What is the difference between A588 and A242?

ASTM A242 (Corten A) and ASTM A588 (Corten B) are both ASTM standards for high-strength low-alloy structural steels with enhanced atmospheric corrosion resistance (weathering steels). While they share similarities, their key differences lie in mechanical properties, chemical composition (especially phosphorus), and intended structural applications:

Here’s a breakdown of the primary differences:

Phosphorus (P) Content:

A242 (Corten A): Allows for higher phosphorus content (historically up to 0.15%, though modern versions are often lower). Phosphorus significantly boosts atmospheric corrosion resistance.

A588 (Corten B): Strictly limits phosphorus (typically max 0.03-0.04%). This improves weldability and fracture toughness, especially in thicker sections, at the expense of some corrosion resistance.

Tensile Strength:

A242 (Corten A): Specifies a minimum tensile strength of 63 ksi (435 MPa) for Grades 1 & 3, and 70 ksi (485 MPa) for Grade K (a newer, stronger variant).

A588 (Corten B): Specifies a minimum tensile strength of 70 ksi (485 MPa) for all grades. This is the most significant mechanical difference.

Yield Strength:

A242 (Corten A): Minimum yield strength of 50 ksi (345 MPa) for material thicknesses up to 0.75 inches (19 mm) in Grade 1/Grade 3, and higher for Grade K (65 ksi min).

A588 (Corten B): Minimum yield strength of 50 ksi (345 MPa) for all material thicknesses covered by the standard (up to 8 inches / 200mm for some grades). Note this applies to thinner sections; yield strength requirements decrease slightly for very thick sections per both standards.

Weldability & Toughness:

A588 (Corten B): Generally considered to have superior weldability and impact toughness (especially at lower temperatures and in thicker sections), primarily due to its lower phosphorus and carbon content.

A242 (Corten A): While weldable, its higher P content can make it slightly more sensitive in critical welding applications, particularly as thickness increases, requiring stricter adherence to qualified welding procedures.

Intended Application Focus:

A588 (Corten B): Developed for structural applications like bridges, buildings, and equipment requiring high strength-to-weight ratio, weldability, and toughness in thicker sections while maintaining good atmospheric corrosion resistance. Dominant choice for demanding structural projects.

A242 (Corten A): Developed initially to provide enhanced atmospheric corrosion resistance. Often used successfully in thinner sections for architectural applications like cladding, fascia, roofing, and sculpture. Grade K was added later to compete better with A588 on strength. More commonly associated with aesthetic and moderate structural use in Europe.

Ductility (Elongation):

A588 (Corten B): Slightly higher minimum elongation requirements (21% min in 8 inches / 20% in 2 inches) compared to traditional A242 grades.

A242 (Corten A): Minimum elongation for Grade 1/Grade 3 is 18% in 8 inches (15% in 2 inches). Grade K matches A588 at 21% in 8 inches.

Material Forms & Thickness Suitability:

A588 (Corten B): Optimized and commonly used for thicker structural sections (plates, shapes, bars up to 8 inches thick/200mm depending on grade).

A242 (Corten A): More commonly associated with thinner material forms like sheet, strip, and plates (especially Grade K).

Summary Table: ASTM A242 (Corten A) vs. ASTM A588 (Corten B)

In Simple Terms:

A588 (Corten B) is the go-to standard for demanding structural applications. Where high strength, good weldability, and toughness (especially in thicker sections and at lower temperatures) are paramount alongside corrosion resistance, use A588/ASTM A588/Corten B.

A242 (Corten A) is often chosen for architectural applications or moderate structural uses in thinner sections. Where maximum corrosion resistance in thin sections is a priority and the highest tensile strength requirements of A588 are not needed (especially for Grades 1/3), A242/ASTM A242/Corten A is suitable. Grade K of A242 bridges the strength gap for thinner sections.

Always consult the specific requirements of your project and the relevant building codes when selecting between these steels. For critical structural projects involving significant welding or heavy sections, A588/A588M is overwhelmingly the preferred specification.

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