Papers by Keyword: Microstructure

Abstract: This paper presents a method for fabricating a porous Ni-Al₂O₃-Al compact using uniaxial double-action pressing, which was subsequently infiltrated with molten aluminium. Al₂O₃ ceramic particles primarily serve to create porosity within the composite compact. Due to the difficulty pressing hard metal powders, aluminium powder was introduced into the Ni+Al₂O₃ mixture to act as a plasticizer, improving the material's compressibility. Experiments indicated that the optimal infiltration temperature was 750 °C with an infiltration duration of 300 seconds. To evaluate the reaction extent among the initial components, a subset of infiltrated samples underwent annealing at 800 °C for 3 hours under an inert argon atmosphere. Both annealed and reference samples were subjected to thermal cycling. The microstructure and thermal stability of the resulting composite materials were analyzed and characterized using scanning electron microscopy with energy-dispersive spectroscopy, respectively.

Abstract: It is well known that the work hardening process of low-carbon steels is highly dependent on the movement and accumulation of dislocations in the crystal grains, which affect the stress and strain magnitudes and their distribution. The aim of this paper is to explain the importance of dislocation movement and density on the temperature, i.e. stress and strain changes during cold plastic deformation of low-carbon steels. Therefore, tests were carried out in this paper using the methods of static tensile testing, thermography, digital image correlation (DIC) and microstructural analysis. The microstructure analysis was carried out using a light and transmission electron microscope (TEM). The transmission electron microscope analysis was performed in two different modes, the TEM and scanning TEM (STEM). The results of static tensile testing, thermography and digital image correlation (DIC) are related to the microstructural changes that occur during the work hardening process of low-carbon steel. At the moment of maximum work hardening (immediately before fracture), significant grain elongation and high dislocation density of low-carbon steel were observed.

Abstract: Austenitic stainless steels (ASSs) are characteristic with a combination of good mechanical and corrosion properties. Therefore, they are used in the primary circuits of nuclear power plants. Under the influence of a corrosive environment containing chloride ions and mechanical loading, the phenomenon of stress corrosion cracking occurs in ASSs. SCC can also be initiated by the surface condition of ASSs. Machining is usually the last stage of production, during which a significant deformed zone with high residual tensile stresses can be created, which can accelerate the initiation of stress corrosion cracking. Research is focused on analyzing the influence of final turning on microstructural changes of the surface-machined layer caused by various turning parameters (e.g.: cutting speed, feed, depth of cut, cutting tool geometry). No significant microstructure changes were observed between the samples by light microscopy, so we focused on transmission electron microscopy (TEM) on thin lamellas prepared using the focus ion beam (FIB) technique. TEM observation confirmed the presence of a deformed zone and a passivation layer. In the case of the sample that was turned with a higher feed and cutting speed, the passivation layer was discontinuous. Such a microstructural change can significantly affect the corrosion resistance of ASS.

Abstract: To prepare bulk single-crystal REBCO superconductors by the new single-direction growth method (SDMG: Single-Direction Melt-Growth), it is necessary to produce a large-area seed of high quality, for example, based on EuBCO. Since the samples prepared by the SDMG method copy the structure of the seed, for the production of large-area seeds it is necessary to optimize the time-temperature regime in order to grow seeds with a suitable structure and composition and minimize structural defects (limiting the amount of subgrains and others). A higher growth rate was used in comparison with the standard growth rates used to produce EuBCO seeds of larger dimensions. The increased growth rate in the crystal growth window reduces the outflow of the melt from the sample, and thus it is possible to achieve a single-crystal sample in the entire volume of the precursor. The samples were produced at different growth rates: 1; 2; 3 and 5 °C/h. The microstructure of the samples was studied by polarized light microscopy and scanning electron microscopy. The size and distribution of Eu211 particles in the sample volume and the subgrain structure were studied on the fabricated samples.

Abstract: This study investigated the impact of as-printed and heat-treated additively produced 2507 super duplex stainless steel (also known as SDSS) on microstructure and hardness distribution. Optical microscopy was used to examine the phase transformations of the steel during the as-printed (untreated) and solution-annealed treatment stages of samples. The relationship between microstructure and hardness distribution (center and edge) was studied. Because the LPBF process cools rapidly, the SDSS shows that the main phase in as-printed samples is ferrite, with 5% austenite. The fully balanced microstructure is forming when the solution annealing is performed, with austenite content about 52%. The hardness of SDSS is strictly related to the material microstructure, where the fully ferritic structure shows higher hardness 50.16–46.18 HRC, while the balanced duplex microstructure reveals lower values 34.58–32.26 HRC.

