Understanding Weld Undercut and How It Can Be Prevented

14 May 2018

If you look up weld undercuts in a technical handbook, there’s a description of the phenomenon, but it’s not a user-friendly explanation. In essence, this weld imperfection is characterized by a sharp recess, a groove that interrupts the joint’s smooth profile. That groove forms at the point where the base metal meets the weld material. Here’s the welding inspector now, on-site to study the problem.

Identifying Weld Undercuts 

The weld dome is smooth. On the edge of that cooling joint, some of the base structural metal has melted away. A groove has formed. The weld zone has been weakened by the loss of toe metal. By the way, the toe of the weld is defined as the area where the weld contacts the base metal. Anyway, if that critically important fusion site is unfilled, then the weld’s structural strength is suspect. Now it’s up to the welding inspector to assess the depth and width of this groove. Does that weld profile fall within acceptable design tolerances?

Understanding the Causative Factors 

Through training and an instinctive grasp of the situation, a welder regulates the equipment. The heat affected zone forms correctly, the hand of the craftsman travels forward, and the weld dome forms properly. What if the travel speed is exceeded? The welder is applying too much heat. The base plate melts, the weld toe parts, and that undesirable groove pops into existence. The welding inspector is measuring the groove. Is he about to report the welder? Possibly, but equipment failure is also a likely culprit here, with electrode damage or equipment current fluctuations causing profile discontinuities.

Preventing Weld Undercuts 

The inspector has assessed the underfilled weld toe. It exceeds the allowable undercut, as specified by the relevant weld codes. In hindsight, the equipment can be repaired and the operator can be retrained to eliminate such work practices. However, hindsight doesn’t help the present defect. Reduce the depth of the grove by grinding its edges, then apply another layer of weld metal. If the problem recurs, perhaps under the hands of a different technician, take the equipment out of service. Maintain the gear, adjust its output, and advise the use of a more suitable electrode.

At the end of the day, operator gaffes and equipment defects are unacceptable. They happen, of course, which is why weld inspectors are always diligently on the move. Just reduce that weld travel speed, turn down the equipment current, and really buckle down after a weld undercut defect has been caught. Found often in fillet and butt welds, this joint discontinuity issue must be eliminated, for these grooves will undermine the structural strength of an otherwise properly applied weld.

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