Welding and Brazing for Pipefitters

Welding and Brazing for Pipefitters

Chapter Introduction

Welding and brazing are essential permanent joining processes in the pipefitting trade. For the Red Seal exam, you must master not only the operating techniques, but also the Canadian standards that govern these processes, welding symbols, qualification testing, and joint preparation calculations. This chapter covers all the required knowledge, with an emphasis on the critical distinctions between processes, welding parameters, and the regulatory requirements of the Canadian Electrical Code, Chapter V for pressurized piping systems.

Fundamental Definitions

Welding is a fusion joining process where metal parts are heated to their melting temperature, with or without filler metal, to form a continuous joint. Brazing is a process where the filler metal melts at a temperature above 450 °C but below the melting point of the base metal. Braze welding is a variant where filler metal is deposited in a joint without melting the base metal, but without capillary action.

The crucial distinction: in welding, the base metal melts; in brazing, it never melts. This difference determines the thermal stresses, the mechanical properties of the joint, and the permissible applications.

Welding Processes for Pipefitters

Shielded Metal Arc Welding (SMAW)

SMAW, also called stick welding, is the most widely used process for field piping. An electric arc is established between a consumable coated electrode and the workpiece. The coating melts to form a protective slag and shielding gas.

Critical parameters:

Common electrodes (AWS A5.1/A5.5 classification):

Table 1 — Characteristics of common electrodes

Gas Tungsten Arc Welding (GTAW/TIG)

GTAW, or TIG welding, uses a non-consumable tungsten electrode and an inert gas (argon or helium) to shield the weld pool. This process produces high-quality welds with no slag, making it ideal for root passes in piping, stainless steels, and non-ferrous alloys.

Parameters:

Advantages: precise weld pool control, no spatter, clean weld, suitable for root passes.

Disadvantages: slow speed, high cost, requires great skill.

Flux Cored Arc Welding (FCAW)

FCAW uses a continuous tubular wire containing an internal flux. It can be self-shielded (no external gas) or gas-shielded (CO₂ or Ar-CO₂ mixture). This process offers high productivity, particularly for large-diameter piping in shop environments.

Parameters:

Gas Metal Arc Welding (GMAW/MIG)

GMAW uses a continuous consumable wire and a shielding gas (CO₂, Ar, or mixtures). Less commonly used in process piping due to difficulty controlling the root pass, it is nevertheless used for large-diameter pipes and plate work.

Transfer modes:

Oxyacetylene Welding (OAW)

OAW uses the flame of an oxygen-acetylene mixture to melt the base metal and filler metal. Although less used for pressurized piping, it remains relevant for brazing, braze welding, and repair work on thin sheet metal.

Flame types:

Joint Preparation and Welding Symbols

Joint Types

The choice of joint type depends on pipe thickness, welding position, and service requirements. For piping, butt joints are the most common.

Table 2 — Butt joint preparation by thickness

The bevel is the angle formed by the edge preparation. The land is the remaining thickness at the root of the joint. The root gap is the space between the two pieces at the base of the bevel.

Welding Symbols (AWS A2.4 standard)

Welding symbols are a standardized graphical language. The reference line is the horizontal line on which symbols are placed. The arrow points to the joint. The basic symbol is placed above or below the reference line depending on whether the weld is on the opposite side or the same side as the arrow.

Basic symbols:

Dimensions: the weld size is indicated to the left of the symbol, the length to the right. A contour symbol (flat, convex, concave) is placed above or below the basic symbol.

Example: a ⌐ symbol with 6 on the left and 50 on the right means a fillet weld with a 6 mm leg over a 50 mm length.

Qualification Testing and Procedures

Procedure Qualification (WPS/PQR)

The WPS (Welding Procedure Specification) is a document that describes the welding parameters: process, base metals, filler metals, positions, preheat, post-weld heat treatment, and electrical parameters. The PQR (Procedure Qualification Record) is the report of the tests performed to validate the WPS.

Essential variables: these are parameters that, if changed, require requalification. For example:

Non-essential variables: may be changed without requalification (e.g., electrode brand name, provided the AWS classification is identical).

Welder Qualification

The welder must be qualified according to CSA W47.1 (steel) or CSA W47.2 (aluminum). Qualification consists of welding a test piece in a given position, then subjecting it to destructive tests (tensile, bend) or non-destructive tests (radiography).

