Q235NH vs Q355GNH – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Q235NH and Q355GNH are two commonly specified Chinese structural steels that engineers frequently compare when designing load-bearing, welded, or pressure-containing components. Typical decision contexts include balancing cost versus required yield strength, selecting material for low-temperature impact requirements, and deciding whether additional microalloying for higher toughness and strength is justified.
The principal technical distinction is that Q355GNH is specified and processed to deliver a higher minimum yield strength and generally contains microalloying or stricter processing controls to improve toughness and strength compared with Q235NH. Because both are non‑stainless structural steels often delivered in normalized or thermomechanically treated conditions, they are directly compared when engineers must trade weldability, toughness at low temperature, formability, and material cost.
1. Standards and Designations
- Common reference standards:
- China: GB/T 700 (general carbon structural steels); GB/T 1591 (low-alloy high-strength structural steel); GB/T 232 (hot-rolled sheets/plates), and related national standards that cover normalized and impact-tested variants.
- International equivalence: there is no exact 1:1 ASTM/EN equivalent, but Q235 ≈ low-carbon mild steels (e.g., A36/A283 family), and Q355 ≈ lower-range HSLA steels in EN (S355 family) and ASTM high-strength structural steels.
- JIS and EN standards may be used for comparative design but do not rename Q‑grades.
- Classification:
- Q235NH: Carbon structural steel (normalized, impact-tested variant).
- Q355GNH: Low-alloy/high-strength structural steel (higher-strength grade, fine-grain or controlled processing indicated by "G", normalized, impact-tested variant).
2. Chemical Composition and Alloying Strategy
The table below shows typical composition ranges commonly referenced in supplier datasheets and national standards. These values are indicative; always confirm with mill certificates or the specific standard version.
| Element | Typical Q235NH (wt%) | Typical Q355GNH (wt%) |
|---|---|---|
| C (Carbon) | ~0.12–0.20 | ~0.12–0.22 |
| Mn (Manganese) | ~0.30–1.40 | ~0.50–1.60 |
| Si (Silicon) | ≤0.35 (typical) | ≤0.50 (typical) |
| P (Phosphorus) | ≤0.045 (max) | ≤0.035–0.045 (max) |
| S (Sulfur) | ≤0.045 (max) | ≤0.045 (max) |
| Cr (Chromium) | ≤0.30 (if present) | Often ≤0.30; may be slightly higher in some specs |
| Ni (Nickel) | Trace to none | Trace to low (occasionally present) |
| Mo (Molybdenum) | Not typical | Trace (possible in specific variants) |
| V, Nb, Ti (Microalloying) | Generally none | May contain microalloying (V, Nb, Ti) in Q355 variants to refine grain and raise strength |
| N (Nitrogen) | Controlled (for toughness) | Controlled (for toughness) |
Notes: - Q235NH is essentially a low‑carbon, low‑alloy steel provided in a normalized and impact-tested condition; composition is kept simple to maximize ductility and weldability. - Q355GNH typically targets higher yield strength through mild increases in carbon and manganese and/or controlled microalloy additions (V, Nb, Ti) and thermo‑mechanical processing to refine grain size and improve toughness without resorting to high alloy content.
How alloying affects performance: - Carbon increases strength and hardness but reduces weldability and ductility when raised significantly. - Manganese increases hardenability and tensile strength and aids deoxidation. - Microalloying elements (Nb, V, Ti) produce fine precipitates that pin grain boundaries, increase yield strength through precipitation strengthening, and improve toughness when processed correctly. - Sulfur and phosphorus are kept low to avoid embrittlement and poor fatigue/weld performance.
3. Microstructure and Heat Treatment Response
Typical microstructures: - Q235NH: Ferrite–pearlite microstructure after normalization. Normalizing refines grain size relative to as-rolled material and improves isotropic toughness compared to non-normalized hot-rolled steels. - Q355GNH: Fine-grained ferrite with a higher proportion of tempered bainite or low‑temperature pearlite depending on processing. If microalloyed and thermomechanically controlled, Q355GNH can exhibit a more refined, uniform ferrite grain size with fine carbide or carbo‑nitride precipitates.
Heat treatment and processing effects: - Normalizing (air cooling from austenite): Both grades benefit from normalization to homogenize microstructure and improve toughness — designated by the “N” in the grade. - Thermo-mechanical rolling (controlled rolling): More frequently used for Q355 variants to achieve higher strength and toughness via grain refinement and precipitation strengthening without significantly increasing carbon content. - Quenching and tempering: Not typically applied to Q235NH; Q355 variants intended for even higher strength might be available in quenched & tempered conditions in other product lines, but that changes the grade designation and supply chain expectations.
Practical implication: - Q235NH is straightforward to heat treat (normalize) and predict microstructure (ferrite–pearlite). - Q355GNH responds to tighter process control and microalloying; the same heat treatment can produce higher yield and better low-temperature toughness due to refined grains and precipitates.
