Q355NH vs Q355B – Composition, Heat Treatment, Properties, and Applications

Introduction

Q355NH and Q355B are two widely used Chinese-designation structural steels within the Q355 family. Engineers, procurement managers, and production planners commonly face the choice between them when specifying plate and section material for bridges, pressure-retaining structures, heavy machinery frames, and welded fabrications. Typical decision drivers include strength vs. toughness trade-offs, weldability and fabrication constraints, and lifecycle protection against atmospheric corrosion versus lowest first cost.

Although both are non-stainless carbon/low-alloy structural steels, a common practical axis of comparison in project selection is surface durability in outdoor or industrial environments. In other words: neither is inherently a stainless or dedicated weathering steel, so atmospheric corrosion performance—driven by chemistry, microstructure, and surface protection—becomes a deciding factor in many specifications. Designers therefore compare Q355NH and Q355B not only on strength and toughness but also on how each responds to coatings, galvanizing, or exposure when uncoated.

1. Standards and Designations

• Principal Chinese standard: GB/T 1591 (or its successor documents) governs Q355-series structural steels. Local mill certificates and delivery standards specify exact requirements.
• Equivalent or related international designations: there is not a direct one-to-one equivalent to ASTM/ASME grades; Q355 is often compared functionally to HSLA steels such as ASTM A572 Grade 50 or S355 in EN, but exact chemistry and testing regimes differ.
• Classification: both Q355NH and Q355B are non-stainless, low-alloy/high-strength structural steels (HSLA category). They are not tool steels, stainless steels, or high-alloy corrosion-resisting grades.

2. Chemical Composition and Alloying Strategy

Table: qualitative presence of alloying and impurity elements for the two grades. Exact mill limits depend on the release standard and thickness; always verify mill test certificates.

Explanation: - Both grades are engineered primarily by controlling C and Mn for strength while limiting P and S as harmful impurities. Microalloying elements (Nb, V, Ti) are often used across Q355 variants to refine grain, raise strength, and maintain toughness without large increases in carbon. - Any intentional additions to improve atmospheric corrosion resistance (for example, small Cu, P, or Cr additions used in weathering steels) are not intrinsic to the base Q355 designation and must be confirmed on the mill certificate. In practice, atmospheric corrosion resistance depends far more on protective coatings and alloying specifically designated for weathering service than on the standard Q355 chemistry.

3. Microstructure and Heat Treatment Response

• Typical as-rolled microstructure: both grades are delivered as hot-rolled plates with a ferrite–pearlite matrix. The relative proportion of ferrite and pearlite and the presence of fine carbides or microalloy precipitates determine strength and toughness.
• Q355B: produced for general structural use with a balance of strength and ductility. Microalloying may be minimal; grain size and inclusion control are typical manufacturing levers.
• Q355NH: the “N” and “H” modifiers commonly indicate normalized and enhanced low-temperature toughness requirements in related Q-grade nomenclature. Normalizing (air cooling from above the transformation range) refines grain size, producing finer ferrite–pearlite microstructures and more uniform properties through thickness.
• Heat-treatment response:
• Normalizing tends to refine grain and improve impact toughness and homogeneity; this is typical for NH-designated steels.
• Quenching and tempering is not the normal processing route for Q355-class plates (these are not quenched-and-tempered steels), although local heat treatment after welding or for special components is possible.
• Thermo-mechanical controlled processing (TMCP) can be used to obtain higher strength and finer microstructure without additional normalization.
• Practical effect: Q355NH variants processed to tighter heat-treatment and grain-size controls typically show improved toughness (especially at lower temperatures) and more consistent through-thickness properties than Q355B processed to standard rolling and cooling schedules.

4. Mechanical Properties

Table: qualitative and commonly quoted typical property expectations. Values and test temperatures depend on thickness and the specific standard: verify mill test certificates for project-critical values.

Interpretation: - Both grades are designed around a yield class of ~355 MPa. The main mechanical distinction is toughness at lower temperatures and through-thickness homogeneity—Q355NH is typically specified when improved impact toughness (e.g., at −20 °C or lower) and normalized processing are desired. - Q355B is a conventional structural grade adequate where low-temperature toughness is not a controlling requirement.

