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

Maximum CEV by Grade and Thickness

Worked Example per AS/NZS 1554.1

Problem: A fabricator welds a Grade 350 column splice using full-penetration butt welds. Flange plates are 28 mm thick. Mill certificate reports: C = 0.16%, Mn = 1.38%, Si = 0.32%, Cr = 0.08%, Ni = 0.07%, Mo = 0.03%, V = 0.05%, Cu = 0.15%. Determine CEV, preheat requirements per AS/NZS 1554.1, and verify compliance with AS/NZS 3679.1.

Step 1 — CEV Calculation per AS/NZS 1554.1 Clause 4.2: CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 CEV = 0.16 + 1.38/6 + (0.08 + 0.03 + 0.05)/5 + (0.07 + 0.15)/15 CEV = 0.16 + 0.230 + 0.032 + 0.015 = 0.437

Step 2 — CEV Limit per AS/NZS 3679.1 Table 3: For Grade 350 section, 28 mm thick (20-40 mm bracket): max CEV = 0.48. CEV = 0.437 < 0.48. Compliance confirmed.

Step 3 — Preheat Determination per AS/NZS 1554.1 Table 4.1: For Grade 350, CEV 0.437, t = 28 mm:

• Low hydrogen electrodes (HD ≤ 5 mL/100g): Minimum preheat 50°C per Table 4.1
• For highly restrained butt weld splice (restraint factor ≥ 0.5): Increase by 25°C → preheat to 75°C
• Interpass temperature: 150-250°C per AS/NZS 1554.1 Clause 4.7

Step 4 — Carbon Content Verification per AS/NZS 3679.1: For Grade 350, 20-40 mm thickness: max C = 0.22%. Actual C = 0.16% < 0.22%. OK. Mn: max 1.60%. Actual 1.38% < 1.60%. OK. P: need mill certificate (max 0.035% per Table 3). S: need mill certificate (max 0.035% per Table 3).

Step 5 — Electrode Selection per AS/NZS 1554.1 Table 4.2: For Grade 350: E48XX (AS/NZS 4855) or E49XX low-hydrogen electrodes. Matching tensile strength required: electrode fu ≥ base metal fu (450 MPa minimum for Grade 350). E48XX provides 480 MPa minimum — matching strength confirmed.

Step 6 — Heat Input per AS/NZS 1554.1 Annex C: Recommended heat input: 1.0-2.5 kJ/mm for Grade 350. Calculated: Q = 0.8 × 28V × 260A / (375 mm/min / 60) = 0.8 × 7280 / 6.25 = 0.93 kJ/mm. Within recommended range. Adequate toughness expected.

Result: CEV = 0.437 < 0.48 (Grade 350 limit for 28 mm). Preheat 75°C (50°C base + 25°C high restraint per AS/NZS 1554.1 Table 4.1). E48XX low-hydrogen electrode. All per AS/NZS 1554.1:2014 Clause 4.2 and AS/NZS 3679.1:2016 Table 3.

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:

1. Determine governing load combination (LRFD or ASD per applicable code)
2. Calculate maximum internal forces (moment, shear, axial)
3. Compute nominal capacity per code provisions
4. Apply resistance/safety factors
5. 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.

Related pages

• AS 4100 code provisions
• Australian steel grades chart
• Steel material properties reference
• Steel design tools directory