Metal Science and Heat Treatment最新文献

The effect of quenching and tempering on the microstructure, phase composition and mechanical properties of steel 0.33C – 1.8Si – 1.44Mn – 0.58Cr is investigated. The structure is studied using scanning and transmission electron microscopy, dilatometry and differential scanning calorimetry. Tensile and impact strength tests are performed. The Rockwell hardness is measured. The processes of formation of carbides in the steel at different temperatures are analyzed, as well as the influence of the structure on the mechanical properties. Quenching yields laths of martensite with carbon-saturated dislocation boundaries due to formation of Cottrell atmospheres. The internal stresses reach 40% of the yield strength σ_0.2. Tempering to a temperature of 280°C causes precipitation of transition η-carbides (Fe₂C) inside the laths, which depletes the martensite matrix of carbon by about 90%. This increases the yield strength by 16% to 1130 MPa despite the lowering of the internal stresses and growth of the width of the laths. Precipitation of cementite chains over the lath and block boundaries occurs in tempering at 500°C, which is accompanied by lowering of the yield strength to 1130 MPa. Calculations of the proportion of carbon atoms in the martensite shows that almost all of the carbon present in the steel goes to formation of cementite. The decomposition of martensite during tempering increases the toughness but has a minor effect on the strength-ductility parameter σ_r × δ (MPa ∙ %).

Processes of deformation and fracture of a natural ferrite-martensite composite under a local high-speed impact are considered. Targets fabricated from steel 14G2 with an isotropic structure of tempered sorbite and from the composite are studied after impact tests. It is shown that the target from steel 14G2 with sorbite structure is pierced through under the concentrated high-power impact of a bullet. When the target is made of the composite, the bullet ricochets. The mechanisms of the processes occurring in the targets under the impact of a bullet are analyzed.

The effect of quenching and aging on the microstructure and mechanical properties of welded joints obtained by laser welding of titanium alloy Grade 2 and aluminum alloy 2024 is studied. Microstructural analysis of the welded joints is carried out using light and scanning electron microscopes. The samples are tested on a slice. It is shown that during heat treatment, the columnar heat-affected zone of aluminum undergoes a significant change, in which the grain width increases from 5 – 7 to 20 – 40 μm, and the length from 30 – 100 to 100 – 200 μm. The shear strength of the welded joint after a heat treatment increases from 120 to 195 MPa, and the microhardness of the columnar heat-affected zone changes from 100 – 111 HV to 145 – 161 HV. The effect of the heat treatment on the distribution of chemical elements in the weld metal is not revealed, whereas the columnar heat-affected zone contains presumably a hardening Al₂Cu phase.

Features of formation of surface layers in steel VKS-10 (13Kh3N3M2VFB-Sh) under thermal and thermochemical treatments, i.e., laser processing with and without surface fusion and subsequent vacuum nitriding, are considered. The effect of preliminary laser treatment on the kinetics of saturation of the surface with nitrogen is studied. It is shown that the preliminary laser treatment without fusion of the surface followed by nitriding provides a higher effective thickness of the layer on steel VKS-10 and higher wear resistance of the surface than the treatment with fusion.

Special features of the process of synthesis of oxide dispersion-strengthened (ODS) ferritic/martensitic steels by powder metallurgy and laser technologies are considered. The process of production of ODS steels is described. The properties of the steels and the possibilities of their additive manufacturing are analyzed.

The results of studies of hot cracks formed in cast shapes from low-alloy steel G21Mn5 are presented. Macroand microstructural analyses are carried out. Fractography studies of samples cut from castings used in the production of railway freight cars are performed. The criteria used to assess the susceptibility of alloys to hot brittleness are considered. It is shown that the orientation of dendritic crystals affects significantly the formation of hot cracks, and the direction of the dendrite growth is determined by the conditions of heat dissipation, which are influenced in their turn by the geometry of the casting. The phenomenon of coalescence under the conditions of misorientation of neighbor dendrites and its effect on the formation of hot cracks is studied. A modified RDG criterion is suggested, which allows for the influence of the geometry of the cast product on the formation of hot cracks through the angle of misorientation of neighbor crystals.

The method of chemical cleaning of the surface of blades of gas turbine units in a solution of orthophosphoric and sulfuric acids using a purpose-designed closed-type autoclave microwave system “Mars 5” and ultrasonic exposure in distilled water is described. The two-stage cleaning involves the chemical action of the acid solution in the autoclave and ultrasonic cleaning in the stage of washing. The effect of acid solutions of various concentrations on the degree of surface cleaning from metal oxides and microcracks in the gas turbine blades is studied. It is shown that when the samples of gas turbine blades are treated in the “Mars 5” autoclave with a solution of acids and with ultrasound in the washing stage, the residual oxygen content reflecting the presence of oxides on the surface of the samples is less than 1%.

A method for obtaining composite surface-alloyed castings from gray cast iron SCh25 by cavityless casting with the use of master alloys containing chromium and ferroboron deposited as a paste layer on the surfaces of the casting molds is considered. The microstructure and the phase composition of the surface layer and the microhardness of the structural components are determined. Mechanical tests of the cast samples for compression and wear are carried out. Self-propagating high-temperature synthesis is shown to occur between the components of the master alloys deposited on the surfaces the molds during the production of the castings. The types and compositions of the borides and carboborides formed in the alloyed layer are determined. The effect of the parameters of the alloyed layer on the properties of the composite castings is analyzed.

The possibility of four-layer (Ni – Cu – Ni – Sn) electroplating and epoxy paint coating of a NdFeB magnet from a hard magnetic alloy used in wind generators is considered. The corrosion-protective effect of various paint coatings on the magnet in aggressive environments is determined. It is shown that the electroplate layuer should have a thickness of at least 20 µm and the epoxy paint coating should be no thinner than 15 µm.

The boundary conditions for describing the mass transfer of carbon from oxygen-free low-pressure atmospheres into steel during vacuum carburizing are analyzed. The effect of the content of carbide-forming alloying elements on the values of the carbon potential of low-pressure acetylene atmospheres is determined depending on the temperature of the process of vacuum carburizing. Temperature dependences of the carbon mass transfer coefficient are derived.