Study of the fine structure of pipe steel after long-term holding at elevated temperature

Abstract

Or aim is to study the processes of changes in the microstructure and fine structure of pipe steel in he course of long-term holding at elevated temperatures (400 °C). By the method of transmission-electron microscopy, we investigated samples 1420 × 25.8 mm in size of the base metal of a pipe of strength class K60 in the initial state and after holding for up to 200 h at a temperature of 400 °C. It is shown that, in the course of long-term holding at 400 °C, we do not observe any noticeable changes in the structure of the ferrite matrix of samples of the base metal of the pipes made of specially developed steel, including the dislocation and subgrain structures of ferrite, as compared with samples without tempering. We recorded the changes observed in the course of tempering at 400 °C in the high-carbon phase of the samples as compared to an initial sample. Indeed, we detect tempering of martensite “islands,” precipitation of cementite particles, and decomposition of austenite into a ferrite–cementite mixture. Carbonitride phases do not undergo any visible changes in the course of tempering and are identical for all investigated samples. The obtained results serve as a microstructural justification of stability of the level of the mechanical properties of the investigated steel as a result of long-term holding at 400 °C.