The grade line carries more weight than it looks
Most fabrication drawings state a grade in a title block and nothing else. The note reads S275, or S355, or mild steel, or it refers to an older national standard that the mill stopped rolling years ago. That single line decides which plate the buyer pays for, which welding procedure the fabricator has to qualify, which impact test the mill has to run, and whether the finished weldment is still fit for service on a cold night.
A buyer who leaves the grade vague usually gets the cheapest material that a supplier believes will satisfy the drawing. That is a rational response to an incomplete enquiry. It also leaves the buyer without control over an input that cannot be corrected once the plate is cut.
This article covers what the designation actually promises, where the number misleads, and what to put in the enquiry so that two suppliers quote the same steel.
Reading an EN 10025-2 designation
For hot rolled structural plate and sections, the European reference is EN 10025-2. The designation carries three pieces of information. S marks structural steel. The number, 275 or 355, is the minimum yield strength in N/mm² for material up to 16 mm thick. The letters JR, J0, J2 or K2 give the impact quality, which fixes the Charpy test temperature and the minimum absorbed energy.
A delivery condition suffix such as +N or +AR may follow. S355J2+N is normalized, and the suffix is not decoration: it constrains how the mill produced the plate and therefore how consistent the toughness is through the thickness.
The impact classes are worth memorising, because they are the part buyers most often lose:
| Subgrade | Charpy test temperature | Minimum energy |
|---|---|---|
| JR | +20 °C | 27 J |
| J0 | 0 °C | 27 J |
| J2 | −20 °C | 27 J |
| K2 | −20 °C | 40 J |
Yield strength is a thickness-dependent number
The 355 in S355 applies to plate up to 16 mm. Above that, the guaranteed minimum falls away in steps, and the same happens with S275. The material properties guidance from the Steel Construction Institute sets out the values by thickness band.
| Nominal thickness | S275 minimum yield | S355 minimum yield |
|---|---|---|
| ≤ 16 mm | 275 N/mm² | 355 N/mm² |
| 16 to 40 mm | 265 N/mm² | 345 N/mm² |
| 40 to 63 mm | 255 N/mm² | 335 N/mm² |
| 63 to 80 mm | 245 N/mm² | 325 N/mm² |
A heavy base plate ordered as S355 and 80 mm thick delivers 325 N/mm², not 355. If the design margin was calculated on the headline number, the margin is thinner than the designer believed. This is a design side error rather than a fabrication one, but it surfaces at the fabricator's door when the mill certificate arrives and the numbers do not match the assumption.
The impact subgrade decides whether the part survives winter
Strength tells you when the part yields. Toughness tells you whether it breaks without warning. Those are separate properties, and a weldment with adequate strength and poor toughness can fail in a brittle manner at a stress well below yield.
Brittle fracture risk rises with thickness, with tensile stress, with stress raisers such as weld toes and cope holes, and as temperature falls. That is why the subgrade is selected against the service temperature rather than against the workshop temperature, and why the choice belongs to the designer. The rules that connect exposure temperature, stress level and limiting thickness sit in EN 1993-1-10.
For a buyer, the practical consequence is a question to answer before the enquiry: where will this structure spend its life, and what is the coldest temperature it will see in service? A machine frame in a heated hall and a crane component stored outdoors through a northern winter do not need the same subgrade, and the price difference between JR and J2 is real.
An underspecified subgrade is also hard to detect later. Chemical composition and yield will match, the part will look identical, and only the mill certificate and the impact test record show what was supplied.
Carbon equivalent and the welding procedure
Weldability is expressed as a carbon equivalent value (CEV), and EN 10025-2 sets mandatory limits for the grades it covers. S275 sits at a maximum of 0.40 for material up to 30 mm and S355 at 0.45, which is why both are routinely welded without preheat at moderate thickness. Thicker sections, higher CEV and high restraint move the fabricator toward preheat and a more conservative heat input.
The welding procedure has to be qualified against the grade, not against a generic carbon steel. ISO 15614-1 covers the procedure qualification test and ties the qualified range to the material group, so a procedure written for S275 does not automatically cover S355 at heavy thickness. The system behind those procedures is separate again: a fabricator working to ISO 3834-2 has welding coordination, qualified procedures and qualified welders in place, which is what a buyer can audit before placing an order.
If impact toughness matters in the finished joint, say so. Weld metal and heat affected zone toughness are separate from plate toughness, and demonstrating them needs an impact tested procedure and often a production test piece.
