THE DEGRADATION OF THE STRUCTURE OF HEAT-RESISTANT STEEL TUBES AFTER LONG-TERM USE IN THE PETROCHEMICAL INDUSTRY

The main reason for limiting the life of furnace coils is heat-aging under the influence of temperature and time factors which reduces long-term strength of the metal. The article presents an experimental analysis of the structure and mechanical properties of furnace coils from corrosion-resistant austenitic steels after various terms of operation and after emergencies. Examination of the samples cut from the coil after 12-48 years of service showed that the mechanical properties of most of them are within the requirements of regulatory documents. We found that a significant decline in long-term strength occurs due to partial or complete decarbonization of the cross-section of chrome steel pipe. The calculations of the depth of decarburizations were done for various life times. A possible way to increase longevity by creating protective diffusion layers, in particular, either chromed or siliconizing was suggested. High alloys, which operate at temperatures up to 800 °C, have another mechanism of degradation, associated with the redistribution of chromium, precipitation of σ-phase, leading to chromium depletion of grain boundaries, intergranular corrosion and brittle fracture. Increasing longevity in this case is possible only through a complete blocking of the surface layer by coating it with specific enamels used for high-temperature alloys, such as Nimonic.