UK Steel Charpy Impact Values -- BS EN 10025-2 Subgrade Selection
Charpy V-notch (CVN) impact test requirements for UK structural steel grades S275 and S355 per BS EN 10025-2:2019, with UK National Annex guidance for toughness selection in building and bridge structures. Covers JR, J0, J2, and K2 subgrades, minimum absorbed energy at specified test temperatures, and the relationship between fracture toughness, service temperature, and steel thickness in UK steelwork design.
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Charpy Subgrade Designation System
BS EN 10025-2 classifies structural steels by their Charpy impact performance at specified temperatures. The subgrade designation follows the format S{minyield}{subgrade}, where the subgrade letter+number indicates the test temperature and minimum absorbed energy:
| Subgrade | Test Temperature | Minimum Energy (CVN) | Typical UK Application |
|---|---|---|---|
| JR | +20 C | 27 J | Internal building steel, non-critical secondary members |
| J0 | 0 C | 27 J | Standard UK building frames, internal office/retail |
| J2 | -20 C | 27 J | External steel, bridges, crane beams, dynamically loaded |
| K2 | -20 C | 40 J | Safety-critical, fracture-critical, thick sections >50mm |
The number after the letter indicates the test temperature: JR (room temperature, +20 C), J0 (0 C), J2 (-20 C), K2 (-20 C with higher energy requirement). The letter R originally stood for "room temperature" and was retained when the French designation system was adopted.
Charpy Test Method per BS EN ISO 148-1
The Charpy V-notch test uses a 10 mm x 10 mm specimen with a 2 mm deep V-notch machined at the centre. The specimen is cooled (or heated) to the specified test temperature, then struck by a pendulum hammer. The absorbed energy in Joules (J) is recorded. Three specimens are tested and the average must meet or exceed the minimum specified value; no single result may fall below 70% of the minimum.
S275 Charpy Values by Thickness and Subgrade
S275JR is the base subgrade for S275 steel, with S275J0 and S275J2 providing progressively better low-temperature toughness. S275K2 is rarely specified -- S355J2 is the preferred alternative for structural applications requiring K2-level toughness.
| Subgrade | Thickness t (mm) | Min CVN (J) | Test Temp (C) | Max CEV (%) | UK NA Notes |
|---|---|---|---|---|---|
| S275JR | t <= 150 | 27 | +20 | 0.40 | Internal only; CEV may limit weldability |
| S275J0 | t <= 150 | 27 | 0 | 0.40 | Standard for most UK building frames |
| S275J2 | t <= 150 | 27 | -20 | 0.40 | External or cold-region steelwork |
| S275K2 | t <= 100 | 40 | -20 | 0.40 | Not commonly stocked in UK; specify S355J2 instead |
S355 Charpy Values by Thickness and Subgrade
S355 is the dominant structural steel grade in UK construction. S355J2 is the standard specification for exposed or safety-critical steelwork; S355JR is restricted to internal applications in heated buildings.
| Subgrade | Thickness t (mm) | Min CVN (J) | Test Temp (C) | Max CEV (%) | UK NA Notes |
|---|---|---|---|---|---|
| S355JR | t <= 150 | 27 | +20 | 0.45 | Internal use only; limited availability in thick sections |
| S355J0 | t <= 150 | 27 | 0 | 0.45 | Building frames, typical internal specification |
| S355J2 | t <= 150 | 27 | -20 | 0.47 | Standard UK external specification |
| S355K2 | t <= 100 | 40 | -20 | 0.47 | Bridges, fracture-critical, dynamic loading |
| S355J2+N | t <= 150 | 27 | -20 | 0.47 | Normalised; preferred for thick sections >40mm |
| S355J2+M | t <= 50 | 27 | -20 | 0.47 | Thermomechanically rolled; CEV typically lower |
The +N suffix indicates normalised delivery condition; +M indicates thermomechanically rolled. Both achieve the same Charpy performance as J2 but through different processing routes. UK steel stockholders typically stock S355J2+N for plate and S355J2+M for open sections.
Fracture Toughness Selection per BS EN 1993-1-10
The UK NA to BS EN 1993-1-10 provides a table-based method for selecting steel subgrade based on:
- Minimum service temperature T_md (C) -- typically -5 C for internal UK buildings, -15 C for external steelwork
- Reference stress sigma_Ed as a fraction of fy(t) -- higher stress demands better toughness
- Element thickness t -- thicker sections require better toughness due to triaxial stress state
- Detail category (moderate or severe) -- notches and weld details increase fracture risk
UK Service Temperature Guidance
| Structure Type | T_md (C) | Minimum Subgrade |
|---|---|---|
| Internal, heated building | -5 | JR or J0 |
| Internal, unheated | -10 | J0 |
| External, sheltered | -15 | J2 |
| External, exposed | -20 | J2 or K2 |
| Bridge -- internal | -15 | J2 |
| Bridge -- external | -20 | K2 |
| Offshore topside | -10 | J2 |
| Cold store (-30 C) | -30 | Special |
Worked Example: Subgrade Selection for a UK Car Park
A multi-storey car park in Manchester has external steel beams (S355, 25 mm flange thickness) with welded moment connections. Determine the required Charpy subgrade.
