Welding, Cutting, and Thermal Processes

Welding, Cutting, and Thermal Processes

Module Introduction

This chapter covers all the welding, cutting, and heat treatment processes that every generalist ironworker must master for the Red Seal exam. In Canada, on-site welding is governed by specific national standards, notably CSA W59 (Welded Steel Construction) and CSA W47.1 (Certification of Welding Companies). You must not only know how to perform these processes, but also understand the metallurgical principles, welding parameters, weld sizing calculations, and applicable regulatory requirements.

Fundamental Metallurgical Principles

Steel Structure and Heat Effects

Structural steel (ASTM A36, CSA G40.20/G40.21) is an iron-carbon alloy typically containing 0.15% to 0.30% carbon. When steel is heated above the upper critical temperature (approximately 900 °C), the crystal structure transforms from ferrite (body-centered cubic) to austenite (face-centered cubic). The cooling rate determines the final microstructure:

The heat-affected zone (HAZ) is the region of the base steel adjacent to the weld that has not melted but whose microstructure has been altered by the heat. The width of the HAZ depends on the heat input (kJ/mm) and the thermal conductivity of the material.

Preheating and Interpass Temperature

Preheating slows the cooling of the weld, allowing hydrogen to diffuse out of the molten metal before solidification. It reduces the risk of cold cracking (hydrogen-induced delayed cracking). The minimum preheat temperature is determined by:

The carbon equivalent formula according to CSA W59 is:

CE = C + (Mn/6) + (Cr + Mo + V)/5 + (Ni + Cu)/15

For steel with CE > 0.45%, preheating is generally required. The interpass temperature (between successive passes) must not exceed the specified maximum temperature (often 250 °C for structural steels), otherwise the toughness of the HAZ degrades.

Heat Input

Heat input is calculated as follows:

Heat input (kJ/mm) = (Voltage × Current × 60) / (Travel speed in mm/min × 1000)

Example: welding at 25 V, 200 A, travel speed of 300 mm/min:

Heat input = (25 × 200 × 60) / (300 × 1000) = 300,000 / 300,000 = 1.0 kJ/mm

Too high a heat input produces a wide HAZ and a loss of strength. Too low a heat input causes rapid cooling and a risk of cracking. Typical values for shielded metal arc welding (SMAW) are 0.8 to 2.5 kJ/mm.

Welding Processes — SMAW (Shielded Metal Arc Welding)

Principle and Equipment

Shielded metal arc welding (SMAW) — also called stick welding — is the most widely used process in site ironwork. An electric arc is established between the consumable electrode and the base metal. The coating melts to form a protective slag that isolates the molten metal from the atmosphere.

Electrode Classification per CSA W48

The Canadian classification system follows the EXXXX format:

Example: E7018 — 70 ksi (490 MPa) strength, usable in all positions, low-hydrogen basic coating, direct current or alternating current.

Common electrodes in ironwork:

Electrode Storage and Handling

Low-hydrogen electrodes (E7018) are hygroscopic: they absorb moisture from the air. A damp electrode introduces hydrogen into the molten metal, causing cracking. Storage rules:

SMAW Welding Parameters

Welding current depends on the electrode diameter:

Arc length should be approximately equal to the diameter of the electrode core. An arc that is too long produces excessive spatter and poor gas shielding. An arc that is too short causes the electrode to stick.

Welding Processes — GMAW and FCAW

MIG/MAG Welding (GMAW)

Gas metal arc welding (GMAW) uses a continuous wire electrode and a shielding gas. In ironwork, the following are mainly used:

Metal transfer modes:

FCAW Welding (Flux-Cored Arc Welding)

Flux-cored arc welding (FCAW) combines the productivity of GMAW with the tolerance of SMAW. The wire contains an internal flux that generates gas shielding and slag. Two types:

FCAW-S (self-shielded wire) is widely used in site ironwork because it tolerates wind and lightly contaminated surfaces. Common wires: E71T-8 (all positions, low hydrogen) and E71T-11 (all positions).

GMAW/FCAW Parameters

Wire feed speed determines the current. General rule: for 1.2 mm wire, every 25 mm/min of wire feed speed corresponds to approximately 10 A. Voltage controls arc length and bead width.

Work angle: maintain a drag (push) angle of 10° to 15° for good deposition. The wire should be directed toward the center of the weld pool.

Welding Processes — GTAW (TIG)

Principle

TIG welding (GTAW) uses a non-consumable tungsten electrode and a separate filler rod. Argon or an Ar/He mixture shields the weld pool. This process produces high-quality welds but at low productivity.

