Welding processes, symbols, joint types, distortion control, and inspection explained for the Red Seal Millwright exam — what millwrights need to know, not what welders need to know.
Welding is a core part of the millwright trade. Many millwrights hold CWB tickets, carry a Red Seal in welding, or came into the trade through a welding background — and SMAW in particular is everyday work on most industrial sites.
Whether you weld daily or only occasionally, the Red Seal exam tests welding knowledge across process selection, weld symbol interpretation, joint preparation, distortion control, inspection, and safety. This guide covers what the NOA expects — the knowledge that shows up on exam day.
The most common manual welding process in industrial maintenance. An electrode (rod) is consumed as it deposits weld metal — the flux coating on the electrode produces shielding gas and slag to protect the weld pool.
Key characteristics:
Electrode classification (AWS E-numbering):
E7018 — the most common structural electrode. Low-hydrogen, all-position, 70,000 psi tensile strength.
Breaking down the designation:
AWS Electrode Designation
Tap any character to decode it
Minimum tensile strength of the deposited weld metal, in thousands of psi. 70 = 70,000 psi.
Position Legend
E7018 — position digit 1
All-position electrode. This is why it's the standard choice for structural and maintenance welding where position varies.
E7024 — position digit 2
Flat and horizontal only. High iron powder content gives a high deposition rate, but only suitable for flat/horizontal work.
E7048 — position digit 4
Similar to E7018 but specifically designed to run vertical-down in addition to the standard positions.
EXAM TIP
The position digit tells you where you can run the electrode. If a question describes overhead or vertical welding, the electrode must have a 1 in the third position. A 2 electrode used out-of-position will produce poor fusion and unacceptable weld quality.
EXAM TIP — STORAGE
E7018 is low-hydrogen — it requires dry, heated storage in a rod oven at 120–150°C. Once removed, electrodes must be used within 4 hours — after that they must be reconditioned. Reconditioning means baking at 230–260°C for a minimum of 2 hours, then returning immediately to a holding oven or using right away. Moisture in low-hydrogen electrodes causes porosity and hydrogen-induced cracking (HICC).
A continuous wire electrode feeds from a spool through a gun, shielded by an externally supplied gas. Higher deposition rate than SMAW, easier to learn, well-suited to production work.
Shielding gases:
100% CO₂
Deeper penetration, more spatter, lower cost
75% Argon / 25% CO₂ (C25)
Smoother arc, less spatter, better appearance — most common in industrial applications
100% Argon
Used for non-ferrous metals (aluminum, copper)
Transfer modes:
Short circuit
Low voltage/amperage, small wire, thin material. Wire touches pool and short circuits, deposits metal. Low heat input.
Globular
Intermediate settings, large irregular droplets, high spatter. Generally avoided.
Spray
High voltage/amperage, fine droplets, smooth arc, high deposition. Flat and horizontal positions only.
Pulse
Controlled spray at low average amperage. Better for thin material and out-of-position welding.
EXAM TIP
Short circuit transfer is used for thin material and out-of-position work. Spray transfer is flat/horizontal only but gives the best quality on thicker material.
Similar to GMAW but uses a tubular wire with flux inside the core. Two types:
Gas-shielded (FCAW-G)
Requires external shielding gas. Better quality, indoor use.
Self-shielded (FCAW-S)
No external gas needed. More portable, works outdoors in wind. Lower quality than gas-shielded.
Higher deposition rate than SMAW, produces slag like SMAW. Common in structural and heavy fabrication.
A non-consumable tungsten electrode creates the arc; filler metal is added separately by hand. Shielded by inert gas (argon or helium).
Key characteristics:
Current type:
DCEN (DC Electrode Negative)
Used for steel and stainless. Deep penetration.
AC
Used for aluminum. The AC arc cleans the oxide layer on aluminum during the EP (electrode positive) half-cycle.
EXAM TIP
TIG on aluminum requires AC current because aluminum oxide has a higher melting point than the base metal — the cleaning action of AC breaks the oxide layer so fusion can occur.
Combustion of acetylene and oxygen produces a flame hot enough to weld or cut steel.
Flame types:
Neutral
Equal oxygen and acetylene. Used for most welding. Identified by a clearly defined inner cone with no feather.
Carburizing (reducing)
Excess acetylene. Has a feathery secondary cone. Used for hardfacing, some cast iron work.
Oxidizing
Excess oxygen. Shorter, harsher-sounding. Used for some brass/bronze welding. Damages steel — avoid on carbon steel.
Cutting
Oxygen cutting works on carbon steel because iron burns in the oxygen stream after being preheated to ignition temperature. Does not work on stainless steel or aluminum.
