Q355NH vs SPA-H – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Engineers, procurement managers, and manufacturing planners frequently choose between Q355NH and SPA-H when specifying pressure-vessel shells, structural plates, or heavy fabrication steels. The decision often balances strength versus weldability, toughness in low-temperature service versus cost and regional availability. Typical decision contexts include pressure-vessel design (where impact toughness and code compliance matter), structural sections (where higher yield saves weight), and fabrication workflows (where welding procedure and preheat requirements drive cost).
The primary practical distinction between the two grades lies in their alloying and processing strategy: Q355NH is a normalized, higher-yield microalloyed/HSLA-type steel optimized for strength and impact toughness after controlled thermal processing, while SPA-H (commonly used as a trade designation for conventional pressure-vessel carbon steel plates such as ASTM/ASME A516 Grade 70 in many markets) is specified for ductility and weldability for pressure-vessel service. Because their chemical compositions are similar in base elements but differ in microalloying and permitted trace elements, performance differences are subtle but important for design and fabrication.
1. Standards and Designations
- Q355NH
- Commonly specified under Chinese standards (GB/T 1591 series) and supplied to meet EN/ISO equivalents in some markets.
- Classification: High-strength low-alloy (HSLA) / structural steel with enhanced notch toughness; "N" indicates normalization; "H" indicates additional impact test requirements (low-temperature toughness).
- SPA-H
- Commonly used commercial/trade name in pressure-vessel sourcing; frequently associated with ASME/ASTM pressure-vessel plate specifications such as ASTM A516 Grade 70 / ASME SA-516 Grade 70 in many supplier catalogs (confirm exact equivalence with the mill certificate).
- Classification: Carbon pressure-vessel steel (conventional carbon steel for boiler and pressure-vessel plates).
Note: Always verify the precise standard referenced on mill test certificates (MTC). Different markets and suppliers may use trade names differently.
2. Chemical Composition and Alloying Strategy
The table below gives typical element ranges commonly encountered in mill certifications and product datasheets. These are representative ranges; actual values must be confirmed from the supplying mill’s certificate.
| Element | Typical Q355NH (representative ranges) | Typical SPA-H (representative ranges, e.g., A516 Gr70) |
|---|---|---|
| C | 0.10 – 0.20 % | 0.16 – 0.28 % |
| Mn | 0.6 – 1.6 % | 0.70 – 1.20 % |
| Si | 0.10 – 0.50 % | 0.10 – 0.35 % |
| P | ≤ 0.035 % | ≤ 0.035 % |
| S | ≤ 0.035 % | ≤ 0.035 % |
| Cr | trace – 0.30 % (if present) | trace – 0.30 % (if present) |
| Ni | trace – 0.40 % | trace – 0.40 % |
| Mo | usually very low or none; up to ~0.10 % possible | trace – 0.10 % |
| V | may contain microalloying (trace, e.g., 0.01–0.12 %) | typically not added |
| Nb (Cb) | may contain microalloying (trace, e.g., ≤0.06 %) | typically not added |
| Ti | possible microalloying (trace) | typically not added |
| B | trace if used for hardenability control | typically not added |
| N | trace levels; controlled for toughness | trace levels |
How alloying affects performance: - Carbon and manganese are the principal strength drivers; higher C increases strength but reduces weldability and toughness if not controlled. - Microalloying elements (Nb, V, Ti) used in Q355NH promote grain refinement and precipitation strengthening, improving yield strength and toughness after controlled thermal processing with minimal increase in carbon equivalent. - Small additions of Cr, Ni, or Mo (more often seen in SPA-H variants or to meet specific toughness requirements) can slightly increase hardenability and corrosion resistance, but these are normally low or absent. - Sulfur and phosphorus are kept low in both grades to protect toughness.
3. Microstructure and Heat Treatment Response
- Q355NH
- Typical processing: controlled rolling followed by normalization (the "N" designation) or thermo-mechanically controlled processing (TMCP) followed by normalization in some cases.
