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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)

Acier	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

Acier	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)

Acier	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	La prévention
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?

Exigence	Choisissez
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