Steel Grade
The steel grade of the casing relates to the tensile strength of the steel from which the casing is made. The steel grade is expressed as a code number which consists of a letter and a number, such as X-80. The letter is arbitrarily selected
to provide a unique designation for each grade of casing. The number designates the minimal yield strength of the steel in thousands of psi. Strengths of API steel grades are given in Table 1.6.
Hardness of the steel pipe is a critical property especially when used in H2S (sour) environments. The L-grade pipe has the same yield strength as the X-grade, but the N-grade pipe may exceed 22 Rockwell hardness and is, therefore, not suitable for H2S service. For sour service, the L-grade pipe with a hardness of 22 or less, or the С-grade pipe can be used.
Many non-API grades of pipes are available and widely used in the drilling industry. The strengths of some commonly used non-API grades are presented in Table 1.7. These steel grades are used for special applications that require very high tensile strength, special collapse resistance or other properties that make steel more resistant to H2S.
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Table 1.6: Strengths of API steel grades. (API Spec. 5CT, 1992.)
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* Elongation in 2 inches, minimum per cent for a test specimen with an area > 0.75 in2.