1. Introduction
Q235 and Q345 are the two most widely used structural steels in China and throughout Asia. Together they account for more than 70 % of all steel consumed in construction, bridges, pressure vessels, machinery, and general fabrication.
This article provides a single, ready-to-use reference that compares:
- Q235A/B/C/D (GB/T 700-2006)
- Q345A/B/C/D/E (GB/T 1591-2018) and their direct international equivalents:
- ASTM A36 / A572 Gr.50
- EN S235JR / S355JR / S355J2
- JIS SS400 / SM490A
All welding recommendations are based on Chinese national standards, AWS D1.1, EN 1011-2, and real-world Asian fabrication practice.
2. Chemical Composition and Carbon Equivalent (CEV / PCM)
| Steel | C max | Si max | Mn max | P max | S max | CEV typ. (IIW) | Typical PCM |
|---|
| Q235A | 0.22 | 0.35 | 1.40 | 0.045 | 0.050 | ≤0.38 | ≤0.22 |
| Q235B/C/D 0.20 | 0.35 | 1.40 | 0.035–0.040 | 0.030–0.040 | ≤0.37 | ≤0.20 |
|
| Q345A/B 0.20 | 0.50 | 1.70 | 0.035 | 0.035 | ≤0.44 | ≤0.24 |
|
| Q345C/D/E 0.18 | 0.50 | 1.70 | 0.030 | 0.030 | ≤0.42 | ≤0.22 |
|
→ Q235 has excellent weldability similar to ASTM A36 / S235JR (no preheat required in most cases).
→ Q345 is comparable to S355JR / A572 Gr.50 (preheat recommended above certain thicknesses).
3. International Grade Equivalents (for specification writing)
| China (GB) | USA (ASTM) | Europe (EN) | Japan (JIS) | Korea (KS) |
|---|
| Q235A | A36, A283 Gr.C | S235JR | SS400 | SS400 |
| Q235B | A36 | S235J0 / S235J2 | SS400 | SS400 |
| Q345B | A572 Gr.50 | S355JR | SM490A | SM490A |
| Q345D | A572 Gr.50 | S355J2 | SM490YA | SM490YA |
4. Recommended Welding Processes and Filler Metals
| Steel | Preferred processes (in order of popularity in China) | AWS / EN filler classification | Chinese common electrodes |
|---|
| Q235 | SMAW → GMAW/MIG → FCAW → SAW | E7018, ER70S-6, E71T-1 | J422 (E4303), J426 (E4316), CHE427 (E5015) |
| Q345 | SMAW → GMAW/MIG → FCAW → SAW | E8018-C3, ER80S-G, E81T1-Ni1 | CHE507 (E5015), CHE557 (E5515), CHE607 |
5. Preheat and Interpass Temperature Requirements (EN 1011-2 & Chinese practice)
| Combined thickness (mm) | Q235 (all grades) | Q345A/B | Q345C/D/E (CEV ≤0.42) |
|---|
| ≤20 | None | None | None |
| 20–40 | None | 50–100°C | 80–120°C |
| 40–70 | 50°C (optional) | 100–150°C | 100–150°C |
| >70 | 75–100°C | 120–175°C | 120–175°C |
Maximum interpass temperature: 250 °C for both steels (to avoid toughness loss).
6. Post-Weld Heat Treatment (PWHT)
| Condition | Q235 | Q345 |
|---|
|
|
|
| Normal fabrication (t ≤ 50 mm) | Not required | Usually not required |
| Heavy sections or restraint | Stress relief 550–620 °C optional | 550–620 °C × 1 h/inch recommended |
| Pressure vessels (GB 150 / ASME) | Follow code | 580–620 °C mandatory in many cases |
7. Typical Welding Parameters (GMAW solid wire ER70S-6 / ER80S-G)
| Steel | Wire Ø (mm) | Current (A) | Voltage (V) | Heat input (kJ/mm) |
|---|
| Q235 | 1.2 | 220–300 | 26–32 | 0.8–1.6 |
| Q345 | 1.2 | 240–320 | 27–34 | 1.0–2.0 |
8. Common Defects and Prevention
| Defect | Main cause with Q235/Q345 | Prevention |
|---|
| Cold cracking | Hydrogen + restraint (Q345) | Low-hydrogen consumables, preheat |
| Lack of fusion | Low heat input, thick plates | Minimum 1.0 kJ/mm, proper weaving |
| Excessive distortion | High heat input on thin Q235 | Skip welding, back-step sequence |
| Lamellar tearing | Through-thickness loading on heavy plate | Use Z-grade steel (Q345D-Z35) if required |
9. Which steel to choose?
| Requirement | Choose |
|---|
| General fabrication, cost critical | Q235B (equivalent to A36/SS400) |
| Higher strength, moderate thickness | Q345B (equivalent to S355JR) |
| Low-temperature service (≤ –20 °C) | Q235D or Q345D/E |
| Offshore or seismic structures | Q345D-Z25/Z35 or Q355J2 |
