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API Casing Coupling:Couplings are always applicated in casing pipes and oil tubings as a kind of connection. There are different steel grade for couplings, which are identified by various colors as following: J55-GreenP110-WhiteC75-Blue C90-Brown N80-RedSpecificationSize (Pipe OD) (in)4-1/2, 5, 5-1/2, 6-5/8, 7, 7-5/8, 8-5/8, 9-5/8, 10-3/4, 11-3/4, 13-3/8, 16, 18-5/8, 20ThreadingShort Round-Thread Casing (STC)Long Round-Thread Casing (LC)Buttress Thread Casing (BC)GradeH-40, J-55, K-55, C-75, L-80, N-80, C-95, AS-95, P-110, L80-13CR
