Australian Steel Grades Guide — Grade 300, 350, 400 per AS/NZS 3678 & 3679.1

Complete reference for Australian structural steel grades covering AS/NZS 3678:2016 (hot-rolled plates) and AS/NZS 3679.1:2016 (hot-rolled sections). Includes comprehensive mechanical property tables for Grades 250, 300, 350, and 400 across all thickness brackets, Charpy V-notch impact designations (L0, L15, and no suffix), Carbon Equivalent Values (CEV) for weldability assessment, and practical grade selection guidance for Australian construction projects.

Quick access: AS 4100 Steel Design Overview | Australian Steel Sections Guide | AS 4100 Base Plate Design | Weld Capacity Calculator

Australian Steel Grade Classification System

Australia uses a grade number system where the grade designation equals the minimum yield strength (f_y) in MPa for the smallest thickness bracket (t <= 12 mm). This contrasts with the ASTM system (A36 = 36 ksi = 250 MPa, A572 Gr 50 = 50 ksi = 345 MPa) and the European system (S235, S275, S355, S460 where the number is f_y in MPa).

Two complementary standards govern Australian structural steel:

Both standards share identical grade designations and largely identical mechanical property requirements, though chemical composition limits may differ slightly to account for different rolling processes.

Grade Summary Table

Grade f_y (t <= 12 mm) f_u Range (MPa) Typical Application CEV (typical)
Grade 250 250 MPa 410 – 540 Secondary framing, non-structural, legacy projects 0.30–0.35
Grade 300 300 MPa 440 – 600 General structural — default Australian grade for beams, columns 0.36–0.40
Grade 300 L0 300 MPa 440 – 600 Primary members requiring Charpy at 0°C 0.36–0.40
Grade 300 L15 300 MPa 440 – 600 Fracture-critical and alpine region applications 0.36–0.40
Grade 350 350 MPa 480 – 620 High-strength for heavy columns, plate girders, crane beams 0.38–0.43
Grade 350 L0 350 MPa 480 – 620 As Grade 350 + Charpy at 0°C 0.38–0.43
Grade 350 L15 350 MPa 480 – 620 Heavy sections in fracture-critical applications 0.38–0.43
Grade 400 400 MPa 540 – 700 Specialised high-strength, crane runway girders 0.40–0.45

300PLUS is the proprietary brand name used by InfraBuild (formerly OneSteel/Arrium) for Grade 300 sections. It meets all AS/NZS 3679.1 Grade 300 requirements and is the default supply grade for all UB, UC, PFC, EA, and UA sections from Australian mills. When you order a UB section in Australia without specifying a grade, you receive 300PLUS.

Thickness-Dependent Yield Strength

A critical nuance of Australian grades is the reduction in yield strength with increasing material thickness. Thicker sections cool more slowly after hot rolling, resulting in a coarser grain structure and reduced strength. AS 4100:2020 Table 2.1 defines the design yield strengths accounting for this thickness effect.

Grade 300 — Full Thickness Range

Thickness t (mm) f_y (MPa) f_u (MPa) Notes
t <= 12 300 440–600 Standard for web and flange up to 12 mm
12 < t <= 20 280 440–600 6.7% reduction from the 12 mm value
20 < t <= 25 270 440–600 Heavy UC flanges, thick plates
25 < t <= 32 260 440–600 310UC158 flanges (t_f = 25.0 mm)
32 < t <= 40 250 410–580 Heavy plate girders >32 mm flange
40 < t <= 50 240 410–560 Very heavy plates
50 < t <= 63 230 400–560 Extreme thickness applications

Grade 350 — Full Thickness Range

Thickness t (mm) f_y (MPa) f_u (MPa) Notes
t <= 12 350 480–620 17% higher than Grade 300 at thin sections
12 < t <= 20 330 480–620 Plate girder web and flange plates
20 < t <= 25 320 480–620 Heavy columns
25 < t <= 32 310 470–620 Thick flange plates
32 < t <= 40 300 460–610 Equivalent to Grade 300 at t <= 12
40 < t <= 50 290 450–600 Very heavy sections
50 < t <= 63 280 440–590 Extreme thickness

Grade 400 — Full Thickness Range

Grade 400 is a high-strength grade used primarily for crane runway beams and weight-critical structures. It is less commonly stocked and may require mill orders with minimum quantities.

