Metal Science and Heat Treatment最新文献

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.

Influence of thermal aging (873 K, 5000 h) on microstructure and tensile-plastic flow behavior of normalized and tempered (N&T) 1.4W – 0.06Ta IN-RAFM steel is investigated. The Ludwigson and Voce equations are used to elucidate the tensile-plastic flow response of the IN-RAFM steel over a wide temperature range of 298 – 873 K. Both the yield strength and the tensile strength are fitted by the initial stress and saturation stress, respectively, as per the Voce constitutive equation. The strain hardening exponent increases under aging due to the increase in the work hardening capability of the aged steel, while the strain hardening coefficient decreases with the aging. TEM specimens extracted from a tensile tested sample are used to correlate the formation and the movement of dislocation debris in the structure of both N&T and thermally aged steel. The absolute value of n_v decreases due to the aging exhibiting a two-stage behavior. The acceleration of the recovery process is lower at the high temperature in the steel subjected to thermal aging as compared to the N&T steel. The Voce relation is used successfully to predict the yield stress and the ultimate tensile strength of both thermally aged and N&T IN-RAFM steel at different temperatures.

Steel 30KhGSA is studied after a quenching and partitioning (Q&P) heat treatment and after traditional quenching. Structural and x-ray analyses are performed and mechanical properties are determined. The Q&P treatment produces a martensite-austenite structure with 10 vol.% retained austenite. The use of Q&P instead of the traditional quenching elevates the impact toughness by a factor of 3 and the resistance to fatigue crack growth by a factor of 2. A long hold at room temperature (for more than a month) reduces the content of retained austenite in the Q&P-steel to 7.5 vol.% and lowers its impact toughness and crack resistance by a factor of 1.5 as compared to the state right after the hardening.