Q355NH vs COR-TEN C – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Engineers, procurement managers, and manufacturing planners frequently face the choice between Q355NH and COR-TEN C when specifying structural steels for outdoor or pressure-bearing applications. The decision typically balances long-term atmospheric performance and maintenance (corrosion resistance) against mechanical requirements, weldability, and lifecycle cost. Q355NH is most often chosen where guaranteed yield/toughness and normalized heat treatment are critical (pressure vessels, welded structures with toughness requirements), while COR-TEN C is selected when reduced maintenance and the development of a protective rust patina under atmospheric exposure are important.
The principal practical distinction is that COR-TEN C is designed to develop a stable protective oxide layer in many atmospheric environments, reducing the need for painted protection, whereas Q355NH is a normalized high-strength structural steel optimized for strength and toughness and typically requires conventional coatings for long-term corrosion protection. Because these steels serve different primary design objectives, they are commonly compared for outdoor structural use versus pressure or welded structural applications.
1. Standards and Designations
- Q355NH
- Principal standard: GB/T 1591 (China) and related national specifications.
- Category: High-strength low-alloy structural steel (HSLA) designed for normalized condition; common for pressure vessels and welded structures.
- COR-TEN C
- Common trade/designation: COR-TEN (Weathering steel) — several proprietary/standard equivalents exist (e.g., ASTM A242, ASTM A588, EN 10025-5 family). "COR-TEN C" is often used commercially to denote a C-class weathering grade with enhanced atmospheric corrosion resistance.
- Category: Low-alloy atmospheric corrosion-resistant structural steel (weathering steel).
Other related standards (informational): - ASTM/ASME: A242, A588 (weathering steels); A36, A572 for structural carbon/HSLA steels. - EN: EN 10025-5 covers weathering steels in the European system. - JIS: JIS G3114 and related may have equivalents. Always confirm the exact standard and grade suffixes when specifying.
2. Chemical Composition and Alloying Strategy
Below are typical composition ranges widely reported in supplier datasheets and standards literature. These are approximate ranges; consult the specific standard or mill certificate for project-level specification.
| Element | Q355NH (typical range, wt%) | COR-TEN C (typical range, wt%) |
|---|---|---|
| C | 0.10 – 0.20 | 0.06 – 0.20 |
| Mn | 0.40 – 1.60 | 0.30 – 1.35 |
| Si | 0.10 – 0.50 | 0.25 – 0.75 |
| P | ≤ 0.025 – 0.040 | 0.03 – 0.07 |
| S | ≤ 0.025 – 0.035 | ≤ 0.025 |
| Cr | trace – 0.30 | 0.30 – 0.60 |
| Ni | trace – 0.60 | trace – 0.60 |
| Mo | trace – 0.10 | typically nil |
| V | trace – 0.10 (microalloying) | typically nil |
| Nb (Nb/Ti) | possible microalloying 0.01–0.06 | typically nil |
| Ti | trace in some heats | typically nil |
| B | trace in some microalloy grades | not typical |
| N | low (control for toughness) | low |
Notes: - COR-TEN grades often include small amounts of Cu (copper) and sometimes P as deliberate alloying elements to promote the formation and stability of the protective patina; Cu is an important contributor to atmospheric corrosion resistance but is not listed in the table above per the requested element set. Typical Cu for weathering steels can be ~0.25–0.75 wt% depending on the specification. - Q355NH frequently uses microalloying (Nb, V, Ti) and controlled nitrogen to refine grain size and ensure impact toughness after normalization. - Alloying strategy: - Q355NH: microalloying + controlled carbon and Mn for strength and hardenability; normalizing gives a fine-grained ferritic–pearlitic or bainitic matrix depending on processing. - COR-TEN C: low-to-moderate alloy additions (Cr, Cu, P) to modify surface oxide composition and slow uniform corrosion under cyclic wet/dry atmospheric exposure.
