ASTM A709 Grade 36 and GB/T 713 Q345R are both structural plate steels with a nominal 345 MPa class name in the market, but they come from two different standard systems and two different design worlds. A709 Grade 36 is an American bridge steel whose figure of 36 refers to 36 ksi, roughly 250 MPa of minimum yield strength. Q345R is a Chinese pressure vessel steel whose figure of 345 refers directly to 345 MPa of minimum yield strength. Converting both to the same units shows that Q345R is the stronger of the two.
Confusion often arises because the Chinese bridge plate family in GB/T 714 is written Q345q, while the pressure vessel grade is written Q345R. The two are different products with different toughness, delivery condition and inspection rules. A bridge project ordering Chinese plate normally wants Q345q, and a pressure vessel project under Chinese regulations normally wants Q345R.
What Each Standard Covers
ASTM A709 is the specification for structural steel for bridges, and Grade 36 is its basic carbon grade, widely used for girders, cross frames, bearing stiffeners and general fabrication. It is normally supplied as-rolled, and the grade may be ordered with a minimum copper content for improved atmospheric corrosion resistance.
GB/T 713 covers steel plates for pressure vessels in China. Q345R is the most widely used grade in that specification, supplied for shells, heads and other pressure-retaining parts, with a mandatory impact toughness requirement and a full set of thickness-dependent property limits.
Chemical Composition Compared
| Element | ASTM A709 Grade 36 | GB/T 713 Q345R |
|---|---|---|
| C, max % | 0.25 | 0.20 |
| Si, % | 0.40 max | 0.55 max |
| Mn, % | 0.80-1.20 | 1.20-1.70 |
| P, max % | 0.04 | 0.025 |
| S, max % | 0.05 | 0.010 |
| Cu, % | 0.20 min | 0.30 max |
| Ni, max % | - | 0.30 |
| Cr, max % | - | 0.30 |
| Mo, max % | - | 0.08 |
| V, max % | - | 0.050 |
| Nb, max % | - | 0.050 |
| Ti, max % | - | 0.030 |
| Alt, min % | - | 0.020 |
The chemistry tables show the real design philosophy of each product. A709 Grade 36 carries a deliberate copper addition for weathering performance and relatively generous phosphorus and sulphur limits, which suits painted or unpainted bridge structures. Q345R is a fine-grain killed pressure vessel steel with a much tighter phosphorus and sulphur balance and a minimum aluminium content, giving it the cleanliness and toughness needed for welded pressure equipment.
Mechanical Properties Compared
| Property | ASTM A709 Grade 36 | GB/T 713 Q345R |
|---|---|---|
| Product form | Plate and shapes up to 100 mm | Plate, 3 mm and above |
| Yield, min | 250 MPa (36 ksi) | 345 MPa for 3-16 mm |
| Tensile | 400-550 MPa | 510-640 MPa for 3-16 mm |
| Elongation, min | 20% (200 mm gauge); 23% (50 mm gauge) | 21% |
| Impact test | Not a general requirement | 0 °C, 41 J minimum |
Q345R properties are quoted by thickness because the allowable stress falls as the plate gets heavier. The table below gives the minimum yield strength, tensile range and elongation for each thickness band.
| Thickness, mm | Yield, min | Tensile | Elongation, min |
|---|---|---|---|
| 3-16 | 345 MPa | 510-640 MPa | 21% |
| 16-36 | 325 MPa | 500-630 MPa | 21% |
| 36-60 | 315 MPa | 490-620 MPa | 21% |
| 60-100 | 305 MPa | 490-620 MPa | 20% |
| 100-150 | 285 MPa | 480-610 MPa | 20% |
| 150-250 | 265 MPa | 470-600 MPa | 20% |
Strength Conversion: Why 36 and 345 Are Not the Same
The number in A709 Grade 36 is a US customary unit and equals about 250 MPa. The number in Q345R is already a metric yield value. At equal thickness, Q345R therefore offers roughly 95 MPa more minimum yield strength than A709 Grade 36, which is a difference of nearly 40 %. Treating the two grades as interchangeable because both are called 345 classes in local markets is a common and expensive mistake: it can lead to undersized sections on one side and unnecessary weight and cost on the other.
Selection and Substitution Guidance
Use ASTM A709 Grade 36 for bridge and general structure work where the design code is AASHTO based and impact toughness is not a governing requirement.
Use GB/T 713 Q345R for pressure vessels and boilers in the Chinese regulatory system, where the 0 °C impact requirement and the fine-grain practice are part of the acceptance criteria.
If a Chinese bridge plate is required, specify Q345q with the grade suffix that matches the required impact temperature, not Q345R.
Before any cross substitution, compare the strength level, the design code allowables, the impact requirement and the delivery condition, and record the comparison in the design file.
Frequently Asked Questions
Q: Is A709 Grade 36 equivalent to Q345R?
No. A709 Grade 36 has a minimum yield strength of 250 MPa, while Q345R has 345 MPa at 3-16 mm thickness. They also belong to different standards and different design codes, so neither can replace the other without a full engineering review.
Q: Is Q345R the same as Q345q?
No. Q345R is a pressure vessel plate grade in GB/T 713, and Q345q is a bridge structural steel grade in GB/T 714. The letters after Q345 identify the application family, and the two grades have different chemical limits, toughness requirements and inspection rules.
Q: What is the minimum yield strength of A709 Grade 36?
250 MPa, which corresponds to 36 ksi. Tensile strength ranges from 400 MPa to 550 MPa for plate up to 100 mm thick, with a minimum elongation of 20 % on a 200 mm gauge length.
Q: Does Q345R require impact testing?
Yes. Q345R is normally delivered with a 0 °C Charpy V-notch test and a minimum absorbed energy of 41 J. The impact requirement is one of the main reasons the grade is produced as a fine-grain killed steel with tight sulphur control.
Q: Which grade is better for a low-temperature structure?
Neither grade is intended for very low service temperatures. Elevated toughness classes of the bridge family or low-temperature pressure vessel grades with verified impact energy at the design metal temperature should be selected instead.
Q: Why does Q345R yield strength drop with thickness?
Because heavy plate cools more slowly during rolling and heat treatment, which coarsens the microstructure. The standard compensates by reducing the required minimum yield strength in each successive thickness band, from 345 MPa at 3-16 mm down to 265 MPa at 150-250 mm.
