If you are comparing A588 Grade A and A588 Grade C, the first thing to know is that the two grades have the same minimum mechanical properties - 50 ksi [345 MPa] yield strength through 100 mm plate, per ASTM A588/A588M.
The original version of this article suggested Grade C is stronger at thicker sections; that is not correct.
What actually separates the two grades is their chemical composition, and what that means for welding, cold forming and weathering behaviour.
This guide explains the differences with data, and tells you how to choose.
Key Takeaways
Same strength, different chemistry: Grade A and Grade C both meet the unified mechanical table of ASTM A588/A588M (Grades A, B, C, K). Strength is not a selection factor between them.
Grade C is leaner in carbon: C ≤ 0.15% vs Grade A's C ≤ 0.19%, with a defined Ni range and a higher Mn cap - which generally improves weldability and cold-forming behaviour.
Both are weathering (COR-TEN-type) grades: Cu–Cr–Ni alloying is designed to form a stable patina, so painting is optional in suitable atmospheres.
Choose by welding programme, project spec and mill availability - not by strength. Confirm every heat with the MTC.
1. What are A588 Grade A and Grade C?
ASTM A588/A588M is the standard specification for high-strength, low-alloy structural steel with a 50 ksi [345 MPa] minimum yield point, developed for atmospheric corrosion resistance. Plates of this type are commonly called weathering steel. The specification currently covers Grades A, B, C and K; this article compares Grade A and Grade C.
Both grades are used in exposed structural applications - bridge girders, building frames, transmission towers, rail cars and shipping containers - where the stable oxide patina removes the need for painting over the design life.
2. Chemical composition: Grade A vs Grade C (corrected tables)
This is where the two grades really differ. The tables below are separately labelled - the original page duplicated the "Grade C" title onto both tables, which mislabelled the Grade A data. Values follow ASTM A588/A588M composition limits.
Table 1 - ASTM A588 Grade A vs Grade C chemical composition (%, max unless shown as a range)
| Element | Grade A | Grade C | What it does |
|---|---|---|---|
| C | ≤ 0.19 | ≤ 0.15 | Core strength element; Grade C's lower cap improves weldability and reduces preheat risk. |
| Mn | 0.80 – 1.25 | 0.80 – 1.35 | Strength and toughness; Grade C's wider range partly compensates its lower carbon. |
| P | ≤ 0.040 | ≤ 0.040 | Contributes to the patina, but capped for weldability and ductility. |
| S | ≤ 0.050 | ≤ 0.050 | Impurity; kept low for ductility and toughness. |
| Si | 0.30 – 0.65 | 0.15 – 0.40 | Deoxidiser and patina contributor; Grade A carries more silicon. |
| Cu | 0.25 – 0.40 | 0.20 – 0.50 | The key patina-forming element in both grades. |
| Ni | ≤ 0.40 | 0.20 – 0.50 | Stabilises the patina and improves toughness; Grade C specifies a defined range. |
| Cr | 0.40 – 0.65 | 0.30 – 0.50 | Patina stability and strength; Grade A carries a higher range. |
| V | 0.02 – 0.10 | 0.02 – 0.10 | Grain refinement and precipitation strengthening in both grades. |
Note: Nb and Mo are not specified for Grade A or Grade C in ASTM A588/A588M - the broken ".." / "…" placeholders on the original page are removed. Composition limits shown are per the standard; actual heat chemistry appears on the MTC.
3. Mechanical properties: one table for all grades
Per ASTM A588/A588M Table 2, the minimum mechanical values apply equally to Grades A, B, C and K. Grade C is not stronger than Grade A at any thickness - the original article's statement was corrected. The values step down only with plate thickness:
Table 2 - Mechanical properties per ASTM A588/A588M (Grades A / B / C / K)
| Plate thickness | Tensile strength, min | Yield strength, min | Elongation in 2 in. [50 mm], min, % |
|---|---|---|---|
| ≤ 100 mm (≤ 4 in) | 70 ksi [485 MPa] | 50 ksi [345 MPa] | 21 |
| 100 < t ≤ 125 mm (4 – 5 in) | 67 ksi [460 MPa] | 46 ksi [315 MPa] | 21 |
| 125 < t ≤ 200 mm (5 – 8 in) | 63 ksi [435 MPa] | 42 ksi [290 MPa] | 21 |
Note: Charpy impact testing is not a standard requirement of ASTM A588/A588M. If your design needs guaranteed impact properties (e.g. low service temperatures), specify the relevant supplementary requirement at enquiry and confirm the agreed values on the MTC.
