Canadian Steel Chemical Composition — CSA G40.21 Limits for 300W, 350W, 350WT
Complete reference for CSA G40.21 chemical composition requirements for Canadian structural steel grades. Composition limits directly affect weldability, CEV (carbon equivalent) calculations, and mechanical properties. Carbon equivalent limits are critical for preheat determination per CSA W59.
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CSA G40.21 Composition Limits — Weldable Grades
Per CSA G40.21-18 Table 1, the heat analysis limits for weldable structural grades are shown below. All values are maximum percentages unless noted.
Grade 300W Composition
| Element | t ≤ 20 mm | 20 < t ≤ 40 mm | 40 < t ≤ 65 mm | 65 < t ≤ 100 mm |
|---|---|---|---|---|
| C | 0.23 | 0.23 | 0.23 | 0.23 |
| Mn | 1.50 | 1.50 | 1.50 | 1.50 |
| P | 0.04 | 0.04 | 0.04 | 0.04 |
| S | 0.05 | 0.05 | 0.05 | 0.05 |
| Si | 0.40 | 0.40 | 0.40 | 0.40 |
300W has the widest composition tolerances, making it the most forgiving grade for welding without special precautions. Carbon content at 0.23% max keeps the CEV manageable.
Grade 350W Composition
| Element | t ≤ 20 mm | 20 < t ≤ 40 mm | 40 < t ≤ 65 mm | 65 < t ≤ 100 mm |
|---|---|---|---|---|
| C | 0.23 | 0.23 | 0.23 | 0.23 |
| Mn | 1.50 | 1.50 | 1.50 | 1.50 |
| P | 0.04 | 0.04 | 0.04 | 0.04 |
| S | 0.05 | 0.05 | 0.05 | 0.05 |
| Si | 0.40 | 0.40 | 0.40 | 0.40 |
350W shares identical composition limits with 300W. The higher yield strength (350 MPa vs 300 MPa) is achieved through controlled rolling and microalloying rather than increased carbon. Niobium (Nb) and vanadium (V) additions up to 0.10% each are permitted for grain refinement.
Grade 350WT Composition
| Element | t ≤ 20 mm | 20 < t ≤ 40 mm | 40 < t ≤ 65 mm | 65 < t ≤ 100 mm |
|---|---|---|---|---|
| C | 0.20 | 0.20 | 0.20 | 0.20 |
| Mn | 1.35 | 1.35 | 1.35 | 1.35 |
| P | 0.030 | 0.030 | 0.030 | 0.030 |
| S | 0.030 | 0.030 | 0.030 | 0.030 |
| Si | 0.40 | 0.40 | 0.40 | 0.40 |
350WT has tighter limits on carbon (0.20% max vs 0.23%), phosphorus (0.030% max vs 0.04%), and sulphur (0.030% max vs 0.05%) compared to 350W. These lower impurity levels are necessary to achieve guaranteed Charpy V-notch toughness at -45 deg C per CSA G40.21.
Carbon Equivalent (CEV) and Weldability
Per CSA W59, preheat requirements are determined using the carbon equivalent formula:
CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
CEV Values for Common Grades
| Grade | CEV max (t ≤ 20 mm) | CEV max (t ≤ 65 mm) | CEV max (t ≤ 100 mm) |
|---|---|---|---|
| 260W | 0.42 | 0.44 | 0.46 |
| 300W | 0.44 | 0.46 | 0.48 |
| 350W | 0.45 | 0.47 | 0.49 |
| 350WT | 0.43 | 0.45 | 0.47 |
| 400W | 0.47 | 0.49 | 0.51 |
| 480W | 0.52 | 0.55 | 0.57 |
| 700Q | 0.55 | 0.58 | — |
Lower CEV values indicate better weldability. For CEV ≤ 0.45, preheat is generally not required for plate thickness under 25 mm. For CEV > 0.45, CSA W59 Table 5.1 specifies minimum preheat temperatures.
Microalloying Elements
CSA G40.21 permits microalloy additions for grain refinement and precipitation strengthening:
| Element | Purpose | Typical Range | Effect on CEV |
|---|---|---|---|
| Nb | Grain refinement | 0.005-0.05% | Minor |
| V | Precipitation strengthening | 0.01-0.10% | Minor |
| Ti | Grain refinement, nitride former | 0.005-0.03% | Minor |
| Al | Deoxidation, grain refinement | 0.02-0.06% | Not calculated |
These microalloys enable 350W to achieve 350 MPa yield without increasing carbon content. The resulting steel has better weldability than a straight carbon-manganese grade of equivalent strength.
