Steel Grades for Heavy Weldments: A36, A572, S235JR and S355 Compared
Most heavy frames are built from one of four grades. They are close cousins, they are mostly interchangeable in intent, and choosing between them is usually a question of which standard your drawing follows and whether you need the extra yield strength.
Here is the comparison, and the cases where it actually changes the design.
The four grades
| Grade | Standard | Min yield | Tensile | Where it is used |
|---|---|---|---|---|
| ASTM A36 | ASTM A36/A36M | 250 MPa (36 ksi) | 400–550 MPa | US default for plate and sections |
| ASTM A572 Gr 50 | ASTM A572/A572M | 345 MPa (50 ksi) | 450 MPa min | US high-strength low-alloy |
| S235JR | EN 10025-2 | 235 MPa | 360–510 MPa | EU general structural |
| S355 (JR/J0/J2) | EN 10025-2 | 355 MPa | 470–630 MPa | EU workhorse for heavy structure |
Practical pairings: A36 ≈ S235JR and A572 Gr 50 ≈ S355. They are not identical, chemistry limits, tolerance classes and impact requirements differ, but for a general weldment a shop will treat them as equivalents, and a drawing that says “A36 or equivalent” will be built in whatever the local mills stock.
In Vietnam and the wider region, S355 and A572 Gr 50 are readily available, as are A36 and S235JR. Nothing on this list is exotic.
What the JR / J0 / J2 suffix means
This is the part buyers most often miss, and it is the one that occasionally matters a great deal.
The suffix specifies notch toughness, the Charpy V-notch impact energy the steel must deliver, and at what temperature:
| Suffix | Impact test temperature | Minimum energy |
|---|---|---|
| JR | +20 °C | 27 J |
| J0 | 0 °C | 27 J |
| J2 | −20 °C | 27 J |
| K2 | −20 °C | 40 J |
If your frame will be loaded in a cold climate, outdoor plant in northern Europe, Korea in winter, a cold store, specify J2. Toughness is what stops a small flaw propagating into a brittle fracture, and it is a property you cannot add later.
If the equipment lives indoors in a temperate or tropical plant, JR is adequate and cheaper.
The failure mode: a drawing says “S355” with no suffix, the mill supplies S355JR because it is the cheapest qualifying grade, and the frame ends up outdoors in a place that sees −25 °C. Nothing is technically wrong with the supply. Write the suffix.
When higher yield strength pays for itself
Moving from A36 to A572 Gr 50, or S235 to S355, buys roughly 40–50% more yield strength at a modest material premium, typically a few percent per tonne.
It pays when the design is strength-governed. If a member is sized by stress, the higher grade lets you thin the section. Less plate to buy, less weld metal to deposit, less weight to lift and ship. On a strength-governed frame this frequently more than repays the premium.
It does not pay when the design is stiffness-governed. Young’s modulus is about 200 GPa for all of these grades. A stronger steel is not a stiffer steel. If your member is sized by deflection, which is common on machine bases, where the tolerance is alignment rather than failure, going to S355 buys nothing at all except a higher invoice.
The test: ask your engineer whether the governing check was stress or deflection. If it was deflection, stay with the cheaper grade and spend the money on section depth instead.
Weldability and preheat
All four grades are readily weldable. The relevant number is the carbon equivalent (CE or CEV), which predicts hardening in the heat-affected zone and therefore preheat requirement.
Rough guidance:
- CE below 0.40: generally weldable with little or no preheat on moderate thickness
- CE 0.40–0.50: preheat likely on thicker sections
- CE above 0.50: preheat and controlled interpass temperature required
A36 and S235JR typically sit at the low end. A572 Gr 50 and S355 run slightly higher. Thickness matters as much as chemistry: the same grade at 60 mm needs preheat where 20 mm does not, because the surrounding cold mass quenches the joint faster.
Get the actual CE from the mill certificate rather than a table. It is printed on the cert, and it is what the WPS should have been qualified against, see AWS D1.1 in plain language.
Weathering and stainless, the two exceptions
Weathering steel (ASTM A588, or S355J0WP) forms a stable oxide layer and can be used unpainted outdoors. It suits bridges and exposed structures. It is a poor choice where the frame is in constant wet contact, where run-off would stain, or where anything bolts to it with dissimilar metal.
Stainless (304/316) appears on skids for food, pharmaceutical and marine service. It is several times the price of carbon steel, it welds differently, it distorts more because of its higher thermal expansion and lower conductivity, and it needs pickling and passivation after welding. Specify it where the process demands it, not for appearance.
Mill certificates
Whatever grade you specify, require EN 10204 3.1 certificates, issued by the mill’s own inspection department, traceable to the heat number, listing chemistry and mechanical properties.
A 2.2 certificate is a statement of compliance with no test results for your specific heat. For a load-bearing frame, ask for 3.1. It costs nothing and it is the document your QA will want to see three years later.
What to write on the drawing
Four lines:
- Grade and standard: “Plate and sections: S355J2 to EN 10025-2” or “ASTM A572 Grade 50”
- Toughness suffix: chosen for the lowest service temperature
- Certification: “Mill certificates to EN 10204 3.1 required”
- A substitution clause: “Equivalent grades acceptable with prior written approval” if you are willing to let the shop use what it can source quickly, which often shortens lead time
Unsure whether your frame is strength- or stiffness-governed, or which grade the region can actually supply on your timeline? Send the drawing and we will come back within 24 business hours.
See also: reading EN 10025 plate designations such as S355J2+N.
See also: S355 structural steel, JR to K2, with yield by thickness, CEV and equivalents in full.