Thickness t (mm) f_y (MPa) f_u (MPa) Notes
t <= 12 400 540–700 Maximum standard rolled section strength
12 < t <= 20 380 540–700 Thin plate girders
20 < t <= 32 360 520–690 Crane runway beam flanges
32 < t <= 50 340 500–680 Heavy crane girders

Charpy Impact Testing and Toughness Designations

Charpy V-Notch (CVN) Test

The Charpy test measures the energy absorbed when a standard notched specimen is fractured by a pendulum impact at a specified temperature. It provides a measure of the steel's resistance to brittle fracture — critical for structures subject to low temperatures, dynamic loading, or with crack-like defects.

Australian grades offer three Charpy designation levels:

Designation Test Temp Min Absorbed Energy (Longitudinal) Application
No suffix N/A Not required Secondary members, interior non-fracture-critical, small structures
L0 0°C 27 J Primary members in most Australian climates, standard commercial work
L15 -15°C 27 J Alpine regions, Tasmania, exposed structures in cold climates

When to Specify Charpy Testing

Per AS 4100:2020 Section 13 (Fracture Control):

For most commercial and industrial buildings in Sydney, Melbourne, Brisbane, Adelaide, and Perth, L0 designation is the standard specification.

CEV and Weldability

The Carbon Equivalent Value (CEV) predicts the hardenability of steel and its susceptibility to hydrogen-assisted cold cracking (HACC) during welding. Lower CEV indicates better weldability with reduced preheat requirements.

The IIW (International Institute of Welding) formula adopted by AS/NZS 1554.1 is:

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

Where all elements are expressed as weight percentages from the mill certificate.

Australian Grade CEV Ranges

Grade Typical CEV Range Weldability Assessment
Grade 250 0.28–0.35 Excellent — no preheat for most thicknesses
Grade 300 0.36–0.40 Good — no preheat typically for t < 30 mm
Grade 350 0.38–0.43 Good to moderate — preheat for t > 25 mm, harder grades
Grade 400 0.40–0.45 Moderate — preheat typically required for t > 20 mm

Preheat Requirements per AS/NZS 1554.1

AS/NZS 1554.1:2014 Table 4.1 specifies minimum preheat temperatures based on CEV, combined thickness, and hydrogen scale of the welding consumable:

CEV Combined Thickness Hydrogen Scale C Hydrogen Scale D
< 0.41 < 30 mm No preheat No preheat
0.41–0.43 30–50 mm 50°C 100°C
0.43–0.45 30–50 mm 75°C 150°C
> 0.45 > 20 mm 100°C 200°C

Combined thickness = sum of thickness of all parts being joined at the weld. Hydrogen scale C = basic (low-hydrogen) electrodes. Hydrogen scale D = rutile or cellulosic electrodes (higher hydrogen risk).

Practical CEV Implications for Common Sections

For a 310UB46.2 Grade 300 (flange t_f = 11.8 mm, web t_w = 6.7 mm), CEV is typically 0.38. Welding the flange to an end plate of equal or lesser thickness (combined thickness ~24 mm) requires no preheat with low-hydrogen electrodes. This makes Grade 300 the most shop-friendly and site-friendly grade for Australian steel fabricators.

For a 310UC158 Grade 300 (flange t_f = 25.0 mm), CEV may be 0.40 at the upper end. Welding to a 20 mm base plate (combined thickness = 45 mm) with low-hydrogen electrodes requires 50°C preheat. If rutile electrodes are used (hydrogren scale D), preheat increases to 100°C. The thicker the section, the more important it is to specify and control preheat.

