Metal Science and Heat Treatment最新文献

The wear of normalized (N) and dual-phase (DP) low-carbon (0.08% C) and medium-carbon (0.42% C) steels under air-jet erosion at 15° and 90° impact angles of alumina particles is studied. The DP steels are produced via intercritical annealing heat treatment of low- and medium-carbon steels at 745°C for various holding times (2, 3, and 4 min) followed by water quenching, which results in formation of a varying amount of martensite in the structure. The results indicate that the hardness and the UTS of the low- and medium-carbon dual-phase steels increase with increase in the martensite content. Material removal occurs by cracking, crater formation, and low-angle cutting at 90°, whereas ploughing and chipping of the material occur at 15°.

The chemical and phase compositions of rolling scale of low-carbon sheet steel at the Izhora Plant are studied experimentally. The results are compared to the data of the scale studies at various metallurgical enterprises reported by other authors. It is shown that the main chemical elements in the scale composition in all the cases are Fe and O plus some Mn, Cr, Al, Ca, and C. The phase compositions of the scale are also similar and contain hematite, magnetite, and wustite. However, the proportion of the phases over cross-section of the scale varies significantly and depends on the oxidation conditions. It is inferred that in order to reuse the scale in steelmaking, its preliminary study is required in each specific case.

The effect of current pulsing during gas tungsten arc welding (GTAW) of titanium alloy Ti – 6Al – 4V on the structure and properties of the weld is studied. Beads on plate welds are prepared on 2-mm-thick Ti – 6Al – 4V sheets using continuous- and pulse-current GTAW(keeping the heat input the same in both conditions). When welding, an indigenously developed shielding setup is used for protection against atmospheric contamination without any external shielding. The weld characteristics are determined by visual inspection, metallographic analysis, and microhardness testing. It is established that the use of current pulsing during GTAW provides finer grains, deeper weld penetration, and increased hardness in the weld zone and in the HAZ. The weld bead has acceptable quality and a silver color on the top and root sides.

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.