V. M. Chernov

Internal friction and brittle-ductile transition in structural materials

The temperature dependences of internal friction (ultrasound decrement) and impact toughness of bcc metals (Fe-12Cr-W-V-Ta-B-C ferritic/martensitic steel, V-4Ti-4Cr vanadium-based alloy) have been studied in a temperature range from 100 to 300 K. The acoustic and mechanical characteristics exhibit correlated variation. The use of nondestructive acoustic techniques for the investigation of mechanisms of the brittle-ductile transition in metallic structural materials is experimentally justified.

Microstructure of EK-181 ferritic-martensitic steel after heat treatment under various conditions

The effect of heat treatment conditions on the microstructure and phase composition of a lowactivated high-temperature ferritic-martensitic EK-181 steel (Fe-12Cr-2W-V-Ta-Ba) is studied. Additional thermal cycling about the austenite-martensite phase transition temperature between quenching and tempering hinders the formation of fine interstitial phases, decreases the phase transformation-induced hardening intensity, retards the formation of a substructure with continuous misorientations, and decreases the brittle-ductile transition temperature.

Impact toughness of EK-181 ferritic-martensitic chromium (12%) steel under loading by concentrated bending

The low-temperature fracture of a high-temperature low-activated ferritic-martensitic EK-181 chromium (12%) steel (RUSFER-EK-181: Fe-12Cr-2W-V-Ta-B) is studied using impact and static concentrated bending tests as a function of the specimen dimensions (standard, small), the type of stress concentrator (V-shaped notch, fatigue crack), and the temperature (from −196 to +100°C). The ductile-brittle transition temperature falls in the range from −85 to +35°C. The temperature dependences of stress-intensity factor KIc and fracture toughness JIc are determined. The severest type of impact toughness tests is represented by tests of V-notched specimens with an additional fatigue crack and two lateral V-shaped notches (three-sided V-shaped notch with a central fatigue crack). The fracture energy of the steel depends on the type of stress concentrator and the specimen dimensions and is determined by the elastic energy and the plastic deformation conditions in the near-surface layers of a specimen, which are controlled by the lateral notches. At the same test temperature, the impact toughness and the fracture toughness are interrelated. Irrespective of the type of specimen (including notches and a fatigue crack), the ferritic-martensitic steel exhibits the same fracture mechanism.

Temperature dependence of the lattice parameter and Debye-Waller factor of a high-chromium pressure-vessel steel

The method of thermal neutron diffraction has been used to study samples of the EK-181 steel at temperatures of 15 to 973 K in an IBR-2 reactor (Joint Institute for Nuclear Research (JINR)). Temperature dependences of the lattice parameter, internal textural stresses (of the third kind), and the Debye-Waller factor of this steel have been calculated from diffraction spectra by the Rietveld method. It has been found that at low temperatures the temperature dependence of the lattice parameter in the EK-181 steel (RUSFER EK-181) differs from the corresponding dependence in pure iron and binary iron-chromium alloys containing 12 and 16% Cr. Also, a broadening of the (200) reflection has been observed in the diffraction spectra of the EK-181 steel and the Fe-12Cr alloy, while it is not detected in the spectra of Fe-16Cr and pure iron.

The microstructural stability of low-activation 12%-chromium ferritic-martensitic steel EK-181 during thermal aging

The results of structural investigations and mechanical tests of low-activation 12%-chromium ferritic-martensitic steel EK-181 after long-term (13500 h) aging at 450°C and 620°C are presented. It is shown that the high thermal stability of steel microstructure ensures that its original short-term mechanical properties are maintained at T ≤ 620°C.

Fatigue Strength of Ferrite-Martensite 12% Chromium Steels EK-181, EP-823 and Vanadium Alloy V–4Ti–4Cr

Static and fatigue strength at room temperature under conditions of repeated stretching of lowactivated ferrite-martensite 12% chromium steels EK-181 (Fe–12Cr–2W–V–Ta heat treatment + aging in lead at 600°C, 3000 h), EP-823 (Fe–12Cr–W–V–Ni–Mo–Nb, annealed condition), and alloy V–4Ti–4Cr (heat treatment + aging in lead at 600°C, 3000 h) were studied. It was shown that for materials there is straight-line dependence between the level of rupture resistance values and fatigue strength. The maximum fatigue limit of 600 MPa appears in steel EK-181 after a standard heat treatment and aging in lead at 600°C, 3000 h, and the minimal one of 300 MPa is observed in vanadium and V–4Ti–4Cr alloys. The fatigue failure mechanism is predominately of ductile character for all materials studied. The fatigue cracking originates near the surface and in some cases clustering of nonmetallic inclusions is the place of origin. The fatigue crack propagation is related to formation of a typical striation relief. Significant distinctions in the fracture surface relief of specimens after standard heat treatment and aging in liquid lead are not observed.

Effect of high-temperature thermomechanical treatment in the austenite region on microstructure and mechanical properties of low-activated 12% chromium ferritic-martensitic steel EK-181

The effect of high-temperature thermomechanical treatment with deformation in the austenite region on the microstructure and mechanical properties in low-activated 12% chromium ferritic-martensitic steel EK-181 (Fe–12Cr–2W–V–Ta–B) has been investigated. This treatment leads to a significant increase (compared to traditional regime of treatment) in the density of dislocations, dispersity, and volume fraction of nanosized particles V(C,N) and, as a consequence, to an increase in the yield strength while maintaining a sufficient reserve of ductility.

Fatigue strength of low-activation ferritic–martensitic high-chromium EK-181 steel

The static and cyclic mechanical properties of low-activation ferritic–martensitic EK-181 (Fe‒12Cr–2W–V–Ta–B–C) steel are studied in the temperature range 20–920°C (static tests) and at 20°C (cyclic tests). The fracture mechanisms of the steel under static tension and fatigue fracture conditions are analyzed by scanning electron microscopy.

Strengthening mechanisms of heat-resistant 12% Cr ferritic-martensitic steels after different modes of heat treatment

The features of microstructure of 12% chromium ferritic-martensitic steels, EK-181 (Fe–12Cr–2W–V–Ta–B) and ChS-139 (Fe–12Cr–Ni–Mo–W–Nb–V–B), after heat treatments providing different levels of strength and plastic properties are investigated. A theoretical analysis of a number of strengthening mechanisms of these steels, depending on the conditions of heat treatment, is carried out. It is shown that dispersion hardening by MX carbonitride nanoparticles is one of the most effective ways of increasing the strength of ferritic-martensitic steels under study.

Microstructure and mechanical properties of heat-resistant 12% Cr ferritic-martensitic steel EK-181 after thermomechanical treatment

The effect of high-temperature thermomechanical treatment (TMT) with the deformation in the austenitic region on the features of microstructure, phase transformations and mechanical properties of low-activation 12% Cr ferritic-martensitic steel EK-181 is investigated. It is established, that directly after thermomechanical treatment (without tempering) the sizes and density of V(CN) particles are comparable with those after a traditional heat treatment (air quenching and tempering at 720°C, 3 h), where these particles are formed only during tempering. It causes the increasing of the yield strength of the steel up to ≈1450 MPa at room temperature and up to ≈430 MPa at the test temperature T = 650°C. The potential of microstructure modification by this treatment aimed at improving heat resistance of steel is discussed.