SMAW, GMAW, GTAW, FCAW, SAW — arc physics, transfer modes, electrode classification, heat input formula, and process selection criteria for structural applications.
Arc Welding Process Fundamentals
The AWS CWI Part A exam tests knowledge of all major arc welding processes: SMAW (covered electrode), GMAW (wire + shielding gas), GTAW (non-consumable tungsten), FCAW (flux-cored wire), and SAW (submerged arc). Each process has distinct shielding mechanisms, polarity requirements, and position limitations.
Heat Input Formula: HI (J/in) = (Voltage × Amperage × 60) / Travel Speed (in/min)
Key Process Limits:
- SAW: Flat (1G) and horizontal (2G) only — granular flux cannot be retained vertically
- GMAW spray transfer: Requires >80% Ar shielding gas + current above transition threshold
- GTAW: AC preferred for aluminum (cathodic cleaning); DCEN for steel
- E6010: DCEP only; deep penetrating, fast-freezing slag — the pipe root electrode
- E7018: Low hydrogen, requires baking at 700–800°F; maximum 4-hour exposure after removal from oven
Confusing DCEP (electrode positive, more heat at electrode) vs DCEN (electrode negative, more heat at work). Arc blow affects DC welding of ferromagnetic materials — switch to AC or reposition work lead to correct.
AWS A5.1 (SMAW electrodes); AWS A5.18 (GMAW solid wire); AWS A5.20 (FCAW); AWS A5.9 (GTAW filler metals); AWS A5.17 (SAW).
Iron-carbon phase diagram, TTT/CCT curves, HAZ formation, preheat requirements, PWHT, and the metallurgical basis of common weld failures.
Metallurgy for Welding Inspectors
The heat-affected zone (HAZ) is the base metal adjacent to the fusion line that experienced temperatures high enough to alter its microstructure without melting. HAZ grain growth reduces toughness. The coarse-grained HAZ (CGHAZ) is the most susceptible region for hydrogen cracking and grain boundary embrittlement.
Carbon Equivalent (CE): CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
- CE < 0.40: Generally no preheat required
- CE 0.40–0.60: Preheat required; calculate per D1.1 Annex I
- CE > 0.60: Special procedures required
Critical Temperatures:
- A1 (eutectoid): 1333°F (723°C)
- A3 (upper critical): 1670°F (910°C) for pure iron
- Martensite start (Ms): Depends on alloy content
Hydrogen Cracking (HAC) Prerequisites — all four must be present:
1. Susceptible microstructure (hard martensite)
2. Hydrogen source (moisture, organic compounds)
3. Tensile stress (residual or applied)
4. Low temperature
Failing to recognize that hydrogen cracking can be delayed — cracks may not appear until 24–72 hours after welding. Do not release hold-points immediately after cool-down on high-carbon or alloy steels.
AWS A2.4 welding symbols, joint types, groove preparations, effective throat calculations, and weld sizing terminology.
Reading and Applying Weld Symbols
AWS A2.4 weld symbols use a reference line with an arrow side (below the line) and other side (above the line). The symbol's position relative to the line indicates which side of the joint receives the weld. A weld-all-around circle indicates a continuous weld around the entire joint perimeter.
Five Basic Joint Types: Butt, Corner, T-joint, Lap, Edge
Effective Throat vs Weld Size:
- Fillet weld: Effective throat = weld size × 0.707 (for equal legs)
- CJP groove: Effective throat = full plate thickness
- PJP groove: Effective throat = groove depth minus reduction (per code)
Symbol Reading Order (left to right on reference line):
weld size | groove type | depth | length-pitch | contour | finish
Confusing the arrow-side rule: the arrow points toward the member that receives the groove preparation in single-bevel and single-J joints (the arrow-side member is prepared). For symmetrical grooves, either member may be prepared.
Classification, causes, and dimensional limits for porosity, slag inclusion, lack of fusion, undercut, overlap, cracks, and lamellar tearing.
Discontinuity Identification and Root Causes
Discontinuities are deviations from the ideal weld. Not all discontinuities are defects — a defect is a discontinuity that exceeds the acceptance criteria of the applicable code. Understanding root causes allows an inspector to identify systemic problems before they create widespread rejection.