- Typical microstructure: fine-grained ferrite-pearlite or ferrite with dispersed fine bainitic islands and nanoscale precipitates (NbC, VN, or TiN) from microalloying. Normalizing produces refined prior austenite grains and improved notch toughness.
- Heat-treatment response: normalization and controlled cooling refine grain size and precipitate distribution, enhancing yield strength and low-temperature toughness without heavy quench-and-temper cycles.
- SPA-H (conventional pressure-vessel carbon steel)
- Typical processing: hot-rolled plate, normalized condition optional depending on supplier/specification.
- Typical microstructure: ferrite-pearlite with relatively coarser grains as compared with TMCP microalloyed steels when not normalized.
- Heat-treatment response: normalization can improve toughness; however, conventional A516-like steels are designed to meet impact requirements through chemistry and rolling practice rather than heavy microalloying. Quenching and tempering is generally not part of the standard supply route for these plates.
Implication: Q355NH’s microalloying plus normalization gives a favorable strength–toughness balance, especially for applications requiring higher yield strength and good low-temperature toughness.
4. Mechanical Properties
Below are representative mechanical property ranges; exact guaranteed values come from the specific standard and the mill test certificate.
| Property | Q355NH (typical) | SPA-H (typical, e.g., A516 Gr70) |
|---|---|---|
| Minimum Yield Strength (MPa) | ~355 MPa (design intent of Q355) | ~240–280 MPa (commonly ~260 MPa) |
| Tensile Strength (MPa) | 470 – 630 MPa (typical range) | 415 – 585 MPa (typical range) |
| Elongation (% on 200 mm or 50 mm) | ≈ 20% (varies with thickness) | ≈ 18–22% (varies with thickness) |
| Charpy Impact Toughness | Specified to low temperatures for Q355NH (e.g., -20 to -50°C) | Specified for low temperatures in pressure-vessel steels (e.g., -20°C), depends on thickness |
| Hardness (HB or HRC) | Moderate; higher than conventional carbon plate due to HSLA strengthening | Moderate; generally lower than Q355NH for same thickness |
Interpretation: - Q355NH provides higher specified minimum yield strength (hence the "355") and, when processed correctly, superior or similar toughness at low temperatures compared with conventional pressure-vessel steels. - SPA-H/A516 Gr70-type steels offer adequate tensile strength and very good ductility and toughness for pressure-vessel service, but lower yield allows thicker or heavier sections for the same load.
5. Weldability
Weldability assessment for both grades focuses on carbon content, hardenability, and presence of microalloying elements. Two commonly used empirical indices:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Lower $CE_{IIW}$ and $P_{cm}$ values indicate easier weldability and lower preheat requirements. Both Q355NH and SPA-H are low-alloy carbon steels with relatively modest carbon equivalents. - Q355NH: microalloying (Nb, V) and slightly higher Mn in some heat lots can increase hardenability locally, which raises sensitivity to rapid cooling and hydrogen cracking in thick welds. As a result, Q355NH may require more cautious welding practices (appropriate preheat, interpass temperature control, low-hydrogen consumables) for thicker sections compared with plain carbon A516-like steels. - SPA-H/A516 Gr70-style plates: generally very weldable using standard procedures; lower yield and fewer microalloying elements make hydrogen-induced cracking less of a concern, but preheat and PWHT considerations depend on thickness, restraint, and service temperature.
Best practice: determine welding procedure specifications (WPS) based on measured CE/Pcm values from the MTC, thickness, and required impact properties; always follow code-prescribed preheat and post-weld heat treatment where applicable.
6. Corrosion and Surface Protection
- Both Q355NH and SPA-H are non-stainless carbon steels. Corrosion resistance in atmospheric or aqueous environments is limited without protection.
- Common surface protection options:
- Hot-dip galvanizing (for moderate corrosion protection; check thickness and base metal suitability).
- Epoxy or polymer coatings, fusion-bonded epoxy (FBE), or multilayer systems for aggressive environments.
