Metal Science and Heat Treatment最新文献

The microstructure of nanostructured bainite formed by austempering in high-carbon silicon steel is characterized by high resolution electron microscopy. The results show that the bainitic sheaves consist of nanoscale subunits. The found kinds of crystal lattice distortions of each subunit in one bainite lath show clear difference. The latest generated subunit has most distortions. As the subunit grows up, the lattice distortion gradually gets relaxed. This indicates that the subunits form with high phase transformation strain energy, which causes severe lattice distortion. The results provide new experimental evidence on subjects relevant to the understanding of the mechanism of formation of bainitic ferrite.

The deformation behavior of duplex stainless steel 2205 is investigated by the method of high-temperature tensile and compressive tests using Gleeble 1500. The results of the tensile tests show that the tensile strength gradually decreases with increase in the deformation temperature; in the temperature range from 1020 to 1260°C the reduction of area is more than 60%. The incoordinate deformation of the two-phase tested steel in the range of 1020 – 1080°C leaves 1100 – 1200°C a suitable range for hot deformation. The results of compressive tests at the temperatures from 1100 to 1200°C show that the flow curve has typical features of dynamic recrystallization; with increase of the deformation temperature or decrease of the deformation rate, the peak stress and strain decrease separately; from this the Zener–Hollomon parameter is determined, which reflects the flow property of the tested steel, and an equation is proposed to describe the hot deformation.

Electron microscopy and x-ray diffraction analysis are used to study the structure of coatings deposited on steel 12Kh18N9T by non-vacuum electron beam cladding of TiC and Ti + C particles. Wear tests of the coated samples are performed under the conditions of friction against fixed abrasive particles and hydroabrasive impact. It is shown that cladding of the Ti + C powder mixture produces composite titanium carbides with dominantly dendritic structure. The reinforcement of the surface layers of steel 12Kh18N9T (AISI 321) with TiC particles doubles its wear resistance.

The structures of metal matrix composites based on the Fe – Cr – Mn – Mo system and fabricated by an aluminobarothermal process with the use of argon or nitrogen as the gas atmosphere in the reactor are compared. Chemical analysis for the content of the metals, nitrogen and oxygen in the samples is performed. The structure is studied in different conditions. The total hardness of the samples and the microhardness of their matrixes are determined. It is shown that the morphology of the reinforcing nonmetallic particles depends on the type of the gas atmosphere in the reactor during the synthesis and remains unchanged after a 2-h hold at 1250°C and water cooling. The determined coefficients of recovery of nitrogen from the nitrided chromium and the data of the diffraction analysis are used to show that the aluminobarothermal synthesis under nitrogen pressure is an optimum variant.

Advanced recent and traditional processes for synthesizing hard alloys based on WC – Co are reviewed. The mechanisms of grain compaction during sintering and the efficiencies of the grain growth inhibitors for repeated precipitation of WC grains are considered. The methods of pressure sintering and rapid sintering are studied. Advantages and disadvantages of different methods are described.

A study of sigma phase formation, its characterization, and its effect on the properties of duplex stainless steel 2205 are presented. The steel has been heat treated according to the mode: heating to a temperature of 850°C, holding during 6 h and cooling in various media, i.e., furnace, air, and water. The optical metallographic methods reveal sigma phases within the ferrite region of the steel microstructure after all the heat treatments; a slightly elevated amount of the phase is detected after cooling with the furnace. Some fine carbides and nitrides are encountered along with the sigma phase. The SEM and EDS characterization confirms that the chemistry of the sigma phase is Fe – 12.2Cr – 2.1Mo. The hardness of the water-quenched steel is 336 HB, which is 47% higher than its hardness before the heat treatment.

The structure and properties of surface layers of billets from steel 12Kh18N9T formed by vacuum electron beam surfacing of powder mixtures are studied. The main alloying component is molybdenum in an amount of 40 wt.%. The structural and phase studies are made using optical and electron microscopes and an x-ray diffractometer. The microhardness and the wear resistance of the deposited layers are determined. The structure of the hardened surface layer is shown to contain phases Mo_0.08Fe_0.92, Mo_0.9Fe_0.1, Mo_0.1Fe_0.9, α-Fe and γ-Fe. The alloying with molybdenum raises the wear resistance of the steel by a factor of 1.8 and the heat resistance by a factor of 6.7.

Oxidation of alloy Ti6242S in the temperature range of 500 – 800°C in an air atmosphere is investigated by x-ray diffraction analysis, nuclear microanalysis, optical and electron microscopy. The effect of the content of oxygen at the oxide/metal interface on the lattice constants of the phases is assessed using XRD. It is shown that the concentration of oxygen in the gas-saturated layer depends on the temperature and the time of the annealing. The results obtained are expected to raise the reliability of prediction of the oxidation behavior of the alloy.

The structure and properties of the surface and of the core of steel 08Kh18N10T-Sh subjected to radial forging and subsequent saturation by the method of ion-plasma nitriding are studied. It is shown that the complex deformation–ion-plasma treatment accelerates the nitriding process and provides a high level of strength of the surface and of the core of steel 08Kh18N10T-Sh. The lower the saturation temperature, the shorter the hold time and the higher the degree of the deformation of the steel, the more deformed α-phase capable to accelerate the saturation process is preserved in the structure.

The study is devoted to pack-powder boriding of ASTM A709 steel in a powder mixture containing boron carbide, potassium fluoroborate and silicon carbide. This surface hardening process is conducted between 1123 and 1273 K for from 2 to 8 h. The formed diiron boride layers are studied by different techniques (scanning electron microscopy, x-ray diffraction (XRD) analysis, glow discharge optical emission spectroscopy (GDOES), Vickers microhardness testing, surface profilometry, Rockwell-C indentation cohesion and pin-on-disc tests). Kinetically, an integral-method-based approach is applied to calculate the boron diffusivities in Fe₂B. The activation energy of the process is deduced and compared to reported data. The same model is ultimately verified by comparing the empirical values of the thickness of the Fe₂B layers obtained at 1223 K and 1273 K for 9 h and the predicted values.