S275 vs S355 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
S275 and S355 are two widely used European structural steels specified in EN 10025. Engineers, procurement managers, and manufacturing planners frequently weigh the trade-offs between cost, strength, weldability, and fabrication demands when choosing between them. Typical decision contexts include: minimizing weight while maintaining safety margins (favoring higher strength), balancing fabrication ease and welding risk (favoring lower carbon equivalents), and managing component cost and availability.
The principal technical distinction is that S355 is specified to a higher minimum yield strength than S275, and many S355 subgrades incorporate microalloying or more stringent toughness requirements, which together influence processing and selection. Because both are low-alloy/HSLA structural steels with similar chemistry windows, they are commonly compared for beams, plates, sections, and welded fabrications where structural performance, toughness, and cost must be balanced.
1. Standards and Designations
- EN: EN 10025 family — S275 and S355 are European structural steel grades (e.g., S275JR, S355J0, S355J2).
- ASTM/ASME: No direct one-to-one ASTM equivalent; similar roles are served by ASTM A36 (lower strength) or ASTM A572 grades (higher-strength low-alloy).
- JIS: Japanese standards do not map directly but have mild steel/structural steels with similar properties.
- GB (China): GB/T structural steel grades have comparable classes; refer to specific material certificates rather than assuming equivalence.
- Classification: Both S275 and S355 are considered carbon-manganese structural steels within the HSLA (High-Strength Low-Alloy) family when microalloyed; they are not stainless, tool, or high-alloy steels.
2. Chemical Composition and Alloying Strategy
The following table shows typical composition ranges for commonly supplied S275 and S355 grades. These are indicative ranges; exact chemical limits depend on the specific EN 10025 subgrade and manufacturer certification. Always verify mill certificates for design calculations.
| Element | Typical S275 (wt%) | Typical S355 (wt%) |
|---|---|---|
| C (Carbon) | 0.10 – 0.20 | 0.12 – 0.22 |
| Mn (Manganese) | 0.60 – 1.50 | 0.60 – 1.70 |
| Si (Silicon) | ≤ 0.55 (typically 0.10–0.35) | ≤ 0.55 (typically 0.10–0.35) |
| P (Phosphorus) | ≤ 0.035 (max specified) | ≤ 0.035 (max specified) |
| S (Sulfur) | ≤ 0.035 (max specified) | ≤ 0.035 (max specified) |
| Cr (Chromium) | trace – not specified | trace – sometimes present |
| Ni (Nickel) | trace – not specified | trace – sometimes present |
| Mo (Molybdenum) | trace if any | trace if any |
| V (Vanadium) | usually none; some microalloy grades contain V | may contain V in microalloy variants |
| Nb (Niobium / Cb) | generally none | often present in controlled amounts for some S355 variants |
| Ti (Titanium) | optionally present as deoxidizer | optionally present in microalloyed material |
| B (Boron) | typically not used | rarely used in structural grades |
| N (Nitrogen) | low, controlled | low, controlled |
How alloying influences behavior: - Carbon and manganese primarily control base strength and hardenability; higher C and Mn increase strength but reduce weldability and formability. - Silicon and small additions of Cr, Ni, Mo can slightly increase strength and hardenability. - Microalloying elements (Nb, V, Ti) enable higher strength via precipitation strengthening and grain refinement without large increases in carbon — beneficial for S355 variants to achieve 355 MPa yield while maintaining weldability and toughness. - Phosphorus and sulfur are kept low to preserve toughness and weldability.
3. Microstructure and Heat Treatment Response
Typical microstructures: - As-rolled S275: predominantly ferrite with polygonal ferrite and some pearlite depending on cooling rate and carbon content. Grain size and ferrite morphology controlled by thermo-mechanical rolling. - As-rolled S355: ferrite-pearlite with finer ferrite grain size in microalloyed variants; precipitation of carbo-nitrides (NbC, VC) in microalloyed grades strengthens by hindering dislocation motion.
Heat treatment response: - Normalizing: Both grades respond to normalizing with modest grain refinement and slightly higher strength and toughness; used rarely for large structural sections. - Quenching & tempering: Not typical for standard S275/S355 structural deliveries; could be applied to specially heat-treated variants but is outside normal EN 10025 practice. - Thermo-mechanical control processing (TMCP): Common for S355 to obtain higher yield and improved toughness without high carbon. TMCP yields finer grains and favorable balance of strength and ductility.
Effect of processing routes: - S355 microalloyed/TMCP material achieves higher yield with minimal sacrifice to weldability and toughness versus simply increasing carbon. - S275 is generally more tolerant of cold forming and less demanding in welding preheat because of lower strength and often lower carbon equivalent.
4. Mechanical Properties
Below are representative mechanical property ranges. Values depend on subgrade (JR, J0, J2), thickness, and processing; treat ranges as indicative and verify against supplied mill test certificates.
| Property | S275 (typical) | S355 (typical) |
|---|---|---|
| Yield strength (MPa, min) | 275 | 355 |
| Tensile strength (MPa) | 410 – 560 | 470 – 630 |
| Elongation (%) | 20 – 25 (depends on thickness) | 18 – 22 (depends on thickness) |
| Impact toughness (Charpy V, J) | JR: 27 J @ +20°C; J0/J2 variants for lower temperatures | JR/J0/J2 available: e.g., 27 J @ +20°C (JR) or 27 J @ 0°C / −20°C (J0/J2) depending on subgrade |
| Hardness (HBW, typical) | ~120 – 180 (varies) | ~140 – 200 (varies) |
Interpretation: - Strength: S355 is the stronger grade by design (higher specified yield and higher tensile ranges). - Toughness: Both grades can be supplied with similar impact energy ratings by selecting appropriate subgrades (e.g., JR vs J0/J2). Thickness and heat treatment determine toughness performance. - Ductility: S275 typically shows slightly higher elongation, making it a bit more forgiving in forming operations. - Hardness correlates with strength; S355 typically reads higher hardness, which affects machining and tool wear.
