When designing and fabricating pressure vessels for the oil, gas and petrochemical industries, material selection is the single most critical decision an engineer will make. The steel must demonstrate consistent mechanical properties, excellent weldability and predictable behaviour under the design temperature and pressure conditions. Among the carbon steel plates available, ASTM A516 (and its ASME equivalent, SA 516) has established itself as the industry standard. This guide dissects the two most commonly specified grades, Grade 60 and Grade 70,  to help technical decision-makers understand exactly where and why each should be applied. Whether you are optimising wall thickness to reduce structural weight or ensuring code compliance for a sour service separator, getting the specification right at the design stage prevents costly over-engineering and catastrophic service failures.

What Steel is Used for Pressure Vessels?

Carbon steel is the most extensively used material for welded pressure vessels operating at moderate to lower temperatures. Its universal acceptance is driven by a unique balance of tensile strength, ductility and weldability that is difficult to match with alloyed or stainless alternatives when considering life-cycle cost and fabrication simplicity. Within this category, the material colloquially known as boiler plate steel is the workhorse. The term boiler plate historically refers to steel plates used for fired and unfired pressure vessels, and it is formally governed by the ASTM A516/ASME SA-516 specification.

These are killed carbon-manganese-silicon steel plates intended primarily for fusion-welded boilers and pressure vessels where improved notch toughness is important. The specification covers four grades (55, 60, 65, and 70), but Grade 60 and Grade 70 dominate practical engineering procurement. Depending on the thickness and the customer’s normalising requirements, SA 516 plates can offer reliable service down to -51°C in impact-tested conditions.

At Brown McFarlane, our technical inventory strategy directly addresses the most demanding fabrication schedules. We routinely supply boiler plate steel in thicknesses up to 250 mm, a stock depth that allows vessel manufacturers to produce heavy-walled reactors, high-pressure separators and large-diameter columns without project-schedule delays. The ability to source all required plate components from a single, technically audited stockholding partner drastically reduces the risk of mixed-material batch issues.

The Technical Difference Between SA 516 Grade 60 and 70

While both grades share the same base specification and exhibit excellent notch toughness, the split in mechanical strength fundamentally changes allowable stress values and, consequently, the finished vessel’s wall thickness.

Tensile and Yield Strength – The Decisive Mechanical Split

The divergence between the two grades is defined by their tensile and yield strength ranges as specified in ASTM A516/A516 M. The numbers “60” and “70” approximate the minimum tensile strength in ksi (thousands of pounds per square inch), but the metric values are critical for international projects.

SA 516 Grade 60

• Tensile Strength: 415 – 550 MPa (60 – 80 ksi)
• Minimum Yield Strength: 220 MPa (32 ksi)

SA 516 Grade 70

• Tensile Strength: 485 – 620 MPa (70 – 90 ksi)
• Minimum Yield Strength: 260 MPa (38 ksi)

This approximately 18% increase in minimum yield strength when moving to SA 516 Gr 70 is what permits design engineers to reduce the required wall thickness of a cylindrical shell or formed head while staying within the allowable stress limits of the ASME Boiler and Pressure Vessel Code, Section II, Part D. In the world of pressure vessel fabrication, reducing wall thickness translates directly to lower material weight, reduced welding consumable costs, easier handling and lighter supporting structures. For a large crude oil separator with a diameter of 4 metres and a design pressure of 50 bar, selecting Grade 70 over Grade 60 can result in a calculated wall thickness saving of 10–15%, which, extrapolated across the full weight of the vessel, delivers significant project capital savings.