Pipe welding positions:

Qualification scope: a welder qualified in 6G is qualified for all positions. A welder qualified in 5G is qualified for 1G, 2G, and 5G, but not for 6G.

Applicable Canadian Standards

CSA B51 — Boilers and Pressure Vessels

CSA B51 governs the design, fabrication, and inspection of boilers and pressure vessels. It references construction codes such as the ASME Boiler and Pressure Vessel Code, Section IX for the qualification of procedures and welders.

Key requirements:

CSA B149.1 — Natural Gas and Propane Installation Code

CSA B149.1 applies to natural gas and propane installations. It contains specific requirements for welded and brazed joints.

Rule 4.6.1: Steel piping joints must be welded according to a procedure qualified to ASME Section IX, or brazed according to a qualified procedure.

Rule 4.6.2: Brazed joints must be made with a filler metal having a melting point above 540 °C.

Rule 4.7.3: Welds must be free of visible defects: cracks, porosity, lack of fusion, pinholes, and slag inclusions.

Canadian Electrical Code, Chapter V

The Canadian Electrical Code, Chapter V applies to electrical installations in classified areas (hazardous locations). For the pipefitter, this concerns welding in areas where flammable gases or vapors may be present.

Rule 18-100: Welding and brazing are prohibited in classified areas, unless a hot work permit is issued and safety measures are taken (gas detection, ventilation, removal of ignition sources).

Rule 18-102: Electric welding equipment must be grounded in accordance with the requirements of Section 10 of the Code.

Welding Calculations

Calculating Fillet Weld Length

For a fillet weld, the leg (l) is the distance from the weld toe to the intersection of the faces. The throat (t) is the minimum distance between the root and the face of the weld. For a full-throat fillet weld:

t = l × sin(45°) = l × 0.707

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

Calculating Weld Cross-Section

The cross-sectional area of a fillet weld (A) is:

A = (l² × tan(θ/2)) / 2

For a 90° angle between parts: A = l² / 2

Example: leg of 10 mm, area = 10² / 2 = 50 mm².

Calculating Deposited Metal Mass

The mass of filler metal required (m) is:

m = A × L × ρ

Where:

Example: weld with a 50 mm² cross-section over 1 meter of length:

m = 50 × 1000 × 7.85 × 10⁻⁶ = 0.3925 kg

In practice, add 10 to 20% for losses (spatter, fumes, slag).

Calculating Preheat

Preheat is required to prevent cold cracking in steels with higher carbon content. The minimum preheat temperature can be estimated using the carbon equivalent (CE) method:

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

Rule of thumb:

Brazing and Braze Welding

Principles of Brazing

Brazing relies on capillary action: the molten filler metal is drawn into the gap between the two parts by surface tension. The optimal gap is 0.05 to 0.15 mm. A gap that is too large or too small reduces joint strength.

Brazing temperatures:

Brazing Filler Metals

Table 3 — Common brazing filler metals

Brazing Fluxes

Flux dissolves metal oxides and protects the surface during heating. The choice of flux depends on the base metal and the brazing temperature.

Table 4 — Brazing fluxes

Brazing Procedure

Frequent errors:

Welding Defects and Quality Control

Common Defects

Table 5 — Welding defects, causes, and remedies

Non-Destructive Testing (NDT)

Destructive Testing

Welding Safety

Hazards and Protection

Electrical hazards: the electric arc operates at low voltage (20 to 30 V) but high current. The welding circuit is under open-circuit voltage of 50 to 100 V. Always use insulating gloves and safety footwear.

Radiation hazards: the arc emits intense ultraviolet and infrared rays. Use a welding helmet with the appropriate filter lens shade (shade 10 to 13 for SMAW, 9 to 12 for GTAW).

Fume hazards: welding fumes contain metal oxides, fluorides, and organic compounds. Ventilate the area and use a fume extractor if necessary.

Fire hazards: spatter and sparks can ignite combustible materials. Maintain a clear work zone of 10 meters and keep a fire extinguisher within reach.

Burn hazards: welded parts remain hot after welding. Use gloves, long sleeves, and a leather apron.

Pitfalls to Avoid

Summary

Final Exam Tips

This chapter covers all the theoretical knowledge required for the Red Seal exam in welding and brazing. Hands-on shop practice and reading the complete standards remain essential for thorough preparation.