Preheat Determination from CEV
Per CSA W59-18 Clause 5.3.3, the minimum preheat temperature is based on CEV and combined thickness:
| CEV Range | t ≤ 20 mm | 20 < t ≤ 40 mm | 40 < t ≤ 65 mm |
|---|---|---|---|
| ≤ 0.45 | None | 10 °C | 50 °C |
| 0.46-0.50 | 10 °C | 50 °C | 95 °C |
| 0.51-0.55 | 50 °C | 95 °C | 120 °C |
| 0.56-0.60 | 95 °C | 120 °C | 150 °C |
For 350W (CEV ≈ 0.45), preheat is required only when the combined thickness exceeds 40 mm, typically at beam-to-column connections with thick end plates or stiffeners. 350WT's lower CEV further reduces preheat requirements.
Hydrogen-Induced Cracking Control
Hydrogen-induced cold cracking (HICC) is controlled by three factors combined: CEV, hydrogen level (from welding consumables), and restraint level. CSA W59 addresses this through:
- Low-hydrogen electrodes: E48XX class with H4 or H2 designations for SMAW
- Preheat: Reduces cooling rate, allows hydrogen diffusion
- Interpass temperature: Maintains minimum thermal cycle
- Post-weld hydrogen release: 50-100 °C hold for 1-3 hours for thick sections
For 350WT used in bridge applications, CEV control to 0.43 max combined with mandatory low-hydrogen welding provides reliable resistance to HICC even in cold-weather field welding.
Composition Effects on Mechanical Properties
| Element | Effect on Strength | Effect on Toughness | Effect on Weldability |
|---|---|---|---|
| Carbon (C) | Strong increase | Decreases | Worsens (increases CEV) |
| Manganese (Mn) | Moderate increase | Improves (up to 1.5%) | Moderate CEV effect |
| Silicon (Si) | Minor increase | Neutral at ≤0.40% | Minor CEV effect |
| Phosphorus (P) | Minor increase | Severely decreases | Worsens hot cracking |
| Sulphur (S) | No effect | Decreases directionality | Worsens hot cracking |
| Niobium (Nb) | Moderate increase | Improves grain refinement | Improves HAZ toughness |
The balance between strength and weldability is achieved through controlled microalloying. The trend in Canadian standards is toward lower carbon (0.18% max for new grades) and more microalloy control.
Frequently Asked Questions
What is the carbon equivalent (CEV) of CSA G40.21 Grade 350W steel? The CEV of 350W is typically 0.43-0.47 depending on thickness and mill source. The typical CEV for t ≤ 20 mm is 0.45. This value is calculated as CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 per CSA W59. At CEV = 0.45, preheat is not required for plate thickness under 25 mm per CSA W59 Table 5.1. For thicker sections (t > 40 mm), minimum 50 °C preheat is recommended.
How does 350WT composition differ from 350W? 350WT has lower maximum carbon (0.20% vs 0.23%), lower phosphorus (0.030% vs 0.04%), and lower sulphur (0.030% vs 0.05%) compared to 350W. The CEV of 350WT is typically 0.40-0.43 vs 0.43-0.47 for 350W. These tighter controls enable the guaranteed Charpy V-notch toughness of 27 J at -45 °C required for cold-weather and fracture-critical applications.
What microalloying elements are used in CSA G40.21 350W steel? CSA G40.21 permits niobium (Nb) up to 0.10%, vanadium (V) up to 0.10%, and titanium (Ti) up to 0.03% in 350W. These microalloys refine the grain structure and provide precipitation strengthening, allowing 350W to achieve 350 MPa yield strength without relying on high carbon content. The resulting CEV remains below 0.45 for standard thickness ranges.
Why does preheat matter for thick CSA G40.21 steel sections? As thickness increases, the cooling rate after welding increases due to the larger heat sink effect. Rapid cooling can produce hard, brittle martensite in the heat-affected zone (HAZ), increasing the risk of hydrogen-induced cracking. Preheat slows the cooling rate, allowing hydrogen to diffuse out of the weld zone. Per CSA W59, the preheat requirement depends on CEV and combined thickness — thicker 350W sections (t > 40 mm) require 50-95 °C preheat.
Related Pages
- Canadian Steel Grades — CSA G40.21 Reference
- Canadian Steel Properties — Fy & Fu Tables
- CSA W59 Welding Procedure Specifications
- CSA S16 Beam Design
- Canadian Beam Sizes — W Shapes
- CSA Weld Inspection — Visual, UT, MPI
- CSA S16 Column Design
- Welded Connection Calculator
This page is for educational reference. Chemical composition data per CSA G40.21-18. Verify heat analysis against mill test certificates before welding. Preheat requirements per CSA W59-18 Table 5.1. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.
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