Q235NH vs Q355NH – Composition, Heat Treatment, Properties, and Applications

Introduction

Q235NH and Q355NH are two widely specified Chinese structural steels used in pressure vessels, boilers, and general structural applications that require normalized (N) heat treatment and sometimes enhanced impact performance (H). Engineers, procurement managers, and manufacturing planners commonly face a choice between these grades when balancing cost, weldability, and the need for higher strength or improved toughness. Typical decision contexts include whether to prioritize lower material cost and easier fabrication (favoring the lower-strength grade) or to reduce section thickness and weight through a higher-strength material (favoring the higher-strength grade).

The practical difference between the two is principally a performance-level tradeoff: Q355NH provides a higher guaranteed yield strength than Q235NH, with consequences for thickness, weight, and toughness requirements. Because they are both carbon (non-stainless) steels with normalization in their processing routes, they are compared frequently for applications where strength, notch toughness, and fabrication behavior must be balanced.

1. Standards and Designations

• Common standards where these steels appear:
• GB/T (China): Q235NH and Q355NH are designations in Chinese national standards for pressure-vessel/structural steels.
• EN (Europe): Roughly comparable to S235 and S355 families (structural steels), but direct substitution requires review of all property requirements.
• ASTM/ASME: Equivalent ASME/ASTM grades are not direct one-to-one matches; ASME pressure-vessel steels such as SA-516 Grade 70 are separate specifications with different chemistry and toughness requirements.

• JIS: Japanese grades are different; substitution needs verification.

• Classification:

• Both Q235NH and Q355NH are carbon/mild low-alloy steels (non-stainless), typically categorized as structural or pressure-vessel steels rather than tool or stainless steels. Q355NH is generally regarded as a higher-strength structural/HSLA-type steel (higher performance level within the structural family).

2. Chemical Composition and Alloying Strategy

Notes: - The “NH” suffix indicates normalized condition and an impact toughness requirement or other thermal processing characteristic rather than large alloy additions. The alloying strategy for both grades emphasizes carbon and manganese adjustments and strict impurity limits (P, S) to ensure toughness and weldability. Q355NH achieves higher yield strength primarily through composition and controlled processing rather than heavy alloying.

How alloying affects properties: - Carbon and manganese are the primary strength contributors: higher C and Mn increase strength and hardenability but reduce weldability and ductility if not controlled. - Silicon is a deoxidizer and has modest strengthening effect. - Microalloying elements (V, Nb, Ti), when present even at low ppm levels, increase yield strength by refining grain size and precipitating carbides/nitrides, improving strength without proportionally degrading toughness. - Impurities (P and S) embrittle and reduce toughness and are therefore tightly limited in these pressure-vessel/structural grades.

3. Microstructure and Heat Treatment Response

Typical microstructures: - As-normalized (N) condition: Both grades are commonly normalized (heat to austenite, then air cool) to develop a fine, relatively uniform ferrite–pearlite microstructure. - Q235NH: Normalizing produces a ferrite–pearlite structure with relatively coarse pearlite content compared with higher-strength steels. The microstructure supports good ductility and acceptable toughness at moderate temperatures. - Q355NH: Normalizing coupled with a slightly different chemistry and possibly controlled rolling/refinement gives a finer-grained ferrite–pearlite with higher dislocation density and sometimes microalloy precipitates. This yields higher yield and tensile strength while preserving toughness.

Heat-treatment routes and responses: - Normalizing (standard for the “N” designation): Improves toughness by grain refinement and provides consistent mechanical properties. Effective for both grades, particularly where impact toughness at low temperatures matters. - Quenching and tempering (Q&T): Not commonly applied to these grades in standard practice for pressure-vessel steels; Q&T will substantially raise strength but also alter toughness and is a different material class. - Thermo-mechanical controlled processing (TMCP): Often used for Q355-class steels to obtain higher strength with good toughness by combining controlled rolling and accelerated cooling; this is a production route rather than an in-shop heat treatment and helps achieve the required higher-strength targets without excessive alloying.

4. Mechanical Properties

Explanation: - Q355NH is the stronger grade by design: higher minimum yield and higher typical tensile strength allow thinner sections for the same structural load. The tradeoff is modestly lower ductility and potentially greater sensitivity to weld-derived hard microstructures unless proper welding procedures are used. - Toughness depends on thickness, normalized treatment, and quality control. When normalized and produced to specification, both grades can meet impact requirements; Q355NH often has tighter process control to meet higher-strength + toughness combinations.

5. Weldability

Weldability considerations: - Carbon content and combined hardenability govern preheat, interpass temperature, and post-weld heat treatment (PWHT) requirements. - Microalloying and manganese content affect hardenability and the risk of cold cracking in the heat-affected zone.

Useful carbon-equivalent and alloying indices (interpretive; apply to qualitative assessment): - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (for cold-cracking susceptibility, interpret qualitatively): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative): - Both grades target low-to-moderate $CE_{IIW}$ and $P_{cm}$ values compared with quenched steels, so they are generally considered weldable with standard consumables and preheat practices. - Q235NH, with typically lower strength and somewhat lower carbon-equivalent, is generally more forgiving in welding—less preheat and less risk of HAZ cracking. - Q355NH, while designed for weldability, may require more conservative welding practice (controlled heat input, possible preheat for thicker sections, and matching filler materials) because its higher strength and slight increase in hardenability can increase susceptibility to hard microstructures in the HAZ if improperly welded.

6. Corrosion and Surface Protection

• Both Q235NH and Q355NH are non-stainless carbon steels; intrinsic corrosion resistance is limited.
• Typical protection methods:
• Hot-dip galvanizing (for atmospheric corrosion protection).
• Paints, primers, and coatings (epoxy, polyurethane systems) for aggressive environments.
• Cathodic protection and cladding (e.g., stainless cladding) for chemical service where corrosion resistance is critical.
• PREN (pitting resistance equivalent number) formula: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• PREN is a stainless-steel corrosion index and is not applicable to Q235NH/Q355NH because these are not stainless alloys and do not rely on Cr/Mo/N-based passive films.
• Clarification: For pressure-vessel or chemical applications requiring corrosion resistance, consider linings, claddings, or selecting stainless or corrosion-resistant alloys rather than relying on Q235NH/Q355NH.

7. Fabrication, Machinability, and Formability

• Cutting: Both grades cut with standard oxy-fuel, plasma, or laser methods; Q355NH may require slightly adjusted parameters because of higher strength and hardness.
• Machinability: Q235NH generally machines more easily due to lower strength and hardness. Q355NH machines acceptably but tool wear can be higher; selection of tooling and feeds should account for higher tensile/hardness.
• Formability/bendability: Q235NH offers better formability and larger bend radii at a given thickness. Q355NH can be formed but may need larger bend radii or controlled forming parameters to avoid cracking, especially if microalloying increases strength.
• Surface finishing: Both take painting, galvanizing, and coating well after appropriate surface preparation.

8. Typical Applications

Selection rationale: - Choose Q235NH when cost sensitivity, high ductility, and ease of fabrication/welding are the primary drivers. - Choose Q355NH when higher yield strength allows reduction in section thickness, delivering weight and material savings, or when code/design requires the higher performance level.

9. Cost and Availability

• Relative cost: Q235NH is typically less expensive