4. Grade A vs Grade C: what actually differs
Table 3 - Side-by-side difference comparison
| Aspect | A588 Grade A | A588 Grade C | Practical impact |
|---|---|---|---|
| Max carbon | 0.19% | 0.15% | Grade C is more forgiving in welding; lower hardenability and preheat risk. |
| Silicon range | 0.30 – 0.65% | 0.15 – 0.40% | Grade A deoxidises with more Si; Grade C leans on Al/V practice. |
| Manganese range | 0.80 – 1.25% | 0.80 – 1.35% | Grade C uses a slightly higher Mn cap to recover strength lost by lower C. |
| Copper range | 0.25 – 0.40% | 0.20 – 0.50% | Both remain firmly weathering grades; patina forms in both. |
| Nickel | ≤ 0.40% | 0.20 – 0.50% | Grade C has a guaranteed Ni floor - steadier patina and toughness. |
| Chromium range | 0.40 – 0.65% | 0.30 – 0.50% | Grade A carries more Cr; both meet the atmospheric-corrosion intent. |
| Vanadium | 0.02 – 0.10% | 0.02 – 0.10% | Identical grain-refining addition. |
| Yield / tensile / elongation | Identical (Table 2) | No strength advantage either way. | |
| Typical position | General-purpose weathering plate, very widely specified | Chosen when lower carbon / defined Ni range matter, e.g. heavy welding | Confirm supply availability per mill. |
5. Application and selection guide
- Typical applications (both grades): bridge girders and trusses, building frames, transmission and antenna towers, rail rolling stock, shipping containers, sign and barrier structures - exposed steelwork where painting is undesirable.
- How to choose between Grade A and Grade C:
- Check what the project specification names. Many structural specs call out "A588 Grade A" or "Grade B" by name. If the spec says Grade C, order Grade C; if it only says "A588", either grade satisfies the mechanical table - ask your engineer.
- Evaluate the welding programme. Heavy, restrained joints with limited preheat control favour Grade C: its 0.15% carbon cap and lower silicon reduce hardenability and hydrogen-cracking risk.
- Compare availability and sizes. Both grades cover plates up to 200 mm in the standard table, but mill rolling programmes differ. Confirm the exact thickness × width × length range, delivery condition (typically as-rolled or normalized) and impact-test options at quotation.
- Verify with the MTC. Every heat ships with a certificate stating actual chemistry and mechanical results; check the values against Table 1 and Table 2 before release.
6. Welding and fabrication notes
- A588 welds readily with SMAW, GMAW, FCAW and SAW using low-hydrogen consumables; follow the project WPS (e.g. AWS D1.1) for preheat and interpass control.
- Grade C's lower carbon content is an advantage in welding - less preheat is generally needed at equal thickness and restraint.
- Control heat input to avoid grain growth in the heat-affected zone, and remove weld slag before the patina forms so the oxide layer develops evenly.
- Cold forming is possible within normal limits; avoid sharp bends at low ambient temperature.
- For the patina to work, the plate needs alternating wet/dry cycles. Avoid permanent immersion, buried service, or heavy chloride splash unless the surface is painted - and remember patina rub-off onto adjacent materials in the first 1–2 years.
7. FAQ
Which is stronger: A588 Grade A or A588 Grade C?
Neither. Per ASTM A588/A588M Table 2, Grades A, B, C and K all meet the same minimum mechanical requirements: 50 ksi [345 MPa] yield up to 100 mm, 46 ksi [315 MPa] for 100–125 mm and 42 ksi [290 MPa] for 125–200 mm. The original article's claim that Grade C keeps higher strength was incorrect; the real differences are chemical.
Is A588 the same as COR-TEN steel?
ASTM A588/A588M is the specification most commonly specified for COR-TEN-type weathering steel plates. Confirm the exact grade correspondence on the Mill Test Certificate.
Does A588 plate need to be painted?
No. A588 is designed to develop a stable protective patina in suitable atmospheric conditions (alternating wet/dry cycles). Painting is recommended for permanent immersion, heavy chloride splash or sheltered indoor exposure.
What thickness range does ASTM A588 cover?
The standard's mechanical table covers plates up to 200 mm (8 in); minimum strengths step down at 100 mm and 125 mm. Mill supply range for each grade should be confirmed at quotation.
Which grade is better for heavy welding?
Grade C's lower maximum carbon (0.15% vs 0.19%) and silicon make it more forgiving in welding and cold forming. Final grade selection also depends on the project specification, availability and the MTC.
Need A588 Grade A or Grade C weathering steel plate?
Send us: grade · size (thickness × width × length) · quantity · delivery condition · supplementary requirements (impact test, through-thickness Z, CEV limit) · delivery port. We will confirm mill availability, price and a realistic schedule.
- Email: alloy@gneesteelgroup.com
- Tel / WhatsApp: +86-15824687445
Gnee Steel (Tianjin) Co., Ltd - No.4-1114, Beichen Building, Beicang Town, Beichen District, Tianjin, China
Typical lead time 30–45 days subject to mill schedule - confirm at quotation. All chemical and mechanical values are verified against the Mill Test Certificate.
