pipe

Piping is the formation of a cavity or series of cavities inside a solidifying metal ingot, usually running along the centerline from top to bottom. As the outer surfaces of the ingot cool and freeze, the interior remains molten longer. The liquid metal shrinks as it cools, but the solid shell cannot collapse inward uniformly, creating a void that can measure anywhere from a hairline crack to a cavity several inches across, depending on ingot size, pouring temperature, and cooling rate.

The defect is most pronounced in large steel ingots, particularly those cast in big open-top molds without protective covers or controlled cooling. Carbon steel ingots are especially susceptible. The shrinkage occurs because molten steel contracts roughly 8 to 10 percent by volume as it solidifies, and the metal at the center cools last, pulling away from the already-solid outer layers. Heavy section ingots cool so slowly that this differential shrinkage becomes severe.

Managing and detecting pipe

Steelmakers combat piping through hotter ingot molds, slower cooling rates, or the addition of exothermic compounds to the top of the ingot to maintain internal temperature. Modern practice often uses hot tops, open-mold casting with ceramic sleeves, or controlled soaking pit cycles. The cavity itself is typically removed during the primary breakdown of the ingot in a blooming mill or forge, where the ingot is compressed and reheated to fuse any remaining voids before rolling or forging to final product.

Ultrasonic or radiographic inspection can reveal piping before the ingot enters hot processing. If undetected and not healed during breakdown, residual pipe defects may persist as internal cracks or weak points in the finished steel. This is why critical applications such as pressure vessels or forged shafts require ingots cast from vacuum-refined metal or other premium practices that reduce the severity of piping.