Features of the structure of steels of various compositions for the manufacture of pipes transporting hydrogen sulfide-containing natural gas

Abstract

The paper presents the results of studies of the structural features of plate rolled products of low-carbon pipe steels 04KhNDB, 05KhGB and 06GNFB differing in the basic composition and content of microalloying elements. In the production of industrial rolled products for each steel under study, the choice of rolling temperatures was made individually, but in the same way from the point of view of structure formation. The completion of the finishing stage of rolling was carried out in the austenite region near the temperature of the beginning of the phase transformation, the beginning of cooling - from the austenite region, in order to form a homogeneous structure and exclude the formation of structural banding. The steels have a similar microstructure after rolling, but the structure of the axial zone, which is basically the weak point in tests in hydrogen sulfide, is different. The microstructure of steel 04KhNDB is almost uniform in thickness and a slight segregation of manganese has little effect on the hardness of the axial zone. In steels 05KhGB and 06GNFB, segregation of manganese and an increase in the proportion of intermediate transformation products in the axial zone leads to the formation of elongated regions with increased hardness. This is most pronounced in steel 06GNFB, in which cracks were revealed after testing for hydrogen cracking in accordance with the NACE TM0284 method. It has been established that with a homogeneous microstructure in the axial zone of rolled products without areas of increased hardness and in the absence of manganese sulfides in it, large inclusions of titanium and niobium carbonitrides do not adversely affect the resistance to hydrogen cracking.