Vol 20, No 2 (2018)

Introduction. Continuous improvement of materials cutting methods provides the appearance of new modifications of technological processes of blanking production, in particular, high precision plasma cutting. However, the equipment manufacturers accompany the proposed technologies with recommendations of processing modes, which are indicative and intended for a certain range of processed materials. The purpose of the paper is to improve the technological schemes of high precision plasma cutting in terms of quantity evaluation of cutting accuracy and surface quality of cut for structural steels, including bimetallic compositions in a specified thickness range. Methods. The evaluation of the accuracy and quality of the cut is carried out in accordance with ISO 9013: 2002. The steel ST3SP and the bimetallic composition “steel ST3 + steel 12H18N10T”, formed by explosion welding are chosen as the study material. Results and discussion. It is established that when HiFocus technology is used for cutting the steel ST3 in the lower thickness range (3 mm), the precision of the cut is not ensured. To increase the accuracy of the cutting the transition to a smaller nozzle size is proposed in this technology. The application of HiFocusplus technology, which is distinguished by the additional swirling of swirling gases, makes it possible to cut materials in a wider range of thicknesses. However, at cutting thicknesses of 4-6 mm, there is an excess of the acceptable deviation of the cut perpendicularity at both its edges. To increase the accuracy of shaping, a reduction in cutting speed is necessary. The efficiency of using Hi Focusplus technology for cutting a bimetallic composition “steel ST3 + STEEL 12H18N10Т” is shown. The optimal cutting scheme is identified with the choice of ST3 steel as the front side. It is established that the maximum cutting accuracy for this composition is achieved at a cutting speed of 1.5 m/min.

Introduction. Aluminum alloys of special purpose are characterized by a certain combination of mechanical, physical, and physical-chemical properties due to operation under strictly defined conditions. In the development of new materials with improved technological properties, much attention is paid to alloys of the Al-Si system of hypereutectic concentration. It is known that the combined modifying agent comprising 2 or more elements outperform each component separately. A large number of ways to modify these alloys with the purpose of grinding the primary silicon crystals and eutectic is developed. Most known technologies are not widely applied in practice, therefore, the development of a method for modifying the melt of hydrogen-containing compounds remains a topical theme. Objective: development of technological method of processing the melt, providing for an increase of hydrogen content, for modifying the structure of the as-cast and obtaining the deformed Al – 15÷30% Si alloys with improved physical and mechanical properties. The parameters of the microstructure in the cast state and after the hot plastic deformation, as well as the mechanical characteristics of the modified alloys, are investigated. The study of the microstructure of the resulting alloys is undertaken. Research methods. Dilatometric tests, mechanical tests for static elongation, as well as metallographic analysis of the investigated alloys are used. The results and discussion. A new method of modification that allows reducing sharply the size of the primary crystals of the siliceous phase, resulting in greatly increased mechanical properties of high-silicon alloys and its deformability is suggested. Application of the developed method allows obtaining the structure of the eutectic type in hypereutectic Al-Si alloys. By obtaining the modified structure, characterized by an increased degree of dispersion of the constituents, a sharp decrease in the dimensions of the primary crystals of the brittle siliceous phase and a favorable change in its shape, plastic deformation of the investigated alloys became possible. It is found that hot deformation has a positive effect on the mechanical properties of Al-Si alloys, especially on its plasticity. It is shown that the complex of physical-chemical properties of deformed semi-finished products exceeds even the properties of sintered aluminum alloys.

Introduction. Boronizing and boroaluminizing are effective methods used to improve the surface properties of machine parts and tools. However, its application in industrial production is often restricted. High brittleness of boronized and boroaluminized layers is one of the restraining factors. Conventional methods of boronizing and boroaluminizing with furnace heating are aimed at the formation of needle and layered structured layers respectively. As a rule, the hardest and most brittle phases are formed on top of these layers, such as FeB and Fe2Al5. The purpose of the work: to study the phase formation sequence in boronized and boroaluminized layers obtained after electron beam treatment in vacuum on the surface of carbon steels. The methods of investigation. Alloying with either boron carbide (electron beam boronizing) or boron carbide and aluminum (electron beam boroaluminizing) is applied. Different modes of electron beam processing are tested: accelerating voltage, beam current and irradiation time. Microstructure, microhardness, element and phase composition of obtained layers are investigated. Results and Discussion. It is established that the phase formation at electron beam alloying with boron carbide occurs according to diagram Fe-B, where iron monoboride FeB is the nucleate phase. FeB iron monoboride crystallizes in the form of rhombic and prismatic crystals and Fe2B appears in the form of rounded dendrites. Thus, FeB crystals come out as being enclosed into Fe2B shells. The remaining liquid crystallizes as a eutectic system during cooling. This pattern formation of layer is also valid for the electron beam boroaluminizing. The only difference is the eutectic’;s composition, which consists of Fe2B phase and solid solutions of aluminum and boron in α-Fe. Generally, the microstructure of obtained layer after electron beam treatment is more preferable than the ones after conventional treatment with furnace heating. The layer structure with hard and brittle FeB surrounded by Fe2B and eutectic lead to an increase in its mechanical properties.

