Materials Science Forum Vols. 636-637

Abstract: Friction Stir Welding (FSW) is a relatively young technology in the field of welding. Its process parameters and their influence on weld quality are currently the topic of intense research. Present work focuses on the influence of the tool plunging force on the mechanical properties, microstructure and surface finish of friction stir welds. Three welds have been produced using the same tool rotation and traverse speed and different tool dimensions and plunging forces. Visual inspection, optical and scanning electron microscopy were undertaken to assess surface and dimensional features as well as to study microstructure. Bend, hardness and tensile tests were carried out to characterise the weld strength. Taken into account the plunging force values of the conducted welds, a relationship between weld properties and calculated welding pressures was created. Favourable welding pressure values are recommended.

Abstract: The goal of this research is to choose the proper method of car body welding. Properties of weld metal deposits depend on many conditions. First of all, this paper attempts to study the role of oxide inclusion sites on the transformation of austenite to acicular ferrite in steel weld metal deposits and their toughness. Safety and exploitation conditions of welded steel structure depend on many factors. The most significant of those factors are connected with materials, welding technology, state of stress and temperature. Because of that a good selection of steel and welding method is crucial to obtain proper steel structure. Car body elements of higher durability are made of low carbon and low alloy steel, very often with small amount of carbon and the amount of alloy elements such as Ni, Mn, Mo, Cr and V in low alloy steel and their welds. In the terms of the kind of steel it is used a proper welding method and adequate filler materials. In the present paper the influence of Mn, Ni, Mo, in WMD on the behaviour of steel structure for low temperature service was tested.

Abstract: The increasing length of oil and gas transportation pipelines, associated with their construction and operating costs, has lead to the development of new steel grades with higher performance. The API 5L- X100 is a new high strength steel for pipeline applications which enables the use of thinner walled pipes, lighter to transport and easier to handle on site, allowing greater operating pressures and reducing overall costs. However, this steel grade has limited ductility. Since advantages largely surpass disadvantages, these materials are being seen adequate for earthquake risk areas and low temperature environment as in the Arctic region. X100 grade is already used in northern Canada and is planed for Japan Sub Sea. Automatic metal arc welding on site is the most common method of welding onshore pipelines in steel grades X65, X70 and X80. The use of high strength steels requires the development of new welding procedures with narrow specifications and the X100 steel has limited weldability. Research is needed to develop appropriate welding procedures, avoiding typical metallurgical problems like cold cracking and toughness reduction in the weld area and to achieve high productivity and economical feasibility. This paper presents results on API X100 steel grade welded by high power fiber lasers. Since these lasers are quite new in the market, an analysis of the laser source, as well as the beam/material interaction is made. The welds produced were investigated for both macro- and microstructural analysis and mechanical properties, contributing to a better understanding of the transformations induced in this material by the thermal cycle associated with laser welding.

Abstract: This study concerns the elemental and microstructural characterization of proto-historic bronze rings from the southwestern Iberian Peninsula. Micro-EDXRF analyses demonstrate that the artifacts are binary bronze alloys (8–13% Sn) with arsenic and lead as the major impurities. Optical microscopy and SEM-EDS allowed the identification of common inclusions (e.g. copper sulphides) and alteration processes (redeposited copper, intergranular and intragranular corrosion). Microstructures consisting of fine dendrites, coarse and/or equiaxial grains were also identified, as well as the presence of (+) eutectoid, deformed inclusions, twinned grains and/or slip bands. The combination of these characteristics allowed establishing the metallurgical procedures (casting, forging and annealing) used in the production of the bronze rings. The identification of different thermomechanical operational sequences indicates that the metallurgical knowledge was well established in the southwestern Iberian Peninsula during those ancient times.

