What is the difference between ASTM A572 Grade 50 and ASTM A709 Grade 50?
While ASTM A572 Grade 50 and ASTM A709 Grade 50 share the same 50 ksi (345 MPa) minimum yield strength and are often produced from the same steel melt, they are governed by different specifications for different purposes. The differences are crucial for code compliance and application.
ASTM A572 Grade 50 is a high-strength low-alloy (HSLA) structural steel specified in the ASTM A572/A572M standard. It is the most common "50 ksi yield" steel in North America, strengthened through microalloying (e.g., with Columbium/Vanadium) during hot-rolling rather than heat treatment. It is primarily used in building construction, general fabrication, and heavy equipment where a balance of higher strength (compared to A36), good weldability, formability, and cost-effectiveness is required. Its key limitation is that impact toughness is not a mandatory requirement unless specifically ordered as a supplementary option.
ASTM A709 Grade 50 is the bridge-specific version of 50 ksi yield strength steel, governed by the ASTM A709 standard and mandated by the AASHTO bridge design code. It shares the same minimum yield and tensile strength as A572 Grade 50 but includes mandatory impact toughness requirements to ensure fracture-critical safety under dynamic loads and temperature variations in highway bridges. It also includes a weathering variant (Grade 50W) for unpainted applications. This grade represents a "qualified" material with stricter quality assurance, making it the legally required and technically appropriate choice for bridge construction, where reliability in severe service conditions is paramount.
1. Application
ASTM A572 Grade 50:
ASTM A572 Grade 50 is a globally known steel specification used in various general structural applications. This steel is usually utilized in the construction of buildings, equipment, and transmission towers. It is also suitable for use as a secondary component of bridges, indicating an important role in diverse construction projects.
ASTM A709 Grade 50:
ASTM A709 Grade 50 is specifically designed for bridge construction. This steel grade was formulated to satisfy the need for bridges to withstand the various stresses, loads, and demanding environmental conditions.
2. Mechanical Properties
ASTM A572 Grade 50
ASTM A572 Gr. 50 is engineered for strength and reliability. It boasts a minimum yield strength of 50 ksi (345 MPa) and a minimum tensile strength of 65 ksi (450 MPa). Depending on the form and thickness, elongation values can range between 18% and 21% over a 2-inch stretch. This signifies a good balance between strength and flexibility.
ASTM A709 Grade 50
ASTM A709 Grade 50 provides significant advantages, particularly in bridge construction. Its tensile strength, yield strength, and elongation values are the same as A572 Grade 50. The difference between the two is that A709 often imposes stricter requirements. This strictness is to ensure that it performs ideally in critical, heavy-duty applications like bridge loading.
3. Corrosion Resistance
ASTM A572 Grade 50
ASTM A572 Grade 50 offers good resistance to atmospheric corrosion due to its low-alloy composition. The composition, which is better than that of standard carbon steel, includes elements like columbium and vanadium that enhance its corrosion resistance. It is important to remember that ASTM A572 Grade 50 is not classified as "weathering steel," unlike some grades mentioned in A709. As a result, ASTM A572 Grade 50 generally requires protective measures like painting or other coatings, especially in harsh environments where prolonged exposure to the elements cannot be avoided.
ASTM A709 Grade 50
ASTM A709 Grade 50 is also widely known for its strength and its ability to withstand various harsh environmental conditions. Unlike A572, it includes "Weathering Steel" options, such as A709 Grade 50W. This grade is enhanced with elements such as copper, chromium, and nickel to help the steel develop a durable, rust-like patina that reduces corrosion. With proper design and use, structures made from Grade 50W do not require painting most of the time, which leads to significant maintenance savings.
4. Certification and Testing
ASTM A572 Grade 50
ASTM A572 Grade 50 requires a mill certification to verify its compliance with the chemical and mechanical property standards stated in the ASTM A572 specification. The standard testing includes the measurement of tensile, yield strength, and elongation. A chemical analysis of the heat is also conducted as an additional test and then the result is reported.
Charpy V-notch (CVN) impact testing can be requested as a supplementary requirement by the purchaser, even if it is not mandatory. In CVN testing, the testing parameters, such as temperature and energy absorption levels, must be mutually agreed upon by both the buyer and the seller.
