Influence of long-term thermal cycling in a corrosive environment on the change in properties and microstructure of SCh20 cast iron

Abstract

The article considers the study of a metal sample of the wall of the working chamber of the compressor cylinder of the model 5G-100-7 made of SCh20 cast iron, which has been in operation since 1992 and is intended for pyrogas compression at a temperature of 110 °C, decommissioned due to the end of its service life. Mechanical tests were carried out as a result of which the mechanical characteristics of the metal were determined, such as tensile strength and Brinell hardness. The chemical composition of the metal was determined and an excess of sulfur content was established, which is explained by metal contamination due to the presence of sulfur compounds in the compressible medium. Pollution of the compressible medium is caused by the imperfection of the technology for cleaning pyrolysis gas from various impurities. The microstructure of the center of the wall and the edge of the sample, which are in direct contact with the compressible medium, has been studied. It has been established that the microstructure of the sample center is typical for gray cast iron. Along the edges of the wall, rounded corrosion pits and corrosion pits are observed. As a result of the analysis of the microstructure of the surface of the wall of the working chamber, it was found that there is a layer on the surface with dissolved graphite inclusions and the formation of pearlite grains with cementite along the boundaries. Under the surface layer, ferrite grains were found, as a result of the decomposition of perlite, and the transition of carbon to graphite with a morphology different from the base metal and cementite along the periphery of graphite inclusions and ferrite grains.