- Painting systems (surface preparation and primer/topcoat selection per environment).
- Stainless-specific indices such as PREN are not applicable to these non-stainless grades. For stainless steels the PREN index is:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
which does not apply here because neither grade is stainless.
Corrosion allowance: for pressure vessels, designers typically include a corrosion allowance and specify coatings and cathodic protection as required by service conditions.
7. Fabrication, Machinability, and Formability
- Cutting: plasma, oxy-fuel, and laser cutting are readily applied to both grades; Q355NH’s higher strength may produce slightly more edge hardness with thermal cutting, but differences are small.
- Bending and forming: SPA-H (A516-like) is generally easier to form due to lower yield strength and good ductility. Q355NH’s higher yield can increase springback and require higher forming forces; however, its refined microstructure still permits good formability when correct procedures are used.
- Machinability: both are moderate; neither is a free-machining alloy. Microalloying in Q355NH can slightly reduce machinability versus plain carbon SPA-H, but effect is small in thick plate and structural work.
- Finishing: surface grinding, shot-blasting, and polishing behave similarly for both grades.
8. Typical Applications
| Q355NH (typical uses) | SPA-H / A516 Gr70 (typical uses) |
|---|---|
| High-strength structural members, bridge components, crane booms, heavy sections where higher yield saves weight | Pressure-vessel shells and heads, low- to medium-pressure boilers, general pressure-retaining equipment |
| Pressure vessels that require higher minimum yield and specified low-temperature toughness (when code permits) | Storage tanks, heat exchangers, and standard boiler plates |
| Offshore secondary structures and platforms where normalized plates with good low-temperature toughness are required | Process industry pressure vessels and piping skirts where conventional vessel plates are specified |
Selection rationale: - Choose Q355NH when you need higher yield for weight savings, or when normalized plate with guaranteed low-temperature impact toughness and HSLA performance is required. - Choose SPA-H/A516-type steel when the application is classic pressure-vessel service with standard code acceptance, simpler welding procedures, and where slightly lower yield is acceptable.
9. Cost and Availability
- Q355NH: widely manufactured and available in China and many Asian markets; availability in other regions depends on import supply chains. Price can be competitive for large-volume purchases and depends on thickness and plate size.
- SPA-H / A516 Gr70: globally available from many mills, especially in North America, Europe, and Asia; well-established supply chain for pressure-vessel plate sizes and thicknesses.
- Relative cost: under similar market conditions, microalloyed Q355NH may carry a modest premium for high-performance lots or stricter impact requirements; conversely, regional supply dynamics often dominate price differences.
Product forms: both grades are available as hot-rolled plates; additional processing (normalizing, milling, cutting) affects lead times.
10. Summary and Recommendation
| Criterion | Q355NH | SPA-H (e.g., A516 Gr70) |
|---|---|---|
| Weldability | Good, but microalloying may require stricter preheat/controls | Very good with standard procedures |
| Strength–Toughness balance | Higher yield, refined microstructure, good low-temp toughness | Good tensile and ductility; lower yield but excellent ductility for vessel service |
| Cost & Availability | Competitive in Asia; depends on mill and processing | Widely available globally; standard for pressure vessels |
Recommendation: - Choose Q355NH if you require higher minimum yield strength and a steel processed (normalized/TMCP) for improved low-temperature toughness and a better strength-to-weight ratio. Q355NH is a good choice for structural applications and some pressure vessels where code acceptance and welding procedure qualification allow its use. - Choose SPA-H (commonly an A516 Gr70-type pressure-vessel plate) if you prioritize established pressure-vessel practice, maximum ease of welding and fabrication under standard WPSs, and broad global availability for conventional boiler and pressure-vessel applications.
Final note: Always confirm the exact standard, composition, and guaranteed mechanicals from the mill test certificate for the specific heat and plate thickness. Choose welding procedures, preheat, and post-weld heat treatments based on measured carbon equivalent indices and the code or customer requirements that govern the finished equipment.