S355JR, S355J0 and S355J2 are the same S355 structural steel grade with the same 355 N/mm2 minimum yield strength; the letters describe impact toughness, how much energy the steel absorbs before it fractures, tested at three different temperatures. This guide is the companion to our overview of structural steel grades S275 vs S355 and goes one level deeper into what those extra letters on a drawing actually mean.
How the Charpy impact test works
Strength and toughness are not the same property. A steel can meet its yield strength on a tensile test and still fracture suddenly, without warning, if it is cold and struck hard enough: that is a brittle fracture, and it is exactly what the sub-grade tests are designed to rule out.
The test used is the Charpy V-notch impact test. A small notched steel sample is cooled to a set temperature, then struck by a weighted pendulum in a single blow. The pendulum measures how much energy the sample absorbed before it broke, recorded in joules (J). A tough sample bends and absorbs more energy; a brittle one snaps cleanly and absorbs less. The test is repeated at the temperature the standard specifies for each sub-grade, and the steel has to hit a minimum energy figure to earn that letter.
The S355 sub-grades at a glance
| Sub-grade | Test temperature | Minimum impact energy | Typical application |
|---|---|---|---|
| S355JR | +20 degrees C | 27 J | General indoor or sheltered steelwork, moderate ambient conditions |
| S355J0 | 0 degrees C | 27 J | Standard exposed UK structures, the common outdoor default |
| S355J2 | -20 degrees C | 27 J | Cold-exposed steelwork, dynamic or fatigue-loaded members, exposed bridges and gantries |
| S355K2 | -20 degrees C | 40 J | The most demanding cold or dynamic applications, a higher toughness margin than J2 at the same temperature |
These same letters apply below S355 too: S235JR, S275JR and S275J0 follow the identical logic. If you are weighing up S235 against S275 and S355, or want the full picture on S275 steel or S355 steel on their own, the toughness letters carry across.
When an engineer specifies each sub-grade
The sub-grade is set by the design, not chosen off a supplier’s shelf. A structural engineer works through Eurocode 3’s toughness selection method (BS EN 1993-1-10), which weighs the lowest service temperature the steel will realistically see, the thickness of the section, and whether the stress it carries is static or repeatedly cycling.
Cold exposure is the obvious driver: steel that sits outdoors through a UK winter, on an exposed frame, bridge or gantry with no cladding to buffer it, is more likely to need J2 than a beam buried inside a heated building. Dynamic or fatigue loading raises the bar too, since a member that flexes repeatedly, such as a crane runway beam or a bridge deck girder, is more prone to fatigue cracks starting a brittle fracture. Thicker sections cool more slowly and behave differently under impact, which the design tables account for directly, and welded connections add a further factor, because the heat-affected zone around a weld can locally reduce toughness. A sheltered internal beam might only ever need JR; an exposed agricultural or industrial frame is more likely to specify J0 as standard; anything genuinely cold-service, heavily dynamic, or part of a steel bridge structure tends to move up to J2 or K2. We fabricate to whatever the engineer’s specification calls for as part of our structural steelwork, and we would always rather build to the drawing than guess.
The common misunderstanding: J2 is not “stronger” than JR
It is a natural assumption that a “higher” letter means a stronger steel, and it is wrong. Yield strength (355 N/mm2 minimum) and tensile strength (470 to 630 N/mm2) are identical across JR, J0, J2 and K2. Nothing about the letter changes how much load the section carries.
What changes is only the proven behaviour at low temperature. Specifying J2 for a job that will only ever sit in a warm, sheltered building adds cost for a toughness margin the structure will never need. Specifying JR for a job that will actually see a cold, dynamic environment leaves the design short of the toughness it was calculated against, which is a genuine risk, not just a paperwork issue. Getting the sub-grade right is about matching the certificate to the service condition, not upgrading “for safety” by default.
Reading the full designation on a drawing
A full steel designation, such as S355J2+N, stacks three pieces of information into one code:
- S355 is the grade: minimum 355 N/mm2 yield strength.
- J2 is the toughness sub-grade: 27 J minimum at minus 20 degrees C.
- +N is the delivery condition, how the mill processed the steel after rolling.
The delivery suffixes break down as follows. +AR means as rolled, delivered straight from the mill with no further heat treatment. +N means normalised, or normalised rolled, where the steel is heat treated (or rolled within a controlled temperature range) to refine its grain structure for more consistent toughness. +M means thermomechanically rolled, a controlled rolling process at reducing temperatures that achieves similar refined properties without a separate heat-treatment step afterwards. Which condition a section needs, on top of the grade and sub-grade, again comes from the engineer’s specification.
What a material certificate shows
For traceability, structural steel is supplied against a mill certificate to BS EN 10204. The type most commonly specified for structural work is a 3.1 inspection certificate, issued and validated by the steel mill itself. It records the heat number for that cast of steel, the chemical composition, and the mechanical test results, including the Charpy impact energy that proves the JR, J0, J2 or K2 rating actually achieved.
Under BS EN 1090 execution, and alongside CE and UKCA marking, that certificate is tracked from raw material through fabrication to the finished piece, so a specific beam or column can, in principle, be traced back to the batch and test results it came from. Background on the wider testing standards sits with the Steel Construction Institute. TC Rowan can supply material traceability on request; ask us when you send over the specification.
Frequently asked questions
What is the difference between S355JR, S355J0 and S355J2?
All three have the same minimum yield strength of 355 N/mm2 and the same tensile strength range. The letters describe impact toughness, tested with a Charpy V-notch test at a set temperature: JR requires 27 J at +20 degrees C, J0 requires 27 J at 0 degrees C, and J2 requires 27 J at minus 20 degrees C. A higher sub-grade proves the steel stays tough at a colder temperature, not that it is stronger.
Is S355J2 stronger than S355JR?
No. Strength, both yield and tensile, is identical across JR, J0 and J2; only the proven impact toughness at low temperature differs. Specifying J2 where JR would do adds cost without a strength benefit, and specifying JR where the design needs J2 risks brittle fracture in cold conditions. The correct sub-grade comes from the structural engineer’s calculation, not a default upgrade.
What does +N or +AR mean after a steel grade like S355J2+N?
These letters describe the delivery condition, how the steel was processed after rolling, not the grade itself. +AR means as rolled, with no extra heat treatment. +N means normalised or normalised rolled, which refines the grain structure for more consistent toughness. +M means thermomechanically rolled, a controlled rolling process that achieves similar properties with less added heat treatment. A drawing calling for S355J2+N is specifying grade, toughness and delivery condition together.
What does a material certificate show for structural steel?
A BS EN 10204 type 3.1 certificate, issued by the steel mill, shows the heat number, the chemical analysis and the mechanical test results for that batch of steel, including the Charpy impact result that proves the JR, J0 or J2 rating. Under BS EN 1090 execution, that certificate is tracked through to the fabricated piece, so the steel in your finished structure can be traced back to its test results. TC Rowan can provide traceability on request.
Not sure which sub-grade your drawings specify, or need the certificate paperwork to go with it? Send us the engineer’s specification and we will supply and fabricate the right steel. Get in touch for a free quote.