Metal Science and Heat Treatment最新文献

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.

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.