Papers by Author: Christos G. Aneziris

Abstract: Impurities and resulting inclusions are an issue when processing higher amounts of scrap during steel making. To increase the recycling rate, the removal of impurities from the scrap in form of inclusions is of great interest. In previous studies was found that inclusions attach primarily on carbon containing refractories, especially if on their surface an interfacial layer (1–3 µm thickness) was formed in-situ. This study investigates the formation mechanism of this in-situ layer in detail by application of computer tomography (CT) measurements on two scales. The large scale CT scans visualized the general appearance whereas the small scale measurement regarded the in-situ formed layer and the attached inclusions in detail. Based on these measurements, previous results and a literature review it was concluded that the layer formed mainly due to carbothermally reduced impurities which moved to the decarburized surface of the refractory in gaseous form and enhanced sintering of the surface region to develop the layer.

Abstract: Metal-matrix composite materials, based on a metastable austenitic stainless steel reinforced with a magnesia partially stabilised zirconia have been prepared by a ceramics-derived extrusion technology. Using this powder metallurgical method enables the shaping of lightweight cellular structures as well as bulk specimens with a variety of steel/ceramic ratios at room temperature. However, the extrusion of composite structures is limited by the uniform cross section throughout its entire length. Joining of these metal-matrix composite preforms after sintering by conventional welding techniques is a challenging task. The presence of ceramic fractions may lead to several complications and the subsequent heat exposure during joining may initiate phase transformations in both metastable components resulting in a deterioration of the mechanical properties of the composite material. An adapted ceramics-derived joining technology allows the combination of varying TRIP-steel/zirconia composite materials. The main features are the machining and joining of the parts in their dry green state at room temperature before their thermal treatment. Thus, the material’s consolidation and the formation of the joint take place simultaneously. The ability of joining different parts offers the possibility to create structures for complex applications and testing conditions. The key to advanced properties of the joining zone are the base materials, the surface treatment of the parts, and the paste used for joining. The joining process of different base materials, the mechanical properties, and the microstructure of sinter-joint samples are presented.

Abstract: The current contribution deals with metal-matrix composites prepared by paper manufacturing technology. In contrast to conventional techniques, this technology is an energy-and cost-efficient process for the shaping of thin sheets using solid powder mixtures. Conventional and pre-ceramic as well as pre-metallic paper-manufacturing have in common that cellulose (pulp) fibres are loaded with inorganic fillers. The present study is focused on the paper web formation using a metastable austenitic steel powder (16-7-3 %Cr-%Mn-%Ni) and a magnesia partially stabilised zirconia powder as inorganic fillers. The paper web formation was investigated. During filtration of the aqueous fibre-filler suspension the steel particles were incorporated in-between the fibre network and steel clusters were formed. Thus, solid retentions of > 90 wt.% were achieved. Calendering had a positive influence on porosity, bulk density, and tensile strength of the green paper sheets. The development of an optimized debinding process is presented and the microstructural changes as well as phase formations during firing are discussed in response to the residual carbon content. The sintered composites attained ultimate tensile strengths of up to 177 N/ mm2 at a total porosity of 66 %. These metal-matrix composites are promising materials for the shaping of light-weight structures.

Abstract: A novel and innovative process for full as well as hollow metal-matrix composite bead fabrication using gel-casting process by alginate gelation is investigated. In particular the influence of four different alginates and various solidifying agents have been investigated regarding the formation and stability of full and hollow beads. The suspension contained a metastable austenitic steel powder (16Cr7Mn3Ni) and zirconia particles as well as different alginates and was added dropwise into water containing different solidifying agents for forming beads. With the aid of sodium and potassium alginate dropped in solution with CaCl₂, full metal beads have been obtained. Hollow beads have been produced using potassium alginate and Ca (OH)₂ as solidifying agent and show a hollow space fraction of 65%. At lower strains (up to 15 %), all zirconia reinforced full steel beads obtain higher specific energy absorption (SEA) in comparison to pure steel beads.

Abstract: Composite materials and micro- and macrostructure designs have been the focus of numerous scientific studies over the past few years according to their crashworthiness [1-3]. Crashworthiness is concerned with the absorption of energy through controlled failure mechanisms and modes that enable a defined load profile during energy absorption [4]. Cellular materials, such as metal foams, are materials which display a unique combination of physical and mechanical properties, e.g. for crash box applications. The defining characteristic of metal foams is a very high porosity, typically in the range of 70 to 90 vol. %. In principle, cellular metals can be manufactured from gas, liquid or solid phases and currently the most advanced methods involve melt-metallurgical processes [5]. Several groups have produced foam structures by using hollow spheres to form the cells of the material [5, 6]. These materials exhibited plateau stresses of 5 MPa and 23 MPa respectively, with volume specific energy absorptions SEA of 2 MJ/m³ and 10 MJ/m³ respectively, up to 50 % strain [6, 7]. By combining ceramics with ductile metals, failure-tolerant metal matrix composites (MMCs) can be created. With regard to application of the MMCs as wear resistant materials in metal forming tools a prolongation of the life time and the resultant reduced equipment downtimes have been achieved by active steel infiltrating of porous zirconia structures with the aid of Ti as activator [8]. A very promising approach concerning zirconia/steel - composite materials with superior mechanical properties has been demonstrated by Guo et al. using a low-alloyed TRIP steel in combination with an Y-PSZ – ceramic [9, 10]. In a previous study honeycomb structures were formed from composites of high-alloyed austenitic stainless TRIP-steel AISI 304 with Mg-PSZ with different mixing proportions due to ceramic extrusion at room temperature and sintering at 1350 °C for 2 h in an 99.9 % Argon atmosphere [11]. One of the most promising manufacturing route to produce open cell composite foams is based on the patent of Schwartzwalder [12] by the replication method using polyurethane sponge as a template. The polymer foam is impregnated in a powder slurry (this first coating contributes as an adhesive porous layer for further coating processes), the ceramic slurry is squeezed out of the functional pores and cold spray coatings are applied in order to eliminate defects out of the squeezing process and reach the critical wall thickness for acceptable mechanical properties. In [13] the authors reported about foams with 90 Vol% high alloyed TRIP-steel and 10 Vol% Mg-PSZ. Up to 50 % compressive strain a remarkable enhancement of the SEA was observed in comparison to comparable structures with TRIP-steel only.

Abstract: In terms of this work formulations of carbon bonded castables based on new binder approaches and nanoadditions will be demonstrated. The new binder system allows the manufacturing of water based magnesia carbon castables with the same properties and chemistry of pressed magnesia carbon bricks. This binder can be also applied in oxide castables offering them high refractoriness and workability during processing. According to the workability nano-additions improve significantly the spreading diameter of carbon castables and as a result their flowability.