EXAM TIP
Know how to identify the three flame types and which application each is used for. The oxidizing flame on carbon steel is a common wrong-answer trap.
Five basic weld joint types — the exam expects you to identify them and know which weld type is appropriate for each.
| Joint Type | Description | Common Weld |
|---|---|---|
| Butt joint | Two pieces edge-to-edge in the same plane | Groove weld |
| T-joint | One piece perpendicular to another | Fillet weld |
| Corner joint | Two pieces meeting at a corner | Fillet or groove weld |
| Lap joint | Two pieces overlapping | Fillet weld |
| Edge joint | Edges of two pieces side by side | Edge weld |
Fillet Weld
Triangular cross-section, used in T-joints and lap joints. Described by leg size. Does not require joint preparation (no bevelling). Most common weld type in structural and maintenance welding.
Groove Weld
Deposited in a groove between two pieces. Used where full penetration or high strength is required. Requires joint preparation (bevelling, spacing).
Plug and Slot Welds
Weld through a hole or slot in one piece into another. Used where fillet welds are not accessible.
Groove preparations:
Weld symbols on engineering drawings follow AWS A2.4 standards. The exam will ask you to read basic weld symbols.
The reference line is horizontal. Everything below the reference line refers to the arrow side of the joint. Everything above refers to the other side.
Weld Symbol Decoder
Tap each part of the symbol to decode it
Fillet Weld Symbol
The triangle indicates a fillet weld. Placed below the line = weld on the arrow side.
Arrow side vs. other side: Symbol below the line = weld on the arrow side. Symbol above = weld on the other side. This is the most frequently tested weld symbol concept.
Basic symbol elements:
Fillet weld symbol
Triangle. Size is the leg dimension. ¼ to the left of the triangle = ¼" fillet weld.
Groove weld symbol
Varies by preparation type (V, bevel, J, U, etc.).
Flag on reference line
Field weld — to be done in the field, not in the shop.
Circle at arrow junction
Weld all around.
EXAM TIP
Arrow-side vs. other-side is the most frequently tested weld symbol concept. The symbol below the line = weld on the side the arrow points to. Symbol above = weld on the opposite side.
Proper joint preparation is critical to weld quality and is tested on the exam.
Fit-up
Joint gaps must be consistent and within specification. Too tight — lack of penetration. Too wide — burn-through, excessive distortion.
Root opening (root gap)
The space between pieces at the bottom of a groove joint. Allows penetration to the root.
Root face (land)
A small flat area at the bottom of a bevel. Prevents burn-through and supports the root pass.
Bevel angle
The angle of the bevel cut. Included angle = both bevels combined. A 60° included angle = 30° bevel per side on a V-groove.
Cleanliness
Remove mill scale, rust, paint, oil, and moisture from the weld zone. Contaminants cause porosity, inclusions, and cracking.
Preheat
Required for high-carbon or alloy steels to prevent hydrogen-induced cracking. In Canada, preheat requirements are governed by CSA W59 and are determined by the steel's carbon equivalent (CEV) and the combined thickness of the joint. The exam tests when preheat is required and why — not the full specification table. Thicker sections, higher carbon content, and cold ambient temperatures all increase the need for preheat.
Heat causes metal to expand and contract unevenly — the result is distortion. Controlling distortion is a core millwright skill when welding fabricated structures or repaired components.
Why distortion occurs
The weld zone heats and expands during welding, but surrounding cool metal restrains it. As the weld cools and contracts, it pulls the base metal with it.
Distortion control methods:
Presetting (pre-bending)
Intentionally bend or offset the joint in the opposite direction of expected distortion before welding. The distortion brings the piece back to flat.
Backstep welding
Deposit short weld segments in the direction opposite to the overall weld progression. Each bead expands locally but the overall progression reduces cumulative distortion.
Balanced welding
Alternate weld passes on opposite sides of a joint or structure to balance contraction forces.
Clamping and fixturing
Mechanically restrain the workpiece during welding. Effective but can cause high residual stress.
Minimum heat input
Use the lowest amperage that gives adequate fusion. Less heat = less distortion.
Weld sequence
Plan the order of welds to allow contraction to work symmetrically. Weld from the center outward, from fixed points to free ends.
Peening
Striking the weld with a hammer while hot stretches the weld bead and reduces contraction. Used between passes on some applications. Not used on root pass or cover pass.
EXAM TIP
Backstep welding is the most commonly tested distortion control method. Know what it is and why it works.