3. Microstructure and Heat Treatment Response
Q355NH - Typical processing: hot-rolled and normalized (the "N" suffix indicates normalized condition), sometimes thermo-mechanically rolled for Q355 variants. - Typical microstructure after normalization: refined ferrite and pearlite with dispersed microalloy precipitates (NbC, VC, TiN) that stabilize grain size and improve toughness. - Heat treatment response: Normalizing (reheating above Ac3 and air cooling) yields improved and more uniform toughness compared with as-rolled material. Q355NH is not usually quenched & tempered; quench/temper routes are available in other grades if higher strength is needed but require careful control.
COR-TEN C - Typical processing: hot-rolled in the as-rolled condition and often left untreated because the weathering behavior is achieved via composition rather than heat treatment. - Typical microstructure: ferrite–pearlite matrix; alloying additions encourage formation of alloy-enriched oxide layers on the surface. - Heat treatment response: Not usually normalized; mechanical properties are governed by chemistry and rolling. Thermo-mechanical treatments are not typical since the primary design aim is atmospheric corrosion performance rather than maximizing strength-toughness via heat treatment.
Implication: Q355NH’s normalized microstructure delivers predictable toughness and is suitable for welded and pressure applications; COR-TEN C relies on surface chemistry and a stable microstructure from rolling for its protective performance.
4. Mechanical Properties
Typical property ranges (approximate; actual values depend on thickness, production route, and standard). Consult mill certificates for project acceptance limits.
| Property | Q355NH (typical) | COR-TEN C (typical) |
|---|---|---|
| Yield strength (MPa) | ~355 (designation basis; ± dependent on thickness) | ~300–355 |
| Tensile strength (MPa) | ~470 – 630 | ~470 – 630 |
| Elongation, % (A) | 20 – 26% (depends on thickness) | 18 – 26% |
| Impact toughness (Charpy V, J) | Good — often specified at -20°C or lower (normalized grade) | Moderate — adequate for many structural uses but check low-temperature requirements |
| Hardness (HB) | ~150 – 200 | ~150 – 190 |
Which is stronger/tougher/more ductile: - Strength: Both are comparable in tensile ranges; Q355NH is specified to provide a minimum yield of ~355 MPa, so it provides predictable yield strength across thicknesses. - Toughness: Q355NH (normalized, microalloyed) is engineered to meet impact requirements and will generally provide superior guaranteed toughness for low-temperature or pressure vessel applications. - Ductility: Both can exhibit comparable elongation, but Q355NH’s controlled processing gives more consistent ductility in welded assemblies.
5. Weldability
Weldability depends on carbon content, carbon equivalent, and microalloying. Two common empirical indices:
-
Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
-
Pcm (weldability parameter): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation (qualitative): - Q355NH: Moderately low carbon and controlled microalloying usually yield a moderate $CE_{IIW}$ and $P_{cm}$, which means typical welding with low-hydrogen consumables and preheat control is feasible. The presence of Nb/V/Ti increases the risk of local hardening in the HAZ, so preheat and post-weld heat treatment (PWHT) must be considered for thick sections or critical applications. - COR-TEN C: Carbon is typically low to moderate; however, alloying elements (Cr, Cu, P) have minor effects on carbon equivalent. Weathering steels can be welded with appropriate procedures, but care must be taken with filler selection to maintain similar corrosion performance at the weld (weld metal may not form the same protective patina). Post-weld stress relief is seldom required unless dictated by design; avoid welding procedures that introduce excessive hydrogen in thick sections.
Practical guidance: - For both grades select filler metals compatible with required mechanical properties and, for COR-TEN C, compatible corrosion behavior (use weathering steel fillers when exposure and appearance are important). - Use preheat when thickness, CE, or service conditions indicate risk of HAZ cracking; follow relevant welding procedure specifications (WPS) and codes.
6. Corrosion and Surface Protection
- COR-TEN C (weathering steel): Designed to form a tightly adherent, stable oxide layer under alternating wet/dry atmospheric exposure that retards further uniform corrosion. The alloying balance (Cu, Cr, P) modifies the oxide chemistry. Weathering steels perform well in many rural and urban atmospheres but are less effective in marine, continuously wet, or highly polluted environments where chlorides or continuous moisture prevent stable patina formation.
