ASTM A515 Grade 70 is a high-strength carbon-silicon steel plate designed for welded boilers and pressure vessels in moderate to elevated temperature service. It is the strongest of the three grades covered by ASTM A515, which also includes Grades 60 and 65. The grade offers a good balance of strength, weldability, and economy, and it is commonly used in power generation and petrochemical industries. This guide covers the composition, mechanical properties, heat treatment, and typical applications of A515 Grade 70.
Material Overview and Designation
ASTM A515 covers carbon steel plates for pressure vessels at ambient and higher temperature service, produced by killed steel practice to ensure internal soundness. The number in the grade name indicates the minimum tensile strength in ksi: Grade 60 has a minimum of 60 ksi, Grade 65 of 65 ksi, and Grade 70 of 70 ksi. The specification is intended for elevated-temperature service, and it does not require low-temperature impact testing. Where low-temperature toughness is needed, a grade such as ASTM A516, which is formulated for that purpose, should be selected instead. Grade 70 is also not intended for sour service environments.
Chemical Composition
The carbon content of Grade 70 is the principal strengthening element and is specified according to plate thickness. The maximum carbon is 0.31 percent for plates 25 mm and under, 0.33 percent for plates over 25 to 50 mm, and 0.35 percent for plates over 50 mm. Manganese is limited to a maximum of about 1.20 percent on heat analysis, phosphorus and sulfur to 0.025 percent maximum each, and silicon is specified in the range of 0.15 to 0.40 percent. The balance is iron. The controlled carbon and manganese levels keep the plate weldable while providing the strength required at elevated temperatures.
Mechanical Properties
The tensile strength of Grade 70 is 70 to 90 ksi (485 to 620 MPa), with a minimum yield strength of 38 ksi (260 MPa). Minimum elongation is 17 percent in 200 mm and 21 percent in 50 mm gauge length. The material provides good elevated-temperature strength for boiler and pressure vessel service, and it is normally supplied in the as-rolled or normalized condition. Normalizing, which involves heating above the critical range and cooling in air, refines the grain structure and improves toughness and uniformity, and is often specified for thicker plates or for more demanding service conditions.
Heat Treatment and Fabrication
Plates are commonly supplied as-rolled or normalized. Welding is performed by conventional processes such as shielded metal arc welding, gas metal arc welding, and submerged arc welding, using filler metals matched to the strength of the base plate. Preheating is recommended for thick sections and highly restrained joints. Post-weld heat treatment may be required by the governing design code depending on thickness, service temperature, and the fabrication route. Because the specification does not require low-temperature impact testing, the design should confirm that the service temperature is within the capabilities of the material before it is specified.
Typical Applications
A515 Grade 70 plates are used in the manufacturing of pressure vessels for the oil and gas industry, including separators, reactors, and storage tanks operating at moderate to elevated temperatures. They are used in the construction of boilers and heat exchangers, where the plate must withstand sustained steam pressure and thermal loads. The grade is also used for industrial storage tanks and other pressure-sensitive equipment, and in applications where high strength and durability are required in high-temperature environments. For services below approximately -18 degrees C, or where low-temperature impact toughness is mandatory, A516 or another low-temperature grade should be selected.
Frequently Asked Questions
What is the tensile strength of A515 Grade 70?
The tensile strength is 70 to 90 ksi (485 to 620 MPa), with a minimum yield strength of 38 ksi (260 MPa).
What is the difference between A515 Grade 70 and A516 Grade 70?
A515 is intended for elevated-temperature pressure vessel service, while A516 is formulated with improved notch toughness for low-to-moderate temperature service. The two grades should be selected according to the operating temperature of the vessel.
What is the maximum carbon content of Grade 70?
The maximum carbon content depends on plate thickness: 0.31 percent for plates up to 25 mm, 0.33 percent from 25 to 50 mm, and 0.35 percent above 50 mm.
Can A515 Grade 70 be used at low temperatures?
No. The grade is designed for ambient and higher temperature service and does not require low-temperature impact testing. For low-temperature service, A516 should be used.
What heat treatment is applied to A515 Grade 70?
Plates are normally supplied in the as-rolled or normalized condition, with normalizing recommended for thicker plates or where improved toughness and uniformity are required.
Is A515 Grade 70 suitable for sour service?
No. The specification does not include the controls required for sour service, so a grade qualified for that environment should be selected for applications involving hydrogen sulfide.