Metal Science and Heat Treatment最新文献

The microstructure and compressive properties of the 7xxx series aluminum alloy (Al – 12.44Zn – 3.22Mg – 1.13Cu – 0.19Zr – 0.12Sr) reinforced with SiC whiskers via squeeze casting are investigated after aging treatments T6-1, T6-2, T6I4-1, and T6I4-2. For this purpose, the effect of four kinds of aging heat treatments on the properties of the aluminum metal matrix composites (AMMCs) is studied systematically through Vickers hardness and room temperature compressive testing. The microstructure is analyzed by optical microscopy, scanning electron microscopy, and x-ray diffraction. The experimental results show that the hardness of the AMMCs after four kinds of aging processes has different degrees of improvement compared to no-aging. After the T6-1, T6-2, T6I4-1, and T6I4-2 aging processes, the hardness of the composites is 325, 278, 306, and 345.8 HV, respectively. The additional T6-2 aging treatment provides the highest compressive strength and compressive strain, about 747 MPa and 7%, respectively, as well as a maximum dislocation strengthening of approximately 86.89 MPa.

An overview of the most used variants of the structural-chemical Schaeffler diagram developed by various researchers is presented. A new version of the Schaeffler diagram and equations for calculating Cr- and Ni-equivalents are suggested, which makes it possible to correctly predict the content of δ-ferrite in the structure of heat-resistant ferritic–martensitic steels of the EP450 – EP823 – EK181 – ChS139 group containing ≥ 10 wt.% Cr, after hot deformation and heat treatment. Based on the factor analysis of the relationship between the physicochemical and structural characteristics of such steels and their mechanical properties, equations are obtained in which the Cr- and Ni-equivalents are allowed for as factors determining the value of the yield strength.

The mechanisms of intragrain plastic deformation occurring in local micro-volumes of metal under the influence of temperature stresses during high-speed heating due to high-energy exposure are studied. The temperature distribution over the thickness of the sample is calculated and its experimental verification is carried out. Samples of steel 12Cr18Ni10, which does not undergo phase transformations during heating and cooling, are examined after a treatment with scanning air-plasma arc. It is shown that the intragrain plastic deformation during heating of austenitic steel is caused by thermoelastic stresses arising in the heated layer and in the volumes bordering it and manifests itself in the appearance of sliding traces in the grains. The mechanisms of the appearance of temperature stresses in the metal are proposed and substantiated.

Calculation of diagrams of components of residual stress tensor arising in the surface layer of a tribomaterial under sliding friction is used to formulate ideas about the role of the medium in creating a plasticizing and strengthening triboeffect. The characteristics of the structural state of the surface layers of brasses, aluminum and tin bronzes during their friction against steel in surface-active lubricants are presented. The conditions for achieving minimum power losses to friction and wear are determined. The role of local diffusion phenomena in quasi-spinodal phase transitions observed in surface micro-volumes of copper alloys during their contact deformation is revealed. It is shown that among industrial copper alloys, brass of the L90 type, aluminum bronzes BrA5, BrA7, and tin bronzes BrOF4-0.25, BrOF6,5-0,15 have the greatest wear resistance in surface-active lubricants. Examples of implementation of the phenomenon of selective transfer in friction pairs are given. Recommendations are developed for its use in friction units of machines and devices.

The effect of heat treatment modes under conditions of phase pre-transformation on the structure and properties of electron-beam-welded joints of titanium alloy VT20 is investigated. It is shown that heat treatment at 980°C for 120 – 150 sec makes it possible to remove residual stresses and to increase the tensile strength of the welded joints to 1230 MPa, as well as to ensure a uniform chemical composition over the volume of the joints to meet the requirements of OST 1 90013–81. The suggested method of heat treatment may be used for local treatment of the heat affected zone and of the welded joint in air environment.

Diffusion kinetics of boron in AISI 1050 steel is investigated. AISI 1050 steel samples are borided for 2, 4 and 6 h at 850, 900 and 950°C by the powder-pack boriding method using Ekabor-II boriding powder. After the powder-pack boriding, about 25.4 – 118.9 μm thick FeB/Fe₂B boride layers are obtained on the surfaces of the samples. The boride layers have a typical tooth morphology and their hardness is 1071 – 1460 HV_0.1. The growth rate constants for the boriding temperatures of 850, 900 and 950°C for the AISI 1050 steel are determined to be 1.522 × 10 ^–12, 2.964 × 10 ^–13 and 6.354 ×10 ^–12 m² · sec ^–1, respectively. The boron diffusion activation energy in the AISI 1050 steel is 162.93 kJ · mol ^–1.

The results of metallographic and x-ray phase studies of 35Kh2N3 steel samples after two-component thermal diffusion saturation with chromium and vanadium in mixtures of different compositions are presented. It is shown that when metallic chromium is replaced by ferrochromium, the structure and the phase composition of the coating change; the amount of carbide and nitride phases decreases and the amount of the Cr – Fe – V solid solution increases. The use of serpentine in the saturating mixture (an emitter of oxygen anions, which creates a flow of charged particles directed to the saturated part) elevates the depth of diffusion of chromium into the metal from 20 to 50 μm. The hardness of the coatings due to chromizing in metallic chromium powder with ferrovanadium and in a mixture of ferrochromium with ferrovanadium amounts to 2000 HV and 1750 HV, respectively.

The effect of the temperature and duration of boriding on the nanohardness and modulus of elasticity of 16MnCr5 steel is studied. Pack-boriding is carried out in a solid medium at temperatures of 1123 – 1223 K for 2–6h using the Ekabor II reagent. The obtained experimental results prove effectiveness of the nanoindendation method for assessing the quality and properties of the borided surfaces of 16MnCr5 steel. It is shown that the values of the nanohardness and of the Young’s modulus of the steel range within 587.5 – 3716.4 MPa and 63.3 – 159.7 GPa, respectively.

The processes of formation of carbide-free bainite and the effect of the temperature-time parameters of isothermal hardening in the range of bainitic transformation on the structure and impact toughness of low-alloy structural steels are studied. A method for nondestructive testing of the impact toughness of medium-carbon steels heat-treated by isothermal quenching is developed on the basis of identified patterns of changes in the properties of retained austenite and in the impact toughens of the steels.