Abstract: During underwater wet welding, the environment has a corresponding effect on the mechanical properties of the weld metal. The use of external electromagnetic action (EEA) during welding is promising for influencing the formation of welded joints and the structure formation in physically inhomogeneous environments. Experimental studies have demonstrated the effectiveness of EEA application in reducing the tendency of weld metal to form pores, enhancing degassing, and lowering hydrogen content etc. The paper presents a metallographic study of the welded joint metal of structural steel (St3) after underwater welding with 12Kh18N9T filler wire, both without and with the use of EEA. Based on calculation methods and predictive modelling, optimal operating modes of the electromagnetic system for an experimental study of the EEA effect during underwater welding have been established and implemented. It has been established that the weld metal mainly has a ferrite-pearlite structure, while an austenitic structure with elongated grains is formed in the weld metal. When using the EEA, the grain structure of the weld metal is refined by an average of 1.5 times with an insignificant decrease in microhardness. In the heat-affected zone (HAZ), in the areas of large grain (I HAZ), recrystallisation (II HAZ) and incomplete recrystallisation (III HAZ), a bainitic structure is formed in the presence of ferrite layers. Under the influence of the EEA, the grain size is refined by 1.2 times in the I HAZ and II HAZ with a decrease in the thickness of ferrite layers and an increase in microhardness by an average of 7 ... 10%. The formation of such a structure will provide a set of strength properties and toughness of the welded joint metal. Research has proven that the technology of wet welding under water using the EEA allows for the production of high-quality welded joints with a high set of physical and mechanical properties of the metal of both welded joints and the HAZ.

Abstract: Phase components of experimental low cost titanium alloys, their substructure and parameters, dislocation structure, features of phase formation in the metal, which differ in alloying systems, were studied using complex research methods. The stoichiometric composition of dispersed phases in the internal volumes of alloy grains was determined by diffraction patterns using transmission electron microscopy. It is shown that in the structure of titanium alloy Ti-2,8Al-5,1Mo-4,9Fe there are dispersed nanoparticles of intermetallic phases of different morphology and stoichiometric composition. These are the phases: Ti₃Al and Fe₂Ti with a size of 10…40 nm; Mo₉Ti₄ - 20…120 nm. Studies of titanium alloy Ti-1,5Fe-O showed the presence in the structure of mainly nanoparticles of oxides: Ti₃O₅ size 10…30 nm and Ti₄Fe₂O, FeTiO₅ (10…90 nm), as well as intermetallics Fe₂Ti (10…40 nm). It is established that the formation of nanoparticles of intermetallic and oxide phases in the thin plate structure of the investigated experimental low cost titanium alloys promotes the formation of the substructure with uniform distribution of dislocation density. This provides a high level of mechanical properties of alloys.

Abstract: Food-grade piping and water transportation systems extensively use dissimilar welding between stainless steel and carbon steel, where cost-effectiveness and corrosion resistance are essential consideration. However, the fusion zone of dissimilar welds often observed microstructural inhomogeneities and hardness changes, thus compromising mechanical qualities and corrosion resistance. This study was seperated in two phases to investigate and optimize dissimilar welding between carbon steel and stainless steel in both plate and pipe applications. Phase 1 studied welding A36 carbon steel plate and A304 stainless steel using gas tungsten arc welding (GTAW) with ER308L filler metal, the effect of post-weld heat treatment (PWHT) holding time on the mechanical and microstructural propertie. PWHT was performed at 650 °C for 20 and 60 minutes. The 20-minute condition yielded an optimal combination of mechanical strength and microstructural refinement, while the 60-minute condition led to grain coarsening and reduced strength. Phase 2 extended the findings to pipe welding applications, adopting the 20-minute PWHT condition. Welding was performed on dissimilar joints between A106-B carbon steel pipe and A312 TP304L stainless steel pipe (2-inch OD) using ER308L and ER309L filler metals under 99.99% argon shielding. Tensile and hardness testing indicated that welds with ER309L offered superior mechanical performance. Microstructural analysis revealed delta-ferrite and stabilized austenite in the fusion zone, with enhanced Cr and Ni concentrations contributing to improved corrosion resistance, as confirmed by electrochemical testing.

