What do A588 Grade A and Grade B each mean?
A588 Grade A and A588 Grade B are two specific grades within the ASTM A588 standard for high-strength, low-alloy structural steel shapes, plates, and bars with improved atmospheric corrosion resistance (commonly called "weathering steel").
Their core identity is the same: both are weathering steels with a minimum yield strength of 50 ksi (345 MPa) that form a protective patina to resist atmospheric corrosion, allowing them to be used unpainted in many applications. The differences lie in their chemical composition (which affects weldability, strength consistency, and corrosion rate) and some mechanical property details.
Core Similarities (A588 Grade A & Grade B)
Yield Strength: ≥ 50 ksi (345 MPa) for material up to 4 inches thick.
Tensile Strength: ≥ 70 ksi (485 MPa).
Primary Application: Unpainted bridges, building exteriors, exposed structural sculptures, and utility structures where the aesthetic weathering patina is desired and maintenance is to be minimized.
Key Feature: Atmospheric corrosion resistance approximately 4 times that of carbon steel (e.g., A36).
Key Differences: A588 Grade A vs. Grade B
| Feature | A588 Grade A | A588 Grade B |
|---|---|---|
| Defining Chemistry | "Copper-Steel" Type. Relies heavily on Copper (Cu) as the primary corrosion-resisting element. Phosphorus (P) is also used. | "Chromium-Silicon" Type. Uses a balance of Chromium (Cr) and Silicon (Si) as primary alloying elements, with Copper. Phosphorus is not a specified element. |
| Key Chemical Limits | Cu: 0.25-0.40% min (critical) P: 0.04% max (can be used up to this limit) C: ≤ 0.19% (for shapes) |
Cu: 0.20% min Cr: 0.40-0.65% Si: 0.30-0.65% P: 0.04% max, but NOT required (typically lower) C: ≤ 0.20% (for shapes) |
| Carbon Equivalent (CEV) | Generally lower due to lower carbon and absence of high Cr/Si. | Slightly higher due to the addition of Cr and higher Si, which increase hardenability. |
| Primary Metallurgical Characteristic | Older, simpler chemistry. Can be more prone to banding (segregation) due to phosphorus, affecting through-thickness properties. | More balanced, modern alloy design. Typically has more uniform through-thickness properties and better weldability predictability. |
| Common Product Forms | Historically used for shapes (beams, channels). Less common today. | The predominant grade for modern production of both shapes and plates. Considered the more reliable, general-purpose weathering steel. |
Why the Different Grades Exist & How to Choose
The grades represent different historical and metallurgical approaches to achieving weathering resistance.
A588 Grade A: An older, "copper-phosphorus" formulation. Phosphorus strengthens and aids corrosion resistance but can make the steel more brittle and prone to segregation during solidification. Its use has declined in favor of Grade B.
A588 Grade B: A newer, "chromium-silicon-copper" formulation. This chemistry provides excellent corrosion resistance without relying on phosphorus, leading to better overall toughness, weldability, and consistency. It is now the industry-standard, default choice for most weathering steel applications.
Practical Selection Guide:
For New Designs (Bridges, Architecture): Specify A588 Grade B. It is the modern, preferred grade with more consistent properties and wider availability.
For Matching Existing Structures: If an existing structure (e.g., an older bridge) was built with Grade A, specifying Grade A for repairs or additions may be necessary for compatibility.
If Not Specified: Most mills and fabricators will default to supplying A588 Grade B when the generic "A588" is called for on drawings.
Summary:
Both A588 Grade A and Grade B are 50 ksi yield strength weathering steels. Grade A is an older copper-phosphorus type, while Grade B is the modern chromium-silicon-copper type that offers better overall weldability and consistency. For all new projects, A588 Grade B (for shapes) or A588 Grade K (for plates) is the unambiguous and recommended choice.
1. What is the primary corrosion-resistant element in A588 Grade A?
The primary element is Copper (Cu), required at a minimum of 0.25-0.40%, which drives the formation of its protective weathering patina. It may also utilize Phosphorus (P) for added resistance.
2. How does A588 Grade A differ chemically from A588 Grade B?
Grade A is a "Copper-Phosphorus" steel, while Grade B is a "Chromium-Silicon-Copper" steel. Grade A relies on Cu and P; Grade B uses Cr, Si, and Cu without relying on P, leading to better weldability and uniformity.
3. Is A588 Grade A still commonly used in new construction?
No, it is largely obsolete for new designs. Modern specifications overwhelmingly prefer A588 Grade B (for shapes) or Grade K (for plates) due to their more consistent properties and phosphorus-free chemistry.
4. What is a key fabrication concern with A588 Grade A?
A potential concern is banding or segregation due to its permissible phosphorus content, which can affect through-thickness ductility and toughness, requiring attention in heavy welding or forming.
Full specification and details are available on request. The above information is provided for guidance purposes only. For specific design requirements please contact our technical sales staff.
