api 5000API 6A flanges of class 5000 and below are often made of A105 carbon steel with a minimum yield strength of 45K, while API 6BX flanges of class 10000api 6a - type - 6b 34.5 mpa (5000 psi) gbt 22513-2013 API 6A-TYPE-6B 34.5MPA (5000PSI)