7305.20 - Casing of a kind used in drilling for oil or gas:

This HTS category, 7305.20, specifically covers casing of a kind used in drilling for oil or gas. These are tubular products made of iron or steel with a circular cross-section, and importantly, their external diameter exceeds 406.4 mm. The defining characteristic for inclusion in this sub-heading is their direct application in the extraction of oil and natural gas, distinguishing them from general-purpose pipes.

Unlike sibling categories such as "Line pipe of a kind used for oil or gas pipelines" (7305.10) or "Other, welded" (7305.31), this sub-heading is not defined by its application in transportation pipelines or by its manufacturing method alone. Instead, the critical differentiator is the *function* of casing: its role in supporting the walls of an oil or gas well during the drilling process and, subsequently, providing a conduit for extraction. While casing is also a type of tube or pipe over 406.4 mm in diameter, its specific use in the oil and gas well construction industry is paramount for classification here.

Under this category, further subdivisions exist to differentiate based on material composition. These subcategories include "Of iron or nonalloy steel" and "Of alloy steel." This distinction is crucial as it allows for the accurate classification based on the specific metallurgical properties of the steel used, which can impact factors such as strength, corrosion resistance, and suitability for demanding downhole environments.