Metal Science and Heat Treatment最新文献

The effect of cold drawing on the evolution of microstructure and texture of near eutectoid steel wire is studied to understand the deformation micromechanism of the wire drawing process. The texture evolution during wire drawing is simulated using a viscoplastic self-consistent (VPSC) model and successfully validated by experimental results. The favorable pearlitic colonies having lamellas aligned along the wire axis are found to undergo thinning and form a fibrous structure. On the other hand, the lamellas oriented perpendicularly to the wire axis are found to undergo bending and kinking in the process of aligning themselves with the wire axis. A (langle 011rangle ) crystallographic texture develops after the wire drawing from the (langle 111rangle ) and (langle 110rangle ) texture of the as-received steel. The VPSC simulation illustrates the relative significance of the {112}(langle 111rangle ) slip in comparison to that of the traditionally used {110} (langle 111rangle ) slip. The VPSC simulation also illustrates presence of about 5 – 6 active slip systems (AVACS) that make it possible to maintain strain compatibility in polycrystalline materials.

The effect of austempering temperatures varied from 300 to 400°C to form bainite in the structure on the corrosion resistance and mechanical properties of two newly developed FeNi steels containing 1 and 2 wt.% Al is investigated. Corrosion tests of the steels are carried out in a 3.5 wt.% NaCl solution. It is found that an increase in the Al content and in the austempering temperature promotes formation of a bainite lath structure in the steels; however, this has an insignificant effect on the hardness. The results of an electrochemical test show that the corrosion resistance of the steel in marine-like atmosphere increases upon increasing the Al content from 1 to 2 wt.%. The increase of the austempering temperature tends to improve the corrosion resistance of both steels, except for the steel with 1 wt.% Al processed at the highest temperature. It is shown that the corrosion resistance of the steels is more affected by the hardness than by the volume fraction of bainite laths in the structure.

Electrode-induction gas atomization (EIGA) is a promising and cost-effective method for producing spherical intermetallic titanium powders used in additive manufacturing. The present study investigates the morphology, microstructure, chemical and phase compositions, nanoindentation properties of a commercial pre-alloyed TiAl-based EIGA alloy powder. The Ti – 48Al – 2Cr – 2Nb powder is characterized by a spherical shape and dendritic structure with the dendrites enriched with Nb and the interdendritic areas enriched with aluminum and chromium. The temperature ranges of the phase transformations in the powder with precipitation/dissolution of phases α₂ , α, γ, B2, (TiNb)Cr₂ are determined using thermal analysis during heating and cooling in an inert atmosphere of argon at a rate of 50 K/min. Development of an oxidation process at the temperatures above 500°C and significant increase in its rate at the temperatures above 900°C is detected.

Impact bending tests of rolled products, base metal and welded joints of pipes manufactured from low-carbon microalloyed steels are performed. The structure of different parts of welded joints is studied. Fractures of impact samples and compositions of nonmetallic inclusions on fracture surfaces are analyzed. The embrittling mechanisms and the causes of scattering of the impact toughness are considered. These may include coarse bainite within a large (deformed) austenite grain, a region near the fusion line with an unfavorable orientation of crystallographic cleavage planes {001} in a HFC welded joint, and large grains of grain-boundary ferrite in the weld and in the HAZ under arc welding. The critical grain size of the α-phase (the maximum fraction), at which the embrittling effect of the nonmetallic inclusions begins to be observed in the steels is 50 – 80 μm.

Metal matrix composites based on Fe – Cr – Mn – Mo – N – C system and obtained by the aluminobarothermic variant of self-propagating high-temperature synthesis (SHS) are studied. The possibility of uniform carburizing of a melt obtained by aluminobarothermic synthesis in the cooling crucible is shown. An artificial neural network model is suggested, which makes it possible to predict the carbon content in the studied composite during carburization in the cooling crucible of an SHS reactor (the average approximation error is 9 – 14% depending on the training method). The results of training of the artificial neural network model using the Adam optimization algorithm and the Levenberg–Marquardt method are compared. It is shown that under the conditions of a limited set of initial data, it is effective to use a perceptron with one hidden layer containing three target neurons and one displacement neuron.

