September 11, 2007
Q: I was recently involved in a discussion about a joint discontinuity. We were trying to determine whether it could be considered undercut or underfill. The weld did not fill the butt joint for the entire V-groove length. Some guys classified the melted-away edge of the joint as undercut; I classified the defect as underfill. Since this joint was never completely filled, can there be undercut?
A: It's great that you use all the right AWS terms when talkin' weldin'. So how do we differentiate undercut from underfill? In a nutshell, undercut occurs in base metal adjacent to the weld, and underfill occurs in the weld metal. Although undercut can occur on both grooves and fillets, underfill will occur only on grooves. Fillets would be considered undersize. It's important to differentiate between the two because your acceptance criteria for each probably is different.
Welding has its own language, and it is important to use that language correctly to prevent confusion among inspectors, welders, engineers, and customers. Welding language is well-laid-out in publication AWS-A3.0, "Standard Welding Terms and Definitions." It's good for all shops to keep a copy available and refer to it often.
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