What Q245R and Q345R Plates Are
Q245R and Q345R are carbon steel plates for boilers and pressure vessels produced to the Chinese national standard GB/T 713, which replaced the older GB 713 edition. In the designation, Q stands for yield strength, the number is the minimum yield strength in MPa, and R marks steel plate for pressure service. Both grades are fully killed and fine-grained, and they are normally supplied normalised or in a controlled-rolled condition in thicknesses from about 3 mm up to 150 mm.
Because the two grades are listed side by side in the same table of GB/T 713, selection is frequently debated. The decision comes down to three factors: chemical composition, strength level, and the operating temperature and pressure of the contained medium.
Chemical Composition Compared
| Element (ladle analysis, %) | Q245R | Q345R |
|---|---|---|
| Carbon, C max | 0.20 | 0.20 |
| Silicon, Si max | 0.35 | 0.55 |
| Manganese, Mn | 0.50-1.00 | 1.20-1.60 |
| Phosphorus, P max | 0.025 | 0.025 |
| Sulphur, S max | 0.015 | 0.015 |
| Acid-soluble aluminium, Alt min | 0.020 | 0.020 |
Q345R is not merely Q245R with a higher strength target. The manganese range is raised to about 1.20-1.60% and the silicon allowance to 0.55%, and that combination provides the extra solid-solution and ferrite-pearlite strengthening behind the higher yield class. Both grades keep the same tight limits on phosphorus and sulphur at 0.025% and 0.015%, which is the main reason both retain useful toughness at low temperature rather than becoming brittle.
Weldability remains good for both. The carbon equivalent of Q345R is generally controlled to about 0.45% for plate up to 60 mm thick, so conventional shielded metal arc, submerged arc and gas metal arc procedures can be used with preheat and interpass control appropriate to the section thickness.
Mechanical Properties Compared
| Property | Q245R | Q345R |
|---|---|---|
| Minimum yield strength ReL, t 3-16 mm | 245 MPa | 345 MPa |
| Minimum yield strength ReL, t 16-36 mm | 235 MPa | 325 MPa |
| Minimum yield strength ReL, t 36-60 mm | 225 MPa | 315 MPa |
| Tensile strength Rm | 400-520 MPa | 510-640 MPa |
| Minimum elongation A | 25% | 21% |
| Charpy V-notch impact energy at 20 degrees C, min | 31 J | 34 J |
The gap in yield strength is roughly 100 MPa at every thickness band, and the tensile range of Q345R is about 110 MPa higher at both ends. Ductility moves in the opposite direction: Q245R carries the higher minimum elongation because its lower manganese and silicon levels leave a softer ferrite matrix. In practice both grades comfortably pass the 27 J minimum absorbed energy that pressure vessel codes expect, and impact testing can also be ordered at lower temperatures when the design condition requires it.
Design and Fabrication Consequences
Wall thickness: because Q345R offers a higher allowable stress, a thinner shell or head can often carry the same design pressure. That reduces plate weight, weld volume and post-weld heat treatment time, and it frequently offsets the higher price per tonne of the grade.
Welding: Q245R tolerates wider welding windows and needs less preheat for thin sections. Q345R demands closer control of heat input and, for thicker sections, preheat and interpass temperature limits to avoid hydrogen-assisted cracking.
Forming and heat treatment: both grades cold form well at reasonable radii, but Q345R work-hardens more quickly, so springback compensation and intermediate stress relief should be planned.
Corrosion allowance: neither grade is an alloy steel, so a corrosion allowance or a stainless cladding layer is needed wherever the medium is aggressive.
Thickness effect: allowable stress always falls with increasing thickness, so the plate thickness assumed in the design calculation must match the thickness band of the certified test data.
How to Select Between the Two Grades
Choose Q245R for moderate design pressure and temperature, for liquefied petroleum gas, natural gas and crude oil service, for small-diameter vessels where plate weight is not critical, and where easy forming and welding are the priority.
Choose Q345R for high-pressure, high-temperature service and for media such as hydrogen, ammonia and methanol, and for large-diameter or heavy-wall vessels where the reduction in plate thickness pays for the stronger grade.
Check the design temperature against the impact test temperature rather than the room-temperature value, and confirm that the certified impact energy applies to the full plate thickness supplied.
Where both grades satisfy the calculation, the deciding factors are usually total fabricated cost, mill availability and the welding procedure already qualified in the shop.
Frequently Asked Questions
Q: Is Q345R simply a stronger version of Q245R?
In practice yes, but not only stronger. Q345R also allows more manganese and silicon, which changes its toughness and welding behaviour, so the welding procedure and heat treatment must be qualified for the actual grade.
Q: What standard covers Q245R and Q345R plates?
Both grades are specified in GB/T 713, the Chinese standard for steel plates for boilers and pressure vessels, which superseded the earlier GB 713 edition while keeping the same grade names and property requirements.
Q: Can Q245R and Q345R plates be welded to each other?
Yes. Dissimilar joints between the two grades are common on vessel shells, nozzles and supports. The welding consumable and preheat should be selected for the higher-strength grade, and the procedure qualification should follow the governing pressure vessel code.
Q: Does the yield strength stay constant as the plate gets thicker?
No. Minimum yield strength steps down with thickness, for example from 345 MPa to 325 MPa and then 315 MPa for Q345R as the thickness increases, because the centre of a thick plate cools more slowly and has a coarser microstructure.
Q: Which grade is more suitable for low-temperature service?
Both are killed, fine-grained steels with low sulphur, and both are capable of passing low-temperature impact testing. The final choice depends on the design metal temperature, the required impact energy and the thickness, so the test temperature must be specified when ordering.
Q: How does plate thickness affect the cost comparison?
Q345R costs more per tonne, but its higher allowable stress usually permits a thinner wall, which lowers the mass of steel, the amount of welding and the heat treatment cycle. On high-pressure vessels the finished cost often favours Q345R.
