Materials Science Forum Vols. 638-642

Abstract: Lubrication in CC moulds controls the shear forces to the sensible strand surface. Since that friction forces are depending on casting speed and the flux behaviour, a study on lubrication have experimentally been carried out when the slag is in contact with liquid steel. Particularly, higher amounts of aluminium in the steel as for pure de-oxidation lead to remarkable redox-reactions and to modified casting slags for the gap between strand and mould. Friction coefficients have been determined and applied to several strand shapes and castings velocities.

Abstract: The production of value added products made of high-alloyed Mn-, Al-, Si-steel grades is coupled to the casting technology. Conventional casting technologies will not fulfill the requirements concerning interaction between steel and casting flux as well as guiding as cast material without stress. Mechanical engineers are invited for further developments in the field of near-net-shape casting technologies to meet above mentioned demands. The Belt Casting Technology seems to be the answer but has to document the potential for an industrial standard process. The main process characteristics of the casting machine are high casting speed, moving mould consisting of a conveyor belt and side dams, complete inertisation and horizontal material flow. The process is already common at the nonferrous metal industry. A labo-ratory caster for steel is located at Clausthal University of Technology.

Abstract: In the present work, TIG and A-TIG welding were compared to show the arc constriction phenomena. A 2D axial symmetric model was developed to simulate the flow behavior in the melting pool. These results were compared to experiments carried out on a stainless steel disc (304L) melted by a stationary heat source. The influence of the Marangoni effect, arc constriction and type of flux is shown. The results point to the importance of the welding parameters and the use of a flux on the characteristics of the weld beads.

Abstract: Socket welds are commonly used to assemble small-bore carbon steel process piping systems because they can be fabricated fairly quickly and are somewhat tolerant of field fit-up issues. In the most simplistic terms, these welds are made by inserting the pipe into a socket and then seal welding using gas tungsten arc welding around the gap between the outside pipe wall and the end of the fitting. This leaves a notch in the root of the seal weld that is open to the process fluid. This notch can act both as a stress riser and a crevice capable of concentrating chemical species. In many process applications, these socket weld notches do not cause any in-service problems. However, in the case of sour service, these notches can become problematic because of the potential for sulphide stress corrosion cracking (SSCC). For SSCC to occur, the right combination of stress, environment, and material susceptibility must be present. Therefore, for socket welds to be used in sour service without the risk of failure, these parameters need to be controlled by proper design and fabrication of the welded joint. Since a crevice is created as a natural by-product of a socket weld geometry, where the SSCC environment will be present, the parameter most readily controlled to minimize susceptibility to SSCC is the microstructure and residual stress of the weld filler metal. As will be shown, this resistance to SSCC can be accomplished by designing a welding procedure specification to include the combination of both a second pass seal weld and by post-weld heat treatment (PWHT). It will also be shown that a complete second pass is crucial to proper tempering and refining the microstructure of the root pass. Without this microstructural tempering and refining by the second pass, the root pass can still undergo SSCC even with subsequent application of PWHT. Examples of SSCC initiating from crack-like weld root defects caused by poor welding techniques and propagating into single-pass socket welds that had been subjected to PWHT will be presented.

Abstract: The effects of post-weld heat treatment on 3.05-mm thick Ti-6Al-4V alloy were investigated using a 4 kW Nd:YAG laser. Two main defects, underfill and porosity, were observed. The use of filler wire reduced underfill defects but slightly increased porosity. No cracks were detected. The as-welded and stress-relief annealed welds had very similar microstructures, hardness, and tensile properties. However, the post-weld solution heat treatment and aging transformed the martensite in the fusion zone into a coarse interlamellar α-β structure, causing a decrease in ductility but a more homogeneous distribution in the hardness of the welds.

Abstract: As a relatively new solid-state joining process, friction stir welding (FSW) may provide a feasible approach to join dissimilar materials such as Mg to Al alloys. In this work, the effects of selected process parameters including work-piece placement, pin tilting angle, and pin location on the quality of dissimilar AA 2024-T3 to AZ31B-H24 butt joints were investigated for the first time. Sound butt joints with low distortion and no solidification cavities or cracks were successfully obtained indicating the potential of FSW to join dissimilar Al to Mg alloys.

Abstract: In the shot peening process, the substrate undergoes large plastic deformation near the surface due to the hit with many shots. A large plastic deformation characterized by a shear droop occurs at the edge of the substrate. When the dissimilar sheets with the edge of the notch geometry are connected without level difference and then the contact area are shot-peened, the sheets can be joined due to the plastic flow generated by a large plastic deformation during shot peening. This method is similar to joining by caulking. The aim of this paper is to investigate the butt joining of high strength steel and dissimilar metal sheets using a shot peening process. The shot velocity and the coverage were controlled in the experiment. The shots used were made of high carbon cast steel and cemented carbide with an average diameter of 0.1 mm. The sheets were high strength steel and aluminum alloys. The influences of processing conditions on the joinability were mainly examined. The joint strength increased with the kinetic energy of shots. Tensile test was also examined to evaluate bond strength. It was found that the present method can be used to enhance the butt joining of high strength steel and dissimilar metal sheets.

Abstract: This work considers the use of Holographic Optical Elements (HOEs) to shape the weld beam and control the microstructure of the weld bead. The beam profiles investigated are a standard Gaussian and an Offset Rugby Post produced by a HOE. Autogenous welds have been produced on plain carbon steel with the introduction of a nickel alloy filler powder, using different energy densities. Cross sections of the welds have been analysed in terms of the weld profile, weld pool shape, HAZ and the extent of the deposit/substrate mixing. Electron BackScatter Diffraction (EBSD) coupled with Energy Dispersive X-ray Spectroscopy (EDS) has been used to study the microstructures developed. The results have shown that by utilising HOE’s the weld pool shape can be modified so that a squarer profile can be obtained. The grain structure within the weld pool can be controlled such that a finer more equiaxed grain structure can be developed when compared with the coarse columnar grains seen with a Gaussian beam with a marked difference in the microstructures in the HAZ.

Abstract: PD7777 published by British Standard Institute in 2000 proposes an additional fracture requirement to the main steel components of the low temperatures storage tank where a partial height hydrostatic test is allowed instead of the full height hydrostatic test required in BS 7777. In the PD7777 a high level (75J) of fracture toughness is required for the austenitic weld metal in 9%Ni steel plate to prevent the re-initiation of ductile fracture from the arrested brittle crack in the weld. This is to report a study that the J-Cv correlation of austenitic weld metal is determined by the experimental data obtained from the actual weld joints in 9%Ni steel plate in order to assess a rational toughness requirement to the austenitic weld metal employed in the 9% Ni made LNG storage tanks. From this study it is concluded that fracture toughness of 75J is too conservative and 50J is adequate. Further some FEM analyses were performed to verify the validity of the weld surrounded by the large amount of heterogeneous base metal in the yield strength distribution.

Abstract: The effect of oxygen content on toughness in the high strength weld metals with full martensitic microstructures was investigated for Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW). Solid and Flux cored two types wires were examined for their influence on the resulting oxygen content in weld metals. It has succeeded in controlling the oxygen contents without changing welding processes. As expected, the increasing oxygen content obviously decreased the upper shelf energy (vEshelf). One of the primary reasons of this tendency is considered that the higher density of oxide makes dimples on the ductile fracture surface smaller. On the other hand, as unexpected, the oxygen content from 7 to 450 ppm had no impact on the fracture appearance transition temperature (FATT), and oxides at the brittle fracture initiation point have not been found. This result supports that the oxides in high strength full martensitic weld metals (Vickers hardness = 360 ~ 430) have no harmful effect on FATT.