Welding and Thermal Cutting

Welding and Thermal Cutting

Chapter Introduction

Welding and thermal cutting are central activities of the ironworker trade (structural and ornamental). The Red Seal exam assesses your ability to interpret weld symbols, select the appropriate process, prepare joints, apply Canadian standards, and execute quality structural welds. This chapter covers all required theoretical and practical knowledge, including weld sizing calculations, welding parameters, applicable codes, and common pitfalls.

Welding Processes and Their Applications

Shielded Metal Arc Welding (SMAW)

Shielded Metal Arc Welding (SMAW), also called stick welding, is the most widely used process in structural ironwork. It uses an electric arc established between a consumable flux-coated electrode and the base metal. The coating melts to form a shielding gas and slag that protects the weld pool from oxidation.

Technical characteristics:

E70XX electrode table (minimum tensile strength: 70,000 psi = 490 MPa)

Rule of thumb: For structural assemblies subject to dynamic loads, use exclusively low-hydrogen electrodes (E7018). Hydrogen causes cold cracking in high-strength steels.

Gas Metal Arc Welding (GMAW/MIG)

MIG welding (Metal Inert Gas) uses a continuous wire electrode and an inert or active shielding gas. In structural ironwork, an argon-CO₂ mixture (75% Ar / 25% CO₂) is primarily used for carbon steel.

Critical parameters:

Transfer modes:

Flux-Cored Arc Welding (FCAW)

FCAW welding uses a tubular wire containing flux. Two variants exist: gas-shielded (FCAW-G) and self-shielded (FCAW-S). FCAW-S is particularly useful on outdoor job sites where wind disperses shielding gases.

Advantages of FCAW-S:

Submerged Arc Welding (SAW)

Submerged arc welding is an automatic process where the arc is submerged under a bed of granular flux. Used for longitudinal and circumferential welds on heavy thicknesses, particularly in the fabrication of welded built-up girders.

Typical parameters:

Resistance Welding

Resistance welding (spot, seam, flash) is used for sheet metal assemblies and attachments. In ornamental ironwork, spot welding is common for grilles and guardrails.

Weld Symbols and Blueprint Reading

Weld Symbol Structure

The standardized weld symbol (CSA W59 standard, based on AWS A2.4) includes:

Essential basic symbols:

Reading rule: If the symbol is below the reference line, the weld is on the same side as the arrow (arrow side). If it is above, the weld is on the opposite side from the arrow (other side). A symbol on both sides indicates a double weld.

Dimensions and Indications

Reading example: A ⌒ symbol below the reference line with "8 × 150-300" means: 8 mm throat fillet weld, 150 mm length, spaced 300 mm centre-to-centre, on the arrow side.

Joint Preparation and Tolerances

Joint Types

Bevel Angles and Root Gaps

Typical preparation table (structural steel, CSA W59):

Rule: The minimum included angle for SMAW is 60° to allow electrode access. For GMAW/FCAW, an angle of 45° to 60° is sufficient.

Cleaning and Surface Conditions

Weld Sizing Calculations

Fillet Weld Strength

The factored resistance of a fillet weld is determined according to CSA S16 (Design of Steel Structures).

Fundamental formula:

Where:

Effective throat:

Linear resistance calculation (per mm of length):

Practical example: A beam transmits a load of 250 kN through an end plate welded on each side. What total length of 8 mm weld is required?

Full Penetration Welds

The resistance of a full penetration weld is equal to that of the adjacent base metal. No throat calculation is necessary, but weld quality must be verified by non-destructive testing (NDT).

Minimum Fillet Weld Size

CSA S16 rule: The minimum size of a fillet weld must not be less than 3 mm, and must not exceed the thickness of the thinner piece minus 1 mm, unless otherwise specified.

Minimum fillet weld size table (CSA S16):

Applicable Canadian Standards

CSA W59 — Welded Steel Construction (Structural Steel)

CSA W59 is the primary reference for structural welding in Canada. It covers:

Key requirements:

CSA W47.1 — Certification of Welding Companies

This standard certifies companies according to their ability to produce compliant welds. Certification divisions include:

CSA S16 — Design of Steel Structures

Standard S16 defines design criteria, including weld sizing rules (Chapter 26). Key rules:

CSA B149.1 — Canadian Electrical Code, Part I

Although primarily electrical, the CSA B149.1 (Canadian Electrical Code, Part I) applies to gas welding installations. Requirements include:

Thermal Cutting

Oxy-Fuel Cutting

Oxy-acetylene cutting is the most common thermal cutting process. The principle relies on the oxidation of iron at high temperature (approximately 870 °C) followed by an oxygen jet that blows away the molten oxide.

Equipment:

Safety:

Cutting parameters:

Plasma Cutting

Plasma cutting uses a constricted electric arc through a nozzle to ionize a gas (air, nitrogen, argon-hydrogen) and create a plasma at very high temperature (15,000 to 30,000 °C).

Advantages:

Limitations:

Torch Cutting: Quality and Tolerances

Common cutting defects:

Cutting tolerances (CSA W59):

Quality Control and Weld Inspection

Non-Destructive Testing (NDT)

Acceptance Criteria (CSA W59)

Surface defects:

Dimensional defects:

Welding Safety

Personal Protective Equipment (PPE)

Ventilation and Fumes

Welding fumes contain metal oxides (iron, manganese, zinc, chromium). Prolonged exposure can cause metal fume fever and lung damage.

Ventilation requirements:

Electrical Hazards

Pitfalls to Avoid

Summary

Final Exam Tips

Mastery of this chapter represents approximately 15 to 20% of the questions on the Red Seal exam for the ironworker trade. Solid preparation on standards, calculations, and symbols will give you a decisive advantage.