Welding, Cutting, and Fabrication

Welding, Cutting, and Fabrication

Module Introduction

This chapter covers all the skills required for the Red Seal exam concerning welding, cutting, and fabrication processes applied to transport trailers. As a trailer technician, you must master not only the basic techniques but also the Canadian standards that govern these operations. You will be assessed on your ability to identify appropriate processes, prepare joints, interpret welding symbols, and apply safety rules.

Welding Processes Used in Trailer Work

Shielded Metal Arc Welding (SMAW)

Shielded Metal Arc Welding (SMAW) remains a common process for field repairs. An electric arc is established between a consumable flux-coated electrode and the workpiece. The coating melts to form a shielding gas and slag that protects the molten weld pool from atmospheric contamination.

Technical characteristics:

Common electrodes:

Rule of thumb: The first digit (6 or 7) indicates the minimum tensile strength in thousands of psi. The last two digits indicate the position and coating type.

Gas Metal Arc Welding (GMAW/MIG)

The MIG process (Metal Inert Gas) uses a continuous wire electrode and a shielding gas. In the trailer context, this process is preferred for repairs on high-strength steel frames and aluminum panels.

Essential parameters:

Transfer modes:

Caution: Spray transfer must never be used in vertical or overhead positions due to the excessive fluidity of the weld pool.

Flux Cored Arc Welding (FCAW)

Flux Cored Arc Welding combines the advantages of MIG and SMAW. The tubular wire contains flux that generates gas shielding and slag. Two variants exist: with external shielding gas (FCAW-G) and without external gas (FCAW-S, self-shielded).

Advantages for trailers:

Disadvantages: Significant fume production, slag removal required between passes, higher wire cost than solid wire.

TIG Welding (GTAW)

TIG welding (Gas Tungsten Arc Welding) uses a non-consumable tungsten electrode and a separate filler metal. This process is used for fine repairs on aluminum, stainless steel, and light alloys in refrigerated or specialized trailers.

Parameters:

Critical point: For aluminum, alternating current is mandatory. The negative half-cycle cleans the aluminum oxide, the positive half-cycle ensures penetration.

Thermal Cutting

Oxy-fuel Cutting

Oxy-acetylene cutting is the most common process for cutting carbon steel. The principle relies on iron oxidation: the metal is heated to approximately 870°C (ignition temperature), then a jet of pure oxygen triggers an exothermic combustion that produces liquid iron oxide.

Required conditions:

Typical settings:

Safety: Acetylene must never be used at a pressure exceeding 103 kPa (15 psi) because it becomes unstable and can detonate spontaneously beyond this pressure.

Plasma Cutting

Plasma cutting uses an electric arc constricted through a nozzle to ionize a gas (air, nitrogen, argon-hydrogen) and create a plasma jet at extremely high temperatures (up to 30,000°C). This process cuts all conductive metals, including aluminum and stainless steel.

Advantages:

Disadvantages: High equipment cost, electrode and nozzle consumption, intense noise and UV radiation.

Carbon-Arc Cutting

Air carbon arc gouging uses a carbon electrode and a compressed air jet to remove metal. This process is used to prepare weld roots, remove defective welds, and create chamfers.

Parameters:

Hazard: This process produces toxic fumes. Adequate ventilation and respiratory protection are mandatory.

Joint Preparation and Welding Symbols

Joint Types

The choice of joint type depends on material thickness, applied load, and access to the joint.

Edge Preparation

Edge preparation is essential to ensure complete penetration. Angles and root openings must comply with AWS D1.1 (steel welding) and D1.2 (aluminum welding) standards.

Common preparations:

Rule: The total bevel angle must be 60° to allow adequate electrode access to the root. A smaller angle prevents complete fusion; a larger angle increases the volume of deposited metal and residual stresses.

Welding Symbols per AWS A2.4

Reading welding symbols is a mandatory skill for the exam. The basic symbol consists of an arrow pointing to the joint, with a horizontal reference line.

