WELDING TEMPERATURE OF THE SPIRAL TUB

​

 The welding temperature of the spiral tube is a key process parameter that determines the strength, toughness, shape and crack resistance of the weld. It must be precisely controlled. Inappropriate temperature will directly result in fatal defects such as incomplete penetration, burn-through, cracks, and coarse grain structure.

1. Weld Pool Temperature (Core)

• Low-carbon steel (Q235B, etc.):1250–1460℃ (sufficient for penetration welding of 3–5mm wall thickness)

• High-strength steel (X70/X80):1300–1500℃ (requires matching preheating/interlayer temperature)
• Temperature Window:Must remain stable above the material's solidification line and below the overburning temperature, ensuring good fusion, no overburning, and no incomplete penetration

2. Preheating temperature (essential for thick-walled/low-temperature/high-strength steel)

• Carbon steel: 80–150℃ (increase to 100–150℃ when the ambient temperature is below 0℃)
• High-strength steel/thick-walled (＞12mm): 150–250℃
• Purpose: To reduce cooling rate, minimize hardened structure, and prevent cold cracking

3. Inter-layer temperature (key for multi-layer multi-pass welding)

• Carbon steel: ≤ 250℃, usually controlled within 80–150℃
• High-strength steel: 150–200℃ (strictly no more than 300℃, to avoid grain coarsening)
• Purpose: To ensure inter-layer fusion, control the microstructure of the heat-affected zone, and reduce residual stress

4. Post-weld slow cooling / heat treatment temperatures

• Hydrogen removal treatment: Hold at 250–350℃ for ≥ 2 hours (for high-strength steel / thick-walled components)
• Annealing: 600–700℃ (improve toughness, eliminate stress)

1. Excessive temperature (excessive heat input)

• Weld formation: Large molten pool, iron overflow → burn-through, weld bead, excessive height, surface depression
• Mechanical properties:Slow cooling → coarse grains, sudden drop in toughness/plasticity, reduced crack resistance
• Defect risks: Prone to hot cracks, overburning, intergranular oxidation, softening in the heat-affected zone

2. Insufficient temperature (insufficient heat input)

• Weld formation: Small molten pool, poor fluidity → incomplete penetration, incomplete fusion, slag inclusion, gas porosity
• Mechanical properties: Rapid cooling → hardened martensite, hardness surges, significantly increased risk of cold cracking
• Mechanical properties:Meets strength standards but has poor toughness, high risk of brittle fracture

3. Temperature fluctuations / unevenness

• Result in inconsistent weld width, uneven weld height, misalignment, stress concentration, and are prone to cause delayed cracks

1. High Frequency Induction Welding (Main Pipe Manufacturing Process)

• Core Adjustment: Excitation Frequency + Power

1. Frequency (100–400 kHz):Frequency ↑ → Skin Effect ↑ → Heating becomes more concentrated and temperature ↑
2. Power:Current/Voltage ↑ → Thermal Power ↑ → Temperature ↑
3. Adjustment Circuit: Change Capacitor C, Inductor L 

• Auxiliary Adjustment:Welding Speed

1. Speed ↓ → Heat Input per Unit Length ↑ → Temperature ↑; vice versa, temperature ↓

• Gap Control: Weld Seam Gap 1–3 mm; Excessive Gap → Insufficient Heat; Too Small → Overheating and Burn Damage

2. Submerged Arc Welding (Large Diameter/Thick Wall Pipes)

• Welding Current (Mostly Affects Penetration Depth)

1. Current ↑ → Penetration Depth ↑, Temperature ↑; vice versa ↓ (Internal Welding: 650–750A, External Welding: 700–800A)

• Welding Voltage (Affects Weld Width)

1. Voltage ↑ → Arc Length ↑, Weld Width ↑, Temperature Distribution More Dispersed; Use Matching Current

• Welding Speed (Affects Total Heat Input)

1. Speed ↑ → Heat Input ↓, Temperature ↓, Weld Width/Overhang ↓; Vice versa ↑
   Wire Feed Speed:

      Wire Feed ↑ → Overhang ↑, Relative Heat Input ↑

3. General auxiliary adjustment methods

• Preheating / Inter-layer temperature control

1. Flame / Electric heating preheating, with real-time monitoring by infrared thermometer
2. Multi-layer welding of thick-walled tubes: Inter-layer temperature not lower than preheating temperature, promptly remove welding slag

• Environmental compensation

1. Low temperature (<0℃): Current +10–15%, wind shelter, slow cooling after welding (wrapped with asbestos)

• Intelligent closed-loop control

1. Infrared temperature measurement → PLC feedback → Automatic adjustment of current/velocity, maintaining temperature within ±10℃

1. Set the baseline: Refer to the process specifications by material/steel thickness to determine the melting pool temperature, preheating/interlayer temperature.

2. Rough adjustment: Set the initial values for current/power, voltage, and speed, and conduct welding tests to observe the melting pool.

3. Fine adjustment:

• Small melting pool, incomplete penetration → Increase current/power, decrease speed
• Large melting pool, burn-through → Decrease current/power, increase speed
• Insufficient melting width → Increase voltage, make minor speed adjustments

4. Monitoring: Use infrared temperature measurement to monitor preheating/interlayer/welding seam cooling, ensuring it remains within the window.

5. Verification: Visual inspection + non-destructive testing (UT/RT), optimize parameters until they reach stable and qualified levels.

• Lack of penetration/undersized fusion: ↑ Current, ↓ Speed, Ensure groove cleanliness, Control gap
• Burn-through/Spatter: ↓ Current, ↑ Speed, Reduce groove bevel, Control wire feeding
• Cold cracks: ↑ Preheating/interlayer temperature, Post-weld decarburization, Reduce cooling rate
• Coarse grains, poor toughness: Control heat input, Avoid overheating, Reasonable interlayer temperature

​