Common Weld Defects
| Defect | Description | Common Cause |
|---|---|---|
| Porosity | Gas pockets in the weld | Contamination, moisture, insufficient shielding |
| Undercut | Groove melted into base metal at weld toe | Excessive amperage, wrong angle |
| Overlap | Weld metal rolls over base metal without fusing | Low amperage, slow travel speed |
| Lack of fusion | Weld metal doesn't bond to base metal or previous pass | Low heat, contamination, poor technique |
| Lack of penetration | Root not fully fused | Low amperage, tight root gap, fast travel |
| Cracks | Fractures in weld or HAZ | High carbon, hydrogen, restraint, rapid cooling |
| Inclusions | Slag or tungsten trapped in weld | Incomplete slag removal, poor technique |
| Distortion | Dimensional change from heat | High heat input, poor sequence |
Heat Affected Zone (HAZ)
The HAZ is the area of base metal adjacent to the weld that was not melted but was heated enough to change its microstructure. The HAZ can be hardened, softened, or sensitized depending on the material. In high-carbon and alloy steels, the HAZ is the most likely location for hydrogen-induced cracking — which is why preheat and low-hydrogen electrodes are specified.
Inspection Methods
Visual (VT)
First and most common. Checks surface defects, dimensions, weld profile.
Dye Penetrant (PT)
Liquid dye applied to surface, penetrates into cracks, developer draws it out. Detects surface-breaking defects only. Works on non-porous materials.
Magnetic Particle (MT)
Magnetic field applied; iron particles concentrate at discontinuities. Detects surface and near-surface defects. Ferromagnetic materials only — not on aluminum or austenitic stainless.
Ultrasonic (UT)
High-frequency sound waves detect internal defects. Operator-dependent, requires trained technician.
Radiographic (RT)
X-ray or gamma ray imaging of the weld. Detects volumetric defects (porosity, inclusions). Requires radiation safety protocols.
EXAM TIP
Know which inspection method detects what and the key limitation of each. MT works on ferromagnetic materials only — carbon steel, low-alloy steel, ferritic and martensitic stainless steels. It does not work on austenitic stainless steel, aluminum, titanium, or copper alloys. For surface cracks on non-ferromagnetic materials, PT (dye penetrant) is the correct method. Note: not all stainless steel is non-magnetic — only austenitic grades are. The exam will specify the type.
Fumes and ventilation
Welding fumes are hazardous. Adequate ventilation or respiratory protection is required. Confined space welding requires forced ventilation and atmospheric monitoring.
Arc flash and UV radiation
The welding arc produces intense UV and visible light. Proper lens shade is required:
SMAW
Shade 10–14 (depending on amperage)
GMAW / FCAW
Shade 10–13
GTAW
Shade 8–13
Oxy-acetylene
Shade 4–6
Fire and explosion
Hot work in areas with flammable materials or gases requires a hot work permit. Fire watch required after welding is complete.
Electrical safety
Never wrap welding cables around your body. Ensure proper grounding. Do not weld in wet conditions without proper precautions.
Compressed gas cylinder safety
Secure cylinders upright, store oxygen and fuel gases separately, cap cylinders when not in use. Never use oil or grease on oxygen fittings — spontaneous ignition risk. Per CSA W117.2, oxygen and fuel gas cylinders must be separated indoors by a minimum of 6.1 m (20 ft), or by a non-combustible wall at least 1.5 m (5 ft) high with a minimum half-hour fire resistance rating.
EXAM TIP
The CSA W117.2 cylinder separation requirement — 6.1 m or a rated firewall — is specific and testable. Know both the distance and the barrier alternative.
Work through these exam-style scenarios. Tap each to reveal the approach.
A weld symbol shows a triangle below the reference line with a ¼ to the left and a 2 to the right. The arrow points to a T-joint. What does this specify?
An E7018 electrode has been left out overnight in a humid shop. What is the risk and what is the corrective action?
A millwright is welding a T-joint on carbon steel plate and notices the weld toe has a groove melted into the base metal. What is this defect called and what caused it?
Why does welding aluminum with GTAW require AC current rather than DCEN?
A fabrication job requires inspection of an austenitic stainless steel weld for surface cracks. Why is magnetic particle inspection not appropriate, and what method should be used instead?
A long weld bead on a flat plate is pulling the ends upward after welding. What technique could have been used during welding to reduce this distortion?
Welding questions on the Red Seal exam reward millwrights who have actually run beads and dealt with real problems — but knowing the theory cold means you won't lose marks on the process and symbol questions.
Related guides:
Last updated: August 2026 · MW Red Seal Millwright Prep is built by a millwright, for millwrights. Content is aligned with the National Occupational Analysis (NOA) for Industrial Mechanic (Millwright) — the same document that structures the Red Seal exam.