Abstract
The effect of high-temperature thermomechanical treatment (HTMT) with plastic deformation by rolling in austenitic region on the microstructure and mechanical properties of 12% chromium ferritic-martensitic steel EP-823 is investigated. The features of the grain and defect microstructure of steel are studied by Scanning Electron Microscopy with Electron Back-Scatter Diffraction (SEM EBSD) and Transmission Electron Microscopy (TEM). It is shown that HTMT leads to the formation of pancake structure with grains extended in the rolling direction and flattened in the rolling plane. The average sizes of martensitic packets and ferrite grains are approximately 1.5–2 times smaller compared to the corresponding values after traditional heat treatment (THT, which consists of normalization and tempering). The maximum grain size in the section parallel to the rolling plane increases up to more than 80 µm. HTMT leads to the formation of new sub-boundaries and a higher dislocation density. The fraction of low-angle misorientation boundaries reaches up to ≈68%, which exceeds the corresponding value after HTMT (55%). HTMT does not practically affect the carbide subsystem of steel. The mechanical properties are investigated by tensile tests in the temperature range 20–700 °C. It is shown that the values of the yield strength in this temperature range after HTMT increase relative to the corresponding values after THT. As a result of HTMT, the elongation decreases. A significant decrease is observed in the area of dynamic strain aging (DSA). The mechanisms of plastic deformation and strengthening of ferritic-martensitic steel under the high-temperature thermomechanical treatments are also discussed.
Highlights
Ferritic-martensitic steels with a chromium content of 9–12% are considered as the candidate structural materials for nuclear and thermonuclear reactors of a new generation.This is due to the achieved complex of physical and mechanical properties, such as high values of heat resistance, low swelling, high resistance to radiation and helium embrittlement and other advantages [1,2,3]
This paper presents the results of investigation of the effect of hightemperature thermomechanical treatment (HTMT) of this steel on the features of its grain and defect microstructure and the carbide subsystem, as well as short-term mechanical properties under tensile testing in a wide temperature range in comparison with traditional heat treatment (THT)
After high-temperature thermomechanical treatment (HTMT), there is an increase in the yield strength in the temperature range from 20 to 650 ◦ C relative to the corresponding values after the THT
Summary
Ferritic-martensitic steels with a chromium content of 9–12% are considered as the candidate structural materials for nuclear and thermonuclear reactors of a new generation.This is due to the achieved complex of physical and mechanical properties, such as high values of heat resistance, low swelling (compared with austenitic steels), high resistance to radiation and helium embrittlement and other advantages [1,2,3]. Both long-term and short-term strength properties of ferritic-martensitic steels are controlled by the elemental composition, microstructure features (dimensions of tempered martensite packets, volume fraction of ferrite, dislocation density), composition, dimensions and spatial distribution of carbide (carbonitride) phases [4,5,6,7,8,9,10,11] Steels of this class are most thoroughly investigated in the structural state after normalization and tempering; this processing is referred to here as a traditional heat treatment (THT). These changes mainly consist of a smaller width of martensitic packets and lamella, an increased dislocation density, a decreased size of coarse phases of the M23 C6 type and an increased dispersion of nanoscale carbides (carbonitrides) of the MX type [11,12,15,16,17,22,23,24,25,26,27,28]
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