4. Mechanical Properties
The following table summarizes typical mechanical property bands commonly associated with the two grades; confirm contracted materials by certificate.
| Property | Q235NH (typical) | Q355GNH (typical) |
|---|---|---|
| Minimum Yield Strength (Rp0.2) | ~235 MPa (naming basis) | ~355 MPa (naming basis) |
| Tensile Strength (Rm) | ~370–500 MPa | ~490–630 MPa |
| Elongation (A) | Higher ductility; e.g., ≥20–26% (varies by thickness) | Lower elongation than Q235NH; e.g., ≥18–22% (varies by thickness) |
| Impact Toughness | Specified as Charpy V-notch at given temperature; normalized for good toughness | Often specified for lower temperatures; improved toughness via microalloying/process control |
| Hardness | Lower (easier machining/forming) | Higher (increased strength; moderate hardness increase) |
Interpretation: - Strength: Q355GNH is the stronger material by design, with a substantially higher minimum yield and higher tensile range. - Toughness: With proper processing and impact testing, both grades can meet toughness requirements; Q355GNH often requires more careful processing to ensure toughness is not compromised by higher strength. - Ductility/formability: Q235NH is generally more ductile and forgiving in forming operations.
5. Weldability
Weldability depends on carbon equivalent and hardenability, plus microalloying and thickness.
Useful empirical formulas: - Carbon equivalent (IIW) commonly used in assessing weldability: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Predicted cold cracking index $P_{cm}$: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Q235NH: Low carbon, limited alloying — generally excellent weldability with low preheat requirements for common thicknesses and reduced risk of hydrogen-induced cold cracking. - Q355GNH: Higher Mn and possible microalloying increase hardenability; this can raise $CE_{IIW}$ and $P_{cm}$ relative to Q235NH, signaling a greater need for attention to preheat, interpass temperature, and hydrogen control when welding thick sections. Proper welding procedure specifications and qualification are recommended. - Microalloying increases strength but can also increase the tendency for local hard zones in weld HAZ if thermal cycles are not controlled.
6. Corrosion and Surface Protection
- Both Q235NH and Q355GNH are carbon (or low‑alloy) steels; they are not stainless and therefore require protective measures for exposed environments.
- Common protection strategies: hot-dip galvanizing, zinc-rich primers, epoxy or polyurethane coatings, cathodic protection for immersed structures, and appropriate surface preparation.
- PREN (pitting resistance equivalent number) is not applicable to these non‑stainless steels. For stainless alloys the index, $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ is meaningful; not relevant for Q‑grades without significant Cr/Mo/N content.
Corrosion selection notes: - Surface treatments add cost but can extend service life significantly; thicker coatings or galvanizing are common for structural members exposed to weather. - For atmospheric or splash zones, consider galvanizing or duplex systems (zinc + paint).
7. Fabrication, Machinability, and Formability
- Cutting: Both grades are readily flame- or plasma-cut; oxy-fuel cutting is common for thicker plate. Q355GNH may require slightly higher energy or slower cutting parameters due to higher hardness.
- Forming and bending: Q235NH has superior formability and springback characteristics; Q355GNH can be formed but requires tighter bend radii rules and more controlled process parameters to avoid cracking.
- Machinability: Low carbon content in Q235NH yields good machinability. Q355GNH, being higher strength and possibly microalloyed, may be somewhat more abrasive on tooling and require slower feeds/cutting speeds.
- Surface finishing: Both accept typical surface treatments; pre- and post-weld grinding and dressing practices are similar, but Q355GNH may show higher hardness in heat-affected zones.
8. Typical Applications
| Q235NH (common uses) | Q355GNH (common uses) |
|---|---|
| General structural elements (beams, channels) where economy and formability matter | Structural members requiring higher load capacity or reduced section thickness (bridges, cranes, heavy frames) |
| Piping supports, non-critical pressure parts where normalized condition and toughness are needed | Welded structures exposed to dynamic loading or where weight savings are needed (offshore platforms, heavy machinery frames) |
| Fabricated components with extensive forming/welding | Components specified for guaranteed minimum yield of ~355 MPa and impact properties at lower temperatures |
Selection rationale: - Choose Q235NH when manufacturing priority is forming, cost efficiency, and good weldability. - Choose Q355GNH when structural weight reduction, higher design stresses, or a higher safety factor on yield is required and when production controls can ensure toughness.
9. Cost and Availability
- Cost: Q235NH is generally less expensive per tonne than Q355GNH due to simpler chemistry and lower processing demands. Q355GNH is typically more costly owing to stricter process controls, higher strength level, and possible microalloy additions.
- Availability: Both grades are widely available in plate, coil, and structural sections in markets where Chinese grades are stocked. Availability by thickness, width, and certified impact testing levels is vendor dependent — Q235 variants are generally more widely stocked.
Procurement tip: - Specify required mechanical tests, impact temperatures, and mill test certificates explicitly; price differences can be offset by reduced fabrication costs (thinner sections) when selecting the higher-strength grade.
10. Summary and Recommendation
| Category | Q235NH | Q355GNH |
|---|---|---|
| Weldability | Very good (lower CE) | Good to moderate (higher CE; more welding controls may be needed) |
| Strength – Toughness balance | Moderate strength, high ductility/toughness | Higher strength with engineered toughness via processing |
| Cost | Lower | Higher |
Recommendations: - Choose Q235NH if you need excellent formability and weldability, lower material cost, and your design loads can be satisfied with a ~235 MPa yield material. - Choose Q355GNH if your design requires a higher minimum yield strength (≈355 MPa), potentially allows thinning of sections for weight savings, and your fabrication processes can accommodate slightly tighter welding and forming controls to preserve toughness.
Final note: Always obtain and review the mill test certificate for the supplied plate or section. Specify required impact test temperature and acceptance levels in purchase documents, and validate welding procedure qualifications when moving from Q235NH to Q355GNH in production.