Q235NH 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 often face the trade-off between cost, weldability, and performance when selecting carbon steels for pressure vessels, boilers, or structural fabrications. Q235NH and SPA-H are two commonly specified grades in East-Asian and international supply chains; both are low-alloy/low-carbon steels intended for welded pressure equipment and general structural use, but they arise from different national standard systems and production philosophies.
The principal distinction is that Q235NH is specified under Chinese national standards for normalized, pressure-vessel-capable plain carbon steel, while SPA-H is a Japanese-style boiler/pressure-vessel steel grade with its own chemical and delivery-condition expectations. Because the two stocks are produced to different standards, they are often compared in procurement and engineering discussions to determine the better fit for strength, toughness, weldability, and surface-protection needs.
1. Standards and Designations
- Q235NH
- Origin: Chinese national standard family (GB).
- Typical standard references: GB/T 1591 (for pressure vessel steels and normalized grades) and GB/T 700 (for general structural steel designation Q235).
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Category: Carbon/low-alloy structural steel intended for pressure-vessel service; delivered in a normalized or thermomechanically-processed condition where the suffix "NH" indicates normalized and improved impact toughness for pressure equipment.
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SPA-H
- Origin: Japanese industrial standards (often seen in JIS/G or equivalent industry specifications for boiler and pressure-vessel plate).
- Typical standard references: Japanese boiler/pressure-vessel plate standards (JIS G3115 or similarly numbered domestic specs — note that naming varies by supplier and historical standards).
- Category: Carbon steel for boilers and pressure vessels; intended to meet higher plate-quality and toughness requirements for welded vessels.
Classification summary: - Both are plain carbon/low-alloy steels (not stainless, not tool steels, not HSLA in the high-strength alloy sense). SPA-H variants may be produced with tighter control on impurities and with slightly different delivery-condition testing compared with Q235NH.
2. Chemical Composition and Alloying Strategy
The two grades are both low-carbon steels with small amounts of alloying and impurity control to meet toughness and weldability requirements. The following table presents representative composition ranges expressed as typical or maximum values commonly used in practice. These figures are representative; always confirm with the mill certificate for specific lot acceptance tests.
| Element | Q235NH (representative) | SPA-H (representative) |
|---|---|---|
| C (carbon) | ~0.12–0.22 % (max ~0.22) | ~0.10–0.18 % (typical max ~0.18) |
| Mn (manganese) | ~0.30–0.80 % | ~0.30–1.00 % |
| Si (silicon) | ~0.02–0.30 % | ~0.01–0.35 % |
| P (phosphorus) | ≤ 0.035 % (controlled) | ≤ 0.025–0.035 % (tightly controlled) |
| S (sulfur) | ≤ 0.035 % (controlled) | ≤ 0.035 % (controlled) |
| Cr (chromium) | trace – up to 0.30 % (if present) | trace – up to 0.30 % |
| Ni (nickel) | trace (typically not added) | trace (typically not added) |
| Mo (molybdenum) | usually not added; trace only | usually not added; trace only |
| V, Nb, Ti (microalloys) | rarely added for Q235NH (not typical) | may include trace microalloying in some SPA-H derivatives |
| B, N | trace; N often controlled | trace; N often controlled |
How alloying affects performance: - Carbon is the primary strength-determining element; lower carbon improves weldability and toughness at the expense of some strength. - Manganese increases hardenability and tensile strength and counters sulfur embrittlement effects (MnS formation). - Silicon at low levels is a deoxidizer and slightly increases strength. - Tight control of P and S is vital for notch toughness and weld integrity; pressure-vessel steels often specify lower maximums than generic structural steels. - SPA-H variants sometimes emphasize stricter impurity control and, in some supplier formulations, controlled microalloy additions to tune strength and toughness.
3. Microstructure and Heat Treatment Response
Typical microstructures: - Q235NH: Delivered normalized (suffix NH), the microstructure is a fine-grained ferrite–pearlite mix produced by reheating and air cooling (normalizing). This refines prior austenite grain size and improves impact toughness relative to as-rolled plate. - SPA-H: Typically delivered in a normalized or normalized-and-tempered condition with attention to uniformity and cleanliness. Microstructure is also ferrite–pearlite but with potentially finer dispersions, depending on rolling and cooling practice and any microalloying.