Common Discontinuities and Primary Causes:
| Discontinuity | Primary Cause | Detection Method |
|---|---|---|
| Porosity | Contamination, moisture, shielding gas loss | VT, RT, UT |
| Slag inclusion | Incomplete slag removal between passes | RT, UT |
| Lack of fusion (LOF) | Insufficient heat, wrong angle, fast travel | UT, RT |
| Undercut | Excessive current, wrong angle | VT, MT |
| Overlap | Low current, slow travel, wrong angle | VT |
| Hot crack | High restraint, sulfur, high heat input | VT, PT, MT |
| Hydrogen (cold) crack | CE, moisture, fast cooling | MT, UT (delayed) |
| Lamellar tearing | Thru-thickness stress, inclusion bands | UT |
Crack Terminology: Longitudinal, transverse, crater, toe, root, underbead, heat-affected zone
Transverse cracks are always more serious than longitudinal cracks — they are oriented perpendicular to the primary stress direction and can propagate across the full weld cross-section. Always escalate transverse crack findings immediately.
Pre-weld, in-process, and post-weld inspection duties; document review; hold points; and the professional obligations of a CWI.
The CWI's Three-Phase Inspection Cycle
A Certified Welding Inspector's duties span three phases: pre-weld verification, in-process monitoring, and post-weld final inspection. Missing any phase compromises the integrity of the record and the structure.
Pre-Weld Checklist:
- Verify WPS is approved and applicable
- Confirm welder qualification (process, position, material, thickness)
- Check base metal ID vs WPS
- Verify preheat compliance (measure with temp sticks or thermocouple)
- Confirm joint preparation meets code (bevel angle, root opening, root face)
- Check filler metal storage (low hydrogen electrodes: controlled storage, exposure time)
In-Process Checklist:
- Monitor interpass temperature
- Verify pass sequence per WPS
- Inspect slag removal between passes
- Check bead width and profile
- Verify voltage, amperage, travel speed are within WPS range
Post-Weld Checklist:
- Visual inspection per code acceptance criteria
- Dimensional verification (size, length, location)
- NDT as required by code and contract
- Document all findings
WPS/PQR/WPQ documentation, quality control systems, nonconformance reporting, and the document hierarchy for structural welding.
WPS, PQR, and WPQ — The Three Documents
The three foundational documents in welding quality control form a hierarchy: the PQR proves a process works, the WPS specifies how to apply it, and the WPQ certifies the welder can execute it.
Document Hierarchy:
PQR (Procedure Qualification Record): The tested record. Records actual welding variables used during procedure qualification testing and test results. Cannot be revised — it is a factual record of what was done and what the specimens showed.
WPS (Welding Procedure Specification): The instruction document. References the supporting PQR(s). Can be revised within the essential variables permitted by code without re-qualification.
WPQ (Welder Performance Qualification): Certifies an individual welder's skill. Process-specific, position-specific, and diameter/thickness range specific. Expires if the welder has not welded with the process in the prior 6 months (per D1.1).
Essential Variables vs Supplemental Essential Variables:
- Essential: Change requires new PQR
- Supplemental Essential (D1.1): Required when CVN toughness testing is specified
- Nonessential: Change allowed with WPS revision only
AWS D1.1 scope and organization; relationship between codes, standards, and specifications; and the CWI's obligation to the applicable document set.
Navigating AWS D1.1 and the Standards Ecosystem
AWS D1.1 Structural Welding Code — Steel is the primary document for structural steel fabrication and erection. It is referenced by IBC and AISC 360. Understanding the code's organization is as important as knowing its content — an inspector who can locate requirements quickly is more effective than one who has memorized isolated rules.
