Q235 vs Q255 – Composition, Heat Treatment, Properties, and Applications

Introduction

Engineers, procurement managers, and manufacturing planners frequently choose between Q235 and Q255 when specifying structural carbon steels for welded frames, plates, and sections. The decision commonly balances cost and ease of fabrication against the need for higher yield strength and in-service performance (for example: higher load capacity or reduced section size). Typical decision contexts include welded structural members, general fabrication, and moderate-duty pressure or storage vessels where both strength and weldability matter.

The principal technical distinction between Q235 and Q255 is their design yield strength: Q255 is specified to a higher minimum yield than Q235. That yield target drives subtle differences in chemical control, processing, and selection trade-offs that make the two grades commonly compared in design and manufacturing.

1. Standards and Designations

• GB (People’s Republic of China): Q235, Q255 (national structural carbon steel grades). These are designated as carbon structural steels.
• Other system equivalents (functional, not identical): Q235 is often compared with ASTM A36 / EN S235JR in structural applications, but direct equivalence requires review of chemistry and mechanical tests.
• Classification: both Q235 and Q255 are plain carbon (low-alloy-free) structural steels, not stainless, tool, or high-strength low-alloy (HSLA) steels in the modern sense—although mill practice may include microalloying or controlled rolling to meet mechanical properties.

Note: Standards and product forms differ (plate, strip, bar, section); always specify the exact standard and product form required in purchase orders.

2. Chemical Composition and Alloying Strategy

The table below summarizes the relative presence of key elements and their roles. These entries describe typical mill practice and relative levels rather than prescriptive standard limits—mill certificates and the applicable standard should be consulted for contract values.

Element	Q235 (typical relative level)	Q255 (typical relative level)	Purpose / Effect
C (Carbon)	Low to moderate (controls strength)	Low to moderate (often controlled to meet higher yield without excessive hardness)	Primary strength control; higher C increases strength and hardenability but reduces weldability and ductility.
Mn (Manganese)	Moderate (deoxidation, strength)	Moderate (may be slightly higher or tightly controlled for yield)	Increases hardenability and strength; helps offset low C for strength.
Si (Silicon)	Low (deoxidizer)	Low (deoxidizer)	Deoxidation agent; small effect on strength.
P (Phosphorus)	Trace (kept low)	Trace (kept low)	Impurity—excess reduces toughness, particularly at low temp.
S (Sulfur)	Trace (kept low)	Trace (kept low)	Impurity—reduces ductility and machinability; Mn-S combinations affect sulfide morphology.
Cr, Ni, Mo, V, Nb, Ti, B	Typically trace / not intentionally added	Typically trace / may include microalloying in some mills	When added intentionally, these control hardenability, grain refinement, and strength (microalloying). Not typical in basic Q235/Q255 unless specified.
N (Nitrogen)	Trace	Trace	Can combine with Al, Ti, Nb to form nitrides; affects toughness and aging.

How alloying affects behavior: - Raising C or increasing alloying (Cr, Mo, V) increases strength and hardenability but reduces weldability and toughness if not compensated by processing. - Mn is the main intentional alloying element in these grades; it balances strength with formability. - Microalloying (V, Nb, Ti) can permit higher yield at lower carbon, improving strength without as much loss of weldability—if present, it should be declared on mill documentation.

3. Microstructure and Heat Treatment Response

Typical as-rolled microstructures for both grades: - Ferrite–pearlite microstructures dominate as-rolled and normalized product forms for carbon structural steels. - Q255, due to its higher yield objective, may show slightly greater pearlite fraction or finer ferrite grain size via controlled rolling or microalloying, but the base microstructures remain ferrite + pearlite in normal commercial processes.

Effect of common processing routes: - Normalizing: refines grain and can produce more uniform mechanical properties; used when better toughness is required. - Quenching & tempering: not typical for commodity Q235/Q255; produces martensitic or bainitic microstructures with much higher strength but is outside normal designation scope. - Thermo-mechanical controlled processing (TMCP): when applied, yields finer grain size and improved strength–toughness combinations while keeping carbon low—this is a common route to raise yield without excessive C.

Implications: - For routine fabrication, both grades are processed to provide predictable ductile ferrite–pearlite behavior. If higher strength is required while maintaining weldability, seek TMCP or microalloyed versions rather than simply increasing carbon.

4. Mechanical Properties

The key guaranteed mechanical distinction is yield strength.

Property	Q235 (typical)	Q255 (typical)	Notes
Nominal Yield Strength (minimum)	235 MPa	255 MPa	These nominal values are the design yield points implied by the grade name.
Tensile Strength	Moderate; depends on product form	Slightly higher or similar; depends on product form	Final tensile depends on thickness, rolling, and heat treatment.
Elongation (ductility)	Good ductility for forming and welding	Comparable but may be slightly lower if higher-strength processing used	Ductility is conditional on chemistry and processing; low carbon improves ductility.
Impact Toughness	Good with appropriate processing	Comparable if processed for toughness; may be more conservative at low temps	Charpy tests depend on product and heat treatment.
Hardness	Typical structural steel hardness	Slightly higher in higher-yield product	Hardness correlates with tensile properties.