Chemical Composition and Weldability

To achieve the higher strength of Grade 70, the ladle chemistry is adjusted primarily through increased carbon and manganese content. Typical heat analysis limits for plates over 12.5 mm thick illustrate this:

• Grade 60: Carbon max 0.23%, Manganese 0.60 – 0.90% (up to 1.50% with reduced Carbon)
• Grade 70: Carbon max 0.33% (or up to 0.35% in thicker sections), Manganese 0.85 – 1.20% (up to 1.50% with reduced Carbon)

A common concern among less experienced specifiers is that the higher carbon equivalent (CE) of Grade 70 might impair weldability. In practice, both grades are classified as having excellent weldability for shop and field fabrication when standard low-hydrogen practices are followed. The difference is easily managed. For Grade 70 plates with thicknesses above 25 mm, a modest preheat (typically 50–100°C) is applied to slow the cooling rate and prevent hydrogen-induced cold cracking. No post-weld heat treatment (PWHT) is mandated by the grade alone, although PWHT will be dictated by the vessel’s design code based on thickness and service environment. The key takeaway for a welding engineer is that switching from Grade 60 to 70 does not require a radical change in fabrication procedure. It simply requires the discipline that any technically competent vessel shop already possesses.

SA 516 Grade 60 vs. Grade 70

There are a few key differences between Grade 60 and Grade 70.

* For each reduction of 0.01 percentage point below the specified maximum for carbon, an increase of 0.06 percentage point above the specified maximum for manganese is permitted, up to a maximum of 1.50%

Application Suitability in Oil & Gas – Why Grade 70 Dominates

In modern oil and gas infrastructure, SA 516 Gr 70 has emerged as the preferred carbon steel plate for a majority of downstream and midstream pressure vessel applications. While Grade 60 still finds use in specific lower-stress or very-low-temperature storage scenarios, the economics of vessel design and the demands of aggressive refinery environments heavily favour the higher-strength grade.

Optimising Wall Thickness in Column and Drum Design

For distillation columns, reflux drums, and high-pressure gas separators, the design pressure rarely justifies the use of expensive alloy-clad plates when a robust carbon-steel shell will meet the mechanical requirements. By specifying Grade 70, the mechanical design engineer maximises the allowable stress (typically governed by the lower of one-third of the tensile strength or two-thirds of the yield strength at the temperature), creating a more slender vessel shell. For the procurement lead, the logistical reality of transporting and lifting a vessel built from thinner Grade 70 plate rather than a thicker Grade 60 equivalent reduces freight classification and crane capacity requirements, thereby removing hidden project costs.

Hydrogen Induced Cracking (HIC) and Sour Service Environments

The single factor that elevates a standard boiler plate steel enquiry into a high-value technical engagement is the presence of wet H₂S (sour service). In upstream gas plants and refinery hydroprocessing units, atomic hydrogen can diffuse into steel walls, recombining with non-metallic inclusions to cause blistering and stepwise internal cracking. Standard SA 516 grades, even when impact-tested, are not inherently resistant to this damage mechanism.

Therefore, when the process fluid contains H₂S above the threshold defined in NACE MR0175/ISO 15156, the specification must shift from standard SA 516 Gr 70 to HIC-resistant SA 516 Gr 70. This material is manufactured with very low sulfur content (typically ≤0.001%), controlled inclusion shape and often a slight chemistry adjustment to ensure the steel passes the rigorous HIC test as per NACE TM0284. Brown McFarlane proactively maintains deep stocks of HIC-tested SA 516 Grade 70 in our UK service centre and through our expanding Spanish market hub, ensuring that fabricators serving the North Sea, West Africa and Mediterranean energy corridors have immediate access to code-compliant, fully certified material without waiting for mill rollings.

Material Grade and Project Lifecycle

By providing full mill certifications, Charpy impact test reports (where ordered), and HIC test certificates, a responsible stockholder, such as Brown McFarlane, removes a layer of quality risk. When you contact our technical team, we will cross-reference your vessel’s design code, MDMT (Minimum Design Metal Temperature) and service condition to ensure the correct boiler plate steel grade is specified. Whether the application is a simple instrument air receiver (Grade 60) or a critical sour gas knockout drum (HIC-resistant Grade 70), the specification must be precise from the very first material requisition.

For guidance on your next pressure vessel material package, contact our technical team. We provide detailed product data sheets and mill certifications for all our boiler plate steel stock.

Request a quote for SA 516 Grade 60/70 boiler plate steel or discuss your technical specification for HIC-resistant plate today.

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