MICROSTRUCTURAL DEFORMATION AND FRACTURE OF REDUCED ACTIVATION FERRITIC-MARTENSITIC STEEL EK-181 UNDER DIFFERENT HEAT TREATMENT CONDITIONS

Abstract

TEM studies were performed to examine the effect of exposing dispersion-strengthened heat-resistant reduced activation 12% chromium ferritic-martensitic steel EK-181 for 3000 h at 600°C to static liquid lead on the steel microstructure in comparison with the steel after traditional heat treatment by quenching and tempering at 720°C. It was found that the steel microstructure has good thermal stability under the specified experimental conditions. Microstructural deformation of EK-181 steel was studied in the neck region of tensile specimens tested at temperatures of 20, 680, 700 and 720°C with and without exposure to liquid lead, and their fracture mechanisms were investigated. As a result of plastic deformation during tensile testing at room temperature, martensite plates and laths near the fracture surface are distorted and fragmented with the formation of new low angle boundaries, and the dislocation density increases. At 680-720°C deformation temperatures, nearly equiaxed ferrite grains are formed, the density and size of second phase particles (M23C6 and MX) increases due to dynamic strain aging, and the dislocation density decreases locally. As the test temperature rises, the degree of martensite tempering increases. At T ≥ 700°C, some dynamic polygonization and dynamic recrystallization are observed. At elevated tensile temperatures, ferrite coarsening is more significant in lead-exposed specimens compared to the traditionally treated material. The plastic deformation and fracture behavior of the steel are largely determined by the test temperature, rather than by the treatment regime.