Chemical Composition

Typical Ladle Analysis Ranges (Weight %)

Element Grade 250 Grade 300 Grade 350 Grade 400
C 0.18–0.22 0.18–0.22 0.18–0.22 0.18–0.22
Mn 0.50–1.20 0.80–1.50 0.80–1.50 1.00–1.60
Si 0.10–0.40 0.10–0.40 0.15–0.50 0.15–0.50
P (max) 0.040 0.040 0.040 0.035
S (max) 0.040 0.040 0.035 0.035
Cr (max) 0.30 0.30 0.30 0.30
Ni (max) 0.30 0.30 0.30 0.30
Cu (max) 0.40 0.40 0.35 0.35
V + Nb 0.03 0.03 0.05 0.08
CEV max 0.43 0.45 0.45 0.47

The manganese content is the primary alloying element controlling strength in Australian grades. Higher grades achieve their increased yield and tensile strength primarily through higher Mn content rather than higher carbon (which would impair weldability). The micro-alloying elements V (vanadium) and Nb (niobium) are present in small quantities to refine grain size, particularly in Grade 350 and 400.

Grade Selection Guide for Australian Projects

Typical Structural Building — Office, Retail, Residential

Specification: Grade 300 L0

All rolled sections (UB, UC, PFC) are supplied in 300PLUS as standard. Plates for base plates, stiffeners, and connection components should also be Grade 300 to AS/NZS 3678. The L0 designation provides a baseline toughness level acceptable for primary structural members. This is the most economical specification and covers 90%+ of Australian building projects.

Industrial Facility — Warehouse, Factory, Processing Plant

Specification: Grade 300 L0 for primary frame, Grade 250 for secondary members

The main portal frame members (rafters, columns) use Grade 300. Purlins and girts may use Grade 250 if cold-formed C or Z sections are specified, or Grade 300 PFC sections. Crane runway beams: specify Grade 350 or 400 depending on span and duty classification. Crane columns supporting heavy EOT cranes (Class C or D per AS 1418) should also use Grade 350.

High-Rise Building (> 20 storeys)

Specification: Grade 350 for columns below transfer level, Grade 300 elsewhere

The lowest 5-8 storeys of high-rise columns may benefit from Grade 350 due to high axial loads. The 17% strength increase can reduce column sizes by one or two serial sizes, saving valuable floor area. Above the transfer level, revert to Grade 300. All grade transitions must be clearly marked on structural drawings and shop drawings — fabricators cannot visually distinguish Grade 300 from Grade 350.

Coastal or Marine Environment

Specification: Grade 300 L15 + corrosion protection

Structures within 1 km of the coast should specify L15 for improved toughness, as the combination of corrosion and low ambient temperature increases fracture risk. Additional corrosion protection (hot-dip galvanising to AS/NZS 4680 or a durable paint system to AS 2312) is required regardless of grade.

Cyclonic Region (Region C and D per AS 1170.2)

Specification: Grade 300 L0 minimum, consider L15 for tension members

Cyclonic wind loading produces high stress reversals. L0 is the minimum for primary framing. For connections subject to stress reversal and members that could experience tension during a cyclone event, L15 provides additional safety margin against brittle fracture during the extreme loading event.

Design Strength vs Nominal Grade

When performing capacity calculations per AS 4100, use the design yield strength f_y from AS 4100 Table 2.1, not the nominal grade number. This is a common source of error:

The Steel Calculator section properties tool automatically applies the correct thickness-dependent f_y based on the section geometry and specified grade.

Comparison: Australian vs International Grades

Australian Grade Nearest US (ASTM) Nearest European (EN 10025) Notes
Grade 250 A36 S235 Legacy grade, rarely used for new work
Grade 300 A572 Gr 50 S275 / S355 Sits between S275 and S355
Grade 350 A572 Gr 55 S355 Closest equivalent to S355
Grade 400 A572 Gr 60/65 S420 / S460 Higher strength bracket

For international projects where Australian steel is procured under a different code, verify that the specified grade meets the minimum requirements of both the procurement standard and the design standard. Grade 300 with CEV <= 0.43 and L0 Charpy satisfies most international structural requirements for temperate climates.