Q355NH vs Q355B – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Engineers, procurement managers, and manufacturing planners commonly face the choice between closely related structural steels when specifying plate, section, or rolled product for welded and exposed structures. The Q355 family includes grades intended for general structural use as well as variants engineered to resist atmospheric degradation and to provide guaranteed low‑temperature toughness; making the correct pick affects lifecycle cost, fabrication approach, and safety margins.
At a high level, Q355B is a general-purpose high‑strength structural steel, while Q355NH is a variant intended for greater resistance to atmospheric corrosion and for assured toughness after standard thermo‑mechanical processing. These differences drive selection tradeoffs between corrosion performance vs. first‑cost, and between routine weldability vs. the need for post‑weld precautions in thicker sections.
1. Standards and Designations
- GB/T (China): Q355 family is defined in GB/T 1591 and related product standards for high-strength low-alloy structural steels. Variant letters (A, B, C, ...; plus suffixes) indicate processing and impact requirements.
- EN (Europe): Rough equivalents are in the S355 family (EN 10025) but there are differences in chemical limits and testing requirements.
- ASTM/ASME (US): No direct one-to-one match; S355-type steels are the closest analogues.
- JIS (Japan): Similar high‑strength structural steels exist, but direct correspondences require cross-reference tables.
Classification: both Q355B and Q355NH are carbon / low‑alloy, high‑strength structural steels (commonly grouped as HSLA — High‑Strength Low‑Alloy steels). Q355NH is a weathering/atmospheric‑corrosion‑resistant variant of that HSLA family.
2. Chemical Composition and Alloying Strategy
Table: Typical compositional emphasis (qualitative ranges). Always confirm mill certificates and the controlling edition of the standard for exact limits.
| Element | Q355B — typical specification notes | Q355NH — typical specification notes |
|---|---|---|
| C (Carbon) | Low to moderate C for balance of strength and weldability (typically limited) | Similar low C; controlled to maintain weldability and toughness |
| Mn (Manganese) | Principal strength‑building element (moderate level) | Similar or slightly higher to support strength after processing |
| Si (Silicon) | Deoxidizer; limited amounts | Similar low levels |
| P (Phosphorus) | Kept low (impurity control) | Kept low, but in weathering steels phosphorus is sometimes controlled to enhance patina formation |
| S (Sulfur) | Kept very low (machinability/quality) | Kept very low |
| Cr (Chromium) | Typically very low or trace | Small additions possible to improve corrosion patina and hardenability |
| Ni (Nickel) | Trace to low | May be added in small amounts for improved toughness and weathering performance |
| Cu (Copper) | Usually minimal | Deliberately added in small amounts (a defining feature of many weathering steels) to enhance atmospheric corrosion resistance |
| Mo, V, Nb, Ti (microalloying) | May be present in microalloyed variants to control strength and grain | May be present to obtain strength and fine-grain microstructure while maintaining toughness |
| B, N | Controlled levels (N often low) | Controlled; nitrogen may be used in alloy design but levels are low |
How alloying affects behaviour: - Carbon and manganese increase strength but raise hardenability and the risk of HAZ hardening; keeping carbon low preserves weldability. - Microalloying elements (Nb, V, Ti) give strength through precipitation and grain‑refinement rather than by increasing carbon. - Small additions of Cu, Cr, and Ni promote the formation of a protective surface patina in atmospheric exposures (weathering performance) and improve low‑temperature toughness when properly processed.
Note: exact numerical composition limits vary by standard edition and specific product callouts; always rely on the certificate of analysis for procurement and weld procedure qualification.
3. Microstructure and Heat Treatment Response
Typical microstructures: - Q355B: produced as hot‑rolled or normalized plates with a mixed ferrite–pearlite or fine bainitic matrix depending on rolling schedule and cooling. Microalloying (if present) promotes fine grain size and dispersion strengthening. - Q355NH: processed with tighter control on rolling and cooling (or normalized) to produce a fine‑grained ferritic/bainitic microstructure optimized for toughness and to support the weathering alloy additions. The patina that provides atmospheric corrosion resistance is a surface phenomenon that develops after exposure.
Response to thermal processes: - Normalizing: Refines grain structure in both grades and improves toughness; Q355NH is commonly supplied normalized or thermomechanically rolled to achieve the required low‑temperature impact properties. - Quenching & tempering: Not typical for these commercial structural grades; would change classification and typical use. - Thermo‑mechanical controlled processing (TMCP): Used to obtain a favorable balance of strength and toughness while minimizing carbon content; TMCP is common across HSLA production and particularly important for Q355NH to ensure the fine microstructure needed for toughness and patina performance.
4. Mechanical Properties
Table: Typical property baseline (use mill certs and standard for contractual values).
| Property | Q355B (typical) | Q355NH (typical) |
|---|---|---|
| Yield Strength (min) | ~355 MPa (guaranteed minimum in longitudinal direction) | ~355 MPa (same nominal minimum) |
| Tensile Strength (typical range) | ~470–630 MPa (depends on thickness and processing) | Similar range; may trend slightly higher due to TMCP and microalloying |
| Elongation (A%) | Generally ≥ 20% (varies with thickness) | Comparable or slightly improved due to controlled processing |
| Impact Toughness | Specified per grade; Q355B impact levels vary by sub‑grade and may be tested at ambient or sub‑ambient temperatures | Q355NH is typically specified with guaranteed low‑temperature impact properties (e.g., tested at sub‑ambient temperatures) |
| Hardness | Moderate (HBW typical for structural plate) | Similar; controlled by processing and chemistry |
Interpretation: - Both grades are designed around the same nominal yield level; mechanical differences are driven by processing and minor alloying. - Q355NH is engineered to deliver better low‑temperature toughness and more consistent properties across thicker sections, while Q355B is a general structural grade where low‑temperature impact performance may not be as tightly guaranteed.
