S235 vs S275 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
S235 and S275 are two widely used European structural carbon steels specified by EN standards. Engineers, procurement managers, and manufacturing planners commonly weigh trade-offs between cost, weldability, formability, and strength when selecting between them. Typical decision contexts include choosing a more economical grade for lightly loaded structures versus a slightly higher-strength grade where section size, weight reduction, or regulatory minimum yield are critical.
The principal technical distinction between S235 and S275 is their minimum specified yield strength: S275 has a higher guaranteed yield than S235. That difference drives selection in load-bearing applications, but both grades are similar in chemistry and processing, so other factors (weldability, toughness, availability, and surface protection) often determine the final choice.
1. Standards and Designations
- EN: EN 10025 series (most common for S235, S275 variants such as S235JR, S235J0, S235J2, S275JR, etc.).
- ASTM/ASME: No direct one-to-one ASTM equivalents; comparable ASTM grades are typically specified based on required properties rather than direct substitution.
- JIS: Japanese standards classify structural steels differently; selection requires property-based matching.
- GB (China): GB standards include structural steels similar in use but not direct designations; match by mechanical and chemical requirements.
Classification by general steel family: - Both S235 and S275 are plain carbon/low-alloy structural steels (not stainless, not tool steel, not high-alloy). Some mill variants may include microalloying (Nb, V, Ti) or thermomechanical processing, which still classifies them as structural steels (often grouped with HSLA when deliberately microalloyed).
2. Chemical Composition and Alloying Strategy
| Element | S235 (typical presence) | S275 (typical presence) |
|---|---|---|
| C (Carbon) | Low carbon; controlled for weldability | Low carbon; may be slightly higher ceiling than S235 |
| Mn (Manganese) | Moderate (primary strength & hardenability influencer) | Moderate; similar or slightly higher limits to achieve target yield |
| Si (Silicon) | Small amounts (deoxidation) | Small amounts |
| P (Phosphorus) | Kept low (impurity) | Kept low |
| S (Sulfur) | Kept low (impurity) | Kept low |
| Cr, Ni, Mo | Not intentionally alloyed in standard grades (traces possible) | Same |
| V, Nb, Ti (microalloying) | May be present in specific thermomechanically processed variants (trace to small %) | May be present in specific variants |
| B (Boron) | Typically not added in standard grades | Typically not added |
| N (Nitrogen) | Controlled; affects aging and toughness | Controlled |
Notes: - EN grades specify maximum limits that depend on thickness and variant (e.g., JR, J0). The general alloying strategy is low-carbon chemistry with manganese to provide yield/tensile targets while maintaining good weldability and formability. Microalloying and thermo-mechanical rolling can be used to raise strength without significant increases in carbon.
How alloying affects behavior: - Carbon increases strength and hardenability but reduces weldability and toughness if elevated. - Manganese contributes to strength and hardenability and counters sulfur embrittlement. - Silicon is mainly a deoxidiser; higher silicon can slightly increase strength. - Microalloying elements (V, Nb, Ti) refine grain size and enable higher yield strengths through precipitation strengthening and controlled rolling without large carbon increases.
3. Microstructure and Heat Treatment Response
Typical microstructure: - As-rolled and normalized S235 and S275 are predominantly ferritic-pearlitic in microstructure. Grain size and pearlite fraction will vary with cooling rate and thermo-mechanical processing. - Thermomechanically rolled or microalloyed variants show finer ferrite grain sizes and dispersed carbides/precipitates (NbC, VC, TiC), providing enhanced yield strength at similar chemical carbon levels.
Heat treatment response: - These grades are designed primarily for use in the as-rolled or normalized condition. They are not typically supplied for quench-and-temper unless specifically ordered as a different grade. - Normalizing (heating above critical temperature and air cooling) refines grain size and homogenizes microstructure, improving toughness. - Quenching and tempering will increase strength and toughness but is uncommon for standard S235/S275 specifications and may alter conformity with EN 10025 requirements. - Thermo-mechanical controlled processing (TMCP) is commonly used to achieve higher strength (e.g., S275 properties) with low carbon by refining grain and distributing microalloy precipitates.
4. Mechanical Properties
| Property | S235 (typical per EN designation) | S275 (typical per EN designation) |
|---|---|---|
| Minimum Yield Strength (ReH) | 235 MPa (designation basis) | 275 MPa (designation basis) |
| Tensile Strength (Rm) | Typical range overlaps structural-steel bands (e.g., several hundreds of MPa) | Typically higher upper range than S235 |
| Elongation (A) | Generally good ductility; elongation typically adequate for forming (varies with thickness) | Slightly lower elongation at comparable thickness due to higher yield, but still good ductility |
| Impact Toughness | Variants (JR, J0, J2) define min. impact energies at prescribed temperatures; generally good at room temp | Similar toughness variants available; selection depends on required impact temperature |
| Hardness | Low-to-moderate hardness typical of structural steels; easily machined | Slightly higher hardness on average due to higher yield |
Explanation: - S275 is the stronger grade by design because its guaranteed minimum yield is higher. The increase in yield often comes with a modest increase in tensile strength and a small reduction in elongation/drawability, but toughness can be matched by selecting the appropriate JR/J0/J2 variant or using normalized processing. - Because both grades are low-carbon, they retain good ductility and impact toughness when supplied to the appropriate delivery condition.
