The Microstructure, Tensile and Impact Properties of Low-Activation Ferritic-Martensitic Steel EK-181 after High-Temperature Thermomechanical Treatment

Abstract

In this work, we study the effect of high-temperature thermomechanical treatment (HTMT) with deformation in the austenite region on the microstructure, tensile properties, impact toughness, and fracture features of advanced low-activation 12% chromium ferritic-martensitic reactor steel EK-181. HTMT more significantly modifies the steel structural-phase state than the traditional heat treatment (THT). As a result of HTMT, the hierarchically organized structure of steel is refined. The forming grains and subgrains are elongated in the rolling direction and flattened in the rolling plane (so-called pancake structure) and have a high density of dislocations pinned by stable nanosized particles of the MX type. This microstructure provides a simultaneous increase, relative to THT, in the yield strength and impact toughness of steel EK-181 and does not practically change its ductile-brittle transition temperature. The most important reasons for the increase in impact toughness are a decrease in the effective grain size of steel (martensite blocks and ferrite grains) and the appearance of a crack-arrester type delamination perpendicular to the main crack propagation direction. This causes branching of the main crack and an increase in the absorbed impact energy.

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

“…A traditional heat treatment (THT) included quenching from T = 1100 • C (holding for 1 h) and tempering at T = 720 • C (for 3 h). A high-temperature thermomechanical treatment (HTMT) was carried out according to the following regime (Figure 1): heating to T = 1100 • C (followed by holding for 1 h) + hot plastic deformation by rolling in the austenite region to a value of ε ≈ 50% (mill rolls were at room temperature) + quenching in water + tempering at 720 • C (exposure for 1 h) [9]. An investigation of internal friction of the EK-181 steel samples after THT and HTMT by the non-destructive acoustic method was carried out using the resonant method of a composite piezoelectric vibrator.…”

“…It has an X-ray ene persive microanalysis system with an INCA energy detector. The results of imp are presented in [9]. The dislocation density was calculated by the secant meth preparation of thin foils for such studies was carried out using a Hitachi FB-2100 ion beam (FIB) system, which allows samples to be cut from the required area of terial (in our case, directly below the fracture surface of impact samples) with s accuracy.…”

“…It has an X-ray energydispersive microanalysis system with an INCA energy detector. The results of impact tests are presented in [9]. The dislocation density was calculated by the secant method.…”

“…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.…”

“…It is necessary to expand the range of their operating temperatures (increasing the upper limit) in the reactor core. It has been shown [9] that the HTMT mode causes the steel grain and carbides refinement. It provides a simultaneous increase (by ≈20%) in the yield strength of EK-181 steel in the temperature range from −196 to 800 • C and its impact strength (with a decrease in the ductile-brittle transition temperature).…”