Abstract: Quasicrystalline ingots of the following compositions were obtained and studied: Al₆₃Cu₂₅Fe₁₂; Al_62.735Cu₂₅Fe₁₂Sc_0.265; Al_62.56Cu₂₅Fe₁₂Sc_0.44. It is shown that 0.01% Sc is contained in the icosahedral quasicrystalline phase (Al_65.1Cu_22.57Fe_12.33Sc_0.01). In addition to this scandium is contained in the intermetallic Al_51.45Cu_37.29Sc_8.23 (hexagonal W phase). It is established that alloying of the quasicrystalline Al-Cu-Fe alloy with scandium in the amount of 0.44 at. % significantly increases the content of the quasicrystalline icosahedral phase in ingots from 50 to 65 vol.% which leads to an increase in the microhardness of ingots. The microhardness of Al-Cu-Fe quasicrystalline ingots in the range of 20 - 400 °C is weakly dependent on temperature and amounts to 7-8 GPa. With increasing temperature, the microhardness value begins to decrease and at 720 °C reaches a value of 0.5 GPa. The study of the electrochemical characteristics of the corrosion process of quasicrystalline ingots showed that alloying of Al-Cu-Fe quasicrystalline alloy with Sc decreased the corrosion rate of non-annealed samples, and simultaneously lowered both the corrosion potential and the pitting potential.

Abstract: Relevance of the work. One of the key challenges currently faced by manufacturers of steel wire rod for welding applications worldwide is achieving maximum plasticity in the metal before delivery to hardware plants. This need arises due to the ongoing technological advancements in hardware plants and the integration of modern equipment capable of producing wire at high drawing speeds with significant degrees of unit deformation. Such processes demand high-quality raw materials (wire rod). Another crucial aspect is ensuring a high-quality surface for the welding wire, free from microcracks and hard inclusions. These defects can negatively impact the welding process by increasing metal spatter, causing electric arc instability, and leading to critical defects in welded joints, such as cavities and cracks. Rational microstructural design is a key principle in producing highly plastic wire rod for the efficient manufacturing of solid-section welding wire from low-carbon alloy steels. The presence of hard components (such as martensite, bainite, or their mixtures) in the wire rod’s structure is undesirable, as they increase the risk of metal failure during intensive cold deformation by drawing. Minimizing these hard phases remains an urgent scientific and practical challenge. The aim of the study is to determine the mechanism of martensite grains formation during slow cooling of low-carbon alloy steel grade CrMoV1Si wire rod, used for producing welding wire, from the austenitization temperature to room temperature. Material and methodology. The material used in this study was steel with the chemical composition (in wt.%) 0.08C–1.30Mn–0.54Si–1.06Cr–0.54Mo–0.24V–Fe_(balance). The samples were subjected to thermal cycles using an automated Gleeble 3800 thermal deformation simulation system. The thermal cycle involved heating the samples to the temperature required for complete austenitization, holding them at this temperature, and then continuously cooling at controlled rates of 0.20, 0.10, and 0.05 °C/s to room temperature. Metallographic analysis was conducted using an optical microscope and a scanning electron microscope. The hardness of individual structural components was measured using a microhardness tester following the standard method. The chemical composition of the phases was determined by energy-dispersive X-ray spectroscopy and Auger electron spectroscopy. Results. The study established an anomalous increase in the volume fraction of austenite shear transformation products in CrMoV1Si steel after continuous slow cooling at rates of 0.20–0.05 °C/s, at the same time, martensite had a relatively high carbon content (up to 1.6 wt.%). The authors attribute this microstructural evolution to dynamic changes in the chemical composition of individual phases during cooling, primarily due to the partitioning of carbon and other alloying elements between the α and γ phases, as confirmed experimentally. Based on the obtained results, a mechanism has been proposed for the formation of high-carbon martensite grains in low-carbon alloy steel wire rod during slow cooling from the austenitization temperature to room temperature.