Australian Steel Chemical Composition — AS/NZS 3679.1 Guide
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Complete reference for chemical composition limits of Australian structural steel per AS/NZS 3679.1:2016 and AS/NZS 3678:2016. Carbon equivalent (CEV) values, elemental limits by grade, and weldability implications for AS 4100:2020 design.
Carbon Equivalent Formula (CEV)
AS/NZS 3679.1 specifies: CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
| Element | Effect | Typical Limit |
|---|---|---|
| C | Strength, hardenability | 0.22% max (t ≤ 12 mm) |
| Mn | Strength, toughness | 1.40% max |
| Si | Deoxidiser | 0.50% max |
| P | Impurity | 0.040% max |
| S | Impurity | 0.040% max |
Maximum CEV by Grade and Thickness
| Grade | t ≤ 12 | 12-20 | 20-40 | >40 |
|---|---|---|---|---|
| Gr 300 (section) | 0.44 | 0.46 | 0.48 | 0.48 |
| Gr 300 (plate) | 0.42 | 0.44 | 0.46 | 0.48 |
| Gr 350 (section) | 0.46 | 0.48 | 0.50 | 0.52 |
| Gr 400 (section) | — | 0.48 | 0.50 | 0.52 |
Worked Example
Problem: Grade 300 plate 16 mm, C=0.18%, Mn=1.25%, Si=0.35%, Cr=0.12%, Ni=0.10%, Mo=0.04%, V=0.06%, Cu=0.12%.
Solution: CEV = 0.18 + 1.25/6 + (0.12+0.04+0.06)/5 + (0.10+0.12)/15 = 0.18 + 0.208 + 0.044 + 0.015 = 0.447. Limit for 16 mm = 0.46. Acceptable. Welding without preheat permitted per AS/NZS 1554.1.
Design Resources
- [[Australian Steel Grades|/reference/australian-steel-grades/]] | [[Australian Steel Properties|/reference/australian-steel-properties/]] | [[Australian Beam Sizes|/reference/au-beam-sizes/]] | [[Australian Bolt Capacity|/reference/australian-bolt-capacity/]] | [[AS 4100 Beam Design|/reference/as4100-beam-design-example/]] | [[All Australian References|/reference/]]
FAQ
What is the CEV formula for Australian structural steel? AS/NZS 3679.1 specifies CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. For Grade 300 sections ≤12 mm, max CEV is 0.44%.
What is the maximum carbon content for Grade 300 steel? Max carbon is 0.22% for t≤12 mm and 0.25% for thicker sections. Grade 300PLUS controls below 0.20%.
Does Grade 400 steel require preheat? CEV 0.45-0.52%, 50-100°C preheat per AS/NZS 1554.1 for t>16 mm in restrained conditions.
Educational Use Only — This reference is for educational and preliminary design purposes only. All structural designs must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) in accordance with AS 4100:2020 and all applicable Australian Standards. Results are not for construction.
Design Applications
Common Design Scenarios
This reference covers structural design scenarios commonly encountered in Australian steel design practice:
- Strength verification: Check member or connection capacity against factored loads per the applicable design code
- Serviceability checks: Verify deflections, vibrations, and other serviceability criteria
- Code compliance: Ensure design meets all provisions of the governing standard
- Connection detailing: Verify weld sizes, bolt quantities, and edge distances
Related Design Considerations
- System behavior: consider the interaction between members and connections
- Load paths: verify that forces can be transferred through the structure to the foundations
- Constructability: check that the design can be fabricated and erected practically
- Cost optimization: evaluate alternative sections or connection types for economy
Worked Example
Problem: Verify a Grade 300 member for the following conditions:
Typical span: 6.0 m | Load: service loads per applicable code | Section: common section in this category
Design Check:
- Determine governing load combination (LRFD or ASD per applicable code)
- Calculate maximum internal forces (moment, shear, axial)
- Compute nominal capacity per code provisions
- Apply resistance/safety factors
- Verify interaction if combined forces exist
Result: Use the results from the Steel Calculator tool to verify design adequacy.
Chemical Composition and Weldability
The chemical composition of structural steel directly affects weldability, which is a critical consideration in Australian steel fabrication.
Carbon Equivalent (CEV)
The carbon equivalent value provides a measure of weldability per AS/NZS 1554:
CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
Recommended limits:
- CEV ≤ 0.41 for excellent weldability (no preheat required)
- CEV ≤ 0.45 for good weldability (preheat may be required above 25mm)
- CEV > 0.45 requires controlled welding procedures and preheat
Grade 300 Chemical Composition (AS/NZS 3679.1)
Maximum allowable percentages for universal sections:
- Carbon (C): 0.22% max — higher C increases strength but reduces weldability and ductility
- Silicon (Si): 0.50% max — deoxidizer, improves strength at the cost of toughness
- Manganese (Mn): 1.70% max — strengthens through solid solution, improves notch toughness
- Phosphorus (P): 0.04% max — must be limited to avoid embrittlement
- Sulfur (S): 0.04% max — must be limited to prevent hot shortness in welding
- Nitrogen (N): 0.008% max — high N reduces toughness and weldability
Frequently Asked Questions
What Australian Standard governs structural steel design?
AS 4100-2020 (Steel Structures) is the primary standard for structural steel design in Australia. It covers all aspects of design including member capacity, connections, serviceability, and fire resistance. The standard uses a limit states design philosophy with resistance factors (φ) applied to nominal capacities. Companion standards include AS/NZS 3679.1 for hot-rolled sections, AS/NZS 1554 for welding, and AS/NZS 4600 for cold-formed steel.
What are the common steel grades used in Australian construction?
The most common steel grades for Australian construction are Grade 300 and Grade 350 per AS/NZS 3679.1. Grade 300 (minimum yield 300 MPa for sections > 12 mm thick) is the standard for general structural applications. Grade 350 (minimum yield 340 MPa for sections > 12 mm) is used where higher strength reduces weight. Grade 400 and Grade 450 are available for specialized applications requiring higher strength-to-weight ratios.
How does AS 4100 compare to AISC 360?
Both AS 4100 and AISC 360 use limit states design (LRFD) principles. Key differences include: AS 4100 uses a single "capacity factor" φ approach rather than separate φ for different failure modes; AS 4100 specifies distinct buckling curves for hot-rolled and welded sections; the moment capacity formula in AS 4100 uses αm factor directly rather than Cb; and AS 4100 has more detailed provisions for slender sections and combined actions. Despite philosophical differences, both codes produce similar results for typical members.