Q345 vs Q355 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Q345 and Q355 are two widely used Chinese-designated structural steels frequently considered in design, fabrication, and procurement for buildings, bridges, pressure parts, and heavy equipment. Engineers routinely balance factors such as strength versus weldability, cost versus margin of safety, and toughness at low temperatures when selecting between these grades.
The central practical difference is that Q355 specifies a higher nominal yield strength than Q345 while remaining within the same family of low-alloy, high-strength structural steels. Because the two grades share similar chemistry and processing routes, the selection is often driven by required yield capacity, thickness/section constraints, impact performance, and cost rather than by radically different material behavior.
1. Standards and Designations
- Primary Chinese standard: GB/T 1591 (high-strength low-alloy structural steels), which includes Q345 and Q355 series.
- Other related standards and cross-reference families (availability and exact equivalence vary by region and application): EN (e.g., S355 family), ASTM/ASME structural grades, JIS; note that direct equivalence between GB and EN/ASTM is not exact and must be validated for each application.
- Classification: both Q345 and Q355 are HSLA (high-strength low-alloy) carbon/microalloyed structural steels (not stainless, not tool steels, not high-alloy steels).
2. Chemical Composition and Alloying Strategy
The Q-series steels are designed to achieve higher yield strength through controlled carbon content combined with microalloying and thermo-mechanical control of the microstructure. Exact limits vary by subgrade (e.g., Q345A/B/C/D/E variants); the table below gives typical composition ranges used for engineering selection and comparison. Always consult the applicable certificate and standard for precise limits.
| Element | Typical Q345 (wt%) | Typical Q355 (wt%) |
|---|---|---|
| C | 0.12 – 0.20 | 0.10 – 0.20 |
| Mn | 0.80 – 1.60 | 0.80 – 1.60 |
| Si | 0.20 – 0.50 | 0.20 – 0.50 |
| P | ≤ 0.035 | ≤ 0.035 |
| S | ≤ 0.035 | ≤ 0.035 |
| Cr | ≤ 0.30 (if present) | ≤ 0.30 (if present) |
| Ni | ≤ 0.30 (if present) | ≤ 0.30 (if present) |
| Mo | ≤ 0.08 (occasional) | ≤ 0.08 (occasional) |
| V | trace (microalloying) | trace (microalloying) |
| Nb | trace (microalloying) | trace (microalloying) |
| Ti | trace (microalloying) | trace (microalloying) |
| B | trace (rare) | trace (rare) |
| N | controlled (low) | controlled (low) |
How alloying affects properties: - Carbon and manganese primarily control strength through solid-solution strengthening and influence hardenability. - Microalloying elements (Nb, V, Ti) promote grain refinement and precipitation strengthening, improving yield strength without large increases in carbon (which preserves weldability). - Low sulfur and phosphorus are specified to preserve toughness and weld quality.
3. Microstructure and Heat Treatment Response
Typical microstructures: - As-rolled or normalized plates of Q345 and Q355 generally show a ferrite–pearlite matrix with refined grains due to microalloying and controlled rolling. Q355 grades, targeting higher yield, may show slightly greater dislocation density and stronger precipitation/hardening effects from controlled thermo-mechanical processing. - Neither grade is primarily supplied as quenched and tempered steels; they are intended as structural steels suitable for fabrication in the as-rolled or normalized condition.
Heat treatment response: - Normalizing: both grades respond to normalizing with grain refinement and modest increases in strength and toughness. Normalizing can be used to homogenize structure for heavy sections. - Quenching and tempering: possible but less common for these steels; Q345/Q355 can be hardened to higher strength levels, but trade-offs in toughness and distortion and need for stricter composition control make Q&T rare for standard structural supply. - Thermo-mechanical controlled processing (TMCP): modern production routes use TMCP to increase yield strength while maintaining toughness, a principal mechanism by which Q355 achieves higher guaranteed yield at similar chemistry.
4. Mechanical Properties
The following are representative typical property ranges as supplied in normalized/rolled condition — final properties depend on temper, thickness, and subgrade.
| Property | Typical Q345 | Typical Q355 |
|---|---|---|
| Specified Yield Strength (typical) | ~345 MPa (nominal target) | ~355 MPa (nominal target) |
| Tensile Strength (Rm) | ~470 – 630 MPa | ~490 – 640 MPa |
| Elongation (A5, % typical) | 20 – 26% (depending on thickness) | 18 – 25% (depending on thickness) |
| Charpy Impact (V-notch) | Specified for subgrades; typically 27 J @ specified temperature (varies) | Similar requirements; may be specified at equal or lower temperature |
| Hardness (HB) | ~120 – 190 HB (varies with heat treatment and thickness) | ~120 – 200 HB (slightly higher possible) |
Interpretation: - Q355 is specified with a higher minimum yield and often a slightly higher tensile envelope; the increase is modest but meaningful for structural design (allowing smaller sections or higher allowable stress). - Toughness (impact) is more a function of subgrade selection (A/B/C/D/E), thickness, and testing temperature than the nominal grade number. Proper subgrade selection is essential for low-temperature applications.