The effect of air quenching and subsequent tempering on the structure, mechanical properties and fracture mechanism of steel 10Kh9K3B2MFBR alloyed with tantalum and boron is studied. Transmission electron microscopy is used to show that after the air quenching, the steel acquires a structure of lath martensite with nanosize particles of (Nb, Ta)(C, N) carbonitride and cementite. Residual film-like austenite morphology is located over the boundaries of laths and blocks. The steel in this state is characterized by a high strength (σ_0.2 = 1020 MPa) and an impact toughness of 24 J ∙ cm². During the low-temperature tempering, dispersion hardening and decomposition of retained austenite with precipitation of cementite chains along the lath boundaries lead to embrittlement. Increase of the tempering temperature in the range of 500 – 750°C is accompanied by a monotonic decrease in the strength characteristics. However, a significant increase in the impact toughness occurs only at tempering temperatures ≥ 780°C. It is concluded that the relatively high tempering temperature required to ensure a satisfactory impact toughness is explainable by enhanced precipitation of particles of a M23C6-type carbide and (Nb, Ta)(C, N) carbonitride, which retards the retrogression processes in the lath martensite structure.

The process of high-temperature oxidation of multicomponent multiphase HP40NbTi alloy at 1150°C with duration of up to 500 hours in air is studied experimentally at the micro level using SEM and x-ray mapping. It is shown that the process occurs under the influence of various interrelated mechanisms. The changes in the composition and structure of the surface multilayer scale and of the subsurface diffusion zone in the alloy after oxidation of various durations are studied consistently. It is demonstrated that after a long-term exposure, conditional equilibrium is established in the alloy subsurface area; the formation of scale and the internal oxidation deplete the diffusion zone of chromium and silicon. This is compensated by the diffusion of Cr, Ni and Fe from the bulk of the metal to the surface. The process slows down with time due to the increase in the depth of the diffusion zone and formation of a “barrier” layer of silicon oxides.

Analysis of published results of thermal fatigue studies carried out by various methods of thermocycling testing is presented. The widely used Coffin’s method and its feasibility for describing the dependence of the durability on the plastic deformation in a cycle (ε_pl) is considered. It is shown that the range of the variation of ε_pl provided by the method is too narrow for covering the processes occurring in the materials during operation of the articles. This fact is confirmed by evaluation of ε_pl in testing of corset samples at the I. I. Polzunov Scientific and Development Association on Research and Design of Power Equipment, the test portion of which repeats the shape of failure areas frequently occurring in actual articles due to thermal fatigue. It is shown that during the testing process, cracks appear already in the first loading cycles and are a manifestation of the structural changes in the material. This is a result of an excessively high ε_pl, which is confirmed by a theoretical estimation. It is recommended to use a similar approach to simulate thermal fatigue in articles and to vary the plastic strain in them by changing the shape of the shoulders of corset samples without changing the thermal regime in testing.

Refractory high-chromium alloy VZh159 (Haynes 282) obtained by the method of selective laser melting (SLM) is studied. C-curves of the precipitation of particles of a topologically closed-packed (tcp) phase are plotted for the temperature range of heat treatment and operation. It is shown that the content of the tcp-particles of unfavorable morphology increases and the ductility of the synthesized material during synthesis in a protective nitrogen environment decreases with increase in the chromium content in the alloy. The intensification of the precipitation of the particles of the tcp-phase is associated with decrease in the solubility of chromium in the solid solution at high temperatures due to elevation of the nitrogen content in the alloy. To stabilize the alloying system, the chromium content in alloy VZh159 is reduced by 1 wt.% with respect to the upper and lower alloying limits as compared to the deformable counterpart, which makes it possible to prevent marked lowering of the ductility of VZh159 fabricatedby SLM.

This investigation focuses on the microstructure characterization as well as the mechanical properties of A713 matrix composites reinforced with TiB₂ and graphene particles. The A713 – TiB₂ – graphene composites are made using the stir casting process. The particle reinforcement with graphene (0.5 wt.%) is maintained, whereas the reinforcement with TiB₂ (2%, 4%, and 6 wt.%) is varied for fabrication of the composites. Optical micrographs, x-ray diffractometry and scanning electron microscopy are used to analyze the microstructure of the composites. The mechanical properties of the composites are analyzed by measuring their hardness and tensile strength. A fractured tensile test specimen is subjected to a SEM analysis to analyze the fracture mechanism.