Vol 59, No 3-4 (2017)

The methods of metallographic, differential thermal and x-ray phase analysis, light and electron microscopy, and measurement of microhardness are used to study the morphological features of the structure of hypereutectic alloy Al – 30 at.% Cu formed due to cooling of liquid phase at a rate of about 2 and about 10⁴ K/sec after superheating above the liquidus. A comparative study of structural and phase transformations in castings is performed.

Plates from alloy AA7020 (Al – Zn – Mg) with a thickness of 4.5 mm are studied after T6 heat treatment and friction stir welding. Tensile and bending tests are performed. The hardness of the welds is measured in transverse direction. The microstructure over cross section of a weld is studied by the methods of light and transmission electron microscopy. The weldability of alloy AA7020 by friction stir welding is shown to be high.

Variants of secondary heating of aluminum alloys are considered, i.e., under quenching of plates in a water tank or on a horizontal quenching unit with water jet cooling, under continuous quenching of strips, and under quenching of tubes in vertical furnaces. Recommendation are given for removal or substantial reduction of the intensity of secondary heating under industrial conditions.

Results of metallographic studies of the structure of steels 38G2S and 37G2F and steels of group D after electromechanical surface hardening of tube specimens over the external diameter are presented.

The laws of formation and growth of carbonitride layer in steels 40G2, 38KhMA, 40KhN2MA, and 09GSF at a temperature of 560°C are studied. Metallographic methods are used to determine its composition, location and length. Equations for computing the lengths of the carbonitride phase and of the layer due to surface cyaniding are derived.

The effect of the rate of cooling on the structure and properties of castings from a high-manganese steel is studied. The level and the sign of the residual stresses, the size of the austenite grains and the microhardness of the austenite, the content and morphology of the carbide phase, and the hardness of the cast steel are determined. The optimum cooling conditions producing the most homogeneous structure in a casting are obtained. Comparative analysis of the effect of the cooling rate and of the microhardness of the austenite on the morphology and size of the carbide phase is performed.

The corrosion behavior of joints formed by TIG and IMIG welding from clad sheets of heat-hardenable aluminum alloy 1151 of the Al – Cu – Mg system is studied. The corrosion tests are performed in an aqueous solution of NaCl in a salt-spray chamber. The welded joints are subjected to a metallographic analysis.

The effect of carbonitriding as a finishing operation of hardening of the thread of drill pipes on the properties of the matrix metal, its temper brittleness in particular, is studied. The cold brittleness is evaluated with the help of tests for impact toughness. Steels for the production of drill pipe locks for operation at negative temperatures are recommended.

Results of a study of the interaction between iron impurity and manganese and aluminum alloying elements during formation of phase composition in alloys of the Mg – Mn, Mg – Al, Mg – Al – Mn, and Mg – Al – Zn – Mn systems are presented. It is proved that this interaction results in introduction of Fe into the intermetallic phase. The phase compositions of model magnesium alloys and commercial alloys MA2-1 and MA5 are studied. It is shown that both manganese and aluminum may bind the iron impurity into phases. Composite Fe-containing intermetallic phases of different compositions influence differently the corrosion resistance of magnesium alloys.

Rolled sheets with a thickness of 60 mm are studied after thermomechanical rolling and after an additional heat treatment. The conditions of the deformation and thermal deformation aging are shown to affect the impact toughness of the low-carbon microalloyed steel. The changes in the critical temperatures of the impact toughness due to increase in the degree of preliminary cold deformation and under additional heating of the steel to 250°C are determined. The parameter of the susceptibility of surface and central (over the thickness) layers of a thick sheet to mechanical aging is computed.

The effect of thin films of copper oxide deposited before nitriding on the phase composition and the kinetics of growth of diffusion layers in carbon steels is considered. The process of formation of an oxide film involves chemical reduction of pure copper on the surface of steel specimens from a salt solution and subsequent oxidation under air heating. The oxide film exerts a catalytic action in nitriding of low- and medium-carbon steels, which consists in accelerated growth of the diffusion layer, the nitride zone in the first turn. The kinetics of the nitriding process and the phase composition of the layer are controlled by the thickness of the copper oxide precursor, i.e., the deposited copper film.

The effects of plastic deformation near the temperatures of the F → A phase transformation and of deformation-induced ferritic transformation (DIFT) on the structure and properties of microalloyed ferritic-pearlitic steels are studied. It is shown that the refinement of the structure of the steels due to the F → Atransformation during heating and due to the DIFT under plastic deformation and subsequent cooling raises the mechanical properties of the metal. Modes of hot rolling are suggested for structural ferritic-pearlitic steel 17G1S-U in order to form an ultrafine-grained structure with elevated mechanical properties.

The microstructure and magnetic properties of permanent magnets fabricated from cast hard magnetic alloys of the Fe – Ni – Al system with cobalt addition of type YuNDK are studied. The causes of failure of such magnets in operation are determined. Recommendations are given for correcting the melting process and the modes of heat treatment of magnets in order to provide the required properties without disturbing the integrity of the magnets.

Amathematical model for computer simulation of structural transformations in an alloy steel under the conditions of the thermal cycle of multipass welding is presented. The austenitic transformation under the heating and the processes of decomposition of bainite and martensite under repeated heating are considered. Amethod for determining the necessary temperature-time parameters of the model from the chemical composition of the steel is described. Published data are processed and the results used to derive regression models of the temperature ranges and parameters of transformation kinetics of alloy steels. The method developed is used in computer simulation of the process of multipass welding of pipes by the finite-element method.