Volume 58, Nº 11-12 (2017)

The kinetics of phase recrystallization and formation of structure in medium-carbon steels under short-term heating at a temperature exceeding Ac₁ and accelerated cooling is studied. The conditions of implementation of two mechanisms of abnormal formation of pearlite differing in the lengths of the diffusion paths of carbon are determined, and the morphology of the pearlite is described.

The kinetics of formation of austenite in steel 08G2B under continuous heating at a rate of 0.3 and 90 K/sec to the intercritical temperature range is studied with the help of x-ray diffraction, dilatometric and calorimetric methods of analysis. The tests are performed for specimens in two initial conditions, i.e., after recrystallization-free controlled rolling with accelerated cooling and after water quenching from 1000°C. It is shown that the α – γ transformation under the conditions studied may be treated as a diffusion-controlled isokinetic reaction occurring in two stages with rapid formation of austenite nuclei primarily over the boundaries of initial austenite grains.

Processes of precipitation of silicides and aluminides in commercial titanium alloys under different modes of heat treatment are studied. The effect of alloying on the types of precipitating particles is considered. The temperature ranges of formation of intermetallics are determined and the possible mechanisms of transformation of particles of different types are discussed. A schematic isothermal diagram of decomposition of metastable phases in refractory titanium alloys is suggested.

Regular features of variation of structure, phase composition and hardness characteristics in a β-phase-based titanium alloy Ti – 10 V – 2Fe – 3Al subjected to isothermal treatment in a melt of lead and tin for 5 sec – 32 h at 250 – 800 °C after a hold at 860 °C are considered. The types of the phases precipitated in the process of isothermal decomposition of the high-temperature β-solid solution and the dependences of the lattice constant of the β-phase and of the hardness on the temperature-and-time mode of the treatment are determined. Adiagram of isothermal decomposition of the β-solid solution in alloy Ti – 10 V – 2Fe – 3Al is plotted.

Special features of formation of structure and properties of seamless pipes from medium-carbon low-alloy steel for oil and gas applications are considered and associated with chemical inhomogeneity of the metal of the pipes.

The effect of grain-boundary precipitates on cryogenic impact toughness of two corrosion steels (standard AISI 316 and a steel with a nitrogen additive) is studied. The steels are aged at 600 – 900°C with a hold of up to 1000 min. The KCV impact toughness at –196°C is determined. It is shown that the impact toughness of the nitrogen-containing steel decreases under cooling after the aging at 700 and 800°C more considerably than that of steel 316 after aging at 800 and 900°C. The causes of the embrittlement of the nitrogen-containing steel are determined.

The effect of deformation on the damping capacity of a copper-base shape memory alloy produced by standard methods is studied. The metallographic analysis is performed with the help of scanning electron microscopy. The internal friction is measured at different temperatures. The damping capacity of the alloy is analyzed and its relation to the mechanical and physical properties is determined.

Some parameters of laser surfacing of self-fluxing nickel alloy PG-12N-01 are considered. Different structures containing a low-melting γ-Ni – Ni₃B eutectic and a γ-Ni – Cr₃C₂ eutectic that crystallizes at a higher temperature and forms the strength skeleton of the coating may form depending on the rate of the surfacing. The effect of the rate of the surfacing on the wear resistance of the coating and on the coefficients of dry friction are determined.

Complex bimetallic inclusions formed in billets from steels 09G2S and K52 during heating for rolling are studied by methods of electron microscopy and local x-ray spectrum analysis. To check the established evolution of oxide inclusions based on aluminomagnesium spinel and other oxide compositions, individual inclusions of manganese sulfide and complex oxide-sulfide inclusions, a method developed by the Severstal’Company and the Karpov NIFKhI for determining the content of corrosion-active nonmetallic inclusions (CANI) as a function of the heating mode is applied to the steels studied. It is recommended to increase the temperature and duration of heating of billets for hot rolling in order to lower the content of CANI and to raise the resistance of the steels to local corrosion.

Amechanism of local formation of crystals of random orientation in growth of single-crystal billets of refractory nickel alloys for blades of gas turbine engines is suggested. Aturn of the lattice is initiated at the place of contact between the initial melt and the surface of hard particles of NbC carbide and/or TiN nitride.

The Thermo-Calc software is used to compute the phase transformations occurring during cooling of alloys. Polythermal and isothermal sections of the phase diagram of the Al – Mg – Si – Fe system are plotted. The phase composition and the structure of aluminum alloys in cast condition and after a heat treatment are studied experimentally.

Wear (pin-on-disk) tests of low-alloy powder steels (with 0.5 and 0.65% C) are performed after vacuum sintering combined with hardening. The studied alloying systems are 0.65% C + 1.5% Cr + 0.2% Mo, 0.65% C + 0.9% Ni + 0.9% Mo + 0.2% Mn and 0.5% C + 4% Ni + 1.5% Cu + 0.5% Mo. The wear resistance of the steels after the hardening sintering is an order of magnitude higher than after the conventional sintering. The wear resistance of the steels increases in the following order: pearlitic, pearlitic-bainitic, and martensitic structures. Accordingly, the hardness grows from 98 to 105 HRB.

Forged low-carbon alloy Co – 27% Cr – 6% Mo – 0.06% C is studied after 2-h plasma nitrocarburizing in a gaseous mixture of 10% CO₂ + 20% N₂ + 70% H₂ at 400, 500 and 600°C. The surface layer and the core are studied by diffractometric analysis and scanning electron microscopy. The microhardness and the tribological characteristics of the alloy are determined. The effect of the temperature of nitrocarburizing on the wear resistance, surface roughness and friction coefficient of the alloy is considered.