Where substitution creeps in
Substitution is the quiet risk in this topic. A drawing that says S275JR can be met by several national grades with similar but not identical properties, and suppliers in different countries will reach for whatever is on the stock list.
The common equivalents are close enough to look interchangeable on a quotation and different enough to matter in a structure. A36 and SS400 guarantee yield around 250 N/mm², below S275, and their impact requirements are not the same. Q235 is lower again. None of them is wrong material. They are simply not what the drawing asked for, and the difference only appears when someone checks.
Two sentences in the enquiry close this gap. State that substitution requires written approval before rolling, and name the certificate you expect with the delivery. An inspection certificate 3.1 to EN 10204 carries the mill's own test results and the heat number, which is what makes the grade claim traceable to a specific plate rather than to a purchase order line.
Price and availability move with the subgrade
Buyers often assume the grade is a small part of the piece price. On plain plate it can be, and on a fabricated assembly the steel is usually a minority of the cost against cutting, fitting, welding, machining and coating.
The subgrade behaves differently from the strength grade. Moving from S275 to S355 adds a modest amount per tonne in most markets. Adding J2 instead of JR narrows the number of mills that roll the plate, and narrows it further at heavy thickness, because the impact requirement constrains the rolling and cooling route. A late change from JR to J2 on a 60 mm plate can therefore change both the price and the delivery date, and the buyer discovers it after the order is placed.
Availability also drives the "equivalent grade" conversation. When a mill has no J2 plate in the required thickness, the substitution offer arrives with a promise that it is just as good. Whether it is good enough is a designer's judgement, and a fabricator cannot make that call on the buyer's behalf.
What changes in inspection
The grade decision dictates the evidence package. For S275JR, a mill certificate and a dimensional check may be enough. For S355J2 at heavy thickness, the buyer should expect the certificate to show the impact results at the specified temperature, the heat number to appear on the plate marking, and the welding procedure to have been qualified on the same material group.
It also changes the weld inspection plan. Higher strength and greater thickness raise the consequence of an incomplete fusion defect, and the joint types that were acceptable with visual and spot penetrant testing may warrant ultrasonic testing. The enquiry should say which joints are full penetration and what proportion is examined, rather than leaving it to the fabricator's standard practice. The article on cast versus welded machine frames covers the same trade-off from the other direction, where the casting route replaces the plate grade question with a casting grade and a defect acceptance level.
The four lines to add to the enquiry
| Item | What to write | Why it matters |
|---|---|---|
| Grade and standard | S275JR to EN 10025-2, or S355J2+N, per part | Removes the supplier's choice of a lookalike grade |
| Impact class and service temperature | Subgrade plus the coldest expected service temperature | Connects the steel to the place it will be installed |
| Certificate | Inspection certificate 3.1 to EN 10204, heat numbers marked on parts | Makes the grade claim traceable after delivery |
| Substitution rule | Written approval required before any plate is cut | Stops the quiet swap that nobody records |
Two further items help on anything over about 30 mm thick: state the delivery condition if normalized plate is required, and ask for the welding procedure reference with the quotation so the qualification can be checked before the order.
Who owns which decision
| Decision | Owner | Why it sits there |
|---|---|---|
| Service temperature and required subgrade | Designer | Comes from the structure's location and load case |
| Yield value used in the calculation | Designer | Must be the thickness-corrected value, not the headline grade |
| Steel grade actually purchased | Supplier, within the approved specification | Buyer approves any deviation before cutting |
| Welding procedure and preheat | Supplier, within a qualified system | Depends on the fabricator's process and equipment |
| Certificate type and traceability | Buyer, in the purchase order | The mill supplies what was asked for |
| NDT method and coverage | Buyer specifies, supplier proposes | Follows from joint type and consequence of failure |
Where to take the question
The grade line is cheap to fix while the drawing is still open and expensive once plate is cut. A buyer who states the grade, the subgrade, the certificate and the substitution rule gets quotations that can be compared line by line instead of a set of prices that may or may not include the same steel.
Realjet fabricates welded structures, machine frames and equipment supports to customer drawings, with welding under an ISO 3834-2 quality system and machining of interfaces after fabrication. The contract manufacturing service page sets out how the scope is fixed, and the structural steel fabrication page shows the kind of assemblies involved. If the service temperature is unusual, mention it in the first message, because it changes the plate enquiry before anything else is quoted.