Explanation: - Q255 is stronger by the design yield criterion; depending on how the steel is produced, Q255 can achieve the higher yield by microalloying and rolling control rather than by raising carbon substantially. When carbon is kept low and microalloying/TMCP used, toughness and weldability can remain acceptable. - Actual toughness and ductility are determined more by processing history and impurities than by grade alone.

5. Weldability

Weldability of carbon steels is strongly controlled by carbon equivalence and local hardenability.

Common carbon-equivalence formula (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

Another index used in Europe: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Lower carbon equivalence (CE) indicates lower risk of cold cracking and reduced preheat/postheat requirements. - Q235, typically produced with low carbon, generally presents excellent weldability for routine welding processes (SMAW, GMAW, FCAW). - Q255, having a higher yield target, can be produced by either slightly increasing carbon or by other strategies (Mn control, microalloying, TMCP). If the supplier achieves higher yield with microalloying/TMCP and keeps carbon low, weldability remains good. If higher carbon is used to reach the yield, CE increases and preheat/postheat and qualified welding procedures become more critical. - Always request CE or Pcm values on the mill certificate and follow applicable welding procedure specifications (WPS). For critical welded structures, perform PWHT recommendations and hydrogen control as needed.

6. Corrosion and Surface Protection

• These grades are plain carbon steels (non-stainless); corrosion resistance is limited to that of unalloyed carbon steels.
• Typical protection strategies:
• Hot-dip galvanizing for atmospheric corrosion resistance.
• Organic coatings (primers, paints, powder coatings) for engineered systems.
• Metallurgical coatings (zinc-rich primers, epoxy overlays) depending on exposure.
• PREN (pitting resistance equivalent number) is a stainless-steel index: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Not applicable to Q235/Q255 because they do not contain sufficient Cr, Mo, or N to be stainless.
• If corrosion resistance is required beyond coated carbon steel, specify stainless or corrosion-resistant alloys rather than relying on Q235/Q255.

7. Fabrication, Machinability, and Formability

• Forming/bending: Q235 has very good formability due to lower yield; Q255 can be formed but may require larger bend radii or more force depending on product and temper.
• Cutting: Same practices apply; oxygen cutting, laser, plasma cutting are routine. Harder or higher-strength versions could induce more tool wear.
• Machinability: Low-carbon steels have moderate machinability; sulfide inclusions or free-machining variants (not standard for Q235/Q255) improve machinability but may reduce impact properties.
• Surface finish and post-processing: Both accept welding, drilling, threading, and standard surface treatments well; post-weld heat treatment is rarely required for typical structural use unless specified.

8. Typical Applications

Q235 — Typical Uses	Q255 — Typical Uses
General structural components (beams, channels, columns)	Structural members where higher yield allows weight or section reduction
Welded fabrications (frames, racks, enclosures)	Heavy frames, cranes, hoisting components with higher design stresses
Plates and sheets for general manufacture, low-load tanks	Applications where modest increase in yield improves margin without changing material class
Pipes and profiles for non-critical service	Machinery parts where slightly higher yield improves life or stiffness

Selection rationale: - Choose Q235 for broad availability, excellent weldability, and lowest material cost for conventional structural components. - Choose Q255 when project requirements specify a higher minimum yield so that section size, weight, or deflection can be reduced while retaining similar manufacturing practices. Confirm whether the supplier achieves higher yield via microalloying/TMCP rather than higher carbon.

9. Cost and Availability

• Cost: Q255 typically commands a modest premium versus Q235 due to higher property requirements or additional processing (TMCP, microalloying). The premium varies by region, mill, and market conditions.
• Availability: Q235 is very common and widely stocked in many product forms. Q255 is less ubiquitous but commonly available from major mills; availability depends on the product form (plate, coil, bar) and regional production.
• Procurement tip: Specify mechanical property certificates and chemistry limits; if tight supply exists, consider qualifying alternate suppliers or accepting equivalent HSLA grades with similar guaranteed properties.

10. Summary and Recommendation

Attribute	Q235	Q255
Weldability	Excellent (low C, low CE typical)	Good to fair (depends on route to higher yield; microalloying/TMCP = good)
Strength–Toughness balance	Standard structural balance	Higher yield; balance depends on processing
Cost	Lower (widely produced)	Slightly higher (higher-yield requirement or processing)

Recommendations: - Choose Q235 if you prioritize maximum weldability, ease of fabrication, lowest material cost, and standard structural performance where a 235 MPa yield meets design requirements. - Choose Q255 if the design requires a higher minimum yield (255 MPa) to reduce section sizes or increase load capacity, and you have verified that the supplier’s chemistry and processing achieve this yield without excessive carbon that would compromise weldability or toughness.

Final procurement guidance: - Always request mill test certificates (chemical composition and mechanical tests), carbon-equivalent values, and details on any microalloying or TMCP processing. - For welded assemblies in critical service, specify required preheat/postheat, hydrogen control, and perform joint qualification using the actual plate/section supplier product. - When corrosion resistance, higher temperature service, or very high toughness is needed, consider alternative steel grades or alloy selections rather than relying solely on Q235/Q255 substitutions.