Austenitic stainless steel HK40 is heat-resistant steel, which is used at high temperatures, used mainly in the form of castings poured by gravity into tubes used in chemical process equipment, metallurgical, petrochemical, steel therefore must resist this oxidation, carburization and sulfatation. One problem in this type of steel is the formation of brittle phases, when subjected to a temperature of 540–900 °C. The aim of this study is to determine the possible presence of M7C3 type carbide, M23C6. The tube has an outside diameter analyzed 4 inches (10.16 cm) and a wall thickness of 0.5 inches (1.27 cm). The samples used for this investigation were cut 5 specimens measures 12.5 mm in diameter and 2.25 mm in length, and underwent a heat treatment at a temperature of 680 °C for 15, 30, 45, 60 and 75 minutes with subsequent cooling in water. The samples were characterized by electron microscopy. This study was performed because the trend in the Petrochemical Industry is to reduce the wall thickness of the tubes to improve heat dissipation and reduce the residual stresses generated in the steel.