Welding joint preparation for thick plates (> 12.5 mm): For arc welding of plates with thickness greater than 12.5 mm, how should the plate edges be prepared to ensure full penetration and sound fusion?

Introduction / Context:
Edge preparation is a critical step in welding thick plates. For plate thicknesses above about 12.5 mm, proper groove geometry is essential to achieve full penetration, limit distortion, and control heat input and weld metal volume. Understanding when to use single-V, U, or double-V/U grooves is a core skill in fabrication design and welding procedure qualification.

Given Data / Assumptions:

• Plate thickness > 12.5 mm.
• Objective: secure full penetration with stable weld quality.
• Common processes: SMAW, GMAW, SAW (principles apply generally).

Concept / Approach:
As thickness increases, a single-sided preparation demands a large groove angle, high heat input, and excessive weld metal. A double-V or double-U groove splits the joint across both faces. This halves the maximum root distance, balances angular distortion, reduces overall weld metal, and improves access for slag removal or interpass cleaning. U-grooves further reduce weld metal due to curved sides but are more expensive to machine; V-grooves are easier to prepare but consume more filler.

Step-by-Step Solution:
Identify the constraint: plate thickness is high, so square butt or single-V alone is inefficient.Select a joint that reduces weld volume and distortion: double-V or double-U.Apply on both sides to allow balanced welding and complete fusion from each face.Conclusion: use a double-V or double-U groove on both sides.

Verification / Alternative check:
Comparative weld volume calculations show double-V reduces deposited metal and heat input by roughly 30–40% versus single-V on very thick plates, while also reducing angular distortion.

Why Other Options Are Wrong:
Do not require bevelling: true only for thin plates, not thick sections.

Single-V/U on one side: workable but less efficient and more distortion-prone.

Double-V/U on one side is ill-posed; double preparations imply both faces.

Backing strip square butt still needs edge opening for thicknesses this large.

Common Pitfalls:
Ignoring distortion control; underestimating weld metal volume and time for single-V on thick plates; neglecting access for back-gouging or cleaning.

Final Answer:
should have a double-V or U-groove on both sides