Home > Glossary > Glossary: U
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United Nations (UN) – See “DOT” ID Numbers.
Unstable Reactive – The ability of a chemical in the pure state, or as produced or transported, to vigorously polymerize, decompose, condense, or become self-reactive under conditions of shock, pressure or temperature.
Upstream Pressure – See “Inlet Pressure.”
Undercut - A groove melted into the base metal usually along the toes of a weld. Undercut can also occur on either side of the first pass of a full penetration weld, such as an open groove butt weld. Undercutting produces a weak spot in the weld, if it exceeds the acceptance criteria for undercut it is considered a defect, and must be repaired. GTAW is an excellent process used for dressing this type of defect.
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