Nanoscale study of ferritic-martensitic steel Rusfer EK-181 after various heat treatments

Abstract

INTRODUCTIONReduced activation ferritic/martensitic chromium steels are promising candidates as structural materials for new generation fusion and fission reactors [1,2]. Of known foreign materials of this class are 9% chro mium Eurofer 97, JLF 1, F82H, CLAM, and 9Cr 2WVTa steels [3][4][5][6][7]. Such materials are developed in Russia, as well, which are capable of withstanding high radiation doses and capable of working at tempera tures of up to 700°C. Special attention is paid to 12% chromium ferritic martensitic EK 181 steel with increased heat resistance that is considerably higher than that of foreign ones at such temperatures [8,9]. An improvement of mechanical properties can be accomplished selecting regimes of alloying and heat treatment. As a result, this steel suffers precipitation hardening [8][9][10].It was shown by transmission electron microscopy technique [8,9], different types of heat treatment of EK 181 steel result in formation of various hardening phases in the material. For example, after conven tional normalizing and tempering (N&T) heat treat ment including quenching at 1070-1100°C and tem pering at 720°C, the size of carbide phases is in the range of 10-200 nm, with the average size of these phases being 75 nm [9]. As follows from the results of atom probe tomography studies of ferritic martensitic EK 181 steel [11], clusters enriched in V, Cr, and N with dimensions ~3 nm are also formed in a matrix after conventional heat treatment.Further improvement of the properties of EK 181 steel is associated with a decrease in the dimension of disperse particles and homogeneity of their distribu tion. In turn, this requires clarification of heat treat ment for a particular material. For optimizing regimes of heat treatment, one needs information on the nanoscale state of EK 181 steel that is formed after different stages of heat treatment.The aim of this work is to study the nanoscale state of EK 181 steel after various heat treatments using atom probe tomography. MATERIALThe chemical composition of EK 181 steel is shown in the table below. A high level of high temper ature strength and radiation resistance of EK 181 steel are known to depend considerably on the regime of heat treatment [9]. Conventional normalizing and tempering (N&T) heat treatment is used to increase the tensile strain up to 700°C. To decrease ductile to brittle temperature transition shift caused by irradia tion, to increase impact strength and tensile strain within brittle region combined heat treatment is used, including termocycling near the onset point of the phase transition α γ [9]. The content of δ ferrite does not exceed 5% after both heat treatments. The microstructure of steel after quenching at 1070-1100°C comprises a martensite phase which is formed as a result of polymorphic transition γ α. A ferrite Abstract-Reduced activation ferritic/martensitic 12% Cr EK 181 steel (Fe-12Cr-2W-V-Ta-B-0.16C) was investigated using atom probe tomography after various regimes of heat treatment: annealing at 800-850°C, conventional n...

Effect of alloying with titanium on the microstructure of an oxide dispersion strengthened 13.5% Cr steel

“…In addition, vana dium participates in the formation of strengthening phases (carbides and carbonitrides). The structural examinations have shown that, under the particular conditions of high temperature heat treatment, a nanostructure, which largely favors a substantial increase in the high temperature strength of a steel, is formed in vanadium alloyed ferritic steels [20,21]. The ultramicroscopic examinations of the ODS mate rials have shown that vanadium participates in the pro cesses of the formation of nanosized oxides.…”