Symbol elements:

Reading rules:

Example: A V symbol with "60°" and "3" indicates a 60° V-groove with a 3 mm root opening.

Applicable Canadian Standards

CSA W59 — Welded Steel Construction

CSA W59 (Welded Steel Construction) is the primary reference for structural steel welding in Canada. It covers qualification requirements for procedures, welders, and operators.

Key requirements:

CSA W47.1 — Certification of Welding Companies

This standard requires companies performing welding on load-bearing structures to be certified. The technician must verify that the company holds a valid certificate before performing structural repairs.

CSA B149.1 — Natural Gas and Propane Code

This standard applies to trailer gas systems (propane tanks, piping). The technician must know the minimum distances between ignition sources (including welding) and propane tanks.

B149.1 Rule: No hot work (welding, grinding, cutting) shall be performed within 3 meters of an unprotected propane tank unless the tank is emptied and purged.

Canadian Electrical Code, Part I

The Canadian Electrical Code, Part I (C22.1) applies to trailer electrical installations. For welding, the requirements concern equipment grounding and protection against electric shock.

Rule 8-200: Welding circuits must be protected by circuit breakers or fuses conforming to manufacturer requirements. Open-circuit voltage must not exceed 100 volts DC or 80 volts AC for manual welding machines.

Weld Calculations

Sizing Fillet Welds

The strength of a fillet weld depends on the throat (the theoretical weld thickness measured perpendicular to the face). For a 45° fillet weld, the throat is calculated as follows:

Throat = 0.707 × leg size

Example: A fillet weld with an 8 mm leg has a throat of 0.707 × 8 = 5.66 mm.

Allowable Stress

The allowable stress in a weld is generally limited to 0.3 × the minimum tensile strength of the filler metal for static loads.

Formula: Allowable load = Throat × Length × Allowable stress

Practical example: A fillet weld with a 6 mm leg (throat = 4.24 mm) over a length of 100 mm with an allowable stress of 150 MPa can support:

Dilution and Deposited Metal

Calculating the number of passes required to fill a joint depends on the preparation volume and the process deposition rate.

Single V-joint volume: V = (width × depth × length) / 2

Typical deposition rates:

Weld Defects and Inspection

Common Defects

Non-Destructive Testing (NDT) Methods

Visual Testing (VT): The first and most important method. Check dimensions, shape, presence of cracks, porosity, etc.

Penetrant Testing (PT): Detects surface cracks in non-porous materials. Use a penetrant, a developer, and a UV lamp if fluorescent.

Magnetic Particle Testing (MT): Detects surface and near-surface cracks in ferromagnetic materials. Requires magnetizing equipment and iron particles.

Ultrasonic Testing (UT): Detects internal defects. Requires a certified operator and perfect contact between the probe and the surface.

Radiographic Testing (RT): Detects internal defects. Requires specialized equipment and radiological safety precautions.

Welding Safety

Personal Protective Equipment (PPE)

Ventilation

Welding fumes contain metal oxides, ozone, and toxic gases. Ventilation must be:

Electrical Hazards

Confined Spaces

Trailer tanks (cargo tanks) are confined spaces. Requirements include:

Trailer Fabrication and Repair

High-Strength Steels

Modern trailer frames use high-strength low-alloy (HSLA) steels with yield strengths of 350 to 700 MPa. These steels require special precautions:

Heat input calculation: Q = (V × I × 60) / (v × 1000)

Where:

Example: 25 V × 200 A × 60 / (300 mm/min × 1000) = 1.0 kJ/mm

Aluminum

Aluminum trailers (tankers, vans) present particular challenges:

Common alloys:

Side Rail Repair

Trailer side rails are critical structural components. Any repair must:

Rule of thumb: Never weld perpendicular to the axis of a side rail across its full height. Transverse welds create stress concentrations that can lead to fatigue cracking.

Pitfalls to Avoid

Summary

Mastering these concepts will enable you not only to pass the Red Seal exam but also to perform safe and durable repairs on transport trailers. Welding practice and symbol reading are skills acquired through experience — take every opportunity to exercise them.