Heat treatment and thermo-mechanical processing effects: - Normalizing (both grades): Refines grain size, improves uniformity, and increases toughness at similar strength levels. Normalizing is the standard route for both grades when specified for pressure-vessel service. - Quenching & tempering: Not typical for Q235NH or SPA-H because these grades are intended as low-to-moderate strength, weldable steels; Q&T would push them into different categories (higher-strength alloy steels). - Thermo-mechanical controlled processing (TMCP): Some modern plates are manufactured by TMCP to achieve better strength–toughness balance without excessive carbon or microalloy additions. SPA-H plates from certain mills may be produced with TMCP to achieve tight toughness requirements at lower alloy content.
4. Mechanical Properties
The following table provides typical mechanical property bands commonly used as acceptance criteria for these grades; actual contractual values should be taken from the applicable standard or the mill test report.
| Property | Q235NH (typical acceptance) | SPA-H (typical acceptance) |
|---|---|---|
| Tensile strength (Rm) | ~370–500 MPa (varies by thickness) | ~380–520 MPa (varies by specification and thickness) |
| Yield strength (Rp0.2 or ReL) | Nominally ~235 MPa (Q235 designation) | Typically somewhat higher than Q235NH in some SPA-H specs; depends on plate thickness |
| Elongation (A%) | ≥ 20% (depends on thickness) | ≥ 18–22% (depends on thickness and spec) |
| Impact toughness (Charpy V-notch) | Specified minimums at given temperature (e.g., 27 J at specified temperature) | Often requires similar or tighter CVN minima and/or lower test temperatures |
| Hardness (HB) | Typically low (soft) — e.g., HB 120–200 range depending on steel and thickness | Similar range; SPA-H may be controlled to slightly lower hardness for better toughness |
Interpretation: - Q235NH is designed around a nominal 235 MPa yield strength (hence the "235"). SPA-H variants are generally specified with similar tensile ranges but can be produced to tighter toughness or slightly higher strength depending on the exact JIS or supplier specification. - Toughness (impact energy at a specified temperature) is heavily influenced by composition (P, S, N), grain size (from normalizing), and plate thickness. SPA-H variants sometimes emphasize more stringent impact requirements for colder-service boilers.
5. Weldability
Weldability depends on carbon content, hardenability elements (Mn, Cr, Mo, V), and impurity control. Two empirical indices commonly used to assess weldability are shown below.
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International Institute of Welding carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
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International carbon equivalent (Pcm) used in some codes: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Because both Q235NH and SPA-H are low-carbon steels, calculated $CE_{IIW}$ and $P_{cm}$ values will generally be low, indicating good weldability with conventional filler metals and moderate preheat practices. - Q235NH’s slightly higher nominal carbon (relative to some SPA-H formulations) can increase the need for preheat in thicker sections or when using high-heat-input procedures. SPA-H’s stricter impurity control and production conditioning often yield marginally better as-welded toughness and lower susceptibility to hydrogen-induced cold cracking. - Microalloying (if present in some SPA-H variants) can marginally increase hardenability, affecting the amount of preheat or post-weld heat treatment required. - In all cases, thickness, joint design, weld restraint, and hydrogen control (filler selection, consumable moisture) are more decisive than grade choice alone. Use weld procedure qualifications (WPS/PQR) and calculate $P_{cm}$ or $CE_{IIW}$ for the specific chemistry to set preheat/post-weld heat treatment limits.
6. Corrosion and Surface Protection
- Both Q235NH and SPA-H are plain carbon steels (non-stainless). They rely on surface protection for corrosion resistance.
- Typical protections: hot-dip galvanizing, zinc-rich primers and multilayer coatings, industrial paints, solvent-borne or epoxy-based linings, and cathodic protection for immersed equipment.
- When corrosion-resistance indices like PREN are referenced, they do not apply to plain carbon steels. For stainless steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This is not applicable to Q235NH or SPA-H because the chromium and molybdenum contents are negligible.
