JR, J0, J2 and K2: Impact-Toughness Suffixes Explained

In a structural steel designation such as S355J2, the “355” and the “J2” describe different things.

• S355 identifies the structural-steel family and a specified minimum yield-strength level for the relevant thickness range.
• J2 describes a Charpy impact-toughness requirement.

The toughness suffix must therefore not be treated as a different yield-strength grade.

The practical decoding table

For commonly used EN structural grades:

The EN 10027 designation system uses J, K and L for different impact-energy levels and R, 0, 2, 3, 4, 5, 6 for the associated temperature symbols. For everyday S235/S275/S355 purchasing, JR, J0, J2 and K2 are among the most common combinations.

S355JR vs S355J0 vs S355J2

These grades share the same S355 strength family, but their guaranteed impact-toughness test temperature changes:

• S355JR: 27 J at +20 °C.
• S355J0: 27 J at 0 °C.
• S355J2: 27 J at -20 °C.

Moving from JR to J0 to J2 therefore gives a guarantee at progressively lower test temperatures. It does not mean the nominal yield strength rises from one grade to the next.

What is different about K2?

S355K2 increases the specified Charpy energy level to 40 J at -20 °C.

So J2 and K2 share the same suffix temperature of -20 °C, but K2 represents a higher impact-energy requirement.

That makes this statement wrong:

“K2 is simply J2 tested colder.”

It is not. The difference is the energy level.

Does J2 mean the steel is safe at -20 °C in every structure?

No.

A Charpy designation is one input into brittle-fracture resistance; it is not itself a complete design-temperature approval. Actual material selection can also depend on:

• product thickness;
• stress level;
• detail category and stress concentration;
• loading rate and fatigue/dynamic effects;
• welding and heat-affected zones;
• applicable design code;
• project-specific minimum design temperature.

The Charpy suffix tells you what the material standard requires for the specified impact test. The structural design rules determine whether that material is suitable for the actual component and service temperature.

Do orientation and specimen size still matter?

Yes.

The shorthand suffix does not replace the detailed test rules in the product standard. The applicable standard defines matters such as:

• sampling position;
• longitudinal or transverse orientation;
• full-size or sub-size specimens;
• number of specimens;
• averaging / individual-value rules;
• test frequency.

For example, published EN 10025-2 product data commonly gives the JR/J0/J2/K2 energies as results based on a set of Charpy specimens, with additional rules where full-size specimens cannot be taken.

Toughness suffixes and delivery conditions are separate pieces of information

A designation such as S355J2+N contains two different concepts:

• J2 = the toughness quality;
• +N = the delivery condition, normalized or normalizing rolled.

Other delivery-condition suffixes can also appear separately from the toughness designation, for example +AR for as rolled and +M for thermomechanically rolled material. These suffixes describe the delivery condition; they do not replace or redefine JR, J0, J2 or K2.

This distinction becomes especially important when comparing S355J2+N with fine-grain grades such as S355N or S355NL. The “N” in S355N belongs to a different grade family and should not be read as shorthand for S355J2+N.

Common purchasing mistakes

1. Ordering only “S355”

That leaves the impact-quality level unspecified. For many structural projects the difference between JR, J0, J2 and K2 matters.

2. Treating J2 and K2 as interchangeable

They have different impact-energy requirements.

3. Assuming J2 defines the minimum design temperature

It defines an impact-test condition, not a universal structural design-temperature limit.

4. Ignoring the product standard

S355J2 can appear in several product contexts. The complete requirement should include the relevant product standard and product form.

5. Confusing toughness with delivery condition

JR/J0/J2/K2 and delivery-condition suffixes such as +AR, +N and +M are separate pieces of the order designation.

MetalMate takeaway

JR, J0, J2 and K2 primarily tell you about Charpy toughness, not yield strength. Read the energy letter and temperature symbol together, then go back to the product standard for specimen orientation, size, frequency and acceptance details.

References

• NEN, EN 10027-1:2016 — Designation systems for steels — Steel names: https://www.nen.nl/nen-en-10027-1-2016-en-226697
• European Commission Joint Research Centre, Preparatory study for iron & steel products — designation system derived from EN 10027-1:2016: https://susproc.jrc.ec.europa.eu/product-bureau/sites/default/files/2026-03/Preparatory%20study%20iron%20%26%20steel_March2026_0.pdf
• Salzgitter Flachstahl, Meaning of steel grade designations: https://www.salzgitter-flachstahl.de/en/news/details/bedeutung-der-stahlsortenbezeichnung-fuer-ausgewaehlte-staehle-aus-dem-lieferprogramm-der-salzgitter-flachstahl-20784.html
• Salzgitter Flachstahl, Non-alloy structural steels — mechanical and impact properties: https://www.salzgitter-flachstahl.de/en/products/hot-rolled-products/steel-grades/non-alloy-structural-steels.html