Victor Geanta

Stainless Steels with Biocompatible Properties for Medical Devices

Stainless steels, commercial as well as with special properties, are the principal metallic materials used for medical devices manufacturing. Stainless steels for medical devices should have superior mechanical properties, as: hardness, wear resistance, tensile strength, elongation, fracture toughness, creep resistance etc. This paper aims to present experimental researches regarding the obtaining in vacuum arc remelting device (VAR) of austenitic and martensitic stainless steels and their characterization from microstructure and microhardness point of view.

Chemical elements diffusion in the stainless steel components brazed with Cu-Ag alloy

The paper presents the study of diffusion of chemical elements through a brazing joint, between two thin components (0.5mm) made of stainless steel 304. An experimental brazing filler material has been used for brazing stainless steel component and then the diffusion phenomenon has been studied, in terms of chemical element displacement from the brazed separation interface. The filler material is in the form of a metal rod coated with ceramic slurry mixture of minerals, containing precursors and metallic powders, which can contribute to the formation of deposit brazed. In determining the distance of diffusion of chemical elements, on both sides of the fusion line, were performed measurements of the chemical composition using electron microscopy SEM and EDX spectrometry. Metallographic analysis of cross sections was performed with the aim of highlight the microstructural characteristics of brazed joints, for estimate the wetting capacity, adherence of filler metal and highlight any imperfections. Analyzes performed showed the penetration of alloying elements from the solder (Ag, Cu, Zn and Sn) towards the base material (stainless steel), over distances up to 60 microns.

Hafnium influence on the microstructure of FeCrAl alloys

Due to their special properties at high temperatures, FeCrAl alloys micro-alloyed with Zr can be regarded as potential materials for use at nuclear power plants, generation 4R. These materials are resistant to oxidation at high temperatures, to corrosion, erosion and to the penetrating radiations in liquid metal environments. Also, these are able to form continuously, by the self-generation process of an oxide coating with high adhesive strength. The protective oxide layers must be textured and regenerable, with a good mechanical strength, so that crack and peeling can not appear. To improve the mechanical and chemical characteristics of the oxide layer, we introduced limited quantities of Zr, Ti, Y, Hf, Ce in the range of 1-3%wt in the FeCrAl alloy. These elements, with very high affinity to the oxygen, are capable to stabilize the alumina structure and to improve the oxide adherence to the metallic substrate. FeCrAl alloys microalloyed with Hf were prepared using VAR (Vacuum Arc Remelting) unit, under high argon purity atmosphere. Three different experimental alloys have been prepared using the same metallic matrix of Fe-14Cr-5Al, by adding of 0.5%wt Hf, 1.0%wt Hf and respectively 1.5%wt Hf. The microhardness values for the experimental alloys have been in the range 154 ... 157 HV0.2. EDAX analyses have been performed to determine chemical composition on the oxide layer and in the bulk of sample and SEM analyze has been done to determine the microstructural features. The results have shown the capacity of FeCrAl alloy to form oxide layers, with different texture and rich in elements such as Al and Hf.

Obtaining and Characterization of Biocompatible Co-Cr as Cast Alloys

Biocompatible metallic materials used for surgical implants in human body should have superior mechanical properties, as: microhardness, wear resistance, tensile strength, elongation, fracture toughness, creep resistance etc. These results can be also obtained by strict setting and controlling of chemical composition, by adding chemical elements that lead at the improvement of the mentioned properties. In this paper has been presented experimental researches regarding the obtaining process of some biocompatible Co-Cr cast alloys, having different chemical compositions, using the method of vacuum induction melting (VIM). After the obtainment process, samples were characterised from microstructure and microhardness point of view.

Dissimilar Brazed Joints Between Steel and Tungsten Carbide

Brazing is a joining process used to obtain heterogeneous assemblies between different materials, such as steels, irons, non-ferrous metals, ceramics etc. Some application, like asphalt cutters, require quick solutions to obtain dissimilar joints at acceptable costs, given the very short period of operation of these parts. This paper presents some results obtained during the brazing of dissimilar joints between steel and tungsten carbide by using different types of Ag-Cu system filler materials alloyed with P and Sn. The brazing techniques used were oxygen-gas flame and induction joining. The brazing behaviour was analysed in cross sections by optical and electron microscopy. The metallographic analysis enhanced the adhesion features and the length of penetration in the joining gap. The melting range of the filler materials was measured using thermal analysis.

Tribological Behavior of Composite Electrodes for Spot Welding

The paper presents some aspects regarding tribological characteristics of composite tops of the electrodes for spot welding. The solution aims to improve the resistance to deformation and high intensity electric current conductibility without loses and excessive local heating. The composite material was obtained by mechanical alloying process, from a mixture of tungsten carbide, chromium, CuNiAl alloy and cooper powder which has been heated in furnace at 1100°C. The embedding process aims to avoid the toxic effects of metallic elements such as Be and Zr, which are usually introduced in cupper alloys for improving the mechanical characteristics. The tribological characteristics and microhardness of the melted zone were measured in order to estimate the strengthening effect obtained by the tungsten carbide presence for different mixture recipe.

Armor with Hyper-Entropic Behaviour and Self-Protection

The paper presents research conducted in order to achieve some self-shielding armor with high and pre-established strength to intense wear abrasion combined with corrosion, in damp environment, with sulfur content, at temperatures up to 500°C. The first stage was dedicated to the elaboration of the concept of armor and the hardfacing technology, using a hyper-entropic filler metal which assures a self-protection characteristic. The samples used in the experiment were mild steel plates covered with weld in the rhomboid form. In the next stage some investigations were conducted in order to manufacture of new welding materials such as Fe-22%Cr-Mo-V deposited by welding in certain circumstances, having the hardness after welding about 30 HRC. This weld deposit becomes harder after a short working time, having average hardness value of 55 HRC. In stage three some exploratory research was done in order to establish the welding parameters for obtaining the self-protection layers of hyper-entropic material. In the final stage the armor element achieved was tested, to determine performance characteristics during working into the mill fan. After 2 weeks, some samples were sectioned in order to measure the hardening effect during working.