- Selection guidance: choose galvanized or coated steels for atmospheric exposure; specify suitable linings or corrosion allowances for internal service (e.g., boilers, tanks). For aggressive chemical environments, select a corrosion-resistant alloy rather than trying to protect plain carbon steel.
7. Fabrication, Machinability, and Formability
- Formability: Low carbon and normalized microstructures in both grades make them readily formable by cold bending, rolling, and press-braking. Thicker plates require appropriate bend radii; normalized condition improves ductility and reduces springback variability.
- Machinability: Low-carbon steels machine reasonably well; machinability depends on hardness and inclusion content. SPA-H plate with tighter cleanliness control can sometimes produce better surface finish and tool life.
- Cutting and thermal processes: Plasma, oxy-fuel, and laser cutting are commonly used. Preheating recommendations should follow weldability assessment for cutting/cold-worked edges.
- Surface finishing: Both accept grinding, shot-blasting, and typical painting systems. For applications requiring tight tolerances or surface quality (e.g., pressure vessel sealing faces), final machining in the normalized state is recommended.
8. Typical Applications
| Q235NH – Typical Uses | SPA-H – Typical Uses |
|---|---|
| General pressure vessel plate for low-to-moderate temperature service where economy and adequate toughness are required | Boiler and pressure-vessel plate where stricter toughness and impurity control are specified |
| Structural components and frames where welding and forming are primary concerns | Pressure vessel shells, boiler drums, and components specifying Japanese-style plate standards |
| Tanks, low-pressure piping supports, and welded fabrications | Applications requiring tighter as-delivered impact requirements or where supplier JIS-type traceability is requested |
Selection rationale: - Choose Q235NH when cost-efficiency and widely available Chinese-standard plate are primary considerations and when specified normalized toughness and weldability meet design requirements. - Choose SPA-H when the purchaser requires a JIS-style boiler/pressure vessel plate with potentially tighter impurity and toughness acceptance criteria, or when matching existing equipment specified to Japanese standards.
9. Cost and Availability
- Cost: Q235NH is often more economical than specialty-imported SPA-H plate because of high domestic production volumes in regions where GB-standard plate is common. SPA-H can be more costly when imported or when produced to tighter cleanliness and testing regimes.
- Availability: Q235NH is widely available from many mills in China and from exporters; SPA-H availability depends on regional production and mill stock. Plate thickness, cut-to-length service, and certification (e.g., mill certification, impact test certificates) affect lead times for both.
- Product forms: Both are commonly supplied as plates; the cost premium increases for thicker plates, tested impact values at lower temperatures, or additional mill certifications.
10. Summary and Recommendation
Summary table (qualitative)
| Attribute | Q235NH | SPA-H |
|---|---|---|
| Weldability | Very good (low C, normalized) | Very good to slightly better in some specs (higher cleanliness) |
| Strength–Toughness balance | Designed around 235 MPa yield and adequate toughness | Comparable tensile properties; often specified to tighter impact criteria |
| Cost | Often lower / good value | May be higher due to specification and test demands |
Recommendations: - Choose Q235NH if: - You need a cost-effective, widely available normalized plate with good weldability for typical pressure-vessel or structural applications. - The design requires a nominal 235 MPa yield class and the specified impact and thickness limits of Q235NH meet the service temperature and toughness demands. - Mill certificates and normalized delivery condition are acceptable for procurement and code compliance.
- Choose SPA-H if:
- Your project calls for material conforming to Japanese-style boiler/pressure-vessel specifications, tighter impurity control, or stricter as-delivered impact requirements.
- You require traceability, specific supplier qualifications, or a plate that may be produced with more precise control over cleanliness and toughness for colder-service or more heavily stressed welded fabrications.
- The slightly higher cost is justified by the need for tighter acceptance criteria or to match existing equipment/material standards.
Closing note: Q235NH and SPA-H are both practical choices for welded vessels and general structural plate work. The right selection depends on the exact specification requirements (mechanical, impact temperature, thickness), weld procedure constraints, corrosion-protection plan, and commercial factors (lead time and cost). Always confirm the exact chemical and mechanical values on the mill test report and perform carbon-equivalent calculations and weld procedure qualifications for the specific lot of material before production.