... 5CT material:J55,K55,N80 oil pipeAPI 5L Line Pipe Size: 4-1/2"~20" Steel grade: J55, K55, N80, L80 ... welded, which makes it stronger than ordinary welded pipe.1. Size: 4-1/2"~20"Steel grade: J55, K55, N80, L80 ... raw materials and examine products in every stage of the production:*Size and surface inspection*Non-destructive test*Thread inspection*Hydrostatic test ...