Two Pressure Vessel Grades, Two Standards Systems
Q345R and A516 Gr 70 are both high-quality pressure vessel steel grades used in oil and gas, petrochemical, power generation and general heavy fabrication. They are specified under different standards systems, so a direct comparison has to start with the designation itself. Q345R comes from the Chinese pressure vessel plate standard family, while A516 Gr 70 is an ASME and ASTM specification for carbon steel plate for pressure vessels and boilers. Both are supplied in the normalised or normalised plus stress-relieved condition for vessel duty, and both are weldable by the usual arc processes.
Reading the Grade Designations
In Q345R the letter Q stands for yield strength, the number 345 means the minimum yield strength is not less than 345 MPa, and R identifies the steel as intended for pressure vessels. The grade therefore tells the designer the guaranteed minimum yield level directly. A516 Gr 70 is named differently: A516 is the specification for carbon steel plates for pressure vessels, and the 70 in the grade refers to the specified minimum tensile strength of 70 ksi, which corresponds to roughly 485 MPa in metric units. A 70 ksi grade is a medium strength plate, whereas Q345R is a low alloy pressure vessel plate whose yield value is written into its name.
Chemical Composition
| Element | Q345R | A516 Gr 70 |
|---|---|---|
| C (%) | 0.20 max | 0.27 - 0.30 max depending on thickness |
| Si (%) | 0.55 max | 0.13 - 0.45 |
| Mn (%) | 1.20 - 1.70 | 0.85 - 1.20 for thin plate, higher for heavy plate |
| P (%) | 0.025 max | 0.035 max |
| S (%) | 0.010 max | 0.035 max |
| Microalloying | Nb, V, Ti, Cu, Ni, Cr added for toughness | No deliberate microalloying |
Q345R is a microalloyed low alloy plate: small additions of titanium and niobium refine the grain structure and improve toughness as well as corrosion resistance, and residual copper, nickel and chromium levels are controlled. A516 Gr 70 is a medium to low carbon steel with no deliberate microalloying addition; its strength comes from carbon and manganese together with controlled rolling and normalising, which keeps the grade economical and easy to weld.
Mechanical Properties
| Property | Q345R | A516 Gr 70 |
|---|---|---|
| Minimum yield strength | 345 MPa for thin plate, stepping down with thickness | 260 MPa (38 ksi) |
| Tensile strength | About 510 - 640 MPa | 485 - 620 MPa (70 - 90 ksi) |
| Elongation | About 21 % minimum | About 21 % minimum in 50 mm |
| Impact testing | Charpy impact specified at 0 °C | Charpy impact only when ordered to a supplementary requirement |
| Delivery condition | Normalised, or normalised and stress relieved | As rolled, normalised, or normalised and stress relieved |
The practical consequence is that Q345R gives a higher yield strength for the same plate thickness, so a Q345R vessel wall can often be thinner than an A516 Gr 70 wall designed to the same load. A516 Gr 70 compensates with simpler chemistry, generous elongation and a very predictable welding response, and its tensile specification is expressed as a window rather than a single minimum.
Temperature, Toughness and Service Conditions
Q345R keeps good impact toughness at low temperature, which makes it a common choice for vessels in cold regions or in low temperature operating conditions, and its normalised delivery condition supports that behaviour. A516 Gr 70 is designed for moderate and lower temperature service where very high pressures and high temperatures are not involved, and it remains one of the most widely used boiler and vessel plate grades for that duty. Neither grade is intended for sustained service in the creep range, where chromium-molybdenum plate grades are used instead.
Applications and Selection Guidance
Boilers, pressure vessels and storage tanks in petroleum refineries and chemical plants are the classic Q345R applications.
Reactor shells, heat exchanger bodies and large storage tanks in general pressure service are typical A516 Gr 70 applications.
Choose Q345R when a higher yield strength and a guaranteed 0 °C impact value let you reduce wall thickness on a heavy vessel.
Choose A516 Gr 70 when the design is governed by weldability, toughness at moderate temperature and easy availability across a wide thickness range.
Confirm the applicable design code and the specified impact test before changing from one grade to the other on an existing vessel drawing.
Both grades are normally delivered on mill test certificates covering chemistry, tensile properties and impact results where applicable, and both are routinely formed by cold rolling or pressing and joined by submerged arc or shielded metal arc welding with a matching consumable.
Frequently Asked Questions
Q: Is Q345R equivalent to A516 Gr 70?
They are similar in tensile range but not equivalent. Q345R guarantees a minimum yield of 345 MPa from the Chinese pressure vessel standard, while A516 Gr 70 guarantees a 70 ksi minimum tensile strength under the ASME and ASTM specification.
Q: What does the 345 in Q345R mean?
The number states that the minimum yield strength is not less than 345 MPa for the standard thickness range, and the R shows that the plate is intended for pressure vessels.
Q: Which grade is stronger?
Q345R has the higher minimum yield strength at about 345 MPa against roughly 260 MPa for A516 Gr 70, although the tensile windows of the two grades overlap.
Q: Does A516 Gr 70 require impact testing?
Impact testing is a supplementary requirement in the A516 specification, so it applies only when it is written into the purchase order, whereas Q345R carries a specified 0 °C Charpy requirement.
Q: Can the two grades be used interchangeably in a vessel?
No. A substitution changes wall thickness calculations, impact requirements and the qualified welding procedure, so it must be approved by the responsible design engineer against the governing code.
Q: Which grade suits low temperature service?
Q345R is the better choice for cold regions and low temperature conditions because it retains good impact toughness, while A516 Gr 70 is aimed at moderate and lower temperature service without very high pressures.
