Key Engineering Materials Vol. 647

Abstract:

The β – Ti alloy with 35 wt. % Nb and 6 wt. % with various oxygen addition (between 0.05 and 0.8 wt. %) has been studied in this work. The alloy was arc melted in a protective atmosphere of helium into water cooled copper crucible. Subsequently the material was thermo-mechanically treated (i.e. homogenized, hot forged, solution treated and cold swaged). Samples of the alloy with the lowest oxygen content were also subjected to aging treatment 450°C for 8 hours. The yield stress R_p0.2, tensile strength (R_m), elongation (A), reduction area (Z) and Young's modulus (E) depending on the oxygen content and on microstructure were studied. Also the microstructure analysis by using the light and electron microscopy has been performed.

Small oxygen addition increases both yield and tensile strength in cold swaged or solution treated conditions. The same can be also said in a lesser extent about Young’s modulus. The decrease in elongation is more distinct for oxygen addition higher than 0.5 wt. %. The oxygen content in this alloy should not exceed this value for practical applications. The increase of strength due to precipitation strengthening leads to significant increase in modulus in comparison with strengthening caused by oxygen addition (when comparing similar strength values achieved).

Abstract: The work analyses microstructures of ZnAl4Cu1, ZnAl27Cu2 and ZnAl40Cu2Si2 alloys in cast condition and after heat treatment in relation to microhardness HV_0.2 and purity of the alloys. As shown the microstructure of ZnAl4Cu1 alloy contained η phase and eutectoid, which consisted phases α + η. ZnAl27Cu2 alloy contained α phase, eutectoid and ε phase, and ZnAl40Cu2Si2 alloy had a similar composition, but moreover contained Si particles. Microhardness in the molten state in ZnAl4Cu1 alloy was HV_0.2 = 94, in ZnAl27Cu2 alloy it was HV_0.2 = 117, and in ZnAl40Cu2Si2 alloy it was HV_0.2 = 142_.

Abstract: The aim of this work was to describe the dependence of microstructure of NiTi shape memory alloy on the conditions of powder metallurgy processing route. The technology consisted of blending of elemental Ni and Ti powders, uniaxial cold pressing and reactive sintering. The effects of reactive sintering temperature, heating rate, holding duration and particle size were determined. The proposed technology can be used as the alternative production route of NiTi to minimize the contamination of the alloy.

Abstract: Nanocrystaline nickel was prepared by selective leaching technology. Consequently, the powder was compacted by spark plasma sintering method. This process is suitable due to its high heating rates, which leads to relatively low thermal exposition of compacted material. The dependence of structure of compacted material on preparation conditions is described in this paper.

Abstract:

Cemented carbides belong among materials with high hardness and wear resistance even at temperatures around 700 °C. These properties are due to carbide composite structure which is formed mainly of tungsten carbide (WC) in combination with a metal matrix (usually cobalt). A synergistic effect that has a positive impact on the final properties is obtained by the combination of hard carbides and a soft matrix. The high hardness of the cemented carbides is associated with a decrease in fracture toughness which in the case of cutting tools is an important property. It is therefore necessary to measure the value of fracture toughness and thus monitor the state of the material. In practice, the fracture toughness of cemented carbides is usually tested by indentation methods of metallographic samples. Therefore, this work focuses on the comparison and optimization of computational models for determining fracture toughness using indentation methods. Eight types of cemented carbides used for the manufacture of cutting tools were tested. Fracture toughness of selected cemented carbides was measured after heat loading.

Abstract:

The present paper explores the impact of laser cladding parameters on the corrosion behavior of the resulting surface. Powders of Inconel 625 and austenitic Metco 41C steel were deposited on steel substrate. It was confirmed that the level of dilution has profound impact on the corrosion resistance and that dilution has to be minimized. However, the chemical composition of the cladding is altered even in the course of the cladding process, a fact which is related to the increase in the substrate temperature. The cladding process was optimized to achieve maximum corrosion resistance. The results were verified and validated using microscopic observation, chemical analysis and corrosion testing.

