Metal Science and Heat Treatment最新文献

Various methods of hardening of machine parts and mechanisms are considered. Volume-surface quenching of steels with reduced hardenability is analyzed. The structure, hardness and toughness of steels subjected to different types of quenching are investigated. It is shown that volume-surface quenching ensures a uniform distribution of hardness from the surface to the base metal of the steels, and their toughness is higher than that of steels subjected to volume or surface quenching. The method of volume-surface quenching is recommended for wide use in the production of vehicle parts.

Using the synergistic effect of the influence of Cr, Mo, Nb, V, Cu, and other elements on the properties of steels, a low-alloy structural steel with high performance properties has been developed. The microstructure, mechanical properties and corrosion resistance of the steel have been studied. The steel structure is multiphase and consists of polygonal ferrite and M/A islands. The yield strength of the steel at 600°C is at least 2/3 of the yield strength at room temperature. During 360 h of the corrosive effect of salt mist, the corrosion rate of the steel gradually decreases with increase of the time, and the structure of the rust layer is transformed from loose to dense. The mechanical properties, heat resistance, corrosion resistance and heat resistance of steel meet the requirements for materials of building structures.

The fatigue resistance characteristics of an aluminum-based experimental dispersion-hardened composite material obtained by internal oxidation are investigated. Fatigue tests are carried out under loading by a “soft” scheme of cantilever bending of cylindrical rotating samples under symmetric-cycle conditions. The structural and deformation features of the behavior of the material during cyclic deformation are studied by optical direct, digital, and scanning electron microscopy. The fatigue resistance characteristics of the composite are scattered little. The mechanism of nucleation and propagation of fatigue cracks is determined. Analysis of a fatigue fracture surface is performed.

The influence of an alternating magnetic field in the austenitic temperature range during structural-wave magnetic resonance treatment (SWMRT)combined with thermocycling treatment (TCT) on the structure, physical and mechanical properties of low-carbon martensitic steel 08Kh2G2NMFB is investigated. The changes in the microstructure after the treatment are determined by metallographic, electron-microscopy and x-ray diffraction analyses. The elasticity parameters after the SWMRT are studied. Steady values of the temperature frequency coefficient are obtained for steel 08Kh2G2NMFB after the SWMRT + TCT in the range of (113 – 116) × 10 ^{– 6} K^{– 1}.

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.