The casing is a special pipe in the completion of oil and gas wells. In the drilling and production process, it plays a role in protecting the wellbore, reinforcing the wall of the well, isolating the oil, gas, water layer in the wellbore and sealing various complex formations. The casing is divided into three types according to its function: surface casing, technical casing and oil casing. Due to the different steel grades of the bushings, the manufacturers have developed their own special steel grades to meet the special needs of users. The casing has a variety of wall thicknesses depending on the needs of the completion. The connecting thread of the casing is usually round threaded, but some manufacturers have developed some special properties to improve the connection thread, such as improving the tensile strength, torsion and bending resistance of the thread, preventing stress corrosion cracking performance, and hermetic sealing performance. Some special threads, such as trapezoidal thread, TM thread, 3SB thread, etc.