What is A516 GR.70 material?

I. General Product Description

A516 Grade 70 is a carbon steel plate specifically designed for pressure vessels and other critical applications. Its key characteristics are excellent notch toughness and good weldability, making it suitable for environments where low temperatures could cause brittle fracture. The "70" denotes a minimum yield strength of 38 ksi (260 MPa) and a minimum tensile strength of 70 ksi (485 MPa).

The ASTM A516 specification is the "Standard Specification for Pressure Vessel Plates, Carbon Steel, for Moderate- and Lower-Temperature Service," which applies to plates intended for use in welded pressure vessels where enhanced notch toughness is important.

Grade 70: This specifies the mechanical properties of the steel. Higher numbers (like Grade 70) indicate higher strength. Common grades include 55, 60, 65, and 70.

II. Chemical Composition

The chemical composition is carefully controlled to achieve its mechanical properties and weldability. It is a relatively low-carbon steel with additions of Manganese and Silicon for strength.

Carbon Equivalent (CE): The carbon equivalent (CE) of A516 Gr. 70 typically ranges from 0.35 to 0.45. This is a calculated value that predicts the hardenability and crack susceptibility of the steel.

Interpretation: A CE below 0.40 is generally considered excellent for welding without preheat. A CE between 0.40 and 0.60 indicates good weldability, but preheat is often recommended to control cooling rates and prevent hydrogen-induced cracking.

Common Formula (IIW): CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15

III. Mechanical Properties

IV. Weldability

Despite its good weldability, standard precautions for carbon steels must be taken.

Hydrogen-Induced Cracking (HIC) / Cold Cracking: This is the most significant risk. It occurs when three factors are present:

Hydrogen: Introduced from moisture in electrodes, humid air, or contaminants (oil, grease, rust) on the base metal.

A Brittle Microstructure: Formed in the HAZ from rapid cooling.

High Tensile Stress: Inherent in welded joints.

Porosity: Can be caused by contaminants on the surface or moisture in welding consumables.

V. Delivery Conditions

1. Standard Delivery Conditions (As-Rolled, Normalized, or Stress Relieved)

A516 Grade 70 is most commonly supplied in one of three conditions, as specified by the purchaser in the order:

As-Rolled (AR): The plate is cooled naturally after the final hot-rolling process. No subsequent heat treatment is performed.

Typical Use: For lower thickness applications where the required mechanical properties are achieved through the rolling process itself. This is the most economical option.

Normalized (N): The plate is heated to a temperature above its upper critical temperature (approximately 1600°F / 870°C) and then air-cooled. This refines the grain structure, improves toughness, and provides more uniform mechanical properties throughout the plate's thickness.

Typical Use: The most common condition for pressure vessel applications, especially for thicker plates (typically over 1.5 inches / 40 mm) where through-thickness properties are critical. It is often a code requirement (e.g., ASME BPVC).

Stress Relieved (SR): The plate is heated to a temperature below its lower critical temperature (typically between 1100-1200°F / 595-650°C) and held for a sufficient time to reduce residual stresses, then slowly cooled.

Typical Use: Applied after forming or welding operations to relieve internal stresses induced during fabrication. Plates are not typically ordered in the SR condition; stress relieving is usually a fabrication step.

2. Impact Testing (Charpy V-Notch)

This is a critical delivery requirement, especially for pressure vessels operating in low-temperature environments.

Notch Toughness: A516 Grade 70 is often ordered to meet supplementary requirement S5 (Charpy V-Notch Impact Test) from ASTM A20/A20M.

Test Temperature: The purchaser must specify the test temperature (e.g., -20°F, 0°F, +40°F, etc.). A common requirement is testing at -20°F (-29°C) or lower for applications like liquefied gas storage.

Energy Absorption: The minimum energy absorption value (in ft-lb or Joules) must be specified. A common requirement is 15 ft-lb (20 J) average for a set of three specimens at the designated test temperature.

3. Other Common Supplementary Requirements (from ASTM A20/A20M)

The purchaser can specify additional requirements, which must be stated on the order. Common ones include:

S1: Vacuum Treatment: Enhances soundness and reduces porosity by degassing the molten steel.

S2: Product Analysis: Allows the purchaser to analyze a sample from the finished product to verify chemical composition.

S3: Simulated Post-Weld Heat Treatment (SPWHT): Test specimens are heat-treated in the lab to simulate the stress-relieving cycle the actual plate will undergo during fabrication. This ensures the mechanical properties will still meet specifications after fabrication.

S4: Additional Tension Test: Requires extra tension tests to provide more data on the mechanical property uniformity.

S7: Ultrasonic Examination: Requires the plate to be inspected ultrasonically to detect internal discontinuities. This is specified per acceptance standards like ASTM A578/A578M (straight-beam) or A577/A577M (angle-beam).

Conclusion

ASTM A516 Grade 70 is a high-quality carbon steel plate renowned for its notch toughness and weldability, making it the industry standard material for constructing pressure vessels that operate at moderate and low temperatures. Its designation ensures it meets strict chemical and mechanical property requirements for safety-critical applications.