Metal Science and Heat Treatment最新文献

Scanning and transmission electron microscopies are used to study the microstructure of the surface layer of plates from steel 20Kh13 (X20Cr13) after a thermomechanical treatment by friction stir processing(FSP) using a carbide-tipped WC – Co tool. The variation of the microhardness over the thickness of the layer obtained is determined. It is shown that the tool material transfers into the strengthened surface layer upon heating to 1100°C and mixes with the substrate. This results in formation of bands alloyed with particles of tungsten carbide (W₂C) 350 µm in size. Two distinct zones of alloying-induced hardening are identified in the surface layer, which is attributed to the changes in the effect of stirring with the spherical tool tip.

The microstructure and phase composition of a pipe welded joint from alloy HP40NbTi of the Fe – 25Cr – 35Ni system formed by a pulsed gas tungsten arc welding (GTAW) process are studied using light and electron microscopy and x-ray spectrum microanalysis. The short-term and long-term mechanical properties of the joint are determined. It is shown that a more uniform temperature field with a lower temperature level during the pulsed welding prevents active development of the NbC → G phase transformation in the HP40NbTi alloy as compared to the standard GTAW process. This contributes to formation of a more homogeneous structure in the weld metal, greater uniformity of strength in different zones of the welded joint, and elevation of its operating properties.

The effect of residual thermal stresses on mechanical properties of composites with magnesium matrix WE43 reinforced with SiCp particles is studied by the method of finite element simulation. The residual thermal stresses (strains) in the SiCp/WE43 composites are concentrated in the matrix near the interface with the SiCp particles. It is shown that the residual stresses (strains) reduce the mechanical properties of the composites. With increase in the size of the SiCp reinforcing particles, the decrease in the elastic modulus and in the yield strength intensifies and amounts to 14.17 and 4.47% on the average, respectively. The effect of polygonal particles on the reduction of the elastic modulus and of the yield strength of the composites is shown to be higher than that of round particles.

The study investigates the influence of Strain-Induced Melt Activation on physical, microstructural and mechanical behavior of Zn – Al alloys. Initially, the alloy displayed a dendritic structure. Following the SIMA process (casting, forging at 250°C followed by partial remelting at 470°C with holding period of 10 and 20 minutes), the dendritic structure transitioned into a globular and spheroidize morphology. This transformation was attributed to the combined effects of forging and disintegration of primary α (Al-rich) phase dendrites, leading to grain elongation. Furthermore, an increase in the distribution of the eutectoid β (Zn-rich) phase accompanied by a reduction in grain size was observed. SIMA process with holding time 10 minutes results in significant improvement in physical, morphological and mechanical properties of Zn – 27Al alloy.

The effect of post-welding heat treatment on the structure and properties of VPTIG (variable-polarity tungsten inert gas) welded joints from aluminum alloy 2219 is studied. The quenching temperature selected using the results of DSC analysis, is 535°C, the hold time is 60 min. The aging parameters are selected using the results of determination of the microhardness of the welded joint after various heat treatments. The aging temperature is determined to be 175°C; the aging duration is 16 hours. This post-welding treatment mode ensures the achievement of maximum values of the mechanical properties of the welded joint. The tensile strength of the joint after heat treatment is 410 MPa, which is about 50% higher than the strength before the heat treatment.

The mechanism of secondary recrystallization in electrical anisotropic (grain oriented) steel is considered. The dependence of the degree of perfection of the “sharp” (left{100right}langle 001rangle ) texture on the effective temperature of the onset of secondary recrystallization is explained by the different influence of the temperature on special boundaries Σ9 and Σ5. The Σ9 boundary preserves its special structure to higher temperatures than the Σ5 boundaries thus ensuring formation of a “sharp” crystallographic texture (left{100right}langle 001rangle ) due to the high mobility in secondary recrystallization. A hypothesis about the role of the inhibitor phase in the process of secondary recrystallization is formulated.

The microstructure and mechanical properties of an Al-based metallic composite reinforced with a steel grid are investigated. The process used to produce such a bimetallic composite consists in combining high pressure and high temperature ensuring the diffusion of active alloying elements. It is shown that the size of the stacked steel grid has a significant effect on the mechanical characteristics of the composites. The aluminum-based composite reinforced with a finer steel grid has higher mechanical properties compared to the composite with a coarse grid, due to the more homogeneous joining of the Al sheets.

The microstructure and mechanical properties of welded joints of Hardox 400 (18KhGT) steel obtained by MAG (metal active gas) welding are studied. Steel ER 120 SG (Mn3Ni2CrMo) welding wire with diameter 1.2 mm is used for the welding. Static tensile tests of the welded joints show that cracks occur outside the weld metal zone, and the tensile strength and the impact energy are higher compared to the base metal. The highest hardness of 407 HV is observed in the weld metal area mainly due to the presence of martensitic structure. Then the hardness decreases consistently in the direction to the heat-affected zone and the base metal.

The effect of reinforcement with titanium diboride (TiB₂) nanoparticles with an average size of 80 nm on the mechanical properties of AZ91D magnesium alloy was studied. The reinforcing TiB₂ particles in an amount of 0.5 and 1.5 wt.% were introduced through the stir-casting process. Microstructural analysis and static tensile tests were performed to evaluate the effect of reinforcing the AZ91D alloy with TiB₂ nanoparticles. It was found that the addition of TiB₂ nanoparticles significantly improves the mechanical properties of the AZ91D alloy. The ultimate tensile strength increases by 3% for the composite with 0.5 wt.% TiB₂ and by 10% for the composite with 1.5 wt.% TiB₂ compared to the unreinforced alloy. The yield strength also increases by 5% and 14% for the composites with 0.5 and 1.5 wt.% TiB₂, respectively. In addition, the microhardness is significantly increased by adding TiB₂. The results obtained showed that reinforcing the magnesium alloy AZ91D with TiB₂ nanoparticles increases the efficiency of its use for the manufacture of massive engineering constructions.

The microstructure and mechanical properties of low-alloy low-carbon hot-rolled steel after various heat treatment modes are studied. After the austenitization in the intercritical range followed by salt bath holding between B_s and M_s temperatures, the tensile strength of 850 – 1150 MPa and elongation of 17 – 22% are achieved in the steel due to the TRIP effect. After intercritical annealing followed by quenching and holding below M_s, the tensile strength of the steel varies from 830 to 1150 MPa with elongation in the range of 20 – 24% as in dual-phase high-ductility steels. After quenching from a fully austenitized condition to below the temperature M_s and a long hold to achieve single stage quenching and partitioning treatment, the tensile strength of the steel is 980 – 1280 MPa, and the elongation is about 12%. The lowest tensile strength of 920 – 1020 MPa with 11 – 16% elongation is obtained after total austenitizing followed by bainitic holding. After some heat treatment modes, the product of the tensile strength by the ductility exceeds 20 GPa · %, which corresponds to the bottom level of the third-generation advanced high-strength steels (AHSS).