Metal Science and Heat Treatment最新文献

A dilatometric study of isothermal bainitic transformation in experimental engineering steels is performed in the temperature range of 300 – 400°C. A method is suggested for calculating the amount of retained austenite in steels after austempering, which is based on the analysis of dilatometric curves. The content of the microstructure constituents formed in the steels during austempering (primary and secondary martensite, bainite, retained austenite) is determined. A generalized kinetic equation is used to analyze the kinetics of the isothermal bainitic transformation. The temperature ranges of formation of bainite with different morphologies and the mechanical properties of the steels after the austempering are determined.

The microstructure and mechanical properties of XAR500 steel are studied after two different heat treatment modes, i.e., after annealing and after quenching followed by tempering. The microstructure of the steel is studied using atomic emission spectroscopy and light microscopy. The mechanical properties of the steel are determined by measuring the HB/HBW hardness and by static tensile tests. An effective annealing mode is recommended for ensuring high technological properties of the steel (deformability and machinability).

X-ray diffractometry and scanning and transmission electron microscopy are used to investigate structural and phase transformations in low-activation chromium-manganese austenitic steel under long-term (500 h) aging at 700°C in quenched and cold-rolled conditions and its mechanical properties are determined. Particles of M₂₃C₆ precipitate after aging over the boundaries of grains and microtwins as well as inside grains on components of microstructural defects and fine particles of MC carbides. The aging lowers the strength properties of the steel. The post-aging structure of the quenched steel contains particles of a FeCrMn σ-phase, which reduces the elongation to 7% in tensile tests at room temperature to 7%. Brittle fracture components associated with the σ-phase appear on the fracture surfaces and are partially preserved at elevated test temperatures (600 – 700°C). The possibilities of adjusting the chemical composition and the treatment modes of novel low-activation austenitic steels aimed at prevention of formation of σ-phase under long-term aging in the operating temperature range (650 – 700°C) are discussed.

Cold-deformed biocompatible alloys Zr_51–xTi₃₁Nb₁₈Hf_x (x =2– 10 at.%) are investigated after 1-h annealing at 700°C and furnace cooling. The structure, phase composition and texture of the alloys are studied using x-rays phase analysis, scanning electron microscopy and electron backscatter diffraction after micro-indentation and tensile testing. A methodology tested on titanium alloys is used to derive a formula for calculating the temperature of transition of zirconium alloys to a single-phase β-range (the temperature of polymorphic transformation T_pt). It is shown that decrease in the hafnium content provides more developed recrystallization and decelerates the decomposition of the β-solid solution during annealing yielding different second phases (ω, α″, α). The effect of the alloying with hafnium on the textural condition of the alloys during annealing is considered. The relation between the structural and phase conditions of the alloys and their physical and mechanical properties is established and recommendations are developed on the hafnium content ensuring a balanced combination of properties.

The most important mechanical properties and the structure of sheets of titanium alloy VT8 after various heat treatments and regular features of its oxidation are studied. Tensile tests of the alloy at room and elevated temperatures and in the mode of superplasticity are conducted. Electron microscope studies and x-ray diffraction phase analysis are performed. It is inferred that sheet alloy VT8 can be used for elements of various structures operating for a long time at elevated temperatures.

The microstructure and the hardness of hot-rolled structural steel 50KhFA (0.50-Cr-V)are studied after the employed and newly developed cyclic annealing modes. It is shown that it is impossible to obtain a 100% granular pearlite structure in hot-rolled steel after annealing from the initial ferrite-pearlite condition with typical distribution of the carbide phases. The suggested novel mode of cyclic annealing is more effective, because it provides a more uniform distribution of granular pearlite over the entire cross-section of the annealed rolled product and a significant decrease in the hardness. The new annealing mode is recommended for metallurgical production.

The structure and tribological properties of self-lubricating Cu – NiP – C coatings deposited on copper substrates by high-pressure cold gas-dynamic spraying are investigated. The coatings are obtained by spraying mixtures of copper and clad graphite powders in an amount of 20, 30 and 40 wt.%. The incorporation of clad particles leads to formation of a lamellar structure and microdefects along the inter-splat boundaries of the copper matrix. Increase in the content of clad particles in the coating causes monotonic growth in its hardness due to shot peening and increases the content of the NiP alloy. Despite the reduction of the coefficient of friction, the wear rate of the composite coatings increases as compared to the pure copper coating. With the increase in the content of clad graphite, the wear mechanism changes from oxidative wear and plastic deformation to delamination and then to abrasive wear.

The effect of temperature-time parameters of isothermal hardening in the bainitic transformation range on the thermal stability of retained austenite and impact toughness of sparingly alloyed structural steels 38KhS (38CrSi) and 60S2 (60Si2) is studied. The amount of retained austenite in the steels is determined by a magnetometric method. The fine structure of foils is investigated. Impact bending tests of samples are carried out. It is shown that the thermal stability of the thin foils of retained austenite at liquid nitrogen temperature correlates with the impact toughness of the steels after the isothermal hardening. A magnetic method for assessing the impact toughness of an isothermally hardened structural steel is suggested.

The effect of the aging mode on the mechanical and electrical properties of the heat-resistant electrically conductive aluminum alloy Al – 0.35Cu – 0.18Sc – 0.06Zr is studied. It is found that the highest set of operational properties of the alloy is achieved after aging at a temperature of 663 K for 15 hours. After aging under this mode, the microstructure of the alloy is characterized by a uniform distribution of dispersed inclusions of secondary phases Al₃(Sc, Zr) 10 – 20 nm in size in the aluminum matrix. Such a structure ensures strengthening of the alloy by the Orowan mechanism, due to which the tensile strength increases to 170 MPa. At the same time, there is a significant increase in the electrical conductivity of the alloy to 60.4% IACS due to a decrease in the distortion of the crystal lattice. The results of heat resistance tests in combination with the plotted Arrhenius curves showed that the effective service life of Al – 0.35Cu – 0.18Sc – 0.06Zr conductors at 503 K reaches 350,400 h.

The process of formation of the structure and properties in billets from heat-resistant nickel alloy EP741NP obtained by hot isostatic pressing (HIP) and the effect of heat treatment (HT) on its characteristics are studied. It is established that the post-HIP billets contain pores and their volume fraction is distributed nonuniformly over cross section. A digital analysis of the microstructure of the material at the periphery and in the center of the billet is used to determine the grain sizes and their anisotropy. The basic mechanical characteristics of the EP741NP alloy after the HIP and the subsequent HT (hardening and three-stage aging) are studied in the temperature range of 20 – 900°C. It is shown that the HT raises the strength characteristics of the EP741NP alloy in the whole of the temperature interval of the tests. The variation of the hardness and of the elastic and shear moduli (E and G) over cross section of the billets is determined after the HIP and subsequent HT.