S355JR vs S355J2 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
S355JR and S355J2 are two widely used grades from the EN 10025 family of structural steels. Both are low-alloy, high-strength structural steels intended for welded constructions, heavy fabrications, and general engineering applications. Engineers, procurement managers, and manufacturing planners commonly weigh cost, weldability, low-temperature performance, and downstream processing when choosing between them.
The principal practical difference between S355JR and S355J2 is their guaranteed impact toughness at different temperatures: JR is tested at ambient temperature, while J2 is specified and tested for tougher performance at sub-zero temperatures. Because many design and fabrication decisions hinge on impact resilience in service, that toughness requirement drives material selection, test burden, and occasionally cost and availability.
1. Standards and Designations
- EN: EN 10025-2 (S355JR, S355J2 are specified here as structural steels).
- ASTM/ASME: There is no direct one-to-one ASTM designation, but S355 equivalents are often compared to ASTM A572 Grade 50 or A36 variants depending on processing and mechanical properties.
- JIS / GB: Local standards in Japan and China have functionally similar structural steels, but designation and testing differ; direct substitution requires property verification.
- Classification: Both S355JR and S355J2 are non-stainless, low-alloy/high-strength structural carbon steels (often treated as HSLA by virtue of microalloying in some variants). They are not tool steels or stainless grades.
2. Chemical Composition and Alloying Strategy
The EN 10025 series specifies maximum element contents rather than a single composition. The table below shows typical maximums and commonly controlled elements for S355JR and S355J2 per EN practice. Exact composition depends on mill practice and any additional microalloying requested.
| Element | Typical control (S355JR) | Typical control (S355J2) |
|---|---|---|
| C (max) | ≤ 0.22 wt% | ≤ 0.22 wt% |
| Mn (max) | ≤ 1.60 wt% | ≤ 1.60 wt% |
| Si (max) | ≤ 0.55 wt% | ≤ 0.55 wt% |
| P (max) | ≤ 0.035 wt% | ≤ 0.035 wt% |
| S (max) | ≤ 0.035 wt% | ≤ 0.035 wt% |
| Cr | Typically ≤ 0.30 wt% (if present) | Typically ≤ 0.30 wt% |
| Ni | Typically ≤ 0.30 wt% (if present) | Typically ≤ 0.30 wt% |
| Mo | Typically ≤ 0.10 wt% (if present) | Typically ≤ 0.10 wt% |
| V, Nb, Ti | Often added in small amounts (≤ ~0.05 wt% each) for TMCP/microalloying | Same, possibly more controlled for J2 when toughness required |
| B | Trace additions possible (ppm) | Trace additions possible |
| N | Typically ≤ 0.012 wt% | Typically ≤ 0.012 wt% |
| Al (deoxidation) | 0.015–0.060 wt% (for control) | 0.015–0.060 wt% |
Notes: - EN 10025 defines mechanical requirements and impact test temperatures for the different subgrades; chemical limits are broad and depend on manufacturer and additional quality classes. - Many S355 products are produced by thermo-mechanical controlled processing (TMCP) and may intentionally include microalloying (V, Nb, Ti) to refine grain size and increase strength without excessive carbon.
How alloying affects properties: - Carbon and manganese primarily control strength and hardenability; higher carbon increases strength but reduces weldability and ductility. - Silicon is a deoxidizer and can slightly raise strength. - Microalloying elements (V, Nb, Ti) refine grain size and give stronger ferrite-pearlite or bainitic microstructures at lower carbon levels. - Low sulfur and phosphorus contents are maintained to preserve toughness and weldability.
3. Microstructure and Heat Treatment Response
Typical microstructures: - Both S355JR and S355J2, in as-rolled or TMCP deliveries, exhibit a ferrite + pearlite matrix with possible bainitic islands depending on cooling rate. Grain refinement from TMCP and microalloying improves yield strength and toughness without much increase in carbon. - The J2 variant is usually produced with slightly tighter controls on cleanliness, nitrogen, and microalloy content plus controlled rolling to meet the lower-temperature impact toughness requirement.
Heat treatment response: - Normalizing: Both respond well to normalizing (austenitize then air cool) — this refines grain size and homogenizes microstructure, improving toughness. Normalized S355 will typically achieve better Charpy toughness than as-rolled material. - Quenching & tempering: While technically possible, quench & tempering is not a standard delivery for the EN S355 grades; it transforms the microstructure substantially (martensite tempered to lower hardness, higher toughness) but is used only when specific properties beyond the EN grade are required. - Thermo-mechanical processing (TMCP): Many S355 products are manufactured by TMCP to achieve high strength with low carbon and good toughness. TMCP produces a fine-grained microstructure that improves strength–toughness balance more effectively than simply increasing carbon. - Stress relief and post-weld heat treatment: For welded structures, minimal PWHT is common for S355, but heavy sections or critical applications may require controlled PWHT depending on joint design and hydrogen risk.
4. Mechanical Properties
EN 10025 specifies mechanical property limits for S355 family. The following table summarizes typical and standardized values; actual results depend on thickness and production route.
| Property | S355JR (typical/specified) | S355J2 (typical/specified) |
|---|---|---|
| Yield strength (Rp0.2, min) | 355 MPa (for many thickness ranges) | 355 MPa (same nominal requirement) |
| Tensile strength (Rm) | 470–630 MPa (typical range per spec) | 470–630 MPa (similar) |
| Elongation (A, min) | ~20% (varies with thickness) | ~20% (varies with thickness) |
| Impact toughness | 27 J at +20 °C (Charpy V) | 27 J at −20 °C (Charpy V) |
| Typical hardness (HBW) | 140–190 HBW (manufacturing dependent) | 140–190 HBW (similar) |
Interpretation: - Static strength (yield and tensile) is effectively equivalent between JR and J2 grades when compared at the same delivery condition; both guarantee a minimum yield of 355 MPa. - The distinguishing mechanical property is impact toughness: S355J2 is guaranteed to maintain ductile behavior down to −20 °C, whereas S355JR is only guaranteed at +20 °C. This does not make J2 stronger in static terms, but it is more resistant to brittle fracture at lower temperatures. - Elongation and hardness ranges overlap; processing route (TMCP, normalization) has a larger influence on toughness and ductility than the JR/J2 suffix alone.