5. Weldability
Weldability is influenced by carbon equivalent and microalloying content. Two common empirical formulas used to assess weldability are:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
and
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Both Q345 and Q355 are formulated to keep carbon content and carbon equivalents relatively low. Microalloying (Nb, V, Ti) provides strength without large increases in carbon, which helps to preserve weldability. - Q355 may have slightly higher hardenability in certain subgrades due to processing and marginally different chemistry; therefore preheat and interpass temperature control may be slightly more conservative for thicker sections of Q355 than for Q345. - For critical welds, follow procedure qualification (WPS/PQR), account for thickness, and use preheat/post-heat according to the calculated $CE_{IIW}$ or $P_{cm}$ and the applicable fabricator/standard guidance. - Hydrogen control, proper filler selection, and control of heat input are key to avoiding cold cracking in both grades.
6. Corrosion and Surface Protection
- Neither Q345 nor Q355 are stainless steels; corrosion resistance is that of low-alloy carbon steels and therefore relies on surface protection strategies.
- Typical protection methods: hot-dip galvanizing, zinc metallizing, organic coatings (paints, powder coating), cathodic protection, and effective design to avoid water traps.
- PREN (pitting resistance equivalent number),
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
is a stainless-steel index and is not applicable to Q345/Q355; these indices are only relevant to austenitic or duplex stainless steels that contain significant Cr, Mo, and N. - For applications exposed to aggressive environments (marine, chemical), choose corrosion-resistant alloys or apply robust coatings rather than relying on Q345/Q355 as-is.
7. Fabrication, Machinability, and Formability
- Cutting: both grades cut well with oxy-fuel, plasma, and laser processes; cutting parameters may be adjusted for thickness and heat input to minimize HAZ effects.
- Forming/bending: formability is good; lower carbon and microalloying help maintain ductility. Minimum bend radii depend on thickness and subgrade; Q355’s slightly higher yield may require marginally larger bend radii for the same forming process.
- Machinability: typical carbon steel machinability; Q355’s slightly higher strength can increase tool wear modestly. Choose cutting speeds and feeds based on tensile properties and section thickness.
- Surface finishing: both accept welding, grinding, and machining; care with residual stresses and distortion for tight-tolerance fabrications.
8. Typical Applications
| Q345 — Typical Uses | Q355 — Typical Uses |
|---|---|
| General structural steelwork: frames, supports, medium-load beams, columns | Heavier structural sections: high-load beams, crane rails, bridge components |
| Fabricated components where cost sensitivity is primary and yield 345 MPa is adequate | Applications where slightly higher yield allows section reduction or higher allowable stress |
| Machinery bases, secondary structural members | Primary load-bearing members, higher-stress welded assemblies |
| Pressure vessels or tanks (when allowed by code and with proper certification) | Structural components in cold regions where higher strength is specified (subject to impact subgrade) |
| Fencing, platforms, general fabrication | Heavy equipment, maritime structures (with coatings), high-stress frames |
Selection rationale: - Use Q345 when standard structural strength suffices and the priority is lower material cost and easier forming. - Use Q355 when designs require a higher guaranteed yield strength to reduce section size or meet higher design stresses, provided weldability and impact requirements are compatible.
9. Cost and Availability
- Cost: Q355 is typically slightly more expensive per mass than Q345 because of higher guaranteed mechanical properties and the processing required to meet Q355 requirements. The premium varies by market, mill, product form, and global steel prices.
- Availability: Both grades are widely produced and available in plate, coil, and section forms. Availability of specific subgrades, thicknesses, and certified test reports depends on mill capability and regional supply chains.
- Procurement tip: specify subgrade, impact temperature, and product form clearly in purchase orders to avoid mismatches and to ensure correct mill certificates.
10. Summary and Recommendation
| Attribute | Q345 | Q355 |
|---|---|---|
| Weldability | Good — broad ductile range; low CE when microalloyed | Good — slightly higher hardenability possible; may require conservative preheat for thick sections |
| Strength–Toughness balance | Moderate strength with good ductility/toughness | Higher yield for the same family; slight trade-off in ductility at same composition but compensated by TMCP |
| Cost | Lower (typically) | Higher (typically) |
Conclusion and guidance: - Choose Q345 if: your design requires a reliable, economical HSLA structural steel where nominal yield ~345 MPa meets design requirements; where forming and ease of fabrication are priorities; and where cost sensitivity and widespread availability are important. - Choose Q355 if: you need the modest but useful increase in guaranteed yield strength to reduce section sizes or increase allowable loads; when the project specification explicitly demands higher yield capability; and when the marginal additional cost is justified by structural or weight savings.
Final note: both Q345 and Q355 are part of the same structural-steel family; the right choice depends on design stress, thickness and welding conditions, toughness requirements, and total life-cycle cost. Always specify the exact subgrade, impact-performance temperature, and required mill test documentation, and validate welding procedures with $CE_{IIW}$ or $P_{cm}$–based assessments for critical fabrications.