5. Weldability
Weldability depends on carbon equivalent and microalloying/hardenability. Two commonly used empirical metrics:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Lower $CE_{IIW}$ and $P_{cm}$ values indicate easier welding, lower preheat requirements, and reduced risk of HAZ cracking. - Q355B, with generally low carbon and limited weathering alloying, is widely regarded as easy to weld with standard filler metals and normal preheat practice for moderate thicknesses. - Q355NH may have slightly higher $CE_{IIW}$ and $P_{cm}$ because of deliberate additions (Cu, small Cr/Ni) and any microalloying; this can necessitate modest preheating, controlled interpass temperatures, or modified welding consumables for thick sections or restrained joints. - In all cases, thickness, restraint, and joint design have a larger influence on the need for preheat than the nominal grade alone. Follow qualified WPS (welding procedure specifications) and use mill certificates to calculate the applicable carbon equivalent for the specific lot.
6. Corrosion and Surface Protection
- Q355B is not corrosion resistant by chemistry and requires protective coatings (paint systems, galvanizing, polymer claddings) or cathodic protection for long service life in exposed environments.
- Q355NH is produced with alloying intended to improve atmospheric corrosion resistance; it forms a adherent weathering patina under many outdoor conditions that reduces the corrosion rate and can lower life‑cycle coating costs in rural, urban, and industrial atmospheres.
PREN (Pitting Resistance Equivalent Number) is specifically relevant to stainless steels:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- PREN is not applicable to non‑stainless carbon or HSLA steels like Q355B/NH; their corrosion resistance is not driven by Cr/Mo/N formulations designed to resist chloride pitting.
- Important limitations: weathering steels are not universally appropriate. In marine (offshore) or chloride‑rich environments, the protective patina may not be stable and additional coatings or corrosion allowance will be required.
7. Fabrication, Machinability, and Formability
- Cutting: Both grades are compatible with standard thermal cutting (oxyfuel, plasma, laser) and mechanical cutting; cutter parameters may need adjustment for thicker weathering steel plate to avoid edge oxidation affecting patina formation.
- Forming/bending: Low carbon and TMCP processing give both grades good formability for structural forming operations; Q355NH’s controlled microstructure helps maintain ductility at lower temperatures.
- Machinability: Neither grade is optimized for machining; machinability is standard for carbon/HSLA steels and influenced by sulfur levels and microalloying. Q355B may be slightly easier where high toughness alloying is absent.
- Finishing: If the design relies on the weathering patina of Q355NH, surface treatment and welding practice should avoid excessive post‑fabrication coatings that suppress patina formation.
8. Typical Applications
| Q355B — Typical Uses | Q355NH — Typical Uses |
|---|---|
| General structural components: beams, columns, plates for buildings and industrial frames | Exposed structural members: bridges, facades, outdoor sculptures where reduced maintenance is desired |
| Fabricated welded structures, cranes, supports where standard corrosion protection will be applied | Highway and railway bridges, weather-exposed tanks, cladding where patina formation is acceptable |
| Industrial machinery frames, containers, platforms | Urban infrastructure, architectural elements, long‑life outdoor structures |
Selection rationale: - Choose Q355B when cost, availability, and straightforward fabrication for coated or enclosed applications are primary drivers. - Choose Q355NH when you require reduced maintenance painting, built‑in atmospheric corrosion resistance in typical outdoor atmospheres, and guaranteed low‑temperature toughness.
9. Cost and Availability
- Cost: Q355NH typically carries a price premium relative to Q355B because of added alloying control and processing to meet weathering and toughness specifications. The premium varies with market conditions and the specific product form.
- Availability: Q355B is ubiquitous in plate and structural forms. Q355NH is widely available but may have longer lead times or be offered by a narrower set of mills depending on thickness and surface finish. Both grades are commonly supplied as plate, coil, and structural sections.
10. Summary and Recommendation
Table: Quick comparison
| Characteristic | Q355B | Q355NH |
|---|---|---|
| Weldability | Very good for typical structural welding | Good, but alloying may require more attention on thick/heavily restrained joints |
| Strength–Toughness balance | Good; meets standard structural requirements | Similar strength; improved guaranteed low‑temperature toughness and HAZ performance |
| Cost | Lower (general structural grade) | Higher (weathering and toughness processing) |
Final recommendation: - Choose Q355B if you need a dependable, cost‑effective HSLA structural steel for applications where atmospheric corrosion will be handled by coatings or where the environment is not aggressive. - Choose Q355NH if your structure is frequently exposed to the atmosphere and you want reduced maintenance and a built‑in corrosion‑resistant surface behavior, or if your application requires guaranteed low‑temperature impact performance and tight control of toughness across thicker sections.
Note: Always specify the exact standard edition, delivery condition (e.g., normalized, TMCP), impact testing temperature and energy, and supply form on purchase orders. For welding, calculate a carbon equivalent for the specific mill analysis and qualify WPS accordingly.