용접하면 왜 뒤틀릴까? 변형 최소화 설계·시공 전략

1. Three Types of Weld Distortion

• Angular distortion: Fillet-welded members pulling toward each other; butt-welded plates bowing.
• Longitudinal shrinkage: Overall length reduction along the weld direction.
• Transverse shrinkage: Gap closure perpendicular to the weld.

2. Pre-distortion (Presetting) Technique

When distortion is predictable, pre-angle the joint in the opposite direction so it returns to position after welding.

• Example: For T-joint fillet welds where vertical plate pulls inward, clamp it tilted 3–5° outward before welding.
• Experience-based: weld sample specimens first, measure distortion, then set pre-distortion accordingly.

3. Weld Sequence Design

• Symmetric sequence: Alternate from center outward on both sides. Welding one side only pulls the structure toward it.
• Backstep welding: Deposit short beads opposite to overall travel direction — shrinkage cancels out.
• Avoid asymmetric welds: Designs with welding concentrated on one side will always distort. Convert to two-sided welds where possible.

4. Jigs and Clamps

• Rigidly constraining members in welding fixtures suppresses distortion — but increases residual stress. Plan accordingly.
• Strongback: Temporarily weld reinforcing members across joints to prevent warping; remove after cooling.

5. Minimize Heat Input

• Reduce number of passes and minimize heat per pass (lower current, faster travel).
• Preheat prevents cracking — it does NOT reduce distortion. In fact, more thermal expansion means potentially more distortion.