Abstract: Cr₂N-Ag nanocomposite thin films, containing 11 wt.% Ag solid lubricant, were deposited on substrates made of Cr-V ledeburitic tool steel Vanadis 6 by reactive magnetron sputtering at a deposition temperature of 500 °C, using pure Cr and Ag targets, in a composite low pressure Ar/N₂ atmosphere. The films are composed of Cr₂N-matrix and Ag particles. The films have an average thickness of 4.7 μm. They grew in a columnar manner, but, individual silver agglomerates were visible along the columnar crystals, also. The average size of silver agglomerates lies well below 50 nm. Annealing of the films at temperatures below the deposition temperature do not changed composition of films, but induce higher precipitation of silver particles on the surface of films at the temperatures 300 and 400°C. On the other hand, film annealed at 500°C manifested more remarkable Ag-redistribution, being represented by the decrease in population density of Ag-particles on the surface. The key parameter for transport Ag lubricants from Cr₂N matrix to the surface is the working temperature. Cr₂N-Ag films could be used for specific tribological applications through proper investigation of the working temperature and conditions.

Abstract: Microstructural analysis and microhardness measurement of experimental repair weld joints were carried out in heat affected zone (HAZ) of the base material (BM) – steel 15Ch2NMFA after weld deposition of several layers by welding metal of Inconel FM 52. A temper bead welding (TBW) technique was used. Required microhardness up to 350 HV0.1 in heat affected zone of base material in distance 0.5 mm from fusion line was exceeded in all three experimental blocks with various welding energy inputs during deposition of three or five weld layers. Metallography showed reaustenitization and structure transformation in HAZ after the deposit of first layer in all three blocks. After deposit of the last layer the structure of HAZ in two blocks was tempered only and one variant of them have been recommended for futher technological tests. Structure of HAZ of the third block was not full tempered even after the last layer deposit.

Abstract: The paper explores laser welding of high-strength CP-W^® 800 steel and forged steel. Dissimilar welds were produced with different welding parameters. Two welding beam diameters were used. The microstructure and mechanical properties of the resulting welds were examined. Metallographic sections were prepared and microhardness and tension tests were carried out. Fatigue tests were conducted on the weld specimens to map the effects of welding parameters on the fatigue behaviour of the dissimilar weld between the CP steel and the forged steel. Greater amount of heat input impairs the strength of the joint but improves its resistance to initiation of fatigue cracks.

Abstract: The use of new construction materials is increasingly on demand for the construction of new power plants, and for the modernization of existing plants that are at the end of their service life span. Steels such as type P/T92 (modified martensitic 9-12% Cr), and low-alloy steels derived from modified steel 2.5Cr1Mo (ASTM marked P/T23) are considered promising alternatives. In the construction of power units, the so-called heterogeneous joints, which most often consist of a combination of low-alloyed materials and highly-alloyed ones, preferably need to be avoided. These welded joints are often the weak links in the overall construction. Knowledge of the behaviour of creep-resistant steel welded joints is very important for the subsequent evaluation of the life span of the units. This study deals with the degradation of heterogeneous welded joints of steel T23 - T92 and 15CH1M1F - P91, using the commercially available welding materials Thermanit MTS 616 (highly-alloyed), Union I P23; ThermanitP23, Böhler P23-IG and Thermanit (FOX) P23 (low-alloyed). In the heterogeneous welds examined, slight loss of strength of the base material was observed during isothermal heat exposure and extension of the diffusion active zones (i.e., Carbon Depleted Zone (CDZ) and Carbon Enriched Zone (CEZ)). These degradation processes caused structural instability of heterogeneous welds. It was found that the use of low-alloyed welding materials showed superior structural stability than highly-alloyed welds. Additional laboratory analyses are warranted due to the extreme service conditions and the high temperature loads in power units.