Multipass Welding: Bead Sequence, Interpass Temperature and Distortion Control
What is Multipass Welding?
Multipass welding involves depositing two or more overlapping beads to fill a joint. It is required whenever:
- The material thickness cannot be penetrated in a single pass
- The groove geometry requires progressive filling
- Quality standards impose heat input control per pass
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Bead Sequence
The deposition sequence directly influences final distortion and mechanical properties of the joint. Three main strategies exist:
1. Progressive Sequence (block by block)
Each pass is deposited from one end to the other before starting the next. Simple, but causes greater accumulated distortion in long joints.
2. Backstep Sequence
Beads are divided into short segments and executed in the direction opposite to overall progression. Reduces longitudinal distortion in long workpieces.
3. Cascade and Pyramid Sequence
Overlapping beads in a stepped arrangement, always starting at the centre or alternating symmetrically. Recommended for deep V and X grooves.
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Interpass Temperature (Tp)
Interpass temperature is the workpiece temperature measured immediately before starting each subsequent pass. Its correct management is an essential variable for:
- High-strength steels: max Tp 250°C (see WPQR)
- Austenitic stainless steels: max Tp 150°C to limit sensitisation
- Carbon steels: typical Tp 200–350°C
Measuring Interpass Temperature
| Method | Advantage | Limitation |
|---|---|---|
| Temperature crayon (Tempilstik) | Simple and economical | ±15°C accuracy |
| Contact thermometer | Direct reading | Requires clean surface |
| Infrared pyrometer | Non-contact | Variable emissivity |
Measurement zone: 75 mm from the fusion zone, on the base material surface (EN ISO 13916).
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Heat Input per Pass
Heat input (Q) controls the thermal cycle of the HAZ:
Q = (U × I × 60) / (v × 1000) × k [kJ/mm]
Where: U = voltage (V), I = current (A), v = travel speed (mm/min), k = efficiency factor
| Process | Factor k |
|---|---|
| MIG/MAG (135) | 0.80 |
| SMAW (111) | 0.80 |
| TIG (141) | 0.60 |
| SAW (121) | 1.00 |
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Distortion Control in Multipass
Preventive Techniques
- Angular pre-setting: tilt the workpiece against the expected distortion direction (e.g., 3–5° for T-joints)
- Mechanical restraint: clamps and jigs during welding
- Symmetrical sequence: alternate sides on double-sided grooves (X, K) — deposit equivalent amounts on each side
- Heat input control: lower Q → smaller HAZ → less distortion
Corrective Techniques
- Cold straightening: presses or hammers (ductile steels, thin plates)
- Flame straightening: localised heating with a torch (curves, flanges)
- Avoid both techniques on high-strength steels without designer approval
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