ASTM A709 Grade 50
ASTM A709 Grade 50 steel for bridge projects requires a dual certification. The certification must cover both the ASTM A709 standard and the AASHTO M270 specification because AASHTO establishes standards to ensure the safety and reliability of our highway and transportation infrastructure.
The certification process involves tests for yield strength, tensile strength, and elongation, as well as a comprehensive chemical analysis like that in A572. Charpy V-notch impact testing is mandatory for this steel grade.
Testing requirements will vary based on the climate zone where the bridge is located, and these zones are either Zone 1, Zone 2, or Zone 3. If a part is considered a fracture-critical member where failure could lead to serious issues, testing requirements become stricter and are done on a plate-by-plate basis.
5. Cost
ASTM A572 Grade 50
ASTM A572 Grade 50 is more affordable than the standard A709 Grade 50. This is all due to its less stringent mandatory testing requirements and simplified certification processes. This grade of steel is mass-produced, making it suitable for a wider range of applications. The price for ASTM A572 Grade 50 plates generally ranges approximately between $410 to $810 USD per ton. If you are going to purchase in smaller quantities from metal suppliers, prices will skyrocket, often reaching between $6 – $8 USD per pound for cut pieces.
ASTM A709 Grade 50
ASTM A709 Grade 50 has higher costs due to several factors, such as the mandatory Charpy V-notch impact testing, stricter quality control measures, and stringent certification processes that often mandate adherence to AASHTO M270.
In terms of price, ASTM A709 Grade 50 plates vary significantly, usually ranging between $610 to $1,650 USD per ton or even more. This fluctuation in price is influenced by the grade of the steel, especially for weathering steel.
Chemical Composition about ASTM A572 Grade 50 and ASTM A709 Grade 50
| Element | ASTM A572 Grade 50 | ASTM A709 Grade 50 |
| Carbon (C) | ≤ 0.23 | ≤ 0.15 – 0.20 (varies by type) |
| Manganese (Mn) | ≤ 1.35 | 0.75 – 1.35 (varies by type) |
| Phosphorus (P) | ≤ 0.030 – 0.040 | ≤ 0.040 – 0.050 |
| Sulfur (S) | ≤ 0.030 – 0.050 | ≤ 0.040 – 0.050 |
| Silicon (Si) | 0.15 – 0.40 | 0.15 – 0.65 (varies by type) |
| Columbium (Cb) | 0.005 – 0.05 (Type 1) | – |
| Vanadium (V) | 0.01 – 0.15 (Type 2) | 0.01 – 0.10 (some types) |
| Cb + V | 0.02 – 0.15 (Type 3) | – |
| Copper (Cu) | ≥ 0.20 | ≥ 0.20 – 0.50 (in weathering grades) |
| Nickel (Ni) | Small amounts may be present | ≤ 0.50 (in some types, especially weathering) |
| Chromium (Cr) | Small amounts may be present | 0.30 – 0.70 (in weathering grades) |
| Molybdenum (Mo) | – | ≤ 0.15 (in some types) |
| Nitrogen (N) | ≤ 0.015 (Type 5) | – |
| Titanium (Ti) | ≤ 0.04 (Type 5) | – |
1. What is the primary advantage of SA572 Grade 50 over A36 steel?
Its main advantage is a 50% higher yield strength (50 ksi vs. 36 ksi), allowing for lighter, more efficient structural designs with significant material savings at only a modest cost increase.
2. How does SA572 Grade 50 achieve its higher strength?
It is a High-Strength Low-Alloy (HSLA) steel strengthened primarily through microalloying with Columbium (Nb) and Vanadium (V) during controlled hot-rolling, without requiring heat treatment.
3. Is SA572 Grade 50 suitable for welding?
Yes, it has good weldability using standard low-hydrogen processes. Pre-heating is recommended for thicker sections, but it is far less complex to weld than quenched & tempered high-strength steels.
4. What is the most common application for SA572 Grade 50?
It is the go-to material for optimized building construction, widely used in columns, beams, and trusses where reducing member size and weight provides economic and performance benefits.
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.
