The structural steel plate grades you'll see most in Canada are CSA G40.21 300W (also called 44W) and 350W (50W), and the US grades ASTM A36 and A572 Grade 50. The CSA grade number is the minimum yield strength in megapascals: 300 MPa for 300W, 350 MPa for 350W. A36 has a minimum yield of 250 MPa (36 ksi) and A572 Grade 50 has 345 MPa (50 ksi). Much plate is certified to more than one grade at a time, which is why mill certificates often list a Canadian grade and a US grade together.
This article explains the grades. For ordering plate cut to size, see our guide to steel plate cut to size in Ontario, and for how plate is priced, steel plate prices in Canada.
Steel plate grade chart
| Grade | Imperial name | Type | Minimum yield strength | Where you'll see it |
|---|---|---|---|---|
| CSA G40.21 260W | 38W | Carbon structural | 260 MPa | Less commonly specified for structural plate |
| CSA G40.21 300W | 44W | Carbon structural | 300 MPa | General plate: base plates, gussets, brackets |
| CSA G40.21 350W | 50W | High-strength low-alloy | 350 MPa | Structural plate where the design needs more strength |
| ASTM A36 | Carbon structural | 250 MPa (36 ksi) | US-origin drawings; general fabrication | |
| ASTM A572 Grade 50 | High-strength low-alloy | 345 MPa (50 ksi) | US-origin drawings; often dual-certified with 350W | |
| CSA G40.21 350A, ASTM A588 | 50A | Weathering steel | 350 MPa (350A) | Exposed steel meant to form a protective rust layer |
| CSA G40.21 Q grades, ASTM A514 | Quenched and tempered | Much higher; per the standard | Heavy-duty, high-strength applications |
The Canadian grade names and their imperial equivalents in this chart (38W for 260W, 44W for 300W, 50W for 350W, 50A for 350A) are as listed in Algoma Steel's published plate grade list. Algoma, in Sault Ste. Marie, is a major Canadian plate producer. A36 values are from the ASTM A36 specification.
Thickness can change the numbers. Under A36, plate over 203 mm (8 in) thick has a lower minimum yield of 220 MPa (32 ksi). For any unusually thick plate, in any grade, check the standard or the mill certificate rather than assuming the headline value.
What the letters and numbers mean
CSA G40.21 grade names are built from a number and one or more letters:
- The number is the minimum yield strength in MPa (300, 350 and so on). The older imperial names use ksi instead (44, 50).
- W: weldable. The everyday structural grades. Chemistry is controlled so the steel can be welded with normal procedures.
- A: atmospheric corrosion-resistant. Weathering steel, which forms a tight rust layer that slows further corrosion when the detailing lets it dry out. Not a choice for buried steel or steel kept wet.
- Q: quenched and tempered. Heat-treated high-strength plate. It needs welding procedures matched to the grade.
- T: impact tested. Added after the other letters (300WT, 350WT). The steel has been Charpy impact tested for toughness, which some projects require for members in cold exposure or under dynamic load.
ASTM grades are named differently. A36 is a specification number, not a strength, and A572 uses a grade number (50) that is the yield strength in ksi.
300W (44W) vs 350W vs A36: which plate to use
For most plate in buildings, the realistic choice is between 300W, 350W and A36, and in practice the grade is whatever your structural drawings specify.
| Question | 300W / 44W | 350W / 50W | A36 |
|---|---|---|---|
| Minimum yield | 300 MPa | 350 MPa | 250 MPa |
| Typical role | General plate work | Plate sized for strength | US-specified work |
| Stiffness | Same as other grades | Same as other grades | Same as other grades |
| Weldability | Weldable grade | Weldable grade | Weldable with normal practice |
Two points worth understanding:
A stronger grade doesn't make plate stiffer. All structural steel has about the same modulus of elasticity, about 200 GPa. A base plate or bracket sized to limit bending or deflection doesn't get any stiffer by switching to 350W. It gets stiffer by getting thicker.