5. Weldability
Weldability is governed primarily by carbon content and carbon equivalent (hardenability). Microalloying and residuals also matter.
Common weldability indices: - International Institute of Welding carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - International Institute of Welding practical parameter: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - S355 typically has a marginally higher carbon equivalent than S275 due to the higher carbon and possible microalloying additions; this increases propensity for weld-induced hard zones and hydrogen cracking if not managed. - Microalloyed S355 grades often rely on low carbon plus Nb/V/Ti additions; this gives higher strength with relatively controlled CE, mitigating excessive welding preheat compared with high-carbon steels. - Practical guidance: for thicker sections or low-temperature service, apply preheat and controlled interpass temperatures per CE or Pcm assessment; use appropriate consumables and post-weld heat treatment if required.
6. Corrosion and Surface Protection
- Neither S275 nor S355 are stainless steels; general corrosion resistance is similar and limited to basic resistance typical of plain carbon-manganese steels.
- Typical protection strategies:
- Hot-dip galvanizing for atmospheric corrosion protection (common for structural elements).
- Paint systems (primer + topcoat) for architectural and marine-exposed structures.
- Metallizing, polymer coatings, or sacrificial anodes for aggressive environments.
- Stainless indices (PREN) are not applicable for S275/S355, as they are not stainless steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This formula is only meaningful for stainless alloys containing significant Cr/Mo/N.
When to consider other materials: - For chloride-rich or chemically aggressive environments, consider corrosion-resistant alloys rather than relying on coatings alone.
7. Fabrication, Machinability, and Formability
- Cutting: Plasma, oxy-fuel, and laser cutting are routine. S355’s higher strength may require slightly higher cutting energies and more attention to heat-affected zone (HAZ) properties.
- Forming/bending: S275, with lower yield and somewhat higher elongation, is generally easier to cold form and can tolerate smaller bend radii. S355 requires larger bend radii or pre-heating for tight geometries to avoid cracking.
- Machinability: Both are not considered free-machining steels; higher strength and hardness in S355 can increase tool wear and reduce achievable feed rates. Use appropriate tooling, speeds, and coolant.
- Finishing: Surface treatment, straightening, and stress-relief practices follow standard structural steel procedures. For welded assemblies, account for distortion control more in S355 due to higher restraint stresses.
8. Typical Applications
| S275 – Typical Uses | S355 – Typical Uses |
|---|---|
| Light structural sections, secondary beams, purlins, brackets, general fabrication where moderate strength suffices and cost is critical | Main beams, heavy structural members, bridge components, offshore jacket structures, high-load crane rails where higher yield and reduced section thickness (weight saving) are important |
| Architectural steelwork, catwalks, platforms, façade supports | High-load frames, heavy plate, structural components requiring higher design stress or reduced thickness |
| General welded fabrications with frequent forming | Fabrications where higher toughness at lower temperatures and higher strength-to-weight are prioritized |
Selection rationale: - Choose S275 when lower cost, easier forming/welding, and thicker allowable sections are acceptable. - Choose S355 when higher yield strength enables thinner sections and weight reduction, or where design codes mandate minimum yield values.
9. Cost and Availability
- Cost: S355 typically carries a premium over S275 due to higher alloy control and additional processing (TMCP, microalloying, testing). The premium varies with market conditions and product form.
- Availability: Both grades are widely available in plate, sheet, rolled sections, and structural profiles. S355 may be slightly more common for heavy plate and high-strength structural applications; S275 is often used for lighter structural sections.
- Lead times: Specialty subgrades (e.g., specific impact ratings, thicknesses, or microalloy variants) can increase lead times; early specification and supplier communication are advisable.
10. Summary and Recommendation
Summary table (qualitative):
| Attribute | S275 | S355 |
|---|---|---|
| Weldability | Good (lower CE) | Good–Moderate (higher CE potential; microalloying helps) |
| Strength–Toughness balance | Moderate strength; good ductility | Higher strength; can maintain good toughness with proper subgrade |
| Cost | Lower | Higher (premium for higher strength) |
Recommendation: - Choose S275 if you need a cost-effective, easily formed and welded structural steel for applications where the 275 MPa yield is sufficient, where fabrication simplicity and lower cutting/tooling wear are priorities, and when weight reduction is not a primary objective. - Choose S355 if your design requires higher yield strength to reduce section thickness or weight, or where higher design stresses and improved toughness (using appropriate subgrades) are required. Use S355 when the program can accommodate slightly higher material cost, tighter welding control (preheat/interpass), and potentially increased machining effort.
Final notes: - Always specify the exact EN 10025 subgrade (e.g., JR, J0, J2, or N/T conditions) and thickness limits in procurement documents. - Request mill test certificates and confirm chemical composition and mechanical properties for design calculations, weld procedure specifications (WPS), and fabrication planning. - For critical welded structures or low-temperature service, evaluate carbon equivalent using $CE_{IIW}$ or $P_{cm}$ and apply preheat or PWHT as indicated by welding codes and standards.