Fracture features of impact samples of low-activation ferritic-martensitic steel EK-181 after high-temperature thermomechanical treatment

Abstract

A comparative fractographic investigation of fracture in the temperature range from −186 to 100°C of the Charpy impact samples is performed for the reactor low-activation ferritic-martensitic steel EK-181 after its high-temperature thermomechanical treatment (HTMT) and traditional heat treatment (THT). The mechanisms of steel fracture are revealed depending on the impact test temperature and treatment mode. On the upper and lower shelves of the impact toughness temperature curve, the steel fractures by the mechanism of transcrystalline ductile dimple fracture and transcrystalline quasicleavage, respectively. In the intermediate region (in ductile-brittle transition area), fracture occurs by a mixed mechanism. The transition temperature to the brittle state is determined, at which the proportion of ductile and brittle fractures is the same (after THT it is −3°C; after HTMT it is −14°C). It is established that HTMT significantly changes the type of fracture of the impact samples in comparison with THT. The microstructure formed during HTMT with hot deformation of austenite leads to the appearance of a crack-arrester type of delamination during the impact tests in the cold brittleness region, favoring an increase in the fracture toughness of the steel at a lower ductile-brittle transition temperature.

The Cold-Brittleness Regularities of Low-Activation Ferritic-Martensitic Steel EK-181

“…The results on the study of the temperature dependence of Young's modulus are consistent with those of the study of the temperature dependence of the strength properties of EK-181 steel after THT and HTMT (during uniaxial tensile tests in the temperature range from −196 to 100 • C) [9,10,12]. It is shown in [9] that as the temperature drops below 20 • C, the yield strength of this steel increases after both treatments (Figure 11a).…”

“…In a fractographic study of the EK-181 steel samples after impact tests in the range from −186 to 100 °C in [9,10], it was established that on the upper shelf of the temperature dependence of impact strength, the steel fractures by the mechanism of ductile dimple transgranular The value of Young's modulus in the states after THT and HTMT differs throughout the entire temperature range under consideration by approximately the same amount (Figure 3a) from those for an undeformed sample and a sample deformed by plastic bending (Figure 3b). For example, at room temperature testing of undeformed samples, HTMT leads to an increase in the elastic modulus of the steel, relative to TMT, by ≈ 8.5 GPa (from E 1 THT ≈ 224.5 to E 1 HTMT ≈ 233 GPa).…”

“…In a fractographic study of the EK-181 steel samples after impact tests in the range from −186 to 100 • C in [9,10], it was established that on the upper shelf of the temperature dependence of impact strength, the steel fractures by the mechanism of ductile dimple transgranular fracture. On the lower shelf, the steel is destroyed by the transgranular quasi-cleavage mechanism, while in the range of ductile-brittle transition of the steel under study, the fracture mechanism changes from ductile to brittle (a mixed type of fracture is observed) (Figure 5).…”

“…Earlier [8][9][10], we discussed the features of the microstructure and mechanical behavior of the EK-181 promising low-activation 12% chromium ferritic-martensitic reactor steel (Fe-12Cr-2W-V-Ta-B) after a traditional heat treatment (THT, quenching + high tempering) and a high-temperature thermomechanical treatment (HTMT, with deformation in the temperature region of metastable austenite) under static and dynamic loads. The use of HTMT is mainly aimed at improving the high-temperature strength properties of ferriticmartensitic steels, including those under creep conditions [5,[9][10][11][12][13][14]. It is necessary to expand the range of their operating temperatures (increasing the upper limit) in the reactor core.…”