Abstract: In this work, the thermodynamic information on aluminium nitride formation and experimental precipitation kinetics data are reviewed. A revised expression for the Gibbs energy of AlN is developed with special emphasis on microalloyed steel. Using the software package MatCalc, computer simulations of AlN precipitation kinetics are performed and compared to several independent experimental results from literature. To mimic the geometrical arrangement of AlN precipitates along austenite grain boundaries, a new model for precipitation at grain boundaries is used, which takes into account fast short-circuit diffusion along grain boundaries as well as the slower bulk diffusion of atoms from inside the grain to the grain boundaries. This is essential for the calculation of AlN precipitation in austenite where nucleation occurs predominantly on grain boundaries. By studying the AlN precipitation at grain boundaries numerically, and by comparison with experimental data, it is demonstrated that the precipitation kinetics of AlN differs significantly from the simulated precipitation kinetics of randomly distributed precipitates assuming spherical diffusion fields.

Abstract: A reduction from 1.0 to 0.3%Si has recently been shown to improve mechanical properties of H11-type hot work tool steels. The present paper shows that an important improvement in toughness can be explained by the effect of Si content on the precipitation sequence of secondary carbides during tempering after quenching. Carbide particle distributions were observed and identified by electron microscopy, allowing to relate the effect of Si on mechanical properties directly to its effect on cementite and subsequent alloy carbide formation during high temperature tempering.

Abstract: In shape memory alloys (SMA), the texture can be an interesting factor influencing the anisotropic physical and mechanical characteristics during the phase transformations. It is well known that the texture significantly influences the stress-strain curve and shape memory strain of NiTi SMA. The aim of the present experiment was to analyze the textural modifications in the Ti-rich Ni-Ti SMA after annealing at moderate (500°C for 30 min) and subsequent low level of cold work reduction (10% thickness reduction). The textural results were obtained by X-Ray Diffraction (XRD) during thermal cycling in three points: (i) at room temperature (B19’ phase, after cold work), (ii) at 180°C (B2 phase), and (iii) at room temperature (B19’ phase, after cooling from 180°C). The phase transformations were characterized by Differential Scanning Calorimetry (DSC) and XRD.

Abstract: In order to obtain a better understanding of mechanisms governing the microstructural evolution of nickel base superalloys during forging, experimental and numerical studies have been undertaken. For the experimental part, isothermal compression tests were performed at 1100°C for several ratio and strain rates to reproduce microstructural evolution during industrial forging. The resulting microstructures were analysed by Electron Back Scattered Diffraction method and Transmission Electron Microscopy to identify the dynamic recrystallization mechanisms. In parallel, numerical studies has been carried out in which a crystalline plasticity modelling implemented in the finite element code Abaqus® coupled to a recrystallization Cellular Automaton code was used to simulate forging. The first model allows us to obtain the local mechanical fields (strain, stress, crystallographic orientation, dislocation density …) and the second one predicts the dynamic recrystallization.

Abstract: 9 – 12%Cr-MoVNb steels are successful materials for the use at higher temperatures. The universal application in turbines and power plants promises an expedient material for fission and future fusion reactors. Different developmental alloys were fabricated, irradiated and mechanically tested to optimize the material properties. Already, after the first irradiation tests could be shown that this type of steel has a reduced swelling and much faster decay of radioactivity than e.g. austenitic Cr-Ni-steels. This inherent property of reduced activation includes a high recycling potential. Further variations in the chemical compositions were carried out to increase the decay of radioactivity and to reduce the hardening and strengthening by neutron irradiation. The state of development today is a 9%Cr-WVTa-alloy, which is called EUROFER. The post-irradiation behavior and the development of material matrix are shown in comparison with further test alloys. The emphasis is to characterize the hardening mechanisms by irradiation at lower temperatures, which appear in helium bubbles, interstitial loops, and α`-precipitates.

Abstract: The aim of present research was to study the effect of the position of the tool relative to the support backing plate of the FSW machine on the formation of defects and on alterations of the microstructure and mechanical properties of friction stir welds in phosphorus-deoxidised copper (Cu-DHP) thin sheets of 1 mm thick. The welds were carried out using position control conditions; distances between the tool and the backing plate of 0.1 mm, 0.075 mm and 0.05 mm were used. The formation of defects like continuous voids along the weld is very influenced by the tool position, though the heat-input plays an important role in the process. Large grain refinement was observed in the nugget of the welds; the change of the relative tool position has little effect on this grain refinement. Substantial hardening was observed in the thermomechanically affected zone (TMAZ) of the welds. The welds exempt of defects, such as continuous voids, attained a little tensile strength overmatch condition.