What is the difference between A516 70 and A572 50?

Confused between ASTM A516 Grade 70 and ASTM A572 Grade 50 steel? This in-depth guide explains the key differences in chemistry, mechanical properties, and applications. You can learn how to choose the right material for your project to ensure safety, compliance, and cost-effectiveness.

As engineers, designers, and procurement specialists, selecting the right material is one of the most critical decisions we make. The choice directly impacts safety, performance, and budget. Two of the most commonly specified carbon steel plates are ASTM A516 Grade 70 and ASTM A572 Grade 50.

At a glance, their mechanical properties can seem similar, but they are not interchangeable. Using the wrong one can lead to catastrophic failure or non-compliance with codes. So, what exactly is the difference between A516 Grade 70 and A572 Grade 50?

This article will break down the key differences to help you make an informed decision for your next project.

1. Primary Application and Governing Standards: The Fundamental Difference

This is the most crucial distinction and the starting point for selection.

• ASTM A516 Grade 70: This steel is governed by the ASME Boiler and Pressure Vessel Code (BPVC). Its primary application is in the fabrication of pressure vessels. The “70” denotes a minimum tensile strength of 70 ksi. It is designed specifically to withstand internal pressure and has excellent notch toughness, especially at moderate and lower temperatures. You will often find it in welded boilers, storage tanks, and other pressure-containing structures.
• ASTM A572 Grade 50: This is a high-strength, low-alloy (HSLA) structural steel. The “50” denotes a minimum yield strength of 50 ksi. It is governed by standards like AISC for structural applications. Its primary use is in bridges, buildings, and other structural frameworks where high strength-to-weight ratio is valuable. It offers good weldability and greater strength than standard carbon steel like A36.

2. Chemical Composition: Chemistry Dictates Performance

The difference in application stems from their chemical makeup.

• A516 Gr 70 is a carbon-silicon steel. The chemistry is carefully controlled, with limits on elements like manganese, phosphorus, and sulfur to ensure weldability and notch toughness. It may also have supplemental Charpy V-Notch impact testing requirements (e.g., A516 Gr 70N).
• A572 Gr 50 is a High-Strength Low-Alloy (HSLA) Columbium-Vanadium steel. It contains small, intentional additions of alloying elements like columbium (niobium) and/or vanadium. These elements provide strengthening through grain refinement and precipitation hardening, allowing it to achieve higher strength with less weight.

3. Mechanical Properties: A Side-by-Side Comparison

While both are strong, their strength profiles are defined differently.

Notice that A572 Gr 50 has a higher yield strength than A516 Gr 70, which is often the governing factor in structural design to prevent permanent deformation.

4. Cost and Availability

Generally, A572 Grade 50 is more cost-effective than A516 Grade 70. The stringent manufacturing process, testing requirements (especially impact testing), and certification needed for pressure vessel applications make A516 a premium product. For non-pressure applications, using A516 where A572 would suffice is an unnecessary expense.

Conclusion: How to Choose Between A516 70 and A572 50

Making the right choice is straightforward if you follow this logic:

• Choose ASTM A516 Grade 70 if: Your component is a pressure vessel, boiler, or any equipment governed by the ASME BPVC that must contain internal pressure. Safety and notch toughness are your top priorities.
• Choose ASTM A572 Grade 50 if: Your project is a bridge, building, or structural frame where the primary goal is to support loads with a high-strength, lightweight material. Cost-effectiveness for structural applications is key.

Always consult the governing design code (ASME for pressure equipment, AISC for structures) and involve a certified materials engineer when in doubt. Using the correct material is not just a recommendation—it’s a requirement for safety and reliability.