Investigation of changes in the structure of heat-resistant iron-containing chromium-nickel alloy as a result of long-term operation

Abstract

Pyrolysis furnaces operate in harsh temperature and power conditions. Radiant coils are the most vulnerable part in their design. The coil pipe operates in two modes: operational - at a constant pressure and temperature not exceeding 1050 °С, and cleaning mode, which is characterized by increased pressure and higher temperature (about 1100 °С) compared to the operating one, which leadsto a thermal force effect. Such operating conditions of coil pipes lead to changes in the structure and properties of the material. In this regard, chromium-nickel steels are widely used for the manufacture of coils, and for operation at higher temperatures (1000-1050 °C), the content of chromium and nickel is increased, obtaining iron-containing chromium-nickel alloys, to ensure a stable austenitic structure. Identification of the dominant damaging mechanism makes it possible to determine the remaining resource and replace the coils in time to prevent possible emergencies. It is obvious that structural changes in the pipe metal will have a decisive influence on the performance of the structure as a whole. This paper presents the results of a study of the influence of some operational factors on the state of the metal of pipes of a standard furnace coil made of heat-resistant iron-containing chromium-nickel alloy. A change in the matrix microstructure in the form of partial homogenization of austenite was revealed. The fact of the occurrence of carbide reactions has been established. The presence of an oxidation zone mainly along the boundaries of dendrites was revealed, and the main types of oxides were determined. The dominant damaging mechanism of radiant tubes during operation has been determined.