5. Weldability

Weldability depends on carbon content, carbon equivalent (hardening tendency), and presence of microalloying elements. Two commonly used indices:

• IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• International Pcm: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation for Q355B vs Q355NH: - Both grades are considered reasonably weldable for standard structural welding procedures when following appropriate preheat/interpass control for thickness. Low-to-moderate carbon and controlled Mn keep carbon equivalents in a range conducive to common welding consumables. - If Q355NH includes deliberate microalloying or if it is delivered normalized, its propensity for HAZ hardening may differ slightly from Q355B. Normalized microstructure can reduce HAZ softening and improve toughness, which often makes post-weld behavior more favorable. - Practical guidance: always calculate the relevant carbon equivalent for the batch and thickness, follow filler-metal manufacturers’ recommendations, and use preheat/interpass controls and PWHT only when calculations or experience indicate risk of cold cracking or compromised toughness.

6. Corrosion and Surface Protection

• Both Q355NH and Q355B are non-stainless steels; they do not form protective passive films like austenitic or duplex stainless steels. Native atmospheric corrosion (rusting) will occur unless protected.
• Typical protection strategies: galvanizing (hot-dip or electro), specification of protective paint systems, polymeric claddings, or the use of sacrificial anodes in marine environments.
• When corrosion resistance indices are relevant (for stainless grades), the Pitting Resistance Equivalent Number (PREN) is used: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ That index does not apply to Q355 steels because Cr, Mo, and N are not present at protective levels.
• Important note: some mills may manufacture variants with small copper or phosphorus additions to improve short-term atmospheric resistance; such variants are not standard Q355B/Q355NH and must be explicitly specified and certified. For long-term uncoated outdoor service in industrial or coastal atmospheres, select a dedicated weathering steel or apply an appropriate mitigation system.

7. Fabrication, Machinability, and Formability

• Forming and bending: both grades are readily cold-formable in the normal plate thickness ranges intended by the standard. Q355NH may allow slightly tighter bend radii or more consistent spring-back due to normalized grain structure.
• Machinability: both are typical carbon/HSLA steels; machinability is average. Microalloying and higher strength may reduce tool life slightly versus low-carbon mild steels.
• Surface preparation and finishing: both accept standard surface treatments—shot-blasting, priming, galvanizing, and painting. For critical cosmetic or corrosion-sensitive applications, surface cleanliness and pre-treatment (e.g., grit blast to specified Sa standards) are essential.

8. Typical Applications

Two-column table listing typical uses and selection rationale.

Selection rationale: - Choose Q355B for standard structural use and where low-temperature toughness is not prioritized and cost is a significant factor. - Choose Q355NH when project specifications require normalized deliveries, improved low-temperature toughness, or tighter through-thickness property control.

9. Cost and Availability

• Relative cost: Q355B is typically the lower-cost option because its production route and testing requirements are more standard. Q355NH deliveries can be priced higher if they require normalization, additional impact testing at low temperatures, or tighter chemistry control.
• Availability by product form: both grades are commonly available as hot-rolled plate, wide-flange sections, and welded tubes—but availability varies by region and mill backlog. Special chemistry or certified normalized plates (Q355NH) may have longer lead times; confirm availability early in procurement.

10. Summary and Recommendation

Summary table (high-level qualitative comparison).

Recommendations: - Choose Q355B if: your project requires a cost-effective, commonly available structural steel with standard strength (≈355 MPa yield), where ambient temperatures and toughness requirements are within normal civil/structural ranges and an uncoated-service lifetime is not a primary concern. - Choose Q355NH if: the specification demands improved low-temperature impact toughness, normalized processing or tighter through-thickness property control (for thick plates or heavy welded fabrications), or when project codes explicitly call out the NH variant for critical welded or cold-service components.

Final practical notes: - Neither grade is a stainless or dedicated weathering material; if long-term atmospheric corrosion resistance is required without coatings, specify a weathering steel or stainless alloy, or design for robust protective systems (galvanizing, multi-coat paint). - Always verify mill test certificates for chemistry, heat-treatment history, mechanical test values, and impact test temperatures before acceptance. For welded structures, compute the appropriate carbon equivalent and follow qualified welding procedures and preheat/interpass controls as indicated by calculation and experience.