Metal Science and Heat Treatment最新文献

The effect of high-temperature annealing pretreatment above Ac_cm on the evolution of bainitic austempering microstructure in Fe – 0.95C – 1.37Si – 1.04Cr – 0.44Mn (wt.%) steel is investigated by differential thermal analysis, x-ray diffraction and high-resolution transmission electron microscopy. It is found that a novel nanobainite with a needle-like morphology forms in the experimental steel under conventional bainitic austempering after annealing pretreatment at 950°C. A thin film crystal of the novel nanobainite with a thickness as small as 50 nm has a body-centered cubic lattice, in which the opposite stacking fault is the main crystal defect. The interface of the novel nanobainite crystal is composed of both coherent and non-coherent areas. The steel containing the novel nanobainite structure has a higher hardness than that with conventional bainite.

The effect of continuous cooling on the development of the bainite transformation process and the mechanical properties of low-carbon (0.17% C) steel alloyed with 1.73% Mn, 1.35% Si and microalloyed with Nb (0.02%) and Ti (0.04%) is studied. The microstructure and phase analysis is carried out at various continuous cooling conditions. The work hardening behavior under various conditions is examined using the Hollomon equation and modified Crussard–Jaoul work hardening model. The steel is subjected to austenitization in the intercritical range and above the A₃ temperature between 790 and 900°C for 5 and 30 min, followed by continuous cooling to form a dominantly bainitic microstructure, which makes it possible to achieve an ultrahigh strength (1100 MPa) at a high ductility (20%). A longer austenitization time contributes to an increase in the strength and ductility of the steel.

The effect of processing on the microstructure and mechanical properties of cast duplex stainless steel 2507 is studied. It is established that the structure of hot-rolled steel contains δ-ferrite and austenite (γ) bands that change due to 50% cold rolling to form microstructural constituents of a more refined and fragmented nature of a duplex (γ +δ) phase. Austenite grains are most effectively refined (up to 5 μm) because of the recrystallization occurring during aging treatment. The cold-rolled steel has maximum strength and minimum elongation. The best combination of strength and ductility is achieved after hot deformation + solution treatment at 1040°C with water cooling and after hot deformation + solution treatment at 1300°C with water cooling followed by isothermal aging at 1000°C. Such processing ensures a good formability of the steel.

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}.