Which of the following is not one of the four weld types?

• A. Surface
• B. Plug
• C. Fillet
• D. Lap

Answer: (D)

The correct response identifies that "Lap" is not considered one of the four primary weld types used in pipefitting. The four standard types of welds are typically categorized as: butt, fillet, groove, and plug.

A lap weld is a type often used in various applications, but it does not constitute one of the fundamental categories of welds like the others do. Instead, lap welding involves overlapping two pieces of material and welding along the edge, which is more specific and not as universally recognized as the other weld types listed.

The other options presented—fillet and plug—are indeed recognized welding types. A fillet weld is often used to join two surfaces at a right angle, while a plug weld involves filling a hole in one piece of metal with welding material from another piece. Understanding these definitions helps distinguish between the common welding types and why "lap" does not fit within the primary categories typically emphasized in pipefitting and welding training.