- Q355NH: Not weathering steel. Long-term durability in atmospheric exposure requires conventional protection: coatings (organic paint systems), galvanizing (hot-dip), or metallization. For corrosive service or marine exposure, specify supplemental corrosion protection.
Use of corrosion indices: - PREN (for stainless grades) is not applicable to non-stainless steels like these, but for reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is used for stainless corrosion resistance and does not apply to Q355NH/COR-TEN C.
7. Fabrication, Machinability, and Formability
- Machinability: Both steels are typical mild/low-alloy grades; machinability is acceptable but Q355NH microalloying and higher strength can reduce chip-breakage friendliness compared with plain carbon steels. COR-TEN C’s alloy additions have minor impact on machinability.
- Formability: Both can be cold-formed; Q355NH’s higher yield and normalized condition may require greater forming force and tighter bend radii control. For complex cold forming, consider annealed or lower-yield alternatives.
- Cutting and finishing: Standard oxy-fuel cutting, plasma, and machining practices apply. Surface preparation for coatings differs: COR-TEN may be left uncoated for aesthetic or functional patina development; Q355NH requires surface preparation for paint or galvanizing.
8. Typical Applications
| Q355NH — Typical Uses | COR-TEN C — Typical Uses |
|---|---|
| Pressure vessel shells and parts where normalized microstructure and guaranteed yield/toughness are required | Bridges, architectural façades, sculptures, and outdoor structures where reduced maintenance and patina appearance are desired |
| Heavy welded structural components (frames, cranes) requiring predictable toughness | Sign posts, storage tanks, railway wagons (certain designs), and outdoor industrial structures |
| Components subject to impact/toughness requirements or regulated design codes | Decorative cladding and building envelopes where weathering finish is specified |
| Fabricated assemblies where heat-treatment and weld procedure control are feasible | Infrastructure in non-marine, non-continuously wet environments where patina forms |
Selection rationale: - Choose Q355NH when load-bearing capacity, guaranteed toughness, and code-driven material qualifications are primary. - Choose COR-TEN C when atmospheric corrosion resistance, low-maintenance aesthetics, and long-term exterior exposure in suitable environments are priorities.
9. Cost and Availability
- Relative cost: COR-TEN C is often priced higher than generic carbon structural steels due to deliberate alloy additions (especially copper and controlled chemistries). Q355NH typically aligns with HSLA carbon-steel pricing but specialty normalized or pressure-vessel-certified plate may command premiums.
- Availability by product form: Q355NH is widely available in plate and structural sections in regions following GB standards; COR-TEN C is widely available in plate and coil for architectural and structural markets but may be less common in some markets and thicknesses — lead times can be longer for specific weathering steel grades or certified mill test reports.
10. Summary and Recommendation
| Attribute | Q355NH | COR-TEN C |
|---|---|---|
| Weldability | Good with standard low-H procedures; microalloying requires attention | Good with appropriate filler choices; weld metal may need to match weathering behavior |
| Strength–Toughness | High, designed (normalized) for guaranteed toughness | Adequate strength; toughness acceptable for typical structural use but check low-temp specs |
| Cost | Moderate (HSLA/pressure-grade premium possible) | Often higher due to alloying; lifecycle savings possible from reduced maintenance |
Conclusions — Choose: - Choose Q355NH if you need predictable yield strength and guaranteed impact toughness, must meet pressure or code-driven welding qualifications, or if the structure will be coated or cathodically protected and not relying on a weathering patina for corrosion control. - Choose COR-TEN C if your project is an outdoor, atmospheric application where the formation of a stable protective patina can reduce maintenance (painting) and the exposure environment is suitable (not marine or continuously wet); also useful for architectural finishes where the weathered appearance is desired.
Final note: Both grades have legitimate, different use cases. For any project-level decision, obtain the exact mill certificates, confirm the applicable standard (and thickness-dependent property tables), and perform a corrosion-environment assessment and welding procedure qualification relevant to the intended service.