Step 1 -- Determine T_md: External, exposed structure in northern England: T_md = -15 C (conservative; Manchester's lowest recorded temperature is -12 C, but -15 C is standard for exposed UK steelwork).
Step 2 -- Determine sigma_Ed / fy(t): For a moment-resisting beam at ULS, the maximum flange stress approaches fy. sigma_Ed / fy(t) = 0.75 x fy / fy = 0.75 (using the 0.75 factor from EN 1993-1-10 for the reference stress calculation).
Step 3 -- Check EN 1993-1-10 Table 2.1: For S355, t = 25 mm, sigma_Ed = 0.75 fy, T_md = -15 C: The table indicates J2 subgrade is required (max thickness for JR at -15 C = not permitted; J0 max thickness at -15 C/0.75fy = 15 mm; J2 max thickness at -15 C/0.75fy = 45 mm).
Step 4 -- Specification: S355J2 to BS EN 10025-2. Minimum CVN = 27 J at -20 C. For the welded moment connections (detail category "severe"), verify K2 is not required: at T_md = -15 C, sigma_Ed = 0.50 fy (reduced stress at the connection due to section over-strength), K2 max thickness = 60 mm. 25 mm < 60 mm, so J2 is adequate.
Answer: S355J2 is suitable. Specify S355J2+N if plates are involved, or S355J2+M for standard open sections.
Carbon Equivalent Value (CEV) and Weldability
The CEV limits in BS EN 10025-2 (0.40 for S275, 0.45-0.47 for S355) are linked to Charpy performance through weldability. Higher CEV = higher hardenability = greater risk of hydrogen cracking in the HAZ (heat-affected zone), which reduces effective toughness.
CEV Formula per BS EN 10025-2
CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
For UK steel, the practical CEV for S355J2 typically ranges from 0.38 to 0.43 for sections and 0.40 to 0.45 for plate. The lower CEV of thermomechanically rolled (J2+M) sections improves weldability without preheat for thicknesses up to ~30 mm.
Design Resources
- UK Steel Grades Reference -- EN 10025-2 grade selection for UK projects
- UK Steel Mechanical Properties -- fy, fu, and elongation tables
- UK Steel Chemical Composition -- CEV and alloy limits
- UK Universal Beam and Column Sizes -- UB/UC section dimensions and properties
- UK Beam Design Guide -- EN 1993-1-1 flexure, shear, and LTB
- UK Connection Design Guide -- EN 1993-1-8 bolted and welded joints
- All UK Steel Design References -- complete library
Frequently Asked Questions
What is the difference between S355JR and S355J2?
The only difference is the Charpy test temperature. Both require minimum 27 J absorbed energy, but JR is tested at +20 C (room temperature) while J2 is tested at -20 C (freezer temperature). The chemical composition of J2 steel is typically cleaner (lower S and P) to achieve the low-temperature toughness, and the CEV may be slightly higher. For UK external steelwork, J2 is the standard requirement; JR is acceptable only for internal steelwork in heated buildings where the steel temperature never drops below approximately +5 C.
When is K2 subgrade required in UK practice?
K2 (40 J at -20 C) is required per the UK NA when EN 1993-1-10 Table 2.1 indicates that J2 is insufficient for the combination of steel thickness, stress level, and minimum service temperature. Typical scenarios: bridge main girders (t > 50 mm), dynamically loaded crane beams in unheated buildings, fracture-critical tension members in stadia and long-span roofs, and thick welded joints in offshore structures. K2 is also specified when the structural consequence of brittle fracture is exceptionally high (Consequence Class 3 per BS EN 1990 Annex B).
How does the UK NA modify EN 1993-1-10 toughness selection?
The UK NA to BS EN 1993-1-10 adopts the recommended procedure without modification but adds UK-specific service temperature guidance: T_md = -5 C for internal heated buildings, -15 C for external steelwork in most of the UK, and -20 C for exposed steelwork in Scotland, northern England, and high-altitude locations. The UK NA also cross-references BS EN 1993-1-10 Table 2.1 to BS EN 10025 subgrades and confirms that for UK building structures to EN 1993-1-1, J2 subgrade is the default specification for all external steelwork.
What does the +N and +M suffix mean for UK steel orders?
+N (normalised) steel is heated above the transformation temperature (approximately 900 C) and cooled in air. This refines the grain structure, improving toughness and reducing the ductile-to-brittle transition temperature. +M (thermomechanically rolled) achieves similar properties through controlled rolling and cooling without a separate normalising heat treatment, resulting in lower CEV and better weldability at the cost of a lower maximum thickness (typically 50 mm). UK practice: specify +N for plate over 40 mm; +M is acceptable and often preferred for sections up to 50 mm.
Related Pages
- EN 1993 Steel Design Overview
- European Steel Properties
- UK Steel Fy/Fu Values
- UK Steel Chemical Composition
- EN 1993 Column Buckling
- EN 1990 Load Combinations
Educational reference only. All design values are per BS EN 10025-2:2019, BS EN 1993-1-10:2005 + UK National Annex, and BS EN ISO 148-1:2016. Verify all values against the current editions of the standards and the applicable National Annex for your project jurisdiction. Impact testing requirements must be confirmed by the project specification and the project's Principal Designer. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY -- NOT FOR CONSTRUCTION without independent professional verification.