Applications in Ironwork

TIG Parameters

The tungsten must be sharpened to a conical point (30° to 60° angle) for direct current. For aluminum in alternating current, a rounded point (ball) is used.

Thermal Cutting

Oxy-fuel Cutting (OFC)

Oxyacetylene cutting is the most common thermal cutting process in ironwork. It works by combustion of iron: the metal is heated to approximately 870 °C (ignition temperature), then a jet of pure oxygen causes a rapid exothermic oxidation that cuts through the thickness.

Conditions required for oxy-fuel cutting:

Carbon steel satisfies these conditions. Stainless steel and aluminum cannot be oxy-fuel cut (chromium oxide and alumina have very high melting points).

Cutting parameters:

Essential safety rule: acetylene must never be used at a pressure greater than 103 kPa (15 psi) — beyond this, it becomes unstable and can detonate spontaneously.

Plasma Cutting (PAC)

Plasma cutting uses an electric arc constricted through a nozzle to ionize a gas (air, nitrogen, argon/H₂) into plasma at 20,000 °C to 30,000 °C. This process cuts all conductive metals, including stainless steel and aluminum.

Plasma cutting produces a narrower HAZ than oxy-fuel cutting and a cleaner cut. However, the equipment is more expensive and requires an electrical power supply and an air compressor.

Manual Torch Cutting — Technique

For a quality straight cut:

Common cutting defects:

Welding Symbols and Drawings

Basic Symbols per CSA W59

The weld symbol consists of a reference line, an arrow, and a base symbol. The position of the symbol relative to the reference line indicates the side of the weld:

Common Weld Symbols

Dimensions on Symbols

Weld Sizing Calculations

Fillet Weld Strength

The strength of a fillet weld is calculated based on the effective throat, which is the minimum distance between the root and the face of the weld. For a 45° fillet weld, the effective throat is:

Throat = 0.707 × Leg

The allowable shear stress in a fillet weld according to CSA S16 (Design of Steel Structures) is:

Fw = 0.67 × Fu × 0.707 × Leg

Where Fu is the specified ultimate strength of the electrode (often 490 MPa for E70XX).

Calculation example: 8 mm leg fillet weld, E70 electrode (Fu = 490 MPa):

Minimum Fillet Weld Size

According to CSA S16, the minimum leg size of a fillet weld depends on the thickness of the thinner material:

Minimum Weld Length

The minimum effective length of a fillet weld is 4 times the leg size. If the calculated length is less, the minimum length of 40 mm must be used.

Applicable Standards and Codes

CSA W59 — Welded Steel Construction

CSA W59 is the reference standard for structural steel welding in Canada. It specifies:

CSA W47.1 — Certification of Welding Companies

CSA W47.1 requires that any company welding structural steel be certified. This certification involves:

CSA W186 — Welding of Reinforcing Bars

For the generalist ironworker working on reinforced concrete structures, CSA W186 covers the welding of reinforcing bars. Rebar welds require specific electrodes (E7018 or E8018 depending on the steel grade) and special precautions regarding temperature.

Canadian Electrical Code, Part I

The Canadian Electrical Code, Part I (C22.1-21) applies to temporary electrical installations on construction sites, including welding stations. Rule 8-200 requires that supply conductors for welding stations be protected by fuses or circuit breakers sized at 125% of the primary rated current of the station.

CSA B149.1 — Natural Gas and Propane Code

CSA B149.1 governs the installation of gas systems, including acetylene and oxygen cylinders on construction sites. Key requirements:

Welding and Cutting Safety

Personal Protective Equipment (PPE)

Ventilation and Fumes

Welding fumes contain metal oxides, gases (CO, NOx, ozone), and organic compounds. Ventilation must be:

Confined Spaces

Welding in a confined space (tank, box, tunnel) requires:

Specific Hazards

Weld Quality Control and Inspection

Visual Inspection

Visual inspection is the first step of quality control. It verifies:

Acceptance Criteria per CSA W59

Non-Destructive Testing (NDT)

Destructive Testing

Common Pitfalls to Avoid

Summary

Self-Assessment Questions

(Answers: 1. Consult the CSA W59 preheat table — generally 100 °C for CE 0.50% and 25 mm; 2. 1.05 kJ/mm; 3. 7.07 mm; 4. 103 kPa; 5. Plasma; 6. 4 hours; 7. Circle at the arrow/reference line junction; 8. CSA W47.1; 9. 6 mm; 10. Alternating current AC)