During the operation of the reaction furnace coils, carburizing processes take place, as a result of which the physicochemical properties of the metal change. This leads to a decrease in the operational characteristics of the pipe material and plasticity, the appearance of internal stresses, degra- dation of the structure, and, as a result, premature failure of the coils. An analysis of the publications showed that in austenitic steels of the coils of the reaction furnaces after long-term operation under conditions of high temperatures and contact with the carbon-containing medium, the development of ferromagnetic properties was registered. In this regard, studies were carried out on the peculiarities of the distribution of magnetic characteristics over the surface of the fragments of pyrolysis furnace tubes made of steel 20X25H20C2, which worked 750, 1300, 8000, 10,000 hours. Analysis of the measurement results showed that in the initial stage of operation, magnetic properties are developed only in local areas. With longer operation, these phenomenon occur over the entire surface of the pipes and local zones become larger, both in quan- tity and in values of the resultant tension of a constant magnetic field. Further continuation of the operation leads to an increase of areas with high values of magnetic parameters. An analysis of the literature has shown that the development of ferromag- netism in austenitic steels, which are operated in the conditions of reac- tion furnaces, can occur by two mechanisms. On the one hand, this is caused by the diffusion of carbon into the surface layers of the metal due to the long-term exposure to a carbon-rich environment, in which various types of carbides formed as a result of carburization deplete alloying ele- ments in the surface section of the reaction tube walls. At the same time, there is another process, the gradual growth of oxides in the internal direc- tion dissolves the original carbides in the subsurface section and leads to deeper carburization, as well as additional depletion of the alloying ele- ments. However, this phenomenon in the coils of reaction furnaces has not been fully studied to date, and this question remains relevant. These studies of magnetic characteristics are also complemented by a consideration of toughness. Analysis of the results of tests on impact bending showed that in areas with a high magnetic field, the impact strength decreases by 2 orders of magnitude compared with the delivery condition. It should be noted that the greatest reduction in toughness is observed in areas with the maximum development of magnetic proper- ties. These results can be applied as a method for determining poten- tially dangerous zones of reaction furnace coils, as well as developing a criterion for rejecting pipes.