In the construction industry, selecting the steel grade is one of the most important steps. This is to ensure both safety and durability in the construction of your project. The two most popular options in the construction industry are the ASTM A572 Grade 50 and ASTM A709 Grade 50 steel. While they may seem similar, possessing high strength, low alloy, and excellent mechanical properties, they both possess unique characteristics that make them ideal for different applications. Differentiating between these two steel grades will greatly aid you in selecting the most suitable one for your next project.
What is ASTM A572 Gr.50?
ASTM A572 Grade 50 is one of the classifications that fall under the ASTM A572 standard, which is specifically designed for high-strength, low-alloy (HSLA) structural steel. This grade is characterized by having a minimum yield strength of 50 kilopounds per square inch (ksi), which is approximately equal to 345 megapascals (MPa), indicating a high tolerance to stress before deformation.
What is ASTM A709 Gr.50?
ASTM A709 Grade 50 is a specific grade also defined in the ASTM A709 standard. This standard, which is known as the “Standard Specification for Structural Steel for Bridges”, specifies the types of steel that are ideal for bridge construction, such as High-Strength Low-Alloy (HSLA) steel and carbon steel. Grade 50 is categorized as HSLA steel, meaning it has a minimum yield strength of 50 kilopounds per square inch (ksi) or 345 megapascals (MPa), the same as A572 Grade 50.
A572 Gr. 50 vs. A709 Gr. 50 Steel
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.
6. Availability
ASTM A572 Grade 50
ASTM A572 Gr. 50 is a widely used steel grade with high strength low alloy level. It is usually available in steel mills, distributors, and service centers in both industrialized and developing countries. This steel grade comes in various forms such as plates and structural shapes, which include beams, columns, channels, angles, and bars. Because of these characteristics, the steel has extensive applications in fields like construction, manufacturing, and infrastructure, which is why many steel suppliers keep A572 Grade 50 a part of their standard inventory.
ASTM A709 Grade 50
ASTM A709 Gr. 50 is primarily used in bridge construction. It is common in areas with significant infrastructure development, like North America, where both ASTM and AASHTO standards are critical. This grade of steel is normally available in plate form because that is the preferred material for bridge construction.
A709 Grade 50 is a special type of steel that is not mass-produced. If you want to procure this type of steel, it is advisable to contact steel suppliers that specialize in construction materials for infrastructure projects or have experience serving bridge fabricators.
Chemical Composition
| 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) | – |
Conclusion:
In conclusion, ASTM A572 Grade 50 and ASTM A709 Grade 50 are both categorized as high-strength steels and have the same mechanical properties. Their difference lies in the fact that they are designed for different applications.
ASTM A572 Gr. 50, which is known for its versatility and cost-effectiveness, is an excellent choice for general structural applications, while ASTM A709 Gr. 50 is specifically engineered for bridge construction to provide enhanced toughness and corrosion resistance. This grade also experiences more stringent certification and testing requirements, which affect its price. The selection between the two types of steel grades should always be based on the specific needs of the project, not what we personally prefer.
