Advanced Materials Forum III

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Authors: A. Paúl, A. Beirante, Nuno Franco, Eduardo Alves, José Antonio Odriozola
Abstract: High temperature phase transformations in EUROFER reduced activation ferritic martensitic (RAFM) steel were studied in-situ by means of X-ray diffraction. Results show that, during slow cooling, the austenite to ferrite transformation takes place around 755 oC. Full transformation of the austenitic phase into pure martensite is observed for cooling above 5 oC/min. This transformation was found in samples annealed at 950 oC for 3 h and quenched in liquid nitrogen. TEM analyses reveal a high concentration of carbides along the grain boundaries of the martensitic structure. The thermal expansion coefficient derived from the measurements was 12.7x10-6 K-1.
Authors: César A.C. Sequeira, Fernand D.S. Marquis
Abstract: The effects of aluminising on the hot corrosion resistance of two nickel-chromium alloys containing 10 and 30 weight percent chromium, respectively, were examined. The Ni/Cr specimens were aluminised by pack cementation in Ar and underwent cyclic hot corrosion testing in Na2SO4- NaCl molten salt. XRD analysis indicated that a NiAl phase formed between the coating layer and substrate. Cyclic hot corrosion test results indicated that the effects of aluminising are more pronounced in the case of the 10 % Cr than in the case of 30 % Cr. The ductile NiAl phase suppressed the potential for crack initiation during thermal cycling of the 10 % Cr specimens, and reinforced the hot cyclic corrosion resistance in molten salt for the 30 % Cr specimens.
Authors: Mónica Reis, Rui Louro, Paulo J. Morais, Leonel Santos, Helena Gouveia
Abstract: The Friction Stir Welding (FSW) process is a quite recent joining method whose particular characteristics yield to materials modifications not yet fully understood. This paper aims to present the research results of a study focused on the modifications induced in an aluminium alloy AA5083 – H111, when processed by FSW to build components for structural applications. The welded samples were firstly analysed by optical microscopy in order to define the different joined zones and to identify defects. Further tests included the measurement of the Vickers microhardness, grain and particle distribution and chemical analysis of the constituents, particularly of the intermetallic second phase particles, by Auger spectroscopy. Special attention has been given to the characterisation of several microstructural “non-homogeneities” like a laminated structure called onion ring and an identified anomaly (in the form of a line), whose presence was evaluated in order to determine its effects on the final joint properties. The results have shown important new details about the relationship between the microstructure and the final joint structural properties, contributing to improve the knowledge about the materials behaviour increasing the potential of utilization of the FSW process.
Authors: C. Steven Wright, M. Youseffi, S.P. Akhtar, T.H.C. Childs, C. Hauser, P. Fox
Abstract: This paper presents the results of a recent comprehensive investigation of selective laser melting (slm) of prealloyed gas and water atomised M2 and H13 tool steel powders. The objective of the study was to establish the parameters that control the densification of single and multiple layers with the aim of producing high density parts without the need for infiltration. Powders were processed using continuous wave (CW) CO2 and Nd:YAG lasers. Relationships between alloy composition, powder particle size and shape, flowability, microstructure (phases present, their size, morphology and distribution), track morphology, post scanned density, surface finish and scan conditions (Laser power, spot size and scan speed) are discussed for single track, single layer and multi-layer (up to 25 layers) constructions. Processing with a Nd:YAG laser with powders placed on substrates rather than on a loose powder bed gave more stable builds than with the CO2 laser. Using the Nd:YAG laser densities up to ~90% relative were possible with H13 powder compared with a maximum of ~70% for M2 in multi-layer builds. Maximum density achieved with CW CO2 processing was only ~60%, irrespective of powder composition. The paper compares the processibility of these materials with stainless steel powders processed to higher densities (up to 99% relative) under similar conditions. The results of the work show that a crucial factor for high density processing is melt pool wettability and this is controlled largely by carbon content; low carbon contents producing better wettability, flatter tracks and higher densities. The significance of this observation for the processing high alloy steels by slm will be discussed.
Authors: Henrique Santos, Joana Leal, Carlos Sá
Abstract: Duplex stainless steels (d2s) are intended to present a microstructure composed of similar volume fractions of austenite and ferrite; this is the phase composition leading to the best compromise of mechanical and corrosion properties. To stabilize the austenite phase, nitrogen is added as an alloying element in the so-called second generation d2s; nitrogen is a cheap element (compared to nickel, the most common austenite stabilizer) that simultaneously increases the strength by a solid solution hardening effect; usually nitrogen contents in the second generation d2s vary from 0.15 to 0.25 weight %. The effect of a 0.14 % nitrogen addition to a cast d2s with a nominal composition corresponding to EN 10283 has been studied to evaluate its austenite stabilizing effect and its influence on the partition intensity of the elements contributing to the pitting corrosion resistance (chromium and molybdenum) between the two phases composing the microstructure. Two cast d2s have been prepared for this study: one without any nitrogen addition and another with 0.14% nitrogen; both had the same nominal composition. Specimens of the two cast d2s have been heat treated in a range of temperatures from 1050 to 1400°C, every 50°C; after a 2 hour soaking the specimens have been cooled in water. It is known that increasing the solution temperature increases the volume fraction of ferrite in the cast d2s microstructure. Both the volume fraction of each constitutional phase (austenite and ferrite) and the respective chromium and molybdenum content have been measured. The results of the volume fraction measurements showed an intense austenite stabilizing effect of the nitrogen. The results of the microanalysis of chromium and molybdenum showed that both elements present an ability to concentrate in the ferrite, although the molybdenum partition is clearly more intense than the chromium partition.
