The chemical composition of casing varies widely, and a variety of compositions and treatment processes are used during the manufacturing process This means that the physical properties of the steel varies widely. The materials which result from the manufacturing process have been classified by the API into a series of “grades” (Table 3). Each grade is designated by a letter, and a number. The letter refers to the chemical composition of the material and the number refers to the minimum yield strength of the material e.g. N-80 casing has a minimum yield strength of 80000 psi and K-55 has a minimum yield strength of 55000 psi. Hence the grade of the casing provides an indication of the strength of the casing. The higher the grade, the higher the strength of the casing.
In addition to the API grades, certain manufacturers produce their own grades of material. Both seamless and welded tubulars are used as casing although seamless casing is the most common type of casing and only H and J grades are welded.