5. Weldability
Key factors: - Low carbon and low alloy content in both S235 and S275 generally give them good weldability when standard procedures are followed. - Higher carbon equivalence or the presence of microalloying elements increases hardenability and potential for cold cracking; therefore preheat and interpass temperatures may be specified for thicker sections or for steels with higher CE or Pcm.
Useful weldability indices:
- Carbon Equivalent (IIW):
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
- Pcm (decreasing weldability):
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - For both grades, CE and Pcm values are typically low compared with high-strength alloy steels, indicating straightforward welding with standard consumables and procedures. However, as thickness increases or when microalloying is present, CE/Pcm rise and appropriate welding controls (preheat, interpass temperature, post-weld heat treatment where required) should be applied. - Specify the correct filler metal to match mechanical properties and to avoid under-matching critical joints. For cyclic or fatigue-sensitive structures, take account of residual stresses and possible hardness in the HAZ.
6. Corrosion and Surface Protection
- S235 and S275 are non-stainless carbon steels; they do not offer intrinsic resistance to atmospheric or aggressive corrosion. Protection strategies include:
- Hot-dip galvanizing for atmospheric corrosion and long-term outdoor exposure.
- Paint systems and primers (e.g., epoxy, polyurethane, zinc-rich primers).
-
Local coatings (spray, brush) or metallizing for repair and touch-up.
-
Stainless metrics such as PREN are not applicable to non-stainless structural steels. For completeness, the PREN formula (used for stainless alloys) is:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is irrelevant for S235/S275 because their chromium, molybdenum and nitrogen contents are very low and not intended to provide corrosion resistance.
7. Fabrication, Machinability, and Formability
- Forming: Both grades are ductile and formable; S235 is generally easier to form due to its lower yield. When tight radii are required, S235 may be preferred unless the design requires the higher strength of S275.
- Bending: Springback is slightly greater for S275 due to higher yield; adjust tooling/back-bending as needed.
- Cutting and drilling: Both machine readily by plasma, oxy-fuel, laser or mechanical processes. Tool wear increases modestly with strength; adjust feeds and tooling when moving from S235 to S275.
- Machinability: Good for both when in normalized/as-rolled conditions; if higher hardness variants or heavily microalloyed products are used, machinability can decline.
- Finishing: Typical surface preparation and priming requirements are similar; welding spatter removal and grinding practices are identical.
8. Typical Applications
| S235 (typical uses) | S275 (typical uses) |
|---|---|
| Light-to-medium structural components: general building sections, frames, supports, railings, mild structural members | Heavier structural members where higher minimum yield is required: chassis frames, crane components, larger beams and columns |
| Cold-formed sections, welded frames where high ductility and good weldability are priorities | Situations requiring smaller cross-sections for the same load or where weight savings are beneficial |
| Non-critical mechanical parts, agricultural equipment, fencing | Moderate-duty mechanical parts, load-bearing components in infrastructure where slightly higher strength improves performance |
Selection rationale: - Choose S235 where forming, cost, and ease of welding are prioritized and where the design yield requirement is satisfied by 235 MPa. - Choose S275 where design codes, load cases, or weight/section optimization demand the higher guaranteed yield (275 MPa) while retaining good weldability and toughness.
9. Cost and Availability
- Relative cost: S275 is typically marginally more expensive than S235 due to its higher yield specification and sometimes tighter manufacturing/process control. The price delta is often modest in mill-product markets.
- Availability: Both grades are widely available in plates, coils, bars, and structural shapes in most markets. S235 is extremely common for general structural stock; S275 is also common, particularly in regions or applications that specify the higher yield limit.
- Product forms: Plate and sheet in a range of thicknesses; long products (angles, channels) and sections; availability by thickness and delivery condition (JR, J0, J2; normalized; TMCP) varies by mill and region.
10. Summary and Recommendation
| Attribute | S235 | S275 |
|---|---|---|
| Weldability | Excellent (very good) | Very good but slightly more sensitive when thicker or microalloyed |
| Strength–Toughness balance | Lower minimum yield; excellent ductility | Higher minimum yield; similar toughness options available with appropriate delivery condition |
| Cost | Typically lower | Typically slightly higher |
Recommendations: - Choose S235 if: - The design’s minimum yield requirement does not exceed 235 MPa. - Ease of forming, lower cost, and maximum ductility are priorities. - You require the broadest standard availability for light-to-medium structural parts.
- Choose S275 if:
- The project requires the higher guaranteed yield of 275 MPa to reduce section sizes or weight.
- Slightly higher tensile/yield properties are needed without moving to higher-alloy steels.
- You prefer a strength margin for structural members while maintaining good weldability and toughness with correct processing.
Final note: When selecting between S235 and S275, always verify the required delivery condition (JR/J0/J2, normalized, TMCP), thickness-dependent limits, and any project-specific constraints (weld procedures, impact temperature, corrosion protection). Match the steel grade to functional requirements (load, fatigue, environment) rather than price alone to avoid rework and ensure long-term performance.