Selection of the optimal composition of steel for manufacturing furnace tubes of catalytic-reforming units

Abstract

Smaller-diameter and, therefore, thinner-walled (up to 5-9 mm wall thickness) furnace tubes are used in modern, large catalytic-reform ing units. In addition, the tube wails may be heated to 620~ Under such conditions, the carburization process of the tube metal has an even greater effect on the tubes' operating reliability because when the tube wall temperature is increased, the carburization rate increases, and when the wall thickness is decreased, the relative amount of carburization increases sharply. Therefore, the problem was set up to select a heat-resistant steel of optimal composition ensuring resistance to hydrogen sulfide corrosion, hydrogen, and carburization. As a result of an investigation of specimens of steels 20K, 12MKh, 12KhM, 15KhSM, 12Kh2MFSR, 1Kh12V2MF, 12Kh13M1, and 15Kh2M2FBS held for a long time (up to 2 yr) in the medium of a catalytic-reform ing unit, at 500-530~ and at a pressure of up to 50 kgf/cm 2, it was found that the steels behaved differently under the same conditions in relation to the composition and content of alloying elements. Steels containing a sufficiently large amount of carbide-forming elements, namely, chromium, vanadium, etc., were carburized. Specimens of steel 15KhSM had the maximum carburization depth (1.5 mm). Steels 12Kh2MFSR and 15Kh2M2FBS were carburized to practically the same degree, and the specimens of steels 1Khl2V2MF and 12Kh13M1 had a thin carburized layer containing an increased amount of carbides. Carburization was not observed on specimens of steels 12MKh, and the metal structure was identical with respect to the cross section of the specimens. Specimens of carbon steels were completely decarburized, and cracks were formed in the metal of the specimens as a result of hydrogen corrosion. The obtained results indicate that the medium of reforming units is carburizing for some steels and decarburizing for other steels. The direction of the reaction depends on the equilibrium constant under the given conditions and the value of the thermodynamic activity of the carbon in the steel. Comparative tests of the carburization tendency of specimens of various steels of experimental and industrial melts were carried out in a medium of technical methane at 600 and 700~ and cycle times of 10% 300, and 1000 h and in an industrial carburization furnace (medium of natural gas) at a carburization temperature of 940~ and a cycle time of 100 h. Specimens with a diameter of 10 mm and length of 40 mm were tested. The depth and concentration of the carburized layer was determined as a result of metallographic and layerwise chemical analyses. It was determined that at all the test temperatures, as the chromium content was increased from 1 to 2%