Root Concavity in Pipe Welding: Causes and Prevention
A pipe weld can look consistent on the outside while an uneven root remains inside. One possible imperfection is root concavity: a recessed area along the internal weld bead that requires assessment before the joint is accepted. root concavity in pipe welding
Preventing root concavity in pipe welding starts with consistent joint preparation, controlled welding parameters, and appropriate backing gas pressure. Understanding when the depression develops also helps identify the right corrective action.
What Is Root Concavity in Pipe Welding?
Root concavity is a shallow depression in the root surface of a butt weld. Instead of forming the intended internal profile, part of the weld root sits below the adjacent pipe surface.
It is sometimes called suck-back. Although commonly associated with TIG welding, the term describes the weld’s geometry rather than a particular welding process.
Root concavity and incomplete penetration are different imperfections. Concavity concerns the root profile; incomplete penetration means the weld has not penetrated to the required depth. Inspection must establish which condition is present.
What Causes Root Concavity?
Weld Metal Shrinkage
As molten weld metal solidifies, it contracts. Shrinkage through the weld thickness can leave a depression at the root.
Incorrect Root Gap
Poor joint preparation can leave the opening too narrow or too wide for the welding procedure. Either condition can make root bead control more difficult.
Excessive Travel Speed
Moving too quickly can increase the likelihood of root concavity. Travel speed must remain appropriate for the joint and the specified welding parameters.
Excessive Back Purge Pressure
During welding with internal gas purging, excessive pressure can push against the molten root. Restricted venting deserves particular attention as the joint closes.
Root Remelting During the Next Pass
The second pass can remelt the existing root bead and change its profile. A root that appears satisfactory initially may therefore become concave during subsequent welding.
How to Prevent Root Concavity
root concavity in pipe welding Follow the Welding Procedure
Use the approved welding procedure specification, or WPS, for joint preparation and welding settings. Avoid applying a universal current, root gap, or travel speed to pipes with different materials and wall thicknesses.
root concavity in pipe welding Check Fit-Up Before Welding
Measure the root opening and check its consistency around the joint. Confirm that the root face, bevel, and alignment meet the procedure requirements before starting.
root concavity in pipe welding Control Purge Pressure
Provide the specified gas outlet and keep it unobstructed. Treat purge flow and internal pressure as separate considerations: increasing flow does not automatically improve root protection.
root concavity in pipe welding Maintain Consistent Root Pass Technique
Keep travel speed and filler addition consistent with the procedure. When the pool becomes difficult to control, check the setup before making several parameter changes at once.
root concavity in pipe welding Review the Following Pass
If concavity develops after the next pass, investigate root remelting. Review that pass’s settings and technique rather than assuming the original root pass caused the problem.
A Practical Troubleshooting Sequence
To narrow down the cause, establish when and where the concavity appears:
Before welding: Record fit-up measurements and purge arrangements.
After the root pass: Inspect accessible internal surfaces.
After the following pass: Check whether the root profile has changed.
When the problem repeats: Compare its location with fit-up variations, welding position, and recorded settings.
This sequence helps separate preparation problems from conditions introduced during welding.
Is Root Concavity Always Rejectable?
Acceptance depends on the applicable code and project specification. A visible depression alone does not establish whether the weld must be rejected.
Required fusion, remaining weld thickness, and the permitted profile must be assessed against the governing requirements. Avoid using a single depth limit for every piping application.