5. Weldability
Weldability is a frequent selection driver for structural steels. Both S355JR and S355J2 are designed for welding, but specific considerations apply.
Key factors: - Carbon content and combined alloying determine hardenability and the risk of cold cracking. Both grades have relatively low carbon (≤ ~0.22 wt%), supporting good weldability. - Microalloying and residual elements (Cr, Mo, V, Nb) increase hardenability and may necessitate preheat or post-weld heat treatment in thick sections.
Useful welded-steel indices: - Carbon equivalent (IIW form): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm martensite-susceptibility: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - For identical nominal compositions, S355JR and S355J2 will have nearly identical $CE_{IIW}$ and $P_{cm}$ values. However, J2 mills often exercise tighter process control (cleaner steel, controlled microalloy additions) to meet low-temperature toughness, which can indirectly improve weldability by reducing impurity-driven hydrogen trapping. - For heavy sections, increased hardenability or higher CE may require preheat or controlled interpass temperatures; weld procedure qualification should reference the specific material certificate. - Post-weld heat treatment (PWHT) is rarely mandated for ordinary S355 welded structures but may be required for welded assemblies with high restraint, thick sections, or when specifications demand it.
6. Corrosion and Surface Protection
- Both S355JR and S355J2 are non-stainless carbon steels — corrosion resistance in atmospheric or aggressive environments is modest.
- Standard protection strategies: hot-dip galvanizing, zinc electroplating (for small components), organic coatings (primers, epoxies), metallization (flame or arc-sprayed zinc/Al coatings), or a combination (zinc-rich primer + topcoat).
- The PREN index is not applicable because PREN is used for stainless steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Choice of coating depends on environment (C1–C5 classification), expected lifetime, aesthetics, and maintenance strategy. Galvanizing is common for structural steel exposed to the elements; S355 substrate does not materially change corrosion-control strategy between JR and J2.
7. Fabrication, Machinability, and Formability
- Cutting: Plasma, oxy-fuel, laser, and waterjet cutting behave similarly for both grades. Hardness and thickness can affect cutting settings.
- Machinability: Low carbon and low alloy content give reasonable machinability; microalloyed or higher-strength TMCP variants may be slightly harder to machine but differences between JR and J2 are minimal.
- Forming and bending: Formability is governed by yield strength and ductility; since both share similar nominal yield and elongation, forming behavior is generally comparable. Cold-forming in very low-temperature environments benefits from J2’s improved low-temperature toughness.
- Welding and fabrication practice: Use qualified welding procedures and consider preheat/temper bead strategies for thick sections or where hydrogen control is required. S355J2 may require additional impact testing/certification for project compliance.
8. Typical Applications
| S355JR — Typical Uses | S355J2 — Typical Uses |
|---|---|
| General structural steelwork: beams, columns, frames for buildings at ambient conditions | Structural members in cold climates: offshore topsides, refrigerated structures, bridges in cold regions |
| General fabrication where ambient-temperature toughness is acceptable | Heavy welded fabrications requiring verified low-temperature toughness |
| Machinery frames, general engineering components | Pressure-retaining structures and equipment operating in colder service (when specified) |
| Standard plates, hot-rolled shapes, and sections for civil works | Structural elements exposed to sub-zero service or increased brittle-fracture risk |
Selection rationale: - Choose S355JR for typical ambient-temperature structural applications where lower procurement/testing cost is desirable. - Choose S355J2 when the design requires verified impact resistance at lower temperatures, or when project specifications mandate a −20 °C toughness rating.
9. Cost and Availability
- Cost: Both grades are commonly produced and therefore generally similar in base price. S355J2 may attract a small premium due to the additional testing and tighter process control needed to certify low-temperature impact performance.
- Availability: Both are widely available in plates, hot-rolled coils, beams, and sections. Lead times depend on size, thickness, and whether a normalized/TMCP or special microalloyed variant is required.
- Long-lead specialty requirements (e.g., normalized delivery for thick plate, or additional chemical control for weldability) can increase cost and lead time for either grade.
10. Summary and Recommendation
| Criterion | S355JR | S355J2 |
|---|---|---|
| Weldability | Very good (low C); standard welding procedures apply | Very good (low C); similar but with tighter mill controls possible |
| Strength–Toughness balance | High strength; ambient temp toughness guaranteed | High strength; improved low-temperature toughness guaranteed |
| Cost | Standard, slightly lower testing burden | Slightly higher (additional impact testing and control) |
Conclusion and guidance: - Choose S355JR if your structure operates at or above typical ambient temperatures, if the project does not require verified low-temperature impact testing, and if minimizing procurement and testing cost is a priority. - Choose S355J2 if the structure will operate in cold environments, if specifications demand impact toughness at sub-zero temperatures (typically −20 °C), or if the design has high restraint/weld-joint brittle-fracture risk where certified low-temperature toughness is required.
Final practical note: Because both grades share the same nominal static strength, the selection often hinges on the impact test temperature requirement and associated supply/testing implications. Always request mill certificates and Charpy test records relevant to the material batch, and qualify welding procedures on the actual product form and thickness you will use.