A thinner plate in a stronger grade is a design change. Because base plate thickness often comes from a bending calculation, a stronger grade can sometimes allow a thinner plate. That trade-off belongs to the engineer, who also has to check welds, bolts and bearing. Our worked example of column base plate design to CSA S16 shows where plate grade enters the calculation.
Mild steel plate and "plate yield strength"
"Mild steel" is a common shop name for low-carbon steel. It isn't a grade, so it doesn't have a guaranteed strength. When someone asks for the yield strength of mild steel plate, the useful answer is the minimum for the grade on the mill certificate:
- A36: 250 MPa (36 ksi), lower for plate over 203 mm thick.
- 300W (44W): 300 MPa.
- 350W (50W) and A572 Grade 50: 350 MPa and 345 MPa.
The measured yield printed on a mill certificate is usually higher than the minimum, but design always uses the specified minimum, not the test result. If plate arrives with no certificate and the job needs one, it can't simply be assumed to be a particular grade.
Types of structural steel plate
Beyond the everyday structural grades, you may come across:
- Floor plate (checker or tread plate). A raised-pattern plate for stair treads, platforms and walkways. See our guide to steel checker plate.
- Weathering plate. Grades such as 350A or A588, used for exposed structures such as some bridges, where the steel is left unpainted.
- Quenched and tempered plate. High-strength plate for heavy equipment and special structures, specified by an engineer with matching welding procedures.
- Abrasion-resistant plate. Hard, wear-resistant plate for chutes, liners and buckets. It is chosen for hardness rather than as a structural grade.
- Pressure vessel plate. Made to separate specifications for boilers and tanks, and certified for that use. Not interchangeable with structural plate.
For buildings, nearly all plate work (base plates, cap plates, shear tabs, gussets, stiffeners and embed plates) is done in the structural grades: 300W, 350W, A36 or A572 Grade 50.
Ordering plate: what to put on the order
Plate gets cut into many small parts, so grade mixups are easy. Put these on the order or drawing:
- The grade, exactly as specified, for example "CSA G40.21 350W" or "ASTM A572 Gr. 50".
- Whether mill test certificates are required. Many commercial and institutional projects require them.
- Thickness, in the units on your drawing. If a metric thickness isn't stocked, any substitution of the nearest imperial thickness is a design change for the engineer to approve.
- Impact testing, if the specification calls for a T grade.
- Cut sizes, holes and edge preparation, ideally as a drawing or DXF.
Our plate cut-to-size guide, linked at the top of this article, lists weights per square metre by thickness and compares cutting methods.
Plate cut, drilled and fabricated
We cut and drill base plates, cap plates and connection plates to your details, and weld them to beams, posts and channels in our CWB certified shop. Shop drawings are prepared in-house, so the grade on your structural drawings carries through to every plate.
Send your drawings or plate list for a free quote, or see the plates and other steel products we supply.
Frequently asked questions
What is 44W steel plate?
44W is the older imperial name for CSA G40.21 grade 300W, a weldable structural steel with a minimum yield strength of 300 MPa (about 44 ksi). Canadian mills still list the two names together, and shops often call it 44W.
What is A36 steel plate?
ASTM A36 is a US carbon structural steel specification for plates, shapes and bars. A36 plate has a minimum yield strength of 250 MPa (36 ksi), or 220 MPa (32 ksi) for plate over 203 mm (8 in) thick, and a tensile strength of 400 to 550 MPa (58 to 80 ksi).
Is 300W the same as A36?
No. They are similar general-purpose structural grades, but 300W has a higher minimum yield (300 MPa against 250 MPa for A36). Plate is often certified to both, but if your drawing calls for 300W, an A36-only certificate does not meet it.
What is the yield strength of mild steel plate?
Mild steel is an informal name for low-carbon steel. For structural plate, the guaranteed minimum yield comes from the grade: 250 MPa for ASTM A36, 300 MPa for CSA G40.21 300W and 350 MPa for 350W.
What do the letters after a CSA G40.21 grade mean?
The number is the minimum yield strength in MPa. W means weldable, A means atmospheric corrosion-resistant (weathering) steel, Q means quenched and tempered, and a T added to the end means the steel is impact tested for toughness.
