Metallurgist最新文献

Axial chemical inhomogeneity in metals is a common issue in the production of thick-sheeted products from microalloyed pipe steels. To study the behavior of liquation zones during rolling, a model has been developed to describe and analyze the stress-strain state throughout the rolled section, including the axial chemical inhomogeneity zone. This mathematical model was implemented using the finite element software Abaqus. This paper investigates the influence of deformation direction (longitudinal and transverse), degree of deformation, and metal temperature on the changes in chemical inhomogeneity during thick-sheeted rolling. It has been found that the degree of deformation has the greatest influence on the transformation of chemical inhomogeneity. The results suggest that rolling with a reduction of at least 10–15% is optimal, since this level of deformation significantly enhances the fragmentation of chemical inhomogeneity. This is achieved by increasing the stress and strain intensities in the axial zone, which helps break the bonds between different areas of chemical inhomogeneity of the metal.

Heat-resistant nickel alloys are promising materials for manufacturing structural elements used in high-temperature nuclear power plants. Nickel alloys are characterized as difficult-to-weld materials; this is one of the main factors limiting their use. This paper presents the results of the development and industrial exploitation of welding technology for heat-resistant nickel alloy grade CrNi₆₂MoCh₂-VI (EC 199-VI). The quality control results of the obtained welded joints are presented. Notably, the welded joints of the CrNi₆₂MoCh₂-VI alloy exhibit resistance to intercrystalline corrosion and demonstrate improved mechanical characteristics.

On the basis of the results of experimental studies aimed at the investigation of the influence of dielectric permittivity of a working liquid on the size characteristics of powders obtained under the conditions of electroerosion metallurgy of the wastes of TN20 tungsten-free hard alloy, it was discovered that the higher dielectric permittivity of water leads to a greater loss of the pulse energy required for its breakdown than in the case of breakdown in ethanol and the formation of powder characterized by a smaller average size of particles, namely, 14.3 and 28.4 μm, respectively. The results of conducted experimental studies enable us to solve the problem of recycling of the wastes of tungsten-free hard alloys and their subsequent utilization, thus reducing the cost of production of cutting tools.

Presently, the main trend in thin-sheet steel production, including the high-strength automobile steel, is to reduce production costs while retaining essential properties and quality. A considerable part of the cost of high-strength steel comes from alloying with expensive chemical elements such as vanadium and niobium. Simply reducing the content of these elements in finished products would compromise the mechanical properties of rolled products, leading to inferior quality and defects. This work, based on statistical analysis for steel grade S355MC, demonstrated the potential to reduce alloying while maintaining the required yield strength by adjusting the hot rolling conditions. It was also revealed that, in addition to the concentration of chemical elements and the hot rolling mode, the thickness of the finished product affects the tensile strength. Therefore, the analysis was performed exclusively on products with a thickness of 4 mm. The study resulted in a regression equation that illustrates the dependence of yield strength on vanadium content and hot rolling parameters.

The paper addresses the importance of using aluminum for the production of drill pipes. It also discusses one of the problems associated with the production of drill pipes having a protective thickening (central upset), such as the formation of a “streak delamination” type defect in the protector formation zone. Pipes with internal end and central upsets made of alloy D16T were selected as study objects. The extrusion process was performed under the following two experimental conditions: workpiece metal temperature—440 °C, punch travel speed—1.4 mm/s (option 1) and workpiece metal temperature—4 00 °C, punch travel speed—0.9 mm/s (option 2). It has been experimentally established that lowering the workpiece metal temperature by 40 °C and punch travel speed by 0.5 m/s during the extrusion process promotes the formation of a homogeneous macrostructure. By simulating the described experimental options of the processes, it was found that a 40 °C decrease in the workpiece metal temperature and a 0.5 m/s decrease in the punch travel speed during the extrusion process lead to a 50% reduction in the maximum deformation rate in the deformation zone, and to a 14% reduction in the degree of deformation. By calculating the stress condition parameter in the deformation zone, it became possible to show that performing the process according to option 2 results in a more favorable stress condition of the metal compared to option 1, which could also contribute to the reduction in the number of “streak delamination” type defects in the protector zone.

The results of structural studies and assessment of the bending strength of a VKNA-4U alloy based on nickel aluminide are presented. It is for the first time that this alloy was produced by spark plasma sintering of mechanically activated powders of the starting components. The mechanical activation of the powders lasted for 1.5, 3.5, and 5 min. X‑ray diffraction analysis was performed to determine the optimal duration (3 min 30 sec) of mechanical activation for this alloy. To obtain a dense low-defect workpiece, the VKNA-4U alloy was sintered at a heating temperature of 1100 °C, a pressure of 40 MPa, and a holding time of 5 min. This material has higher ultimate bending strength at room temperature (σ_bend = 1215 ± 65 MPa) than the strength of Ni₃Al intermetallic compound produced in similar conditions and microhardness 690 ± 25 HV.

A method has been proposed to enhance the mechanical properties of hot-finished sintered steels, which consists of additional plastic deformation with a degree of deformation, at which the intracrystalline bonding of the material is preserved. The effect of ultrafine silicon nitride particles on the structural formation and properties of hot-finished sintered steels is analyzed.

The influence of laser welding modes on the structure and properties of butt joints formed for the models of cast sample—cast sample, wrought sample—wrought sample, as well as a hybrid joint of a cast and a wrought sample of Al3Ca0.5La1Mn aluminum-calcium alloy, was studied. The influence of welding parameters on the quality of the weld was evaluated using structural and mechanical tests. The optimal welding parameters were identified as follows: laser power of 1400–1600 W, welding speed of 20 mm/s, focal length of 193 mm, and argon flow rate supplied to the welding zone of 15 L/min. This resulted in the formation of a high-quality welded joint with low porosity and the absence of cracks. The variation in the mass fraction of alloying elements in the process of laser welding of aluminum-calcium alloys was revealed. Under optimal laser welding modes, calcium evaporation in the melting zone is approximately 6.5–10.3%. The obtained joints exhibit a factor of safety of 0.71–0.83 of the ultimate strength of the base metal.

Laser surface alloying is a method used to modify a metal surface to produce a thin surface layer with improved mechanical properties. The results of tests on surface carburizing and boriding of steel 20 using a laser beam are presented. Before laser irradiation, the surface of the material was coated. Two types of coating were used: suspensions of graphite powder and boron carbide. Steel 20 was carburized and borided by melting the coating and the substrate. This processing produced various microstructures in the surface layer, depending on the type of coating used and the processing parameters. The microhardness of the alloyed layers was measured along the depth axis. The surface layer consists of an alloying zone and a heat-affected zone. Some specimens carburized by surface alloying displayed cracking and porosity due to carbon enrichment and high solidification rates. The laser-borated surface has a remelting zone consisting of an eutectic mixture of iron borides FeB, Fe₂B, and Fe₃B. The dense boride zone located near the surface, while the content of the eutectic mixture of borides and martensite increased toward the base metal. The microhardness of the borated layer was 1200 HV. The alloy layers were tested for abrasive wear.

The article presents a method for increasing the operating time of a cyclic unit by combining the operations of torch and gas-dynamic gunning through addition of magnesium briquetted flux.