AWS D1.1 (2020) Clause Organization:
- Clause 1: General Requirements
- Clause 2: Design of Welded Connections
- Clause 3: Prequalification of WPSs
- Clause 4: Qualification (procedure and performance)
- Clause 5: Fabrication
- Clause 6: Inspection
- Clause 7: Stud Welding
- Clause 8: Strengthening and Repairing
- Annexes A–P: Supplemental information
Code vs Standard vs Specification:
- Code: Mandatory requirements adopted by reference into law (IBC adopts D1.1)
- Standard: Consensus document with requirements (may or may not be mandatory by reference)
- Specification: Material/product requirements (ASTM A36, ASTM F3125)
AWS D1.1; AWS D1.2 (Aluminum); AWS D1.3 (Sheet Steel); AWS D1.5 (Bridge); AWS D1.6 (Stainless); AASHTO/AWS D1.5M; ASME Section IX (pressure).
VT, MT, PT, RT, UT — capabilities, limitations, applicable standards, and acceptance criteria for each method.
NDT Method Selection and Application
AWS D1.1 requires visual testing (VT) on all welds and specifies supplemental NDT requirements based on connection type and engineer designation. Each method has distinct capabilities that make it suited for specific discontinuity types.
NDT Method Comparison:
| Method | Best For | Limitations | Standard |
|---|---|---|---|
| VT | Surface discontinuities, dimensional | Surface only; requires good lighting | AWS D1.1 Cl. 6 |
| MT | Surface and near-surface in ferromagnetic metals | Ferromagnetic only; surface prep needed | ASTM E709 |
| PT | Surface discontinuities in non-porous materials | Surface only; no depth info | ASTM E165 |
| RT | Volumetric flaws — porosity, slag | Cannot detect fine cracks, LOF parallel to beam | ASTM E94 / E1032 |
| UT | Volumetric and planar flaws; thickness | Operator skill dependent; surface condition | AWS D1.1 Annex K |
Geometric Unsharpness (Ug) in RT: Ug = Fd/D where F = source-to-object distance, d = source size, D = object-to-film distance. Maximum Ug per code.
Tension, bend, impact (CVN), hardness, and macroetch tests used in procedure and performance qualification.
Qualification Test Specimens
Procedure qualification testing (PQR) requires mechanical testing to confirm that the welded joint meets the mechanical property requirements of the applicable code. Performance qualification (WPQ) requires bend tests to confirm the welder's ability to produce sound welds.
PQR Required Tests (D1.1):
- Reduced-section tension (2 specimens): Meets or exceeds base metal UTS
- Root bend and face bend (2 each) OR side bend (4): No open discontinuity > 1/8"
- CVN impact if required by engineer (lateral expansion + energy absorbed)
- Macro-etch for ESW/EGW processes
Charpy V-Notch (CVN) Test:
- Tests notch toughness and ductile-to-brittle transition temperature
- Results: Energy absorbed (ft-lb or J) and lateral expansion (mils)
- Test temperature specified by engineer or owner
- Supplemental essential variable in D1.1 when CVN is required
Bend Test Acceptance: Maximum 1/8" (3 mm) open discontinuity in any direction. Corner cracks up to 1/4" acceptable if no slag inclusion is visible.
Essential and nonessential variables, qualification ranges, position groups, and expiration/renewal requirements under AWS D1.1.
Qualification Scope and Ranges
A WPS qualifies within a range of essential variables. A welder qualifies within a specific process, position group, and material/thickness range. The inspector must verify that the work being performed is covered by current, applicable qualification documents.
Welder Qualification Position Groups (D1.1):
- 1G/1F (flat) qualifies: 1G/1F only
- 2G (horizontal groove) qualifies: 1G, 2G
- 3G (vertical groove, uphill) qualifies: 1G, 2G, 3G, 3F
- 4G (overhead groove) qualifies: 1G, 4G, 4F
- 3G + 4G qualifies: All positions
- 6G (pipe, 45°) qualifies: All groove positions for pipe
WPS Essential Variables (D1.1 Table 4.5 — SMAW):
- Change in F-number grouping of filler metal
- Change in A-number (weld deposit chemistry)
- Increase in base metal thickness beyond qualified range
- Change in position outside qualified group
- Change from PWHT to no PWHT or vice versa
- Preheat: More than 100°F (55°C) decrease
Welder Continuity: 6-month maximum between uses of process to maintain qualification (D1.1 Cl. 4.26).