Authors: João M.G. Mascarenhas, Manuela Oliveira, C. Steven Wright
Abstract: This paper presents the results of an investigation into the sintering behaviour of three vanadium enriched variants of T42 high-speed steel. Powders were prepared by water atomization with vanadium and carbon contents of 6-8 wt% and 2.2 -2.7 wt%, respectively. These were annealed, die pressed and sintered in vacuum. All three alloys were sintered to full density giving “as –sintered” microstructures comprising globular MC carbides dispersed in a martensitic matrix. Optimum sintering temperatures were in the range 1240 - 1250°C with lower optimum temperatures associated with higher carbon levels. Sintering characteristics are correlated with phase diagrams calculated using ThermoCalc™ software and TCFe2000 database. The implications for the design of sinterable vanadium containing high-speed steels are discussed.
Authors: Luís G. Reis, Bin Li, Manuel de Freitas
Abstract: For a safe and reliable design of components, it is needed to study the effects of multiaxial loading and particularly the non-proportional loadings on the fatigue damage. The objective of this paper is to evaluate and compare the additional hardening effects of proportional and non-proportional loading paths. Low-cycle fatigue behaviour of three structural steels: CK45 (ferritic-perlitic microstructure) normalized steel, 42CrMo4 (bainitic microstructure) quenched and tempered steel and stainless steel (austenitic microstructure) X10CrNiS 18 9 are studied under different proportional and non-proportional loading paths and different levels. A series of tests of biaxial low-cycle fatigue composed of tension/compression with static or cyclic torsion were carried out on a biaxial servo-hydraulic testing machine Instron 8088. The experiments showed that the three materials studied have very different additional hardening behaviour, under multiaxial cyclic loading paths. The local cyclic stress/strain states are influenced by the multiaxial loading paths due to interactions between the normal stress and shear stress during cyclic plastic deformation. The microstructure is an important key and has a great influence on the additional hardening. The additional hardening effect is dependent of the loading path and also the intensity of the loading.
Authors: Pedro Teixeira, Altino Loureiro, Dulce Maria Rodrigues, José Neves
Abstract: This paper presents the results of an investigation aimed at correlating the gradient of microstructures with the mechanisms of fracture and with J-toughness in the heat-affected zone (HAZ) of submerged arc welds on a high strength structural steel. The microstructure of the weld metal is composed of refined acicular ferrite while the microstructure of the coarse grained heataffected zone (CGHAZ) is basically formed by coarse bainite with a small proportion of martensite and primary ferrite in the grain boundary of the large prior austenite grains. A significant decrease in toughness is observed with an increasing incursion of the fatigue crack front into the CGHAZ. The degradation in toughness is definitely brought about by the CGHAZs of the welds, the size and distribution of these zones being relevant factors in the weld performance.
Authors: D. Braga, Amilcar Ramalho, Pedro Nuno Silva, Albano Cavaleiro
Abstract: Steels continue to have a preponderant role in mechanical components under all type of wear solicitations namely, abrasion. The ability of micro-scale abrasion test for evaluating the properties of bulk materials has been widely demonstrated. However, only recently this technique was especially developed to characterize thin-coated materials. This study presents results obtained in micro-scale abrasion tests performed on different low and high alloy steels. These steel samples underwent thermal and chemical (nitriding) treatments with the aim of enhancing their surface hardness. Nitriding parameters were varied so as to obtain different structures (with and without formation of a “white layer” of iron nitrides (ε-Fe2-3N or γ’-Fe4N compound layer). Test conditions such as normal load and concentration of the abrasive medium (SiC particles in distilled water) were changed in order to obtain a 2 or 3 body wear contact type. Results obtained allowed to compare the specific wear rate ks for the different steels and treatments tested as well as to relate the influence of surface hardness and test parameters on the wear mechanisms.
Authors: António Rodrigues, Altino Loureiro, António Castanhola Batista
Abstract: The aim of this research is to study the effect of the welding conditions and of an activating flux on the microstructure of the melted material of autogeneous A-TIG welds, made on austenitic stainless steel AISI 304. The increase of heat-input coarsened the microstructure, changed its morphology and decreased the δ-ferrite content retained in the microstructure. The use of a TiO2 activating flux does not significantly affect the microstructures. A thin layer tends to form in the surface of the welds, which microstructure and ferrite content are different from those observed in its core.

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