Introduction. Modern engineering copes with different tasks associated with the modification of the structure of the surface layers of metallic materials using high-temperature heating sources. Structural transformations that occur during this treatment make it possible to increase the strength, corrosion and tribological behavior of metals. Titanium and its alloys are widely used in modern industry, but its distribution is limited by a high coefficient of friction and low resistance to wear. An insufficient attention is payed to the problem of titanium and its alloys hardening with the use of high-temperature sources of heating. Analysis of the works related to high-speed heating of titanium-base alloys showed that the laser beam is most often used as a source of surface heating. The Ti-6Al-4V titanium alloy predominantly performs the function of the base material. The samples obtained by surfacing powder mixtures containing titanium diboride (TiB2) and boron carbide (B4C) possess high hardness and wear resistance. However, the thickness of the coatings formed this way does not exceed 1 mm. To produce modified layers of increased thickness it is rational to use the method of electron beam treatment of materials in air. The aim of the work is to study the possibility of cladding of a powder mixture containing boron carbide to modify surface layers of cp-titanium by the method of non-vacuum electron beam treatment. Materials and Methods. Cp-titanium is used as the base material. Plates of base material were treated with a highly concentrated electron beam discharged into air. Powder mixtures with different content of boron carbide powder (10, 20 and 30 wt. %) were used to form particles of the high-strength phase in the surface layers. Modified materials were analyzed by optical and scanning electron microscopy. Studies of abrasion resistance were carried out under friction conditions on fixed and loosely fixed abrasive particles. Results and discussion. The mechanical and tribotechnical characteristics of modified titanium layers are largely determined by structural transformations occurring in the surface layers of the material. The treatment of a titanium alloy with a high-concentration electron beam in air allows obtaining modified layers with a thickness of more than 1 mm. Cladding of a powder mixture containing boron carbide leads to the formation of high-strength particles in the surface-alloyed layers, which have a significant effect on the properties of the base material. Addition to the cladding mixture 10 wt. % of a boron carbide powder allows obtaining qualitative layers containing finely dispersed particles of titanium monoboride and titanium carbide. The volume fraction of the high-strength phase in these layers is ~ 20%. Increasing the concentration of boron carbide in the original powder mixture to 30 wt. % leads to the formation in the structure of modified layers of large primary crystals of titanium boride and titanium carbide of dendritic morphology. An increase in B4C concentration also leads to an increase in the volume fraction of the strengthening phase to 40-44%. A characteristic feature of these samples is the presence of conglomerates of fine particles in the lower coverage zone. The average microhardness of the hardened layers reaches 4 250-6 400 MPa. In the conditions of friction on fixed of abrasive particles, the maximum wear resistance exceeds 2.4 times the same index of the reference sample was recorded during the testing of the alloy obtained by cladding the mixture with 30 wt. % B4C. The same samples showed an eightfold increase in the wear resistance values when the abrasive particles were loosely attached to the material.

Introduction. The development of low-carbon martensitic steels was preceded by the development of low-pearlitic, pearlite-free or bainitic steels. Both groups of steels did not require liquid cooling media for heat treatment, and the strength was at the level of 400-600 MPa. The bainite structure had a higher strength, but bainitic steels have significant drawbacks due to its manufacturability and relatively low viscosity, because it is difficult to avoid the appearance of upper bainite during heat treatment. Modern bainitic steels have strength of 1500 MPa, but it is still difficult to achieve the required reliability characteristics. With a Cr/C ratio greater than 35 wt. % (8 at. %), bainite transformation in low-carbon steels (0.04-0.1% C) is not observed, and such steels are referred to as low-carbon martensitic steels. In the work, steels marked with 07H3GNM, 15H2G2NMFB, 27H2G2NMFB are studied. Objective of the work is to determine the composition, morphology and mechanical properties of low-carbon martensitic steels with nonmetallic inclusions. To assess the effect of the martensite structure on the mechanical properties of low-carbon martensitic steels with strong carbide-forming elements after complete quenching and from intercritical temperature range is also the work objective. Methods of research. To study the structure, a microscope “Olympus GX-51”, a scanning electron microscope “Tescan MIRA3” with energy-dispersive analysis adapter were used. The fine structure and morphology of the phases were studied by transmission and scanning electron microscopy. Foils obtained by electropolishing were used for transmission electron microscopy. Electropolishing was carried out at temperatures close to 0 °C, in an electrolyte of 80% glacial acetic acid and 20% perchloric acid. A fine structure was examined on JEM 200CX and SM 30 microscopes at an accelerating voltage of up to 200 kV. The value of the parameter IC was determined in accordance with GOST 25. 506 - 85, according to the results of tests for static bending of the samples with a crack, type 4, with dimensions 5x10x60 mm. Mechanical properties during tensile tests were determined in accordance with GOST 1497-84, impact strength – according to GOST 9454-78. Critical points were established using differential scanning calorimetry and confirmed by dilatometric studies. Heat treatment of steels included quenching 950 °C, tempering 250 °C in the first case, and quenching from intercritical temperature range in the second. Results and discussion. The main inclusions in low-carbon martensitic steels were aluminum oxides, FeO, MnO, SiO2 oxides, and elongated sulfides (FeS, MnS), which form is close to globular. In steels with strong carbide-forming elements, carbides contained an increased amount of niobium and vanadium. Investigation of the destruction of samples with the structure of low-carbon martensite containing nonmetallic inclusions showed that the main reason for the decrease in viscosity with increasing carbon content is the increase in the fraction of the plate component. In the construction of a model for the destruction of steels with a rack and plate structure of martensite, it proceeded from the additive contribution to the strength of various morphological forms of martensite and the leading role in initiating the destruction of the impermeable interfaces for dislocations of the plate component.