rectangle placed on the side opposite the groove symbol.         |
|                      An 'R' placed inside indicates backing strip must be removed later.|
| * SPACER STRIP     : A rectangle centered across the reference line inside a groove.    |
+-----------------------------------------------------------------------------------------+

Surface Contour and Finish Methods

Welds on boiler drums, steam piping, and pressure vessels must frequently be contoured to prevent stress risers or allow ultrasonic examination:

  • Flush / Flat Contour: A straight horizontal line placed above or below the weld symbol.
  • Convex Contour: An outward-curved arc.
  • Concave Contour: An inward-curved arc (common on cosmetic or fatigue-resistant fillet welds).
  • Finish Method Letter Designators: Placed adjacent to the contour symbol to specify the mechanical finishing process:
    • G: Grinding (e.g., grind flush for ultrasonic testing).
    • M: Machining (e.g., lathe or mill facing).
    • C: Chipping (pneumatic chipping hammer).
    • R: Rolling.
    • H: Hammering / Peening.
    • U: Unspecified mechanical method.
              FLUSH CONTOUR (Grind)              CONVEX CONTOUR (Machine)

                     G  <-- Finish Method               M
                   ----- <-- Flush Symbol             (---) <-- Convex Symbol
                    \ /                                \ /
                   --v--                              --v--

5. Nondestructive Examination (NDE) Symbols

AWS A2.4 governs symbols for Nondestructive Examination (NDE), utilizing standardized letter codes positioned on reference lines identical to welding symbols.

NDE MethodLetter CodeIndustrial Boilermaker Application
Visual ExaminationVTBaseline inspection of fit-up, root pass fusion, undercut, and weld reinforcement profile.
Liquid Penetrant ExaminationPTDetection of surface-breaking flaws, root pass hot cracks, and non-magnetic stainless steel welds.
Magnetic Particle ExaminationMTDetection of surface and slightly subsurface cracks in ferromagnetic carbon steel shells and nozzles.
Radiographic ExaminationRTVolumetric examination (X-ray / Gamma-ray) of full penetration butt welds in drums, pipes, and headers.
Ultrasonic ExaminationUTVolumetric high-frequency sound wave flaw detection and Phased Array (PAUT) of heavy-wall vessels.
Leak TestingLTHydrostatic, pneumatic, or bubble-leak testing of vessel shell courses and tubesheet roll joints.
Electromagnetic / Eddy CurrentETHeat exchanger and boiler tube internal wall thinning and pitting inspection.
+-----------------------------------------------------------------------------------------+
|                            NDE SYMBOL PLACEMENT CONVENTIONS                             |
+-----------------------------------------------------------------------------------------+
| 1. ARROW SIDE   : Code below reference line -> Test performed on arrow side surface.     |
| 2. OTHER SIDE   : Code above reference line -> Test performed on other side surface.     |
| 3. BOTH SIDES   : Codes above and below line -> Test performed on both plate surfaces.   |
| 4. NO SIDE / ALL: Code centered on reference line -> Volumetric method (RT/UT) examining |
|                   the entire cross-section of the weld joint.                           |
| 5. EXTENT (%)   : Percentage (e.g., 'RT 100%' or 'UT 25%') placed to the right of code. |
| 6. SPOT COUNT   : Number of spot examinations in brackets (e.g., 'RT [2]') or length.   |
+-----------------------------------------------------------------------------------------+

6. Realistic Trade Scenario: Downcomer Nozzle Welding & NDE Callout

+-----------------------------------------------------------------------------------------+
|                                 FIELD TRADE SCENARIO                                    |
+-----------------------------------------------------------------------------------------+
| Blueprint Detail: Downcomer Nozzle (N1) to Boiler Steam Drum Shell Course               |
| Joint Callout: Multi-tier reference line attached to an arrow pointing to the nozzle.  |
|                                                                                         |
| Symbol Interpretation & Field Execution Plan:                                           |
| 1. Lowest Reference Line (Step 1): Single-bevel groove symbol on Arrow Side with a      |
|    broken arrow pointing to the vessel shell plate; dimensions read '1-1/2 (1-3/4)',    |
|    root opening '1/8"', groove angle '45 deg', and tail reads 'WPS-STM-401 (GTAW/SMAW)'.|
|    -> Action: Machine a 45-degree bevel 1-1/2" deep on the drum shell cutout (broken   |
|       arrow member). Set a 1/8" root gap. Deposit GTAW root and hot pass, followed by   |
|       SMAW fill passes achieving 1-3/4" effective throat fusion.                        |
| 2. Middle Reference Line (Step 2): Fillet weld symbol on Other Side (inside drum) with  |
|    dimension '1/2"' and flush contour 'G'.                                              |
|    -> Action: Boilermaker inside drum deposits a 1/2" reinforcing fillet weld on the    |
|       inside shell-to-nozzle junction and grinds it smooth/flush to prevent flow drag.  |
| 3. Top Reference Line (Step 3): Centered NDE code 'RT 100%' and Arrow Side 'MT'.        |
|    -> Action: Perform 100% volumetric radiographic testing across full nozzle weld      |
|       thickness, followed by magnetic particle testing on the exterior weld surface.    |
+-----------------------------------------------------------------------------------------+
Test Your Knowledge

An AWS welding symbol displays an arrow-side and other-side fillet weld with the notation '3/8 3 - 8' on both sides in a staggered configuration. What does the notation '3 - 8' specify to the boilermaker?

A
B
C
D
Test Your Knowledge

When reading an engineering blueprint for a pressure vessel nozzle installation, the welding symbol features a sharp break (crank) in the arrow line pointing directly toward the vessel shell plate rather than the nozzle forging. What does this broken arrow signify?

A
B
C
D
Test Your Knowledge

On an AWS groove welding symbol for a thick-walled boiler header, what is the precise distinction between the dimensions formatted as '1/2 (5/8)' located to the left of the groove symbol?

A
B
C
D
Test Your Knowledge

A blueprint detail for a high-pressure steam line joint displays a multi-tier reference line. The lower reference line specifies a single-V groove weld with backing, and the upper reference line contains the centered designation 'RT 100%'. What operational sequence must the craftsperson follow?

A
B
C
D
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6.2 Piping Schematics, P&IDs, Isometric Drawings & Spool Sheets

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