Two Pressure Vessel Steels
SA516 Gr.70 and SA612 are both ASME pressure vessel plate specifications, but they serve different design niches. SA516 Gr.70 is a carbon steel plate for moderate- and lower-temperature service, widely stocked and available in heavy thicknesses up to 200 mm. SA612 is a higher-strength, aluminum-killed fine-grain steel with a minimum yield strength of 345 MPa, designed for medium-thickness pressure vessels where weight reduction matters, such as LPG spheres and mobile tankers.
Chemical Composition
The two grades differ in composition limits. SA516 Gr.70 allows carbon up to 0.27-0.31 percent depending on thickness, manganese 0.85-1.20 percent, silicon 0.15-0.40 percent, and phosphorus and sulfur each up to 0.035 percent. SA612 controls carbon to a maximum of 0.25 percent, manganese 1.00-1.50 percent, silicon 0.15-0.50 percent, phosphorus and sulfur each up to 0.025 percent, and requires aluminum-killed fine-grain practice with aluminum at least 0.020 percent and niobium up to 0.02 percent. The lower carbon and impurity limits of SA612 give it a lower carbon equivalent and improved weldability, while the fine-grain practice improves toughness.
Mechanical Properties
SA516 Gr.70 has a minimum yield strength of 260 MPa (38 ksi) and a tensile strength of 485-620 MPa (70-90 ksi), with minimum elongation of 17 percent on a 200 mm gauge or 21 percent on a 50 mm gauge; Charpy impact testing is optional, typically 27 J. SA612 offers a minimum yield strength of 345 MPa (50 ksi) and a tensile strength of 570-725 MPa (83-105 ksi), with minimum elongation of 16 percent on a 200 mm gauge, a standard Charpy requirement of 27 J average at -40 degrees Celsius, and hardness limited to 207 HB. The modulus of elasticity is about 200 GPa for both grades.
Thickness Availability and Design Impact
SA516 Gr.70 is available up to 200 mm thickness and is excellent for heavy-plate applications such as thick-wall vessels and large shells. SA612 has a standard maximum thickness of 25 mm under the ASTM/ASME specification, which can be extended to 6-150 mm through technical agreement, and is therefore best suited to medium-thickness, optimized designs. Because of its higher strength, SA612 allows wall thickness to be reduced by about 16 percent for the same design pressure, lowering weight and material cost in transportable vessels.
Applications by Industry
In oil and gas, SA516 Gr.70 is used for large storage tanks, thick-wall separators, and columns, while SA612 suits LPG storage spheres, fractionation towers, and mobile tankers. In petrochemical service, SA516 Gr.70 appears in reactors, absorbers, and general process vessels, whereas SA612 is chosen for medium-pressure reactors and chemical process columns. In power generation, SA516 Gr.70 is used for boiler shells, steam drums, and tube sheets, and SA612 for boiler components, steam generators, and high-pressure water pipes. Both grades serve heat exchangers and industrial gas storage, with SA612 preferred where low-temperature impact toughness is specified.
Frequently Asked Questions
What is the main difference between SA516 Gr.70 and SA612? The strength level and design intent: SA516 Gr.70 has a minimum yield of 260 MPa and suits heavy, thick-wall vessels, while SA612 has a 345 MPa minimum yield and suits lighter, medium-thickness vessels.
Can SA612 replace SA516 Gr.70? Only after design verification. The higher strength allows thinner walls, but thickness availability and impact requirements differ.
What are the impact test requirements? SA516 Gr.70 impact testing is optional, typically 27 J; SA612 requires 27 J average at minus 40 degrees Celsius as standard.
Which grade has better weldability? SA612 has a lower carbon equivalent, but preheating may be required for thicker sections; SA516 Gr.70 generally needs no preheating.
Where is SA612 commonly used? LPG storage spheres, fractionation towers, mobile tankers and compressed gas receivers.
What thickness range is SA516 Gr.70 available in? Up to 200 mm, with excellent industrial availability for heavy plates.
